PROCEEDINGS SECOND SYMPOSIUM on the GEOLOGY of the SOUTHEASTERN COASTAL PLAIN Edited by Daniel D. Arden, Barry F. Beck, and Eleanore Morrow Sponsored and hosted by Earth Science Department Georgia Southwestern College Americus, Georgia March 5-6, 1979 Published by Department of Natural Resources Environmental Protection Division Georgia Geologic Survey 53 INFORMATION CIRCULAR ~ , The papers prese nted in this Information Ci rcul ar represe nt the opinions of the respective authors. No manuscripts were revie wed by the manageme nt of t he Georgia Geologic Survey for technical accuracy. The Georgia Geologic Survey does wish to stress that the petroleum potential for those strata underlying the veneer of Coastal Plai n deposits is not addressed in this volume. PROCEEDINGS SECOND SYMPOSIUM on the GEOLOGY of the SOUTHEASTERN COASTAL PLAIN Edited by Daniel D. Arden, Barry F. Beck, and Eleanore Morrow Atlanta 1982 Sponsored and hosted by Earth Science Department Georgia Southwestern College Americus, Georgia March 5-6, 1979 Published by Department of Natural Resources Joe D. Tanner, Commissioner Environmental Protection Division J. Leonard Ledbetter, Director Georgia Geologic Survey William H. Mclemore, State Geologist 53 INFORMATION CIRCULAR SYMPOSIUM COMMITTEES Daniel D. Arden, General Chairman Technical Program : Barry F. Beck Harland E. Cofer Publicity and Printing : Barry F. Beck Richard Little Housing A. Richard Henderson Frank B. Jones Local Arrangements: J. Philip Manker Walter K. Mathews James W. Russell Smoker: A. Richard Henderson William Tietjen Registration ; Dowe P. Ca rter Jack C. Carter 0. Jay Cliett Max T. McKinney TABLE OF CONTENTS Page I. STRATIGRAPHY AND PALEONTOLOGY Kevnote Paper Biostratigraphy, sea level fluctuations, subsidence rates, and petroleum potential of the Southeast Georgia Embayment. C. Wylie Poag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Palynostratigraphy of the basal Cretaceous un1ts of the eastern Gulf and southern Atlantic Coastal Plains. . Raymond A. Christopher. .. .. . . . . . . . . . . .. .. . ... . .. . . . .......... . . .. .. . ..... 10 Paleoenvironment and age of kaolin deposits in Andersonville , Georgia, district. Harland E. Cofer, Jr. and Norman Frederiksen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 o. Late Mesozoic and Paleocene geology of the Georgia Coastal Plain. Howard R. Cramer and D. Arden . . 38 Sedimentology and paleoenvironmental analysis of the Upper Cretaceous Tuscaloosa and Eutaw Formations in western Georgia. William J. Frazier .. .. . .. . . .. .......... . .. . . . ..... . ... . . . ..... 39 Paleocene to Middle Eocene depositional cycles in eastern Alabama and western Georgia. Thomas G. Gibson ....... .... . ..... . . . . . . .. ....... . ....... ... .... . .... .. ..... ... . . . . .. . . 53 A stratigraphic framework for Cretaceous and Paleogene margins along the South Carolina and Georgia coastal sediments , G.S. Gohn, L.M. Bybe/1, R.A. Christopher, J.P. Owens, and C.C. Smith ... 64 Upper Eocene stratigraphy of eastern Georgia. Paul F. Huddlestun and John Hetrick . . .... .. . . . . . .... 7 5 Upper Jurassic Smackover petroleum geology of southwest Alabama. Ernest A. Mancini ........ ..... . .. . ... . . . . . . . . ..... . ..... .. . .. . . ..... . . . ... . .. . .. .. ... ... 76 Structural control of Jurassic sedimentation in Alabama and Florida. James A. Miller . . . . . ............. 88 Foraminiferal biostratigraphy and paleoecology of the marls associated with the Tuscahoma Sand (Paleocene/Eocene) of southwest Alabama. Gary E.' Oliver . . . .. .... . . .. . .... . .. .. . .. .. ..... . . . 88 Lithofacies and depositional cycles in Upper Cretaceous rocks, central Georgia to eastern Alabama . Juergen Reinhardt .. .. . . . .. . . . . .. . .. . ... . .................... .. . . .. . . .. . ... . . .. .. .. .... . 89 Micropaleontology ~f the Twiggs Clay, Georgia Coastal Plain . Walter Schmidt and S. W. Wise, Jr. ................... . . . ... . ...... . ..... . . . ...... . ... . . . . .. 97 Sedimentological tools for identifying depositional environments . William F. Tanner ............. . . .. .114 Environmental implications of palygorskite (attapulgite) in Miocene of the southeastern United States, Charles E. Weaver and Kevin C. Beck . .. . . . ...... ... ... . ... .... . .... . ...... . ... ... ... . . .. . .118 Depositional environments in the Jurassic Norphlet Formation in south. Alabama. Richard P. Wilkerson ... ..... .... . ... . ... . ... . . .. ..... . . .. . . . ... ... . .. . ..........126 (continued) iii Page II. HYDROLOGY AND ENVIRONMENTAL GEOLOGY Keynote Paper Ground water from the Coastal Plain for the future. Philip E. LaMoreaux .... . .... . .. . ...... .. ... ... 132 Hydrochemical processes in Coastal Plain aquifers. William Back, Mary Jo Baedecker, and Warren Wood ....................... ; ... ... . ..... ..... ........... .... ...... .. . ..... . . . 136 Environmental geologic mapping in Florida. Ernest W. Bishop and Walter Schmidt . . ........ . . ...... . 137 Development and implementation of water resource investigations utilizing surface resistivity geophysi- cal techniques in the Suwannee River Water Management District, Florida. David W. Fisk ......... 138 Hazardous waste disposal in the southeastern United States-an imminent problem . Richard C. Fountain and Arthur W. Hayes .................................. .. . . . ..... . .... 139 The geohydrology of the Gulf Trough-a preliminary investigation. CarolS. Gelbaum and Ju)ian Howell ..... .. .. . .... ..... ... ....... .. ........ .. . . .. . ............ .... ...... .. . . . 140 Geological evaluation of potential pipeline corridor sites along the Georgia coast. Vernon J. Henry, .. Robert T. Giles, and James L. Harding .. . . ... . ....... . ...... . .. . ........... . ... . . .... ..... 154 Relict fresh ground water of the U .S. Atlantic contintental shelf: and unevaluated buffer in present-day saltwater encroachment. Francis A. Kohout .................................... I 70 Altitude of the Floridian aquifer in Northwest Florida. Tom Kwader ... ............... . ..... .. .. .... 177 Long-term hydrologic stability of Gulf Coast salt domes and mines. Joseph D. Martinez and Madhurendu B. Kumar ............. .. . .... . .... . ........ . ......... . 177 The stratigraphy of the Florida-Hatteras shelf and slope and its relationship to the offshore extension of the principal artesian aquifEH. Charles K. Paull and William P. Dillon ............. .. .. ... . .... 178 Geologic hazards and constraints to petroleum exploration and development on the southeastern U.S. Continental Shelf, slope, and Blake plateau . Peter Popenoe .................................. 182 Geological investigations for dredging operations, FBM Submarine Support Base, Kings Bay, Georgia . Earl R. Titcomb . ..................................................................... 182 Aquifer potential of the shallow sediments of the coastal area of Georgia, Thomas W. Watson . ......... 183 Ill. STRUCTURE, TECTONICS, AND GEOPHYSICS Pre-Cretaceous rocks beneath the Georgia Coastal Plain. Timothy M. Chowns .... .. ..... ........... 196 Drainage patterns, buried volcanic necks, and plate tectonics in the south Georgia Coastal Plain. James E. Clark, Jr. ......................................................... . ........... 196 Heat flow and the geothermal resource potential in the eastern United States. John K. Costain ........ 196 Summary of development of the continental margin off Georgia based on multichannel and single- channel seismic-reflection profiling and stratigraphic well data. William P. Dillon and Charles Paull .197 Geochemistry of pre-Cretaceous rocks beneath the Coastal Plain of Georgia and South Carolina. David Gottfried and Michael W. Higgins ................................................... 201 Joints and minor faults in the Georgia Coastal Plain . Michael W. Higgins and Sam M. Pickering, Jr. .................................. ..... ................ . ........... 201 Seismicity in Georgia. Leland Timothy Long ................................................... 202 Hydrochemistry of formation fluids in onshore and offshore strata in the Southeast Georgia Embayment: Frank T. Manheim and Charles K. Paul/ ... .. ......... ..... ..... . .... .. . ....... .. .. ........ 211 The effects of Cretaceous and younger faulting on Coastal Plain rocks around Augusta, Georgia. David C. Prowell ... ... .... .. . ...... . . . .................. . . . ... . ....... . ....... ......... 219 iv INTRODUCTION Five years ago, the Earth Science Department at Georgia Southwestern College sponsored a "Symposium on the Petroleum Geology of the Georgia Coastal Plain." The symposium was well attended and the proceedings volume , which was published by the Georgia Geological Survey as Bulletin 87, was so well received that it quickly sold out and has been out-of-print for several years. Enco uraged by th at success and spurred by the initiation of offshore drillin g In ttle Southeast Georgia Embayment, we decided that 1979 was a g o o d tl me. tp re vi ew o ur kn o wl edge of A tlantic and Gulf Coastal Plain geology. Recent exp loratory dri lling and seis mic prospecting have provided much new data wit h 'w hich to in terp ret t he comp licated picture of subsurface coastal plain stratigraphy and basement structure. The increasing population and industrial expansion of the Atlantic and Gulf Coastal Plains, however, have als o p laced ne w st ra in s o n the geoh ydrologic system and created a multiplicity o f prob lems in env ironme ntal g eo logy which were only anticipated five year s ago. Thu s, the 1979 sym posiurn was not confined to petroleum geology, but was expa nded by the add iti on of a larg e, well-attended session on environmental geology and hydrology. Geologic investigations in the Southeast are continuing at a high level of activity , and new Info rmat ion is constantly developing which indicates that our past con ce pts of t he stratigraphy and structure of this region may have been greatly o versimplifi ed . We predict that the next symposium will have even greater sc op e th an t he 1979 session. Daniel D. Arden, Barry F. Beck , and Eleanore Morrow - The Editors I. STRATIGRAPHY AND PALEONTOLOGY 2 Keynote Paper BIOSTRATIGRAPHY, SEA LEVEL FLUCTUATIONS, SUBSIDENCE RATES, AND PETROLEUM POTENTIAL OF THE SOUTHEAST GEORGIA EMBAYMENT C. Wylie Poag U.S. Geological Survey Woods Hole, Massachusetts ABSTRACT The foraminiferal record in the COST GE-1 well, Southeast Georgia Embayment, permits the recognition of discrete biozones, paleoenvironments, and depositional hiatuses, and thereby provides a basis for estimating the magnitude of sea-level fluctuations, sediment accumulation rates, and subsidence rates of the Georgia-Florida continental shelf. The age of sedimentary rocks in the GE-1 well ranges from Early Cretaceous (Valanginian?) to Pleistocene, and a variety of paleoenvironments are represented (e .g., terrestrial, continental shelf, upper continental slope). Eight major hiatuses represent intervals of erosion and nondeposition and are correlative with low stands of global sea level. Conversely, the deepest water paleoenvironments correspond to high stands of global sea level. Generally, the rates of sediment accumulation fluctuated in unison with changing paleoenvironments. For example, accumulation was most rapid (5 .0 to 6.4 cm / 1000 yrs) in terrestrial or shallow marine biotopes that developed during the Early Cretaceous, the middle and late Eocene , and the middle Miocene. Accumulation rates were slowest (1.3 to 2.5 cm/1000 yrs) in deep marine environments during the Late Cretaceous, early Oligocene and Pleistocene. Two complementary methods of calculating subsidence rates at the GE-1 well site indicate the significant effect of sediment loading. Periods of most rapid subsidence correspond to periods of rapid sediment accumulation, whereas periods of uplift of the shelf correspond to episodes of erosion or nondeposition. Subsidence was greatest during the Cretaceous and early Tertiary, and uplift has been dominant since the middle Oligocene. A poor potential for significant petroleum production in the Southeast Georgia Embayment is attributed to the low geothermal gradient coupled with the shallow burial of potential source beds that resulted from the numerous intervals of erosion and nondeposition during Cenozoic time. INTRODUCTION The first deep stratigraphic test well (COST G E-1) in the Southeast Georgia Embayment was drilled by the Ocean Production Co. between February 22 and May 31, 1977. The well site (fig, 1) is located approximately 74 nautical miles east of Jacksonville, Fla., at lat. 30 37'08" N and long . 80 17'59" W near the edge of the continental shelf (water depth,41.5 m; total depth of well,3966 m). Two preliminary reports concerning the structure, stratigraphy, petrography, organic chemistry and petroleum potential of the GE-1 well have been published by the U.S. Geological Survey (Amato and Bebout, 1978; Scholle , 1979) . The purpose of this report is to review the stratigraphic sequence penetrated by the GE-1 well, to present paleoenvironmental interpretations, to relate these results to global sea-level cycles and to basin subsidence, and to comment briefly on the petroleum potential of the Southeast Georgia Embayment. GEOLOGIC SETTING The COST GE-1 well was drilled on the western side of a narrow basement ridge that separates the Triassic rocks of the Southeast Georgia Embayment from Triassic and Jurassic rocks in the Blake Plateau Basin (Dillon and others, 1979, and this volume ; my figs. 1, 2). However, the Cretaceous and Cenozoic rocks penetrated by the GE-1 well are structurally continuous with their equivalents in the Blake Plateau Basin. Seismic profiles show that the 3279 m of Cretaceous and Cenozoic rocks penetrated by the G E-1 well are part of a flat-lying, structureless sequence of evenly bedded rocks that crosses the basement ridge without perturbation and thins gradually landward and basinward from the GE-1 well (Dillon and others, 1979, and this volume; my fig. 2) . The basement ridge was penetrated a total of 687 m and consists of metamorphosed sedimentary and igneous rocks interbedded with volcanic rocks. Radiometric dating of this metamorphic basement yielded an average date of 355 3 m.y . (Late Devonian; Simonis, 1979) . The COST GE-1 well, located in the east central part of the Southeast Georgia Embayment, is the only offshore drill hole in the embayment that has penetrated pre-Cretaceous rocks. Cenozoic rocks, however, have been sampled in three additional core holes located toward the northern, western, and southern margins of the embayment (AMCOR 6002, JOIDES 1, and JOIDES 2; my fig. 2; Schlee, 1977; Poag and Hall, 1979). 3 Figure 1. Location of drill holes and major geologic structures superimposed on bathymetric map of the Georgia-Florida continental margin. (COST = Continental Offshore Stratigraphic Test; AMCOR = Atlantic Margin Coring Project; J = JOIDES= Joint Oceanographic Institutions for Deep Earth Sampling) . 1------- ~25 Km - - - -- - - i w E GE-l Figure 2. Schematic stratigraphic section based on a seismic reflection profile through the COST GE-1 well. SOUTHEAST GEORGIA EMBAYMENT BLAKE ~ PLATEAU .._____)" BASIN -- ----- 4 STRATIGRAPHY The stratigraphic record of the COST GE-1, AMCRO 6002, JOIDES 1 and JOIDES 2 bore holes has been described on the basis of micropaleontology, lithology, and petrography (Amato and Bebout, 1978; Poag and Hall, 1979; Valentine, 1979; Rhodehamel, 1979; Halley, 1979). The sedimentary section at COST GE-1 consists of 1000 m of Cenozoic rocks resting disconformably on 700 m of Upper Cretaceous rocks, which in turn lie disconformably on 1600 m of Lower Cretaceous rocks (fig . 3). Two thirds of the Cenozoic section is Eocene in age; at least six regional hiatuses represent interruptions in the deposition of the remaining Cenozoic strata (fig. 3) and can be traced regionally in the other core holes and by means of seismic reflection profiles (Pauli and Dillon, 1979; Poag and Hall, 1979). All of the Upper Cretaceous stages except the Cenomanian are represented in the COST GE-1 well . However, an erosional interval in the early Paleocene apparently removed the upper Maestrichtian; the lower Coniacian and upper Turonian rocks also are missing . Microfossils as old as Aptian in age have been identified in the GE-1 well (Amato and Bebout, 1978; Poag and Hall , 1979; Valentine, 1979) . 1have extrapolated sedimentation rates in order to estimate Barremian through Valanginian ages for the lowest Cretaceous rocks. These estimates agree with the seismic interpretation that Jurassic rocks are not present at this well site (Dillon and others, 1979) . AGE LITHOLOGY L :I:MES'lO>IE white, tan, dolomitic, cherty CU\.YS"'':M: , foUDS'IONE' (f) (CHALK? ) w0::: gray Iw - ~ ~ SANilS'IIl'lE' LIMES"'UUE OOI.CMITE, z E.Vl\PORITE 0 I- gray - LLJ IJ) IJ) EUROPEAN STAGE MAESTRICHTIAN PROVINCIAL STAGE NAVAR ROAN POLLEN ZONE CA-6 I MA-l UPPER z ~ z <( 0.. ::;; LOWER <( u IJ) :::l 0 LuLJ <( I- LLJ au:: SANTONIAN a:: IJ) LLJ :::l 0.. 0 0.. LLJ u :::l <( I- LLJ a:: u CONIACIAN TURONIAN CENOMANIAN IJ) :::l 0 LLJ z u ~ <( Ill I- __J LLJ <( a:: u UPPER MIDDLE. LOWER a:: LLJ it APTIAN 0 TO BARREMIAN __J TAYLOR AN AUSTINIAN EAGLEFORDIAN ? WOODBINIAN ? WASHITAN ? FREDERICKSBURGIAN 1 TRINITIAN 1 NUEVO LEONI AN CA -5 CA -4 CA -3 CA -2 -c v B 1--- A IV Ill c II B A I Figure 2. Palynological zonation of the Cretaceous System of the middle Atlantic states. Palynological characteristics of zone IV in the Coastal Plain of the southeastern United States In general, assemblages from pollen zone IV in both outcrop and subsurface samples contain an abundance of moderately to well-preserved palynomorphs, consisting of a wide variety of pteridophyte spores, gymnosperm pollen, angiosperm pollen, and in samples from Alabama, a few dinoflagellate cysts and acritarchs. Many of these forms range into younger zones and are not considered biostratigraphically useful. The forms discussed below were not all found together in every assemblage examined from zone IV, but all have been found useful guides to the zone at more than one locality. The biostratigraphically significant palynomorphs from zone IV of the eastern Gulf and southern Atlantic Coastal Plains are: Nevesisporites sp. (pl. 1, fig. 1). This species is characterized by a narrow cingulum, laesurae that bifurcate at their ends, and thin, closely spaced cicatricose striae on the proximal surface that are oriented obliquely to the rays of the laesurae. Nevesisporites sp. occurs with a low relative frequency in all samples in which it has been observed, but its distinctive morphology and consistent occurrence make it a potentially useful guide to zone IV. Equisetosporites spp. and Welwitschiapites sp. (pl. 1, figs. 2-7). Although only a few specimens of Equisetosporites (pl. 1, figs . 2-5) have been observed in samples from zone V, polyplicates, represented by these two genera, are more diverse and generally more abundant in zone IV than in the overlying zones. In addition, Welwitschiapites sp. (pl. 1, figs. 6, 7), with its numerous thin ribs ornamenting its outer exinal layer, appears to be restricted to zone IV, especially in samples from the outcropping Tuscaloosa Group (or Formation). Carolina torosus (Reissinger) Klaus 1960 emend. Cornet and Traverse 1975 (pl. 1, fig. 8). This species is consistently present in samples from zone IV. In the stratigraphically higher zones, however, Carolina is rarely found, and where present, the genus is represented by a morphotype that is one-half to three-quarters of the diameter of those from zone IV. Fraxinoipollenites rotundus sensu Phillips &Felix 1971 (pl. 1, fig. 9), "Retitricolpites" geranioides Couper sensu Brenner 1963, (pl.1, figs. 10, 11), and Tricolpites wilsonii Kimyai 1966 (pl. 1, fig. 12). These forms are among the more commonly occurring and morphologically distinctive tricolpate and tricolporates pollen types that are restricted to zone IV. ?Ajatipol/is tetraedralis (Bolchovitina) Krutzsch 1970 (pl. 1, fig. 13). A species of small permanent tetrad with poroid apertures arranged according to "Garsides Law" appears to be restricted to zone IV and therefore serves as a guide to that zone. The species closely resembles the form described as Ajatipol/is tetraedralis (Bolchovitina) Krutzsch 1970, which has been referred to as cf. Ajatipollis tetraedralis by Doyle and Robbins (1977). Atlantopol/is verrucosa (Groot & Groot) Goczan, Groot, Krutzsch & Pacltova 1967 (pl. 1, fig. 14) and Complexiopol/is spp. (pl. 1, figs. 15-21). The most characteristic microfloral elements of zone IV are the distinctive Normapolles forms referred to the general Complexiopollis and Atlantopol/is. Atlantopollis is represented by a single species. A verrucosa (Groot & Groot) Goczan, Groot, Krutzsch & Pacltova 1967 (pl. 1, fig. 14). This species has not 12 been recorded from zone V. Most of the species of Complexiopo/lis from zone IV are as yet undescribed, the single exception being C. funiculus Tschudy 1973 {pl. 1, fig. 16). Although the genus Complexiopollis occurs throughout zones IV and V, most species from the two units can be clearly differentiated on the combined basis of two morphologic characteristics. First, the majority of Complexiopo/lis species from zone IV exhibit some type of low sculptural elements (pl. 1, figs. 15, 18, 19); typically, this type of sculpturing is absent in species from stratigraphically higher zones (see pl. 2, figs. 23-27). Second, the exine of most species from zone IV possess a discernible columellate layer, at least in the region of the exogerminal (.pl. 1, figs. 15-21 ), if not around the entire grain. This columellate layer, if present, is not visible with light microscopy in species from zone V. The only species of Complexiopo/'lis from zone IV that has been observed in zone Vis C. funiculus (pl. 1, fig. 16). although it is not known if the species ranges throughout the zone. Palynological characteristics of zone V in the Coastal Plain of the southeastern United States Pteridophyte spores and gymnosperm pollen are found infrequently or sporadically in zone V, thereby limiting their biostratigraphic utility. Angiosperm pollen is much more diverse and has a higher relative frequency in zone V than in zone IV. As a result of these two factors, I have based my palynological characterization of zone V exclusively on angiosperm pollen; more specifically, it is characterized by the occurrence of several morphologically d isti net tricol pates, tricol porates, tri porates, and Normapolles: Nyssapo/lenites sp. (pl. 2, figs. 1, 2). The species is characterized by its tetrahedral shape, subtriangular amb, micropunctate surface, and nexine that thins abruptly at the colpi margins to produce a distinctive "border" surrounding the colpi. Doyle (1969a, fig. 5h, i) illustrated a similar form from the "Cliffwood beds" of the Magothy Formation of New Jersey, which he referred to as "Tricolporate type 5." Porocolpopollenites spp. (pl. 2, figs . 3-6) . At least two species of Poroco/popollenites can be delineated in zone V. Both forms are characterized by their oblate shape, circular amb, brevitricolporate apertures, scabrate surface, and a weJI-developed "postvestibulum" at each aperture. P. sp. A (pl. 2, figs. 5, 6) is the smaller of the two morphotypes, has an equatorial diameter that ranges from 20 to 30J.J.m, and an exine that is 1.5 J.J.m thick. The equatorial diameter of P. sp. B (pl. 2, figs, 3, 4) ranges from 30to 45 11m, and its thinner exine measures 1 Jlm. Porocolpopollenites sp. A and P. sp. B have previously been reported from the Eutaw Formation of western Alabama by Leopold and Pakiser (1964) as Porocolpopollenites orbiformis Thomson & Pflug 1953, and Porocolpopollenites Thomson & Pflug 1953 (some unnamed species), respectively. Doyle (1969a) and Doyle and Robbins (1977) illustrated a form similar to Porocolpopo/lenites sp. B from the South Amboy Fire Clay Member of the Raritan Formation of New Jersey. Tricolporate type 34 (pl. 2, figs. 7, 8). A wide range of morphologic variation is displayed by the forms referred to here as Tricolporate type 34. All are oblate, and have a triangular amb and reticulate ornamentation. The equatorial diameter of most forms is relatively uniform, ranging from 18 to 23 J1m. However, the nature of the reticulum varies considerably . The lumina may be small and circular, or large with irregular or polygonal shapes; the muri may be thick or thin, low or high. At present, I do not know if it would be advantageous to treat the morphotypes as a single species or as a series of closely related species because sufficient stratigraphic work has yet to be carried out. Forms similar to Tricolporate type 34 were reported by Leopold and Pakiser (1964, pl. 8, figs. 73-77) from the Eutaw Formation of western Alabama as Pollenites kruschi (R. Potonie) "asp. pseudolaesus" (R. Potonie) Thomson & Pflug 1~53. Doyle (1969a, fig. 5j, k) also illustrated a similar form from the Magothy Formation of New Jersey, which he referred to as "Tricolporate type 6." Cupuliferoidaepol/enites sp. (pl. 2, fig. 9). This species, with its unsculptured exine is characterized by its sexine, which is thickest in the mesocolpia and thins toward the colpi margins. Cupuliferoidaepollenites sp. is generally a rare element of the zone V microflora. Tricolpites sp. (pl. 2, figs. 10, 11). This species, with its finely reticulate sexine, shows a nexine that is thickest in the mesocolpia, thinning toward the colpi margins. The species is common to abundant in assemblages from zone V. Myrtacidites sp. (pl. 2, figs. 12, 13). This species lacks surface ornamentation, and characteristically shows a separation of nexine and sexine in the vicinity of the pore. Myrtacidites sp. is present in most samples from zone V, but whereas it is a common element in samples from the basal part of the zone, it is rare in samples from the upper part. 13 Tricolporate type 69 (pl. 2, figs. 14, 15). Like Myrtacidites sp., this species exhibits a distinctive exinal structure, with a relatively thick nexine separated from a thin sexine by a distinct columellate layer. In most specimens, the columellate layer is thickest near the aperture margins. Tricolporate type 69 is common in samples from the basal part of zone V. but is rare in the upper part of the zone. Holkopol/enites spp. (pl. 2, figs. 16-22). I have recognized several morphotypes of this genus in zone V, the most commonly occurring of which is a large (25to 30J.Lm) form with a thick nexine in which the channeling of the nexine is more random than in typical representative of the genus (for example, the type species. Holkopollenites chemardensis Fairchild in Stover, Elsik & Fairchild, 1966). This species closely resembles the morphotype referred to by Wolfe (1976) as CP3E-1 from the Campanian-aged Merchantville, Woodbury, Englishtown, Marshalltown,, Wenonah, and Mount Laurel Formations of the middle Atlantic states. Although the forms from zone V of the southeastern Coastal Plain may eventually prove to be conspecific with those of Wolfe, the amb of the specimens from zone V appear to be more triangular than those described by Wolfe. Other, less commonly occurring forms of Holkopollenites include smaller oblate forms with a triangular amb (pl. 2, figs. 20-22). All species of Holkopollenites occur throughout zone V. Triporate and Normapolles forms (pl . 2, figs. 2327; pi. 3, figs. 1-23). The most dramatic difference between the palynological composition of zones IV and Vis seen in the triporate and Normapolles elements of the microflora. These pollen types are more abundant and more diverse in zone V than in zone IV, with only one species occurring in both zones [Complexiopo//is funiculus Tschudy 1973 (pl. 1' fig.16)]. The genus Complexiopollis is the most commonly occurring Normapolles in zone V. and is represented by a wide variety of forms (pl. 2. figs. 23-27). Most of these species can be differentiated from those in zone IV by their reduced or lack of ornamentation, and by the absence of a discernible columellate layer. Many of these forms range into higher biostratigraphic zones; with the exception of Complexiopo//is abditus Tschudy 1973 {pi 2. fig . 24) and C. sp. D (pl. 2, fig. 25); however, none have as high a relative frequency as they do in zone V. Leopold and Pakiser (1964) illustrated several of these Complexiopollis species from the Eutaw Formation of western Alabama (their pl. 8, figs. 21-44, 47-52); Doyle (1969a) illustrated three forms from the South Amboy Fire Clay Member of the Raritan Formation and from the Magothy Formation of New Jersey (his fig. 4a, c, d); Doyle and Robbins (1977) illustrated two forms from the South Amboy Fi1e Clay of New Jersey (their pi 7, figs. 21, 23-25). In addition to Complexiopo//is, a number of other Normapolles and triporate forms are commonly found in zone V, but are absent in zone IV. Among these are Pseudoplicapollis spp. {pl. 3, figs. 1-4), Minorpo//is spp. (pl. 3, figs. 5-7), Santalacites sp . (pi 3, fig. 8), Momipites spp. (pl. 3, figs. 9-12), and several representatives of an undescribed genus characterized by its convexly triangular amb, small, weakly developed annuli, and a narrow interloculum or "Schichtfuge" (pl. 3, figs. 13, 14). Other Normapolles that occur as very rare elements in zone V include Plicapo//is spp . (pl. 3, figs. 15, 16), Praecursipo//is plebius Tschudy 1975 (pl. 3, figs. 17, 18), Labrapollis spp. {pl. 3, figs. 19-21 ), Trudopo//is sp. (pl. 3, fig. 22), and Osculapo//is sp. (pl. 3, fig. 23). Age of pollen zones IV and V In all respects, the basal palynologic zone recognized in the eastern Gulf and southern Atlantic Coastal Plains fits the criteria for inclusion in pollen zone IV (fig. 2) of Doyle (1969a; Doyle and Robbins, 1977). This zone has also been referred to as the Complexiopo//is Zone by Habib (1977) and the Complex iopol lis-A tlantopo/1 is A sse m b Iage-Zone by Christopher (in press). Pollen zone IV has been considered as old as middle Cenomanian (Wolfe and Pakiser, 1971; Christopher, 1977b), and as young as early Turonian (Doyle, 1969b; Doyle and Robbins, 1977) . However, recent unpublished palynological investigations of the Eagle Ford Group of Texas suggest that zone IV is present within its Britton Formation, and corresponds to the Eaglefordian part of the Rotalipora cushmani-greenhornensis subzone (foraminifers) of Pessagno (1969). Pessagno considered the R. cushmani-greenhornensis subzone to be late Cenomanian in age, and it follows that pollen zone IV is also late Cenomanian. A comparison of the microfloral assemblages from zone V with those described by Leopold and Pakiser (1964) from the McShan and Eutaw Formations of western Alabama suggests a strong biostratigraphic correlation between these units. I conclude, therefore, that there is very little, if any, age difference between the McShan and Eutaw Formations at their type localities in western Alabama and zone V reported on here. Leopold and Pakiser (1964) considered the Eutaw Formation to be pre-Senonian in age, on the basis of the absence of structurally advanced Normapolles that typify the Senonian of Europe. They suggested that the McShan and Eutaw Formations, together with the underlying Tuscaloosa Group, are of preSenonian (latest Cenomanian and Turonian) Age, and (or) of Coniacian Age. 14 In terms of its microfloral composition and relative frequencies of occurrence of the biostratigraphically significant pollen types, zone V of the eastern Gulf and southern Atlantic Coastal Plains bears some similarity to pollen subzone V-A (fig. 2) as described by Christopher (1977a, b; in press) . On the basis of the concurrent ranges of selected Normapolles genera in New Jersey and Europe, Christopher considered subzone V-A to be late Turonian in age, which would indicate a similar age for zone V described in this report. However, recent unpublished investigations of the Cretaceous System of Texas suggest that biostratigraphic equivalents of the Eutaw Formation (that is, zone V reported on here) occur in the Austin Group of Coniacian and Santonian Age, rather than in the Eagle Ford Shale of late Cenomanian and Turonian Age. On the basis of studies of calcareous nannofossils, Charles C. Smith (personal commun., 1978) supported an Aus~ tinian equivalency of the Eutaw Formation, and further suggested that the Eutaw Formation of eastern Alabama and western Georgia may be early to middle Santonian in age. In summary, the basal or older biostratigraphic zone described in this report can be correlated with pollen xone IV of late Cenomanian (middle Eaglefordian) Age, and the upper or younger zone with subzone V-A of early to middle Santonian (early to middle Au~tinian) Age. In the eastern Gulf and southern Atlantic Coastal Plains, these zones appear to be separated by a hiatus representing the Turonian and all but possibly the uppermost part of the Coniacian (upper part of the Eaglefordian and basal part of the Austinian). THE RELATIONSHIP OF ZONES IV AND V TO THE BASAL OUTCROPPING AND SUBSURFACE LITHOLOGIC UNITS OF THE EASTERN GULF AND SOUTHERN ATLANTIC COASTAL PLAINS The updip section Palynologic examination of a number of samples from near the fall line between Alabama and North Carolina (that is, sample localities 1-14, fig. 1) reveals the following about the relationship between pollen zones IV and V and the basal outcropping lithologic units of the area: 1. Zone IV occurs in the outcropping Tuscaloosa Group (or Formation) between Alabama (localities 1 and 2) and extreme western Georgia (locality 4), whereas the Eutaw Formation of this region can be placed in zone V (locality 3; data from Leopold and Pakiser, 1964) . 2. Throughout central and eastern Georgia (localities 5-9), outcropping units mapped as Tuscaloosa contain assemblages that can be assigned to zone V. Even in the shallow subsurface of the Savannah River area, as revealed in samples from a core drilled on the Savannah River Nuclear Power Plant (locality 10), the basal Coastal Plain deposits can be placed in zone V. 3. The Cape Fear (locality 13), the Middendorf (localities 11 and 12), and the basal part of the Black Creek Formations of the Carolinas (locality 13) all contain zone V pollen assemblages. 4. Only in northernmost North Carolina are zone IV assemblages again found in the updip section. The assemblages were found in a shallow core drilled in Halifax County, North Carolina (locality 14); they occur in a lithology that Brown and others (1972) assigned to their Unit F, which they regarded as Fredericksburgian to Washitan in age. It is not surprising that two distinct pollen zones can be recognized in the Tuscaloosa Group (or Formation) and Eutaw Formation of Alabama and extreme western Georgia, as lithologic and other nonpalynologic paleontologic differences between the marginal or nonmarine Tuscaloosa and the marine Eutaw suggest two disconformable periods of deposition (Eargle, 1946, 1948; Monroe and others, 1946; Applin and Applin, 1947; Conant, 1967; Brett, 1967; Drennen, 1953; Stephenson, 1956; Sohl, 1964, and others). However, throughout central and eastern Georgia, the basal outcropping Coastal Plain units are nonmarine in origin, and for this reason they have been traditionally mapped as, and correlated with, the Tuscaloosa of Alabama. Recent palynologic investigations (Scrudato and Bond, 1972;Cousminer,1973, Cousminer and Terris, 1973; Tschudy and Patterson, 1975; this study) suggest that the "Tuscaloosa" of central and eastern Georgia includes lithologies that range in age from Santonian (i.e., zone V of this report) to middle Eocene. No zone IV assemblages have been reported from this unit, and biostratigraphic correlation of the "Tuscaloosa" of central and eastern Georgia and the Tuscaloosa of Alabama does not appear to be demonstrable. Rather, the oldest "Tuscaloosa" of central and eastern Georgia appears to be biostratigraphically correlative with the Eutaw Formation of Alabama. Throughout the Carolinas, where the basal outcropping Coastal Plain units have also been mapped as Tuscaloosa (Cooke, 1936; Stephenson and others, 1942; Spangler and Peterson, 1950; Conley, 1962; and others), the names Cape Fear, Middendorf, and Black Creek Formations are now 15 commonly applied to the basal Coastal Plain units (Heron, 1960; Heron and Wheeler, 1959, 1964; Heron and others , 1968; Swift and Heron, 1969; and others). Except for the upper part of the Black Creek Formation , which has yet to be studied in detail, all these units can be assigned to zone V. I do not mean to imply that all the lithologic units in which zone V assemblages occur are facies of one another and are not superimposed , because additional work may result in a subdivision of zone V that will allow detection of time stratigraphic differences among them. At present, however, palynologic evidence suggests that deposition of the Eutaw, Cape Fear, Middendorf, and basal part of the Black Creek Formations , and some of the units mapped as "Tuscaloosa" in central and eastern Georgia are not far removed from one another in time . The reappearance of zone IV assemblages in northern North Carolina (locality 14, fig. 1) is somewhat anomalous. As discussed above, zone IV has not been detected elsewhere in the updip section of central gnd eastern Georgia, South Carolina, or Notth Carolina . In addition, studies in Virginia suggest that only Lower Cretaceous desposits are present in outcrop in the southern part of the State. Detailed lithostratigraphic and biostratigraphic studies of this area are apparently needed for a better understanding of how the northern North Carolina units relate to Coastal Plain deposits to the north, south, and downdip. The downdip section Although zone IV does not occur in the updip section of South Carolina and most of Georgia, it does occur in a number of coastal wells in these states (localities 15-22, fig. 1). (A more complete discussion of the lithostratigraphic and biostratigraphic relationships among these and other wells is presented by Gohn and others, this volume). A palynologic correlation of the Cretaceous rocks from these eight wells is presented in figure 3. Figure 3 indicates that a thin, unfossiliferous sand occurs at the base of the sectior1. The sand pfnches out both to the north and south, leaving deposits of zone IV age resting directly on basement rocks. If any lower Cretaceous rocks occur in the coastal areas of South Carolina and Georgia, they are represented by this thin sand . Zone IV thins from south to north and apparently pinches out in northern South Carolina (between localities 15 and 16). A second barren interval, represented by a series of oxidized sands and clays, overlies deposits of zone IV age throughout all but the southernmost part of the section . Here, the oxidized sands and clays are absent, leaving dep(9sits of zone V age resting directly on those of zone IV age. Lithologically, desposits of zone IV age and the upper barren interval correspond to both the lower and upper members of the Atkinson Formation of Applin and Applin (1967). 22 21 20 s.cJN .o. 17 16 15 POST-ZONE V UPPER CRETACEOUS TO -1381 TO -1 3 10 IOOmL 50 BOkm 0 0 40 Figure 3 Biostratigraphic cross section of the subsurface Cretaceous System of coastal South Carolina and Georgia. The section extends from basement to the Cretaceous- Tertiary boundary. Locality 15 = Calabash Well, Brunswick County, North Carolina; locality 16 =Myrtle Beach-10th Avenue Well, Horry County, South Carolina; locality 17 = Penny Royal Well, Georgetown County, South Carolina;locality 18 = U.S .G .S. Clubhouse Crossroads Core No.1 . Dorchester County, South Carolina;locality 19 = Fripp Island Well, Beaufort County, South Carolina; locality 20 = Pan American-Union Camp Paper No.1 Well, Glynn County, Georgia, locality 21 = Humble-Union Bag Camp Paper ST No. 1 Well, Glynn County, Georgia; Locality 22 = Pan American-Union Camp B-1 Well, Camden County, Georgia. 16 Deposits of zone V age are continuous and maintain a fairly uniform thickness throughout the section. Lithologically, deposits of zone V age correspond to the unit described by Applin and Applin (1967) as "beds of Austin age." In all wells examined, rocks of zone V age are overlain by Cretaceous deposits of Campanian to middle Maestrichtian Age. SUMMARY AND CONCLUSIONS To summarize the updip relationship between pollen zones IV and V, we note that apparently zone IV occurs in the outcropping Tuscaloosa Group (or Formation) of Alabama and extreme western Georgia. Deposits of zone IV age plnch out and are overridden by younger lithologic units of zone V age somewhere in extreme western Georgia, and zone IV does not reappear in the updip section until it reaches the northernmost part of North Carolina. This relationship indicates that the basal outcropping Coastal Plain units of most of Georgia and the Carolinas are of Santonian or younger age and are not biostratigraphically correlative with the Cenomanian-aged Tuscaloosa of Alabama. However, inasmuch as zone IV does occur throughout most of the coastal areas of South Carolina and Georgia, its landward limit must be somewhere between the outcropping Cretaceous units and the coastal regions of Georgia and South Carolina as depicted in figure 4. I cannot determine at this time if this line represents the depositional extent of sediments of zone IV age, or if deposits of zone IV age once extended farther inland and were eroded during the hiatus between zones IV and V (that is, the Turonian and Coniacian). ,J _I VA ~ - liNI.l -,-;;c-c- f' "' r - ~' / ,. ) -/-- ~ --J,......---;f'sc EXPLANATION N 1 ~ DELINEATES CRETACEOUS THE SYSTEM APPROXIMATE LANDWARD EXTENT OF POLLEN ZONE IV IN THE SUBSURFACE OF GEORGIA AND THE CAROLINAS Figure 4 . out c rop pattern of the Cretaceous System of the eastern Gulf and southern Atlanti c Coastal Plain showing the position of the pinch out of pollen zone IV in the subsurface of Georgia and the C arolina s. 17 The results of a palynologic examination of a well located in extreme southwestern Georgia (locality 23, fig . 1) deserve mention at this time, although the well has yet to be tied into either the updi p or coastal subsurface sections. This well, located in the Southwest Georgia Embayment, penetrated 2163 m of Coastal Plain sediments before bottoming in Paleozoic rocks. Lithologic correlation with nearby wells suggests that lithologies below 1030 m can be assigned to the Lower Cretaceous as described by Applin and Applin (1964, 1965). However, four cutting samples from depths between 1666and 1945 m contain several guide fossils to zone IV. No forms known to be restricted to the Lower Cretaceous were observed. If we assume down-hole contamination is not the reason for the occurrence of zone IV palynomorphs at these depths, the Lower Cretaceous is apparently either absent or restricted to the basal 213m of the well . If the Lower Cretaceous is absent in southwestern Georgia, as it apparently is along the coast, then it raises the question of whether most, if not all, of the subsurface units in Georgia mapped as Lower Cretaceous are in reality of zone IV (late Cenomanian) age. REFERENCES CITED Applin, E.R., and Applin, P.L., 1964, Logs of selected wells in the Coastal Plains of Georgia: Georgia Geol. Survey Bull. 74, 229p. Applin, P.L., and Applin, E.R ., 1947, Regional subsurface stratigraphy, structure, and correlation of middle and early Upper Cretaceous rocks in Alabama, Georgia and north Florida: U.S. Geol. Survey Oil and Gas lnv. Prelim. Chart 26. _ _,1965, The Comanche Series and associated rocks in the subsurface in central and south Florida: U.S. Geol. Survey Prof. Paper 447, 84 p. __,1967, The Gulf Series in the subsurface in northern Florida and southern Georgia: U .S. Geol. Survey Prof. Paper 524-G, 34 p. Brenner, G.J., 1963, The spores and pollen of the Potomac Group of Maryland: Maryland Dept. Mines, Water Res. Bull. 27, 215p. Brett, C.E., 1967, Upper Cretaceous equivalents in Georgia and the Carolinas, in Jones, D. E., ed., Geology of the Coastal Plain of Alabama: A guidebook for the 80th Ann. Mtg., Geol. Soc. America, New Orleans, La., p. 18-25. Brown, P.M., Miller, J .A., and Swain, F.M ., 1972, Structural and stratigraphic framework, and spatial distribution of permeability of the Atlantic Coastal Plain, North Carolina to New York: U .S. Geol. Survey Prof . Paper 796, 79p. Christopher, R.A ., 1977a, The stratigraphic distribution of Normapolles and triporate pollen in zones IV, V, and VII of the Raritan and Magothy Formations (Upper Cretaceous) of New Jersery (abs.): Am. Assoc. Strat. Palynologists 10th Ann. Mtg ., Abs . with Program, p. 7-8. __,1977b, Selected Normapolles pollen genera and the age of the Raritan and Magothy Formations (Upper Cretaceous) of northern New Jersey, in Owens, J.P ., Soh!, N.F., and Minard, J .P., eds., A field guide to Cretaceous and Lower Tertiary beds of the Raritan and Salisbury embayments, New Jersey, Delaware, and Maryland: Guidebook prepared for Ann. AAPG/SEPM Convention, Washington, D.C., June 12-16, 1977, p. 58-69. _ _,Normipolles and triporate pollen assemb- lages from the Raritan and Magothy Formations (Upper Cretaceous) of New Jersey: Paly- nology, v. 3, in press . Conant , L.C., 1967, The pre-Selma Cretaceous strata, in Jones, D.E. (ed .), Geology of the Coastal Plain of Alabama : A guidebook for 80th Ann . Mtg ., Geol. Soc . America, New Orleans, La., p. 4-11. Conley, J.F., 1962, Geology and mineral resources of Moore County, North Carolina: North Carolina Div. Mineral Res. Bull. 76, 40 p. Cooke, C.W., 1936, Geology of the Coastal Plain of South Carolina: U.S . Geol. Survey Bull. 867, p. 1-196. Cornet, B., and Traverse, A ., 1975, Palynological contributions to the chronology and stratigraphy of the Hartford Basin in Connecticut and Massachusetts: Geoscience and Man, v. 11' p. 1-33. Cousminer, H .L., 1973, Paleogene palynology of basal Coastal Plain sediments, Irwinton district, Georgia (abs.): Geol. Soc. America Abs. with Program, v. 5, no. 7, p. 584-585. Cousminer, H.L., and Terris, L., 1973, Palynology of Paleocene clays from Georgia (abs.): Am. Assoc. Strat. Palynologists, 5th Ann . Mtg ., Abs. with Program, p. 72-73. Doyle, J.A., 1969a, Angiosperm pollen evolution and biostratigraphy of the basal Cretaceous formations of Maryland, Delaware, and New Jersey (abs.): Geol. Soc. America, Abs. with Program, Part 7, p. 51 . __,1969b, Cretaceous angiosperm pollen of the Atlantic Coastal Plain and its evolutionary significance: Arnold Arboretum Jour., v. 50, no. 1, p. 1-35. Doyle, J.A., and Robbins, E.l. , 1977, Angiosperm pollen zonation of the continental Cretaceous of the Atlantic Coastal Plain and its application to deep wells in the Salisbury embayment : Palynology, v. 1, p. 43-78. Drennen, C.W., 1953, Reclassification of outcropping Tuscaloosa Group in Alabama : Am . Assoc. Petrol. Geol. Bull., v. 37, p. 522-538. Eargle, D.N ., 1946, Correlation of the pre-Selma Upper Cretaceous formations between Tuscaloosa County, Alabama, and Neshoba County, Mississippi: U .S. Geol. Survey Oil and Gas . lnv. Prelim . Chart 20. __,1948, Correlation of pre-Selma Upper Cretaceous rocks in northeastern Mississippi and northwestern Alabama: U.S. Geol. Survey Oil and Gas lnv. Prelim. Chart 35. 18 Goczan, F., Groot, J.J . Krutzsch, W., and Pacltova, B., 1967, Die Gattungen des "Stemma Normapolles Pfl. 1953b" (Angiospermae) . Neubeschreibungen und Revision europaischen Formen (Oberkreid bis Eozan): Palaontol. Abhandl., v. 2, no. 3, p. 427-633. Gohn, G.S., Bybell, L.M ., Christopher, R.A., Owens, J .P., and Smith, C.C., this volume, A stratigraphic framework for Cretaceous and Paleogene sediments along the South Carolina and Georgia coastal margins. Habib, D., 1977, Compqrison of Lower and Middle Cretaceous palynostratigraphic zonations in the western North Atlantic, in Swain, F.M., ed ., Stratigraphic micropaleontology of Atlantic basin and borderlands : Elsevier Scientific Publ. Co., Amsterdam, The Netherlands, p. 341-367. Heron , S.D. , Jr ., 1960, Clay minerals of the outcropping basal Cretaceous beds between the Cape Fear River, North -Carolina , and Lynches River, South Carolina, in Clays and Clay Minerals, 7th Nat'l. Conf .: New York, Pergamon Press, p. 148-161. Heron, S.D., and Wheeler, W.H., 1959, Guidebook for Coastal Plain field trip featuring basal Cretaceous sediments of the Fayetteville area, North Carolina : Geol. Soc . America, Southeastern Section, 1959, 20p . _ _ ,1964, The Cretaceous formations along the Cape Fear River. North Carolina: Atlantic Coastal Plain Geol. Assoc., 5th ann . field excursion, 53p . Heron, S.D., Swift, D.J.P., and Dill, C.E., Jr., 1968, Graded rhythmic bedding in the Cape Fear Formation, Carolina Coastal Plain : Sedimentology, v. 11, p. 39-52. Kimyai, A. , 1966, New plant microfossils from the Raritan formation (Cretaceous) in New Jersey: Micropaleontology, v. 12, no. 4, p. 461-476. Klaus, W., 1960, Sporen der karnischen Stufe der ostalpinen Trias : Jahrb. Geol. Bundesanstalt (Wien) Sonderbd ., v. 5, p. 107-183. Krutzsch, W., 1970, Zur Kenntnis fossiler disperser Tetraden-pollen: Palaontol. Abhandl., v. 38, p. 399-433. Leopold, E.B ., and Pakiser, H . M ., 1964, A preliminary report on the pollen and spores of the pre-Selma Upper Cretaceous strata of western Alabama: U.S. Geol. Survey Bull. 1160-E, p. 71-95. Monroe , H.W., Conant, L.C., and Eargle, D.N ., 1946, Pre-Selma Upper Cretaceous stratigraphy of western Alabama : Am . Assoc . Petrol. Geol. Bull., v. 30, p. 187-212. Pessago, E.A., Jr., 1969, Upper Cretaceous stratigraphy of the western Gulf Coast area, Mexico , Tex as and Arkans as: Geol . Soc. America Mem . 111 , 139 p. Phillips , P.P., and Felix . C.J ., 1971 , A study of Lower and Middle Cretaceous spores and pollen from the southeastern United States. II. Pollen: Pollen et Spores, v. 13, no. 3, p. 439-473 . Scrudato , R.J ., and Bond, T.A., 1972, CretaceousTertiary boundary of east-central Georgia and west-central South Carolina: Southeastern Geology, v. 14, no. 4, p. 233-239. Sirkin, L.A., 1974, Palynology and stratigraphy of Cretaceous strata in Long Island, New York, and Block Island, Rhode Island: U.S. Geol. Survey Jour. Res., v. 2, no. 4, p. 431-440. Sohl, N.F., 1964, Pre-Selma larger invertebrae fossils from well core samples in western Alabama: U.S. Geol. Survey Bull. 1160-C, p. 5564 . Spangler, W.B., and Peterson, . J .J., 1950, Geology of Atlantic Coastal Plain in New Jersey, Delaware, Maryland and Virginia: Am. Assoc. Petrol. Geol. Bull., v. 34, no. 1, p. 1-99. Stephenson, L.W., 1956, Fossils from the Eutaw formation, Chattahoochee River region , Alabama-Georgia: U.S. Geol. Survey Prof. Paper 274-J, p. 227-250. Stephenson, L.W., King, P.B., Monroe, W.H., and Imlay, R.W., 1942, Correlations of the outcropping Cretaceous Formations of the Atlantic and Gulf Coastal Plain and Trans-Pecos, Texas: Geol. Soc. America 1 Bull., v. 53, no. 3, p. 435-448. Stover, L.E., Elsik, W.C., and Fairchild, W.W., 1966, New genera and species of early Tertiary palynomorphs from the Gulf Coast: Univ. Kansas, P'aleontol. Contributions , Paper 5, p. 1-10. Swift, D.J.P., and Heron, S.D., Jr., 1969, Stratigraphy of the Carolina Cretaceous: South- eastern Geology, v. 10, p. 201-245. Thomson, P.W. , and Pflug, H.D., 1953, Pollen und Sporen des Mitteleuropaischen Tertiars: Palaeontographica, Abt. B, v. 94, p. 1-138. Tschudy, R.H., 1973, Complexiopollis pollen lineage in Mississippi embayment rocks : U.S. Geol. Survey Prof. Paper 743-C, o. C1-C15. _ _ , 1975, Normapolies -pollen from the MISSISsippi embayment: U.S. Geol. Survey Prof. Paper 865, 42 p. Tschudy, R.H., and Patterson, S.H., 1975, Palynologic evidence for Late Cretaceous, Paleocene, and early and middle Eocene ages for strata in the kaolin belt, central Georgia: U.S. Geol. Survey Jour. Res., v. 3, no. 4, p. 437-445. Wolfe, J.A., 1976, Stratigraphic distribution of some pollen types from the Campanian and lower Maestrichtian rocks (Upper Cretaceous) of the middle Atlantic States: U.S. Geol. Survey Prof. paper 977, 18 p. Wolfe, J.A., and Pakiser, H.M ., 1971, Stratigraphic interpretations of some Cretaceous microfossil floras of the middle Atlantic states : U.S. Geol. Survey Prof. Paper 750-B, p. B35-B47. 19 EXPLANATION OF PLATE 1 Biostratigraphically important sporomorphs from pollen zon e IV in the eastern Gulf and southern Atlantic Coastal Plains . Figure 1 Nevesisporit es sp. Figures 2-5 Equisetosporites spp. Figures 6, 7. Welwitschiapites sp. Figure 8. Carolina torosus (Reissinger) Klaus 1960 emend. Cornet and Traverse 1975. Figure 9 Fraxinoipoflenites rotundus sensu Phillips & Felix 1971 . Figures 10. 11. "Retitricolpites" geranioides Couper 1960 sensu Brenner 1963. Figure 12. Tricolpites wilsonii Kimyai 1966. Figure13. ?Ajatipoflis tetraedralis (Bolchovit ina) Krutzsch 1970. Figure 14. At!antopol/is verrucosa (Groot & Groot) Goczan, Groot, Krutzsch & Pacltova 1967. Figures 15, 17-22. Complexiopoflis spp. Figure 16. Complexiopoflis funiculus Tschudy 1973. EXPLANATION OF PLATE 2 Biostratigraphically important sporomorphs from pollen zone V in the eastern Gulf and southern Atlantic Coastal Plains. Figures 1, 2. Nyssapoflenites sp . Figures 12. 13. Myrtacidites sp. Figures 3, 4 Porocolpopo/lenites sp, B Figures 14, 15. Tricolporate type 69. Figures 5, 6. Porocolpopol/enites sp. A Figures 16-22. Holkopoflenites spp. Figures 7, 8. Tricolporate type 34. Figures 23. 26, 27 . Complexiopoflis spp. Figure 9 Cupuliferoidaepol/enites sp . Figure 24. Complexiopof!is abditus Tschudy 1973. Figures 10. 11. Tricolpites sp . Figure 25 Complexiopollis sp . D. EXPLANATION OF PLATE 3 f3iostratigraphically important sporomorphs from pollen zone V in the eastern Gulf and southern Atlantic Coastal Plains. Figures 1-4. Pseudoplicapoflis spp. Figures 5-7. Minorpol/is spp Figure 8. Santalacites sp Figures 9-12. Momipites spp. Figures 13, 14 Representatives of an undescribed genus characterized by its convexly triangular amb, small weakly developed anuli, and narrow interloculum or "Schichtfuge." Figures 15, 16 Plicapol/is spp . Figures 17, 18 Praecursipollis plebius Tschudy 1975. Figures 19-21 Labrapoflis spp . Figure 22 Trudopoflis spp. Figure 23. Osculapo!lis sp . 20 t 8 3 2 , 4 PLATE 1 5 15 ' 12 ~. . . .. :- ~ 13 16 17 19 20 18 ---20um 21 21 PLATE 2 10 11 13 14 15 16 17 18 19 20 21 24 25 23 ----20um 22 PLATE 3 2 5 7 9 10 .. .. 15 ~f. 16 19 - - - - 20um 23 17 18 PALEOENVIRONMENT AND AGE OF KAOLIN DEPOSITS IN THE ANDERSONVILLE DISTRICT, GEORGIA Harland E. Cofer, Jr. Georgia Southwestern College Americus, Georgia and Norman Frederiksen U.S. Geological Survey Reston, Virginia ABSTRACT The kaolin deposits of the Andersonville district of Georgia are thick and relatively pure; they grade laterally and vertically into sandy kaolin and sand. The sedimentological and mineralogical character of the clay and the enclosing sand suggests deposition in a marine shallowwater to estuarine environment. Palynomorph and invertebrate fossil assemblages from strata adjacent to the kaolin beds support the suggested depositional environment of the kaolin and the subsequent subaerial formation of bauxite. Movement on the Andersonville and related faults probably influenced basinal topography and served to maintain a restricted circulation in the basin . Sporomorph biostratigraphy limits the accumulation of kaolin and the formation of the bauxite deposts to early Wilcox (late Paleocene) time. INTRODUCTION The objective of this paper is to summarize the sedimentological, mineralogical, structural, and paleontological evidence observed in the sediments of the Andersonville district and to fit this evidence into a paleoenvironmental framework. The ultimate objective is to explain the sequence of events leading to the formation of the kaolin-bauxite deposits within the time frame provided by palynological data. The authors make no attempt to cite the voluminous literature pertaining to the origin of kaolin deposits of the region or of the Andersonville area. Concepts and ideas contained within this literature have been freely used . LOCATION AND GEOLOGIC SETTING The Andersonville, Ga ., kaolin-bau xite district is about 40 km south of the Fall Line which separates the largely unconsolidated sediments of Mesozoic and Cenozoic age from the older crystalline rocks of the Piedmont. The district marks the westernmost extent in Georgia of commercial kaolin deposits that occupy the narrow outcrop belt of Upper Cretaceous and lower Tertiary rocks trending northeastward across the State (fig. 1). Bauxite is asso- r I J~~ I I _ I r~L_ I _r' I I I \ I I ~ 1 \ : I .... I _t_ ____ - -1 . DOOLY r i r' I ~ SCHlEY a.-- :- SUMTER-- ~}~{:::'{fttil\ I - 1I1.. _..,.., .........IMlR ICUS r-'ptAINS I I SCALE K:i."I JMMIIJMI[]Mr:l\====:==:!:1:==:::jiO Kll 0 Ml Tl RS -F~ uII Probablf Lull CRlTACIOUS & lARlY HRTIARY ROCKS ---, I \_, Figure 1. Location of the Andersonville district and major faults of the area. 24 ciated with other deposits in the kaolin belt but is of mineable quality only in this district. At the present time. bauxite and kaolin are mined and processed for the manufacture of refractories and commercial alum near Andersonville. GENERAL STRATIGRAPHY The formation names used in the report are those adopted by Zapp (1965) for lithologically distinct units in the district (fig. 2). The stratigraphy was based in part on paleontologic evidence and in part on lithostratigraphic similarity of strata to beds of known age in eastern Alabama. The present authors include the thin, freshwater to brackish-water lateral equivalents of the marine Tuscahoma as part of that formation; Zapp apparently included these sediments in the Nanafalia in the central part of the district. The Providence Sand of Late Cretaceous age is the oldest outcropping unit in the district. As exposed, it consists of about 6 m of nonmarine, coarse-grained, cross bedded arkosic sand containing clay clasts. In the subsurface, the arkosic sand grades rapidly downward into lignitic, pyritic sand and thence to marine sand. The Providence is overlain by the Clayton Formation of the Midway group (lower Paleocene). The Clayton Formation consists of about 19 m of calcareous, gray clayey sand and sandy clay which contain thin limestone beds and abundant oyster biostromes and detrital shell lenses. Overlying the biostromal sand and clay is a variable thickness of dark greenish-gray, micaceous, glauconitic, phosphatic, silty montmorillonitic clay, which contains a planktic foraminiferal assemblage but no bottomdwelling fauna. Unconformably overlying the Clayton is a thick section (18 m) of micaceous, kaolinitic sand that locally grades laterally and vertically through sandy kaolin into relatively pure lenticular beds of clay. These kaolin beds may in turn enclose thin lenses of bauxite and bauxite clay . In the subsurface to the west and south, the kaolinitic sand of the Nanafalia is represented by dark-gray, lignitic sand and silt containing scattered thin light-colored kaolin lenses as much as a meter thick. This unit lacks calcareous fossils but has been correlated with the Nanafalia Formation of Alabama because of its lithologic character and stratigraphic position (Zapp, 1965). The Nanafalia Formation is difficult to distinguish from the overlying Tuscahoma Formation by using sporomorphs, and some strata assigned to the Nanafalia in the Andersonville district may possibly be correlative with part of the Tuscahoma to the west. The Nanafalia represents the lowermost part of the Wilcox Group (upper Paleocene). Conformably overlying the Nanafalia is the Tuscahoma Formation which is also in the lower part of the Wilcox. It consists of laminated sands and silts which may vary in character laterally within 100m from a fossiliferous, glauconitic, silty marine sand to a lignitic, pyritic, silty kaolinitic clay. The formation ranges in thickness from 0 to 10m. Crossbedded deltaic and fluvial sands, which become progressively more marine in character toward the top, unconformably overlie the Tuscahoma Formation. The uppermost sand layers are weakly cross bedded to cross laminated, are bioturbated, and locally are calcareous and contain a microfauna typical of the Claiborne Group (middle Eocene) (Grumbles, 1957). The maximum thickness of the unit is about 18m south of Sweetwater Creek. The uppermost Tertiary unit in the district is a silty, clayey sand, which is locally cherty and is presumably derived from the weathering of calcareous sediments of Claiborne and Jackson age that crop out south and east of the area. A veneer of Quaternary alluvium covers two broad terraces at elevations of 91 m and 103 m adjacent to the Flint River, which bounds the district on the east. STRUCTURE The dominant structural feature of the area is the slightly arcuate west-trending Andersonville fault (fig. 1). The fault plane is nearly vertical, and the maximum displacement of 30m has been measured on the top of the Clayton Formation. The southern (seaward) block has moved upward relative to the northern (shoreward) block. A seismic profile along the Flint River and gravity profiles across the fault indicate that it extends to depth (Rountree and others, 1978). The regional northeast strike and gentle (4 m/km) southeastward dip of the strata as determined on the Clayton surface are interrupted by the Andersonville fault (fig. 3). In a broad area north and south of the fault, the strike of the formation is east-west and dip varies from horizontal to about 2.5 m/ km. Farther south the dip steepens to 15 m/km. In the immediate vicinity of the fault, the dip is reversed and, in places, is as steep as 60. In the vicinity of Mountain Creek, abrupt changes exist in dip of the formations; a seismic profile, gravity data, vertical escarpments in alluvium, and conspicuous lineaments also indicate the presence of faulting . In part, the structural relief of the Clayton surface is a result of post-Paleocene movement on the Andersonville fault; however, lithologic variation in the sediments of the Nanafalia and Tuscahoma suggests intermittent movement during early Wilcox time. We believe that the basinal topography at 25 GROilP FORMATION CH PHIC LO C I l. -...0.. w z w u 0 w -...... 0 .....!! .. I E 0 ........... .. D ~ w JJ: z w u ....:.. ,., 0 w ..J <{ a. -...... .-..-..... :IE ~'<::::'. >. - ' ';',t.'~ ;... ,~ , ', " ~ ~ ,.. , .I 3 C!1!JZJ) , .. 51 11 R0 CK DESCRI PT I 0 N r1RE.E.fliSH KAOLINITIC AND MONTMOR I LLON IT I C RICH SHALE RED TO REDDISH BROWN CROSS-BEDDE~ QUARTZOSE SANDSTONE, CROSS-BEDDING POORLY DEVELOPED IN UPPER PORTION BECOMING WELL DEVELOPED IN MIDDLE PORTION, BIOTURBATION PRESENT IN MIDDLE AND UPPER PORTIONS. WEAKLY TO STRONGLY LAMINATED CLAYEY SANDY SILTSTONE OCCASIONALLY BECOMING GLAUCONITIC AND FOSS ILl FEROUS, BASAL PORT I ON IS r.;RAY PYRITIC AND CARBONACEOUS COARSE GRAINED, LOCALLY CHANNELED SANDSTONE CONTAINING CLAYBALL CONGLOMERATE, MEDIUM TO COARSE GRAINED KAOLINITIC MICACEOUS SANDSTONE ENCLOS lNG LOCAL ACCIIMULATIONS OF KAOLIN CONTAINING BAUXITIC MASSES DARK GRAY MARINE SHALE CONTAINING Q.S.IR.EA Cl!li.t!I! U t!MiG l.dli.I/.1 OCCASIONALLY r,RADES LATERALLY TO AN ARGILLACEOUS LIMESTONE OR A GRAY ARENACEOUS SHALE CONTAINING PELECYPOD FRAGMENTS, Cll ::> 0 w u <{ 1- w ....... a: u ..0.~ ... a: w a. a. .::> LIGHT RED TO TAN MICACEQIIS o SLIIC SANDSTOI~E. GRADES UPWARD TO A LIGNITIC AND PYRITIC SANDSTONE, L-----------------~L-- 129 Figure 2. Stratigraphic column of the Andersonville district. 26 SCALE It I LDM [ llRS - _EXPLANATION .... Faults ~ State Roads ~ Structural Contours ij) THKJO Location of Samples Referred to in Text Contour Interval 10 ft (3.05 m) Figure 3. Structural contour map of the top of the Clayton Formation and location of samples cited in text. the onset of deposition of the Nanafalia was similar to that of the subsurface map of the top of the Clayton (fig. 3) with the downdropped block of the Andersonville fault restored to the level of the adjacent surface. Rountree and others ( 1978) proposed a major fault extending west -northwest from 3 km south of Mountain Creek on the Flint River to near Concord Community, Sumter County. The present authors believe the trend of the proposed fault is more nearly west-southwest and that it extends from this point on the Flint River to the vicinity of Plains, Ga. (figs. 1, 3). SEDIMENTOLOGY OF ROCKS OF EARLY WILCOX AGE Coarse, angular, micaceous sand overlies the marine clays of the Clayton Formation. Clasts of montmorillonitic clay derived from the Clayton commonly are present in the lower few centimeters of the sand overlying this formation. Where the lowermost part of the Nanafalia is clayey and plastic , clasts appear as ovoid or irregular patches of dark-colored montmorillonitic clay embedded in sandy kaolinitic clay. In a few localities, a dark-gray , lignitic, clayey sand overlies the Clayton and grades upward into kaolin. 27 A slightly modified computer-generated isopach map of nearly pure kaolin (< 15 percent sand and silt) indicates a gross northwest trend in clay-body distribution (fig . 4) The trend becomes more nearly east-west in the vicinity of the Andersonville fault. Thick kaolin accumulations are present where the slope of the Clayton surface is relatively flat and become thinner over steeper slopes (fig. 3) . At clay depocenters, kaolin and sandy kaolin occupy almost the entire section. Scattered incursions of sand and silt into the clay and the spreading of clay into sand depositional areas result in a lateral interfingering of clay and sand, accentuating the lenticular shape of the deposits. In the south and southwest part of the district, suspended clay appears to have moved down the steep basinal slope and to have become intercalated with the sands. Deposition of clays as gravity slides or turbidity currents probably producecfthe non bedded and poorly sorted character of the sandy clay lenses associated with the more marine facies of the Nanafalia in this part of the studv area. Kaolin varies from massively bedded, uniformly colored, gray-white clay composed of nearly pure kaolinite to finely or coarsely laminated clay in which the bedding is marked by concentrations of silt-size and fine sand-size mica and quartz. The bedding may be accentuated by deposition of iron oxides, sulfides, or carbonates by percolating ground water. Locally, carbonized and mineralized plant remains in the form of rootlike structures, twig-size fragments, and grass-bladelike impressions several centimeters long and a few millimeters across are present in otherwise pure kaolin (fig. 5A). Commonly associated with these plant remains are clay casts or molds that have been termed by others as bryozoan casts or filled burrows (Buie, 1978). These reach several centimeters in length and may be branching, curved, straight, or anastomosing (fig. 58) . The plant remains and casts both are present at multiple horizons and are widely distributed in the district. Casts, particularly the branching and anas- ---Andersonville Fault ~ State Roads ~ Streams Isopac Interval 10 ft ( 3. 05 m) SCALE ~-~S=====I MI LES 1- .J IILOM[T15 Figure 4. Modified computer generated isopach map of kaolin containing 15 percent or less coarse clastics . 28 B Figure 5A. Carbonized root structures in otherwise massive kaolin . Figure 58 . Branching of clay-filled burrows in massive kaolin . Figure 5C. Contact between kaolin and fresh-water swamp deposits in mine face. Figure 50. Mud cracks in kaolin infilled with overlying marine-to-brackish water deposits of the Tuscahoma Formation in mine face. 29 tomosing types, are always present in clays that contain root structures; similar features are found in the absence of identifiable plant debris, however. In a few localities, a kaolin lens may contain a layer a meter or more thick that is rich in organic matter. Typically, relatively pure or slightly sandy kaolin changes within a few centimeters into a highly lignitic clay (fig. SC). This clay is typically sandy and silty and contains small rip-up clasts of white kaolin well above the base. The top part of the layer rich in organic matter is fine grained and grades into the overlying kaolin by increase of clay relative to plant debris and some interlamination in the final few centimeters . The deposits tend to be elongated, to be somewhat sinuous, to cover less than a hectare in area, and to grade laterally into intertonguing clayey sand surrounding the kaolin lens. No large wood fragments. stumps, or roots were observed in the deposits and no root or rootlike structures extend into the underlying kaolin. In most cores and mine exposures. the top of the kaolin beds contain cracks that extend downward for a few centimeters and are infilled by material from the overlying Tuscahoma sediments. The dimensions, shape, and distribution of the filled cracks, both in cross section and plan view, resemble those of mud cracks (fig. 50). Near the top of the kaolin beds that are overlain by nonmarine Tuscahoma sediments, large, infilled, chambered burrows most probably made by insects extend 10 or more centimeters into the clay (fig. 6A). Bauxite and bauxitic clays are typically composed of pisolitic-structured intergrowths of kaolinite and gibbsite (fig. 6B), but a few bauxite samples are nonpisolitic. The "structureless bauxite'' is a fine-grained micro-oolitic intergrowth of the two minerals. Bauxitic materials form tabular bodies a few centimeters to a few meters thick within kaolin lenses and grade into kaolin laterally and vertically. Multiple lenses of bauxite are present within a single kaolin deposit. Bauxite and bauxitic clay "horizons" appear to parallel the dip of the Nanafalia only generally. Their elevations in adjacent kaolin lenses may or may not be equivalent. Bauxite is not present in all lenses and is generally absent in southern parts of the district, where the enclosing sands have a more marine character. The Tuscahoma Formation conformably overlies the sand and clay of the Nanafalia. It is composed of fine- or medium-grained sand which becomes interlaminated with silt toward the upper part of the formation . In the subsurface, glauconitic, fossiliferous sands and silts reach a maximum thickness of 9 min the southern part of the district, but they thin rapidly across the structural high south of the Andersonville fault and become nonmarine. The formation pinches out over part of the Andersonvill~ uplift and is absent in the northern and northeastern part of the district. A marine incursion of Tuscahoma on the downdropped block of the Andersonville fault indicates that the formation was deposited near sea level. The lateral transition from a glauconitic sandy silt to a black kaolinitic silt containing abundant carbonized wood fragments takes place within 100 min the Wilburn Mine north of Sweetwater Creek at location WL 1 (fig. 3). In this exposure, a channel originating in the Tuscahoma and filled by Tuscahoma sediments cuts well into the underlying Nanafalia. Sporomorph (spore and pollen) assemblages from the sediments of the transition zone (WL 1) and the channel (WL 1C) suggest different source areas for the pollen (table 1). The assemblage from the channel is more diverse than that of the adjacent sediments, suggesting that the channel drained a broad area supporting several plant communities. Furthermore, the transition-zone assemblage contains a much higher proportion of marine to brackish-water microplankton (dinoflagellates and acritarchs), whereas the channel assemblage contains a higher proportion of presumably terrigenous fungal spores (table 1). In general, the channels are most abundant in the vicinity of the Andersonville fault, and the drainage patterns of the channels are directed toward the downdropped block of the fault and are marginal to the marine sediment facies. MINERALOGY OF ROCKS OF EARLY WILCOX AGE The principal clay mineral of the Nanafalia Formation is kaolinite, but montmorillonite, illite, gibbsite, and chamosite are present in many kaolin deposits. The crystallinity of the kaolinite is variable. According to Hinckley's (1963) indexing method, it varies from moderately good to poor (1.4 to 0.04) . The lower parts of all kaolin lenses are somewhat less well crystallized than the top parts (fig. 6C). Kaolin having the highest crystallinity usually is present in interstices of pisolitic bauxite and in-transition (bauxitic) clay horizons (Flock, 1966). Kaolin crystallinity is also high in the upper-middle parts of lenses containing no significant gibbsite enrichment . In most deposits where the Tuscahoma is absent and Claiborne sands overlie the kaolin, the crystallinity of the upper few decimeters of kaolin is also moderatlely good. Books and stacks of kaolin are small; vermicular books are almost totally lacking except in those areas of highest crystallinity (fig. 60). Montmorillonite content of the kaolin varies from 1 percent to 30 percent and montmorillonite is most abundant in the topmost meter of many deposits 30 Figure 6A Large chambered (insect?) burrow in kaolin infilled with brackish water deposit of Tuscahoma Formation. Figure 6B. Photomicrograph of pisolitic and oolitic bauxite composed of about 85 percent gibbsite and 15 percent kaolin. x25 Figure 6C. SEM photograph of kaolin of low c rystallinity index with "swirl" pattern . x3000 Figure 6D . SEM photograp11 of "moderately w ell crystallized" kaolin with small kaolin book . x5000 31 TABL 1 -- SPOROMORPH ASSEMBLAGES FJELD NO. wu wuc AX72 FVl LS95 58B THJ24 FNl PALYNOLOGY NO, Rl492D Rl531 R1702D R1702F R1532 R1702C R1492C R1702B ELEVATION IN FEET 280 380 430 323 315 290 262.5 317-322 AsCULIIOITES ClRCYMS!RIATIIS Figure 2. Composite stratigraphic column of lower part of Upper Cretaceous Series in western Georgia "spaced pisolithic laterites" of Uganda (McFarlane, 1976, plates 1, 2, and 3). Spaced pisoliths are usually indicative of residual soils (McFarlane, 1976) . The presence of unaltered quartz veins within the paleosoil which extend down into recognizable gneissic saprolite confirms this view . Some pisoliths are present in basal Tuscaloosa sediments. The Tuscaloosa is overlain disconformably by the Eutaw Formation. The contact is sharp, appears to have relatively little relief, and is usually easy to recognize in the Chattahoochee Valley area, where Tuscaloosa maroon and greenish-gray mudstones are characteristically overlain by course, crossbedded, Ophiomorpha-burrowed sands of the Eutaw (fig. 3B) . The Tuscaloosa Formation in southwest Georgia is equivalent to the upper part of the Tuscaloosa Group in Alabama, specifically correlative with the unnamed upper member of the Coker Formation and the overlying Gordo Formation (Drennen, 1953). To the east, the Tuscaloosa is correlated with the lower part of the Middendorf Formation in eastern Georgia and South Carolina and with the Potomac Group in Maryland. Based on palynological studies (Christopher , this volume) the Tuscaloosa has been correlated with the Britton Formation of the Eagle Ford Group in Texas and is therefore considered to be of late Cenomanian Age. Lithologies The Tuscaloosa Formation in southwest Georgia is composed of two distinct lithogies : a crossbedded, conglomeratic, arkosic arenite and a mottled, silty mudstone. In most exposures, mudstone overlies arenite across a continuous, gradational contact, forming a fining-upward sequence and separated from vertically adjacent finingupward sequences by local disconformities. Crossbedded, conglomeratic, arkosic arenite The most common Tuscaloosa lithology in the study area is mature to submature arkosic arenite . Mean grain size ranges from coarse to fine sand and sorting is generally poor to moderate; skewness is almost always positive. Graphs of cumulative grainsize frequency distributions plotted on probability paper show the dominance of both suspended and saltated loads (see other examples in Visher, 1972) . Quartz-grain shapes range from nearly equant to elongated and usually are very angular to subangular, the degree of roundness increasing proportionally with increasing grain size. This roundness trend continues into the gravel fraction , most of which is very well rounded to subrounded. Tuscaloosa gravels are composed mainly of metaquartzite clasts whose textures suggest at least partial derivation from metaquartzite exposures in the Pine Mountain area north of Muscogee County. An excellent example of this is the presence within the Tuscaloosa of rounded pebbles of itacolumite nearly identi c al to in situ exposures on Pine Mountain; since itacolumite is a relatively rare lithology, it provides a good indicator of sediment dispersal patterns and provenance. Feldspar grains were origi- 41 Figure 3 Features of the Tuscaloosa Formation. A, Nonconformity separating Tuscaloosa pebbly sand from gneissic saprolite . B, Disconformity between Eutaw sand and underlying Tuscaloosa mudstone. C, Fining-upward sequences: note sharp, irregular, erosional contract of coarse, crossbedded sand on structureless mudstone (arrow) . D, Multistory sand body; lower arrow indicates contact with underlying mudstone; upper arrow indicates local disconformity separating very coarse sand from fine to medium sand below. nally subangular and generally coarse, but most have undergone chemical alteration and are now primarily kaolinite. In many areas, altered feldspar grains were deformed during compaction, being squeezed around adjacent quartz grains, resulting in a kaolinitic epimatrix. Only where early cementation prevented significant compaction can the original character of the feldspars be observed. Heavy minerals comprise from 0.2 to 1.4 percent of framework grains and consist of magnetite, zircon, kyanite, garnet, andalusite, ilmenite, and tourmaline (listed in order of abundance) . Sands range from almost unconsolidated to welllithified; cement type varies, being siliceous in some samples and ferruginous in others . The kaolinite epimatrix is responsible for moderate consolidation in many samples . Primary sedimentary structures include largescale, high-angle tabular crossbeds, large-scale, high-angle trough (festoon) crossbeds, small-scale trough crossbeds (not observed in coarse sand), and horizontal bedding . These are usually arranged vertically within a fining-upward sequence with the large-scale tabular cross beds near the bottom, overlain by trough crossbedding whose scale decreases upward, coinciding with decreasing sediment grainsize (fig. 3C). Horizontal bedding, when observed, is usually associated with large-scale tabular crossbeds near the base of a fining-upward sequence . Penecontemporaneous slump structures, large dislocated mud blocks(up to one meter in diameter and usually associated with the coarsest sediment at the base of a fining-upward sequence), and smaller, 42 rounded mud-clasts are present in many exposures. Secondary structures include rare pyrite nodules and barite concretions (Eargle, 1955). Over 300 determinations of crossbedding dip directions have been made of Tuscaloosa arenites. Distribution of dip directions from a given exposure are generally unimodal, but dispersion about the mean is high. Vector means for the various exposures studied range from southeasterly to westsouthwesterly, but the modal direction (and, significantly, the regional vector mean) is approximately southerly. Mottled, silty mudstone. Upper parts of Tuscaloosa fining-upward sequences typically consist of maroon and mediumgray mottled silty mudstone. The mean grain-size varies from coarse silt to fine silt, but sorting is usually poor and skewness is positive. Quartz dominates the mineralogy of the silt fracti'on. Sedimentary structures are generally absent and vague compositional and color mottling is the most typical characteristic of the sediment. Even in those exposures where obvious mottling is absent, few depositional structures are present and deposits are typically massive. In several exposures, there are numerous, small, vermiform burrows with faint internal lamination. Penecontemporaneous slump structures are encountered in a few exposures. Of special note are siderite spherules about one millimeter in diameter which weather to red or brown spots of iron oxide (Eargle, 1955). Mudstones contain the only fossils to be found within the Tuscaloosa in the study area. Especially interesting are well-preserved leaf imprints found in thick mudstones beneath the Eutaw contact in Upatoi Creek Valley. Many of these were described by Berry (1923) who cited them to champion the Late Cretaceous age of the Tuscaloosa Formation. Lignitic wood fragments and well-preserved palynomorphs are also present (see Christopher, this volume). Stratigraphic relations of lithologies. Silty mudstones ove'rlie arkosic arenites, making a conformable stratigraphic sequence in which grain size decreases vertically and in which sedimentary structures vary from tabular and trough crossbedding in the sands of the bottom to mottling and burrowing in the muds at the top. Each finingupward sequence is separated from those above and below by sharp, irregular, erosional disconformities (fig. 3C). Such fining-upward sequences are the hallmark of the Tuscaloosa Formation in western Georgia (fig. 4). In some exposures, two fining-upward sequences contact each other without an intervening mudstone (fig. 30). The contact is recognized by the abrupt change in grain size and the presence of mud clasts directly above the erosional surface. Such an association of sandy fining-upward sequences without mudstone is termed a multistory sand body (Friedman and Sanders, 1978). Another exception to the usual fining-upward pattern is seen in lens-shaped mudstone bodies immediately adjacent to "normal" fining-upward sequences. Characteristically, the mudstone lenses contain coarse lag gravels overlain directly by structureless, sandy mudstone. Paleoenvironmental Interpretation Based on the evidence summarized below, the Tuscaloosa Formation in southwest Georgia is interpreted to have formed as alluvial deposits on a newly formed coastal plain. The evidence is taken from both the sedimentologic and paleontologic characteristics of the unit and is presented in summary fashion below. 1. The fining-upward sequences themselves constitute perhaps the best evidence of fluvial deposition because they are considered to represent the vertical succession of strata formed by the progradation of point bars and associated overbank envi ron ments over thalweg deposits during stream migration. Such fining-upward, pointbar sequences have been discussed by numerous authors (for example, McGowen and Garner, 1970; Visher, 1972; Friedman and Sanders, 1978) and are believed to typify fluvial deposition in a meandering stream environment. 2. Variations in point-bar sequences are also significant. Multistory sand bodies are formed where one stream eroded deeply into deposits of a previous fluvial cycle, removing muddy top stratum deposits (and, in the process, creating mud clasts by slumping of blocks off the cut bank and into the thalweg) and allowing the prograding point bar to deposit its sands directly on sands of the previous point bar. Mudstone lenses with basal lag-gravels probably represent "clay plugs" formed by the filling-in of river channels abandoned by stream migration, such as cut-off meanders and ox-bow lakes (Friedman and Sanders, 1978). 3. Festoon crossbedding (according to Visher, 1972) is indicative of confined, unidirectional flow and is therefore supportive (but not by itself conclusive) evidence of fluvial deposition. 43 A en 0:::: (I) w 1- w ~ 8 l (2) I J Figure 4. Fining-upward sequences with the Tuscaloosa Formation. Exposure A . is located on both sides of Lindsay Creek By-pass at the intersection with Macon Road. Exposure B is located on Wildcat Road just south of Buena Vista Road on the Fort Benning military reservation. 4. Crossbedding dip directions suggest a southward-flowing paleocurrent system moving down a regional paleoslope in the same manner as present streams. Sediment dispersal patterns agree with this conclusion, indicating a source area to the north. This may be seen not only in the itacolumite clasts of Tuscaloosa gravels, but also in the heavy mineral fraction of the sands. Their mineralogy suggests a high-rank metamorphic and plutonic igneous source, such as the Piedmont province. 5. Siderite, found in some Tuscaloosa top stratum muds, provides further evidence of alluvial origin. Siderite is only formed in freshwater environments characterized by strongly reducing chemical conditiot;~s such as might be encountered in stagnant backswamp areas. It is not likely to be marine (Berner, 1971). 6. The presence of well-preserved terrestrial plant fossils, especially delicate leaves, suggests a non marine environment. Also, the complete lack of any marine fossils whatsoever supports the fluvial interpretation. EUTAW FORMATION Stratigraphy The Eutaw Formation overlies the Tuscaloosa disconformably. In the Chattahoochee Valley area, it is approximately 38m thick but thins toward the east, being about 30.5 m thick in the Harmony Church area of Fort Benning and 26m thick east of 44 Fort Benning (Eargle, 1955). In the study area, the Eutaw consists of four lithofacies, three of which define a lower, crossbedded sandy member which is best developed in updip areas, especially in outliers on the tops of hills in southern and central Muscogee County, where it comprises most of the formation (being 26m thick in the Buena Vista Road area). This crossbedded sandy member thins to the south, being approximately 4.5 m thick on U.S. 27 just south of Upatoi Creek and absent altogether 4.8 km to the east in Ochillee Creek Valley. The fourth lithofacies is gray, bioturbated, fossiliferous, muddy sand and sandy mud which forms the majority of the Eutaw in most of its main outcrop belt in Upatoi Creek Valley. To the east, the lower sandy member thickens until it makes up the entire formation in the Flint River area. East of the Flint, the Eutaw is indistinguishable from overlying units (Eargle, 1955). The Eutaw is overlain disconformably by the Blufftown Formation. Monroe (1947) considered the entire Eutaw in western Georgia to correlate with the Tombigbee Sand Member of the upper Eutaw in western Albama , but diagrams in Copeland's (1972) discussion of Alabama's Coastal Plain stratigraphy show correlation with the majority of the Eutaw in its type section. Stephenson (1957) considered the molluscan fauna of the Eutaw in the study area to be ancestral to that in the Snow Hill Member of the Black Creek Formation in North Carolina and to be correlative with the upper part of the Austin Chalk in Texas. Christopher (this volume) uses palynology to demonstrate correlation with the basal part of the Austin Group, suggesting a Coniacian or Santonian Age. Eutaw Lithofacies Lithologically, the Eutaw Formation in west Georgia is very complex. From eight to twelve different lithologies have been recognized , based on grain size, sedimentary structures, and fossils. I have grouped these into four lithologic associations, or lithofacies, which will be briefly described in the following paragraphs. Their stratigraphic relations are illustrated in figure 5. Lithofacies I. The lowermost Eutaw lithofacies, found only in northern parts of the outcrop belt, is characterized by fine to coarse, poorly to moderately sorted, unskewed to positively skewed , slightly feldspathic quartzose sand. As a general rule, grain size decreased upward within this unit. Heavy minerals make up less than one percent of the sand and include hematite, ilmenite, zircon, hornblende, magnetite, rutile, tourmaline (two different varie- ties), and others (listed in order of abundance). Comparison with Tuscaloosa heavy-mineral suites s.uggests that Eutaw sands were at least partially derived from reworking of Tuscaloosa sands, revealed especially by the abundance of hematite in the Eutaw which occurs in the Tuscaloosa not as a detrital mineral but as a cementing agent. Quartz grains are generally sub-equant to prolate and roundness ranges from subangular to very angular. Feldspars are severely altered. This lithofacies typically is yellowish gray (5Y 2/6), very dusky red purple (5RP 2/2) and shades of bright orange not specifically on the rock color chart. Primary sedimentary structures include welldeveloped, large-scale, wedge-shaped, tabular crossbedding, usually overlain by large-scale trough crossbedding (pi cross-stratification of Allen , 1963), followed by horizontal laminations (fig. 6A). "Herringbone" crossbedding is commonly observed above the horizontally laminated strata. Reactivation surfaces are encountered occasionally (fig. 6B) as well as other types of compound crossbedding. Well preserved, distinct Ophiomorpha burrows are sparsely distributed throughout this lithofacies. Polychaete burrows are also found occasionally. Thin, silty claystone lenses are interbedded with crossbedded medium to fine sand in the upper part of this lithofacies, sometimes as "clay drapes" between cross-laminations. Typically, thick clay lenses are overlain by crossbedded sand containing abundant clay clasts. Thin, discontinuous, peat beds are rarely found associated with bioturbated sand and thin clay lenses near the top of the unit; in one locality, a dislocated slump block of peat is found associated with large-scale, tabular crossbedding. Crossbed dip directions are usually bimodally distributed with modes approximately 180 apart, the dominant mode usually being directed southeasterly. Secondary sedimentary structures include thin iron-oxide hardpans and hollow ironoxide concretions ("Indian paint pots"). Paleoenvironmental Interpretation of Lithofacies I. Lithofacies I is interpreted as representing deposition in tidal channels, perhaps associated with a tidal delta. Herringbone crossbedding, reactivation surfaces, and bimodally distributed crossbed dipdirections indicate reversing-flow conditions. The overall fining-upward character of the sediment and associated change in sedimentary structures from large-scale tabular and trough forms at the bottom to horizontal laminations and small-scale crossbedding at the top is similar to the stratigraphic sequence produced by the migration of tidal channels (Kumar and Sanders, 1974) . Bioturbated sands with clay lenses and peat beds at the top suggest 45 N ~ w aw:: (.) s ~ w w a:: (.) LITHOFACIES :rsz: D LITHOFACIES ][ 1[ LITHOFACIES I V.E.'" 80 0 2 KILOMETERS Figure 5. Stratigraphic cross section and inferred correlations of Eutaw lithofacies in southern Muscogee and Chattahoochee Counties. Vertical lines indicate outcrop control of cross section. channel abandonment and the development of a marsh environment. I have interpreted the strongly stained character of these upper sands and clays to be due to oxidation of dispersed organic materials and various divalent metal ions, originally deposited under the reducing conditions typically encountered in marsh environments. Abundant clay clasts were formed during storms which ripped up marsh deposits and the dislocated peat block mentioned above is believed to have slumped off the cut bank of a tidal channel during its migration. Ophiomorpha burrows indicate normal marine, shallow subtidal to low intertidal conditions (Weimer and Hoyt, 1964). Lithofacies II The second lithofacies lies disconformably on the first in northern parts of the study area (fig. 6C) and constitutes the lowermost part of the Eutaw in its main outcrop belt, lying directly on the Tuscaloosa. It is characterized by medium to very coarse, moderately sorted, quartzose sand with iron oxide cement scattered through the unit as thick hardpans, irregular layers. and other masses. Grain size decreases upward within this unit but does not pass into a stained clay-lens and peat-bed aspect as in Lithofacies I. Rather, grain size decreases steadily, grading continuously into Lithofacies Ill. Heavy mineral suites contain tourmaline, rutile, and zircon, suggesting that ferromagnesian minerals seen in the first lithofacies have been removed by intrastratal solution possibly providing some of the material for iron-oxide cementation. Color ranges from moderate reddish brown (10R 4/6) to light brown (5YR 6/4) with little of the color variability of Lithofacies I. 46 Figure 6. Features of the Eutaw Formation. A, Vertical section through tidal channel sequence in Lithofacies I, B, Reactivation surfaces within coarse sand in Lithofacies I (object at lower right is head of entrenching tool). C, Sharp, disconformable contact (arrow) of Lithofacies II overlying Lithofacies I. D, "Ball and pillow" structures in updip exposure of Lithofacies Ill. E, Hummocky crossbedding and burrowing in Lithofacies Ill interbedded with Lithofacies IV . F, Horizontal bedding and mud clast conglomerates in Lithofacies Ill. 47 This lithofacies is characterized by large-scale, trough crossbeds and less abundant tabular cross beds . Scale of crossbedding decreases upward in the unit, gradually being replaced by massive, unstructured sediments at the top. Ophiomorpha burrows are especially common at the base of the unit, where they may be so abundant that individual burrows are not distinguishable within a maze of burrows. The number of Ophiomorpha burrows decreases toward the top of the Iithofacies where they disappear altogether. Crossbed dip directions box the compass with no obvious modal direction. The environmental interpretation of this lithofacies is drawn not only from its specific characteristics but also from the character and stratigraphic relations of the two overlying lithofacies, which form (with Lithofacies II) an essentially continuous, conformable "package" of strata. Consequently, discussion of paleoenvironments will be postponed until the remaining lithofacies have been described. Lithofacies Ill. The third lithofacies lies conformably on the second across a continuous, gradational contact in northern areas where it forms one to two-meterthick tabular layers. These layers are traceable over fairly large areas and can be shown to thin progressively toward the south wh ere they are intercalated with layers of the fourth lithofacies, usually with an abrupt, erosional, lower contact and a relatively sharp but mi xed, gradational, upper contact. Lithofacies Ill is characterized by fossi Iiferous, fine to very fine, moderately to well sorted, silty, micaceous, quartzose sand. Heavy minerals compose up to three percent of the sediment and consist of ilmenite, rutile, zircon, almandite, tourmaline, and rare glauconite. Fine, carbonaceous, plant debris is observed on bedding planes, especially in downdip areas where Lithofacies Ill is mainly represented by thin layers within Lithofacies IV . Color is generally light olive gray (5Y 6/ 1) but in places approaches very light gray (N8). Carbonaceous laminae are generally shades of brown. In updip areas, the third lithofacies is typically massive or contains well-developed, large, "balland-pillow structures (fig. 60). In other areas, it contains horizontal laminations and very low-angle crossbedding of the type called "hummocky crossstratification" by Harms (1975). This type of bedding has the following characteristics: "(1) lower bounding surfaces of sets are erosional and commonly slope at angles less than 10 degrees ... , (2) laminae above these erosional set boundaries are parallel to that surface, or nearly so, (3) laminae can systematically thicken laterally in a set, so that their traces on a vertical surface area are fan-like and dip diminishes regularly, and (4) the dip directions of erosional set boundaries and of the overlying laminae are scattered" (Harms, 1975, p. 87). In addition, bounding surfaces between sets are typically convex upward (Harms, 1975, fig. 5-5) . Figure 6E is a photograph of Lithofacies Ill showing these characteristics. Frequently associated with hummocky crossbedding are thin beds of intraformational conglomerate whos e clasts are sub-equant, subrounded "balls of medium dark-gray (N4) silty clay, similar to some clay beds in Lithofacies IV (fig. 6F) . Distinct burrowing is often observed, especially at the tops of layers and some can be recognized as the escape burrows of a callianassid-shrimplike organism (Ron Taylor, personal commun., 1979). This lithology is frequently very fossiliferous with numerous molds and casts of bivalves (Cardium , Protarca, and Corbu/a, to name a few), gastropods, and cephalopods , along w ith rare sharks' teeth and fish scales. The fossils are typically found on bedding plane surfaces and are uniformly small (regardless of their genera), suggesting hydrodynamic sorting during transportation. The shells are generally unrounded, however, and thus have not been transported far. Lithofacies IV. The fourth lithofacies composes the bulk of th e Eutaw Formation in its main outcrop belt in Upatoi Creek Valley . It is intercalated with the third lithofacies, and individual beds thicken as Lithofacies Ill beds thin in the downdip direction. In most of the study area, Lithofacies IV is a fossiliferous, poorly sorted, silty, clayey, very fine to fine, micaceous, quartzose sand, but the average grain size decreases gradually downdip so that on the south side of Upatoi Creek the unit is composed of very fine sandy, clayey siltstone or even silty claystone . The primary texture of the sediment is speculative since the entire lithofacies is strongly and p ervasively bioturbated . What may have originally been alternating layers of very fine sand and silty clay are now layers that are completely intermixed and churned up. The sediment contains up to two percent muscovite mica, and heavy minerals include glauconite, rutile, almandite, ilmenite, tourmaline, hematite, zircon, and magnetite. Glauconite grains are generally fine to very fine sand size and have a subrounded appearance. The sediment ranges in color from medium light gray (N6) to dark gray (N3). Notice has already been taken of the completely bioturbated character of the sediment which results in a distinctly to indistinctly mottled appearance. In some instances, individual trace fossils can be 48 observed and are dominantly horizontal in their orientation although they may be seen to move up and down through the strata along their horizontal course. Some of these burrows are recognizable because of the presence of clean, white sand within generally dark-gray mud, suggesting sand infilling of open burrows. Calcareous concretions, occasionally with septarian-type shrinkage cracks, occur in downdip areas, usually associated with very fossiliferous horizons. The septarian aspect of these concretions is taken to imply relatively low sedimentation rates because the shrinkage necessary to crack the sediment could not have occurred under high lithostatic (and hydrostatic) stress in the deeper subsurface. This lithofacies is abundantly fossiliferous (see Stephenson, 1957) with numerous bivalves (especially Exogyra upatoiensis, Ostrea cretacea, Gryphaea wratheri, Anomia argentaria, Cardium sp. Nucula sp., etc .), gastropods, cephalopods (for example, Placenticeras, benningi), abundant sharks' teeth , fish vertebrae, and unidentified bone material. Unlike the fossils in Lithofacies Ill, those in Lithofacies IV range widely in size and some appear to be in life position. Carbonaceous plant debris and pieces of lignite (from small "twigs" up to half-meter long "logs") are found dispersed through the lithofacies, but lignitic pieces are "water-worn" in appearance and plan debris (like that in Lithofacies Ill) is composed of very small fragments with no recognizable leaf prints . Paleoenvironmental interpretation of Lithofacies II, Ill, and IV. The upper three lithofacies are considered together because they constitute a stratigraphically and genetically related unit. I interpret them as representing deposition in progressively deeper subenvironments of the shore zone, the record of a marine transgression. Lithofacies II is considered to have been deposited on the low-tide terrace of the upper shoreface (or swash zone) just below mean low-water mark (Davis, 1978). The dominance of trough crossstratification implies megaripple migration (see relation of bed forms to shore geometry in Clifton, and others, 1971) and subordinate tabular crossstratification represents deposition from sand waves during slight fluctuations of hydraulic conditions, especially depth of flow. Strongly polymodal paleocurrent data suggest highly variable flow conditions of the wave-, tide-. and storm-influenced upper shoreface (Selley, 1968; see also the paleocurrent model for low-tide terrace deposits in figure 34, Davis, 1978). The abundance of Ophiomorpha burrows at the base of Lithofacies II coincides in this model with the area just below low tide, the same area preferred today by Callianassa major, the most likely candidate for the "Ophiomorpha - animal" (Weimer and Hoyt, 1964) . Vertical change in grain size and in scale of cross stratification indicates progressively deepening water as sea level rose, increasing the distance from shore. Lithofacies II grades upward into Lithofacies Ill, which is believed to represent deposition on the lower shoreface . Harms (1975, p. 87) characterizes lower shoreface sediments as consisting of "very fine to fine sandstone with tabular beds of nearly horizontal stratification or hummocky crossstratification. . .. The sandstone contains more mica and carbonaceous fragments than does the upper, swash zone facies." Hummocky crossstratification is believed by Harms (1975) to have been formed by strong storm-wave surges over the lower shoreface, a speculation supported in Eutaw deposits by the association of mud-clast intraformational conglomerates and hydrodynamically sorted bioclasts. Burrowing within Lithofacies Ill deposits provides supportive evidence for this model as does the presence of fine carbonaceous debris on bedding planes and as discrete laminae instead of dispersed through the sediment as might be expected if the source of the debris were nearby. The high degree of sorting of Lithofacies Ill sand and relatively high proportion of heavy minerals suggest constant winnowing and reworking of the sediment by waves . The constantly downdipthinning and continuous, tabular nature of Lithofacies Ill strata may also be considered as evidence for the proposed model, since the shoreface is a geographically continuous zone characterized by water-depth asymmetry away from land . As water depth continued to increase during transgression, the lower shoreface was progressively less affected by wave agitation, passing gradually into the transition zone between the shoreface and the shallow shelf. The transition zone, represented in Eutaw strata by Lithofacies IV, is characterized by intensely bioturbated sandy, silty, clayey sediment (Harms, 1975) and a cosmopolitan marine faunal assemblage. Lithofacies IV fossil assemblages do not appear to have been transported any significant distance and are characterized by high diversity and moderate density, an ecological situation typical of open marine environments. The relatively large amount of carbonaceous debris is supportive evidence of the transition zone interpretation. Campbell (1971) reports that carbonaceous particles exceeded ten percent of total grains in some transition zones within the Gallup Sandstone. Such carbonaceous material was probably carried by ebbtidal currents or storm currents and allowed to settle out relatively far from land where wave 49 energy was less. The geometry of Lithofacies IV strata is also significant. They thicken in the downdip direction, implying that it was progressively better developed with increasing distance from land. The presence of thin intercalated layers of Lithofacies Ill-type sediment probably represents storm deposits, formed by waves of great length which scoured the bottom and momentarily initiated lower-shorefacelike conditions. Gradual decrease in grain size in the downdip direction implies gradual deepening of water and the change to a true graded shelf environment. Such true shelf deposits do not exist in surface exposures of the Eutaw Formation in Georgia; however, they are found in Alabama (Reinhardt, this volume). Eutaw Paleogeography It remains now to assemble the specific depositional environments discussed above into a coherent reconstruction of Eutaw paleogeography. I believe that the Eutaw Formation in western Georgia represents a barrier-island complex formed during rising sea level. However, all of the original barrier-complex facies have not been retained in the stratigraphic record. Subaerial facies were removed by erosion as the transgressing sea pushed the barrier complex landward, producing the disconformity between Lithofacies I and Lithofacies II. This barrier retreat and concomitant erosion resulted in the deposition of subtidal shore-zone sediments (Lithofacies II, Ill, and IV) directly over subtidal back-barrier sediments (Lithofacies I). This model presumes the former existence of more extensive back-barrier facies north of the present Eutaw outcrop belt. These deposits have been removed by post-Eutaw erosion, either during pre-Biufftown emergence or, more likely, during a more recent period of emergence. Complete preservation of barrler-cornplex deposIts In transgress ive stra tig.raphi c sequences is rare, but l1as been reported {for examp le, Hobday and Tanl I NP14 Tallahatta 50 NP13 >-a-:-' 1--NP12 ~ I-- NP11 I-- NP10 Non Deposition ---~ Bashi ~ ------ ---- -- 55 UJ z UJ u NP9 -------------- Tuscahoma 1--- w NPB ~- -~ --- -- -- ~ ~ ~ ~ ____. ~ Nanafalia ____,;r I NP5 ~---------------- 0 NP4 GO UJ ....1 1--1----- <( a.. NPJ -- ~ a: - I ... 0 0 0 ~ _! 0 z "w-' Ia::I>:. PROVINCIAL STAGE 2 ~ NE 0" :>l' li ( ; I I ! ' [ ~ fl:AI I j . - . ? ' AU\~ H OF) z " u (5 a: "' > ::> ~ >a: II. >a: -<( I- a: UJ I- gzz a : Uz J UJ 1---izz"--- ? 0 a>:<( 0 ~ 0 z UJ ">w- I I II I l I II II I I II III I I II 7"" 2 ~ !;! t---:!- z z<( a: 0 m ~ 0 >- 3 III I f.-- ~ z "OF) w l_l I I I I zUJ UJ 0 0 UJ _! <( 0. a: II. >- z "z>w- <( i': z "z 0 I II z ? I I .. " z iii <( OF) ~I . . I . . I .. ............ 1---z .. .. . . . . . .. . . . 0 ~ ~ . ...... !;! ::; J, ~ 1111111 lllllllllll llllll lll :( ~ ~~ rt-ln :::-:-:-:-:-:-:-:-:- i~ I t2 -.-.-::: _::::::::T >a: <( II. a: UJ 0. C/) 0. ::::> ::::> ~ . .. : . .. .. .. ~ o _ r-'- . . . .... . ~ z ..... .. ... . <( 2 fz2 <( OF) 0 ? ln+rTnT1......... "T''TIT1T1.......1 0 UJ 0 <( ~ ~ I- UJ a: . . . ... . . . 7 ntlt.lm mr f w LW~: _ j 0 ~1 ~I m~ -a~":' 0 II. ~ <- ' I I-. _! Figure 3. Geologic column showing the stratigraphic distribution of Cretaceous and Paleogene depo sitional sequences onshore in the Southeast Georgia Embayment. Thicknesses of strati graphic units and durations of ages are not im plied. North (right)to-south (left) distribution of terrigenous clastic facies (stippled pattern) and carbonate facies (limestone pattern) is shown. Moderate to long periods of nondepo sition and (or) erosion are shown by vertical lines. 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Research Spec. Pub . 12, 68 p. Stephenson, L.W., 1914, A deep well at Charleston , South Carolina: U.S. Geol. Survey Prof. Paper 90-H, p. 69-94. Swain, F.M., and Brown, P.M., 1964, Cretaceous Ostracoda from wells in the southeastern United States: North Carolina Div. Mineral . Resources Bull. 78, 55 p. Thierstein, H.R., 1976, Mesozoic calcareous nanno- plankton biostratigraphy of marine sediments: Marine Micropaleontology, v. 1, p. 325-362 . Valentine, P.C., 1979, Regional stratigraphy and structure of the Southeast Georgia Embayment, in Scholle, P.A., ed., Geological studies of the COST GE-1 well, United States South Atlantic Outer Continental Shelf area: U.S. Geol. Survey Circ . 800, p. 7-17 . Valentine, P.C ., and Poag , C. W., 1976, Cross sections, showing regional stratigraphi c relationships, in Hathaway, J.C ., and others, Preliminary summary of the 1976 Atlantic margin coring project of the U.S. Geological Survey: U.S. Geol. Survey open-file rept. 76-844, p. 198-205. Wolfe, J.A., 1976, Stratigraphic distribution of some pollen types from the Campanian and lower Maestrichtian rocks (Upper Cretaceous) of the middle Atlantic states: U.S. Geol. Survey Prof . Paper 977, 18 p. Zack, Allen, 1977, The occurrence, availability, and chemical quality of ground water, Grand Strand area and surrounding parts of Horry and Georgetown Counties, South Carolina: South Carolina Water Resources Comm . Rept. No.8, 100 p. Zupan, A .J., and Abbott, W.H., 1976, Comparative geology of onshore and offshore South Carolina, in Hathaway, J.C., and others, Preliminary summary of the 1976 Atlantic margin coring project of the U.S. Geological Survey, U.S. Geol. Survey open-file rept. 76-844, p. 206-214. 74 UPPER EOCENE STRATIGRAPHY OF EASTERN GEORGIA Paul F. Huddlestun and John H. Hetrick Georgia Geologic Survey Atlanta, Georgia ABSTRACT The upper Eocene Barnwell Formation is herein raised to group status. Three formations of t!le Barnwell Group are recognized: the Clinchfield Sand, the Dry Branch Formation (new name), and the Tobacco Road Sand. Four members of the Clinchfield Formation are recognized: the Riggins Mill and Treadwell Members of eastcentral Georgia, and the Albion and Utley Members of eastern Georgia. The Dry Branch Formation is divided into two members, a lower Twiggs Clay Member and an upper Irwin ton Sand M ember. An unn amed c alcareous sand / sandy limestone subdivis ion of ll1 e Dry Bran c h is recognized In th e shal low subsurface of Screven and no HI1 ern Eff i ng l1am Counties , Georg ia The Sandersville limes tone Is recog ni zed as a member or th e Tobacc o Road Sand. The three upper Eocene rock groups in the southeastern United States may best be viewed as lithosomes: a sand lithesome (Barnwe ll Group of 111e south Atl antic Coastal Plain). a clay lithesome (Yazoo Group olthe Gulf Coastal Plain), and a carbonate lithesome (Ocala Group of the Florida banks) . These li l110somes tntertongue in central and eastern Georgia. Two tongues of the Ocala Group are recognized: the lower tongue, the Tivola Limestone, is present only in the western part of eastern Georgia and is equivalent to the lower part of the Dry Branch Formation with which it intergrades both laterally and vertically. The upper tongue, the Ocmulgee Formation (new name. previously Cooper Marl) is equivalent to the Tobacco Road Sand. The Twiggs Clay, thick and predominating the Dry Branch Formation in the west, is very thin to absent in the east. The Twiggs Clay probably represents the easternmost tongue of the Yazoo Group of the Gulf Coastal Plain For practical usage in Georgia, the Twiggs Clay is considered a part of the Barnwell Group. Three minor sedimentary cycles are represented in the Barnwell Group: the Clinchfield represents the first sedimentary cycle, the Dry Branch the second cycle, and the Tobacco Road the last cycle. Sedimentation in each of these cycles was terminated by a relative lowering of sea level or stillstand . The greater concentration of cut and fill channel sands in the middle of the Dry Branch may indicate a third very minor event. 75 UPPER JURASSIC SMACKOVER PETROLEUM GEOLOGY OF SOUTHWEST ALABAMA Ernest A. Mancini State Oil and Gas Board of Alabama and the University of Alabama, University, Alabama The f o l low i n g is a vers i on o f a pa pe r to b e pu bli sh ed in J o urn al o f H y d ro logy, Sp ec ial Geoche mi cal V olume . ABSTRACT Upper Jurassic Smackover deposition in southwest Alabama was primarily controlled by the Mississippi Interior Salt, Manila, and Conecuh Basins and closely approximated carbonate sedimentation in the Persian Gulf. Each of these basins has distinctive lithofacies and faunal assemblages. Early salt movement resulted in local variations in carbonate sediment distribution, and pre-Jurassic paleo-highs, such as the Wiggins Uplift and Conecuh Arch , also modified carbonate sedimentation. Throughout most of southwest Alabama, the Smackover Formation consists of a lower member which overlies the Norphlet Sandstone and an upper member which is overlain by the Buckner Anhydrite. Where present, the lower Smackover member includes stromatolitic laminated and bioturbated mudstone, fossiliferous wackestone and packstone, and/or dolomite.The upper member usually consists of oolitic, fossiliferous grainstone, bioturbated, pelletal, or fossiliferous mudstone to packstone, stromatolitic mudstone, and/ or dolomite. Petroleum traps in southwest Alabama are principally combination traps involving favorable stratigraphy and salt anticlines, faulted salt anticlines, or extensional faults associated with salt movement. Reservoir rocks include oolitic grainstones, leached and dolomitized wackestones, packstones, and grainstones, dolomitized stromatolitic mudstones, and granular dolomite. Porosity is facies-selective and is preserved chiefly in lithofacies of the upper Smackover member. The algal stromatolites that characterize the lower Smackover member and are interbedded with upper Smackover lithologies throughout most of southwest Alabama make excellent petroleum source rocks. The .flanks of the Wiggins-Conecuh trend and updip Smackover grainstones associated with salt structures are excellent areas for petroleum exploration in southwest Alabama . The key to successful prospecting is the delineation of traps associated with salt movement and identification of original high to moderate-energy lithofacies that have been leached and/ or dolomitized. INTRODUCTION Petrol eu m ex ploration has in creased rap id ly In south west Al ab ama since t he disco very of petroleu m in the G i lbert own Fi eld in C h octa w Coun ty in 1944. In 1967, only 10 pro d uc ing oil and gas f ie lds ex isted in Al aba rna , and by 1977, the number of producing fields in southwest Alabama alone increased to 31 (Masingill and others, 1977) . The value of oil , condensate, and gas produced in the State during 1977 approximated 192 million dollars (Masingill and others, 1977). The primary exploration target in southwest Alabama has been the Upper Jurassic Smackover Formation (fig . 1). PERIOD GROUP OR FORMATION COTTON VALLEY GROUP HAYNESVILLE FORMATION () UJ UJ ~ SMACKOVER FORMATION 11:: :-::,> NORPHLET FORMATION LOUANN SALT "(") WERNER ANHYDRITE UJ UJ ~ 11:: EAGLE MILLS FORMATION 1- Figure 1. Jurassic and probably Triassic subsurface stratigraphy in southwest Alabama . The geologic factors that make a successful Smackover petroleum prospect include the petroleum trapping mechanism, petroleum reservoir, petroleum source rock, and relationship between hydrocarbon migration and structural deformation. Regional geologic trends must be understood before a successful Smackover exploration strategy can be formulated . This research determines the regional stratigraphic and structural relationships associated with Smackover deposition and deformation in southwest Alabama (fig . 2) . Reg ional relationships are identified through subsurface geological study utilizing well logs and core materials . Establishment of the regional trends will help in delineating the geologic processes controlling Smackover petro- 76 Platform Lagoon 0 c 2 0 c 0 " "u" ~ 0 c ~ 0 c 0 "'tl , c 0 3 iO Ill Open Platform ti c 0 ~ Ill Basin Figure 3. An idealized model for carbonate deposi- tion in a "shallow shelf" depositional framework indicating environments of a... deposition and textures of carbonate rocks a... accumulating in the various environments. (/) (/) slope. Carbonate lithofacies are not protected by a shelf-margin barrier and tend to be distributed in bands which parallel the coastline and reflect the Brewton greater wave and current activity near the shore. Patch reefs may be developed on local topographic FLORIDA paleo-highs. Recent examples of the "ramp" frame- 0 Miles 20 N 1 work are the Campeche Bank (Yucatan Peninsula) and the Trucial Coast of the Persian Gulf. Ancient analogs are the Jurassic Cotton Valley Formation of the northwestern Gulf of Mexico and the Jurassic Smackover Formation from south Texas to Florida (Ahr, 1973). Figure 2. Location map of study area. leum accumulation . Emphasis has been placed on the Mobile County area because of the recent petroleum discoveries in that area (McCaslin, 1975; Oil and Gas Journal, 1975). GEOLOGIC SETTING Carbonate deposition commonly takes place on either a "shallow shelf" (fig. 3) or a "ramp" (fig. 4) depositional framework . The "shallow shelf" model involves a nearly flat platform and a clearly defined shelf-slope break. Lithofacies are governed by topography, with grainstones and boundstones occurring on shoal areas and mudstones and wackestones occurring in lagoons or behind barriers. Generally, a continuous reef occurs at the shelf edge. Modern examples of the "shallow shelf" framework include Florida and the Bahamas. Ancient analogs are the Cretaceous Edwards of Texas, Cretaceous El Abra of Northern Mexico, and the Permian Capitan of West Texas and New Mexico (Ahr, 1973). The "ramp" model is an inclined platform that extends basinward without a pronounced break in Offshore Figure 4. An idealized model for carbonate depostion in a "ramp" depositional framework indicating environments of deposition and textures of carbonate rocks accumulating in the various environments. The lithofacies distribution associated with the "ramp" model is affected by topographic highs which develop on the platform. These topographic highs may be a result of paleo-highs or salt movement. In the Persian Gulf, Purser (1973) found that lithofacies patterns are controlled primarily by the distance between a salt structure and the shoreline. The highest wave and current activity occurs on the crest of the salt structures, with a progressive decrease of energy off the structure. Structures located basinward tend to have high-energy carbonate lithofacies concentrated at their crests and lower-energy lithofacies distributed in concentric 77 bands around the structure as a function of depth and decreasing energy. Topographic highs near the coast usually have high-energy lithofacies on the basinward side and low-energy lithofacies on the shoreward side. Smackover accumulation in southwest Alabama closely approximated present-day carbonate sedimentation in the Persian Gulf. The Louann Salt was primarily responsible in forming the ramp surface. A regional structure map drawn on top of the Smack- over illustrates a gentle south to southwest dip for the formation (fig . 5). This trend is locally interrupted by extensional faults, such as the PickensGilbertown , Foshee-Pollard, and Jackson-Mobile, and by salt anticlines, such as the Chatom and Klepac. Early salt movement resulted in local variation in carbonate sediment distribution. Pre-Jurassic paleo-highs, such as the Wiggins Uplift and Conecuh Arch, also modified carbonate sedimentation (fig. 6). APPROXIMATE UPDIP LIMIT OF SMACKOVER FORMATION NP SMACKOVER NOT PRESENT .._.__.__.__, FA U L T CONTOUR INTERVAL = 500 FT 10 20 Ml Figure 5. Regional structure map on top of the Smackover Formation for southwest Alabama. Faults have been defined by Gary V. Wilson , Alabama Geological Survey. 78 N 1 CONTOUR INTERVAL ~50FT 0 10 20 Ml Figure 6. Regional isopach map of Smackover Formation for southwest Alabama (modified from Wilson, fig. 6, 1975). In the area of study, three probable basins, the Mississippi Interior Salt, the Manila, and the Conecuh Basins, are recognizable (fig. 7). These basins have been delineated on the basis of thickness of the Smackover Formation (fig. 6) and the depth to basement and basement configuration as indicated by geophysical gravity anomalies (Wilson, 1975, fig. 4) . The most westerly basin , which occurs primarily in Washington and southern Choctaw Counties, is an extension of the Mississippi Interior Salt Basin of eastern Mississippi. The Manila Basin of eastern Clarke, western and southern Monroe, and northern Mobile and Baldwin Counties, is named after the town of Manila, which is located near the apparent center of the basin in eastern Clarke County (figs. 2 and 7). The most easterly basin, the Conecuh Basin as defined by Sigsby (1976), occurs in Escambia and southwestern Baldwin Counties and extends into northern Florida. The Wiggins-Conecuh trend separates the Conecuh and Manila Basins (fig. 7). Smackover carbonates thin to less than 100ft over this structural trend (fig. 6). Seismic refraction and gravity data suggest that the Wiggins-Conecuh trend acted as a stable basement high throughout Smackover deposition (Wilson, 1975) . The composition of this basement complex consists of granitic, basaltic, and volcanic rocks (Neathery and Thomas, 1975; Neathery, 1979, personal commun). The basement complex is believed to be a continuation of the Piedmont structural trend (Neathery and Thomas, 1975). The Manila Basin appears to be separated from the Mississippi Interior Salt Basin by a basement paleo-high or salt ridge (fig. 7) . Smackover carbonates thin to less than 100ft over this trend located in northern Mobile, eastern Washington, and western Clarke Counties (figs. 6 and 7) . In the Manila 79 INTERIOR ? ) SALT ( BASIN t l I \ I .--- ...... I / \ '\ BASIN FLORIDA + APPARENT BASEMENT H16H .......-,. APPROXIMATE BASIN OUTLINE - - - AREA OF KNOWN THICKEST SMACKOVER 0 10 20 MI. Figure 7. Paleogeographic map for southwest Alabama during Smackover deposition. Basin and the eastern part of the Mississippi Interior Salt Basin the Smackover exceeds 400 and 550ft, respectively. STRATIGRAPHY AND LITHOFACIES The Jurassic section normally present in the subsurface of southwest Alabama is shown in figure 1. The contact between the Smackover and underlying Norphlet Formation in Mobile County is conformable, grading upward from dolomitic sandstone to a silty dolomite. The Smackover conformably underlies the Buckner Anhydrite Member of the Haynesville Formation. The contact is gradational and generally grades from dolomite through interbedded dolomite and anhydrite to anhydrite. In the study area, the Smackover Formation can be divided into a lower and upper member, each of which contains distinctive carbonate lithologies. Where present the lower member includes stroma- tolitic, laminated and bioturbated mudstone, fossiliferous wackestone and packstone and/or dolomite. The upper member usually consists of oolitic, fossilferous grainstone, bioturbated, pelletal, or fossiliferous mudstone to packstone, stromatolitic mudstone and/or dolomite. The contact between these stratigraphic units is gradational. Smackover lithofacies are distinctive for three basins, suggesting that depositional conditions var- ied among them. The lower member represents intertidal and subtidal lithofacies that were depos- ited during a marine transgression which was prob- ably initiated during late Norphlet time. The upper Smackover member represents supratidal to subti- dal deposition. Although numerous shoreline oscil- lations are evident during deposition of this lithofa- cies, the overall trend in most of the study area is regressive. Deposition of the Buckner Anhydrite completes the regressive carbonate phase. 80 The lithofacies observed in the Placid Oil Jackson No.1 (281ft of core), 4 mi north of Toxey, Choctaw County, Ala., are typical lithologies for the Mississippi Interior Salt Basin (fig. 8). In this core, the Buckner consists of anhydrite interbedded with dolomite which is characteri stic of supratidal deposition. The upper part of th e Smackover is a dolomitic, oncoloidal, fossiliferou s grainstone interbedded with dolomitic packstone with c alcite replacing anhydrite laths. Underlying these carbonates is a fossiliferous, bioturbated packstone which grades into an oo litic, fossiliferous, crossbedded grainstone containing terrigenous grains. These lithologies Indicate Intertid al and subtidal deposition. The next unit is a fossiliferous, pelletal , bioturbated, argi llaceous, lami nated wackeston e which probably accum ul ated in a subtidal en vironment. The lower part of the core consists of an oolitic, oncoloidal, fossiliferous grainstone interbedded with fossiliferous, bioturbated, stromatolitic mudstone and pelletal, oncoloidal wackestone. These lithologies primarily represent intertidal to subtidal deposition. The mudstone, however, was occasionally subaerially exposed, as evid ence d by mudcracks and collapse solution-brecciation features. The lower Smackover, as observed in well cuttings (~72ft) from this dry hole, is an argillaceous, laminated mudstone which probably accumulated in intertidal and subtidal environments. The Smackover lithofacies present in Choctaw County are sim ilar to those described by Badon {1974) for Clarke County, Mississippi , and by Wake lyn (1977) for Perry and Stone Counties, Mississippi. In Clarke County, Mississippi, upper Smackover lithofacies consist of a peritidal dolomite, a nearshore high-energy oolitic grainstone, and a subtidal low-energy pelletal mudstone (Badon, 1974). In Perry and Stone Counties, the upper Smackover includes interbedded grainstones, packstones and wackestones, which were deposited in supratidal to subtidal environments (Wakelyn, 1977), whereas laminated mudstone of the lower Smackover represents subtidal deposition (Wakelyn, 1977). The Buckner in both of these areas consists of interbedded dolomite and anhydrite which accumulated in a supratidal environment (Badon, 1974; Wakelyn, 1977). The fauna found in the Jackson No. 1 core is also similar to that described by Badon (1974) and Wakelyn (1977) for eastern Mississippi. These lithol ogic and faunal similarities suggest that the carbonate rocks deposited in western Al abama and eastern Mississippi accumulated in the same depositional basin, the Mississippi Interior Salt Basin. The Smackover in the Manila Basin is predominantly a dolomitized, laminated mudstone and wackestone, which probably accumulated in intertidal and subtidal environments. Fossils are rare to absent in cores from the Getty C.D. Broughton no. 3 (50ft of core), 7 .5 mi northwest of Uriah, Monroe County, and Getty Blacksher Estate No.1 (60ft of core), 1 mi northwest of Uriah, Monroe County. The scarcity of taxa in this basin compared to the Mississippi Interior Salt Basin suggests environmental conditions were more hosti le. These faunal differences and distinctive lithofacies imply that the Manila Basin was isolated from the Mississippi Interior Salt Basin during Smackover deposition. The lithofacies present from top to bottom in the Exxon L&N Railroad No . 1 core (287ft), 1.5 mi. west of Pollard, Escambia County, Ala., include an intertidal bioturbated, argillaceous, laminated, pelletal dolomitic mudstone, intertidal to subtidal fossiliferous, pelletal, dolomitic packstone and wackestone, and an intertidal laminated, argillaceous, pelletal mudstone interbedded with dolomite (fig. 9). These lithofacies are similar to those described by Ottmann and others (1973) and Sigsby (1976) for the Jay trend located within the Conecuh Basin. Sigsby (1976) reports that the upper Smackover member in this basin is represented by pelletal dolomite and mudstone, and the lower Smackover member includes a fossiliferous mudstone, pelletal mudstone, pelletal oncoloidal mudstone, and dolomite. Fossils were observed in the L&N Railroad No. 1 core and have been reported by Sigsby (1976) from other cores in the basin. Faunal diversity in the Conecuh Basin is higher than in the Manila Basin but lower than in the Mississippi Interior Salt Basin. The faunal differences and distinctive lithofacies suggest that the Conecuh Basin was separated from the Manila Basin during Smackover deposition. In the Chunchula-Hatters Pond area, probably only the upper Smackover member is present. The laminated mudstone characteristic of the lower Smackover member was not observed in any of the 16 cores studied from this area. Lithofacies typical of these fields are found in the Getty Peter Klein No. 1 (104ft of core), 12 mi northeast of Mobile. These lithofacies include a granular dolomite interbedded with a massive dolomite, a bioturbated wackestone, and a massive, bioturbated dolomite (fig. 10). Fossils were very rare or absent in the cores studied from Mobile County. Smackover deposition in Mobile County was dominated by the Wiggins Uplift. The Smackover is absent on the crest of this structure (fig. 5) and is much thinner (50-150 ft) along its northern flank compared to ty pical Smackover thickness in southwest Alabama (figs. 6 and 11) . The apparent absence of the lower member and the upper member not attaining normal thickness for this area suggests that the Wiggins was a positive feature during early Smackover time. Immediately west of Mobile County in Stone and Perry Counties, Mississippi, the Smackover is 800 to 900ft thick, and the laminated mudstone lithofacies of the lower Smackover member is well developed in these counties (Wakelyn , 1977). The reduced thickness of the Smackover section in the Chunchula-Hatters Pond area, therefore, is probably due to nondeposition of carbonate sediments. 81 ~Limestone oolomite D Sandstone [g} Anhydrite " ~Granular Dolomite ~ Grainstone ~Packstone ~Wackestone ~Mudstone ~Anhydritic I .Il l :Ir Dolomitic ~Calcitic B Argillaceous liD Oil-Stained [TI Fossiliferous ~Oolitic ~Pelletel ~ Oncoloidal ~ lntraclastlc W Bioturbated EJ Stromatolitic G Nodular Anhydrite Feet "I "I " A A A -t03 00 OiL JC ~ -!--' ~~l- +- E -1031~ f (;) ",-'\ Iii :J: :z:l I 6 - t0330 A. f) Anhydrite, Interbedded w/ Dolom1te Grainstone, OolitiC, lnlerbedded w/ Packstone Packstone, Fossiliferous BUCKNER -1o3e ..... r-: -1o3eo . -1037!5 -10390 -1040!5 . A A G . j I _...... " en " :J: - 10420 . ..... - -1043~ - 10480 -1 04.05 '_.&_ - 10-4180 ..... . Cil -o~e -101!510 A. t) ...... c. ...., -10G2& --10&40 ..... ""' """' r>. -10&&1> _l ....._ I c. !) l 0 - 105?0 _l Ir " I 0 I"" II I ..... v _j} 'V "'-/ Grainstone, Oolitic, X-Bedded Wackestone, Argillaceous Grainstone, Oolitic Mudstone, Bioturbated Grainstone, Oolitic lfnufl an 61HIInlahl1 c.. C3ralnstone, Oolitic M ild I OI't aml o lltl l'lr "'o "' Ot:~ 1 11e Mudstone, Stromatolitic W c. het l '9 " Fo$llfrou Mudstone, Stromatolitic Wackestone, Interbedded w/ Grainstone Mudstone, Stromatolitic Wackestone, Interbedded w/ Grainstone SMACKOVER Figure 8. Carbonate lithologies identified in the Placid Oil Jackson No.1 core, 4 miles north of Toxey, Choctaw County, Ala. 82 ~ Fa J 1\ ~ ft. A \~ ... A ~ ' -- Anhydrite Mudstone, Anhydritic Mudstone I BUCKNER - 1&.:!'2'0 Mudstone, Bioturbated Feet ~ A-""""-"AA ,..,.. ,.....,...,...,... ,...,...~,...,.....,.../" ,_[ !i:::ll _,...or A A l -~'o ~ I ~ I ~ I ~I Mudstone Mudstone, Argillaceous Mudstone Mudstone, Argillaceous Mudstone I I I I I I I I SMACKOVER -18020 / 5/ 7-- / _.._ / / ) 7 _.._ j x :r. /"' " / 1 -18030 I JL I 1~ I I I I I _, -18040 T () I T s 7 1 7 / 1 1/ I 1/ -18050 / / 11 I I I -18060 0w 0 ' ' I l'i':J,----,-----~ tJ ~~40~; 1j ! Mudstone, Laminated Wackestone Packstone, Dolomitized I -18070 I Waeke.stona Packstone, Dolomitized I I -m{! J( ~ i ;1 Mud1111ttn lii..I'I"'j naJ.d I I Dolomite _,_~H~~!wh~l I I Mudstone, Laminated Interbedded w/ Wackestone if. !tD "' -ISOJOft 1 Dolomite Mudstone, Laminated I I I I ~ ...I ..I ...'I Mudstone, Arg1llaceous Sandstone I NORPHLET -18080 I ....._ I I I / CP 7 / A" I I -18090 7 7 _.._ 7 j ep j I / / A"'- / -18100 I "-"I I I I/ / CD 7 7 ; -18110 . 1 1 1 7TTI 7 11 1 / "' " / 7 . . . . -18120 1 1 1 :0: E' ~~~ Anhydrite Dolomite, Bioturbated Dolomite. Massive Wackestone Dolom1te. Granular No Recovery Dolom1te. Mass1ve Dolomite, Granular Sandstone, Dolomitic BUCKNER SMACKOVER NORPHLET Figure 9. Carbonate lithologies identified in the Exxon L&N Railroad No. 1 core, 1.5 mi west of Pollard, Escambia County, Ala. See lithologic legend for figure 8. Figure 10. Carbonate lithologies identified in the Getty Peter Klein No. 1 core, 12 mi northeast of Mobile, Hatters Pond Field, Mobile County, Ala. See lithologic legend for figure 8. HJW N ' 209 .. 0 206 2 0 2293 0 2322 0 1950 0 1886-B 0 2120 0 2350 0 Rw : N 2'!160 0 2 461 0 '207 1 0 I ~ N 1 0 Wf:LL LOCATION ANO Pf:,.MIT NUMIIE.. I I Plf...CI!MI!NT SALT . - ,.11!1-0 OUTl.INf. s J CONTOU.. INTI!..VAL z: 2D I"T 0 , "' ' L---....1 t A e..1. ,.., ft Figure 11 . Isopach map of Smackover Formation for Chunchula-Hatters Pond area, Mobile County, Alabama (modified from Mancini and Stow, fig . 8, 1977). 84 PETROLEUM EXPLORATION moldic dolomite porosity is the leached product of dolomitized pelletal grainstone. lntercrystalline Petroleum traps in southwest Alabama are principally combination traps involving favorable stratigraphy and salt anticlines, faulted salt anticlines, or extensional fault traps associated with salt movement or with the updip limit of Louann Salt deposition. In the Mississippi Interior Salt Basin, Washington and Choctaw Counties, traps are primarily salt anticlines or faulted salt anticlines. In the Conecuh Basin, Escambia County, petroleum is principally trapped by a combination of extensional faulting and salt movement. Such a trapping mechanism is common throughout the Jay trend area (Sigsby, 1976).1n the Manila Basin, southern Monroe County, domal paleo-highs possibly related to igneous intrusions form the petroleum trap. The petroleum traps in Mobile County involve a combination of trapping mechanisms. The Chunchula structure appears to be a moderate-relief salt anticline (fig . 12), and the Hatters Pond structure is a faulted salt anticline associated with salt movement along the west side of the Jackson-Mobile fault system (Mancini and Stow, 1977). dolomite is the result of leaching and dolomitization of pelletal wackestone and packstone, and leached matrix porosity represents dolomitization of algal stromatolites. Sigsby (1976) estimates that 90 percent of the effective porosity in the Jay trend area is a result of leaching and dolomitization of pelletal lithofacies. lntercrystalllne poros1ty IS dominant in the Peter Klein No. 1 core . It is difficult to discern the original composition and texture of the reservoir in the Chunchula-Hatters Pond area because of the severity of the dolomitization process. The Smackover (fig . 5) is absent near the crest of the Wiggins, but is productive on the northern flank of the uplift, suggesting the Wiggins was a positive feature during Smackover deposition. Employing Purser's (1973) model for the Persian Gulf, moderate energy lithofacies probably were deposited on the flanks of the Wiggins in concentric bands around this basinward structure. Therefore, the original lithology of the reservoir may have been a pelletal packstone or wackestone. Stratigraphy plays an integral part in forming a Smackover petroleum trap. Smackover thickness does not appear critical , as indicated by the Smackover isopach map constructed for the ChunchulaHatters Pond area (fig. 11 ). In fact, 8 mi northeast of these fields, the Smackover increases in thickness and wildcats drilled on structural highs in this area have been noncommercial. The carbonate lithofacies in northwest Mobile County are mudstones The algal stromatolites that characterize the lower Smackover member and are interbedded with upper Smackover lithologies throughout most of the study area make excellent petroleum source rocks. Although these rocks do not appear to be present in the Chunchula-Hatters Pond area, they were penetrated in wildcats drilled about 8 mi northeast of those fields in Mobile County. with typical reservoir-forming packstones and grainstones being rare to absent. Reservoir rocks in southwest Alabama include oolitic grainstones, leached and dolomitized wackestones, packstones, and grainstones, dolomitized stromatolitic mudstones, and granular dolomite. Porosity is facies selective and is preserved chiefly in lithofacies of the upper Smackover member. Preservation of original porosity and the formation of secondary porosity are controlled by diagenetic alteration . Dolomitization and leaching are the most critical processes in the formation of secondary porosity. In the Jackson No. 1 core, porosity is both primary oolitic and oncoloidal interparticlate and secondary moldic, resulting from leaching and dolomitization of oolitic and oncoloidal grainstones. Badon (1974) reports that the principalS mackover reservoir rocks in Clarke County, Mississippi, are oolitic grainstones with primary depositional interparticle porosity. Porosity in the Exxon L&N Railroad No.1 core is secondary, resulting from leaching and dolomitization of pelletal packstone and wackestone. Ottman and others (1973) state that the major porosity types in the Jay trend area are grain moldic dolomite, intercrystalline dolomite, and leached matrix. Grain Integrating the regional structural, stratigraphic, and lithofacies relationships with the style of petroleum trapping mechanisms and reservoir and source rock distributions, a successful Smackover exploration strategy can be formulated for each of the basins in southwest Alabama. Exploration in the Mississippi Interior Salt Basin should involve prospecting for grainstones on the basinward side of salt structures and for updip oolitic grainstones associated with salt movement. In the Conecuh Basin petroleum stored in dolomitized and leached pelletal packstones and trapped by extensional faulting and salt movement should continue to be an excellent petroleum target. Prospecting in the southern part of the Manila Basin should be directed toward exploring for dolomitized reservoir rocks associated with paleo-highs. The flanks of the Wiggins-Conecuh trend through Mobile and Baldwin Counties are an excellent area to explore for petroleum as demonstrated by the recent discoveries in Mobile County. The key to successful prospecting along this trend is the delineation of traps associated with salt movement and identification of original high-to moderate-energy lithofacies that were deposited on the flanks of structures and that have been preserved or have undergone favorable diagenetic alteration. The process that is probably the most critical for porosity enhancement is dolomitization. 85 Ptw r "' N 1 1978 0 WfLL LOCATION AND Pt.RMIT NUMSlfof I I ~lf.RCf.ME.NT SALT - lltLO OUTliNt CONTOU~ INTtRVAL :H) ' I I Mt ~-""" RIW Figure 12 . Structure map on top of the Smackover Formation for Chunchula-Hatters Pond area, Mobile County, Alabama (modified from Mancini and Stow, fig. 3, 1977) . 86 CONCLUSIONS 1. Smackover accumulation in southwest Alabama closely approximated present-day carbonate sedimentation in the Persian Gulf. The Louann Salt was primarily responsible in formin g th e ramp surface. Carbo nate depos it ion was affected by ea rly salt moveme nt and paleohighs, such as the Wigg ins- Conec uh tre nd . 2. T hree pro ba ble basins are present in so utl1west Al abama. T hese include t he Mi ss issippi Interior Salt, the Manila, and the Conecuh Basins. The Wiggins-Conecuh trend separated the Conecuh and Manila Basins during Smackover deposition. The Manila Basin appears to have been separated during Smackover times from the Mississippi Interior Salt Basin by an apparent basement paleo-high or salt ridge. Smackover lithofacies and faunal assemblages are distinctive for each of the three basins. 3. The Smackover Formation can be divided into lower and upper members. Where present, the lower member includes stromatolitic laminated and bioturbated mudstone, fossiliferous wackestone and packstone and/or dolomite. 4. Petroleum traps in southwest Alabama are principally combination traps Invo lvin g favorab le stratigraphy and salt inc lines, fa ulted salt anticlines, or extensional faults assoc iated wit h sa lt movement. Reservoir rocks inc lude oolitic gra instones, leached and dolomitized wackestones, packstones, and grainstones, dolomitized stromatolitic mudstones, and granular dolomite. Porosity is faci es- se lective and is preserved c hi efly In lit hofac ies of the upper Smackover member. T he algal stromatolites that characterize the lower Smackover member and are interbedded with upper Smackover lithologies make excellent petroleu m source rock s. 5. The flanks of the Wi gg ins- Conec uh t re nd and updip Smackover grainstones assoc iated with salt structures are exce ll ent areas tor petro leum ex plorati o n in sout hwest Al abama. Th e key to successful prospectin g is t he de lineati on of traps assoc iated with salt movement and ide nt ification of o ri gin al high to moderate energy lit hof ac ies that have been leac hed and/o r dolomitized. ACKNOWLEDGMENTS This research was done principally under Alabama State Oil and Gas Board Contract No. OGB 77-6011 . Research support was also supplied by the Geology and Geography Department and Mineral Resources Institute, The University of Alabama. I acknowledge the State O il and Gas Board an d Geological Survey of Al abama for th e use of c ores and well logs. I t han k Kendall P. Hanby and John M. Masingill of th e A laba ma Stat e Oil and Gas Boa rd, Charles W. Copeland and Gary V. Wilson of the Alabama Geological Survey, and D. Joe Benson, Geology and Geography Department, The U niversity of Alabama, for informative discussions dealing with the Upper Jurassic of south Alabama. I express appreciation to D. Joe Benson and Gary V. Wilson for their constructive criticsm of the manuscript. REFERENCES Ahr, W.M ., 1973, The carbonate ramp: an alternative to the shelf model: Gulf Coast Assoc. Geol. Sacs. Trans., v. 23 ;. p. 221-225 . Badon, C.W., 1974, Petrology and reservoir potential of the Smackover Formation, Clarke County, Mississippi: Gulf Coast Assoc. Geol. Sacs. Trans., v. 24 , p. 163-174. Mancini, E.A., and S.H. Stow., 1977, Preliminary delineation of geologic trends and processes controlling petroleum accumulation in the Upper Jurassic of southwestern Alabama. Univ. Alabama Min . Res. Inst. Tech. Rept. No. 3, 12 p. Masingill, J.H., Hall, P.F., and Tolson, J.S., 1977, The petroleum industry in Alabama, 1977: Alabama, 1977: Alabama State Oil and Gas Board Oil and Gas Rept. 3, 34 p. McCaslin, J.C., 1975, Jurassic finds multiply in southwest Alabama: Oil and Gas Jour., v. 73 , no. 33, p. 157. Neathery, T .L., and Thomas, W.A., 1975, PreMesozoic basement rocks of the Alabama Coastal Plain: Gulf Coast Assoc. Geol. Sacs. Trans, v.25, p.86-99 . Oil and Gas Journal, 1975, Alabama's deep Jurassic play spreads: Oil and Gas Jour., v. 73 no. 33, p. 34-35. Ottmann , A.D ., Keyes, P.L., and Ziegler, M.A., 1973, Jay Field- a Jurassic stratigraphic trap: Gulf Coast Assoc . Geol. Sacs. Trans. v. 23, p. 146-157 . Purser, B.H., 1973, Sedimentation around bathymetric highs in the southern Persian Gulf, p. 157-178: In Purser, B. H., ed ., The Persian Gulf, New York,Springer-Verlag. Sigsby, R.J., 1976, Paleoenvironmental analysis of the Big Escambia Creek-Jay-Blackjack Creek Field Area: Gulf Coast Assoc. Geol. Sacs. Trans., v. 26, p. 258-278. Wakelyn, B.D., 1977, Petrology of the Smackover Formation (Jurassic): Perry and Stone Counties, Mississippi: Gulf Coast Assoc. Geol. Sacs. Trans., v. 27, p. 386-408. Wilson, G.V., 1975, Early differential subsidence and configuration of the northern Gulf Coast basin in southwest Alabama and northwest Florida: Gulf Coast Assoc. Geol. Sacs. Trans., v.25, p.196-206. 87 STRUCTURAL CONTROL OF JURASSIC SEDIMENTATION IN ALABAMA AND FLORIDA James A. Miller U .S. Geological Survey Atlanta, Georgia ABSTRACT The extent and thickness of Jurassic strata in Alabama and Florida w ere m appe d as part o f a U .S. G eol og ica l Surve y regio na l geohyd ro lo gi c st udy. Roc ks eq uivalent to t he Wern er, Lo uan n, No rp h let , S mackover, Haynesville, and C otto n Va ll ey sequ ences of M i ss issip p i and Arkansas have bee n recog nized and , except fo r the Wern er equ ival e nt, are herei n exte nded into panh ahdle Florida. The predominantly fluvial Jurassic sequence in Alabama and Florida was interrupted at least twice by barred-basin conditions which resulted in the deposition of extensive evaporite deposits, and at least once. during Smackover time, by a major marine transgression. The updip limits and isopach map trends of the Jurassic units mapped appear to have been controlled by a major northwest-trending, right-lateral wrench fault, the dominant master shear in a regional wrench-fault system active throughout Jurassic time. FORAMINIFERAL BIOSTRATIGRAPHY AND PALEOECOLOGY OF THE MARLS ASSOCIATED WITH THE TUSCAHOMA SAND (PALEOCENE/EOCENE) OF SOUTHWEST ALABAMA Gary E Oliver University of Alabama University, Alabama ABSTRACT T o better del ineate t he Pa leocen e/Eoce ne boundary in the eastern Gulf Coast area, the foraminifers of the marls associated with the T uscahoma Sand were studied from seve n samp led loca lities in the type are a in southwes t Alabama. Planorotalites pseudomenardii ((3olli} occurs in the lower two unnamed marls. Th e occu-rrence of this plank tonic fo raminifer plac es thes e marls in the Planorota/ites pseudomenardii Range Zo ne as defined by Stainforth and others (1975). T he Bells Landing and Greggs Land in g Ma rl s, wh ich lie above th e unna med ma r ls, are characterized by Morozovella velascoensis (Cushman), Morozovella acuta (Toulmin), Morozovella aequa (Cushman & Renz) , P/anorotalites chapm ani (Pa rr ), Pse udohastigerina wilcoxensis (Cushman & Ponton) . and .'lcarinina mckannai (White) . The presence of M. velascoensis and absence of P. pseudomenardii p laces these marls in the uppermost Pa leocene Morozovella velascoensis Interva l Zone as defined by Stainforth and others (1975) The Paleocene/ Eocene boundary, therefore, occurs within the Tuscahoma Sand between the Bells Landing Marl, w hi ch is th e upp ermost mar l of t he Tuscahoma Sand, and the Bashi Marl which forms the base of the overlying Hatchetigbee Formation. Benthic foraminiferal popu lations indicate that the lower two marls accumu lated in a middle neritic environm e nt. Th e Bells Landing Marl and Greggs La nding M arl were deposited in middle to inner neritic conditions. 88 LITHOFACIES AND DEPOSITIONAL CYCLES IN UPPER CRETACEOUS ROCKS, CENTRAL GEORGIA TO EASTERN ALABAMA Juergen Reinhardt U.S. Geological Survey Reston, Virginia ABSTRACT The Upper Cretaceous rocks of the eastern Gulf Coastal Plain are composed largely of siliciclastic sediments , which were deposited in marginal-marine and shelf environments. At the eastern margin of the basin (central Georgia) and in updip areas farther west, the outcropping deposits consist dominantly of fluvial feldspathic sand and kaolinitic clay and grade to open-shelf glauconitic sand , marl, and chalk both downdip and westward into the basin (central and western Alabama). Changes in the distribution of lithofacies are most notable in marginalmarine environments along the basin margins; the sequences of lithofacies indicate four major marine transgressions during the Late Cretaceous. Sedimentation apparently began during the Cenomanian along the Piedmont margin when the Tuscaloosa Formation was deposited. The overlying Eutaw Formation (Santonian) resulted from the first marine transgression and regression. This unit is composed dominantly of cross bedded quartz sand containing Ophiomorpha burrows, laminated carbonaceous silt to fine sand, and local accumulations ofOstreacretacea valves. The Eutaw sediments record the encroachment of a shallow sea onto an unconsolidated shoreline and the subsequent formation of a barrier-island complex. The second trangression took place during the early Campanian and "drowned" the Eutaw barrier system. The resulting Blufftown Formation consists largely of innershelf glauconitic sand and shelly marl. Near the top of the Blufftown, the unit shoaled and was subsequently transgressed. A final "drowning" of the barrier-bar complex (Cusseta Sand) took place during the latest Campanian. After this fourth transgression, inner-shelf, massive glauconitic sand (Ripley Formation) was deposited. The timing of marine transgressions was probably the major factor in determining the distribution of lithofacies during the Late Cretaceous in the eastern Gulf Coastal Plain. Local fluctuations in water depth, circulation, water chemistry, and sediment supply controlled the composition of lithofacies and the distribution of small-scale (less than 10m thick) cycles within the basin. Controls on the large-scale (tens to hundreds of meters thick) cyclicity of transgressive and regressive phases of these Upper Cretaceous deposits were global and resulted from major changes in the world's water budget. INTRODUCTION During the past three years, the U.S. Geological Survey, in a project entitled "West Georgia Coastal Plain", has been attempting to identify and describe the stratigraphic and structural elements in the Upper Cretaceous and lower Tertiary rocks in the Chattahoochee Valley of Georgia and Alabama. A first statement of results from the Tertiary part of this project is presented by Gibson (this volume) and should be viewed as a companion to this paper. Beyond attempting to formulate a detailed history for the eastern margin of the Gulf Coastal Plain, the specific missions of the project have been: 1) to identify tectonic elements that might be reactor hazards within the area and, more recently, 2) to identify the nature and level of uranium and thorium concentrations within the sedimentary sequence. In this paper, an attempt is made to integrate lithofacies analysis with a time stratigraphy developed by U.S. Geological Survey biostratigraphers from surface and shallow subsurface rocks in the eastern Gulf Coastal Plain. The paper will outline the major lithologies that compose the Upper Cretaceous section and suggest the range of continental, marginal-marine, and marine environments that the lithologies represent. Further, the vertical organization of the sediments into depositional cycles will be documented and, finally , the overall patterns of sedimentation will be related to sea-level changes during the Late Cretaceous. The problems being addressed in this region at the edge of the Gulf Coastal Plain are three-dimensional. The discussion of updip-downdip changes will, however, be somewhat limited; vertical sequence within the Chattahoochee Valley will be the focus of this paper. Acknowledgements Many thanks go to Raymond A. Christopher, Norman F. Soh I, and Charles C. Smith for the biostratigraphic support that has made this study possible. Special thanks go to James P. Owens for his encouragement and constructive criticism of my work in the Coastal Plain. G.S. Gohn, J.P. Owens, N.F. Sohl (all U.S.G .S.), and M.S. Friddell (Georgia Geological Survey) reviewed an earlier version of this manuscript; their comments have substantially improved this paper. James Estabrook, Dennis Duty, and Art Donovan have given excellent field support during the past three years. 89 GEOLOGIC FRAMEWORK The map presented in figure 1 is a compilation modified from mapping by Eargle (1950, 1955) and Monroe (1941). This figure serves to present the gen eral distribution of Upper Cretaceous units in the study area as they have been mapped with relatively little modification since the early reports by Smith and Johnson (1897) in Alabama and Stephenson (1911) in Georgia. The units as presently defined from Tuscaloosa Formation to Providence Sand are thick packages of clastic sediment that show considerable variation in composition and te xture within the outcrop area and in the shallow subsurface. Our view of these rocks is necessarily biased by the present outcrop pattern , which is largely controlled by the drainage pattern of the Chattahoochee River (see fig . 1) and its dissection of the landscape. The sedimentary environments in which the Upper Cretaceous rocks were deposited range from continental for the Tuscaloosa to predominantly marginal marine and marine for the Eutaw, Blufftown, Cusseta, Ripley, and Providence. Simply stated, Upper Cretaceous shorelines, or more specifically, barrier-island chains, trended generally east-west. The distribution of facies in the units younger than Tuscaloosa is largely controlled by this paleogeographic constraint. Given the outcrop patterns (fig . 1), it is apparent that more continental facies will be represented toward central Georgia, and more open-marine facies will be represented in the Chattahoochee Valley . From the Flint River west, the depositional basin also opens to the west and southwest (Herrick and Vorhis , 1963) . LITHOFACIES Continental facies The Tuscaloosa Formation in the eastern Gulf Coastal Plain can be traced in outcrop from just west of Macon, Ga., to the type section in western Alabama (see Christopher, this volume). In the central part of the Gulf Coast (downdip) the section is thicker, and the stratigraphy and lithofacies are more complex (Monroe and others, 1946). In the TERTIARY ROCKS PROVIDENCE RIPLEY I ... CUSSETA l!H BLUFFTOWN [;:'::. .;-:.'! EUTAW TUSCALOOSA I. . CRYSTALLINE ROCKS ; - , ..-,--L.,'-r--N C !. \ s c. ALA. r GA \ 20 MILES f -- - - . - -- - - ' 0 20 KILOMETERS Figure 1. Generalized geologic map of Upper Cretaceous units (modified from Eagle, 1950, 1955; and Monroe, 1941 ). Note the NE-SW trend of the outcrop belt between the Flint and Chattahoochee Rivers. C = Columbus, GA; E = Eufaula, AL; B =Butler, GA; JC =Junction City, GA; AK =Alabama Kraft locality; CB = Chimney Bluff locality. 90 eastern Gulf Coast, the Tuscaloosa is characterized by kaolinitic, arkosic sand (probably equivalent to the Gordo Formation of western Alabama) and medium-scale, unidirectional, trough crossbeds. The sand beds commonly contain intraformational clay clasts, which are typically brick red and locally show conspicuous reduction rinds. The reduced state of the iron on the outer parts of the clasts indicates a change of oxidation state from the initial environment in which the clay was deposited (subaerial flood plain or flood basin) to its final depositional site in a fluvial channel. Thickly bedded red to mottled clays interfinger with the crossbedded sands and are lithologically similar to beds defined as the Vick Formation in Alabama (Conant, 1946) . In the Chattahoochee area, all the Tuscaloosa lithologies are clearly fluvial in character (except as defined by Ray , 1958) in outcrop and are marine only considerably downdip and to the west (Applin, 1964; Soh!, 1964) . Similar (though less kaolinitic) high-energy unidirectional sands are present near the base of the Eutaw Formation in the northeastern part of the study area in the vicinity of Junction City and north at Butler, Ga. (see fig. 1). Interbedded thin to medium-thick clay beds provide abundant clay clasts to the channel fills that are generally less oxidized than clasts in the Tuscaloosa . Marine facies The basal part of th e Eutaw Fo m1a tio n some wh at downdip, near the Cl1att al1ooc hee River, is also composed of cross bedd ed q uart z sa nd and lo c all y abundant clay clasts. T he re are, 11owev er, key differences in the sedimentary structures : (1) strong bimodality in crossbed directions within single outcrops and, (2) extremely abundant Ophiomorpha nodosa burrows, indicating near-shore, openmarine sedimentation (Weimer and Hoyt, 1964; see also Frey and others, 1978). Well-defined, clay-lined channels occur within this part of the Eutaw, apparently preserving tidal channel fillings within a barrier-island complex. The high-energy barrier-bar facies is commonly overlain by a poorly bedded micaceous, clayey sand interval containing extremely abundant wood fragments (large pieces are commonly bored) and a bivalve fauna, which suggest a slightly to moderately restricted back-barrier environment (N.F. Sohl, written commun., 1977). For such a vertical sequence to be deposited, the barrier system had to migrate seaward or prograde; this could reflect either a local or a regional marine regression. This brief discussion of vertical sequences introduces the concept of lithologic transitions and repetitions or cycles. Sedimentary cycles were first des- cribed for the Cretaceous rocks in this region by Eargle (1950). Cycles reflecting sea-level changes are best developed in the marginal marine sediments near the Chattahoochee drainage. Two types of cycles , regressive and transgressive, will be described from this general area. Sediments deposited during regressive cycles (fig . 2) constitute the bulk of the Upper Cretaceous sedimentary pile in the Chattahoochee region . The entire Eutaw Formation, for example, following the initial transgression across the Coastal Plain composed of Tusca loosa san ds and clays, represents a s ingl e re gressi ve cy cle. T he great thickness of these regressi ve cyc les (co mmonly greater than 50 m) or even th e thi c kn ess o f a single lithofacies (as much as 30m) means that the record of an entire cycle cannot be seen in a single outcrop, but records of four such cycles are preserved within the total Upper Cretaceous section. The cycles may have resulted largely from progradation due to sedimentation rate exceeding basin subsidence or a general slow sea-level drop. This contrasts sharply with the view that sea level may have dropped rapidly several times during the Late Cretaceous (Vail and others, 1977). The Regressive Cycle. The major lithologies that constitute an ideal regressive cy c le rep rese nt p rogressively more co ntine nta l facies upw ard in the cy c le and are ge nera lized o n f ig ure 2. From base to to p, th e idea l cy c le is compose d o f lith ofacies f rom in ne.r-s hell, baaie rbar, open- bay, and rest ricted-l ag oo n en vironm ents. The inner-shelf lithologies vary from clayey shell marl to very fine glauconitic sand. These lithologies grade to marls and chalks of the Selma Group in central and western Alabama (Monroe, 1941). Bedding is typically massive and commonly bioturbated rather than distinctly burrowed. Variable amounts of shell debris or whole shells are scattered through the shelf lithologies; locally, shell lags delineate bedding planes , storm layers, and omission surfaces . In outcrop, distinct concretion bands are seen within the thick sequences of shelf lithologies (lower part of the Blufftown and Ripley Formations) . Toward the top of this facies. bedding becomes thinner, and the amount of carbonaceous debris increases considerably . Faunal diversity and preservation of shell material decrease correspondingly. The barrier-bar facies is characterized by trough crossbedded, Ophiomorpha-burrowed, quartz sand. Bedding features are smaller in scale and are progressively more modified by crustacean and polychete worm burrows from updip to downdip areas . Clay drapes and clay clasts are common lithologic components that delineate bedding tea- 91 tures. This lithofacies is best preserved within a regressive cycle at the base of the Eutaw in the Chattahoochee Valley, at the base of the Cusseta Sand, and in the "continental facies" of the Providence Sand. The open-bay, or back-barrier facies, is characterized by lenses of well-sorted fine quartz sand containing planar-bedded shell lags in a highly bioturbated clayey sand . The faunas in this lithology can vary considerably and may be difficult to distinguish from a lower shore-face assemblage (N.F. Sohl, written commun., 1978). Locally high concentrations of wood and woody debris are another of the distinguishing characteristics of back-barrier environments. This lithofacies is especially welldisplayed in the Eutaw Formation west of the Chattahoochee River and in the Blufftown considerably east of the Chattahoochee. Another well-represented back-barrier lithofacies is considered the result of sedimentation in a restricted-lagoon environment . This lithofacies is dominated by thick clayey intervals, containing thin sand beds laminated by carbonaceous debris ("coffee grounds" ). The sand interbeds may contain a rather abundant infauna preserved only as burrows, which disrupt the carbonaceous laminae. Shell debris and molds are not typically preserved in this lithofacies. Locally, linguloid brachiopods and whole leaves are, however, well preserved. The Eutaw and the Cusseta, especially east of the Chattahoochee, contain rather thick intervals representative of this lithofacies. In an idealized cycle, a fluvial component could also be added at the top, but vertical transitions from marginal-marine to fluvial lithologies are not well preserved in the Chattahoochee Valley sections. The lateral transitions from fluvial to marine lithofacies are much better preserved . The basin fill as a function of marine regression is only part of the cyclic record as seen in vertical sequences. REGRESSIVE CYCLE >50 M THICK RESTRICTED lAGOON CLAY AND THIN SAND OPEN BAY MOTTLED CLAYEY SILT TO SAND BARRIER BAR OR INLET FilliNG CROSS-BEDDED SAND INNER SHELF GLAUCONITIC SAND TO SHELL MARL Figure 2. The generalized regressive cycle that includes the bulk of the Upper Cretaceous sedimentary units near the eastern margin of the Gulf Coastal Plain. Inferred depositional environments and predominant lithologies are shown; see text for discussion of lithofacies. 92 The Transgressive Cycle. Numerous well-exposed sections document transgressive events. For example, at Chimney Bluff (see location, fig . 1) on the Chattahoochee River, the transition upward from a barrier-bar sand looks superficially like a transition to a back-barrier lithofacies. The channeled top of the sand is filled with abundant carbonaceous debris, coarse quartz grit and pebbles in a sandy clay matrix. This very poorly sorted unit (ravinement unit of fig. 3) is overlain by massive open-marine clays. Farther west and downdip, the documentation of barrier drowning is not possible, but a vertical transition from massive clean quartz sand to overlying glauconitic silt and micaceous marl records the effect of transgression on sediment size and composition within the innershelf environment. TRANSGRESSIVE CYCLE < 20 M THICK INNER SHELF MASSIVE CLAY AND SILT RAVINEMENT UNIT POORLY SORTED PEBBLY SAND TO CLAYEY SAND BARRIER BAR CROSS-BEDDED SAND BACK BARRIER CROSS-LAMINATED SAND TO LAMINATED CLAY AND SILT Figure 3. The generalized transgressive cycle as preserved within barrier-island destructional events. Note that units are not to scale. Cycles recording sedimentation during a transgressing sea are not well preserved in the geologic record (Fisher, 1961; Swift, 1968; Ryer, 1977). However, the record of a transgressing sea has been noted as thin sheet sands (Kraft, 1971) and as sedimentary packages up to 750 m thick (Hobday and Tankard, 1978). A somewhat idealized cycle based on field examples in the Chattahoochee Valley is presented in figure 3. The lithofacies are generally similar except for the ravinement unit and thicknesses of the individual units. Two obvious differences in the regressive and transgressive cycles can be noted: (1) the vertical arrangement of lithofacies and, (2) the scale of the cycles. The scale of the transgressive cycles (less than 20 m thick) enables us to see the record of an entire cycle at a single locality. For example, along the Southern Railroad Spur into Alabama Kraft Paper (location figure, fig. 4), sediments from an entire transgressive cycle are preserved. Within this exposure, three sharp lithologic transitions may be seen. Ripple cross-laminated and channeled micaceous silt (fig . 4, unit A) at the base contains very abundant carbonaceous debris (back-barrier). This unit is overlain by a thin crossbedded sand unit (fig. 4, unit B) containing well preserved Ophiomorpha nodosa burrows (barrier). The sand unit is sharply overlain by a massive very poorly sorted pebbly sand to clay (fig . 4, unit C, ravinement unit). The top of the exposure is made up of a fissile, poorly bedded, clay to silty-clay unit containing a diverse bivalve and gastropod fauna (inner shelf) . The study of transgressive cycles as an integral part of the sedimentation at the eastern margin of the Gulf Coastal Plain has relied on access to good exposures and the recognition that "upward deepening" as well as "upward shallowing" patterns are repeated in the Upper Cretaceous section near the Chattahoochee River. Further, the interpretation of the ravinement unit as the result of barrier-island destruction in these cycles is central to the risingsea model. Although thin (less than 10m thick) alternations of the major lithofacies may have resulted from local bottom topography, circulation, and sediment supply, systematic vertical changes on larger scales (typically 15-50 m thick) resulted from what can be recognized as oceanic changes (see for comparison, Poag, this volume). This paper does not attempt to critically evaluate the control on these changes (see for example, Fisher and Arthur, 1977), but the Earth's albedo has been seen as central to similar cycles during the Cenozoic (Berger, 1979). GEOLOGIC SUMMARY Combining the various lithologic elements and the two types of depositional cycles described above, we can document a history of sea-level change as seen in the Upper Cretaceous stratigraphy near the margin of the eastern Gulf Coastal Plain. Figure 5 summarizes schematically the pattern of deposition . The record of deposition is tied biostratigraphically to European stages from samples in the project area. The absolute ages for the European stages are from Van Hinte (1976) and may be modified somewhat (J. Hazel, written commun., 1978). The pattern of deposition shown for the Tuscaloosa is largely inferred (note the dashed line in fig. 5) from farther west (Monroe, and others, 1946). 93 Figure 4. Railroad cut near Alabama Kraft Paper, Omaha Quadrangle, Ala. The sequence preserves an entire transgressive cycle as generalized in figure 3. A= back barrier deposits; 8 =poorly preserved barrier bar sands; C = ravinement unit; D = open marine clays. The lighter unit in the upper part of unit Dis largely a weathering profile plus a thin terrace gravel. Photo courtesy of N.F. Sohl. Apparently only the end of Tuscaloosa deposition is recorded at the eastern margin of the Gulf Coastal Plain (see also C hristopher, this volume). After a substantia l mterval ol' nondepos itiOn (about 8 m.y .), the sea transgressed across a broad coastal plain margin leaving only a thin rework ed surface. Most of the Eutaw Formation is the record of a marine regression that continued with only minor fluctuations until the sea transgressed near the beginning of the Campanian. The basal sands of the Bl.ufftown Formation record the landward migration of barrier bars. In much of the area, these barriers were actually breached, and the ravinement has been preserved. The considerable thickness of marls and glauconitic sands in the lower part of the Blufftown is the record of a major marine transgression . A thin regressive phase separates the lower part from the upper part of the Blufftown. A second wellpreserved ravinement unit marks the later Campan- ian transgressive event (Alabama Kraft section, fig. 4). The transition upward from the Blufftown to the Cusseta represents a shoaling event; the regression is recorded by a flood of coarse clastic sediments caused either by sea-level retreat (changing stream gradients) or by progradation of a deltaic system. as favored by Hester (1968). The transition to the shelf sediments of the Ripley (another major marine transgression) is poorly seen in outcrop, but a transgression cycle is indicated through the Cusseta-Ripley contact. The Ripley-Providence transition is within a regressive cycle and looks much like the Blufftown-Cusseta contact in scale and composition. The Providence is unconformably overlain by the Clayton Limestone of Paleocene (Danian) age; the gap in sedimentation probably represents about 3 m.y. 94 w 0 <( I- V) w 70 SEA lEVEl lOW HIGH NON-DEPOSITION PROVIDENCE RIPLEY ~USSETA 75 CA BlUFFTOWN 80 EUTAW 85 NON-DEPOSITION 90 L ________ _ 95 CE TUSCALOOSA -------~ 100 - TRANSGRESSION - REGRESSION Figure 5. Schematic representation of sea-level fluctuations during the Late Cretaceous as inferred from the deposits in the eastern Gulf Coastal Plain . CONCLUSIONS The sedimentary column of Upper Cretaceous rocks near the margin of the eastern Gulf Coastal Plain has been discussed in terms of major lithofacies and their vertical organization. I have described, in a general manner. the main lithologic components within cycles that resulted from changes in sea level. These patterns have been applied to the stratigraphic column, and the dynam ics of this stratigraphy have been presented. The cycles discussed in this paper are of two types : transgressive and regressive. The transgressive cycles are thin and record the effect of a rapid and, in some places, a destructive event. Regressive cycles represent the bulk of the Upper Cretaceous basin fill and result either from progradation or from an actual gradual lowering of sea level. The complexity of what happened through Late Cretaceous time near the margin of the Gulf basin presents us with an ideal opportunity to document the interplay of continental sedimentation, which results largely from onshore tectonics, and eustatic sea-level changes, which result from a variety of factors involving the world water budget. At least during the last 20 m.y. of the Late Cretaceous, the nature and distribution of sediments were largely controlled by eustatic sea level changes. REFERENCES Applin, E.R., 1964, A microfauna from the Coker Formation, Alabama: U.S. Geol. Survey Bull . 11600, p. 65-70. Berger, W.H., 1979, Deep-sea drillings and major themes of ocean evolution; Am. Assoc. Petroleum Geol. Bull . v. 63, p. 419. Conant, L.C., 1946, Vick formation of preTuscaloosa age of the Alabama Coastal Plain : Am . Assoc . Petroleum Geol. Bull ., v. 30, p. 711-715. Eargle, D.H., 1950, Geologic map of the Selma group of central and eastern Alabama: U.S. Geol. Survey.Oil and Gas lnv. Prelim. Map 105. _ _1955, Stratigraphy of the outcropping Cretaceous rocks of Georgia: U.S. Geol. Survey Bull. 1014, 101 p. Fisher, A.G., 1961, Stratigraphic record of transgressing seas in light of sedimentation on the Atlantic Coast of New Jersey: Am . Assoc. Petroleum Geol. Bull ., v. 45, p. 1656-1666. Fisher, A.G., and Arthur, M.A., 1977, Secular variations in the pelagic realm: in Cook, H .E., and Enos, Paul, eds., Deep-water carbonate environments, SEPM Special Pub. 25, p. 19-50. Frazier, W.J., 1976, Origin of septarian concretions in the Cretaceous Blufftown Formation of Georgia's Coastal Plain: Geol. Soc. Am. Abs. w. Progr., v. 8, p. 177. Frey, R.W., Howard, J.D ., and Pryor, W.A., 1978, Ophiomorpha : its morphologic, taxonomic and environmental significance: Paleogeography, Paleoclimatology, Palaeoecology, v. 23, p. 199-229. Herrick, J.M., and Vorhis, R.C., 1963, Subsurface geology of the Georgia Coastal Plain: Georgia Geol. Survey lnf. Circ. 25, 78 p. Hester, N.C., 1968, The origin of the Cusseta Sand (Cretaceous): Unpub. Ph.D dissertation, Univ. Cincinnati, 219 p. Hobday, O.K ., and Tankard, A.J ., 1978, Transgressive-barrier and shallow-shelf interpretation of the lower Paleozic Peninsula Formation, South Africa: Geol. Soc. America Bull ., v. 89, p. 1733-1744. Kraft, J.C., 1971 , Sedimentary facies patterns and geologic history of a Holocene marine transgression: Geol. Soc.America Bull., v. 82, p. 2131-2158. 95 Monroe, W.H., 1941, Notes on deposits of Selma and Ripley age in Alabama: Alabama Geol. Survey Bull. 48, 150 p. Monroe, W.H., Conant, L.C., and Eargle, D.H., 1946, Pre-Selma Upper Cretaceous stratigraphy of western Alabama: Am. Assoc. Petroleum Geol. Bull., v. 30, p. 596-607. Ray, L.L., 1958, Geology with engineering data, 1:25,000 in Engineer intelligence study E15211: Terrain study of Fort Benning and vicinity,, U.S. Army Corps Engineers, p. 47-62 . Ryer, T.A., 1977, Patterns of Cretaceous shallow water sedimentation, Coalville and Rockport areas, Utah: Geol. Soc. America Bull., v. 88, p. 177-188. Smith, E.A., and Johnson, L.C., 1897, Tertiary and Cretaceous strata of the Tuscaloosa, Tombigbee and Alabama Rivers: U.S. Geol. Survey Bull., no . 43, 189 p. Sohl , N.F ., 1964, Pre-Selma larger invertebrate fossils from well core samples in western Alabama: U.S. Geol. Survey Bull. 1160C, p. 55-64. Stephenson, L .W., 1911, Cretaceous (rocks of the Coastal Plain of Georgia), in Veatch , J .O., and Stephenson, L .W., Preliminary report on the geology of the Coastal Plain of Georgia: Georgia Geol. Survey Bull. 26, p. 66-215 . Swift, D.J. P., 1968, Coastal erosion and transgressive stratigraphy: Jour. Geol., v. 76, p. 444456. Vail, P.R., Mitchum, R.M. , Jr., and Thompson, S., Ill, 1977, Seismic stratigraphy and global changes of sea level, part 4: Global cycles of relative changes of sea level: in Payton, C.E., ed., Seismic stratigraphy-application to hydrocarbon exploration, AAPG Mem . 26, p. 83-97. Van Hinte, J .E., 1976, A Cretaceous timescale: Am. Assoc. Petroleum Geol. Bull., v. 60, p . 498,516. Weimer, R.J., and Hoyt, J.H., 1964, Burrows of Callianassa major Say, geologic and shallow neritic environments: Jour. Paleontology, v. 38, p. 761-767. 96 MICROPALEONTOLOGY OF THE TWIGGS CLAY, GEORGIA COASTAL PLAIN Walter Schmidt Florida Bureau of Geology Tallahassee, Florida and Sherwood W. Wise, Jr. Department of Geology Florida State University Tallahassee, Florida ABSTRACT Fossil diatoms, foraminifera, ostracods and calcareous nannofossils of the Twiggs Clay Member of the Dry Branch Formation (Eocene: Barnwell Group) of eastern Georgia have been studied in surface samples and well cuttings. Paleoecologic data derived from these microfossils coupled with macrofossil data suggest that the Twiggs Clay was deposited in an inner con-tinental shelf province. In the past, major portions of the Twiggs have been considered unfossiliferous due to the mode of preservation of the siliceous microfossils. Scanning electron microscopy of many samples shows that these often occur as molds and casts. This type of preservation would be lost in any standard microfossil washing procedure. The Twiggs Clay can be assigned to the lsmolithus recurvus calcareous nannofossil zone of most authors. This, in conjunction with its stratigraphic position somewhat below the Eocene/Oligocene contact, suggests that Twiggs time spans the middle part of the late Eocene. INTRODUCTION The Twiggs Clay of Georgia (fig. 1) has long been of interest to Coastal Plain geologists because of its commercial deposits of opaline claystone ("fuller's earth") and the problematic environments of deposition they represent. By latest interpretation, the Twiggs Clay is considered a member of the Dry Branch Formation of the upper Eocene Barnwell Group (Huddlestun and Hetrick, this volume). In general, updip sections of the Twiggs tend to be sandy and opaline, whereas downdip sections are calcaerous and montmorillonitic (Carver, 1972b). Southward, the Twiggs becomes more calcareous and gradually merges into the Ocala limestone in the west and into marl and shell beds in the east. Not surprisingly, a wide variety of environments of deposition has .been identified or postulated for the Twiggs. These range from estuarine conditions in the updip locality near Grovetown, Ga. (Berry, 1924) to those labeled "very near shore marine" (Darrell, 1974). "normal marine" (Shearer, 1917). "quiet, low energy" (Pickering, 1970). "regressive" (Carver, 1966, 1972a). '.'transgressive" (Weaver and Wise, 1974), and "peculiar" (Cushman, 1945) for the rest of the outcrop belt, depending primarily on the type of evidence examined by each investigator and where the unit was examined. Similarly, the question of origin of the opaline claystones has also elicited a wide range of opinions. Cooke (1943). LeGrand (1962), King (1962). and Noble (1962) considered the Twiggs Clay a bentonite or weathered volcanic ash, an opinion challenged by Pickering (1970) and Weaver and Wise (1974) on the basis of the abundance and mode of preservation of various microfossils found within the unit. Pickering (1970) speculated that the Twiggs Clay was precipitated as colloidal particles formed by the action of saltwater on normal terrestrial clay minerals. Weaver and Wise (1974, 1975; also Wise and Weaver, 1974) suggested that the silica of the opaline claystones (opai-CT of Jones and Segnit, 1971) was derived from the dissolution and reprecipitation of biogenous silica. In light of the above, the present paper examines the microfauna and flora of the Twiggs Clay with special interest in paleoenvironmental aspects of the assemblages. Groups treated are diatoms, calcareous nannofossils, foraminifers;, and ostracods. Systematics for all illustrated specimens are given in Schmidt (1977) . Palynomorphs are not included but have recently been examined by Darrell (1974). In addition, a lithologic analysis of our samples has been completed (Schmidt, 1977). and will be presented in a subsequent publication. In general, there has been little microfossil work done on the Twiggs Clay. Cushman (1945) recorded a foraminiferal fauna from a locality near Sandersville, Ga.; Herrick and Vorhis (1963) listed many additional species; and Pickering (1970) noted some foraminifers along with macrofossils near Clinchfield, Ga. Darrell (1974) reported in abstract on palynomorphs from Houston and Washington Counties, Georgia. 97 oo N l u"oo' n1 oo EAST CENTRAL GEORGIA 25 '--S~CA~"---' MILES EXPLANATION /-COUNTY BOUNDARIES ~TWIGGS CLAY OUTCROP BELT ADAPTEO f'AON lt,G. RIC ti AAO~C I U9) AND R.E,CARIJ ER (1 91Z) e20 OUTCROP SAMPLES 0 7 SUBSURFACE SAMPLES ... . Figure 1. Study area and sample location map. Among some of the other known localities where the Twiggs Clay crops out are a number previously considered unfossiliferous. Apparently, the major reason for these localities being unworked and thought to be unfossiliferous was the mode of preservation of the siliceous microfossils, particularly the diatoms and sponge spicules. These occur primarily as molds and, therefore , cannot be extracted by standard washing and sieving techniques. All of our sample localities are shown in figure 1 and listed in appendix I. Samples referred to in this paper are keyed to those localities by number. Where more than one sample was collected from a locality, the locality number is followed by a hyphenated sample number as indicated in appendix I. The distribution of fossils by type at each locality as determined by this study or as reported in the literature is indicated in table 1. No samples, however, from localities 9, 13, and 26 were examined for fossils. Instead, only published mineralogical data exist for these sites. FOSSIL DIATOMS Scanning electron microscopy of fracture surfaces of the opaline claystones revealed siliceous microfossils which occur as molds and casts in a number of samples. These fossils occur mostly in samples which contain the highest percent of Si02. The opaline material, after X-ray diffraction studies, proved to be opai-CT which, on free growth surfa- ces, exhibited characteristic lepispheres (Wise and Kelts, 1972; see also Wise and Weaver, 1974; Weaver and Wise, 1974). Most of the microfossil molds are of marine diatoms including pen nates and large and small centrics. Sponge spicules and a number of unidentified organic molds (possible radiolarian remains) are also common. From the apparent abundance of microfossil remains it would appear the opaline claystones represent highly altered diatomite deposits rather than ash beds. Most microfossils in the clay of these locales have been completely destroyed by dissolution, and only the reprecipitated opai-CT remains, preserving in some cases the shape of fossil skeletons. Early diagenesis of these siliceous clays probably followed a pattern recently postulated for deep-sea siliceous ooze diagenesis, that is, in situ dissolution of biogenous opal with silica reprecipitated inorganically as authigenic opai-CT (Wise and Weaver, 1974). In places where the opal content of a clay is extremely high , molds of microfossils may not be preserved. Significantly, once diagenesis begins, practically all available biogenous opal in the affected material may be converted to opai-CT with little trace remaining of the original substance. Thus, most of the microfossils originally deposited in the material probably were not preserved , not even as molds, but were destroyed by in situ dissolution following burial. The silica released by dissolution, however, was subsequently reprecipitated to form a significant proportion of the rock matrix and constitutes the principal lithifying agent in the material. 98 TABLE I Distribution of Fossils by Sample Locality Sample Locality I 234 Diatoms X 1 -1 - Calcareous Nannofossils XX X 1- - Foraminifera Ostracods -X X X 1 -1 - - 0 X XX Other Fossils X 0 X 5 6 - 0 7 8 . 9 10 II 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30- X X :t X - r- X X - - 0 0 0 0 -0 - - 0 6. - 0 0 0 '-- 0 X 0 Other Fossils = sponge spicules, molluscs, sharks teeth (all marine) X = fossil groups identified in this study 0 = fossils reported by Pickering (1970) at or near the sample lo c illitv 0 = fossils reported by Carv e r (1972) at or near the sample lo c al itv 6 =fossil s reported by Cushman {1945) at or near the sample 1ocal1tv Blank .. no fossil remains found Sample Preparation To find arid identify diatom frustules in this opaline clay, two methods proved to be most useful over the other conventional methods generally in use today. The first was to make smear slides for the light microscope, and the second was fracture surface observation in the scanning electon microscope. It was discovered that a number of diatom frustules did remain intact, undissolved, and needed only to be separated from the surrounding matrix. The procedure for diatom preparation generally follows well-known microfossil separation techniques; however, modification had to be made to best suit these particular samples. For a list of procedures used, see appendix II. Generally, species identification was best carried out on the scanning electron microscope. Other methods of sample preparation proved to be of little value and were not pursued . Thin sections were prepared using epoxy immersion but yielded no significant results. A low-power binocular microscope was used for surface observation, but only a few diatoms are large enough to be see.n by this method. Marine Ecology Diatoms are found wherever there is moisture and sufficient light for photosynthesis; therefore, the widest range of variation and environmental factors is encountered. Many species enjoy a wide distribution, and it is difficult to decide whether they are oceanic or neritic . It is also common to find truly oceanic forms associating with neritic planktonic forms. It seems these divisions, therefore, should be accepted for some species only in general terms. Within the Twiggs Clay, the number of individuals (microfossils) present is not large enough to w.arrant a complete statistical study to determme detailed temperature and ecologic niche. However, there is among the fossil remains a fairly diverse number of species which, when only the most abundant forms are used, suggest a narrow range of environments which can be compared with other fossil groups to infer the paleoecology during Twiggs time. A brief analysis of the environment represented by diatoms from the Twiggs Clay can be made after considering each species separately. Coscinodiscus radiatus Ehrenberg (pl. 1, figs. 1, 2): this species is reported as marine neriticoceanic, warm water. C. oculus iridis Ehrenberg (pl. 1, figs. 3,4): living representatives of this species are widely distributed throughout all oceans; considered an oceanic, pelagic species. Actinoptychus senarius (Ehrenberg) Ehrenberg (pl. 2, figs. 1-3): A senarius is a cosmopolitan species found in neritic and oceanic plankton. A. splendens (Shadbolt) Ralfs (pl. 2, fig. 4): species considered a common littoral form, often near coasts. Trigonium barbadense Greville (pl. 2, fig. 8): the ecology ofT. barbadense is hard to define; however, a temperate to tropical climate is called for. This species also is one of a few that appears to have a short stratigraphic range (upper Eocene to lower Oligocene) which would suggest high stratigraphic value. Para/ia su/cata (Ehrenberg) Cleve (pl. 2, fig. 7): this species lives attached to the substrate for reproduction although it may be found occa- 99 sionally in plankton. The species then is littoral and is said to frequent brackish waters. Odontella aurita (Lyngbye) Agardh (pl. 2, fig. 9): 0. aurita also lives in chains attached to the substratum . A neritic or littoral coastal species, it is sometimes found floating free. Eupodiscus radiatus Bailey (pl. 3, figs. 1, 2) : common in neritic seas . From the species present, certain environments are apparent. With the exception of Coscinodiscus oculus iridis, they all have in common a neritic or littoral domain and most are planktonic from warm marine waters. The fairly pure aspect of the clay or, more correctly, the lack of detritus and other continentalassociated material may point to a marine environment a fair distance offshore. This may be verified by the presence of Odontella aurita and Paralia sulcata, both bottom dwellers and probably not able to withstand a great deal of wave action. Their benthic life style, however, limits the water depth since they must receive enough light to photosynthesize. The presence of Coscinodiscus oculus iridis reinforces an offshore interpretation since it is an oceanic pelagic species. One species, Paralia sulcata, can be associated with fresh or brackish water. From this variety of environmental domains, it would appear we are dealing with an offshore coastal assemblage. An inner shelf province where many niches tend to overlap would fit the assortment of specific ecologies represented. CALCAREOUS NANNOFOSSILS As previously stated , a calcareous facies of the Twiggs exists as the member trends southwestward. This fact was originally apparent when fossil mollusks, foraminifera, and ostracods were noted within the clay, and again later when the clay was prepared for X-ray diffraction, and both wellordered and disordered calcite were found to be present. It is now known from this study that in addition to these calcareous fossils, there exists a wellpreserved calcareous nannofossil assemblage. The coccoliths were obtained from exposures of Twiggs Clay in Houston County, Georgia. These localities (Nos. 2-4 in appendix 1) were visited_. during the Geological Society of America Southeastern Meeting and Field Trip, 1974 (Huddlestun and others, 1974). Locality 2 provides an essentially complete section of the Twiggs, whereas the others are partial sections . There was little variation in the distribution of coccoliths within these sections. The coccoliths were observed first while scanning a fracture surface for diatom molds. A routine calcareous nannofossil preparation was then made with good results. The following coccolith species were identified by light microscopy (listed by species epithets; bibliographic references are provided by Loeblich and Tappan, 1966, 1968, 1969, 1970a, 1970b, 1972, 1973, and Wise 1973): Discoaster barbadiensis Tan Sin Hok, 1927 Reticulofenestra bisecta (Hay, Mohler, and Wade) Roth, 1970 Coccolithus eopelagicus (Bramlette and Riedel) Bramlette and Sullivan, 1961 Coccolithus formosus (Kamptner) Wise, 1973 Helicopontosphaera lophota (Bramlette and Sullivan) Haq, 1971 Sphenolithus moriformis (Bronniman and Stradner) Bramlette and Wilcoxon, 1970 Pontosphaera multipara (Kamptner) Roth, 1970 Transversopontis obliquipons (Deflandre) Hay, Mohler, and Wade, 1966 Coccolithus pelagicus (Wallich) Schiller, 1930 lsthomolithus recurvus Deflandre, 1954 Reticulofenestra reticulata (Gartner and Smith) Roth, 1972 Discoaster saipanensis Bramlette and Riedel, 1954 Discoaster tani Bramlette and Riedel, 1954 Reticulofenestra umbilica (Levin) Martini and Ritzkowski, 1968 The above species are useful in helping to define the age of the Twiggs Clay as well as in providing limited ecological information. Using other fossil evidence, Cooke and Shearer (1918) long ago assigned a Jackson Age (late Eocene) to the Twiggs. Since then, a number of other studies also placed it within this interval. The major criterion used was Foraminifera along with the stratigraphic position of the unit within the coastal plain sediments. The coccolith data are in agreement with this general assessment, and place the Twiggs within the lsthomolithus recurvus Zone of Hay and others (1966), defined as the interval from the first occur- 100 renee of/. recurvus to the last occurrence of Discoaster barbadiensis (fig . 2). In the Gulf Coast, the Cocoa Sand, the Pachuta Marl , and the Shubuta Marl of the Yazoo Group, studied by Levin (1965) and Levin and Joerger (1967), belong to this zone. Huddlestun and Hetrick (this volume) believe the Twiggs Clay probably represents the easternmost tongue of the Yazoo Group of the Gulf Coastal Plain, but for practical purposes, include it in the Barnwell Group. Other authors have defined an lsthomolithus recurvus Zone or Subzone more narrowly, placing it between the first evolutionary occurrence/. recurvus and the first occurrence of Sphenolithus pseudoradians (Roth and others, 1971 ; Martini , 1971) . This corresponds to zone NP 19 of Martini (1971). The Twiggs Clay may well fall within this more narrowly defined interval which is middle late Eocene; however, evidence for this is somewhat incomplete. While it is true no Spheno/ithus pseudoradians were noted in our samples, neither was its evolutionary precursor, Sphenolithus radians, found . The absence of S.pseudoradians , therefore, is only considered suggestive of a middle late Eocene age. The position of the Twiggs Clay within the sequence of coastal plain sediments can also be considered in this evaluation. The Twiggs is a lower facies of the Barnwell Group which is upper Eocene. Since we know the clay does not continue to the uppermost Eocene and from the nannofossil evi- dence it is younger than early late Eocene, it can be reasonably inferred that Twiggs time occurred during the middle part of the late Eocene as indicated in figure 2. Although ecologic information is limited for the calcareous nannofossils, Bukry (1973) has made a few generalizations. He states that during the late Eocene, there was little evolution in the warm-water genus Discoaster which developed the greatest number of species during intervals of warmest temperature in the early to middle Eocene and middle Miocene. The reduced Oiscoaster speciation and the generally long-ranging zones in the late Eocene and Oligocene indicate a cooltemperature interval. He further notes that Reticulofenestra reticulata is a nearshore species whereas /sthmolithus recurvus is a cool-water marker. This indicates a somewhat temperate or subtropical rather than a strictly tropical environment. TRENDS OF FORAMINIFERAL DISTRIBUTION Table II lists foraminiferal species identified here and by others (Cushman, 1945; Herrick and Vorhis, 1963; and Pickering , 1970). Combined with this are environmental interpretations based on generic data given by Murray (1973). From the species present, the ancient environment can be recognized , after noting the individual ... gw ...J -:lEzw u 0 ... - a:<.!) lt ...J oO ...J TWIGGS DEPOSITION ... ...J Q Q ::1 Figure 2. Nannofossil range chart showing approximate time of Twiggs Clay deposition. 1. lsthmolithus recurvus 4. Reticu/ofenestra umbilica 2. Discoaster tani nodifer 5. Disc oaster barbadiensis 3. Coccolithus formosus 6. Discoaster saipanensis 101 TABLE ll Foraminiferal fauna from the Twi~~s Clay with some genera li z ed ec oloqi ca l data from Mu rray (197 3 ) . The dat a are 8iH'1.G'J ir. 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ICto'dh,.fil n ~ tJl ~&,r,JiiH) b rt B 9 n :3 ;) 0 3 - -~ ..1, , -:- , J fl 11 e H .IBvl s tni fnutmsqnJ e l c ., ~ndo~ g( v 'J- i I ia - I ~L !! .lutt alot .H-::tt"'~ f.t unl ll e ' I l va.r~-~~rqligefi\i ll b l:-:1 .J ~ Hl- I ,-_! s___ b\ 1.u?,s..h9l]\tHa.jIt\ 0 . \ 2i I I . >111"1! t' ' '( 1t l G ;::i 6 \J '~'\ \J::t9\ 2.\.J (\\\\(!fCI,[\\ (, \ .r IB ~ l.a~ qcI 11t I <' lcos 11 t r,, l.t>H 5< ,; ' 9 1 lsrlW 9 fn02 f') c~rjf; ~)J_bfl l. _1,__. h C')<.MJ ~?P. , fl\1 /1' I \''"F1) !1 :) i q01j ylbi'll2 fi n.F> rlt 'c!r1tE;1 -1.)()20, o?.\..11\!\\on~l(IC\G lo e::J;t911U:J~lo 121it sdi bnB ?..\1\1 (tH?.r initlr.M l\8t .e:nrilo bnr, rltof'-l) zm,\\)iJIOb n ( t \8 ir11t1f;M io 8 r CJ~1 CHlOS ot 2bnOQ29llO:'l cirli -:ls(-~;r1!.1i~lo'Sr)il;Cftilbaferf:1mlar;.>l:,irrle/'!st,lnemlirbulbdiutriYhWzse1ia\&rl:tmJ!iler9nttwW, cV'rGiuCf1.btVBtri8op1Ge.lQir~liW if>Vlstni t:>enileb 0T'-~rl"1:Swlro\T; .e>tslqrno:mi J.BriVhlrnOe ei c=mU 10i oorr9biva .'t9V9Worl 1_. laevis ('lont a qru) 919W 2.1\B\\.)~;-IOGI.JGi:.(\ ?..11\\ l\lon ,3 r\Ci2. on 9LI'Il 2i ti 9 1i rlW JAA3:'11Li!IMAAO=i =tO 20Li13RT ~bul ina gibb a DlfPOilf't.JS'tfl~ (Von 11u e n s te r ) yH>noitulove 2ti 2sw '1''H1.~iflll .r.:slqrni32. wo ni beion 8 f~~r7~~J~e)mr f''cffi~\'\'31;-, t~?lP,fffi\l')r\'9 {\f~e,i C~-1 Cc9.fEIQ.)81 q -flO;) ylno zi ,s1ote1~)rlt ,zno\\w;1o'nuezq 2lo s;JnecdB .sg.s ensoo3 etst eibbim s to svJt2;3QQU2. b919bic. Hyposaline to normal marine sediment, cold to snuronit\Jliwo-'(~~011. o~ei~NT ertl to noitic.oq erlT 9li oels ns::J 2tnsmibe2 nir,!q lstzoJ to eoneups2 1ewol B ai 2l_)QiwT oriT .noit:,;u\sVf:J Bir1! ni bs1sbieno ~l .sn:'3;:J03 1sqqu zi rbirlw quo18l\swms8 eril to 2oiJst srq~eJ%Wl~{~t~(~i-J;''t1t(l ~~~~~siY"tfma INo111~tB'\IIti 't:fjn i2 -iva lieaolonnsn edt mmt bns ene:Jo3 teom1eqqu Sed iment and veqetation, l-30"C, 0-50 m, tidal marshes and laqoons, nearshore ~~mal marine , muddy sediment, temperat J l1elf 0-800 m, Bolivina ddy sediment, bathyal to marqinal mari e Oiscorbis D. assulata u'$ h". J" ~.'!.!.~ J .!!..lLVl ~"' '- Eponides ~or~al marine, muddv sediment, cold, 100[ 4500 m, ~~ter shelf to bath~al ~~rmal marine, vegetation, >l2C, 0-50 m inner shelf 2JJIWT q ~ o(l ime.\1 ~Jlli;Q '1~qera L , she 1f " I ll (l rt~ 'l marlr"' mu d dy s~ M i m~ot , ca lrl_ t o l~mperate, ~ - > JfJOO 111, 's he l l f o bti thva 'l Marine planktonic Cibicides . lobatulus ~ormal marine, clinginq to ve t1 etation, s ones, shells, r 1!-tr.. ,arc.Ln: l i.L tr.o. p.:U:.al~ r)- >. lOQO.. t~. ~J1e 'l to bathyal C. mississippiensis (Cu shma nJ .notliaoqeb ye1:::.. aggi NT lo ~1mii 'J ismi x01q qs " iwon<: lH; ,i:-J e gnr;t ircP.olo:-trtsv~ C. planoconvexus Cushman and tT:3Mr_\[\'l\.l !-i\l?r.>t\e'lo\uo\',eR -~ 01~1J\IJ0 \.n ?.IHI\\IOII\r'lc,l ~ Globi'!er i na sp . Zlc-!lfJ\bnchsc\ l!llc.GO:lGIG .2 212.\191\bJ,\L?. \C\ZhO:w.\tJ (j '\81\bO\ \r11:.1 \9\2.60::1::>.10 .S Mar\1.\Jc_l)'lMb\o~\\~111\Ct :<)O~) _[ species ecology and any trends which may exist. In After looking at the entire spectrum of foraminif- a general sense, porcelaneous species of foraminif- era present, the depth ranges of the more abundant era are characteristic of the inner part of the conti- species yield a distinct depth correlation . There are nental shelf of modern oceans (Bandy, 1956; Bandy many more species present which represent an and Arnal, 1957). Arenaceous foraminifera occur in inner shelf biofacies than any other faunal zone. It is shallow waters and on the continental shelf in this zone, ranging in depth from a few feet to (Zalesny, 1959). Hyaline species, also present in the Twiggs sa'mples in small numbers. seemto be less nearly 200ft, ,: that the Twiggs Clay w.as deposited. . restricted, being found in abundance near shore ;:~_s The presence of some bathyal and estuarine well as in bathyal depths. forms simply points out that this environment dur- ing Twiggs time may not have been isolated from The great marjority of forams present in the adjacent facies, but accepted mixing of species and Twiggs ~l.ay. a~e , porcela;neous, : with .auenaceous . , lithologies from neighboring provinces. This situa- and hyaline forms being less common. This would immediately place th~' assemblage ecologic:ally iri tion may have been introduced by relatively minor ''II ~Ctl.tatiohs of the slraridli ne position wftn concom- the inner continental shelfran~e. ~ - , \, . ,, )!., i , : .. .: ,, itant migratipn of,facies belts. ,.:!:I i I , , , ... I, ' . . Another observation of a general nature was rC7P,~rtedpy . k~e~s p954} anp Bandy (r195,6), ~eali~~ w,i_~h1 ,the ,lnt'~ri~r' St~u~t ur,e, ~t ~hef?.st. T h ~y, repor~ 1 a that those I pef'jera Wl1h slrn le lnterl rs e fre- fn qt.iently v'Mi -ab un~~nt sHoal'' w1aterY,' w Mrea$ gknera wn;~ ' . c?mrlfe~ te'd i'n\erib:r ' s.t'r_ug~u~-~~ ' ~re mdst chara1ctrr~~ti c ~~ bafh~~.' Cl~pths.N'9 i:J0';11' n~~l style, however, 1s not1cei:lb le 1n hie Tw 19Q asse.molage. Thus, ana lysis of this parameter doe's lidt seem to be applicable ,in this case. It Is not poss ible at th e present lil)le to generalize oCnI ~S~ ~o~f' Wb rl (Jt: fol-'a'mi ~n_lltt,elte"ad, eopetd'hadui~~e!!tl hbe~rHeoh~assobfenle1nf' 1ln~slu~fPf-Jtl cl~nt 'sn)dy df 'f'i{u_nas ',f dm tl11s partic't.l lar''pol t' 91 vi ~w.. 1hert:~ 1 has, ' hdw'ever, 'been' 'a1 cc5m ifa;tibn 6t H3'rhP,eratufe a~~ JdeR H(ahge~ o~ afisi ~r.flca Jlt~l?.~~ cle's' f~om tNe areas 'Jvhf:rr,e pparen(fy rellat:) le'st u- dl e~ ' hav~ :o\:!eH ade: -rh'i~- h gr ~~e~ ~Bt:?he:: ~x Plll eg'e1r' ' (~96U).1 sahdy ' 0 '956), ~ d M'u'rray' (1973).. li$'l'n'g' thei daia,'c hkrahgn~l1t,i.(I:eg1t~'JJd'1o~~p~~lc'5o)byt~.(e,rqt),1aao'r'!tnfs-?i~tn,ss~o\s\~1atf1eh~1lats~ie:)n~e.lp,i1t~ct~YitMah.hs~~ude{rste,1'i~a_s(p'b'(,mu1e~,bppftt,ris1no~htd,rrne~a~~.n,tqtruf19!wrfn,1~ne.o)y~"h.s1,f'rt'1o1.i~~nnxa,~1'myyor,~t.ioE.rel,Qd~Vn1~(e,].~rt lit. I,. I fl _ I'J, I From the studies of Hulings an d Purl (1964). on y tera'~oe.d:m~jaq.'~. e~ytrpnf11~fl!~ l w , s~line tap,rs fyavc~ii,Ec1sh C\fOoi';Jl~iq,q,o~es1i,dq.eelrierq ~'iHg~n~J a,rd~st~ ~~.ir~ .~~_n~ (less' than 35ft) nerrt1 c ssern p tY.J?,ip~' ~~~~~pw plaQe w1th h.lg h,er vy,~.te,r ~'i\11 n- itl~s. Of the' a'd tt1 ey lassifled as to bio~-'lcl~s, o qly three genera are found as fossil s in the 'tiNig1gs m~teria l . These are ,C~s l)manidea, 1-f~micythef,e and C w yth ate e r rneettrait1icaalls1soef'mwbhl ich ~r~ age. :fMS!dered ~ sn.allow 103 Figure 3. Cytheretta a/exanderi Howe and Chambers. Sample 30; x 45. Scanning electron micrograph . Figure 4. Cushmanidea laevigata Puri. Sample 2; x 70. Scanning electron micrograph. Figure 5. C. laevigata Puri. Sample 2; x 70. Hinge of right valve. Scanning electron micrograph. SUMMARY The diatoms present represent a cross section of living requirements including brackish water species, shallow water neritic species and oceanic pelagic species. At only a few locations were vertical sections of any substantial extent available. These locations, however, showed no significant faunal change in the intervals sampled. From such a variety of environmental domains it would appear we are dealing with an em bayed coast that may have fresh water input. Other faunal evidence yields similar results. Foraminifera species are present, associated with estuaries, with inner continental shelf, and both outer shelf and bathyal depths. The majority of forams represent the inner shelf biofacies and it is in this zone, ranging in depth from a few feet to nearly 200 ft in depth, that the Twiggs Clay was deposited. However, we emphasize that no one environmental interpretation answers all questions dealing with the Twiggs Clay history. There are two species of foraminifera present from the shore (a brackish environment), three species from the middle and outer neritic zone, and three species from the bathyal zone. Clearly, the western end of the outcrop belt was deposited offshore in quieter water than in the Bibb County area, which is further updip and accepted considerable sediment influx (Schmidt, 1977). The eastern portion of the study area represents almost no clastic input but is certainly also inner shelf marine. In addition to the fossils noted, a few echinoid and bryozoan remains have been found along with sponge spicules and a shark tooth. This wide array of marine organisms indicates a normal marine environment. Another ramification of this study was the time zone set up from the coccolith zonation. It was pre- viously known that the Twiggs Clay is late Eocene in age, and this was verified using published foraminifera ranges . However, by using a first occurrence and two extinction datum levels of three coccolith species, the time of Twiggs Clay deposition can be more accurately defined. It is now considered middle late Eocene. ACKNOWLEDGEMENTS This paper is based on a Master of Science thesis (Schmidt, 1977) submitted to the Department of Geology, Florida State University. We are grateful to Mr. Sam M. Pickering, Jr., who kindly made available the well-cutting and core collections of the Georgia Geological Survey. Dr. S. Duncan Heron also made available samples from his private collections. David M. McCollum, Dr. William I. Miller, Dr. Andrew M. Gombos, and Dr. Lyman D. Toulman assisted in the identification of various microfossils. Dr. Miller critically reviewed portions of the manuscript which was typed by LaVerne Lamb. Dr. Miller also took many of the scanning electron micrographs and Dr. Charlene R. Miller and Dennis S. Cassidy assisted with the photography. 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Zales.ny, E.R., 1.959, ;F:orar;ninif,er.Q.I ~cq!ogy of;Santa Monica Bay, Californ.ia: MicropaleoQtplogy, v. ,5, Pt)01-126, _ . '. I: ' I \.- J '. 1 J I I I A'PPENDIX I SAMPL.:E 'LOCA'IJTJES' '1 I ~ 1 I '.1 t arr '4' 1 R1 ich ' H~f 'Q so utn..t. o( 'Coitu nt4y1 ~ (~''_ dqhed) , ,nll! ~' - ~aI!~J. 4?, .7 ac cess road 1-l j 111 ~olu, ht}h east of Kn? villE:), Ga. Quarry ~bpu_t 0.~ rp1 'sout,h ~n access road. E* po.sed" flere ar~ about sp tt of secti6n vVfth the1 _~P.~er' 2'd'ft o'elrigfw'i'gg1s'Ciay. 1 u C:l~r'ne~HVire fab'o 12d(t1ofOca lalim'estone I t u nvd e( r w hh I :htr le j: ab6ut io I ,I I trpf:CIInc'hfie,la S'and . I I I 'I .,j r r A 1 ~ '" t 1 ' f ll r1 1 11 2. Stop . No . 4, G._$. f- ., F i~ l. . Gu.iqle BQQk JNp . 12 ,' 1 (H~oJd ~ I Ej$ ti,J.fl aJld Qth ~rs,. 1974), ,frql]l1 1~ . $ . ~ . ,,r f$oytil~ast{~r1q, M e~tl ng_ Atla nta , 1974,, ,i.;i,me- ,s-~.one Q\J.~WY pn. ,t1h.e east sjde of the Rl3rr&~ - E i ko lioad about 2 mi sou t h o1f l?en:y. .oity ,Jirn its . i;:X.r19l?.~P. 1 t'le e , are ap,ovt 7g 1fct of the Tw. i gg~ CI;ry._. I I ! ' anp: o.. ~- 9(.~1~9pdp_N,l,1~?s;t5u ,_G, ~ .S ,A . o ,Fje ld ~r.eJ~ ~,U(de !39dk .-r9J 4) , f rom 1 ~0- s ,) 2 .A. 1 . '~TGalwofbi'.iugtoglg'n~she~u)Cs.a.sd1e~.ra~'ba0Moteeu'ett.e~r;-xl3Lnp.a94:.b1,.s,m.(Ae-Hcit'il'esr'iorn~ey?ta't.h~id11e/o97cf{14phH:.ppaae.lr6~:rt,tneec~v{el?tltPNlneSef, we'M side St: th:e h\g'. l'lw'ay',' ~l 1 ' '' . I I !I ' ' \ '' ,. ' I ' 4~. StOpNo. 3'/ G.S.A.~,Field : Guid 'e:Book No. 12 (HliddlestUiti a'nd bth.ers , l97<4). from G:S.A. Southeaster-n. Meeting, Atlanta, 191.4!. On U.S. Route 341 o: o.3 mi -sbUth;'of' Clinchfield.Geo- logic . sectio'ri e~posed in Medusa .Portland Cement Go., west quarry. ,The Twiggs Clay here measmes about 105 fL r I 11 5. Road cuts along Georgia State Highway 247 between Big Indian Creek and the top of the first drainage divide to the south. Section of Twiggs about 90 ft thick; however, hard to measure due to covered intervals and dip approximati'ons. 6. Subsurface sample from Georgia Geologic Survey~ Well No. 472. Location: 4 mi south of Pulaski-Bieckley County line, east side of U.S. Highway 26. In this well the Twiggs is undiffe~- . entiated and only the Barnwell Formation is identified. I! l 7. Subsurface sample G.G.S : Wel'l No. 195.' Loca- ,tion: northeastern part of city; 0,5 mi east of of Highway '129 in City Cochran . lri this well, the Twiggs Clay is present and divlded into two . I partsby a tongue of the Ocala Limestone (Tiv- 'ol a Tongue~ ; The total th ickriess is ~bout 85ft._ , I :, I i' ) 8. ! Road cut about 7 mi north of Cochran in Sleck- leyCounty , ori Houte 87. Here the'1wiqgs' Ciay. is eroded and : indurated, a thickness a n o-btai'ne'd . ' '' 1 9. ' Location from Carver (19T2a) southern Twiggs County (m ineralogy ' data only). 1 , :. r ,, 10. Subsurface sample, G.G.S. Well No. 360. Loca- . tion: about 3 ,5 mi east of Huber; 2.5 rni east of -U:S. H'i.ghway 129, arid l.5 mi southof a cross- roads .at machine stop; In this well theBarnwell is identified ; however, the clay'Jn theupper 25 ; .ft of the formation may be Ti.-viggs .. ' . " . , I ,: 'I ' 1 ~ 1. -A road cut on the south tbank of -Ugly creek about 2.2 mi south of Myricks Mill, on an unnumbered dirt road. Here tl:le clay is fairly pure !:>'Lif only partly exposed. 12. 'A road :cut along . a road from Fitzpatrick to Myric!ks Mill Pond . on th-e north side of the :roa'd abouH :5 mi friom' Route 80. ' I ;: 13. Locntion from Carve:r: ' (1 972a); northern Twiggs County (mineralogy data only) . 'I'' ' I ' :' I ' 14.' ' Pikes l?eak Station '(type locale) . About 2.6 mi south of Dry Branch Station on U.S. Highway 1, .: 80, a raiJroad -cut which e.Xposes ab'out 100ft .. Seven sam'ples were collected herte about every 10 ft with No . l startin'g. at'the base (1 to 7) . Also near Pikes Peak an exposure'oh the land of the Georgia Kaolin co ~ about 15-20 ft exposed: On Route 80 coming s6uth f-rom Dry Branch (about 1. 8 mi) at crest of -hill, a good cut exists on the east side of ti;Je' road along the railroad . 106 15: Stbp No.1 of.the G.-S:Ao. Field Guid~ I Book No .. 12 (Huddlestl!ln cihd 'others, 1974), from G.S.A. Southeastern Meeting, Atlanta, 1974. Section exposed on west side of Interstate Route 16 on top and sides of .:~ il,l above ,Oc mulgee River Valley, about 8.2 mi south from Ocmulgee ,1 1 fftiivthwick[3,rid,g~ 1 TheTvvigg,~ ,.. , . CIR''-Y ~~ r 13. is ' q.bo,w~ . .: . ?5 J ; r. ~ -. ! . I :-. \ ,. ! ! , i 16. Road cuts -at Mattie Wells School ; Ot:l' Georgia State Highway 49, northeast of Macon, 2.2 mi NE of the Bibb-Jones County line. Total thick- ness here approximately 20 ft; however, the base is not exposed . Six samples collected starting at the base about every 3ft (1 to 6). Also in south Jones County.~a location from Carver (1972a) (mineralogy data only). 25: I At Wrensi Ga., mine of GeorgiaiTenhessee ' Minin~ 'and Chem'ical Co. " At it he mine, the :T..,.hggs thickness-is about40 ;fr :wit'h base not '' exposed. Seven samples ohe every 6' f.t stantihg at were base. collected (Ho i7). ' ab~ out .. I . I ( ) I i ' I p' I ' ~ j ' : I 1 I : - 26. Location from Carver (1972a) near Stellaville, ' Ga:."(rnineralo!jy data dnly). ,: r, i Ca~v~r (f~7~afn8anHarlem, ' : f I I ; r :._1 1 i -) J 1; '27. Lo'cation fr.om , , , ~ :' ~ 1 ~ ! , . _.. r , Ga. (mineralogy data). !. , i ~,.;r. : ,; 28 . Subsurface sample G.G.S. Well No. 371. Location: Silver Crest and Fleming Heights, Gracewood. In this well about 30ft of clay occur in the Barnwell Formation. 17 . Subsurface sample, G.G.S. Well No. 529. Loca- 29 . tion : southwestern part of Wilkinson County, near Danville. In this well the Barnwell Forma- tion is identified and its clay member is proba- bly Twiggs Clay, about 40ft thick. ~- ;i1 t ... Stop No. 1 on Georgia Geological Society Third Annual Field Trip, Oct. 4-5, 1968 (Herrick and Counts , 1968). Southeast of Augusta, U.S. Highway 25 going south, turn right on Windsor Spring Road, go about 1.6 mi to road cut on both sides of road. Here Twiggs Clay is about 18. Subsurface sample G.G .S. Well No . 441. Location : within the City of lrvinton, which is in north-central Wilkinson County. In this well! 35 ft thick. Two samples were collected, August 1 and 2, with one being at base.. the second about 15ft higher. only the Barnwell is identified ; however, about 4G' ft:.of ctay 'wilhin: the -formation is proba'tily' ': . ~p,,, "~t~th'~e~':~1~2s~rs~~~i~~!rfn~ng~~~~~:1'~JJ~: Twiggs . ,1 I il I ' T l)r I' ,,,, 11G[iffin~ ~~oatt La ~llf D~1S~TJ1J;l(e.,p rpvid,e_q 1b1Y, !i)r. 19. ,South of Gordqn about 1.6 mi on Georgia S. Duncan Heron (his samp le No. A- 148) . a ' Highv\i~y 1' 8; ditch 1on th'e west sltl eb'fl hl{ tl~d ~ 1Y1111 )I 1') 1 11 I I\ .g , 111 1 exposes a felfl.feet p,t}'{Vig:Q? C; I,~Y.:: , , '. ,: .. . 1, , 20. Road cut on Georgia Highwa.y .57, abou.t 1.~ . mi 1 ::~ 1 ,.: .,. j \, . ,::' ,) i ' 1; ~ : ~ I l i , ' ;, . ' ::\ ', i I i ~ : \ , '...: , ' ,,,_:1' southeast of Toomsboro. Here about 10-12 ft ~~eM~;:a~sxe d(194T6h,~. ~- 1~1c)~ 1 e IS referenced in ~; , . 'SMEAR SLIDE APPENDIX II PREPARATION FOR DIA- ' ( : ' ) : L ' i : ~ ' : ! -' ' :. I '' ' I ' ' I I ;.) ~ I 21 . Subsurface sample G.G.S. Wei I No. 438. Location : within tt'le ,city of . Duh>:lin ~ +rlere,comly the .TOMS FBOM THE TWIGGS CLAY ' '~ ' . I 1 ', ( ,-~: ) ' / ,' I \h ., ,' ', ', \ 1 ,' ,I .' i' . ': \ i i t l ! :! , . p~~m: . 1, ,, .$.~~;~~ .9 ,3rl?f?,l,~, (qre.P,~)P..~Qrka .ble ,si.z,r), Barnwell Formation is identified and appar- thrs rncreases the surface area. ently no TwFggs ' Ciay1 'is! present b'ec.M.Jse ': .. ,, i'''. . . ', , . :~,: : :<:,. -.;. .";: ,., ~ ., , , ,..; mostly limestone and marl compose the Step 2. Dry sample completely. I ;s'' e' c.' t'lo~ n., l \j l ': ~~: : ', ,~ u :i:;\ ,- ~ . :.(.: ~r' , t ~ l J :FI_, 1 ,! , !L .', .. il. \....., ' ... ..... '.' " ,. ._,.... : . ..:'/ .q \'> '\\ ~."'./\ :. :: i. 1: li'l Step 3. Remove from heat. 22. In creek gully just east of Buft~!6' Creek . tPJ~kt it rr 1'1 l Olli'! .f <" ;- .,: . :;;. ,:: ' . , , , , , ,,\'' , ~ _, ::.; .: ;;r;.yrn . of Sandersvi lie on Route 24 ap.q,utrB ,m,i,. south on a dirt road about 0.8 mi, g ully alo hg Ste 111 ' f ~- - Whil e ~ample is still hot, add Stoddard (1' 1" sol l1'crn,''Vahs'al,:kerc's'ene: ar p'etrbleum .road. ,, ' I . . , :: , , ,, ,T rH 1 1 !J li,nu , ~~~ - i ~~t~, ,9f ,~em.~ swt. A,l,lo.vy~ to ~~}}~_,~o 4 23. Road cut on northeast ' side df'roa'd;.: ap pwxi~ < mately 4.3 mi southeast of Deepstep on the Sandersvilltfrda'd,' % mi 1sckrth 'cif Keg Cre:ek. 24. Subsurface sample, G.G.S. Well No. 554. Location : Within the City of Louisville . In this well the Barnwell is identified and is mostly marl and limestone, clay absent. \,lrl~ Step 5. 1 r - rJ Step 6. ; 1;: =,' J<'.l .' \ )f~,r ~, l ~ ;.~ \ \\.' \ . :. , ~ Decant Stoddard. '1,': tr :! : ., V' :',} ~ , ; ; :, , ., # , , ~:, :.' :i J\ : ; Boil sample approximately one hour in H20 + H202 + Calgon or sodium pyrophosphate-about4: 1: 1/2 teaspoon. Step 7. Sieve in 6311 sieve . Step 8. If further cleaning is needed, a centrifuge in distilled water at 3000 rpm for a few minutes may help (repeat if needed) . Step 9. Put sample back in beaker and add concentrated HC I (enough to get bubbles). Step 10. When bubbling has stopped, wash sample again in 63J.l sieve (at this point, if a great deal of clay remains, repeat steps 2 through 7) . Step 11 . Mount sample using Hyrax or other suitable mounting medium. POINTS TO PONDER Always use distilled water in diatom preparation. In many areas, tap water contains diatom tests which may contaminate your sample. PLATES PLATE 1 Figure 1. Coscinodiscus radiatus Ehrenberg. Sample 25-7 at Wrens, Ga.; x 550. Transmitted light micrograph. Figure 2. C. radiatus Ehrenberg. Sample 25-2 at Wrens ; x 450. Transmitted light micrograph. Figure 3. Coscinodiscus oculus iridis Ehrenberg . Sample 29-1 near Augusta ; x 650. Transmitted light micrograph . Figure 4. C. oculus iridis Ehrenberg. Sample 25-2 at Wrens, x 450. Transmitted light micrograph. Figure 5. Coscinodiscus sp . Sample 16-2, x 350. Scanning electron micrograph . PLATE 2 Figure 1. Actinoptychus senarius Ehrenberg. Sample 16-4; x 1100. Scanning electron micrograph. Figure 2. A. senarius (Ehrenberg) Ehrenberg. Sample 4; x 1025. Scanning electron micrograph. Figure 3. A. senarius (Ehrenberg) Ehrenberg . Sample 25-2 ; x 900. Transmitted light micrograph. Figure 4. Actinoptychus splendens (Shadbolt) Ralls in Pritchard. Sample 30; x 350. Scanning electron micrograph . Figure 5. Rhaphoneis sp . Sample 25-2, x 4,000. Transmitted light micrograph . Figure 6. Rhaphoneis sp. Sample 25-6; x 1,600. Transmitted light micrograph . Figure 7. Paralia sulcata (Ehrenberg) Cleve. Sample 25-7, x 1,400. Transmitted light micrograph. Figure 8. Trigonium barbadense Greville. Sample 25-5; x 1,500. Transmitted light micrograph . Figure 9. Odontella aurita (Lyngbye) Agardh . Sample 25-5; x 1,600. Transmitted light micrograph. 108 PLATE I CD 109 PLATE II 110 PLATE 3 Figure 1. Figure 2. Figure 3. Figure 4., 5. Figure 6., 7. Figure 8., 9. Figures 10.-12. Figures 13.-15. Figures 16.-18. Figures 19.-21 . Figures 22.-24. Figures 25.-26. Eupodiscus radiatus Bailey. Sample 4, x 215. Scanning electron micrograph. Eupodiscus radiatus Bailey. Sample 14-2, x 310. Scanning electron micrograph. Note: two frustules, one case, one mold. Stephanopixis sp.? Discoaster barbadiensis Tan Sin Hok. Sample 3; x 2,000. 1) Transmitted light; 2) Phase contrast. Discoaster saipanensis Bramlette and Riedel. Sample 3; x 1,800. 3) Transmitted light: 4) Phase contrast. Discoaster tani Bramlette and Riedel. Sample 3; x 1,500. 5) Transmitted light: 6) Phase contrast. Coccolithus formosus (Kamptner) Wise. Sample 3; x 2,100. 9) Transmitted light; 10) Phase contrast; 11) Crossed nicols. Reticulofenestra reticulata (Gartner and Smith) Roth. Sample 3; x 2,200. 1) Transmitted light: 2) Phase contrast; 3) Crossed nicols. Coccolithus pelagicus (Wallich) Schiller. Sample 2; x 2,200. 4) Transmitted light; 5) Phase contrast; 6) Crossed nicols. Transversopontis obliquipons (Deflandre) Hay, Mohler, and Wade. Sample 3; x 2,000. 7) Transmitted light; 8) Phase contrast; 9) Crossed nicols. Discolithia meltipora (Kamptner) Roth. Sample 4; x 1,500. 10) Transmitted light; 11) Phase contrast; 12) Crossed nicols. lsthmolithus recurvus Deflandre. Sample 3; x 1,500. 7) Transmitted light; 8) Phase contrast. PLATE 4 (All figures are scanning electron micrographs) Figure 1. Textularia hockeyensis Cushman and Applin. Sample 3; x 35. Figure 2. Nodosaria ewaldi Reuss? Sample 1; x 110. Figure 3. Nonion sp. Sample 3; x 200. Figure 4. Lagena laevis (Montagu) Williamson. Sample 2; x 170. Figure 5. Nonion inexcavatum Cushman and Applin. Sample 3; x 60. Figure 6. Bolivina jacksonensis Cushman and Applin. Sample 3; x 65. Figure 7. Cibicides lobatulus (Walker and Jacob) Cushman. Sample 3; x 45. Figure 8. Cythereis montgomeryensis Howe and Chambers. Sample 3; x 40. Figure 9. C. montgomeryensis Howe and Chambers. Sample 4; x 90. Figure 10. C. montgomeryensis Howe and Chambers. Sample 3; x 90. Hinge of left valve. Figure 11. Hemicythere punctata Puri. Figure 12. H. punctata Puri. Sample 4; x 180, Hinge of left valve. Figure 13. H. punctata Puri. Sample 3; x 90. Figure 14. Trachyleberis citrusensis Puri. Sample 3; x 110. A complete specimen. 111 PLATE Ill li' .... . f t ~ 112 PLATE IV 113 SEDIMENTOLOGICAL TOOLS FOR IDENTIFYING DEPOSITIONAL ENVIRONMENTS William F. Tanner Regents Professor of Geology Florida State University Tallahassee, Florida ABSTRACT Marine terraces of probable Pliocene age, at elevations of 80 m, 50 m, and 35 m, west of Tallahassee, Fla., have been identified on geomorphological grounds as barrierand-lagoon sets. The present paper uses sedimentological methodology to see if the same identification could be achieved were there no useful geomorphological evidence. Grain-size probability plot characteristics (surf break, tail of fines, dune hump) , a variability diagram (variability of sample means vs. variability of sample standard deviations), a plot of skewness vs . kurtosis, and the algebraic sign of the sample skewness, are all used to good effect. Bedding, crossbedding, ripple marks, fossils and facies change data were not available. Therefore, this study simulates a common coastal plain problem: to determine the site of deposition of more-or-less uniform sands where fossils are absent and exposures are poor. The sedimentological interpretation identifies the coastal setting correctly, indicates moderate rather than high or low breakerzone energy levels, suggests a barrier island location, and strongly supports the notion of local dunes, but does not identify the lagoons. Three marine terraces west of Tallahassee, Fla., are located at altitudes of about 80, 50 and 35 m above MSL (fig. 1). They are tentatively considered to be between 2 and 10 m.y. old. Gremillion, Huddlestun and Tanner (1964), on the basis of geomorphology, identified them as barrier-and-lagoon sets, but inadvertently assigned a Pleistocene age (an older age was intended at that time). It is the purpose of the present paper to show how sedimentological techniques can be used to help in the identification of such deposits. No crossbedding, ripple marks, or other such features were found to aid in the identification; there are no known fossil localities or visible facies changes, and exposures are generally poor. The mean grain size (115 samples; Goetschius, 1971) for the entire suite of three terraces and two lagoons is 1. 770 phi units, (0.29 mm), and the mean standard deviation for the suite is 0.87 phi units, with the sensitivity associated with quarter-phi sieving for 30 min. (Mizutani, 1963). The settling tube was not used (Coleman and Entsminger, 1977). These data do not suffice to identify the depositional site, being common values for coastal, alluvial and eolian deposits. FLORIDA GADSDEN COUNTY 10 I KM Figure 1. Map of sample area, showing three high terraces, probably all pre-Pleistocene in age, representing former sea-level positions at (approximately) 35 m, 50 m, and 80 m. Map modified from Gremillion and othe rs, 1964 (in which the word " Pleistocene" was inadvertently used). Thirty-three of the probability plots (fig. 2) show reasonably clear examples of the surf break (Tanner, 1966), and the tail-of-fines is present in many of these. This combination indicates low-tomoderate wave energy in the breaker zone . Fourteen of the sample size distributions are essentially Gaussian, and in the light of the preceding sentence, they probably indicate high-energy beach and nearshore conditions. Twenty-one of the probability plots show the dune hump (Stapor and Tanner, 1975). A few of the plots show evidence of both surf and eolian activity. Positive skewness values (closely related to the dune hump) were found in 79 samples, and negative skewness (commonly associated with the surf break) was found in 36 samples. This method of study does not identify or eliminate the possibility of river deposition. 114 0.1 1 2 15 10 20 30 50 70 eo 90 98 42 49 0 ~ SB 2 Figure 2. Representative grain size plots, showing two examples of the "surf break" (SB), one example of the "dune hump" (DH) and two examples of the "tai I of fines" (TF) . If the eye is held close to paper level, and to one side of the lower right-hand corner of the figure, DH will become obvious . Sample 49 is perfectly Gaussian. This combination of four samples indicates coastal dunes and moderately high surf conditions . More than 125 other samples, from the same study area, showed only variations of what has been presented here. A plot of the standard deviation (i.e., variability) of the sample means, against the standard deviation of the sample standard deviations, may be instructive. On such a plot, high-energy agencies tend to impart great variability to both parameters, whereas lowenergy agencies produce only small variability. More specifically, mountain and hill-country streams give maximum values for variability of both means and standard deviations, whereas eolian transport provides the smallest values for both. The sequence of agencies, from maximum to minimum, and with considerable overlap, is: river, offshore wave, swash, wind (fig. 3). Sediments in lower reaches of alluvial rivers overlap markedly the offshore and high-energy beach environments, whereas dune and low-energy beach sands overlap somewhat. Variabilities have been computed for both means and standard deviations for five sets of samples (the three terraces, and the two lagoons) . All five sets fall in the "high energy beach, offshore wave, and coastal plain alluvial river" category (fig. 3). All five sets are closely bunched together on the variability diagram. An independent study of additional samples, collected at new locations and analyzed without knowledge of the previous work, verified the variability chart position of the 50-m terrace (within 20 percent for variability of means; within 40 percent for standard deviations; on the variability diagram, which extends across three orders of magnitude, this is close agreement). The lack of a dune position on the chart does not deny wind work, but puts it no higher than second rank. 0.1 .......... ~(v ........ <:;).:::) O.OIL-~~~-~ LL----~--L-l-~~UU~--~W~F~T Figure 3. The variability diagram: the variability (standard deviation) of sample means has been marked along the horizontal ordinate, and the variability (std. dev.) of sample standard deviations has been shown along the vertical ordinate. The upper right hand corner is an area of relatively great variability, and the lower left hand corner an area of relatively little variability. This particular version has been prepared for medium sand, and sizes not too far removed from it, and is not applicable for coarse silt or for gravel of any size. The number fields for marine agencies are indicated on one side of the diagonal strip, and for nonmarine agencies, on the other side. In addition, dots outline the swash-and-dune, as well as pure dune, areas occupied by beach ridges. This chart does not permit the unequivocal identification of single agencies and sites; on the other hand, it does permit reasonably close assessment of the general setting. Sample suites are indicated for the 80-m, 50-m, and 35-m terraces, and for the 50-m and 35-m lagoons. Any one of these suites (or all five of them) is (are) shown, on this chart, to belong in the dune-beachestuary-lower-alluvial setting. They cannot represent hill country or mountain streams, nor can they represent inland dunes. Although glacial features are not shown here, it is clear that glacial deposits, if present, cannot be significant. 115 Once the beach-and-dune interpretation had been made, a plot of skewness and kurtosis values for 40 samples from the 50-m terrace showed that seven occupy positions generally assigned to beaches, five more fall in a "beach ridge or coastal dune" category, and the rest indicate wind work (fig. 4) . The mix of positive and negative skewness values, given in a previous paragraph, provides the same interpretation, but with slightly less effort. SK -o.s 0 0 .6 i', I '- '-' 2 K* B E 1.0 0.8 0.6 0 0 0 0 0 0 Oooo 0 N E WPT Figure 4. A plot of skewness (Sk) versus kurtosis (K*) for modern sediments in the Florida Panhandle coastal zone, used as a base for examining moment measures (third and fourth) from the study area. K* is kurtosis-minus-two (if computations are so designed that K=3 for the Gaussian distribution) or kurtosis-plus-one (if Gaussian K is calculated as zero). Modern coastal dune and beach samples studied are covered without exception by the indicated number fields (shown by outlines only); all modern beach ridge samples fall in the area outlined by dashed lines, consistent with the observation that beach ridges are ordinarily constructed by some combination of wind and swash. Many workers have constructed plots of two or more moment measures, with only modest success, the exceptions for individual samples having been fairly numerous; therefore this plot must be used with caution . However, the interpretation of "beach, dune and beach ridge" origin seems to be moderately well supported, and can be accepted with confidence provided other indicators show the same thing . In the present case, a river origin cannot be ruled out by the Sk-vs-K diagram, but is largely eliminated by the probability plots. Using only the sedimentological information, one should conclude: Primary agency: offshore waves, surf-andswash, and/or coastal rivers in the lower reaches. Secondary agency: wind. First restriction : not a delta of the Mississippi or Alabama River type. Second restriction : wave energy was low-to- moderate much or most of the time, and probably had an average value in that range , but was high energy part of the time. Third restriction : the over-all mean and standard deviation do not permit much relief. This collection of sedimentological statements can be streamlined by saying : coastal and nearshore sands, deposited on a gently sloping bottom, in the vicinity of a river mouth but not in a humid-climate delta (much clay and fine silt) . The "gently sloping bottom" suggests a further simplification: probably barrier-and-lagoon sands. The chronology here is terrace type (highest is oldest), rather than standard stratigraphic (highest is youngest). If the geomorphological expression were not known, this might be a source of confusion as to the details of the historical sequence, but should not modify the conclusions given above, which are based on the entire sample suite, regardless of location. Looking at relative locations, which can be considered in terms of the direction toward the sea (a standard piece of information in the coastal plain), one can note the following changes: finer mean grain size seaward, more variability of the mean seaward, best sorting (numerically smallest standard deviation) in the middle of the land-to-sea line rather than at either end, and best evidences of eolian effects toward the land. Less clear, but suggestive, is the fact that grain size decreases toward the east, except for a small reversal near the eastern edge of the area. One might wish to deduce, from the information in the previous sentence, littoral transport from west to east. If crossbedding or ripple mark data were available (Tanner, 1962, 1965) , it might be possible to confirm or refute this interpretation without recourse to the geomorphological information, which suggests west-to-east littoral drift. Without the geomorphological expression, one probably could not identify the lagoons, which were merely topographically low strips in the sand sheet, having no distinctive sedimentological characteristics at any of the sample sites. The ages of the three terraces cannot be known with precision . Traditionally, marine terraces have been considered to be Pleistocene. The ironstonedating method of Maxwell (1971a), done by Maxwell 116 on the 9-m and 6-m terraces farther south, yielded an age of about 400,000 yrs.lf a published alluvial terrace curve from western Alabama (Maxwell, 1971 p) can be extrapolated in the project area, the 35 to 80 m terraces are roughly 1 to 4 m.y. old. A study of Huddlestun (personal commun.) indicated that these three terraces may be between 2 and 8 m.y. old. Published marine terrace curves, pertinent to the study area (Tanner, 1968),suggest ages perhaps as great as Miocene. All of these different approaches are thought to give rough agreement: probably Pliocene, but clearly pre-Pleistocene. At many coastal plain localities, geomorphological evidence is not available and exposures may be so poor that crossbedding, ripple mark and other stratigraphic information cannot be obtained. Under this circumstance, sedimentological procedures, such as those outlined here, may be useful in determining the environment of deposition, or at least in narrowing the number of choices to an acceptable minimum. Tanner, W. F., 1962, Upper Cretaceous coast of Georgia and Alabama: Georgia Min. Newsletter, v. 15, nos. 3, 4; p. 89-92 . _ _, 1965, Cretaceous shoreline across the South : The shale shaker, v. 15, p. 118-125. _ _, 1966, The surf "break": key to paleoge- ography? Sedimentology, v. 7, p. 203-21 o: _ _, 1968, Tertiary sea level symposium: Introduction, in Tanner, W.F. ed., Tertiary sealevel fluctuations: Paleogeography, Paleoclimatology, Paleoecology, v. 5, no. 1, p. 7-14. REFERENCES Coleman, Craig, and Entsminger, Lee, 1977, Sieving vs. settling tube: a comparison of hydrodynamic and granulometric characteristics of beach and beach ridge sands: in Tanner, W.F. ed. , Coastal sedimentology, Geology Dept., Florida State Univ. , Tallahassee, p. 229-312. Goetschius, D.W., 1971,Preliminarysedimentological and geomorphological study of certain high terrace sands between the Ochlockonee and Apalachicola rivers, Liberty and Gadsden Counties, Florida: Unpub. M.S. thesis, Florida State Univ., Tallahassee, 98 p. Gremillion, L.R., Tanner, W.F., and Huddlestun, P.F., 1964, Barrier-and-lagoon sets on high terraces in the Florida Panhar:~dle : Southeastern Geology, v. 6, p. 31-36. Maxwell, R.W. ,1971a, Preliminary ionium date from marine terrace, Florida: Coastal Research, v. 3, no. 5, p. 9-10. _ _,1971b,Origin and chronology of Alabama river terraces: Trans., Gulf Coast Assoc. Geol. Sacs., v. 21. p. 83-95. Mitzutani, S., 1963, A theoretical and experimental consideration on the accuracy of sieving analysis: Jour. Earth Sciences, Nagoya Univ., v. 11 , no. 1, p. 1-27. Stapor, F.W., and Tanner, W.F. , 1975, Hydrodynamic implications of beach, beach ridge and dune grain size studies: Jour. Sed. Petrology, v. 45, p. 926-931. 117 ENVIRONMENTAL IMPLICATIONS OF PALYGORSKITE (ATTAPULGITE) IN MIOCENE OF THE SOUTHEASTERN UNITED STATES Charles E. Weaver and K.C. Beck Georgia Institute of Technology School of Geophysical Sciences Atlanta, Georgia ABSTRACT During early Mioce~ time palygorskite formed in the southeastern United States in shallow, brackish-water coastal lagoons. It altered from montmorillonite by the addition of Si and Mg. It formed in a humid, subtropical to tropical climate that was modified by ocean currents controlled by the movement of continental plates. It is unlikely that the palygorskite formed in a normal marine environment. The Miocene sediments of the southeastern United States contain commercial deposits of palygorskitesepiolite and phosphate. These minerals formed in shallow-water coastal environments. Other minerals that formed in these environments include dolomite, calcite, opalcristobalite, and zeolite. The purpose of the study was to determine the environmental conditions under which these minerals formed, particularly palygorskite (Weaver and Beck, 1977). Sediments were deposited in shallow water in a mildly tectonically active hinge area separating the Atlantic Ocean and the Gulf of Mexico. Montmorillonite is the dominant clay mineral in the Tertiary of the Atlantic and Gulf Coastal Plains. However in the upper Oligocene and lower Miocene sediments of northern Florida, southeastern Georgia, southern South Carolina, the Georgia Shelf, and the Blake Plateau, palygorskite and sepiolite are commonly the dominant clay mineral. An isopach map of the Miocene and upper Oligocene Tampa Formation (fig. 1) shows the major structural features in the area: OcalaSuwannee Uplift, Atlantic and Apalachicola Em- ALABAMA z <( I.J.J u 0 u i- z <( Figure 1. _.J i- Generalized Miocene paleogeographic map 1-----i 20 KM <( showing two positive areas separated by two depocenters. The Suwannee Uplift is an older feature than the Ocala Uplift. Narrow sill separated Apalachicola Embayment from CONTOUR INTERVAL trough area of Atlantic Embayment. !00'=30. 5 M 118 bayments, Gulf Trough. During at least part of Miocene time the Gulf Trough was separated from the Gulf of Mexico by a shelf or ridge and was open to the Atlantic Ocean. Trangression over a karst topography started in the late Oligocene when the Tampa limestone an d dolomite were deposited. Palygorskite formed in the shallow-water feather edge of transgression. Montmoril lonite was deposited in the open marine environment. A regressive phase occurred towards the en d of early Miocene. A seco nd palygorskite horizon was deposited in the coastal environments of the retreating sea (fig. 2). >- (.!) 0 ...J 0 I I- <>t- ...J u 0 Fullers Earth (Pre Chi po la) Figure 3 is a cross section in the northern part of the area along the Savannah River. Palygorskite and sepiolite are restricted to lower Miocene beds and stop abruptly at t he lower- midd le Miocene boundary . Numerous other cross sections show that the bedded palygorskite is restricted to the upper Oligocene and lower Miocene formations . The middle Miocene sediments are characterized by the presence of marine diatoms and opalcristobalite (fig. 4). Phosphate pebbles are concentrated at the lower-middle Miocene boundary and mark the beginni ng of the middle Miocene transgression. This boundary, characterized by phosphate and clay pebbles, can be traced overmuch of the area. This is th e period of time when much of the phosphate was concentrated in the coastal area. Figure 5 is a north-south cross section down the center of the Trough. Palygorskite (stippled) is present in the upper Oligocene and lower Miocene beds. The commercial palygorskite clay beds occur in both the lower Miocene and middle Miocene sediments (southern part of section). The latter deposits are detrital and were derived from the authigenic lower Miocene deposits. 12 (/) 24 a:: w 1w-- 2! z :r 1-- Q. w 0 36 Figure 6 shows the general lithologic units of the lower Miocene. Coarse, high energy, gravelly deposits occur in the center of the Atlantic Embayment. These were deposited in an estuarine environment at the mouth of the ancestral Altamaha and Suwannee Rivers. The deposits are flanked by shallow-water brackish dolomite (limpid variety) and dolomitic palygorskite beds. Relatively pure clay beds occur to the northwest of the Ocala High and extend southeast into the embayment. The environment was shallowwater marine to brackish. The distribution of palygorskite and sepiolite in the lower Miocene is quite extensive (fig. 7). Sepiolite is concentrated shoreward of the palygorskite and was apparently formed under less saline conditions. The Atlantic Embayment and Trough contain detrital palygorskite. Much of the lower Miocene palygorskite was depos- ited as 0.3 to 3 m discontinuous beds in brackish- water or schizohaline environments. Montmorillonite To rreya occurs in the continental and marine sediments, and was also deposited in shallow brackish-water lagoons. There, Si and Mg were supplied by sea water, 48 and under relatively high temperature and subtropi- Mi ocene cal conditions, the montmorillonite was converted to Ti dal Lagoona l Upper Oligocene Tampa palygorskite. Most of the montmorillonite did not go into solution. 57 Oysters abundant LAKE TALQUIN CORE w- 6890 Figure 2. Transgressive-regressive sequence in core from northeast edge of Apalachicola Embayment. Florida. Figure 8 is a sketch of a typical core from the mining area. Deposition started with the deposition of a marine barrier sand. Behind this was a shallow lagoon in which palygorskite formed . As regression continued, a classical soil zone was formed on montmorillonitic river flood plain deposits. This was followed by 119 EFFIO EOCENE Z Zeolite B Biotite 5 KM MIDDLE MIOCENE LOWER MIOCENE OLIGOCENE Figure 3. Northwest-southeast cross section along the Savannah River extends from northern Effingham County to near the coast. The Miocene shows an off-lap sequence. The uppermost sand unit is post-Miocene. The top of the Oligocene in the easternmost well may be at the top of the coquina or the top of the underlying massive limestone. Light stipple zone contains sepiolite; dark stipple, palygorskite. Eastern two wells are not plotted to scale. GGS-394 is 26 km east of EFF-3 and GGS-717 is 19 km east of GGS-394. 120 CLAY MINERALOGY 0 BPL 12 24 Figure 4. Core hole Effingham no. 3 showing lithology, mineralogy and phosphate content. en 0:: UJ 1UJ :!: ~ 36 :J: 1a.~ B =biotite; I= illite; 48 K = kaolinite; M = montmorillonite; P = palygorskite; and S = sepiolite. 61 ..",_,.'.. "" .~ .",' "' 0 a; 0 E !!! (D(!) "0 ' e"n ' ""' .., ... .m.. ~- cn> (00 "I'"I" "'!:::: """'' "N ' "~' (!) """' "'(!) I<) """'''(!) (!) ... ~ "g ' I FLA : GA . --1 l-t AnQie to Line l)f SecfioJn I<) !!! "'(!) (!) .,_,. """"''' ~ I' t I Suwannee 0"' Diatoms- Sponge Spicules Figure 5. Southwest- northeast cross section extending from St. George Island through Embayment and Trough t o Savann ah River. Section shows distri bution of clay minerals. White areas in lower part of section Indicate no data; In the upper part of section, wh ite indicates where kaol inite is the predom inant clay. Authigenic lower Miocene commercial clay beds occur between sections W6890 and GGS494; d etrital mi ddle Mi ocene commercial clay beds GG5205 and GGS175. Stipple= palygorskite, horizontal = montmorillonite. 121 N N Bose I Limestone MIOCENE -20KM LOWER MIOCENE CLAYS Figure 6. Distribution of major lithologic units in the lower Miocene. Mixed area to the southwest contains beds of palygorskite,but overall lithology is complex. Palygorskite is major clay in the dolomitic sediments. Figure 7. Map showing distribution of palygorskite and sepiolite in the lower Miocene. Montmorillonite is the dominant clay in unlabeled areas. Dotted line indicates location of concentration of detrital palygorskite in middle Miocene sediments. TRANSGRESSION Meters 0 1 I ,.1..1.. ~~ .....,.. 2 Manne Clay Burrowed Lagoonal Ti da l Figure 8. Lithology of MC-1 core from La Camelia Mine, Florida. Blank intervals contain relatively pure homogeneous palygorskite clay beds. General environments and direction of shore-line movement is indicated. rapid transgression and a second period of regression. A second clay bed was deposited in a lagoonal environment. Palygorskite formation stopped as the sea again transgressed the area. The presence of abundant mud cracks, limpid dolomite, and limited faunal data indicate that the palygorskite formed in a restricted brackish environment. Most of the palygorskite consists of short (1Mm) fibers and was formed directly from the montmorillonite. A minor amount grew from solution and developed 10 to 20:Mm fibers. Figure 9 is a cross section of a middle Miocene detrital clay deposit. The deposits are generally lens shaped and contain abundant clay clasts and pebbles. The clay was deposited in a marine environment (diatoms) between submerged lower Miocene beaches. A soil zone is present at the base of the deposit and is presumed to mark the lower-middle Miocene boundary. Many of the clay pebbles in the deposit are rich in apatite. Many of the phosphate pebbles in the Miocene were formed by apatite replacing palygorskite clay pebbles. Palygorskite formed in the transitional area between the marine and continental environments. There are many references to a marine origin for palygorskite, but no direct or positive evidence. Adjacent beds may be marine, but there is no reported fossil evidence that the palygorskite beds themselves are marine. Palygorskite is abundant in many deep-sea cores; however, the marine palygorskite is nearly always offshore from continental palygorskite deposits and has textural features that indicate it is detrital. Why does palygorskite not form in all brackish lagoons? What were the conditions in the southeastern United States that caused it to develop in the late Oligocene and what change in conditions caused it to stop being deposited at the end of early Miocene? Thermodynamic calculations confirm that palygorskite formation is favored by a high concentration of Si and to a lesser extent, Mg. High Mg favors the formation of corrensite, mixed-layer chloritemontmorillonite. One concern is why there was no Mg-rich corrensite associated with the Mg-rich palygorskite and sepiolite. The high Mg concentrations needed for the formation of corrensite can best be obtained in an evaporitic environment. Figure 10 shows the general distribution of some authigenic minerals through time. Corrensite and dolomite are abundant in the Paleozoic and early Mesozoic. As Mg-rich corrensite decreases in abundance, Mg-rich palygorskite increases in abundance, as does kaolinite. Various studies indicate that in North America, Europe, and presumably North Africa, the Paleozoic and Early Mesozoic climate was warmer and much drier than today. This would favor the development of evaporitic conditions and the formation of corrensite. Beginning in the late Mesozoic, rainfall increased. This increased rainfall caused more intense weathering and the formation of kaolinite. It apparently also created more brackish water conditions in the fringing marine environments. This favored the formation of palygorskite in areas where corrensite formed in drier periods. Coastal marine palygorskite deposits range from Triassic to Miocene. Most deposits fringe the Tethys and South Atlantic Oceans. Phosphate deposits show a similar distribution and commonly are associated with palygorskite deposits. Upwelling cold ocean waters are 13ssumed to be the source of the P; they would also be a source of Si for the formation of palygors.kite. The temporal data suggest that climate, that is, temperature and possibly more important, humidity, determine whether or not palygorskite will form. The 123 8 A c D 0 j .. . . .. MONTMORILLONITE 0 -- 0 (/) a: LLJ 1LLJ ~ z 12 :I: 1Cl. LLJ 0 o 0 0 . o .. 0 Ci I) d . . " =. 35 . . ,o p o45 c> = c . c 8 o ~ Cl 0 c:l 0 45 .. . ""': 30 . . .. oa. . . oso- o o . 15 o O O .... . oo :r. r 0 . . . . d , . , , e;:::::l (J . 0 . ~=:: o . . . : o :. : ; .. c'c' o o 0 . .. .. . -:-- . . ~ o ..... .. 30 % Polyoorskite and Sepiolite , ~: C Coarse Sand a Cloy Clost CHEROKEE CO. MINE lOOM Figure 9. North-south line of section (4 cores) through Cherokee Company Mine, northwestern Thomas County, Georgia. Cenoz. l 30 Mesozoic I Paleozoic Tot aI Carbonate LuIJ z 20 .q Q z ::::> .!Dq LIJ .~... 10 .q _J LIJ 0:: 0 0 -- -- ------ Dolomite ---- .,.,.,.,.,., ...... / I ~~~ II \ I1 \ Corrensite -like / I I ,1'/ . ~ \ , . . - - - - ...... '-/ , ............ ..... ..,:> eQl w 0 ::::> 1- i= 10 "') NORPHLET FORMATION LOUANN SALT ('. 0 WERNER ANHYDRITE (f) (f) < a:: EAGLE MILLS FORMATION 1- Figure 1. Generalized subsurface stratigraphy in South Alabama. 126 a_ a_ (/) (/) Brewton FLORIDA 0 Miles 20 N 1 Figure 2. Location map of study area. of the Appalachian structural front which were positive features at the time of Norphlet deposition. These include the Wiggins Uplift, the Conecuh Ridge, and the Pensacola Ridge (Wilson, 1975). Sedimentary basins in the study area are the Mississippi Interior Salt Basin, the Conecuh Embayment, and possibly a third basin to the north of the Conecuh Ridge (Wilson, 1975). The structure map on the top of the Norphlet Formation gives its overall orientation in south Alabama (fig. 3). It is present at its shallowest depths as far north as Wilcox County at a depth of 5,000 ft. It trends in a general northwest-southeast direction, dipping generally to the south, with its deepest penetration in Mobile and Baldwin Counties , at depths over 19,000 ft. The Norphlet Formation is not present in Conecuh and western Escambia Counties over the Conecuh Ridge. This feature is composed of igneous and metamorphic rocks of the Appalachian Piedmont (Neathery and Thomas, 1975), and was probably a "high" during Norphlet times . The Conecuh and Pensacola Ridges were probably major sediment contributors to the Norphlet Formation . The formation is also thin or not present over the Wiggins Uplift in Mobile and Baldwin Counties. The Wiggins Uplift was probably not an important sediment contributor. It was, however, important as a positive structural element and may have partially separated the sedimentary basin which extended from Mobile through Escambia Counties. Faulting of the Norphlet Formation has occurred in several areas. Several fault systems trend in a general northwest-southeast orientation, and one (Mobile Graben) trends north-south. These faults are extensional in nature and related to movements in the underlying Louann Salt (Sigsby, 1976) . The faults shown in figure 3 were identified by Wilson and Kidd (1977, Alabama Geological Survey Smackover Structure Map) . ENVIRONMENTS OF DEPOSITION Since most of the data obtained for this study are from Escambia and Mobile Counties, a representative section from each area will be described and discussed . The lower black shale member of the Norphlet Formation is present in some areas of Escambia County. It was analyzed tor pollen and spore and kerogen content, and found to be practically barren of both (Schwab, 1979, personal commun .). This is probably an indication of the harsh environmental conditions which existed at least in lower Norphlet times. The Exxon L&N Railroad core from central Escambia County provides the thickest cored interval in the eastern part of the study area, but does not contain the lower black shale (fig . 4) . The base of the core consists of about 32ft of red sandy siltstone in horizontal, discontinuous laminae and containing occasional laminae of medium sand. An 18-ft interval of interbedded red silt and medium- to coarse-grained sand overl ies the lower siltstone. Above this lithofacies are 34ft of feldspathic sublitharenite which is gray, fine- to coarse-grained, and contains slightly inclined planar laminae. The feldspathic sublitharenite grades vertically into a 7-ft interval of gray, fine-grained, well sorted, massive subarkose. Directly overlying the subarkose are finely laminated carbonate mudstones of the Smackover Formation. The sequence appears to be continuous from the red beds at the base into the carbonates at the top, with no evidence of erosion or n o n d e p o s i t i o n . The presence of updip conglomerates in a core from Wilcox County indicates that alluvial deposition was taking place in the area. The lower red sandy siltstone in the L&N Railroad core represents the initial clastic influx into the area , and was probably deposited in the distal part of an alluvial fan . Migrating channels or braided streams prograded 127 ........,. FAULT CONTOUR INTERVAL =500FT 0 10 20 mi . Figure 3. Structure map on top of Norpnlet Formation for South Alabama. into the area, leaving an alternating sequence of fine silt and coarse sand. Petrographic analysis shows that the feldspathic sublitharenite consists of 72% quartz, 10% feldspar, and 18% rock fragments, most of which are igneous and metamorphic. Texturally, it is submature and is silt to coarse sand size, moderately well sorted, and subangular. The coarse nature of its laminae and the high percentage of feldspar and rock fragments suggest that deposition took place near the source area. Sigsby (1976) found Norphlet Formation isopachs thicken and thin in a northeast-southwest orientation, indicating the influence of Appalachian structure. The Conecuh and Pensacola Ridges, therefore, were probably the important sediment co:-~tributors in this area. The feldspathic sublitharenite grades vertically into a massive, clean .subarkose, which was probably deposited in a lower-shoreface marine environment. This subarkose consists of 62% quartz, 7% feldspar, 21% calcite cement, and 7% rock fragments, which are igneous, metamorphic, and sedimentary. It is fine grained, well sorted, subrounded, and texturally mature. Directly overlying the subarkose are finely laminated carbonate mudstones that were deposited in subtidal to intertidal environments. The upper subarkose indicates the beginning of a transgressive phase which resulted in the reworking of earlier deposited sandstones. This transgression continued into early Smackover time. The subarkose is probably the shoreward equivalent of subtidal Smackover Formation carbonates. 128 L&N RAIL.i;:0:_~__,1.0 Mi l , " '-----'"p'-___.,,.,__,,.'!____Y '"m'"' A--A' LINE OF GEOLOGIC SECTION (?A~ ' . '1 .231 WELL AND GGS IDENTIFICATION STUDY AREA '"' NU,MBE ; I _.'J ..:~' -~;~~ ~ )()V)f~ ~ \ .,_;/ ', 'I ) I I / . .......\.,. ". \ j. I \ \ .,_ ~ - ~- ' !....., ...JJ -, I J ''-' ,I r_ , _/'\;;"\'' _ 1 ,, 1 ' __ l r I I I l I I 83 ., . ,, - ~~ ~ ' @ .. ' ';\ \I -'.r E) \ - ,. -.- '-...... r I J-' ~~ t - -1 z I, ~~ 'J~ 3 1 ' ,, J ~ ,/ I . ' " ( - ' ez Figu re 1. Map of study area showing axial trace of the Gulf T rough, with locations of wells and geologic sections discussed in this report. GEOLOGY Scattered oil test wells show that a wedge of Coastal Plain sediments ranging in thickness from 3500 to 6000 ft underlies the area where the Gulf Trough is found (Herrick and Vorhis, 1963) . However, most wells in the study area do not penetrate rocks older than the Ocala limestone of late Eocene age. Accordingly , the configuration of the Gulf Trough and its effect on sedimentation are best known in Ocala and younger rocks The stratigraphic units discussed in this paper are, from oldest to youngest, the Ocala Limestone, the Suwannee Limestone of Oligocene age, and Miocene to Holocene sediments. Eocene Series Ocala Limestone Lithology and Porosity The Ocala Limestone is generally a bioclastic, skeletal limestone consisting of the remains of bryozoa. coralline algae, and larger foraminifera bound by a minimal amount of micritic to fine crystalline calcite cement. Certain diagnostic foraminifera are frequently found at or near the top of the Ocala Limestone. These include Lepidocyclina ocalana Cushman. Operculinoides oc a/ana Cushman ,1\stero cyc/ina sp., and Heterostegina oc a/ana Cushman . The bioclastic Ocal a appears to be an exceptionally porous unit with visible porosity estimated to be on the order of 30 percent . 141 _,...2oo......_ POTENTIOMETRIC CONTOUR- Shows altitude at which water level would hove stood in ti9h1ly cased wells. Contour level varies. Datum is mean sea level. ~_:\--lf------4--..IHI--+---+-+-u- .... ""' I?~~,~.?.____'._P_fL--~L-....J"fl MILES Figure 2. Potentiometric surface of principal artesian-Florida aquifer in southeastern Georgia and northeastern Florida, January-May 1976 . After Hester and others, 1977. So uth of the G u l f T rough , in southern Co lq ui tt, Broo ks an d Thomas Co unties. the Oca la can be di vided into uppe r and lower parts based on grada- tio nal lith ol ogic changes. The lo wer porti o n Is biom- icrite or fi nely g ranula r li mesto ne . w hi c h c ontai ns sparse faun a representing few species , grading upward into the bioclastic porous limestone desc ribed above . Lipidocyc/ina ocalana Cushman is common in the lower port ion , but is much sm aller than normal. . The lower unit is very dense, lacks appreciable visible porosity, and is not found northwest of the Gulf Trough . Samples from wells within the trou gh, su c h as GGS 3186 in northwestern Thomas County show that th e Ocal a Limestone has a micriti c to microg ranular texture and somet imes conta ins Int ra clasts. Wells loc ated w ith in t he tro ugh in Too mbs County (fo r ex ample G GS 146) show that the Ocala contains minor clay an d spa rse mica in add itio n to the more usua l constitue nts. Faun a are not always p rese nt (fo r examp le GGS 3186) eith er because the deposi ti ona l e n vi r onme nt w as unsui t ab l e o r because alterat ion du r ing diage nesi s has destroyed them . The Ocala Limestone is dolomitized to varying degrees at many localities. North of the trough, dolomitization is minimal with the dolomite occurring as scattered euhedral crystals at random horizons . On the south si de of th e trou gh, in southwestern Georgia, the dolomite occurs in almost a cyclical fashion altern at ing with beds of limestone . Most dolomite beds range in th ickness from 5 to 50ft and the degree of dolomitization varies from complete replacement with no textural and faunal preservation to partially dolomitized beds containing chalky fossils . Laterally these horizons are not continuous except for a dense dolomite near the top of the Ocala, which produces a distinctive signature on the neutron log. 142 On the south side of the trough, in southwestern Georgia, the lower part of the Ocala contains scattered gypsum nodules and some thin beds of nodules of chert. The upper part contains intergranular gypsum near the gradational boundary with the lower unit. No chert or gypsum has been found north of the trough. Thickness Trends The Ocala Limestone is completely penetrated by only a few wells in the study area, especially within th~ trough, and therefore, a detailed isopach map of th1s un1t cannot be prepared. Certain thickness trends, however, can be observed. The Ocala Limestone appears to be thickest in southern Colquitt, Thomas and Brooks Counties. For example, in Thomas County (GGS 132, fig. 3, cross section C-C') and in GGS 3188, the Ocala is more than 700ft thick. The exact thickness in many wells is uncertain because the basal Ocala is in gradational contact with the middle Eocene and is difficult to distinguish from that unit. The Ocala in southwest Georgia tends to become a dense, nonfossiliferous micrite, losing its typical bioclastic character altogether. In Bulloch County, in the northeastern portion of the study area, the Ocala ranges in thickness from 80 to 200ft and thickens gradually to more than 240 ft toward the coast near Savannah (see fig. 3, cross section A-A') In the central portion, in Ben Hill and Irwin Counties, the thickness of the Ocala ranges from 250 to 375 ft. Southeast of the trough in Coffee County (GGS 51 0), the Ocala is 440ft thick. Structural Surface The configuration of the structural surface of the Ocala Limestone, shown on figure 4, closely resembles that of the Suwannee Limestone (fig. 6). Several elongate basins trending northeastward are evident and become more pronounced on the maps of the younger (Suwannee) formation. . The Ocala surface, northwest of the trough, is dissected by two broad drainage basins, one in Worth County and the other on the border of Tift and Irwin Counties. Each basin terminates at its widest point, where it intersects, at right angles, an elongate basin within the trough. South of the trough in Thomas, southern Colquitt and western Brooks Counties, the Ocala surface has more than 100ft of topographic relief relative to the surface eastward toward Brooks and Lowndes Counties and is at least 300ft higher than the Ocala surface in the trough. From this high the Ocala slopes gently eastward into Brooks and Lowndes Counties. Sever (1966) noted a high on the Ocala surface in approx- imately the same area and attributed the elevated surface to arching. The data obtained during this study do not support the theory of arching. Howev.er, the Oc~la is relatively thick (600 to 700ft) in th1s area, wh1ch may suggest a depositional basin. In the eastern half of the study area, the Ocala dips southeastward toward the Southeast Georgia Embayment. Oligocene Series Suwannee Limestone The most widespread Oligocene formation found in the study area is the Suwannee Limestone of late Oligocene age. Rocks of Vicksburg age have been found in isolated wells, but are of little regional significance in the context of this report. Therefore, only the Suwannee Limestone will be discussed here. Lithology and Porosity The Suwannee Limestone is white, granular to nodular in texture, and usually contains abundant foraminifera. The limestone is always recrystallized and is partially replaced by dolomite to varying stages at random localities. The Suwannee Limestone is usually very porous. Diagnostic foraminifera are commonly found at or near the first appearance of white limestone in cuttings and core. Included among these are Pararotalia mexicana Nutall, Lepidocyclina sp., Numulites sp., and Oictyoconus sp. Abundant molluscan and echinoid fragments are found at many localities. In the Gulf Trough the fauna and texture of the Suwannee are not well preserved. Samples studied from the southwest segment of the trough show the Suwannee there to have the poorest preservation. The limestones in this area have been micritized and they have very little visible porosity. To the northeast alteration is not so severe; the limestone texture is not completely changed to micrite and fauna are less sparse. The result is a limestone with more apparent porosity. Isopach Map Outside the Gulf Trough the Suwannee Limestone is generally 100 to 200 ft thick, as shown in figure 5. In areas of local relief, thicknesses fall outside this range. For example, south of the trough the Suwannee thins to less than 100 ft in Thomas County, then thickens eastward to 220ft in Brooks County. The thin veneer of Suwannee overlies the thick basin sequence of Ocala Limestone that is shown as a topographic high in figure 4. The Suwannee thickens eastward into Brooks and Lowndes Counties where the underlying Ocala is topographically low. The Suwannee is more uniformly thick northeast of Brooks County, and gradually thins to less than 20ft in Effingham County. 143 400' A EXPLANATION RECENT TO MIOCENE OLIGOCENE { SUWANNEE LIMESTONE) 30d zoo' tod SEA LEVEL -1001 -zoo' ooo' -4oo' D UPPER EOCENE (OCALA LIMESTONE) D MIDDLE EOCENE {CLAIBORNE GROUP) - 5 0 01 -soc' 6 11 GGS WELL IDENTIFICATION NUMBER 400' 3 00' 0~--~--Z--~3 MILES zod VERTICAL SCALE GREATLY EXAGGERATED too' SEA LEVEL tOO' zoo'- 400' c 3 0 0'- 2 0 Ci too SEA LE VEL " 10 0' -zoo' -:~oo' - 4 00' - ~o o' -e oo' 7 0 0' - a oo - 9oo' -t o oo' -:;oo' -GOO' 70 0' soo' "90 0' Mitchell County !lulloch County A' 400' 300' 2001 1001 SEA LEVEL too' -zoo' -300' -4001 -~oo ' -soo' 9' 4 00 ' 300' 200 ' 100' SEA LEVEL -i oo ' -2oo -30 01 -400' -~oo ' -soo' TD 700 ' sao' --9 0 0 1 T homas County /' / ' '----- c' 400' 300' zoo' too' SEA LEVEL -too' - 2001 "300' -4001 --5oo ' -soo' -roo' 800' -9001 1ooo' Figure 3. Three representative geologic sections across the axis of the Gulf Trough . 144 EXPLANATION STRUCTURE CONTOUR-- Showo __ tOO-- attitude of top of the Ocala Limestone. - Dashed where approximately located. Contour interval 100 feet. Datum is lftlan sea level .,. FAULT-- Dashed wher. approzimately ~ 411" lo~oted. U,upthrown aidei D,downthrown 0 side. ? ' -......,. ~ . SU I RKE~. . . / ........._____c!.3 ........... _y ./ \ ~ . i N~ -~'1l,- .~ _/~-~ \2\.o.,o EF~~--c \ : :~~,~,. , ~_ ..... _ ... lr- -l ) V 7 ). l- . f t..Jifu//1 ' - ' ' r ~~ /" ) {, ;I'/ ' (..._ ,_ , ' ~ (.'J1 A "i I ,H r 1 ~ j 3z ~ Ul ~ ~ l.y (J Cl 3'' ~ i / ;:;"SS-9-'-i--V.rC/ "UT" : ( BrBB ' \_ / \ \WILKINSO !...- ---- -.-------(, ,.--Z .__J.I I , , . _____, . "'i /')- .\."'"'. -'.'"'", ,.CRAWFORD ' I , I. " '"t '" ",.') MU~SCOGEE J::r.~ ,y-. \~./.-, ' ~---""1r'I ""-r.,-,!"'J" "r \. ./1,-;o"_.r",,J,".).,,\..\r."'"/'\'"'"\~./'. , , 1.. , , , I /'f"' ( /~a~ -~ " N U l 1 ...... l, \... ~-~--1 ~1--~- LE~ , . J I '' I r- rSCHLEY'L , N "l/ ,-"-." -.,.-')/ " \ I. ~ . ' ' TREUTU:-:N:r-.- /"A I Bu L L "( . \' EFFINGHAM' 7' . e ~:-~ .''r"-~~"'><', f~~,X ~~ ~- " ( I OUil'f.IANI' . ' n I ~-~. , . ,, " 1--- .J < 0 M' ''"' <' ' J. ' , c - - - - ' ' ' ' . ' I *~-~-~~I. CI :"""" ' i...; ( '> ""' 'l' r "-'r' .>.-'7.'_' ,"" / 1io0 ' . 1: \ :if{Y" ' .s( ANS\------- - -\ I. ., Io ' <-"" ~"-L IR '_;..- ,J'"o ., . '- '- o rATTNAc- 7 . 32' ~' ~ I \ "'""' . l \ \ [__r ___r-,j_ _ r . ~ ~ iv (; r~-'1"""' ,.To,,:":~, I ,7/-f - ; - - - - " ' )_ _ _ ! I E A R L y B A K E R_,.,..r' I ' \ .r3 ! I ~- 'r---~l~' 0 M I L l ER ( >MIT. ti ..1.._ --;~ _-JL I . - () (; .... !-... ..'.,?""' ' I as -- ---Y___ ---~~-----2 -~---.l--1.~ ) i~oOI -, ! Tl OMA . I r L- -.,,~; C L I N C H j 1 \ LOWNOEs_ J . , BROOKST ,l L_,,[_~- __ I , L , \ r --, ECHOLS : _ i I -----r: 31' "- '? I 84' . 83 ! \ i I~OL.J..L..._:Oi____2J0IO~--3'0-~~~-40 MILES , _ . 82' Figure 5. Thickness of the Suwannee Limestone, Gulf Trough area. The Suwannee Limestone is thickest in the Gulf Trough , particularly in four large elongate basins. The basin in Coffee County is a downfaulted block, where the Suwannee attains a thickness of 500 to 600ft (for example GGS 1825). In a narrow band adjacent to the southside of this basin, the Suwannee is absent, and the Ocala Limestone is unconformably overlain by Miocene sediments (for example GGS 51 0). The shaded area on figure 6 shows the band where the Suwannee is absent. Structural Surface A map of the structural surface of the Suwannee Limestone (fig. 6) clearly shows the configuration of the Gulf Trough. A chain of deep elongate basins extends northeastward from Decatur and Grady Counties in southwest Georgia to Toombs and Tattnall Counties in the eastern Coastal Plain. The northernmost basin becomes shallow and less depressed in a northeastern direction and finally disappears in Candler and Bulloch Counties. The deep, elongate basins are separated by shallower interbasin areas. A good example of such an interbasin area is in central Colquitt County. In Mitchell, Worth and Tift Counties the Suwannee Limestone surface is dissected by postOligocene erosion. The contour pattern on figure 5 indicates a southward-flowing drainage system. Less well-developed drainage can be seen on the northwest side of the trough in Toombs and Montgomery Counties. These drainage features, like those on the Ocala surface, terminate at the trough, where they intersect elongate basins. On the south side of the trough, drainage patterns indicating a northward flow are not found. Certain trends on the south side of the trough are also of interest. The surface of the Suwannee slopes gently to the northeast at the rate of about 3.3 ft/mi from Thomas through western Atkinson County. In central Atkinson County, the slope changes direction to the southeast. Topographic relief on the surface of the Suwannee throughout the study area generally corresponds to relief on the Ocala Limestone surface. During the late Oligocene, faulting occurred in several places. Faulting shaped the Toombs County basin, resulting in an abnormal thickness of Suwannee on the downthrown side of the faults that transect the basin. These fault traces are most apparent on the map showing the configuration of the top of the Ocala Limestone, since they offset the Ocala sediments. However, the fault traces are nearly healed by sediments accumulated during Suwannee time and faulting is not obvious on the map of the Suwannee structural surface. Block faulting is thought to be the explanation for the thick section of Suwannee Limestone previously noted in the basin in Coffee County. The fault traces bounding this basin are shown on the maps of the Ocala surface and Suwannee thickness (figs. 4 and 5), and may be implied on the structure contour map of the Suwannee where the surface of the unit is depressed. Miocene to Holocene Series Lithology and Porosity Miocene to Holocene sediments,as grouped collectively in this report, include the Tampa Limestone, the Hawthorn Formation, and post-Hawthorn sediments. The Tampa is the earliest Miocene unit recognized in well cuttings in this study. The Tampa overlies the Suwannee Limestone in much of the southwest portion of the report area. The earliest Miocene unit in the northeastern section of the study area is lithologically similar to the Tampa, but whether it is correlative in age is yet to be determined . The Hawthorn Formation is believed to overlie the Tampa throughout the study area. It is in the Hawthorn that certain thick sediment sequences occur that appear to be unique to the trough. No units younger than the Hawthorn Formation are distinguished in this report. In most of the southwest portion of the report area, the basal Miocene is predominantly a white arenaceous limestone, sometimes dolomitized and phosphatic and containing sparse macro-shells. However, a significantly different basal Miocene lithology occurs in the deep elongate basins in this area. In these basins, the basal sediment~ (GGS 3179, for example) are poorly sorted sands with varying amounts of silt and minor phosphate. Overlying these sands are black waxy clays occurring as beds several feet thick that are bedded with silt and fine-to medium-grained sand.To the northeast in the Toombs County basin, the basal Miocene along the trough's flanks and in the immediate surrounding area is a gray arenaceous limestone which is occasionally dolomitic and locally very phosphatic with abundant bivalves, barnacles and gastropods. The black waxy clays characteristic of the southwestern basins are not found in the basal Miocene of the Toombs County basin. One of the constituents of the basal Miocene unit within the trough is a breccia composed of fragments of the Suwannee Limestone. These angular clasts of Suwannee Limestone occur in an arenaceous limestone matrix. In many of the samples examined, reworked Oligocene fauna are found in basal Miocene sediments. 147 EXPLA~ATION -200--- STRUCTURE CONTOUR-- Shoon attitude of top of Suwannn Limeatone DoMed when~ appr01.imately loeat1d. Contour interval 100 feet. Datum is m-.on Mt:J lev.a. ~ SUWANNEE LIMESTONE ABSENT - - u - 0 - FAULT-- Dashed where approamot~ly located , U1 upthrown side; D1 downthr0wn side. /~ t -.__ \ . /R. .I.UCGHUSMT0A1r0~ \---tL~~r'-<; -"------I--~ /"~ / i ~-> 1 . . - ~ I - ..rrI I BALOW IN _, I \ > , f \" I ' . J E"FFE.R'SON'\ ""' I " B u R K ~ - E ~ / - 33" r ('.._ ('11fJP4.-H:.pTNAL 'r' . oWEBSTER[ S U M T E R ~ "y=- -~ \ 5 ! . 1 ..... ,..,..v, / 7 i I V r-.' , ~; 3 c.. , ""'..~~)r - "~t..N~i A~T H,J ?" af;l--"32" c.j:o:. ~ ~ ~ i _~6>'S---,.L/// l::.L7;:'T___ r-~ ,I v '< F ~ - _J , _; ~: 83" 10 0 10 20 30 40 MILES 82" Figure 6. Configuration of the top of the Suwannee Limestone surface in the Gulf Trough area. Within the Gulf Trough, Miocene to Holocene sediments are abnormally thick as depicted in figure 7, in a zone approximately 10 mi wide which extends the entire 235 mi length of the trough. The series of narrow elongate basins contain the thickest accu mulation of sediment. The Miocene to Holocene units thin in all directions from the elongate basins. The deep, elongate basins are very steep sided. In all probability they are fault bounded. For example, on cross section B-B' (fig. 3) the suggestion of faulting is very strong. On this cross section, GGS well 3179 in Colquitt County has a very thick Miocene to Holocene section, while the wells on each side of it show that the top of the Suwannee is hundreds of feet higher. Faulting during the early Miocene was not restricted to shaping the Gulf Trough. The southeastern Coastal Plain appears to be significantlyaffected by an extensive offset which trends southeastward through Jeff Davis, Bacon and Pierce Counties. This offset is herein named the Big Sat ilia fault for the nearby Big Satilla Creek which it parallels. The Gulf Trough forms the northwest boundary of the fault, which may extend farther eastward toward the coast of Georgia. The Ocala Limestone and Suwannee Limestone are offset vertically 150 to 200ft by the fault (see figs. 4 and 6). The type of faulting is thought to be high-angle gravity or block faulting which may have a lateral component. Repetition of beds has not been found. The surface of the Suwannee Limestone in the Toombs County basin is 200 ft lower than the Suwannee surface in the Coffee County basin on the southwest side of this major fault. This discrepancy may be related to fault movement. As well as can be determined from existing well data, the placement of the Big Satilla fault is within a 1 mi range of the dashed line shown on figure 6. Miocene strata dip east and southeast across the fault, forming a monocline. Structural Hypotheses Structural development of the Gulf Trough began prior to the Oligocene Epoch. However, due to sparse well data, direct evidence of exactly when the trough began to develop is lacking. Indirect evidence from well data outside the trough suggests that the trough may have been a viable feature during late Eocene time. Lithologic trends of the Ocala . Limestone suggest that the trough may have been an area of separation between depositional environments or may have become an obstacle to the late Eocene transgressive sea. South of the trough, in the soutwest portion of the study area, the Ocala can be divided into a bioclastic, porous upper unit and a micritic, non-porous lower unit, with a significant amount of gypsum occurring at the boundary of these units and below. On the north side of the trough, only the upper porous unit is found. Subsidence occurring during Oligocene time allowed a thick sequence of Suwannee Limestone to be deposited in the trough. Downwarping was steady and moderate over a long period of time, as shown by the fact that the Suwannee within the trough is free of clastic components and the strata are not disrupted. However, the Suwannee in the trough is generally fine grained, and contains fewer biota than normal. High-angle gravity or block faulting occurred after Suwannee deposition and downwarping, and prior to post-Suwannee erosion. Examples are the Coffee County basin and the southwestern part of the Toombs County basin, where anomalously thick sequences of Suwannee Limestone are found. After faulting, erosion removed all the Suwannee on a portion of the southeastern, upthrown block bordering the Coffee County basin and on the northwestern upthrown block of the Toombs County basin. Downwarping and gravity faulting continued with greater intensity into the early Miocene. Erosion of the Suwannee Limestone was one result of this intensified tectonic activity. Almost concurrently, the subsiding basins were infilled with sediment. At first, the sediment supply came primarily from older rocks northwest of the Gulf Trough via rivers that incised larger valleys on the north side of the trough. The Suwannee Limestone contributed sediment both as fragments and as dissolved carbonate. The Suwannee south of the trough may not have been actively eroded away as that to the north, although locally the Suwannee was completely eroded away. The thick accumulation of Suwannee in the Coffee County basin may indicate that the unit was originally thicker over its entire depositional extent than the Suwannee isopach map shows. However, there is not enough evidence from other localities to support the idea that the Suwannee was originally 600 to 700ft thick. After a period of erosion, most of the Suwannee was submerged by shallow water and covered with a thin veneer of white arenaceous limestone. Rapid subsidence accompanying erosion resulted in the Gulf Trough being filled in Miocene time with thick accumulations of poorly sorted sands, silt and beds of black waxy clays in the southeast basins and thick accumulations of white arenaceous macrofossiliferous limestone in the northeast basin. The deep basins contain over 700ft of Miocene to Holocene sediment, while in the less downwarped interbasin areas, only 400 to 500ft of sediment of this age are present. The elongate trough basins are probably bounded by disconnected faults rather than a single long fault extending the entire length of the GulfTrough. Eventually, the trough was completely filled with sediment as tectonic activity declined. This infilling healed or disguised many fault traces. 149 EXPLANATION LINE OF EQUAL THICKNESS OF RECENT -2.00--- THROUGH MIOCENE SEDIMENTS--- Dashed where approaimotely located. lrdwwal 100 feet _... , / "--"'v- MACON.... '" -<) ---~ . \ MUSCOGEE J-r.r--r T A Y L 0 fl. ,COLUMSUS / ~ ~ ,f" '. -_..J I /~ /AUGUST.J""? ---..J I \---6l~~y /"t / i-' R < .IC,'HM-ONI~D'-'~~- - I (__ L-_., \J I .- 1 ---- ' JEFFE-RSON'. "'!. B U R K E ' ' y ---z---33' I , ~ ---~ I '! \ . 1 JE' r KIN' SCREVE N~ 3z. .'. .l.. 1 ~msTER~ suMTER I\--- -- -J > , I ) /I )''\. /( TAn rAu:? \ "' / /W{f~/ 'U T '- ~ .......... . -~ J.. " t;c~-Hf..ATH~32' r (J1 0 ~~ ~ ~ I"" 7-?/- I J (J ' !.... ~ ~ "!.... ~ 83" ~ 10 0 10 20 30 40 MILES I 82 Figure 7. Thickness of Miocene through recent sediments, Gulf Trough area. The Big Satilla fault occurred during the period of intensive tectonic activity that took place during the early Miocene. The time of movement of this fault is placed after the erosion of the Suwannee and during the deposition of early Miocene sediments. HYDROLOGY The area of study, like much of the Georgia Coastal Plain, is underlain by a thick sequence of late Eocene to early Miocene limestone strata that is hydraulically interconnected and is referred to as the principal artesian aquifer. Clastic rocks and carbonate rocks which belong to the Lisbon Formation (middle Eocene) form the lower confining bed of the principal artesian aquifer in the study area. The upper confining bed consists of clastic rocks and carbonate rocks of low permeability, all of which are part of the Miocene to Holocene Series. Within the Miocene to Holocene Series, the Hawthorn Formation comprises a confining or semi-confining unit for the principal artesian aquifer, although the Hawthorn is known to locally produce adequate supplies of water for domestic purposes. The Tampa Limestone and its equivalents which underlie the Hawthorn are locally considered hydraulically connected to the aquifer. Water Availability Distribution Water-well drillers have reported consistent problems associated with obtaining water from wells in the GulfTrough. A major problem is the great depth to which holes must be drilled (over a thousand feet in some cases) before an adequate source of water is encountered. Even wells that reach the principal artesian aquifer at great depths often yield little water, sometimes 50 gpm or less. Outside of the trough, in the southwestern portion of the study area, conditions are more "normal"; wells drilled to an average depth of 400ft in the principal artesian aquifer yield as much as 3200 gpm. In the southeastern part of the study area wells yield up to 1250 gpm, and in the northeastern portion 1000 gpm can be obtained. The distribution of ground-water availability in the study area is shown on figure 8. This map illustrates the distribution of the specific capacity indices of municipal, industrial and domestic wells. Maximum indices are used in areas with a high concentration of wells. The shaded zone on figure 8 represents the narrow, steep gradient anomaly shown on the potentiometric map (fig . 2). The water-availability map was constructed by plotting the specific capacity index (Davis and DeWiest, 1966) for each well selected. Specific capacity of a well is defined as the yield per unit drawdown measured in gallons per minute per foot, (gpm/ft) Since each of the wells used for the map does not penetrate the aquifer to an equal depth, the specific capacity for each well was divided by the depth of hole open to the aquifer in the well to obtain a unit value-the specific capacity index, expressed in units of gpm/ft2. This value provides a useful method of comparing water availability in large areas where many wells penetrate the aquifer to different depths. A later publication will present the tabular data for the wells used to construct figure 8. The symbols on the map represent ranges of the specific capacity. For example, a well in northwest Thomas County (located within the Gulf Trough) yielded 175 gpm with 94ft of drawdown. The well penetrates 548 ft of the principal artesian aquifer. Its specific capacity index is: 175 gpm 94 ft = .003 g pm/ft 2 548ft a This figure signifies a very low production well. On the other hand , well in central Thomas County (located outside the trough) yielded 3200 gpm with 9ft of drawdown and an aquifer penetration of 242 tt, with a specific capacity index of 1.47. This is considered an excellent well. Wells can be divided into two categories based on specific capacity: those outside the Gulf Trough and those within its boundaries. For wells located outside the Gulf Trough , the specific capacity indices increase from the northeast to the southwest. To the northeast in the Toombs-Tattnaii-Bulloch County area, the specific capacity index ranges from 0.01 to 0.9. In the central part of the study area values ranging from 0.1 to 0.9 are common. Farther southwest in the Tift-Thomas-Grady County area, specific capacity indices as high as 10.0 are often found. For wells located within the Gulf Trough , specific capacity indices are everywhere lower than those outside. Wells within the trough show a marked decrease in specific capacity index from northeast to southwest, a trend opposite that shown by wells outside the trough. The lowest yielding wells in the entire study area are located in the southwestern segment of the Gulf Trough. The specific capacity index increases southward, away from the trough . On the south side of the trough in northern Colquitt County, the index ranges from 0.01-0 .09; southward toward central Colquitt County, the values increase from 0 .1 to 9.0. This trend indicates a change in aquifer permeability with the higher permeability occurring away from the trough. Such a permeability change would occur if less permeable material were downfaulted forming a subsurface barrier which would impede the southeastward flow of ground water. 151 EXPLANATION SYMBOL RANGE OF SPECIAC CAPACITY INDEX (GPII/112) 0 0 .001- 0 .009 0 0 01 - 0.09 0 0 .1 - 0.9 .... 0 1.0 - 9.9 ----- --- - -----------10-.-0----9-9-.-0---- ZONE OF POTENTIOMETRIC ANOMALY MUS-~OGE~7-,:r..r-T ~cowloi67JS ! ' \., T A y L 0 r R\ . ; __ ..J I ).. / J _' 'T u~ T_~,..!'MARtON r ~L -;~ - [ MAC ~- ~ -.1 ['- - ' (I i l. 32. -"~ ~ r ~ -- ... . ~ .. l \=- - - -1 /. ~ t '- ~/-'-<-'\. _......-....:-- ~/R "AIJGlJ STA r ICHI\IOI ' ' - .---:( /r'l / 1 I '-( r .-/. "-J j - v;r t I_ BALDWIN \. JEFFERSON aUR K , .. .-7 1 ~ t'"='""-lx ':KJ),.-....,..1 I-C J t ' ' N ALL..c I 7\:, , ~ ~ t.C"Vi::"l ' T1H Jtr:JtY .J--- 32 01 N ~ ~ iv ~ C) 31. \ 85" ~ ';.._ ~ I b :: ~ --q- -- 'X;= - -"--~ lj , "'" '-} 31" ;.._ I L-1 "' r I N CH i ! , B RO O K S - '-) LOWNOES l r 1 ~ -- ' --- .i __l_ __ I \ - 7s/ -- - -j.--- - \ '~-~-I' E IC H 0 . . . ., L CHARLTOA ~, --- --) , :~. .,""'-... "'" 84" 83" 10 0 10 ( : 't / ~ ~ 82" Figure 8. Map showing specific capacity index, or water availability distribution, in the Gulf Trough area . In effect, the Gulf Trough behaves as just such a barrier to ground-water flow. In Colquitt and southern Tift Counties and southwest to Decatur County, thick Miocene clastics help form this barrier. Water movement across the deeper basins in the trough is probably minimal; most water probably flows across the trough interbasin areas, where Miocene sediments are thinner. In addition, many wells drilled in the deep basins are completed in the Miocene; very few wells penetrate the upper surface of the Suwannee Limestone, which comprises the upper part of the aquifer. The barrier effect of the trough is shown on the potentiometric map (fig. 2). North of the trough, water moves laterally eastward and westward toward major rivers rather than to the southeast across the trough. Within the trough itself more water is able to flow across the deep basin in the northeastern area because, unlike the other deep basins, this one is filled with a thick sequence of early Miocene arenaceous limestone that contains abundant macrofossils, has moderately developed secondary porosity and is hydraulically connected to the principal artesian aquifer. In addition, the Suwannee Limestone is more porous in this area of the trough than in the southwest. Therefore, ground-water availability from the northeastern area is reflected by more moderate specific capacity indices. SUMMARY The Gulf Trough is a long, narrow geologic feature that was produced by a combination of depositional and structural conditions. The trough is approximately 235 mi long, about 10 mi wide, and trends N53 E from Decatur through Bulloch Counties. A chain of elongate, narrrow, deep basins exists with the Gulf Trough. These basins are separated by interbasin areas which are topographically higher than the basins but are considerably lower in elevation than the area outside the trough. Development of the GulfTrough began, at least as early as Oligocene time, with subsidence of several elongate basins and infilling of these basins with a thick sequence of Suwannee Limestone. Faulting occurred in limited areas, forming parts of two basins. During the early Miocene, development of the trough accelerated as downwarping and faulting deepened the basins that formed during the Oligocene. Erosion of the Suwannee Limestone surface resulted in the development of surface drainage and contributed to the sediment supply for the trough. Erosion left the Ocala Limestone exposed along the south rim of the Coffee County basin and left thin veneers of Suwannee Limestone rimming the other basins. Movement along the Big Satilla fault occurred during the early Miocene, after the Suwannee was eroded. The result is that the eastern third of the study area is downthrown 150 to 200ft with respect to the southwest portion of the study area. During the middle Miocene, as the tectonic activity declined, , the basins were completely filled and fault scarps were healed over by sediments. The GulfTrough is coincident with a steep potentiometric gradient anomaly on the potentiometric map of the principal artesian aquifer. Both the trough and the anomaly extend from Decatur County in southwest Georgia northeastward to Bulloch and Effingham Counties,wherethey disappear. The Gulf Trough creates the potentiometric anomaly and, by acting essentially as a vertical boundary to ground-water flow, is responsible for reduced ground-water availability. Several problems remain to be solved, including: 1. The determination of the mechanisms that created the Gulf Trough. 2. The quantification of the effects of the Gulf Trough on the aquifer such as permeability, transmissivity, recharge and water quality. 3. The redefinition of the geologic and hydrologic data by drilling test wells and conducting aquifer tests. REFERENCES CITED Dall, W.H., and Harris, D., 1892, Correlation Papers: Neoc~ne: U.S. Geol. Survey Bull. 84,349 p. Davis, S.N., and DeWiest,R., 1966, Hydrogeology: New York, John Wiley and Sons, Inc., 463 p. Herrick, S.M., 1973, Subsurface geology of the Ocala Limestone, Cooper Marl, and Suwannee Limestone in Georgia: Unpub. rept., on file at Georgia Geol. Survey, Atlanta, Ga. Herrick, S.M ., and Vorhis, R.C., 1963, Subsurface geology of the Georgia Coastal Plain: Georgia Geol. Survey Info. Circ. 25, 80 p. Hester, W.G., and others, 1977, Potentiometric surface of principal artesian-Fioridan aquifer in Southeastern Georgia and northeastern Florida, January-May 1976: U.S. Geol. Survey, openfile map. Sever, C.W., 1966, Reconnaissance of the ground water and geology of Thomas County, Georgia: Georgia Geol. Survey Info. Circ. 34, 14 p. 153 GEOLOGICAL EVALUATION OF POTENTIAL PIPELINE CORRIDOR SITES ALONG THE GEORGIA COAST Vernon J. Henry and Robert T. Giles University of Georgia Department of Geology and Marine Institute, and Skidaway Institute of Oceanography Savannah, Georgia and James L. Harding University of Georgia Marine Extension Center Savannah, Georgia ABSTRACT In anticipation of the onshore impacts that will result from offshore oil and gas production, the Georgia State Office of Planning and Budget, in 1977, instigated a highresolution seismic and sidescan sonar survey of the Georgia coast as a first step in evaluating optimal locations for pipeline corridors. The principal objective of the survey was to determine the presence and nature of physical and geological hazards and constraints to pipeline emplacement and stability in the upper 15m of the estuarine/nearshore zone to a distance offshore of 5 km. Areas containing a high degree of risk, or hazard, to pipeline stability were eliminated as potential pipeline corridors, while areas containing moderate to low risks, or constraints, must be given due consideration in developing engineering design criteria or in planning alternative routes. Except for possible shallow faults or karst features 3 km seaward of the south end of Jekyll Island and in the Brunswick River, approximately 1 km southeast of the Sidney Lanier Bridge (U.S. Highway 17), conditions considered hazardous to pipeline stability are limited to (1) major sounds and rivers where severe bottom scour and/or bottom instability occurs in ebb deltas and inlet throats, and (2) shoreline segments with a history of significant erosion. INTRODUCTION Contracted by the Georgia Office of Planning and Budget in 1977, this study centered on a highresolution seismic and sidescan sonar survey of the coast from approximately 5 mi offshore into inlets and estuaries for the purpose of obtaining information on the physical and geological characteristics of the surface and shallow subsurface deposits. Such data are directly applicable to pipeline route siting decisions which would be required of the S!ate of Georgia as a result of outer continental shelf petroleum activity. Objectives were to identify or infer geologic constraints and potential hazards, such as soil instability, active bottom scour, faults, cut and fill structures, buried river channels, shallow rock outcrops, shoreline erosion, and mobile bottom features, such as sand waves. Specific geological characteristics identified in the present study that are considered to be potential hazards and constraints to pipeline route siting are discussed. The surveys were designed to narrow the scope of later phases of evaluation so that any area identified by the initial seismic/sonar surveys as being hazardous could be eliminated from further study and consideration as a pipeline corridor. ACKNOWLEDGEMENTS Gregory J. Nash ably assisted in all facets of the field operations and with preparation of maps and figures. The facilities and services provided to the University of Georgia Marine Geology Program by the Skidaway Institute of Oceanography are gratefully acknowledged. METHODS AND PROCEDURE Geological field data were acquired during the period April- December, 1977. Seismic/sonar instrumentation consisted of a high-resolution seismic profiling system (EG&G Model 255 UNIBOOM), a sidescan sonar system (EG&G Mark IB), and a 3 cu in. air gun system (Bolt, Associates). The UNIBOOM and sidescan sonar were operated along track lines approximately 1.5 and 3 mi offshore of the barrier islands and parallel to the regional strike of the coastal deposits. Selected lines were run perpendicular to the shoreline along regional dip. Track lines in the sounds and tidal channels behind the islands were chosen on the basis of prior coverage and accessibility. The air 154 gun survey was carried out along the Atlantic Intracoastal Waterway as well as along a track approximately 5 mi offshore from St. Marys Entrance Channel to Wassau Sea Buoy . Research vessels used in the survey were the 22m KIT JONES and the 14m SPARTINA, operated by the University of Georgia Marine Institute on Sapelo Island, and the 24 m BLUE FIN operated by the Skidaway Institute of Oceanography, Savannah, Ga. Inshore navigation was by sight fixes on topo map-identified land/ cultural features and channel markers, and by radar ranging and Loran A off the island fronts. The identification of geological constraints and hazards resulting from the sidescan sonar and UN IBOOM surveys is presented in maps 1-4. Unless deeper structural hazards were present, only the top 15m of the UN IBOOM data were utilized in preparing maps 3 and 4, as depth of pipeline burial would be well within this interval. A summary of hazards and constraints is given in map 5. EXPLANATION OF TERMS USED IN EVALUATION OF DATA General Discussion The terms" constraint" and" hazard" relate to geological conditions as well as long- and short-term energy levels affecting the physical environment. A constraint is considered to be a geological or physical condition for which engineering criteria can be established and incorporated into the pipeline design so as to safely and economically mitigate detrimental effects. Examples of such constraints are active bottom features, such as sand waves, and the variation in geotechnical properties in sediments associated with buried river channels and cut and fill structures that result from channel migration. Hazards, on the other hand, are geological or physical conditions that impose a relatively high degree of risk to pipeline stability and integrity. Hazardous conditions would involve faulting, slumping or slope failure, and s~vere scour. The length of time over which an .event occurs and the rapidity at which it is initiated are important factors in determining constraints or hazards to pipeline stability. Short-term events such as faulting, storm scour, and shoreline erosion present severe hazards compared with longer-term events associated with the movement of large sand waves, the development of mid-island shoals, and geotechnical conditions, such as differential compaction . Hazards Scour Scour refers to the removal of bottom sediment by wave and/or current action. Storms can cause immediate and marked increases in horizontal and vertical stresses superimposed on the normal or steady-state conditions. Even under relatively low energy conditions, the cumulative effects of waves and currents may eventually expose a buried pipeline. Scour potential increases where the bottom is composed of easily erodible materials and/or is topographically irregular. Scour can undermine a pipeline causing it to rupture-a condition described as " spanning" (Kreig, 1965). Faults A fault is a hazard because, even if inactive, it represents a plane or zone of weakness along which movement of unpredictable occurrence and extent could be initiated by even minor seismic activity or subsidence associated with ground-water removal. Although consolidated sediments typically present in the shallower portions of coastal plain formations can undergo considerable movement without fracturing, such displacement could cause pipeline rupture. Constraints Mobile Bottom Features-Sand Waves and Giant Sand Waves Small sand waves (wave length< 1Om, amplitude< 30 em) indicate the presence of lower velocity currents associated with wave action and/or normal tidal currents, both of sufficient velocity to place unconsolidated material in motion resulting in minor bottom scour. Large sand waves (wave length> 10 m, amplitude >30 em) indicate significant scour by higher velocity currents associated with storms and possibly spring tide conditions. Large sand waves and giant sand waves (wave lengths> 100 m) represent ongoing and/or periodic processes associated with strong tidal and/or storm currents. Because their presence infers mobility as well as an adjacent area of significant scour, the larger sand waves are considered to be a constraint with regard to route selection. Neither the frequency nor the rate of movement of these features is known for the Georgia Bight. Shallow Rock or Other Resistant Material Depending on the intensity of lithification, the occurrence of shallow rock along a potential pipeline route could be merely a constraint to burial of the pipeline, or could actually prevent burial because of the costs involved. In any case, the most serious condition would exist at the boundary between the rock or other resistant material, such as dense clay layers, and adjacent unlithified or unconsolidated sediment where abrupt changes in grain size and composition could cause differential compaction and consequent bending or rupturing of the pipeline. 155 Subsurface Sedimentary Structures Several types of large sedimentary features common to coastal deposits exhibit wide variability in grain size, mineralogy, water content, chemical composition, and compactibility. Such geotechnical conditions can result in significant changes in bearing capacity along a pipeline route leading to differential compaction and consequent spanning of the pipeline. High-Angle Bedding-Areas of high-angle beds are considered to be potential constraints because this type of bedding indicates rapid deposition with little or no reworking after deposition. Large contrasts in bearing capacity almost certainly exist between the relatively poorly ~orted stream deposits and the containing strata. Also, the bearing capacity of the gravels, sands, and clays that commonly comprise alluvial deposits in this region can vary markedly over short horizontal or vertical intervals. Buried River Channels-Buried stream channels are considered to be potential constraints due to large contrasts in bearing capacity between the rei- atively coarse, probably high water content, deposits and adjacent, often finer-textured, sediment. Also, as with the high-angle beds, the texture of the channel deposits can vary significantly over short horizontal or vertical intervals, causing differences in bearing capacity that could result in fatigue failure or flotation of the pipeline. Cut and Fill Structures-Cut and fill structures are formed by lateral migration of stream channels. Interlayering of sand and clays and other textural contrasts can result in differential compaction along the pipelong route. OCCURRENCE AND DISTRIBUTION OF HAZARDS AND CONSTRAINTS Bottom Survey Using Sidescan Sonar Sidescan sonar records (sonograms) augmented by UN IBOOM and fathometer profiles were used to map areas of scour, sand waves, and outcrops of resistant material (see maps 1 and 2). GOPB Project-UUAJanuary 1978 Norlh S ope.Jo Sa\lnd 156 MAPl BOTTOM SURVEY I SIDE SCAN SONAR POTENTIAL HAZARDS AND CONSTRAINTS ............._. SAND WAVES llen'illh ovr 100m I W $ SAND WAVES llenQih 10- 100m I .~ ....S .....C ....O . UR ~ OUTCROPS OF RESISTANT MATERIAL ~ ~ FIGURE LOCATION SCALE 10 1$ GOPil Project-UGAJanuary 1978 Bollvillo River Doboy Sound Allomoha Sound --'t St Andrew Sound MAP 2 BOTTOM SURVEY I SIDE SCAN SONAR POTENTIAL HAZARDS AND CONSTRAINTS Sl. Meryl River FLA. Scour Severe scour is present in ebb tidal deltas, at bends and intersections of tidal rivers and streams, and within the sounds and entrance channels to depths of over 25m . Severe scour is present also in each of the major sounds. An example of scour in Sapelo Sound is shown in figure 1. The shifting or G) FIGURE LOCATION SCALE -=-=~====~5----1~0==~5 kilometer Entrance migration of major ebb and flood tidal channels associated with the tidal deltas also produces scour of the channel walls. This is a normal condition which may be significantly intensified during severe storms . This type of scour is inferred by bathymetry, the presence of large bedforms, and by channel migration. Documentation of the latter process is based on the comparison of historical and contemporary hydrographic charts by Nash (1977) . 157 Mobile Bottom Features-Sand Waves and Giant Sand Waves Small sand waves (megaripples) with wave lengths less than 10 m were the most common bedform encountered. In offshore areas they were usually associated with and often irregular sand sheets (figs. on 2, 3t,haensdu4rf)a. cTeheoflalottwe~ features are less than 0.5 m relief. Small patches of megaripples not associated with sand sheets occur- red randomly and relatively rarely. Fields of meg- aripples were commonly present in tidal rivers. Neither megaripples nor the sand sheets are consi- dered to constrain pipeline emplacement or stabil- ity. Large bedforms were found associated only with tidal rivers, sounds, and ebb tidal deltas. Giant sand waves were found in St. Marys Entrance, St. Andrew Sound (fig. 5), Doboy Sound, Sapelo Sound (fig. 6), and the Brunswick River. Large sand waves were found in all sounds, entrance channels, and larger rivers. They are particularly well developed in Sapelo Sound, Altamaha Sound (fig. 7), and in St. Catherines Sound (fig. 8). The larger bedforms associated with sounds and entrance channels displayed a variety of geometric shapes and orientations. Smaller sand waves were commonly superimposed on larger sand waves with the two size groups in differing orientation. Sand transport in these areas is obviously controlled by complex current systems. Outcrops of Resistant Material Areas of outcrop were commonly observed where scour or dredging operations have exposed rock or other resistant strata, such as dense clay or marl. Outcrops of resistant material were primarily identified in the southern half of the Georgia coast including dredged portions of the Turtle and Brunswick Rivers (fig. 9), the Intracoastal Waterway south of St. Simons Sound and the Kings Bay entrance channel, portions of the Satilla and St. Marys Rivers, Cumberland Sound and the St. Marys Entrance Channel. Most of the outcrops in the region from Brunswick to St. Marys are thought to be limestones and marls of the Pliocene Charlton Formation (Woolsey, 1977). Subbottom Survey Using High-Resolution Seismic Technique High-resolution subbottom profiling (UNIBOOM and air gun) was used to identify subsurface structures and sedimentary features defined as hazardous or posing constraints to pipeline emplacement or stability (see maps 3 and 4). GOPB Projcci-UGAJanuary 1978 - 10 meter dapl.fto Norlh Bellville River - ...: .q- S t. Cclherines Sound Sapelo Sound 158 MAP 3 SUB BOTTOM SURVEY I UNIBOOM AND AIR GUN POTENTIAL HAZARDS AND CONSTRAINTS ~ ~ W.Y.%'-1 tWB! ~ HIGH ANGLE FORESETS DUfiii: C Ct'I~ NN f"-5 CUT &. Fill POSSIBLE FAULT FIGURE LOCATION S c; ALE ~~~0==~----'0==~' kilometer GOPO Pruj~c!-UGA January 1978 Doboy AUomoho Sound SI Andrew Sound MAP4 SUBBOTTOM SURVEY I UN I BOOM AND AIR GUN P 0 T E N.T I A L H A Z A R D S AND CONSTRAINTS HIGH ANGLE FORESETS BURIED CHANNELS CUT !I FILL POSSIBLE FAULT FIGURE LOCATION SCALE Sl Mory1 River FLA. High-Angle Beds High-angle beds with ~ips up to60 m/km underlie the Georgia coastline within 15 m of the bottom between the Vernon and Satilla Rivers. These features probably represent foreset beds of extensive coastal deltas that were active during the Pliocene (Woolsey, 1977). The zone of foreset beds ranges in thickness from 26 m in St. Catherines Sound and Doboy Sound to a maximum of 30 m and 34 m in Sapelo Sound (fig . 10) and Altamaha Sound , respectively. South of Village Creek, the foreset trend is located predominately offshore and appears to extend southward along the Florida coast as reported by Meisburger and Field (1975). In the Altamaha Sound and off Cumberland Island the top of the foresets is deeper than 15 m below the bottom. 159 A shallower, somewhat thinner zone of highangle beds that cut into the underlying foreset beds also is present beneath the Georgia coast. These beds appear to be Quaternary deltaic deposits, occur infrequently, and are best developed near the mouth of the Hampton River beneath Village Creek (fig. 11) and behind Sapelo Island where the zones are 30 m and 20 m thick, respectively. Buried Stream Channels Buried stream channels are present both offshore and beneath the salt marshes and estuaries behind the barrier islands along the entire Georgia coastal zone (see maps 3 and 4 and fig. 12). Offshore, buried stream channels of Pleistocene (?)age occur extensively between Tybee Roads and WassawSound (fig.13). Channels from0.5 km to 1.5 km wide and cut to 10 to 25 m below the bottom underlie3 to20 m of horizontally bedded sediments interspersed with cut and fill structures. Buried channels are found from near Wassaw Sound entrance (just south of House Creek on Little Tybee Island) to more than 6 km from shore. A large channel incised to 25 m below the bottom of Wassaw Sound is located just north of Wassaw Island (fig. 14) and several buried channels, generally 2 km or less from shore, are present between St. Catherines and Altamaha Sounds. Numerous channels are present about 1 km north and south of McQueens Inlet near the center of St. Catherines Island. Depths range from 2 m to 23 m below the bottom, the deepest being about 2 km from shore. Between St. Andrew Sound and St. Marys Entrance, several buried channels are present within 2 km of shore and extend to less than 20 m below the bottom. Numerous buried channels occur in the vicinity of Stafford Shoals near the center of Cumberland Island. On either side of the shoals, the channels are overlain by 2m to 20m of horizontally bedded sediment. Few buried channels are present between St. Simons Sound and Cumberland Sound, although several less than 30m deep occur behind Cumberland Island. Cut and Fill Structures Although most cut and fill structures occur in deposits beneath the streams and estuaries behind the island, they are common in Doboy Sound, Altamaha Sound, St. Marys Entrance, St. Catharines Sound, and the area offshore of Little Tybee and Tybee Islands where they extend to less than 15 m below the present bottom and in zones generally less than 1 km in lateral extent. The Duplin River behind Sapelo Island is underlain by cut and fill structures that occur throughout a zone more than 2 km wide and extend to 12 m below the present bottom (fig. 15). Cut and fill structures beheath Village Creek generally extend 8 m to 20m below the bottom and are superimposed on the deeper buried channels and high-angle foreset beds (fig. 16). Structures extending 10 m to 12 m below the bottom occur behind St. Simons Island in both McKay and Frederica Rivers. South of St. Simons Sound, cut and fill structures are rare except along the St. Marys and Satilla Rivers. Possible Faults Two possible faults were found in the survey area, one on the Brunswick River just southeast of Sidney Lanier Bridge at Brunswick (fig. 17), and the other 2.5 km offshore from the south end of Jekyll Island (fig. 18). In the Brunswick River locality, acoustic penetration was poor, only about 20m, and multiple reflections resulting from rock cropping out in the channel bottom obscured acoustic returns below this depth. In the Jekyll Island locality, a zone of broken and reverse-dip reflectors more than 1.2 km wide and extending to at least 45 m below bottom was present. However, there was no indication of an extension of this feature in any direction. Both features may be solution collapse structures associated with underlying calcareous rocks. SUMMARY OF DISCUSSION OF PIPELINE CORRIDOR EVALUATION Areas considered to contain sufficiently hazardous geological and physical conditions to warrant elimination as potential pipeline corridors are shown on map 5. Except for the two sites indicated as possible faults, hazardous areas are limited to the outer portion of sounds, the inlet throats between the islands, the entrance channels and the ebb tidal deltas seaward to the 6m contour. The severe scour associated with migrating channels and the large sand waves present were the principal criteria used in evaluating these areas. The probable effects of storms and hurricanes on these areas were also considered. Severe scour also is present in the more restricted localities of bends and intersections of tidal streams, and significant scour occurs within a 5 km inshore zone in association with sand waves and other large mobile bedforms. However, these conditions are not only localized, they are scattered throughout the coastal zone and must be dealt with on a site-specific basis. Furthermore, areas of scour in tidal rivers, streams, and creeks are predictable and can be taken into account in route planning. 160 MAP 5 SUMMARY OF HAZARDS AND CONSTRAINTS Hazards e Po..lble Flult = = Areas of Severa Scour - Sounda, Inlets, and Ebb Tldll Deltas Constraints Areas with or Underlain by High Angle Foreaet Bedding, Burled River Channels, and Cut and Fill Structures Areas Underlain by Rock< 15 m Below Surface Areas Free from Slgnlflcant Constraints Marsh Areas Urban Areas 161 Geological and physical constraints do not singularly prohibit pipeline emplacement or stability. They should, however, be given due consideration in pipeline design and in considering alternative routing of pipelines. Also, such constraints could reinforce ecological or other considerations to the extent that a coastal segment is declared unsuitable as a pipeline corridor. Based on historical trends of shoreline erosion, pipeline landfall should not be made on the northern one-third of the Georgia barrier island or at any points where a breakthrough has occurred or is likely to occur, such as on the southern portion of the Cumberland Island. Relative to pipeline corridors across salt marsh areas, it is recommended that any such routes be sited adjacent to the existing causeways connecting Jekyll-St. Simons Islands and Tybee Island to the mainland in the Brunswick and Savannah areas, respectively. REFERENCES CITED Kreig, J.C., 1965, Criteria for planning an offshore pipeline: Jour. Am. Soc. Civil Eng., Pipeline Div., v.91, p.15-37. Meisburger, E.P. and Field, M.E., 1975, Geomorphology and sediments of the Florida Atlantic inner continental shelf, Georgia to Cape Canaveral: U.S. Army Corps of Eng., CERC Tech. Memo 54, 119 p. Nash, G.J., 1977, Historical changes in the mean high water shoreline and nearshore bathymetry of South Georgia and North Florida: Unpub. thesis, Univ. Georgia, Dept. Geology, Athens. Woolsey, J.R., 1977, Neogene stratigraphy of the Georgia coast and inner continental shelf. Unpub. dissertation. Univ. Georgia, Dept. Geology, Athens. Figure 1. Severely scoured area in Sapelo Sound. Note the presence of sand waves in the deeper portion of the scour trough. Ebb flow is toward right of figure. Distance between scale lines is approximately 25m. Location of figure is shown on Map 2. 162 Figures 2. & 3. Central portion of Cumberland Island on Stafford Shoal. Irregular migratory sand sheets have developed down current from scoured areas (B). Megaripples (A) with wave lengths generally less than 1 m are superposed on the sand sheets. Scour troughs have depths of less than 1 m. Distance between scale lines is approximately 25 m. Location of figures is shown on Map 2. 163 Figure 4. North end of Wassaw Island showing irregular, sinuous sand sheets. Darkened areas are caused by the presence of megaripples (A) on the surfaces of the sand sheets resulting in strong acoustic signature. Scoured areas (B) are present. Distance between scale lines is approximately 25 m. Location of figure is shown on Map 1. Figure 5. Giant sand waves (wave lengths up to 107 m and heights up to 2.5 m) 3 km off the north end of Cumberland Island within St. Andrew Sound ebb tidal delta. Note the strong ebb tide-oriented asymmetry with subparallel megaripples (wave lengths up to 4 m) superimposed on the flanks . Ebb flow is toward right of figure. Distance between scale Iines is approximately 25 m. Location of figure is shown on Map 2. UN IBOOM interference pattern is shown at A. 164 Figure 6. Giant sand waves in Sapelo Sound with distinct ebb tide asymmetry. Large megaripples are superimposed on the sand waves . Ebb flow is toward right of figure. Distance between scale lines is approximately 25 m. Location of figure is shown on Map 1. Figure 7. Sand wave field with strong ebb tide asymmetry in Altamaha Sound . Megaripples occur in a subparallel alignment on the flanks of the larger waves . Ebb tidal flow is toward right of figure . Distance between scale lines is approximately 25 m. Location of figure is shown on Map 2. 165 Figure 8. Entrance channel of St. Catherines Sound. Sand waves (wave lengths up to 17 m and heights up to 2 m) and megaripples (wave lengths 4-8 m) with different orientations are shown indicating a complex tidal current pattern. This section of record was taken crossing the scoured entrance of channel. The depth at the center of the channel is approximately 18m. Ebb flow is into the figure. Distance between scale lines is approximately 25 m. Location of figure is shown on Map 1. Figure 9. Outcrops of resistant material (A) , probably Pliocene limestone, exposed by scour and/or dredging in the Brunswick River near the Sidney Lanier Bridge. A small field of sand waves occurs in the upper righthand corner of the record. Distance between scale lines is approximately 25 m. Location of figure is shown on Map 2. 166 Figure 10. Sap_elo Sound entrance channel abeam buoy N"8". West to east transect-left to right on profile. Highangle beds extend to within 10 m of the bottom. Distance between scale lines is approximately 8 m. Location of figure is shown on Map 4. Figure 11 . Figure 12. Figure 11 . Village Creek, St. Simons Island, about2.5 kmfrom entrance into Hampton River. Transecteasttowestleft to right on profile. Pliocene foreset beds overlain by Pleistocene(?) cut and fill deposits. Foreset beds extend to within 10 m or less of the present bottom . Distance between scale lines is approximately 8 m. Location of figure is shown on Map 4. Figure 12. Hudson Creek at Meridian dock, Sapelo Island. Transect northwest to southeast- right to left on profile. Buried stream channel approximately 50 m wide, cut to a depth of nearly 20m below the present bottom and overlain by up to 7.5 m of modern sediment. Scour to the left of buried channels is 7 m deep. Distance between scale lines is approximately 8 m. Location of figure is shown on Map 4. 167 Figure 13 . Wassaw Sound just north of northeast corner of Wassaw Island . Transect west to east- left to right on profile. Buried channel cut into Pliocene strata to a depth of over 35m below present bottom and overlain by up to 20m of sediment containing Pleistocene(?) cut and fill structures. Distance between scale lines is approximately 8 m. Location of figure is shown on Map 3. Figure 14.. Offshore of Tybee Island 16 km east of the north end of Wassaw Island. Transect approximately northeast to southwest- right to left. Buried channels cut to 30m below the present bottom and overlain by up to 10 m of nearly horizontal younger Pleistocene/Holocene sediment. Distance between scale lines is approximately 8 m. Location of figure is shown on Map 3. Figure 15. Duplin River near intersection with Barn Creek, Sapelo Island. Transect north to south- right to left on profile. Quaternary cut and fill structures to depths near 10 m below the present bottom. Distance between scale lines is about 8 m. Location of figure is on Map 4 . 168 Figure 16. Village Creek near Musgrove Plantation, St. Simons Island . Transect nearly north to south -left to right on profile. Pliocene foreset beds overlain by Pleistocene(?) foreset beds. Incised channels and cut and fill structures overlie the younger foresets. Distance between scale lines is approximately 8 m. Location of figure is on Map 4. Figure 1i . t:lrunswick River 1.3 km southeast of Sidney Lanier Bridge (Hwy. 17). Transect northwest to southeastleft to right on profile. Possible fault or filled sink hole affecting a zone about 100 m wide . Distance between scale lines is approximately 8 m. Location of figure is shown on Map 4. -- ~ ~ -- -- - -- --- . --- - - : --"' - .:: ..:;~--- - -- --- --- - - - - ---- - - ;- - - - . .: - - - -: - :. - - :.,. - - -~- - ~ - - - - ~ ~ :~,-~ ;-r- - - - - - - - - - - - - :-- ..: - - - - - - - - - - ' Figure 18. St. Andrew Sound , south end of Jekyll Island, about3 km offshore. Transect north to south- right to left on profile. Possible fault extends from 7 m to at least 40 m below the bottom . Distance between scale lines is approximately 8 m. Location of figure is shown on Map 4. 169 RELICT FRESH GROUND WATER OF THE U.S. ATLANTIC CONTINENTAL SHELF: AN UNEVALUATED BUFFER IN PRESENT-DAY SALTWATER ENCROACHMENT F.A. Kohout U.S. Geological Survey Woods Hole, Massachusetts ABSTRACT Although extensive water-resources investigations have been made on the mainland of the Atlantic seaboard, observations made during the U.S. Geological Survey AMCOR (Atlantic Continental Margin Coring) Project of 1976 indicate that our knowledge of the offshore extent of freshwater aquifers is primitive . A line of 5 test holes off the New Jersey coast showed that relatively fresh ground water (less than 5 g/kg chloride) forms a lens about 200m thick extending about 100 km offshore. Similar conditions appear to exist in some aquifers off Massachusetts , Long Island (New York), Delaware, Maryl and, the Caro linas, Georgia, and Florida. Apparently, aquifers underlying the continental shelf were extensively flushed of salt water during low stands of Pleistocene sea level. As sea level rose and rapidly flooded the shelf' about 8000 years ago, the fresh water was trapped beneath clay-confining beds. An implication of present findings is that this relict fresh ground water presently serves as a temporary buffer to present-day saltwater intrusion in coastal well fields wherein heavy pumpage has caused water levels to decline to as much as 35m below sea level. There exists a great need for identifying the offshore location and nature of the fresh saltwater transition zone, the thickness and offshore ex ten t ott he aqu lfers, and the properties of their overlying confining beds to defi ne better the Initial and boundary conditions needed for predictive mathematical modeling of sa ltwater encroachment adjacent to coastal well fields. INTRODUCTION Possibly the most spectacular exhibit of the occurrence of fresh ground water beneath the Atlantic continental shelf was obtained during the JOIDES (Joint Oceanographic Deep Earth Sampling) drilling project of 1965. Figure 1 shows flow from test hole J-1 8, 40 km east of Jacksonville, Fla., taken from the deck of the Caldrill drilling vessel (Kohout, 1966 , fig. 6). The well flowed vigorously from the drill pi!:Je at 5 m above sea level after penetrating Eocene limestone about250 m below sea level. The chloride content of the water was about 0.7 g/kg. The pressure head was measured by closing in the well and carrying an attached garden hose up the derrick until the flow stopped at about 10 m above sea level (Kohout, 1966; G.W. Leve and R.L . Wait, oral commun. 1965). This is the only known groundwater pressure-head measurement that has ever been made for a test hole drilled on the continental margin of the United States. In 1976, the drilling of 22 test holes during the U.S. Geological Survey AMCOR (Atlantic Continental Margin Coring) Project, for the first time, provided some indication of the offshore extent of relatively fresh ground water in subsea-floor aqui fers. The test holes were drilled to a rnaximum depth of 305m below the sea floor. Salinity of interstitial water was determined from 175 samples of pore water squeezed from sediment cores with a hydraulic press. The technique is described by Manheim {1966) . The refractive index of the water varies with sa linity and quick eva luation was accomp lished on shipboard shortly after core recovery with an EN DECO refractometer1. The original salinity values were reported in Hathaway and others (1976);the remaining water was heat sealed in a polyethylene pipe for later chemical analysis. In 1976 and 1977, test holes were drilled through freshwater aquifers to basement rocks on Nantucket Island and Martha's Vineyard Island (respectively, 64 and 32 km off the New England mainland). This paper utilizes the offshore salinity information, augmented by data from onshore wells, to provide a preliminary view of the probable extent of relatively fresh ground water under the continental Shelf. Geologic Framework In general, sediments of the Atlantic continental margin thicken seaward and the aquifers do not crop out on the shelf. The sediments are mainly clastic at the north and grade increasingly to carbonates south of Cape Hatteras, N.C. Geologic formations of Jurassic, Cretaceous, and Tertiary age, generally cal led coastal plain sediments, typically dip seaward to form a thickening wedge of sediments overlying crystalline basement rocks. Pleistocene sediments that mantle the shelf tend to be fine grained in the south with increasing amounts of sand and gravel off the New England coast where g~acial outwash influenced deposition. General Considerations and Definitions 1. The continental shelf is defined as extending from the shore seaward to the 200-m isobath. Any use of trade names and trademarks is ror descriptive purposes only and does not constitute endorsement by the US Geological Survey 170 Figure 1. The natural freshwaterflow from JOIDES test hole J-1 B, 40 km offshore from Jacksonville, Fla ., being sampled by G .W. Leve, U.~ . Geological Survey. The top of the pipe protrudes through the drilling hole in the center of the Caldrill drilling vessel to about 5 m above sea level. The bottom of the drill pipe was set opposite Eocene limestone about 250 m below sea level at the time of the photograph . Photo by R.L. Wait, U.S. Geological Survey. 2. A gradational zone of diffusion usually exists between fresh water and seawater in a coastal aquifer. Salinity in the zone of diffusion ranges from low values in fresh ground water (defined as containing less than 1 g/ kg of dissolved solids) to seawater of about 35 g/ kg (35,000 ppm) . 3. The Environmental Protection Agency (EPA) has designated total dissolved solids of 10,000 ppm (1 0 g/kg) as the boundary for protection of freshwater aquifers (U .S. House of Representatives, 1974). Zones of higher dissolved solids can be used for injection of industrial wastes, whereas aquifers conta ining less than 10,000 ppm are to be protected as potential water sources or feed stock for desalting plants . The 10 g/ kg salinity boundary occurs fairly close to the middle of the zone of diffusion where maximum salinity gradient takes place-the concentration depth curves usually becoming asymptotic toward freshwater and also asymptotic toward seawater. The 10 g/ kg boundary is hydrologically significant, has quasi-legal status as used by EPA, and, therefo re, is considered useful as a mapping criterion for indicating areas of major water-resources interest in this documentation . 4. Water-table aquifers are eliminated from consideration because of their local nature and because discharge is generally complete at distances less than 1 km from shore. On the other hand, artesian aquifers protected by relatively low permeability confining beds can exend long distances offshore beneath the continental shelf. Low stand of sea level during the Pleistocene glacial maximum uncovered large areas of the shelf and infiltration of rainfall took place over extended periods of time. Rise of sea level rapidly flooded the shelf about 8,000 years ago (Emery and Garrison, 1967; Emery and others, 1967; Milliman and Emery, 1968; Dillon and Oldale, 1978). 171 REGIONAL DISTRIBUTION OF GROUND WATER SALINITY UNDER THE CONTINENTAL MARGIN A map showing minimum salinity observed in test holes drilled through sediments underlying the conti.nental' shelf is shown in figure 2. In general, very low salinity (less than 1 g/kg) occurs at distances less than about 16 km off the DelawareMaryland-New Jersey coast, but as far as 120 km off the Florida coast (Manheim and Horn, 1968). In the AMCOR test holes, the presence of fresh water in aquifers underlying the Continental Shelf was observed as a sharp decrease of salinity through the clay-confining bed before the drill penetrated more permeable sand and gravel of an underlying aquifer. The salinity profile for hole 6008 (inset, fig. 2) is typical of sites where an aquifer containing relatively fresh water underlies and is protected from rapid vertical intrusion of seawater by a low permeability confining bed. Based on the minimum salinity in the observed profile, the value above the line adjacent to the hole location is salinity in g/kg; the value below the line is the depth below sea level at which the minimum salinity was observed. To augment the offshore core-hole data, water-quality data from selected onshore observation wells are plotted adjacent to closed circles in figure 2. The depth of the well screen is below the line and the salinity of water obtained from that depth is above the line. Data for inshore wells are plotted generally inside the coastline, and for offshore test holes, outside the coastline. Aquifers Off the New England Shore The general pattern of decreasing salinity below the floor of the continental shelf was observed on the landward side of Georges Bank about 250 km off the Massachusetts coast (inset graphs, fig. 2) . Caving, or sloughing, of Pleistocene glacial-outwash sand and gravel into the hole stopped the drill at relatively shallow depth. However, minimum salinity of about 22 g/kg in hole 6017 and 27 g/kg at the bottom of hole 6019 (general salinity of seawater in this area is about 33 g/kg; Bigelow and Sears, 1935; Bumpus, 1965) suggests that low salinity water also might be present under Georges Bank in this far offshore area (inset graphs, fig. 2). It should be noted that glacial ice with attendant ice-marginal lakes and streams occupied this part of the continental shelf in Pleistocene time. In 1976, a test hole drilled at Nantucket Island, 64 km off the New England coast, found fresh ground water in coastal plain sediments to a depth of about Figur~ 2. Map showing estimated position of the 10 g/kg isohaline line in ground water underlying the U .S. Atlantic continental shelf. Based on minimum salinity in the observed profile, the value above the line adjacent to the hole location is salinity in g/kg; the value below the line is the depth below mean sea level (msl) at which the minimum salinity was observed. For onshore observation wells, the salinity (above the line) is related to the depth of the well screen (below the line). 172 500 m below msl (mean sea level) (Kohout, and others. 1977a). The coastal plain sediments thin and pinch ou t toward shore an d the crystalline rocks of the New Eng land mai nl and lack the recharge potential and hydrau lic conti nuity needed to account for the deep fresh water at Nantucket. Also, the shallow water-table aquifer underlying Nantucket lacks the head to provide recharge to the deep artesian system. A deep test hole subsequently drilled at Martha 's Vineyard Island , 32 km offshore, had a head of +5 .3 m compared to a head of +7 .3 m msl at Nantucket Island (Kohout and Delaney, 1979). The landward hydraulic gradient suggests that a freshwater artesian system, charged up during low stand of sea level in Pleistocene time, is now decaying as fresh groundwater is squeezed out by the advancing seawater(Collins, 1978 ; Kohout and Delaney, 1979). Aquifers Off Long Island, New York The circumstances are somewhat different at Long Island, N.Y . Although the island has no connection with mainland recharge areas, local precipitation infiltrates through Pleistocene deposits underlying the middle and northern parts of Long Island and moves vertically downward into the underlying Magothy and Lloyd Sand Member of the Raritan Formation, thence horizontally toward both north and south coasts. The Magothy has been heavily exploited in the New York City area and is affected by saltwater encroachment in coastal areas. The Lloyd Sand is fresh at a depth of 365 to 460 m below sea level at the south shore. The extent of fresh water in the offshore reaches of the aquifer is unknown, but the south shore beach communities depend on ground-water supplies obtained from the Lloyd Sand. Aquifers Off the New Jersey-Maryland Coast A line of five AMCOR test holes on a transect across the continental shelf east of the New Jersey coast permitted construction of a cross section through the lens of relatively fresh ground water and the transition zone to seawater. The cross section showed that low chlorinity water (less than 1 g/kg) extended about 10 km offshore and that relatively fresh ground water (less than 5 g/kg) formed a flatlying lens extending more than 100 km offshore (Hathaway and others, fig. 5, 1979). Overlying the lens of relatively fresh water is an extremely sharp chlorinity gradient increasing upward to the chlorinity of seawater at the sea floor. The high gradient occurs in low permeability clay in the upper part of the Miocene deposits and in Pleistocene deposits. The clay serves as a conf ini ng bed for the underlying permeable beds of the Kirkwood Formation. Water in the Kirkwood Formation was under artesian pressure on the mainland prior to intensive ground-water development in the early 1900's. Water levels in the offshore part of the aquifer underlying the barrier beach (particularly the "800-foot sand" at Atlantic City) are now (1978) drawn down to as much as 30 m below sea level by heavy pumping (Richard L. Walker, USGS, Trenton, N.J., oral commun., 1979). Clearly, the integrity of the confining bed is very important as an impediment to rapid vertical infiltration of seawater into barrier beach and mainland well fields. An implication of the present work is that saltwater intrusion would have occurred long before now if it had not been for the existence of the offshore freshwater lens-the extent of which was not even guessed in early studies that expressed concern about saline encroachment (Barksdale and others, 1936). The circumstances off the New Jersey coast also prevail off Ocean City, Md. AMCOR hole 6008 (inset, fig . 2) had freshened very sharply during penetration of the offshore extension of the Ocean City-Manokin aquifer (equivalent to the Miocene Kirkwood Formation of NewJersey) when the drill pipe became stuck and the hole lost in sandy aquifer material. Aquifers of Miocene and Cretaceous age tend to contain inr,reasingly salty water southward toward Cape Hatteras (Brown and others, 1972) . However, Tangier Island, just below the Maryland-Virginia border in the middle of Chesapeake Bay (fig . 2), obtains artesian fresh water (0.7 g/ kg salinity at 314 m depth) from the Upper Cretaceous Mattaponi Formation . At Ocracoke on the Outer Banks of Cape Hatteras, an artesian aquifer yields water of about 3 g/kg salinity from a depth of 182m. Therefore, the 10 g/ kg boundary .is placed seaward of Cape Hatteras. Aquifers Off South Carolina and Georgia Geologic formations thin and their outcrops (or subcrops) are displaced seaward around the nose of the Cape Fear Arch, whose axis runs northwestsoutheast approximately coincident with the North Carolina-South Carolina border near its intersection with the shoreline. Sediments which are predominantly clastic to the north change increasingly to carbonates southward from the Cape Fear Arch . Predominantly carbonate Oligocene and Eocene rocks form a regional aquifer extending southward from South Carolina and Georgia, where it is called the principal artesian aquifer, into Florida, where it is called the Floridan aquifer. 173 An underlying aquifer in Cretaceous rocks contains fresh water at increasingly greater depth southwestward along the South Carolina shorefrom 244m a.t Myrtle Beach , 890 mat Parris Island , to 944 m at Savannah, Ga. These data suggest that fresh water should extend far offshore. Howev.er, AMCOR test holes 6004 and 6002 showed an increase of salinity with depth from that of normal sea water (35 g/kg to about 41 g/kg (inset graphs, fig. 2). This increase above the normal salinity of sea water has been associated with the presence of evaporite deposits in deeper strata in other areas (Manheim and Hall, 1976). The 10 g/kg line is deflected landward around these possible areas of evaporite deposition and then seaward east of the GeorgiaFlorida State line. The Floridan Aquifer The Floridan aquifer is a very thick section of carbonate rocks, capped by clay confining beds of variable thickness, that underlies all of Florida and parts of Alabama, Georgia, and South Carolina. The relationships of fresh water and saltwater are complex and are different at different points around the long coastline of Florida. As noted previously, JOIDES J-1 B encountered a freshwater flow at about 300 m below sea level at a distance of 40 km offshore from Jacksonville, Fla. Water flowed from a zone that is equivalent to the Eocene Ocala, Avon Park, and Lake City Limestones ott he Floridan aquifer on the mainland. Based on the original piezometric surface near Jacksonville, Stringfield (1965, p. 161) calculated that there should be enough head remaining 100 km from the shore to cause fresh water to discharge at a depth of 500 m below sea level. This prediction was supported by data from JOIDES hole J-2. Pore water squeezed from middle Eocene cores decreased in salinity to 1.8 g/kg at 220m below sea level (Manheim, 1967, fig. 5). The correlation of low salinity with sediments equivalent to the top of the Floridan aquifer on the mainland provides good evidence that relatively fresh ground water is present and that discharge may be occurring at the edge of the continental shelf 120 mi east of Jacksonville, Fla. Southward from this location, confining beds of the Miocene Hawthorn Formation become thinner and are breached by sinkholes. Saltwater encroachment occurs in the Floridan aquifer in the St. Augustine to Daytona Beach area (Leve, 1968). A sinkho le 465 ft. (142 m) deep ca lled Red Snapper Sink is locat ed about 40 m offshore southeast of St. Augustine, Fla. (Wilcove, 1975). The high oxygen content of seawater deep in the sinkhole, and the results of a dye-dispersion test performed by a SCUBA diver, suggest that there was a slight downward velocity of seawater and that Red Snapper Sink, Fla. may be a point of seawater intrusion into the Floridan aquifer (Kohout, and others, 1977b). Southward from Daytona Beach, confining beds thicken and the Floridan aquifer deepens to 30004000 ft (914-1220 m) and possibly deeper at Key Largo in the Florida Keys. The upper part of the aquifer is the main source for thousands of municipal and irrigation water wells throughout the central and coastal parts of Florida.The deeper parts of this aquifer are occupied by seawater, particularly along the lower east coast where wells greater than about 900 m are used for injection of secondary treated sewage into cavernous Eocene limestone and dolomite. The 10 g/kg boundary has been placed outside of the 200m isobath from West Palm Beach to Miami because of the probability that discharge is occurring from the shallow parts of the aquifer below about 300 m along the continental slope of the Straits of Florida . It has been hypothesized that cold seawater may be migrating inland in the deep part of the aquifer on the inland-flow part of a convective flow cell motivated by geothermal heat (Kohout and others, 1977c, and other papers In this special report on the geothermal nature of the Floridan Plateau) . It is probable that the deep parts of the aquifer in the Florida Keys may extend to depths greater than 5000 ft (1525 m) below sea level. During the drilling of an oil-exp loratory well at Marquesas Key near Key West . " 300% returns' of water were reported in th e drillers log . The quality of the water is unknown because samples were not collected during the drilling operations. The excess return of fluid suggests that an artesian flow may have been encountered far offshore, west of Key West. Submarine springs occur off the west coast of Florida from Naples to St. Petersburg. About 12 mi offshore from Naples, Fla., the outflow from a warm submarine spring (97 F) has salinity similar to seawater. concentration (Kohout, and others, 1979). Geothermally heated saline water upwells through a sinklike depression at a depth of about 65ft (19m) below sea level. Water-quality data is not available from oil-exploratory drilling in this offshore area. The aquifers are extremely poorly understood except that they likely consist of highly permeable cavernous limestones overlain by confining beds. The western shelf of Florida was exposed land during low stand of sea level and was subject to freshwater recharge and probable development of caverns and sinkholes. The circu mstances are so obscure as to prevent proper hydrologic or ge.ologlc eval uation . The 10 g/kg boun dary has been placed at a distance of about 40 km from shore.entirely on speculation . Data are sorely needed in the entire western part of the Floridan Plateau , which extends several hundred ~ilometers off the west coast of Florida to the position of the 200 m isobath. 174 SUMMARY At Nantucket Island, about 40 mi (64 km) off the New England coast, fresh ground water was found. in coastal plain sediments to a depth of about 1500 ft (500 m) below sea level. The coastal plain sediments thin and pinch out before reaching shore and the crystalline rocks of the New England mainland lack the recharge potential and hydraulic continuity needed to account for the deep fresh water at Nantucket. These observations led to recognition that aquifers underlying the continental shelf were extensively flushed of salt water during low stands of Pleistocene sea level, and that fresh water was trapped beneath clay confining beds when sea level rose and flooded the Shelf about 8000 years ago. The USGS AMCOR Project of 1976 confirmed that the observations at Nantucket were not unique. A line of 5 test holes off the New Jersey coast showed that relatively fresh ground water (less than 10 g/kg salinity) was present in a lens about 200 m thick extending about 100 km offshore. Similar conditions appear to exist in some aquifers off Long Island (New York), Delaware, Maryland, the Carolinas, Georgia, and Florida. Salinities lower than seawater also occur under Georges Bank off New England. In this paper, salinity data from the offshore core holes have been augmented by data from selected onshore observation wells to formulate a map showing distribution of ground-water salinity under the U.S. Atlantic continental shelf. The EPA criterion of 10 g/kg salinity for protection of freshwater aquifers has been adopted to indicate areas of major waterresources interest. Mathematical models or saltwater intrusion usually assume that dynamic equilibrium has already been established between fresh water and saltwater prior to development of wells for extraction. However, little is known of the whereabouts of the freshsalt transition zone or how fast the salt front might be moving towards centers of withdrawal. Further, the clay confining beds that overlie these aquifers in some offshore areas of the shelf are of singular importance in preventing rapid vertical influx of saltwater into heavily pumped well fields, some of which have drawdowns of more than 30m below sea level. Mathematical models of saltwater encroachment require definition of the initial and boundary conditions of aquifers in order to produce reliable predic- tions of the intrusion progress. Our present knowl- edge of the offshore location and nature of the fresh-salt transition zone, the thickness and off- shore extent of aquifers, and the properties of their overlying confining beds can be appropriately de- scribed as primitive. The primary purpose of the AM COR drilling program was calibration of seismic profiles related to offshore oil and gas production. The data obtained on water-resources aspects were, for the most part, obtained as a result of serendipity. Considering that some coastal well fields have drawdowns well below sea level, the potential for saltwater encroachment is great and is not being confronted realistically. Obtaining the necessary information for calibrating predictive mathematical models is feasible if offshore drilling programs-are formulated specifically for water-resources objectives. REFERENCES Barksdale, H.C., Sundstrom, R.W., and Brunstein, M.S., 1936, Supplementary report on the ground-water supplies of the Atlantic City region: New Jersey Dept. Conser. and Econ. Devel., Special Rept. 6, 139 p. Bigelow, H.B., and Sears, M., 1935, Studies of the waters on the Continental Shelf, Cape Cod to Chesapeake Bay, II. Salinity: Papers Phys. Oceanography and Meteorology, v. 4, no. 1, p. 94. Brown, P.M. , Miller, J .A., and Swain, F.M ., 1972, Structural and stratigraphic framework, and spatial distribution of permeability of the Atlantic Coastal Plain, North Carolina to New York: U.S. Geol. Survey Prof. Paper 796. Bumpus, D.F., 1965, Residual drift along the bottom on the Continental Shelf in the Middle Atlantic Bight Area: Limnology and Oceanography, v. 10, Supp., p. R50-R53. Collins, M.A., 1978, Comment on "Fresh ground water stored in aquifers under the Continental Shelf: Implications from a deep test, Nantucket Island, Massachusetts," by Kohout and others, 1977: Am. Water Resources Bull. v.14 no. 2, p. 484-485. Dillon, W.P., and Oldale, R.N., 1978, Late Quaternary sea-level curve: Reinterpretation based on glaciotectonic influence: Geology, v. 6, no. 1' p. 56-60. Emery, K.O., and Garrison, L.E., 1967, Sea levels 7000 to 20,000 years ago: Science, v. 157, no. 3789, p. 684-687. Emery, K.O ., Wigley, R.L., Bartlett, A.S., Rubin, Meyer, and Barghoorn, E.S., 1967, Freshwater peat on the Continental Shelf: Science, v. 158, no.3806, p. 1301-1307. 175 Gill, H.E., Seaber, P.R., Vecchioli, John, and Anderson, H.R., 1963, Evaluation of geologic and hydrologic data from the test-drilling program at Island Beach State Park, New Jersey: New Jersey Dept. Conserv. and Econ. Devel., Div. Water Policy and Supply, Water Resources Circ. 12,25 p. Hathaway, J.C., Schlee, J.S., Poag, C.W., Valentine, P.C., Weed, E.G.A., Bothner, M.H., Kohout, F.A., Manheim, F.T ., Schoen, R., Miller, R.E., Schultz, D.M ., 1976, Preliminary summary of the 1976 Atlantic Margin Coring Project of the U.S. Geological Survey: U.S. Geol. Survey Open-file Rept. No. 76-844, 217 p. Hathaway, J.C., Poag, C.W ., Valentine, P.C., Miller, R.E., Schultz, D.M., Manheim, F.T., Kohout, F.A., Bothner, M.H., and Sangrey, D.A., 1979, The U.S. Geological Survey core drilling on the U.S. Atlantic Shelf: Science, v. 206, no. 4418. Kohout, F.A., 1966, Submarine springs: Encyclopedia of earth sciences series: New York, Reinhold Publ. Corp., v. 1, p. 878-883. Kohout, F.A., and Delaney, D.F., 1979, Reply to discussion by Michael A. Collins on "Fresh ground water in aquifers under the Continental Shelf: Implications from a deep test, Nantucket Island, Massachusetts," by Kohout, and others, 1977: Am. Water Resources Bull. v. 15, no. 1, p. 252-254. Kohout, F.A., Hathaway, J.C., Folger, D.W., Bothner,M.H ., Walker, E.H., Delaney, D.F., Frimpter, M.H., Weed, E.G .A ., and Rhodehamel, E.C., 1977a, Fresh ground water stored in aquifers under the Continental Shelf: Implications from a deep test, Nantucket Island, Massachusetts: Water Resources Bull. v. 13, no. 2, p. 373-386. Kohout, F.A., Henry, H.R., and Banks, J.E., 1977c, Hydrogeology related to geothermal condi- tions of the Floridan Plateau, in Smith, D.L., and Griffith, G.M., eds., The geothermal nature of the Floridan Plateau: Florida Bur. Geolo~y, Special Pub. no. 21, p. 1-42. Kohout, F.A., Leve, G.W., Smith F.T., and Manheim, F.T., 1977b, Red Snapper Sink and ground water fl,ow offshore northeastern Florida: Int. Assoc. Hydrogeologists, Mem. 12, Proc. 12th Inti. Congress, Karst Hydrogeology, Univ. Alabama Huntsville Press, p. 193. Kohout, F.A., Munson, R.C., Turner, R.M., and Royal, W.R., 1979, Satellite observations of a geothermal submarine spring off Florida west coast: 5th Wm. T. Pecora Memorial Symp ., Satellite Hydrology, Am. Water Resources Assoc., Program and Abs., p. 9-7, in press. Leve, G.W., 1968, The Floridan Aquifer in northeast Florida: Ground Water (NWWA), v. 6, no. 2, p. 19-29. Lusczynski, N.J., and Swarzenski, W.V., 1962, Fresh and salty ground water in Long Island, New York: Proc. Am. Soc. Civil Engr., Jour. Hydraulics Div. 38 (HY4): p. 173-194. Lusczynski, N.J., and Swarzenski, W.V., 1966, Salt water encroachment in Southern Nassau and Southeastern Queens Counties, Long Island, New York: U .S. Geol. Survey Water-Supply Paper 1613-F, 76 pp. Manheim, F.T., 1966, A hydraulic squeezer for obtaining interstitial water from consolidated and unconsolidated sediments: U.S. Geol. Survey Prof. Paper 550-C, p. C256-C261. Manheim, F.T., 1967, Evidence for submarine discharge of water on the Atlantic Continental Slope of the United States, and suggestions for further search: New York Acad. Sci. Trans. ser. 2, v. 29, no. 5, p. 839-852. Manheim, F.T., and Hall, R.E., 1976, Deep evaporitic strata off New York and New Jersey-evidence from interstitial .water chemistry of drill cores: U.S. Geol. Survey Jour. Research, v. 4, no. 6, p. 697-702. Manheim, F.T., and Horn, M.K., 1968, Composition of deeper subsurface waters along the Atlantic continental margin: Southeastern Geology, v. 9, no. 4, p. 215-236. Milliman, J.D., and Emery, K.O., 1968, Sea levels during the past 35,000 years: Science, v. 162, no.3858, p. 1121-1123. Perlmutter, N.M., Geraghty, J.J., and Upson, J.E., 1959, The relation between fresh and salty ground water in southern Nassau and southeastern Queens Counties, Long Island, New York: Econ. Geol. v. 54, p. 416-435. Soren, Julian, 1971, Ground-water and geohydrologic conditions in Queens County, Long Island, New York: U.S. Geol. Survey WaterSupply Paper 2001-A. Stringfield, V.T., 1965, Artesian water in Tertiary limestone in the southeastern states: U.S. Geol. Survey Prof. Paper 517. United States House of Representatives, 1974, Endangerment of drinkin water sources: House Report 93-1185, p. 32. Wilcove, Raymond, 1975, The great Red Snapper Sink: NOAA Magazine, v. 5, no. 2, p. 46-47. 176 ALTITUDE OF THE FLORIDAN AQUIFER OF NORTHWEST FLORIDA Tom Kwader Northwest Florida Water Management District Havana, Florida 0 10 20 30 40 MILES All contours in feet below sea level. ABSTRACT A detai led c o nto u r map dep ic ti ng th e top oi th e Fl o ri dan aqu ifer of nort11wes t Flori da has recentl y bee n comp le ted. In th is area the Floridan aq uifer consists of an oHiapping seque nce of Terti ary limes tones ~ a n glng i n age fro m Plioce ne to u pper Eocen e. Th e surfa ce of the aqui fer is app rox imately first order, but ranges g reatl y in altit ude. In th e recharg e area or no rth ern Jac kson and Ho lmes Cou nti es , th e aqui fe r Is approxi mat ely 150 It above mea n sea level. bu t dips sl1n.rp ly to thesou ti1West and lies more than 1400 ft below mea n sea level in t he Pensaco la area . Most of the data used were obtained from well cuttings and co res described and flied at the Florida Bureau of Geology. Li tho lo g ic descri pti ons also w ere corre ll'! led with num erous geop hysica l lo gs on fi le at l he No rthwest Flo ri da Water Managem ent District and the U.S. Geo log ical Su rvey. Ove r 600 data poin ts were co nt ou red i n th e final map wit h the ai d o f the U.S. Geologi cal Survey CalCamp 2-D contouring programs. LONG-TERM HYDROLOGIC STABILITY OF GULF COAST SALT DOMES AND MINES Joseph D. Martinez and Madhurendu B. Kumar Louisiana State University Baton Rouge, Louisiana ABSTRACT Th e long-term hydrol.og ic stability O'f Gul f Coast sa lt dom es and mined open in gs Is a pa rameter that must be co nside red i n an assess ment of th e poten ti al utili ty o't these fea tures for the sto rage and/or dispo sa l of 11 ighlevel radi oactive was tes. One of th e to xic com po nents, Plut o nium . has a lo ng half- life whic h req ui res iso latio n o f thi s mater ial for periods poss ibly as lo ng as 25 0,000 yea rs. An assessment of hydrologic stability requires a determination of the presence or absence of dissolution now and in the future. If dissolution is an active process , rates 177 and patter ns mu st be Iden ti fi ed. Evidence of dissol ution. w hic h involves th e exterior or t11e do me. is provi ded by th e prese nce o f sal ine plu mes in adjacent aq ui fers. Suc h plum es may be located by subsurface stl ldles o r new exp lo rato ry w ells. An equa lly imp ortant obj ec tive is th e d eve lo pme nt o f an und erstanding of wat er leaks In some existi ng min es . Th is Wi ll p rovid e a bas is fo r predi ctin g the hydrolog ic integrit y of mined openi ngs developed for the iso lation of radi oac tiv e w ast es.Data and tentative conc lusions are presented in this progress report. THE STRATIGRAPHY OF THE FLORIDA-HATTERAS SHELF AND SLOPE AND ITS RELATIONSHIP TO THE OFFSHORE EXTENSION OF THE PRINCIPAL ARTESIAN AQUIFER Charles K. Pauli and William P. Dillon U.S. Geological Survey Woods Hole, Massachusetts ABSTRACT The stratigraphic units that contain the principal artesian aquifer onshore in northern Florida, Georgia, and South Carolina were traced by means of seismic reflection profiles to the area under the Florida-Hatteras shelf and slope, and contour and isopach maps were made. The existing hydrologic data suggest that the offshore extension of the aquifer contains fresh water as tar eastward as the she lf break off northern Florida, but that the fresh water does not extend very far under the shelf off central Georgia. A seaward extension in the salinity bouhdary within the aquifer off southern Georgia corresponds to a depocenter delineated by isopachs of Eocene and Oligocene units. The axis of this depocenter correlates with a Tertiary facies boundary onshore. SEISMIC STRATIGRAPHY In 1976 the U.S. Geological Survey collected more than 5,000 km of single channel seismic reflection profi les covering the Florida-Hatteras shelf, slope, and inner Blake Plateau (fig. 1) (Pauli and Dillon, 1979) . An analysis of these data indicated that three regionally traceable unconformities are in the Tertiary section. Seismic stratigraphic units, bounded by unconformities, were correlated with stratigraphic data from offshore boreholes: The JOIDES well J-1, J-2, J-5, and J-6 (JOIDES, 1965) ; the U.S. Geological Survey AMCOR wells 6002, 6004, and 6005 (Hathaway and others, 1976) : COST GE-1 (Scholle, 1979); the Coast Guard Tower Well (McCollum and Herrick , 1964); and wells along the coast. The major unconformities appear to separate Cretaceous units from Paleocene, Paleocene from Eocene, and Oligocene and older strata from Miocene and younger strata. We made structure contour and isopach maps of units (figs. 2 A-F) by using interval velocities obtained from multichannel seismic profiles in the area (Dillon and others, 1979). The Paleocene rocks were deposited in a fairly uniform blanket of sediments 100m thick or less (fig. 2 C). Eocene and Oligocene sediments are more than 700 m thick in a triangular basin off southern Georgia (fig. 2 E) . Miocene and youhger sediments are about 50 m thick on the shelf and thicken to more than 200m at the shelf break (fig . 2 F). 340 .,. N I 33 s.c 31 29 620 -+-Location of offshore wells .& Location of dredge hauls of Cretaceous ooe , ' .,: 76 Figure 1. Location of seismic reflection profiles and dril hole data used in the analyses. OFFSHORE EXTENT OF THE AQUIFER Onshore In northern Florida, Georgia, and South Carolina, the principal artesian aquifer is generally contained within Eocene and, to a lesser extent, Oligocene rocks, and sealed above and below with the Miocene and Paleocene units, respectively (Counts and Dansky, 1963). The offshore extensions of these same Eocene and Oligocene units are traced in figure 2 E. Only six boreholes are on the continental shelf off the southeastern United States from which hydrologic data were collected and which penetrate the unit corresponding to the principal artesian aquifer onshore (J- 1, J-2, J-5. COST GE-1, 6002, and 6004, fig. 1). Within the aquifer unit of J-1 and J- 2, water containing less than 1 ppt (parts per thousand) salinity was found (Manheim, 1965; Kohout, 1978). Data on the salinity of water within the aquifer unit at COST GE-1 and J-5 well are ambiguous. The available data suggest that water containing less than 20 ppt salinity is present; this 178 A 8 ... "' 32' 32' .. ~ .. ,.. .. .. ... .. . ....... , 29' lll!;;;.;------:!:~----"!::-------:'=----:!:-:-''-------:71::-.----7;6' 82' ..... Contours in .-lr E) no Cenozoic present Top of Paleocene c D 33' , . 30' . .. - 29' !:::,.;---~~---"!..:,.::-----:,::.: Isopach of Po/eoc6/HI Sedimenls .: ----+-,.f.' ---:71:::-.- - -,;.,, B oulcropping Poleocene strata [;m no R:Jleocene ~I E ... --, ,----~--.---,---,.---,--.'.:';"" ... t:/-> ./',...::::::::>: <~_.../ ... . . ,./ F ... 32' ,. =-- ~ 30' . .. ~!;;;.,~ . -~~~~~=------=,~.---~~~ -~n:~.---~~ lscpach of Oligocene a>d Eocone Sedimenls Isopach of /lloogM8 ond _ , Sedimenls .. ....: Figure 2. Structure contour and isopach maps of surfaces and units inferred to be Cenozoic age. Map A is an isopach map of Cenozoic sediments. Maps B and D are structure contour maps of reflectors which correlate with unconformities separating Paleocene and older strata from Eocene and younger strata, and an unconformity which separates Oligocene and older strata from Miocene and younger strata. Maps C, E, and Fare maps showing thicknesses of units bounded by the Cenozoic unconformities, and correlate with sediments of Paleocene, Eocene, and Oligocene age and Neogene to Recent age, respectively . Map E is an isopach map of the unit which corresponds with the principal artesian aquifer onshore. 179 amount indicates a freshwater influence at these locations (Manheim, this volume). In water from the AM COR wells 6002 and 6004, salinities were greater than 40 ppt (Hathaway and others, 1976); ()uch values are greater than those of seawater,l and probably indi1cate the diffusion of salts from brines at greater depths (Manheim and Paull, this volume). The boundary between Tertiary units to the west that contain water of less than 10 ppt salinity and those to the east that contain water of higher salinity is shown in figure 3. Water of less than 10 ppt salinity is considered potentially usable and is to be protected as a resource, by definition of the Environmental Protection Agency (U.S. House of Representatives, 1974). An estimated boundary between Tertiary units that contain water of less than 1 ppt salinity, the limit of potable water, and those that have water of higher salinity is also shown in figure 3. This boundary has a seaward extension off the southern Georgia shelf. The position of the seaward extension of lowsalinity values in interstitial water corresponds to the axis of a Tertiary depocenter offshore (fig. 3). The axis of the depocenter beneath the shelf appears to be coli near with the deepest part of the Southeast Georgia Embayment onshore (Applin and Applin, 1967), which is shown by a thickening in the Tertiary units. This boundary forms a facies boundary between a predominantly carbonate province to the south and a mixed carbonate-clastic province to the north (Chen, 1965). The wells on the shelf suggest that this facies boundary may continue offshore. The seaward curve in salinity contours, indicating low-salinity waters beneath the outer shelf, suggests that increased flushing by artesian flow has taken place south of the depocenter; this flushing is perhaps related to more permeable rocks in the southern complex. CONCLUSION The unit that corresponds to the principal artesian aquifer onshore was traced seaward beneath the continental shelf . Within this unit, the boundary between the Tertiary rocks that contain relatively fresh water and those that contain salty interstitial water (Kohout, 1979, and Kohout, this volume) protrudes seaward between 30 and 31 o N. Thus, most of the area of the continental shelf south of southern Georgia appears to be underlain by a resource of fresh water. This boundary between fresh and salty interstitial waters corresponds to a facies + N I 0 32 : : 31 30 FL. ... . ... l%o . --10/oo - - Eocene 8 Oligocene Isopach ........... 29~~.------~----~~~~.~-----~L-------~~~----~7=7-. ------=7~ Figure 3. The isopach map of Eocene and Oligocene sediments indicates a basin under the shelf that has an axis extending onshore toward southern Georgia. The seaward extension of strata containing low-salinity water appears to be bounded by the axis of the Tertiary depocenter. 180 change across the axis of a Tertiary depocenter from a predominantly carbonate province to the south to a mixed carbonate-clastic province to the north. We speculate that this change in rock type may be accompanied by a change in permeability that has allowed more extensive flushing of the principal aquifer. This unit should be protected during offshore petroleum developrpent until we have adequate hydrogeologic information to trace the aquifer precisely. REFERENCES CITED Applin, P.L., Applin, E.R ., 1967, The Gulf Series in the subsurface in northern Florida and southern 'Georgia: U.S. Geol. Survey Prof. Paper524 G, 35 p. Chen, C.S., 1965, The regional lithostratigraphic analysis of Paleocene and Eocene rocks of Florida: Florida Geol. Survey Bull . 45, 105 p. Counts, H.B., and Dansky, E., 1963, Salt-water encroachment, geology and ground-water resources of Savannah area, Georgia and South Carolina: U.S. Geol. Survey, WaterSupply PGper 1611, 100 p. Dillon, W.P., Paul!, C.K., Buttler, R.T., and Fail, J.P., 1979, Structure and development of the Southeast Georgia Embayment and northern Blake Plateau: Preliminary an1alysis: Am. Assoc. Petroleum Geol. Mem. 29, p. 27-41. Hathaway, J.C., Schlee, J.S., Poag, W.C ., Valentine, P.C., Weed, E.G.A., Bothner, M.H, Kohout, F.A., Manheim, F.T., Schoen, R., Miller, E., and Schutz, D.M., 1976, Preliminary summary of the 1976 Atlantic Margin Coring Project of the U.S. Geol. Survey: Open-file Rept. 76-884, 217 p. JOIDES (Joint Oceanographic Institutions Deep Earth Sampling Program). 1965, Ocean drilling on the continental margin: Science, v. 150, p. 705-716. Kohout, F.A., 1978, Map depicting probable extent of low salinity ground water in aquifers b~neath the Continental Margin of the United States: Unpub. rept., 28 p., 2 figs. _ _, 1979, Relict fresh ground waters of the Continental Shelf: An unevaluated butter in present-day saltwater encroachment, this volume. Manheim, F.T., 1965, Composition of deeper subsurface waters along the Atlantic Continental MarQin: Southeastern Geology, 4, no.4, p, 215-236. Manheim~ F.T., and Paul!, C.K., 1979, Hydrochemistry of formation fluids in onshore and offshore strata in the Southeast Georgia Embayment, this volume. McCollum, M .J., and Herrick, S.M., 1965, Offshore extension of the upper Eocene to stratigraphic sequence in southeastern Georgia: U.S. Geol. Survey Prof. Paper 501-C, p. C-61 - C-63. Paul!, C.K., and Dillon, W.P., 1979, The subsurface geology of the Florida-Hatteras Shelf, Slope and inner Blake Plateau: U.S. Geol. Survey Open-file Rept. 79-448, 94 p. Scholle, P.A., 1979, Geological Studies of the COST GE-1 Well, United States South Atlantic Outer Continental Shelf Area: U.S. Geol. Survey Circ. 800, 114 p. U.S. House of Representatives, 1974, Endangerment of drinking water sources: House Report 93-1185,32 p. 181 GEOLOGIC HAZARDS AND CONSTRAINTS TO PETROLEUM EXPLORATION AND DEVELOPMENT ON THE SOUTHEASTERN U.S. CONTINENTAL SHELF, SLOPE, AND BLAKE PLATEAU Peter Popenoe U.S. Geological Survey Reston, Virginia ABSTRACT The U.S. Geological Survey and the Bureau of Land Management have been conducting environmentalhazards studies on the southeastern continental shelf, slope, and Blake Plateau since mid-1976. These studies include high-resolution seismic profiling of the shelf, slope, and northern Blake Plateau; depl6yment of bottom-instrument packages on the shelf to study sediment movement and its causes; coring and sampling programs to define sediment size, composition, and lithofacies; clay mineralogy; trace-metal analyses of sediments; composition and concentration of seston; side-scan sonar, and underwater television studies of reefs and hardgrounds; and submersible observations of marine habitats and uni:que bottom features . Several studies, such as the trace metal analyses of sediments, are intended to establish baseline levels. These studies 1ndi- cate a relatively pollution-free environment. Regional reflection studies indicate that very little faulting is associated with shelf or Blake Plateau sediments although several large faults are present on the continent al slope, One slump was defined on the Florida-Hatteras slope near 32 N, 79 W, and an attempt to core this slump showed it to be covered by Jag phosphorite nodules. Many geologic features on th e shelf, slope, and Plateau are constraints to commercial petroleum development, but planning and engineering can mitigate environmental damage to or from these features. These features include areas of scour or mass movement, channeling or steep topography, cut and fill structures, and reeflike features and hardgrounds which serve as marine habitats. GEOLOGIC INVESTIGATIONS FOR DREDGING OPERATIONS, FBM SUBMARINE SUPPORT BASE, KINGS BAY, GEORGIA Earl F. Titcomb, Jr. U.S. Army Corps of Engineers Savannah, Georgia ABSTRACT The establishment of the Navy's Fleet Ballistic Missile Support Base at Kings Bay, Ga., has required extensive dredging to provide access and anchorage facilities for the Navy's nuclear submarines. The existing channel from the Fernandina Harbor entrance at Cumberland Sound through the sound and up the Intercoastal Waterway to Kings Bay required both widening and deepening. Within Kings Bay, extensive dredging was required to provide adequate space for support facilities such as dry dock and tender boat anchorage. The Georgia portion of the work was handled by the Savannah District of the Corps of Engineers, while the Florida portion was done under the Jacksonville District. The dredging presented numerous challenges. The quantities of material to be excavated in the Savannah District's four mile portion alone exceeded eight million yards. In addition to the very large quantities, hard "limestone" and dolomite were found in several areas. This presented special problems si nee the cost of excavation of the rock is much greater than for unconsolidated materials. Use of suitable sands from the required excavation for renourishment of Fernandina Beach beaches is being planned. The materials unsuitable for nourishment use are being spoiled in dike areas near Kings Bay or disposed of offshore. Disposal of unsuitable material, both on and offshore, required extensive special treatment to minimize environmental impacts. In order to properly evaIuate the subsurface materials, 100 borings were made by Savannah District and an equal number by Jacksonville District. In addition, side scan sonar, proton magnetometer and seismic reflection profiling studies were conducted to assist in interpolation of data between borings and to locate any artifacts which might have been located within the expanded area. The borings were made from either conventional barges or Jacksonville District's jack-up drill barge. Where consolidated materials were encountered, the materials were drive sampled, using either split tube or solid tube barrels. The drilling was particularly difficult in the more exposed areas of the Intercoastal Waterway and Fernandina Harbor. No historic artifacts were found in the proposed expansion area during the magnetometer survey. Hard limestone was found to be concentrated in the northern portion of Kings Bay. In the southern portion of the Bay and in the Intercoastal Waterway, the depths to limestone were deeper or the limestone was not found. This made siting of the initial facilities in the southern portion of Kings Bay desirable. Future plans for expansion of the facilities are being considered by the Navy. These will involve additional dredging and construction of more permanent onshore facilities. 182 AQUIFER POTENTIAL OF THE SHALLOW SEDIMENTS OF THE COASTAL AREA OF GEORGIA Thomas Watson Georgia Department of Natural Resources Georgia Geologic Survey Atlanta, Georgia ABSTRACT In the coastal area of Georgia, there are three distinct aquifer systems. For many years, the principal artesian aquifer (a deep sequence of limestones) has been the primary source of all ground-water withdrawals in southeastern Georgia. The principal artesian aquifer of coastal Georgia is overlain by sediments ranging in thickness from less than 200 to more than 600ft. These sediments can be subdivided into two shallow aquifer systems. The surface aquifer system is a series of laminated sands and clays, attaining a maximum thickness of 70ft near the coast. These sediments are predominantly fine grained and have a low hydraulic conductivity. The surficial aquifer is underlain by the secon.d groundwater system, a 400-500 ft thick series of Miocene clays and sandy clays with lens-shaped bodies of sand, gravel, and thin carbonate layers. Although the Miocene series varies widely in hydraulic conductivity, and the lower strata act a3 a confining bed overlying the principal artesian aquifer, it is commonly used as an aquifer. Wells with yields of over 200 gallons per minute (gpm) are being used by several industries in the coastal area. Shallow aquifers have less aquifer capacity than the principal artesian aquifer. As alternative sources of good quality ground water, they have a potential of from 50,000 to over 300,000 gal/day, depending on location and well construction. Induced recharge to the principal artesian aquifer through the use of connnector wells may be a feasible method to take advantage of water in the surface aquifer. Water quality of the shallow aquifers varies with depth and lithology. In the surface aquifer, water quality varies from very low total dissolved solids to slightly alkaline, moderately hard water with total dissolved solids averaging less than 200 milligrams per liter (mg/1). Water from the Miocene sediments is somewhat more uniform in quality, with total dissolved solids averaging about 250 mg/1. The surface aquifer is extremely vulnerable to contamination. Although some contamination is inevitable as population increases, careful planning of potential sources of pollution would minimize the spread of contaminated ground water. The high clay content of theM iocene series reduces the vulnerability of these sediments to surface contamination. Here, as in the surface aquifer, care in planning would minimize the effects of pollution. INTRODUCTION The principal artesian aquifer of Georgia is a series of carbonate strata long recognized as one of the most productive aquifers in the world. For many years, the principal artesian aquifer has been the primary source of all large ground-water withdrawals in the coastal area. Heavy ground-water withdrawal and subsequent lowering of water levels have allowed saline water intrusion into the fresh water zone of the principal artesian aquifer in Brunswick (Wait and Gregg, 1973, p. 1). Current levels of withdrawal in the Savannah area may be approaching the limits of safe levels of water withdrawal (Counts and Krause, 1976, p. 3). As competition for available fresh water becomes more intense, ground water from sediment overlying the principal artesian aquifer will almost surely play an important role in the future development of coastal Georgia. This report is intended to provide a basis for further research and to guide general water-management decisions. PURPOSE Future management of the ground-water resources of coastal Georgia may include redistribution of water withdrawals to minimize cones of depression in the principal artesian aquifer. Another resource management technique that is being considered is increasing recharge to the freshwater zones of the principal artesian aquifer. Both practices can include the use of secondary aquifers such as the Miocene and surface aquifers, collectively referred to as the shallow aquifer system. The purpose of this study was to evaluate aquifer potential of the shallower sediments overlying 'the principal artesian aquifer. This information is necessary in determining to what extent the shallow aquifer may be used to augment the principal artesian aquifer. Augmentation through the use of secondary aquifers would help solve the problem of potential or existing salt water intrusion in specific areas of the coast by changing the hydrostatic pressure differential between fresh water and salt water bodies in the principal artesian aquifer. Using these shallower sediments as aquifers is not a new idea. Interviews conducted in the study area indic.ate that 183 as many as 80% of al l suburban and rural homes have at least one well drawing waterfrom a shallow aqui fer . Herrick ( 1971), Counts and Dansky (1963), Callahan (1964), and others , al l acknow ledged the ground-water pote ntial of the Neoge ne sediments. However, extremely high well yields and high artesian water levels of the deeper principal artesian aqu ifer have obscured the signif ica nce of shallow aq uifers. Specific objectives ot shallow aquifer evaluation were: a physical description of the water-bearing strata Including grain size and sorting; mapping the thickness and distribution of potent ial aqui f ers; and, determination of general c11emical characteristics of the shallow ground water. ~-- -- --- --r I ., GEOGRAPHY The 21 counties included in this report tie entirely within the Coastal Plain Province of Georgia (see fig. 1). Topography ranges from well-drained. gently rolling hills to relatively flat, featureless plains and swampland. Major population centers are Savannah in Chatham County and Brunswick in Glynn County. The economy of this part of the State depends largely upon forestry and the pulp and paper manufacturing Industry. Approximately 77% of the report area is forested with commercial stands of loblolly and slash pine. Several large wood pulp and paper mills are Included In the study area. Additional regional income Is derived from manufacturing, shipping, commercial fishing, and agriculture. Climate of the coastal region is mild with an average annual temperature of nearly 70 degrees F. Rainfall averages from 45 to 50 in . annually. Of the total rainfall, approximately 71% is returned to the atmosphere by plant transpiration and evaporation from surface water, 27% is returned to the ocean by rivers and streams, and 2% of the rainfall percolates downward to recharge the deep aqu ifers such as the principal artesian (Callahan, 1964, p. 50). 1t is significantto note that near ly one third of the annual rainfall of the coastal area, that portion which is returned to the ocean by rivers and streams, percolates through the sands of the surficial aquifer before being intercepted by stream systems. Although annual rainfall Is abundant, localized agricultural droughts can have serious results . Much of the annual rainfall is the result of scattered thundershower activity. Although a wea1her station may receive an averag e amount of precipitation over an entire year, several weeks of drought resulting in severe crop damage may occur during the hot growing season. For example, in Wayne County in the central portion of the study area, total rainfall for 1977 was only 2% below normal. However, the months from April through August averaged 42% below normal (U.S. Department of Commerce, 1977). Figure 1. Location of study area. ANALYTICAL METHODS Much of the subsurface data for this report came from the exam ination of rotary drill cuttings from varIous sources. Lithologic togs of 84 wells were prepared from cuttings on file with the Georgia Geologic Survey. In addition, cuttings and geophysical logs from 500 exploratory wel ls drilled by the Exxon Corporation were used by the study team during the course of this investigation. Geophysical logs of boreholes indicate the various lithologic units, their degree of saturation, and to some extent, the chemical nature of water contained by the sediments. Geophysical logs and descriptions of well cuttings provided a general framework for identifying areas of particular interest. Specifically, the initial phases of the Investigation sought to identify saturated sand layers, their depth below land surface, thickness, and lateral extent. Hydraulic conductivity, the degree of ease with which water can move through a sediment, can be estimated from a description of the grain size, shape, and sorting. For this purpose, a hollow-stemmed auger and split-spoon sampling device were used to obtain an undisturbed sample for analysis. Core samples from 137 test holes were used, of which 76 were drilled specifically for this study. The remainder were donated by mineral-exploration companies. Hydraulic conductivity estimates obtained from grain size and sorting data. multiplied by aquifer thickness, provided estimates of aquifer potential (Lohman, 1972, p. 52). 184 When completing a water well, the contractor often measures the specific capacity of the well, the response of the water level in the well to pumping. Specific capacity figures can be used to estimate aquifer potential, especially when the geology of the aquifer is known (Brown, R.H., 1963, p. 336). Both grain-size analysis and specific capacity tests were used to determine potential water-yielding characteristics of the shallow aquifers. The results of both methods were in close agreement. Ground-water chemistry samples for this study were collected according to methods described by Brown and others (1974, p. 6-12) to insure uniformly accurate results. Thirty ground-water samples were collected for this report, and chemical analyses were performed by the Georgia Geologic Survey or the U.S. Geological-Survey. GEOHYDROLOGY The sediments overlying the principal artesian aquifer can be divided into two distinct aquifer systems. These systems have dissimilar aquifer characteristics, and will be discussed individually. Figure 2 shows two generalized cross sections which illustrate the general relationship of the aquifers found near the coast. Miocene Aquifers The older of the two series lies directly above the principal artesian aquifer. This group of sediments was deposited in shallow seas which covered much of South Georgia during the Miocene epoch. Miocene sediments are thickest in Wayne County where they attain a thickness of over 500 ft as shown in figure 3. The Miocene section can be divided into lower, middle, and upper units, the lower being mostly carbonate rock which is hydraulically connected to the principal artesian aquifer. Middle and upper Miocene units are hydraulically separated from the principal artesian aquifer by thick sequences of dense, greenish-gray, sandy clay. 1nterlayered with the clay are lens-shaped bodies of sand, which are of interest because of their aquifer potential. Although individual sand lenses are not continuous throughout the study area, data indicate that they are most common in the thicker portions of the Miocene units and generally occur at depths greater than 200ft below land surface. Below this depth, sand lenses often reach a thickness of 50 to 80 ft, and extend laterally for over a mile. Texture of the Miocene sands varies somewhat. The coefficient of sorting is generally high, indicating poor sorting, with sands and gravels mixed. Median grain size is approximately one millimeter in diameter. No apparent trends linking grain size with depth or.location were evident from the data examined during this study. Surface Aquifer Overlying the Miocene series are sediments of the surface aquifer system, the youngest shallow aquifer. These sediments form a relatively thin layer of sands gravels, and clays, extending from the surface to a depth of approximately 80ft. Figure 4 shows the distribution and thickness of this aquifer. Data indicate that the surface aquifer system consists primarily of sand and thin layers of white to reddish-brown clay. Layers of pure sand or clay are generally no more than a few inches thick. Layers of sand-clay mixtures are usually less than 5 ft thick. Sand in the surface aquifer is fine grained and well sorted in most instances. Two notable exceptions to this general description were observed. First, flood plain deposits of major rivers in the study area commonly exhibit a wide range of grain sizes from sand to fine gravel. The Altamaha River in Wayne County, for example, has an extensive system of medium-to coarse-grained alluvial deposits. These deposits are more than 120ft thick in some places. Second, in the central portion of the study area at depths of from 30to 50ft below land surface, there is an extensive layer of coarse and fine gravel (see fig. 4). The thickness of this sand layer varies from 5 to 15 ft, and it is laterally persistent, occurring at the base of the surf,ace se-diments- In Wayne, Glynn, Long, and Mcintosh Counties. GROUND-WATER USE AND AVAILABILITY The surface aquifer receives recharge directly from rainfall which percolates through the sediments. In most areas of coastal Georgia the surface water table is quite shallow. During dry months the base flow of streams and rivers of the coastal area is maintained by ground water discharging from the surficial aquifer. The coastal area has very sandy soils which are actually part of the surface aquifer in many cases. The result is a very high rate of rainfall infiltration. Recharge to the Miocene aquifers is the result of percolation from the surface aquifer, and in some areas, discharge from the principal artesian aquifer. Although clay layers conduct ground water very slowly, thickness and density of the clays are variable throughout the study area causing recharge rates to be variable as well. 185 A' ECHOLS COUNTY \ 100 fl, .:........-- SEA LEVEL . . - =P-_ A EFFINGHAM ,.(/) :I: COUNTYj r r 0 :IE ,. D c "TI 1'1 "' PRir-JCIPAL ARTESIAN AQUIFER 8' HORIZONTAL SCALE Ok:=:::a:=::=:::::2j;OS:=:==::=::=:::::4t;0=:==:=3:::=::::::::;j60 MILES EXPLANATION ~LIMESTONE []sAND 1--J CLAY 8 SAVANNAH RIVER-., ,.rn :I: SURFACE AQUIFER r r 0 ,.:E MIOCENE D SERIES c "TI 1'1 "' PRINCIPAL AR TESI AN AQUIFER Figure 2. Generalized cross sections showing stratigraphic relationship of shallow aquifers and the principal artesian aquifer. 186 EXPLANATION ar' -400- LINE OF EQUAL THICKNESS OF SEDIMENT-Interval 50 feet DATA POINT 83 __' l . \, I 82 Figure 3. Thickness of unconsolidated Miocene sediments. At present, the shallow aquifer system is almost exclusively used as a source of water for domestic wells. In the central portion of the study area where the shallow sediments are thickest as shown in figure 5, it is estim'ated that more than 80% of all domestic wells tap the shallow aquifer system. The great depth of the principal artesian aquifer in this area makes the shallow aquifer system economically more attractive for most people. In the counties along the coast, many dwellings also have a well tapping only the surface aquifer. These shallow wells are most commonly used as a secondary water supply for yard irrigation rather than for drinking water. However, many rural homes use water from the surface aquifer for all water needs . Surface aquifer wells range from 10 to 80ft. Figure 6 shows the variations in estimated waterwell potential of the Miocene and surface aquifers combined . Water-well potential is expressed as the estimated yield in gallons per day of a properly constructed well penetrating the full thickness of the saturated sand or gravel aquifers. Proper well spacing to prevent interference between two or more wells is assumed . A well. designed to make the best use of shallow aquifer potential would be receiving water from several zones. 187 EXPLANATION AREA WHERE COARSE SAND AND FINE GRAVEL ARE PREDOMINATE , LINE OF EQUAL THICKNESS - J O - OF SEDIMENT- Interval 10 feet DATA POINT 0 20 60 MILES \, _.\ 82 " Figure 4. Distribution and thickness of sediments comprising the surface aquifer. Modified from Herrick, 1965. GROUND-WATER DEVELOPMENT TECHNIQUES There are several reasons why the shallow sand and gravel aquifers are not used more exte nsively for co mm ercial and agricultural wells. Developing a large capacity well in a san d and gravel aqu ifer req uires that t he well con tractor have specific equipment and experience Man y water well contractors in coastal Georgia have never constructed a well in a sand and gravel aquifer, largely because there was no need to do so. In addition, well construction and maintenance in a sand and gravel aquifer can be more expensive than simply drilling an open hole in a limestone aquifer. Care must be used to avoid well collapse and sand in the water. Finally, low hydraulic conductivity of the shallow aqu ifers In some areas of coastal Georg ia li mits ground- water flow and availability. Essenti ally , constructing a large capacity well in the sands and gravels of coastal Georgia has offered no apparent disadvantage over the use of the principal artesian aquifer, which produced water in seemingly unlimited quantity. 188 EXPLANATION 8 1" -400- LINE OF EQUAL THICKNESS OF SEDIMENT -Interval 50 feet DATA POINT ' \ 83" ) ' ' ' 82. Figure 5. Total th ickness of unconsolidated shallow sed iment. Well construction techniques are available that can elim.inate well collapse and make good use of "tight" aqurfers of apparent low productivity. These special techniques have been used successfully in other ~reas of the country where water is much less abund- ant.than in coastal Georgia. By taking advantage of optrmum well construction technology, more ground water could b~ obtained from the shallow aquifer system, thus helprng to reduce drawdown in the principal artesian aquifer. Current Well Construction Methods One type of shallow well commonly used in the study area is called a "rock" well. In constructing a "rock" well, a hole is drilled until a resistant layer is encountered. Resistant layers, usually thin limestones or dolomitic limestones, are common at the top of the Miocene Series. Drilling continues through the rock layer, and compressed air is used to create a hollow or reservoir immediately beneath. Casing is then set from the surface to the "rock". 189 EXPLANATION Estimated wa'ter yield, in gallons per day, available to properly constructed wells topping shallow aquifer systems . Projected yields ore based on hydraulic conductivity and thickness of the aquifer material. Adequate spacing of wells is assumed . ~ ~. ; - - " '" . > : / "-"\. . H { \.EF~FoIN,oGoHoAJ~ J r --~. I \ 1 CLiNCH 50,000 \ CHARLT f .---\_ , -.... ~ ., 83" 0 20 40 60 MILES 82" Figure 6. Estimated minimum well yield of shallow aquifers. "Rock" wells are a popular variety of domestic well because they are economical to construct and yield from 2 to 10 gpm . However, the reservoir, which is not screened , can and often does collapse, greatly reducing pumping capacity. "Rock" wells are usually no more than 80-ft deep; however, a 200-ft deep "rock" well used by a seafood processing plant in Brunswick has been in use since 1969, delivering slightly over 100 gpm. The best method for preventing the collapse of wells in unconsolidated aquifers is the use of well screens . Well screens are preferable to simply perfo- rating the casing, because the size of openings in a screen can be selected to complement the sand size of a specific aquifer. This allows maximum surface area of the aquifer to be exposed to screen, while minimizing the quantity of sand which enters the well. The screen is welded or threaded onto the string of well casing at intervals to correspond with the sand aquifers. If the sand aquifer is very fine grained, or if the grain-size sorting is poor, a technique known as filter packing can be used to enhance well yield . When a driller constructs a filter pack, the hole is purposely 190 I EXPLANATION I I TOOMBs( AREA WHERE HEAD DIFFERENTIAL / t BETWEEN SHALLOW AQUIFER AND PRINCIPAL ARTESIAN AQUIFER WOULD I TAT T 1 +---'--=~'r----"--lr- ~- ALLOW RECHARGE THROUGH CONNECTOR WELLS -,c c I N .,.I I ~ ' E C H 83 0 20 40 GO MILES Figure 7. Well construction techniques. drilled quite large, the casing and screen are installed as needed, and the annular space between the formation and the screen is filled with sand or gravel caretully graded tor size. The grain size and sorting of the ti Iter pack are chosen to keep sand in the aq uiter from being pumped into the well. Perhaps the most common type of shallow domestic well in the study area is known as a driven or jetted well. Driven wells are usually installed by homeowners to serve as a secondary water supply. A drive point, or slotted galvanized pipe with a pointed tip, is driven into the surface sands, often with the help of water pressure from a garden or portable pump. Driven wells are nearly always less than 40 tt-deep and no more than 2 in. in diameter. They generally provide from 1 to 5 gpm of water. Driven wells are common even where municipal water is available, and are used chiefly tor yard irrigation . The main drawback to driven wells is that they are vulnerable to surface contamination. Figure 7 shows examples of various well construction methods. Exampl es B through E are in use in coastal Georgia, with D and E being the most common construction techniques. 191 Potential Ground-Water Development Techniques Well capacity can be increased by enlarging the well diameter. Water in large diameter wells can be pumped from storage in the well bore, and pumpage can exceed the volume of water flowing into the well bore for short periods. A large diameter well also exposes a large surface area of the aquifer facilitating the flow of water into the well. A pond can be thought of as a shallow, large diameter, uncased well. T.he pond-well technique is successfully used for agncultural irrigation in the Waycross area under geohydrologic conditions similar to those found over a ~id e port ion of t his report area. T he pond Is ~es1g ne d t o ho ld enough water to operate an irrigation sy stem for a predetermined lengthor time. Water In the p ond is dep leted during the irrigat ion and allowed to recover be fore the next ir rigat ion. Calculations (Beck, 1978, p. 19) have shown that ponds with a radius of 50ft and a depth of 15ft below the water table are capable of yielding 100,000 gal- /day . Initial construction costs are reasonable depending upon location. ' A In addition to conventional withdrawal of ground water from wells, induced recharge by means of connector wells could be an alternative use for the surface aquifer. Rainfall in the coastal area is rapidly absorbed by the surface aquifer. However, dense clay layers in the Miocene series prevent much of the water stored in the surface sands from percolating down to recharge the principal artesian aquifer. The main recharge areas for the principal artesian aquifer are to the west of the study area, where the confining layer is thin . Connector wells within the study area would link the surface aquifer with the principal artesian aquifer, bypassing the Miocene clays. Water from the surface aquifer would then flow downward by gravity into the principal artesian aquifer. In effect, a connector well hastens the natural recharge process. In order for induced recharge to take place, the water table of the surface aquifer must be at a higher elevation than the artesian pressure level in the principal artesian aquifer. This situation exists throughout much of the coastal area. Figure 8 shows the portion of the study area where the hydraulic head differential is suitable for induced recharae. c E (Limestone) EXPLANATION A. CONNECTOR WELL CONDUCTING ~ATER FROM S.URFACE AQUIFER TO PRINCIPAL ARTESIAN AQUIFER. DIRECTION OF WATER FLOW IS SHOWN BY ARROWS. B. MULTIPLE AQUIFER WELL WITH THREE SCREENED INTERVALS AND ONE FILTER- PACKED INTERVAL . C. PONDWELL UTILIZING SURFACE AQUIFER . D. "ROCK" WELL DRAWING WATER FROM RESERVOIR EXCAVATED IN SURFACE AQUIFER . E. DRIVEN OR JETTED WELL . Figure 8. Map showing the area where connector wells may be feasible. 192 Calculations based on hydraulic conductivitY, of the Miocene clay layers (Callahan, 1964, p. 24; Counts and Donsky, 1964, p. 77) and measured hydraulic head potential indicate that approximately 48,000 gal/day/sq mi reach the principal artesian aquifer by natural percolation in the coastal area . This is equivalent to only 1 in . of rainfall per square mile per year. Using a method described by Lohman (1972, p. 23), estimates of recharge via gravity-flow connector wells show that approximately 19,000 gal/day could be injected into the principal artesian aquifer through one connector well. This is a 40% increase in the calculated natural recharge per square mile. A series of 3 or 4 connectcJr wells in a square mile could more than double the natural recharge in a specific area. Connector wells are a potential management option. Their use in the Brunswick area, for instance, could help decrease head differential in the principal artesian aquifer, which would in turn slow the intrusion of brackish water. However, in any situation where human activity interferes with a natural process, results can be detrimental and difficult to anticipate. For instance, the greatest danger with artificial recharge is the introduction of interior quality water into an aquifer. At this time, additional research into the economic and technical feasibility of connector wells in Georgia seems justified. GROUND-WATER QUALITY The chemical quality of ground water is the result of chemical interactions between the aquifer materials and the water they contain. Rainwater is usually quite low in dissolved solids and neutral to slightly acidic. That portion of rainfall which percolates downward changes its chemical composition even as it moves through the soil zone. Si l)~e ground-water movement is quite slow, commonly-measured in teet per year, the ground water has time to adjust to the chemical properties of the aquifer material. Thirty water samples were collected for this study to determine the range in chemical quality. Laboratory analyses were performed either by the Georgia Geologic Survey or the U.S. Geological Survey. Table 1 shows selected summary results for the samples evaluated. Quality of water from Neogene sediment is good through most of the study area, based on Department of Natural Resources, Environmental Protection Division , 1977, Sate Drinking Water Standards. This applies only to the major inorganic dissolved solids included in the analyses. 'Bacterial and organic chemical determinations were not performed at this time. In general, the quality of water is a relative indication of its source; that is, the level of dissolved solids increases with depth. Water from the surface aquifer is usually lower in dissolved solids than is water from the Miocene aquifers. Total dissolved solids average less than 100 parts per million (ppm) from swrface sediments, while water from Miocene sediments averages slightly over 300 ppm . Table 1. Summary of ground-water quality analyses. Constituent or Physical Property TEMPERATURE oc SILICA Si02 IRON Fe CALCIUM Ca MAGNESIUM Mg SODIUM Na POTASSIUM K BICARBONATE HC03 SULFATE so4 CHLORIDE Cl FLORI DE F NITRATE N03 DISSOLVED SOLIDS CARBONATE HARDNESS CONDUCTIVITY MICROMHOS FIELD pH ANALYTICAL RESULTS(!) ~~.,_,l'li\)"' "''(,<:;)\~~ "''~'1'/i\)~ 23.8 21.9 20.5 42.0 20.7 4.0 2.6 0.6 0.0 65.0 29.5 1.6 38.0 10.3 0.0 146 20.5 2.5 22 .0 4 .0 0.1 348 133 0 .0 164 25 0 175 25.2 I 1.2 0.3 0.0 35d2) 312(2) 0 .0 s4s21 204 30 319 121 4 720 401 51 7.65 6.80 4.63 (I) Valu411 in milliQrams per liter, except Temperature (Degrees Celsius); Conductivity (Micromhos@ 25 Cl; and pH. f2lvalues Qreater than recommended lim its. 193 Water from the shallow aquifers is of three chemical types: (1) Water from the Miocene aquifer is generally a moderately hard calcium-bicarbonate water; (2) Samples from the surface aquifer were a sodium bicarbonate type; and (3) Some wells in the surface aquifer yielded sodium chloride water, although the concentration was usually not in excess of safe drinking water standards. Maintaining the chemical quality of water in the shallow aquifers will be a ser ious challenge. Although the surface and Miocene aquifers are not extensively used at present, they contain a substantial reserve of water available for future use. The surface aquifer is especially vulnerable to contamination because permeable sands which form the aquifer are found at, or close to, land surface. With increased development of the coastal area, a certain amou nt of contami natio n to the surface aqu ifer is proba bly unavoidable, especially from non.-point pollution resu lting from su rface runoff. However, damage to su rface aquifers from identifiabl e point sou rces of poll ution is need less, and can be prevented. Locati on of potential points of contamination should be based on complete geologic and hydrologic data, including the thickness and extent of local clay layers and a water-table contour map. The geology of the Miocene aquifer makes it less susceptible to contamination. Substantial thicknesses of clay surrounding the lenticular sand aquifers will slow the recharge of surface water and allow natural filtration and absorption to take place. Care should be taken to prevent needless pollution of the Neogene aqu1fers. Complex relationships between surface water and ground water can lead to pollution in unexpected places, and a polluted shallow aquifer is useless, as well as dangerous to the health of those dependent upon shallow ground water. Brown R.H.. 1963, Estimating the transmissibility of ~n artesian aquifer from the specific capacity of a well: in Berital, Ray, comp., Methods of determining permeability, transmissibility, and drawdown: U.S. Geol. Survey Water-Supply Paper 1536-1' p. 336-338. Callahan, J.T., 1964, The yield of sedimentary aquifers of the coastal plain southeast river basins: U.S. Geol. Survey Water-Supply Paper 1669-W, 56 p. Counts, H.B., and Donsky, Ellis, 1963, Salt water encroachment, geology and ground water resources of Savannah area, Georgia and South Carolina: U.S. Geol. Survey Water-Supply Paper 1611, 100 p. Counts, H.B. and Krause, R.E., 1976, Digital model analysis of the principal artesian aquifer, Savannah, Georgia area: Water Resources Investigations 76-133, U.S. Geol. Survey Open-file rept., 4 p. Folk, R.L., 1965, Petrology of sedimentary rocks: Univ. Texas, 159 p. Georgia Department of Natural Resources, 1977, Rules for safe drinking water: Chap. 391-3-5, Environ. Protection Div., Atlanta. Herrick, S.M., 1965, A subsurface study of pleistocene deposits in coastal Georgia: Georgia Dept. Mines, Mining and Geology , Int. Clrc. 31 , 8 p. _____, 1971 , Well logs of th e coastal pl ai n of Geor g ia: Georgia Geol. Su rvey Bull . 70, 460 p. Lohm an, S.W.. 1972, Ground water hydraulics: U.S. Geol. Survey Prof. Paper 708, 67 p. Masch, F.D., and Denny, K.J ., 1966, Grain size distri- bution and its effect on the permeability of unconsolidated sands: Water Resources Research, v. 2, no. 4, p. 665-667. Wait, R.L., and Gregg, D.O., 1973,. Hydrology and chloride contamination of the principal artesian aquifer of Glynn County, Georgia: Georgia Dept. Nat. Resources, Georgia Geol. Survey, Hydrologic Rept. 1, 93 p. U.S Department of Commerce, 1977, Climatological data, Georgia, Annual Summary: Nat'l Oceanic and Atmospheric Admin ., Environ. Data Service, Asheville, N.C. SELECTED REFERENCES Beck, B.F., 1978, The feasibility of using ponds as shallow wells in the coastal area of Georgia: unpub. report prepared for Environ. Resources Center, Georgia lnst. Technology and Georgia Dept. Nat. Resources, 22 p. Brown, Eugene, Skougstand, M.W., and Fishman , M.J., 1974, Methods for collection and analysis of water samples for dissolved minerals and gases: Techniques of water resources investigations, Book 5, Chap. A 1, 160 p. 194 Ill. STRUCTURE, TECTONICS AND GEOPHYSICS ' 195 PRE-CRETACEOUS GEOLOGY BENEATH THE COASTAL PLAIN OF THE SOUTHEASTERN STATES, U.S.A. T .M. Chowns Department of Geology West Georgia College Carrollton, Georgia ABSTRACT The most prominent feature of a paleogeologic map of the Fall Line unconformity beneath the southeastern coastal plain is a complex Triassic graben extending southwestwards from the vicinity of Charleston, South Carolina to Apalachicola, Florida. The graben occupies an area of approximately 50,000 square miles and contains at least 6000 feet of continental red beds with substantial volumes of basaltic lava and minor intrusives. Radiometric ages for the latter cluster around 180 m.y. and indicate rifting associated with the initiation of the Gulf of Mexico and Atlantic Ocean. The northern flank of the graben consists of medium-to high-grade metamorphic rocks, which represent the subsurface continuation of the Pied- mont province. The southern flank comprises a sequence of unmetamorphosed to slightly metamorphosed felsic pyroclastic deposits and Lower Ordovician-Middle Devonian clastic sediments which extend southwards into peninsular Florida and appear to be underlain by a Precambrian granitic basement. At least part of the felsic volcanic terrane is Cambrian or Late Precambrian in age, but radiometric dates show a wide range of ages and it is possible that more than one volcanic association is present. Stratigraphic and paleontologic affinities suggest that this sequence was originally associated with the African craton and therefore lies on the eastern margin of the Appalachian orogen. DRAINAGE PATTERNS, BURIED VOLCANIC NECKS, AND PLATE TECTONICS IN THE SOUTH GEORGIA COASTAL PLAIN James E. Clark, Jr. Law Engineering Testing Company Atlanta, Georgia ABSTRACT The flow patterns of the major streams and rivers in the Coastal Plain of South Georgia were studied by (1) Landsat-1 Imagery, (2) gravity surveys conducted on one km . spacing, (3) deep well borings, and (4) PotassiumArgon age date determinations on basalt and rhyolite cuttings from those borings. It was concluded that the underlying volcanic rock is a volcanic neck at a depth of approximately 4000 feet. This volcanic neck controls the radial drainage patterns in the Seventeen Mile River drainage basin near Douglas, Ga. (Coffee County, Latitude 31 o 45' N, Longitude 82 48' W). The gravity survey across the Ocmulgee River near Lumber City, Ga. (Jeff Davis C.ounty), suggests a series of normal faults. Additional gravity surveys across the Ocmulgee River near Jacksonville, Ga. (Telfair County), and the Altamaha River near Baxley, Ga. (Appling County), can be modeled as near basement faults and/or basic igneous intrusions. The Potassium-Argon dates of the volcanic rocks indicate Triassic and Jurassic emplacement. The age and lithology of the volcanic neck, the inferred normal faults and the intrusions can be related to continental rifting during the initial breakup of Laurasia and the formation of the Atlantic Ocean . HEAT FLOW AND THE GEOTHERMAL RESOURCE POTENTIAL IN THE EASTERN UNITED STATES John K. Costain Virginia Polytechnic Institute and State University Blacksburg, Virginia Abstract not available. 196 SUMMARY OF DEVELOPMENT OF THE CONTINENTAL MARGIN OFF GEORGIA BASED ON MULTICHANNEL AND SINGLE-CHANNEL SEISMIC- REFLECTION PROFILING AND STRATIGRAPHIC WELL DATA William P. Dillon and Charles K. Pauli U.S. Geological Survey Woods Hole, Massachusetts INTRODUCTION 81 eo 79 78 The basement of the continental margin off the southeastern United States was formed by the pro- 35 cesses of rifting, which modified pre-existent basement, and continental drifting, which created new 0 100 200KM --Seismic Profile TD5 basement. The present continental margin framework resulted from subsidence of this basement accompanied by reef growth and clastic-sediment accumulation, controlled in part by ocean-current patterns, sea-level fluctuations, rates of input of ter- I rigenous and biogenous sediments, etc. The U.S. Geological Survey has collected a grid of 9000 km of deep penetration, common-depth-point, seismic reflection profiles on the U.S. southeastern contin- ental margin for purposes of studying the structure, development, and petroleum potential of this region. Profile TD-5 (fig. 1) best exemplifies the structure of the continental margin off Georgia, and will be used with knowledge gained from the rest of the grid to characterize the development of that area. This profile passes across three drillsites; therefore, the stratigraphy along this line is better known than that along any other profile across the eastern U.S. continental margin. Profile TD-5 is discussed in Dillon and others (1979a) and Dillon and others (1979c) . Stratigraphic data for the COST GE-1 drillsite are presented in Poag (this volume) and in a series of papers in 29 Scholle (1979); for the DSDP390drillsite in Benson, Sheridan, and others (1978); and for the ASPC well in Poag (1978). Other deep structural studies of the region have been reported by Buttler and others 27 (1978), Dillon and Paull (1978), Shipley and others (1978), Buttler and others (1979), Dillon and others (1979b), and Klitgord and Behrendt (1979) . Struc- 2s ture of Upper Cretaceous and Cenozoic strata has been analyzed by Paull and Dillon (1979). REGIONAL STRUCTURE AND NATURE OF BASEMENT The continental margin area oft the southeastern United States is associated with three zones of basement subsidence, shown by stippled patterns in figure 1. These are the Blake Plateau basin, Carolina trough, and the Southeast Georgia Embayment (Kiitgord and Behrendt, 1979). The Blake Plateau Figure 1. Location of seismic profile discussed and physiography of the U.S. southeastern continental margin, contour interval 200 m. Major structural features are labeled in bold italics, with zones of subsidence shown in stippled pattern. 197 basin, in the southern part of the area, displays basement depths of 8 to 14 km in an irregular pattern. North of the Blake Plateau basin and paralleling the edge of the continent from about 31 .5 N to Cape Hatteras (35 N), the elongate Carolina trough contains depths of 10-11 km to basement . The Southeast Georgia Embayment is a gently subsided, eastward-plunging depression recessed into the continent between two platform areas-the Carolina platform on the north and the Florida platform on the south. Profile TD-5 extends from the outer part of th e Southeast Georgia Embayment, across the deepest part of the Blake Plateau basin at its northern end, and eastward to the deep sea (fig. 1). The profile typifies the structure of the continental margin at the Blake Plateau basin, where a deep broad basin underwent maximum subsidence near its landward side. The seaward side of this continental margin depositional basin was closed off by reefs that acted as sediment dams. Erosion of these reefs by deep-sea processes has left an escarpment more than 3 km high at the seaward side of the Blake Plateau (Blake Escarpment, fig . 1). Basement beneath the Blake Plateau basin is probably formed by large amounts of continental basement fragments, mantle-derived intrusive and extrusive mafic rocks, and considerable quantities of included sediments; this is referred to as transi tional basement. Such basement is characterized by broad, low-amplitude magnetic anomalies and a crustal thickness of generally 15-20 km , intermediate between continental and oceanic thicknesses. DEVELOPMENT OF THE CONTINENTAL MARGIN Rifting between North America and Africa began in Triassic time. During the rift stage, continental basement was fractured and extended, and intrusion and extrusion of mafic, mantle-derived material took place. Eventually , as the continents moved apart, the rift stage was followed by the drift stage, in which new oceanic crust was formed from such mafic material. However, off the southeastern United States, these two stages were separated by an intermediate stage in which the transitional basement was formed. The zones of deepest subsidence of continental margin basins seem to be underlain by this transitional basement. The spreading center that was active during earliest opening of the ocean, when the Blake Plateau basin transitional crust was forming, ceased its activity at about 170 or 175 million years (m .y.) ago when a new spreading center developed to the east. This new spreading center has been active to the present time. Figure 2 is a diagrammati c presentation of the inferred stages of development of the continental margin off Georgia at the location of line TD-5. The NORTH AMERICA AFRICA 140M.Y. END OF == ~RIDGIAN 140 .. 170 116M.Y END OF NEOCOMIAN 100 M.Y. END OF EARLY CRETACEOUS ~ENT Fig u re 2. Pro posed stages in t he develo pment of t he con t ine nt al margin o ff Georgia as inte r pre t ed from profile T D-5. In each idealized section, the de posits form ed bet wee n t he ti m e indicated and the previous stage are stippled. Crosses repre sent con t i n en t al base me nt. "vees" represen t oceanic baseme n t, and a mixture of the two symbols represents tra n sit io nal ba seme nt . T he ho ri zons w hi c h represent the sea floor at a stage are labeled wi th the ir age in la ter st ages as t hey a re progress ive ly more deep ly bur ied. first diagrammatic section (170 m.y.) shows the continental margin at the time of the spreading-center jump. In figure 2, the two centers of spreading are shown by the symbol of diverging arrows, the right hand set representing the newly developing spreading center. By this time (170 m .y .), a well-defined post-rift erosional surface had formed and probably was being covered by sediments deposited in the newly formed Blake Plateau basin. Off South Carolina, northern Georgia, and southern North Carolina, a layer giving very strong reflections and high refraction velocities, probably of volcanic rock, covers the post-rift erosional surface. The renewed volcanic activity may have occurred because of stresses associated with the spreading-center jump. 198 After the spreading-center jump, truly oceanic crust began to accrete, and the eastern U.S. continental margin became progressively farther west of the spreading center as the zone of new oceanic crust became wider. At 140 m .y . (fig . 2), the ocean was broader and reefs began to function as sediment dams. These reefs, now beneath the outer Blake Plateau, were initiated on the African side before the spreading-center jump. The largest part of continental margin basin subsidence occurred before 140 m .y. (approximately the end of the Jurassic) . The pattern of this early subsidence was irregular; present basins and troughs subsided more than adjacent areas. Conversely, later subsidence was much more uniform across the entire continental margin region . By 116 m.y. one of the developing reefs became dominant, and its growth and, eventually, erosion of its front formed the present Blake Escarpment. The Early Cretaceous (about 140 to 100 m.y. ago) was a period of extensive formation of back-reef and carbonate bank deposits on the outer Blake Plateau . Meanwhile, the shoreline oscillated across the inner Blake Plateau , as shown by the marine-continental facies boundary estimated from reflection characteristics of the strata. At about the end of Neocomian time, the main escarpment reef died and a new reef formed, growing during Aptian and Albian before it, too, died. This step-back of the reef occurred along much of the outer Blake Plateau . The reef, as a continuous feature, does not extend north of the Blake Spur (fig . 1), although the seaward part of the Lower Cretaceous shelf is marked by probable patch reefs and carbonate banks. In th e Late Cretaceous and Paleocene (approximately 100-55 m.y. ago , fig . 2), rising ocean level and continued regional subsidence left the former shelf as a broad submerged plateau that extended across the present Blake Plateau and outer continental shelf. Fine sediments, characteristic of outer shelf or slope depths, accumulated. The Cenozoic history of the continental shelf south of Cape Hatteras is summarized in figure 3. Near the end of the Paleocene, a major erosional event took place that left an irregular unconform'ty in a linear belt 100-km wide beneath the present outer continental shelf and inner Blake Plateau, and that is believed to have been caused by the initiation of the ancestral Gulf Stream. The erosional surface was buried in the Eocene by a seaward progradation of the shelf. This first Tertiary she!f progradation ended in the Oligocene with another regional erosive event, but one that did not produce as great erosional relief as the earlier Paleocene episode. The Oligocene erosion may have been related to a worldwide lowering of sea level. At about this time there also occurred an extensive episode of erosion in the deep sea that strongly affected the continental s.lope off the southeastern United States . That erosion probably removed considerable amounts of material from the Blake Escarpment south of the Blake Spur. The Oligocene unconformity beneath the present continental Shelf is covered by a second Tertiary shelf progradation (fig . 3, Present). This progradational wedge is smaller than the Eocene wedge and apparently was delimited to seaward by the flank of the Gulf Stream. The Gulf Stream flow has been instrumental in forming the Tertiary depositional pattern; it has prevented deposition on the inner Blake Plateau by preventing progradation of the shelf toward the east. A very thin layer of Tertiary deposits has accumulated on the outer Blake Plateau seaward of the main flow of the stream (fig. 2, Present); these are chiefly pelagic oozes and not continentally derived sediments (Charm and others, 1969). Fi9u re 3. Proposed stages in the deve lopment of the Florida-Hatteras she lf off Georgia during Cenozoic time. Symbols are used on ly to indicate deposits of specific ages and do not represent litho logy. 199 SUMMARY The continental margin off Georgia probably began to form with rifting , mafic intrusive and extrusive activity, and rapid sediment deposition which led to development of a transitional basement. Early subsidence was rapid for the basement beneath the present Blake Plateau basin, and the Uppe r Jurassic deposits form the thickest unit. Reefs acted as sediment dams at the seaward side of the basin. Near the end of the Neocomian, the reefs died, but a new reef formed slightly to landward and continued to form a sediment dam until the end of the Early Cretaceous . Subsequently , the Blake Plateau has been a moderately deep water environment (several hundred meters) until present. The Gulf Stream became significant on the Blake Plateau near the Paleocene-Eocene boundary, and since then has prevented the shelf sediments from prograding across the plateau . BIBLIOGRAPHY Benson, W.E., Sheridan, R.E., and others, 1978, Initial reports of the Deep Sea Drilling Project, v. 44: Washington, U.S. Government Printing Office, 1005 p. Buttler, R.T., Shipley, T.H ., and Watkins, J.S. ," 1978, Blake continental margin seismic section: Am. Assoc. Petroleum Geol. Seismic Sec. No.2 Buttler, R.T., Watkins, J .S., and Dillon, W.P., 1979, Geology of the offshore Southeast Georgia Embayment, U.S. Atlantic Continental Margin, based on multichannel seismic reflection profiles: in Watkins, J .S., Montadert, L., and Dickerson, P.W., eds., Geological and geophysical investigations of continental margins: Am . Assoc. Petroleum Geol. Mem. 29, p . 11-25. Charm, W.B., Nesteroff, W.D., and Valdes, Sylvia, 1969, Detailed stratigraphic description of the JOIDES cores on the continental margin off Florida: U.S. Geol. Survey Prof. Paper 581-D, 13 p. Dillon, W.P. and Pauli, C.K., 1978,Seismic-reflection profiles off coasts of South Carolina and Georgia: U.S. Geol. Survey Misc. Field Studies Map MF-936. Dillon , W.P., Pauli, C.K ., Buttler, R.T. and Fail , J.P., 1979b, Structure and development of the Southeast Georgia Embayment and northern Blake Plateau: Preliminary analysis: in Watkins J.S ., Montadert, L., and Dickerson, P.W ., eds. , Geological and geophysical investigations of continental margins: Am. Assoc. Petroleum Geol. Mem . 29, p. 27-41. Dillon, W.P., Pauli, C.K., Dahl, A.G. and Patterson, W.C., 1979a, Structure of the continental margin near the COST GE-1 well site from a common depth point seismic reflection profile: in P.A. Scholle, ed., Geological studies of the COST GE-1 well, United States South Atlantic Outer Continental Shelf area, U.S. Geol. Survey Circ. 800, p. 97-107. Dillon, W.P., Poag, C.W., Valentine, P.C., and Pauli, C.K., 1979c, Structure, biostratigraphy, and seismic stratigraphy along a CDP seismic profile through three drillsites on the Continental margin off Jacksonville, Florida: U.S. Geol. Survey, Misc. Field lnv : . Map, MF1090. Klitgord, K.D., and Behrendt, J.C., 1979, Basin structure of the U.S. Atlantic Continental Margin : in Watkins, J.S., Montadert, L., and Dickerson, P.W., eds ., Geological and geophysical investigations of continental margins : Am . Assoc. Petroleum Geol. Mem. 29, p. 85-112. Pauli, C.K. Dillon, W.P., 1979, The subsurface geology of the Florida-Hatteras Shelf, Slope, and Inner Blake Plateau : U.S. Geol. Survey Open-file Rept. 79-448, 94 p. . Poag , C.W., 1978, Stratigraphy of the Atlantic Continental Shelf and Slope of the United States: Ann . Rev . Earth Planet Sciences, v. 6, p. 251-280. Poag, C.W., Biostratigraphy, sea level fluctua~ions, subsidence rates and petroleum potential of the Southeast Georgia Embayment: this volume. Scholle, P.A. , ed., 1979, Geological studies of the COST GE-1 well , United States South Atlantic Outer Continental Shelf area: U.S. Geol. Survey, Circ. 800, 114 p. Shipley, T.H. , Buttler, R.T., and Watkins, J.S., 1978, Seismic stratigraphy and geologic history of Blake Plateau and adjacent western Atlantic continental margin: Am . Assoc. Petroleum Geol. Bull. , v. 62, no. 5, p. 792-812. 200 GEOCHEMISTRY OF PRE-CRETACEOUS ROCKS BENEATH THE COASTAL PLAIN OF GEORGIA AND SOUTH CAROLINA David Gottfried and Michael W. Higgins U.S. Geological Survey Reston, Virginia Abstract not available. JOINTS AND MINOR FAULTS IN THE GEORGIA COASTAL PLAIN Michael W. Higgins U.S. Geological Survey Reston, Virginia and Sam M. Pickering, Jr. Yara Engineering Company Sandersville, Georgia Abstract not available. 201 SEISMICITY OF GEORGIA Leland Timothy Long Georgia Institute of Technology Atlanta, Georgia ABSTRACT Earthquakes in Georgia occur in the Coastal Plain, Central Piedmont and folded Appalachian provinces. Six earthquakes are known to have occurred in the Georgia Coastal Plain. The most recent event occurred on December 27, 1976, near Reidsville, Ga., where it was felt with intensity V. On August 31, 1886, the Coastal Plain of South Carolina, near Charleston, was the location of the earthquake which caused the highest intensity of shaking in Georgia. Intensities of VIII were experienced near the South Carolina-Georgia border and the level of shaking decreased toward the northwest in Georgia to intensity V. The seismic activity in the Central Piedmont occurs in a zone which extends northeast from Central Georgia into South Carolina. Eight events have been felt in central Georgia and two near the South Carolina border in the Central Piedmont. The largest three of these events were felt with intensity VI. The folded Appalachians of northwest Georgia have experienced three events from within Georgia and others from adjacent areas of Alabama and Tennessee. The observed activity of that portion of the Southern Appalachian Seismic Zone implies that one event of intensity V should be expected approximately each ten years. No geologic fault in Georgia has been found to be associated with seismic activity. Large geologic faults can be found throughout Georgia, but they are considered inactive. INTRODUCTION Earthquakes can be one of the most terrifying of the natural phenomena. Catastrophic loss of life and mass destruction of property are typical of major earthquakes that occasionally strike in populated regions of the world . However, the chance of experiencing an earthquake varies significantly from region to region. The historical record of earthquakes and their locations shows that most earthquakes occur in well-defined belts or zones that mark the boundaries of the earth's rigid crustal plates. Away from the plate boundaries and in the interior of the plates, the seismic activity is significantly lower. Georgia and the southeastern United States lie within the interior of the North American Plate. Earthquakes at plate boundaries are adequately explained by the relative movement of the plates in accordance with the theories of plate tectonics (Wilson, 1971; Isacks and others, 1968). Earthquakes occurring in the interior of plates are not as easily explained. Where mountainous continental crust occurs in the interior of a plate, a minor level of seismic activity is often observed, but the relation of this activity to plate tectonics is not always clear. Some zones of seismic activity may be related to landward extensions of major transform faults or to zones of weakness in continental crust (Sykes, 1978). Perhaps, also, local stresses related to vertical motion within plates contribute to earthquake activity. The seismic activity levels in the southeastern United States vary with location (Bollinger, 1973a) . For example, the Southern Appalachians are notably more active than the surrounding areas. The activity level for some of these surrounding areas is so low that the 100 to 200 years of historical data do not allow satisfactory evaluation. In the Coastal Plain, the seismicity data may even appear to be contradictory in that the Charleston area has experienced many more earthquakes than any other Coastal Plain site. Ultimately, the relative seismic activity rates will be explained by physically measurable entities such as crustal thickness, crustal rigidity, vertical crustal movement or horizontal stresses. At this time we can define only some of the differences in seismic activity rates and some of the characteristics of earthquakes in more active areas. DATA BASE 0 nly since the installation of the WWSSN stations ATL, OXF, BLA and SHA in the early 1960's has it been possible to adequately document the occurrence of earthquakes in the southeastern United States. Instrumental recordings of earthquakes allow measurement of earthquake magnitude. Magnitude is a single number which quantifies the excitation by an earthquake of certain seismic waves or ground vibrations. Magnitude can be determined to within the error of measurement from seismograms recorded at any distance. The magnitude scale is logarithmic and hence each increase of one unit corresponds to a tenfold increase in the amplitude of the ground vibrations. Richter (1935) developed the definition of magnitude and a function to correct for the normal attenuation of seismic energy with distance from the epicenter, (the point on the earth's surface above the zone of faulting that caused the earthquake vibrations). Hence, magnitude is often called "Richter Magnitude". At a magnitude of approximately 4.5, slight damage may 202 occur near the epicenter. At approximately 5.5 moderate damage may occur. and above 6.5 the damage can be considerable . The August 31, 1886, earthquake near Charleston, S.C. is the only event in the southeastern United States to possibly exceed a magnitude of 6.5. Accounts of earthquakes occurring before the 1960's relied heavily on reports by people experiencing the earthquakes or on documentation of damage perpetrated by the earthquake. Such reports are evaluated according to the empirical Modified Mercalli scale for earthquake intensity (see Appendix I). Intensity measurements are intended to specify the severity of the earthquake motion at a given point by its effect on people, structures and landscapes. Intensity data provide essential information in studies of earthquakes because they provide data on earthquake shaking and damage potential not currently available from the very limited distribution of earthquake recording instrumentation. A standardized questionnaire is given in Appendix II. Instructions in the evaluation of intensity data can be found in Richter (1958). Care must be exercised in interpretation of some felt reports. If not obtained immediately, intensity surveys can be influenced by news reports or hearsay. Also, reports of very local earthquakelike vibrations are occasionally reported along the Georgia Coast, but these are generally attributed to supersonic: aircraft or military tests offshore. A location plot of southeastern United States earthquakes (fig . 1) does not directly reveal distinct trends within Georgia. The larger Georgia earthquakes are listed in Appendix Ill (from Stover and others, 1978). Only the central Georgia area near Milledgeville indicates an apparent concentration of activity. In fact, the two major previously identified seismic trends in the southeastern United States only border on Georgia. If one follows the usual definitions or seismic zones in the southeastern United States (Bollinger, 1973b), the South CarolinaGeorgia Seismic Zone, which has an axis that trends northwest, covers the northeastern third of Georgia and extends in width to the northeast across South Carolina. The Southern Appalachian Seismic Zone, (from Bollinger, 1973b) which has an axis that trends northeast, crosses only the northwest corner of Georgia. These seismic zones were drawn entirely on the basis of historical concentrations of earthquake epicenters, as there exists no clear relation so Figure 1. Location plot of recent and historic seismic activity in the Southeastern United States, adapted and updated from Bollinger (1975). 203 between faults or other near-surface geologic structures and earthquakes in the southeastern United States. In fact, no fault in Georgia has yet been found to be associated with seismic activity. Prominent geologic faults and zones in which multiple faults exist can be found throughout North Georgia. They have also been interpreted from geophysical data in the Coastal Plain of South Georgia but are considered inactive today. EARTHQUAKES FELT IN GEORGIA The earthquakes felt within Georgia have epicenters both inside and outside of Georgia's borders. Numerous small events near Georgia's borders have been felt within Georgia at intensity level of V or less. Two great earthquakes, the Charleston, S.C. earthquake of August31 , 1886, and the three largest New Madrid, Mo., earthquakes of 1811 and 1812 were felt across Georgia. Within Georgia, earthquakes occur in three general areas which correspond approximately to the Coastal Plain, Piedmont, and folded Appalachian physiographic provinces (fig. 2). Earthquakes in these three provinces will be discussed separately by province since the crustal structures in each province influence the manner of stress release by earthquakes. With existing data, the three provinces can be characterized according to the earthquake mechanism for events that occur within their boundaries as well as according to the crustal structures that host the earthquake. EARTHQUAKES IN THE COASTAL PLAIN Coastal Plain sediments cover the southern half of Georgia and extend into the neighboring states: southeastern Alabama, Florida and the southeastern half of South Carolina. The crustal structure beneath the Coastal Plain sediments is characterized by remanent features of extension .during the early opening of the Atlantic ocean . That is, the crust is generally thinner as a consequence of the extension and is intruded by extensive mafic volcanic rock units characteristic of incipient rift zones. The thinning and intrusives distinguish the crust of the Coastal Plain from the thicker crust of the Piedmont Province. In particular, examination of magnetic anomaly maps show that the Piedmont Province lithologies do not extend indefinitely under the Coastal Plain sediments. The southern boundary of the Piedmont lithologies (Popenoe and others, 1978) is assumed to define the northern boundary of the seismically active part of the Coastal Plain. Three or perhaps six small earthquakes are suspected to have occurred in the Georgia Coastal /7l t ~~--*-~~ ~~~~~=-+- 11991v109:'LIC: OG ... , __ /'::,. Seismic Stafon ./Year) ilnte?s'fY.,Eu thquake : IV : \ / \ PIEDMONT !'::.. 1R~1EG"G~BG ~lfrl4t,5~felt4t~~~-' V C~94:!-1 ;/ '"Era-" ATL 19l.. ! .E ... 0 CIJ >- 100 0 - '-Ill c CIJ w > -Qj lL -0 10 ... CIJ .c E z:J ~ +- 2 1055 t' '6 10 Years ( t) 100 (1986) Figure 4. Rate of occurrence of earthquake s in 10-year intervals following the 1886 Charleston earthquake. N is the number of events during a 10year interval following after the main event. almost identical to the value of 1.14 obtai ned for the Great Alaskan earthquake (Page, 1968). The fit to the Omori equation shows that the current rate of activity at Charleston is entirely consistent with an aftershock sequence. Hence, without other constraints, the possibility exists that the tectonic input at Cha rl eston is mi nimal , the Charleston area may not be un ique, and an intenstiy X earthquake might be ex pected anywhere in the Coastal Plain . Ultimately, the statistical approach presented above will be modified when a mechanism for earthquakes in the Coastal Plain is accepted and proven correct. A uniform probability of occurrence is not realistic because the crust in the Coastal Plain is heterogeneous in composition, nonuniform in thickness and, as such, can not be expected to react uniformly to any applied stresses. A spat ial association of earthquakes with mafic intrusions has been noted, and explanations for such an association (based on stress amplification or the particular material properties of the intrusives) have been proposed (Long 1977; Campbell 1978) . If shown to be correct, this association could limit the probable location of significant events in the Coastal Plain to major centers of mafic intrusives. EARTHQUAKES IN THE CENTRAL PIEDMONT The central portion of the Piedmont extends northeast from central Georgia and generally follows the axes of the Carolina Slate belt and Charlotte belt. The seismicity follows a zone of thinner crust interpreted as a late Precambrian or early Cambrian rift (Long, 1979). The crust is on the order of 33 km thick (Kean, 1978) . The historical earthquake epicenters in this zone in Georgia are concentrated in two general areas: central Georgia and Clark Hill Reservoir area. The most active area appears to be central Georgia, with eight felt events with intensities from Ill to VI. These historical epicenters show a possible northwest trend. Two events of intensity VI have been felt in the Clark Hill Reservoir area. In the central Piedmont an earthquake has been felt approximately once each 10 years for the past 100 years . The seismic activity in central Georgia and the Clark Hill Reservoir area has been monitored in detail by recently installed seismic recording stations. In central Georgia, the general northwest trend has been confirmed and additional epicenters of small unfelt events (often called microearthquakes) have been located about Lake Sinclair. In the Clark Hill Reservoir area the most obvious center of activity has been the aftershock zone of the August 2, 1974, earthquake. Two other microearthquake epicentral zones have been identified near the reservoir. Through seismic monitoring from 1973 to the present and associated geophysical studies in the Clark Hill Reservoir area (Denman, 1975; Bridges, 1976; Marion , 1977; Guinn, 1978; Dunbar 1978) a general model for the Piedmont Province earthquakes has been developed. The observations are consistent with these earthquakes being caused by a release of stress along existing planes of weakness such as joints near the surface. The stresses could be derived from minor flexure of the crust, stress amplifi cation ln inh om oge neou s geolog ic units, erosion al un load lng o r thermal perturbati o ns . The strain rel ease cou ld be fac ilitated by strength deterioration thro ugh weath ering processes or (I n some areas) water-related changes caused by reservoir impoundment. The implication of this mechanism is that the fault area for these earthquakes will be limited by the depth penetration of joints or other planes of weakness and hence will be limited to depths less than about 4 km. A maximum magnitude of about 5.5 is implied by limitations on a reasonable fault radius and stress drop (Long , 1976) . 206 EARTHQUAKES IN THE SOUTHERN APPALACHIANS The seismically active part of the Southern Appalachians affects only the northwestern corner of Georgia. The southeastern edge of this zone in Georgia has been defined by the southeastern extent of Paleozoic sediments or, equivalently, by the Cartersville or Great Smoky fault. Few earthquakes are documented in Georgia in this zone, but some have occurred just across the border in Alabama and Tennessee. The Conasauga, Tenn ., earthquake of November 4, 1976, was typical of these events. It had a maximum intensity ofV to VI. A focal mechanism for this event indicates a probable thrust-type movement on a northwest-trending fault plane. Similar focal mechanisms are obtained for other earthquakes in eastern Tennessee, implying that tne stresses are horizontal and compressional and trend NE-SW. Few depths of focus are available, but most are estimated to be in the range of 4 to 8 km. This depth range places the earthquakes below the Paleozoic sediments and in the crystalline basement. Considering the historical recording of about 10 events in the last 100 years, one event of intensity V or greater should be expected approximately each 10 years in northwestern Georgia and adjacent areas of Alabama and Tennessee. SEISMIC SURVEILLANCE IN GEORGIA All the seismic stations except AMG shown on figure 2 are maintained by the School of Geophysical Sciences at the Georgia Institute of Technology . Financial support from the Nuclear Regulatory Commission, the Corps of Engineers, Georgia Power and Georgia Tech have made this net possible. AMG is supported by Georgia Southwestern College. Data for the Georgia Institute of Technology stations are all recorded in the laboratories of the School of Geophysical Sciences by means of telemetry from the remote sites. Information on recent or local earthquakes can be obtained by contacting the School of Geophysical Sciences. The National Earthquake Information Service of the U.S. Geological Survey in Golden, Colo., normally provides information on large regional or worldwide earthquakes by issuing bulletins to the news services. ACKNOWLEDGEMENT The research which has made this paper possible was supported by the Nuclear Regulatory Commission through grant number AT (49-24)-0210. The Georgia Power Company and the U.S. Army Corps of Engineers have suported the operations of the seismic stations. Appreciation is extended to Carl W. Stover and G .A. Bollinger for their comments and assistance with the manuscript. BIBLIOGRAPHY Bollinger, G.A., 1973a,Seismicity of the southeastern United States: Seismological Soc. America Bull.,v. 63, p. 1785-1808. - -- 1973b, Seismicity and crustal uplift in the southeastern United States: Am. Jour. Sci.,v. 273-A, p. 396-408. _ _ _, 1975, A catalog of Southern United States earthquakes -1754 through 1974: Virginia Polytech. lnst. and State Univ. Res. Div.,Bull. 101, 68 p. Bridges, S. R., 1976, Evaluation of stress drop on the August 2, 1974, Georgia-South Carolina earthquake and aftershock sequence: unpub. M.S. thesis, Georgia lnst. Tech., Atlanta, Ga., 103 p. Campbell, D.L ., 1978, Investigation of the stressconcentration mechanism for intraplate earthquakes: Geophys. Res. Lett., v. 5, p. 477-479. Denman, H.E., Jr., 1975, Implications of seismic activity at the Clark Hill Reservoir: unpub. M.S . thesis, Georgia lnst. Tech., Atlanta, Ga., 103 p. Dunbar, D.M. , 1978, A seismic velocity model of the Clark Hill Reservoir area: unpub. M.S. thesis, Georgia lnst. Tech., Atlanta, Ga., 59 p. Guinn , S.A., 1978, Earthquake focal mechanisms in the southeastern United States: unpub. M.S., thesis, Georgia Inst. Tech., Atlanta, Ga., 150 p. lsacks, B.D., Oliver, J ., and Sykes, L.R., 1968, Seismology and the new global tectonics: Jour. Geophys. Res., v. 73, p. 5855-5899. Kean, A.E., 1978, A refraction crustal study of the southeastern United States: unpub. M.S. thesis, Georgia lnst. Tech., Atlanta, Ga., 68 p. Long, L.T., 1974, Bouguer gravity anomalies of Georgia in Symposium on the petroleum geology of the Georgia Coastal Plain, Georgia Geol. Survey, Bull. 87, p. 141-166. - - - 1979, The Carolina slate belt--Evidence of a continental rift zone: Ge.ology, v. 7, p. 180-184. _ _ _, 1977, Maximum "induced" earthquake, Clark Hill Reservoir Area: Design earthquake report, Geological and seismological evaluation of earthquake hazards at the Richard B. Russell Project, U. S. Army Engineer Dist. Corps of Engineers, Savannah, GA., Sect. B, 15 p. _ _ _, 1976, Speculations concerning southeastern earthquakes, mafic intrusions, gravity anomalies, and stress amplification: Earthquake Notes, v. 47, no. 3, p. 29-35. Marion, G.E., 1977, A spectral analysis of microearthquakes that occur in the southeastern United States: unpub. M .S. thesis, Georgia lnst. Tech., Atlanta, Ga., 154 p. Page, Robert, 1968, Aftershocks and microaftershocks of the Great Alaska Earthquake of 1964: Seismological Soc. Am. Bull ., v. 58, p. 1131-1168. 207 Popenoe, P., and Zeitz, 1., 1977, The nature of the crystalline basement beneath the Coastal Plain of South Carolina and southeastern Georgia: U.S. Geol. Survey Prof. Paper, 1028-1, p. 119-137. Richter, C.F., 1958, Elementary seismology: San Francisco, Calif., W.H. Freeman Co., 768 p. Scholz, C. H., 1968, Microfractures, aftershocks, and seismicity: Seismological Soc. Am. Bull., v. 58, p. 1117-1130. Stover, C.W., Reapor, B. G., Algermissen, S.T., Long, L.T., 1978, Seismicity map of the State of Georgia: U.S. Geol. Survey Misc. Field Studies Map. Sykes, L.A., 1978, Intraplate seismicity, reactivation of existing zones of weakness, alkaline magnetism and other tectonism postdating continental fragmentation: Rev. Geophys. Space Phys., v. 16, p. 621-688. Taber, Stephen, 1914, Seismic activity in the Atlantic Coastal Plain near Charleston, South Carolina: Seismological Soc. America Bull ., v. 4, p. 108-160. Tarr, A. C., 1977, Recent seismicity near Charleston, South Carolina, and its relationship to the August 31, 1886, earthquake: U.S. Geol. Survey Prof. Paper, 1028-D, p. 43-57. Wilson, J.T., 1971, Continents adrift: Readings from Scientific American, San Francisco, Calif., W.H . Freeman Co., 172 p. APPENDIX 1: Modified Mercalli Intensity Scale of 1931 (Abridged) I. Not felt except by a very few under especially favorable circumstances. II . Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing. Ill. Felt quite noticeably indoors, especially on upper floors of buildings, but many people do. not recognize it as an earthquake. Standing motorcars rock slightly. Vibration like passing truck. Duration estimated. IV. During the day, felt indoors by many, outdoors by few. At night some awakened. Dishes, windows, and doors disturbed; walls make creaking sound. Sensation like heavy truck striking building. Standing motorcars rocked noticeably. V. Felt by nearly everyone; many awakened. Some dishes, windows, etc., broken; a few instances of cracked plaster; unstable objects overturned. Disturbance of trees, poles, and other tall objects sometimes noticed. Pendulum clocks may stop. VI. Felt by all; many frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster or damaged chimneys. Damage slight. VII. Everybody runs outdoors. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable in poorly built or badly designed structures. Some chimneys broken. Noticed by persons driving motorcars. VIII. Damage slight in specially designed structures; considerable in ordinary substantial buildings, with partial collapse; great in poorly built structures. Panel walls thrown out of frame structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well water. Persons driving motorcars disturbed. IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb; great in substantial buildings, with partial collapse. Buildings shifted off foundations. Ground cracked conspicuously. Underground pipes broken. X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations; ground badly cracked. Rails bent. Landslides considerable from river banks and steep slopes. Shifted sand and mud. Water splashed (slopped) over banks. XI. Few, if any (masonry), structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipelines completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly. XII. Damage total. Waves seen on ground surfaces. Lines of sight and level distorted. Objects thrown upward into the air. 208 APPENDIX II U.S. DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY EARTHQUAKE REPORT Form Approved OMB No. 42-R1700 Please answer this questionnaire carefully and return as soon as possible. 1. Was an earthquake felt by anyone in your town or zip code area recently? 0 Not felt: Please refold and tape for return mail. 0 Felt: Date Time _ _ __ DAM 0 PM 0 Standard time 0 Daylight time Name of person filling out form _ _ __ _ __ _ _ __ _ _ _ _ _ __ _ _ Address--- - - -- - - -- - - - - -- - - - - - - - - - -- - City State C o u n r y ' - - - - - -- - - -- Zip code ___ _ __ _ _ _ _ __ _ If you felt the earthquake, complete the following section. If others felt the earthquake but you did not, skip the personal report and complete the community report. PERSONAL REPORT 2a. Did you personally feel the earthquake? b. Were you awakened by the earthquake? e. Were you frightened by the earthquake? 10 Yes 10 Yes 30 Yes 0 No 0 No 0 No d. Were you at 40 Home SO Work 60 Other? e. Town and zip code of your location at time of earthquake f. Check your activity when the earthquake occurred : 10 Walking 80 Sleeping 90 Lying down 10 O Standing g1\.) II 0 Driving (car in motion) g. Were you 12 0 Sitting 14 0 Inside or llO Other 150 Outside? h. If inside. on what floor were you? 1600 Continue on to next section which should include personal as well as reported obser\lations. COMMUNITY REPORT Check one box for each question that is applicable. 3a. The earthquake was felt by 0 No one 170 Few 180 Several b. This earthquake awakened c. This earthquake frightened 0 No one 0 No one 210 Few 250 Few 220 Several 260 Several 190 Many 130 Many 170 Many 100 All? 140 All? 180 All? 4. What outdoor physical effects were noted in your community? Parapets or cornices fallen 290 Yes 0 No Trees and bushes shaken 300 Slightly 310 Moderately Standing vehicles rocked Moving vehicles rocked 330 Slightly 360 Slightly 34 0 Moderately 37 0 Moderately Ground cracks 390 Wet 40 0 Steep slopes ground Landslides 410 Small 430 Largo Underground pipes 440 Broken 45 0 Out of service 32 0 Strongly 35 0 Strongly 380 Strongly 410 Dry and level ground Water splashed onto sides of lakes, ponds, swimming pools Elevated water tanks 460 Yes 470 Cracked 0 No 480 Twisted 490 Fallen (thrown down) Air coolers 500 Displaced 510 Rotated 510 Fallon Railroad tracks bent Stone or brick fences Tombstones 530 SUghtly 550 Cracked 580 Displaced 540 Greatly 560 Fallon 590 Cracked 57 0 Destroyed 600 Rotated Chimneys Highways or streets Sidewalks 610 Fallen 62 0 Cracked 63 0 Twisted 640 Fallen 650 Broken at roof line 660 Bricks fallen 67 0 Cracked slightly 68 0 Large cracks 69 0 Displaced 70 0 Cracked slightly 710 Large cracks 72 0 Displaced Continued on the reverse side 5. What indoor physical effects were noted in your community? Windows, doors, dishes rattled 73 0 Yes 0 No Buildings creaked 74 0 Yes 0 No Building trembled (shook) 75 0 Yes 0 No Hanging pictures 760 Swung 77 0 Out of place 780 Fallen Water in small containers 790 Spilled 800 Slightly disturbed Windows 810 Few cracked 820 Some broken 830 Many broken 6a. Did hanging objects, doors swing? 0 No b. Can you estimate direction? 0 No 840 Slightly 86 0 V iolentiY 87 0 North/South 890 Other 850 Moderately 88 0 East/West 7a. Were small objects (dishes, knick-knacks, pictures} 0 Unmoved 910 Overturned 920 Fallen, not broken b. Was light furniture 0 Unmoved 95 0 Overturned 94 0 Sh;fted 96 0 Fallen, not broken c. Were heavy furniture or appliances 0 Unmoved 990 Sh;fted 900 Shifted 930 Broken? 970 Broken? 980 Overturned 1000 Broken? 8. Indicate effects of the following types to interior walls if any ~ Plaster 1010 Cracked 102 0 Fell Dry wall 1030 Cracked 1040 Fell Ceiling tiles 1050 Cracked 106 [J Fell 9a. Check below any damage to buildings or structures. Foundation 107 0 Cracked 108 0 Destroyed Interior walls Exterior walls 1090 Split llOO Fallen 1120 Hairline cracks 1110 Separated from ceiling or floor 1130 Large cracks 114 0 Bulged outward 1150 Partial collapse 1160 Total collapse Building 117 0 Moved on foundation I18 0 Shifted off foundation b. What type of construction was the building that showed this damage? 1190 Wood 1230 Brick 1200 Stone 124 0 Cinderblock 1210 Brick veneer 12G Other 1250 Reinforced concrete c. What vJas the type of ground under the building? 126 0 Don't know 127 rJ Sandv soil 128 0 Marshy 129 0 Fill 130 0 Hard rock Ul 0 Clay soil 132 0 Sandstone, limestone, shale d. Was the ground: Ill 0 Level 134 U Slopmg 1350 Steep? e. Check the appro;~C;imate age of the building: 1350 Built before 1935 1370 Built 1935-65 1380 Built after 1965 10a. What percentage of buildings were damaged? Within 2 city blocks of your location 0 None 1400 Many(about 50%) b. In area covered by your zip code 0 None 143 0 Many (about 50%) 119 0 Few (about 5~~-i 1410 Mc.st (about 75%) 141 0 Few (at:tout 5%) 144 LJ Most (ab o ut 75%) 11 a. Were springs or well water disturbed? b . Were ri\lers or lakes changed? 145 0 Level changed 147 0 Muddied 1480 Yes 0 No 145 0 Flow disturbed 0 Don't know [ J Don't know 12a. Was there earth noise? 0 No 149 0 Faint 150 0 Moderate 151 0 Loud b. Direction of noise 1510 North 113 0 South 154 0 East 155 0 West c. Estimated duration of shaking 156 0 Sudden, sharp 1110 Long (less than 10 sees) (30-60 sees) 1580 Short (10-30 sees) 1590 Other 13. What is the approximate population of your city/town? 160 0 Less than 1,000 161 0 10,000 to 100,000 161 0 1,000 to 10.000 163 0 Over 100,000 Or are you in a 1~ 0 Rural area? This community report is associated with what town or zip code? Thank you for your time and information. Refold this card and tape for return mail. APPENDIX Ill: Chronological Listing of Earthquakes for the State of Georgia (from Stover and others, 1978) DATE YEAR MONTH DAY 1826 1872 1875 1875 1884 1885 1903 1909 1912 1912 1913 1914 1928 1933 1943 1958 1963 1964 1964 1964 1965 1965 1965 1965 1969 1969 1969 1971 1973 1974 1976 OCT 15 JUN 17 JUL 28 NOV 02 MAR 31 OCT 17 JAN 24 OCT 08 JUN 20 OCT 23 MAR 13 MAR 05 MAY 23 JUN 09 JUL 29 APR 08 OCT 08 FEB 18 MAR 07 MAR 13 APR 07 JUL 22 NOV 08 NOV 08 MAY 05 NOV 04 NOV 08 APR 16 OCT 08 AUG 02 DEC 27 ORIGIN TIME(UTC) H Ms 20 00 23 05 02 55 . . 10 00 22 30 01 15 10 00 01 15 .. 05 . . 20 05 ' . 10 15 11 30 . . 04 30 17 . . 06 01 43.4 09 32 11 .6 18 03 00.1 01 20 18.1 07 41 10.2 23 55 33.3 12 58 01.0 13 04 11 .5 17 14 . . 18 58 23 01 52 07 31 .. 13 38 .. 08 52 09.8 06 57 13.9 LAT. ( N. ) 32.0 33.1 33.1 33.8 33.1 33.0 32.1 34.9 32.0 32.7 34.5 33.5 30.8 33.3 33.4 31.5 3 3 .9 34.8 33.9 33.2 33.9 33.2 33.2 33 .2 33.9 33.2 3 3 .9 33.9 33.9 33.87 32.22 ( w .) 81.1 83 .3 83.3 82 .5 83 .3 83.0 81.1 85 .0 81.0 83.5 85 .0 83.5 83.3 83.5 82 .0 83.5 82 .5 85.5 82.5 83.4 82.5 83 .2 83.2 83 .2 82 .5 83.2 82 .5 82.5 82.5 82.49 82.46 *INST- instrumentally recorded unfelt event. INTENSITY MM v Ill VI Ill IV VI v v IV IV VI Ill IV Ill Ill INST* v INST* v INST INST INST INST INST INST INST INST INST VI v Savannah Central Georgia Central Georgia Clark Hill Reservoir Central Georgia Central Georgia Savannah Northwest Georgia Savannah Central Georgia Northwest Georgia Central Georgia Coastal Plain Central Georgia Augusta Coastal Plain Clark Hill Reservoir Northwest Georgia Clark Hill Reservoir Central Georgia Clark Hill Reservoir Central Georgia Central Georgia Central Georgia Clark Hill Reservoir Central Georgia Clark Hill Reservoir Clark Hill Reservoir Clark Hill Reservoir Clark Hill Reservoir Coastal Plain 210 HYDROCHEMISTRY OF FORMATION FLUIDS IN ONSHORE AND OFFSHORE STRATA IN THE SOUTHEAST GEORGIA EMBAYMENT F.T. Manheim and C.K . Pauli U.S. Geological Survey Woods Hole, Massachusetts ABSTRACT Investigations of formation-fluid salinites in a transect from western Georgia to the edge of the Blake Plateau off the Georgia coast show surprisingly similar hydrochemical features offshore and onshore. A fresh brackish wedge of ground water (<25 g/kg total dissolved solids) lies beneath the shelf to a depth of about 900 m. On land, brackish waters extend to a maximum depth of about 1.2 km below sea level in Lowndes County, Georgia . In deeper horizons, hypersaline brines ~?100 g/kg) occur in Lower Cretaceous (?) strata. These strata have a pronounced evaporitic (anhydritic) character in the offshore segment. Strong salinity gradients in interstitial waters signify buried evaporite deposits at drill sites beneath the Blake Plateau. INTRODUCTION Until relatively recently, hydrologists in the United States focused their main attention upon shallow ground waters containing potable and nearpotable waters. Petroleum exploration firms generally limited their interests in fluid-bearing strata to prospective petroleum-producing (reservoir) beds. Consequently, data on regional hydrochemical patterns, migration paths, permeability, and sources of chemical constituents in sedimentary basins as a whole were rare. Increased interest in use of subsurface strata as waste-disposal sites, in problems involving seawater encroachment, and in extension of the concept of "freshwater resources" to water containing as much as 10 g/kg total dissolved solids (see Kohout, this volume) has increased interest in the composition and distribution of deeper saline fluids in sedimentary strata. Manheim and Horn (1968) summarized subsurface hydrochemical data from a shoreline transect extending from Long Island to the Florida Keys and provided a map of inferred salinity distributions from surface to igneous-metamorphic basement. The southeastern Atlantic region from South Carolina to northern Florida was shown to be the site of some of the most complex hydrochemical features along the U.S. Atlantic seaboard. The deepest freshwater horizons extended more than 1000 m below sea level in the region of South Carolina north of Parris Island. In contrast, in southernmost Geor- gia, hypersaline brines (i.e., brines containing at least 50-100 g/kg total solids) were found at depths below 700 m. The distribution of such hypersaline brines, which approached saturation in NaCI beneath much of Florida, was linked to the distribution of evaporitic strata (Manheim and Horn, 1968). Evaporitic facies mainly characterized by dolomiteanhydrite occur in Paleocene rocks in Florida and southern Georgia (Chen, 1965), and thicker strata including some salt are found in Lower Cretaceous and Jurassic evaporites in Florida on land. Salt also occurs in diapirs offshore north of the Blake Plateau (Grow and others, 1977) and has been sampled during oil drilling in the Baltimore Canyon Trough (Oil and Gas Journal, 1978). Recently, Brown and others, (1979) evaluated in detai I the deep-well waste-storage potential of Mesozoic aquifers in Georgia and South Carolina. They estimated formation-fluid salinity ranges by using electrical-log analysis and water analyses. We have taken advantage of these data, have further analyzed some of the electrical logs, and have incorporated hydrochemical data from available offshore drill holes and other onshore drill holes to prepare cross sections of subsurface "salinity" (total dissolved-salt content). Table 1 contains a list of sites utilized; figure 1 shows their location in Georgia and Florida and offshore regions. Our purpose is not to determine fine-scale chemical variations, but to discern broad regional trends that might shed light on fluid history, fluid migration, and fundamental geochemical and hydrochemical processes. METHODS All available sources of information on the composition of formation fluids in the study area (fig. 1) were utilized. The most accurate data are potentially those obtained from analysis of (1) drill stem tests or other (for example, reverse flush) fluid tests on permeable strata in land wells (no oil-producing wells exists in the study area), and (2) pore water extracted (by "squeezing") from cores of unconsolidated and partly consolidated rocks at depths as great as 300 m beneath the sea floor (Manheim, 1967; Manheim and Horn, 1968; Kohout, and others, 1979). The validity of drill-stem test data is normally governed by the care used in sampling fluids to minimize the 211 TABLE 1. List of boreholes utilized in study. Borehole No. LOW-1 LOW-2 LOW-3 CAL-1 D0-1 Ml-1 COL-1 EC-2 EC-5 SCR-1 SAV JAX ST.M . COLIS. SUN GE-1 T Borehole Name Hunt Petroleum, J.T. Stalvey, No. 1, Lowndes County, Georgia Hunt Petroleum, Langsdale No. 1, Lowndes County, Georgia Hunt Petroleum, E.N . Murray No. 1, Lowndes County, Georgia Sowega Mineral, J.W. West No.1, Calhoun County, Georgia J.R. Sealy, Reynolds No.1, Dougherty County, Georgia Stanolind, J.H. Pullen No. 1, Mitchell County, Georgia R.T. Adams, D.G. Arrington No. 1, Colquitt County, Georgia Hunt Petroleum, Superior Pine Co., 2, Echols County, Georgia Humble Oil, Bennett and Langsdale No. 1, Echols County, Georgia Boenwell Drilling Co., McGain-Pryor No. 1, Screven County, Georgia Savannah Port Authority, Chatham County, Georgia Jacksonville Test Wells, USGS (U.S. Geological Survey), Duval County,Fiorida St. Mary A. Oil/Hilliard Turpentine Co., Nassau County, Florida Colonel's Island Test Well, USGS, Glynn County, Georgia Sun Oil Co., Powell, Volusia County, Florida COST (Continental Offshore Statigraphic Test) Well GE-1 Tenneco wildcat well 1 ,2,3, 4,5,6 6002 6004 JOIDES (Joint Oceanographic Institutions Deep Earth Sampling Project) sites AMCOR (Atlantic Margin Coring Project) Site 6002 AMCOR Site 6004 *Reinterpreted in part from original logs. Source of Information Brown, et. al., 1979* Do . Do. Do . Do. Do . Do. Do. Do. Do. Do. Leve, 1961, cited in Manheim and Horn, 1968 Manheim and Horn, 1968 H. Gill, USGS, 1979, written commun . Manheim and "Horn, 1968 Scholle, 1979 L. Poppe, 1979, written commun. A. Johnston, USGS, Reston, 1979, oral commun. Manheim, 1967; Manheim and Horn, 1968 Kohout, et al., 1979 Do. 212 influence of drilling mud. A minimum criterion for "good" water-test samples is that chloride analyses on sequential fluid samples (either flowing water or successive pipe stands) reach a constant value asymptotically. The squeezing-and-analysis methodology was used extensively in Deep Sea Drilling Project studies (Manheim and Sayles, 1974; Manheim, unpub. data) . Wireline samples from which external drill-fluid-contaminated zones had been removed were squeezed through filter paper in a stainless steel press, and recovered fluids were analyzed by microchemical techniques. Data are available from all JOIDES (Joint Oceanographic Institutions Deep Earth Sampling) and AMCOR (Atlantic Margin Coring Project) sites (see fig . 1 and table 1). Much less accurate but indispensable is the technique of estimating fluid resistivity and, through it, formation-fluid "salinity" by quantitative electricallog interpretation. For the older drill holes on the continent, the only available method is simple estimation from the spontaneous potential (SP) log, as has been done by Manheim and Horn (1968) and Brown and others (1979). The technique is described in standard logging references (Schlumberger Well Surveying Corp., 1978, and documents cited therein). 1) SSP= -K log Rmf/Rwe where K is a constant dependent on temperature, Rmf is resistivity of mud filtrate, Awe is the apparent resistivity of formation fluid, SSP is the static spontaneous potential (SP). The SSP is derived from the departure in millivolt scale units from shale baseline of the SP curve: it is corrected where possible for the effects of thin beds, mud resistivity, and fluid invasion of the formation. "Salinity" or the salinity of NaCI solutions having resistivities corresponding to observed resistivities can then be calculated from Rwe We have used the Arps-Hamilton log analysis slide rule or calculator program (for example, Schoonover and Fertl, 1979). The SP data are particularly useful in deposits containing fresh and brackish strata, as the errors (to 50%) are still smaller than the salinity variations; the salinity values vary by four orders of magnitude. In our study areas, the simpler SP methods give rise to serious errors for the deeper strata containing saline water and especially for those containing significant proportions of clay colloids. Further, these methods are not applicable in carbonate strata. For the GE-1 well (Scholle, 1979) that reached 4004 m depth below sea floor about 140 km seaward of Jacksonville, Fla., we used alternative methods utilizing deep induction (resistivity) and porosity logs. The basis for such calculations is as follows: Figure 1. Location of drill holes utilized in study. Abbreviations as shown in table 1. Bathymetric contour interval, 200 m. 2) F = Rt/Rw and F a0m 3) where F is the "formation factor", Rt is the true formation resistivity, Rw is the true formation-fluid resistivity, 0 is porosity, and a and m are constant for given types of strata . 213 An empirical formula (SchlumbergerWell Surveying Corp., 1978) for formation-fluid resistivity, given 100% water saturation, can be expressed as: 4) where V sh is volume percent of shale within the formation, Rsh is the "shale" resistivity. The V sh and Rsh figures can be obtained from induction-, neutron-, and formation-density logs by cross-plot techniques described in Schlumberger Well Surveying Corp. (1978) and references cited therein. We were most fortunate to have both porosity and interstitial-salinity values as well as resistivity values for AMCOR sites 6002 and 6004. These permitted direct calibration of equations 2 and 3 for Tertiary strata penetrated by nearby GE-1. For deeper strata, the estimations are based upon cross-plot data and equation 4. The "salinity" data (NaCI-equivalents) are subject to variation in ionic ratios. These variations are not discussed here, but the error they introduce is less than the uncertainty attributable to log analysis. STRATIGRAPHIC NOTES The geologic cross sections upon which the salinity data are superimposed (figs. 2 and 3) are based on available literature (Toulmin, 1955; Herrick and Vorhis, 1963; Chen, 1965; Applin and Applin, 1967; Maher, 1971; Cramer, 1974; Hathaway and others, 1979, Schlee, 1977; Dillon and others, 1979; and Scholle, 1979). We divide the stratigraphy of southern Georgia into the units: Neogene to Holocene, Eocene and Oligocene, Paleocene, Upper Cretaceous, Lower Cretaceous(?), and igneous and metamorphic basement. The Neogene to Holocene unit is composed of predominantly clastic sediments, and the carbonate content increases to the southeast. The Eocene and Oligocene unit is dominantly carbonate except f_or some of the lower Eocene in southwestern Georgia, where clastic deposits dominate. The Upper Cretaceous and Paleocene units are generalized as marl but carbonate (chalk) and evaporitic facies (anhydrite-dolomite) increase southeastward in the Paleocene. The lithologies of the Lower Cretace- ous(?) are different in the Southeast and Southwest Georgia Embayments; the basins are probably connected only by a thin basal sand. The unit thickens to more than 800 m of unfossiliferous, immature sandstone in southwest Georgia, where its age is questionable (Gohn and others, this volume) but has historically been labelled as Early Cretaceous. The Southeast Georgia Embayment contains a thick and variable sequence of Lower Cretaceous limestone, anhydrite, sandstone, and shale over metamorphic basement (Scholle, 1979). Paleoenvironment Except for anhydrite in the Paleocene and traces of evaporite units in the lower Eocene, all sediments of the Upper Cretaceous and Cenozoic were deposited in normal marine conditions. The lack of distinguishing fossils in the Lower Cretaceous(?) of the Southwest Georgia Embayment renders paleoenvironment identification difficult. In the offshore Southeast Georgia Embayment, the presence of evaporitic strata including anhydrite probably indicates hypersaline deposition. DISTRIBUTION OF FORMATION SALINITY The main salinity features are delineated in figures 2 and 3 by lines of equal "salinity" (total dissolved-salt content), or isosalines, at concentrations of 1, 5, 25, 50, 100, and 200 g/kg. These values may be converted to parts per million by multiplying by 1000. The isosalines are superimposed on transects A-A' and B-B', whose locations are shown in figure 1. In interpreting the diagrams, we note that the isosalines may be subject to error, particularly where salinity distributions are complex, as between Echols and Mitchell Counties, Ga., (between Ml-1 and EC-2, figs. 1, 3), or in the deeper parts of the offshore basin. There may be inliers of different salinity and complex microstructure that cannot be depicted at the scale used here. Moreover, the salinities derived from the SP log in the deeper Southeast Georgia Embayment may be in error because of clay coloids and may understate true formation-fluid salinity. Major relationships are evident from figures 2 and 3: 1. Strata on land have been infiltrated by meteoric (fresh) water (identified as water having a salinity less than 1 g/kg) to depths between 350m in Colquitt County, Georgia (COL-1 ), to more than 500 m near Jacksonville, Fla. (JAX, fig. 3). Farther northward near the South Carolina-Georgia border 214 A I 0: (.) ~ (/) (/) N A' Q SEA lEVEl .......... (/) 0:: w 1w - ~ 0 2:::::! ~ I 1- 0w.... 0 3 4 Figure 2. Salinity-stratigraphy transect, A-A' (see fig. 1 for location). Contours are in total dissolved solids, g/kg. Abbreviations for wells are as in figure 1. Stratigraphic intervals are NEO-HOL, Neogene to Holocene; EO-OLIGO, Oligocene and Eocene; Paleo, Paleocene; UK, Upper Cretaceous; LK?, units historically referred to as being Lower Cretaceous; Dev. met, Devonian metamorphic rocks; Paleoz met, Paleozoic metasedimentary rocks. Partial limestone symbols in Upper Cretaceous and Paleocene strata represent marls and some evaporitic (anhydritic) sediments. The Neogene to Holocene section is variable but is predominantly of clastic lithology. B I() w I I I c 0 N w I g31: I g31: I() I (w.) .., N I (.) w :::E 1-' (/) X <[ 0: 0 J: (/) .., 100 g/kg) are limited to Lower Cretaceous(?) strata on land. A lens of such fluids appears to extend from Calhoun County to Lowndes County, Georgia, in transect B-B' (CAL-1 to LOW-5, fig. 3); below the lens is less saline water. However, at these depths, the SP values yield only semiquantitative "salinity" values, because the "greasy" (high-claycontent) nature of the varicolored micaceous sands reported in this zone affects the well logs. 6. Offshore, in the GE-1 well, very high salinities are identified in Lower Cretaceous strata, consistent with the presence of evaporitic rocks (anhydrite). Toward the base of the section, low permeabilities render interpretation of salinities more difficult and less reliable, even with the aid of the porosity methods. 7. Previous results (Manheim and Horn, 1968) have shown that along the Atlantic margin, salinity commonly decreases just above the basement. In this study, we have detected indication of this phenomenon at sites in eastern Georgia and possibly in COST GE1. On the other hand, indications in some sites (LOW-1 and St. Mary Hilliard, transect B-B') are that salinity levels of 50-100 g/kg continue into crystalline or metamorphosed sediments. DISCUSSION The existence of fresh water beneath the Atlantic Ocean, to the edge of the continental Shelf, has been well documented in the JOIDES and AMCOR drill holes (Manheim, 1967; Kohout and others, 1979; Kohout, this volume; Pauli and Dillon, this volume). New electrical-logging data from the COST GE-1 well confirm the existence of brackish water in the upper 900 m. Moreover, a special agreement by Tenneco Oil Co. permitted the U.S. Geological Survey to run a dri 11-stem test at about 350-m depth in a wildcat well (T, fig. 1) about 85 km seaward of Jacksonville, Fla . A drill-stem test confirmed presence of brackish water having less than half the salinity of seawater (R. Johnston, 1979, oral commun.). The distribution of lithologic and stratigraphic units and formation-fluid salinities shows clearly that few if any "paleosalinities" remain in deeper subsurface porous strata. All surficial strata have had the original seawater solutions permeating the interconnected pores of marine sediments flushed out by fresher waters of meteoric origin. We suggest that in the depth range where freshwater influence diminishes sharply, the dominant salting influence is frequently not seawater, but brine of hypersaline origin. These brines once originated from seawater, but they have been modified by processes involving secondary interactions with solid phases (Braitsch, 1971; Carpenter, 1978) during deposition and burial. Evaporite brines may permeate not only contemporaneous sediments, but also underlying strata to depths of several kilometers or more (e.g., see Manheim and Schug, 1978). Another source of salt is inclusions and microlayers of rock salt that are associated with original anhydritic rocks and are later dissolved, thereby contributing to total formation-fluid salinity. Not infrequently, anhydritic strata are characterized by brine concentrations approaching saturation with respect to NaCI, even though no appreciable salt bodies occur in the strata. In previous papers, the senior author and his coworkers (Manheim and Sayles, 1974; Manheim and Hall, 1976) pointed out that significant increases in salinity of interstitial waters as depth increases in oceanic strata nearly always point to 216 presence of evaporitic strata at depth. As the present data indicate, "evaporitic strata" need not mean massive halite. Anhydritic rocks also incorporate a sufficient reservoir of brine salt to permit upward diffusion to influence fluids of overlying strata during geologic time. The new information can be applied to the interstitial salinity gradients shown in figures 2 and 3 to infer the extension of Lower Cretaceous(?) evaporitic facies to JOIDES holes 4, 5, and 6 (fig . 1, 3). Pre-existing hypersaline concentrations (if any) in earlier, deeper strata would have merged with the saline-water concentrations contributed by the latest evaporitic sequence and would probably not be discernible in the study area. A further inference may be drawn from the saline gradients. If inorganic ions (Na, Cl, etc.) can move upward through the strata in response to concentration gradients, then light hydrocarbons dissolved or otherwise entrained in pore fluids may likewise be able to migrate through the strata. More detailed delineation of saline gradients may help map zones of diffusive or other permeability and "calibrate" surficial hydrocarbon anomalies. REFERENCES Applin , P.L., and Applin, E.R., 1967, The Gulf Series in the subsurface in northern Florida and southern Georgia: U.S. Geol. Survey Prof. Paper 524-G, 24 p. Braitsch, 0., 1971, Salt deposits: Their origin and composition : New York-Heidelberg, Springer-Verlag, 297 p. Brown, P.M., Brown, D.C., Reid, M.S., and Lloyd, O.B., 1979, Evaluation of deep-well, wastestorage potential of Mesozoic aquifers in the southern part of the Atlantic Coastal Plain, Georgia and South Carolina : U.S. Geol. Survey Prof. Paper, in press. Carpenter, A.B., 1978, Origin and chemical evolution of brines in sedimentary basins: Oklahoma Geol. Survey Circ. 79, in press. Chen, C .S., 1965, The regional lithostratigraphic analysis of Paleocene and Eocene rocks of Florida: Florida Geol. Survey Bull . 45, 105 p. Cramer, H.R., 1974, Isopach and lithofacies analysis of the Cretaceous and Cenozoic rocks of the Coastal Plain of Georgia, in Stafford, L.P., ed., Petroleum geology of the Georgia Coastal Plain Symposium: Georgia Geol. Survey Bull. 87 , p. 21-43. Dillon, W .P., Pauli, C.K., Buttler, R.T., and Fail, J.P., 1979, Structure and development of the Southeast Georgia Embayment and northern Blake Plateau, Preliminary Analysis, in Watkins, J.S ., Montadert, L., and Dickerson, P.W., eds., Geological and geophysical investigations of continental margins: Am. Assoc . Petroleum Geol. Mem . 29 , p. 27-41. Gohn, G.S., Christopher, A.A., Smith, C.C., and Owens, J.P., 1978, Preliminary stratigraphic cross sections of Atlantic Coastal Plain sediments of the southeastern United States, Pt. A, Cretaceous sediments along the South Carolina coastal margin: U.S . Geol. Survey Misc. Field Studies Map, MF-1015-A. Grow, J.A., Dillon, W.P., and Sheridan, R.F., 1977, Diapirs along the Continental Slope off Cape Hatteras (abs.): Soc. Explor. Geologists, 47th Ann.lnternatl. Mtg., Calgary, Alberta Program, p. 51. Hathaway, J.C., Poag, C.W., Valentine, P.C., Miller, R.E., Schultz, D.M., Manheim, F.T., Kohout, F.A ., Bothner, M.H ., and Sangrey, D.A ., 1979, U.S. Geological Survey core drilling on the U.S. Atlantic Shelf: Science, v. 206 , no. 4418, p. 515-527 . Herrick, S.M., and Vorhis, R.C., 1963, Subsurface geology of the Georgia Coastal Plain: Georgia Geol. Survey Int. Circ. 25 , 78 p. Kohout, F.A., Manheim, F.T., and Bothner, M.H., 1979, Hydrology and water chemistry, in Hathaway and others, U.S. Geological Survey core drilling on the U.S. Atlantic Shelf: Science, v. 206, no. 4418, p. 515-527 . Lefond , Stanley J. , 1969, Handbook of world salt resources: New York, Plenum Press, p. 82-84. Maher, J .C., 1971 , Geological framework and petroleum potential of the Atlantic Coastal Plain and Continental Shelf: U.S. Geol. Survey Prof. Paper 659, 98 p., 17 pis. Manheim, F.T. , 1967, Evidence for submarine discharge of water on the Atlantic Continental Slope of the southern United States, and suggestions for further search: New York Acad. Sci . Trans., Sec. II, v. 29, p. 839-853 . Manheim , F.T., and Bischoff, J.L., 1969, Geochemistry of pore waters from Shell Oil Co. drill holes on the Continental Slope of the northern Gulf of Mexico: Chem. Geology, v. 4, p. 63-82 . Manheim, F.T ., and Hall, R.E., 1976, Deep evaporitic strata off New York and New Jersey: Evidence from interstitial water chemistry of drill cores: U .S. Geol. Survey Jour. Research, v. 4, p. 697-702. Manheim, F.T., and Horn, M.K., 1968, Composition of deeper subsurface waters along the Atlantic continental margin: Southeastern Geology, v. 9, p. 215-236 . Manheim, F.T., and Sayles, F.L., 1974, Composition and origin of interstitial waters of marine sediments, based on deep sea drill cores, in Goldberg, E.P., ed ., The seas, v. 5, ch . 16, p. 527-567. 217 Manheim, F.T., and Schug, D.M., 1978, Interstitial waters of Black Sea cores: California Univ., Scripps lnst. Oceanography, LaJolla, 1978, Initial reports of the Deep Sea Drilling Project, Volume XLII ***: Natl. Sci. Foundation, Washington, D.C., p. 637-651 . Oil and Gas Journal, 1978, HO&M plugs third Baltimore Canyon dry hole: Oil and Gas Journal, v. 76, no. 38, p. 72 . Schlee, J.S., 1977, Stratigraphy and Tertiary development of the continental margin east of Florida: U.S. Geol. Survey Prof. Paper 581-F, 25 p. SchlumbergerWell Surveying Corp., 1979, Houston Office, oral communication. Schlumberger Well Surverying Corp., 1978, Log Interpretation Charts: Schlumberger Well Surveying Corp., Houston, Tex., 82 p. Scholle, P.A., 1979, Geological studies of the COST G.E.-1 Well, United States South Atlantic Outer Continental Shelf area: U.S. Geol. Survey Circ. 800, 114 p. Schoonover, L.G., and Fertl, W.H., 1979, How to find temperature, Rw and salinity with hand calcu- lators: Oil and Gas Journal, v. 77, p. 109-111. Toulmin, L.D., 1955, Cenozoic geology of Georgia: Am. Assoc. Petrolem Geol. Bull., v. 39, p. 207-235. 218 THE EFFECTS OF CRETACEOUS AND YOUNGER FAULTING ON COASTAL PLAIN ROCKS AROUND AUGUSTA, GEORGIA David C. Prowell U.S. Geological Survey Reston, Virginia Mr. Prowell's article, based on his symposium presentation and the following abstract, was published in GEOLOGY, 1978, val. 6, p. 681-684. ABSTRACT The Belair fault zone is a series of northeast-trending en echelon reverse faults that cut the inner margin of the Atlantic Coastal Plain near Augusta, Georgia. The effects of the faulting during the early Cenozoic have been evaluated by recent geologic mapping of the Coastal Plain strata. The mapping has shown that the fault zone is at least 15 miles (24 km) long. The Coastal Plain sediments around Augusta traditionally have been mapped as a basal fluvial unit (Tuscaloosa Formation) unconformably overlain by a sequence of nearshore marine sediments (Barnwell Formation). The Tuscaloosa consists of two distinguishable sedimentary sequences as far south of Augusta as Wrens, Georgia. The lower unit is characterized by a coarse basal gravel and an unusual heavymineral fraction, whereas the upper unit is characterized by thick kaolin deposits of commercial value. Palynological analysis indicates that the lower unit is Late Cretaceous (Santonian) in age, whereas the upper unit is middle Eocene (Claibornian) . Both units are considerably younger than the Tuscaloosa Formation (Cenomanian) of western Georgia and Alabama. The Barnwell Formation unconformably overlies the Tuscaloosa in the Augusta area; it can be divided into three mappable units, which are, from oldest to youngest: (1) Twiggs Clay Member; (2) Irwinton Sand Member; and (3) an unnamed sand member. The Twiggs Clay Member is a carbonaceous clay that has been dated by palynology as late Eocene (Jacksonian). At Augusta, the Twiggs is a barrier lagoon deposit, but farther downdip it has offshore characteristics. The Irwinton Sand Member is a fine quartz sand that is the nearshore facies equivalent of the Twiggs . Transgression of the late Eocene sea has resulted in part of the Irwinton being conformably deposited on the Twiggs. Unconformably overlying the Irwinton is an unnamed medium to coarse marine sand whose base is marked by a thin layer of ovoid pebbles. Shark teeth in this unit date it as Oligocene to early Miocene. Movement along the Belfair fault zone has caused about 100 feet (30 m) of vertical offset of the base of the Tuscaloosa Formation since initial deposition. The basal unconformity of the Barnwell Formation has been displaced about 40 feet (12m) since its formation in the late Eocene. The succession of fault movements along the Belair zone from the Late Cretaceous until the early Tertiary affected both the thickness and distribution of the Coastal Plain strata in the Augusta ,region. For example, the absence of certain sedimentary units on the upthrown block of the fault zone is probably the result of uplift and erosion . Such evidence indicates that faulting elsewhere in the Atlantic Coastal Plain has influenced the configuration of sedimentary strata and should receive attention by regional mappers. 219