I . I SEDIMENTARY ENVIRONMENTS IN THE PALEOZOIC ROCKS OF NORTHWEST GEORGIA Compiled by T.M. CHOWNS Guidebook 11 Published by the Geological Survey for the 7th Annual Field Trip of the Georgia Geological Society GEORGIA DEPARTMENT OF NATURAL RESOURCES Joe D. Tanner, Commissioner DIVISON OF EARTH AND WATER RESOURCES THE GEOLOGICAL SURVEY OF GEORGIA Sam M. Pickering, Jr.. State Geologist ATLANTA 1972 GEORGIA GEOLOGICAL SOCIETY President Secretary Treasurer Councilor Councilor Officers 1971-72 Robert C. Vorhis J. Marion Wampler Joseph B. Murray J. Marion Wampler Howard R. Cramer President President-Elect Secretary Treasurer Councilor Councilor Officers 1972-73 Robert E. Carver J. Marion Wampler Roger Austin Joseph B. Murray Howard R. Cramer Norman Herz 1972 Field Trip Committee Howard R. Cramer, Chairman T. M. Chowns Lewis Lipps William H. McLemore Doral Mills Joseph B. Murray Robert C. Vorhis ii CONTENTS Introduction 1 Depositional Environments in the Upper Ordovician of Northwest Georgia and Southeast Tennessee, T. M. Chowns . . . . . . . . . . . . 3 Molasse Sedimentation in the Silurian Rocks of Northwest Georgia, T. M. Chowns . . . . . . . . . . . . . . . . . . . . . . . 13 Trace Fossils from the Ringgold Road Cut (Ordovician and Silurian), Georgia, Robert W. Frey and T. M. Chowns . . . . . . . . . . . . . . . 25 Sedimentary Environment of the Armuchee Chert, Northwest Georgia, W. E. Nunan . . . . . . . . . . . . . . . . . . . . . . . . . 57 Depositional Environments of the Tuscumbia-Monteagle-Floyd Interval in Northwest Georgia and Southeast Tennessee, William H. McLemore 69 Road Log. 75 Appendix . 95 iii Acknowledgments This guidebook was prepared for the 7th annual field trip of the Georgia Geological Society, 13-14 October, 1972. The Georgia Geological Society gratefully acknowledges the assistance of the Georgia Geological Survey, under the direction of S.M. Pickering, Jr., in the publication of the guidebook. Special thanks are due to Lynda Stafford who edited the manuscripts for publication, and to Ron Owens who assisted in draughting the figures. J. E. Elkins prepared the photographs for the road log. We also thank the Southern Railway System for generously underwriting the Friday evening smoker. iv ~'~/O m. Forn yo Introduction T. M. Chowns This guidebook is intended as a summary and progress report of recent research into some of the sedimentary environments encountered in the Paleozoic succession in northwest Georgia. It does not attempt an overall review of the stratigraphy of the area for which the reader is referred to the work of Butts and Gildersleeve (1948), Cressler (1963; 1964a; 1964b; 1970) and Croft (1964). In particular attention is directed towards strata of Upper Ordovician, Silurian, Devonian and Mississippian age. Studies have been mainly restricted to the northwestern part of the Valley and Ridge Province because of poor exposure southeast of the Rome Fault, and as a result details of the lowermost strata (Cambrian, Rome and Conasauga formations; Cambro-Ordovician, Knox Group) are still poorly known. Studies have commenced on the Middle Ordovician, Chickamauga Limestone in Alabama (Wilson, 1971 ), but the various facies of the equivalent Taconic mollasse wedge to the east remain to be unraveled. For completeness the reader should consult Ferro, et al. (1972) for a stimulating reappraisal of Mississippian and Pennsylvania sedimentary environments. Table 1 summarizes the stratigraphy in the northwest part of the Valley and Ridge Province and indicates the major sedimentary environments. The stratigraphic column reflects the shifting interface between cratonic carbonate sequences and terrigenous molasse facies derived from the Appalachian orogen. An appreciation of the importance of transitional tidal flat, tidal marsh, barrier island and lagoonal environments in these miogeoclinal sediments is one of the main results of recent research into ancient sedimentary environments. References Butts, C., Gildersleeve, B., 1948, Geology and mineral resources of the Paleozoic area in northwest Georgia: Georgia Geol. Surv. Bull. 54, 176 p. Cressler, C. W., 1963, Geology and ground-water resources of Catoosa County, Georgia: Georgia Geol. Surv. Inf. Circ. 28, 19 p. Cressler, C. W., 1964a, Geology and ground-water resources of the Paleozoic rock area, Chattooga County, Georgia: Georgia Geol. Surv. Inf. Circ. 27, 14 p. Cressler, C. W., 1964b, Geology and ground-water resources of Walker County, Georgia: Georgia Geol. Surv. Inf. Circ. 29, 95 p. Croft, M.G. 1964, Geology and ground-water resources of Dade County, Georgia: Georgia Geol. Surv. Inf. Circ. 26, 17 p. Ferm, J. C., Milici, R. C., Eason, J. E., 1972, Carboniferous depositional environments in the Cumberland Plateau of southern Tennessee and northern Alabama: Tennessee Div. Geol. Report of Investigations 33, 32 p. Wilson, A. 0., 1971, Petrology of parts of the Black River age strata of the Chickamauga Limestone, Etowah and DeKalb Counties, Alabama: in Drahovzal, J. A., and Neathery, T. L., eds., The Middle and Upper Ordovician of the Alabama Appalachians : Alabama Geol. Soc. Guidebook, p. 79-100. 1 System Pennsylvanian Mississippian Devonian Silurian Ordovician CambroOrdovician Cambrian Stratigraphic units Crab Orchard Mountains Fm. 1,100+' Gizzard Fm. 200-350' Pennington Fm. Bangor Limestone Hartselle Fm. Monteagle Limestone Tuscumbia Limestone Fort Payne Fm. 1000-2,700' Chattanooga Shale 10-25' Red Mountain Fm. 150-1,300' Sequatchie Fm. 150-250' Chickamauga Ls . 1,400-2,300' Knox Group 3,500-3,650' Weisner, Shady, Rome, and Conasauga fms. Sedimentary Environments Humid, terrigenous, tidal marsh, lagoon and barrier island sediments (Molasse facies) Shallow marine shelf and semi-arid tidal flat carbonates with pro-molasse terrigenous wedges. Arid tidal flat, carbonate deposits passing upwards into shallow marine shelf carbonate sediments. Terrigenous promolasse sediments to the south and east. Terrigenous, shallow, euxinic sea. Terrigenous, promolasse slope and platform deposits with barrier islands and lagoons. Shallow marine carbonates passing eastwards into terrigenous tidal flat and alluvial deposits (molasse facies). Shallow marine and tidal flat carbonates. Table 1. Stratigraphy of the Valley and Ridge Province 2 DEPOSITIONAL ENVIRONMENTS IN THE UPPER ORDOVICIAN OF NORTHWEST GEORGIA AND SOUTHEAST TENNESSEE T. M. Chowns University of Georgia Introduction In northwest Georgia the Sequatchie Formation (Upper Ordovician) is a group of gray and red calcareous sandstones, siltstones and shales with subordinate limestones, which occupies the interval between the Chickamauga Limestone (Middle Ordovician) and the Red Mountain Formation (Lower Silurian) (Butts and Gildersleeve, 1948, p. 33; Milici and Smith 1 969, p. 25). The unit is between 150 and 200 feet thick and has always been mapped as part of the Red Mountain Formation although it could probably be mapped separately, as it is in the Valley and Ridge of Tennessee (Fig. 1 ). When traced northwestwards across the Vall y and Ridge Province into Sequatchie Valley the formation undergoes a spectacular facies change (see for example Thompson, 1970a, 1971 , p. 71; Drahovzal and Neathery , 1971, p. 18-25, 50-52). While red sandstones and siltst ones predominate in t he east (Rocky Face Mountain, Whiteoak Mountain), gray sandy and silty limestones predominate in the west (Lookout Valley , Southern Sequatchie Valley), and the Sequatchie Formation passes into the Inman, Leipers and Shellmound 1 formations (Fig. 2). The red-bed facies which is the most distinctive feature of the Sequatchie Formation, is highly diachronous. In the northern part of Sequatchie Valley red beds are restricted to the Shellmound Formation, while in the southern part they occur in th e Inman F ormation (Milici and Wedow, in preparation). SimHarly, in Lookout Valley red beds occur only in the lower part of the Sequatchie (Inman Member), but to the east they are foun d at higher an d higher horizons as first Leipers, and then Shellmo und members pass into red beds . The Sequatchie Formation is in fact merely the youngest of a series of red-bed t ongues (near-shore facies), which prograded westwards across the southern Appalachian basin into gray marine limestones during Middle and Late Ordovician times (Allen and Lester, 1957). Age of the Sequatchie Formation Ulrich (1 914 p. 614) first proposed the name Sequatchie for dep sits of Richmond age in t he Southern Appalachians, apparently having in mind a type section at Inman Tennessee2 (Ulrich, 1914, p . 612, 647; Burchard, 1913 , p. 39). However, it is clearly erroneous to prescribe rigid time boundaries to a diachronous red-bed sequence, and it now seems probable that rocks which have been mapped as Sequatchie in Tennessee and Georgia include strata of Edenian, Maysvillian and Richmondian age. The contacts of the formation, both with the Catheys Formation below and the Red Mountain Formation above, appear to be completely conformable so that there is pro bably an unbroken succession of sediments ranging in age from Middle Ordovician to Early Silurian in the study area. 1 The Shellmound is a new formational name proposed by Milici and Wedow (in preparation) for gray calcareous shales and siltstones, with limestones which occur between the top of the Leipers Limestone and the base of the Red Mountain Formation in southeast Tennessee. 2 Ulrich never published a formal description of the Sequatchie Formation and thus his intentions must be interpreted from some oblique references made in a paper read before the 12th International Geological Congress in 1913 (Ulrich 1914, p. 614, 612, 647) and from some short notes published by Burchard (1913, p. 32-43). 3 :::::: ~ :::::::: '{7 l -- I ~ mftas ~ ""' MADDOX GA~::::::::::::::::~ IJI~!~}~~~I~~t:~ ~:~:~=~:tt\E 0 Measured Sections e Measured Sections with Probable Outcrops of Hooker Seams Hooker Seams (McCallie.l908) Fig. 1. Location map for measured sections in the Sequatchie Formation, showing the distribution of the Hooker ironstone seams. Stippled area: Silurian- Pennsylvanian r ocks; blank: Cambrian- Ordovician. Correlation In the complex problem of interpreting Middle and Upper Ordovician facies changes correlation is critical. Figure 3 which provides the framework for the following environmental interpretations is based on measured sections spaced as closely as possible. The correlations shown are lithologic rather than biostratigraphic but approximate time equivalence is inferred. The top of the Sequatchie Formation is drawn beneath the first ledges of Red Mountain sandstone (barrier-sandstone facies), the base above the last calcarenites and calcirudites of the Catheys Limestone. Within the formation correlations are based on gross lithology and sedimentary structures rather than on color variations. In western outcrops the distinctive lithology of the Leipers Limestone serves to divide the succession while in the east it is the argillaceous character of the Shellmound Member which allows it to be differentiated from the more arenaceous Inman-Leipers equivalents. Correlation at the base of the Shellmound is also facilitated by a bed of chamosite-hematite oolite which may be recognized in several strike belts. It occurs in southern Sequatchie Valley, throughout Lookout Valley, and can be traced as far east as Pigeon Mountain; a total distance of 25 miles across strike (Fig. 1 ). Lithofacies "Juniata" Facies In the most easterly exposures where Ordo-Silurian strata are exposed (Rocky Face Mountain, Horn Mountain) the Red Mountain Formation passes gradationally downwards into dark red unfossiliferous siltstones and sandstones which are approximately equivalent to the Sequatchie Formation further west (Zone+ 5 of Allen and Lester, 1957, p. 16). In detail these beds may be divided into a series of fining-upward cycles, with massive, coarse-grained, poorly sorted sandstones passing upwards into siltstones and shales with thin sandstone interbeds. The overall appearance of these beds, therefore, suggests alluvial sedimentation; the massive sandstones being laid down as meandering stream deposits and the siltstones and shales as overbank deposits. The name "Juniata" facies is used for these alluvial beds to emphasize a similarity to the fluviatile Juniata Formation of the central Appalachians which is stratigraphically equivalent to the Sequatchie Formation (Woodward, 1951, p. 387-408; Rogers, 1953, p. 98; Thompson, 1970b). Red Sequatchie Facies Ulrich (1914, p. 614) described the Sequatchie Formation as red shale and argillaceous limestone and it is clear from his notes (in Burchard, 1913, p. 32-43) that, although he defined the formation on the basis of a Richmond age he considered red beds to be a principle lithology. These red beds include fine grained calcareous sandstones, siltstones and shales, grading into impure fine grained pelmicrites The red color is of primary origin and arises from the presence of thin hematite coatings on the grains. Thus the finer the grain size the darker the red coloration. Chemical analyses carried out by Weidemann (unpublished manuscript) suggest values for Fe+++ ranging from 2 or 3 percent in the sandstones to 4 or 5 percent in shaly siltstones. Although in some areas reddening is concordant with primary sedimentary structures, in many places localized bleaching during diagenesis has tended to blur color boundaries. In particular, Wiedemann draws attention to the bleached halos and Liesegang rings which develop around burrows in bioturbated red beds, as well as to bleached zones along extension fractures. In both cases bleaching resulted from the reduction and removal of the hematite stain; in the first case during early diagenesis, in the second at a later stage. The red Sequatchie facies is characterized by a very distinctive suite of sedimentary structures including graded beds, laminations, ripple cross-laminations and desiccation structures. Graded units are thin to medium bedded with scoured bases and consist of fine sandstone or pelmicrite passing upward into shale. Desiccation structures include mudcracks, breccias and birdseyes. Many of the shale units at the top of the graded beds show polygonal mud cracks, and fragments of these polygons occur commonly in the sandstones. 5 LLANDOVERIAN BRASSFIELD ROCKWOOD RED MOUNTAIN RICHMONDIAN? SHELLMOUND SHELL MOUND MAYSVILLIAN? LEIPERS LEIPERS VsEQUATCHIE (.0 EDENIAN? < INMAN SHERMAN IAN? CAT-HEYS CATHEYS CATHEYS CENTRAL BASIN SEQUATCHIE VALLEY AND EAST VALLEY RIDGE Fig. 2. Schematic diagram showing the stratigraphic relationship between Upper Ordovician rocks in southeastern Tennessee and northwest Georgia. Stippled area: littoral facies; blank: open marine facies. Body fossils are rare, although some thin beds of ostracod biomicrite occur in places. Otherwise skeletal remains are restricted to abraded fragments of bryozoans and brachiopods, which occur with scattered limestone pebbles in tenuous lenses at some intervals. Trace fossil markings are common and include Skolithos, Tigillites, Teichichnus and Chondrites. Considered overall the evidence is overwhelming that the red Sequatchie facies was deposited in a supratidal (high-tidal-flat) environment. In particular, the coincidence of graded beds with desiccation structures indicates periods of rapid suspension sedimentation separated by long intervals of subaerial exposure. Such is the case on supratidal flats which undergo only periodic flooding during spring tides and storms. In the intervals between inundations, during dry periods, fine grained tidal muds may be subjected to intense desiccation and provide resiliant mud polygons which are readily eroded and redistributed pn the next flood tide. During storm tides lime clasts and skeletal debris may be transported from intertidal or subtidal areas and left stranded on the supratidal flats. Beds of ostracod biomicrite were probably deposited in shallow ponds temporarily impounded on the flats. Vertical burrows like Skolithos and Tigillites are characteristic of littoral sediments. Since the dominant direction of sediment transport on tidal flats is landwards the red Sequatchie facies must have been supplied with sediment from gray beds offshore (to the west). The red color of the facies was therefore produced on the supratidal flats as a result of subaerial oxidation. Gray Sequatchie Facies This facies includes not only calcareous sandstones, siltstones and shales similar to those .in the red facies (except for color), but also a variety of pellet and skeletal limestone lithologies. The gray Sequatchie facies is therefore more calcareous than the red facies, shows a greater range of lithologies, and was deposited in a greater variety of environments. In general, the facies passes eastwards into supratidal sediments and westwards into marine deposits (Fig. 3) so that transitional environments may be expected. Intervals of flaser bedded, fine grained sandstone, siltstone and shale are characteristic and show the same combination of graded beds and desiccation structures noted in the previous facies. However, in the absence of hematite staining the sandstones are medium gray and the shales greenish gray (total iron 1-4 percent according to Weidemann). These also appear to be tidal-flat sediments, but in contrast to the supratidal facies, bedding is thinner, desiccation cracks and breccias are less well developed and skeletal debris is more abundant. The rarity of burrow structures and the presence of massive convoluted load casts on the bases of some sandstones suggest rapid sedimentation. Thin beds of finely laminated calcareous siltstone and calcilutite possibly represent algal mats. The evidence is therefore consistent with deposition on intertidal flats (low-tidal-flats) where flooding took place daily and prevented subaerial reddening. Repeated scouring and deposition produced thin bedding except in local depressions (e.g. abandoned tidal channels) where thicker beds of waterlogged sandstone accumulated and foundered on underlying substrates. Closely associated with these sediments are units of poorly sorted, cross-bedded, sandy, skeletal calcarenite and calcirudite which are interpreted as the lag deposits of migrating tidal channels. Basal contacts are sharp and calcirudites pass gradationally upward into normal tidal-flat lithologies. Skeletal fragments, mainly bryozoans and brachiopods, are usually broken and abraded, but in some places branching bryozoan colonies occur in growth position. Obviously, these animals were able to establish themselves in the more protected reaches of the tidal distributary system. Rounded intraclasts of shale and dolostone, sometimes imbricated, are a common occurrence. Cross-bedding is in the form of shallow troughs, which mark the passage of low amplitude, curvedcrested megaripples. The thickness of these units varies from less than a foot up to 30 feet, and generally increases in the western part of the study area where the main arteries of the tidal distributary system are exposed. 7 ANDERSON HILL HOOKER RINGGOLD GREEN GAP = SHELLMOUND ~ LEIPERS INMAN J...-. --J.... r=:=:::=1 Red Sequatchie t=:::l Facies r,:t ~ - - - - - - - - - - - - - - - ::_-_ -_:: ~ Gray Sequatchie ~ Facies nmlLeipers ~Facies == w :r: -0 . 1- C() 0 0 z ..)~}<< <{ _.J 7-:::::::::::::::::::::: _.J I-' -l ANDERSON HILL NW TIFTON IA 200L ~ --. -- Ft. ESTELLE L ";?" R ...................... awaa..:.. ::> -w z _.J <( 0 0 ~ cO a: > w 0 :: l !l! [!![!lll 0w:: 3: 0 z <{ 0 _.J _j _j SE 0 Miles 5 Fig. 3. Regional facies relationships in the Silurian of southeast Tennessee and northwest Georgia. For location of section line see Figure 2. A close examination of the lower unit as exposed at Ringgold is particularly instructive . The sequence may be divided into three subfacies as follows; a) The base of the succession consists predominantly of shale with inter beds of siltstoue and fine grained sandstone (30 percent). Approximately 40 percent of these interbeds are graded with sharp bottom contacts and gradational tops. The remaining beds show both gradational tops and bottoms. Although shells are poorly preserved, the interval is fossilifero us throughout, and contains traces assigned to a mixed Cruziana-Zoop hycos assemblage. b) In the overlying subfacies sandstones increase in importance, making up approximately 35 percent of the succession. The sandstones are fine to very fine grained and nearly all are graded (90 percent). Sharp basal contacts are frequently scoured with abundant tool marks as well as some flutes and grooves. An analysis of sedimentary structures within the sandstones reveals that while the lower parts were deposited in the upper flow regime (parallel laminations), the upper parts were deposited in the lower flow regime. Sole marks, and sedimentary structures of this kind , known as Bouma sequences (see Fig. 6), are characteristic of sandstones laid down as sheet deposits by periodic currents of waning velocity (e.g. t urbidites, tidalites and storm deposits). The sole marks suggest currents flowing towards the west (i.e. seawards) normal to the depositi onal strike (Fig. 4). Again this interval is fossiliferous throughout with lebensspuren of a mixed Cruziana-Zoophycos assemblage. c) In the upper part of the alternating sandstone and shale facies only about 50 percent of the sandstones are graded, the remainder have sharp contacts top and bottom and resemble the hematitic beach sands of the barrier sandstone facies. They are often crossbedded and were deposited as migrating sand waves or ripples rather than as periodic sand sheets . This suggests that this subfacies was deposited above wave-base while the lower subfacies were deposited beneath. The shales associated with these sandstones are distinctly more fissile than those associated with the graded sandstones indicating that the winnowing of fine and coarse sediment is more complete. The overall percentage of sandstones in this unit fluctuates between 24 percent and 62 percent in a series of cycles. A distinctive feature of this subfacies is the occurrence of massive load casts or flow rolls on the bases of some of the thicker sandstones. Such structures indicate heavy loading of waterlogged sediments and hence rapid sedimentation. They also suggest gravitational instability. This unit is fossiliferous like those below, but whereas shells are restricted to the bases of the graded sandstones they occur throughout the cross-bedded sandstones in this interval. Like the sediments the trace fossils are of mixed origin with representatives of both the Cruziana and Skolithos assemblages. Taken as a unit the alternating sandstone and shale facies reveals a coarsening-upward cycle which culminates in the hematitic sandstones of a regressive barrier sequence. The combined evidence of periodic currents and gravitational instability strongly suggests that the graded sandstones are turbidites. A closer examination of the Bouma sequences shows .that the turbidites. become increasingly proximal towards the top of the succession (see Appendix and Fig. 4). Both sole markings and load casts suggest a westerly dipping paleoslope. These observations are interpreted to indicate that the lower alternating sandstone and shale unit at Ringgold represents the deposits of a westward prograding slope, referred to here as a promolasse slope. The three subfacies described above are then interpreted as: a) Promolasse and lower promolasse slope deposits b) Middle and upper promolasse slope deposits c) Slope break and promolasse platform deposits in front of a barrier island. Similar sequences have been described in Devonian and Cretaceous shelf and slope sediments by Walker (1971) and Hubert et al. (1972). Turbidity currents appear to have been initiated by gravitational instability and slumping at the slope break, the cyclic development of shales and sandstones immediately above and below this level being the result of successive phases of slumping and rebuilding: By contrast with the sandstones of the overlying barrier facies, the turbidite sandstones are finer grained, less well sorted and without hematite grain coatings. Apparently the barrier system was extremely effective in retaining coarse grained terrigenous sediment 18 Fig. 4. Columnar section of Ordovician and Silurian rocks exposed in the road cuts on I-75 at Ringgold Gap, Georgia. BED NUMBERS PERCENTAGE SANDSTONE 0 2.5 50 WITH GRADED AB~;; I~Dt:X BEDDING (A+ VzB) 75 0 50 I I 10I0 0 I I 50 I I 1I00 47 z 0 1i) PRO MOLASSE (/) SLOPE IaL: l a (.!) BASIN IaL: l 46 z <1: 0w::: > 0 z 0 <1: _.J _.J 451 0w::: Cl.. Cl.. ::::> .... CX) <..0 (j) Z PROMOLASSE Q PLATFORM ~ (PROTECTED IaL: l EMBAYMENT) (.!) z z(/) 0 <( a: ~ I- 44 R R I R R ::?! a:: 0 R LL BARRIER ISLAND 43 r:::::::::::::::::::::::::::::::::::::::::::::: z (BEACH a WASH OVERS) I 4 2 ~ z ::::> 0 ::?! i,~ ";t -~. -, "'.................... --- ....... -~- ::>I --- ~ ; l -(' .-,........../_. /.... /' '/ / , I -~ / ....... .(...".... ' i,. z 0 <1: w aw:: 0::: > 0 0 LAGOON 41 ~--------ir Fe :;:;:;:;:;:;:::::::;:::::;::::::::::::::;:yFe _.J _.J w 1i) (/) IaL:l (.!) (BEACH, TIDAL DELTA a TIDAL CHANNEL) I:: I _.J 0 IaL: l 0 PROMOLASSE ::?! PLATFORM ------138 cO a:: w ~ 0 PRO MOLASSE SLOaPE 137 _.J BASIN Fe 35 (/) PROMOLASSE PLATFORM g~:::~~:~~D~LL~~eTAI:: 1u::::r:::::u::u::::::::::u::::m a WASHOVERS) 32 31 ~ LAGOON I z<1: w z ------- 25 f-- ~ 0 ~ INTERTIDAL FLAT ~<1: ~ ~ f-- f-- ~1i) ------- ~---=-----=--=--===..::: ~ U 8 ~ (J) lL f3 a: SUPRATIDAL FLAT 3 ~~~~=~~~~ ~=~=====-==-::~~ t=,-_-_,_- ---- ~--:----_-:-.-.:-_--_--_--::..-_-_-:-..-..: ' I '-1r-----+---------~ A '"1:,...._.._... I i ;> ---,,1. \ ..... < < _-._ ) __.<... .. ' - . \ ''.:. s. -----,.,.. ><- ...... ; i i !'; /' \ -~ ,-.-.",.'. ...> ~' ,\ I I ~Alternating Sandstone-Shale Facies c:::J Red Shale Facies D Gray Shale Facies 100' b/J Quartz Sandstone Facies zI I <1: z z <1: _.J (>J-) Of-- Cl.. w>ygenation and salinity, all of which r late (ideally at least) to water depth 0r distance from shore (e. g., Farrow, 1966). Seilacher's scheme consists of the following "assemblages" and their salient (but intergradational) environment implications. Scoyenia-nonmarine clastics, such as continental "red beds." Glossifungites--stable, coherent substrates; appreciable water turbulence; generally in the littoral or shallow infralittoral zones. Such conditions are not found commonly; one example from our present Georgia coast is the ancient salt marsh mud cropping out on the oceanward side of Cabretta Island (Frey and Howard, 1969, p. 434-435, Pl. 1, Fig. 2; Pl. 4, Fig. 3). Skolithos-unstable, shifting sediments; comparatively rapid deposition or erosion; appreciable currents or water turbulence; generally in the littoral or shallow infralittoral zones. Cruz iana-well-sorted silts or sands, commonly thinly interbedded; moderately stable substrat s; intermediate water turbulence; generally in the infralittoral and cir alittoral zones. Zoophycos-impure silts and sands; stable substrates; little water turbulence; generally in the circalittoral to bathyal zones, in places free of turbidites. Nereites-pelagic sedimentation, mostly mud; substrates oridinarily stable, with virtually no water agitation, except for periodic turbiclity flows generally in the bathyal to abyssal zones, commonly developed between turbidite deposits. Although these assemblages ar nam d for characteristic genera of trace fossils, individual genera rarely have inequivocaJ significance (Frey and Howard, 1970); varieties of both Sholitho and Zoophycos have been found in the Cruziana assemblage, for example, and Skolithos assemblages in the Pleistocene of Georgia and Florida are d minated by Ophiomorpha (Weimer and Hoyt, 1964; Frey and Mayou, 1971). 38 Furthermore, Cruziana-the crawling trace of trilobites-can hardly be dominant in the Cruziana assemblages of Mesozoic and Cenozoic rocks! Therefore, these assemblages must be approached in terms of such things as trace fossil abundances, behavioral patterns represented, and substrate interrelationships, rather than by simply consulting 'checklists' of generic names. Thus, whatever the variations among them, vertical dwelling structures predominate in the Glossi{ungites and Skolithos assemblages, and horizontal or inclined dwelling and feeding structures and crawling traces predominate in the Cruziana assemblage. Dwelling structures of any kind are rare in the Zoophycos and Nereites assemblages, which are dominated by horizontal feeding structures and grazing traces. The Scoyenia, G/ossifungites, and Nereites assemblages definitely are not present at Ringgold. Tbe SJwlithos and Cruziana assemblages are best developed, although certain of the genera have also been reported from the Zoophycos assemblage. Based purely upon characteristics of individual genera, a typological approach to differentiating thes assemblages is indicated below; however, our final interpretation of the assemblages is illustrated in Figure 4. TRACE FOSSILS Tigillites Skolithos Monocraterion Diplocra terion Arenicolites sp. A Arenicolites sp. B Arenicolites sp. C Trichophycus sp. A Trichophycus sp. B Chondrites sp. A Chondrites sp. B Chondrites sp. C Dictyodora Palaeophycus Planolites sp. A Planolites sp. B Curvolithus Gyrochorte Cruziana Aulichnites "Fraena" sp. A "Fraena" sp. B CHARACTERISTIC ASSEMBLAGES Skolithos Cruziana Zoophycos X X X X X X X x X X X X X X X X X X X X X X X X X X X X X Planolites, being a "catch-all" taxon , is ambiguous in meaning; it occurs even in the Scoyenia and Nereites assemblages. Chondrite , a more distinctive trace fossil, is also a notorious facies-crossing type, although Chamberlain (1971 b, Fig. 6) attached speciaJ significance to it in the Carboniferous of Oklahoma. Dictyodora could well occur in the Zoophycos assemblage but is by no means uniquely indicative of it. In spite of such difficulties, however, the overall aspect of trace fossil distributions at Ringgold agrees favorably with environmental conclusions drawn from sedimentological and stratigraphic data (Fig. 4). In Unit 32, for exampl , the ratio of dwelling structures to feeding structures is about 5:1, whereas in Unit 38 the ratio is about 1:1 , with crawling traces also present; these distributions of behavioral patterns might be expected under the 39 postulated current-energy regimes. A thorough study of the trace fossils, especially from the standpoint of animal-sediment relationships, etc., would undoubtedly yield more refined data. SUGGESTIONS FOR FURTHER STUDY Our survey of trace fossils from the Ringgold road cut should obviously be followed by a comprehensive investigation, not only with respect to this classic stratigraphic section but also through the Catheys, Sequatchie, and Red Mountain Formations elsewhere in Georgia, Alabama, and Tennessee. Only with this kind of information at hand may we evaluate the real facies relationships and the behavioral and paleoecological significance of the trace fossils. Perhaps less exciting but equally important is the need for concerted studies on interrelated trace fossil genera, with a view toward resolving the numerous taxonomical ambiguities noted on previous pages. 40 REFERENCES Allen, A. T., Jr., and Lester, J. G., 1953, Animal tracks in an Ordovician rock of northwest Georgia, in Short contributions to the geology, geography, and archaeology of Georgia: Georgia Geol. Survey Bull. 60, p. 205-214. ___ 1954, Contributions to the paleontology of northwest Georgia: Georgia Geol. Survey Bull. 62, 167 p. Bandel, Klaus, 1967, Trace fossils from two Upper Pennsylvanian sandstones in Kansas: Univ. of Kansas Paleont. Contr., Paper 18, 13 p. Bather, F. A., 1925, U-shaped markings on estuarine sandstone near Bleawyke: Yorkshire Geol. Soc., Proc., n. s., v. 20, p. 185-199. Boyd, D. W., 1966, Lamination deformed by burrowers in Flathead Sandstone (Middle Cambrian) of central Wyoming: Univ. Wyoming Contr. Geology, v. 5, p. 45-53. Chamberlain, C. K., 1971a, Morphology and ethology of trace fossils from the Ouachita Mountains, southeast Oklahoma: Jour. Paleontology, v. 45, p. 212-246. 1971b, Bathymetry and paleoecology of Ouachita geosyncline of southeastern Oklahoma as determined from trace fossils: Amer. Assoc. Petrol. Geol., Bull., v. 55, p. 34-50. Chisholm, J. 1., 1970, Teichichnus and related trace-fossils in the Lower Carboniferous at St. Monance, Scotland: Geol. Surv. Great Britain, Bull. 32, p. 21-51. Crimes, T. P., 1970, The significance of trace fossils in sedimentology, stratigraphy, and palaeoecology, with examples from Lower Palaeozoic strata, in Crimes, T_ P., and Harper, J. C. (eds.), Trace fossils: Geol. Jour., Special Issue 3, p. 101-126. Desio, Ardito, 1940, Vestigia problematiche paleozoiche della Libia: Universitil Milano, Instituto Geologia, Paleontologia, Geografia fisica, ser. P., No. 20, p. 47-92. Dubois, Paul, and Lessertisseur, Jacques, 1964, Note sur Bifungites, trace problematique du Dlvonien du Sahara: Soc. geol. de France, Bull. (7}, VI, p. 626-634. Farrow, G. E., 1966, Bathymetric zonation of Jurassic trace fossils from the coast of Yorkshire, England: Palaeogeogr., Palaeoclimatol., Palaeoecol., v. 2, p. 103-151. Frey, R. W., 1970, Trace fossils of Fort Hays Limestone Member of Niobrara Chalk (Upper Cretaceous), west-central Kansas: Univ. Kansas Paleont. Contr., Article 53,41 p. 1971, Ichnology-the study of fossil and Recent lebensspuren, in B. F. Perkins (ed.), Trace fossils, a field guide to selected localities in Pennsylvanian, Permian, Cretaceous, and Tertiary rocks of Texas, and related papers: Louisiana State Univ., School Geosci., Misc. Publ. 71-1, p. 91-125. Frey, R. W., and Cowles, John, 1969, New observations on Tisoa, a trace fossil from the Lincoln Creek Formation (mid-Tertiary) of Washington: The Compass, v. 47, p. 10-22. Frey, R. W. and Howard, J.D., 1969, A profile of biogenic sedimentary structures in a Holocene barrier island-salt marsh complex, Georgia: Gulf Coast Assoc. Geol. Sacs., Trans., v. 19, p. 427-444. 41 1970, Comparison of Upper Cretaceous ichnofaunas from siliceous sandstones and chalk, western interior region, U.S.A., in Crimes, T. P., and Harper, J . C. (eds.), Trace fossils: Geol. Jour., Special Issue 3, p. 141-166. 1972, Georgia coastal region, Sapelo Island, U.S.A.: Sedimentology and biology. VI. Radiographic study of sedimentary structures made by beach and offshore animals in aquaria: Senckenbergiana maritima, v. 4 (in press). Frey, R. W., and Mayou, T.V., 1971, Decapod burrows in Holocene barrier island beaches and washover fans, Georgia: Senckenbergiana maritima, v. 3, p. 53-77. Gardet, Gustave, et al., 1956, Sur un Problematicum du Lias infe'rieur de la Haute-Marne: Siphonites heberti de Sap.: Soc. G~ol. de France, Bull. (6), VI, p. 997-1000. Goldring, Roland, 1962, The trace fossils of the Baggy Beds (Upper Devonian) of North Devon, England: PaHi.ont. Zeitschr., v. 36, p. 232-251. 1964, Trace-fossils and the sedimentary surface in shallow-water marine sediments, in van Straaten, L. M. J. U. (ed.), Deltaic and shallow marine sedimentation: Develop. Sedimentology, v. 1, p. 136-143. Hallam, Anthony, 1970, Gyrochorte and other trace fossils in the Forest Marble (Bathonian) of Dorset, England, in Crimes, T. P., and Harper, J. C. (eds.), Trace fossils: Geol. Jour., Special Issue 3, p. 189-200. Hallam, Anthony, and Swett, K., 1966, Trace fossils from the Lower Cambrian Pipe Rock of the northwest Highlands: Scottish Jour. Geol., v. 2, p. 101-106. Hlintzschel, Walter, 1962, Trace fossils and problematica, in Moore, R. C. (ed.), Treatise on invertebrate paleontology, Pt. W, Miscellanea: Geol. Soc. Amer. and Univ. Kansas Press, p. W177-W245. 1965, Vestigia Invertebratorum et Problematica; Fossilium Catalogus. I: Animalia, pars 108, 142 p. 1966, Recent contributions to knowledge of trace fossils and problematica, in Rhodes, F. H. T., etal., Treatise on invertebrate paleontology, Part W, Conodonts, conoidal shells, worms, trace fossils: comments and additions: Univ. Kansas Paleont. Contr., Paper 9, p. 10-17. Hattin, D. E., and Frey, R. W., 1969, Facies relations of Crossopodia sp., a trace fossil from the Upper Cretaceous of Kansas, Iowa, and Oklahoma: Jour. Paleontology, v. 43, p. 1435-1440. Heinberg, C., 1970, Some Jurassic trace fossils from Jameson Land (east Greenland), in Crimes, T. P., and Harper, J. C. (eds.), Trace fossils: Geol. Jour., Special Issue 3, p. 227-234. Hertweck, GUnther, 1970, The animal community of a muddy environment and the development of biofacies as effected by the life cycle of the characteristic species, in Crimes, T. P., and Harper, J. C. (eds.), Trace fossils: Geol. Jour., Special Issue 3, p. 235-242. 1972, Georgia coastal region, Sapelo Island, U.S.A.: Sedimentology and biology. V. Distribution and environmental significance of lebensspuren and in-situ skeletal remains: Senckenbergiana maritima, v. 4 (in press). 42 Howard, J.D., and Elders, C. A., 1970, Burrowing patterns of haustoriid amphipods from Sapelo Island, Georgia, in Crimes, T. P., and Harper, J. C. (eds.), Trace fossils: Geol. Jour., Special Issue 3, p. 243-262. Howell, B. F., 1943, Burrows of Skolithos and Planolites in the Cambrian Hardyston Sandstone at Reading, Pennsylvania: Publ. Wagner Free Inst. Sci., v. 3, p. 3-33. 1945, Skolithos, Diplocraterion, and Sabellidites in the Cambrian Antietam Sandstone of Maryland: Publ. Wagner Free Inst. Sci., v. 20, p. 33-40. 1957, New Cretaceous scoleciform annelid from Colorado: Palaeontological Soc. India, D. N. Wadia Jubilee No., v. 2, p. 149-152. Martinsson, Anders, 1965, Aspects of a Middle Cambrian thanatotope on Oland: Geol. Foreningens I Stockholm Forhandlingar, v. 87, p. 181-230. 1970, Toponomy of trace fossils, in Crimes, T. P., and Harper, J. C. (eds.), Trace fossils: Geol. Jour., Special Issue 3, p. 323-330. MUller, A. H., 1962, Zur Ichnologie, Taxiologie und Okologie fossiler Tiere: Freiberger Forschungshefte, Heft C 151, p. 5-49. 1971, Zur Kenntnis von Asterosoma (Vestigia invertebratorum): Freiberger Forschungshefte, Heft C 267, p. 7-17. Osgood, R. G., Jr., 1970, Trace fossils of the Cincinnati area: Palaeontographica Americana, v. 6(41), p. 281-444. Purdy, E. G., 1964, Sediments as substrates, in Imbrie, J., and Newell, N.D. (eds.), Approaches to paleoecology: New York, John Wiley & Sons, p ..238-271. Ruchholz, Kurt, 1967, Zur Ichnologie und Fazies des Devons und Unterkarbons im Harz: Geologie, v. 16, p. 503-527. Seilacher, Adolf, 1960, Lebensspuren als Leitfossilien: Geologisch. Rundschau, v. 49, p. 41-50. 1964a, Biogenic sedimentary structures, in Imbrie, J., and Newell, N.D. (eds.), Approaches to paleoecology: New York, John Wiley & Sons, p. 296-316. 1964b, Sedimentological classification and nomenclature of trace fossils: Sedimentology, v. 3, p. 253-256. 1967a, Fossil behavior: Scientific American, v. 217, p. 72-80. 1967b, Bathymetry of trace fossils: Marine Geology, v. 5, p. 413-428. 1970, Cruziana stratigraphy of "non-fossiliferous" Palaeozoic sandstones, in Crimes, T. P., and Harper, J. C. (eds.), Trace fossils: Geol. Jour., Special Issue 3, p. 447-476. Seilacher, Adolf, and Meischner, D., 1964, Fazies-Analyse im PaUiozoikum des OsloGebietes: Geologisch. Rundschau, v. 54, p. 596-619. Selley, R. C., 1970, Ichnology of Palaeozoic sandstones in the southern desert of Jordon: a study of trace fossils in their sedimentologic context, in Crimes, T. P., and Harper, J. C. (eds.), Trace fossils: Geol. Jour., Special Issue 3, p. 477-488. Simpson, Scott, 1957, On the trace-fossil Chondrites: Geol. Soc. London, Quart. Jour., v. 112, p. 475-499. 43 Weimer, R. J., and Hoyt, J. H., 1964, Burrows of Callianassa major Say, geologic indicators of littoral and shallow neritic environments: Jour. Paleontology, v. 38, p. 761-767. Wilckens, Otto, 1947, PaHiontologische und geologische Ergebnisse der Reise von KohlLarsen (1928-29) nach SUd-Georgien: Abhandl. Senckenberg. Naturf. Gesellschaft, No. 474, 75 p. 44 Plate 1. Trace fossils from the Red Mountain Formation at Ringgold. A, B. Tigillites, vertical exposures. C,D. Skolithos, vertical exposures. E,F. Monocraterion, vertical exposures. G,H. Diplocraterion, vertical exposures. Shallow, intense bioturbation at top of H. I- L. Arenicolites; I and J vertical exposures, K and L sole views. I. Species A. J. Species C. K, L. Species B. M - P. Trichophycus sp. A, sole views. 46 Plate 1 - Trace Fossils 47 Plate 2. Trace fossils from the Red Mountain Formation at Ringgold. A-D . Trichophycus sp. B. A. Bottom of spreite bearing radial scratch marks (scale in em). B. Cross section through spreite (scale as in A). C. End view of specimen having three branches. D. Latero-ventral view of specimen in C, illustrating upward turn of top branch and oblique, elongate scratch marks. E - I. Chondrites, sole views. E, I. Species A. F. Species B. G. Species C. H. Profusion of Chondrites and other small burrows on ripple marks. 48 Plate 2 - Trace Fossils 49 Plate 3. Trace fossils from the Red Mountain Formation at Ringgold. A - C . Dictyodora, sole views. Sequence of photos illustrates progressively more intense bioturbation. (Specimen in A collected by Norman King:) Trichophycus sp. A in middle and lower part of C. D- F. Palaeophycus, sole views. D. Slightly collapsed burrow, crossed at left by Trichophycus sp. A. E. Partly to wholly collapsed burrows, some branched. F. Collapsed burrow, near penny. Large burrow at bottom is "external mold" of either Trichophycus or Planolites; such "molds" seen commonly at Ringgold. G. Trichophycus sp. A, sole view. H - K. Planolites, sole views. H, I. Species A; poorly preserved and slightly iron stained in I. J, K. Species B. 50 J K Plate 3 - Trace Fos~ih 51 Plate 4. Trace fossils from the Red Mountain Formation at Ringgold and Summerville. A, B. Planolites sp. B, sole views. Those in A associated with a priel (=washout) cast. c,n. Gyrochorte, sole views. E. Gyrochorte-like trail, sole view. F. Curuolithus, sole view. G,H. Aulichnites, sole view. I. Cruziana, sole view (scale in inches). From Summerville. J- L .. "Fraena ", sole views. J, K. Species A. L. Species B. 52 53 Plate 5. A. B. c. D. E, F. G,H. I. J. K. Trace fossils from the Red Mountain and Catheys Formations at Ringgold. "Fraena" sp. B, sole view. ?Asterosoma, sole view. ?Arthrophycus, sole view. "Secondary burr~<>ws," probably in Tigillites; vertical exposure. "Mineral-filled" burrows, possibly Planolites sp. A; sole views. ?Hormosiroidea, ?sole view. From the Catheys. G. Burrow cast. H. Burrow mold. Trichophycus sp. A, showing upward-turning ends of burrows (cf. Fig. 2H); sole view. Tigillites, sole view. ?Sabellarifex; these irregular burrows otherwise resemble Skolithos. Vertical exposure. 54 c Plate 5 -Trace Fossils 55 SEDIMENTARY ENVIRONMENT OF THE ARMUCHEE CHERT, NORTHWEST GEORGIA W. E. Nunan Department of Geology University of North Carolina at Chapel Hill Abstract The Lower Devonian Armuchee Chert was deposited on a shallow marine shelf as thick accumulations of sponge spicules and spiculitic micrite. Chertification of the rocks occurred very' early in diagenesis, commencing immediately after deposition. Processes of chertification followed two courses which were related to the unstable opaline silica of the sponge spicules. "Felted" mats of sponge spicules inverted from opal into chalcedony and microcrystalline quartz, while the spiculitic micrite was repla ed by silica from enriched interstitial waters to form chert. The source for the silica in these rocks is attributed to the solution and inversion of abundant spicules "freed" upon death of profuse sponge populations on the shelf. Rosenfeld's (1954) suggestion that the Chattanooga Shale served as a source for the silica in the Armuchee rocks is disputed. Introduction The Armuchee Chert is a body of siliceous sedimentary rocks which is present in a limited area in the folded Appalachians of northwestern Georgia and northeastern Alabama (Fig. 1). Exposures of the Armuchee occur on many of the ridges between the Cartersville Fault southwest of Cedartown (Cressler, 1970) and Taylor Ridge up to the vicinity of Maddox Gap in the northern part of Walker County, Georgia. In Alabama, the formation is exposed on the southeast flank of Bogan Mountain in Cherokee County. The Armuchee is Onesquethaw in age (Nunan and Lipps, 1968) and is considered by many to be a facies equivalent of part of the Frog Mountain Sandstone of Alabama (Hayes, 1902; Rosenfeld, 1954; Kiefer, 1970; Cressler, 1970; Nunan, 1971). In northwestern Georgia, the Armuchee unconformably overlies the Middle Silurian Red Mountain Formation. The chert is everywhere overlain by a thin section of Frog Mountain Sandstone except possibly in some areas of Polk County. In this area, Cressler (1970) suggested that the Frog Mountain is present only locally as thin sand interbeds within the Armuchee and that the chert is in direct contact with Mississippian rocks. The upper part of the Armuchee is interbedded with the Frog Mountain at most exposures (Nunan, 1971). The purpose of this report is to suggest a possible origin for the silica of the Armuchee Chert and to propose a sedimentary environment for the accumulation of the sediments. The origin of the many varieties of chert has been much disputed in the past. Several schools of thought have developed concerning the origin of the silica and the time of introduction of silica into siliceous rocks. Some workers have contended that many cherts were formed by the direct precipitation of silica into siliceous oozes on the ocean floors and were subsequently lithified. Others have suggested organic processes participate in the sequence of chert formation. Elsewhere t he evidence suggests that limestones have been replaced by silica leacl'led from adjacent sedimentary units, for example,overlying volcanic ashes (Allen and Lester, 1957). In 1946, Bramlette discussed possible processes for the alteration of diatomaceous sediments into chert. One of the mechanisms is the dissolution of relatively unstable, abundant, opaline skeletons of diatoms and the re-precipitation of the silica in the more stable mineralogical varieties of chalcedony and microcrystalline quartz in close proximity to the source diatoms. He observed that remains of siliceous skeletal elements of microfossils in rocks as old as pre-Cambrian are commonly preserved only in cherty rocks. 57 Jackson I \ ( ( / ~ ) -.~;._~ ~ ~ _ __ ~ \~ _} ~ ~0 <:;;/" 0)_: D ::----."'>- .--- - ,..r _ .. I I Cherokee IJ Floyd . I - -'___,;:.."- --LI --, I . _/L~ - Po lk ,~_ . ,_...} I / _,-J ....-. - - - I Haralson 1 1 Cleburne -\ I ~ ~ / OUTCROP TRACE OF DEVONIAN STRATA 1..0 -N- OUTCROP AREA OF ~ . ARMUCHEE CHERT 0 10 20 30 40 MILES 0 10 20 30 40 50 60 I I I I I I I KILOMETERS NUNAN 1972 Fig. 1. Location map of outcrop pattern of Devonian strata in northwestern Georgia and northeastern Alabama (after Kiefer and Dennison, 1971; and Cressler, 1970). The origin of the silica in the Armuchee Chert was investigated by Rosenfeld (1954) who attributed it to leaching of the overlying Chattanooga Shale and subsequent replacement of an original sandy rock by the silica. I present evidence that argues for another hypothesis on the origin of the Armuchee Chert. Stratigraphy The Armuchee Chert was named by Hayes (1902) for exposures near the community of Armuchee, Georgia in northern Floyd County. He believed that the chert represented an offshore sediment equivalent in age to the Frog Mountain Sandstone of Alabama. The lower contact of the Armuchee Chert is apparently paraconformable to the underlying Middle Silurian Red Mountain Formation (Oliver and others, 1961). The contact is sharp and marked by an abrupt change in lithology from the sandstone and shale of the Red Mountain into the light grey chert of the Armuchee. The upper contact of the Armuchee Chert is variable in position due to an intertonguing relationship with the Frog Mountain Sandstone (Nunan, 1971). The Armuchee is conformably overlain by a few meters of Frog Mountain at all exposures known to me. Cressler (1970) suggested that there are places in Polk County, Georgia, where the Armuchee may be in direct contact with Mississippian rocks. The Frog Mountain Sandstone is unconformably overlain by the Chattanooga Shale of Late Devonian age (Conant and Swanson, 1961) in most of the area northwest of the Rome Fault. It is possible that the Floyd Shale rests on the Frog Mountain Sandstone in parts of southwestern Floyd County with the Chattanooga Shale being absent (Nunan, 1971 ). In Polk County, the Chattanooga is not present and Cressler (1970) showed the Frog Mountain and Armuchee to be in contact with the Fort Payne Chert and the Floyd Shale. The Armuchee Chert is variable in thickness throughout the outcrop area. The thinnest sections are along Taylor Ridge near Gore, Georgia, (approximately 7.8 meters thick), on Dick Ridge (about 4.7 meters thick) in the gap through which West Armuchee Creek flows (Nunan, 1971), and in Polk County where it is about 2 meters thick near Elders Lake (Cressler, 1970). The thickest sections are on Horn Mountain at Calhoun Gap (21.5 meters), along the northern end of Lavender Mountain near O'Brian Gap (15.6 meters), and on Turkey Mountain (15.5 meters). The chert is interbedded in its upper portions with tongues of the Frog Mountain Sandstone at most exposures northwest of the Rome Fault (Nunan, 1971). At several localities in the lower part of the Armuchee, the bedding in the chert is accentuated by thin, lense-shaped partings of sandstone and shale. Another very characteristic feature is the almost ubiquitous occurrence of knobby protuberances on bedding surfaces (Figs. 2A and B). The sandstone-chert relationships described by Nunan (1971), the equivalent ages of at least parts of the two formations (Kiefer, 1970; Nunan and Lipps, 1968) and the reported occurrence of chert beds in the lower part of the Frog Mountain near Tannehill, Alabama (Kiefer, 1970) are strong evidence that the Armuchee Chert and Frog Mountain Sandstone are facies equivalents of each other. Petrography The chert rocks that comprise the Armuchee are recrystallized spiculites and chert which replace spiculitic micrites. The spiculites consist of "felted" masses of sponge spicules that have inverted from original opal chalcedony and microcrystalline quartz (Figs. 3A and B). The spicules are predominantly of the monaxon variety, or perhaps broken fragments of other types, but a few fish hook (Fig. 4), club (Fig. 5), cross (Fig. 6), and other shapes are present. Central canals are preserved in many of the spicules (Figs. 4 and 5). Fine quartz sand and silt account for as much as 20 percent of some of the spiculites. Dolomite rhombs, replaced by silica, may comprise as much as 25 to 30 percent of the rock. These are commonly coated with limonite. Many of the thin-sections have scattered, rhomb-shaped holes which may be molds of dolomite crystals that have been removed during telogenetic weathering. Glauconite grains account for as much as 10 percent of the rock in places. Fossils, other than sponge spicules, are rare in the spiculites. 59 Fig. 2A. Cross-section of a sponge (?)individual showing influence on the bedding surface which is towards the top of the photograph. Fig. 2B. "Knobby" protuberances common on bedding surfaces of the Armuchee Chert (two of these knobs have been planed off) .. 60 The silicified micrites are of two types, fossiliferous micrite (Figs. 6 and 7) and burrowed micrite (Fig. 8). Fine quartz sand and silt comprise up to 10 percent of these rocks. Glauconite grains may account for as much as five percent but is usually less. Fossils include bryozoan (Fig. 7), gastropod, and brachiopod fragments . Sponge spicules are scattered throughout (Figs. 6 and 7), but the original content may be higher than is now recognizable. Faint rod-shaped forms that may be inverted spicules are present in many of them (Fig. 7). Chertified dolomite rhombs occur in the micrites, and they are usually limonite stained. In the fossiliferous micrites, these rhombs may be as large as 0.2 mm (Fig. 7). In the bioturbated micrites, rhombs are much smaller and are concentrated around peripheries of burrowed material (Fig. 8). Paleogeography The Armuchee Chert occurs in a northeast-southwest belt which semi-parallels the ' 0" isopach that Kiefer (1970) determined for the Frog Mountain. It is along strike with the Jemison Chert in Olin ton County Alabama, which occupies a position just south of where Kiefer (1970) shows the Frog Mountain to be absent, but also just south of where he shows the smallest grain sizes to be present in the Frog Mountain. The age of the Jemison is inaccurately known, but it is possible that it is contemporaneous with the Armuchee (Dennison, personal communication~ 1972). Between the Jemison and the Armuchee outcrop areas, Kiefer shows a significant thickening of the Frog Mountain (Fig. 10). The author suggests that the Armuchee and Jemison cherts were deposited as shal1ow water shelf sediments, possibly contemporaneously, and that the extreme thicknesses of Frog Mountain Sandstone between the two chert bodies may be due to a rather weak delta development by the streams carrying debris off of the Nashville Dome to an Early Devonian sea. Diagenesis Diagenesis of the primary sediments which eventually became the rocks of the Armuchee Chert began almost simultaneously with sedimentation. Disaggregated opaline sponge spicules, disarmed of their protective organic coverings, would be very unstable in the water into which they fell. Solution may have begun almost immediately after death consequently increasing the amount of silica in the water. Many of the spicules probably escaped solution by entrapment in micrite or by concentration of the spicules by the action of currents (Figs. 3A and 3B). In these cases, alteration of the opaline spicules to chalcedony and microcrystalline quartz (Fig. 9) became a process of inversion, analogous to the aragonite-calcite inversion so common in the shells of many invertebrate groups. Bramlette (1946) postulated a similar sequence of events for the evolution of siliceous rocks in the Monterey Formation in California. Silicification of the micrite and their contained fossils probably result d from replacement of the calcite by silica from silica-enriched interstitial water. Fossils in the matrix have indistinct boundaries indicating that they may have been partially djssolved during the process of silicification (Fig. 7). Scattered dolomite rhombs replaced portions of the siliceous sediments. Iron may have been introduced, or concentrated, at this time because most of the relict dolomite rhombs are rimmed with limonite . There is evidence that partial dolomitization occurred both prior to and after silicification, with perhaps more than one period of chel'tification. Rosenfeld (1954) thought the silica in the Armuchee rocks was due to leaching of the overlying Chattanooga Shale, and concentration in the underlying Armuchee by replacement of an. original sandstone rock. There are a number of arguments against such a method for deriving the silica and formation of the cherty rock. First, there is the ubiquitous presence of sponge spicules in the Armuchee rocks, thus providing a source for the silica in the primary sediments. Second, if the Armuchee 61 Fig. 3A. "Felted" mat of recrystallized sponge spicules, crossed nicols . O'Brian Gap, spiculite facies. Fig. 3B. "Felted" mat of recrystallized sponge spicules, plain light. Calhoun Gap, spiculite facies. 62 Fig. 4. Fish hook-shaped spicule and associated spicules floating in a silicified silty micrite. Crossed nichols. Bogan Mountain, Alabama, micrite facies. Fig. 5. Club-shaped spicule, crossed nicols, same thin-section as Figure 4 . 63 Fig. 6. Cross-shaped spicule, plain light. Same specimen as Figure 4. Fig. 7. Bryozoan fragment, chertified dolomite rhombs, and several spicules in a chertified micrite matrix, plain light. Micrite facies. 64 was originally a sandstone, why would such leaching and replacement action effect only that sandstone, and not the underlying Red Mountain Formation (which is quite sandy) or the Armuchee's lateral equivalent, the Frog Mountain Sandstone? Third, there is no evidence that the rocks in the Armuchee Chert were ever sandy sediments. Fourth, the Chattanooga is not present in Polk County, and could not have been the source for the silica in that area. Sedimentary Environment Petrographic evidence suggests that the Armuchee Chert was deposited as thick concentrations of disaggregated sponge spicules and as layers of spiculitic micrite on a relatively shallow shelf. The sponges from which the spicules were derived must have been very abundant in ~he shallow sea. Bryozoans, brachiopods, gastropods, and other benthic sessile organisms lived on the ocean floor. While trilobites (Nunan and Lipps, 1968) crawled around scavenging for food, burrowing organisms disrupted many of the sediments. Stratigraphic relationships of the Armuchee with the Frog Mountain Sandstone also make a shallow water interpretation for the sedimentary environment tenable. The Frog Mountain sands were considered by Kiefer (1970) to have been deposited in shallow marine water along the south edge of the Nashville Dome. Intertonguing sandstone and chert beds in the upper part of the Armuchee, and sandstone and shale lenses in the lower part of the formation strongly suggest that environments of deposition of the Frog Mountain and the Armuchee were contiguous and closely related. Summary The Lower Devonian Armuchee Chert was deposited on a shallow marine shelf a few kilometers seaward from the strand line. The sediments were originally thick accumulations of sponge spicules and associated spiculitic, fossiliferous micrite. The terrigenous clastic content constituted less than 20 percent of these sediments and consisted of fine quartz sand and silt. Chertification of the spiculitic sediments probably followed a course of inversion of opal to chalcedony and microcrystalline quartz soon after deposition. The micritic sediments were silicified shortly after deposition by a process of replacement of the micrite by silica derived from silica rich, interstitial water. The source of the silica in this water is due to an increase in silica in the ocean waters as a result of solution of "freed" sponge spicules in the immediate area, and entrapment of these silica rich ocean waters by the sediments at the time of sedimentation. Dolomite, in the form of scattered rhombs, replaced as much as 25 percent of the rocks. Most of these rhombs were subsequently replaced by chert. Acknowledgments This study grew out of a study of the stratigraphy of the Lower Devonian rocks of northwestern Georgia which was completed as a Master's thesis as part of the requirements for a degree from Emory University. My special thanks go t o Dr. H. R. Cramer for introducing me to the Armuchee and for his patience and per~uasion as my advisor throughout my work on these rocks. The following people also deserve a note of appreciation for their help and encouragement : Dr. P. K. Birkhead , Clemson University, for first calling my attention to the presence of sponge spicules in these rocks; Dr. Lewis Lipps, Shorter College, for supplying many paleontologic specimens; Drs. J. M. Dennison and D. A. Textoris, University of North Carolina, for encouragement and many enlightening discussions; and Dr. Textoris, T. M. Gathright, Jr., Virginia Division of Mineral Resources, and L. B. Howell, University of North Carolina for photographic assistance. Dr. Textoris critically reviewed the manuscript. Special thanks are due my wife, Elizabeth, for clerical assistance and moral support during this study. 65 Fig. 8. Chertified bioturbated micrite, negative print. The white areas are concentrations of limonite-coated silicified dolomite rhombs. Plain light. O'Brian Gap. Fig. 9. Chalcedony and microcrystalline quartz. Crossed nicols. O'Brian Gap, micrite facies. 66 TENNESSEE I \t.._, I t- ' _ _r-' I----, _J'-.._ -- 0~ JEMISON SEDIMENTS '0' ISOPACH LINE OF FROG MOUNTAIN SANDSTONE COARSEST FROG MOUNTAIN SEDIMENTS ARMUCHEE SEDIMENTS 0 10 20 30 40 0 10 20 30 40 so 60 I I I I I I KILOMETERS PALINSPASTIC BASE NUNAN 1972 Fig. 10. Paleogeographic setting of the Armuchee sediments. (Frog Mountain and Jemison data after Kiefer, 1970; base after Kiefer and DE,.~ -o- 1,000 N....... ~ ~ (9 -~ (D 0 .....LI--:M7:ii:le:;s-- I 0 25 Figure 2. Cross-Section A-A'. Cross-Section of the Tuscumbia-Monteagle-Floyd Interval in northwest Georgia and southeast Tennessee. Section A-A' is expanded 25 miles to accomodate thrust faulting. but they were not identified by genus. Lycopsid type plant fossils (Edward Stanley, personal communication, 1969) were found by the author in the uppermost Monteagle Limestone two miles east of Ringgold, Ga. The Floyd Shale is predominantly a brown to black, fissile, silty, carbonaceous, calcareous shale with intercalated beds of siltstone and limestone. lllite is the main clay mineral, with some kaolinite(+ 10%). Fossils are generally rare, and those that do occur are generally fenestrate bryozoa. Depositional Environment Because of numerous similarities (oolites, lime muds, dolomitization, cross-bedding, carbonate banks and reefs, etc.) between the rocks of the Monteagle and modern sediments of the Bahama Banks and Florida Bays, the Monteagle is interpreted to have formed in a similar environment. Many of the Mississippian carbonate rocks have direct analogues in the modern sediments. Ferm and Ehrlich (1967) say that the Floyd Shale probably represents deltaic front and pro-delta deposits . The aut hor is in agreement wit h t his in terpretation , but would like to qualify it somewhat. The great thickness and lack of shallo w water sedimentary structures (ripple marks , mud cracks, etc.} see m to indicat e t hat the Floyd was deposited rat her rapidly in deeper waters . The carbonaceous material and paucity of f ossils appear t o indicate that the general dep ositional environment was unfavorable to bent honic life and probably reducing. A modern environment showing several similar features is the offshore area of the Texas and Louisana Gulf Coast. References Blatt, H., Middleton, G., and Murray, R., 1972, Origin of sedimentary rocks: Prentice-Hall, Inc., Englewood Cliffs, N. J., 634 p. Butts, C., and Gildersleeve, B., 1948, Geology and mineral resources of the Paleozoic area in northwest Georgia: Georgia Geological Survey Bull. 54, 176 p. Ferm, J. C., and Eh rlich, R., 1967 , Petrology and stratigraphy of the Alabama coal fields, in Ferm, J. C. , Ehrlich , Rober t , and Neathery , T. C., ed ., A field guide to Carboniferous detrital rocks in north ern Alabama : Geol. Soc. America Guidebook for the 1967 Annual Field Trip of the Coal Division, University, Alabama, p. 11-15. Friedman, G. M., and Sanders, J. E., 1967, Origin and occurence of dolostones, in Chilingar, G. V., Bissell, H. J., and Fairbridge, R. W., ed., Carbonate Rocks, Volume I: Elsevier, Amsterdam, p. 267-317. Tiling, L. V., 1964, Penecontemporary dolomite in the Persian Gulf: Am. Assoc. Petrol. Geologists, v. 48, p. 532-533. McQueen, H. S., Hinchey, N. S., and Aid, K., 1941, The Lincoln Fold in Lincoln, Pike, and Ralls Counties, northeastern Missouri: Kansas Geol. Soc. Guidebook, 15th Ann. Field Conf., p. 99-110. Peterson, M. N. A., and von der Borch, C. C., 1965, Chert : Modern inorganic deposition in a carbonate-precipitating locality: Science, v. 149, p. 1501-1503. 73 ITINERARY 74 ROAD LOG - FIRST DAY Mileage Between Points Cumulative 0.0 0.0 0.3 0.3 0.4 0.7 0.3 1.0 6.4 7.4 1.6 9.0 8.3 17.3 Remarks . Leave the Albert Pick Motor Inn, Chattanooga, Tennessee on U.S. 41, driving south. Chattanooga Creek. Turn right onto 26th Street. Turn onto the entrance ramp of 1-24 South. Junction with 1-75. Head south on 1-75. Georgia-Tennessee boundary. Stop 1 - Ringgold road cut. STOP ONE -RINGGOLD ROAD CUT (T. M. Chowns and Robert W. Frey) The road cuts on 1-75 at Ringgold expose strata from Middle Ordovician (Chickamauga Limestone) to Mississippian (Floyd Shale) in age and provide the best section of the Silurian Red Mountain Formation in the state. Part of the section is scheduled as a geologic monument (Chowns, 1971). The Red Mountain Formation is one of the principaLridge formers in the western part of the Valley and Ridge Pwvince. Here, at Ringgold Gap, the formation_dips at about 15 and strikes approximately N 10 E. The Silurian cuesta is in two parts with a main escarpment to the west,.capped by the Whiteoak Mountain Sandstone, and a lesser escarpment to the east, consisting of alternating sandstones and shales at the top of the formation (Fig. 1 ). The complete section is given in Figure 4 and in the appendix, but may be summarized as follows: Red Mountain Formation (968') Sequatchie Formation (184') Catheys Formation Chattanooga Shale 4th road cut (east) Alternating sandstones and shales (21 7 ) - 3rd road cut i ll Red shales with thin sandstones (288')- 2nd road cut Gray shales with thin ironstones (158 ') - gap Massive crossbedded, red, quartz sandstones (73') Alternating sandstones and shales (232') Gray shales, siltstones and limestones- 1st road cut (west) Shellmound Member (104') Fine grained red sandstones-Inman- Leipers Member (80') Alternating fossiliferous limestones and shales (188' seen) 75 CHATTANOOGA SH. MISSISSIPPIAN RED MOUNTAIN FM . SEQUATCHIE FM. CHICKAMAUGA LS. !jjjj!jj!f!f!;::- ~::;-::"?:!j:::~ :=- ..- --~ /~ ~ ~~~z~ INTERSTATE 75 I \0 r--. I ONE MILE I VERTICAL EXAGGERATION 2 E::::)Aiternating Sandstone I:::::::J Shale Facies r:-1 Red Shale L:_JFacies D Gray Shale Facies r:D Qua~tz Sandstone WillFoc1es ~ Gray Sequatchie ~Facies ~Red Sequatchie ~Facies Figure 1. Geologic cross-section of strata exposed in the Ringgold road cuts. -..J -..J Figure 2. Alternating limestones and shales of the Catheys Formation, Ringgold (scale in feet). 00 r- Figure 3. Contact between the Catheys Formation and the Sequatchie Formation, Ringgold; a) Reddened catheys limest<:mes; b) Catheys limestone interbedded with red siltstones; c) Sequatchie red siltstones with rolled bryozoan fragments; d) Unfossiliferous Sequatchie red siltstones (scale in feet) . Figure 4. Red and gray calcareous siltstones and fine grained sandstones of the red Sequatchie facies , Ringgold (scale in feet). Figure 5. Red and gray mottled calcareous siltstone from the red Sequatchie facies, Ringgold. Figure 6. Graded bedding and dessication breccias from the red Sequatchie facies, Ringgold. Note vertical cracks are joints, not dessication cracks. 79 Figurro 7 . Ca s Lof !llll de rete ks in fine grnin ed calcaTe u us sands Lone from Uw rPcl Seq l.l