INTERPRETATION OF THE SEISMIC STRATIGRAPHY
OF THE
PHOSPHATIC MIDDLE MIOCENE ON THE GEORGIA CONTINENTAL SHELF
by J~A. Kellam
and V.J. Henry Geology Department Georgia State University

STATE OF GEORGIA DEPARTMENT OF NATURAL RESOURCES
J. Leonard Ledbetter Commissioner
ENVIRONMENTAL PROTECTION DIVISION Harold F. Reheis Assistant Director
GEORGIA GEOLOGIC SURVEY W.H. Mclemore State Geologist

CARTOGRAPHERS Mary Turlington John Hayden
prepared in cooperation with the U.S. Department Interior, Minerals Management Service

1986

Atlanta

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GEOLOGIC ATLAS 4

I I

TABLE OF CONTENTS
Introduction Seismic Tracklines and Boring Locations in the Study Area Cross Sections from Seismic Profiles Structure-Contour of the Top of the Oligocene-Age Sediments
Structure-Contour of the Base of the Middle Miocene-Age Sediments Structure-Contour of the Top of the Middle Miocene -Age Sediments Isopach of the Middle Miocene-Age Sediments Bathymetric Map

1 2 3- 4 5
6 7 8 9

INTRODUCTION

PLATE 1

Introduction
Mineral exploration in Georgia in the 1960's revealed the presence of extensive phosphorite deposits under the marshland and barrier islands in Chatham County, Georgia. This phosphorite, the Tybee Phosphorite Member of the Coosawhatchie Formation (Huddlestun, in press) also was noted in a local offshore boring, the Savannah Light Tower test hole located about 10 mil es east of Tybee Island. The phosphate concentration in the Tybee Phosphorite Member, as high as 29 .7% P20 5 (Zellars and Williams, 1978) and averaging 10% P20 5 (Furlow , 1969; Zellars and Williams, 1978), is roughly comparable to the phosphorite currently being mined in Florida and North Carolina. Studies during the 1960's demonstrated the economic feasibility of mining these phosphates from under the marshes (Cheatum, 1968). However, the marshes provide a unique ecological habitat and are important nutrient sources. They are economically more va luable in an unaltered state. For this reason, it was not deemed advisable to mine the marshes. Nevertheless, if the Tybee Phosphorite Member is present offshore in commercially exploitable deposits, the recents development of more envi ronmentally amenable subsea mining techniques may estab lish the Tybee Phosphorite Member as a valuab le resource for the future.
This atlas is prepared by the Georgia Geologic Survey with two objectives: (1) to delineate, through interpretation of seismic data, the stratigraphic framework of the phosphate-bearing strata, and 2) to evaluate the relationship of these strata to the unoerlying Pri ncipal Artesian Aquifer System, the major source of fresh water to coastal Georgia. The maps and cross sections produced were based on the compilation and integration of several earlier studies on the subsurface stratigraphy of the Georgia continental shelf. These previous studies relied heavily on interpretation of high resolution seismic reflection data and corre lation of the seismic data with the few lithologic logs or cores available on the Georgia continental shelf.
Phosphorites
Ph osphorite deposits generally consist of igneous apatites, guano, and the depositional products of marine environments. Presently, almost al l the economic phosphorite deposits are of shallow marine origin (Bushinsky, 1964; McKelvey, 1967). The Tybee Phosphorite Member of the Miocene Coosawhatchie Formation is also believed to have been formed in a shallow marine environment (Huddlestun, in press).
The element phosphorus predominantly
occurs in apatite, Ca3 (P04b a common com-
ponent of igneous rocks. After being weathered out, phosphorus is transported to the sea as phosphate (P04-3), or adsorbed on iron compounds, aluminum hydroxides and clays, or carried in dissolved organic compounds.
Phosphorus has a very low solubility in seawater (McKelvey, 1967). As a result it is a limiting nutrient for life in the ocean. Ocean water is generally almost saturated with phosphorus as a result of its low solubility, so that it is continuously ino rganically precipitated . A large percentage of this phosphorus is util ized by planktonic organisms, and much of it is eventually deposited by settling of these organisms after their death. In shallow water, deposition can occur before the phosphate can be dissolved, through chemical reactions, or be utilized by other organisms (Bushinsky, 1964; Riggs, 1979a; Birch, 1980; Wallace, 1980; Riggs, 1984). Deposition also commonly occurs through direct precipitation of phosphorus in regions of upwelling. Upwelling brings cold, phosphate-rich bottom water into contact with warm surface water. In the higher temperature and pH of surface water phosphorus is less soluble and thus precipitates out of solution (Bushinsky, 1964;Riggs, 1979b, 1984). Direct mineral replacement from sea water is aided by the presence of limy sediments, as concretions on calcareous material, such as skeletal fragments or fecal pellets, and as a result of the replacement of calcium carbonate with calcium phosphate (Ames, 1959; Birch, 1980; Wallace, 1980). Concentration is also aided where clastic or carbonate sedimentation is slow enough that the phosphate is not"diluted" by non-phosphatic material and where subsequent transport is restricted enough to prevent dissipation (Riggs, 1979b; Odin and Letol le, 1980).
The principal uses for phosphates in the United States are as fertilizer or feed supplements. Most of the domestic production of phosphate (87-91%) and about35%oftheworld's production comes from deposits in Florida and North Carolina (Zellars-Williams, 1978; Stowasser, 1983), from the upper Miocene/lower Pliocene Bone Valley Formation and the lower/middle Miocene

Pungo River and Pliocene Yorktown Formations, respectively. The "total identified resources in recoverable product tons" (Zellars and Williams, 1978) for the Tybee Phosphorite Member was estimated to be 3125 million short tons, 34% of the total for the North Carolina to Florida coastal plain. It must be recognized that any definitive statement on total recoverable ore will require extensive additional investigation, on and off-shore.
Regional Geology
General statement
The Atlantic continental margin is described as a passive trailing edge of the continent and has been relatively stable tectonicall y from the Cretaceous to the present. Tectonic activity has occurred only as tilting , subtle warpi ng , and minor faulting (See Figure 1). The Georg ia continental shelf occupies a broad, shal low reentrant about 70-80 miles wide. Water depth at the shelf break is 150-200 feet. This contrasts with the usual shelf break of about 300-350 feet. (Figure 2) . The Georgia shelf is bordered on the west by a series of Pleistocene to Holocene barrier islands and tidal inlets.
The Coastal Plain and continental shelf of Georgia consist of a series of seaward dipping and variably thickening sedimentary wedges of early Cretaceous to Holocene age. The "basement" consists of Precambrian and Paleozoic igneous and metamorphic rocks overlain by lower Mesozoic clastics. The Jurassic/Cretaceous contact is located at an approximate depth of 3,500 to 4,500 feet below sea level beneath the coastal area.
The major structural feature of the region is the Southeast Georg ia Embayment, bounded by the Cape Fear Arch at the north and the Peninsular Arch to the south (see Figure 1). The embayment opens and deepens to the southeast. Beaufort Arch and "Midd le Shelf High" (ldris, 1983) are northeast-to- southwest trend ing highs which have been interpreted as being depositional in origin and influenced by differential compaction over underlying structures (Foley, 1981; Popenoe, personal communication). Foley (1981) shows a "M id She lf Low" in the southern portion of the study area. He interprets it as a deep trough bounded by a Miocene scarp to the west and the "Outer Shelf High" to the east. This low shallows northward, and deepens and broadens to the south. This "Outer Shelf High" trends north-south with positive relief increasing northward in the study area. Foley (1981) interpreted the geometry of the upper Miocene sediments overlying the Outer Shelf High as indicating deposition on an inclined and uplifted surface of middle Miocene sediments. Stratigraphy
The Neogene stratigraphy of the Coastal Plain of Georgia has been the subject of extensive study since the 1950's, involving ground water and aquifer investigations, and the definition of stratigraphic units. For consistency the recent and comprehensive study by Huddl estun (in press) is used throughout this study. (See Table 1) .
Although the Neogene of the Georg ia Coastal Plain has received detailed study, little biostratigraphic control is available for the adjacent continenta l shelf. Borings and high resolution seismic data were obtained in the vicinity of the Savannah Light Tower (Porter and Associates, 1962; McCollum and Herrick, 1964). Deep dril ling from the 1965 J .O.I.D.E.S. expedition was used to correlate on- and offshore stratigraphy east of Jacksonville, Florida (Bu nce and others, 1965; Schlee and Gerard, 1965). Other core information concerning offshore stratigraphy was obtained from the AMCOR 6002 boring (Hat haway and others, 1976; Hathaway and others, 1979), COST GE-1 (Scholle, ed., 1979), and the Georgia Geologic Survey core Cumberland Island 1 (GGS -3426).
Studies describing shall ow high resolution seismic data obtained in the study area include Woosley (1977) , Henry (1983), Henry and others (1978), Foley (1981), and Kellam (1981) on fi le with the Geology Department of Georgia State University. Seismic data obtained on the U.S. Geologic Survey R/V Gillis cruise of August 1979 (Popenoe, 1983), provided for direct corre lation with the J.O.I.D.E.S and Cost GE-1 borings and were particularly useful in tying together the studies in the northern portion of the study area with those in the south. Th ese data revea l a regional continuity of Coastal Plain and continental shelf stratigraphy and establish the basis for a generalized correlation of seismic stratigraphy of the study area to regional biostratigraphy.

FIGURE 1 STRUCTURE OF THE GEORGIA CONTINENTAL SHELF
80 30'

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approx imate location of axis of feature

0

15

Nautical miles

After Woo lsey 1977; Fo ley 1981; ld ris 1983

TABLE 1 GENERALIZED STRATIGRAPHIC NOMENCLATURE OF THE GEORGIA INNER CONTINENTAL SHELF

QUATERNARY PLIOCENE

NORTHERN undiffe rentiated
DUPLIN MARL

SOU THERN
undiffere ntiated DUPLIN MARL EQUIVALENT

MIOCENE upper
middle

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Ebenezer Member

Charlton Member
Berryv i ll e Clay
Member

und ifferen tiated
Ea st
Coosaw -hatchie Berryville Clay Formation Member

lower
OLIGOCENE EOCENE

Marks Head Formation
Parachucla Formation
Lazaretto Creek Formation Oca la Formation
Santee Formation

Ocala Format ion & Equ ivalent Fro m Huddlestun (in pre ss)

FIGURE 2 LOCATION MAP

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Bea ufort Arch

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As shown in Table 1, Tertiary stratigraphy in the study area ca n be divided into four seism ic st ratigraphic sequences: 1) the late Oligocene Lazaretto Creek Formation and the age equivalent portion of the Cooper Marl; 2) the lower Miocene, consisting of the Parachucla and Marks Head Formation s; 3) middle Miocene, Coosawhatchie Form ation; 4) post midd le Miocene, consisting of undifferentiated upper Miocene, Pliocene (Duplin Formation equivalent, and younger und ifferentiated sediments), and undifferentiated Quarternary sediments. The ages of the units and the hiatuses separating them, based on seismic reflector picks and correlation with co res, show good correlation with accepted Tertiary eustatic sea levels and changes proposed by Vail and others (1977).
The Tybee Phosphorite Member is the basal unit of the middle Miocene Coosawhatchie Formation. Due to its relative t hin ness and a paucity of core and boring information from th e shelf, it is not possible to identify the Tybee Phosphorite Member as a unique reflector on the seism ic records. Since it is the basal member of the Coosawhatchie Formation, its geographic extent is more or less represented by the structurecontour of the base of the midd le Miocene (Plate 6).
The Principal Artesian Aquifer System is a comp lex of Eocene, Oligocene, and Miocene uni ts. Facies changes occur within these units on the shelf, as seen in the avai lable litholog ic data. As a result, identification of the "top" of the aquifer and its seaward extent is generally con jectural. A clarification ofthe stratig raphic relationsh ip between the phosphorite and the aq uifer system is necessary before development of the mineral potential can begin. It is extremely important to protect this valuable aquifer and its aq uitard . Breach ing of the aquifer could result in contamination by seawater.
METHODS
This study utilized seismic stratigraphi c crosssections from several previous stud ies as presented on Plate 2.
Fi rst the various interpretations were reconciled to insure th at the picks made for specific horizons, correlated across the study area. In order to provided a unified interpretation, addition al information was obtained by exam ination of previously uninterpreted seismic records acqui red from the U.S. Geological Survey. After all ava ilab le data were reconciled, the prominent reflectors were transposed to stratigraphic crosssections at a common scale.
Plates 3 and 4 are cross-sections chosen to provide a three-dimensional picture of the stratigraph ic framework of the Georg ia continental shelf. Isopach and structure-contour maps were prepared using the data from the cross-sections (Plates 5-9).
SOURCES
Ames, Jr., L.L., 1959, The genesis of carbonate apatites, Economic Geology, v. 54, p. 829-841.
Birch , G.F. , 1930, A model of precontemporaneous phosphatination by di agenetic and authegenic mechanisms from the western margin of southern Africa, in Marine Phosphorites; A symposium, S.E.P.M. spec. publ. 29, p. 79-100.
Bunce, E.T., and others, 1965, Ocean dril ling on the continental margin, Science, v. 150 , no. 3697, p. 709-716 .
Bushinsky, G.l., 1964, On shallow water origin of phosphorite sediments, in Developments in sed imentology, L.M.J.V. vanStraaten, Elsevier, p. 62-70 .
Cheatum, E.L., and others, 1968, Report on the proposed leasing of state-owned lands for phosphate mining, University System of Georgia, 125 p.
Foley, F.D., 1981, Neogene seismic stratigraphy and deposition al history of the lower Georgia coast and continental shelf, unpublished MS thesis, University of Georgia, 81 p.
Furlow, J.W., 1969, Stratigraphy and economic geology of the eastern Chatham County phosphate deposit, Georgia Geol. Survey Bull. 82,40 p.
Hathaway, J.C ., and others, 1976, Preli minary summary of the 1976 Atlantic Margin Coring Project, U.S. Geol. Survey Open-fi le Report 76-844, 218 p.
_ _ __ , and others, 1979, U.S. Geological Survey core drilling on the Atlantic shelf, Science, v. 206, no. 4418, p. 515-527.
Henry, V.J., 1983, Final Report, Ocean Bottom Survey of the U.S. South Atlantic OCS region, report for the U.S. Geological Survey, Contract No. 14-08-0001-06266, 99 p.

Henry, V.J., and others, 1978, Geological evaluation of potential pipeline corridor sites along the Georgia coast. Final Report, phase I, Georgia Office of Planning and Budget, 101 p.
Huddlestun, P.F., in press, A revision of the Iithostratig raphic units of the coastal plain of Georgia, Georgia Geologic Survey Bull. 104
ldris, F.M ., 1983, Cenozoic seismic stratigraphy and structure of the South Carolina lower coastal plain and continental shelf, PhD dissertation, Univ. of Georgia, 159 p.
Kel lam, J.A., 1981, Neogene seismic stratigraphy and depositional history of the Tybee Trough area, Georgia/South Carolina, unpublished MS thesis, Un iversity of Georgia, 111 p.
McCollum, M.J., and Herrick, S.M ., 1964, Offshore extension of the upper Eocene to Recent stratigraphic sequence in southeastern Georgia, U.S. Geol. Survey Prof. Paper 50 1-C, p. 61 C-B3C.
McKelvey, V. E. , 1967, Phosphate deposits, U.S. Geol. Survey Bull. 1252-D, p. 1-21 .
Odin, G.S. and Letolle, R., 1980, Glauconitization and phosphatization environments: A tentative comparison in Mari ne Phosphorites; A symposium, S.E.P.M. spec. publ. 29, p. 227238,
Pauli , C.K., and W.P. Dillon, 1980, Structure, stratigraphy, and geologic history of the Florida/ Hatteras Shelf and inner Blake Plateau, A m. Assoc. Pet. Geol. Bull. v. 64, no. 3, p. 338-358.
Popenoe, P., 1983, High resolution seismic reflection profile collected August 4-28, 1979, between Cape Hatteras and Cape Fear, North Carolina and off Georgia and northern Florida, U.S. Geol. Survey Open-File Report 83-582,4 p.
Porter, N ., and Associates, 1962, Hydrographic and geologic surveys for offshore structure project, Savannah, Georgia, unpublished report , 17 p.
Riggs, S.R., 1979a, Petrology of the Tertiary phosphorite system of Florida, Economic Ecology, v. 74, p. 195-220.
_ ___, 1979b, Phosphorite Sedimentation in Florida - a model phosphogenic system, ibid. p. 285-314.
- - - -, 1984, Paleoceanographic model of Neogene phosphorite deposition , U.S. Atlantic Continental Margin, Science, v. 223, no. 4632, p. 123-131 .
Schlee, J. , and Gerard, R., 1965, Cruise report and preliminary core log M/V Caldrilll-17 April to 17 May 1965, J.O.I.D .E.S. Blake Panel Report, unpublished report, 64 p.
Scholle, P.A. (ed.). 1979, Geolog ical studies of the COST GE-1 well, United States south Atlantic outer continenta l shelf area, U.S. Geol. Survey Circ . 800, 114 p.
Stowasser, W.F., 1983, Phosphate rock, Minerals commodity profiles, Bureau of Mines, U.S. Dept. of Interior, 18 p.
Un published data on fi le at the Geology Dept. of Georgia State University.
Vail, P.R., Mitchum, C.M., and Thompson, S., 1977; Global cycles of relati ve changes in sea level. In Payton, C .E. (Ed.) Seismic stratigraphy -applications to hydrocarbon exploration . American Assoc. Pet. Geol. Memoir 26: p. 83-98.
Wa llace, R.J., 1980, The origin and diagenes is of the phosphate deposit in the middle Miocene Hawthorne Format ion in north east Chatham County, Georgia, unpubl ished MS th esis, Universi ty of Kansas, 71 p.
Woolsey, J.R., 1977, Neoge ne stratigraphy of the Georgia coast and inner co ntinenta l shelf , unpublished PhD dissertation, University of Georg ia, 222 p.
Zellars, M.E., 1978, The genesis and occurrence of Tert iary phosphorite in the Southeastern United States, Zellars- Will iams Inc., 5 p.
Zellars-Wi ll iams, Inc., 1978, Eva luat ion of the phosphate deposits of Georgia, North Carolina, and South Carolina using the minerals avail abi lity syste m, U.S . Dept. of Interior contract, 65 p.
- - -1979, Phosphates - Offshore Georgia and South Carol ina, U.S. Geol. Survey, unpublished report, 30 p.

The data for this study consists of a compilation of information obtained from previous studies of the shal low stratigraphy of the continental shelf of Georg ia. The high resolution seismic data used in these studies were gathered by a cooperative effort of the Un iversity of Georgia Marine Geology program at the Skidaway Institute of Oceanography, Savannah, Georgia, and the U.S. Geological Survey at Woods Hole, Massachusetts .
Seismic profiles from the studies were correlated, integrated, and compared with lithologic data from the six test holes or borings located in or adjacent to the study area. In areas where data from previous studies was not available, uninterpreted data were examined and correlated with existing produced showing structure-contour and isopach information pertain ing to the phosphatic strata and aquifer system under th8 continental shelf of Georgia.

SEISMIC TRACKLINES and
BORING LOCATIONS IN THE STUDY AREA
+

PLATE 2

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SOURCES

EXPLANATION

Bunce, E.T. , and others, 1965, Ocean drilling on

the continenta l margin, Science, v. 150, no.

Kellam 1981

3697, p. 709 -716 . Foley, F. D., 1981, Neogene seismic stratigraphy

+

and deposit ional history of the lower Geor-

This st udy (Data from Popenoe, 1983)

gia coast and continental shelf, unpublished

. ..:
+
:

\ \

\

\

\

\

\

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MS thesis, University of Georgia, 81 p.

Foley 1981

Hathaway, J.C., and others, 1976, Prel iminary summary of the 1976 Atla ntic Margin Coring Project, U.S. Geol. Survey Open-fi le Report 76-844, 218 p.
____ , and others, 1979, U.S . Geological Survey core drill ing on the Atlantic shelf,

Henry 1983 Woolsey 1977

... ........

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......

......

......

, \ I

Science, v. 206 , no. 4418, p. 515-527. Henry, V.J., and others, 1978, Geological evalua-

Henry and others 1978

tion of potential pipeli ne corridor sites along

the Georgia coast. Final Report, phase I, Georgia Office of Planning and Budget, 101

Used as cross section lines in this study

p. Kellam, J.A., 1981 , Neogene seismic stratigraphy

0

and depositional history of the Tybee Trough

area, Georgia/South Caro lina, unpublished

MS thesis, University of Georgia, 111 p.

T est hole, core or boring

/ . ..:

McCollum, M.J., and Herrick, S.M., 1964, Offshore extension of the upper Eocene to Recent stratigraphic sequence in southeast-

SCALE 1A49,659

0

5

10 Nautical miles

/

OAMCOR 6002

ern Georgia, U.S. Geol. Survey Prof. Paper

501-C, p. 61 C-63C. Schlee, J., and Gerard, R., 1965 , Cru ise report
and pre liminary core log M/V Caldrill l - 17 April to 17 May 1965, J.O.I.D .E.S. Blake Panel Report, unpublished report, 64 p. Scholle, P.A. (ed .), 1979, Geological studies of the COST GE-1 wel l, United States south Atlantic outer continental she lf area, U.S. Geol. Survey Circ. 800, 114 p. Unpublished data on file at the Geology Dept. of Georgia State University. Unpublished data on f ile at t he Georgia Geologic Survey, Atlanta. Woolsey, J.R., 1977, Neogene stratigraphy of the Georgia coast and inner continental shelf, un published PhD dissertation, Un ivers ity of Georgia, 222 p.

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STUDY AREA LOCATION

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Base from Nati ona l Oceanic and Atmosph er ic Adm inistrat ion Char leston Light t o Cape Canaveral navig<:~tion map, 1984

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CROSS SECTIONS FROM SEISMIC PROFILES

PLATE 3

A ''TSe~a lGeve~l -G=----------------------------------------------------------F--f-' ~~------------------------~A'

Plates 3 and 4 present a series of cross sections derived from high resolution seismic information. These cross sect ion s were chosen to provide a representative grid of stratigraphic sections oriented roughly parallel and perpendicular to the coast and to the structural trend of the continental shelf of Georgia.
One of the few significant features on this portion of the shelf is that the north-south trending Beaufort Arch is seen as very subtle arching of Oligocene to middle Miocene strata in cross sections D-D' and E-E'. The major "structural" influ ences in the southern half of the mapped area are the shore-parallel "Mid Shelf Low" and "Outer Shelf High" (Foley, 1981 ), (see cross-sections F-F, G-G, Plate 4, this atlas). These features involve Oligocene to Middle Miocene strata, but may be both depositional and erosional rather than structural in origin.
Oligocene deposits consist of calcareous oozes deposited in a deep marine environment such as the outer shelf and slope. Generally where it is detectable with high resolution techniques, the Oligocene carbonates are represented on seismic reco rds by a few discontinuous internal reflectors. This is due either to homogeneity of its constituents or to the attenuat ion of t he acoustic signal , or both . The upper surface of the Oligocene is hummocky on the seismic records, as seen east of Tybee Island (Kellam, 1981) (see Plate 5, this atlas). The presence of this extensive erosion suggests subaerial exposure during late Oligocene time.
The lower Miocene strata consists of shallow marine, interbedded terrigenous clay and sand grading to the east and south into a calcareous argillaceous "ooze" like that found in the AM COR 6002 boring (Hathaway and others, 1976). The clastic facies, on seismic records , shows a discontinuous internal banding indicating the interbedding of sand and clay which resu lted from continual minor fluctuations of sea level. The surface of the lower Miocene also shows a hummocky surface, apparently the effect of subaerial erosion.
Middle Miocene strata consists of phosphatic, shallow marine, terrigenous clay and sa nd in the Savannah Light Tower boring, the GGS- 3426 core, and the J.O.I.D.E.S. J-1 boring, but grade in

SOURCES
Bunce, E.T., and others, 1965, Ocean drilling on the continental margin, Science, v. 150, no. 3697 , p. 709-716.
Foley, F.D., 1981, Neogene seismic stratigraphy and depositional history of the lower Georgia coast and continental shelf, unpublished MS thesis, University of Georgia, 81 p.
Hathaway, J.C., and others, 1976, Preliminary summary of the 1976 Atlantic Margin Coring Project, U.S. Geol. Survey Open-file Report 76-844, 218 p.
- - - - and others, 1979, U.S. Geological Survey core drilling on the Atlantic shelf, Science, v. 206, no. 4418, p. 515-527 .
Henry, V.J., and others, 1978, Geological evaluation of potential pipeline corridor sites along the Georgia coast. Final Report, phase I, Georgia Office of Planning and Budget, 101 p.
Huddlestun, P.F., in press, A revision of the lithostratigraphic units of the coastal plain of Georgia, Georgia Geologic Survey Bull. 104
Kellam, J.A., 1981, Neogene seismic stratigraphy and depositional history of the Tybee Trough area, Georgia/South Carolina, unpublished MS thesis, University of Georgia, 111 p.
McCollum , M.J., and Herrick, S.M., 1964, Offshore extension of the upper Eocene to Recent stratigraphic sequence in southeastern Georgia, U.S. Geol. Survey Prof. Paper 501-C, p. 61 C-63C.
Schlee, J., and Gerard, R., 1965, Cruise report and preliminary core log M/V Caldrilll-17 April to 17 May 1965, J.O.I.D.E.S. Blake Panel Report, unpublished report, 64 p.
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.
Unpublished data on file at the Geology Dept. of Georgia State University.
Unpublished data on file at the Georgia Geologic Survey, Atlanta.
Woolsey, J.R., 1977, Neogene stratigraphy of the Georgia coast and inner continental shelf unpublished PhD dissertation, University of Georgia, 222 p.

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--.--------.:.-::-:::---_------_--:--==_--~=-=~---:----=--:----:--:-~:---.-:-~-.--:---~.-:----_:::-:-2:--::.---~~--~-7---;------~:_---_:---~:-:----==--=::;:::-:::;-_-;_:_;----_--;.--_---_~--.--_~---_---:_---:-_--=..--::-------=-----:---;-------:--~=-~-~-----:----'----?-----~--=-----.-----:-----:-----:-----.----7---:---:---:-=---=---:----~-_:-_-: -:-----=_Q-=p_---=---.......:. _:_ -:_:_ ~~------ -----~----

--
--

-J~0--- ~ ;:---~:-::-------Q-~-----:-:-----~-----~-'11-.-

40--- p

"

60-

- -- -

p

80 -- - - ----r;~ ~

1------- - ~

100- ------- - - - -
120 I /0/E?
140-1/

p
M----

-

--o-M -----..-

?-----

? --

UM.~ / - - -

----- ~~? mM M IM

-

-

-

-

-

-

-
-

-

-

/
-~----

:-:-:-~~-- ~-~=-~~-~-~~-=-~==:::::==========:o~::i:-:u=~m-M~-~:M:J--m~--l-=l-~~=J~!==~=t==--;-~=;-=~=;~~~-;--~--~~~~~~:--~-:-~=-~1=-lr--

IM

--
-----1M __________ --- -

0

- --- ------ - - - - -- _ _ __ _..,.--------- 0 -

1-

160 -

....

to a variably siliceous, clastic, calcareous clay in

the AM COR 6002 (Foley, 1981). The terrigenous

facies exhibits a very characteristi c banding on seismic records . This banding is strong, continuous, and laterally extensive, making the middle Miocene easily identifiable. The upper surface of

8'

Sea level

E-E'

D-D'

8

'

the Middle Miocene strata is an extensive erosion

surface. It is possible that winnowing and concen-

tration of phosphatic material (in topographic

lows such as stream chan nels or scour holes) are the result of the same processes that produced the erosion surface. This erosion is seen in the northern portion of the study area in the form of the Tybee Trough (cross section (D-O"). To the

--- --- ----

IM ----------- a--------

-

___ _ _ _ _ - - - - - ---'1M~ ,____

. .. ___ _ ----- ----- -

0

/

------ ___ -~----- -- ---

_..,---
1-

south erosion is manifested by the erosional

scarp landward of the Mid Shelf Low and by the ancestral Altamaha River (Foley, 1981) (see Plate

140-

_/

7, this atlas).

The top of the middle Miocene generally dips

160

to the east and south (Plate 7). Middle Miocene

strata are much closer to the surface in the

northern portion of the study area, and crop out

in places. Additionally, much of the overburden

has been removed from the crest of the Beaufort

Arch. The middle Miocene has been removed

from the nose of the arch (cross section D-D'). To the south and east, Pliocene sediments
overlie the Miocene in an on-lapping depositional relationship (cross sections A-A', B-B', C-C ', this

c" G-G' Sea level

F-F'

c'

plate) . Upper Miocene sed iments are considered to be present only as a wedge deposited on the landward flank of the "Outer Shelf High" (cross section F-F') and probab ly as wedges on lapping at the eastern extent of the mapped area, at the shelf break (cross sections D-D' and E-E').
The Tybee Phosphorite Member of the middle Miocene Coosawhatchie Formation is seen under Tybee Island, in the Savannah Light Tower boring, and in the G.G.S. 3426 core on Cumberland Island. It is not present in the three southe rn

40- ---------------------~- ~ ~ 20-~--~-----~-----------==::::::-----------------~-------------

-

p

Q p

100

-

120

0/E?

~ ----=----- -- - ------ - - - - - -

___--------~P;-----~~==~~~~~~-~-~

-- rnM ------

--------- '-~

------ ~!--~--;; ~~ '-=:==- ===:::::::1:M::::::-:-:-_----/. - ------ -----'----- --- -

offshore borings. The phosphatic Middle Miocene sediments, in which t he Tybee Phosphorite Member is contained, are most accessible in the

140
160-

northern portion of the study area, where they

occur in relative ly shallow water, under only a

thin layer of overburden. Also, in this northern area the phosphorite zone and the aquifer system appears to be separated by a substantial aqui-

VERTICAL _Sc;A L E GR E A TL Y EXA GGERATED VE RTICAL SCAL E IN M ETERS

clude, the Marks Head Formation of early Mio-

D AT U M IS M E AN SEA LE V I.:. L

cene age. A final decision on the commercial and

environmental feasibility for development of the

phosphate resource will require the gathering of

add itional litho log ic and stratigraphic informa-

tion.

C ' _ ~S~e~"~'~~ve~l ___________________________________________________________________________________________________________________________________________________________________________________~

c

PLATE 4

120

-

1-

140
-

160

D Sea level
20
40 ~
60
-------
80
100

E

Sea level

Q

--- - 20 - -----,.<.R.

-

-

40

~mM

~

LM?

60

0?

80 -

- 100

F Sea level

20

-......________--------

p--

40 60

'-.....' -----

p
M-

-

80
100 --- ----- -- ----

-

G Sea level

B-B'

A-A'

0'

0

lM

0

C-C'
I
-- 9p mM =-r= -- --I-M0--------
C- C'

B-B'
I

-=----

-

s.---....~ ._- -

--.

-

-

.

-

-

- -- - -
~

- ~- ~- - - -

-~

- p p -- -- ---- -

-

-

"/m ' M
- ---

-

-

--

lM

~ 0

B-B'

~ IM mM

p
---- ~...~~M~,---------------------- mM? 1M
~
0
C-C'

_-_--__-_-_-_-_-_------~~~:=::_-;~_-=;===:.____=====~=-~-- -
IM ? 0?
B-B'

--
-- ---- ------- -- ---.~~..
mM lM

10M - - - -

A-A'
I

Q
p
- --

- -

--

--

- --

,o"::_Q_ mM

-- ---

--

~ ~--

JM

- -----

-lcM,-

a:
o"0 '

A-A'

~
~ IM

E'

f-

- - - =--------

---- ~ --

~7r& s- ~=- ==- ==~~

--

f-

F'

uM uM

mM
- ----IM -- -~ --- IM---- -- ---

- ---- ----

0

A-A'

p
Q

---

--- --

' ~ M--------- -- --------------

--- ---
G'

60

80

100

~ mM? - -

120

140

TRACKLINE LOCATIONS

Tybee I sla~

D

c?E
_)
)
SIsaa~n.e..d. ~
g F
)
J
u m b e r l) n d Island
G

B"
A"

A

D' E'

F'

N

G'

1

~ 0/E?
LOC ATION MAP N
1

- - -------- ------ ~- - - - - - ---- -
VERTICAL SCALE GREATLY EXAGGERATED VERT ICAL SCA L E IN METERS DATUM IS MEAN SEA LEVEL

- - - - -------- 9___
p
----
--.

0

100

Miles

This plate presents structure-contours on the top of the Oligocene sediments on the continental shelf of Georgia. This surface is important in that it gives an indication of the top of the aquifer underlying the phosphoritic zone. It is recognized that the Principal Artesian Aquifer of the Coastal Plain is a complex system of sediments ranging in age from Eocene to Miocene. At this time, data are insufficient to precisely locate fresh water-bearing strata under the continental shelf. The elevation of the top of the Oligocene gives a reasonable indi cat ion of the relationship of the potential aquifer system to the Middle Miocene phosphorite-bearing zone.
In the northern portion of the study area, the top of the Oligocene reflector correlates with the top of the Lazaretto Creek Formation (Huddlestun , in press), a san dy limestone/ca lcareous sand id ent ified in the Georgia Geologic Survey test well on Skidaway Island, and in the Savannah Light Tower test boring 10 miles east of Tybee Island. To the south, in the AMCOR 6002, J.O.I.D.E.S. J-1 , and COST GE-1 test holes, Oligocene-aged sediment is an argillaceous, calcareous "ooze" which correlates with the Cooper Marl, with little potential as an aquifer. In the southwest portion of the study area, the Oligocene is absent. The Ocala Limestone is directly overlain by th e lower Miocene Parachucla Formation. The base of the Parachucla Formation in this region is composed of interlayered terrigenous clay and limestone/ marl.
The regional "structure" is reflected in the structure-contour map of the "top of the Oligocene", with the Beaufort Arch along the coast, th e "Middle Shelf High" and the Mid Shelf Low and Outer Shelf High. The "Mid Shelf Low" and "Outer Shelf High" referred to by Foley (1981) appear to be components of the Eocene-age Suwannee Channel proposed by Pinel and Popenoe (1985). On this plate, th e "Mid Shelf Low" represents Oligocene sediments overlying the inlet axis of th e channel, while the "Outer Shelf High" represents those overlying the northern fl ank as described by Popenoe (1985). Some evidence of the erosional nature of the late Oligocene/early Miocene hiat us can be seen. On seismic sections (Plates 3 and 4), the erosion surface often shows a hummocky character of a scale too small to be defined on this map. Several depressions interpreted as being karstic in nature (Kellam, 1981) are seen in the north ern portion of the study area.

SOURCES
Bunce, E.T., and others, 1965, Ocean drilling on the continental margin, Science, v. 150, no. 3697, p. 709-716 .
Foley, F.D. , 1981, Neogene seismic stratigraphy and depositional hi story of the lower Georgia coast and continental shelf, unpublished MS thesis, University of Georgia, 81 p.
Hathaway, J.C., and others, 1976, Preliminary summary of the 1976 Atlantic Margin Coring Project, U.S. Geol. Survey Open-file Report 76-844, 218 p.
_ _ _ , and others, 1979, U.S. Geological Survey core drilling on the Atlantic shelf, Science, v. 206, no. 4418, p. 515-527.
Henry, V.J., and others, 1978, Geological evaluation of potential pipeline corridor sites along the Georgia coast. Final Report, phase I, Georgia Office of Planning and Budget, 101
p.
ldris, F.M., 1983, Cenozoic seismic stratigraphy and structure of the South Carolina lower coastal plain and continental shelf, PhD dissertation, Univ. of Georgia, 159 p.
Kellam, J.A., 1981, Neogene seismic stratigraphy and depositional history of the Tybee Trough area, Georgia/South Carolina, unpublished MS thesis, University of Georgia, 111 p.
McCollum, M.J., and Herrick, S.M., 1964, Offshore extension of the upper Eocene to Recent stratigraphic sequence in southeastern Georgia, U.S. Geol. Survey Prof. Paper 501-C, p. 61 C-83C.
Pi net, P.R., and Popenoe, P., 1985, A scenario of Mesozoic-Cenozoic ocean circulation over the Blake Plateau and its environs, Geol. Soc. Am. Bull. v. 96, p. 618-829.
Schlee, J., and Gerard, R., 1965, Cruise report and preliminary core log M/V Caldrilll- 17 April to 17 May 1965, J.O.I.D.E.S. Blake Pan el Report, unpublished report, 64 p.
Scholle, P.A. (ed.), 1979, Geolog ical studies of the COST GE-1 well, United States south Atlantic outer continental shelf area, U.S. Geol. Survey Circ. 800, 114 p.
Unpublished data on file at the Geology Dept. of Georgia State University.
Unpublished data on file at the Georgia Geologic Survey, Atlanta.
Woolsey, J.R ., 1977, Neogene stratigrap hy of the Georgia coast and inn er continental shelf, unpublished PhD dissertation, University of Georgia, 222 p.

EXPLANATION
7 5 - Structure Contour- Shows li ne of e levation
below mean sea level of erosion surface of Ol igoce ne-age sed iments. Con tour interval is 5 meters . Dashed whe re approximate. Datum is mean sea level.

SCALE 1:449,659

0

5

10 Nau t ical miles

___ - --~- --- - -- --""~,
SOU TH CARO LINA

STRUCTURE-CONTOUR MAP
of the
TOP OF THE OLIGOCENE-AGE SEDIMENTS

I II I II

~

\

I

+

I I I

/I I I I III I

+

I

I
@1 I I I

I

80

I

--- 80
as - - - -

/
/
~

I I
I I

I

I

I

I

I

/Q
0

I

\

I

\

\

\

\

\

\
\ -t

+

I I I
I I I I I

------

\

.....__ ....._....._
-- - - - -- 720--- ------
""" -- \

\

\

-\ \

I

\ \

J

\ \

I

-\ \ I

II I
I I I -1 I
I I

I I \

/ /
/
........ /

J

+

PLATE 5
32 31 30'
31

or.EAN
STUDY AREA LOCATION
Base from National Oceanic an d Atmosphe r ic Administration Char lesto n Light to Cape Canavera l nav igation map, 1984

(0)
135>'

./

/

I

/ /
I

/ I I

80 30'

80

I

STRUCTURE-CONTOUR MAP
of the
BASE OF THE MIDDLE MIOCENE-AGE SEDIMENTS I

This plate presents structure-contours of the base of the middle Miocene Coosawhatch ie

SOURCES

Formation on the continental shelf of Georgia.

Foley, F.D., 1981, Neogene seismic stratigraphy

The basal un it of the Coosawhatch ie Formati on

and depositiona l history of the lower Geor-

in th e coastal area, recently given the name

gia coast and continenta l shelf, unpub li shed

"Tybee Phosphori te Member" by Huddlestun (in

MS thesis, University of Georqia, 81 p.

press), contains considerable phosphorite.

Hathaway, J.C ., and others, 1976, Preliminary

According to Huddlestun (in press), the Tybee

summary of the 1976 Atlantic Marg in Corin g

Phosphorite Member averages 20 feet in thickness in coastal Chatham County with a thickness

Project, U.S. Geol. Survey Open-fil e Report 76-844, 218 p.

of 33 feet under southern Ty bee Island. This unit

---~. and others, 1979, U.S. Geological

thins to 1-2 feet in northwestern Chatham Cou n-

Survey core drilling on the Atlantic shelf,

ty. It is about 7.5 feet thick in coastal Bryan

Science, v. 206, no. 4418 , p. 515-527.

County and 9 feet thick in the G .G.S. 3426 core

Henry, V.J., and others, 1978, Geological evalua-

on Cumberland Island. The phosphorite has

tion of potential pipeli ne corridor sites along

been reported, in a simi lar thickness (about 30 ft.) to that under Tybee Is land, in the Savannah Light Tower test hole (McCollu m and He rrick, 1964).

the Georgia coast. Final Report, phase I, Georgia Office of Planning and Budget, 101 p. Huddlestun , P.F., in press, A revision of the

A subsurface feature of potential interest in

lithostratigraphic units of the coastal plain of

t he developm ent of the phosphorite resou rce

Georgia, Georgia Geologic Survey Bull. 104

+

occurs east of the Savannah Light Tower. This

Kel lam, J.A. , 1981, Neogene seismic stratigraphy

feature is the Tybee Trough. Believed to be the

and depositional history of the Tybee Trough

buried remnant of a barrier island/tida l inl et

area, Georgia/South Carolina, unpubl ished

comp lex, the T ybee Trough is manifested in the

MS thesis, University of Georg ia, 111 p.

subsurface as a cluster of channels cutting

McCollum, M.J., and Herrick, S.M., 1964, Off-

middle Miocene sed im ents (Kell am, 1981). Seis-

shore extension of the upper Eocene to

mic evidence of cut-an d-fill and other comp lex

Recent stratigrap hic seq uence in southeast-

fi ll structures could rep resent winnowing and

ern Georgia, U.S. Geol. Survey Prof. Paper

concentration of phosphatic material. Some of these chan nels are as much as 120-130 feet deep. On the surface of the middle Miocene, this channe ling is evident as a c lu ster of negative reli ef features (Plate 7). However, negative features on Plate 6 appear to be inherited from lows on the un derlying surface.
As a general trend, the base of the middle Miocene can be seen to deepen sout hward from a minimum of less than 82 feet adjacent to Tybee Island to a maximum of more than 395 feet in the Mid Shelf Low in t he southern port ion of the study area.
In the northern portion of the study area, about 20 miles east of the northern end of Tybee Island, a previously unmapped high is defined. Unpublished data obtained from the Georgia

501 -C, p. 61 C-63C. Schlee, J., and Gerard, R., 1965, Cruise report
and preliminary core log M/V Caldrilll- 17 April to 17 May 1965, J.O.I.D.E.S. Blake Panel Report, unpublished report, 64 p. Scho lle, 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. Unpublished data on file at the Geology Dept. of Georgia State University. Unpublished data on file at the Georgia Geologic Su rvey, Atlanta . Woolsey, J.R., 1977, Neogene stratigrap hy of the Georgia coast and inner continental shelf, unpublished PhD dissertation, University of Georgia, 222 p.

State University Geology Program shows an

apparently large scale high separated by a saddle

from the ou ter shelf hig h . At present, a shortage

T

of data in the area and to the north, on the South

Carolina shelf, proh ibits c larification of the gene-

tic nature of this feature.

EXPLANATION
8 5 - Structure Contour - Shows line of equal evaluation below msl o f base of middle Miocene-age sediments. Contour interval is 5 meters. Dashed w here approximate. Datum is mean sea level.

SCA LE 1 :44 9,659

0

5

10 Nautic<:~l mi l es

I I
j I
I I
'o\j
+

/

'-. ......_

/
/

. . . _ _ _ _ _ 45

I

/

0 <) ~

/ / U"\)1. ~

'-....-...._____/

- so - / / /

/
- - 55 _ .........

0

60 ---~

~
C]

- - - - - - - - - - - _ / 75

-
/
/
/
/
/
I
,l~{~///
~~ ~ covo

-
+

-

-

~

80

+ \

--------~

-------

-

..96'
" \ \ \ \ \

"" ------ ------_---- / ....................
__ --------- ......

......_
......_ ..... ~......

/
/ / ........ /
____ ........ /

__ _ ~ ---- --"~-,
+

GEORGIA
ATLANTIC OCEAN
FLOR IDA

/
f
/
'/ y /
;v/
...J- ........ ...J.__....l..-_J- __

STUDY AREA LOCATION
Base from Nat 1onal Oceanic and Atmosp heric Admin ist ration Cha rleston Light to Cape Canave ra l nav igation map, 1984

80 30'

PLATE 6
3 1 30' 30 30'

This plate shows structure contours on the upper surface of the Middle Miocene sediments on the continental shelf of Georgia. The Middle Miocene is represented by the Coosawhatchie Formation onshore and on the continental shelf. It consists primarily of phosphatic clay and sand, with clay dominating in the offshore (the Berryville Clay Member), grading to sand landward (Ebenezer Member) (Huddlestun, in press) . The formation is phosphatic to varying degrees, but in specific areas and strata, phosphate may be the dominant lithology. The Tybee Phosphorite Member (Huddlestun, in press) is a stratum containing economic grade phosphate, found under portions of the coasta l counties and extending under the continental shelf of Georgia.
The Tybee Trough is a subsurface feature of potential interest in the development of the phosphorite resource. Th e Trou gh occurs east of the Savannah Light Tower boring. The Tybee Trough is believed to be the buried remnant of a barrier island tida l inlet comp lex, and is manifested in the subsurface as a cluster of channels cutting middle Miocene sediments (Kellam, 1981). Some of these channels are as much as 120- 130 feet deep. Due to the spacing of tracklines it is not possible to trace the orientation of these channels. They are evident as a cluster of negative features on the surface of the middle Mioceneaged sediments.
In both Florida and South Carolina phosphorite deposits. have been found to occur as concentrations of lag material in channels cut into phosphatic sediments (Riggs and Freas, 1965; Gibson, 1967). Seismic reflectors interpreted as showing cut-and-fill and other complex fill structures are seen in the Tybee Trough area seismic li nes. These features cou ld be composed of reworked concentrations of phosphatic material winnowed from older Miocene phosphatic sed iments.

Th e extensive erosion occurring after the deposition of the Coosawhatchie Formation is evident as an irregular "hummocky" reflector, representing the top of the Middle Miocene. Additionall y, evidence of erosion is seen 1n the truncation of internal reflectors. This erosion also is evidenced by the absence of Middle Miocene deposits on the nose of the Beaufort Arch in the northwestern corner of the study area. The southwestern portion of the mapped area contains an erosional scarp of relatively high relief cut into the middle Miocene-age strata (Foley, 1981). The scarp in turn is bisected by a large re-entrant east of St. Simons Island. This re-entrant, which connects with the Mid Shelf Low, is believed by Foley (1981) to be the channel of the ancestral Altamaha River.
In addition to the shore-parallel limb of the Beaufort Arch, a subtle high can be seen, passing through the Tybee Trough area. The other sign ificant feature w ith pos itive relief seen on the structure-contours of the middle Miocene is the "Outer Shelf High", parallel to and seaward of the "Mid Shelf Low".
Following the trend of underlying Tertiary sediments, the middle Miocene sediments plunge to the east and southeast, with a rapid "break" in the southeastern corner of the mapped area. The top of the middle Miocene ranges in depth from less than 66 feet in the northwest, to 330 feet in the Mid Shelf Low, to greater than 510 feet in the extreme southeast. In approximately the northern third of the area mapped, the middle Miocene is directly overlain by a veneer of Quaternary sediments, but in isolated areas appears on seismic sections to crop out. South of a line running approximately east of Ossabaw Sound, middle Miocene strata plunge to the south and are on lapped by Pliocene-aged strata as much as 265 feet thick (in the Mid Shelf Low).

SOURCES

Bunce, E.T., and others, 1965, Ocean drilling on

the continental margin, Science, v. 150, no. 3697, p. 709-716.

EXPLANATION

Foley, F.D., 1981, Neogene seismic stratigraphy and depositional history of the lower Georgia coast and continental shelf, unpublished MS thesis, University of Georgia, 81 p.
Gibson, T.G., 1967, Stratigraphy and paleo-

7 0 - - Structure Contour - Shows line of equal elevati on belo w mean sea level of eros ion surface of middle Miocene-age sediments. Contour interval is 5 meters. Dashed wh ere approximate. Datum is mean sea level.

environment of the phosphatic Miocene strata of North Carolina, Geol. Soc. Am. Bull. v. 78, p. 631-650. Hathaway, J.C., and others, 1976, Preliminary ~ summary of the 1976 Atlantic Margin Coring ~ Project, U.S. Geol. Survey Open-file Report 76-844,218 p,
- - - and others, 1979, U.S. Geological Survey core drilling on the Atlantic shelf, Science, v. 206, no. 4418, p. 515-527.

No middle Miocene presen t
Middle Miocene thi ckne ss reduced by chan nel ing. Boundaries are approx imate.
Outcrop - Seism ic data ind icates apparent outcrop on sea floor of middle Miocene.

Henry, V.J., and others, 1978, Geological evaluation of potential pipeline corridor sites along the Georgia coast. Final Report, phase I, Georgia Office of Planning and Budget, 101 p.
Huddlestun, P.F., in press, A revision of the lithostratigraphic units of the coastal plain of Georgia, Georgia Geologic Survey Bull. 104

SCA L E 1 A49,659

0

5

10 N au t ica l mi l es

Kellam, J.A., 1981, Neogene seismic stratigraphy and depositional history of the Tybee Trough area, Georgia/South Carolina, unpublished MS thesis, University of Georgia, 111 p.
McCollum, M.J., and Herrick, S.M., 1964, Offshore extension of the upper Eocene to Recent stratigraphic sequence in southeastern Georgia, U.S. Geol. Survey Prof. Paper 501-C, p. 61 C-63C.

SO UTH CAROLI NA

Riggs, S.R., and Freas, D.H., 1965, Stratigraphy and sedimentation of phosphorite in the central Florida phosphate district, preprint 65H84, Ann. mtg. Am. lnst. of Mining, Metallurg. and Pet. Eng. Inc., Chicago, 17 p.

ATLA N 1 'JC O C EA. N

Schlee, J., and Gerard, R., 1965, Cruise report and preliminary core log MIV Caldrilll -17 April to 17 May 1965, J.O.I.D.E.S. Blake Panel Report, unpublished report, 64 p.
Scholle, P.A. (ed.) , 1979, Geological studies of the COST GE-1 well, Un ited States south Atlantic outer continental shelf area, U.S. Geol. Survey Circ. 800, 114 p.
Unpublished data on file at the Geology Dept. of Georgia State University.
Unpublished data on file at the Georgia Geologic Survey, Atlanta.
Woolsey, J.R., 1977, Neogene stratigraphy of the Georgia coast and inner continental shelf, unpublished PhD dissertation, University of Georgia, 222 p.

STUDY AREA lOCATION
Base f r om Nati onal Ocea n ic and A t mospheric Admin i st rat ion Cha r lesto n Li ght to Ca pe Canave ra l n av i ga ti on mup , 1984

STRUCTURE-CONTOUR MAP
of the
TOP OF THE MIDDLE MIOCENE-AGE SEDIMENTS

PLATE 7

/ /

+

32

+

----
-------

------

+

+

ou l O P

outc rop

35 ..,outcw p

outc mp

outcmp

/

/

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/

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/ / / /

I

I

I I I
I
I
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40

out cr- op
40_/

r}

~~

\...50/ 45

' ---- \

~

~

50 - - - 55 - - - -

-- -- - - --- --. ___ 60 - - - -

_...--

I

I I

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--

_...-- / /1 I I

_...-------..ss

....-- ------.----- - - - - - / / / / /II I

/

'\

/65/ ---

--/ / / /

' /

10/

----

-- ---- I / / / 75 ....--
--- ----------- / / / gO

-----

_....

-

/

/

'/,'0 / -

/////

/

/

-/

/

---- gO/ - _..-

g'0 / /

+

.-.
31

I
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I I

30 30'

This plate represents the thickness of th e middle Miocene (Coosawhatchie Formation) on the continental shelf of Georgia. The middle Miocene consists primarily of phosphatic sands and clays of th e Coosawhatchie Formation with clay (Berryville Clay Member) dominating offshore and grading shoreward into the predominantly sandy Ebenezer Member (Huddlestun, in press). The overlying Berryville Clay Member averages about 50 feet under Tybee Island. The Tybee Phosphorite Member, the basal unit of the Middle Miocene in the coastal area, reaches a maximum thickness of 33 feet under Tybee Island. The Tybee Phosphorite Member averages 20 feet thick in eastern Chatham County and is present in varying amounts in borings along coastal Georgia, including 7.5 feet in Bryan County and 9 feet in the G.G.S. 3426 well on Cumberland Island. The Berryville and Tybee Phosphorite Members are present in thicknesses similar to eastern Chatham County in the Savannah Light Tower test hol e 10 miles east of Tybee Island. At a correlative stratigraphic position in the three southern borings (J.O.I.D.E.S. J-1, COST GE-1, and AMCOR 6002) lithologies are described as phosphatic in varying amounts but no actual phosphorite zone is present.

The Tybee Phosphorite Member cannot be reso lved as a definite series of reflectors on seismic sections, either as a result of its relative t hinness or due to its compositional similarity to adjacent strata. Also, because of the scarcity of lithologic data on the continental shelf, it is not possible to estimate the areal extent or thickness of the phosphorite unit at present. Available lithologic evidence does suggest a limit of the possible extent to the north and west of a line between the J.O.I.D.E.S. J-1 and AMCOR 6002 borings.
The Middle Miocene sediments on the continental shelf range in thickness from 0 to greater than 175 feet. Discounting the effects of erosion, the middle Miocene thickens to the east and south. It is completely planed off the Beaufort Arch in t he north and thins on the crest of the "Outer Shelf High". Effects of extensive erosion can be seen in the southwest portion of the study area, with the "Mid Shelf Low" bordered on the west by a prominent eros ional scarp (Foley, 1981).
In the northern portion of the study area, east of Tybee Island, a cluster of negative structures can be seen which are channeled features in the Middle Miocene (cross section D-D', Plate 4). These are interpreted by Kellam (1981) to represent the remnant of a barrier island/tidal inlet complex. These channels, some as much as 120130 feet deep, are potential sites for the deposition of winnowed and concentrated phosphate material.

SOURCES
Bunce, E.T., and others, 1965, Ocean drilling on the continental ma rg in , Science, v. 150, no. 3697' p. 709-716.
Foley, F.D., 1981, Neogene seismic stratigraphy and depositional history of the lower Georgia coast and continental shelf,' unpublished MS thesis, University of Georgia, 81 p.
Hathaway, J.C., and others, 1976, Preliminary summary of the 1976 Atlantic Margin Coring Project, U.S. Geol. Survey Open-file Report 76-844, 218 p.
_ ___ , and others, 1979, U.S. Geological Survey core drilling on the Atlantic shelf, Science, v. 206, no. 4418, p. 515-527.
Henry, V.J., and others, 1978, Geological evaluation of potential pipel ine corridor sites along the Georgia coast. Final Report, phase I, Georgia Office of Planning and Budget, 101 p.
Huddlestun, P.F., in press, A revision of the lithostratigraphic units of the coastal plain of Georgia, Georgia Geologic Survey Bull. 104
Kellam, J.A., 1981, Neogene seismic stratigraphy and depositional history of the Tybee Trough area , Georgia/South Carolina, unpublished MS thesis, University of Georgia, 111 p.
McCollum, M.J., and Herrick, S.M., 1964, Offshore extension of the upper Eocene to Recent stratigraphic sequence in southeastern Georgia, U.S. Geol. Survey Prof. Paper 501-C, p. 61 C-63C.
Schlee, J ., and Gerard, R., 1965, Cruise report and preliminary core log MIV Caldrill l - 17 April to 17 May 1965, J.O.I.D.E.S. Blake Panel Report, unpublished report, 64 p.
Scholle, P.A. {ed.), 1979, Geological studies of the COST GE-1 well, United States south Atlantic outer cont inental shelf area, U.S. Geol. Survey Circ. 800, 114 p.
Unpublished data on file at the Geology Dept. of Georgia State University.
Unpublished data on file at the Georgia Geologic Survey, Atlanta.
Woolsey, J.R., 1977, Neogene stratigraphy of the Georgia coast and inner continental shelf, unpub lished PhD dissertation, University of Georgia, 222 p.

EXPLANATION
15 - - Line of Equal T hickness- Shows thickn ess of middle M iocene-age sediments. Contour interval Smeters. Dashed where approximate:
Middle M iocene thickness reduced by channeling. Boundaries approximate.

SCALE 1:449,659

0

5

10 Nautical rn i le s

STUDY AREA LOCATI ON
Base f rom Nationa l Oceanic and Atmospher ic Administra t ion C harle~ ton L ight to Cape CanavNa l navigat i on map, 1984

ISOPACH MAP
of the
MIDDLE MIOCENE-AGE SEDIMENTS

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81

45 - - - - - - - - - - -
+

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80 30'

PLATE 8

+

32"

--

+

31 " 30'

+

,

The bathymetric surface of the continental shelf of Georgia is generally flat and featureless with a southeastward gradient of 2 ft!mi to th e sheli I:Jreak at about 33 fathoms (200 feet). Topography can best be described as gently undulatory. Low relief features , suc h as giant sand waves and a ridge and swale topography, are responsible for the sinuous character of the contour lines.
The lithology of the seafloor in the mapped area consists of a veneer of unconsolidated Pleistocene to Recent sediment. Only a few widely dispersed, low relief outcrops exist on the continental shelf of Georgia. These outcrops consist of lithified material of Pliocene and middle Miocene age.

BATHYMETRIC SURFACE
+

+

EXPLANATION

21 -

Lin e of equal elevation in fathom s sh ows

bath ymetric sur1ace below msL Da t um is

mea n sea leve l.

SC A L E 1 :449 ,659

0

5

10 Naut ical m iles

SOURCE
National Oceanic and Atmospheric Administration, 1984, Charleston Light to Cape Canaveral navigation map, 25th edition, scale 1:449-659.

SO UT H CAR OLI NA
G EO R G I A

- - -- - - -

A TL ANTJC OCEA N

STUDY AREA LOCATION
Base f r om Na t i ona l O ce an ic and Atm o sp her ic A d m in ist rat ion Char les to n Li gh t to C~pe Dlnavera l naviga t ion map, 1984

G

PLATE 9