Seismic investigation of the phosphate-bearing, Miocene-age strata of the continental shelf of Georgia

SEISMIC INVESTIGATION OF THE
PHOSPHATE-BEARING, MIOCENE-AGE STRATA
OF THE CONTINENTAL SHELF OF GEORGIA
Vernon J. Henry, Jr. and Jeffrey A. Kellam
Department of Natural Resources Environmental Protection Division
Georgia Geologic Survey

Cover Photo: Lowering uniboom seismic transducer from deck of research vessel Blue Fin.
Photo courtesy of Georgia State Univerait:y Geology Department.
~

SEISMIC INVESTIGATION OF THE
PHOSPHATE-BEARING, MIOCENE-AGE STRATA OF THE
CONTINENTAL SHELF OF GEORGIA
By Vernon J. Henry, Jr. Department of Geology Georgia State University
and Jeffrey A. Kellam
Department of Natural Resources J. Leonard Ledbetter, Commissioner Environmental Protection Division Harold F. Reheis, Assistant Director
Georgia Geologic Survey William H. McLemore, State Geologist
Study was partially funded by Minerals Management Service
of u.s. Department of Interior
1988
Bulletin 109

CONTENTS

Abstract

1

Acknowledgements .............................................. 1

Introduction .................................................. 2

Location

2

Objectives .................................................. 2

Phosphorites .................................................. 4

Background and Previous Work .................................. 5

Data Acquisition .............................................. 5

Seismic Data

5

Bore Hole Data ........................................... 7

Data Reduction and stratigraphic Analysis

9

. Regional Geology ..............................................
General Statement .............. .... ....... ......... .....

9 9

Regional Structural Elements and Topographic Features

11

Floridan Aquifer

14

General Description

14

. Stratigraphy and Lithology . ....... . ........ ........ 14

Regional Stratigraphy

15

Paleogene ..........~ ................................ 15

Eocene ...................................... 15

Oligocene

15

Neogene

16

Miocene .................................... 16

Lower Miocene

16

Middle Miocene

18

Upper Miocene

25

iii

Pliocene ....................................... 25

Quaternary ..................................... 28

Discussion

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General Statement ........................................ 28

Offshore Areas Recommended for Further Study

32

. ... .. General Statement

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. . . . . . . . . . . . . . . . . . . . . . . . . .

32

..... New and Preliminary Data from the TACTS Borings

32

. Recommended Exploration Areas ....... ............... 33

Summary and Conclusions ....................................... 36

References Cited .............................................. 37

Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11a
b
Figure 12a b
Figure 13

ILLUSTRATIONS

Location of study area and test well

and core drill sites ......... \

3

Seismic tracklines ............................. .

6

Stratigraphic correlation chart.................

10

Regional geology and structure..................

12

Seismic section depicting the "Sea Island

Escarpment" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

13

Structure-Contour of the top of the

Oligocene-age sediments.........................

17

Structure-Contour of the base of the

middle Miocene-age sediments...................

19

Representative seismic section..................

21

Structure-Contour of the top of the

middle Miocene-age sediments ................

22

Isopach of the middle Miocene-age sediments.....

23

Seismic sections depicting tidal inlet

channeling in middle Miocene-age sediments,

Tybee Trough. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 4

Seismic sections depicting channeling in

middle Miocene-age sediments, Tybee Trough

26

Representative cross sections derived from

seismic sections .............................. .

27

Representative cross sections derived from

seismic sections...............................

29

Potential sites for further investigations for

phosphorites .................................. .

34

v

Table 1 Table 2

TABLES

References to test wells and borings

used in this study ............................ .

8

Correlation of seismic stratigraphy with

biostratigraphy in offshore test wells

and borings .................... ~ .............. .

31

vi

ABSTRACT
Seismic stratigraphy previously developed for Neogene deposits on the Georgia continental shelf was correlated with well-defined, onshore lithostratigraphy of the Miocene Hawthorne Group. These strata regionally contain economically significant quantities of phosphate. Furthermore, they serve as the confining unit for the Eocene/Oligocene Floridan Aquifer.
The offshore seismic stratigraphic framework was based also on lithologic data from widely separated borings on the Georgia continental shelf. From this information, stratigraphic profiles and structurecontour and isopach maps were constructed that depict Neogene formational contacts and show both structural and topographic features that could be associated with the accumulation of phosphate.
Extensive exploratory drilling is necessary to verify the proposed stratigraphy and to confirm the presence of phosphorite in those deposits interpreted to be of Miocene age. Recommended drilling targets include the Beaufort High in the north portion of the study area; lower-most Pliocene deposits along the base of the Sea Island Escarpment, a late Miocene erosional feature; a mid-shelf middle Miocene topographic high; and, several areas of the shelf in which Miocene through Quaternary deposits are extensively channeled.
ACKNOWLEDGEMENTS
The valuable assistance of Dr. Paul Huddlestun of the

Georgia Geologic Survey and Dr.

Peter Popenoe of the u.s.

Geological survey in reconciling

the seismic data with biostrati-

graphic information, is grate-

fully acknowledged.

Their

helpful discussions and sugges-

tions during the course of the

study were most appreciated.

Camille Ransom and Bryan Hughes

of the South Carolina Water

Resources Commission provided

access to the information

obtained from the Port Royal

Sound borings. The valuable

assistance provided by Leslie

Jones Rueth, who participated in

the data gathering, interpreta-

tion of seismic records, and

writing as part of her Masters

thesis research, also is

gratefully acknowledged.

The substantive contributions of Dr. Peter Popenoe, Dr. Roger Amato of the Minerals Management Service and Dr. Bob Woolsey of the Mississippi Mineral Resources Institute, as external reviewers, are most appreciated as were those of Dr. Earl Shapiro, Dr. Bruce O'Connor and Mr. Jerry German, who served as internal reviewers for the Georgia Geologic Survey.
Finally, thanks are due to the Minerals Management Service of the U. s. Department of the Interior, through the University of Texas, Department of Geology, for funding the study; to the Skidaway Institute of Oceanography for providing laboratory facilities and logistical support; and to Kay Crane, Administrative Coordinator of the Georgia State University Department of Geology, and her successor, Tracy Roberts, for ably typing the manuscript.

1

INTRODUCTION
The data used in this study were obtained as part of an ongoing stratigraphic investigation of the Georgia coast and inner continental shelf. The study was conducted during the period 1984-1987 by the Georgia State University Department of Geology under contract with the Georgia Department of Natural Resources Geologic Survey for the u.s. Department of the Interior Minerals Management Service. Mineral exploration in Georgia in the 1960's revealed the presence of extensive phosphorite deposits under the marshlands and barrier islands in Chatham County, Georgia (Furlow, 1969). This phosphorite, contained in the Tybee Phosphorite Member of the Coosawhatchie Formation (Huddlestun, 1988) also was noted in a local offshore boring, the Savannah Light Tower (SLT) test hole located about 11 mi east of Tybee Island. The phosphate concentration in the Tybee Phosphorite Member, as high as
29.7% P205 (Zellars-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 valuable 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 recent development of more environmentally amenable subsea mining

techniques may establish the Tybee Phosphorite member as a valuable resource for the future.

LOCATION

The area of investigation,

shown in Figure 1, is located

along the inner portion of the

Georgia coast and on the

continental shelf adjacent to

Georgia and South Carolina from

Port Royal Sound, South Carolina

to st. Mary's Inlet on the

Georgia-Florida border.

The

seaward boundary is approximate-

ly 92 km (50 njmi) east of the

Georgia-South carolina shoreline

in water depths of approximately

50 m (165ft).

OBJECTIVES
The principal objective of this investigation was to tie the well-defined, onshore lithostratigraphy of the Miocene-aged rocks of coastal Chatham County (Furlow, 1969) to the seismic stratigraphic framework previously developed for the Georgia continental shelf (Kellam and Henry, 1986). The major goal of this study was to develop a comprehensive seismic stratigraphic framework of Neogene deposits, particularly those of the Miocene Hawthorne Group, which are known to contain economically significant quantities of phosphate in Florida and North Carolina. Also, as these strata serve as the confining unit for the Eocene/Oligocene Floridan Aquifer, the identification of the Hawthorne Group offshore was considered important in predicting potential impacts that may be incurred by subsequent mining operations.

2

t
1
Charles:Y
--vv>
AT ROYAL SOUND

_GJ.ORG!A
FLoRio/\

0

30 MILES

I

I 11

1
1

1
1

0

50 KILOMETERS

Figure 1. Location of study area and test well and core drill sites.
3

The stratigraphic interpretations made in this study are based on high resolution seismic reflection data recently collected on and in the GeorgiaSouth Carolina shelf and estuaries, as well as on a compilation of similar data collected from 1977 to 1981, in previous surveys over the same area. Correlations were made with lithologic cores or logs from several borings located on the Georgia-south Carolina coast and continental shelf as shown in Figure 1.
PHOSPHORITES
Phosphorite deposits generally consist of igneous or metamorphic apatite, guano, or the depositional products of marine environments. Presently, almost all the economic phosphorite deposits are of shallow marine origin (Bushinsky, 1964; and McKelvey, 1967). The Tybee Phosphorite Member of the Miocene Coosawhatchie Formation is also believed to have been formed in a shallow marine environment (Huddlestun, 1988). The most important characteristics of phosphate sedimentation are summarized by Slansky (p.159-161, 1986).

The element phosphorus

predominantly occurs in apatite,

Ca 5 mon

(mP0in4o)r3

c(oFm,OpHo,nCeLn,tC0o3f),

a comigneous

and metamorphic rocks. After

being weathered out, phosphorus

is transported to the sea as the

phosphate ion (Po 4 - 3), or adsorbed on iron compounds,

aluminum hydroxides and clays,

or carried in dissolved organic

compounds.

Phosphorus has a very low

solubility in seawater (McKel-

vey, 1967). As a result it

generally is considered to be

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 inorganically

precipitated.

A large per-

centage of this phosphorus is

utilized by planktonic or-

ganisms, 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, 1984; Birch,

1980; and Wallace, 1980).

Deposition also commonly occurs

through direct precipitation of

phosphorus in regions of

upwelling oceanic waters.

Upwelling brings cold, phos-

phate-rich bottom water into

contact with warm surface water.

In the higher temperature and pH

of surface water, phosphorus is

less soluble and thus precipi-

tates out of solution (Bushin-

sky, 1964; and Riggs, 1979b,

1984). Direct mineral replace-

ment from sea water is aided by

the presence of limy sediments,

such as calcium carbonate

skeletal fragments or fecal pel-

lets which can be replaced by

calcium phosphate (Ames, 1959;

Birch, 1980; and 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,

1979a; and Odin and Letolle,

1980).

4

The principal uses for

phosphate in the United States

are as fertilizer or feed

supplements.

Most of the

domestic production of phosphate

(87-91%) and about 35% of the

world's production comes from

deposits in Florida and North

Carolina (Zellars-Williams,

1978; and Stowasser, 1983), from

the upper Miocenejlower Pliocene

Bone Valley Formation and the

lowerjmiddle Miocene Pungo River

formation, respectively. The

11total identified resources in

recoverable product tons"

(Zellars-Williams, 1978) for the

Tybee Phosphorite Member was

estimated to be 3.125 million

short tons, 34% of the total

estimated reserves for the North

Carolina to Florida coastal

plain. A definitive statement

on total recoverable ore will

require extensive additional

investigation, on and offshore.

BACKGROUND AND PREVIOUS WORK

Early studies of the

geology of the Coastal Plain of

Georgia and South Carolina

include those by Sloan (1908) ;

Veatch and Stephenson (1911);

Cooke (1936); Richards (1945);

Siple (1956); and Malde (1959).

The aquifer systems of the

Coastal Plain were the focus of

study by Wait (1965); Counts and

Donsky (1963); McCollum and

Herrick (1964); and Miller

(1986). Arora (1984) defined

the stratigraphy and areal

extent of the Floridan (Prin-

cipal Artesian) Aquifer in

Georgia.

Investigations of

phosphate deposits in coastal

regions also provided consider-

able subsurface detail of

Cenozoic stratigraphy (Malde,

1959; Pevear and Pilkey, 1966;

Furlow, 1969; Harding and

Noakes, 1978; Zellars-Williams,

Inc., 1979; and Wallace, 1980).

Herrick and Vorhis (1963)

and McCollum and Herrick (1964)

provided a general stratigraphic

framework for coastal Cenozoic

sediments.

Significant con-

tributions to the Neogene

stratigraphy of the Coastal

Plain and inner continental

shelf of Georgia have been made

by Akers (1972), particularly

Huddlestun (1973, 1982, and

1988) and Weaver and Beck (1977)

based on lithostratigraphic and

biostratigraphic correlation of

planktonic foraminifera.

Utilization of a dense network

of previously collected high-

resolution seismic reflection

profiling on the Georgia coast

and inner continental shelf

(Figure 2) enabled previous

investigators to identify

Neogene seismic stratigraphic

units and generally interpret

depositional environments (Henry

and others, 1973; Woolsey and

Henry, 1974; Woolsey, 1977;

Henry and others, 1978; Henry

and others, 1981; Foley, 1981;

Kellam, 1981; Henry, 1983;

Idris, 1983; Kellam and Henry,

1986; and Henry and Rueth,

1986).

DATA ACQUISITION

Seismic Data

The instruments used in the
acquisition of surface and subsurface seismic data include the following:

a. ,; EG&G model 23o subbottom
profiling system;

b. Bolt model 600B, one cubic
inch airgun with profiling system;

c. ORE 3.5 kHz tuned transducer;

5

1>.

6

~.

~.

TRACKLINE LOCATION MAP
U.S. Geological Survey - - - - - Georgia State University - - - - -
SOUTHEASTERN U.S. ATLANTIC OCEAN BOTTOM SURVEY
'1,.'1) SHALLOW HIGH RESOLUTION
SEISMIC TRACKLINES

0-~50 KILOMETERS

~

0

-

110 NAUTICAL MILES

Figure 2. Seismic tracklines location map. Study area is shaded.
6

d. EG&G model 234 Engineering Recorder and EPC 3200 graphic recorder;
e. Northstar Loran C was used for navigation and trackline location.
The research vessels used for the data acquisition were
the Kit Jones and the ~ .Ei..n
of the Skidaway Institute of Oceanography, Savannah, Georgia, and the Gilliss and Fay under lease, at the time, to the U.S. Geological Survey Office of Marine Geology, Woods Hole, Massachusetts.
The acoustic signals from the towed sound source pass through the subbottom and are reflected at impedance discontinuities within the sedimentary column such as compositional changes or unconformities. The maximum penetration attained in the study area was approximately 100 meters ( 328 ft) . Reflections from the surface (seabed) and subsurface acoustic discontinuities are picked up by a hydrophone streamer which is towed through the water. The signals are filtered to decrease noise and fed to a graphic recorder which produces a continuous record. In addition to the high resolution seismic data collected along the shaded track lines shown in Figure 2, seismic lines connecting nine test wells drilled in Port Royal Sound, South Carolina to the Savannah Light Tower (SLT) were utilized in this study (see below).
Bore Hole Data
The seismic records were analyzed to determine key reflectors related to apparent

changes in lithology and/or

erosional surfaces (i.e., forma-

tional contacts).

Seismic

reflectors assumed to represent

formational contacts were traced

to and from existing bore hole

control points shown in Figure 1

and referenced in Table 1.

Additional information was

obtained from nine test wells

drilled in, and just seaward of,

Port Royal sound, South Car-

olina, during the summer of 1984

by the South Carolina Water

Resources Commission (SCWRC), in

cooperation with the u.s.

Geological survey, to determine

the hydrologic and geologic

characteristics of the Floridan

Aquifer in the Hilton Head

Island and Beaufort/Parris

Island area.

stratigraphic

interpretation was provided by

Bryan Hughes of the SCWRC and

Paul Huddlestun of the Georgia

Geologic survey (Henry and

Rueth, 1986).

In Georgia, the stratig-

raphy from borings 1n the

vicinity of the Savannah River

in Chatham County, and the

Savannah Light Tower (SLT),

located approximately 17 km (11

mi) offshore, was described by

Furlow ( 1969) . More recently,

the Neogene stratigraphy along

the lower Savannah River out to

the SLT and AMCOR 6002 was

revised by Huddlestun (1988)

from examination of fauna and

lithology from borings along the

river and at the SLT.

A

stratigraphic profile extending

from the GGS 3426 well on

Cumberland Island to AMCOR 6002,

also prepared by Huddlestun

(1988), was used along with the

logs from J-1 and COST GE-l to

correlate the seismic data in

the southern position of the

study area (Table 1, Figures 1,

6 , 7 , 9 and 1 0 ) .

7

Table 1. References to test wells and borings used in this study.

LQcation/Desiqnation
Onshore; Port Royal Sound Savannah River-Test Boring Chatham County (Chatco)
GGS 3426
Offshore; Savannah Light Tower (SLT)
AMCOR 6002 COST GE-l JOIDES 1 (J-1)

Reference
Henry and Rueth, 1986
u.s. Army Corps of Engineers, 1984
Furlow, 1969; Huddlestun, 1973, 1982, 1988
Martinez, 1981; Huddlestun, 1988
McCollum and Herrick, 1964; Furlow,l969; Huddlestun, 1988
Hathaway and others, 1979 Scholle, 1979 Bunce and other, 1979; Schlee and
Gerrard, 1979

8

DATA REDUCTION AND STRATIGRAPHIC ANALYSIS
Seismic interpretation is most reliable where bore hole data are of sufficient density to provide the necessary control for correlation with seismic reflectors. In the offshore study area, such control points are few and far between. Therefore, interpretations relied primarily on the traceability of key reflectors and identification of seismic signatures considered to be representative of characteristic sedimentary or separating, structures within biostratigraphic units.
Following analysis and interpretation of the records, seismic lines were photographically reduced and transferred to graph paper, manipulating vertical and horizontal scales to facilitate the most advantageous presentation of the profiles. Reflectors for various units may or may not be seen on each seismic record and the resulting cross sections. However, key horizons can often be picked from the intersecting seismic lines and interpolated from that point.
Generally, the seismic systems were apparently capable of resolving bed thickness of 12 meters at a frequency range 400-1500 Hz. An average sound velocity of 1.5 kmjsecond has been assumed, as is common practice in high resolution seismic studies (EG&G, 1971). A correlation chart comparing seismic stratigraphic contacts with lithostratigraphy, and depicting the relationship of the phosphatic unit to the aquifer, is shown in Figure 3.

REGIONAL GEOLOGY
General Statement
The Atlantic continental margin has been relatively stable tectonically from the Cretaceous to the present. Tectonic activity has occurred only as tilting, subtle warping, and minor faulting. The Georgia continental shelf occupies a broad, shallow reentrant about 113-129 km (70-80 mi) wide. Water depth at the shelf break is 46-61 m (150-200 ft). This contrasts with the usual shelf break of about 91-152 m (300-500 ft). The Georgia Continental 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 deposits. The Jurassic/Cretaceous contact is located at an approximate depth of 1067-1372 m (3,500-4,500 ft) below sea level beneath the coastal area.
According to Paull and Dillon (1980), Popenoe and others (1987) and Popenoe (1988), the present continental shelf-slope is a Cenozoic constructional feature significantly controlled by the position of the Gulf Stream relative to sea level fluctuations. Since the beginning of the Cenozoic, the shelf-slope

9

SEISMIC STRATIGRAPHY AS DETERMINED BY SEISMIC CORRELATION WITH REGIONAL STRATIGRAPHY

SYSTEM Qua ternary

SERIES
Pleistocene to
Recent

STRATIGRAPHIC NOMENCLATURE

NORTH Undifferentiated

SOUTH Unditferentia ted

SEISMIC CHARACTERISTICS
Thin blanket with weak internal reflectors, discontinuous bedding and shallow buried channels

GENERALIZED RELATIONSHIP TO FLORIDAN AQUIFER UPPER
CONFINING UNIT AND OCCURRENCE OF PHOSPHORITE

Plio c ene

Duplin Marl

Duplin Marl

Complex channel Iill, discontinuouS sheet, lenses with few weak internal reflectors

.....

0

Strongly banded, prograding loresets

Phosphate-bearing units 15-40%

distinguish upper unit, conformable strong reflector separates units

Bone phosphate of lime

Miocene 1-----------1
Tertiary

I

~. .h m d

I -- - - - -

j__

r //1/F///i :~~~a~in~~~~::~et5d:~c~:;~n5u~~:dbanding,

Upper confining unils

Eocene

Ocala Group Santee Formation

Ocala Group and
EQuivalent

r---- - - - - - -- - - - -- 1 - - - - '~~ ~~

Few, weak, discontinuous subparallel internal reflectors generally seism cally transparent

Floridan aQuifer units

Seismically transparent, with very lew internal reflectors visible

Figure 3. Stratigraphic correlation chart. Compiled from Henry and Rueth, 1986; Huddlestun, 1988.

has experienced net progradation.
Regional Structural Elements and Topographic Features

Three principal coastal plain ~tructures which were generally thought to be caused by deformation of the basement complex between North Carolina and Florida, are 1) the Cape Fear Arch; 2) the Southeast Georgia Embayment; and 3) the Peninsular Arch (Figure 4). Recent work by Klitgord and others (1984), Dillon and Popenoe (1988) and Popenoe (1988) discuss the two arches as the Carolina Platform and Florida Platform, respectively, separated by a minor sag basin, the Southeast Georgia Embayment. The platforms are dominantly
positive features underlain by continental crust. The Southeast Georgia Embayment and the Southwest Georgia Embayment are underlain by Triassic basins, which may explain why these areas have undergone more subsidence than the less fractured platforms.

The Peninsular Arch is a Mesozoic structure and is a
product of continental breakup that initiated the latest opening of the Atlantic. According to Popenoe (1988) the Cape Fear Arch is a corner of the Carolina Platform caused by
an offset in continental crust across the Blake Spur Fracture Zone Offshore, making the structure appear as an "arch."

Of more immediate impor-

tance to this study are three

relatively small-scale buried

topographic features:

the

Beaufort High/Outer Shelf High,

the Sea Island Escarpment, and

the Inner Shelf Low. Also known as the Beaufort Arch, the Beaufort High has been described by Huddlestun (1988) as being a low, broad structural high extending south-southwestward from Beaufort County, South Carolina, and eastern Chatham County, Georgia, onto the continental shelf where it has been traced as far south as offshore Cumberland Island by Foley (1981). This feature is essentially a topographic expression of the Miocene Deposits on the Georgia Continental Shelf and identified by Foley (1981) as the Outer Shelf High. Because the overlying Pliocene and Quaternary deposits and the underlying Oligocene deposits are flat-lying, ebe feature is considered to be erosional, rather than structural in origin.
The Sea Island Escarpment, the name proposed by Huddlestun (1988) , was first described by Woolsey and Henry (1974) from high-resolution seismic records which show the feature to extend from southern coastal Chatham County southward under the present barrier islands to cumberland Island where it curves offshore and can tentatively be traced under the inner shelf as far south as Cape Canaveral (Figure 5). Woolsey (1977) and Foley (1981) suggest that the escarpment was cut by waves and/or currents between middle Miocene and Pliocene time and buried by prograding inner continental shelf deposits during the late Pliocene. According to Huddlestun (1988) the large-scale clinoforms shown in Figure 5, are upper Pliocene Raysor - equivalent shelly sands that both overlie and occur seaward of the escarpment. Lower Pliocene Wabasso beds

11

---

Blake Plateau

mcg Blake Bahama

~

Basin

.0

-3
<0
;:)

I

N

Figure 4. Regional geology of the southeastern United States and continental shelf. study area is shaded. Modified from Paull and Dillon, 1980.
12

:-

,. .; ..Miocene~.;;,,

f :: -~ 5:0~m -.1..

.....
w

. . . o" ~'l).c;;e i

,!:
.~.
.,."'
J

)

J""""'"'ll\

Figure 5. Seismic section depicting the "Sea Island Escarpment." Section located under st. Simon's Island.

appear to occur only seaward of the feature.

The Inner Shelf Low descri-

bed by Foley (1981) is a trough-

like feature, open to the south,

and bounded on the west by the

Sea Island Escarpment and on the

north and east by the Beaufort

High/Outer Shelf High.

The

trough is filled to overflowing

with Pliocene deposits (see

above) that pinch out to the

north, thin to the east and

west, and thicken to the south.

The topographic and stratigraph-

ic relationships among and

between these three features is

shown in Figure 12a, Profile FF'

and Figure 12b, Profiles BB' and

B'B".

Floridan Aquifer

A brief description of the units comprising the Floridan Aquifer is included because consideration for the protection of this aquifer system arid its confining layers is essential prior to any mining of the overlying phosphatic units. It is recognized that the Floridan Aquifer of the Coastal Plain is
the principal source of groundwater for southeast Georgia.
For this reason preservation of the aquifer and confining strata are of the utmost importance.

General Description

Limestones of middle Eocene to early Miocene age compose the Floridan Aquifer which constitutes, as a whole, the most prolific artesian aquifer in
Georgia (Herrick and Wait, 1956; and Arora, 1984) (Figure 3). This aquifer also was termed "Principal Artesian Aquifer" by Warren (1944) due to the fact
that it serves roughly threefifths of the total area of the

coastal Plain of Georgia, as well as southeasternmost South Carolina and central and northern Florida (Thomson and others, 1956; Herrick and Wait, 1956; and Miller, 1986).

The Floridan Aquifer is thickest al.ong the coast and in the southern part of Georgia and thins to the north and west pinching out near the Fall Line and east of the Chattahoochee River (Thomson and others,
1956) .

Stratigraphy and Lithology

The Floridan Aquifer in

Georgia is primarily composed of

the upper Eocene Ocala Group

which consists of several

hundred meters of permeable

limestone.

The Suwannee

Limestone, an overlying section

of undifferentiated carbonate

rocks of Oligocene age, is so

similar in its water-bearing

properties that the Ocala Group

and Suwannee Limestone are

regarded as one water-bearing

unit.

onshore, the lower boundary
of the Floridan Aquifer is the contact between the 1imestones of the Ocala Group and the sand and clay of the Claiborne Group of middle Eocene age. The upper boundary onshore is defined by the uppermost Tertiary limestone, usually the top of the suwannee Limestone or Ocala Group (Arora, 1984).

Throughout much of the study area the Hawthorne Group of Miocene age serves as the upper confining layer of the aquifer (Figure 3) As this unit is under consideration for the potential mining of phos-
phates, definition of the Miocene confining unit offshore

14

is vital to avoid breaching and contamination of the aquifer.
Regional Stratigraphy
Paleogene
Eocene
Eocene deposits are the most voluminous of the Cenozoic section in the Southeastern United States with a thickness of nearly 500 m (1640 ft) at the Cost GE-l well on the continental shelf off southern Georgia (Scholle, 1979). The section thins northward towards Charleston where deposits are 130 m (427 ft) thick (Gohn and others, 1979).
Within the depth range of seismic profiles obtained in this study, only upper Eocene deposits are detectable. Upper Eocene units in Georgia are represented by the Ocala Group which have been described as a gray to buff, slightly glauconitic, fossiliferous, sandy limestone (Herrick and Vorhis, 1963; and McCollum and Herrick, 1964) . The lower part of the Cooper Formation and the Cross Formation (formerly Santee Limestone) are considered as the upper Eocene lateral equivalent in South Carolina (Hazel and others, 1977; and Idris, 1983).
The homogeneous, porous nature of these upper Eocene limestones enable them to store and transmit large quantities of water. It is therefore considered as the primary unit of the Floridan Aquifer in Georgia and South Carolina.
In seismic profiles, the upper Eocene is represented by sparse, discontinuous internal reflectors with weak, irregular

signatures.

The erosional

unconformity at the Eocene/Olig-

ocene contact, which produces a

relatively strong acoustic

reflector, is traceable on a

regional scale, particularly in

the northern part of the study

area, where it is nearer to the

surface. In the southern part

of the area it is more deeply

buried and near the limit of

resolution of the seismic data.

This erosional surface probably

represents a period of subaerial

exposure resulting from a

eustatic fall in sea level (Vail

and others, 1977).

Oligocene

In the portion of the con-

tinental shelf adjacent to Tybee

Island, the top of the Oligocene

reflector correlates with the

top of the Lazaretto Creek

Formation (Huddlestun, 1988), a

sandy limestone/calcareous sand

identified in Chatham County,

and in the SLT test boring 11 mi

east of Tybee Island. To the

south, in the AMCOR 6002, JOIDES

J-1~ and COST GE-l test holes,

Oligocene-age sediment is an ar-

gillaceous calcareous "ooze"

which correlates with the Cooper

Marl. In the extreme southwest

portion of the study area, the

Oligocene is absent according to

Huddlestun (1988).

Also,

reflectors correlated with the

Oligocene strata in the SLT

boring could not be carried to

the borings in Port Royal Sound

located in the extreme northern

part of the study area.

The Ocala Group is directly overlain by the lower Miocene Parachucla Formation. The base of the Parachucla Formation in this region is composed of interlayered terrigenous clay
and limestone/marl. Lithologic similarities between Oligocene

15

and Eocene deposits make it
difficult to seismically distinguish the two units and, as previously stated, they are hydrologically considered as one unit.

Oligo~~ne deposits are

represented in seismic profile

by sparse, discontinuous

reflectors of variable inten-

sity. According to Hathaway and

others (1979), early Oligocene

units represent deposition in an

outer shelf or deeper marine

environments.

A structure-

contour map of the top of the

Oligocene is presented in Figure

6.

The hatchured contours

denoting topographic lows or

"holes are probably solution

related.

Neogene

Miocene

Miocene deposits in coastal Georgia and South Carolina are represented by the Parachucla Formation and Marks Head Formation of early Miocene age and the Coosawhatchie Formation of middle Miocene age. Units of late Miocene age are present only as discontinuous lenses on the inner Georgia shelf accord-
ing to Woolsey (1977) and Foley (1981) .

Lower Miocene

The lower

Miocene is comprised of two

formations; the Parachucla

Formation of the lower part of

the lower Miocene and the Marks

Head Formation of the middle

lower Miocene (Huddlestun, 1982,

1988). These units range in

total thickness from about 5 to

27 m (45 to 90 ft) across the

study area (Kellam and Henry,

1986).

Huddlestun (1988), described the Parachucla Formation as generally a phosphatic, calcareous, argillaceous sand with limestone and dolostone locally dominating the lithology. Although the phosphate content of the Parachucla is variable, it is consistently less phosphatic than the overlying forma-
tions.

The Parachucla Formation is

disconformably or paraconform-

ably overlain by the Marks Head

Formation which is composed of

slightly dolomitic, phosphatic,

sandy clays and argillaceous

sands. The formation exhibits a

tendency to fine in a seaward

direction. While the Marks Head

Formation is less phosphatic

than the overlying middle

Miocene units, phosphate is a

characteristic component. A bed

of dolomitic clay marks the top

of the Marks Head Formation

where it is overlain disconfor-

mably by the Coosawhatchie For-

mation.

The Parachucla and

Marks Head Formations serve as

the upper confining units for

the Floridan Aquifer on the

coast and continental shelf off

Georgia.

The characteristic seismic signature of lower Miocene units consists of closely-spaced, parallel reflectors of weak to moderate strength (Woolsey, 1977; Foley, 1981; and Kellam, 1981) . Reflectors within the Marks Head display prograding foresets of possibly deltaic origin (Woolsey, 1977).

A shallow shelf and restricted marine deltaic depositional environment dominated the lower Miocene following a transgression in early Miocene
time (Woolsey, 1977; and Hathaway and others, 1979). A

16

CONTOUR INTERVAL 5 METERS
GE1e
Figure 6. structure-Contour of the top of the Oligocene-age sediments. Topographic lows denoted by hatchures are probably karstic features.
17

eustatic drop in sea level
during the middle Miocene resulted in a regression and subsequent subaerial erosion of the lower Miocene deposits (Vail and others, 1977) . A fairly prominent seismic reflector
marks the erosional surface between lower Miocene and middle Miocene units.

Middle Miocene

Deposits of

middle Miocene age on the

coastal plain and continental

shelf of Georgia and southeast

South Carolina are represented

by the Coosawhatchie Formation

described by Huddlestun (1982,

1988).

The Coosawhatchie

Formation was previously known

as the Coosawhatchie Clay Member

of the Hawthorne Formation

(Heron and others, 1965).

The Coosawhatchie Formation consists of phosphatic clay, sandy clay, argillaceous sand and phosphorite. Huddlestun (1988) divided the formation into four members, three of which are represented in coastal and continental shelf deposits in the study area: the Tybee Phosphorite Member, the Berryville Clay Member and the Ebenezer Member. These members are not always readily distinguishable in seismic reflection profiles.

The lithology of the basal Tybee Phosphorite Member is quartz sand and phosphorite with small amounts of clay and dolomite (Huddlestun, 1988). The phosphorite is generally composed of well-rounded, black, brown or amber grains that range in size from 0.1 mm to 1 mm (0.003937 to 0.03937 in) (Woolsey, 1977; Wallace, 1980; and Huddlestun, 1988). The phosphorite is typically as-
sociated with fraqments of fish

bones and teeth. Phosphorite concentrations within the Tybee Phosphorite Member range from 12 to 40% BPL (Wallace, 1980). In coastal Chatham County, commercial-grade phosphorite is present within the Tybee Phosphorite Member (Furlow, 1969). The Tybee Phosphorite Member averages 6 m ( 20 ft) in thickness in coastal Chatham County with a thickness of 10 m (33 ft) under southern Tybee Island. This unit thins to .30.61 m (1-2 ft) in northwestern Chatham County. It is about 2 m (7.5 ft) thick in coastal Bryan County and 3 m (9 ft) thick in the G.G.S. 3426 core on Cumberland Island. In the SLT test hole the phosphorite has been reported to be approximately 9 m (30 ft) thick, a similar thickness to that under Tybee Island.
The Tybee Phosphorite Member is conformably overlain by the Berryville Clay. An olive-gray, phosphatic, variably calcareous, microfossiliferous, silty clay, the Berryville Clay makes up the entire Coosawhatchie section on the continental shelf of Georgia according to Huddlestun (1988). Where the Tybee Phosphorite Member is absent, the Berryville Clay Member disconformably overlies the Marks Head Formation. The Berryville Clay Member grades laterally westward into the sands of the Ebenezer Member. Figure 7 depicts structurecontours of the base of the middle Miocene-age sediments, representing the potential stratigraphic location of the Tybee Phosphorite Member.
The Ebenezer Member is described as a gray to olivegray, slightly phosphatic, argillaceous, fine to medium-

18

0

10 NAUTICAL MILES

I

I

CONTOUR INTERVAL 5 METERS

GE1

Figure 7. structure-Contour of the base of the middle Miocene-age sediments.
19

grained sand (Huddlestun, 1988). The Ebenezer sand is considered as moderately to poorly phosphatic following the trend for
decreasing phosphate content upwards in the middle Miocene section. In coastal areas, the Ebenezer Member constitutes the upper part of the Coosawhatchie Formation. Where lower Miocene units are absent, the Coosawhatchie Formation represents the upper confining layer for the underlying Oligocene and Eocene beds of the Floridan Aquifer.

As a whole, the middle

Miocene displays seismic

signature characteristics that

are easily identifiable within a

seismic profile. The strong

reflectors that exhibit a

closely-spaced, parallel

"banding" are readily traceable

throughout the coast and

continental shelf (Woolsey,

1977; Foley, 1981; Kellam, 1981;

and Henry, 1983). A representa-

tive seismic section is pre-

sented in Figure 8.

The

characteristic middle Miocene

banding can be easily distin-

guished, as well as the erosion-

al nature of the middle Miocene-

post-middle Miocene contact.

Figure 9 depicts the structure-

contours of the top of the

middle Miocene-aged sediments.

The Outer Shelf High, aligned

north-south through AMCOR 6002;

the Sea Island Escarpment

delineated by the close contour

gradient under the coastal

barrier islands; and the Inner

Shelf Low, expressed as a well-

developed trough between those

features, are particularly well

shown in this illustration.

Figure 10 is an isopach of the

middle Miocene-age sediments,

depicting the thickness of the

phosphatic unit.

With a eustatic rise in sea

level during the middle Miocene,

marginal to open marine condi-

tions prevailed (Alt, 1974; and

Mar~inez, 1981).

Upwelling

conditions operative during the

early part of the middle Miocene

time are suggested by high

phosphate concentrations and the

high biological activity in-

ferred by the abundance of

vertebrate remains (Furlow,

1969; Abbott, 1974; and Alt,

1974). Winnowing of phosphate-

bearing units, resulting in

phosphorite concentration, is

attributed to periodic storm

episodes (Howard and Reineck,

1972; and Woolsey, 1977).

Woolsey (1977) suggested that

increased sedimentation rates

during the middle part of the

middle Miocene prevented the

degree of phosphatization and

concentration that took place in

the earlier part of the middle

Miocene.

A period of non-

deposition and erosion resumed

as the sea-level dropped in the

late Miocene concurrent with a

Messinian glacial period

(Berggren and Haq, 1976). The

resultant erosion surface

characterizes the Miocene/Plio-

cene contact. The relatively

rapid drop in sea level may have

resulted in cutting the promi-

nent erosion scarp of the Sea

Island Escarpment. The presence

of extensive sets of clinoforms

seaward of the scarp suggests

deltaic outbuilding during the

Pliocene (Woolsey, 1977; Henry

and others, 1978; Foley, 1981;

and Huddlestun, 1988).

A subsurface feature of potential interest in the development of the phosphorite resource occurs east of the SLT. This feature is the Tybee Trough (Figure 11a) . Believed to be the buried remnant of a barrier island/tidal inlet complex, the

20

PLIOCENE
SOmsec 40m
I_
Figure 8. Representative middle Miocene seismic section. From R/V GILLISS, cruise GS-7903-6, line 1P; 12 mi east of cumberland Island.
21

32.0

0

/

/

I

/

I

/

3130'

31
----.J'10 NAUTICAL MILES
CONTOUR INTERVAL 5 METERS
Figure 9. Structure-contour of the top of the middle Miocene-age sediments.
22

I

+

31 "

0

10 NAUTICAL MILES

CONTOUR INTERVAL 5 METERS
GE1e
.so

Fiqure 10. Isopach of the middle Miocene-age sediments.

23 '

Tybett Trouth

,

t

tl

_ l.ower Mlohn,~
t
1
tL GOOm

Location of Tybee Trough

I
r
j
Figure lla. Seismic section depicting tidal inlet channeling in middle Miocene-age sediments, Tybee Trough.
24

Tybee Trough is manifested in the subsurface as a cluster of channels cutting middle Miocene sediments (Figure 11b) (Kellam, 1981). Seismic evidence of cutand-fill and other complex fill structures could represent winnowing and concentration of phosphatic material. Some of these channels are as much as 37-40 m (120-130 ft) deep in the southern portion of the study
area.

As a general trend, the

base of the middle Miocene can

be seen to deepen southward from

a minimum of less than 25 m (82

ft) adjacent to Tybee Island to

a maximum of more than 120 m

(395 ft) in the southern portion

of the study area. Scholle

(1979) reported middle Miocene

phosphatic sediments at -105m (-

544ft) to -218m (-719ft) in the

COST GE -1 well and Mannheim and

others (1980) reported middle

Miocene phosphatic sediments (up

to 23% well at

P2d0e5p)thsi

n

the AMCOR of between

6002 -27m

(-89ft) and -67m (-221ft)

(Hathaway and others, 1976.)

UPPer Miocene - Upper Miocene deposits are poorly represented on the Georgia coast and shelf
with only a few discontinuous lenses of upper Miocene sediments occurring on the inner
shelf (Hathaway and others, 1976; Woolsey, 1977; and Foley, 1981). Upper Miocene units are absent in coastal areas as an apparent result of subaerial exposure following regression of middle Miocene seas.

Pliocene

Other than middle Miocene deposits, Pliocene deposits are the thickest and most widespread

deposits of the Neogene in the

Georgia coastal area. Pliocene

deposits on the Georgia coast

and continental shelf are

represented primarily by the

Duplin Formation (Raysor

Formation equivalent according

to Huddlestun, 1988).

The

Duplin Formation consists of a

well-sorted, variably shelly,

calcareous, fossiliferous sand

and marl that is locally pebbly

and gravelly. Pliocene deposits

are thickest on the inner

continental shelf in the south-

ern portion of the study area in

the Inner Shelf Low but thin and

pinch out to the north on the

Beaufort High. Also, they thin

westward at the edge of the Sea

Island Escarpment and eastward

across the top of the Outer

Shelf High. (See Figure 12a

Profile F-F'.)

The Pliocene section is easily recognizable in seismic profiles as it overlies the prominent reflector of the middle Miocene erosion surface. Pliocene units are characterized by thin-bedded, intertonguing units and seaward prograding foresets (Woolsey, 1977; Henry
and others, 1978; and Kellam,
1981).

In early Pliocene time, a eustatic rise in sea level
resulted in the transgression of what has been termed by Colquhoun (1971) as the "Duplin Sea." Reworked Miocene sands and gravel were deposited as the basal clastic sediments of the Raysor Duplin Formation. By the close of the early Pliocene, a restricted basin and estuarine environment had developed
following inundation of the
present coastal region. A high sedimentation rate is inferred by the presence of extensive prograding foresets deposited

25

~

~.;,

Sea $uttace"

Otn$ t

t "
~

,,
:r

Figure llb. Seismic sections depicting channeling in middle Mioceneage sediments.
26

REPRESENTATIVE CROSS-SECTIONS PERPENDICULAR TO SHORE
TYBEE

20 ~ ~ ~~s~~~~~~:j E'
60 100
60 100

100 140

-

TRACKLINE LOCATIONS

Tybee Isla::/

c:?E
.)

)

St8s lMa'!_~ lOl

0'

J F C'
}

N
1

LOCATION MAP
N
1

0

100

Milas

KEY
a - Quaternary

p - Pliocene

uM - upper Miocene

mM- middle Miocene

LM - lower Miocene

M - Miocene

0 VE -

Oligocene
approximately 1:100

Figure 12a. sections.

Representative cross sections derived from seismic

27

immediately seaward of the Sea Island Escarpment during the retreat of the "Duplin sea" during the middle Pliocene. This regression was part of a eustatic drop in sea level associated with the middle Pliocene glacial event (Berggren and Van Couvering, 1974) and resulted in the subaerial erosion and extensive channeling of Pliocene deposits.

Quaternary

Quaternary deposits on the

inner continental shelf of

Georgia and South Carolina are

represented by a thin sheet of

unconsolidated sands (Pilkey and

others, 1981).

In seismic

profile, these sediments are

characterized by weak, dis-

continuous horizontal reflectors

and generally strong reflectors

showing numerous cut and fill

structures incised into un-

derlying Pliocene deposits

during the retreat of the Silver

Bluff Sea (Woolsey, 1977; Henry

and others, 1978; Foley, 1981;

and Kellam, 1981).

Woolsey

(1977) grouped Holocene deposits

into basal sand, barrier island

and estuarine facies of similar

character to late Pleistocene

deposits of related origin.

DISCUSSION

General Statement

In analyzing seismic reflection profiles taken on the inner continental shelf of Georgia and south carolina, six major seismic units were identified. In the Port Royal Sound area only four units were recognized as a result of the pinching-out or erosion of
Oligocene- and Pliocene-age sediments. Eocene and, in some cases, Oligocene deposits were

not traceable in the extreme

southern portion of the study

area due to depth limitations of

the seismic system.

Regional stratigraphic

correlation was determined from

information obtained from drill

sites in the area (Table 1).

While six stratigraphic units

were identified in the seismic

profiles, analysis was

concentrated on middle Miocene-

age deposits considered as most

relevant to this study.

In order to delineate the

stratigraphic relationship and

lateral extent of these Miocene-

age deposits, a series of cross

sections were prepared from

seismic tracklines parallel and

perpendicular to the coastline.

These schematic cross sections

are presented in Figures 12a and

12b. The middle Miocene is

recognized by characteristically

strong, continuous, parallel

reflectors traceable throughout

the study area.

Large-scale

clinoforms are seen as seaward

prograding reflectors. To the

north, the prominent banding is

less distinguishable possibly

due to the presence of the thin-

bedded Berryville Clay Member.

The other members of the Coosaw-

hatchie Formation were tent-

atively identified in seismic

records near bore holes, where

ground truth could be

established, but contacts could

not be regionally traced with

confidence.

The top of the middle

Miocene deposits is indicated to

be an erosional surface that

ranges in depth from sea floor

outcrops off the Savannah River

to over 100 m (328 ft) below

mean sea level off st. Simons

Island between the Sea Island

Escarpment and the Outer Shelf

High (see Figure 9). On the

latter feature in the vicinity

of AMCOR 6002 boring, the middle

28

G-G'
A11 Sea Level
140

REPRESENTATIVE CROSS-SECTIONS

PARALLEL TO SHORE

F-F'

A'

-- ____...-- - - - -
A

140 - - G- G'
8" Saa Level

F-F'

e'

140

100 140
G- G'
C"
100 140

- 0 ---==-

---= m
~---1-M =..:::-- ;/-
------ 0 /

140 .

Figure 12b. sections.

Representative cross sections derived from seismic
29

Miocene is indicated to be less than 45 m (148 ft) below mean sea level. The middle Miocene reflectors thus follow the trend of increasing depth and thickness to the south on the inner shelf, but rise closer to the seabed on the outer shelf.

The thickness of the middle Miocene _ranges from zero in the vicinity of Hilton Head Island to over 40 m (131 ft) on the Outer Shelf High east of Sapelo Island (see Figure 10). Off Cumberland Island, seismic data indicate that the deposits thin to less than 20m (66ft). Seaward of the Outer Shelf High the deposits appear to thicken significantly but seismic definition of the Middle, Lower and Oligocene contacts is uncertain due to decreasing record quality, perhaps related to both lithologic change and increasing depth.

Table 2 was prepared to

show the relative depths to key

stratigraphic horizons based on

seismic picks and picks using

index microflora, microfauna and

lithology.

All things con-

sidered, the two techniques

compared reasonably well. The

latter method is constrained

from often having little or no

choice in sample depth selection

if the core is not continuous,

recovery is poor or the sample

condition less than optimum. As

a result most, or many of the

samples examined are not at the

formational contacts but may

come from well above or below,

resulting in interpolated

contact depths.

Also, in

comparing picks of shallower

horizons, errors in converting a

datum related to sea level

(tidal stage, water depth) or

drill rig elevation relative to sea floor depth (Kelly bushing elevations, sample recovery depth) can produce depth estimates that suffer by comparison.

It is important to point

out that good record quality and

unique seismic signatures of

most of the Neogene formations

in the study area provide for

high confidence in assessing the

regional occurrence and contin-

uity (i.e., geometry) of

stratigraphic boundaries and

target formations. However, the

seismic method, velocity

assumptions, and instrumentation

have an inherent potential for

error, particularly in accurate-

ly determining th.$ depth to, and

thickness of, a g1ven reflector,

or set of reflectors.

An

estimate of the margin of error

is plus or minus 2m (7ft), or

more, under particularly adverse

circumstances.

Poor record

quality will result from noise

caused by water surface wave

action, system deployment

geometry, and vertical and

horizontal variations in

acoustical character of subsur-

face stratigraphy that cause

acoustical opaqueness or absorp-

tion, high reflectivity, and

ringing and multiples that mask

or block data. Because acoust-

ical impedance of strata varies

with texture, lithology, water

content and the presence of

biogenic gas, facies changes

between distant boring control

points can cause problems in

accurately tracing seismic

formational contacts. In any

case, the use of high resolution

seismic surveys provides the

best means for determining

regional stratigraphic framework

on which to base site-specific

investigations such as core-

drilling.

30

Table 2. Correlation of seismic stratigraphy with biostratigraphy in

offshore test wells and borings.

(Depths in meters below

mean sea-level).

Stratigraphic Marker

Top Middle Miocene

SLT

literature

22m

(see Table 1)

this study

20m

(Figures 6, 7, 8 & 9)

Top Lower Miocene

literature

32m(varies)

this study

24m

Top Oligocene

literature

35m

this study

32m

AM COR 6002 67m
60m

Well-Boring

Cost

GE-l

J-1

140m

45-124m

>140m

65m

97m

no data no data

90m

no data no data

110m 115m

190m 145m

124m 130m

31

Offshore Areas Recommended for Further Study
General Statement
Throughout the u.s. Southeast Atlantic Coastal Plain and Continental Shelf, Miocene formations are characterized by high phosphorite concentrations (Riggs and others, 1982). Riggs (1984) suggests that primary formation and deposition of phosphorites is controlled by structure and topography. In the Florida and North Carolina phosphogenic provinces, the greatest concentrations of phosphorite were determined to have accumulated in shelf environments around the nose and flanks of large-scale (firstorder) structural highs such as the Ocala and Mid-Carolina Platform Highs (Riggs, 1984). Smaller scale (second-order) structural andjor topographic highs controlled phosphorite deposition in more localized areas. The results of this study indicate that the occurrence of phosphate on the Georgia and southern South carolina shelf is directly related to Neogene topographic features and their contemporaneous sea levels and oceanographic and estuarine processes.
New and Preliminary Data from the TACTS Borings
A recent report edited by Mannheim (1988) describes the status of ongoing studies of samples from three of eight foundation borings drilled on the mid- to outer-shelf area of Georgia in 1984 for the u.s. Navy Tactical Air Command Test Site (TACTS). The borings were made along three shore-parallel lines, respectively located

30nm, 45nm and 60nm seaward of the coastline. Three evenly spaced borings were made on the inner and outer 1ines, and two borings were equally spaced along the middle line. Greatest depth of sample recovery, although not continuous, ranged from 91m (300ft) to 102m (336ft) below the sea floor.

The borings are strategically located with regard to this study and any future investigations. Borings A, D and G (inner line, N-S) are along the western margin of the outer Shelf High; borings c and F (middle line, N-S) are on the crest of the High and borings B, E and H (outer line, N-S) are along the eastern margin of the
High.

A cooperative study of the

TACTS borings by the u.s.

Geological survey, the Georgia

Geologic Survey and Georgia

State University was initiated

in early 1988, sponsored by the

u.s. Minerals Management

Service, the u.s. Geological

Survey and the U.S. Bureau of

Mines. Preliminary information

of lithology, biostratigraphy,

phosphate distribution and

sediment chemistry of samples

from borings B and H (outer

line) and D (inner line)

strongly indicate that sig-

nificant phosphorite development

is present along the margins of

the Outer Shelf High. Values as

high were

as 21.6% present at

P2oor5

(45.2%BP2) near major

unconformities such as at the

top of the upper Middle Miocene

(Serravallian) (Mannheim, 1988).

A detailed study of the chemis-

try and biostratigraphy of the

borings, followed by a high-

resolution seismic survey

linking them to the existing

stratigraphic network, is

32

essential in planning a resource investigation strategy and, ultimately, an accurate economic and environmental evaluation of the resource.
Recommended Exploration Areas
Previous studies have determined that ore-quality phosphorite deposits exist within the study area (Furlow, 1969; Wallace, 1980; and Huddlestun, 1988). While most investigations have concentrated on the Chatham County are,a, new evidence cited in the previous section strongly suggests that a significant phosphorite matrix extends offshore. In order to verify the seismic stratigraphy proposed in this study and to determine the occurrence and grade of phosphate within the Neogene deposits, an exploratory core drilling program is necessary. Based on topographic expression, water depth, subbottom depth and proven phosphatic nature of the middle Miocene deposits, the most promising sites for initial exploration are delineated as Areas 1, 2 and 3 on Figure 13.
The Beaufort High extends southwestward from the Carol ina Platform into coastal Georgia. While Riggs (1984) stated that phosphorite formation was minimal in the Southeast Georgia Embayment, the presence of Miocene phosphorite deposits along the Georgia coast suggests that the Beaufort High was of sufficient relief to provide the necessary environment for the deposition of phosphorites. Miller (1980) proposed that upwelling currents rose along the flanks of a structural high and primary phosphate accumulated near a "hinge line" such as the axis of an arch. The

phosphorite deposits located in the Chatham County area on the shoreward flank of the Beaufort High would appear to support this theory. For this reason, the middle Miocene sediments on the southeastern flank of the high would be a primary candidate for further seismic exploration and drilling.
Specifically, suggested drilling targets are located in the northern half of Area 1 in the vicinity of Tybee Trough and the SLT (shown in Figure 13) . In this location, the middle Miocene sediments are near the seabed and, although only a few meters thick in the immediate vicinity of the Tower, thicken seaward and to the south.
In studies of the phosphatic Pungo River Formation of North carolina and "Hawthorne Formation" of Florida, Miller (1980) suggested that phosphate was concentrated in negative structural features (troughs) situated between two structural highs. Exploratory drilling is recommended, therefore, to determine the occurrence of phosphate in the Inner Shelf Low in which the thickness of the Pliocene section ranges from zero in the northern portion to 45 m (148 ft) in the southern portion. The erosional boundary between the middle Miocene and Pliocene may be a zone of phosphorite concentration.
Regions of extensive channeling are suggested areas for further investigation of phosphorite deposits. In the vicinity of Tybee Trough in Area 1, Kellam (1981) identified numerous channels cut into the Miocene section (see Figure 11). These channels were apparently formed during a period of

33

0 5 10

I

I

Nautical Miles

-~
.cCD:.
..Cl)
.C..D.
:::s 0
Jacksonville
Figure 13. Potential sites for further investigations for phosphorites.
34

subaerial erosion when an

extensive stream network existed

on the exposed shelf.

We

believe that phosphates from the

middle Miocene units were win-

nqwed from finer sands and clays

and concentrated in these

channel bottoms as lag deposits.

The filled portion of Tybee

Trough ranges from 1 to 5 km

(0. 6 to 3 mi) in width and is

over 30 m (98 ft) in subsurface

topographic relief. The high

density of channels in this

region makes it a prime target

for test drilling for phosphatic

channel deposits.

Also within Area 1, several large channel systems are present in the southern portion where, east of Ossabaw Island, a channel, or estuary, approximately 5 km (3 mi) in width and 25 m (82 ft) deep is incised into the top of the Middle Mioc-
ene. This feature also should be considered for investigation of phosphoritic channel lag deposits.

In Area 2 numerous deep

channels and scour troughs are

present in Miocene through

Quaternary deposits.

These

features appear to be developed

on the Outer Shelf High in

association with barrier islands

and tidal inlets formed during

successive Neogene sea level

stillstands. Also within Area

2, seismic records indicate that

middle Miocene deposits come to

within 15 m (49 ft) of the sea

bed in 32.3 m (98 ft) of water.

AMCOR 6002 boring, located on

the flank of this feature,

substantiates these elevations.

Also, analyses of middle Miocene

sediments in the 6002 well for

phosphorite by Mannheim and

others (1980) showed zones of

P20s of greater than 18%.

The region west of Areas 1 and 3 includes the Pliocene deposits characterized by foreset bedding developed over a late Miocene erosional scarp, the Sea Island Escarpment. These deposits range in thickness from 20 m (66 ft) along the edge of the scarp to 50 m (164 ft) in the central portion of the study area. During the early Pliocene, middle Miocene sediments were probably reworked and deposited as the basal unit of the Pliocene. Evidence of phosphorite in Pliocene deposits from coastal well cores suggests that phosphorite from the middle Miocene was incorporated into Pliocene sediments.
In Area 3, the shallow channels involving Pliocene and Quaternary deposits are suggested for exploratory drilling on the basis of possible phosphorite concentration in the calcareous Charlton Formation of Pliocene age as indicated from core drilling on Cumberland Island (McLemore and others, 1981) .
It is important to understand that the areas suggested for exploratory drilling were selected wholly on the basis of their potential to contain economic deposits of phosphorite as suggested by this study. The proposed drilling program is important not only to test the model suggested by this study, but to provide knowledge on the occurrence, distribution, economic value and environmental constraints that will be needed to make rational and timely management decisions concerning any future use of these or other mineral resources.

35

SUMMARY AND CONCLUSIONS

1. Regional correlation of Neogene stratigraphic units of the Georgia coast and continental shelf was .achieved through the use of high resolution seismic profiling and available well core data.

2. Within the penetration limits of the UNIBOOM system, depositional units identified on the coast and continental shelf represent upper Eocene through
Quaternary sediments. These units are bounded by unconformities resulting from subaerial exposure during eustatic low sealevel stands.

3. On the Georgia shelf, Miocene deposits form a relatively low-relief topographic high, the Beaufort High and its southern extension, the Outer Shelf High, which parallels the coast to at least as far south as
Cumberland Island.

4. A major topographic

feature, the Sea Island

Escarpment, is cut into the

middle Miocene sequence and

has controlled deposition

of the overlying Pliocene

sediments.

The buried

scarp trends in a NNE-SSW

direction parallel to the

coast.

5. The Inner Shelf Low extending into Areas 1 and
3 is a topographic trough

located between the Outer Shelf High and the Sea Island Escarpment and filled with Pliocene deltaic and open shelf deposits. Core drilling in this feature is highly recommended.
6. On the Georgia shelf, topographic, rather than structural features appear to control phosphorite deposition. The southeastern flank of the Beaufort High in the vicinity of the SLT is considered as a primary site for exploration on the premise that phosphorite deposits tend to accumulate on the nose and flanks of topographic highs. Drill cores have already established the presence of oregrade phosphorite in the coastal region of this zone. The Outer Shelf High (Area 2) and the adjacent Inner Shelf Low (Area 1) also are primary targets for investigation.
7. Extensive channeling characterizes Pliocene and middle Miocene units in the Inner Shelf Low and Outer Shelf High. Where these channels cut into middle Miocene sediments, channel lag deposits may contain significant phosphorite.
8. The three recommended target areas were selected on the basis of seismic stratigraphy developed with little verification from bore hole data. In order to establish a detailed biostratigraphic framework and to confirm the occur-

36

renee, distribution and tenor of phosphorite deposits in the target areas, an exploratory
drilling program based on the results of this study and completion of the TACTS boring analyses is recom-
mended.

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43

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