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A brief overview of Miocene lithostratigraphy, northern Florida and eastern Georgia ( FGS: Open file report 31 )
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 Material Information
Title: A brief overview of Miocene lithostratigraphy, northern Florida and eastern Georgia ( FGS: Open file report 31 )
Series Title: ( FGS: Open file report 31 )
Physical Description: 6 p. : ill., map ; 28 cm.
Language: English
Creator: Scott, Thomas M
Florida Geological Survey
Publisher: Florida Geological Survey
Place of Publication: Tallahassee
Publication Date: 1990
 Subjects
Subjects / Keywords: Geology, Stratigraphic -- Miocene   ( lcsh )
Geology -- Florida   ( lcsh )
Geology -- Georgia   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Statement of Responsibility: by Thomas M. Scott.
Bibliography: Includes bibliographical references.
General Note: Cover title.
Funding: Digitized as a collaborative project with the Florida Geological Survey, Florida Department of Environmental Protection.
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Source Institution: University of Florida
Holding Location: University of Florida
Rights Management:
The author dedicated the work to the public domain by waiving all of his or her rights to the work worldwide under copyright law and all related or neighboring legal rights he or she had in the work, to the extent allowable by law.
Resource Identifier: aleph - 001751891
oclc - 25643176
notis - AJG4830
System ID: UF00001030:00001

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Table of Contents
    Title Page
        Title Page
    Introduction
        Page 1
        Page 2
    Lithostratigraphy
        Page 2
        Page 3
        Page 4
    Conclusions and references
        Page 4
    Figures
        Page 5
        Page 6
        Copyright
            Main
Full Text










State of Florida
Department of Natural Resources
Tom Gardner, Executive Director




Division of Resource Management
Jeremy Craft, Director




Florida Geological Survey
Walt Schmidt, State Geologist and Chief








Open File Report 31


A Brief Overview of Miocene Lithostratigraphy-Northern
Florida and Eastern Georgia

by

Thomas M. Scott


Florida Geological Survey
Tallahassee, Florida
1990


OTY F FLOPUInA L"IlThIES







A BRIEF OVERVIEW OF MIOCENE LITHOSTRATIGRAPHY-NORTHERN
FLORIDA AND EASTERN GEORGIA

THOMAS M. SCOTT
Florida Geological Survey, 903 West Tennessee St.,
Tallahassee, Florida 32304-7795


INTRODUCTION
The Miocene Epoch was a time of dramatic chan-
ges in the depositional scheme of the southeastern
United States. Carbonates, which dominated during
the Paleogene, were no longer the dominant sediment
type deposited in the marine environments. An influx
of vast quantities of siliciclastic sediments beginning in
the earliest Miocene first mixed with the carbonates
then overran the carbonate-producing depositional
environments. This influx of siliciclastic sediments ap-
pears to have been the result of renewed uplift in the
Appalachian Mountains and the resultant increase in
the erosional rate (Stuckey, 1965; Scott, 1988). The
timing of this important epeirogenic episode is not well
documented. However, it is reasonable to assume that
the change occurred during the Late Oligocene to
earliest Miocene, since most of the underlying Lower
Oligocene and older Paleogene sediments are car-
bonates and the Upper Oligocene and oldest Miocene
sediments are a mixture of carbonates and siliciclastics.
The Gulf Trough and Southeast Georgia Embayment
(Figure 1) provided an effective barrier to the limited
siliciclastic supply reaching the marine environment
prior to the uplift (Scott, 1988). With a major increase
in the supply of siliciclastic sediments reaching the
marine environment, the Gulf Trough and the
Southeast Georgia Embayment began to be filled. As
the influx of the siliciclastic sediments was carried into
the basins, some of the sediment began to be moved
south of these features onto the Florida Platform.
With an increasing supply of siliciclastics reaching the
Florida Platform, these sediments effectively forced
carbonate deposition to occur further south.
While the change from carbonate deposition to
,iliciclastic deposition appears to occur quickly in a
vertical sequence, the area distribution of the change
is significantly different. By the earliest Miocene, first
in eastern Georgia and then south, in northern Florida,
carbonate sediments are mixed and interbedded with
siliciclastics with the proportion of siliciclastics in-
creasing upscction. In southern Florida, carbonates
continued to dominate deposition through the Early
Miocene. However, in southern Florida, siliciclastic-s
are more commonly intimately mixed and interbcddcd
with carbonates along the eastern portion of the penin-


sula than along the west coast. This suggests the influx
of siliciclastics was driven by the active longshore drift
system associated with the Atlantic Coast. By Middle
Miocene time, deposition over the entire Florida Plat-
form was dominated by siliciclastics with only localized
occurrences of carbonates. This pattern of deposition
continued through the Late Miocene and into the
Pliocene. During the later Pliocene and in the Pleis-
tocene, carbonate deposition reoccurs in portions of
southern Florida as the siliciclastic sediment supply
began to dwindle.
The Miocene Series deposition included the for-
mation of a number of unusual minerals. The most
notable of these minerals is carbonate fluorapatite or
francolite, the dominant phosphate mineral present in
the Miocene sediments of the southeastern Coastal
Plain. Within the sediments of the southeastern Coas-
tal Plain, the Miocene time is unique in the abundance
of francolite formed. The formation of francolite re-
quires a specific, as yet not well understood, set of
depositional conditions (Riggs, 1984). Post-deposi-
tional reworking and concentration of francolite has
resulted in the formation of economically important
phosphorite deposits in northern and central Florida
(Scott, 1988). Concentrations of phosphate occur in
the subsurface of eastern Georgia and northern
Florida within the Coosawhatchie Formation, Haw-
thorn Group (Figure 2). In Georgia, this zone of phos-
phorite has been named the Tybee Phosphorite
Member, Coosawhatchic Formation (Huddlestun,
1988). A similar zone of phosphorite, possibly
equivalent to the Tybee Phosphorite Member, occurs
at the base of the Coosawhatchie Formation along the
southern rim of the Jacksonville Basin (Figure 1) in
northern Florida (Scott, 1988).
Dolomite occurs widely in the Miocene sediments
of north Florida. It is present but less abundant in the
eastern Georgia Miocene sediments. In northern
Florida, virtually all of the limestones in this section
have been diagenetically altered.to dolostonc.
One other important mineral occurring in the
Miocene sediments of this area is palygorskite (also
known as attapulgite). This Mg-rich clay mineral re-
quires uncommon depositional conditions to form and
as such its distribution is really restricted (Weaver


'.!1tPA4Y








and Beck, 1977). Although there are no known
economically important deposits of palygorskitc in
northeastern Florida and eastern Georgia, this clay
mineral occurs disseminated throughout the Miocene
sediments (Hetrick and Friddell, 1984).
The Hawthorn Group sediments constitute the
entire Miocene section to be considered in this text.
These sediments were deposited under a variety of
shallow, marine conditions. Huddlestun (1988) dis-
cusses the environment of deposition as having water
depths ranging from very shallow, near sea level to "at
least middle neritic" depths as indicated by the faunas
associated with these sediments. Puri and Vernon
(1964) stated that the Hawthorn sediments had "been
a dumping ground for alluvial, terrestrial, marine, del-
taic and marine beds of diverse lithologic units...."
Current thought is that these Miocene sediments are
of marine origin with subsequent modifications due to
erosion, reworking and diagenetic processes (Hud-
dlestun, 1988; Scott, 1988).
The Miocene sediments of Florida and Georgia
have attracted the attention of geologists for many
years. The cumulative understanding of these sedi-
ments is described in detail in Huddlestun (1988) and
Scott (1988); The reader is referred to these refer-
ences for complete descriptions of the units compris-
ing the Hawthorn Group.

LITHOSTRATIGRAPHY
The currently recognized lithostratigraphic
framework of the eastern Georgia and northern
Florida area is primarily the work of Huddlestun
(1988) (Figure 2). Scott (1988) utilized Huddlestun's
Georgia framework extending it into northern Florida
with only minor changes (Figure 2). The entire north-
ern Florida-southern Georgia area is part of the same
Atlantic coastal depositional regime of the
Southeastern Georgia Embayment and the surround-
ing positive and negative areas (Figure 1). In the
western portion of northern Florida and central-
southern Georgia, there is a transitional change to a
Gulf of Mexico coastal depositional regime. Also in
central Georgia, the depositional environments are
dramatically influenced by the Gulf Trough (Huddles-
tun, personal communications, 1988, 1989, 1990). The
most notable changes from east to west arc the
dramatic reduction in phosphate occurrence and the
general increase in clay content of the Lower and
Middle (?) Miocene sediments across northern
Florida into the panhandle.
The lithostratigraphic nomenclature currently ac-
cepted and utilized by the Florida Geological Survey
and the Georgia Geologic Survey is that of Huddlcstun
(1988) and Scott (1988). The Hawthorn is a group


consisting of, in ascending order, the Parachucia For-
mation (in Georgia) and its equivalent Penncy Farms
Formation (in Florida), the Marks Head Formation,
the Coosawhatchie Formation and its equivalent
Statenville Formation.

Lower Miocene Series
Parachucla and Penney Farms Formations
Huddlestun (1988) reintroduced and revised the
name Parachucla as a formation in Georgia. It consists
of a lower member, the Tiger Leap, and an upper
member, the Porters Landing. The Parachucla For-
mation occurs widely in Georgia. Huddlestun (1988)
indicates that the Parachucla extends into South
Carolina on the north and into Florida on the south.
The occurrence of Parachucla sediments in Florida
appears very limited (Scott, 1988). The lower
Parachula Formation has not been recognized as oc-
curring in northern Florida (Huddlestun, personal
communication, 1988). However, upper Parachucla,
Porters Landing Member sediments have been recog-
nized in outcrop along the upper Suwannee River in
northern Florida (Portell. 1989) and in one core in
Nassau County, Florida near the Georgia-Florida bor-
der (Huddlestun, personal communication, 1986).
The upper Parachucla grades laterally into the Penney
Farms Formation in southern Georgia and northern
Florida (Scott, 1988). These formations charac-
teristically vary in color from greenish gray to bluish
gray in the siliciclastic-rich sediments and tan to
greenish gray in the carbonate-rich zones.
The Tiger Leap Member of the Parachucla For-
mation is a lithologically heterogenous unit consisting
of carbonates interbedded with siliciclastics (Huddles-
tun, 1988). The carbonates are characteristically lime-
stone but locally vary to dolostone. The siliciclastic
sediments are composed of clayey quartz sands. Phos-
phate is a minor component disseminated throughout
the unit. The Tiger Leap Member is variably fos-
siliferous.
The Porters Landing Member is predominantly a
siliciclastic unit composed of sands and clays. It is
slightly phosphatic, nonfossiliferous to variably fos-
siliferous, occasionally gravelly and locally contains
carbonate in the matrix or as discrete beds (Huddles-
tun, 1988). This member becomes more carbonate-
rich to the south in Georgia and grades into the Penney
Farms Formation in northern Florida (Scott, 1988).
The Penney Farms Formation, named by Scott
(1988), is a very heterogenous unit consisting of inter-
bedded carbonates, mostly dolostone, and siliciclas-
tics. The carbonates are sandy, occasionally clayey,
phosphatic and variably fossiliferous. Fossils are
generally preserved only as molds and casts. The sands








and Beck, 1977). Although there are no known
economically important deposits of palygorskitc in
northeastern Florida and eastern Georgia, this clay
mineral occurs disseminated throughout the Miocene
sediments (Hetrick and Friddell, 1984).
The Hawthorn Group sediments constitute the
entire Miocene section to be considered in this text.
These sediments were deposited under a variety of
shallow, marine conditions. Huddlestun (1988) dis-
cusses the environment of deposition as having water
depths ranging from very shallow, near sea level to "at
least middle neritic" depths as indicated by the faunas
associated with these sediments. Puri and Vernon
(1964) stated that the Hawthorn sediments had "been
a dumping ground for alluvial, terrestrial, marine, del-
taic and marine beds of diverse lithologic units...."
Current thought is that these Miocene sediments are
of marine origin with subsequent modifications due to
erosion, reworking and diagenetic processes (Hud-
dlestun, 1988; Scott, 1988).
The Miocene sediments of Florida and Georgia
have attracted the attention of geologists for many
years. The cumulative understanding of these sedi-
ments is described in detail in Huddlestun (1988) and
Scott (1988); The reader is referred to these refer-
ences for complete descriptions of the units compris-
ing the Hawthorn Group.

LITHOSTRATIGRAPHY
The currently recognized lithostratigraphic
framework of the eastern Georgia and northern
Florida area is primarily the work of Huddlestun
(1988) (Figure 2). Scott (1988) utilized Huddlestun's
Georgia framework extending it into northern Florida
with only minor changes (Figure 2). The entire north-
ern Florida-southern Georgia area is part of the same
Atlantic coastal depositional regime of the
Southeastern Georgia Embayment and the surround-
ing positive and negative areas (Figure 1). In the
western portion of northern Florida and central-
southern Georgia, there is a transitional change to a
Gulf of Mexico coastal depositional regime. Also in
central Georgia, the depositional environments are
dramatically influenced by the Gulf Trough (Huddles-
tun, personal communications, 1988, 1989, 1990). The
most notable changes from east to west arc the
dramatic reduction in phosphate occurrence and the
general increase in clay content of the Lower and
Middle (?) Miocene sediments across northern
Florida into the panhandle.
The lithostratigraphic nomenclature currently ac-
cepted and utilized by the Florida Geological Survey
and the Georgia Geologic Survey is that of Huddlcstun
(1988) and Scott (1988). The Hawthorn is a group


consisting of, in ascending order, the Parachucia For-
mation (in Georgia) and its equivalent Penncy Farms
Formation (in Florida), the Marks Head Formation,
the Coosawhatchie Formation and its equivalent
Statenville Formation.

Lower Miocene Series
Parachucla and Penney Farms Formations
Huddlestun (1988) reintroduced and revised the
name Parachucla as a formation in Georgia. It consists
of a lower member, the Tiger Leap, and an upper
member, the Porters Landing. The Parachucla For-
mation occurs widely in Georgia. Huddlestun (1988)
indicates that the Parachucla extends into South
Carolina on the north and into Florida on the south.
The occurrence of Parachucla sediments in Florida
appears very limited (Scott, 1988). The lower
Parachula Formation has not been recognized as oc-
curring in northern Florida (Huddlestun, personal
communication, 1988). However, upper Parachucla,
Porters Landing Member sediments have been recog-
nized in outcrop along the upper Suwannee River in
northern Florida (Portell. 1989) and in one core in
Nassau County, Florida near the Georgia-Florida bor-
der (Huddlestun, personal communication, 1986).
The upper Parachucla grades laterally into the Penney
Farms Formation in southern Georgia and northern
Florida (Scott, 1988). These formations charac-
teristically vary in color from greenish gray to bluish
gray in the siliciclastic-rich sediments and tan to
greenish gray in the carbonate-rich zones.
The Tiger Leap Member of the Parachucla For-
mation is a lithologically heterogenous unit consisting
of carbonates interbedded with siliciclastics (Huddles-
tun, 1988). The carbonates are characteristically lime-
stone but locally vary to dolostone. The siliciclastic
sediments are composed of clayey quartz sands. Phos-
phate is a minor component disseminated throughout
the unit. The Tiger Leap Member is variably fos-
siliferous.
The Porters Landing Member is predominantly a
siliciclastic unit composed of sands and clays. It is
slightly phosphatic, nonfossiliferous to variably fos-
siliferous, occasionally gravelly and locally contains
carbonate in the matrix or as discrete beds (Huddles-
tun, 1988). This member becomes more carbonate-
rich to the south in Georgia and grades into the Penney
Farms Formation in northern Florida (Scott, 1988).
The Penney Farms Formation, named by Scott
(1988), is a very heterogenous unit consisting of inter-
bedded carbonates, mostly dolostone, and siliciclas-
tics. The carbonates are sandy, occasionally clayey,
phosphatic and variably fossiliferous. Fossils are
generally preserved only as molds and casts. The sands







and clays are phosphatic, often dolomitic and rarely
fossiliferous. Phosphate is commonly found in greater
concentrations in the Penncy Farms Formation than in
the Parachucla.
The greatest thickness of the Parachucla Forma-
tion is reported as 120 feet (37 m) in the northern
portion of the Southeast Georgia Embayment (Hud-
dlestun, 1988). The greatest thickness of the Penney
Farms Formation is more than 155 feet (47 m) in the
Jacksonville Basin (Scott, 1988).
The Parachucla Formation disconformably over-
lies the Oligocene Suwannee Limestone and, where the
Suwannee is absent, the Eocene Ocala Limestone. It
is overlain disconformably (?) by the Marks Head
Formation throughout most of Georgia. However,
near its northern limit in Georgia, it is disconformably
overlain by the Pliocene Cypresshead Formation
(Huddlestun, 1988).
The Penney Farms Formation disconformably
overlies the Ocala Limestone throughout most of
northern Florida. In a very limited area near its
western limit, the Penney Farms Formation lies discon-
formably on the Suwannee Limestone. Except where
the younger units have been removed, this unit is over-
lain disconformably by the Marks Head Formation.
Where erosion has removed the younger units of the
Hawthorn Group, it is overlain by undifferentiated
sediments of presumed Plio-Pleistocene age.
The age of the Parachucla and Penney Farms
Formations is Early Miocene (Aquitanian) (Huddles-
tun. 1988). The lower member of the Parachucla, the
Tiger Leap, is older than the Penney Farms Formation
which appears to equate in age with the Porters Land-
ing Member. The age of the Penney Farms Formation
is based on very limited paleontologic evidence and
lithologic correlation with the upper Parachucla For-
mation (Scott, 1988; Huddlestun, 1988).

Marks Head Formation
Huddlestun (1988) reintroduced the name Marks
Head Formation for the sediments of the middle por-
tion of the Hawthorn Group. It consists of variable
mixtures of siliciclastics and carbonate with variable
phosphate content. In Georgia, the unit contains most-
ly siliciclaslics with scattered carbonate beds (Hud-
Jlestun, 1988). In Florida, the Marks Head Formation
is the most lithologically Variable unit of the Hawthorn
Group consisting of interbedded phosphatic siliciclas-
tics and carbonates (Scott, 1988). The siliciclastics are
quartz sands with variable clay, dolomite and phos-
phate contents and clays with variable sand, dolomite
and phosphate contents. Siliciclastic and carbonate
intraclasts are very common in these sediments in
northern Florida (Scott, 1988, 1989). The charac-


teristic colors in these sediments range from greenish
gray to olive green in the siliciclastics and tan to
greenish gray for the carbonates.
The maximum thickness of the Marks Head For-
mation is 139 feet (42 m) in northeastern Georgia. In
Florida, the maximum recorded thickness is 130 feet
(40 m). The Marks Head Formation overlies the
Parachucla and Penney Farms Formations disconfor-
mably. It is disconformably overlain by the
Coosawhatchic Formation in most of the area. Where
the Statenville Formation replaces the Coosawhatchie
Formation, the Marks Head is disconformably overlain
by the Statenville. In areas where the youngest Haw-
thorn Group sediments are missing, the Cypresshead
Formation or undifferentiated sediments lie on the
Marks Head.
The age of the Marks Head Formation is'con-
sidered to be late Early Miocene (Burdigalian) based
on the occurrence of planktonic foraminifera in the
Georgia sediments (Huddlestun, 1988). These sedi-
ments in Florida are inferred to be late Early Miocene
based on the lithologic correlations with the Georgia
sediments (Scott, 1988).

Middle Miocene Series
Coosawhatchie Formation
Huddlestun (1988) formally named the
Coosawhatchie Formation for the upper sediments of
the Hawthorn Group in eastern Georgia. He recog-
nized five members of the Coosawhatchie Formation.
the Tybee Phosphorite Member, the Bcrrwille Clay
Member, the Ebenezer Member, the Mcigs Member
and the Charlton Member. With the exception of the
Meigs Member, these members occur in easternmost
Georgia. In northern Florida, sediments thought to be
equivalent to the Tybee, Berryville and Ebenezer
Members have been noted but not named. The
Charlton Member is recognized over a large area of
north Florida (Scott, 1988).
The Coosawhatchie Formation is predominantly a
siliciclastic unit containing varying amounts of car-
bonate as discrete beds and sediment matrix. It ap-
pears that the Coosawhatchie sediments become more
carbonate-rich in northern Florida (Scott, 1988).
Clays and sands dominate the lithology, occurring in
widely varying admixtures that include phosphate and
carbonate. Phosphate is most common in the Tybee
Phosphorite Member ard its equivalents but also is
scattered throughout the formation. The carbonates
present in the Coosawhatchie Formation are charac-
teristically limestone in most of eastern Georgia and
become primarily dolostone in northern Florida (Hud-
dlestun, 1988; Scott, 198). These sediments vary in
color from light gray to olive gray.








The Coosawhatchie Formation attains a maximum
thickness of 284 feet (87 m) in the Southeast Georgia
Embayment of southeasternmost Georgia. It is
laterally equivalent and gradational with the Statcn-
ville Formation. The Coosawhatchie Formation lies
disconformably suprajacent to the Marks Head For-
mation. It occurs disconformably subjacent to the
Cypresshead Formation and undifferentiated sedi-
ments of Plio-Pleistocene age.

Statenville Formation
The Statenville Formation was named by Hud-
dlestun (1988) for the interbedded phosphatic sands,
dolostones and clays of the Hawthorn Group as ex-
posed along the Alapaha River near Statenville, Geor-
gia. This unit is currently known to occur in a limited
area of southern Georgia and northern Florida. The
phosphorite deposits mined in northern Florida
belong in the Statenville Formation.
Quartz sands predominate in much of the unit.
The sands are often very phosphatic and variably
clayey, commonly with thin clay and dolostone beds.
The proportions of clay and dolostone beds are highly
variable. The clays and dolostones contain variable
amounts of sand and phosphate. The Statenville For-
mation is the most distinctively bedded unit of the
Hawthorn Group, commonly being crossbedded. The
sediments range in color from light gray to yellowish
gray and olive gray.
The Statenville Formation's thickness variability is
not well known. Scott (1988) recognized a maximum
of 87 feet (26.5 m) of Statenville sediments in a core
hole in central northern Florida. It is laterally
equivalent and gradational with the Coosawhatchie
Formation. The Statenville Formation lies disconfor-
mably on the Marks Head Formation or an unnamed
sand and dolostone of the Hawthorn Group in north-
ern Florida and Georgia. In Florida, it is overlain by
undifferentiated sands and clays of Plio-Pleistocene
age. In Georgia, it may be overlain by the undifferen-
tiated sediments or, in a limited area, the Miccosukee
Formation.

Conclusion
Sediments of the Hawthorn Group, Miocene of
the southeastern Atlantic Coastal Plain form a very
interesting and complex lithostratigraphic sequence.
These sediments have attracted much attention due to
their interesting mineralogy and hydrogeologic
properties. In our attempts to decipher the geological
history of the coastal plain we have discovered many of
its complexities. These sediments are not well under-
stood in many ways and there are many avenues of
research available. The next major step in our inves-


tigations is to determine the geological framework of
the offshore, continental shelf areas.

REFERENCES
HETRICK, S. M. AND M. S. FRIDDELL. 1984.
Clay mineralogy of the Hawthorn Group: Georgia
Geological Survey Open File Report 84-7, 91 p
HUDDLESTUN, P. F. 1988. A revision of the
lithostratigraphic units of the Coastal Plain of
Georgia: Georgia Geological Survey Bulletin 104,
162 p.
PORTELL, R. W. 1988. Fossil invertebrates from the
banks of the Suwannee River at White Springs,
Florida: in Southeastern Geological Society
Guidebook Number 30, October 7, 1989.
PURI, H. S. AND R. O. VERNON. 1964. Summary
of the geology of Florida: Florida Geological
Survey Special Publication 5 (Revised), 312 p.
RIGGS, S. R. 1984. Paleoceanographic model of
Neogene phosphorite deposition, U.S. Atlantic
continental margin: Science, v. 223, n. 4632. p.
123-131.
SCOTT, T. 1988. The Lithostratigraphy of the
Hawthorn Group (Miocene) of Florida: Florida
Geological Survey Bulletin 59, 148 p.
STUCKEY, J. L. 1965. North Carolina: Its geology
and mineral resources: North Carolina Depart-
ment of Conservation and Development, 550 p.
WEAVER, C. E. AND K. C. BECK. 1977.
Miocene of the southeastern United States: A
model for chemical sedimentation in a peri-
marine environment: Sedimentary Geologv, v. 17.
p. 1-234.








The Coosawhatchie Formation attains a maximum
thickness of 284 feet (87 m) in the Southeast Georgia
Embayment of southeasternmost Georgia. It is
laterally equivalent and gradational with the Statcn-
ville Formation. The Coosawhatchie Formation lies
disconformably suprajacent to the Marks Head For-
mation. It occurs disconformably subjacent to the
Cypresshead Formation and undifferentiated sedi-
ments of Plio-Pleistocene age.

Statenville Formation
The Statenville Formation was named by Hud-
dlestun (1988) for the interbedded phosphatic sands,
dolostones and clays of the Hawthorn Group as ex-
posed along the Alapaha River near Statenville, Geor-
gia. This unit is currently known to occur in a limited
area of southern Georgia and northern Florida. The
phosphorite deposits mined in northern Florida
belong in the Statenville Formation.
Quartz sands predominate in much of the unit.
The sands are often very phosphatic and variably
clayey, commonly with thin clay and dolostone beds.
The proportions of clay and dolostone beds are highly
variable. The clays and dolostones contain variable
amounts of sand and phosphate. The Statenville For-
mation is the most distinctively bedded unit of the
Hawthorn Group, commonly being crossbedded. The
sediments range in color from light gray to yellowish
gray and olive gray.
The Statenville Formation's thickness variability is
not well known. Scott (1988) recognized a maximum
of 87 feet (26.5 m) of Statenville sediments in a core
hole in central northern Florida. It is laterally
equivalent and gradational with the Coosawhatchie
Formation. The Statenville Formation lies disconfor-
mably on the Marks Head Formation or an unnamed
sand and dolostone of the Hawthorn Group in north-
ern Florida and Georgia. In Florida, it is overlain by
undifferentiated sands and clays of Plio-Pleistocene
age. In Georgia, it may be overlain by the undifferen-
tiated sediments or, in a limited area, the Miccosukee
Formation.

Conclusion
Sediments of the Hawthorn Group, Miocene of
the southeastern Atlantic Coastal Plain form a very
interesting and complex lithostratigraphic sequence.
These sediments have attracted much attention due to
their interesting mineralogy and hydrogeologic
properties. In our attempts to decipher the geological
history of the coastal plain we have discovered many of
its complexities. These sediments are not well under-
stood in many ways and there are many avenues of
research available. The next major step in our inves-


tigations is to determine the geological framework of
the offshore, continental shelf areas.

REFERENCES
HETRICK, S. M. AND M. S. FRIDDELL. 1984.
Clay mineralogy of the Hawthorn Group: Georgia
Geological Survey Open File Report 84-7, 91 p
HUDDLESTUN, P. F. 1988. A revision of the
lithostratigraphic units of the Coastal Plain of
Georgia: Georgia Geological Survey Bulletin 104,
162 p.
PORTELL, R. W. 1988. Fossil invertebrates from the
banks of the Suwannee River at White Springs,
Florida: in Southeastern Geological Society
Guidebook Number 30, October 7, 1989.
PURI, H. S. AND R. O. VERNON. 1964. Summary
of the geology of Florida: Florida Geological
Survey Special Publication 5 (Revised), 312 p.
RIGGS, S. R. 1984. Paleoceanographic model of
Neogene phosphorite deposition, U.S. Atlantic
continental margin: Science, v. 223, n. 4632. p.
123-131.
SCOTT, T. 1988. The Lithostratigraphy of the
Hawthorn Group (Miocene) of Florida: Florida
Geological Survey Bulletin 59, 148 p.
STUCKEY, J. L. 1965. North Carolina: Its geology
and mineral resources: North Carolina Depart-
ment of Conservation and Development, 550 p.
WEAVER, C. E. AND K. C. BECK. 1977.
Miocene of the southeastern United States: A
model for chemical sedimentation in a peri-
marine environment: Sedimentary Geologv, v. 17.
p. 1-234.













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