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FLRD GEOLOSk ( IC SUfRiW


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State of Florida
Department of Natural Resources
Elton J. Gissendanner, Executive Director




Division of Resource Management
Charles W. Hendry, Jr., Director




Florida Geological Survey
Steve R. Windham, Chief









Open File Report 10

Geology of Sarasota County, Florida

by

Kenneth M. Campbell


Florida Geological Survey
Tallahassee, Florida
1985





































3 1262 04543 6382






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Geology of Sarasota County, Florida

by Kenneth M. Campbell

Florida Geological Survey

Tallahassee, Florida

1985

Physiography



Several authors have discussed the physiography of the Florida peninsula.

For the purposes of this report, White's (1970) classification will be utilized.

The vast majority of Sarasota County lies within the Gulf Coastal Lowlands. Two

small areas in the northeast portion of the county fall within the boundaries

of the DeSoto Plain. Barrier islands and lagoons are developed along most of

the county's gulf coast.



Elevations within the county range from mean sea level along the gulf coast

and the lower Myakka River to a maximum of approximately 100 feet in the extreme

northeast portion of the county (immediately south of Verna). Elevations

increase almost imperceptably from the west and southwest toward the northeast.

The topography tends to be flat, with relatively steeper slopes in the vicinity

of streams (Lane, 1973).



Gulf Coastal Lowlands



The prominent topographic features of the Gulf Coastal Lowlands within

Sarasota County are scaxps and terraces developed during Pleistocene sea level

stands. Healy (1975a) shows four terraces within Sarasota County. The Pamlico





Florida Bureau of Geology Library
903 W. Tennessee St.
allahassee, F6 32804








terrace is found at elevations of approximately 8-25 feet above mean sea level,

the Talbot at 25-42 feet, the Penholloway at 42-70 feet and the Wicomico at

70-100 feet. The scarps which separate these terraces are, for the most part,

poorly defined.



With the exception of the coast at Venice, the Sarasota County gulf coast

consists-of barrier islands, spits and lagoons. These barrier islands were

formed during the last 4000 to 5000 years, since sea level became reasonably

stable. The barrier islands seen today represent the latest adjustment to

changing conditions during this period of time. Barrier islands change size,

shape and position in response to both short-term and long-term conditions.

Barrier islands can be "welded" together, split into segments, become attached

to the mainland or even disappear completely. The method of formation and the

original location may be obscured.



Barrier islands require an abundant sand supply. Since present sea level

has stabilized there has been very little new sand being added to the barrier

islands in this area. The result is that portions of these barrier islands are

being eroded. Most of the sand lost to erosion is being redeposited as spits at

the ends of the barrier islands, in the lagoons or offshore.



DeSoto Plain



The DeSoto Plain is a very flat area located primarily in Manatee, Hardee,

DeSoto, Highlands, Glades and Charlotte counties. Only a small portion of the

DeSoto Plain extends into the northeast corner of Sarasota County. That portion

of the DeSoto Plain present in Sarasota County consists primarily of the








relatively steeper slope between the very edge of the plain and the inland edge

of the Gulf Coastal Lowlands. White (1970). believes that the DeSoto Plain is a

submarine plain probably formed under Wicomico seas (70 100 feet above present

sea level).

DRAINAGE



The majority of Sarasota County is poorly drained. Many swamps, marshes,

and ponds are present throughout the county. The Myakka River and its tribu-

taries are the major streams in the county, although there are numerous small

streams. Healy (1975b) shows the majority of Sarasota County as an area of

.artesian flow. The ground water table is at or near the land surface

throughout much of the county. Natural drainage systems have been channelized

and extensive ditch systems constructed to improve drainage.



STRATIGRAPHY



Surface and near surface sediments in Sarasota County consist of quartz

.sand, consolidated and unconsolidated shell beds, clays, limestone and dolomite.

These sediments range in age from Oligocene (38-22.5 million years ago) to

Holocene (10,000 years ago to present).



Oligocene Series



Suwannee Limestone



The Suwannee Limestone is present throughout Sarasota County in the

subsurface. It is generally broken down into two units.







The upper unit of the Suwannee Limestone is a creamy white to light

yellowish grey limestone containing darker dolomitized zones (Lane, 1973).

The undolomitized portions are variably packstone to wackestone, poorly to well

indurated and variably recrystallized. The upper unit is highly fossiliferous,

containing abundant poorly preserved foraminifera, molluscs, echinoids and

corals. Moldic and vuggy porosity is common.



The lower unit of the Suwannee Limestone is generally a pale grey to light

yellow calcilutitic limestone. The lower unit is typically softer, more calci-

lutitic, and less porous and fossiliferous than the upper unit and may contain

finely divided pyrite (Lane, 1973).



The top of the Suwannee Limestone is encountered at approximately 350 feet

below mean sea level in the northeastern-most part of the county and dips to

the south and west. In the southern-most part of the county, the top of the

Suwannee Limestone is encountered at depths of approximately 650 feet below mean

sea level.



The thickness of the Suwannee ranges from approximately 150 feet to over

350 feet. The thinnest portion is in extreme southwestern Sarasota County while

the thickest portions are in the vicinity of Sarasota and in the eastern portion

of the county (Lane, 1973).
















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Miocene Series



Hawthorn Group



Scott (1985) has raised the Hawthorn from formation status to group status.

The Hawthorn Group will include those sediments which in the past have been

included in the Tampa, Hawthorn and Bone Valley formations. In Sarasota County,

the Hawthorn Group consists of the Arcadia Formation which is overlain by the

Peace River Formation.

Arcadia Formation



The Arcadia Formation (Scott, 1985) in Sarasota County consists of, in

ascending order, the Tampa Member and an unnamed upper member. The Tampa Member

overlies the Suwannee Limestone and is lithologically similar to the type Tampa

Formation of King and Wright (1979) but has a slightly greater phosphate content

(1-3 percent) and greater areal limits (Scott, 1985). The Tampa Member is a

white to tan-colored, quartz sandy limestone with a carbonate mud matrix.

Varying amounts of clay are usually disseminated throughout the rock (King and

Wright, 1979, Scott, 1985). Some beds within the Tampa Member contain more than

50 percent quartz sand. Dolomite is relatively uncommon within the Tampa Member

(King and Wright, 1979; Scott,-1985). The Tampa Member is recognizable

throughout most of northern Sarasota County, however the unit becomes indistinct

due to a facies change in southern Sarasota County.



The upper (unnamed) member of the Arcadia Formation includes those sediments

which in the past have been referred to as the "Hawthorn carbonate unit" (Scott,

1985). Lithologically, these sediments consist of white to yellowish-grey,







quartz sandy, phosphatic, sometimes clayey, dolomites and limestones (uncommon).

Occasional beds of carbonate rich quartz sand and thin clay beds are present.



The Arcadia Formation is present throughout Sarasota County in the

subsurface. The top of the Arcadia Formation is encountered at approximately

mean sea level in northeastern Sarasota County to just over 100 feet below mean

sea level in the southern-most part of the county. The Arcadia dips gently to

the south-southeast. The thickness of the Arcadia ranges from approximately 300

feet to over 500 feet. The thickness increases from the northeast and east to

the west, southwest and south (Scott,'1985).



Peace River Formation



The Peace River Formation (Scott, 1985) in Sarasota County consists of

those sediments which have been described as "upper Hawthorn clastics."

Lithologically, these sediments consist of yellowish-grey to light olive green

interbedded phosphatic sands, clayey sands, clays and dolomite stringers. The

Peace River is present throughout the county, with the exception of the imme-

diate area of Sarasota.



The top of the Peace River Formation is found at or near mean sea level

(+ 30 feet) throughout much of the county (Scott, 1985). In northeastern Sarasota

County, however, the Peace River is encountered at 50 feet above mean sea level.



The thickness of the Peace River ranges from zero in the vicinity of

Sarasota to 110 feet in the eastern-most part of the county. The formation

thickness to the east and south (Scott, 1985).






&







Anastasia Formation



The Anastasia Formation (probably late Pleistocene age) is present at

"Point of Rooks", Siesta Key. The Anastasia forms a prominent outcrop which

extends along the beach for about 1 mile. The outcrop consists -of casehardened,

bedded coquina composed primarily of mollusc shells and fragments (Lane, 1973).

Wave action erodes and undercuts this material into large slabs. The resistance

to wave erosion of this material accounts for the prominent seaward projection

of the coastline at this point.



Holocene Series



Deposits of Holocene age (10,000 years ago to present) are primarily

limited to present day stream flood plains, beaches, intertidal swamps and

marshes, inland swamps, marshes and lakes. These sediments consist of sand,

silt, clay and organic materials.







Pliocene-Pleistocene Series


Undifferentiated Surficial Sands and Shell



Surficial deposits of Pliocene-Pleistocene age (5.3 to .01 million years

ago) blanket the county. Throughout the majority of the county, the surficial

deposits consist of sandy, silty and clayey, variably indurated shell beds

overlain by a thin veneer of clean quartz sand. The thickness of these deposits

is typically 15-30 feet. The shell beds are not present in the eastern and

western portions of the northern third of the county (Lane, 1973). The shell

beds generally are 5-6 feet thick, but range from 2-15.feet thick (Lane, 1973)

and are often iron stained. Iron cemented hardpan is common.



Clean quartz sand of Pleistocene age (1.6 to .01 million years ago) forms a

veneer over the shell beds. or the Peace River Formation where the shell beds are

absent These deposits consist of unconsolidated very fine to medium grained

(1/16-1/2 mm) quartz sand. The sands are white to light brown in color and

contain trace amounts of phosphate sand and limestone or shell fragments.



The clean quartz sands were deposited by Pleistocene seas at various sea

levels. The scarps and terraces discussed under Physiography are developed in

these sands. The maximum thickness of surficial sands is 45-50 feet in the

vicinity of Verna.






Bibliography

Healy, H. G., 1975a, Terraces and Shorelines of Florida, Florida Bureau of
Geology, Map Series #71.

1975b, Piezometric Map With the Area of Artesian Flow of the
Floridan Aquifer in Florida, Revised 1975, Florida Bureau of Geology Map
Series #4 Revised.

King, K. C. and R. Wright, 1979, Revision of the Tampa Formation, West-Central
Florida, Transactions, Gulf Coast Association of Geological Societies, vol.
XXIX, pp. 257-261.

Lane, B. E. 1973, Geology of Sarasota County, unpublished Florida Bureau of
Geology Manuscript, 19 p.

Scott, T. M., 1985, The Regional Lithostratigraphy of the Hawthorn Group
(Miocene) of Florida, Florida Bureau of Geology Bulletin, manuscript in
preparation.

White, W. A., 1970, Geomorphology of the Florida Peninsula, Florida Bureau of
Geology Bulletin #51, 164 p.















































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