Geology of Sumter County ( FGS: Open file report 7 )

Material Information

Geology of Sumter County ( FGS: Open file report 7 )
Campbell, Kenneth M. ( Kenneth Mark ), 1949-
Florida Geological Survey
Place of Publication:
Tallahassee, Fla.
Florida Geological Survey
Publication Date:
Physical Description:
12 leaves : ill., maps ; 28 cm.


Subjects / Keywords:
Geology -- Florida -- Sumter County ( lcsh )
Sumter County ( local )
City of Crystal River ( local )
City of Ocala ( local )
City of Apopka ( local )
City of Vernon ( local )
Limestones ( jstor )
Sediments ( jstor )
Crystals ( jstor )
Sand ( jstor )
Peat ( jstor )
non-fiction ( marcgt )


General Note:
Florida Geological Survey open file report 7
Statement of Responsibility:
by Kenneth M. Campbell.

Record Information

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:
38575152 ( oclc )


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"Geology of Sumter County"


Geology of Sumter County

Kenneth M Campbell
Florida Geological Survey
Tallahassee, Florida


Florida Bureau of Geology Library
903 W. Tennessee St.
Tallahassee, FL 32304


Several authors have discussed the physiography of the Florida peninsula.

For the purposes of this report, White's (1970) classification will be uti-
lized. The majority of Sumter County lies within the Western Valley'and Tsala

Apopka Plain. Other major physiographic features of Sumter County include the

Sumter and Lake uplands (figure 1).

Western Valley

The Western Valley is a large irregularly shaped low area which is

bounded on the west by the Brooksville Ridge and on the east by the Sumter

and Lake uplands. The Western Valley is connected to the Central Valley by

the Lake Harris Cross Valley. The Lake Harris Cross Valley is an east-west

trending gap separating the Sumter and Lake uplands. Elevations within the

Western Valley range from approximately 40-100 feet.

--Tsala Apopka Plain

The boundaries of the Tsala Apopka Plain are the Brooksville Ridge on the

west and the Withl-acoochee River on the east. The plain forms the lowest and

flattest portion of the Western Valley (White, 1970). Tsala Apoka Lake occu-

pies the northern portion of the plain. Elevations range from approximately

40 feet (Tsala Apopka Lake) to about 75 feet. Tsala Apopka Lake is thought to

be a relict of a much larger lake which occupied most of the Tsala Apopka

Plain (White, 1958).

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Cooke, C. W., 1945, Geology of Florida, Florida Geological Survey Bulletin 29,

345 p.

Vernon, R. 0., 1951, Geology of Citrus and Levy Counties, Florida, Florida

Geological Survey Bulletin 33, 256 p.

White, W.A., 1958, Some Geomorphic Features of Central Peninsular Florida,

Florida Geological Survey Bulletin 41, 92 p.

1970, The Geomorphology of the Florida Peninsula, Florida Bureau

of Geology Geological Bulletin 51, 164 p.



The Oligocene age (23.7-35.6 million years ago) Suwannee Limestone, which

overlies the Ocala Group throughout much of peninsular Florida is not present

in Sumter County. If the formation was originally deposited in the area, it

has since been removed by erosion. The Miocene Hawthorn Group sediments have

also been removed by erosion. Some of the undifferentiated sediments (quartz

sands and clays) are phosphatic and are considered to be residual material of

the Hawthorn Group (Scott, personal communication 1984). Previous authors

have assigned these materials to either the Hawthorn or Alachua formations

(Cooke, 1945; Vernon, 1951).

The general lithology of the undifferentiated surficial sediments is

variable from fine to coarse grained sand to clayey sand, sandy clay and clay.

Clay content generally increases downward. Limestone and phosphatic limestone

fragments are often found in the lower part. Peat or organic rich sediments

are found at the surface in some parts of the county.



Crushed limestone is the major mineral commodity produced in Sumter

County. The several companies in operation within the county are mining pre-

dominantly from the late Eocene Crystal River Formation. These quarries are

located in the central portion of the county.

All limestone mined in the county is mined from open pit quarries.

Generally, overburden must be removed by bulldozers prior to mining. In some

areas, the limestone is soft enough that bulldozers equipped with a claw can

rip the rock loose. If harder rock is encountered, drilling and blasting are

necessary to fracture the rock. Where mining extends below the water table

and pits remain flooded, draglines are utilized in mining. After mining, the

material is transported by truck to processing plants to be crushed and stock-

piled. The primary products produced are dense road base material and

agricultural lime.


Two companies are currently mining peat from Holocene age deposits within

Sumter County. These deposits are located east of Oxford near the Lake

County boundary and near the Withlacoochee River southwest of Tarrytown.

Mining is accomplished by clearing the surface of vegetation, pumping to

dewater the peat, then excavating the peat with a dragline. The peat is then

shredded and stockpiled to dry. All of the peat produced is utilized for

various horticultural purposes such as landscaping and potting soils, although

some of the peat is suitable for energy applications.

Limestone of the lower Ocala forms the bedrock in the Tsala Apopka Plain

area of northern and western Sumter County. The lower Ocala unconformably

overlies the Middle Eocene Avon Park Limestone and conformably underlies the

Crystal River Formation where that formation has not been removed by erosion.

The boundary between the Crystal River and the lower Ocala is transitional.

Where the Crystal River Formation has been removed by erosion, the lower Ocala

is overlain by Miocene to Holocene plastic sediments.

Crystal River Formation

The general lithology of the Crystal River Formation is described by

Vernon (1951) as a white or cream colored, soft, very massive, friable coquina

of large foraminifera set in a pasty calcite (calcilutite) matrix. In

southern Sumter County, the calcilutite matrix becomes predominant.

The Crystal River Formation is abundantly fossiliferous, commonly forming

a coquina of large foraminifera. The most common fossil types include forami-

nifera (large and small) echinoids, molluscs, bryozoa and oysters.

The Crystal River is conformably and gradationally underlain by the lower

Ocala. In Sumter County, the Crystal River Formation is unconformably

overlain by Miocene to Holocene plastic sediments. The Crystal River forms

the bedrock in Sumter County wherever it is present.

Miocene to Holocene Series

Undifferentiated surficial sands and clays

The surficial sediments throughout most of Sumter County consist of

quartz sand, clayey sand and clays. The thickness of these surficial sedi-

ments ranges from just a few feet to almost 100 feet thick.


Surface and near surface sediments in Sumter County consist of quartz

sand, clay, peat, limestone and dolomite. The sediments discussed below range

in age from Middle Eocene (40-45 million years ago) to Holocene (10,000 years

ago to present).

Eocene Series

Avon Park Limestone

The Middle Eocene Avon Park Limestone is the oldest formation to crop out

in Florida. The Avon Park is present in the subsurface throughout Sumter

County, but is not exposed within the county.

Within Sumter County, the Avon Park may be either limestone or dolomite.

Generally the uppermost 30-70 feet of the formation consists of limestone,

underlain predominantly by dolomite or dolomitic limestone. The limestone of

the Avon Park typically is white, cream or brown in color, poorly to well

indurated, calcilutite (clay and silt sized) to very fine grained. The

limestone may be thin bedded to relatively structureless and commonly contains

organic material as flecks and thin seams. Where the formation is dolomitized

it is microcrystalline to fine grained, unconsolidated to well indurated,

euhedral to subhedral dolomite crystals.

The Avon Park Limestone unconformably underlies the Ocala Group through-

out the county, with the possible exception of two small areas. Vernon (1951)

mapped two areas, one north of Center Hill, the other southeast of Webster,

where the Ocala Group has been removed. Supporting data, however, is limited

and the configuration of these areas is unknown. In these areas the Avon Park

Limestone is overlain unconformably by Miocene to Holocene plastic sediment.
Characteristic fossil groups found in the Avon Park Limestone include

echinoids, foraminifera, bryozoa, molluscs, and corals. These groups are

indications of a shallow marine depositonal environment.

Ocala Group

The Ocala Group consists three formations, which in ascending order are,

the Inglis, Williston and Crystal River formations. For the purposes of this

report, the Inglis and Williston are not differentiated. These two formations

are discussed as the lower Ocala Group. Essentially all of Sumter County is

underlain by limestone of the Ocala Group.

Lower Ocala Group

The lower Ocala Group in Sumter County is a white to cream or tan colored,

granular and variably chalky or recrystallized limestone. Where not exten-

sively recrystallized, the lower Ocala is a poorly to well indurated calcarenite

composed in large part of miliolid foraminifera. Cement may be either

calcite or dolomite. The lower portion is commonly dolomitized. Dolomitic

sections consist of poorly to well indurated microcrystalline (silt sized) to

very fine grained, euhedral dolomite crystals.

The lower Ocala limestone is abundantly fossiliferous, commonly forming a
foraminiferal coquina. The most common fossils include miliolid foraminifera,

other foraminifera, molluscs and echinoids.

Brooksville Ridge

The Brooksville Ridge forms the western boundary of the Western Valley.

The ridge is present only in a small portion of west central Sumter County in

the vicinity of Nobleton. The Brooksville Ridge trends north to south and has

elevations which range from 70-200 feet. The southern part of the ridge runs

through central Citrus, Hernando, and Pasco counties, to the west of Sumter


The Brooksville Ridge is composed of a core of limestone which is

overlain by clayey sands, sandy clays and clays which are in turn overlain by

Pleistocene sands. The clays and clayey sediments have limited downward per-

colation of ground water, thus limiting the amount of dissolution of the

limestone core of the ridge. The result is that the Brooksville Ridge stands

high relative to the Western Valley and the Tsala Apopka Plain.

Sumter & Lake Uplands

The Sumter and Lake uplands occupy the northeastern corner and part of

the eastern boundary of Sumter County. The two uplands are separated by the

Lake Harris,Cross Valley. In general, the elevation of the two uplands

decreases in a northerly direction. Elevations within Sumter County range

approximately ,from 50-100 feet in the northern part and approximately 75-140

feet in the southern part.

The Sumter and Lake uplands are similar to the Brooksville Ridge in com-

position: a limestone core overlain by clayey sediments, in turn overlain by

Pleistocene sands. Dissolution of of limestone has been limited by the clayey

sediments resulting in the relatively high elevations.