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OPEN FILE REPORT 7
"Geology of Sumter County"
OPEN FILE REPORT 7
Geology of Sumter County
by
Kenneth M Campbell
Florida Geological Survey
Tallahassee, Florida
1984
Florida Bureau of Geology Library
903 W. Tennessee St.
Tallahassee, FL 32304
PHYSIOGRAPHY
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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REFERENCES
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.
CC
SR
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.
ECONOMIC GEOLOGY
Stone
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.
Peat
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.
STRATIGRAPHY
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
County.
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.
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