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Florida aquifer system
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FLRD GEOLOSk ( IC SUfRiW
[year of publication as printed] Florida Geological Survey [source text]
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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 16
Geology of Union County, Florida
Frank R. Rupert
Florida Geological Survey
t'~' L i-I: V?
Union County is situated within the Northern Highlands geomorphic
province of White (1970). This province spans north Florida from eastern
Bradford County westward into Alabama. The topographically high clayey
sand hills comprising this zone are thought to be dissected remnants of a
much more extensive highland plain, possibly an ancient delta, which
covered much of the Gulf Coastal Plain (White, 1970). Within Union County,
land surface elevations vary from approximately 50 feet above mean sea
level (MSL) in stream valleys at the southern edge of the county, to about
165 feet MSL in the flat plains of the central and northern areas. Large
sandy swamps, bays and shallow swampy lakes cover much of north-central
Union County; the numerous drainage streams in this area are generally
sluggish, flowing in poorly-defined channels. Along the western and
southern edges of the county, adjacent to the larger streams such as
Olustee Creek, Swift Creek, and the Santa Fe River, the small dendritic
drainage creeks are more deeply incised in channels in the surrounding
terrain. This results in a series of steeply-sloped ravines cut in the
otherwise flat topography along the western and southern edges of Union
County. Fifty-foot bluffs border the wide flood plain of the Santa Fe
River near Worthington Springs and along Olustee Creek, between Union and
The major lakes in Union County are Lake Butler, Palestine Lake, and
Swift Creek Pond. These lakes are generally shallow, with low swampy
shorelines and sand or mud bottoms; all have outflow channels which are
tributaries to Olustee Creek or the Santa Fe River (Clark, et al, 1964).
Union County is underlain by hundreds of feet of alluvial and marine
sands, clays, limestones and dolomites (Clark et al, 1964). The oldest
rock penetrated by water wells is limestone of the Middle Eocene Epoch (42
to 49 million years before present) Avon Park Formation. Undifferentiated
surficial sands and clays of Pliocene to Holocene Age (5 million years old
and younger) are the youngest sediments present. The Avon Park Formation
and the younger overlying limestone units are important freshwater
aQuifers, and the discussion of the geology of Union County will be con-
fined to these Eocene age and younger sediments. Figure 1 shows the geolo-
gic cross section locations, and figures 2 and 3 illustrate the underlying
stratigraphy of Union County.
AVON PARK FORMATION
The Avon Park Formation (Miller, 1986) as it occurs under Union County
is typically a dense, tan to dark brown, porous dolomite, frequently inter-
bedded with tan, gray, or cream-colored limestones and dolomitic limestones
of varying hardness (Clark, et al, 1964). Foraminifera are the dominant
fossils present, although dolomitization has destroyed or altered many of
the contained fossils. The Avon Park Formation is a component of the
Floridan aquifer system and underlies Union County at depths ranging from
400 to 600 feet below land surface (Florida Geological Survey in-house well
Marine limestones of the Ocala Group (Puri, 1957) unconformably
overlie the Avon Park Formation under all of Union County (Clark, et al,
.ESTIN W-57 SWIFT CREEK POND
0 1 2 3 4 5
i 1| i I i --
0 2 4 6 8
-e CROSS SECTION LOCATION
rtil IRF 1 IINION COUI NTY GEOLOGICAL CROSS SECTION LOCATIONS
0 1 2 3 4
0 2 4 6
VERTICAL EXAGGERATION IS 210 TIMES HORIZONTAL SCALE
FIGURE 2: GEOLOGICAL CROSS SECTION A A'
0 0 MSL
PLIOCENE TO HOLOCENE
AVON PARK FORMATION
0 1 2 3 4 5
I II I i
0 2 4 6 8
VERTICAL EXAGGERATION IS 210 TIMES HORIZONTAL SCALE
FIGURE 3: GEOLOGICAL CROSS SECTION B B'
------- -----II----- ----~~rmLUL-DIULUIILS~II*~P
1964). The Ocala Group is comprised of three formations; in ascending
order, these are the Inglis Formation, the Williston Formation, and the
Crystal liver Formation. These formations are differentiated on the basis
of litholoqy and fossil content. Typically, the lithology of the Ocala
Grouo grades upward from alternating soft and hard, white to tan fossili-
ferous limestone and dolomitic limestone of the Inglis and lower Williston
formations into white to pale orange, abundantly fossiliferous, chalky
limestone of the upper Williston and Crystal formations. Foraminifera,
molluscs, bryozoans, and echinoids are the most abundant fossil types
occurring in the Ocala Group sediments. Thickness of the Ocala Group sedi-
ments under Union County averages about 250 feet. Depths to the top of the
Ocala Group range from about 40 feet just west of Worthington Springs, to
nearly 300 feet near Raiford. The porous and cavernous nature of the Ocala
Group limestones make them important freshwater-bearing units of the
Floridan aquifer system. Many drinking water wells in Union County
withdraw water from the Crystal River Formation.
The Oligocene age (24 to 37 million years 4. P.) Suwannee Limestone
(Cooke and Mansfield, 1936) overlies the Ocala Group sediments under most
of Union County west of Lake Butler (Clark et al, 1964). In general, the
Suwannee Limestone consists of tan, white, or cream-colored marine
limestone, frequently dolomitic and coquinoid in portions and varying con-
siderably in hardness. In some wells, the Suwannee Limestone is lithologi-
cally similar to the )cala Group limestones, and is identified primarily
on the last occurrence of the foraminifera Dictyoconus cookei. The
thickness of the Suwannee Limestone ranges between 20 and 40 feet, and the
beds may be discontinuous in the subsurface; this unit is not known in
wells east of Lake Rutler (Clark, et al, 1964). In north Florida, the
Suwannee Limestone is a freshwater-bearing unit of the Floridan aquifer
Phosphatic sands, clays, limestones and dolomites of the Miocene age
(5 to 24 million years B. P.) Hawthorn Group (Scott, in preparation) uncon-
formably overlie the Suwannee Limestone in western Union County; east of
Lake Butler, the Hawthorn Group sediments lie directly upon the Ocala Grouo
limestones. The Hawthorn Group is predominantly a series of marine depo-
sits, consisting of variable and interbedded lithologies, and characterized
by phosphatic sands, granules and pebbles. Although not differentiated to
date within Union County, three formations of the Hawthorn Group are
distinguishable in surrounding counties; in order of decreasing age, these
are: the Penney Farms Formation, interbedded phosphatic quartz sand, clay
and carbonate; the Marks Head Formation, thinly and complexly interbedded
phosphatic clays, sand, and carbonate; and the Coosawhatchie Formation, a
green to tan phosphatic quartz sand with varying amounts of clay and dolo-
mite. The Hawthorn Grouo sediments have a generally northeastward dip, and
range in thickness from about 50 feet in parts of western Union County to
at least 260 feet in the northeastern portion of the county near Raiford.
The thick, relatively impermeable clays within the Hawthorn Group are the
primary confining beds for the underlying Floridan aquifer system.
Pliocene to Holocene age undifferentiated sands form a thin veneer over the
Hawthorn Group sediments in most of Union County, although the larger river
valleys in the southern and western parts of the county may cut down into
PLIOCENE TO HOLOCENE UNDIFFERENTIATED
Undifferentiated quartz sands and clays comprise.the surficial sedi-
ments over most of Union County. These unfossiliferous deposits are vir-
tually impossible to age-date, and include the unnamed reddish coarse
clastics, the relict Pleistocene (2.8 million to 0.1 million years B. P.)
marine terrace sands, and Holocene age (0.1 million years to present)
aeolian, lacustrine and alluvial deposits.
Groundwater is water that fills the pore spaces in subsurface rocks
and sediments. This water is derived principally from precipitation within
Union and nearby counties. The bulk of Union County's consumptive water is
withdrawn from groundwater aquifers. Three main aquifer systems are pre-
sent under Union County. In order of increasing depth, these are the sur-
ficial aquifer system, the intermediate aquifer system, and the Floridan
aquifer system (Southeastern Geological Society Ad Hoc Committee on Florida
Hydrostratigraphic Unit Definition, 1986).
SURFICIAL AQUIFER SYSTEM
The surficial aquifer system is the uppermost freshwater aquifer in
Union County. Sediments comprising this aquifer are primarily the sands
and thin limestone layers in the uppermost part of the Hawthorn Group as
well as the overlying Pliocene to Holocene Age sands. On average, the sur-
ficial aquifer system is about 40 feet thick over most of Union County
(Clark, et al, 1964). The surficial aquifer system is unconfined and its
upper surface is the water table. In general, the water table elevation
fluctuates with precipitation rate and conforms to the topography of the
land surface. Within Jnion County, the water table is normally 10 feet or
less below land surface. Recharge to the surficial aquifer system is
largely through rainfall percolating downward through the surficial sedi-
ments, and to a lesser extent by upward leekage from the deeper aquifers.
Water naturally discharges from the aquifer by evaporation, transpiration,
springflow, and by downward seepage into the lower aquifers. The surficial
aquifer system y1lds water of suitable quality for consumptive use and is
normally tapped by shallow dug or sand point wells. Due to the relatively
thin units comprising this aquifer, however, only limited amounts of water
are available before local water table lowering occurs.
INTERMEDIATE AQUIFER SYSTEM
The intermediate aquifer system is comprised of water-bearing sand and
limestone layers within the Hawthorn Group. Low permeability clays above
the sand and carbonate layers generally confine the intermediate aquifer
system under artesian conditions. Water yield from this aquifer varies
locally with the quantity of sand and the porosity of the carbonate; in
some areas, the Hawthorn Group carbonates are very dense, yielding little
water. Recharge to the intermediate aquifer system consists chiefly of
downward leakage from the surficial aquifer system and upward seepage from
the Floridan aquifer system in areas where the piezometric surface of the
Floridan aquifer system is higher than that of the intermediate system.
Numerous rural and domestic wells draw water from the intermediate aquifer
system, and as with the surficial aquifer system, the volume of water
available depends largely on local thickness of the aquifer units.
FLORIDAN AQUIFER SYSTEM
The Floridan aquifer system is comprised of several hundred feet of
Eocene to Oligocene age porous marine limestones, including the Avon Park
Formation, the Ocala 3roup, and Suwannee Limestone. It is by far the most
productive aquifer in Union County. In extreme southwestern Union County,
the upper part of the Floridan aquifer system is unconfined, and is under
water table conditions; in the remainder of the county, the aquifer is con-
fined by low permeabilty clays of the overlying Hawthorn Group, and is
under artesian conditions. West of Lake Butler, the Suwannee Limestone
comprises the upper unit of the Floridan aquifer system. East of Lake
butler, the Crystal River Formation of the Ocala Group is the uppermost
unit. County-wide, depth to the Floridan varies between 75 and 325 feet
(Florida Geological Survey in-house well data). The Floridan aquifer
system Is an important freshwater source throughout Florida, and many deep
domestic wells and most municipal and industrial supply wells draw from
Recharge to the Floridan aquifer in Union County occurs primarily as
downward leakage through the confining beds from the shallower aquifers
(Clark, et al, 1964). In southwestern Union County, the Hawthorn Group is
thin or absent, and direct recharge through downward percolation occurs.
Water leaves the Floridan aquifer system through natural movement down gra-
dient (westward) and subsequent discharge through springs, lakes, and along
the Santa Fe River.
At present, no mineral commodities are being mined on a commercial
basis in Union County. In general, the potential for commercial mineral
production in this county is low. The following discussion of the major
mineral commodities is intended to provide an overview of the mining poten-
tial for each mineral.
A number of shallow private pits in Union County are worked for fill
sand. These sand deposits are concentrated in.the unconsolidated Pliocene
to Holocene age surficial sediments covering most of the county. The
unnamed variably-colored clayey coarse clastics, believed to be equivalent
to the Miccosukee and Citronelle Formations to the west, characteristically
contain fine to coarse grained quartz sand and localized gravel beds.
These clayey sands are utilized as roadbase in counties to the south where
the clay content is higher. Commercial sand production from these sedi-
ments in Union County would require extensive washing to remove the clay
matrix; the economics of this procedure would probably preclude mining in
Phosphatic sediments of the Hawthorn Group underlie most of Union
County. The phosphate occurs as tan to black sand, granule, and pebble
sized grains, and generally compriseing up to 25 percent by volume of the
Hawthorn Group sediments. Most well lithologic logs reveal phosphate grain
content to be highly variable, ranging on average between one and ten
percent; the higher phosphate percentages occur at depths in excess of 60
feet in wells near Raiford (Florida Geological Survey in-house well data).
The variable nature of these deposits, as well as the excessive depth to
the higher phosphate concentrations make the phosphate mining potential low
in Union County.
LIMESTONE AND DOLOMITE
Union County is underlain by extensive deposits of Eocene to Miocene
age marine limestones. However, the excessive thickness of the overlying
Hawthorn Group siliciclastics and the Pliocene to Holocene undifferentiated
surficial sediments puts most limestone too deep for commercial mining. In
the extreme southwestern corner of Union County, Ocala Group limestones are
present wihtin 40 feet of the surface; however, this depth may still be
beyond the range for economic mining, and the compositional quality of this
rock for industrial use is untested.
Peat is an organic deposit formed from rapid accumulation of decaying
vegetation. To date, no commercial mining of peat occurs in Union County.
Although unproven, the areas of highest peat potential are the shallow,
swampy regions of the northern and central parts of the county.
Clay and clayey sand deposits occur in the upper Hawthorn Group sedi-
ments as well as the undifferentiated Pliocene to Holocene surficial sedi-
ments over most of Union County. Except for private dirt fill pits, there
has been no commercial exploitation of these deposits. The suitability of
these clays for industrial and commercial use is, as yet, untested. To the
east in Putnam County, and in counties to the south, the red clayey sands
and sandy clays formerly referred to as unnamed coarse clastics are used
extensively as road material.
Clark, W. E., Musgrove, R. H., Menke, C. G. and Cagle, J. W., 1964, Water
resources of Alachua, Bradford, Clay and Union Counties, Florida:
Florida Geological Survey, Report of Investigations no. 35, 170 p.
Cook, C. W. and Mansfield, W. C., 1936, Suwannee Limestone of Florida
(abstract): Geological Society of America Proceedings, 1935, p.
Miller, J. A., 1986, Hydrogeologic framework of the Floridan aquifer system
in Florida and in parts of Georgia, Alabama, and South Carolina:
U. S. Geological Survey, Professional Paper 1403-B, p. 25-27.
Purl, H. S., 1957, Stratigraphy and zonation of the Ocala Group: Florida
Geological Survey Bulletin 38, 248 p.
Scott, T. M., (in preparation), The lithostratigraphy of the Hawthorn
Group (Miocene) of Florida- Florida G aoogica;lSurvey Bu1e+in 5q.
Southeastern Geological Society Ad Hoc Committee on Florida hydrostrati-
graphic unit definition, 1986, Hydrogeological units of Florida:
Florida Bureau of Geology, Special Publication no. 28, 9 p.
White, W. A., 1970, Geomorphology of the Florida peninsula: Florida
Geological Survey, Bulletin no. 51, 164 p.
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