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FLRD GEOLOSk ( IC SUfRiW


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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

by

Frank R. Rupert


Florida Geological Survey
Tallahassee, Florida
1987


t'~' L i-I: V?



















































SCIENCE
LIBRARY







UNION COUNTY

GEOMORPHOLOGY
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

Columbia Counties.

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).







STRATIGRAPHY

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.


EOCENE SERIES

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

data).


OCALA GROUP
Marine limestones of the Ocala Group (Puri, 1957) unconformably

overlie the Avon Park Formation under all of Union County (Clark, et al,




BAKER COUNTY

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-e CROSS SECTION LOCATION







rtil IRF 1 IINION COUI NTY GEOLOGICAL CROSS SECTION LOCATIONS


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SUWANNEE
UMESTONE


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VERTICAL EXAGGERATION IS 210 TIMES HORIZONTAL SCALE


FIGURE 2: GEOLOGICAL CROSS SECTION A A'


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2 u-

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0 MS


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GROUP


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PLIOCENE TO HOLOCENE
r- UNDIFFERENTIATED


HAWTHOFRN


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SUWANNEE
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GROUP


AVON PARK FORMATION


MILES
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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
-River .
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.


OLIGOCENE SERIES

SUWANNEE LIMESTONE

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

system.

MIOCENE SERIES

HAWTHORN GROUP

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

Hawthorn section.

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
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

9








(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.
I1







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
this aquifer.

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.

MINERAL RESOURCES
At present, no mineral commodities are being mined on a commercial

basis in Union County. In general, the potential for commercial mineral

IC








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.

SAND

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

Union County.


PHOSPHATE

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

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

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.








REFERENCES


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.
71-72.

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.