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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 17

Geology of Bradford County, Florida

by

Frank R. Rupert


Florida Geological Survey
Tallahassee, Florida
1987





































3 1262 04543 6275







L RAr Y





LIBRARY




















GEOLOGY OF BRADFORD COUNTY, FLORIDA

by .

Frank Rupert



OFR-17



Florida Geological Survey
Tallahassee, Florida
1987






BRADFORD COUNTY

GEOMORPHOLOGY

Bradford County lies in the Northern Highlands physiographic province

of White (1970). This province spans north Florida from the eastern edge

of Bradford County westward into Alabama. Characterized by a series of

topographically high and gently rolling clayey sand hills, this province is

thought to be a stream-dissected remnant of a once more extensive highland

plain covering much of the Gulf Coastal Plain (White, 1970).

Skirting the eastern edge of Bradford County is a topographic feature

named the Trail Ridge (see Figure 1). The Trail Ridge is an elongate,

north-south trending series of quartz sand hills rising abruptly above the

swampy plain of eastern Bradford County and reaching nearly 220 feet above

mean sea level (MSL). Its crest roughly parallels the Bradford-Clay county

line, and the ridge on average extends less than one mile into Bradford

County (Clark et al., 1964). The bulk of the Trail Ridge lies in neighbor-

ing Clay County, where it reaches 10 miles in width.

Elsewhere in Bradford County, land surface elevations vary from

approximately 60 feet MSL in the swampy Santa Fe River Valley in the

westernmost tip of the county, to 175 feet MSL in southeastern Bradford

County east of the town of Hampton. Over most of the county, the terrain

is generally flat, with large swampy areas and shallow lakes. Creeks and

streams are numerous but sluggish, and flow in poorly-defined channels.

The predominant surficial sediments are quartz sands and clayey sands.

Along the Santa Fe River at the southwestern edge of the county and along

the New River bordering the western edge, the tributary streams are more

deeply incised in the surrounding terrain. Here, tributary streams flowing

into the larger river valleys have cut ravines into the resistant clayey







sands. Steep-sloped bluffs also border the wide valley floors of both the

Santa Fe and New rivers in western Bradford County.

The Santa Fe River is the largest stream in the Bradford County area

and forms the Bradford-Alachua county boundary. The river begins in Santa

Fe Lake, a large shallow body in southeastern Bradford County and

northeastern Alachua County. Flow is westward, where it receives flow from

Hampton Lake, the Sampson River, draining Lake Sampson, and from the New

River which comprises the Bradford-Union county line. The New River forms

at the confluence of numerous small creeks in northern Bradford County, and

drains the highland areas in the northern and western portions of the

county.


STRATIGRAPHY

Bradford County is underlain by hundreds of feet of marine sands,

clays, limestones and dolomites (Clark et al., 1964). The oldest rock

penetrated by water wells is limestone of the Eocene age (37 to 54 million

years before present, B.P.) Avon Park Formation. Undifferentiated sur-

ficial 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 this discussion of the geology of Union County will be confined to

these Eocene age and younger sediments. Figure 1 shows the stratigraphic

cross section locations, and Figures 2 and 3 illustrate the underlying

stratigraphy of Bradford County.


EOCENE SERIES

AVON PARK FORMATION

The Avon Park Formation (Miller, 1986), as it occurs under Bradford

-2








BAKER COUNTY


MILES
0 1 2 3 4 5
I ,, I I
0 2 4 6 8
KILOMETERS


LAKE


ROWELL


HM---TO
I HAMPTON
L---J


A 4C
SC


* WELL


u 4


-- CROSS SECTION LOCATION


c
O


-L
*. SANTA FE
S 'SWAMP

^.v


FIGURE 1:
BRADFORD COUNTY GEOLOGICAL CROSS SECTION LOCATIONS


W-5


10 >-
0z

I


















0
n > o 0

) 0. m
Sot
I I U


MSL


60 1-200


--250



--300



- -350


VERTICAL EXAGGERATION IS
210 WMES HORIZONTAL SCALE


AVON PARK FORMATION


0 1 2 3 4 5

0 2 4 6 8
KILOMETERS


T.D. 607 FEET


.ELL NUMBERS SHOWN ARE FLORIOA GEOLOGICAL SURVEY WELL ACCESSION NUMBERS


FIGURE 2: GEOLOGICAL CROSS SECTION A A'


LU
I- i
uJ w
2 .L

60 200



150
40-

100


20
50


0+0


* -50



* -100



* -150












0
(o w
(0
w -
LLU-


-300


-350


-400


S-450


L -50 T.D.-723 FEET

VERTICAL EXAGGERATION IS 210 TIMES HORIZONTAL SCALE


MILES
0 1 2 3 4 5
I p I I
0' 2 4 6 8
KILOMETERS


AVON PARK
FORMATION


WELL NUMBERS ARE FLORIDA GEOLOGICAL SURVEY WELL ACCESSION NUMBERS


FIGURE 3: GEOLOGICAL CROSS SECTION B B'


0 0%3?


opp`'9
ocp~'LC






County, is typically a dense, tan to dark brown, porous dolomite, frequently

interbedded with tan, gray, or cream-colored limestones and dolomitic lime-

stones 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 the top of this unit underlies Bradford

County at depths ranging from 400 to 700 feet below land surface (Clark et

al., 1964; Florida Geological Survey in-house well data).


OCALA GROUP

Marine limestones of the Ocala Group (Purl, 1957) unconformably

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

1964). The Ocala Group is comprised of three formations; in ascending

order, these are the Inglis Formation, the Williston Formation, and the

Crystal 1iver Formation. These formations are differentiated on the basis

of lithology and fossil content. Typically, the lithology of the Ocala

Group grades upward from alternating hard and soft, 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
limestones of the upper Williston and Crystal Formations. Foraminifera,

mollusks, bryozoans, and echinoids are the most abundant fossil types

occurring in the Ocala Group sediments. Thickness of the Ocala Group sedi-

ments under Bradford County average about 250 feet. The permeable and

cavernous nature of the Ocala Group limestones make them important fresh-

water-bearing units of the Floridan aquifer system. Many drinking water

wells in Bradford County withdraw water from the Crystal River Formation.







OLIGOCENE SERIES

SUWANNEE LIMESTONE

The Oligocene age (24 to 37 million years 1. P.) Suwannee Limestone

(Cooke and Mansfield, 1936) occurs as discontinuous erosional remnants over-

lying the Ocala Group sediments under the extreme western tip of Bradford

County from the town of Brooker westward (Clark et al., 1964; Florida

Bureau of Geology in-house data). In general, the Suwannee Limestone con-

sists of tan, white, or cream-colored marine limestone, frequently dolomi-

tic and coquinoid in portions and varying considerably in hardness. In

some wells, the Suwannee Limestone is lithologically similar to the Ocala

Group limestones, and is identified primarily on the last occurrence of the

foraminifera Dictyoconus. 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 Brooker (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 quartz sands, clays, limestones and dolomites of the Mio-

cene age (5 to 24 million years B. P.) Hawthorn Group (Scott, in prepara-

tion). unconformably overlie the Suwannee Limestone remnants or Ocala Group

in extreme western Bradford County; east of Brooker, the Hawthorn Group

sediments lie directly upon the Ocala Group limestones. The Hawthorn Grouo

is predominantly a series of marine deposits, consisting of variable and

interbedded lithologies, and characterized by phosphatic and quartz sands,

granules and pebbles. Three formations of the Hawthorn Group are distin-







guishable in Iradford County; in ascending order 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 dolomite. The

Hawthorn Group sediments have northeastward dip and range in thickness from

about 100 feet in western Bradford County to at least 300 feet in the
northeastern corner of the county near the state prison. The thick, rela-

tively impermeable clays within the Hawthorn Group are the primary con-

fining beds for the underlying Floridan aquifer system. Pliocene to

Holocene age undifferentiated sands form a veneer over the Hawthorn Group

sediments in most of Bradford County, although the larger river valleys in

the southern and western parts of the county may cut down into the Hawthorn

section.


PLIOCENE TO HOLOCENE UNDIFFERENTIATED

Undifferentiated quartz sands and clays comprise the surficial sedi-

ments over nost of Bradford County. These unfossiliferous deposits are

virtually impossible to age-date, and include the unnamed reddish coarse

clastics, the relict Pleistocene (2.8 million to 0.1 million years 3. 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 precipitalon within

Union and nearby counties. The bulk of Bradford county's consumptive water

is withdrawn from ground-water aquifers. Three main aquifer systems are

present under Bradford County. In order of increasing depth; these are the








surficial 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

Bradford 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 Pleistocene marine terrace sands. On average, the

surficial aquifer system is about 40-feet thick over most of Bradford

County (Clark et al., 1964). The surficial aquifer system is unconfined

and its upper surface is the water table. In general, the water table ele-

vation fluctuates with precipitation rate and conforms to the topography of

the land surface. Within Bradford 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

sediments, and to a lesser extent by upward leakage from the deeper

aquifers. Water naturally discharges from the aquifer by evaporation,

transpiration, springflow,.and by downward seepage into the lower anuifers.

The surficial aquifer system yields water of suitable quality for consump-

tive 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 deeper water-bearing

sand and limestone layers within the Hawthorn Group. Low permeability

clays above the sand and limestone layers generally confine the intermediate






aquifer system under artesian conditions and separate It from the overlyino

surficial aquifer system. Water yield from this aquifer varies locally

with the quantity of sand and the porosity and permeability of the

limestone; in some areas, the Hawthorn Group limestones are very dense,

yielding little water. Recharge to the intermediate aquifer system con-

sists chiefly of downward seepage from the surficial aquifer system and

upward seepage from the Floridan aquifer system in areas where the poten-

tiometric 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 Group, and the Suwannee Limestone. It is by far the

most productive aquifer in Bradford County. The Floridan aquifer system is

confined by low permeability clays of the overlying Hawthorn Group, and is

under artesian conditions. West of Brooker, discontinuous beds of Suwannee

Limestone comprise the upper unit of the Floridan aquifer system. East of

Brooker, the Crystal River Formation of the Ocala Group is the uppermost

unit. County-wide, depth to the Floridan varies, on average, between 75

and 300 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 Bradford County occurs primarily

as downward leakage through the confining beds from the shallower aquifers

(Clark et al., 1964). Water leaves the Floridan aquifer system through

natural movement downgradient northwestwardd) 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 Bradford County. In general, the potential for commercially

feasible mineral production in this county is low. The following

discussion of the major mineral commodities is intended to provide an over-

view of the mining potential for each mineral.


SAND
A number of shallow private pits in Bradford 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 equi-

valent to the Miccosukee and Citronelle formations to the west, character-

istically contain fine to coarse grained quartz sand and gravelly sand.

Similar unnamed clayey sands are utilized as roadbase material in counties

to the south. Commercial production of these sands would require extensive

washing to remove the clay matrix; the economics of this procedure would

probably preclude commercial mining in Bradford County. White quartz sands

of the Trail Ridge fringe the eastern edge of Bradford County. These sands

are commercially mined in adjacent Clay County, and may offer industrial

potential.


PHOSPHATE
Phosphatic sediments of the Hawthorn Group underlie most of Bradford

I.'








County. The phosphate occurs as tan to black sand, granule, and pebble

sized phosphorite (P205). Scott (1983) analyzed the P205 content of the

Hawthorn Group sediments in four Bradford County cores. The composite P205

percentages were found to range from a low of 0.1 percent to a maximum of

13.5 percent, with a county-wide average of only 3.5 percent (Scott, 1983).

Since the minimum economic concentration of P205 is approximately 28 per-
cent (Cathcart and Patterson, 1983), the phosphate mining potential is low

In 3radford County.


HEAVY MINERALS

Economic deposits of heavy minerals, primarily ilmenite, rutile, leuco-

xene, staurolite, zircon, and monazite, are presently mined on the Trail

Ridge in nearby Clay County. Borehole sample data presented in Spencer

(1948) indicate that composite percentages of heavy minerals in the Trail

Ridge sands drop from approximately 4.0 percent in the currently-mined area

of Clay County, to between 1.0 and 1.5 percent on the western flank of the

ridge in Bradford County. These relatively low concentrations in Bradford

County preclude economical mining with existing technology.

LIMESTONE AND DOLOMITE

Bradford 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 undif-
ferentiated surficial sediments puts most limestone at too great a depth

for commercial mining.


PEAT

Peat is an organic mineral commodity formed from rapid accumulation of








decaying vegetation. This commodity is currently being mined by two com-

panies near Keystone Heights in nearby Clay County (Campbell, 1986). To

date, no commercial mining of peat occurs in Bradford County. Although

unproven, the areas of highest peat potential are the shallow, swampy

regions in central Bradford County and in the Santa Fe Swamp in the south-

eastern corner of the county (Davis, 1946; Bond et al., 1986).

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 Bradford County. Except for private borrow 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


Bond, P., Campbell, K. M., and Scott, T. M., 1986, An overview of peat in
Florida: Florida Geological Survey, Special Publication no. 27, 151 p.

Campbell, K. M., 1986, The industrial minerals of Florida: Florida Geolo-
gical Survey, Information Circular 102, 94 p.

Cathcart, J. B., and Patterson, S. H., 1983, Mineral resource potential of
the Farles Prairie and Buck Lake roadless areas, Marion County, Florida:
U. S. Geological Survey, Map Series MF-15918.

Clark, W. E., Musgrove, R. H., Menke, C. G. and Cagle, .1. W., 1964, Water
resources of Alachua, Bradford, Clay and Union Counties, Florida:
Florida Geological Survey, Report of Investigations no. 35, 170 p.

Cooke, C. W. and Mansfield, W. C., 1936, Suwannee Limestone of Florida
(abstract): Geological Society of America Proceedings, 1935, p.
71-72.

Davis, J. H., 1946, The peat deposits of Florida: Florida Geological
Survey, Bulletin no. 30, 247 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 1403-8, p. 25-27.

Purl, H. S., 1957, Stratigraphy and zonation of the Ocala Group: Florida
Geological Survey Bulletin 38, 248 p.
Scott, T. M., 1983, The Hawthorn Formation of northeastern Florida, Part II,
Characterization and beneficiation of the northeastern Florida phos-
phate-bearing Hawthorn Formation: Florida Bureau of Geology, Report
of Investigation no. 94, p. 41-90.

(in preparation), The lithostratigraphy of the Hawthorn
Group (Miocene) of Florida: Florida Geological Survey, Bulletin no.
59.

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.

Spencer, R. V., 1948, Titanium minerals in Trail Ridge, Florida: U. S.
Bureau of Mines, Report of Investigations 4208, 21 p.

White, W. A., 1970, Geomorphology of the Florida peninsula: Florida
Geological Survey, Bulletin no. 51, 164 p.