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OPEN FILE MAP SERIES NO. 84


The Floridan aquifer system (Southeastern Geological Society,
1986) is Florida's principle artesian freshwater aquifer system.
It extends from southern South Carolina southward through
southern Georgia and eastern Alabama into Florida. In
Florida, it functions as a potable water source for the region
extending from the west-central Florida panhandle southward to the
south-central peninsula. In this region the Floridan aquifer
system provides the sole source of water for municipalities,
rural homesteads, and agricultural operations (Bush and Johnson,
1988).


The Floridan aquifer system is largely contained within
carbonate rock and occurs in strata of differing age and varying
lithologic character. The formal geological units containing the
aquifer in the SRWMD include the Middle Eocene Avon Park Formation,
the Upper Eocene Ocala Limestone, the Lower Oligocene Suwannee
Limestone, and the Lower Miocene St. Marks Formation (Miller,
1986). Although not part of the aquifer system, basal carbonates
of the Miocene Penny Farms Formation of the Hawthorn Group may, in
some parts of the District, be in hydrologic continuity with the
underlying Floridan aquifer system. Deposition of the Eocene
through Early Miocene units occurred primarily under
temperate-to-warm shallow marine conditions. Most units are
comprised of skeletal marine limestones with varying micrite and
siliciclastic content. Post depositional dolomitization of the
original limestone has occurred in some areas, and rock color
commonly can vary with local ground water conditions. The
carbonates are generally overlain by variably thick Middle Miocene
through Recent siliciclastic sediments.


MAPPING METHODS
The map illustrates the structural top of the highly permeable
carbonate rock containing the upper Floridan aquifer system in the
Suwannee River Water Management District (SRWMD).
It was produced at the Florida Geological Survey (FGS) under a
cooperative contract with the SRWMD, and was funded by the FGS and
the Department of Environmental Protection's Ground Water Quality
Monitoring Program. Subsurface data were obtained from lithologic
descriptions of well cuttings and cores on file at the FGS. The
map reflects data available through 1990. Well data were manually
reviewed for errors in interpretation or other inconsistencies.

Formational tops in the FGS database represent the subjective
picks of a number of different workers over a period of many years.
Every effort was made to insure reasonably consistent lithologic
and paleontologic criteria for identifying the uppermost rock units
of the Floridan aquifer system. In the northern and north central
portions of the SRWMD, the overlying Hawthorn Group contains a basal
carbonate unit which is most likely in hydrologic continuity with the
Floridan aquifer system. When present, the top of this unit
was used in constructing the map. Elsewhere in the District, only
the structural tops of strata belonging to the Avon Park Formation,
Ocala Limestone, Suwannee Limestone, or St. Marks Formation were
used.

The original data were digitally extracted from the FGS Well
File Database using software specifically written for the purpose
by the senior author. This software permitted direct importation
of the well locations and top of rock elevations into the Autodesk
Incorporated program AutoCad, on IBM compatible computers. Well
data were digitally plotted on individual county basemaps provided
by the SRWMD, and later merged into a district map.
Variation in the true positions of wells may affect the map's configuration.
The well data points were hand contoured at 10 feet contour intervals,
and the isolines were digitized into the basemap files using AutoCad.

The base grid for the map is the Florida State Plane
Coordinate System (east), a Transverse Mercator projection. County
boundaries and township and range grids were imported into AutoCad
from the SRWMD Arclnfo system.


SHAPE AND EXTENT OF THE TOP OF THE FLORIDAN AQUIFER SYSTEM
The configuration of the structural top of the Floridan
aquifer system has likely been influenced by a number of factors.
These include regional structural features, the nature of the
original depositional surface, karst activity, and post-
depositional erosion by both submarine and subaerial agents. As
shown by the map, the surface of carbonate rock comprising the
Floridan aquifer system is a complex series of noses, troughs,
pinnacles, and depressions.
The primary structural feature influencing the carbonates of
the Floridan aquifer system in the SRWMD is the Ocala Platform
(Scott, 1988), a broad, positive, dome-like platform centered under
western Levy County. The crest of the platform trends northwest to
southeast through the west-central part of the District. Middle
Eocene carbonates of the Avon Park Formation occur near the land
surface in a small area along the crest of this feature. Late
Eocene Ocala Limestone comprises the uppermost rock unit over much
of the platform in the western portion of the SRWMD, with younger
Oligocene Suwannee Limestone carbonates and overlying younger units
lapping onto the flanks of the structure. These units generally


dip away from the crest of the platform into adjacent negative
features in three predominant directions: west-southwestward into
the Apalachicola Embayment, northeastward towards the Southeast
Georgia Embayment, and southward into the South Florida Basin.
A broad, generally flat, gulfward sloping karst plain is
developed on the structural surface of these onlapping
Miocene, Oligocene and Eocene carbonates in the coastal
portions of Jefferson, Taylor, Dixie, and Levy Counties.
Elevations of the top of rock in this region lie at approximately
mean sea level (MSL) near the coast, gently rising to about 50 feet
above mean sea level inland in Gilchrist and Lafayette Counties.
In the northern portions of Madison and Hamilton Counties the
Oligocene Suwannee Limestone, comprising the top of the Floridan
aquifer system in this area, rises gently northward toward Georgia.
Here the elevation of the top of the Suwannee Limestone varies, in
general, between sea level and about 80 feet above MSL.
Elevations on the highly irregular structural surface in the
central portion of the SRWMD typically range between 10 and 100
feet above MSL. The shallow crest of the Ocala Platform extends
eastward in the SRWMD to an approximate line extending from eastern
Hamilton County, through central Columbia County and western Union
and Bradford Counties, into northeasternmost Alachua County.
Northeastward from this line, the elevation of the structural top
of the Floridan aquifer system declines markedly from approximately
50 feet below MSL at the eastern edge of the Ocala Platform to
depths in excess of 370 feet below MSL in eastern Baker County, as
it dips into the trough of the Jacksonville Basin of the Southeast
Georgia Embayment. The Suwannee Limestone is absent in this
region, and Ocala Limestone comprises the top of rock along the
eastern flank of the platform.
ACKNOWLEDGEMENTS
The authors wish to thank Ron Ceryak and Nolan Col of the
Suwannee River Water Management District for their review and
helpful suggestions during the evolution of this map. Thanks are
also due Tom Scott and Walter Schmidt of the FGS for their
reviews of the draft map. Finally, special thanks are extended to
Nettie Enright for her countless patient hours of editing and AutoCad
work which culminated in a final useful product.
REFERENCES
Bush, P., and Johnson, R., 1988, Ground-water hydraulics, regional
flow, and ground-water development of the Floridan aquifer
system in Florida and in parts of Georgia, South Carolina, and
Alabama: U.S. Geological Survey Professional Paper 1403-C, 80
p.
Miller, J., 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,
91 p.
Scott, T.M., 1988, The lithostratigraphy of the Hawthorn Group
(Miocene) of Florida: Florida Geological Survey Bulletin 59,
146 p.
Southeastern Geological Society, 1986, Hydrogeological units of
Florida: Florida Geological Survey Special Publication 28, 8


Top of Rock


of the


Three dimensional
mesh surface showing the
shape of the top of rock surface
of the Floridan aquifer system.


3E


4E


5E


Floridan Aquifer System


in the


Suwannee River Water


Management District, Florida

BY

DAVID ALLISON, MARK GROSZOS,
AND FRANK RUPERT, P.G. # 149

1995


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