OPEN FILE MAP
AN ISOPACH MAP OF THE HAWTHORN GROUP
SUWANNEE RIVER WATER MANAGEMENT
SMark Groszos and Frank Rupert, P.G. No. 149
Florida Geological SurveyALL
Walter Schmidt, State Geologist and Chief
ID Division of Resource Management Department of Natural Resources Tallahassee, Florida
The Hawthorn Group (Scott, 1988) is a lithologically diverse, predominantly
subsurface unit underlying much of the eastern panhandle and peninsular Florida.
It extends from the Apalachicola River in the central panhandle, eastward to the
Atlantic Ocean, northward into Georgia, and southward the length of the
peninsula. The Hawthorn Group contains economic deposits of phosphate. It also
contains an intermediate aquifer system and is important as an intermediate confining unit to the underlying artesian Floridan aquifer system, the state's
primary freshwater aquifer (Southeastern Geological Society Ad Hoc Committee,
Within the Suwannee River Water Management District (SRWMD), the Hawthorn
Group is composed predominantly of Miocene age interbedded sands, clayey sands,
clays, and carbonates. It generally contains variable amounts of phosphate in
the form of sand, granules, or pebbles, which is mined in eastern Hamilton
County. In many areas the Hawthorn Group is fossiliferous, commonly containing
marine invertebrate and vertebrate fossils and sometimes terrestrial vertebrate
fossils. Locally, the Hawthorn Group overlies the Eocene, Oligocene, and Miocene
limestones comprising the Floridan aquifer system. It is overlain by variable
thicknesses of Pliocene to Recent age undifferentiated sands and clayey sands.
The Hawthorn Group is a significant component in the hydrogeologic framework
of the SRWMD. It contains the intermediate aquifer system, consisting of waterbearing limestone, dolomite, and sand strata. Wells in rural areas of the
eastern SRWMD commonly draw from this intermediate aquifer system.
Low permeability clays near the base of the Hawthorn Group function as an
aquiclude to the underlying Floridan aquifer system. In some areas, carbonates at the base of the Hawthorn Group are in hydrologic continuity with the Floridan
aquifer system. Clays at the top of the Hawthorn likewise serve as lowpermeability barrier to downward movement of water from the overlying surficial
The present map was produced at the Florida Geological Survey (FGS), under
a contract with SRWMD funded by the Florida Department of Environmental
Regulation's Ground Water Quality Monitoring Program. Subsurface data were obtained from lithologic descriptions of well cuttings and cores on file at the
FGS, and driller's logs provided by the SRWMD. The map reflects data available
in 1988. Each well or driller's log was reviewed for errors or other
inconsistencies. A number of wells (5 to 10 percent, were discarded due to
anomalous or questionable data, missing intervals, or because they did not
penetrate the entire Hawthorn Group section.
Formational tops and bottoms 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 criteria for identifying the sediments
composing the Hawthorn Group. In some cases, cores were spot checked to confirm
previous worker's picks. One significant lithologic criteria in recognizing the
Hawthorn Group is the presence of phosphate. Phosphate is commonly reworked into
younger sediments, possibly biasing the true position of the top of the unit, and
hence affecting the isopach configuration. In wells where this was recognized
as a problem, the top of the Hawthorn Group was delineated based primarily on
other lithologic criteria or lithology changes.
The original data were digitally-extracted from the FGS Wellfile Database
using software specifically written for the purpose by Dr. David Allison. This
software permitted direct importation of the well locations and Hawthorn
thickness data into the Autodesk, Incorporated program AutoCad, on IBM compatible
computers. Well data were initially plotted on individual county basemaps,
provided by the SRWMD, and later merged into a District-wide map. While most of
the well locations are not field checked, they are based on the best available
information. Variation in the true positions of wells may affect the map's
configuration. The well data points were hand contoured at a 20 feet contour
interval, and the isolines digitized into the basemap files using Autocad.
Hatchured lines are used to indicate thinner areas within the main body of
the Hawthorn Group unit. They do not provide specific information concerning the
geometry of the upper or lower surfaces of the Hawthorn Group.
The base grid for the map is the Florida State Plane Coordinate System
C east), a Transverse Mercator projection. County boundaries and township and range grids corners were imported into Autocad from the SRWMD Arcinfo files.
This map represents the thickness of only the Hawthorn Group sediments. It
should not be confused with or interpreted as a Floridan aquifer system
overburden isopach. In most areas, an interval of undifferentiated sand and
clays is present above the Hawthorn Group.
This map is currently produced on demand from the FGS Autocad files. It is
anticipated that this format will allow the most flexibility in continually
updating the isopach map as new data becomes available, while maintaining
publishing economy in this era of scaled-down budgets.
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Shape and Extent
The present configuration of the Hawthorn Group unit was influenced by a series of structural features and erosional processes. Deposition of the Hawthorn Group sediments during the Miocene occurred under shallow marine conditions, on an unconformable, karstic limestone surface (Scott, 1988). Fluctuating sea levels during the Miocene formed a variety of paleoenvironments, ranging from occasional exposures of dry land to shallow lagoon and prodelta. In the southwestern portion of what is now the SRWMD, a structural high named the Ocala Platform trends northwest-southeast under present day Levy County. Hawthorn sediments were probably deposited against the flanks of this feature, extending over it by the Middle Miocene. Erosion by subsequent sea level highstands removed the Hawthorn from the crest and areas to the southwest of the feature. Northeast of the SRWMD, a structural low named the Jacksonville Basin (Goodell and Yon, 1960) accumulated nearly 500 feet of Hawthorn Group sediments. The Hawthorn Group sediments dip and thicken into this basin.
Infilling of paleokarst depressions and sinks in the underlying carbonates by Hawthorn sediments, or deposition over karst highs undoubtedly influenced thickness locally. Post Hawthorn karst activity, coupled with both subaerial and submarine erosion could have further modified the unit in many areas. High-standing Plio-Pleistocene seas eroded and reworked Hawthorn sediments in much of the southwestern portion of the SRWMD. In general, the Hawthorn Group is absent in counties in the coastal counties of the SRWMD today. Streams have cut down into the Hawthorn Group, in some case producing excellent exposures of the unit, such as those along the stretch of the Suwannee River between Hamilton and Columbia Counties.
Within the SRWMD, the Hawthorn Group sediments are highly variable in thickness, generally ranging from 0 feet along a line approximately parallel to and about 40 miles inland of the present Gulf coastline, to a maximum of just over 400 feet in northeastern Bradford County. In Jefferson and Madison Counties, the Hawthorn attains a maximum thickness of about 100. Maximum thicknesses generally increase to the northeast. In eastern Hamilton County, central Columbia County, and Union County, it reaches about 140 feet in thickness. At Gainesville, in Alachua County, the unit reaches 160 feet thick. The Hawthorn rapidly thickens to the northeast, as it dips into the trough of the Jacksonville Basin. A well in northeast Bradford County penetrated 410 feet of Hawthorn Group sediments, the thickest recorded section in the SRWMD.
Well Number, Unit
and Location Point
Contour Line (Relative to NGVD) (Dashed contour lines indicate where contour data were extrapolated.)
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