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Summary of the Hydrology of the Floridan Aquifer System In Florida and In Parts of Georgia, South Carolina, and Alabama

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- Geologic Setting
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Hydraulic Properties of the Aquifer System
Regional Flow System
Groundwater Development
Groundwater Chemistry
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By Richard H. Johnson and Peter W. Bush
Professional Paper 1403-A

Hydrogeology

GEOLOGIC SETTING

The Coastal Plain physiographic province of the southeastern United States is underlain by a thick sequence of unconsolidated to semi consolidated sedimentary rocks that range in age from Jurassic to Holocene. These rocks thicken seaward in the study area from a featheredge where they crop out against older metamorphic and igneous rocks of the Piedmont and Applachian provinces to a maximum measured thickness of more than 21,100 ft in Mobile County in southern Alabama and a projected thickness of more than 25,000 ft in south Florida. Coastal Plain rocks generally dip gently toward the Atlantic Ocean or the Gulf of Mexico, except where they are warped or faulted on a local to subregional scale. Coastal Plain sediments were laid down on an eroded surface developed upon igneous intrusive rocks, low-grade metamorphic rocks, mildly metamorphosed Paleozoic sedimentary rocks, and graben-fill sedimentary deposits of Triassic to Early Jurassic age (Barnett, 1975; Neathery and Thomas, 1975; Chowns and Williams, 1983).

The poorly consolidated Coastal Plain rocks are easily eroded. Where they consist of carbonate rocks, the strata are partially dissolved by percolating water, resulting in the development of karst topography where such rocks are at or near the surface. Accordingly, the topography developed in much of the study area is characterized by extensive slightly dissected plains, low, rolling hills, and widely spaced drainage. A series of sandy marine terraces of Pleistocene age has been developed in much of the area.

Coastal Plain rocks in the project area can be separated into two general facies: (1) predominantly clastic rocks, containing minor amounts of limestone, that extend southward and eastward toward the Atlantic Ocean and the Gulf of Mexico from the Fall Line that marks the inland limit of the Coastal Plain; (2) a thick continuous sequence of shallow-water platform carbonate rocks that underlies southeast Georgia and all of the Florida peninsula (Miller, 1986). In north-central Florida and in southeast Georgia, where these clastic and carbonate rocks generally interfinger with each other, facies changes are both rapid and complex. In general, the carbonate facies of successively younger units extends progressively farther and farther updip, encroaching to the northwest upon the clastic rocks in an onlap relation--at least until the end of Oligocene time. Miocene and younger rocks form a predominantly clastic facies that, except where removed by erosion, covers the older carbonate rocks everywhere. The various stratigraphic units within both the clastic and carbonate-rock areas are separated by unconformities that represent breaks in sedimentation.

Cretaceous rocks generally crop out in a band adjacent to the crystalline rocks and folded strata of the Piedmont and Appalachian physiographic provinces. Rocks of Tertiary age, whose carbonate facies form most of the Floridan aquifer system, crop out in a discontinuous band seaward of the Cretaceous rocks and are also exposed in an area in west peninsular Florida. Still farther seaward is an exposed band of predominantly clastic rocks of Miocene age, which forms an upper confining unit in the Floridan aquifer system. Miocene rocks generally separate the Floridan from Pliocene and Quaternary strata that are mostly sands and form surficial (unconfined) aquifers.

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