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Florida Karst I

Hydrogeologic framework of the Floridan Aquifer

Aquifer foundation

During the late Permian (approximately 180 million years ago), the closure of the Iapetus Ocean sutured the Gondwana allocthenous terrain of the Sewanee Basin and Oceola Granite to the Appalachian province and other terrain blocks that now compose the Piedmont. This formed the base of the Floridan Platform. During the Jurassic (approximately 165 million years ago) failed rift systems developed along the weak point of the Permian suture zone resulting in negative topographic features bounded by normal faulting.

This failed rifting, known as the Georgia Channel System or Suwannee Strait, served to isolate the Florida/Bahamas Platform from the continent and therefore the source of siliciclastics sediment from the eroding Appalachian Mountains. This provided the setting for over 100 million years of carbonate sediment deposition.

Between the Jurassic and the Mid-Cretaceous (between 165 and 125 million years ago), the Florida/Bahamas Carbonate Platform was a "gigaplatform" and was likely the largest of its kind in the history of the earth. At its maximum extent, it stretched from the western Gulf of Mexico, through Florida and the Bahamas, parts of Cuba, and along the east coast to Nova Scotia. The Florida/Bahamas portion of the gigaplatform was a shallow shelf rimmed carbonate platform with restricted interior flow. Accelerated evaporation caused the deposition of anhydrites and salts. Cycles of anhydrite deposition form the lower confining units of the Floridan Aquifer System.

Floridan Aquifer System

During the Mid-Cretaceous (approximately 125 million years ago), sea level fell precipitously allowing significant subaerial exposure on the platform. Subsequent sea level rise inundated the platform again, and re-initiated carbonate sedimentation; however, the western margin of the platform became a "catch up" depositional environment. One possible cause is that rapid sea level rise removed the western margin from the euphotic zone. Another possible scenario is that changing ocean currents in the proto-Caribbean caused an Ocean Anoxic Event (OAE) killed the carbonate factory on the western platform margin. The end result was a transformation of the western margin of the platform into a distally-steepened carbonate ramp and the development of the Peninsular Arch, a pervasive depositional high in Florida stratigraphy.

Several sea level fluctuations during Tertiary were responsible for significant subaerial and karstification surfaces on Eocene and Oligocene carbonates. These units, the Avon Park Formation, Ocala Formation, and Suwannee Limestone, comprise the Floridan Aquifer System hydrostratigraphic unit. Generally, the aquifer system is only a few hundred feet thick in the north and over 3000 feet thick in south Florida. The aquifer is generally thinner on the Peninsular Arch and the Ocala Platform depositional highs. Several sea level fluctuations during the end of the Oligocene (approximately 30 million years ago) were responsible for significant subaerial erosion and karstification surfaces in central and northern Florida. In West Central Florida, the Sewanee Limestone was completely removed by erosion and Oligocene karst surfaces are present on the underlying Ocala Formation. At the crest of the Ocala Platform, the Ocala Formation was also removed, exposing the Avon Park Formation.

Confining units

Once sea level rose and stabilized in the early Miocene, siliciclastics were able to fill the Georgia Channel System and invade the carbonate platform to the south. Rejuvenated sedimentation from the Appalachian Mountains is hypothesized to be the source of these siliciclastics, which infilled the exposure surfaces of the Sewanee Limestone and Ocala Formation. Today this marks a major unconformity in the sedimentary record. Siliciclastic deposition has continued through the modern. Thicker deposition in the southern and central peninsula confined the Floridan Aquifer System. Thinner deposition combined with several periods of subaerial exposure produced extensive semi-confined and unconfined regions in West Central Florida. It is these areas that are of direct interest to karst research in Florida.

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