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Gulf Coastal Plain
THE GULF COASTAL PLAIN
by Christopher Werner
The Gulf Coastal Plain includes southwestern Citrus, western Hernando, western Pasco, Pinellas and northwestern Hillsborough counties. Most of the caves developed within this region are located within Hernando and Pasco counties. The Gulf Coastal Plain lies in the coastal plain physiographic province (Wetterhall, 1964). It is bounded in the east by the Brooksville Ridge which trends north-westerly. Surface drainage within the Gulf Coastal Plain is poorly developed. Much of the drainage is underground. The Floridan aquifer is exposed at the surface in which Tampa, Suwannee, and/or Ocala Limestone is exposed. These limestone units make up the Floridan Aquifer in this region.
The west flank of the Brooksville Ridge drops steeply to the Wicomico terrace at an altitude of 35m. From here toward the west the broad sinkhole plain is terminated fairly close to the coast at an altitude of approximately 8m at the Pamlico escarpment. The Pamlico terrace extends westward and becomes submerged below present sea level. These escarpments are paleo-shoreline terraces during the past 110,000 years. The Wicomico terrace is associated with the Cody scarp in the Suwannee River Basin and the Woodville Karst Plain.
The Gulf Coastal Plain has interbedded sand and clay deposits of Plio-Pleistocene age overlying a very thick sequence of carbonates. The Tampa Limestone of Miocene age underlies the undifferentiated surficial sands. The Tampa Limestone is a white to gray, sandy, fossiliferous limestone. The Suwannee Limestone is a fossiliferous, yellow to white, fine-grained limestone of Oligocene age (Wetterhall, 1964). The Suwannee Limestone is very porous and contributes to much of the potable water extracted from the aquifer.
The Ocala Limestone underlies the Suwannee Limestone and is subdivided into three distinct formations: the Crystal River Formation, the Williston Formation and the Inglis Formation. The Crystal River and the Williston Formation are both soft, chalky, white to tan coquinoid limestones (Wetterhall, 1964). The Crystal River Formation harbors many of the deep cave systems within the Gulf Coastal Plain.
A continuous surficial aquifer system does not exist north of Pasco County (Trommer, 1993). The surficial aquifer is dissected and discontinuous as discrete lenses overlying the Floridan aquifer. The Floridan Aquifer System is usually exposed at or near the surface over much of the Gulf Coastal Plain. The regional groundwater flow within the Gulf Coastal Plain is west toward the coast. The Upper Floridan receives rapid recharge from the overlying thin unconsolidated sand and clays (Trommer, 1993). Secondary porosity is highly developed along fractures and result in the formation of underground conduit systems composed of large interconnected rooms. Some of the cave systems develop here are Diepolder and Eagle's Nest.
Most of the water that enters the groundwater system find its way to the coasts where it emerges as springflow (Wetterhall, 1964). Spring discharge usually increases as precipitation increases during the wet season. Hard layers within the limestones allow for solution conduits to form and concentrate flow at a single spring source as opposed to larger areas of seepage (Wetterhall, 1964). Many of the springs are very large first order magnitude springs, while others are located several hundred meters offshore. Many of the cave systems are influenced primarily by the tides, so caution must be exercised when diving.
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