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Geological Setting 

The GeoPark lies within the Atlantic Coastal Plain (ACP). The ACP is characterized by very low relief and typically has a shallow water table.

The ACP in Florida is underlain by a blanket of Miocene and post-Miocene siliciclastic deposits that overlies a thick sequence of Tertiary carbonates. This carbonate platform includes the Floridan aquifer.

The Floridan Aquifer is composed of Eocene to Miocene limestone and dolostone. The maximum freshwater-saturated thickness is about 700 m, and averages 300-400 m. The Eocene and Oligocene rocks are relatively pure carbonates, but Miocene units contain considerable siliciclastic material. The siliciclastic units of the Miocene Hawthorn Formation and the weathering residuum of the "dirty" Tampa limestone member form a semiconfining unit over the early Tertiary carbonates. Quaternary aeolian activity has reworked some of the shallow marine sediments, creating an uppermost layer of very fine, very well sorted sand.

Karst

Although west-central Florida does exhibit some classical fluvio-karst features, the siliciclastic cover, especially the dune sands, has subdued the surface expression of all but the largest features. Many karst features are relict, apparently formed when the water table was considerably lower than its present position. Many of the first-order springs of west Florida discharge through conduits formed by downward moving water when the water table was 100 m or more below land surface.

The insoluble residue of the Tampa Limestone and the fine-grained units of the Hawthorn Formation form a stiff clay cap over the underlying limestone. This cap is competent enough to bridge small cavities, but collapse of the clay into larger, pre-existing cavities creates an upward raveling through the unconsolidated sediments, forming a cover-collapse sinkhole. The result can be a vertical column of surficial sand extending from the surface into the limestone, through the semiconfining layer. The cover collapse can be slow, episodic or catastrophic. At the USF GeoPark, there is one mappable subsidence feature for every 625 m2 (25 x 25 m) of land surface.

The USF area is an active cover-collapse sinkhole region. The USF campus has experienced more than 30 cover-collapse events in its years of operation. This results in a frequency of about one cover-collapse sinkhole per square mile per year. The small cover-collapse feature at well 4A in the GeoPark has collapsed three times since about 1980.

Hydrogeology

Because of the presence of the cover-collapse features and the downward hydraulic gradient, lateral flow of shallow ground water is limited. At the GeoPark, and probably in much of the Tampa Bay region, shallow ground water flows only a few tens of meters or less until it either discharges into a surface water body or finds a cover-collapse sinkhole, through which it then recharges the Floridan Aquifer.

By far, the greatest volume of recharge at the GeoPark moves through the karst conduits and not through the semiconfining layer. The study site is 12,000 m2. The sand columns contribute 16 m3/day of recharge; in contrast, about an estimated 1 m3/day is estimated to leak through the semiconfining layer. Thus the sand columns constitute about 1% of the surface area of the site, but contribute 95% of the recharge to the underlying Floridan Aquifer.

In terms of recharge, these cover-collapse features appear to be the dominant hydrostratigraphic features in west-central Florida.

*The history and karst characteristics overview of the GeoPark is adapted from a historical overview prepared by Len Vacher, Ph.D.

REFERENCES

• Parker, J. W., 1992, Surficial Aquifer Hydrogeology in a Covered-Karst Terrane, Masters Thesis, OB体育官网, 228pp.
  
  
• Stewart, M., Parker, J., 1992, Localization and Seasonal Variation of Recharge in a Covered Karst Aquifer System, Florida, USA, International Contributions to Hydrogeology, vol. 13, Spinger-Verlag, pp. 433-460.