91社区

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 91社区 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 91社区n aquifer.

The 91社区n 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 91社区 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 91社区 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 91社区n 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 91社区n Aquifer.

In terms of recharge, these cover-collapse features appear to be the dominant hydrostratigraphic features in west-central 91社区.

*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, 91社区, 228pp.
  
  
• Stewart, M., Parker, J., 1992, Localization and Seasonal Variation of Recharge in a Covered Karst Aquifer System, 91社区, USA, International Contributions to Hydrogeology, vol. 13, Spinger-Verlag, pp. 433-460.

The GeoPark was born in the Fall of 2001, when Bob Bretnall (founding President of the Geology Alumni Society) and Chuck Connor in his first semester as Chair of the Department of Geology met with Steve Gift (Director of Facilities and Planning and head architect of USF) and others to propose that the plot of empty land south of Parking Lot 19 on the west side of campus be put aside as a resource site for on-campus geological teaching and research. The site, an exquisite example of the covered karst (see geological discussion by Mark Stewart that follows) that typifies and bedevils west-central 91社区, had long been the subject of field study by Mark Stewart, Sam Upchurch, and Sarah Kruse, and their students and classes.

Following that initial meeting, the site was officially designated the Geology Alumni Society GeoPark, with the intention that it continue to be a focal point for on-campus teaching and research. In the grand plan, the site joins with the Botanical Gardens to anchor the west end of a greenway that will extend all the way to the east end of campus.

The 4th annual Geology Alumni Society Equipment Expo inaugurated the GeoPark on Feb 9, 2002. More than a hundred geologists and environmental scientists from the community visited more than a dozen exhibits and demonstrations, several of which featured geophysical surveys of the sinkholes that characterize the site. The 5th annual Geology Alumni Society Equipment Expo, on Feb 14, 2003, added a symposium of speakers under an exhibition tent. In 2004, the Expo will be the North American Environmental Field Conference and Exposition put on by the Nielsen Environmental Field School, Inc., on Jan 14-16, at the nearby Embassy Suites with field demonstrations at the GeoPark.

A short walk from the SCA building, the GeoPark continues to increase in its role as a demonstration site for a variety of courses including hydrogeology, geophysics, and geomorphology. The utility of the site is increased by the results of two MS theses: Diane Bloomberg, 1987 ("Cone-penetrometer exploration of sinkholes: Stratigraphy and soil properties") and John Parker, 1992 ("Surficial aquifer hydrogeology in a covered-karst terrane"). In addition, each Expo adds more wells, more geophysics lines, and more experience that becomes part of the teaching resource.

A central feature of the GeoPark now is "The Rock," a six-ton sample of the Ocala Limestone from Lecanto Quarry in Citrus County. Donated by Crystal River Quarries, Inc., and arranged and delivered to the site by alumni Gabrielle Enos, Tom Scott, Dave DeWitt, and Bob Bretnall (courtesy of the 91社区 Geological Survey and SWFWMD), The Rock provides an on-campus look at the 91社区 aquifer which is penetrated by some of the wells on the site. 

In 2021, a southern portion of the GeoPark was developed into what is now Magnolia Fields - a recreation-use sports field. In 2022, the GeoPark became the third property under the ECORE System. The GeoPark remains an integral part of the greenway that is hosted by the other environmental properties managed under the ECORE System at USF.