DEVELOPMENT OF SINKHOLES IN A THICKLY COVERED KARST TERRANE

A swarm of aquifer drawdown-induced sinkholes developed in eastern Hillsborough County, Florida (west-central Florida), during a major freeze in 2010. The sinkholes resulted in millions of dollars in losses and revised our thinking about how sinkholes form in a terrain normally considered to be at low sinkhole risk owing to thick, clay-rich cover. The Miocene Hawthorn Group includes up to 140 m of interfingering expansive clay, sand and sandy clay, and carbonate strata. The lower Hawthorn Group Arcadia Formation is primarily carbonate and is up to 100 m thick. The upper Hawthorn Group Peace River Formation contains more clay and sand with minor amounts of carbonate and is up to 40 meters thick. A rapid drawdown of up to 20 m in potentiometric surface of the underlying Floridan aquifer resulted in mobilization of water-saturated clays within the Hawthorn Group. Subsidence resulting from dewatering and loss of support/buoyancy caused development of new sinkholes and reactivation of clay-filled sinkholes that had developed as early as the Miocene. These sinkhole failures have caused a reassessment of sinkhole development mechanisms in the thickly covered karst of west-central Florida.