In north-central Florida, sinkholes at the surface are generally related to the dissolution of two host limestone units, the Ocala and Suwannee Limestones. The Eocene-age Ocala Limestone was deposited between 40 and 28 million years ago (ma). Over time, sections of the rock have been recrystallized into dolomite, CaMg(CO₃)₂. Ocala Limestone is generally tan to cream, highly fossiliferous lime mud preserved as packstone to wackestone (Scott, 1992). Above the Ocala Limestone is the Oligocene age Suwannee Limestone (28 ma-24 ma) only present as scattered deposits in low topographic areas and typically absent on the topographically high areas. Figure 5 shows the surface distribution of these units.

Overlying the Ocala and Suwannee Limestones is the Hawthorn Group (Miocene 24-5.3 ma). This group is composed of massive impermeable clay and dolomite units. Interbedded with these impermeable units are sands, sandy clays and fractured carbonate units (Miller, 1986). Except where thin or breached, the Hawthorn Group is the main semiconfining unit to the Floridan aquifer in north-central Florida. The thickness, stratigraphic position and confining nature of the Hawthorn Group determines the form and function of sinkholes. The Hawthorn Group is absent from the structural highs such as the Ocala Uplift to the east of the study area and the Sanford High (Fig. 6). It maintains a thickness of 9 to 18 m (30-60 ft) across the St. Johns Platform and thickens to over 46 m (150 ft) over the Jacksonville basin (Mallinson and others, 1994). In most of the sites profiled, the potentiometric surface of the Floridan aquifer lies below lake surface. This condition creates a downward gradient which allows water to permeate through the Hawthorn Group sediment from the lake into the limestone units below. Additionally, breaches in the Hawthorn Group allow enhanced surface groundwater interaction. An example of catastrophic breaching occurred in the late 1800s when a sinkhole collapsed and drained the former Alachua Lake, thereby creating Paynes Prairie (Pirkle and Brooks, 1959b).