GEOLOGY OF THE UPPER FLORIDAN AQUIFER: A LITHOLOGIC, BIOSTRATIGRAPHIC, AND PETROGRAPHIC ANALYSIS OF THREE CORES IN THE GREATER SAVANNAH REGION

To better understand the geology of the Upper Floridan aquifer (UFA) in the greater Savannah area, the USGS drilled three continuous cores; one updip and two along the coast: Pineora (updip, Effingham Co., GA), Cockspur Island (coastal, Chatham Co., GA), and Palm Dunes (coastal, Beaufort Co., SC). We studied the lithology and biota, both macroscopically and microscopically, in order to characterize the variability of the UFA in the study area.

In the Pineora core, the base of the UFA is at 380 ft, within the limestones of the Ocala Group, and is delineated by a change in porosity. The top of the UFA is at the contact between the Suwannee Limestone at 270 ft, and Miocene age fine-grained siliciclastic sediments of the Parachucla Formation. Preliminary calcareous nannofossil biostratigraphy indicates the UFA to be latest Eocene (NP21) to late Oligocene (NP23-25?) in age. Petrographic analysis shows that the UFA is predominantly a foraminiferal-skeletal to foraminiferal-bryozoan dolomitized grainstone, with both dolomitic and calcitic cement; it is dominated by interparticle porosity.

In the Cockspur Island core, the base of the UFA is at 350 ft, marked by a change from calcitic/dolomitic limestone below to calcite-dominated Ocala Group limestone above. The top of the UFA is at 111.5 ft, between semi-indurated siliciclastic mudstone of the Parachucla Formation below and siliciclastic sediments of the Marks Head Formation above. Preliminary biostratigraphy indicates the UFA is late Eocene (NP18?) to late Oligocene (NP23-25?) in age. This core is dominated by intraparticle porosity, with average percentage of void space at 28%, and is classified as a bryozoan-rich grainstone.

The base of the UFA in the Palm Dunes core is at the base of the Ocala Group (280.5 ft). The top of the aquifer is within the Hawthorn Group at 80 ft, at the contact between a moldic quartz sand below and unconsolidated siliciclastics above. Nannofossils record an age of middle Eocene (NP16) to early Oligocene (NP23) for the UFA, and an unconformity at 147 ft indicates a hiatus of approximately 10 my. Petrographic analysis shows the UFA to be a calcite-cemented sandstone, transitioning to a bryozoan-rich dolomitized grainstone dominated by interparticle porosity. Based on analyses, the UFA crosses lithologic and age boundaries in the study area.