PRELIMINARY CENOZOIC LITHOSTRATIGRAPHY OF THE COCKSPUR ISLAND CORE, FORT PULASKI NATIONAL MONUMENT, CHATHAM COUNTY, GEORGIA

The Cockspur Island core was drilled by the U.S. Geological Survey in February 2010 at Fort Pulaski National Monument (Chatham County, Georgia) in the Atlantic Coastal Plain province near the mouth of the Savannah River. Coring started at a ground elevation of 8.0 ft above sea level, and reached a total depth of 1,017.3 ft. The top 7.0 ft were not cored. The core was described in order to determine its lithologic units. Samples for calcareous nannofossils, dinoflagellates, and calcium carbonate content were taken throughout the core. One sample of an oyster shell was taken at 31.0 ft depth for radiocarbon dating. These data in conjunction with a gamma ray log were used to correlate the lithologic units of the core with the stratigraphic formations of Huddlestun (1988, 1993, and 1986).

The core shows two major lithologies: Eocene carbonate strata (834.7 ft thick) from 1,017.3 to 182.6 ft depth, and Oligocene to Holocene siliciclastic strata (175.6 ft thick) from 182.6 to 7.0 ft depth. This contact at 182.6 ft is an unconformity that is interpreted as having been caused by a sea level fall during a period of major global climate change.

Detailed description of the core has revealed that it may be preliminarily subdivided into distinct lithologically descriptive units at the following depths:

7.0 to 52.7 ft: Siliciclastic sand and mud without phosphate sand [Holocene Satilla Formation].

55.0 to 89.3 ft: Siliciclastic sand and mud with 1-10% phosphate sand [Miocene Tybee Phosphorite Member of the Coosawhatchie Formation].

89.3 to 182.6 ft: Siliciclastic sand and mud without phosphate sand [Oligocene Lazaretto Creek Formation, Parachucla Formation, and (or) Marks Head Formation?].

182.6 to 479.8 ft: Carbonate sand consisting of beds without identifiable macrofossils, and beds with abundant bryozoans, gastropods, and (or) bivalve fragments. No quartz, glauconite, or phosphate [Upper Eocene Suwannee Limestone equivalent and (or) Ocala Group?].

480.0 to 1,017.3 ft: Carbonate sand, with varying amounts of glauconite, phosphate, quartz sand, chert nodules, and rare to abundant burrows (Middle to Lower Eocene Lisbon Formation-equivalent and (or) Tallahata Formation?].