PRELIMINARY SEDIMENTOLOGICAL AND STRATIGRAPHIC ANALYSIS OF CARBONATE DEPOSITS FROM A NEW CORE IN THE PLEISTOCENE GROTTO BEACH FORMATION AT THE GULF SITE, SOUTHERN COAST OF SAN SALVADOR ISLAND, BAHAMAS

This presentation summarizes progress to date on analysis of carbonate deposits from a 54 mm-diameter, 34 m-deep core drilled into the Grotto Beach Formation at The Gulf site on the south coast of San Salvador Island, Bahamas. Below the upper 7+ meters of eolian dune and beach grainstones, the entire core is composed of coral-bearing subtidal deposits. The core does not represent the entire subtidal portion of the Cockburn Town Member because underlying Grotto Beach eolianites were not penetrated, but it contains four reef-coral concentrations, separated by skeletal-ooid-peloidal grainstones (peri-reefal sands) or scalloped hardgrounds, which provide insights into reef facies and reef developmental history during the Pleistocene Last Interglacial. Radiometric dating is underway to better constrain core stratigraphy.

To evaluate reservoir properties of these deposits, core-plug samples were analyzed for porosity, permeability and grain density, and made into petrographic thin sections. Fiji/ImageJ software was used to analyze porosity in high-resolution scans of thin sections with blue epoxy, and selected slides were point-counted for porosity determinations. Radiographs of selected core segments were recorded on an ITRAX core scanner, and gamma-density data were collected on a GeoTek core logger to document relative variations in bulk density. A small subset of samples was also CT-scanned.

Petrographic analysis to date also focused on quantifying the amount of primary porosity within different corals (e.g., Acropora cervicornis, Orbicella annularis, Colpophyllia natans) and on documenting porosity modifications by the processes and products of bioerosion by lithophagid bivalves and clionid sponges. The distribution, type and abundance of encrusters such as red algae, foraminifera, serpulids, and microbial coatings on corals were also examined. Results were compared with data from outcrops of the Eemian (MIS 5e) Cockburn Town Fossil Reef on San Salvador’s west coast and from Devil’s Point and Matthew Town Marina on Great Inagua Island, Bahamas, to evaluate environmental conditions during the last interglacial that resulted in thick coral encrustation and to elucidate the role of bioeroders and encrusters in modifying reservoir properties of reefal rocks.