COMPARATIVE POROSITY ANALYSIS OF CARBONATE CORAL REEF DEPOSITS WITH THICK MICROBIAL ENCRUSTERS: NEW INSIGHTS FROM X-RAY COMPUTED TOMOGRAPHY (CT) IMAGING OF CORE SEGMENTS FROM PLEISTOCENE (EEMIAN, MIS 5E) COCKBURN TOWN MEMBER, GROTTO BEACH FORMATION ON SAN SALVADOR ISLAND, BAHAMAS

Two segments of 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 were scanned at the High-Resolution X-ray CT Facility, University of Texas, Austin and analyzed using Dragonfly software. Comparison between Acropora cervicornis coral encrusted with 10 cm-thick laminated microbialite from -9.3 m and the same type of coral surrounded by skeletal-peloidal-ooid sand from -13 m provides insights into the role of encrusters in reducing porosity of coral-bearing subtidal carbonate deposits.

This study complements previous analyses that aimed at evaluating reservoir properties of cored subtidal deposits, overlain by 7+ m of eolian and beach grainstones, and capped by a Holocene-Pleistocene boundary terra rossa paleosol. 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 determination. 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.

CT-scan analysis of the core segment with thick microbialite revealed porosity of 3.8% as compared with 14.5% for the segment with non-encrusted coral in sand. Since porosity of A. cervicornis was similar between these two samples (10.7% and 11.8%, respectively), the difference in overall core segment porosity stems from contrasting properties of the surrounding deposits: well-lithified micritic microbialites vs friable carbonate grainstone. These results support and expand the range of standard core-plug measurements, which indicated that the heterogeneous subtidal deposits have variable and generally lower porosity and permeability (15.5-48.5% and ~2-1600 mD, respectively) in comparison with overlying beach and eolian grainstones (40-50% and ~4500-19000 mD). CT scans also augment petrographic observations of the distribution of various pore types, ranging from small intergranular voids in sandy deposits and primary porosity within coral skeletons to larger, cm-scale secondary porosity created by bioerosion.