MODIFICATION OF PLEISTOCENE CARBONATE REEF DEPOSITS BY BIOEROSION OF CORALS: INSIGHTS FROM NEW CORE DATA, SAN SALVADOR ISLAND, BAHAMAS

This research focuses on bioerosion of corals present within a new 54 mm-diameter core through a Pleistocene (Last Interglacial; MIS5) coral reef from the Cockburn Town Member of the Grotto Beach Formation at The Gulf site on the south coast of San Salvador Island, Bahamas. The main research objectives are to document evidence of bioerosion of the fossil corals and to determine the impact of bioerosion in producing secondary porosity within the coral structure and in generating carbonate sediment. Thin-section image analysis was used to quantify the amount of primary porosity within different corals present in the core and the alteration of porosity by the processes and products of bioerosion, specifically by lithophagid bivalves and clionid sponges, with implications for modifications of reservoir properties of these reefal rocks.

Image analysis of high-resolution scans of petrographic thin sections included use of Adobe Illustrator to document the distribution of various components within the coral structure: corallites (coral skeletal material), primary and secondary porosity, carbonate cement (acicular aragonite), carbonate sediment (micrite and skeletal, peloidal and ooid sand), and angular coral silt to very fine sand (up to 80 microns in diameter). Fiji/ImageJ software was then used to quantify the abundances (% surface area) of these individual components.

The area occupied by corallites and the amount of primary porosity within the coral structure depend on the type of coral: e.g., Acropora cervicornis can have ~15% primary porosity, while Colpophylia natans may have more than 40%. The amount of secondary porosity, mainly in the form of lithophagid bore holes, can be extensive and modify nearly 75% of the coral structure. Cement is a minor component (usually <1%), but carbonate sediment has infilled up to 60% of the pores within corals. Observed transitions from unaltered to severely fragmented corallites and the presence of coral silt/fine sand fragments in small, dense clusters and scattered in the sediment infilling and surrounding the corals, suggest that they are "chips" made by clionid sponges boring into the coral surfaces. Such bioerosion chips can occupy up to 5% of a sample, and even though volumetrically relatively small, they are an ubiquitous and important component of these reefal deposits.