Distribution, origin and significance of thick microbial encrusters on corals: Pleistocene examples from the Bahamas and Turks and Caicos Islands

Up to ~15-cm-thick microbialites of both laminated and clotted morphologies encrust Acropora cervicornis coral branches in outcrop and core from Cockburn Town on the west coast of San Salvador, Bahamas, immediately below the Devil’s Point Discontinuity (DPD) that separates two stages of the Last Interglacial (Eemian/MIS 5e) reef development. The DPD is not pronounced in a core from The Gulf on the island’s south coast, but the core contains A. cervicornis (U/Th age ~111–119 kya), with slightly younger (~104–108 kya +/-1.5–2 kyr) microbialites of similar thickness. Up to ~5-cm-thick microbial encrusters of uncertain age were also present in storm-transported coastal boulders with Pocillopora palmata corals from The Gulf and Cockburn Town, and A. cervicornis from west Providenciales (Turks and Caicos Islands), but microbialites were not observed elsewhere, including excellent exposures of MIS 5e reefs on West Caicos Island and at Devil’s Point on the west coast of Great Inagua, Bahamas.

The documented distribution relative to MIS 5e highstand together with stable isotope data suggest that formation of thick microbialites was favored by lower sea level and increased restriction and nutrification in a nearshore back-reef setting. We continue to test these hypotheses through microbial biomarker analyses while recognizing that the uneven distribution of microbialites likely also reflects local availability of suitable coral substrates and conditions that eliminated competition with eukaryotes.

Petrographic, ImageJ and point-counting analyses of blue-epoxy thin sections, standard core plug analyses, and X-ray computed tomography (CT) scans reveal the significance of well-lithified, micritic microbialites in permeability reduction of porous, coral-bearing subtidal carbonate rocks. Ongoing comparative analyses also utilize mercury injection capillary pressure (MICP) method in porosity analysis as well as fluid flow simulations to further investigate pore connectivity and better characterize impacts of thick microbial encrusters in modifying reservoir properties of coral-rich deposits.