PETROGRAPHIC ANALYSIS OF ENCRUSTERS ON CORALS FROM PLEISTOCENE REEFS ON SAN SALVADOR ISLAND, BAHAMAS

Coral reefs of the Cockburn Town Member, Grotto Beach Formation (Eemian; MIS 5e), San Salvador Island, Bahamas, provide an important source of information about paleoenvironmental conditions and sea-level fluctuations during the last interglacial highstand. The reefs contain branching corals encrusted by red algae, foraminifera, and microbialites of laminated (stromatolitic) and clotted fabric. In situ exposures of the Cockburn Town Fossil Reef (CTFR) and storm-deposited boulders in The Gulf (TG) area on the southern coast of the island were analyzed petrographically to determine the distribution and composition of encrusters.

The top of CTFR is ~3 m above modern sea-level, and the reef is separated by an erosional discontinuity into Reef I and II. Reef I corals are heavily encrusted and surrounded by medium- to very coarse-grained ooid-peloid-skeletal (mollusks, green alga Halimeda, benthic foraminifera) grainstone, while Reef II corals lack thick encrustation. Lying directly on Reef I corals are thin red algal laminae, associated with encrusting foraminifera and serpulid tubes, followed by up to 4-5 cm thick stromatolites on the upper sides of corals and up to 9-10 cm of clotted microbialites engulfing the coral branches. Stromatolites incorporate fine-grained ooids and peloids and some mollusk fragments within the micritic laminae. Clotted microbialites have patches of micrite with irregular fenestrae, medium- to coarse-grained ooids, peloids, and skeletal fragments, and may be crudely laminated. Encrusting foraminifera (Planorbulina, Carpenteria, Gypsina, Homotrema) are also present within microbialites, and macroborings are evident in corals and microbialites. In contrast, the TG area has only the top of the reef exposed at low tide, and the examined boulders suggest that Pocillopora palmata may be more common than Acropora cervicornis in comparison with CTFR. TG corals have slightly thicker algal laminations (up to 3.5 mm thick), similar distribution of stromatolites, but only small areas of clotted microbialites. The presence of encrusting foraminifera and macroborings indicates that the microbial encrusters enhanced the rapid and firm lithification of the reefs, which impacted their taphonomy and reservoir properties.