DISTRIBUTION AND NATURE OF MICROBIALITES IN THE COCKBURN TOWN FOSSIL REEF (PLEISTOCENE, GROTTO BEACH FORMATION), SAN SALVADOR ISLAND, BAHAMAS: PRELIMINARY FIELD AND PETROGRAPHIC RESULTS

The Sangamonian (MIS 5e) Cockburn Town reef (Grotto Beach Fm.) on San Salvador Island, Bahamas, is an important source of data for the debate regarding rapid sea-level changes during the last interglacial highstand. Although Cockburn Town reef microbialites have been previously identified, details of their distribution remained unexplored. Preliminary re-examination of these deposits reveals a complex distribution of microbialites in relation to major and minor discontinuities within the reef deposit.

A widespread erosional hardground surface separates Reef I and II developmental phases. Exposures of coral floatstone and framestone well below this hardground surface exhibit pristine coral material as well as fragments with 1-5 mm-thick skeletal encrustations typical of normal post-mortem colonization in modern Bahamian reefs. Higher Reef I strata exhibit branching corals and fragments with discrete upward-thickening microbial coatings of laminated micrite-skeletal silt that cover early skeletal crusts. Microbial coatings increase in abundance and in thickness upward, from a few thin coatings that comprise 2-5% of the rock, to 30-80mm thick coatings on most branches. Immediately below the Reef I-II boundary hardground, both laminated and clotted microbialite forms microbial-skeletal boundstone that coalesces between coral branches to comprise 40-50% of the rock (and locally > 80%). In one area, thin drapes of skeletal-ooidal grainstone form shingled partitions within the dense microbial-skeletal boundstone and indicate episodes of lateral accretion.

In contrast to Reef I deposits, most of the corals and coral fragments in overlying (Reef II) deposits examined lack microbialite coatings, and display little or no skeletal encrustation, abrasion or corrosion. Still, Reef II strata are truncated by two hardgrounds (one mid unit and one capping the reef deposit that is part of the major regression leading into the last glacial). Assuming the reef coral to microbialite transition developed in response to environmental changes associated with sea-level drop, then the Cockburn Town reef microbialites and hardgrounds support previous interpretations that at least one (and perhaps more) short-term sea-level fluctuation occurred during the last interglacial highstand.