Clotted microbialites in Late Pleistocene coral-rich, shallow-marine carbonates from the Bahamas: identification, distribution, and abundance

In field examination of cm-thick encrusters on corals from Late Pleistocene shallow-marine carbonate deposits in the Bahamas, laminated fabrics can be easily recognized as microbial in origin (i.e., stromatolites), especially in well exposed outcrops. However, in 50-mm-diameter core segments, clotted or thrombolitic microbialites are more difficult to identify, especially in the absence of associated stromatolites and coral substrates.

Up to ~15-cm-thick microbial encrusters are present on Eemian (MIS 5e or Last Interglacial) Acropora cervicornis coral branches from the Cockburn Town Member of the Grotto Beach Formation exposed at Cockburn Town on the west coast of San Salvador Island, and in cores from the same locality and The Gulf on the island’s south coast. Dead coral fragments were first coated with mm-scale skeletal encrusters (coralline red algae, serpulids, foraminifera) and then overlain by microbialites consisting of: 1) stromatolites on the upper sides of corals; and 2) clotted microbialites on lower surfaces and engulfing the stromatolites.

Examination of thickly encrusted corals revealed several characteristics that can be used as reliable criteria for identifying clotted microbial fabric: 1) presence of micritic patches that incorporate carbonate sand; 2) abundant fenestrae; 3) common encrusting foraminifera; and 4) crudely laminated fabric. Re-examination of the cores using these criteria indicated more extensive microbialite development than previously thought, with clotted microbialites constituting up to ~12% and 17% of all subtidal MIS 5 deposits in The Gulf and Cockburn Town cores, respectively. Applying these criteria to samples previously collected from the Cockburn Town exposures has yielded similar results, with a few select intervals exceeding this percentage range. Having a better understanding of the microbialite distribution and abundance within these deposits allows us to form more concrete hypotheses about paleoenvironmental conditions that result in the formation of thickly encrusted corals.