191-3 Were Contributions of Microbial Communities to Late Pleistocene Reef Deposits Underestimated? An Ongoing Case-study of Eemian Carbonates from the Bahama Archipelago

Session: Aspects of Carbonates (Posters)

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Abstract:

Microbialites encrusting corals within reef, back-reef and shoreward subtidal deposits in parts of the Eemian (MIS 5e) Cockburn Town Member of the Grotto Beach Formation of The Bahamas have been documented, beginning with outcrop and abandoned quarry exposures of the well-known Cockburn Town Fossil Reef (CTFR) on the western coast of San Salvador Island, and continuing with rock cores from a location slightly inland of CTFR (CT core) and another from the south margin of the island at The Gulf (TG core). Laminated micritic, upward-expanding encrustations are easily recognized as microbial in origin (stromatolites), especially in well-exposed outcrops. Clotted (thrombolitic) microbialites also are present, but more difficult to identify, especially in core samples without associated stromatolites and coral substrates. Careful examination of thickly encrusted Acropora cervicornis fragments revealed several characteristics that can be used as reliable criteria for identifying clotted reefal microbialites within the Cockburn Town Member: 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 revealed much more extensive microbialite development than previously thought. Clotted microbialites constitute up to ~12% and 17% of all subtidal MIS 5 deposits in the TG and CT cores, respectively. Applying these criteria to samples previously collected from the quarry exposures has yielded similar results, with a few select intervals exceeding this percentage range. Together, laminated and clotted microbialites significantly contributed to Cockburn Town reef, back-reef, and lagoonal deposits, and their abundance clearly was initially underestimated. In addition, the influence of microbialites in enhancing early lithification and porosity occlusion had a profound impact on erosional durability of these deposits, and was at least partially responsible for the development of sharp internal erosional discontinuity surfaces (hardgrounds).

     The distribution of two microbialite types in these deposits raises questions. What was the timing of their development within these deposits? What were the conditions and/or microenvironments necessary for their development? Although we have established a hypothesis concerning stromatolite development (timing and conditions) within CTFR deposits that fits the current evidence, the timing and cause/microenvironments of the surrounding and overlying clotted microbialite development remains to be determined. Hypotheses concerning clotted microbialite development within the CTFR interval are presented for consideration.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9394

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