INSIGHTS INTO DEVELOPMENT OF REEF MICROBIALITES FROM DETAILED EXAMINATION OF OUTCROPS AND CORES IN EEMIAN (MIS 5E) PARTS OF THE COCKBURN TOWN MEMBER (GROTTO BEACH FM), SAN SALVADOR ISLAND, BAHAMAS

Microbialites from the Eemian/MIS 5e fossil reef exposed at Cockburn Town quarry (western San Salvador Island) are a key feature that distinguishes the two stages of reef development within the uppermost reef interval of the Cockburn Town Member. Detailed examination of a 34m rock core drilled through the reef intervals near the southern island/platform margin in 2019, as well as recent re-examination of a 15m rock core collected in 1997 a few meters landward of the quarry exposures, provide additional insights into the areal extent and distribution of microbialites, the nature of the “Devil’s Point Discontinuity” (DPD) separating reef Stages I (lower) and II (upper), and to response of nearshore shallow subtidal carbonate platform settings to climate-driven sea-level change.

Microbialites are only recognized within Stage I boundstones. Stage I deposits also reveal that: 1) reef boundstones formed primarily around branches of Acropora cervicornis; 2) corals were directly coated with a diverse array of skeletal encrusters (coralline red algae, serpulids, and 3+ genera of foraminifera - including Homotrema), and then overlapped by micritic microbialite coatings with some foraminifera (notably Carpenteria); 3) microbialite coatings within reef boundstones become progressively thicker and more extensive upward toward the DPD; 4) both outcrop and cores largely comprise crudely stratified boundstone layers separated by thin carbonate sand (grainstone) drapes; 5) these couplets laterally flank rare unstratified boundstone masses but display limited vertical aggradation; 6) grainstone drapes thicken toward the lower flanks of each couplet in outcrop; and 7) grainstone drapes are generally thicker in the core sections, likely reflecting more landward proximity to shoreface sands.

Presence of identical boundstone-grainstone couplets in both western and southern platform cores suggest that these facies are not just a local response to environmental change. Patterns of boundstone development favor a short-term sea-level drop for the origin of Stage I microbialites and the DPD. Under such conditions, reef boundstone development was likely facilitated by episodic storm coral fragmentation and transport, nearshore nutrification during post-storm run-off, and subsequent beach shoreface progradation.