DEPOSITIONAL HISTORY OF EEMIAN REEF DEPOSITS (COCKBURN TOWN MEMBER, GROTTO BEACH FORMATION) ON SAN SALVADOR ISLAND, BAHAMAS

In addition to coral geochronology and the mid-reef “Devil’s Point” erosional discontinuity, smaller-scale depositional features within the Eemian (MIS 5e) Cockburn Town Member fossil reef deposits yield important insights into paleoenvironmental changes potentially associated with short-term sea-level fluctuation during the last glacial maximum. On San Salvador Island, lower reef (Reef I) exposures in Cockburn Town quarry exhibit a facies transition from coral floatstone (with minor coral bafflestone/framestone) to coral-microbial bindstone, whereas the upper reef (Reef II) is dominated by coral-mollusc-corallinacean rudstone (with essentially no microbialite). Although subtle horizontal discontinuities are common within Reef I, isolated areas display several inclined grainstone-draped discontinuities that define decimeter-scale lateral accretion surfaces. Each lateral accretion deposit contains branching coral fragments coated by micritic stromatolites (laminated microbialite) and poorly-laminated/clotted sandy microbialites (typically 30-80 mm thick). Microbialite-coated corals are separated by discontinuous, mm- to cm-scale drapes of ooid-peloid-skeletal grainstone similar to foreshore deposits. Reef I features suggest the following history of reef development: 1) establishment of branching coral stands in a bank-barrier setting; 2) development of backreef and lagoonal subenvironments; 3) subsequent restriction of those subenvironments, likely from a minor sea-level drop or a stillstand; 4) coating of coral clasts by microbial communities thriving in the backreef; 5) repeated introduction of new coral clasts via storms from reef-front settings followed by microbial coating of the new rubble; and 6) gradual migration of foreshore sand draping the shallow backreef. Repeated episodes of Reef I microbialite development, microbialite abundance and thickness, and the truncation of Reef I by the “Devil’s Point” discontinuity, indicate a significant period of backreef restriction followed by erosional truncation (sea-level drop). In contrast, the pristine coral preservation and the internal discontinuities of Reef II rudstones suggest episodic but rapid accumulation of reef debris in an unrestricted, energetic nearshore setting.