QUATERNARY SEALEVEL CHANGE ON SAN SALVADOR, BAHAMAS: ESR DATING WITH CORALS AND MOLLUSCS

Sealevel curves are best developed on tectonically stable coastlines, but require accurate dates for the units which define the sealevel heights. On San Salvador, aeolianites with associated ooids and terrestrial molluscs preserve transgressive and regressive phases associated with Quaternary high seastands, while fringing reefs mark the highstands. Lowstands saw no deposition, but terra rosa paleosols did develop. To build a late Quaternary sealevel curve for San Salvador, 19 marine molluscs and corals were dated from two new locations in the Quaternary Grotto Beach Formation. These supplement dates from 11 other locations around the island from the transgressive or regressive dunes, and corals from the highstand reefs in the Grotto Beach and the Rice Bay Formations dated by ESR (electron spin resonance). Across the island, sediment dose rates range from as low as 296 ± 36 μGy/y in the reefs to as high as 745 ± 58 μGy/y in the regressive dunes, as determined by NAA analyses of U, Th, and K in associated sediment. Cosmic dose rates were calculated by assuming geologically rapid changes in water depths or sedimentary cover thicknesses as determined from geologic contexts. U concentrations in the molluscs ranged from 0.2 to 2.2 ppm, but corals averaged 1.5-3.5 ppm. At Pigeon Creek Quarry E, a transgressive ebb-tidal delta developed above the pre-MIS 5 terra rosa paleosol. Preliminary ages from the Pigeon Creek Quarry suggest that the ebb-tidal delta was active for > 10 ky, which suggests that the delta was active for somewhat longer than the currently accepted duration for the MIS 5e highstand. Both MIS 5a and 5c highstands allowed molluscs and/or small corals to be added to the reef tops at the Sue Point and Cockburn Town fossil reefs. For San Salvador, the combined data set suggests that the high seastand in MIS 5e reached at least 3 m above current sealevel. In MIS 5a, sea level rose to at least 1-2 m above current levels. In MIS 5c, sealevel equalled modern sealevel.