TRACKING SEALEVEL WITH CORALS AND MOLLUSCS: ESR DATING on SAN SALVADOR ISLAND, BAHAMAS

Sealevel curves are best developed on tectonically stable coastlines, but require accurate dates for the units which define the sealevel heights. On San Salvador, eolianites with associated ooids and terrestrial molluscs preserve transgressive and regressive phases associated with Quaternary high seastands. Fringing, lagoonal patch, and small bank margin barrier reefs grew during the highstands, while the lowstands saw no deposition. At 11 locations around San Salvador, terrestrial molluscs (Cerion) from the transgressive or regressive dunes, and lagoonal bivalves (Codakia) and corals from the highstand deposits in the Quaternary Grotto Beach and the Rice Bay Formations were dated by ESR (electron spin resonance). Sediment dose rates were calculated from NAA analyses of U, Th, and K in associated sediment. These range from as low as 296 ± 36 μGy/y in the reefs to as high as 745 ± 58 μGy/y in the dunes. Cosmic dose rates were calculated by assuming geologically rapid changes in water depths or sedimentary covers as determined from each sample's current geologic context.

All the loose coral heads dated from the Holocene Rice Bay Formation, ranged from 3.9 ± 0.3 to 7.1 ± 0.4 ka (OIS 1). These were probably moved by storms into the modern lagoons. In situ corals from the Cockburn Town Reef, part of the Cockburn Town Member reef facies in the Grotto Beach Formation, range in age from 125 ka to 115 ka, correlating with OIS 5e. To check for temporally mixed molluscs samples, multiple random grab samples were analyzed from each locality where sufficient shells were available. Minimum ages for the Cockburn Town Member (Grotto Beach Formation) regressive phase ranged from 58.5 ± 4.6 to 74.9 ± 7.6 ka, when corrected for their cosmic doses (OIS 3-4).