MICROFACIES, MINERALOGY, AND STABLE-ISOTOPE RECORD OF THE EAST BEACH LAGOON SEDIMENT, SAN SALVADOR ISLAND, BAHAMAS

San Salvador is a small intraoceanic carbonate platform located in southwestern Bahamas. It is characterized by shallow-marine platform carbonates with predominantly mud-lean, high-energy facies rimming the platform; inland saline lakes and ponds also contain grainy (skeletal) hurricane deposits, as well as low-energy muddy carbonates and thrombolites. This study focuses on the carbonate sand haloes rimming the patch reefs of the East Beach, a high-energy lagoon along the northeastern, windward margin of the island. A total of 13 sediment samples were collected while scuba-diving at depths of up to 5 meters. The frequency analysis of microfacies data was done on grain mounts by point counting of 250 grains per grain mount. Bulk sediment samples were powdered and then analyzed for their mineralogy using the standard X-Ray Diffraction (XRD) method. Carbon and oxygen isotopes were obtained from powdered carbonate sediment samples; the isotopes were also obtained from the separated and powdered skeletal grains, including green algae (Halimeda incrasata, H. lacrimosa), gastropods, echinoids, corals, bivalves, foraminifera and red algae.

The results of the XRD analysis of powdered off-reef, pure carbonate sediment (aragonite + calcite = 100%) indicates predominance of aragonite over calcite. Aragonite is the dominant mineralogy in 80% of the samples; in those samples aragonite content ranges from 54-73%. Where dominant, the calcite content ranges from 63-67%. Overall, the average aragonite to calcite ratio is 1.28. The studied calcite is high in magnesium, with values ranging from 14.3-16.4 mole% (mean 15.5 mole%). The frequency analysis of microfacies data indicates that the major sediment producer is Halimeda (20% of all grains), followed by red algae (11%), foraminifera (5%), bivalves (4%), gastropods (3%), and corals and echinoids (<1%). Non-skeletal grains comprise 56% of all grains, and are predominantly peloids. There appears to be a correlation between an increase in calcite content and abundance of bivalve fragments. Oxygen isotope values range from -0.28 to -2.29 ‰VPDB, with mean value of -0.99 ‰VPDB; carbon isotopes range from -0.73 to 3.18 ‰VPDB, with mean value of 2.27 ‰VPDB. The isotope values of the bulk sediment are clustered, while the individual skeletal grains exhibit greater ranges of values.