PETROGRAPHIC COMPOSITION AND CARBONATE MINERALOGY OF MODERN SUBTIDAL SEDIMENTS FROM TWO SUBTROPICAL LAGOONS, SAN SALVADOR ISOLATED PLATFORM, BAHAMAS

Calcareous algae dominate biotic communities in humid peritidal tropical and subtropical environments, and have arguably been a major producer of carbonate mud throughout the Phanerozoic. This study focuses on modern carbonate sand and mud from the small isolated carbonate platform of San Salvador Island, Bahamas, with a goal of quantifying the input of sediment produced by calcareous algae. A total of 24 sediment samples were collected from the sea floor of two shallow, normal-to-metahaline lagoons: the tide-influenced north branch of Pigeon Creek, and the wave-influenced French Bay. In each lagoon, the samples were collected for petrographic and carbonate mineralogy analyses at 100ft intervals, along four 300-ft-long transects 300 ft apart.

Petrographic analysis of collected sediment samples was carried out on impregnated grain mounts; a minimum of 250 grains per thin-section were counted. The data for the Pigeon Creek indicate that the lagoonal sediments are composed of 6.2-58.7% grapestones, 11.4-35.8% peloids, 10.2-30.0% ooids, 5.0-20.6% foraminifera, 0.75-16.5% gastropods, 0.0-7.8% Halimeda, and 0.0-12.0% of unidentified skeletal grains. The grapestones are the most abundant grain type (~26%); their abundance suggests that the currents in the north arm of Pigeon Creek are insufficient to move or rework sand-size material on a regular basis, resulting in predominance of grains adhering to each other in aggregate clumps cemented by aragonite. The point count data for French Bay indicate that the sea-floor sediments are composed of 20.5-37.2% foraminifera, 4.7-24.4% Halimeda, 2.5-24.4% peloids, 0.8-19.2% ooids, 5.2-18.1% grapestones, 6.4-17.3% gastropods, and 1.7-6.4% of unidentified grains. The petrographic composition is compatible with a high-energy setting and wave-induced turbulence.

Mineralogical analysis was accomplished using X-ray diffraction of powdered samples under CuKa radiation between 24° and 32° 2θ. The XRD data indicates that the Pigeon Creek sediments consistently contain high values of aragonite (83%) versus magnesium calcite (17%). The French Bay data indicates a more equal split, with 58% aragonite and 42% calcite.