30 Sediments, Carbonates (Posters)

Sunday, 9 October 2011
9:00 AM - 6:00 PM
Hall C

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

Paper number: 30-3
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

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.