Carbonate Sedimentation in a Reef-Enclosed Lagoon, North Sound, Grand Cayman Island.

摘要

Sediment samples from environments of North Sound, Grand Cayman Island, B.W.I. were used to assess variability in constituent grain types, carbonate mineralogy, and HCl leachable cations (Mg, Na, Sr) as function of sedimentary environment. From the high-energy conditions near the reef to the quiet-water interior of the sound, four major environments were contrasted: (1) reef shoal, (2) shore zone, (3) grass plain, and (4) restricted lagoon. Constituent grains--coral, coralline algae, and composite grains--are most indicative of reef-shoal sediments. The shore zone is characterized by lithoclasts and cryptocrystalline grains. Occurrence of Halimeda and Foraminifera is proportionally higher in the grass plain, and a relatively high percentage of mollusk fragments distinguishes sediments of the restricted lagoon. Mineralogical differences in the sediments of the four environments generally reflect types and relative percentages of constituent grains. Three carbonate mineral phases were recognized (X-ray diffraction): aragonite, high Mg-calcite, and low Mg-calcite. Concentrations of the HCl leachable cations Mg and Sr are generally covariant with concentrations of high Mg-Calcite and aragonite, respectively. However, not all aragonite is high in Sr. Na concentrations appear to be controlled, not by mineralogy, but by biologic species. Differences in mineralogy and cation concentrations with respect to grain size occur in all major environments.