Sediments on the southern shelf of the Virgin Islands were investigated as a model of carbonate deposition on an open shelf. To determine the origin of the carbonate sands and to relate origin to environmental processes, surface and core samples were studied. Analysis of surface samples for composition shows that the sand has been derived mainly from calcareous algaes and mollusks by in-situ biogenic production. Zonation of the dominant sand producers is related to the present environmental setting where water depth has the greatest influence.

Dating of cores as long as 5 m by the C-14 method indicates accumulation rates of slightly less than 1 mm/year for the last 5,000 years in the areas of the thickest sand. Faunal studies show that the climate during the last 5,000 years was similar to the present-day climate. The only changes in environmental conditions appear to have been an increase in water depth and a concurrent change in the patterns of water movement. Areas protected during lower sea level have experienced a disappearance of the subtidal barnacle, Balanus venustus, and a gradual increase in the amount of coralline algae in the sediment beginning approximately 1,500 years ago. The disappearance of barnacles probably was caused by the introduction of open marine conditions into a previously protected lagoonal area. However, the articulate coralline algae increased in the quiet waters on the lee side of the offshore islands because of a niche created by deepening waters.

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