ABSTRACT

The surface sediments of Card Sound, Florida, and of the adjacent continental shelf differ considerably. The Sound is almost completely separated from the shelf by the northern end of the Florida Keys. The shelf sediments are entirely carbonate, composed primarily of aragonite, high-magnesium calcite, and lesser amounts of low-magnesium calcite. Grain size generally decreases seaward but increases with proximity to the series of coral knolls which parallel the shore landward of the present living reef. In the trough between the knolls and the Keys, sediments are better sorted and higher in calcite. High-magnesium calcite increases toward the coral knolls.

In contrast, the presence of detrital quartz in Card Sound creates a dual-component sediment, with low-magnesium calcite being the predominant carbonate mineral, although some aragonite and high-magnesium calcite also occur. The sediments are poorly sorted, and mean grain size and sorting decrease both down the long axis of the Sound and toward the Keys. The noncarbonate fraction is mainly well-sorted quartz. Both the amount of quartz and the size of the individual grains decrease toward the southwest. The distribution of quartz in several size fractions shows that sediment transport is primarily by waves and wave-generated currents whose direction and magnitude are seasonal and, secondarily, by tidal currents. The facies change from a calcareous quartz sand to calcarenite is transiti nal over a 10-mile interval.