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, and both the amount and grain size decrease southwestward. 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 transitional over a 10-mile interval.

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