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Light-colored carbonate sands and dark-colored muddy clastic sands are deposited in distinct and sharply defined bands across the narrow (less than 3 km, 1.9 mi) and steep insular shelf of northern Puerto Rico. The narrow bands of clastic sands are found adjacent to the mouths of each of the rivers. These bands are in textural equilibrium with the present-day physical conditions on the shelf and are deposited as a result of successive hurricane-related flooding events. Dominant sand-sized components are quartz, feldspar, and rock fragments. The rate of sedimentation in the intervening cross-shelf strips of carbonate sediment is much lower, and the shelf sediment cover here is not in textural equilibrium with the physical environment. Dominant sand-sized components are mol usk, algae, coral, and bryozoan fragments. The boundary between carbonate and clastic bands is very sharp, often occurring within 100 m (330 ft).
The small carbonate fraction of the rapidly deposited clastic sediment and the particles making up the slowly deposited carbonate sediment exhibit strong differences in physical condition. The clastic calcareous fraction is fresh in appearance, highly angular if fragmented, and has original coloration. The relict calcareous material is old appearing, commonly stained and rounded, and has a dull luster. Polished surfaces and highly rounded grains characterize adjacent calcareous beach sands. The striking difference in the physical condition of the carbonate grains in clastic and carbonate sediments is a function of differing lengths of sea-floor exposure.
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