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Abstract The insular shelf off Parguera, Puerto Rico, is 8 to 10 km wide. The shelf break at 20 m depth is a marginal barrier reef. The hard rock surface is covered with massive corals and Acropora cervicornis. The barrier is dissected by numerous narrow channels through which sand is transported over the shelf edge. The upper slope has a 43° gradient where channels are present, and is vertical where channels are absent.

Between the shelf edge and the coast, two elongate reef systems divide the shelf into an inner, middle, and outer shelf. The outer shelf, from the marginal barrier to the Margarita – Media Luna line, is covered with a thin veneer of biogenic sands. These are a mixture of recent and relict deposits.

The backreef lagoons of the middle and inner shelf are similar, and are characterized by finegrained poorly sorted sediments. The fine material comes from winnowing of the backreef apron and from in-situ bioerosion. Up to 10% of the calcareous fraction is contributed by coccoliths.

The backreef apron of Enrique is composed of moderately sorted sands composed of Halimeda, coral, coralline algae, and mollusk fragments. This area is relatively barren, although Thalassia covers part of the area and there is evidence of abundant burrowing activity. In contrast, a large part of the backreef of Media Luna is covered with A. cervicornis thickets. The difference in coral abundance is clearly related to available wave energy.

Zonation of the forereef is similar to other Caribbean coral reefs. However, the Lithothamnium ridge found in other reefs is replaced by Millepora. On Enrique, the zones are sharply defined and narrow in depth range, probably because of the low amplitude of available wave action. The zonation on Media Luna is less well defined, but there is a development of spur-and-groove structures.

The shelf is at a fairly uniform 20-m depth. Seismic records and bathymetry indicate that the shelf is of structurally controlled subaerial origin with limited modification by post-submergence reef development and sedimentation. The reefs have formed either on sand ridges or on the south limb of a syncline of Cretaceous rocks. Growth of 20 m in approximately 6,000 years gives a rate of 0.3 cm/year for the reefs.

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