Bathymetry and bedrock topography show that the bulk of Looe Key National Marine Sanctuary (lower Florida Keys) occupies a shallow (0–7 m), broad (1–2 km wide) ridge between a landward trough (Hawk Channel, <15 m deep) and the outer margin dropoff (to >30 m) south
of Looe Key Reef. Average sediment thickness in the Sanctuary is 5.7 m, whereas directly behind the reef it is 12–15 m. Prevailing direction of sediment transport is from east to west, parallel to the platform margin; however, coral rubble has accumulated immediately behind Looe Key
Reef, and coral-rich sands have collected both landward and seaward of the reef. Based on seismic reflection and C14 data, rate of sediment accumulation in the Sanctuary has been 1–2 m/1,000 years during the past 7,000 years. The sediment is composed principally of coral,
mollusc, and Halimeda fragments with Halimeda being subordinate to the other two. This observation contradicts the accepted view that sand-size fractions in the Horida reef tract contain more Halimeda than other types of grains, but it conforms to a trend of decreasing
Halimeda content from north to southwest along the tract. As this trend is accompanied by a gradual increase in the proportion of dead and dying (relative to living) reefs from north to southwest, it appears to reflect greater production of coralline sand (rather than slower Halimeda
growth) that results from a marked acceleration of bioerosion of abundant dead reefs in the area.
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