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ABSTRACT: The Paradox of Drowned Reefs: A Caribbean Example Mapped Using SeaMARC II Side-Scan Sonar

David Grote, Paul Mann

Three models for the drowning of carbonate platforms and associated fringing coral reefs include (1) rapid submergence below the euphotic zone by tectonic subsidence and sea level rise; (2) excess nutrients in the water; and (3) burial by prograding marine siliclastic sediments. To examine these mechanisms on a regional scale, we mapped drowned barrier reef tracts around the active carbonate banks of the Nicaraguan Rise using SeaMARC II sidescan sonar, 3.5 KHz, and digital single channel reflection techniques. The reef tracts exhibited high sonar backscatter and were prominently displayed on sidescan images. Characteristic features of the reef tracts include (1) uneroded and slightly sinuous mounds that crop out on the sea floor and closely following bathymetric contours; (2) reef mounds that typically occur in stairstep sets of two to three terraces; (3) water depths at the crest of the reef mounds that range from 1050 to 1500 m; and (4) reef mounds that extend for 1200 km around the base of the slope (depth 1300 m) of an active carbonate platform in a moderately active, intraplate setting (Pedro Bank) and along the crest of a submerged fault block in a highly active, interplate setting (Bay Islands Ridge, ridge crest depth at 1600 m). Because these newly discovered reef tracts have not been dredged, their ages and compositions remain unknown. Based on the observed seafloor outcrop, regional extent, and approximate correlation in water depth of the reef tracts, mechanisms 1 and 2 appear to be the most likely drowning mechanisms.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990