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ABSTRACT: Sponge-Algal Bioherms on the Western Platforms of the Nicaraguan Rise, Southwestern Caribbean

Pamela Hallock, A. C. Hine

The western platforms of the Nicaraguan Rise represent modern carbonate depositional environments dominated by calcareous and coralline algal sedimentation. Coral reefs are conspicuously absent. Seismic reflection data indicate bank-margin bioherms with 20-30 m local relief occurring at water depths of 30-50 m. Rock dredging and grab sampling at depths of 20-60 m recovered cobble-sized algal encrustations associated with coarse Halimeda sands and gravels, as well as coarse, poorly cemented mudstones dominated by disarticulated Halimeda segments set in poorly sorted sand-size matrix. Currents as strong as 100 cm/sec, observed on the banks and in channels between banks, appear to limit accumulation of finer sediments. Although algae are the dominant sediment producers, vide observations acquired by a remotely operated vehicle revealed that sponges play an active role in binding sediments and providing substrata for a variety of encrusting organisms. Topographic upwelling associated with the acceleration of the Caribbean Current over the Nicaraguan Rise apparently stimulates sponge-algal growth, while suppressing coral-reef development.

Algal and sponge-algal bioherms were dominant carbonate buildups throughout much of the Paleozoic and into the Mesozoic. Crustose coralline algae are the major binders and encrusters of Late Cretaceous through Holocene buildups. Thus, Nicaraguan Rise bioherms, composed of calcareous and coralline algae, with their sponge, bryozoan, small coral, and foraminiferan companions, may provide modern analogues to similar biohermal assemblages in ancient limestones.

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