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ABSTRACT: Multiple Megabreccia Shedding from Low-Relief Carbonate Platforms in an Active Tectonic Setting: Nicaraguan Rise, Southwest Caribbean Sea

Albert C. Hine, S. D. Locker, H. T. Mullins, L. Tedesco, P. Hallock

Processed, single-channel, digital seismic reflection profiles from small air guns coupled with 3.5 kHz profiles and rock dredges reveal the presence of up to four buried, vertically stacked megabreccia sequences that formed from multiple collapse of an adjacent carbonate platform margin. These sequences dominate the slope facies for the east/north margins of Arawak Bank, a low-relief (250 m), deep (30-40 m) carbonate platform situated on the northern Nicaraguan Rise within 150 km of the tectonically active Caribbean/North American plate boundary, the Cayman Trough. Our field area south of this tectonic boundary consists of six carbonate platforms segmented by current-dominated open seaways.

The megabreccias are acoustically chaotic (numerous point-source diffractions), topographically irregular, basin-thinning (70 m proximal thickness) seismic units separated by a thin, discontinuous, parallel-laminated seismic facies. The most recent megabreccia is fan shaped, lies exposed on the sea floor, and nearly extends across a 10-km-wide, 300-m-deep open seaway. The megabreccia is 20 km wide and 120 m thick adjacent to a 120-m-high erosional escarpment. Large, individual point-source reflections indicate that some blocks may be 50 m in size. Rocks are (1) well-cemented, angular chalk-block breccias; (2) poorly cemented, mixed skeletal packstones, and (3) well-cemented Halimeda packstones (35,910 Ma C14 date).

This study indicates that carbonate platforms need not have immense relief to generate megabreccias and catastrophic margin failure. The close proximity to an active plate boundary probably is responsible for multiple, massive failure of this low-relief bank edge.

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