Strongly Contrasting Modes of Slope Failure and Erosion along the Carbonate Margins of Three Open Seaways; Northwest Nicaraguan Rise

HINE, A. C., S. D. LOCKER, P. HALLOCK, University of South Florida, St. Petersburg, FL, and H. T. MULLINS, Syracuse University, Syracuse, NY

Single channel seismic lines and dredge hauls from three subparallel, closely spaced, open seaways segmenting four carbonate platforms along the northwest Nicaraguan Rise in the Caribbean Sea reveal that each linear basin has a distinct style of slope erosion and basin infill. These roughly north-south trending open seaways are each about 125 km long, but vary in width (9-20 km) and depth (200-800 m).

The westernmost seaway is undergoing complex infilling involving the displacement of large (100 m in diameter) shelf-margin/slope blocks and the establishment of Halimeda bioherms on the tops of these features. The basin and lower slopes of this seaway are also fed by relatively small debris flows and coarse turbidites. The center seaway contains enormous mass-flow megabreccias that may extend to the base of the opposing channel slope. Individual mass-flow

megabreccias extend 16 km downslope, 28 km alongslope, and are up to 100 m thick. These deposits contain individual blocks measured to be 400 m across and stand greater than 100 m high. The easternmost seaway has the greatest relief (350-800 m), yet has the fewest mass-wasting deposits. Slope facies are relatively thin and lie unconformably upon horizontal reflections interpreted to be shallow-water carbonates. This stratigraphic relationship suggests an early, enormous removal/loss of carbonate-platform margin rocks and sediments.

We do not yet fully understand why the slopes along these three seaways have behaved so differently. Undoubtedly, varying combinations of different structural history, sediment type, sediment production and transport, cementation, and current strength have played a role.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)