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Abstract: High Resolution Surficial Geology of the Louisiana Middle to Upper Continental Slope

Harry H. Roberts

Newly acquired high resolution acoustic data and 3D-seismic profiles coupled with direct sea floor observational data and sampling by research submersibles have provided a new appreciation of the local-to-mesoscale complexity of the ocean floor within the middle and upper continental slope depths (primarily 200-1000 m). This geological complexity is superimposed on regional bathymetry dominated by salt diapirs and adjacent intraslope salt/shale withdrawal basins. While the sea floor of intraslope basins is largely characterized by relatively featureless depositional topography, surficial geology is extremely variable on the tops and flanks of salt diapirs. Dome tops are eroded and commonly covered with a coarse sediment lag characterized by both biogenic carbonates and di genetic clasts. Intense faulting associated with diapirs exposes uplifted, tilted, fractured, and sometimes folded slope sediments. These faults provide avenues for the transport of fine-grained sediments, formation fluids, brines, and hydrocarbons to the sea floor. Depending on transported products and flux rate sea floor features range from massive mud volcanoes (> 1km diameter) to carbonate mounds (commonly > 10m) and hardgrounds (by-products of microbial degradation for hydrocarbons). Below water depths of about 500m moderate to slow vertical flux of hydrocarbons frequently results in the formation of hydrates at or near the sea floor. Hydrate mounds and families of mounds have surface features that reflect brine and fluid mud expulsion, authigenic carbonate formations, and ben hic communities that depend on chemical constituents of the hydrate complex. Local over-steepening of diapirs, hydrate mounds, and large mud vents causes slope instabilities resulting in a wide range of mass movement features. On the upper slope, salt dome tops are sites of biogenic carbonate buildups formed during lowered sea level during the late Pleistocene.

AAPG Search and Discovery Article #90955©1995 GCAGS 45th Annual Meeting and Gulf Section SEPM, Baton Rouge, Louisiana