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High Resolution Surficial Geology of the Louisiana Middle-to-Upper Continental Slope
Harry H. Roberts
Newly acquired high resolution acoustic data coupled with direct sea floor observational data and sampling by research submersibles have provided a new appreciation of the local-to-mesoscale geologic complexity of the ocean floor in middle and upper continental slope depths (depths 200-1000m). This relatively small-scale geologic complexity is superimposed on regional geologic features dominated by salt diapirs and adjacent intraslope salt/shale withdrawal basins. While the sea floor of intraslope basins is largely characterized by 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 clasts of diagenetic origin. 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 (< 1m to > 10m relief) and hardgrounds (byproducts of microbial degradation of hydrocarbons). On the uppermost slope, salt dome tops are sites of biogenic carbonate buildups and carbonate veneering formed during periods of lowered sea level during the late Pleistocene. Below water depths of about 500m moderate-to-slow vertical flux of hydrocarbons frequently results in the formation of gas hydrates at or near the sea floor. Hydrate mounds and families of mounds possess surface features that reflect brine and fluid mud expulsion, authigenic carbonate formations, and benthic communities that depend on chemical constituents of the hydrate complex. Local over-steepening of diapirs, hydrate mounds, and large mud vents causes slope instability resulting in a wide range of mass movement features from those associated with surface creep to huge submarine landslides.
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