Abstract: Surficial Geology of the Middle and Upper Continental Slope, Northern Gulf of Mexico: The Important Role of Episodic Fluid Venting

Harry H. Roberts

The continental slope of the northern Gulf of Mexico is perhaps the most complex such environment in today's oceans. Large-scale topography of the middle slope to the shelf edge is dominated by knolls and basins created by shallow subsurface salt diapirs and intervening salt withdrawal basins. Superimposed on this regional framework is a diverse array of geologic features and associated sediment types. Recent studies using high resolution acoustic data, 3D-Seismic surface amplitude maps, and direct observation/sampling from manned research submersibles have verified the importance of episodic venting of fluid and gas on surficial slope geology. Deep-cutting faults activated by massive sediment input during periods of cyclic sea level lowering during the Pleistocene provid the avenues of vertical transport to the slope sea floor. Under rapid flux conditions, massive volumes of fluid mud are distributed in sheets and core-like buildups. Below the stability window for gas hydrates (approx. 500 m water depth) gas-charged fluids develop hydrate mounds and mound complexes apparently under moderate flux conditions. Chemosynthetic communities and authigenic carbonates are common components of these features. Slow flux conditions favor gas-charged fluids of low sediment content. Hardgrounds and mounds of authigenic carbonates (mostly Mg-calcite, aragonite, and dolomite) and less common exotic minerals like barite. Radiometric dating suggests that major expulsion events follow shelf-edge delta development during periods of lowered sea level during the Pleistocene hen sediments are rapidly deposited in the slope province.

AAPG Search and Discovery Article #90950©1996 AAPG GCAGS 46th Annual Meeting, San Antonio, Texas