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Abstract: Evidence of Episodic Delivery of Fluids and Gases to the Seafloor and Impacts of Delivery Rate on Surficial Geology (Gulf of Mexico)

ROBERTS, HARRY H., Coastal Studies Institute, Louisiana State University, Baton Rouge, LA, 70803

Summary

Repeated observations and samplings of hydrocarbon seep-vent related surficial features of the Louisiana-Texas continental slope indicate that episodic release of fluids and gases at the seafloor has produced a spectrum of geologic features. This feature spectrum has developed both from the products that arc released and relative differences in rates of delivery. Observational data collected by manned submersibles and age-dating of shell debris trapped in seep-derived authigenic carbonates suggest a broad spectrum of venting end seepage frequencies. Long time scales for venting and seepage appear to be modulated by sedimentary loading and coincident reductions in hydrostatic pressure during periods of falling to low sea level. It is theorized that (a) reductions in hydrostatic pressure result in destabilization of surface and near-surface gas hydrates, and (b) sedimentary loading down slope from shelf-edge depocenters forces salt deformation accompanied by fault movements that breach seals on overpressured zones causing fluid and gas release, Short-term episodes of venting-seepage (interannual to intraannual) observed during the present period of high sea level, appear related to local activation of faults by salt adjustment and destabilization of surficial gas hydrates from warm-water Loop Current intrusions and eddies. Resulting seafloor features show a clearer relationship to venting-seepage rate than frequencies at which hydrocarbons, formation fluids, and sediment are delivered to the seafloor. Rapid delivery of fluids (including sediment) and gases promote mud-prone features such as mud volcanoes with little evidence of lithification or complex chemosynthetic communities. Slow seepage, in contrast, leads to mineral-prone features like authigenic carbonate mounds and barite cones/chimneys. Intermediate rates promote the formation of gas hydrates in water depths below 500 m. These areas develop complex seep-related communities, scattered authigenic carbonates, and variable seafloor topographies.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah