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

ABSTRACT: Geologic Features of the Northern Gulf of Mexico Continental Slope: A High Resolution View

The role of salt tectonics on creating domes and interdome sedimentary basins of the northern Gulf of Mexico continental slope is well known. However, an understanding of higher resolution sea floor features is now developing because of new technology and emphasis on geohazards data collection in support of deep water oil and gas production. High resolution seismic, side-scan sonar data, and direct observations/sampling by research submersibles have provided new insight into the dynamic nature of the slope. New data sets emphasize that the slope is both an area of rapid deposition and erosion. Thick sedimentary sequences (clays to gravels) are trapped in interdome basins while dome crests of the mid to upper slope displayevidence of having been stripped of 10's to >100 m of sedimen ary overburden. A variety of exposed and buried mass movement features/deposits are associated with steep slopes at the shelf edge and dome flanks (up to 200). Faults that offset the modern sea floor are common. Scraps in excess of 600 and up to 20 m relief have been observed via submersible. Local sediment failures occur along the fault scarps. Faults also function as avenues of transport for formation fluids, gases, crude oil, and fine-grained sediment to the sea floor. Sea floor features resulting from the seepage of these products include mud volcanoes of many sizes, hydrate mounds, carbonate buildups, and productive chemosymbiotic communities. Mud volcanoes are built from the expulsion of large volumes of fluid mud commonly mixed with hydrocarbon gases and crude oil. Fault intersect ons are usually the sites of large (>1 km diameter) mud expulsion features. Flux of fluid mud and hydrocarbons is usually so rapid that biodegradation and associated processes do not lead to chemosynthetic community development and authigenic carbonate formation. In contrast, below 500 m water depth hydrate mounds slowly flux gas, crude oil, and fluid mud to the surface where biodegradation of hydrocarbons and sulfate reduction support productive chemosymbiotic communities and precipitation of authigenic carbonates. The carbonates range from surface crusts to massive limestone structures incorporating the remains of chemosymbiotic molluscs. These unusual carbonate buildups are now known to occur from the shelf edge to the toe of the slope. Viable chemosynthetic communities of tube wor s and mussels appear restricted to water depths below approximately 300 m while seep-related bacteria (primarily Beggiatoa, the water/sediment interface form) occur over the slope's total depth range. Studies using high resolution acoustic data coupled with 5 years of submersible observations and sampling have emphasized the dynamic and complex nature of the modern continental slope of the northern Gulf of Mexico at scales below the resolution of most previous investigations.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.