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

Abstract: Surficial Fluid/Gas Expulsion Features on the Gulf of Mexico Continental Margin with Special Emphasis on Gas Hydrates

The complex geologic framework of the northern Gulf of Mexico continental margin has been inherited from the rapid influx of siliciclastic sediment deposited over thick units of Jurassic salt. Faulting associated with the formation of depositional accommodation space through salt deformation, evacuation, and near-continual salt adjustment to sedimentary loading provides numerous avenues for fluid and gas migration to the modern seafloor. As a consequence, the surficial geology is punctuated with a spectrum of fluid/gas expulsion features.

Flux rate and fluid characteristics are interpreted as important determinants of modern seafloor geology and biology. Qualitatively, under conditions of rapid flux of sediment-rich fluids, mud volcanoes (up to 1 km wide and 50 m high) and extensive sheets of extruded mud result. Slow flux (seepage) promotes the formation of  ¹³C-depleted carbonate hardgrounds, stacked carbonate slabs, and moundlike carbonate buildups (frequently >20-m relief). Barite and other more exotic minerals are also being precipitated under special conditions. In water depths greater than ~500 m rapid to moderate vertical flux of hydrocarbon gases and fluids results in the construction of relief features composed of gas hydrates, ejected mud, and authigenic carbonates.Areas of near-surface hydrates frequently support complex chemosynthetic communities with associated hardgrounds containing calcareous remains of mussels, clams, and associated gastropods. Short-term episodes of venting are probably regulated by fault movement, probably controlled by local salt adjustment. Destabilization of gas hydrates by oceanographic processes also causes short-term episodic gas expulsion. These events occur with intraannual to interannual frequencies. Longer gas hydrate stabilization-destabilization cycles in continental margin settings are forced by hydrostatic loading and unloading at the frequency of sea-level change.

Geologic complexity of the present seafloor below the shelfedge presents a wide variety of geohazards for oil and gas activities. With our current understanding of vent/seep-related geohazards, gas hydrates are the most difficult class to identify and predict their behavior using standard geophysical survey and geotechnical techniques.