ROBERTS, HARRY H., Coastal Studies Institute, Louisiana State Univ., Baton Rouge, LA; and PAUL AHARON, Department of Geology and Geophysics, Louisiana State Univ., Baton Rouge, LA

ABSTRACT: Cold Seep Carbonates of the Northern Gulf of Mexico Continental Slope: A Synthesis of Submersible Investigations

The northern Gulf of Mexico's complex continental slope includes vast areas of rugged sea floor characterized by discrete carbonate buildups of various scales (<1 m to >20 m relief) and coarse carbonate-rich sediments. These carbonates are most frequently associated with regional topographic highs imposed on the slope by the combined effects of sediment loading during periods of lowered sea level and compensatory vertical salt movement. Intricate fault systems associated with these salt diapirs provide the primary migration routes for hydrocarbon gases, crude oil, brines, and formation fluids to the modern sea floor. The cold seeps resulting from this process function as islands of productivity for communities of chemosymbiotic organisms, and special settings for the precipitati n of authigenic carbonates.

Carbonate hardgrounds and buildups in water depths ranging from 120 m to >2000 m have been sampled since 1988 with the aid of manned submersibles Pisces II, Johnson Sea-Link, and ALVIN. The buildups consist predominantly of highly porous and fractured chemical carbonates composed of aragonite, Mg-calcite, and dolomite containing variable proportions of shell debris from chemosymbiotic fauna, plus fine-grained siliciclastics. Anomalous {13}C-depleted values of the chemical carbonates, ranging from -18.5 to -54 o/oo PDB, are indicative of mixed carbon sources derived from bacterial oxidation of methane, crude oil biodegradation, sulfate reduction, and overlying sea water bicarbonate. Chemosymbiotic mussels and clams whose calcareous remains contribute to the growth of carbonate build ps yield {13}C compositions ranging from -6.3 to 0.2 o/oo PDB. In general, the isotope{18}O compositions, ranging from 2.8 to 4.44 o/oo PDB, correspond to the ambient downslope seabed temperature decline.

Authigenic carbonates collected from seeps near the present shelf edge are "diluted" with biogenic carbonate, the product of low stand reef development. Fossil-poor carbonates of the upper and middle slope are interpreted as having formed in the shallow subsurface and exhumed by the combined processes of uplift (by salt migration) and physical erosion. Middle to lower slope carbonates generally have a Mg-calcite pelloidal matrix with acicular to botryoidal aragonitic cements in voids and contain chemosymbiotic mollusc shells. It is now being recognized that carbonates formed from seep-driven processes on the Gulf of Mexico continental slope create hardgrounds and discrete buildups that are excellent analogs for many problematic buildups in ancient deepwater siliciclastic rocks.

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