Large volumes of siliciclastic sediments, input especially during periods of lowered sea level, and compensating salt tectonics have produced a continental slope that is arguably the most complex in today's oceans. Faults associated with deformation of salt and shale provide the primary migration routes for hydrocarbon gases, crude oil, brines, and formation fluids to the modern sea floor. Since the mid 1980s, it has become increasingly clear that this process has an extremely important impact on the geomorphology, sedimentology, and biology of the modern continental slope.

Hydrocarbon source, flux rate, and water depth are important determinants of sea floor response. Under rapid flux conditions, mud volcanoes (to 1 km wide and 50 m high) result, and hydrate hills (rich with authigenic carbonates), carbonate lithoherms, and isolated communities of chemosymbiotic organisms with associated hardgrounds represent much slower flux responses. In numerous moderate- to low- flux cases, cold seep products support islands of productivity for communities of chemosymbiotic organisms that contribute both directly (shell material) and through chemical byproducts to the production of massive volumes of calcium-magnesium carbonate in the form of hardgrounds, stacked slabs, and discrete moundlike buildups (commonly >20m). These seep-related features occur over the full depth range of slope (>2200m). Authigenic carbonates occur as porous aragonite, M_g-calcite and dolomite which commonly contain shell debris from chemosymbiotic fauna, and fine-grained siliciclastics. Anomalous δ¹³C depleted values (-18.5 to 54% PDB) for carbonates indicate mixed carbon sources derived from bacterial oxidation of methane, crude oil biodegradation, sulfate reduction, and overlying sea water bicarbonate. Chemosymbiotic mussels and clams, who% calcareous remains contribute to the growth of carbonate buildups, yield δ¹³C compositions ranging from -6.3 to 0.2% PDB. In general the δ¹³C compositions, ranging from 2.8 to 4.44% PDB, correspond to the ambient downslope sea-bed temperature decline.

Upper slope authigenic carbonates are "diluted" with biogenic carbonate, the product of lowstand reef development. Fossil-poor carbonates of the upper and middle slope are commonly dolomitic, formed in the shallow subsurface, and exhumed by uplift (salt) and physical erosion. Middle to lower slope carbonates generally have a M_g-calcite pelloidal matrix, with acicular to botryoidal aragonitic cements in voids, and contain shell debris. Seep-related carbonates of the Gulf of Mexico continental slope, as well as those formed through degassing of accretionary prisms along active margins, are now thought to create hardgrounds and discrete buildup that are excellent analogs for many problematic carbonate buildups in ancient deepwater siliciclastic rocks.

End_of_Record - Last_Page 9---------------