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Abstract

Intense faulting in the northern Gulf of Mexico slope province results from complex interactions between subsurface salt and the deposition of large volumes of sediment. Many of these faults provide pathways for subsurface fluids and gases to migrate from deep petroleum-generating zones to the modern seafloor. These migration pathways are concentrated along the margins of intraslope basins where they are directed by a spectrum of salt geometries. Both geological and biological responses are highly variable and dependent on rate of delivery as well as on fluid and gas composition. Qualitatively, rapid expulsions of gas-charged fluids (including fluidized sediment) result in the deposition of sediment sheets or, mud volcanoes. Both products of rapid expulsion vary greatly in scale. The sheet-like flows may be localized or extend over many square kilometers of the slope while mud volcanoes vary from < 1 m to several km in diameter. Hydrocarbons associated with rapid flux systems reflect little biodegradation during migration. Sediment samples from these seafloor expulsion areas frequently contain hydrocarbons that are remarkably similar to those that are produced from the parent deep subsurface reservoirs that are directly connected to the surface by faults. High accumulation rates, thin depositional units, and limited hydrocarbon storage capacity characterize sediments of rapid flux systems. Lucinid-vesycomyid clams and bacterial mats are the chemosynthetic communities that dominate in these settings.

At the other end of the flux rate spectrum, slow hydrocarbon seepage results in lithification and mineralization of the seafloor. Microbial utilization of hydrocarbons promotes the precipitation of 13C- depleted Ca-Mg carbonates as by-products. These products occur over the full depth range of the slope. Mounded carbonates can have relief of up to 30m, but mounds of 5-10m relief are most common. Mound-building carbonates represent mixed mineral phases of aragonite, Mg-calcite, and dolomite with Mg-calcite being the most common. Barite is another product that is precipitated from mineral-rich fluids that arrive at the seafloor in low-to-moderate seep rate settings. Hydrocarbons analyzed from these slow-flux settings are highly biodegraded and chemosynthetic organisms are generally limited to bacterial mats.

Below water depth of approximately 500 m, intermediate flux settings seem best exemplified by areas where gas hydrates occur at or very near the seafloor. These environments display considerable variability with regard to surficial geology and on a local scale have elements of both rapid and slow flux. However, this dynamic setting apparently has a constant supply of hydrocarbons to promote gas hydrate formation at the seafloor even though oceanic temperature variation (primarily on the upper slope) cause periodic shallow gas hydrate decomposition. In the northern Gulf, gas hydrates contain both thermogenic and biogenic gas. The presence of these deposits provides the unique set of conditions necessary to sustain dense and diverse chemosynthetic communities.

The cross-slope variability of seafloor response to fluid and gas expulsion is not well known. However, present data indicate that the expulsion process is highly influenced by migration pathways dictated by salt geometries that change downslope from isolated salt masses to canopy structures to nappes.

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