Abstract

Surficial sediments and sea-floor features of the northern Gulf of Mexico continental slope record dramatic episodic venting and slower seepage of formation fluids, brines, crude oil, gases, and fine-grained sediments. Faults, activated by massive sediment input during periods of Plio-Pleistocene falling to low sea levels accompanied by compensating salt movement, provide avenues of vertical transport to the continental slope surface. Many of these faults cut thick sedimentary sequences that frequently contain geopressured zones which provide the driving force for fluid and gas expulsion. Flux rate and fluid characteristics are interpreted as important determinants of modern sea-floor 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. These sediments contain old and displaced microfaunas. Muds are frequently extruded with large volumes of crude oil and gas (both biogenic and thermogenic). In water depths greater than approximately 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 with near-surface hydrates frequently support complex chemosynthetic communities with associated hardgrounds containing calcareous remains of mussels, clams, and associated gastropods. Slow flux (seeps) promotes the formation of 13 C-depleted carbonate hardgrounds, stacked carbonate slabs, and moundlike carbonate buildups (frequently >20-m relief). Barite along with carbonate has been found in areas where rapid flux of sediment-carrying fluids has waned in the recent past and mineral-rich brines are now being ejected. Crests, chimneys, and cones of these minerals are found in such settings. Both recent numerical simulations of fluid release from geopressured zones and direct observations at the sea floor support a pattern of episodic venting. Short-term episodes of venting are probably regulated by fault movement, perhaps controlled by local salt adjustment. Destabilization of gas hydrates by oceanographic processes also causes short-term episodic gas expulsion. These events occur with interannual to intra-annual 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. Radiometric dating suggests that other long-term episodes of venting and major venting expulsion events are also probably modulated by sea- level change, time scales of thousands of years in response to low-stand sedimentary loading. Results of this summary of vent-seep-related phenomena suggest that sediment input from the shelf margin associated with cyclic Plio-Pleistocene falling to low sea-level periods, followed by major listic fault and salt adjustments to a new sediment load have activated venting-seepage throughout this period and perhaps longer.

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