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Abstract

Models of burial diagenesis in the Gulf of Mexico sedimentary basin must explain a wide variety of phenomena, including: (1) uranium mineralization in volcanogenic Oligocene sandstones where the sandstones overlie growth fault zones in Eocene units, (2) lead-zinc mineralization in salt dome caprocks, with dissolved lead and zinc being known only from saline formation waters locally present in deeply buried Mesozoic reservoirs, (3) discharge of NaCl at the land surface, contributing to the dissolved chloride load of rivers, (4) natural seepage of oil and gas, which also leads to “vent” marine communities and IJC-depleted CaCO,-cemented zones on the continental shelf and slope, (5) the presence of hydrocarbons above, and not uncommonly displaced laterally by ten’s or hundred’s of kilometers from mature source rocks, and (6) allochthonous, non-metalliferous saline water in Cenozoic clastic units. Fluid movement along faults is important in most, if not all of these processes.

Convection within the self-fractured overpressured zone is inferred, based on the volumes of water necessary to: (1) remove Si02 and CaC03 from mudrocks and emplace authigenic quartz and calcite cements in sandstones, (2) transfer K20 from feldspar dissolution in sandstones into mudrocks, where it is consumed by illitization, (3) remove sufficient volumes of hydrocarbons from “lean” Gulf Coast mudrocks as kerogen maturation proceeds and transport the hydrocarbons to permit the accumulation of significant volumes of oil and gas.

Because the basement rocks beneath Gulf sediments are probably undergoing prograde metamorphism and devolatiUzation, material transfer into the sedimentary basin from the basement is inferred. The rate at which water (and C02) are added to the sedimentary basin from underlying rocks can potentally affect not only the volumes of water available for diagenesis but can maintain geopressures (and convection) within the sedimentary section long after pressure would normally decay back to hydrostatic values if compaction were the only operative process.

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