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 IJ C-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 Si0 2 and CaC0 3 from mudrocks and emplace authigenic quartz and calcite cements in sandstones, (2) transfer K 2 0 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 C0 2 ) 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.

Abstract Anomalously high porosities (20 -29%) in deeply buried (4.9-5.2 krn) Miocene sandstones at Picaroon field, offshore Texas are largely a result of porosity enhancement by dissolution of calcite cement , Dissolution is not important in equivalent strata nearby, including Doubloon field, where reservoir porosities are generally less than 18%. Doubloon reservoir sands contain moderately saline (TDS = 63-74 gil) ,"NaCl-typc" water characterized by low concentrations of Ca and other cations , enrichment in δ 18 O (+7.8%0 SMOW) and radiogenic Sr ("SrI86Sr = 0.71109). Formation waters of this type are common throughout the Gulf Coast Terti ary section . In contrast, Picaroon waters have salinities of 151 to 243 gil TDS , Ca concentrations of 13 to 22 gil, heavier in δ 18 O (+ 8.0 to + 9.3%0 SMOW) and less radiogenic Sr ("Sr! .6sr= 0.70992 to0.71023). In addition, Picaroon water samples contain unusually high concentrations of other cations such as Sr, Ba, Fe, Pb and Zn. Picaroon sandstones contain late diagenetic fracture-filling ankerite, barite and sphalerite. Ankerite is in oxygen isotopic equilibrium with formation water at temperatures indicated by fluid inclusions (> 147°C) and has a similar Sr isotopic composition ("Sr!,6S r = 0.70970). Picaroon brine s are interpreted to be allochthonous to the Miocene section, because they have elemental and isotopic compositions similar to waters produced from Mesozoic reservoirs in south Texas and central Mississippi. The association of these waters with high quality reservoirs at Picaroon suggests a potential link between deep sources of fluids and carbonate dissolution . A model is proposed in which hOI , acidic water s from the underlying Mesozoic section are injected along major faults into Picaroon sands, resulting in significant porosit y enhancement. Fluid flow is likely episodic, driven by periodic buildup and release of geopressures.

Abstract Authigenic calcian dolomite is a common but rarely abundant (≤20%) component of Neogene deep-water (475-2,767 m) carbonates peripheral to the Florida-Bahamas Platform. Dolomite concentrations as high as 57% of the carbonate fraction occur in the Miocene of west Florida, however, and as much as 86% dolomite has been found in a hardground from the Bahamas. Dolomite occurs principally as pore-filling euhedral rhombs (5–20 μm) that precipitated in situ, as well as by replacement of calcite at disconformities. Stable isotope ratios (oxygen and carbon) suggest dolomite precipitation from deep, cold, seawater-derived fluids, and trace-element (Sr) concentrations suggest strontium-rich aragonitic(?) precursors. Preliminary 87 Sr/ 86 Sr data suggest substantial lag times for dolomite precipitation and contamination by "old" strontium. Because of the high diagenetic potential of periplatform carbonates, Bahamian deep-water dolomites appear to be a natural consequence of shallow subsurface (<60 m) burial diagenesis. In contrast, carbonate ramp slope sediments from west Florida, which have a much lower initial diagenetic potential, are punctuated by discrete concentrations of authigenic dolomite, which may represent paleoceanographically controlled dolomite "events." Overall, our data indicate that deep-water dolomite has had little difficulty in precipitating from normal marine-derived fluids.