Abstract

A study of sediment alteration on the west flank of the West Hackberry dome, Louisiana Gulf Coast, has documented the existence of ∼5 x 1010 kg of authigenic calcite-pyrite cement in Miocene sands at depths of 1.4 to 2.1 km in a 1.5 by 1.5 km area adjacent to the dome. The Sr, C, and S isotopic compositions of the cements support the hypothesis that Ca and S were derived from dissolution of salt-dome anhydrite and that carbonate was derived by thermochemical oxidation of methane and by sulfate reduction, possibly at temperatures as low as 70 °C. Constraints on the maximum aqueous concentrations of Ca that could be produced by dissolving diapiric salt require that >5 x 109 m3 of aqueous fluid, equivalent to a fluid volume/ pore volume ratio of >250:1, were involved in destroying salt, transporting Ca and SO4, and precipitating these cements. The presence of such cements requires a dynamic subsurface mass-transport regime involving either large volumes of fluid or fluids that are extensively recirculated.

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