DIAGENETIC EVIDENCE FOR MASSIVE EVAPORITE DISSOLUTION, FLUID-FLOW, AND MASS-TRANSFER IN THE LOUISIANA GULF-COAST

A study of sediment alteration on the west flank of the West Hackberry dome, Louisiana Gulf Coast, has documented the existence of approxima tely 5 x 10(10) kg of authigenic calcite-pyrite cement in Miocene sand s at depths of 1.4 to 2.1 km in a 1.5 by 1.5 km area adjacent to the d ome. The Sr, C, and S isotopic compositions of the cements support the hypothesis that Ca and S were derived from dissolution of salt-dome a nhydrite and that carbonate was derived by thermochemical oxidation of methane and by sulfate reduction, possibly at temperatures as low as 70-degrees-C. Constraints on the maximum aqueous concentrations of Ca that could be produced by dissolving diapiric salt require that >5 x 1 0(9) m3 of aqueous fluid, equivalent to a fluid volume/pore volume rat io of >250:1, were involved in destroying salt, transporting Ca and SO 4, and precipitating these cements. The presence of such cements requi res a dynamic subsurface mass-transport regime involving either large volumes of fluid or fluids that are extensively recirculated.