FREE THERMOHALINE CONVECTION BENEATH ALLOCHTHONOUS SALT SHEETS - AN AGENT FOR SALT DISSOLUTION AND FLUID-FLOW IN GULF-COAST SEDIMENTS

Basinward migration of Jurassic salt in the U.S. Gulf of Mexico has re sulted in the emplacement of large allochthonous salt sheets into shal low Miocene to Holocene sediments. Although comparatively little direc t information is available on the environment below these salt bodies, it is reasonable to suppose that the formation; of dense brines by di ssolution of the base of these sheets may induce free thermohaline por e fluid convection within the sediments below. We derived equations wh ich make it possible to quantitatively estimate rates of dissolution o f these subsurface salt structures. From these calculations and by geo logically realistic numerical simulations it can be shown that free co nvection beneath allochthonous salt sheets has the potential for being a significant mechanism for both salt dissolution and mass transport, even if the underlying sediments have permeabilities as low as 10(-17 ) m(2) (0.01 mD). The calculated maximum Darcy fluxes and rates of sal t dissolution rapidly increase with sediment permeability. When the ve rtical permeability of the underlying sediment is 10-(17) m(2) (0.01 m D), salt is dissolved from the base of the sheet at an average rate of 3-5 m m.y.(-1) The corresponding fluid velocities are such that over a 10 m.y. period the integrated fluid flux in the underlying sediments would be similar to 10(4) m(3) m(-2). By comparison, integrated fluid flux for compactive expulsion is < 10(3) m(3)m(-2). Thus, for the off shore sediments of the Gulf of Mexico, thermohaline convection beneath an allochthonous salt sheet is a significant driving mechanism for fl uid flow with potentially important implications for heat and mass tra nsport, diagenesis, and salt tectonics.