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

Abstract

ABSTRACT Basinward migration of Jurassic salt in the northern Gulf of Mexico has resulted in the emplacement of large allochthonous salt sheets into shallow Miocene to Holocene sediments. Although comparatively little direct information is available on the environment below these salt bodies, it is reasonable to suppose that formation of dense brines by salt dissolution at the base of these sheets may induce free thermohaline pore fluid convection within the sediments below. From geologically realistic numerical simulations we show that free convection 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 0.01 mD. Calculated maximum Darcy fluxes and salt dissolution rates rapidly increase with sediment permeability. When the vertical permeability of the underlying sediment is 0.01 mD, salt is dissolved from the base of the sheet at an average rate of 3-5 m/My. Corresponding fluid velocities over a 10 My period indicate an integrated fluid flux in the underlying sediments of ~ 104 m3/m2. Salt dissolution also may affect the geodynamic evolution of the salt-sediment system. For example, dissolution at the bottom surface of the salt sheet is maximum at the roots of downwelling plumes and minimum where upwelling plumes reach the salt. The salt and the overlying sediments subside to fill up the space previously occupied by dissolved salt. This creates a small depression or basin on the upper surface of the salt sheet above each downwelling plume. The width of the basin is comparable to the width of the underlying groundwater convection cell, and the depth of each basin is proportional to the difference between salt dissolution rates above downwelling and upwelling plumes. Numerical simulations of pore water convection beneath salt sheets indicate that the difference in dissolution rates is about 5 m/My for a vertical permeability of underlying sediment of 0.01 mD. Such dissolution depressions may act as loci of subsequent sediment loading, and operate in concert with salt tectonics to create mini-basins or salt withdrawal basins. Such calculations will provide a basis for: 1) predicting rates of salt dissolution and removal in actual field settings; 2) interpreting patterns of sediment deformation and sedimentation resulting from salt dissolution; 3) determining the potential effects of variable pore water salinity on subsalt seismic response; and 4) predicting the heat, solute transport, and diagenetic regimes which should exist below a dissolving salt sheet.