Free thermohaline convection beneath allochthonous salt sheets: An agent for salt dissolution and fluid flow in Gulf Coast sediments

Abstract

Basinward migration of Jurassic salt in the U.S. 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 the formation of dense brines by dissolution of the base of these sheets may induce free thermohaline pore fluid convection within the sediments below. We derived equations which make it possible to quantitatively estimate rates of dissolution of these subsurface salt structures. From these calculations and by geologically realistic numerical simulations it can be shown 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 10−17 m2 (0.01 mD). The calculated maximum Darcy fluxes and rates of salt dissolution rapidly increase with sediment permeability. When the vertical permeability of the underlying sediment is 10−17 m2 (0.01 mD), 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 ∼104 m3 m−2. By comparison, integrated fluid flux for compactive expulsion is <103 m3 m−2. Thus, for the offshore sediments of the Gulf of Mexico, thermohaline convection beneath an allochthonous salt sheet is a significant driving mechanism for fluid flow with potentially important implications for heat and mass transport, diagenesis, and salt tectonics.