A. Sarkar et al., FREE THERMOHALINE CONVECTION BENEATH ALLOCHTHONOUS SALT SHEETS - AN AGENT FOR SALT DISSOLUTION AND FLUID-FLOW IN GULF-COAST SEDIMENTS, J GEO R-SOL, 100(B9), 1995, pp. 18085-18092
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.