The role of salt in gravitational failure of passive margins

Rowan, M. G., Rowan Consulting, Boulder, CO; B. C. Vendeville, Bureau of Economic Geology, University of Texas at Austin, Austin, TX; and F. J. Peel, BHP Petroleum (Americas), Houston, TX

Abstract

Passive margins with salt basins typically undergo gravitational failure above the salt detachment. The deep water, distal provinces are dominated by contractional tectonics that balances proximal extension and downdip translation of the overburden. Failure is driven by a combination of gravity gliding above a basinward-dipping detachment and gravity spreading of a sedimentary wedge having a seaward-dipping bathymetric surface. Continued deformation is driven primarily by shelf and upper slope sedimentation, which maintains the bathymetric slope and the resulting gravity potential, and by increased basinward tilting. Deformation is retarded or halted by distal thickening of the overburden, caused by the folding itself or by lower slope and abyssal sedimentation. Differences in deep water deformation along various passive margins, such as the northern Gulf of Mexico or offshore west Africa, can be explained in part by differences in sedimentation, loading subsidence, thermal subsidence, and cratonic uplift.

Salt not only provides the detachment for gravitational failure of the margin, but it also can reduce the gravity potential in other ways. The bathymetric slope can be decreased by proximal subsidence into salt and distal inflation of salt. The inflated salt, as well as existing diapirs or salt walls, can be squeezed laterally, thereby accommodating significant shortening. This, in turn, drives further diapirism and/or lateral salt extrusion. Extruded salt may amalgamate to form extensive salt canopies, so that further gravitational failure may take place largely on shallow, allochthonous detachments rather than on the autochthonous salt level.