Salt-Related Accommodation in the Gulf of Mexico Deepwater:

Withdrawal or Inflation, Autochthonous or Allochthonous?

By

ROWAN, MARK G.

Rowan Consulting, Inc., Boulder, CO

Salt-controlled bathymetric relief provides accommodation for the deposition of reservoir sands in slope minibasins of the northern Gulf of Mexico. But is minibasin-flank relief controlled more by underlying salt withdrawal or surrounding salt inflation? And in the case of withdrawal, is it primarily from allochthonous or autochthonous salt?

Although salt withdrawal is certainly a major component in creating accommodation, salt inflation can be more important in many areas. The most obvious, but by no means unique, example is the Sigsbee Escarpment, where there can be over 1 km of relief that is entirely due to inflation of allochthonous salt. Inflation of shallow salt is caused not just by vertical loading of the source layer, but also by lateral loading of the shallow salt itself during shortening. Salt inflation is most common above the basinward portions of linked allochthonous detachment systems, where contraction is a dominant process.

The majority of slope minibasins west of Mississippi Canyon have traditionally been interpreted as forming due to evacuation of allochthonous salt, but a recent model (Hall, 2000) suggests instead that autochthonous salt deflation is largely responsible for thick accumulations of upper Miocene to Pleistocene sediments in areas such as eastern Garden Banks. While early inflation and late deflation of the Louann salt are certainly common processes, they are not as widespread as this model implies. Many of the minibasins interpreted as primary are in fact floored by allochthonous welds, and subsalt geometries show that the minibasins formed by withdrawal of allochthonous, not autochthonous, salt.