TECTONOSEDIMENTARY PROCESS INTERACTION ALONG PASSIVE MARGINS WITH SALT

The interaction between sedimentation and salt tectonics along passive margins such as the northern Gulf of Mexico and offshore west Africa occurs at a variety of scales. At the basin scale, the deposited sediment load is the major factor driving gravitational failure of the margin, which is accommodated by linked extension and contraction above the salt décollement. Gravity gliding and spreading are facilitated by a basinward tilt of the detachment but are controlled more by the bathymetric slope. Shelf aggradation and upper-slope progradation increase the gravitational instability and drive basinward translation of the passive-margin sequence, whereas shelf-margin canyon incision and lower-slope and abyssal bypass sedimentation decrease the instability and retard or halt margin failure.

At an intermediate scale, salt withdrawal and diapirism result in sea-floor topography that largely controls sediment transport and deposition. Evolving geometries of salt bodies and intervening minibasins set up shifting patterns of sediment fairways, barriers, and depocenters and strongly influence facies distribution within the minibasins. Different styles of diapirs have varying effects: those that grow passively due to sediment loading form local bathymetric highs; those that accommodate contraction by lateral squeezing of the diapir flanks form broader, higher bathymetric features; and those that accommodate extension form local bathymetric lows.

At a more detailed scale, the interplay between salt-flow rates and sedimentation rates controls the growth of individual diapirs. The rate of salt movement is determined primarily by the existing net differential load between the subsiding minibasin and the diapir overburden and thus changes only gradually, whereas the sedimentation rate varies with higher frequency due to both allogenic and autogenic processes. Thus, the bathymetric relief decreases and increases cyclically as the sedimentation rate exceeds or lags behind the inflation of the salt high. Maximum relief is attained during times of slowest sedimentation and leads to slumping and erosion of the bathymetric scarps and break-out and further extrusion of the salt.