Salt-Sediment Interaction During Diapir Growth

Mark G. Rowan, Rowan Consulting, Inc., Boulder, CO

Most salt diapirs spend much of their history growing at or near the sea floor or ground surface, thereby creating topographic relief over the diapirs. This relief not only influences sediment transport and deposition adjacent to the diapirs, it also controls the degree of deformation of flanking strata. In this talk, I present the results of collaborative research, carried out with Kate Giles and Tim Lawton of New Mexico State University, investigating exposed diapirs in La Popa basin, Mexico, and subsurface analogs observed on seismic data.

Deformation is caused by near-surface drape folding of a thin, onlapping stratal wedge, not by drag folding or shear during diapir rise. The interplay between gradually varying salt-rise rates and episodic sedimentation results in fluctuating topography, with both the vertical relief and the width of the topographic halo extending beyond the diapir edge affected. During times of relatively slow sedimentation, relief builds up until the topographic scarp fails, truncating strata and generating local debris flows. Faster sedimentation then onlaps the eroded scarp and decreases the relief until the cycle is repeated.

The interplay between salt-rise rates and sedimentation rates controls the degree of bed rotation, the width of the deformation halo, the severity of angular truncation, and the amount of stratal thinning. If there is a long period of rapid sedimentation before deposition slows down, a broad, thick wedge (or halokinetic sequence) develops with significant thinning, overturned basal strata, and an upper unconformity that is highly angular adjacent to the diapir. In contrast, rapid diapir rise coupled with overall slow sedimentation results in stacked halokinetic sequences that are thin and narrow. If there is no stratigraphic cover draped over the diapir edge, for example in subaerial environments, there is no folding and strata simply abut the diapir with no thinning or deformation. Finally, radial faults are more common where there is both significant folding and a high degree of plan-view curvature of the diapir-sediment interface.