Flip-Flop Salt Tectonics

Quirk, Dave, Robin Pilcher, Amerada Hess Corporation, Houston, TX

A common observation in areas of extensional salt tectonics is that overburden strata flanking a dome, wall or vertical weld are distinctly asymmetric.

In the simplest examples (salt rollers), strata tilt down and thicken towards the dome, in contrast to strata on the other side which tilt up and thin or pinch-out towards it, analogous to syn-rift strata in tilted fault blocks. The strata on the up-tilted (footwall) side are concor­dant with the top of the underlying salt whereas the contact between the down-tilted (hang­ingwall) side is actually a growth fault which extends into the overburden at the highest point of the salt structure.

In many petroleum basins, salt walls show the same asymmetric geometries at specific stratigraphic levels but change in polarity over time. In other words, what was the hanging­wall side at the onset of structuration flips to become the footwall higher up in the section, often reverting back to being the hangingwall at an even younger level. The inverse is true on the opposite side of the wall. Later, if the area is affected by contraction, the wall may squeeze to form a weld but the flip-flop geometry is preserved.

These structures are formed by a process known as extensional footwall doming which is an efficient way of moving large volumes of viscous fluid upwards through the crust whilst accommodating thick-skinned or thin-skinned extension. The process profoundly affects the location of reservoirs and traps, the migration of hydrocarbons and the effective­ness of top seal.