Numerical analysis of how sedimentation and redistribution of surficial sediments affects salt diapirism

Abstract

Two-dimensional finite-element models are used to study how sedimentation and redistribution of sediments on the upper surface affects the development of subsurface salt diapirs. A rising diapir creates a bulge flanked by topographic lows in a generally accumulating sedimentary pile. We find that the rate at which this topography is flattened by erosion and redeposition controls the style of diapirism. This is because the redistribution of material from topographic highs to flanking lows is equivalent to changing the effective forces acting on the salt. Redistributing a potential topography modulates diapiric growth rate. The main effects of including surficial sediment redistribution in numerical models of diapirism are: 1. (1) diapirs grow 10–100 times faster; 2. (2) diapirs may rise above their level of neutral buoyancy and extrude; 3. (3) diapirs assume “finger” or “stock” like shapes rather than “mushroom” or balloon-on-string shapes; 4. (4) layers in the surrounding sediments remain nearly horizontal and only steepen sharply near the diapir. In effect, the rate of redistribution of surficial overburden strongly controls the mode of diapirism. Sediment redistribution (referred to as erosion for brevity) is modeled using a one dimensional diffusion equation. We show the results of two different erosion rates: infinitely slow (no erosion) and extremely fast (which redistributes surficial sediments but does not remove them from the system). We show that the shapes of model diapirs rising beneath surfaces subjected to rapid erosion simulate salt diapirs in the Gulf of Mexico. Columnar diapirs indicate rapid deposition on the shelf and plug-like diapirs slow sedimentation on the abyssal plane. Diapirs rising beneath surfaces with negligible erosion have the “mushroom” shapes interpreted for salt diapirs in central Iran.