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Internal Deformation of Layered Evaporites: Flow Profiles and Deformation Styles

M. P. A. Jackson and Timothy Dooley
Bureau of Economic Geology, The University of Texas at Austin, University Station, Box X, Austin, TX 78713

Continual improvements in seismic acquisition and processing reveal internal layering within many salt sequences. Layering makes it more problematic to pick top and base of salt, which can result in a flawed velocity model. Physical models illustrate deformation within an evaporite sequence having internal layers and both thin and thick prekinematic roofs. The models were driven by gravity acting on a gently tilted sequence. The models show that strain in evaporites can be far higher than in the overburden without the need for a roof thrust along the top of salt. The apparently simple process of updip salt deflation and downdip inflation requires not only lateral flow but also large and variable strains within the salt sequence. Where the roof was very thin or absent, Couette flow dominated. The base of salt was anchored by frictional drag, so the free surface at the top of salt advanced faster than the base. In contrast, where the roof was thick, asymmetric Poiseuille flow dominated because of boundary drag against the roof. Beneath a thick roof vertical variations in downdip flow and strain produced highly disharmonic folding. Within the salt sequence, more-mobile layers preferentially thinned in the extensional zone and thickened in the contractional zone. The intervening less-mobile layers thinned homogeneously, which caused extension to be greatly underestimated. Vertical variations in strain are caused by mobility differences between individual layers and by the boundary effects of the roof and floor.

 

AAPG Search and Discovery Article #90158©2012 GCAGS and GC-SEPM 6nd Annual Convention, Austin, Texas, 21-24 October 2012