The Great Pliocene Salt Squirt - Mechanics of Folding Along the Down-Dip Limit of Salt, Gulf of Mexico

J. K. Snow
Exploration, ENI Petroleum, Houston, TX

Seismic interpretation across the frontal fold belt was used to sequentially reconstruct 14 horizons from Present to end Cretaceous. The top, base, and pinchout of both shallow and deep allochthonous salt were well imaged. Paleobathymetry was reconstructed similar to present seafloor gradients. Cover shortening and deep-salt area, recorded for each time step, show salt flow into the frontal anticline exceeded shortening during Miocene folding. During Pliocene folding, expulsion of deep salt exceeded shortening. Similar effects are observed in other analogues.

The geometry of buckle folding above a mobile substrate requires that material initially moved into the fold core is expelled during continued shortening. This can be explained by two mechanical drivers: 1) horizontal buckling of cover forces salt into the fold core; or 2) excess halostatic fluid pressure forces vertical flexure of cover. Geodynamic models for a stiff isostatically supported layer predict a large difference in flexural wavelength for vertical or horizontal loading.

The observed short wavelength folds (~ 10 km) can be modeled as horizontal buckles nucleated in early Paleogene time and controlled by the elastic thickness of Cretaceous strata. The observed long wavelength Miocene fold (~50 km) cannot be similarly modeled. A simple model of continuous plate flexure resulting from excess halostatic pressure effectively predicts the observed wavelength. The halostatic pressure model further predicts that deep salt will rise to an isostatically compensated elevation well above regional when released through emergent salt diapirs. Reconstruction shows more than a kilometer increase in bathymetric relief associated with emplacement of shallow salt. Extensive, nearly concordant contacts observed below shallow salt imply almost catastrophic release of excess halostatic pressure in the Pliocene.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas