Deep-water, salt-cored foldbelts: Lessons from the Mississippi Fan and Perdido foldbelts, northern Gulf of Mexico

Abstract

The Mississippi Fan and Perdido foldbelts are Tertiary deep-water contractional provinces located at the basinward limit of the Jurassic Louann Salt in the northern Gulf of Mexico. Both formed at different times in response to updip sedimentary loading and extension during gravity spreading of the continental margin above the salt layer. The structural styles are characteristic of salt-cored foldbelts, consisting of linear, generally symmetric detachment folds with regular wavelengths and relatively minor, high-angle reverse faults. However, there are significant differences between the two foldbelts: (1) fold wavelengths, amplitudes, and segment lengths are all greater in the Perdido foldbelt; (2) the Mississippi Fan foldbelt is more asymmetric (basinward-vergent); (3) fold geometries are more rounded in the Mississippi Fan foldbelt and more angular in the Perdido foldbelt; (4) the fold envelope is tilted basinward in the Perdido foldbelt but is subhorizontal in the Mississippi Fan foldbelt; (5) domal structures are found in the Perdido foldbelt; and (6) antecedent folds, highly-rotated fault blocks, and deeply-rooted diapirs are present in the Mississippi Fan foldbelt but absent in the Perdido foldbelt. Variations in the thickness and mechanical stratigraphy of the folded multilayers explain the wavelengths and fold geometries. The asymmetry, fold-envelope orientation, and occurrence of domal structures are ascribed to the facies and original thickness distribution of the evaporite layer, which in turn was controlled by the underlying rift-basin geometry. Finally, preexisting salt structures from an early phase of deformation account for the short fold-segment lengths, the rotated fault blocks, and the presence of diapirs in the Mississippi Fan foldbelt.