Computerized balanced cross-section LOCACE to reconstruct an allochthonous salt sheet, offshore Louisiana

Abstract

The evolution of an allochthonous salt sheet underlying the Mississippi fan is quantified using the two-dimensional LOCACE balanced section program. A first set of tests shows that the shape of the principal normal fault associated with the salt sheet is inconsistent with a roll-over geometry when using flexural slip or simple 120° linear shear criteria with a fixed footwall geometry. The next test with corrected fault geometry allows coherent restoration at the early Lower Miocene using the flexural slip criterion. In two-dimensional reconstructions, the area of salt cannot be balanced, i.e. the area available for salt is considerably wider than the actual salt area shown on the initial section. Thus either salt was dissolved or, perhaps more likely, salt moved outside the plane of section calling for a three-dimensional volumetric reconstruction procedure. The final interpretation is geometrically consistent and suggests that since early Lower Miocene time a large amount of salt moved out of the plane of section forming the allochthonous salt masses. An attempt to approximately balance salt volumes in three dimensions suggests that the amount of salt escaping the system is at least 18%.