The active Sigsbee allochthon is a wedge-shaped mass of Jurassic salt displaced basinward 50-100 miles over Miocene to Holocene sediments. The allochthon thickens upslope from the base of the slope to a midslope position, where it may exceed 20,000 ft. in thickness. Recent drilling of wells through allochthonous salt on the shelf suggests a regional relationship of this salt to the active Sigsbee allochthon. The model suggested by this relationship predicts a continuous evolution and progradation of a salt allochthon from the late Miocene to the present, with three stages of development: (1) active inflation and thrusting of the salt wedge; (2) active deflation and extension of the salt wedge (diapiric cannibalization of allochthon); and (3) deflation of the salt wedge (buoyancy-driven diapirism). Because the entire salt allochthon has continuously prograded into the basin, these three stages also correspond, respectively, to the general stages of development of the salt allochthon of the present lower slope, upper slope, and other shelf. The evolution of the salt allochthon has been the major control on the paleobathymetric and accommodation history of the shelf edge and slope.

The overall geometry of sediments deformed by the allochthon is a regional syndepositional slump. To a large degree, the modern Sigsbee allochthon and its predecessors controlled the spatial distribution of facies across the shelf edge and slope from the late Miocene to the Holocene. The sediments associated with the evolving allochthon can be divided into gross tectonostratigraphic units, which aids prediction of paleoenvironments and facies of sediment deposited across the slope and basin floor.

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