Deformation And Origin Of The Sigsbee Scarp-Lower Continental Slope, Northern Gulf Of Mexico

Abstract

ABSTRACT Multichannel seismic data from the Texas Louisiana lower continental slope between Alaminos Canyon and De Soto Canyon (94.5°W to 87.5°W) provide additional new data for interpreting the salt tectonics and geologic history of the area. This entire lower slope is underlain by large masses of salt which are being deformed by gravity forces as well as by the loading effect of thick Pliocene-Pleistocene depocenters along the upper slope and the Mississippi Fan. The terrace-like lower slope between 94.5°W and 91°W along the southward bulge of the Sigsbee Scarp is underlain by large masses of salt, the top of which is relatively flat with poorly developed salt structures. This lack of salt structures is probably related to a thin sedimentary section overlying the salt mass. The Sigsbee Scarp in this area is underlain by tongues of high-velocity, mobile material (salt or salt mixtures) that are thrust basinward up to 12 km over the flat-lying Pleistocene and older continental rise sediments. The overall southward bulge of the lower slope suggests a lateral and downslope component of salt movement. It is still not clear from the seismic data, however, whether the salt masses represent 1) salt in place pushed up and thickened by additional salt squeezed from beneath the advancing clastic wedge or 2) large allochthonous lobes of salt thrust into and over younger sediments for distances of 50 to 75 km. Data from the central lower slope and its extension beneath the Mississippi Fan suggest salt movement here is mainly vertical in nature. The Sigsbee Scarp is formed by the seaward most of a series of salt ridges that have uplifted and deformed Pleistocene sediments. The Scarp continues eastward beneath the Mississippi Fan but loses its topographic expression and is buried by the Fan sediments. The better development of salt structures on top of the salt in this area probably is due to the increased loading effect of a thicker sedimentary cover. The vertical nature of the salt movement and the possible involvement of older strata in the salt deformation suggest the salt is moving upward from an original salt basin deep below the lower slope and has not flowed laterally for any significant distance. Beneath the upper Mississippi Fan, anticlinal structures are cored at relatively shallow depths with lobate wedges of high-velocity, mobile material, probably salt or salt mixtures. The wedges appear to be thrust over underlying sediments and in some cases appear to be almost completely detached. These wedges apparently represent some type of downslope movement of mobile material, but the exact mechanism is not yet clear. Salt deformation along the entire lower slope is quite young and is probably continuing today. A deep salt structure below the Mississippi Fan, however, suggests a period of earlier salt movement (possibly late Tertiary-early Pleistocene.