ABSTRACT

Features observed in and around Mississippi Canyon Block 798 (MC798) imply a diverse geology and stress regime. This character is illustrated by a seismic dip section from northwest to southeast that starts at the continental shelf break, crosses the upper continental slope and an intra-slope basin, and proceeds down the western flank of Mississippi Canyon to the canyon's thalweg. The boundaries of the intra-slope basin are defined by two diapiric salt structures.

Numerous near-surface, faulted blocks are observed on seismic profiles obtained in the vicinity of the intra-slope basin using various energy sources: boomer, chirp, and G.I. gun. Features that appear to be mud diapirs are located within the basin, and cluster near its southeastern boundary. One of these features has been documented to contain gas hydrates. Heat-flow was measured in this basin. One value, obtained from the base of a diapir, is 10 times the value obtained at the diapir's top. The high value suggests that warm fluids are flowing from an open fault that extends possibly as deep as the Louann Salt. Features observed on nearby profiles appear to be fluid expulsion pipes as much as a mile (1.5 km) in diameter.

Heat-flow data indicate that the sea floor in MC798 lies within the hydrate stability zone and that the zone may be 1600 ft (500 m) thick. Sediments near the sea floor are characterized by relatively weak, but interpretable, seismic reflections. The weak reflectivity suggests cementation by gas hydrates. In one region, shallow sediments are underlain by chaotic, sharply stronger reflectivity typical of free gas.

Boomer and chirp profiles image vertically oriented structures within the surficial sediments. It has been suggested that these structures mark pathways of small-scale fluid expulsion, the fluid possibly being produced by dissociation of gas hydrates. The pathways may coincide with sites of weakness produced by gravity-induced tension.

The tectonic character of the area is complex and dynamic. Overall areal motions are gravity-driven subsidence with evidence of corresponding high local thermal gradients, buoyant salt, mud diapirism and fluid expulsion. The shallow, regional geology thus provides detailed input for margin-spanning geologic models of the evolution of the area. These regional models, qualitative at present, can become quantitative as more and more detailed studies are input into a regional synthesis.