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Abstract

Abstract: The stratigraphic evolution of the Quaternary mass-transport deposits (MTDs) in the Mensa and Thunder Horse intraslope basins, Mississippi Canyon, northern deep Gulf of Mexico, was interpreted based on based on 378 square miles (970 square km) of 3-D seismic data in water depths ranging from 5300 to 6500 feet (1617 to 1983 m). Seven depositional sequences were defined in the study area between 1.3 Ma to the present. Allochthonous salt systems had bathymetric expression and influenced sediment thickness and location of depositional systems. Six MTDs are present in five of the depositional sequences. MTDs overlie erosional boundaries—up to 30 m of the underlying section has been eroded at the base of the deposits. These deposits consist primarily of chaotic, rotated, and thrusted seismic reflections. They vary in size and areal distribution from elongated to more equidimensional.

The oldest MTD is in sequence 1, overlies the 1.3 Ma condensed section, and underlies a series of five east-trending channels. This MTD has an easterly trend and represents the initial deposition after a major reorganization of the slope system. In the underlying Miocene-lower Pleistocene sequences, channels trended from the northwest to southeast. Sequences 2 and 3 consist of seven additional channels that trend primarily from west to east. The second MTD is present in sequence 3, trends to the southeast, and truncates four channels. A series of stacked condensed sections (ca. 0.6 to 0.08 Ma) form a thin unit and separate sequences 3 and 4. Sequence 4 consists primarily of hemipelagic and overbank deposits. Four MTDs are present in the sequences 5-7. Multiple sets of these deposits have channelized into and stacked on one another. These MTDs appear to have been sourced primarily from the west, similar to the channels in the underlying sequences.

This case study illustrates the many variations in MTDs that are present in the same intraslope setting. These variations can occur in their size, shape, thickness, seismic facies, the amount of erosion at their base, and their timing of formation within different positions of sea level.

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