Abstract: Submarine-Slope Failures and Downslope Mass Movement of Sediment

By

James M. Coleman

Deltaic and continental-shelf deposits off the Mississippi River delta are deformed severely and slumped seaward by many types of submarine-slope failures. The major forms of deformation include (a) peripheral-rotational slumping, (b) differential weighting and diapirism, (c) radial-tensional faulting, (d) mass wasting and flowage induced by wave motion and degassing, (e) mud flows, (f) shelf-edge arcuate slumping, and (g) contemporaneous faulting (growth faults). Rotational slumps are subparallel with offshore contours and cause shallow-water sediments to migrate downslope into deep water. These slump blocks have longitudinal dimensions of ~60-600 m and lateral dimensions of 200 to >600 m; some blocks have moved 800 m downslope in a period of one year. Differential loading by coarse bar sands overlaying weak marine clays results in large-scale diapirism (vertical scale of intrusion, 200 to 700m). Deep-seated flowage of clays from beneath the delta stresses continental-shelf deposits, causing radial-tensional grabens. Prodelta and interdistributary clays contain high percentages of biochemically formed methane gas, and passage of storm waves produces bottom-pressure perturbations which cause entrapped gas to migrate vertically upward with a resulting loss of sediment strength. Weak, shallow-marine clays creep down and plunge over the edge of the continental shelf (mudnoses display heights up to 80 m). Large-scale arcuate slumping scars the shelf break and slump planes cut up to 500 m of sediment. This slump material is of major importance in the development of large-scale growth faults on the upper continental slope. High-resolution seismic lines and side-scan sonar records show the distribution and mechanisms involved.

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