Sedimentation in Mississippi Trough, Gulf of Mexico: ABSTRACT

Abstract

Interpretation of high-resolution seismic data indicates that the Mississippi Trough was eroded, and then partially filled, by submarine gravity flows. These flows occurred primarily during late Quaternary stages of lowered sea level. Present-day mass transport appears to be a combination of seafloor creep and low-velocity turbid-layer flow. An acoustically chaotic seismic facies characterizes the deeper parts of the trough fill. Visible reflectors are discontinuous, wavy, and subparallel; they commonly disappear into almost reflectorless, seismically homogeneous units. Sediments deposited lateral to the chaotic facies are commonly composed of continuous, parallel reflectors. These strong reflecting units were deposited as onlapping fill and ponded sediments, and many are confined to semienclosed depressions within the trough walls. These various seismic-facies units represent the freezing-in stage of submarine-canyon sedimentation, and may result from separate submarine debris flows. The deposits of the Mississippi Trough debris flows are fine-grained sediments generated by mass failures of oversteepened deposits which occurred at the mouth of the ancestral Mississippi River. High-energy gravity flows thoroughly mixed the depositional material, leaving little bedding to produce coherent seismic-reflector patterns. Debris flows generated lower velocity gravity flows, which moved independently and lateral to the main flows. The onlapping fill deposits and sediment ponding were deposited by lower velocity flows. Recent sediments, which were sampled by piston coring along the axis of the trough, are rapidly deposited, hemipelagic, olive-gray silty clays. Sedimentary structures are limited to scattered, very thin laminae and thin beds. Clay sedimentation has been continuous during the late Quaternary, as is revealed by the clays containing mixtures of indigenous planktonic Foraminifera, displaced shallow-water microfauna, and terrigenous mineral grains. Large diapirs controlled the position and flow direction of the main erosional channel. Small feeder channels were eroded into the walls of the trough. Trough-wall sediments cover a steep erosional escarpment; they have moved downslope by slump and creep failure. An isopach map of the canyon fill, above erosional surfaces, outlines a linear channel-fill deposit. End_of_Article - Last_Page 449------------