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A continuous, meandering, leveed channel traverses the Mississippi fan from the continental slope to the abyssal plain. Using water-gun seismic reflection profiling, 3.5- and 4.5-kHz profiling, and SeaMARC I side-scan sonar, we surveyed a 30-km (16-nmi) long channel segment midway between the slope break and channel terminus, where the channel bends through a tight meander with a 2.8-km (1.5-nmi) radius of curvature. At the entrance to each meander bend, the outer levee is unusually low, similar to the crevasses observed in rivers. The levees are constructed of an acoustically opaque unit and draped with an acoustically laminated unit; these are interpreted as coarser and finer grained overbank deposits, respectively.
A series of high-amplitude seismic reflectors underlying the channel axis are interpreted as coarse sediments deposited from the base of turbidity currents. When last active, the channel was more than 100 m (300 ft) deep, but it has been filled to the brim by acoustically transparent units, leaving a levee crest/thalweg relief of as little as 5 m (16 ft). These channel-filling units are interpreted as debris flows. The upper surface of the debris flows is sculpted by flowline-parallel side-scan lineations where the flow was unimpeded and by arcuate ridges transverse to the flow where bathymetric obstacles constrained the flow.
We infer a three-phase depositional history. During the Wisconsinian lowstand, the channel/levee complex was built by simultaneous turbidity current deposition of coarse sediment in the channel base and finer sediment on the levees. The meandering planform and crevasses at meander bends developed at this time. Then, as sea level rose, the volume and grain size of the sediment supply decreased. Overbank deposition became less frequent, finer grained, and thinner bedded. Finally, two or more debris flows moved downchannel, clogged the channel, and prevented any further development as a conduit for sediment transport.
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