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5 - Breaching the Levee of a Channel on the Mississippi Fan
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- By David C. Twichell, U.S. Geological Survey, Woods Hole, Massachusetts, William C. Schwab, U.S. Geological Survey, Woods Hole, Massachusetts, Neil H. Kenyon, Institute of Oceanographic Sciences, Southampton, United Kingdom, Homa J. Lee, U.S. Geological Survey, Menlo Park, California
- Edited by James V. Gardner, United States Geological Survey, California, Michael E. Field, United States Geological Survey, California, David C. Twichell
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- Book:
- Geology of the United States' Seafloor
- Published online:
- 25 January 2010
- Print publication:
- 13 August 1996, pp 85-96
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Summary
Abstract
GLORIA images of the youngest channel on the Mississippi Fan indicate that it has not been a stable feature, but instead has shifted its course several times. A detailed study of a site of channel shifting found a complex stratigraphy that resulted from one episode of channel avulsion. The channel avulsion appears to have been initiated by a large mass flow that choked the channel below the point where the levee was breached and additionally spilled a large volume of material through the breach in the levee onto the adjacent fan. Subsequent flows were redirected through this breach in the levee and built a channel-levee complex over the mass movement deposits. A second phase of mass movement resulted from another large mass flow that came down the channel and triggered the collapse of part of the newly developed levee. In this case, locally derived levee sediment was mixed with allochthonous sediment from farther up the fan and was spread northward from the levee. This localized study suggests that fan stratigraphy is complex and variable at several scales, not just at the scale resolved in seismic stratigraphic studies (Weimer 1989) and that large mass flows capable of choking the channel system have been an important mechanism in redirecting sedimentation on the Mississippi Fan.
Introduction
Channels on deep-sea fans are conduits through which sediments are transported to the distal part of the fan (see Mutti and Normark 1987 and Shanmugam and Moiola 1991 for summaries).
13 - Ground-Truth Studies of West Coast and Gulf of Mexico Submarine Fans
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- By Homa J. Lee, U.S. Geological Survey, Menlo Park, California, Robert E. Kayen, U.S. Geological Survey, Menlo Park, California, Brian D. Edwards, U.S. Geological Survey, Menlo Park, California, Michael E. Field, U.S. Geological Survey, Menlo Park, California, James V. Gardner, U.S. Geological Survey, Menlo Park, California, William C. Schwab, U.S. Geological Survey, Woods Hole, Massachusetts, David C. Twichell, U.S. Geological Survey, Woods Hole, Massachusetts
- Edited by James V. Gardner, United States Geological Survey, California, Michael E. Field, United States Geological Survey, California, David C. Twichell
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- Book:
- Geology of the United States' Seafloor
- Published online:
- 25 January 2010
- Print publication:
- 13 August 1996, pp 221-234
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- Chapter
- Export citation
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Summary
Introduction
The use of sidescan sonar technology has greatly expanded in recent years. One impediment to interpreting sidescan sonar images, which are a representation of the amount of sound backscattered from the seafloor, is the incomplete understanding of the physical meaning of acoustic backscatter intensity variations. Ground-truth studies can help us to understand the causes of variations in backscatter. We need to measure physical and geometric properties of seafloor sediment and correlate them with variations in sidescan sonar acoustic backscatter. We present in this paper comparative ground-truth studies of two deep-sea fan depositional lobes. We show that sediment lithology influences sidescan sonar images, but that the relation between backscatter intensity and sediment grain size is not uniquely defined.
Some of the seafloor characteristics that are potential causes of variations in acoustic backscatter intensity are surface roughness, variations in sediment composition, grazing angle of insonification, and seafloor slope, including topographic variability (Urick 1983). The influence of each of these and the subbottom depth range over which sediment compositional variations are important will vary with the characteristics of the sidescan system, including frequency, pulse length, bandwidth, time-varying gains, and footprint size. The number of variables needs to be kept to a minimum in order to simplify a ground-truth study. The distal parts of deep-sea fans are good locations for such studies because they tend to have nearly horizontal seafloor surfaces. Thus, the topographic variability – bottom slope effect can be ignored.