--> Abstract: Late Pleistocene Sedimentation on the Distal Mississippi Fan: Mass Transport Mechanisms, by W. C. Schwab, D. C. Twichell, H. J. Lee, C. H. Nelson, and N. H. Kenyon; #91012 (1992).

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ABSTRACT: Late Pleistocene Sedimentation on the Distal Mississippi Fan: Mass Transport Mechanisms

SCHWAB, W. C., and D. C. TWICHELL, U.S. Geological Survey, Woods Hole, MA, H. J. LEE and C. H. NELSON, U.S. Geological Survey, Menlo Park, CA, and N. H. KENYON, Institute of Oceanographic Sciences, Wormley, Surrey, U.K.

GLORIA and SeaMARC 1 sidescan-sonar images and cores of the distal end of the Mississippi Fan show that channelized mass transport

was the dominant mechanism of transport of silty and sandy sediment on this deep-sea fan. These mass flow sediments were rapidly deposited once outside of channels, which are <300 m wide and <2 m deep on the distal reaches of the fan. The mass flow units sampled are graded sand and chaotic silt deposits. Graded sand units are up to 30 cm thick, composed of silty sand to sandy silt, and are thought to represent deposition from a turbulent suspension. Chaotic silt deposits are up to 115 cm thick, are composed of a nongraded to poorly graded sandy silt matrix, contain clasts of clayey silt and silty clay up to 5 cm in diameter, and typically contain wood fragments. Because the relatively soft mud clasts within the chaotic silt beds could not have withstood a turbulent transport mec anism without disintegrating, it is thought that the chaotic silt beds were emplaced by generally nonturbulent debris flows. Lacking alternative triggering mechanisms, it is thought that both turbidity currents and debris flows originated from slope failure in the shelf/upper slope water depths of the Mississippi margin. However, transportation of these relatively thin debris flow beds approximately 500 km across sea floor gradients of as low as 1 m/km defies mechanical theory.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)