SCHWAB, W.C., and D. C. TWICHELL, U.S. Geological Survey, Quissett Campus, 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, UK

ABSTRACT: Upper Pleistocene Mass Flow Deposits on the Distal Mississippi Fan: Mass Transport Mechanisms

SeaMARC 1A sidescan-sonar images and cores of the distal reaches of the Mississippi Fan show that channelized mass transport was the dominant mechanism of transport of silty and sandy sediment. These 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 comprise two distinct types of deposits, graded sand and chaotic silt. Graded sand units are up to 32 cm thick, composed of silty sand to sandy silt, and are thought to represent deposition from a turbulent suspension (turbidity currents). Chaotic silt deposits are up to 118 cm thick, composed of a non-graded to poorly graded sandy silt matrix, contain clasts of clayey silt and silty clay up to 5 cm in diameter, and typically contain wo d and plant fragments. As the relatively soft mud clasts within the chaotic silt beds could not have withstood a turbulent transport mechanism without disintegrating, it is thought that the chaotic silt beds were emplaced by non-turbulent debris flows. Lacking alternative triggering mechanisms, it is thought that both turbidity currents and debris flows were caused by slope failure in the shelf/upper slope water depths of the Mississippi margin. However, transportation of these relatively thin debris flow across approximately 500 km of sea-floor with gradients as low as 1 m/km defies mechanical explanation; the flowing material must develop and maintain some critical level of excess pore-water pressure for the duration of the flow.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.