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

ABSTRACT: Upper Pleistocene Stratigraphy and Sedimentation on the Distal Mississippi Fan

SeaMARC sidescan-sonar images and cores from the distal Mississippi Fan define the types of deposits found at the ends of channels on a large mud-rich fan. In sonar images a slightly sinuous channel was traced 120

km across the distal fan to where it splays into several smaller channels (less than 50 m wide and 2 m deep) that terminate in areas having high acoustic backscatter. The high-backscatter areas have abrupt rather than gradational edges. Crosscutting relations of the various channels and high-backscatter areas suggest that this area is divided into many smaller depositional areas.

Cores revealed three principal upper Pleistocene sedimentary facies: (1) gray mud; (2) graded sand (7-32 cm thick) with fine sand at the base of the beds; and (3) chaotic silt (10-118 cm thick) composed of sandy silt containing clay clasts. The distribution of the two coarser facies coincides with the areas of high backscatter on the SeaMARC images whereas the muds coincide with areas of low backscatter.

The small channels at the distal edge of the fan indicate channelized transport, and the abrupt edges of the high-backscatter deposits imply an abrupt transition from transport to deposition. The graded sand and chaotic silt beds suggest that both turbulent and nonturbulent mass flows contributed to sedimentation along the distal edge of this fan. The resultant deposits are a series of discontinuous lenses of sand separated by muds; the deposits are not a continuous sheet of sand like those inferred from ancient fans.

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