McHUGH, CECILIA M., and WILLIAM B. F. RYAN, Lamont-Doherty Geological Observatory, and Department of Geological Sciences, Columbia University, Palisades, NY

ABSTRACT: Sedimentary Features Reveal Mass-Wasting and Gravity Flow Processes on the Monterey

Channelized and overbank flow of turbidity currents sweeping through the entrenched Monterey deep-sea channel have generated erosional and depositional features along its walls, floors, and across adjacent levees. These features, investigated with side-looking sonar, multibeam bathymetry, submersibles and a towed camera sled, include fields of sediment waves, overbank channels, gullies, terraces, slump scars, mega-ripples, scours, and a meandering thalweg channel. Overbank flow-stripping occurs where the channel path is straight, but is accentuated along sharp channel bends. Megawaves populate the outside levee as far as 15 km from the channel. Their crests, up to 12 km long, trend sub-parallel to the channel; their wavelengths reach 3 km. Commonly a gully is present on the lee side o each megawave near the mid-point of its crest. Gullies are heads of a tributary system of overbank channels which feed a trunk on the fan surface. Overbank flow contributes to the exhumation of partly buried channels abandoned by changes in the path of the Monterey channel. Overbank flow-stripping is extensive where the downstream gradient and relief of the channel walls and floors abruptly decrease in the canyon/fan transition zone. Here, mega-ripples and scour features are found on the channel floor.

Within the confines of the channel, deflection of turbidity current flows leads to the preferential erosion of one channel wall over the other, a thalweg channel that hugs the outside wail of bends, development of terraces on the inside of bends, and occurrence of large slumps near bends with talus deposition on the channel floor beneath. Terrace levels are continuous for tens of kilometers showing distinct episodes of preferential channel entrenchment at the outside of bends.

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