--> Abstract: Large Kilometer-Scale Erosional and Depositional Bedforms as a Result of Turbidity-Current Overflow from the Monterey Channel, by William Normark, Andrea Fildani, Svetlana Kostic, and Gary Parker; #90039 (2005)

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Large Kilometer-Scale Erosional and Depositional Bedforms as a Result of Turbidity-Current Overflow from the Monterey Channel

William Normark1, Andrea Fildani2, Svetlana Kostic3, and Gary Parker4
1 United States Geological Survey, Menlo Park, CA
2 ChevronTexaco, San Ramon, CA
3 National Center of Earth Surface Dynamics, Minneapolis, MN
4 University of Minnesota, Minneapolis, MN

Overbank deposits along the upper Monterey fan valley exhibit large-scale sediment waves and erosional scours in multibeam bathymetric and seismic-reflection profiles. These features are best developed south of the incised Shepard Meander, where the Monterey East deposit occupies an area, 30 km long, which lies 170 m above the floor of the modern Monterey channel. Within the deposit, there is no continuous channel axis, but rather a channel-like feature formed by a series of deep scours (steps) that are 2.5 to 4.5 km wide, 2 to 5 km long, and as much as 120 m deep. The scours are aligned through the center of an arcuate sediment-wave field. Radiocarbon dating of sediment cores indicates flow-stripping of turbidity currents that are at least 200-m-thick in the main Monterey channel, even during the late Holocene. The series of scour-shaped depressions and sediment waves of the Monterey East deposit can be explained as upstream-migrating cyclic steps locked in sequence by hydraulic jumps. Numerical simulations of flows stripped from the tops of currents passing through the Shepard Meander reveal that pool-like scours are net erosional, whereas sediment waves are net depositional features. According to these simulations, the characteristics and number of scours and sediment waves depend highly upon the characteristic size of sediment carried by the turbidity currents, the inflow velocity, depth and concentration of stripped overflows, as well as the ability of the bed sediment to resist erosion.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005