HR: 16:45h
AN: H54B-04    [Abstracts]
TI: Characterizing Turbidity Current Flow Conditions From Turbidite Grain Size Distributions, Capistrano Formation, San Clemente, CA
AU: * Huntington, K W
EM: kateh@gps.caltech.edu
AF: California Institute of Technology, Division of Earth and Planetary Sciences, Pasadena, CA 91125, United States
AU: Mohrig, D
EM: mohrig@mail.utexas.edu
AF: The University of Texas at Austin, Department of Geological Sciences, Jackson School of Geosciences, Austin, TX 78712, United States
AB: Turbidity currents exert fundamental controls on the evolution of continental margins, yet the physical properties of these flows remain poorly constrained because they are difficult to observe directly in nature or model accurately in experiments. We develop a method to characterize turbidity current flow conditions using grain size and morphologic observations of single-event deposits from the Miocene Capistrano Formation near San Clemente, CA. Observed deposit styles include beds that onlap channel walls indicating time transgressive deposition from a basally charged flow and drape morphologies indicating deposition from flows that concentrate sediment above preexisting topography. While spatial variations in particle size within turbidite channel cross sections exhibiting onlap morphologies provide flow-condition information at a single point through time, particle size variations in drape deposits provide instantaneous vertical flow-structure constraints. We interpret the distribution of fines that does not vary with elevation in the deposits (D<300 microns) to represent the fraction that was fully suspended at some up-dip location and remained distributed throughout the current at the deposition site. Upward fining of the coarse tail of the distributions in an onlap deposit indicates progressive removal of the coarsest particles in suspension from a decelerating current through time. A simple sediment transport model produces vertical sediment concentration profiles as a function of flow conditions that are consistent with deposit grain-size variation. Modeled profiles indicate current velocities of up to 8.4 m/s and deposition times of minutes to hours, and place constraints on the down-channel spatial evolution of the flows. Landscapes are shaped by the net effect of many erosional and depositional events that occur at varying frequencies and magnitudes. Application of this approach to a large number of turbidites would represent a first step towards characterizing the population of possible flow conditions in order to understand how turbidity currents and submarine topography interact.
DE: 1861 Sedimentation (4863)
DE: 1862 Sediment transport (4558)
DE: 3022 Marine sediments: processes and transport
DE: 4219 Continental shelf and slope processes (3002)
DE: 4558 Sediment transport (1862)
SC: Hydrology [H]
MN: 2007 Fall Meeting