HR: 1330h
AN: H52A-1150    [PDF]
TI: Constraining Depositional Slope From Sedimentary Structures in Sandy Braided Streams
AU: * Lynds, R M
EM: lynds@uwyo.edu
AF: University of Wyoming, Department of Geology and Geophysics, PO Box 3006, Laramie, WY 82071 United States
AU: Mohrig, D
EM: mohrig@mit.edu
AF: Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Sciences, Bldg. 54-814, 77 Massachusetts Avenue, Cambridge, MA 02139 United States
AU: Heller, P L
EM: heller@uwyo.edu
AF: University of Wyoming, Department of Geology and Geophysics, PO Box 3006, Laramie, WY 82071 United States
AB: Determination of paleoslopes in ancient fluvial systems has potentially broad application to quantitatively constraining the history of tectonics and paleoclimate in continental sequences. Our method for calculating paleoslopes for sandy braided streams is based upon a simple physical model that establishes depositional skin-frictional shear stresses from assemblages of sedimentary structures and their associated grain size distributions. The addition of a skin-frictional shear stress, with a geometrically determined form-drag shear stress results in a total boundary shear stress which is directly related to water-surface slope averaged over an appropriate spatial scale. In order to apply this model to ancient fluvial systems, it is necessary to measure the following: coarsest suspended sediment size, finest grain size carried in bed load, flow depth, dune height, and dune length. In the rock record, suspended load and bed load can be accurately assessed by well-preserved suspended load deposits ("low-energy" ripples) and bed load deposits (dune foresets). This model predicts an average slope for the North Loup River near Taylor, Nebraska (modern case study) of 2.7 x 10$^{-3}$. The measured reach-averaged water surface slope for the same reach of the river is 1.37 x 10$^{-3}$. We suggest that it is possible to calculate the depositional slope of a sandy fluvial system by a factor of approximately two. Additionally, preliminary application of this model to the Lower Jurassic Kayenta Formation throughout the Colorado Plateau provides a promising and consistent evaluation of paleoslope in an ancient and well-preserved, sandy braided stream deposit.
DE: 1815 Erosion and sedimentation
DE: 1854 Precipitation (3354)
DE: 1860 Runoff and streamflow
SC: Hydrology [H]
MN: 2003 Fall Meeting