Lynds, Ranie M.1, David Mohrig2, Paul L. Heller1
(1) University of Wyoming, Laramie, WY
(2) Massachusetts Institute of Technology, Cambridge, MA
ABSTRACT: Grain-size Distributions of Associated Bedforms: Resolving Depositional Slope in Sandy Braided Rivers
Constraining the paleoslope of fluvial systems can aid in establishing the paleoclimatic and tectonic history of continental sequences. We have developed a method to determine depositional slope from sedimentary structures in sandy braided stream deposits, based upon the modern North Loup River near Taylor, Nebraska. Our method builds 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. 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).
Applied to the North Loup River, the model predicts an average transport slope of 2.7 x 10-3, comparable to the measured reach-averaged water-surface slope of 1.37 x 10-3. We suggest it is possible to calculate the depositional slope of a sandy fluvial system by a factor of approximately two. Preliminary application of this model to the Lower Jurassic Kayenta Formation exposed throughout the Colorado Plateau yields a regionally consistent estimate of paleoslope and promises to be a valuable method for constraining paleoslopes in ancient, sandy braided stream deposits.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.