From Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.

Bureau Seminar, November 19, 2004

Sandy Channel-Filling Deposits as Quantitative Indicators of Sediment and Water Flux for Ancient Terrestrial Environments

David Mohrig
MIT

Abstract:

Determining fluxes of sediment and water using the channel-filling deposits of ancient fluvial systems has potentially broad, but largely untapped, application to quantitatively constraining the history of tectonics and paleoclimate in continental sequences. To date, most studies have tied sedimentology to the distribution of proxy indicators, such as types of fossils, paleosols, and isotopic abundances. A purely sedimentary approach would be useful because many geologic deposits lack suitable proxy indicators. Furthermore, if our goal is to understand the alluvial stratigraphic response to climate change, we should be able to observe direct indicators of changing climate, especially changes in sediment and water flux, within the deposits themselves. This presentation outlines a method for reconstructing these fluxes using deposits made up of particles that moved as suspended load in paleochannels. Data from modern rivers show that the Bagnold criterion applied to D99 for the suspension can be used to estimate values of skin-friction shear velocity in agreement with direct. Best candidates for unmodified suspension deposits are?measurement accumulations laid down in low-velocity zones, either within the primary channel itself, in near-slack water pools in the lees of bars, or bank reentrants where the core flow has separated from the channel margin. Measurements from these deposits on modern river bottoms and in ancient channel fills indicate that the Bagnold condition applies equally well to the sedimentary record, providing the foundation for producing quantitative estimates of sediment and water flux. The methodology for calculating these properties will be summarized, and an application to an ancient fluvial system will be discussed.