Paper No. 12
Presentation Time: 11:10 AM
RECONSTRUCTING THE BACKWATER REACHES OF PALEO-RIVERS AND THEIR INFLUENCE ON FLUVIAL FACIES DISTRIBUTION, CAMPANIAN LOWER CASTLEGATE SANDSTONE, UTAH
PETTER, Andrew L., St. Anthony Falls Laboratory, University of Minnesota, Minneapolis, MN 55414, STEEL, Ronald J., Dept. of Geological Sciences, The University of Texas at Austin, 6.114, 1 University Station C1100, Austin, TX 78712, MOHRIG, David, Jackson School of Geosciences, The University of Texas at Austin, 2275 Speedway, Stop C9000, Austin, TX 78712-1692 and NITTROUER, Jeffrey, Earth Science, Rice University, Rice University MS-126, 6100 Main Street, Houston, TX 77005, alpetter@umn.edu
The Campanian Lower Castlegate Sandstone provides an exceptional record of downstream transitions in fluvial deposition from the thick proximal, sand-rich type section dominated by amalgamated channel-filling barforms to the distal pinchout with isolated mud-filled channels and thin overbank deposits. Paleo-hydraulic analyses along a downstream-oriented outcrop transect in concert with observations of facies and architectures have revealed the stratal signature of backwater hydraulic conditions in the distal reaches of Lower Castlegate paleo-rivers. Backwater hydraulic conditions result from the influence of the receiving basin on river flow, and occur where the elevation of the channel bed is below mean sea level. The backwater reach exerted a significant influence on downstream fining and sorting within Lower Castlegate paleo-rivers, and consequently, backwater hydraulics conditions are shown to have a profound impact on the morphodynamics of Lower Castlegate paleo-rivers by controlling the downstream distribution of bed material (sand). This study marks the first recognition of the effects of backwater conditions on the rock record.
The backwater reach of the Lower Castlegate paleo-river system is recognized by downstream increase in paleo-flow depths which indicates divergence of the channel bed and water surface. The upstream end of the backwater reach coincides with the downstream limit of particles transported only as bedload as shown by grain-size distributions of bar material within the Lower Castlegate. As volumes of bed material decreased downstream into the backwater reach, the paleo-river style changed from braided to meandering and avulsion frequencies and rates of lateral channel migration decreased. Thus, the main factor controlling the spatial variability of facies was downstream loss of bedload from transport by channel aggradation influenced by backwater conditions. This study shows the importance of reconstructing regional-scale transport conditions within the rock record, and demonstrates how the backwater reach may be used as a length scale for quantitative facies prediction within economically important fluvial strata such as the Williams Fork Formation.
