HYDRODYNAMICS OF FLUVIAL SANDSTONES IN THE CRETACEOUS UPPER WILLIAMS FORK FORMATION, PLATEAU CREEK, CO

Laterally extensive (up to 10km) amalgamated sandstones at the base of stratal packages identified in this formation are analyzed quantitatively for hydrodynamic analysis. The amalgamated sandstones represent multi-storey channel-belt sand bodies. Bar form deposits within the amalgamated sandstones contain cross-bed sets averaging 29 cm in thickness. Using an empirical approach, cross-bed set thicknesses are used to calculate dune height and flow depth. Paleoflow depth estimates range from 2 to 8 meters. Flow depth for the 90^th percentile is estimated at 7 meters for the trunk channel. Using grain size and paleoflow depth, a mean slope of 0.0004 is calculated. Sediment transport mechanisms were calculated from grain size and flow depth. These hydrodynamic calculations, along with interpretations from previous studies by others on sinuosity and width-thickness calculations, suggest a suspended-mixed load fluvial dominated system for the multi-storey channel-belt sand bodies. Analyses of channel fill width-thickness ratios place the study area within the backwater effects. These quantitative findings contest previous qualitative interpretations for the Williams Fork Formation as deposits of braided rivers. The Williams Fork Formation at the study area is reinterpreted as deposits of low sinuosity meandering rivers with a distributive morphology. Detrital zircon dating of the formation boundaries, 74.09 ± 0.48 and 70.1 ± 1.8 Ma helps establish a maximum depositional age for this formation. Detrital zircon analyses suggest a shorter time of deposition for the Williams Fork Formation (1.7-6 Ma) than earlier estimates based on palynologic dates.