PALEOCHANNEL HYDRAULICS, GEOMETRIES, AND ASSOCIATED ALLUVIAL ARCHITECTURE OF EARLY CRETACEOUS RIVERS, SEVIER FORELAND BASIN, WYOMING, USA

Early Cretaceous foreland-basin fluvial deposits throughout Wyoming record important interactions between structural deformation, subsidence, basin physiography, and sediment accumulation associated with the early stages of the Sevier orogeny. Knowledge of the rivers and their deposits is critical to understanding these interactions. This study quantitatively reconstructs the paleohydraulics and geometries of these rivers. Results are important for evaluating controls on alluvial architecture and can be applied to basin modeling studies.

Paleohydraulics were reconstructed by applying field data to a physical model that predicts flow, bed topography, and sediment transport in curved fluvial channels at bankfull stage. Paleohydraulic reconstructions were conducted for Cloverly A interval (forebulge depozone) paleochannels and those of the overlying Bechler (foredeep)/Cloverly B (forebulge)/Lakota L2 (backbulge) interval from west to east across Wyoming. Most Cloverly A interval rivers were meandering. Some key characteristics are: widths 50-120 m, depths 4-11 m, discharges 64-640 m³s^-1, slopes 1.81-6.13 x 10^-4, sediment transport rates 0.43-4.17 x 10^-2 kg m^-1s^-1. Paleoflow was dominantly NNE. Most Bechler/Cloverly B/Lakota L2 rivers were meandering. Key characteristics are: widths 30-180 m, depths 4-12 m, discharges 40-1070 m³s^-1, slopes 0.58-5.43 x 10^-4, sediment transport rates 0.17-4.08 x 10^-2 kg m^-1s^-1. Paleoflow was dominantly NNE. The highest discharges were associated with rivers farthest from the mountain belt. Some channel slopes were affected by syndepositional structures within the basin.

Compacted sediment accumulation rates for the foredeep (10^-2 mm yr^-1) are an order of magnitude greater than those for the forebulge and backbulge depozones (10^-3 mm yr^-1). Foredeep deposits are dominated by overbank and lacustrine mudstones, and channel deposits tend to be isolated. Forebulge and backbulge channel deposits tend to be laterally and vertically connected forming conglomerates and sandstones with lateral extents on the order of 10's of km to >100 km. Basin-wide differences in alluvial architecture are attributable to these variable sediment accumulation rates, although in some localized areas the alluvial architecture was more influenced by syndepositional structures.