EARLY CRETACEOUS FLUVIAL SYSTEMS WITHIN THE SEVIER FORELAND BASIN, WYOMING, AND THE EFFECTS OF TECTONISM ON DEPOSITION

Early Cretaceous, foreland basin fluvial deposits throughout Wyoming record interactions between orogenesis, subsidence, sediment accumulation, basin physiography, and syndepositional structural deformation associated with the early stages of the Sevier Orogeny. Lower Cretaceous strata show the former existence of a thick foredeep depozone to the west (the Gannett Group), a thin forebulge depozone throughout central Wyoming (the Cloverly Fm.), and thickening into a backbulge depozone in the Black Hills to the east (the Lakota Fm.). Basin subsidence is attributable not only to orogenic loading, but to dynamic loading as well. Two major unconformities associated with these deposits indicate reductions in sediment supply possibly related to mountain-belt processes.

Most rivers within the basin were meandering. Paleoflow in central and eastern Wyoming was generally northward, indicative of basin-axial rivers. Paleoflow in western Wyoming was eastward, indicative of basin-transverse rivers. Rivers were 4-14 m deep and 32-180 m wide, with discharges on the order of 41-1073 m³s^-1. Some lateral and temporal variations in the scales and discharges of rivers may reflect the effects of an orographic rain shadow. The above results have implications for the reconstruction of the recently identified Early Cretaceous continental-scale drainage system.

The alluvial architecture of thick foredeep deposits contrasts markedly with that of stratigraphically equivalent, much thinner deposits farther east associated with the forebulge and backbulge depozones. Foredeep deposits are dominated by overbank and lacustrine mudstones, and channel deposits tend to be isolated, with limited lateral extents on the order of 10's of meters. Forebulge and backbulge channel deposits tend to be laterally and vertically connected forming sandstones and conglomerates with lateral extents on the order of 10's of km to >100 km. Quantitative simulations of channel-deposit proportions indicate that basin-wide differences in alluvial architecture are attributable to observed differences in sediment accumulation rates, which reflect variable subsidence rates of the different depozones. Additionally, in some areas of the fore- and backbulge depozones, alluvial architecture was controlled by local syndepositional structures.