Abstract

Quantitatively reconstructed paleochannel hydraulics (e.g., channel discharges, slopes, velocities), geometries (e.g., widths, depths, sinuosities, channel patterns), and alluvial architecture record the local effects of syndepositional faults and folds. These effects are evaluated from Lower Cretaceous rocks in three areas: (1) the northern Black Hills, where the axes of syndepositional folds trend roughly perpendicular to paleoflow, (2) the northwestern Black Hills, where the trend of a syndepositional fault is oblique to paleoflow, and (3) the Wind River basin, where the trends of syndepositional faults are nearly parallel to paleoflow.

In each area, channel-belt deposits are laterally and vertically connected where local subsidence rates were high, producing thick, laterally extensive sandstones. Areas of lower subsidence rates are characterized by isolated or laterally connected channel-belt deposits, and thinner sandstones. Surface deformation controlled the positions of some rivers because of the development of antecedent drainages, or by directing rivers toward areas of maximum subsidence. Some reconstructed channel slopes, which range from 0.62 × 10-4 to 5.43 × 10-4, record surface deformation. The three-dimensional alluvial architecture was simulated using the model of Mackey and Bridge (1995), together with quantitatively reconstructed paleochannel parameters, in order to provide quantitative insight as to possible conditions and processes that produced the observed alluvial architecture. The effects of deformation on channel slopes, together with depositional topography, were important in controlling avulsion and resultant alluvial architecture. The most important condition that governed the alluvial architecture was comparable rates of local subsidence and sediment accumulation.

Paleochannel reconstructions indicate that the rivers were 48-180 m wide and 4.4-13.6 m deep, and had discharges of 64-1073 m3s-1. Channel deposits all appear to represent point bars and associated channel fills of meandering rivers. Sinuosities were comparable, ranging from 1.1 to 1.4, and show no consistent trends with other channel parameters. Hence, with the exception of two paleochannels (out of the thirteen that were reconstructed) differences in hydraulic parameters such as slope and sediment transport rate had no discernible effect on channel pattern or sinuosity.

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