Abstract

Point-bar deposits in the Clear Fork Group (Lower Permian) are exposed both in cross section and in plan as exhumed arcuate accretionary ridges. A thin basal lag of locally derived caliche and mudstone fragments is overlain by a 2 to 3 m thick epsilon cross-stratified unit which fines upward from very fine sandstone at the base to mudstone at the top. Sandstone beds in the epsilon units are almost entirely cross-laminated and were deposited by straight-crested ripples, most of which are oriented at 40 degrees to 65 degrees to the main channel direction. The point bars were deposited by small, variable-discharge, perennial streams with high sinuosity. The straight-crested ripples migrating on the point bar surfaces are oblique rather than perpendicular to the near-bed flow direction. The cause of such considerable obliquity was primarily a large transverse gradient in local downstream sediment transport rate and, to a lesser degree, components of transverse near-bed fluid velocity due to helicoidal flow.--Modified journal abstract.

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