ABSTRACT

Point-bar deposits in the Clear Fork Group (Lower Permian) of north-central Texas 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° to 65° to the main channel direction.

The Clear Fork 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.