Pedimentation versus debris-flow origin of plateau-side desert terraces in southern Utah

Abstract

Plateau-side terraces in arid areas around the world are commonly described as pediment remnants, although, in many cases, they may have been formed by debris-flow deposition. Pediments do exist in the area of the Aquarius and Kaiparowits Plateaus of southern Utah; however, many alluvial terraces that were classified by previous workers as pediments are actually formed of thick deposits of sediment released into valleys by episodic landslide events. Pedimentation may have implications of base level stability and aridity that are very different from the implications of landsliding, so that the identification of pediments where none exist, as has occurred for some places in the Aquarius and Kaiparowits areas, may result in erroneous interpretations of geologic history. Although both pediment and debris-flow depositional terraces have smooth, alluvium-covered upper surfaces, the two can be distinguished by the shape of the underlying bedrock surface and the process of formation. The distinction is that pedimentation produces smooth relief through erosion whereas wet debris flow produces smooth relief through deposition, although transitional phases exist. Most terraces on the flanks of the Aquarius and Kaiparowits Plateaus reflect periods of valley filling when wet debris flows rapidly deposited large quantities of sediment. The sources of the wet debris-flow sediment were large lobes of landslide debris that carried down sediment from high on the plateau sides. The relation of younger terraces to landsliding is clear, because the terrace surfaces are graded to the toes of slide lobes, but the origin of older terraces is less obvious, because older lobes of slide debris have generally been removed by erosion.