Assessing the Influence of Mountain Slope Morphology on Pediment Form, South-Central Arizona

Abstract

Pediments are common desert landforms. While the term "pediment" is used to describe subaerial bedrock platforms (here called "bedrock pediments"), it is frequently used to describe the hybrid suballuvial bedrock platforms (here called alluvial slopes) as well. Important distinctions exist between these landforms. For example, whereas bedrock pediments are mantled with a thin layer of in situ and perhaps alluvial debris, alluvial slopes tend to imitate alluvial fans, cone-shaped deposits of alluvium made by ephemeral mountain streams depositing sediment on piedmonts. Mountain slopes backing bedrock pediments and alluvial slopes located in the Phoenix, Arizona, region exhibit disparate morphologies that allow a distinction to be made between bedrock pediments and alluvial slopes developed at their base. Mountain slopes above bedrock pediments tend to exhibit larger clast sizes mantling the slope, less vegetation, steeper slopes, more abrupt piedmont angles, and fewer joints in bedrock outcrops. Mountain slopes above alluvial slopes, in contrast, exhibit smaller clasts, greater vegetation, gentler slopes and piedmont angles (i.e., more concave), and increased joint density in outcrops. Drainage areas feeding bedrock pediments also tend to be smaller compared to drainage areas feeding alluvial slopes. These results suggest that backing mountain slope morphology discriminates the occurrence of bedrock pediments and alluvial slopes in the study area, and further analyses of field data provide an indication of controls on the development of these landforms.