How do pediments form? A numerical modeling investigation with comparison to pediments in southern Arizona, USA

Pediments are gently sloping, low-relief bedrock erosional surfaces at the bases of mountain ranges. Pediments tend to form more readily in arid climates and in weathering-resistant lithologies, but the processes responsible for pediment formation are still not widely understood after more than a century of debate. In this paper, I investigate the behavior of a coupled numerical model for the evolution of mountain ranges and their adjacent piedmonts that includes bedrock erosion in channels, soil production and erosion on hillslopes, and the flexural-isostatic response of the lithosphere to erosional unloading. For relatively small values of the flexural parameter, erosion of the mountain range leads to sufficient flexural-isostatic tilting of the adjacent piedmont that a suballuvial bedrock bench is exhumed to form an erosional surface on the piedmont. In addition, slope retreat at the mountain front and subsequent tilting of the abandoned surface can contribute to pediment formation by lengthening the pediment in the upslope direction. The rate of erosion on the piedmont must also be greater than or equal to the rate of soil production, thereby creating an erosional surface that has, at most, a thin veneer of soil or regolith. The rate of soil production depends primarily on climate and lithology, with lower soil production rates associated with more arid climates and more resistant lithologies. The model predictions are compared to morphometric analyses of pediments in the southwestern United States and to the detailed morphology of two classic pediments in southern Arizona.