Dip, layer spacing, and incision rate controls on the formation of strike valleys, cuestas, and cliffbands in heterogeneous stratigraphy

Abstract

AbstractLandscapes developed over heterogeneous stratigraphy exhibit a spectrum of landforms from dramatic cliffbands to hogbacks, depending on the dip and spacing of the layers. In deeply incised landscapes, a single cliffband may consist of multiple resistant layers, whereas similar stratigraphy elsewhere is separated by strike valleys into individual cuesta benches or hogbacks. This paper presents a geometric analysis, informed by a numerical landscape model, to explain the conditions for development of a strike valley floored by erodible rocks. The results define a threshold incision rate below which strike valleys are more likely to form; this threshold incision rate is proportional to the stratigraphic spacing of cliff-forming layers and a trigonometric function of dip angle. The analysis also yields a time scale for the adjustment of structural landforms to changes in regional incision rate, which is a function of dip angle and the coupling between cliff retreat rate and escarpment height. In example landscapes of the Colorado Plateau, this time scale is likely much longer than that of documented variations of incision rates due to late Quaternary climate and land-use changes. The transitional state of escarpments in layered rock may therefore contain information about regional downcutting rates over time scales different from those recorded by the fluvial network. The utility of such features will require better understanding of the coupling between incision of a foot slope and the retreat rate of the cliff above in different kinds of rocks.