RETREAT RATES AND CONTROLS ON THE FORM OF HARD-CAPPED CLIFFS IN PLATEAU COUNTRY

The retreat of hard-capped cliffbands represents the erosional response of the Plateau landscape to changes in local base level, i.e., incision of major rivers such as the Colorado River. Despite many treatments of the mechanisms and morphologies of scarp retreat, the rates of backwearing of cliffbands have not been quantified, and the dominant controls on those rates have not been explored. We measure spatially and temporally averaged scarp retreat rates in areas such as the Book Cliffs of CO and UT and model the evolution of idealized scarps. Retreat rates are determined using a modification of the cosmogenic basin-averaged erosion rate method that exploits the paucity of quartz sand in the shale substrate relative to the sandstone caprock. Measured rates, along with field and GIS-based observations, constrain parameters in a simple numerical model we use to assess the dominant controls on scarp morphology and retreat rate. The model simulates retreat of hard-capped cliff profiles through time in 1D, based on the assumption that first-order channels on the soft rock plinth dictate the pace of cliff retreat and the forms it generates. Erosion is assumed to be proportional to stream-power, and thus directly proportional to upstream drainage area and local slope (dz/dt = kA^mSⁿ). A specific-elevation boundary condition mimics a local or regional base level. Cliff retreat is allowed by knickpoint migration and/or landsliding; debris-armoring of the channel is captured. In the simplest case, the erosion-law constant (k) varies depending on the type of material at each point on the surface (hard rock, soft rock, or debris). In this case, cliff retreat is analogous to simple knickpoint retreat, with higher erodibility below than above the knickpoint. This generates realistic morphologies, which vary along with retreat rate as drainage area declines through time. The results imply that specific scarp morphology is related to the relative rates of caprock backwearing and fluvial erosion of the soft rock plinth. The numerous feedbacks between morphology, mass wasting, and debris armoring of the channels have not been fully explored, and will require further attention to the physics and rates of processes involved in scarp retreat.