Retreat Rates And Controls On The Form Of Hard-Capped Cliffs In Plateau Country
Ward, Dylan J 1 ; Anderson, Robert S 2
1 University of Colorado Dept. of Geological Sciences and INSTAAR
2 University of Colorado Dept. of Geological Sciences and INSTAAR
The retreat of hard-capped cliffbands represents the erosional response of the Plateau landscape to changes in base level, i.e., incision of major rivers such as the Colorado River. Treatments of the mechanisms and morphologies of scarp retreat have lacked quantitative constraint on rates of backwearing of cliffbands and the dominant controls on those rates. 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. Measured rates, along with field and GIS observations, constrain parameters in a 1D numerical model we use to assess the dominant controls on scarp morphology and retreat rate. The model simulates retreat of cliff profiles through time, driven by incision of channels on the soft rock plinth. Retreat proceeds by knickpoint migration and/or landsliding; debris-armoring of the channel is captured. Realistic morphologies are generated in the simplest case by spatially varying model fluvial erodibility depending on the substrate (hard rock, soft rock, or debris). Retreat rate and morphology vary as drainage area declines through time. Exploring feedbacks between morphology, mass wasting, and debris armoring of the channels will require further attention to the physics and rates of processes involved in scarp retreat.