Desert pavement dynamics: Numerical modeling and field-based calibration

Desert pavements are widely used as a relative surface-dating tool because they are progressively better developed on surfaces ranging from thousands to hundreds of thousands of years in age. Recent work, however, has highlighted the dynamic nature of pavements and undermined their use as surface-age indicators. Quade (2001) proposed that latest Pleistocene vegetation advances destroyed all Mojave Desert pavements above 400 m elevation, making all such pavements Holocene in age. In an effort to reconcile young-pavement evidence with their widespread use as Pleistocene surface-age indicators, we developed a numerical model based on the classic conceptual model in which pavements co-evolve with their underlying eolian epipedons over millennial timescales. In this co-evolutionary process, fine-grained eolian deposition and A_v-horizon development within the eolian epipedon promotes surface clast motion and pavement development, enhancing the eolian-sediment-trapping ability of the pavement in a positive feedback. Model results illustrate the multi-scale nature of pavement dynamics: pavements may require tens of thousands of years to fully develop from a newly abandoned alluvial surface, but may heal over timescales of decades to centuries if a mature eolian epipedon is present. As such, there is no inconsistency between rapid pavement healing and a Pleistocene age for the underlying alluvial surface. To calibrate the model, we conducted surficial geologic mapping and pavement-sedimentological analysis on two desert piedmonts. Our study areas include both proximal and distal fan environments, illustrating the role of parent-material texture in controlling the mode of pavement formation. Using available geochronology, our work provides a rigorous calibration of pavement formation rates in our study areas and provides evidence supporting the use of pavements as local relative surface-age indicators over Holocene to late Pleistocene timescales.