ASPECT CONTROLS ON EROSION RATES AND BASIN MORPHOLOGY IN THE MOJAVE DESERT

In the eastern Mojave Desert, east-west oriented drainage basins have significant morphological asymmetry between north- and south-facing slopes. North-facing slopes are characterized by curvilinear profiles, thick colluvium with well-developed soils, and dense native perennial grasses. South-facing hillslopes have more irregular profiles and are mantled primarily by thin, patchy colluvium. The south-facing slopes have more exposed bedrock (44%) than north-facing slopes (17%), less dense grasses, and debris aprons at the base of the slopes (3-5 meters in thickness). Throughout the basin, bedrock slopes are rougher and steeper than slopes mantled by colluvium. On south-facing slopes there are isolated patches of thicker (2-3 m) colluvium. These colluvial remnants have well-developed A and B horizons with illuvial clay, pedogenic, and bimodal grain size distributions of the fine fraction. The presence of extensive pedogenic carbonate (in soils where the bedrock is calcic-poor leucocratic and meta-granitic rock) and bimodal distributions indicate that a significant portion of the fine colluvial material is dust. OSL ages of the eolian fines in the colluvial remnants indicate that the coarse regolith was generated prior to ~18 ka and has remained stable with minimal bioturbation. Contrastingly, OSL ages of eolian fines incorporated into the more extensive colluvium on the north-facing slopes indicates that regolith generation has been more steady during the Holocene. The colluvial remnants suggest more extensive soil cover in the past. We hypothesize that during the Holocene erosion outpaces regolith production on south-facing slopes. This change is potentially related to increases in erosion caused by enhanced overland flow generated on the slopes with more bedrock and less dense grass. Additionally, samples have been collected from colluvial remnants, bedrock outcrops, and surface clast transects on the north and south-facing slopes for cosmogenic radionuclide analyses.