CLIMATIC CONTROLS ON THE TIMING OF HILLSLOPE SOIL FORMATION AND EROSION IN THE EASTERN MOJAVE DESERT

In the eastern Mojave Desert, east-west oriented drainage basins composed of coarse crystalline Early Proterozoic bedrock emerge from the northwest flank of the New York Mountains piedmont. Curvilinear hillslopes are composed of both bedrock and colluvium. Soils formed in colluvium support native perennial grasses sustained by monsoonal summer precipitation. The soils and associated grasses are more extensive on the mesic northerly aspects. On more xeric southern aspects, slopes are dominated by very thin colluvium, minimal soil development, and desertscrub. Thicker colluvium, with soils similar to those of the northern aspect slopes, are preserved as isolated remnants. The fine fraction of soils formed in colluvium is consistently bimodal (modes = 75 and 400-600 μm). Eolian additions are the source of the silt-sized particles; however, the sand-sized material is likely the product of bedrock weathering. The sand-sized grains in the colluvium and crystals in the bedrock are similar in size, suggesting weathering by fracturing along crystal boundaries and granular disintegration. Both silt- and sand-sized particles are small enough to be translocated downward in soil profiles. The onset of translocation of both eolian silt and weathering-derived sand into the soils was determined using OSL. On north-facing slopes, ages of both silt and sand indicate steady accretionary and inflationary soil profile development through the late Pleistocene and into the midddle Holocene (~15-3 ka). On southern aspect remnants, sand incorporation occurred 23-18 ka and but eolian silt accumulated later (19-13 ka). The OSL ages in the sand indicate bedrock weathering during the late Pleistocene that slowed in the Holocene. The high standard deviation of these ages is likely the result of complex environmental variables that control the timing of weathering of individual bedrock grains. Colluvial remnants, debris aprons, and an extensive 1.5 m fill terrace in the axial drainage indicate that south aspects were once more extensively mantled by colluvium and associated soils, which has eroded during the Holocene. Soil morphology and OSL ages (~3.5 ka) of dust incorporated into terrace sediments after surface stabilization indicate that aggradation occurred during the middle Holocene.