ASPECT AND CLIMATIC CONTROLS ON ECOGEOMORPHIC RELATIONSHIPS AND LANDSCAPE EVOLUTION IN THE MOJAVE DESERT

Steep, low-relief terrain in resistant leucocratic granite and metagranite terrain provides the opportunity to explore relationships between Pleistocene climatic change, vegetation, and landscape evolution in the Mojave Desert. Differences in vegetation, soil cover, and slope morphology between south-facing and north-facing slopes are apparent throughout the study area. North-facing slopes are extensively vegetated with warm season tufted grasses including Little Galleta, Black Grama, and Desert Fluff. Thick colluvium (> 1 m), covers the north-facing slopes producing curvilinear slope forms. The silty loam soils formed in the colluvium also contain illuvial clay and pedogenic carbonate indicating that a substantial amount of the fine fraction of the soil is derived from dust. Conversely, south-facing slopes have sparser vegetation cover of predominantly Big Galleta, Rabbit Brush, and Cholla. The colluvium is thin (0-0.5 m), patchy, and interspersed with bedrock exposures. Soil development is minimal in the gravelly colluvium. Discontinuous remnants of thicker colluvium with well-developed soils are exposed on southern slopes. These colluvial remnants are similar to north-facing slope soils, suggesting that more extensive soil covered the south-facing slopes in the past. Cooler effectively wetter conditions in the Late Pleistocene likely resulted in enhanced physical weathering and accumulation of colluvium via dust and bedrock weathering. As climate warmed and dried during the Pleistocene-Holocene transition, south-facing slopes developed microclimates inhospitable to most grasses. Decreased root strength initiated stripping on south-facing slopes and alluvial fan aggradation at the base of slopes. Weakly developed soils in alluvial fan deposits ~2 m above the modern channel are evidence of this most recent episode of stripping. There are also, however, alluvial fan deposits ~4 m above the modern channel with soil development greater than that observed on hillslopes. These deposits suggest that episodes of colluviation and stripping occurred repeatedly during the Pleistocene.