HILLSLOPE EVOLUTION IN A HIGHLY COMPLEX SEMIARID LANDSCAPE

It is generally understood that landforms with deep weathering are transport limited with gentle, rounded slopes and are weathering-limited with steeper, more angular slopes when there is a shallow regolith. However, on hillslopes of an intermediate nature, changes in factors affecting the rates of weathering and sediment removal can change a hillslope or hillslope segments between these two modes. As a drainage is cut into alluvial fan deposits, the changing slope angle and aspect drives changes in microclimate, in turn affecting soil forming processes and vegetation distribution. Field observations show north facing hillslopes with more mesic vegetation and greater dust entrapment. All hillslopes formed in alluvium have classic transport limited forms. After the drainage has cut into the underlying sedimentary bedrock layers of carbonate and siliclastic varieties and steep slopes develop, the north-south asymmetry is heightened. North facing slopes remain transport limited, but south facing slopes are segmented with slopes above the alluvium-bedrock contact transport limited and slopes below weathering or detachment limited. Soil observations have shown greater carbonate and clay accumulation on north facing hillslopes, with greater subsurface chemical weathering especially in carbonate. Slope forms appear stable until the alluvial cap is exhausted, exposing the carbonate bedrock. At this point the north-south asymmetry is much reduced with both slopes appearing weathering limited. However, the north facing slope retains a thicker colluvial mantle and regolith. Exposed carbonate bedrock is weathering resistant and outcrops, eroding as dust enters cracks to topple blocks downslope. Exposed siliclastic bedrock is susceptible to erosion from overland flow and retreats as the carbonate is removed, leading to concave slope form. When the carbonate cap is exhausted, the landform erodes to a low mound just above base level.