Topographic Signatures of Geomorphic Processes at Desert Piedmonts

Geomorphic processes operating on desert piedmonts leave their mark in the topography of the piedmonts, measured at a range of scales. For example, at the km to 10's of m scale, slope, curvature, and dissection give clues to the relative importance of erosional, aggradational, and transport processes. When a desert piedmont is dominated by erosional and transport processes, a pediment is formed. Conversely, when aggradational processes dominate, alluvial fans are formed. The relative rates of uplift vs. aggradation determine the slope of the fan and whether the fan is dissected. Changes in uplift rate or climatic conditions can lead to isolation of the currently forming fan surface through entrenchment and construction of another fan either further from the mountain front (decreased uplift or increased runoff) or closer to the mountain front (increased uplift or decreased runoff). Fitting 3-dimensional surfaces to alluvial fan units allows parameters for the apex position, slope, and radial curvature to be compared with unit age. The topographic signature of individual fan units will help in regional correlation of fan surfaces and may assist in separating the effects of climate and uplift. At scales of m to cm, processes such as salt weathering, aeolian deposition, and desert pavement formation dominate. The lithologies that make up the piedmont determine their susceptibility to processes of disintegration due to salts (e.g. coarse-grained rocks) or dissolution (e.g. limestones). The composition of the source areas also helps determine the relative importance of debris and mud flows. Resistant lithologies tend to form desert pavement surfaces with time, caused by infiltration of aeolian dust below a monolayer of resistant pebbles to cobbles. This leads to local relief of a few cm, while surfaces composed of easily weathered lithologies have much lower relief at that scale. Work performed under contract to NASA.