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Impact of lithology on the form of soil mantled landscapes

We are investigating the linkages between lithology, weathering and the form of soil mantled topographies. In particular we seek to understand 1) if lithologic variations are expressed in the form of hillslopes and 2) if weathering of poorly consolidated rocks can increase the erosional resistance of materials exposed at earth’s surface. This study utilizes field observations and measurements, numerical modelling, and high resolution topographic data with a focus on the Gabilan Mesa, a location that previous studies have utilized as an ideal soil mantled landscape. In the Gabilan Mesa, shallow marine deposition of the Pancho Rico Formation generated cyclic fluctuations in lithology - variations that are visible on hillslopes as undulations in slope superimposed on classical convex forms. This is despite a continuous mantle of soil, which longstanding wisdom and widely applied rules for the transport of soils on hillslopes predict to buffer topography from underlying lithologies (Gilbert, 1877; Dietrich et al., 2003). These variations in slope were first recognized by Dohrenwend (1979), who termed the features ‘shadow bedding’, as they are most readily identified during low light conditions. We propose that variations in underlying lithology affect the depth scale of soils (e.g., Heimsath et al., 2005) and that this in turn influences rates of transport down hillslopes (Braun et al., 2001; Anderson, 2002). The coupling of lithology dependent soil production and depth dependent transport of those soils allows for geologic structures to be expressed in the form of landscapes. The act of weathering can in turn affect the resistance of earth’s surface to erosion by overland flow, extreme examples being the carbonate cementation of calcretes and the development of gruss. In the Gabilans we are investigating how the act of weathering influences the strength of two end-member lithologies: poorly consolidated Pancho Rico Formation and Salinian granite. We hypothesize that the production of soil can act to armor weak bedrock, which gives rise to a threshold response when erosion rates exceed soil production rates. Such a threshold in the response of landscapes to variations in erosion rate provides a mechanism for the reorganization of drainage networks, a process that appears to be active in the Gabilan Mesa.