Soil Development Over Mud-rich Rocks Produces Landscape-scale Erosional Instabilities in the Northern Gabilan Mesa

Abstract:

Whether landscapes respond to climatic and tectonic perturbations through steady, continuous adjustment or by short-lived, transiently unstable acceleration of erosion rates is fundamental to understanding landscape dynamics and the impact that land-use change may have on watershed sediment budgets. We document the geomorphic response of catchments in the Northern Gabilan Mesa (NGM), where it appears that over geomorphic timescales, well-developed soils may provide more resistance to incision than the mudstones from which they were derived. While measures of tensile strength reveal that intact mudstone is stronger than overlying soils, slaking removes the internal strength of these mudstones. This produces materials in the critical zone that are weaker than the overlying soils. This local configuration appears to impact large-scale landscape dynamics in the NGM, in that once the cohesive soils are locally removed, gullies that incise into the underlying, weaker slaked rock undermine the cohesive soil mantle and propagate headward as a series of arroyos predominantly hosted on south-facing slopes. These arroyos are more frequent in landscapes underlain by shallow marine mudstones and sandstones than sandstones and conglomerates. On the walls of arroyos, the development of relief and the exposure of rapidly disintegrating bedrock is accompanied by the dominance of transport by granular flows. Finally, arroyos often extend farther upslope than do channels without bedrock exposures, ultimately causing an increase in drainage density. Therefore in a situation where cohesive soils overlie a zone of weaker weathered material, incision through the cohesive layer represents a threshold that initiates a transient wave of incision that rapidly undermines and removes large volumes of bedrock and soil. This conceptual model is affirmed by simple one-dimensional geomorphic models that explore the conditions under which such features may be produced.