Patterns of weathering in sedimentary bedrock across a sequence of repeating ridges and valleys

Abstract

Bedrock weathering plays a fundamental role in liberating nutrients, generating porosity, and producing soil. Compared to the mobile soil layer, little is known about the structure of bedrock weathering profiles across hillslopes and the extent to which observations from one hillslope can be scaled to represent other hillslopes of similar topographic form. Here, we compare patterns of bedrock weathering and water storage across a sequence of repeating ridges and valleys in the upturned sedimentary Great Valley Sequence of the eastern Northern California Coast Range. Deep drilling, downhole logging, and analysis of recovered samples reveal a common upslope thickening weathering profile across a sequence of ridges and valleys. The maximum weathering thickness at the divide is comparable across hillslopes with a nearly two-fold difference in hillslope length, suggesting that the thickness of weathering may not scale with hillslope dimensions. Within channels that bound hillslopes, bedrock cores are relatively unweathered within centimeters of the ground surface, while at ridges, the depth to the base of bedrock that is pervasively fractured and oxidized is about 7 m. Relative to fresh bedrock, matrix chips near the top of the weathered profile at the ridges have increased porosity by roughly 10% and experienced chemical denudation mass loss of 5 - 10%. Our observations reveal a pervasively weathered and fractured layer, which coincides with the oxidation front and is accompanied by unsaturated dynamic water storage. We show how hillslopes sharing the same lithology, vegetation, climate, and tectonic history also share patterns of bedrock weathering. Our data provide constraints on the mechanisms that couple the evolution of the land surface and the propagation of weathering fronts.