Spatial Variation in Bedrock Weathering Under a sequence of ridges and valleys in the Great Valley Sequence of Northern California revealed through geophysics and deep drilling

Abstract:

Bedrock weathering in the critical zone influences water storage and nutrient availability. However, direct measurements necessary to interpret critical zone structure cannot be densely sampled along hillslopes. Borehole analysis coupled with near-surface seismic refraction can directly characterize bedrock properties as well as capture spatial variation in subsurface structure at the hillslope scale. The alternating ridge-valley landscape of the upturned Great Valley Sequence in the Northern California Coast Range is an ideal setting to analyze critical zone structure because of its simple sedimentary lithology and topography. Weathering characterization conducted in fourteen boreholes distributed across ridgetops, channel-bottoms, and hillslopes revealed two weathering fronts. The shallow front (6-8 m below ridgetops) is marked by an abrupt increase of mechanical strength and a decrease in chemical weathering. This front also corresponds to the depth extent of pyrite oxidation and of seasonally dynamic rock moisture storage. The deeper front (~20 m) at ridgetop is a more gradual transition from discretely fractured and chemically weathered material to bedrock with rare, cemented fractures. Vertical velocity profiles measured via shallow seismic refraction conducted over borehole locations show corresponding weathering front locations. A high seismic velocity gradient at a 6-8 m depth coincides with the abrupt mechanical strength increase at the shallow weathering front. Low velocity above this depth could be attributed to chemical depletion and/or volumetric strain. A gradual increase of seismic velocity at ~20 m depth is consistent with the deeper weathering front. Across hillslopes, seismic velocity reinforces borehole observations of upslope-thickening weathering profiles. Seismic profiles also strengthen limited borehole evidence that north and south-facing hillslopes have similar structures at depth, despite differences in soil thickness and vegetation between the hillslope aspects. This work demonstrates that seismic refraction improves characterization of weathering fronts across hillslopes, where borehole data are sparse. Joint analysis of the two datasets can further assess the roles of chemical depletion and volumetric strain in critical zone development.