Mapping variations in bedrock weathering with slope aspect under a sedimentary ridge-valley system using near-surface geophysics and drilling

Abstract

The transformation of fresh bedrock into weathered bedrock and mobile soil in the subsurface critical zone is facilitated by changes in chemical composition, material strength, and porosity with depth. These processes dictate how landscapes store and release water to trees and streams (Brooks et al., 2015). Documenting the structure of the critical zone, including the thickness and subsurface topography of different materials, is therefore crucial to quantifying water storage (

Geologic and Tectonic Controls on Deep Fracturing, Weathering, and Water Flow in the Central California Coast Range

“…and the velocity at the interface between fractured rock with oxidation and unoxidized fractured rock was 2.4 ± 0.3 km s −1 . This is consistent with previous work in weathered sedimentary rocks in California which identified velocities of 1.3 ± 0.2 km s −1 at the saprolite‐weathered bedrock interface and 2.0 ± 0.4 km s −1 at the interface between fractured rock with oxidation and unoxidized fractured rock (Hudson Rasmussen et al., 2023). The velocity at the deeper interface is on the low end of lab measurements of unweathered sandstones (2.4–5.6 km s −1 ) (Mavko et al., 2009).…”

Symmetry in Hillslope Steepness and Saprolite Thickness Between Hillslopes With Opposing Aspects

“…Specifically, despite the expectation that NFS should have steeper slopes and a thicker saprolite compared to SFS, it is possible that the actively eroding landscapes of central coastal California require longer geologic time under the current climatic conditions to produce the expected topographic and subsurface asymmetries (Figure 9b). This is supported by previous studies that have demonstrated that delayed geomorphic adjustments to climatic fluctuations can lead to complex contemporary observations of soil erosion, soil thickness and topographic gradients (Heimsath et al., 1999; Hudson‐Rasmussen et al., 2023; Hughes et al., 2009).…”

“…The max vertical velocity gradient best corresponds to the depth at which the seismic velocity ranges from 1,200 to 1,400 m/s (Figure S3 in Supporting Information S1). This range is similar to velocities used to distinguish the saprolite‐weathered bedrock transition in sandstones and mudstones (1,300 m/s; Hudson‐Rasmussen et al., 2023) and granitic gneiss (1,400 m/s; Flinchum et al., 2019). We therefore inferred that the transition between the saprolite and weathered bedrock occurred within the range of the max velocity gradient, the depth to the 1,200 and 1,400 m/s seismic velocity contour.…”

“…Thus, we used vertical velocity gradient profiles across our study transect to identify potential transitions in CZ structure (i.e., porosity, lithology). We calculated the thickness of the saprolite along the survey transect by subtracting the land surface elevations from the average depth of the highest vertical velocity gradient and the corresponding seismic velocity contour, which has previously been shown to represent a transition to saprolite (Flinchum et al., 2018; Hudson‐Rasmussen et al., 2023). We then binned the data into 5 m intervals (horizontal resolution of seismic data) and compared the difference in the calculated saprolite thickness between the max velocity gradient method and corresponding contour method between hillslope aspect.…”

“…Hudson‐Rasmussen et al. (2023) combined seismic refraction geophysical surveys with geochemistry data from Pedrazas et al. (2021) to investigate subsurface weathering between opposing slopes within a rain‐dominated oak woodland underlain by sedimentary rocks.…”