Spatial patterns of bedrock weathering at the hillslope scale inferred via drilling and multi-scale geophysical methods

The critical zone (CZ) comprises the near-surface region of the Earth where biological, physical and chemical weathering processes form porous soil and weathered bedrock from initially fresh bedrock. The structure of the CZ plays an integral role in near-surface processes including water cycling, however this subsurface structure is unmapped relative to surface topography, outside of isolated boreholes, road cuts, and landslides. Here, we characterize this structure and its variability within the Coastal Belt of the Franciscan in the Northern California Coast Ranges to test the hypothesis that there exists a relationship between weathering profile structure and topography. We compare spatially detailed data from boreholes from the Eel River Critical Zone Observatory (ERCZO) with surface seismic refraction and electrical resistivity surveys to develop the most probable seismic velocities associated with interfaces between (1) saprolite and weathered bedrock and (2) weathered and fresh bedrock. P-wave velocities calculated via regression ranged from 350-610 m/s for saprolite and soil, 610-1910 m/s for weathered bedrock, and > 1910 m/s for fresh bedrock. Extending these results to three hillslopes lacking borehole data reveals a common weathering profile structure, whereby weathering depth increases upslope across hillslopes and weathering thickness is roughly parallel to the ground surface along ridgelines. The maximum thickness of weathering, at the topographic divide, appears to scale with hillslope length. Significant seismic anisotropy, reflecting dominant bedding or fracture orientation, is evident in a subset of transects. Electrical resistivity tomograms reveal a consistent, shallow 2-8 m resistive layer associated with the weathered bedrock vadose zone underlain, in many cases, by a conductive zone which we propose reflects a seasonal groundwater system within weathered bedrock. Our seismic and electrical imaging therefore reveals a consistent structure across different hillslopes. In this tectonically active environment, we find that weathering profiles penetrate deeply beneath hillslopes in a broadly systematic pattern. This suggests consistency in weathering process across the landscape that operates at the hillslope scale where channels remain fresh and ridgetops show deep weathering.