Using shallow seismic surveys to test theory for critical zone depth variation across hillslopes

Abstract:

The elevation of unweathered bedrock beneath hillslopes (Zb) is a dynamic subsurface boundary. Theory suggests that the position of Zb across a hillslope could be influenced by hydraulics of groundwater, regional stress state, and physical and chemical weathering processes. Although this subsurface region plays an important role in the ecosystems by influencing the water storage and runoff, direct measurement of Zb along hillslopes and the hydraulic properties within this region reminds difficult. Here we aim to use geophysical tools to probe Zb and understand the dominated process that controls Zb in soil-mantled hillslopes. Our study area is in a predominantly soil-mantled hill landscape north of the Cortina Ridge near Williams, California. Most of the Mesozoic sedimentary Great Valley Sequence bedrock here is north-south in strike and ~50° east in dip. We use Light Detection and Ranging (LiDAR) based digital elevation model to characterize surface geomorphology and shallow seismic refraction to image subsurface structure along and across the north-south oriented ridges. Preliminary cosmogenic dating in sediment deposits in the valley shows ~0.1 mm/yr long-term exhumation rate. By assuming steady state and a nonlinear sediment transport law, the estimated soil diffusivity is ~1.8-2.2×10^-2 m²/yr. Results of shallow seismic refraction here show a change of velocity gradient when the seismic velocity is ~2000 m/s. The depth of this velocity transition could indicate the depth to Zb, implying a change in rock properties such as fracture density, rock moisture, chemistry, etc. Our surveys that run along strike and across hillslopes show that the depth to Zb (and thus the weathering zone thickness) generally increases upslope to the hillslope divide. Surveys perpendicular to the strike along local ridges reveal depth variations due to differences between shale, sandstone and conglomerate. We use an analytical model for groundwater drainage in fresh bedrock to predict Zb depth along seven hillslopes with clear Zb indications. We find that saturated hydraulic conductivity of ~10^-11-10^-12 m/s successively predicts the general trends. This result suggests that a long-term hydrologic interaction with bedrock controls the boundary between weathered-to-unweathered bedrock along hillslopes in this region.