Shallow critical zone architecture of a headwater sandstone catchment quantified using near-surface geophysics

Abstract:

The composition and structure of Earth’s surface and shallow subsurface control the flux of water, solutes, and sediment from hillslopes into rivers. Additionally, bedrock weathering profiles and the stratigraphy of soil and colluvium preserve a record of past surface processes. However, landscapes often exhibit heterogeneity in critical zone architecture that is difficult to capture with remote sensing and costly to characterize through direct measurement in soil pits or drill cores. Here we present results from a multifaceted approach to quantifying spatial variability in critical zone architecture using a suite of geophysical surveys. We focus on Garner Run, a first order sandstone catchment in the Susquehanna Shale Hills Critical Zone Observatory situated in the valley and ridge province of central Pennsylvania, 80 km southwest of the last glacial maximum ice limit. Geomorphic mapping of Garner Run indicates pervasive modification by Pleistocene periglacial surface processes, but the extent to which these processes are recorded in weathering profiles and colluvial deposits is unclear. Through the use of shallow geophysical techniques, including cross-valley transects of seismic refraction, multiple frequency ground-penetrating radar (GPR), and electrical resistivity tomography (ERT), we image spatial patterns in subsurface architecture at a range of scales (10-1,000 m horizontal; 1-50 m depth), and high spatial resolution (cm). By using diverse subsurface methods, we highlight structural (dip-slope) and aspect controls on weathering zone thickness, as well as spatial variations in the depth of colluvium that are consistent with surficial observations. Additionally, our results are consistent with and leverage geologic interpretations based on a 10 m drill core across the entire catchment, and serving as a template for studying modern critical zone processes.