Influence of contrasting aspect, lithology, and vegetation on saprolite genesis in complex terrain: Reynolds Creek Critical Zone Observatory

Abstract:

This study employs a variety of geophysical, biological, hydrological, and pedological methods to expand on the understanding of how contrasting aspects, lithologies, and vegetation influence critical zone structure and evolution. We performed shallow seismic refraction (SSR) and time-lapse electrical resistivity tomography (ERT) surveys across two geologically distinct valleys within the Reynolds Creek Critical Zone Observatory in southwestern Idaho. We also quantified vegetation density, soil pH, and subsurface stratigraphy (by manual sampling) across opposing north-facing (forested) and south-facing (unforested) aspects to better understand the relationship between lithology, vegetation, seasonal moisture dynamics, and saprolite genesis within the critical zone. The first study sub-site, Upper Johnston Draw, resides on late Cretaceous granitic bedrock associated with the Idaho Batholith. The second study sub-site, Upper Sheep Creek, resides on Miocene basaltic bedrock. In the granitic Upper Johnston Draw there is a sharp contrast in depth to unweathered bedrock (regolith thickness) between the north-facing aspect (average depth of 18.6 m) and the south-facing aspect (average depth of 8.2 m). In the basaltic Upper Sheep Creek there is only a marginal contrast in depth to unweathered bedrock between the north-facing aspect (average depth of 14.4 m) and the south-facing aspect (average depth of 12.0 m). These observed relationships between the contrasting lithologies of Upper Johnston Draw and Upper Sheep Creek, coupled with our time-lapse ERT surveys, vegetation density tests, soil pH tests, and subsurface augering data, provide new understanding about the causes of symmetry or asymmetry in saprolite development on north-facing and south-facing slopes. Specifically, these findings suggest that abiotic chemical weathering via hydrolysis may be the dominant control creating the symmetrical pattern of saprolite genesis (north vs. south aspects) observed within the basalt-parented site. While conversely, within the critical zone of the paired granitic site, ecohydrologically-influenced chemical weathering via oxidation and organic acidification may instead be the dominant control creating the distinct asymmetrical pattern of saprolite genesis observed.