QGG PETER BIRKELAND AWARD IN SOIL GEOMORPHOLOGY: SOIL AND HILLSLOPE PROCESSES IN THE SE PIEDMONT, USA

Long-held paradigms of Piedmont landscapes emphasize steady-state in-situ weathering of bedrock into deep soil profiles (NRCS 2014). However, a reevaluation of soils and stratigraphy of sites first described, in part, in historical literature (Eargle 1940) suggests that the Piedmont is in fact a highly dynamic landscape characterized by: discrete periods of massive erosion and deposition from ~40ka to ~150ka (Ferguson 2020, Richter et al. 2020); a notable lack of last-Glacial-aged deposits; and common early- and late- Holocene deposits with a consistent lack of middle-Holocene-aged deposits. To date, however, there are few records of hillslope soils and sediments that would help to elucidate these overall PIedmont landscape evolution processes.

The goal of this study is to characterize four toposequences - including adjacent valley deposits - in two first order watersheds of the SE Piedmont where the entire Holocene source-to-sink system is presumably still intact. The hillslopes are underlain with similar lighologies (felsic metavolcanics), but vary in aspect (N & S) and in proximity to major rivers. Soil profiles, sedimentology and stratigraphy were described for 5 soil pits per hillslope (~1.5 m deep); and samples from all pits were analyzed for total organic carbon and particle size.

Observations reveal common unconformities, identified using buried soils (Birkeland 1999), both between different sediment packages and between sediment and underlying saprolite on the hillslopes. North-facing slopes have colluvial packages (poorly sorted, occasionally containing leached horizons) overlying soil derived from saprolite that are almost twice as thick as their south-facing counterparts in both watersheds. We also observe more developed, redder soils with a higher argillic clay content and an absence of buried soils in hillslope positions with south aspects near major watershed divides, suggesting these landscape positions promote deeper weathering profiles and relatively thinner mobile regolith, a finding consistent with previous modeling based on Piedmont landscapes (Brecheisen et al. 2021). In sum, Piedmont hillslopes appear to be marked by episodic and varied erosion, transportation, and deposition during the Holocene and Late Pleistocene, similar to their valley-bottom counterparts.