HOLOCENE EVOLUTION OF LOW-ORDER WATERSHED HILLSLOPES AND SOILS IN THE SE PIEDMONT, NC USA

Recent investigations of low-order watershed valley bottom sediments in the southeastern U.S. Piedmont suggest alternating periods of alluvial fan and valley bottom aggradation and stability with evidence of valley bottom accumulation throughout the Holocene (Opalka et al., 2022) and late Pleistocene back to ~130ka, (Eargle, 1940; Richter et al. 2020). However, long-held paradigms of Piedmont landscapes emphasize steady-state in-situ weathering of bedrock into deep soil profiles (e.g. NRCS 2014). To date, there are few records of hillslope soils and sediments that would help to reconcile Piedmont landscape evolution processes.

This study characterizes 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 watersheds are underlain with similar lithologies (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). Samples from all 21 soil pits were analyzed for total organic carbon, particle size, pedogenic iron content, bulk elemental analysis, and samples were collected for radiocarbon dating where available.

Higher numbers of observed buried soils on both N- and S- aspect slopes indicate toposequences located proximal to major waterways have experienced greater landscape instability compared to soils on major drainage divides that are more deeply weathered with higher argillic clay content and a higher incidence of the absence of buried soils. North-facing slopes are comprised of poorly sorted colluvial sediment overlying residual soil that is almost twice as thick as that of south-facing slopes in both watersheds. 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– a finding contrary to long-held paradigms of a stable landscape dominated by in-situ bedrock weathering.