REVISITING HILLSLOPE SEDIMENT: HOLOCENE AND LATE PLEISTOCENE LANDSCAPE DYNAMICS IN THE PIEDMONT UPLANDS OF SE NA

Recent and historic investigations of low-order watersheds in SE US Piedmont indicate episodic formation of hillslope sediment (HSS), gully incision, and valley bottom aggradation during the Holocene and late Pleistocene back to ~135ka. These observations oppose long-held paradigms of Piedmont landscapes emphasizing in situ weathering of bedrock into deep soil profiles and steady-state regional erosion. Here we present data documenting the ubiquitous nature, dynamics, and storage of HSS in the SE Piedmont. This study considers two contexts of HSS in first-order drainages of the Piedmont: 1) four toposequences in two watersheds with middle Holocene to modern aged deposits; and 2) two sediment-filled paleo-gullies containing deposits mostly from ~55ka to ~135ka. In both contexts, sediment occurs in distinct allostratigraphic units defined by disconformities and unconformities between sediment packages and underlying saprolite.

In the Holocene toposequences, HSS thickness and preserved stratigraphy vary with aspect and hillslope position – both within any single slope and within the regional landscape. The Pleistocene paleo-gullies contain well-preserved pollen and plant macrofossils and are of similar scale (up to 15 m) and extent as the well-documented historic gullying of the late 19th and early 20th century. As sediment-filled paleo-gullies become erosionally resistant, modern gullies have formed in easily erodible saprolite. Similar to arroyo formation in the SW US, gullying in the SE Piedmont is not uniquely an anthropogenically induced process. HSS is present over much of the Piedmont landscape –from uplands to lowlands– and has been moving continually but not constantly downslope since the Late Pleistocene. Periods of hillslope aggradation are punctuated by episodes of erosion and in some cases severe valley-bottom incision. These observations prompt a reevaluation of Piedmont soils and landscapes, indicating their formation histories are dynamic and polygenic.