Alluvial fan aggradation in low relief, humid-temperate landscapes, Uwharrie National Forest, North Carolina Piedmont

Abstract

We describe the soils, stratigraphy, distribution and ages of alluvial fans in the North Carolina Piedmont to understand factors contributing to their formation in this low relief, humid-temperate setting. We find that those contributing factors (i.e., contributing source basin area and rock type; and climate) parallel, but do not precisely mimic those of arid and semi-arid climate fans. Digital elevation model (DEM) and field observations revealed that about 25% of 209 mapped basin outlets contain alluvial fans. Fans are all < 0.01 km2 in area, with a contributing basin area threshold of ~0.1km2 above which fans are not commonly observed. Fans issuing from basins underlain by more easily weatherable and erodible argillites are generally larger than those that contain more resistant rhyolites. Radiocarbon dates of fan sediments are dominantly late Holocene (13 ages < 3000 ka; two ages between 7000–10,000 ka), with notably no late-Pleistocene aged fans. We thus infer that fan aggradation is predicated on watershed characteristics – and climates – that favor high sediment supply to discharge ratios. Detailed soil and sediment descriptions from pits located on 15 fans reveal that approximately half exhibit cumulic over-thickened soil profiles forming dominantly in sheetflood deposits, providing evidence of ongoing, relatively slow deposition. Low iron ratios measured from those fan soils suggests recycling of more developed soils from basin hillslopes. Occasional matrix-supported debris flow deposits in some fans provide evidence of much less common, rapid (0.5–1 m) fan accumulation. While acknowledging a paucity of detailed climate records in the Piedmont precludes identifying precise mechanistic relationships, our mapping, soils and sedimentologic data suggest that climates favoring modest hillslope erosion without extreme runoff lead to periods of fan aggradation in this transport-limited landscape. Overall, this work documents significant rock-type modulated, climate-driven Holocene landscape evolution in the south-eastern US Piedmont predating legacy sediment erosion and deposition that has gone largely underappreciated.