ALLUVIAL FANS IN A HUMID-TEMPERATE, LOW RELIEF LANDSCAPE: EXAMPLES FROM THE UWHARRIE NATIONAL FOREST, NC

Well documented in arid environments, alluvial fans in humid-temperate, low relief landscapes have often been overlooked. We present a description of soils, stratigraphy and chronology of fans in the North Carolina Piedmont to understand factors contributing to their formation in this setting, and to compare them to their arid region counterparts.

Digital elevation models and field observations reveal that approximately 25% of 209 mapped 1^st and 2^nd order drainage basin outlets contain alluvial fans. These fans are all <0.01 km² in area, and are observed issuing from contributing basin areas of <0.1 km². Alluvial fans issuing from basins underlain by erodible argillites generally have greater surface areas than those underlain by more resistant rhyolitic rock. Radiocarbon dates of fan sediments are predominantly late Holocene (13 ages <3000 ka; two ages between 7000-10000ka), with no ages recorded from the late Pleistocene. We infer that fan aggradation here is based on basin characteristics and climates that likely favored high sediment supply and low discharge. Detailed soil and sediment descriptions – following methods from Birkeland 1999 – from soil 15 soil pits excavated on fan surfaces reveal approximately half exhibit cumulic, over-thickened soil profiles forming dominantly in sheetflood deposits. This suggests ongoing, slow aggradation of these alluvial fans in the Holocene. Infrequent matrix-supported debris flow deposits in fans provide evidence that rapid fan aggradation has occurred, albeit less commonly. Low pedogenic iron ratios measured from these fan soils indicate potential recycling of more developed soils from hillslopes higher in the contributing basins.

Though there is a paucity of detailed climate records in the southeastern Piedmont, our mapping, soils and sedimentologic data suggest that climates here favor modest hillslope erosion without extreme runoff, leading to periods of fan aggradation and brief soil development in this transport-limited landscape. The contributing factors (e.g. basin characteristics and climate) are similar, but do not exactly mirror those in arid regions. In conclusion, our work provides a record of bedrock controlled, climate driven Holocene landscape evolution in the southeastern US Piedmont which predates legacy sediment erosion and deposition.