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.

Introduction to the Women in Geomorphology special issue

“…• alluvial fan dynamics (Opalka et al, 2022;Quock et al, 2022;Regalla et al, 2022;Schoch-Baumann et al, 2022).…”

“…Remote sensing ( 26) Agostini et al, 2022;Atwood & West, 2022;Davies et al, 2022;Davis et al, 2021;Fadul et al, 2022;Gervasi et al, 2021;Hendrickx et al, 2022;Hodge & Buechel, 2022;Hooke, 2022;Hovenga et al, 2021;Konlechner & Hilton, 2022;Lefebvre et al, 2022;Marchetti et al, 2022;McKenzie et al, 2022;Mol & Grenfell, 2022;Opalka et al, 2022;Rabanaque et al, 2022;Regalla et al, 2022;Reitman et al, 2022;Sampietro-Vattuone et al, 2021;Schoch-Baumann et al, 2022;Thompson et al, 2022;Vah et al, 2022;Valentine et al, 2022;Zhang et al, 2022 Physical experiments (8) Bako et al, 2022;Bywater-Reyes et al, 2022;Demiral et al, 2022;Friedrich et al, 2022;Negreiros et al, 2022;Rachelly et al, 2022;Roelofs et al, 2022;Vah et al, 2022 Numerical models ( 11) Agostini et al, 2022;Bywater-Reyes et al, 2022;Cullen et al, 2022;Glover et al, 2022;Hodge & Buechel, 2022;Kooijma...…”

“…Country of lead author a References Goodman et al, in press;Guiney & Lininger, 2022;Hinshaw et al, 2022;Hovenga et al, 2021;Lane et al, 2022;Lininger et al, 2021;McKenzie et al, 2022;McKeon et al, 2022;Mossa & Chen, 2022;Opalka et al, 2022;Patton et al, 2022;Quock et al, 2022;Rathburn & Ely, 2021;Regalla et al, 2022;Reitman et al, 2022;Shandonay et al, 2022;Valentine et al, 2022;Wohl, 2022;Zhang et al,…”

“…Field (26) Atkins et al, 2023;Bernatek-Jakiel & Nadal-Romero, 2023;Biausque et al, 2022;Brooks et al, 2022;Bywater-Reyes et al, 2022;Chin, 2022;Gervasi et al, 2021;Glover et al, 2022; Goodman et al, in press;Guiney & Lininger, 2022; Gurnell et al, 2022;Harvey et al, 2022;Hinshaw et al, 2022;Hooke, 2022;Konlechner & Hilton, 2022;Kooijman et al, 2022;Lane et al, 2022;Lefebvre et al, 2022;Marchetti et al, 2022;McKeon et al, 2022;Mol & Grenfell, 2022;Mossa & Chen, 2022;Opalka et al, 2022;Patton et al, 2022;Quock et al, 2022;Regalla et al, 2022;Reitman et al, 2022;Sampietro-Vattuone et al, 2021;Schoch-Baumann et al, 2022;Souza Fonseca et al, 2022;Spiske et al, 2022;Thompson et al, 2022;Uphues et al, 2022;Valentine et al, 2022;Van Soest et al, 2022;Wohl, 2022;Zhang et al, 2022 (Continues)…”

“…Many of the major research directions in contemporary geomorphology are reflected in this collection of papers. With respect to fluvial environments, including alluvial fans, individual papers examine use of images to detect sediment motion during physical experiments (Vah et al, 2022); comparison of grain‐size distributions from field measurements and remotely acquired images (Marchetti et al, 2022); physical experiments on bedload saltation under differing conditions (Demiral et al, 2022); numerical modelling of critical shear stress for grain entrainment based on imagery of bed‐rock channels (Hodge & Buechel, 2022); biogeomorphology (Bywater‐Reyes et al, 2022; Gurnell et al, 2022); channel morphology and classification (Lane et al, 2022), hydromorphology of ephemeral streams (Rabanaque et al, 2022) and interpretation of geomorphic unit assemblages (Fryirs & Brierley, 2022); flood effectiveness (Gervasi et al, 2021); channel evolution, terraces and sediment sources (Guyez et al, 2022) and the influence of sediment supply on gravel‐bed river widening (Rachelly et al, 2022); hydrological variation and meander morphodynamics (Hooke, 2022); the dynamics of wood in river corridors (Guiney & Lininger, 2022; Gurnell et al, 2022; Wohl, 2022) and numerical modelling of wood‐related processes (Friedrich et al, 2022); numerical model validation (Negreiros et al, 2022) and numerical modelling of groundwater‐driven network evolution (Cullen et al, 2022); base‐level controls and landscape evolution (Fadul et al, 2022); long‐term dryland channel response to urbanization (Chin, 2022); watershed restoration and management, including human alterations of process and form (Atkins et al, 2023; Davis et al, 2021; Goodman et al, in press; Gurnell et al, 2022; Hinshaw et al, 2022; Mossa & Chen, 2022; Sampietro‐Vattuone et al, 2021; Valentine et al, 2022); and alluvial fan dynamics (Opalka et al, 2022; Quock et al, 2022; Regalla et al, 2022; Schoch‐Baumann et al, 2022). Papers addressing process and form in coastal environments include examinations of dune morphodynamics and evolution (Hovenga et al,...…”