Holocene erosion, sedimentation, and stratigraphy at Raven Fork, Southern Blue Ridge Mountains, USA

摘要

Holocene colluvial and alluvial stratigraphy and a radiocarbon chronology are presented for the valley of the lower three kilometers of Raven Fork, a mountain stream draining 194 km~2 of high relief (1.3 km) terrain of the Southern Blue Ridge Mountains in western North Carolina, USA, which is in a region that lacks good chronological data. Lower hillslopes, alluvial/ colluvial fans, alluvial bottomlands (first terrace and floodplain), and the modern stream channel are landforms described with respect to soils, stratigraphy, and sedimentary structures. Standard methods for subsurface investigations (core holes, excavation units, exposures) are used in conjunction with extensive archeological excavations and cultural chronologies. Radiocarbon ages from each landform are used to calculate long-term-average rates of sedimentation. Results indicate that the first half of the Holocene experienced somewhat more rapid rates of hillslope sedimentation (0.3 to 1.1 mm/yr) than the last half of the Holocene (0.1 -0.2 mm/yr) on footslopes, toeslopes, and alluvial/colluvial fans prior to historic time. We suggest that these subtle differences in the rates of sedimentation were driven by changes in global paleoclimate that favored a high frequency of heavy rainfall, including tropical storms and/or severe thunderstorms and more (and possibly larger) floods during the first half of the Holocene. Prehistoric rates of vertical accretion on the first terrace (T1) ranged from 0.1 to 0.8 mm/yr between about 10,000 and 3000 calendar years ago, and incision below T1 formed the late Holocene floodplain beginning at about 6000 years ago. We suggest that this incision is linked to a reduction in the supply of sediment and a reduction in the magnitude of floods. Historical rates of sedimentation on all parts of the depositional landscape (2.0-2.7 mm/yr on hillslopes and fans and 5.8-6.5 mm/yr on floodplains) were about an order of magnitude greater than prehistoric rates. We attribute these rates to human impacts, such as timber harvest and land clearing, which caused accelerated erosion. We attribute the abundance of fine-grained sediment in streams of the Southern Blue Ridge province, which is atypical in many mountain streams around the world, to the regionally widespread mantle of saprolite as a source of sediment to the fluvial system. Holocene sedimentation on all depositional landforms in the valley led to sedimentary burial of archeological materials, which highlights the need to consider site burial on lower hillslopes and terraces for evaluation of the cultural resources in the Southern Blue Ridge Mountains. These findings show that the entrenched condition of the Raven Fork channel was inherited from the middle Holocene and can be considered a "natural" state for this mountain stream, casting doubt on the negative connotation that is often assigned to entrenched channels.