Weathering and soil development on fan surfaces as a function of height above modern drainageways, Roan Mountain, North Carolina

A relative-age chronosequence has been established for fan surfaces on the west and south piedmont slopes of Roan Mountain, North Carolina, based on measurements of weathering and soil-development indices at 20 sites. In addition to standard soil properties, measurements were made of amphibolite clast weathering-rind thickness, maximum surface boulder size, and mean size and sorting of a 10 kg whole-sediment sample from the C horizon. Soils lacking Bt horizons occur only at high altitudes at or near the heads of young fans. Excepting these locations, the youngest fan surfaces have soils with Bt horizons generally less than 60 cm thick, hues of 10-7.5YR, maximum clay contents less than 35 percent, rind thicknesses less than 1–2 mm, and maximum boulder diameters of 1–2 m. Soils of the oldest fan surfaces have Bt horizons 75–200 cm thick, hues of 5-2.5YR, rind thicknesses of 6–14 mm, maximum clay contents of 40–60 percent, and maximum boulder diameters less than 0.75 m. Although the youngest fan surfaces are close to the level of modern drainageways, the oldest stand as much as 20 m above these drainageways and are obviously relict features. An index of fan height above modern drainageways was devised that combines height above lateral drainageways with height above the main stream of the valley into which the fan debouches. Many weathering and soil-development properties correlate significantly with this height index, including weathering-rind thickness (r=0.86), Munsell hue (−0.65), maximum percent clay (0.59), graphic mean (phi units) of whole-sediment particle-size analysis (0.54); maximum boulder diameter (−0.52); maximum percent free iron in B horizon (0.50), and thickness of Bt horizon (0.49). A soil weathering/soil development index using these parameters shows a 0.82 correlation coefficient with the height index. A cluster analysis shows that the 20 sites can be placed into three distinct groups, suggesting that the formation of fan surfaces may be episodic on long; time scales. A comparison of the fan map with a county soils map shows that the oldest fan surface is fairly consistently mapped as Dillsboro series, but that the younger fan surfaces show no consistent relationship to soils mapping.