Pedogenesis and geomorphic implications of soils developed on Blue Ridge alluvial fans, Virginia

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Virginia Tech


Alluvial fans along the western footslopes of the Blue Ridge province, Virginia range over a linear distance of at least 200 km and spread westward up to 4.8 km. Certain soils formed on the fans appear old and highly weathered. Little detailed physical, chemical, or mineralogical data is available about the soils. Concepts about their origin, age, and genesis are vague. This research was conducted in Botetourt, Augusta, Rockingham, and Page counties, Virginia with the intent to gain information about soil properties and to clarify genetic concepts. The objectives were to 1) characterize selected alluvial fan soils distributed along mountain footslopes, 2) show two depositional units compose the sola by using multivariate data analysis procedures, 3) determine the genesis of the soils, and 4) find out if weathered rock bulk density (prb) and free Fe (Fer) are different among sites and, if so, what soil and/or rock properties correlate to ρrb.

Transitional horizons, fragipans, and weakly to moderately developed argillic horizons compose the upper Depositional unit (unit A). Reticulate redoximorphic features and highly weathered quartzite rocks are found in the 2Bt horizons (unit B) where clay approaches 60 percent at 2m.

Up to 25 pedons were sampled within a 50 m² grid at four Sites. Paired samples from the pedons were analyzed by principal component analysis (PCA). The PCA loadings show Significant soil properties contribute about equally to the first principal component. The second principal component loadings show clay-free properties best define the discontinuities. Discriminant functions clearly separate unit A from unit B (P < 0.001).

Clay and free Fe abruptly increase at the discontinuity. Hydroxy-interlayered vermiculite (HIV) dominates unit A clay fractions; whereas, kaolinite is abundant in unit B. Gibbsite is higher in unit B than unit A. Weathered ρrb is not different among five sites on a bajada within the central part of the study area, but it is different from five other Sites on either end of the study area. Free Fe variation among rocks within sites accounts for 70.6% of the variation, but the variation among sites is only 12.7%. Weathered ρrb most strongly, although negatively (r = -0.76), correlates to clay-fraction HIV.

Time and climate are considered the most influential factors controlling soil development on the Blue Ridge fans. A substantial but unknown amount of time passed before unit A was deposited over unit B. Clay percentages, clay mineralogy, and chemical properties indicate the soils formed from materials additional to the quartzite rocks. Vestiges of these materials are no longer present in the soils. Rock weathering patterns suggest the soil weathering environment has changed. The fan soils are similar to other soils in Virginia dated to late Miocene. The alluvial fan soils are probably related to the dated soils, and they may also relate to other soils reported on transported deposits elsewhere in the southern Appalachians.