SOILS AND STRATIGRAPHY OF TRIBUTARY ALLUVIAL FANS IN THE UWHARRIE NATIONAL FOREST, NC, USA

In the streams of the Southeastern US, sediment is the largest non-point source pollutant. Although they store large quantities of sediment, numerous alluvial fans of less than 5000 m²in size remain unstudied in the Piedmont province of the Southeastern US. This, and the general lack of knowledge regarding Piedmont soils, leaves a gap in history of landscape evolution and climate. Using data from alluvial fan soils in the Uwharrie National Forest, four hypotheses were tested: Alluvial fan depositional sequences occur with recorded periods of climate change; alluvial fan sediment size and sorting depends on rock type; alluvial fan sediment size and sorting depends on drainage basin area; alluvial fan sediment size will be comparable to studied fan patterns, where sediment sizes fine down-fan.

The study area contains two major rock types, meta-volcanic and meta-sedimentary. Alluvial fans were mapped along two tributary streams of the Uwharrie River in this area using a combination of topographic data in ArcMap and field observations. Supplementary field observations of each fan were taken including surface clast measurements, vegetation type, vegetation cover, fan channel incision, surface relief, and GPS coordinates of the fan apex. Representative fans of rock type and drainage basin area were chosen for soil pit placement. Samples from each horizon were collected and used for particle size analysis, pedogenic iron testing and clast counts. Charcoal samples provided radiocarbon age dates for the pits. A summation method was used to provide sediment volume estimates for the mapped alluvial fans.

The results indicated that alluvial fans do follow similar deposition patterns to those found in prior studies. It appears most likely that rock type has a greater influence in this region over drainage basin area. Generally, the alluvial fan deposits in meta-sedimentary basins were thicker compared to the meta-volcanic basins. Whether this is due to the sporadic nature of fan deposition in humid-temperate climates or pertaining mainly to rock type necessitates more study. The radiocarbon ages correlate with recurrence intervals of deposition seen in other studies, however more radiocarbon dating is needed across the area to confirm whether these correlate to periods of climate change rather than isolated weather events.