WEATHERING CHARACTERISTICS AND AGES OF DEBRIS-FLOW DEPOSITS AT GRAVES MILL, VA

Debris-flow fan deposits in the upper Rapidan River Basin record a long and complex history of fluvial and mass movement events. During a convective storm in June 1995, 775 mm of precipitation in 16 hours triggered over a thousand debris flows and left many stream channels and debris fans deeply incised. Radiocarbon ages of prehistoric debris-flow deposits exposed in these incisions indicate a debris-flow recurrence interval in the region of 2,000 to 3,000 years since the Wisconsinan glacial maximum. Soil chronosequence studies from surface mapping and trenching of a fan 1 km west of Graves Mill, VA reveal debris-flow deposits of various ages. The youngest and topographically lowest debris-fan site lacks significant soil pedogenesis, shows Munsell colors of 10YR, and a clay content of only 3 percent. By contrast, a second site has a 1.6 m thick debris-flow deposit with a solum thickness of 0.9 m, Munsell colors of 5YR, and an argillic horizon that contains 29 percent clay. This deposit contains greenstone clasts with weathering rinds of less than 2 mm and has a radiocarbon age of 18,490 +/- 60 YBP. A trench at the oldest site revealed two distinct debris-flow deposits distinguished by a greater concentration of cobbles in the lower deposit. The upper unit has a 1.0 m thick argillic horizon, a 2.5YR Munsell color, and a clay content of 72 percent. The lower unit has an argillic horizon that exceeds 0.8 m, a 10R Munsell color, and a clay content of 40 percent. Surface boulders are notably absent from the site, and nearly all in-situ cobbles are extensively saprolitized. Comparison of XRD analyses of the two deposits shows substantially higher concentrations of kaolinite and illite in the older deposit. Future research, including cosmogenic dating of soils, should help elucidate ages of older debris-flow deposits that clearly exceed the limits of radiocarbon dating techniques.