PROGRESSIVE GREENSTONE CLAST WEATHERING IN DATED QUATERNARY DEBRIS FLOWS, MADISON COUNTY, VIRGINIA
Five 2-m deep backhoe pits dug into cobble or boulder diamicts were placed close to those used in the original study. Two pits contained two debris flow events of distinctly different ages, so analyses were made of seven exposed strata. The youngest deposit, dating from 1995, was brown (10YR) with relatively low amounts of clay (32%) in a thin soil profile. The oldest, estimated to be at least 500,000 years old, was red (10R) with relatively high amounts of clay (59%) in a thick profile. The sediments in surfaces of intermediate age, estimated as 18.5Ka, 75Ka, and 140ka in age, were generally yellowish brown (10YR to 7.5YR) with intermediate amounts of clay (39-53%) in well developed argillic horizons. When examined in geomorphic and stratigraphic order, greenstone clasts in the youngest and intermediate-age deposits exhibited a progressive increase in average rind width (1.1-4.8mm); greenstone rinds in the oldest deposit averaged 14.6mm. The geomorphic and stratigraphic positions, and statistical distinctions between the data, suggest that deposits at this site form four distinct age groups. The progressive changes in relative-age indicators also suggest that the average thicknesses of greenstone rinds might be used to estimate the approximate age of debris flow deposits up to at least 150Ka in the central Blue Ridge of Virginia. However, analyses of data from greenstone rinds, clay content, and geomorphic positions of the Qf3 fans at this site suggest that the reported age of these deposits (est. 75Ka) is too old.