USE GREENSTONE WEATHERING RINDS TO ESTIMATE AGE OF DEBRIS FANS, BLUE RIDGE MOUNTAINS, CENTRAL VIRGINIA, U.S.A

Distinct weathering rinds occur on greenstone cobbles buried in Quaternary debris flow deposits in the Blue Ridge of central Virginia. In Madison County, Eaton and others (2003) used carbon-14 and beryllium-10 analyses to estimate the ages of five surfaces (Qf1-Qf5) in one debris fan complex. Bradley and others (2012) measured weathering rinds on greenstone clasts excavated from backhoe pits on those surfaces. Comparisons of these rind data with dates obtained from the same fan surfaces, along with soil color and Bt horizon clay data collected by previous workers, suggested a clear progression of weathering-related features with age. They proposed a regression equation using the average thicknesses of greenstone rinds to estimate the age of debris flows formed during the late Pleistocene.

By making the presumption that weathering rates on buried greenstone clasts will vary across the region uniformly as conditions change through time, one can apply this equation to greenstone-dominated debris fans previously studied in central Virginia. Ryter (1989) mapped two sets of greenstone-dominated debris fans in Augusta County adjacent to Back Creek. The younger fan soils were relatively brown (5YR) with maximum Bt clay 24%; greenstone rinds averaged 4.3 mm. The older fans were more orange (2.5YR) with maximum Bt clay 37%; greenstone rinds averaged 9.0 mm. Youngblood (1998) mapped three sets of debris fans formed by Stoney Creek in Nelson County. The lowest surface contained brown (10YR) soils with low Bt clay (10-30%); greenstone rinds averaged 3.5 mm. The intermediate fans had orange (7.5YR to 2.5YR) soils with high clay (60-70%) and rinds that averaged 6.5 mm. The highest fans were red (10R) with very high clay (70-80%) and rinds of 15 mm. Application of the Madison County fan equation suggests the surface deposits of the two Augusta County fans may be approximately 100Ka and 280Ka. Using the same logic, the Nelson County fans may have formed during episodes ending approximately 65ka, 180Ka, and 500Ka.