COSMOGENIC ISOTOPE BURIAL DATING REVEALS 1.5 MILLION-YEAR-OLD FAN DEPOSIT IN BLUE RIDGE MOUNTAINS OF NORTH CAROLINA

Burial dating based on the differential decay of cosmogenic 10Be and 26Al in quartz yields an age of 1.45 ± 0.17 Ma isotope for a fan remnant on the west slope of Rich Mountain in Watauga County, NC, in agreement with an age of greater than 780 ka indicated by sediments with reversed paleomagnetism. Previously, relative-age dating of fan deposits in the study area was accomplished by means weathering-rind thickness on amphibolite clasts. Of 118 sites at which weathering-rind thickness was measured, this site (Z6) has the third greatest mean rind thickness (15.1 mm). Thus, it essentially represents the oldest generation of fans in the study area. The absence of still older fans may result from the removal of such fans by erosion; Z6 is a low, rounded ridge, representing the last stage in erosional destruction of a somewhat larger fan. An alternative explanation, however, may be that prior to this time fans were sparse, and that the 1.45 Ma age represents the beginning of prominent fan building on piedmont slopes in this area. Such a change in depositional regime conceivably could result from the onset of glacial climates in the early Pleistocene. Vegetation changes or more intense freezing may have resulted in a coarsening of rock debris supplied from hillslopes, debris that was more difficult to move than the finer material previously supplied, resulting in the accumulation of deposits on the piedmont. This explanation receives some support from the coincidence of the age of Z6 with the age (1.5 ± 0.3 Ma) recently obtained for the southern advance of the ice sheet that formed the Ohio River.