HOLOCENE TO EARLY PLEISTOCENE PIEDMONT DEPOSITS IN A HUMID TEMPERATE CLIMATE: SOUTHERN BLUE RIDGE MOUNTAINS, USA

Coarse piedmont gravels on the western footslopes of Rich and Snake Mountains, Watauga County, North Carolina, are only a few meters in thickness and overlie thick saprolite. Relative-age dating based mainly on weathering-rind thickness of amphibolite clasts was used to map piedmont surfaces into six relative-age classes. The map pattern of surface ages is dramatically different from those reported previously for alluvial fans and pediments. Map polygons tend to be elongate downslope, approximately parallel to streams heading in the uplands, and to have fairly constant widths - there is little tendency for a fan shape. The cross-piedmont order of surface ages is essentially random. This pattern, together with observations on the present erosional activity of piedmont streams, suggest that the processes responsible for abandoning a fan surface and establishing another at a lower level operate chiefly in the cross-piedmont direction. Apparently, new fan surfaces are created by a process of stream entrenchment accompanied by lateral erosion and stream migration. The lack of paired terraces indicates that entrenchment takes place only at the margins of young piedmont deposits. The probable explanation for this is that streams migrate away from the margins of bouldery young surfaces toward areas that are more erodible (e.g., saprolitized bedrock or highly weathered older deposits.) The piedmont deposits may be very long-lived. Burial dating based on the differential decay of cosmogenic 10Be and 26Al in quartz yielded an age of 1.4 Ma for one deposit with one of the greatest mean weathering-rind thicknesses (15.1 mm). This age is supported by the previous finding of reversed paleomagnetism in sediments from the same section, indicating a minimum age of 780 ka. The relative-age of piedmont surfaces is reflected by the form of the transverse profile. Profiles of young surfaces show only slight transverse convexities, while older surfaces show pronounced convexities.