TESTING THE ROLES OF CLIMATE, TECTONICS, AND BEDROCK LITHOLOGY IN THE LATE CENOZOIC INCISION HISTORY OF THE NEW RIVER

The New River is the only major river that drains to the Gulf of Mexico while cutting through the Blue Ridge, Valley and Ridge, and Cumberland Plateau physiogeologic provinces of the Appalachians. In the Virginia Valley and Ridge, the New River has developed a sequence of unpaired fill-cut and strath terraces where it passes from erodible carbonate underlying wide valleys to resistant siliciclastics in a series of tight ridges. The few dated New River deposits are along this reach of the river (e.g. Granger et al., 1997) and indicate a major period of aggradation followed by one or more periods of rapid downcutting, possibly as a reaction to Quaternary climate change or regional drainage reorganization. The specific nature and timing of these events are unknown. Cosmogenic exposure dating of terraces at several locations and vertical levels allows preliminary reconstruction of the New River's incision history, which can be compared to climate events in eastern North America. Mapping of fluvial terraces and related bedrock straths and logging of two surface to bedrock cores through 40 meters of fill provide a framework for interpreting these ages. Comparison of weathering indices, such as relative degrees of quartz grain dissolution, with cosmogenic ages allows calibration of relative dating techniques for this area. This makes further incision history interpretation possible based on prior soil chronosequences of Harris et al. (1980) and Mills and Wagner (1985). Downstream differences in terrace configuration and incision pattern suggest differential incision in the study area. Dating of similar surfaces in different reaches of the river will help to clarify whether these differences may be due to tectonic tilting or are due simply to lithologic controls on erosion and deposition; prominent knickpoints exist where the river crosses very resistant lithologies such as the Tuscarora Sandstone. The river's overall incision rate and longitudinal profile may be regulated by its ability to cut through the rock in these few short reaches. Examination of active erosion processes, as evidenced by potholing, fluting, and quarrying features on exposed bedrock at these locations, clarifies their role in the New River's incision pattern.