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 three 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 carbonates to resistant siliciclastics. The few dated deposits along this reach of the river (e.g. Granger et al., 1997) indicate a major period of aggradation followed by one or more periods of downcutting, possibly as a reaction to climate change or regional drainage reorganization. Cosmogenic ¹⁰Be exposure dating of several terrace 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 cores through 40 m of fill provide a framework for interpreting these ages. Preliminary results indicate a slowing of the New River's incision rate from approximately 120 m/Myr between 0.6 Ma and 0.3 Ma to about 28 m/Myr between 0.3 Ma and the present. The more rapid rate may be related to the evacuation of over 100 m of alluvial fill from which the upper terraces in the study area are cut. The slower modern rate is related to the erodibility of resistant lithologies that outcrop in bedrock-floored reaches of the river. Comparison of soil weathering indices with cosmogenic ages allows further interpretation of incision history based on prior soil chronosequences (e.g. Mills and Wagner, 1985). Cosmogenic exposure ages obtained from higher fill-cut terraces imply that these surfaces are about half as old as previously suggested based on soil development. Young cosmogenic ages could be an artifact of erosion from dated surfaces, especially on older, higher terraces. However, soil profiling suggests that erosion from these surfaces is not extreme, so the cosmogenic ages are regarded as reasonable minimums. Downstream differences in terrace configuration suggest differential incision. Dating of similar surfaces in different reaches of the river and examination of active erosion features on resistant bedrock will help to clarify whether these differences in the New River's incision pattern may be due to tectonic tilting or are due simply to lithologic controls on erosion and deposition.