A NEW CHRONOLOGY OF RIVER INCISION ACROSS THE UPPER CUMBERLAND RIVER SYSTEM, TENNESSEE AND KENTUCKY, USA, FROM COSMOGENIC 26AL AND 10BE IN CAVE SEDIMENTS

Rivers across the unglaciated Ohio River basin are almost without exception deeply entrenched. The Upper Cumberland River is an excellent example, with more than 400 km of entrenched meanders as it crosses the Appalachian Plateau uplands. Constraining the timing of incision events has not been possible in the past, given scant alluvial material that was not amenable to known dating techniques. We offer a new chronology of river incision and landscape denudation based on the technique of dating buried sediments using cosmogenic 26Al and 10Be, which we use to date the onset of incision at 14 multilevel caves distributed alongside the Cumberland River and its tributaries. These caves exhibit characteristics indicating a common history related to river entrenchment. Each contain large, abandoned upper-level passages connected to lower, active levels by narrow canyons. The upper-level passages once carried water and sediment from plateau uplands to a stable water table controlled by the Cumberland River. A wave of incision migrated up the Cumberland River and its tributaries, lowering the water table at each of these major caves in turn and causing the sequential abandonment of upper-level conduits in favor of lower levels. Sediments left behind in upper levels represent the onset of incision, and can be interpreted in much the same way as alluvium-mantled strath terraces. The sequential abandonment of cave passages rules out regional entrenchment due to epeirogenic uplift and points instead to knickpoint migration. A single incision pulse older than 2 million years traced upstream along the Upper Cumberland River basin implicates either an older drainage reorganization than that of the Ohio River (near 1.5 Ma) or eustasy as the cause of entrenchment. Using the burial-dating technique to date the onset of incision, we find episodic incision to have occurred at least twice over the past three million years.