FIVE MILLION YEARS OF APPALACHIAN LANDSCAPE EVOLUTION PRESERVED IN CAVE SEDIMENTS

The ability to precisely date quartzose cave sediments between one-half and five million years of age using the differential decay of cosmogenic ²⁶Al and ¹⁰Be has opened new lines of study for karst geomorphologists, including landscape denudation and fluvial processes over geologic timescales. In the unglaciated Appalachian Plateaus of Tennessee and Kentucky, the relative chronology of episodic river incision and base level stability is well documented thanks to over a century of carefully mapped upland surfaces, inset straths, and fluvial gravels. Constraining the timing of these events has been difficult in the past due to unsuitable dating methods and poorly preserved surface materials. Today, cosmogenic burial dating of clastic sediments in abandoned cave passages show that a select number of multilevel caves on the western margin of the Cumberland Plateau were an active part of the regional hydrology during the Pliocene. These caves contain in situ sediments equivalent to terrace deposits that preserve a record of multilevel development in response to episodic entrenchment of the Upper Cumberland River into the Eastern Highland Rim. The highest cave passages above the modern river level formed prior to ~5 Ma, and contain sediments that correspond with the deposition of upland (“Lafayette-type”) gravels on the Highland Rim by a low gradient Cumberland River. Initial incision of the Cumberland River into the Highland Rim occurred after ~3 Ma, followed by a stillstand at ~2.3 Ma that resulted in development of the Parker strath and a second level of cave passages. A second episode of accelerated incision at ~2 Ma downcut the Parker strath, followed by a brief stillstand that left a major terrace beneath the Parker strath and formed a third level of cave passages. Shorter cycles of incision and aggradation beginning at ~1.3 Ma followed, forming discontinuous terraces above the modern flood plain and marking the end of well-developed cave levels. Widespread aggradation at ~0.8 Ma filled the lower levels until removal by incision to the modern channel elevation. Incision episodes are interpreted as a response to eustasy at ~2.5 Ma and to drainage reorganization of the Ohio River at ~1.5 Ma. Aggradation at ~0.8 Ma is interpreted as a response to climate change.