THE INFLUENCE OF CLIMATE CHANGE AND RIVER INCISION ON CAVE DEVELOPMENT IN THE UNGLACIATED OHIO RIVER BASIN

A number of cave systems in the unglaciated Ohio River basin contain very large phreatic tubes now found abandoned some 40-80 meters above the modern water level. These large, nearly level passages apparently formed during a long period of base-level stability, and were abandoned upon river incision. Cosmogenic nuclide dating of quartz-bearing cave sediments by radioactive decay of ²⁶Al and ¹⁰Be reveal that four of these large caves on the Cumberland escarpment, Tennessee, were abandoned after the initiation of Laurentide glaciation at about 2.4 Ma. Sediment ages range from approximately 1.8 Ma near the Cumberland River to about 0.8 Ma on upstream tributaries. These ages are consistent with the timing of cave passage abandonment on the Green River, Kentucky, and the New River, Virginia. The pattern of cave passage abandonment along the Cumberland River suggests that a knickpoint may have propagated up the river and its tributaries causing cave passages to be abandoned in sequence. Further evidence of river stability followed by rapid incision comes from patchy but thick deposits of highly weathered alluvial gravels found 40-60 meters above entrenched meanders of the modern river. Both the terraces and the caves may indicate that the onset of glaciation and re-routing of major rivers along the ice sheet margin generated a pulse of river incision that propagated through the entire Ohio River basin. The large, high-level phreatic tubes that are characteristic of this region probably formed during the Miocene and Pliocene, when a warm and stable climate promoted slow river incision and a stable water level that was conducive to extensive cave development.