A RE-EVALUATION OF THE TIMING OF MAMMOTH CAVE DEVELOPMENT AND FORMATION OF THE OHIO RIVER (Invited Presentation)

One of the seminal contributions of Art and Peg Palmer and Dick Powell was the recognition that caves and karst landscapes record the regional geomorphic history of erosion and river incision, particularly in the Interior Low Plateaus of Kentucky and Indiana. In this tectonically stable part of the country, the landscape reflects long periods of base level stability punctuated by rapid pulses of river incision that left terraces and karst plains on the surface and multilevel caves underground. One of the key events driving river incision regionally was large-scale drainage reorganization due to advance of the Laurentide Ice Sheet. The ice sheet dammed the preglacial Teays River, causing it to spill across drainage divides into the preglacial Ohio River, thus forming the modern Ohio River near the ice sheet margin. This prompted rapid incision near the old drainage divide and potentially drove regional incision due to increased drainage area. This idea that rapid river incision is expressed in regional cave development was supported by cosmogenic nuclide dating at Mammoth Cave in the late 1990’s, which showed that the large upper levels of the cave were abandoned due to rapid base level lowering of the Green River near 1.5 Ma, attributed at the time to an incision pulse driven by formation of the modern Ohio River.

We will present a re-examination and re-analysis of old data from Mammoth Cave as well as new data from other sites across the region to show that the story may not be quite so simple. Incision of the Green River at Mammoth Cave predates formation of the modern Ohio, which likely occurred during a major ice advance near 1.3 Ma. This suggests that the preglacial Ohio River and its tributaries (the Tennessee, Cumberland, Green, and Salt Rivers) entrenched in response to incision of the Mississippi River that occurred sometime after deposition of the Upland Complex (Lafayette) gravels in the Late Pliocene. In this case, we would expect to see that ages of multi-level caves across the region reflect the time-transgressive passage of knickpoints up the major river systems, as previously observed for Cumberland-style caves in Tennessee using cosmogenic nuclide dating. Future dating of caves and terraces can test this idea, and may show that deep incision was limited to the region near and upstream of the preglacial drainage divide.