MISSING MID-WISCONSINAN ICE: INSIGHTS FROM A NEW OSL CHRONOLOGY OF OUTWASH EVENTS IN THE LOWER OHIO RIVER VALLEY

The Pleistocene geomorphic history of the Ohio River drainage basin has been profoundly influenced by the advance and retreat of continental ice sheets. Where the Ohio River Valley is narrow, outwash terraces are discontinuous and most are not well preserved. Where the lower Ohio River valley traverses the Illinois Basin, however, the valley is broad and contains a suite of extensive and well preserved outwash terraces. Coal is abundant in the outwash and charcoal is rarely preserved, so dating terraces using radiocarbon is challenging. Consequently, these terraces only had relative ages based upon landscape position and soil development, so the highest and oldest terraces, lacking a Sangamon interglacial paleosol, were presumed to be composed of last glacial maximum outwash. More than sixty luminescence ages were acquired from terraces and underlying outwash along a 100 km section of the lower Ohio River above its confluence with the Wabash River. Luminescence ages of the two highest terrace levels were mid Wisconsinan. The highest terrace level ranged from 52.8 ± 11.2 ka to 43.7 ± 2.3 ka. The ages of the next terrace level ranged from 40.5 ±2.7 ka to 30.7 ±6.6 ka, and the last glacial maximum terrace level ranged from 21.6 ± 3.4 to 16.8 ± 2.3. These ages suggest ice was advanced into the Ohio River basin during the mid-Wisconsinan, which is counter to previous research that suggests the basin was ice free. The lack of data for ice in the basin could be due to erosion by the last glacial maximum advance or possibly the fact that this time period is near the limits of radiocarbon dating.