RADIOCARBON CONTROL FOR THE ADVANCE OF THE GREEN BAY LOBE TO ITS LATE WISCONSIN (MIS 2) MAXIMUM POSITION AT DEVILS LAKE, SOUTH-CENTRAL WISCONSIN

Currently there are few reliable numerical age estimates that constrain the timing of the maximum extent of the Laurentide Ice Sheet in the midcontinent, a problem that largely reflects the scarcity of radiocarbon dates that closely constrain late Wisconsin glacial events. To improve our understanding of the timing of the last glacial advance and retreat, recent research (e.g., Attig et al., 2011; Carson et al., 2012) has begun dating lacustrine sediment deposited in a range of environments along the margin of the Green Bay lobe in south-central Wisconsin. The specific geomorphic settings of these lacustrine deposits allow unequivocal correlation of the sediment to discrete late Wisconsin glacial events, thus providing chronologic control that has previously been lacking.

While previously published data from this research program addresses the timing of onset of retreat of ice from the last maximum position, new data is shedding light on the timing of the end of ice advance to its maximum extent (locally known as the Johnstown phase). The Baraboo Hills in south-central Wisconsin are formed by a doubly-plunging anticline of the Precambrian Baraboo quartzite. Devils Lake gorge cuts through the south range of the Baraboo Hills. The gorge was blocked at both ends by late Wisconsin ice, creating a lake during the glacial maximum and the lower, modern, Devils Lake during post-glacial time. We collected a 9.1-m core into laminated silty lacustrine sediments immediately south of Devils Lake; the base of the core is 9.2 m higher than modern lake level, suggesting that the sediment could only have been deposited when sediment and ice were completely blocking both ends of the gorge. Three radiocarbon dates from plant macrofossils in an organic-rich zone near the base of the core range between 20,480 +/- 100 14C yr BP (24,890 – 24,050 cal yr BP) and 19,100 +/- 80 14C yr BP (23,290 – 23,060 cal yr BP), indicating that the Green Bay Lobe had advanced to its maximum position by that time. These dates represent the first direct absolute age control for the timing of the end of the Green Bay lobe’s advance to its late Wisconsin maximum position, and one of few such chronologic controls along the southern Laurentide ice sheet.