REFINING THE LATE GLACIAL CHRONOLOGY OF THE GREEN BAY LOBE OF THE LAURENTIDE ICE SHEET USING ICE-MARGINAL LACUSTRINE SEDIMENTS

Currently there are very 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 datable carbon along the glacial margin. To improve our understanding of the timing of the last glacial advance and retreat, recent research has begun dating ice-marginal lacustrine sediments: Attig et al. (2011) applied OSL dating to sediments deposited in a small lake in the Baraboo Hills that was impounded by ice only at the late Wisconsin maximum, and Carson et al. (2011) applied a combination of OSL and radiocarbon dating to sediments deposited in small slackwater lakes in south-central Wisconsin that were impounded by late Wisconsin outwash.

Both of those datasets address the timing of onset of retreat of ice from the last maximum position. Sediments collected from within the Devils Lake Gorge in the Baraboo Hills are providing insight into the timing of ice advancing to its maximum position. Late Wisconsin ice blocked both ends of the gorge, creating a lake during the glacial maximum and the lower, modern, Devils Lake. We have collected a 9.1-m core into lacustrine sediments immediately south of Devils Lake; the base of the core is 9.2 m higher than modern lake level, suggesting that the sediments could only have been deposited when ice was completely blocking both ends of the gorge. Three radiocarbon dates from the bottom 2.4 m 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. Combined with the previous studies, these data further constrain the timing of late Wisconsin advance and retreat of the Green Bay Lobe.