OPTICAL AGES FROM ICE-MARGINAL LAKE DEPOSITS IN THE BARABOO HILLS INDICATE THE GREEN BAY LOBE WAS AT ITS MAXIMUM EXTENT ABOUT 20,000 YEARS AGO

During the last several decades an appreciable body of work has been dedicated to evaluating the chronology of glacial events in the upper Midwest. This undertaking has been hampered by the lack of closely-limiting radiocarbon ages for nearly all late glacial events in Wisconsin, and has been particularly difficult in dating deposits along the western margin of the Green Bay Lobe (GBL). The lack of reliable radiocarbon ages stems from the presence of permafrost which limited the growth of vegetation in the region, and therefore, the potential to use radiocarbon to constrain the timing of late glacial events. The timing of the maximum extent of the GBL has remained obscure.

Therefore, to determine when the GBL reached its maximum extent, we used optically stimulated luminescence to date sediment from two ice-marginal lakes that were dammed against the eastern Baraboo Hills. These lakes could only have existed during and very shortly after the GBL had reached its maximum extent. Six optical ages from samples collected at an exposure of lacustrine sediment in South Bluff Basin averaged 20.1 ka. Six ages from lacustrine samples collected from drill holes and an outcrop in Feltz Basin averaged 19.7 ka. In both of these basins samples were very well behaved and in general showed good agreement from the multiple ages produced at each site. We also collected samples from cores in lake sediment and ice-marginal outwash fans in two other ice-marginal lake basins in the eastern Baraboo Hills. Because these lakes likely existed well after the last glacial maximum, age control from them provides minimal ages for the maximum extent of the GBL. These age estimates are generally younger, and show greater variability compared to the other sites, but do show general agreement with the ages from the Feltz and South Bluff Basin sites. Collectively, our optical ages indicate the GBL stood at its maximum extent in the Baraboo Hills about 20,000 years ago.