OPTICALLY STIMULATED LUMINESCENCE AGES FROM FOUR ICE-MARGINAL LAKES IN THE BARABOO HILLS, WISCONSIN

The timing of the maximum extent of the western margin of the Green Bay Lobe during the last glacial maximum has remained obscure because of the lack of closely limiting radiocarbon dates. In an attempt to better constrain this timing we collected a total of eleven samples for optically stimulated luminescence dating from sediment that was deposited in four ice-marginal lakes that formed during or shortly after the maximum extent of the Green Bay Lobe in the Baraboo Hills. Sediment deposited in a lake near the top of the South Bluff was sampled from outcrop, and sediment deposited in lakes in Steinke Basin, at the Tower Road site, and at the Roznos Meadow site was sampled using a truck-mounted drill and split-spoon corer.

Four samples from near the bottom to near the middle of a 16 m sequence of lake sediment at the South Bluff site yielded calendar age estimates of 26.4 ± 5.3, 20.8 ± 2.3 , 15.1 ± 1.4 and 21.7 ± 1.8 ka. Three samples from the Tower Road site yielded ages of 22.2 ±1.7 ka from the top of a 10 m sequence, and 20.6 ± 2.2 ka from the bottom of the sequence. A sample from the middle of the sequence yielded an age of 17.2 ± 1.4 ka. Two samples from near the top of the lake sediment at the Steinke Basin site yielded ages of 17.6 ± 1.7 ka and 23.9 ± 2.2 ka. Two samples of lake sediment from the Roznos Meadow site yielded ages of 17.3 ± 1.6 and 13.2 ± 1.4 ka. The age estimates from these last two sites are in reverse stratigraphic order.

Our age estimates can be discussed as two groups, those younger than 17.6 ka (n=5) and those older than 20.6 ka (n=6). We believe that the five younger age estimates, that tend to be out of stratigraphic sequence with other age estimates, are inaccurate because the samples were taken from heterogeneous sediments and long-term dose rates were likely not correctly estimated. At this point in our project we suspect that the group of older age estimates, those ranging from 26.4 to 20.6 ka, which were taken from uniform sediment where dose-rate values are more easily estimated, provide a better indication of the timing of the maximum extent of the Green Bay Lobe in the Baraboo Hills.