Optical Ages from Eolian, Lacustrine and Fluvial Sediment in the Glacial Lake Wisconsin Basin

Glacial Lake Wisconsin (GLW) was formed when the Green Bay Lobe of the Laurentide Ice Sheet blocked the Wisconsin River at the east end of the Baraboo Hills. The surficial deposits in the basin are dominated by eolian sand sheets that are re-worked from and directly overlie glacial Lake Wisconsin sediment. In addition, approximately two percent of the landscape is covered in large (15 m + high) dunes that were last active between 14-10 ka based on optically stimulated luminescence dating. Here we present a chronology for the sand sheets, underlying GLW sediments, and remnant outwash trains that are preserved along the edges of the Wisconsin River valley. By doing so, we hope to determine the geologic and climatic factors responsible for dune and sand sheet formation in this setting. Further, we aim to better constrain the timing of Green Bay Lobe retreat by directly dating GLW deposits and overlying eolian sediments. Eight of ten optical ages presented here indicate that GLW existed between 19.3-15.5 ka. Age estimates from eolian sand sheets that cap GLW sediment cluster between 18.1-14.7 ka (n = 5) and 12.4-11.7 ka (n = 3). The older eolian ages likely reflect deflation of GLW sediments just after drainage. However, the younger ages closely overlap those taken from the high relief dunes, suggesting local re-activation of the sand sheet during dune construction. Two samples from the top of the Wisconsin River Love Terrace are 9.1 and 9.8 ka, and likely do not represent the entire occupation of the outwash trains. Collectively, these ages indicate that the Green Bay Lobe had reached its maximum extent before about 19 ka and had retreated to the east end of the Baraboo Hills by about 15.5 ka.