ASSESSING THE DEGLACIAL CHRONOLOGY OF THE GREEN BAY LOBE: EVIDENCE FROM GLACIAL LAKE OSHKOSH

Sediment deposited in large proglacial lakes along the southern margin of the Laurentide Ice Sheet records a complex series of events that occurred as ice lobes receded from their maximum extent during the late Pleistocene, ca. 21 ka ago. One such lake, glacial Lake Oshkosh, formed as the margin of the Green Bay Lobe receded northward from east-central Wisconsin. The level of the lake was controlled by the relationship of the ice margin to four outlets that opened in succession to the north.

Thick sequences of varved lake sediment and till recovered from seven rotosonic cores within the basin indicate a dynamic ice margin. Dating of lacustrine sediment using optically stimulated luminescence (OSL) indicates that glacial Lake Oshkosh initially existed for 1000 years, from ca. 16.5 - 15.5 ka. The recession of the lobe was interrupted by at least two readvances into the basin. The first readvance of the Green Bay Lobe occurred ca. 15.0 ka as determined by radiocarbon analyses of buried organic material. The lobe blanketed the landscape with a layer of red, clayey till and constructed an extensive moraine. With warmer climate conditions and subsequent recession of the lobe, the lake basin had drained by ca. 14.2 ka, and boreal forest (Two Creeks interval) grew on the lake plain. Part of this forest was overridden with a second, less extensive incursion of the lobe into the basin ca. 13.5 ka. With this readvance, glacial Lake Oshkosh formed for a third and final time, drowning parts of the forest. It appears that this readvance was relatively short-lived because the sediment is thinner than 1 m, and moraines and subglacial bedforms are poorly developed.

During the late phases of glacial Lake Oshkosh, an extensive field of parabolic sand dunes formed adjacent to the lake basin. The geometry of the dunes indicates that sand was transported from north to south. Preliminary results from OSL dating of the dunes indicate that they formed c. 12.0-14.2 ka and were partially reactivated ca. 100 years ago reflecting agricultural practices.

The results of this study will help define the Deglacial chronology of the Green Bay Lobe and its interaction with a large, complex ice marginal lake.