HISTORY OF THE GLACIAL LAKE OSHKOSH BASIN, WISCONSIN

Glacial Lake Oshkosh, one of the largest ice-marginal lakes in Wisconsin during the most recent glaciation, formed as the Green Bay Lobe of the Laurentide Ice Sheet receded northward from east-central Wisconsin. The highest and most extensive level of this lake initially drained westward into the upper Mississippi River valley via the Wisconsin River. As the Green Bay Lobe continued to recede, lower and less extensive levels of glacial Lake Oshkosh formed at outlets opened across the Silurian escarpment. These outlets not only incised deep channels into the landscape, but also captured the lake drainage into the Lake Michigan basin. Subsequent readvances of the Green Bay Lobe back into Wisconsin resulted in reactivation of the outlets and refilling of the lake resulting in a thick (up to approximately 150 m), complex sequence of lake sediment and till that may contain the most comprehensive record of late-glacial climate change in Wisconsin.

At numerous locations within the basin, continuous core has been collected using rotosonic-drilling techniques. The cores revealed moderately thin till layers (2 to 5 m) bounded by thick sequences of lake sediment (3 to 35 m) that consist primarily of clay and silt couplets. In one core, more than 800 couplets were observed that ranged in thickness from 2 to 500 mm. In the cores collected to date, these fine-grained sediments gradually grade upward into coarser silt and sand that is interpreted to represent a regression of glacial Lake Oshkosh due to the opening of lower outlets. Many of these coarser units are capped abruptly by fine-grained sediment as the Green Bay Lobe readvanced southward blocking lower outlets and refilling the basin. This sequence is repeated several times in some cores and appears to be consistent with our current understanding of the lake basin.