DETAILS OF GLACIAL LAKE OSHKOSH STRATIGRAPHY AND HISTORY REVEALED THROUGH GEOLOGIC MAPPING

Detailed geologic mapping has been a useful tool in refining our understanding of glacial Lake Oshkosh in east-central Wisconsin. Glacial Lake Oshkosh formed beyond the margin of the Green Bay Lobe of the Laurentide Ice Sheet in northward-draining the Fox River lowland. Glacial Lake Oshkosh sediment defines a depositional basin that covered ca. 10,000 square kilometers. Although regional till stratigraphy of the basin was well known, the extent and stratigraphy of lake sediment in the basin were only generally known when our multi-year project mapping glacial Lake Oshkosh began. Better knowledge of the distribution of lake sediment has important implications; for example, these fine-grained deposits are important aquiclude in the regional hydrostratigraphy. Completion of our mapping has yielded a better understanding of the extent and chronology of the lake. Mapping shows a larger lake extent than was known previously, and this expanded Lake Oshkosh is a better fit to glacial isostatic models. New radiocarbon and optically stimulated luminescence dates help refine the sequence of glacial and lacustrine events in the basin. Three lake cycles occurred in the latter part of the Wisconsin Glaciation between about 29,000 and 13,000 cal yr BP. During the intervals between these lake cycles, the Green Bay Lobe receded far enough to open the Sturgeon Bay lowland, and the water level in the Green Bay/Fox River lowland equilibrated with that in the Lake Michigan lowland. Levels of glacial Lake Oshkosh were controlled by the extent of the Green Bay Lobe and the outlets that were exposed beyond the ice margin. The elevations of these outlets can be correlated with glacial Lake Oshkosh shoreline features such as beach ridges and washing limits. When the Green Bay Lobe extended southward beyond the north end of modern Lake Winnebago, glacial Lake Oshkosh spilled to the southwest into the Wisconsin River. A series of outlets across the Niagara Escarpment along the lake's eastern shore were occupied whenever the Green Bay Lobe margin was located between modern Lake Winnebago and Sturgeon Bay.