SEDIMENTARY RECORD OF LATE GLACIAL EVENTS IN THE FOX RIVER LOWLAND, EAST-CENTRAL WISCONSIN

Geologic mapping of the Fox River lowland over the past several years has provided a better constrained late glacial history of the Green Bay Lobe and glacial Lake Oshkosh. Because of rapid population growth and development in the region, the maps will provide important information for making land-use decisions. New data from the subsurface come from cores of 13 rotosonic boreholes drilled in a buried bedrock drainage system. Cores provide a more continuous record of fluctuations of the Green Bay Lobe and of Lake Oshkosh than do cores drilled outside the buried valley. Radiocarbon dates and correlation with published stratigraphic information indicate that the sediment dates to the last part of the Wisconsin Glaciation between 25,000 and 11,000 years ago. During this period, Lake Oshkosh formed along the western flank of the ice and drained when the ice receded into the Green Bay basin. Sediment of the Holy Hill and Kewaunee Formations was deposited at this time.

The Holy Hill Formation contains yellowish brown, sand- and silt-rich till. In the area where the Holy Hill Formation is at the surface, several lithologically indistinguishable till units are present in the cores, separated by meltwater-stream sediment. The till units reflect either minor glacial fluctuations during the later part of the Wisconsin Glaciation or perhaps earlier glaciation.

The Kewaunee Formation contains four members; tills of these members are reddish brown and clay-rich. The cores contain up to three till units within the Kewaunee Formation resting on either bedrock or the Horicon Member. Considerable thicknesses of glacial-lake and meltwater-stream sediments are interstratified with till. In some of the cores, several sequences of varved lake sediment indicate the presence of several phases of Lake Oshkosh for periods of as much as 1,500 years. Varve thicknesses range from a few millimeters to 50 cm or more. Thickness differences are related to proximity to sediment source as well as intensity of summer melt. Correlation of varves across cores may enable us to tease out the signal of summer melt intensity; this would be an important addition to our understanding of the late glacial environment.