THE DEGLACIAL CHRONOLOGY OF THE GREEN BAY LOBE, WISCONSIN

The late Pleistocene, 30,000 to 10,000 cal yr BP, was a period of rapid environmental change in Wisconsin as climate warmed and ice lobes of the Laurentide Ice Sheet receded. Except for the well-dated Two Creeks Forest Bed that was buried by readvance of ice around 13,200 cal yr BP, the chronology of geologic events in east-central Wisconsin is poorly known because of limited occurrence of organic material for radiocarbon analyses.

Recently, 18 rotosonic cores were collected from a basin located within the terminal moraine of the Green Bay Lobe. This basin, once occupied by glacial Lake Oshkosh, is filled with lake sediment and interbedded layers of till of the Kirby Lake and Middle Inlet Members which were deposited by readvances into the basin. In many of the cores, lake sediment displays upward coarsening sequences that reflect opening of lower outlets and lowering and eventual draining of the lake. There are three such sequences, two of which are commonly truncated by till consisting primarily of reworked lake sediment. Based on radiocarbon and optically stimulated luminescence (OSL) dating of over 40 samples, we interpret those sequences to have resulted from the initial formation of early glacial Lake Oshkosh in front of the receding ice lobe as early as 19,800 cal yr BP, followed by two major readvances of the Green Bay Lobe into the basin about 16,500 and 13,600 cal yr BP, respectively. The timing of formation of early glacial Lake Oshkosh is consistent with some recent OSL dating on local lake sediment located near the near the terminal (Johnstown) moraine indicating that the Green Bay Lobe was at its maximum extent prior to 21,000 cal year BP. This is similar with the maximum extent projected for some other lobes along the southern margin of the Laurentide Ice sheet including the Lake Michigan and Superior Lobes. In addition, the first readvance of the Green Bay lobe appears to be coeval with the maximum extent of the Des Moines Lobe (Clayton and Moran, 1992). The post Two Creek readvance that deposited the Middle Inlet till occurred during the later part of the Bölling-Alleröd (13,000 to 14,700 cal yr BP) warm period when climate was cooling before the start of the Younger Dryas.