LANDSCAPE ACTIVITY AND LOESS DEPOSITION DURING DEGLACIATION OF THE GREEN BAY LOBE, SOUTHERN WISCONSIN

Peoria Loess accumulated on stable land surfaces across the mid-continent during the late Wisconsin glaciation. Peoria Loess in the Mississippi Valley is estimated to have been deposited from 25,000 to 13,000 yrs ago (Bettis et al., 2003, Quat. Sci. Rev. 22:1907-1946), and the Green Bay Lobe is estimated to have melted out of southern Wisconsin by ca. 14,000 yrs ago (Clayton and Attig, 1997, Pleistocene Geology of Dane Co. WI, WGNHS Bull.95). Loess thickness of >1 m has long been recognized on upland land surfaces glaciated by the Green Bay Lobe, indicating that loess deposition occurred following deglaciation despite the short overlap in time. However, there is little documentation of landscape processes during the time of loess deposition. This presentation investigates soils and stratigraphic evidence of the environmental setting in which loess was accumulating following deglaciation of the Green Bay Lobe. Data include stratigraphy, particle size, and clay mineralogy from scattered observations, published reports and soil surveys, and three hillslope transects where soils have been studied in detail. Loess recognizable as a distinct unit overlying glacial sediment occurs at summit elevations, while slopes often have thin (or barely recognizable) loess cover that thickens at footslope positions or in drainageways. Soil profile characteristics support an interpretation that much of the loess was quickly redistributed down hillslopes soon after it was deposited. In instances where hillslopes are identified as beaches of a proglacial lake (Lake Scuppernong), no silty cover exists, indicating loess was eroded by waves and became part of the muddy offshore sediment. Smectitic clay mineralogy of offshore sediment best matches loess mineralogy and differs from the illitic till of the Green Bay Lobe. This indicates that loess was reworked by hillslope and near-shore processes into Lake Scuppernong, and loess accumulation had ceased before full incision of the Rock River and the drainage of Lake Scuppernong.