PROVENANCE, AGE, AND PALEOENVIRONMENTAL SIGNIFICANCE OF THE CENTRAL WISCONSIN LOESS SHEET

The central Wisconsin loess sheet provides an excellent opportunity to refine the minimum limiting age of permafrost degradation proceeding the last glacial maximum in north central Wisconsin. Loess thickness exceeds a meter in central Clark County, thinning northeastward to approximately 40 cm in western Marathon County. Silt loam-textured on its eastern margins, the loess sheet coarsens westward, crossing three different substrata (i.e. fine-grained sandstone residuum, sandy Bakerville till, and silty Edgar till), grading into a very fine and fine sand mantle, presumably also eolian in origin. The loess is not directly glaciogenic, as large meltwater valleys do not occur in the area. Provenance of this loess is likely the fine-grained, Cambrian-aged sandstones and siltstones, usually interbedded with shale, which crop out in western Clark County, at the far western edge of the study area. This bedrock-dominated landscape probably experienced instability and high rates of mass wasting, as did the rest of the Driftless Area, when permafrost began to degrade, subjecting newly exposed sediment to deflation. Based on three OSL dates, permafrost in north central Wisconsin likely began to degrade around 13.3K yr BP, allowing for loess entrainment and subsequent deposition to occur.