Thursday, 9 October 2008: 8:35 AM
George R. Brown Convention Center, 332BE
The relatively thin central Wisconsin loess sheet provides an excellent opportunity to refine the minimum limiting age of permafrost degradation following the last glacial maximum in north central Wisconsin. Loess thickness exceeds a meter in northern and central Clark County, thinning southeastward and eastward to approximately 40 cm in western Marathon County. Silt loam-textured on its eastern margins, the loess sheet coarsens westward and northwestward, as it crosses three different, pre-Late Wisconsin substrata (i.e. fine-grained sandstone residuum, sandy Bakerville till, and silty Edgar till). The very fine and fine sand textures of the loess sheet on its western edges are presumably also eolian in origin. Thickness and particle size data suggest two sources of loess: (1) the ice-walled lake plains on and behind the Late-Wisconsin terminal moraine to the northwest of the loess sheet, and (2) sediment derived from fine-grained, Cambrian-aged sandstones and siltstones, which crop out in western Clark County, at the far western edge of the loess sheet. Because the loess did not originate from broad, proglacial, meltwater valleys, the age of loess can constrain regional paleoenvironmental changes (i.e. regional permafrost degradation) rather than variations in discharge and sediment load from the ice sheet upstream. Instead, thawing of a permafrost dominated landscape would have produced large quantities of freshly exposed sediment for deflation, including numerous ice-walled lake plains northwest of the loess sheet. OSL ages indicate loess entrainment and deposition occurred between approximately 15.2 and 12ka; therefore, permafrost degradation also occurred during this time span.