Characterization and paleoenvironmental significance of the north central Wisconsin loess sheet.

摘要

In this thesis, I investigate the relatively thin loess sheet in north central Wisconsin. Loess thickness on uplands exceeds 70 cm in northern and central Clark County, thinning to the southeast and east, to 35 cm in western Marathon County. Silt loam-textured on its eastern margins, the loess sheet coarsens towards the west and northwest, such that, in western Clark County, the loess mantle is comprised of very fine and fine sands. It mantles at least three different substrates; from west to east they are (1) fine-grained, Cambrian sandstone residuum, (2) sandy Bakerville till, and (3) silty Edgar till. Thickness, particle size data, and silt mineralogy point to two distinct sources for this loess: (1) the Late-Wisconsin terminal moraine to the northwest (including the numerous ice-walled lake plains on and behind the moraine) and (2) fine-grained, Cambrian-aged sandstones and siltstones, which crop out at its western edge. Because of its environmentally sensitive source areas, the timing of loess deposition may be able to constrain regional paleoenvironmental changes. It is likely that large quantities of freshly exposed sediment would have been made available for deflation by the thawing of a permafrost-dominated landscape, e.g., the destabilization of slopes in the areas dominated by dissected Cambrian sandstones and the draining of numerous ice-walled lake plains. Three OSL ages for central Wisconsin loess indicate that loess entrainment and deposition occurred between approximately 15.2 and 12 ka; thus, regional permafrost degradation likely occurred during this time span.