Characteristics and paleoenvironmental significance of a thin, dual-sourced loess sheet, north-central Wisconsin

摘要

We provide the first extensive data on the characteristics, distribution, and origin(s) of a thin but extensive loess sheet in north-central Wisconsin, USA. Unlike the relatively thick loess deposits downwind of broad meltwater valleys that reflect ice sheet dynamics, thin loess sheets far from such rivers may provide unique information about local paleoenvironmental change due to their smaller, and often environmentally sensitive, source areas. Silt loam textured and thin (<35 cm thick) on its eastern margins, loess in north-central Wisconsin thickens (to >70 cm) and coarsens towards the west and northwest, such that, on its western margins, the loess mantle is dominated by very fine and fine sands. Collectively, data on loess particle size distributions, thickness, and silt mineralogy suggest that this loess sheet had sources in two distinct and disjunct landscapes: (1) the late Wisconsin terminal moraine to the northwest, and (2) the sandstone-dominated landscapes to the west and southwest. Post-glacial thawing of these permafrost- affected landscapes probably led to draining of ice-walled lake plains on and behind the moraine, as well as destabilization of slopes on the sandstone landscape; in both cases, exposing large quantities of sediment for deflation. Our study makes two significant contributions to aeolian research: (1) we determine that the north-central Wisconsin loess sheet, unlike almost all others, has two distinctly different and disjunct loess sources, and (2) neither of these two loess sources fall within the traditional mode of ‘‘glacial meltwater valley.’’