Loess, eolian sand, and colluvium in the Driftless Area

ABSTRACT Loess mantles upland summits across much of the Driftless Area of southwestern Wisconsin and its origin and paleoenvironmental significance has been a focus of research since the nineteenth century. Although the Driftless Area was ice free through the many glaciations of the Quaternary, long-term preservation of loess was limited by post-depositional erosion across much if not all of this highly dissected landscape, erosion that was likely accelerated under periglacial conditions during glaciations when ice sheets were nearby. Loess preserved today includes four members of the Kieler Formation and two older loesses known only from one locality. The Peoria Member, the youngest, thickest, and most extensive member of the Kieler Formation, was deposited during and just after the peak of the last glaciation. Its main sources include both the Mississippi River valley and the Iowan Erosion Surface and glacial outwash surfaces farther west in Iowa and Minnesota. More research is needed on the relative contribution from each of these sources to the Peoria Member, and on the sources of older loess units. Eolian sand, often forming dunes, covers extensive low-relief landscapes in the northern Driftless Area, the Mississippi and Wisconsin River valleys, and smaller areas elsewhere in the region, overlying sandstone bedrock, stream terraces, and the former bed of Glacial Lake Wisconsin. These sands are stabilized by vegetation today but were active during and just after the period of Peoria Member deposition. Thus, large areas of eolian sand acted as surfaces of transport where loess did not accumulate but rather was conveyed far downwind of its sources. Colluvium that is a mixture of bedrock-derived sediment and loess covers bedrock slopes throughout the Driftless Area. A variety of geochronologic, geomorphic, and stratigraphic evidence supports the hypothesis that this colluvial mantle formed mainly in the cold, periglacial environment of the last glaciation, with only limited modification during the Holocene. A new research effort, incorporating modern geochemical and geochronological techniques, could provide important insights on the processes that originally produced the colluvial mantle and those that are acting on it today.