MAPPING AND CHARACTERIZING THIN LOESS DEPOSITS AT THE NORTHEASTERN MARGINS OF THE NORTH AMERICAN LOESS SHEET

This paper reports on the distribution, thickness and textural characteristics of thin, patchy loess in the western Upper Peninsula of Michigan. Here on this high-relief, bedrock-controlled landscape, uplands are often mantled with ~ 30-60 cm of loess, and usually underlain by sandy glacial sediment. On this recently glaciated landscape, loess was likely sourced from both distant and local areas, including ground moraines, outwash plains, and the floodplains of meltwater streams. Where the loess is thin, it appears that pedoturbation processes have likely mixed some of the lower, sandy materials up, into the loess. For this reason, most of the loess has a bimodal continuous textural curve, with a primary modal particle-size within the 25-75 µm fraction (the loess) and a secondary mode in the 250-500 µm fraction (sand, mixed into the loess from below). Moreover, these loess deposits vary in loess thickness, silt and sand contents, and modal particle size, across short distances. We have identified four loess regions, or sections, in this area; each has unique characteristics that set it apart. We suggest that the loess region of the western Upper Peninsula is an assemblage of several smaller, overlapping loess sections, each of which had different source areas. Thus, loess textures and thicknesses usually vary between sections. Our research (1) recognizes multiple source areas for loess, (2) develops and interprets textural data on thin loess, and (3) documents the effects of mixing within these loess deposits.