Using PCA to Characterize and Differentiate the Character of Loess Deposits in Wisconsin and Upper Michigan, USA


Peter Scull and Randall J. Schaetzl


Data on loess thickness, as well as a variety of particle size fractions, were determined for 875 samples taken from several loess sheets across Wisconsin and Upper Michigan. Because the particle size characteristics of these samples varied widely within and between the loess sheets, we used principal components analysis (PCA) as a way of isolating the major textural components or signals that exist across the various loess sheets. The purpose of this research was to examine and interpret these principal components or particle size signatures, common to the different loesses, in order to better understand the loess sheets' character, formation mechanisms and likely sources. Our initial assumption — that many of these loesses varied markedly from the classical “silty loess” — was supported by the particle size data and the PCA. Although component 1 was interpreted as thick, silty loess dominated by medium silts, component 2 was mainly composed of very fine sand and coarse silt and is better viewed as “coarse loess.” Components 3 and 4 were less texturally homogeneous and may reflect mixing between the loess and the underlying sediment, especially where the loess is b ~ 40 cm thick. Alternatively, some of the loess samples in components 3 and 4 can be interpreted as poorly sorted versions of sandy eolian sediment, grading downwind to more traditional, siltier loess. Our work is the first to describe, map, and (informally) name the many small loess sheets in the upper Great Lakes region. This research demonstrates that many of the loesses here do not have silt loam textures as often described in the literature. Instead, loesses can be fine–sandy, and others, especially those that have bimodal particle size signatures, may reflect various amounts of post-depositional mixing.