THE GRAIN-SIZE VARIABILITY OF LOESS – IT’S NOT JUST EOLIAN SILT ANYMORE!

Early definitions and descriptions of loess defined/described it, simply, as eolian silt. Most people still think of loess that way. But as loess has become more widely studied, and especially because of detailed mapping of loess deposits in Europe and the Great Lakes region, it has become increasingly evident that a wide variety of fine- and medium-textured eolian sediments occur in nature. Cover sands grade into sandy loess which grades into silty loess, and all manner of “in-betweens” also occur. No longer can we say that loess is simply eolian silt. Loess is now viewed as a rich and texturally diverse eolian sediment, but still silt-dominated. In this poster, we bring forward a variety of detailed grain-size curves from over 3200 loess samples across Wisconsin and Michigan. To develop these curves of various loess “types”, grain-size data from loess samples were input into a spatial factor model, which was then used to identify defining components across the suite of grain-size profiles. Formal model selection identified a five-factor model as the most parsimonious, explaining >98% of variance, and thus, capturing important data characteristics. Most of the loess in the study area has bimodal grain-size curves, with an average of 24.9% sand. Most of the bimodality is associated with mixing of sand into silty loess, both syndepositionally and post-depositionally. Other loess deposits have a more complicated and elusive genesis. Key to their interpretation is the grain-size mode, which is larger at sites nearer to the loess source. Thus, even among the silt-dominated loess deposits, the wide range of grain-size curves provides opportunities to evaluate paleoenvironments of deposition.