PRELIMINARY INVESTIGATIONS OF LOESS IN WESTERN WISCONSIN: EVIDENCE FOR MULTIPLE SOURCE AREAS?

As part of a larger study, our goal is to use geochemical, mineralogical and textural data to characterize the various, often disjunct, loess sheets of Wisconsin and northern Michigan, as a first step toward the identification of their source areas. To this end, we have sampled 361 locations across Wisconsin, each of which is an upland site identified from the local soil survey as having soils developed in loess. Although only about a third of our data are available at this time for detailed spatial and statistical analysis, we are nonetheless able to extend several preliminary conclusions, based on fuzzy K-means analysis of geochemical data derived from field samples: (1) Mississippi Valley loess is clearly distinct from other sources further inland (east), but only south of its confluence with the St. Croix River, (2) the Chippewa River probably served as a loess source in the regions near Eau Claire and Chippewa Falls, but this influence is muted and blurred near its confluence with the Mississippi, and (3) the loess in south-central Wisconsin (either side of a line from Beloit to Tomah) is geochemically distinct from its Mississippi counterpart to the west, although a source region for this loess has not yet been ascertained. We continue to examine and test the hypotheses that the various loess sheets scattered across Wisconsin and Michigan have (had) at least one of the following types of sources: (1) valley train outwash deposits, (2) recently exposed and perhaps also geomorphically unstable drift surfaces, and (3) recently exposed glacial lake beds or outwash plains. We also are examining whether the soils in some areas, mapped as being formed in loess over drift, are in fact developed in a silty biomantle over silt loam drift.