SPATIAL AND TEXTURAL ANALYSES OF SOILS CONFIRM LOESS MANTLE ON THE OUTWASH PLAINS OF SOUTHWESTERN MICHIGAN

Soils on many of the outwash plains in southwestern Michigan have loamy upper profiles, despite being underlain by coarse-textured outwash. This upper, loamy material has long been puzzling to soil scientists and Quaternary geologists alike. We analyzed the spatio-textural characteristics of this loamy-textured sediment to ascertain its genetic and sedimentologic origin(s). The loamy material displays textural bimodality, i.e., it has clear silt and sand peaks. Because the sand peaks align with those in the outwash below, we concluded that the upper material has had sand from below mixed into it, forming loamy textures from an initially siltier sediment. By applying a textural filtering operation to the data, we were able to estimate its original textural characteristics. Filtered data indicated that nearly all of the loamy sediment was initially silt loam-textured, as is typical for loess. Field data showed that the upper, loamy material is thickest slightly east of a large, broad, N-S trending valley (the Niles-Thornapple Spillway) that was variously a proglacial lake and a valley train carrying meltwater. The loamy sediment gets thinner, better sorted, and finer in texture moving east from this channel. We conclude that the loamy mantle on many of the adjacent outwash plains is silt-rich loess that was derived from the Niles-Thornapple Spillway and its major tributaries. The Spillway was active for approximately 500 years, between ca. 17.3 and 16.8 k cal. yrs ago. At this time, a large network of tunnel channels had developed beneath the stagnant Saginaw lobe. Meltwater from these channels mined the silt-rich Saginaw lobe till and funneled it into the Spillway, making it a prodigious silt source.