MAGNETIC-SUSCEPTIBILITY AND MINERAL ZONATIONS CONTROLLED BY PROVENANCE IN LOESS ALONG THE ILLINOIS AND CENTRAL MISSISSIPPI RIVER VALLEYS

Magnetic susceptibility (MS) patterns have proven useful for regional stratigraphic correlations of zones within thick, oxidized Peoria and Roxana Silts along the Illinois and Central Mississippi River valleys for more than 350 km. Variations in MS of C horizon loess are controll ed by silt-sized magnetite content and are interpreted to reflect chan ges in sediment provenance due to fluctuations of the Superior and Lak e Michigan glacier lobes and the diversion of the Mississippi River to its present course. Grain size distributions and scanning electron mi croscopic observations indicate that stratigraphic changes in MS are n ot significantly influenced by eolian sorting or diagenetic dissolutio n, respectively, Three compositional zones (lower, middle, and upper) are delineated within Peoria Silt which usually can he traced in the f ield by MS, the occurrence of clay beds, interstadial soils, and/or su btle color changes, These zones can be correlated with, but are genera lly of more practical use than, previously studied dolomite zones (McK ay, 1977) or clay mineral zones (Frye et al., 1965). However, mineralo gical analyses can help to substantiate zone boundaries when in questi on, MS and compositional zones may indirectly record a climatic signal , primarily through the effect that global cooling has had on ice lobe fluctuations in the Upper Mississippi drainage basin. (C) 1998 Univer sity of Washington.