Deglacial History and Paleoenvironments of Northeastern Illinois

1132w020DiamictonDiamicton; silty clay loamSilt; sticky, plastic40 6080Depth (feet)Clay and siltlake sediment135Silt and diamictonDiamicton and sandg?Diamicton; silty clayloam; hard; pebbly200211217g?Diamicton; siltyclay; pebbly; stickySilt with clay; massiveto bedded; very hardSandDiamicton; silty clayloam; reddish brown264 TDBedrock; dolomiteFigure 16 Schematic lithologic log and natural gamma-ray log of boring WAD 02-05, Wadsworth,7.5-minute quadrangle.swimmers. Underlying this “Anthropocene” sediment (Zalasiewicz et al., 2008) is gray, marlygyttja and marl with abundant ostracodes that live today in deep waters rich in dissolved oxygenyear-round. These species are absent in the lake today. The post-settlement changeover toless species-rich assemblages is likely due to anthropogenic additions of limiting nutrients suchas phosphorus and nitrogen that enhance primary production (diatom activity) in the lake andconcomitant bacterial metabolism that result in anoxia, especially during the summer prior to fallturnover.GLACIAL HISTORYThe glacial history of northeastern Illinois is based largely on the relative position of lithostratigraphicunits and cross-cutting relationships among landforms and buried land surfaces (i.e.,moraines, lake plains, flood channels, and paleosols) and their ages. The ISGS has adopteddiachronic time classification for deposits associated with the last glaciation in northeastern Illinois(Figure 17; Hansel and Johnson, 1992, 1996; Johnson et al., 1997). This system replaced,or was offered as an alternative to, the chronostratigraphic scheme of Willman and Frye (1970).Both diachronic and chronostratigraphic systems use type sections and material referents todefine units and establish events. An event in this context refers to either a point in time or span27