Deglacial History and Paleoenvironments of Northeastern Illinois

Drift ThicknessDrift thickness in northeastern Illinois ranges from nil (where bedrock is exposed at the land surface)to about 120 m under the Marengo Moraine (Piskin and Bergstrom, 1975). Other morainesare formed of thinner drift from ca. 50 to 60 m thick. In general, the glacial drift thins to thesouth. Drift is absent along reaches of all major drainages such as the Illinois, Kankakee, Fox,and Des Plaines rivers.Stratigraphic SuccessionIn the study area, there is evidence for two major advances of the Lake Michigan lobe with anintervening interglacial period when there was warmer climate and significant soil accretion,bioturbation, and weathering. The older Illinois Episode ice sheet and its meltwater depositedthe Glasford and Pearl Formations, and the Teneriffe Silt from about 200,000 to 130,000 yr BP(Johnson, 1976; Curry and Follmer, 1992; Curry and Pavich, 1996; McKay, 2008). In northeasternIllinois, the Sangamon and Farmdale Geosols are superposed weathering profiles developedin late Illinois Episode glacial drift and Alton Subepisode (Wisconsin Episode) loess,respectively (Curry, 1989; Curry and Pavich, 1996). Michigan Subepisode (Wisconsin Episode)glacial sediment, dating from about 29,000 to 14,670 cal yr BP (24,000 to 12,500 C-14 yr BP),overlie the soil complexes, including sorted sediment of the Mason Group, and diamicton of theWedron Group (Figure 10; Hansel and Johnson, 1992, 1996).The thickness and general character of lithostratigraphic units are portrayed in cross sectionsacross Kane County (Figures 11a,b) and McHenry County (Figure 11c). For deposits of the lastglaciation, at the highest level of classification is the Mason and Wedron groups which differentiatesorted from very poorly sorted glacigenic sediment. The Mason Group is subdivided into depositsof laminated silt and clay (Equality Formation), uniform and/or weathered silt (Peoria andRoxana silts), and stratified or cross-bedded sand or sand and gravel (Henry Formation). Reflectingthe importance of regionally important glaciogenic successions, several units are stratigraphictongues which have the same rank as members. The inclusion of tongues in the newstratigraphic scheme allows for the repetition of units at the formation level at a single locality.Banner Formation. The oldest named Quaternary lithostratigraphic unit in the region is correlatedwith the pre-Illinois Episode Lierle Clay Member of the Banner Formation (Willman and Frye,1970). Of the more than 100 borings to bedrock, this unit was identified twice, once in northernmostMcHenry County (Curry and Pavich, 1996), and once in northern Kendall County (Curryet al., 1988). At these sites, the Lierle Clay Member is one m-thick, and composed of gleyed,leached clayey diamicton and sesquioxide (organic iron - manganese) concretions. These featuresformed in an ancient pedogenic environment. The Lierle Clay Member also contains abundantclay minerals such as mixed-layer kaolinite/smectite, which is further indication of weathering.As it is elsewhere in the region, the Lierle Clay is considered part of the Yarmouth Geosol(Grimley et al., 2003).Glasford Formation. Deposited during the Illinois Episode, the Glasford Formation is generallythe oldest Quaternary unit observed in the study region. It generally occurs above bedrock andbelow sediment of the Mason or Wedron Groups. The Glasford Formation consists of generallygray to reddish brown diamicton with lenses of sand and gravel and local inclusions of fine sand,silt, and clay. The Glasford pinches out to the south and east. Several till members of the GlasfordFormation have been traced from Boone and Winnebago Counties in the Rockford area toMcHenry and Kane counties (Graese et al., 1988; Kempton et al., 1985).15