Origin of glacial ridges (OIS 6) in the Kaskaskia Sublobe, southwestern Illinois, USA Nathan D. Webb a, ⁎, David A. Grimley a , Andrew C. Phillips a , Bruce W. Fouke b, c a Illinois State Geological Survey, Prairie Research Institute, University of Illinois Urbana–Champaign, 615 E. Peabody Drive, Urbana, IL 61801, USA b Department of Geology, University of Illinois Urbana—Champaign, 1301 W. Green Street, Urbana, IL 61801, USA c Institute for Genomic Biology, University of Illinois Urbana—Champaign, 1206 West Gregory Drive, Urbana, IL 61801, USA abstract article info Article history: Received 18 January 2012 Available online 20 July 2012 Keywords: Kaskaskia Basin Ridged Drift Illinois Illinois Episode Hagarstown Member Till fabric Glacial ridges Sedimentology Moraine Ice-walled channel The origin of Illinois Episode (OIS 6) glacial ridges (formerly: ‘Ridged Drift’) in the Kaskaskia Basin of south- western Illinois is controversial despite a century of research. Two studied ridges, containing mostly fluvial sand (OSL ages: ~150 ± 19 ka), with associated debris flows and high-angle reverse faults, are interpreted as ice-walled channels. A third studied ridge, containing mostly fine-grained till, is arcuate and morainal. The spatial arrangement of various ridge types can be explained by a glacial sublobe in the Kaskaskia Basin, with mainly fine-grained ridges along the sublobe margins and coarse-grained glaciofluvial ridges in a paleodrainage network within the sublobe interior. Illinois Episode till fabric and striation data demonstrate southwesterly ice flow that may diverge near the sublobe terminus. The sublobe likely formed as glacial ice thinned and receded from its maximum extent. The Kaskaskia Basin contains some of the best-preserved Illinois Episode constructional glacial landforms in the North American midcontinent. Such distinctive features probably result from ice flow and sedimentation into this former lowland, in addition to minimal postglacial erosion. Other similar OIS 6 glacial landforms may exist in association with previously unrecognized sublobes in the midcontinent, where paleo-lowlands might also have focused glacial sedimentation. © 2012 University of Washington. Published by Elsevier Inc. All rights reserved. Introduction A system of prominent glacial ridges and knolls, previously termed the Ridged Drift (Leverett, 1899), occurs commonly in the Kaskaskia Basin of southwestern Illinois (Fig. 1) but sparsely in other areas of Illinois glaciated during the Illinois Episode (marine oxygen isotope stage [OIS] 6). The Kaskaskia ridge system is continuous to fragmentary, with fea- tures as much as several km long, a few km wide, and 10 to 50 m in relief above the surrounding till plain. These landforms have significance be- cause they are perhaps the best-preserved glacial landforms from the penultimate glaciation in the Midcontinent USA. Previous researchers have interpreted the ridges as dominantly morainal (Leverett, 1899; MacClintock, 1929; Willman et al., 1963; Phillips, 2008), eskers or ice-walled channels (Ball, 1940; Jacobs and Lineback, 1969; Burris et al., 1981), crevasse fills (Leighton, 1959; Leighton and Brophy, 1961), or as some combination thereof (Stiff, 1996; Grimley and Webb, 2009). The former interpretations have varied considerably due, in part, to con- trasting sedimentological observations in various geographic portions of the basin. Early investigations lacked the benefit of comparison to key modern glacial process studies (e.g., Banerjee and McDonald, 1975; Warren and Ashley, 1994; Boulton et al., 1999; Mäkinen, 2003) and did not focus on detailed sedimentological aspects or ice-bed conditions. Additionally, till clast fabric determinations, a standard proxy used to infer ice-flow direction (Kjær and Krüger, 1998; Benn, 2004), have been limited in southwestern Illinois to Lineback (1971), who interpreted mainly SW ice-flow directions in the middle to upper Kaskaskia Basin (Fig. 1). Stri- ations are rarely observed in southern Illinois (Fig. 1) due to limited bedrock outcrops and the dominance of soft Pennsylvanian shale and mudstone. Recent surficial mapping and research in the lower Kaskaskia Basin (Phillips, 2004, 2005; Grimley and Phillips 2006; Webb, 2009; Grimley, 2010; Grimley and Webb, 2010) has documented important exposures and test cores in ridge deposits that reveal detailed sedimen- tary successions, as well as outcrops of similar-aged till in the surround- ing plains. The objective of the present study was to combine the findings of these recent sedimentological observations along with regional ice-flow directional data from till clast fabrics and striations in order to reevaluate the origin of Illinois Episode glacial ridges in southwestern Illinois. Most localities studied are in the lower Kaskaskia Basin, farther southwest than most previous research (Fig. 1). A core from the crest of Ralls Ridge, a prominent landform near the maximum extent of Illinois Episode glaciation, along with two sand and gravel pit exposures in ridges and several till fabric sites in areas adjacent to the ridges (Fig. 1) afforded key observations for this study. Detailed site descrip- tions can be found in Webb (2009). Previous hypotheses on the Kaskaskia ridge system Leverett (1899) hypothesized that the Kaskaskia ridge system marked the western border of a lobe persisting in southern Illinois Quaternary Research 78 (2012) 341–352 ⁎ Corresponding author. E-mail address: ndwebb2@illinois.edu (N.D. Webb). 0033-5894/$ – see front matter © 2012 University of Washington. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.yqres.2012.06.005 Contents lists available at SciVerse ScienceDirect Quaternary Research journal homepage: www.elsevier.com/locate/yqres