GEOMORPHOLOGY OF THE ILLINOIS EPISODE GLACIATION (OIS 6) IN ILLINOIS

The Laurentide Ice Sheet advanced considerably farther south and west in Illinois during the Illinois Episode (marine oxygen isotope stage (OIS) 6; ~ 160 – 135 ka peak extent) than during the Wisconsin Episode (OIS 2; ~ 30 – 15 ka peak extent). Thus, the Illinois Episode landscape beyond the last glacial margin is relatively undisturbed from its original form in most areas, with only a cover of last glacial loess on uplands and limited postglacial fluvial dissection. During the Illinois Episode, an ancestral Lake Michigan Lobe likely reached its southernmost extent of the Pleistocene about 150 – 140 ka, a time of maximum global ice volumes. Recent field research, geologic mapping, and analysis of high resolution (LiDAR) and regionally compiled digital elevation maps have led to new observations and ideas related to OIS 6 landforms. Illinois Episode moraines in Illinois are notably more narrow, discontinuous, and widely spaced, than last glacial moraines in northeastern Illinois. A terminal moraine, notable in western Illinois and into southeastern Iowa, is lacking in southern Illinois. Subglacial lineations on the landscape include a continuum from drumlins and megaflutes to mega-scale lineations as much as 20 km long. During deglaciation, the OIS 6 Lake Michigan Lobe likely separated into sublobes (Embarrass, Kaskaskia, Springfield, and Rockford) as the ice sheet thinned and basal conditions became wetter. The landscape of each sublobe is distinct for reasons not fully understood. In north-central Illinois (Rockford Sublobe), megaflutes and drumlins occur in an area of thick glacial drift (> 20 m). Western Illinois (Springfield Sublobe) features megascale glacial lineations, subtle drumlin fields and at least three traceable moraines. Crag-and-tail forms, possibly influenced by buried Paleozoic bedrock obstacles, and subtle but widespread lineations, are apparent in southeastern Illinois (Embarrass Sublobe). In southwestern Illinois (Kaskaskia Sublobe), possible ice streaming into the Kaskaskia River Basin was likely affected by the regional topography, a relatively soft and fine-grained substrate, and subglacial hydrology. This sublobe exhibits multiple eskers and numerous kames in its interior, a marginal moraine with ice-marginal channels on its western side, and push moraines at its terminus.