Large Lakes; Big Floods; the Deglacial History of the Illinois River Valley

During the Quaternary Period, the geomorphology of the Illinois River valley evolved as its function varied over time from proglacial stream valley, subglacial discharge locus, sluiceway for jökulhlaups (large meltwater floods), and slackwater lake basin. Here, we will focus on the latest events that sculpted the valley, starting with deglaciation of the Lake Michigan lobe, an ice stream from the south-central part of the vast Laurentide Ice Sheet. Evidence of large discharge events along the Illinois River valley include eye-shaped islands (lemniscate loops), scoured uplands, moraine-breaching channels, and fluted bedrock surfaces. The deglacial chronology of the Lake Michigan lobe indi-cates two temporal windows when meltwater was prevalent. The oldest window is 22.0 to 18.5 cal ka, and the floods associated with this period are known as the Kankakee Torrents. The oldest radiocarbon age that is unequivocally related to a large deglacial flood is about 18.9 ka; it was obtain from tundra plant fossils archived in basal lake sedi-ments that fill the Oswego channel. The channel was formed by overflow of a proglacial lake across the Marseilles Morainic System. Heading downstream from the Oswego overflow channel, the next well-documented site that docu-ments flooding is at Emiquon. Here, lacustrine sediments suggest two periods of high water; and earlier phase dating from about 18.4 to 17.5 ka, and a later phase from about 15.8 to 13.1 ka. During the early lake phase, water depths may have been as great as 15 m or more. The earlier phase corresponds temporally with a readvance and decay of the Lake Michigan lobe while it formed the Woodstock Moraine. The later lake phase at Emiquon is likely associated with erosion of the Chicago Outlet across the Valparaiso/Lake Border morainic systems. This erosion event formed channels at Emiquon that eroded to bedrock, but are nested in older lake sediment. Slackwater lake depth was controlled, in part, by the Savanna-Deer Field terrace located at the mouth of the Illinois River. Hiatuses in the radiocarbon ages obtained from organics (needles, wood fragments) from Emiquon match OSL ages from dune sands at Manito, Illinois, dating at about 16.6 ka and 12.3 ka. The relationship of these relatively moist and dry events to global climate change is not clear, likely due to very steep ecotone/climatic gradients leading away from the decaying ice sheet. Another poorly contrained/understood phenomenon related to slackwater lake conditions is the effect of isostatic rebound, especially as the forbulge pulsed across the region starting at 23 ka, and peaking at about 10 ka.