LARGE MELTWATER EVENTS ASSOCIATED WITH THE LAKE MICHIGAN LOBE DURING THE LAST DEGLACIATION, ILLINOIS, USA

Evidence of large discharge events in Illinois include lemniscate loops and moraine-breaching channels. The deglacial chronology of the Lake Michigan lobe indicates 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 second window is associated with erosion of the Chicago Outlet at about 14.7 cal ka. The period previous to the first window includes an advance phase of the Lake Michigan lobe beginning at 25.6 cal ka, culminating at the glacial maximum at 23.0 cal ka, and retreat north of Illinois by 22.0 cal ka. The radiocarbon chronology of fossils archived in two ice-walled lakes in northeastern Illinois indicate that the first temporal window was marked by extremely cold periglacial conditions that limited or halted summer melting of the active layer. An ice-walled lake plain near Woodstock, IL, (42.26°N, -88.41°W) has yielded 7 AMS ages that include two clusters, one from 21.9-21.5 cal ka, and the other 18.7-17.8 cal ka. The paucity of ages from about 22.0-18.5 cal ka is replicated at one additional site near Hampshire, IL, and also occurs spatially in bottom dates of ice-walled lake deposits throughout Illinois. Interestingly, the breaching of the Marseilles Moraine near Oswego, IL, occurred near the end of the temporal window at 18.82 cal ka. The combined evidence suggests that subglacial meltwater was stored during the early part of the temporal window as the toe of the Lake Michigan lobe froze year-round. Meltwater was released, perhaps catastrophically, during the later stages of the temporal window. We agree with early workers who indicated that meltwater contributions included those from the Lake Michigan, Huron-Erie, and Saginaw lobes.

New radiocarbon ages from slackwater lake sediment deposited within the main trunk of the Illinois River valley near Havana, IL (40.34°N, -90.10°W) indicate at least two valley lake phases based on 12 AMS ages. The oldest phase dates from 18.05-17.57 cal ka, and the younger phase, 14.50-13.20 cal ka. Both lake phases were preceded by channelized scour events. There is little doubt the younger event is related to erosion of the Valparaiso Moraine via the Chicago Outlet. The older channel fill is consistent with late-stage discharge of the Kankakee Torrents.