The deglacial history of the Lake Michigan lobe in Illinois, USA, fleshed out by chronologies associated with ice-walled lakes

The onset of deglaciation of the Lake Michigan lobe (LML) in the western Great Lakes region of North America is well-defined by radiocarbon ages from terrestrial plant material that was buried by proglacial lake sediment and till. A small portion of the lobe began its final retreat during the Marengo Phase at about 24,780 C-14 yr BP (29,630 cal yr BP) [CalPal]; most of the lobe began its final retreat during the Shelby Phase starting at about 19,350 C-14 yr BP (23,010 cal yr BP)[Calib 5.02]. New radiocarbon ages from tundra plant fossils encased in the deposits of ice-walled lakes allow estimation periods of ice stagnation vs. margin advance as well as constraining the age of important events during the late Wisconsin Episode. For example, radiocarbon ages from fossils in ice-walled deposits indicate the LML margin began its final retreat from Illinois at the onset of the Mackinaw lake phase by about 13,650 C-14 yr BP (16,250 cal yr BP). Located on the Marseilles Morainic System, the oldest dated ice-walled lake deposit known in Illinois began to form at 18,210 C-14 yr BP (21,680 cal yr BP). From its end moraine located near Peoria to Chicago on the shores of southern Lake Michigan, the LML extended about 230 km. Collectively, the ages indicate that the outer 100 km of the LML margin retreated about three times faster than the inner 130 km (7.5 x 10-2 km/yr vs. 2.4 x 10-2 km/yr). Changes in ice thickness and dynamics, debris concentration, climate, and regional drainage are some of the likely factors that affected the difference in rate. The existing data provide conservative estimates for the formation of two moraines, the lake-border Deerfield Moraine (720 cal yrs) and the Tinley Moraine (850 cal yrs). Periods of stagnations, marked by radiocarbon ages from fossils in the ice-walled lake sediments, include about 320 years (Deerfield Moraine), 520 cal yrs (Tinley Moraine), 500 cal yrs (Woodstock Moraine), and 710 years (Ransom Moraine). These and other ages indicate that ice stagnation occurred during at least 45% of the 5,430 year period when these and intervening moraines formed.