CONSTRAINTS ON THE AGE OF THE LAKE MICHIGAN LOBE (LAST GLACIATION) IN ILLINOIS, USA: FROM START TO FINISH

Recent estimates of the annual transport rate of debris entrained by the outer 100 km of the Lake Michigan Lobe range from about 800- to 1,400 m³ per meter-width of ice flow. The flux values are among the highest estimated for Earth’s sedimentary basins, and are conservative because only proximal facies (mostly till and proglacial lacustrine sediment) were considered. The estimates are based on a refined chronology of deglaciation based on basal AMS ages from rhythmically laminated facies of sediment successions that comprise ice-walled lake plains. In Illinois, the basal ages of the laminated facies of ice-walled lake deposits are 22,500 to 16,000 cal yr BP. The time component of the flux estimates is the difference in age of basal ice-walled lake successions of laterally adjacent moraines. The conservative flux value (800 m³ per meter width) does not consider the time it takes for the ice margin to retreat and readvance. The more realistic value of 1,400 m³ per meter-width of ice flow estimates only when glacier activity was actively depositing proximal sediment. The sediment volume is determined from detailed mapping and subsurface data that constrains the 3-D geometry of the uppermost glaciogenic succession comprising the older moraine.

Equally important to understanding the deglacial chronology of the Lake Michigan lobe is the age of its maximum extent. Large wood fragments buried (and in some cases truncated) by glaciogenic sediment of the last glaciation are coveted radiocarbon “trophies”. Small and obscure fossils of tundra plants likely associated with ice-proximal environments are typically overlooked. For example, at Charleston Quarry (39.52°N, -88.;10°W), about 16 km up-ice of the terminal Wisconsin Episode moraine, C-14 ages of tundra plant fossils in proglacial lake sediment are about 1,000 years younger than the age of tree stumps buried by the same lacustrine unit (24,000 vs. 23,000 cal yr BP). The difference in age affects estimates of sediment flux needed for forming the oldest outer moraines in Illinois by a factor of 2.