DETERMINING SEDIMENT TRANSPORT RATES DURING DEGLACIATION OF THE LAKE MICHIGAN LOBE, NORTHEASTERN ILLINOIS

An annual transport rate of about 400 m³ per meter-width of ice flow was estimated for the outer 100 km of the Lake Michigan lobe by Johnson et al.(1991). Basal age control was from several radiocarbon ages of wood fragments in the Farmdale Geosol. The upper age control was weakly defined due to the paucity of material for radiocarbon ages. Here, we provide a flux estimate for formation of the Tinley Moraine in northeastern Illinois. Our results are based on radiocarbon ages of tundra plant fossils identified from deposits of ice-walled lakes.

Based on basal radiocarbon ages from ice-walled lake successions, a conservative estimate of the time span when the Tinley Moraine was formed is 810 years, from 18,100 to 17,290 cal yr BP. The older age is derived from material from three successions on the older Woodstock Moraine; the younger age, from material from one succession on the Tinley Moraine. Cross sections from northernmost Illinois and southernmost Wisconsin provide a volume estimate of 7.2 x 10⁵ m² per meter width of the Wadsworth Formation beneath and west of the Tinley Moraine. Based on these values, the annual flux was about 890 m³ per meter-width of ice flow. A less conservative approach is to use the youngest age of the ice-walled lake succession on the Woodstock Moraine of about 17,570 cal yr BP, which yields a time span of 530 yrs, and a flux of 1,360 m³ per meter-width of ice flow. Hence, our estimates are from two to three times greater than the flux estimate discussed above, which is expected in part because we are estimating sediment flux during a single moraine building event instead of a longer period that includes ice stagnation and retreat.