SUPERPOSED ICE-WALLED LAKE DEPOSITS, NORTHEASTERN ILLINOIS

A complex of ice-walled lake plains occurs in and around Woodstock, Illinois. One ice-walled lake plain stands out from the rest (-88.4113˚W, 42.2571˚N). It is nearly circular, about 1.1 km across, with an unusual central kettle about 0.4 km across. Five cores of this landform have been sampled. Facies architecture, radiocarbon ages of entombed tundra plants, and geomorphology collectively indicate two stages of ice-walled lake development. Gray, silty clay diamicton of the Yorkville Member (Lemont Formation; Livingston Phase) underlies the glaciolacustrine complex forming the ice-walled lake plain, but sediment cores sampled adjacent to the landform reveal patches of dolomite-rich, pebbly sandy loam diamicton of the Haeger Member (Lemont Formation; Woodstock Phase) that cover the fine-grained Yorkville unit. The margin of the ice-walled lake plain is covered by about 1.7 m of well-sorted, fining-upward medium sand. The sand pinches out approaching the kettle’s edge. The underlying fossiliferous lacustrine sediment is as much as 7.5 m thick.

The two stages of development are reflected in five radiocarbon ages (each with < 30 yrs sigma-one error) of Dryas integrifolia found in the lacustrine faces. The first stage lasted from about 21,870 to 21,460 cal yr BP during deglaciation of the Livingston Phase. The second stage occurred from about 18,720 to 17,870 cal yr BP during deglaciation of the Woodstock Phase. The lack of ¹⁴C ages spanning from about 21,460 to 18,720 cal yr BP also is observed from the composite of more than 40 radiocarbon ages associated with ice-walled lakes in Illinois. The hiatus is also observed in 15 ¹⁴C ages of plant fossils from the nearby De Kalb mounds. The lack of physical evidence for the nonconformity in sediment cores such as clay mineral alteration or changes in bedding or grain-size suggests that the active layer did not thaw; the landscape was physically and chemically inert during this time of extremely cold summer temperatures.