THE DE KALB MOUNDS OF NORTHEASTERN ILLINOIS REVISITED

“Glacial doughnuts” are surficial features observed throughout the interior of North America. We have mapped these distinctive circular to semicircular features on aerial photography and shaded relief maps of DEMs in the “type” area where doughnuts were dubbed the De Kalb Mounds. The doughnuts are from 30 m to more than 5 km across and form subtle mesas 1.5 to 7 m above the surrounding surfaces formed of glacial deposits. The typical doughnut succession includes < 1 m of outwash, 1-6 m of fossiliferous, rhythmically bedded lake sediment, 0.3 to 3 m of sand and gravel, and about 1 m of loess. The lake sediment contains ostracodes (Cytherissa lacustris, Limnocythere friabilis, L. herricki, Heterocypris incongruens), pelecypods, chironomids, rare vertebrate fossils, and the remains of tundra plants such as arctic blueberries which have yielded AMS C-14 ages of 16,770 ± 90 (ISGS-A-0108) and 17,610 ± 270 yr BP (ISGS-A-0104). The geometry of some deposits require that at least 7 m of ice once buttressed the doughnuts; ostracode ecology further suggests the ice was about 17 m thick. This indicates that many, if not all, doughnuts formed as ice-walled lakes. The geometry of the underlying till surface provides further hints on the origin of these features. The till/lake fill contact is typically higher in the interiors than on the sides of doughnuts; both are higher than the adjacent till surface. This suggests that the surface of the ice exposed to the atmosphere lowered to the point where solar energy, absorbed by the emerging basal debris-rich layer, caused local melting of the overlying ice. In this scenario, the doughnuts are thermokarst features formed by melting glacial ice (but not permafrost). The origin of the lake fill may provide further clues to the origin of glacial doughnuts. Each rhythmite typically consists of a 1-cm bed of silt and laminae of very fine sand; the thickest known succession includes about 500 couplets . The mineralogy is similar to the underlying illite-chlorite rich till, but unlike the smectite-rich loess. The data thus far suggests the doughnut lake fill consists of redeposited sediment of either aeolian or local glacial origin.