The DeKalb mounds of northeastern Illinois as archives of deglacial history and postglacial environments
B. Brandon Curry, Michael E. Konen,
Timothy H. Larson, Catherine H. Yansa, Keith C. Hackley, Helena Alexanderson,
and Thomas V. Lowell
The type DeKalb mounds of northeastern Illinois, USA (42.0N, −88.7W), are
formed of basal sand and gravel overlain by rhythmically bedded fines, and
weathered sand and gravel. Generally from 2 to 7 m thick, the fines include
abundant fossils of ostracodes and uncommon leaves and stems of tundra plants.
Rare chironomid head capsules, pillclam shells, and aquatic plant macrofossils
also have been observed. Radiocarbon ages on the tundra plant fossils from the
type region range from 20,420 to 18,560 cal yr BP. Comparison of radiocarbon
ages of terrestrial plants from type area ice-walled lake plains and adjacent
kettle basins indicate that the topographic inversion to ice-free conditions
occurred from 18,560 and 16,650 cal yr BP. Outside the type area, the oldest
reliable age of tundra plant fossils in DeKalb mound sediment is 21,680 cal yr
BP; the mound occurs on the northern arm of the Ransom Moraine (−88.5436W,
41.5028N). The youngest age, 16,250 cal yr BP, is associated with a mound on
the Deerfield Moraine (−87.9102W, 42.4260N) located about 9 km east of Lake
Michigan. The chronology of individual successions indicates the lakes persisted
on the periglacial landscape for about 300 to 1500 yr.