DEGLACIAL LAKES IN NORTHEASTERN ILLINOIS: GENESIS, AGE, PALEOENVIRONMENTS, AND IMPLICATIONS FOR CHRONOLOGIES

The onset of deglaciation in the southern Great Lakes region has classically been based on radiocarbon ages of bulk sediment, and more recently, on wood fragments from basal deglacial sediment in kettles and bogs. Recent detailed mapping reveals that fossils contemporaneous with the oldest deglacial deposits are found not in the deepest postglacial sediment in depressions, but in sites that today typically occupy intermediate topographic positions between moraine crests and drainageways: slackwater lakes, ice-walled lakes, and in some cases, along the margins of kettles. The age difference between fossils found at the base of (younger) kettle and bog successions and (older) slackwater/ice-walled lake sites range from about 1,000 to 2,500 yrs. The difference in age at most sites is attributed to the layer of sand and gravel that insulated the ice associated with kettles whereas slackwater and ice-walled lakes were open to the atmosphere. In northeastern Illinois, four characteristics are usually associated with the oldest deglacial deposits: 1) floral remains of tundra vegetation (Dryas integrifolia, Vaccinium uliginosum alpinium, Salix herbacea and other species), 2) two or more of the following ostracodes: Cytherissa lacustris, Limnocythere friabilis, L. pseudoreticulata, Candona subrectangulata, and Heterocypris sp., 3) illite-rich clay mineralogy identical to the underlying till unit, and 4) a topographic position 1 to 7 m above present-day base level. In areas with ice-walled lakes, younger fossiliferous lake sediment rich in silt and expandable clay minerals form low terraces. In other areas with kettles, similar expandable-rich sediment, rich in fossil conifer wood fragments and needles, comprise the basal deglacial succession. Some (most?) kettles are ringed by deposits of bedded silt with tundra fossils like those described above. The geometry of the deposits suggest an early ice-walled lake phase followed by collapse of the lake-bottom interior, lower lake levels, and narrowed sediment focusing into the newly formed kettle basin. The major implication of these observations is that deglacial chronologies should be based (if possible) on radiocarbon ages of fossil tundra plants; ages from tree fossils may postdate the occurrence of glacial ice by as much as about 3,000 years.