PROGLACIAL LAKES IN NORTHEASTERN ILLINOIS; DEGLACIATION CHRONOLOGY AND PALEOENVIRONMENTS

Lacustrine sediment and fossils were analyzed from several sites in NE Illinois to revise the chronology for local deglaciation and to reconstruct limnologic and landscape changes. The revised chronology is based on the AMS C-14 dating of terrestrial plant remains; fossil ostracodes and plant macrofossils were used in our paleoenvironmental reconstructions. Our data indicate that the Huron-Erie Lobe ceased to interact with the Lake Michigan Lobe at about 17,500 RCYBP; this age marks the end of activity of the Princeton Sublobe of the Lake Michigan Lobe, and initial activity of the Joliet Sublobe and formation of the Minooka Moraine during the mid-Livingston Phase. The age of the end of Livingston Phase is about 16,500 RCYBP. At each field site used in our reconstructions, fossils were recovered from rhythmically bedded lacustrine sediment. Tundra plants, such as Dryas integrifolia (arctic dryad) and Armeria maritima (sea pink), along with a few temperate plants (e.g., Rorippa sp. (yellow cress)) probably colonized gravel floodplains and their leaves and seeds were washed during floods into nearby proglacial lakes. Ostracodes associated with these tundra-like environments include Heterocypris incongruens, Limnocythere friabilis, L. herricki, and Cytherissa lacustris (in order of inferred decreasing salinty). Picea mariana (black spruce) appeared during the latter part of the Livingston Phase and co-existed for a time with Salix arctica (arctic willow) and other tundra and temperate plants.

No fossiliferous material has yet been found to date the beginning of the Crown Point Phase (and, hence, onset of activity of the Harvard Sublobe and formation of the Woodstock Moraine). Rapid ice retreat from the Woodstock Moraine is evident from landforms produced by large floods in the Elgin-Algonquin reach of the Fox River. Radiocarbon ages of plant remains washed from lake sediment that collected in depressions on the surface of the Woodstock Moraine indicate the flooding occurred at about 14,900 RCYBP, and before 14,000 RCYBP. These floods, known as the Fox River Torrent, may have joined the Kankakee Torrent of the Illinois River valley. The plant macrofossils also indicate that a spruce-sedge parkland was established by this time.