CHRONOLOGICAL EMBELLISHMENTS TO THE CALUMET AND NIPISSING PHASES: SOUTHWESTERN LAKE MICHIGAN REGION

Radiocarbon ages determined from several sites fill in details regarding the temporal framework of late Quaternary and Holocene lacustrine and paludal settings in the southwestern Lake Michigan lake region. In Lansing, Illinois, a rich assemblage of fossils was encountered in basal beach-ridge deposits of Lake Chicago. Buried by 3 m of laminated and cross-stratified littoral sand, the lower 10 cm was rich in fossil pondweed and bulrush seeds, fragments of charcoal, wood, cones, needles, and coal. Separate samples of each component yielded radiocarbon ages between 12.6 and 12.7 cal ka, except for the coal (> 55 cal ka) and needles (12.4 cal ka). The two finite age clusters match probability-distribution curves of 24 radiocarbon ages of Calumet phase fossils in this region. The presence of 12.65 and 12.4 cal ka fossils in the same deposit suggests two periods when high water conditions allowed burial and preservation of plant fossils. The ages correspond with positive-value δ¹⁸O probability-density peaks indicative of greater meltwater contribution to the Gulf of Mexico. Collectively, this suggests that the Calumet beach at this location mixed younger, less-dense needles with denser, older organics during the last rise in lake level of the Calumet Phase, which culminated with deposition of littoral sands.

Our second example documents the age of the poorly-dated Chippewa Low and better-documented Nipissing transgression. Overflow of Lake Michigan during late Nipissing transgression eroded a wide channel in the vicinity of Palos Park, Illinois. Within this channel at Moraine View Community College, a succession of paludal marl and peat sandwiched between lake sediment documents local conditions during the Chippewa Low. The onset of the low is represented by ages from the base of the marl (11.3 cal ka; all ages are of wood, seeds, or papery insect nest material; all σ₁< 30 yr). Starting at 6.8 cal ka, Lake Michigan rose about 6.4 m in 400 yrs (0.016 m/yr), drowning peat beds at 171.9 to 178.3 m. Our youngest age of the Nipissing transgression, 6.3 cal ka, compared to unpublished OSL dune ages in the Wentworth field of 5.1 to 4.1 ka, indicate the mean rate of rise slowed to 0.007 m/yr, but some of the rate decrease may be attributed to erosion or non-deposition during high lake stages.