DATING THE ONSET OF THE LAST GLACIATION IN EASTERN MINNESOTA

An AMS radiocarbon date of 28,580+220 years BP has been discovered on wood and in-place roots from a core taken near Stacy in Chisago County, eastern Minnesota. This finding creates a maximum date for the Emerald phase, the first advance of the Superior lobe in the Late Wisconsinan. The dated horizon is at a depth of 151 feet in the midst of an interval from 143 to 173 feet of noncalcareous, interbedded silt to sandy silt and medium- to coarse-grained sand. The first organic silt bed occurs at about 146 feet, and the last bed at about 164 feet. The value/chroma is mostly 4/2-4/1 with some 4/3 beds, whereas the underlying leached sandy till is 5YR5-4/4, likely indicating some hiatus. The bedded sediment is overlain by oxidized (5YR4/3), calcareous, dense sandy till attributed to the Emerald phase. This till is overlain by unoxidized (5YR4/2), thick gravelly sand and till of the St. Croix phase.

A similar date (28,590+350 radiocarbon years BP) was derived from peat buried below 3 feet of colluvium at a site in southeastern Minnesota, presumably recording the onset of slope destabilization caused by an increasingly cold climate.

Pollen counts from the Stacy core are dominated by spruce, whereas pollen from a split-spoon sample of an organic silt layer at a site in Arden Hills, Ramsey County, Minnesota, 28 miles to the southwest, lacks spruce pollen and is dominated by pine and wormwood (Artemisia), indicating a warmer climate. The layer sampled at Arden Hills has a radiocarbon date of 32,300+2,000 years BP, and is overlain by 130 feet of mostly sand and gravel of the St. Croix/Emerald phase, and in turn overlies about 250 feet of Superior provenance sand and gravel. The thick sand and gravel below the organic silt at this site and the red sandy till below the organic section in the Stacy core may have been deposited by the Early Wisconsinan Superior lobe advance that contributed reddish sediment to the Roxana Silt to the southeast.