PALYNOLOGICAL AND RADIOCARBON EVIDENCE FOR DEGLACIATION EVENTS IN THEGREEN BAY LOBE, WISCONSIN

A new and significant site of organic silty sand has been found beneat h the Valders till at Valders Quarry in northeastern Wisconsin. This i s now the earliest known late-glacial site associated with red till ic e advances in the western Great Lakes area. Leaves of terrestrial plan ts washed into a small depression provide a date of 12,965 +/- 200 yr B.P. (WIS-2293), which is significantly older than the Two Creeks Fore st Bed (ca. 11,800 yr B.P.). Percentage and concentration pollen diagr ams suggest that the site was open and distant from a closed Picea for est. No wood or Picea needles have been found. This date is statistica lly indistinguishable from 12,550 +/- 233 yr B.P., the mean of three d ates for the end of inorganic varve sedimentation at Devils Lake, 160 km southwest at the terminus of the Green Bay Lobe. Assuming that the Green Bay lobe vacated its outermost moraine in the interval from 13,0 00 to 12,500 yr B.P., only a short time was available for retreat of t he ice margin over 350 km, drainage of red sediment from Lake Superior into the Lake Michigan basin, readvance of over 250 km, retreat of at least 80 km, and advance to this site. The time for these events appe ars to have been too short to resolve by current radiocarbon technique . This extremely rapid collapse of the Green Bay lobe has a calibrated age of about 15,000 cal yr B.P., about that of the dramatic warming s een in the Greenland ice cores. (C) 1996 University of Washington.