THE LATE WISCONSINAN GLACIATION AND DEGLACIATION IN THE AITKIN AND UPHAM BASINS: AITKIN COUNTY, MINNESOTA

The late Wisconsinan glaciation and deglaciation of the Laurentide Ice Sheet in north-central Minnesota is marked by complex interlobate and lacustrine sediment sequences. We collected data in Aitkin County, Minnesota between 2016 and 2018 which provides additional insight into a complex history of multiple ice lobes and their associated glacial lakes. When the terminal Mille Lacs moraine was deposited approximately 17,000 years ago during the Automba phase of the Superior lobe, ice appears to have formed two confluent separate lobes, one around the western snout of the Aitkin basin and another to the west of the basin represented by modern-day lake Mille Lacs. Between the two, a distinct, discontinuous interlobate moraine developed. Additional recessional ice margins formed in the eastern part of the county as ice retreated eastward into the Lake Superior basin. As this ice retreated, its meltwater, along with meltwater from the Brainerd-sublobe and St. Louis sublobe, flowed into glacial lakes Aitkin I and Upham I in the basin between the ice lobes. Both massive and interbedded red, brown and gray laminated lake sediment were deposited in these lakes. It appears that glacial lake Aitkin I existed for more than 350 years based on varve counts in rotary-sonic core. The Aitkin and Upham basins were eventually filled with ice from the St. Louis sublobe as it advanced from the northwest approximately 14,000 to 14,500 years ago. Glacial lakes Aitkin II and Upham II formed as the St. Louis sublobe retreated, occupying the same basin as the initial lakes. Field evidence suggests these younger lakes were less extensive than their predecessors.