CONSTRAINING THE HISTORY OF GLACIAL LAKE GRANTSBURG

Glacial Lake Grantsburg formed in east-central Minnesota and northwest Wisconsin USA when the Grantsburg sublobe of the Des Moines Lobe dammed the St. Croix River during late Wisconsin deglaciation of the region. Rhythmically bedded silts and clays are interpreted as varves and suggest a short-lived lake (<100 years) (Johnson and Hemstad, 1998). Under the grey varved Grantsburg sediment is a layer of deltaic and fluvial sand, which was formed by the ancestral St. Croix River. Beneath these fluvial sediments are red varved silt and clay originally thought to be from Glacial Lake Grantsburg. More recent findings however, suggest it is from a separate lake, Glacial Lake Lind that formed in front of the retreating Superior Lobe (Johnson and Hemstad, 1998). Age control for the timing of Glacial Lake Grantsburg and other glacial events in the region are poorly constrained by a few radiocarbon ages. In order to provide a more accurate age control on the timing of lake formation and ice advance and retreat optically Stimulated Luminescence (OSL) samples were collected from sediments above and below the Glacial Lake Grantsburg clays at two sites. Age results are still pending, however preliminary OSL ages suggest that the lake drained by 11ka.