SINGLE-GRAIN OSL CHRONOLOGY OF ICE RETREAT AND FORMATION OF GLACIAL LAKE BENSON, WEST-CENTRAL MINNESOTA

Glacial Lake Benson formed in the Minnesota River lowland south of the Big Stone Moraine as the Late Wisconsin Des Moines Lobe retreated north of a small moraine located between the cities of Clarkfield, Echo and Clara City Minnesota. The 3900 km² lake basin is defined by the presence of varved lake sediment with occasional dropstones indicating at least 44 years of lake sedimentation, a series of deltas at the mouths of rivers entering the basin, at least two strandlines along the northern margin of the lake basin and a series of outlet channels to the south. Despite significant research on the timing of lake stages of Glacial Lake Agassiz immediately to the north, little age control is available for Glacial Lake Benson. Moreover, there is limited age control for ice positions between the terminal Bemis Moraine and recessional Big Stone Moraine, limiting constraint on the timing and style of ice retreat of the Des Moines Lobe, interpreted as a potential ice stream or surging lobe of the Laurentide Ice Sheet.

In order to provide age constraint for the retreat of the Des Moines Lobe and formation of Glacial Lake Benson we collected seven samples for optically stimulated luminescence (OSL) dating from proglacial outwash, deltaic deposits linked to specific lake levels and ice-contact and supra-glacial deposits linked to ice margin positions. Samples were analyzed using the single-aliquot regenerative-dose technique on single grains of quartz sand. Results from all samples indicate evidence for partial bleaching of the luminescence signal, as expected for samples from glacial environments, and therefore ages were calculated using a minimum age model. Preliminary OSL results range from 14-16 ka, with the exception of one sample interpreted to have been from a crevasse fill that is producing an older age. Results are discussed in relation to the progressive stages of ice retreat, formation of Glacial Lake Benson and its lake-level stages, and subsequent ice retreat to the position of the Big Stone Moraine and initial development of Glacial Lake Agassiz.