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

While reconstructions of ice-retreat and drainage reorganization are important for understanding linkages between meltwater discharge and cooling events during deglaciation, age control for ice-margin positions has been largely limited by the scarcity of material for radiocarbon dating in pro-glacial settings. This study investigates the chronology of ice retreat recorded in ice-contact, deltaic and pro-glacial fluvial deposits along the southern margin of the Des Moines lobe in the upper Minnesota River valley, USA. Samples for single-grain optically simulated luminescence (OSL) dating were collected from deposits that mark key ice-margin positions associated with the Big Stone Moraine and lake-level positions associated with glacial Lake Benson, a short-lived pro-glacial lake that formed immediately prior to glacial Lake Agassiz. Geomorphic mapping in the region has identified deltas associated with two lake levels in glacial Lake Benson and several north-south oriented valleys with underfit streams that mark and ice-marginal drainages that formed during ice retreat in relationship to the outer and inner Big Stone Moraine. Single-grain OSL results constrain the timing of ice retreat within the lake basin and suggest the higher delta level was occupied at ~14.8 ka and lake level dropped to the lower, stable phase of glacial Lake Benson at 14.4-14.0 ka. Results are discussed in relationship to the timing of formation and drainage of glacial Lake Agassiz and style and rates of ice retreat in the region.