Optically stimulated luminescence ages from the Lake Agassiz basin in Manitoba

Geomorphic analysis and optically stimulated luminescence (OSL) ages from undated Lake Agassiz beaches and adjacent fluvial sediments on Riding Mountain in Manitoba provide insight into their early history. New OSL ages of 14.5±2.4 and 13.4±0.7 ka on the oldest (Herman to Norcross) beaches of Lake Agassiz near the Canada-U.S. border indicate that the Laurentide Ice Sheet (LIS) retreated from that part of the Agassiz basin by ~14.5 ka. To the north along Riding Mountain, the Herman strandlines are absent, and OSL ages on the oldest beach there average 12.9 ka, which links it to the younger Norcross-Tintah strandlines. In adjacent Riding Mountain, OSL ages and geomorphological relationships of a large abandoned glacial spillway >200 m above the oldest beaches of Lake Agassiz indicate that this channel predates retreat of the LIS and formation of beaches in this part of the Agassiz basin, with ice remaining in this area until after 14.5 ka. OSL ages on the Gimli beach 170 km to the east are >3000 yr older than conventional assignments, suggesting that it formed during the Moorhead low-water phase 12.8–10.6 ka. Luminescence ages support the conclusion that the Campbell beach formed ~10.9 ka near the end of the Moorhead low-water phase.