LUMINESCENCE DATING OF GLACIAL LAKE GREAT FALLS, MONTANA, U.S.A

Stratigraphic sequences related to glacial Lake Great Falls are exposed along the Missouri River valley. They help to determine the timing and extent of continental glaciation on the Great Plains of North America. Near Holter Lake, Montana, laminated silts of glacial Lake Great Falls are buried by sands. An infrared-stimulated luminescence (IRSL) age of 13.2±0.9 ka was obtained for the lake silts (sample UW355). Anomalous fading, or loss of thermally stable signal through time, is evident. Thus, this age can only be taken as a minimum; correction for fading provides an age with a large standard deviation (35.4±18.3 ka). Optically-stimulated luminescence (OSL), using the single aliquot SAR method, gave an age of 17.1±1.4 ka for the lake silts. Multi-aliquot OSL analysis gave an age of 14.7±1.1 ka for the overlying sand (sample UW356), while single aliquot analysis gave an age of 13.1±0.5 ka. Based on these measurements, a Late Wisconsinan (oxygen isotope stage 2) assignment does not seem unreasonable for the Holter Lake sequence. A stratigraphic sequence near Hower Coulee, Montana, contains fluvial gravels and sands that are buried by silts (the “lower lake”) of glacial Lake Great Falls. The lower lake silts lie below till of the Laurentide Ice Sheet (LIS). The till is overlain by a set of deposits collectively referred to as the “upper lake.” Luminescence measurements of the lower lake suggest the sediments were deposited during the early part of oxygen isotope stage 2. Single-aliquot OSL ages range from 24.6±4.39 ka (UW469) to 14.5±2.03 ka (UW468), while multi-aliquot IRSL ages after correction for fading range from 24.7±6.78 ka (UW468) to 17.6±2.24 ka (UW469). There are two ages on sandy and silty lithofacies of the upper lake sequence, using the single-grain leading edge technique: 10.5±3.33 ka (UW454) and 14.3±3.66 ka (UW467). These ages imply that the deposits of the upper lake sequence likely date to the last part of oxygen isotope stage 2. Sediments of glacial Lake Great Falls were deposited in a lake formed when the Missouri River was blocked by a lobe of the LIS. Direct dating of sediments using luminescence measurements appears to support a late Wisconsin age for these stratigraphic sequences. This implies that a lobe of the LIS advanced into northern Montana and reached the present-day location of the Missouri Valley during oxygen isotope 2.