NEW CHRONOLOGICAL DATA ON THE FURTHEST EXTENT OF THE SOUTHEASTERN FLATHEAD LOBE OF THE CORDILLERAN ICE SHEET FROM LUMINESCENCE STUDIES OF GLACIO-DELTAIC AND MORAINE DEPOSITS, FLATHEAD VALLEY, MONTANA

The timing of the Cordilleran ice sheet in this part of northwestern Montana has historically been particularly difficult to determine, largely due to the lack of suitable materials for dating by various techniques. We visited a number of sites bordering the southern end of Flathead Lake, with previously documented glacial sediments consistent with deposition at the furthest extent of the Flathead lobe. At two sites - Dayton Creek Delta and Elmo Moraine - we were able to sample suitable sediments for luminescence analyses. At the Dayton Creek Delta site a series of cross-bedded fine sand foreset deposits were exposed in an extensive sand and gravel quarry; we collected 6 samples in a stratigraphic sequence from near to the top to the bottom of the exposed foreset sequence. The Elmo moraine is an extensive lateral moraine exposed by a road-cut; here we collected 2 samples from cross-bedded fine sands.

We prepared 1-mm-diameter aliquots of quartz (~16 grains) for luminescence analyses, and used a single-aliquot regenerative-dose protocol and central age or minimum age dose-distribution models. Environmental dose-rate was assessed using ICP-MS/-OES data and calculated cosmic dose-rate. Two of the Dayton Creek Delta samples had insufficient quartz for analyses; from the top to the bottom of the stratigraphic sequence the remaining 4 samples have preliminary OSL ages of 18.9 ± 1.0 ka, 21.6 ± 1.3 ka, 23.8 ± 1.5 ka, and 33.0 ± 3.0 ka. These ages are very consistent to global LGM chronologies of periods of growth to maximum extent (e.g., 33.0-26.5 ka), and periods maintained at maximum position before retreat (e.g., 26.5-20.0 ka). In contrast, the Elmo moraine samples have significantly older preliminary ages of 105.7 ± 14.7 ka and 116.3 ± 8.2 ka; rather than an anticipated LGM feature there is evidence in the literature to suggest that this moraine is a composite of LGM and older deposits, and our samples are consistent with a period of glaciation that predated the LGM. However, the quartz luminescence data is more difficult to interpret for the Elmo moraine samples, and it is not clear whether the samples are overestimated early Wisconsin or significantly older in age. These two samples are being investigated further to see if this can be resolved.