EXTENT AND GLACIAL HISTORY OF THE CORDILLERAN ICE SHEET IN NW MONTANA: USING OSL TO DATE GLACIAL SEDIMENTS FROM THE SOUTHEASTERN FLATHEAD LOBE OF THE CORDILLERAN ICE SHEET

With the retreat of ice sheets in the past, global sea levels rose at the rate of >1cm per year, with terrestrial ice sheets retreating gradually and marine ice sheets collapsing. The Cordilleran Ice Sheet (CIS) was one of the major late Quaternary ice sheets. The CIS initially developed in southern Alaska but spread south during the Fraser Glaciation, reaching its maximum extent in southern British Columbia and northern Washington. The chronology of the western lobes of the CIS have received much attention, but not the easternmost lobes. We propose to build on earlier quartz luminescence analyses of glacio-deltaic and moraine deposits to constrain the age of the furthest southeasterly extent of the CIS at the Flathead Lobe in northwestern Montana. During recent fieldwork, samples were collected from Quaternary moraine deposits, glacial lake deposits, glacial lake outwash, and glacial till from localities consistent with the position of the furthest extent of the Flathead Lobe. Objectives of the study include using small aliquot and single-grain luminescence analysis of quartz or feldspar from glacial sediments; reconstructing environmental radioactivity via an on-site, lab, and modeling approaches; refining the glacial chronology and placing the reconstruction in the regional climate history. The chronological record of ice-sheet retreat for the last deglaciation and last interglaciation permits several conclusions on the past behavior of ice sheets in a warming climate. This presentation will discuss preliminary results of luminescence analyses from the glacial sediments collected during recent fieldwork.