DEVELOPING A DEPOSITIONAL CHRONOLOGY FOR BEACH RIDGES IN THE SOUTHERN LAKE AGASSIZ BASIN

Routing changes from glacial Lake Agassiz to different oceans have been implicated in triggering dramatic changes in Northern Hemisphere and even global climate by altering ocean circulation patterns as recorded in cores from ice sheets, caves and ocean basins. However, independent chronologic evidence for meltwater routing from the Lake Agassiz basin and outlet spillways remain sparse. In recent years complex models have been put forward regarding the number, chronology, and history of outlets from glacial Lake Agassiz. Testing these models will require an extensive and pertinent geochronology. However, no reliable radiocarbon dates have come from strandlines graded to the southern outlet above the Campbell Beach. Optically Stimulated Luminescence (OSL) dating provides a means to determine depositional ages directly from sediment grains and construct a chronology that can be directly linked to formation of beach ridges and variations in lake levels. We have sampled littoral deposits along two transects in North Dakota for OSL dating. The complete set of samples represent strandlines between the Herman and Campbell Beach levels, inclusively. Single-aliquot regenerative dose data collection procedures were used as well as dose distribution analysis methods. Our preliminary results for the upper beaches (Upham through Herman) range from 12ka to 15ka. In this presentation we will discuss methodological considerations that have permitted these age determinations as well as the geomorphic and stratigraphic context for the samples. We are confident that important knowledge will be gained by generating strandline chronologies for Lake Agassiz, and our preliminary analysis indicates that OSL dating will be an important contributor to paleogeographic reconstructions of glacial Lake Agassiz.