A CHRONOLOGY FOR GLACIAL LAKE AGASSIZ BEACHES ALONG UPHAM'S TYPE SECTION: PRELIMINARY OSL DATING RESULTS

Large freshwater outbursts from glacial Lake Agassiz at the close of the Pleistocene have been implicated as a trigger mechanism for the Younger Dryas global cooling phenomena. A direct chronology of beach ridge formation and, thereby, shoreline occupation is needed to test temporal relationships between lake level changes in Lake Agassiz and climate change events. Optically Stimulated Luminescence (OSL) dating provides a means to determine depositional ages for glacial Lake Agassiz shoreline deposits. This study focuses on the four major strandlines of the Lake Agassiz southern basin named by Warren Upham in his 1895 USGS monograph. His type section paralleled present day Minnesota Highway 9 and shoreline complexes were assigned the names of the closest towns. They are, from the interior of the basin outward, Campbell, Tintah, Norcross, and Herman. Three criteria were used to select sampling locations for this study; proximity to the type section, topographic expression, and soil composition. All samples were dated using single-aliquot regenerative dose data collection procedures as well as dose distribution analysis methods as detailed by Lepper et al. (Geology 35:667-670, 2007). This presentation will compare the OSL dating results with previously published geochronological interpretations by Teller (Quat. Sci. Rev. 20:1649-1659, 2001), Fisher (GSA Bulletin 117:1481-1496, 2005), and Lepper et al. (Geology 35:667-670, 2007). Many of the depositional ages obtained from this study agree with traditional Lake Agassiz beach ridge chronologies, however, potentially informative inconsistencies exist between past interpretations and the present OSL data set.