WHAT BEACH IS IT? GEOMORPHOLOGICAL OR GEOCHRONOLOGICAL CORRELATION OF LAKE AGASSIZ SHORELINES IN NORTHWESTERN MINNESOTA, USA

The shorelines of Glacial Lake Agassiz have been named, correlated, and have had a relative chronology established since Upham’s seminal work in 1895. Thankfully researchers for over a century have sought to expand and refine our knowledge of the lake, its signature on land, and its relationship to global climate variations rather than just accepting the status quo during their time. Absolute dating of Glacial Lake Agassiz shorelines has been slow and sparse because the shoreline depositional environment is not conducive to the survival and preservation of organic matter for radiocarbon dating. Optically stimulated luminescence (OSL) dating provides a new and successful means to establish chronologies for these shorelines. Recent OSL dating along the eastern margin of Glacial Lake Agassiz in Pennington, Marshall, and Roseau counties of northwestern Minnesota has produced shoreline chronologies that are not consistent with geomorphologically correlated strandline complexes. Geomorphological correlation of strandlines relies on correction for glacial isostatic rebound to relate modern evaluations to their paleo-elevations. Unfortunately, there are a variety of reasons why elevation corrections may be dubious around a basin as large as Lake Agassiz’s and over the Herman to Camplbell timeframe (>3500 yr). Therefore, evaluating the existing correlation of shorelines is justifiable. Geochronology provides a means to correlate shoreline deposits that is independent of pre-existing assumed relationships. This presentation will review the current OSL shoreline age data set and point out inconsistencies between geomorphological and geochronological correlations that have resulted from several undergraduate student-driven studies in the three counties listed above. The results of our research reinforce the need to validate geomorphological correlations with geochronology. The amount of consistent OSL age data for the shorelines is growing to a point where a willingness to think beyond today’s status quo is required.