TESTING PAST CORRLATIONS OF LAKE AGASSIZ’S TINTAH SHORELINE IN NORTHWESTERN MINNESOTA WITH NEW GEOCHRONOLOGY

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 was not conducive to the survival and preservation of organic matter for radiocarbon dating. Optically stimulated luminescence (OSL) dating has provided a successful means to establish chronologies for these shorelines. Recent OSL dating along the eastern margin of Glacial Lake Agassiz in Red Lake, Pennington, Marshal, 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 shoreline evaluations to a paleo-water plain. Unfortunately, isostatic rebound models may not capture small-scale changes along the margins of a basin like that of Lake Agassiz in which ice and water loads in and adjacent to the basin were dynamically changing during the late Pleistocene and early Holocene. Therefore, evaluating the existing correlation of Lake Agassiz’s shoreline complexes is justifiable. Geochronology provides a means to correlate shoreline deposits that is independent of pre-existing assumed relationships. This presentation will review the current Tintah shoreline OSL age data set and point out inconsistencies between geomorphological and geochronological correlations that have been revealed by several undergraduate, student-driven studies in the counties listed above. The results of our research reinforce the need to validate geomorphological correlations with geochronology. The amount of OSL age data for Lake Agassiz’s shorelines is growing to a point where a willingness to think beyond today’s status quo is required.