NEW AGES FROM A POTENTIAL PRE-AGASSIZ BEACH RIDGE TRANSECT IN NORTHWESTERN MINNESOTA

Lake Agassiz existed from 14.1 to 8.2 ka and covered much of central Canada, northern portions of Minnesota, and a lobe that comprises the modern Red River Valley, although not all of this area was inundated at the same time. The importance of Lake Agassiz is its association with the southern margin of the Laurentide Ice Sheet at the end of the Pleistocene glaciation and its potential role as a driver in global climate change. The drawdown or discharge of a large volume of water from Lake Agassiz has been hypothesized as one potential cause of the onset of the Younger Dryas climate interval. Research done in recent years by Lepper and others on the geochronology of the strandlines has produced robust ages for three of the four widely recognized strandline complexes of Lake Agassiz; those being the Herman (14.1 ± 0.3 ka), Norcross (13.6 ± 0.2 ka), and Campbell (10.5 ± 0.3 ka). The age of the Tintah complex has proven to be more challenging to pinpoint. The dilemma of the Tintah’s age and the complexity of proglacial lake history in the region only increased after research done by a student team last year dated a presumably “Tintah” beach ridge to 16.4 ka. At this time, 16.4 ka, currently accepted glacial models indicate the glacial ice should have been covering this area of Minnesota. Building on last year’s unexpected findings, the goal of this year’s project was to gain additional context for the 16.4 ka beach ridge by examining samples along a ridge crossing transect in the same area of northwestern Minnesota. Satellite imagery with integrated soil survey data as well as previously published digital beach ridge maps and elevation profiles were used to choose sample locations. Once in the field, samples were collected from exposures in sand and gravel quarries or from hand excavated pits. Samples were processed and dated using OSL SAR techniques on quartz sand. This presentation focuses on the age results and our emerging data set for a potentially unidentified pre-Agassiz glacial lake in northwestern Minnesota.