MAPPING LAKE AGASSIZ BEACHES, MEASURING THEIR ISOSTATICALLY-INDUCED SLOPES, AND ESTIMATING THEIR AGES USING LIDAR DEM DATA

1 m spatial resolution LiDAR DEM tiles, with a vertical resolution of 0.001 m, from a large area in the Lake Agassiz basin in North Dakota were imported into ArcGIS. High-resolution DEM mosaics and hillshade maps were prepared in order to define a series of Lake Agassiz strandlines. Although many of these beaches had been identified and named more than a century ago by Warren Upham, LiDAR images clearly reveal their complexity, and identify additional small and low relief ridges not previously mapped. We measured precise elevations along the crests of six of the best developed strandlines—in sequence from highest elevation to lowest: Campbell, McCauleyville, lower Blanchard, Emerado, upper Ojata, and lower Ojata beaches. Other strandlines in the sequence are more poorly developed and discontinuous, and include the Herman, Norcross, and Tintah complex, upper two Blanchard, two Hillsboro, and two Gladstone strandlines, many of which have very low relief (<1 m). We calculated the isostatically-induced north to south slopes on the six best-developed strandlines. The steepest gradient is on the uppermost (Campbell) beach in the sequence and the gradient on each beach below that it is progressively lower, confirming that these beaches were deposited in sequence from oldest to youngest as the level of Lake Agassiz declined. There were no anomalous beach gradients in the sequence which was deposited between ~9400 ¹⁴C (10,629 cal) yrs BP and 8700 ¹⁴C (9644 cal) yrs BP. These gradient calculations support the long-standing conclusion that the Ojata beach formed after the higher-elevation Campbell beach was deposited 9400 ¹⁴C yrs BP, rather than >10,000 ¹⁴C years ago as some have suggested because of the age of organics below the Ojata beach. Those organics and the subaerial surface on which they lie must have survived the Moorhead transgression to the Campbell beach, and the Ojata beach must have been deposited over the preserved older organic unit as lake level declined. We use a simple extrapolation model to estimate the ages of beaches in the sequence.