Mapping and measuring Lake Agassiz strandlines in North Dakota and Manitoba using LiDAR DEM data: Comparing techniques, revising correlations, and interpreting anomalous isostatic rebound gradients

Abstract

Based on light detection and ranging (LiDAR) digital elevation model (DEM) data at 1 m spatial resolution, Lake Agassiz strandlines were mapped and their elevations determined in northeastern North Dakota and southern Manitoba. Two different techniques for determining strandline elevations from LiDAR imagery are compared: one based on measuring the crests of constructional beaches, and the other using the elevation of slope breaks along the shorelines defined by shadows on the LiDAR hillshade map. Our measurements on Lake Agassiz strandlines show that previous correlations of younger strandlines around the Pembina delta, accepted for more than a century, are incorrect; the Gladstone, Ojata, and Emerado strandlines south of the delta are offset by 10 m from their namesakes to the north and so are equivalent to the Ojata, Emerado, and Hillsboro strandlines, respectively. Furthermore, anomalously low gradients for the Ojata and Emerado strandlines around the Pembina delta are interpreted as resulting from postdepositional compaction of underlying fine sediment. Elevations on the isostatically rebounded strandlines confirm that older (higher) ones have steeper northward-rising elevations, and that gradients on those strandlines can be used as proxies for relative age.