SURVEY AND CHARACTERIZATION OF RELICT SHORELINE SEDIMENTS IN THE LAKE SUPERIOR BASIN, GRAND PORTAGE NATIONAL MONUMENT, MINNESOTA

At the end of the last glacial maximum, meltwater from the retreating Laurentide Ice Sheet resulted in higher-than-present water levels in the Lake Superior basin. As lake levels fell over time, shoreline features, such as beaches and terraces, remained on the landscape as ‘strandlines.’ Advances in remote sensing and mapping technologies have allowed for increasingly thorough surveys of strandlines and their associated paleolakes throughout the Great Lakes. However, remotely characterizing the sedimentological nature of these features—which is important for understanding depositional conditions and post-depositional changes—remains a challenge. For this project, remote sensing and field methods were used to survey and sample a relict lakeshore in Grand Portage National Monument, in northeastern Minnesota. Geographic Information Systems (GIS) and Light Detection and Ranging (LiDAR) imagery were used to identify and map visible strandlines, as well as the approximate locations of paleoshorelines based on previously-modeled elevations (e.g. Farrand, 1960). During the summer of 2019, a portion of the Paleolake Algoma shoreline (~3,000 cal. yr. B.P.) within the Monument was surveyed in order to ground truth its presence and location, and collect sediments for further analysis. Sixteen transects were established along the modeled shoreline, and five shovel tests were dug along each transect. Basic sediment descriptions were completed in the field, and sandy sediments—identified as likely shoreline-related—were collected. Shovel test locations were mapped with a Trimble GeoXT GPS unit, and converted to a shapefile in ArcMap. Grain size distribution analyses of the sand fraction were conducted on samples using a sieve shaker and laboratory balance, in order to discern differences in sediment characteristics throughout the study area. These data will be incorporated into GIS for spatial and statistical analyses, to determine whether spatially significant variations exist. This could indicate that depositional conditions, such as proximity to wave action, differed along the shoreline.

Reference

Farrand, W.R., 1960. Former Shorelines in Western and Northern Lake Superior Basin. University of Michigan, Ann Arbor, MI, p. 266 (PhD dissertation).