PALEOLAKE SEDIMENT INVESTIGATIONS ALONG THE ALGOMA SHORELINE IN GRAND PORTAGE NATIONAL MONUMENT, MINNESOTA

At the end of the last glacial maximum, meltwater from the Laurentide Ice Sheet filled the Lake Superior basin to levels higher than the present. Over time, as lake levels fell, shoreline features, such as beaches and terraces, remained as strandlines on the landscape. Advances in remote sensing and mapping technologies have allowed for detailed surveys of strandlines throughout the Great Lakes, but in some locations, obvious topographic evidence is not visible, even with high-resolution digital elevation models. Remotely characterizing the sedimentological nature of these features also remains a challenge.

For this project Geographic Information Systems (GIS) were used to map paleolake shorelines in Grand Portage National Monument, Minnesota. Ground truthing was conducted to confirm the presence of shoreline sediments along the topographically-subtle Paleolake Algoma (~3,000 cal. yr. B.P.) shore. Visible strandlines were identified throughout the park from Light Detection and Ranging (LiDAR) imagery, and were preliminarily correlated to paleolakes based on previously modeled elevations. Shapefiles of shoreline locations were created based on estimated elevations.

Paleolake Algoma was selected as the study area because visible shoreline features are not present where it intersects the park. A shovel test survey was completed in transects along the modeled shoreline, and sediments were described in the field. Sand and gravel sediments were identified as likely shoreline-related, and were collected. Grain size distribution analyses of the coarse fraction were conducted to assess whether samples were deposited in high or low energy environments. Results are being mapped with GIS to determine if spatial patterns in grain size exist throughout the study area.

Survey results show shoreline sediments present in all transects, indicating the modeled elevation for the Algoma shoreline in this area is accurate. Preliminary analyses reveal coarse materials (gravel – coarse sand) are the dominant grain size in most samples, indicating relatively high-energy deposition. However, grain size variation among samples shows no clear spatial patterns. We plan to conduct similar analyses for future studies along paleoshorelines in the park to better characterize shoreline sediments throughout Grand Portage.