24-1 Spatial and Sedimentological Analyses of Paleolake Sediments along the Algoma Shoreline in Grand Portage National Monument, Minnesota

Session: Lake Sedimentary Records of Past Climate and Environment (Posters)

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Abstract:

At the end of the last glacial maximum, the Lake Superior Basin experienced higher-than-present levels due to meltwater influx from the Laurentide Ice Sheet. As lake levels subsequently 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 many 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) and sediment analyses were used to map and characterize a segment of a paleolake shoreline in Grand Portage National Monument, Minnesota. The topographically-subtle Paleolake Algoma (~3,000 cal. yr. B.P.) shore was selected as the study area because visible shoreline features are not present where it intersects the park. Shapefiles of the Algoma and adjacent paleolake shoreline locations were created based on previously-estimated elevations, and though strandline identification using Light Detection and Ranging (LiDAR) imagery.

In 2019, a shovel test survey was completed in transects along the modeled Algoma shoreline, and sediments were described in the field. Sand and gravel sediments were identified as likely shoreline-related, and collected. Grain size distribution analyses of the coarse fraction were conducted, and these results were incorporated into ArcGIS Pro to spatially analyze whether variations in grain size, or depth to shoreline sediments, are present throughout the study area. Grain size distribution patterns could suggest differences in depositional energy along the shoreline. While analyses are ongoing, preliminary results show shoreline sediments present in all transects, indicating the modeled elevation for the Algoma shoreline in this area is accurate. Additionally, coarse materials (gravel – coarse sand) are the dominant grain size in most samples—indicating relatively high-energy deposition. However, obvious spatial variation in sediment characteristics is not present, indicating that depositional conditions may have been relatively uniform throughout the small study area. Future assessments covering a larger area, but with lower spatial resolution, may be more likely to show variation in depositional energy.  

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