QUATERNARY STRATIGRAPHY, GEOMORPHOLOGY, AND PRELIMINARY SURFICIAL MAPPING ALONG THE ST CROIX NATIONAL SCENIC RIVERWAY, BURNETT COUNTY, WISCONSIN

Surficial geologic mapping provides important information needed for resource management by National Park System (NPS) planners. To increase the awareness of geologic resources in the parks the NPS initiated the Geologic Resources Inventory to provide park-specific reports and digital map data. In Wisconsin, NPS contracted with the Wisconsin Geological and Natural History Survey to provide a surficial geologic map database in NCGMP09 format of previously unmapped property within and adjacent to the St. Croix National Scenic Riverway.

This poster presents interim results along the St. Croix River in Wisconsin’s Burnett County based on field observations, mapping from surrounding counties, and LiDAR-derived 3m DEMs. The field component of this project to date includes one rotosonic core, 47 Geoprobe cores, and observations at several gravel pits. The study area contains several lithostratigraphic units including glacial and glacio-fluvial deposits of the Copper Falls Formation, laminated silty clay deposited in Lake Lind (Sunrise Member), glacial deposits of the Trade River Formation, and laminated silty clay deposited in Lake Grantsburg (Falun Member). These occur in varied geomorphic settings dominated by outwash surfaces formed during melting of the Superior Lobe of the Laurentide Ice Sheet. Much of the landscape experienced subsequent eolian modification and peat deposition.

Our core sampling largely confirms relationships among the lithostratigraphic units previously recognized. However, we were able to recognize several thin (1m thick and less) glacio-lacustrine silty clays similar to the Falun Member that occur at higher elevations and possibly higher stratigraphic positions. These thin deposits likely play a role in groundwater flow and understanding their depositional environment will better constrain their continuity in the subsurface. It is evident that a proglacial lake (i.e. Lake Grantsburg) formed at the margin of the Grantsburg Sublobe in Wisconsin as previously suggested. However, it is unclear whether other discontinuous smaller lakes formed across the proglacial landscape during deglaciation. Future work for this project in quadrangles further east in Washburn and Sawyer Counties will continue to refine our understanding of the history of the Superior Lobe in Wisconsin.