QUATERNARY GEOLOGIC MAPPING IN THE ST. CROIX SCENIC RIVERWAY: A FRESH LOOK AT GLACIAL LAKES LIND AND GRANTSBURG

In partnership with the National Park Service (NPS), the Wisconsin Geological and Natural History Survey is conducting fieldwork to produce 1:100,000 scale mapping of the surficial geology of the previously unmapped areas along the St. Croix Scenic Riverway. The mapping area includes quadrangles in Pierce, Burnett, and Washburn Counties, WI. The primary goal for the NPS is to be able to merge this new mapping with previously-completed mapping in Wisconsin and Minnesota to produce a seamless surficial geologic map of the entire park unit. The WGNHS is providing the mapping expertise to achieve the NPS goals. Additionally, WGNHS staff is leveraging the opportunity to re-examine the Quaternary geology and history of the area.

Previous work in northwestern Wisconsin (Johnson et al., 1999; Johnson, 2000) described deposits from multiple ice-marginal lakes associated with the retreat of the Superior Lobe and the subsequent advance and retreat of the Grantsburg Sublobe of the Des Moines Lobe. As also recognized nearby in Minnesota, these deposits have been interpreted to represent glacial Lake Lind and glacial Lake Grantsburg, two separate lakes that existed sequentially in the same general area along the modern St. Croix River. In Wisconsin, these deposits have previously been described primarily from well construction reports, limited outcrop exposures, and limited split-spoon drilling.

In order to produce the new surficial geologic mapping and to re-evaluate the regional glacial history, we are combining field mapping with interpretations based on aerial photography and digital elevation models. While outcrop exposures of unconsolidated sediment will be an important component of this work, the new mapping and research is also incorporating rotosonic and Geoprobe coring. A single rotosonic core will provide continuous sample collection through the entire Marine Isotope Stage 2 (MIS 2) unconsolidated sediment package, and several dozen Geoprobe cores are being sited to specifically target glacial Lake Lind and Grantsburg sediment. This will lead to a more complete understanding of the nature and distribution of these sediments in Wisconsin, providing the opportunity to refine our understanding of the deglacial history of the St. Croix valley.