EVIDENCE FOR AN OUTBURST FLOOD DRAINAGE OF GLACIAL LAKE GRANTSBURG AND ITS ROLE IN THE FORMATION OF THE ST. CROIX RIVER VALLEY, WI/MN, USA

The St. Croix River valley (SCRV) of MN/WI, USA, is an ideal case study to understand geomorphic and sedimentologic impacts of outburst floods during regional climatic change and deglaciation. The SCRV is the product of dyssynchronous advance and retreat of glacial lobes and multiple proglacial lake drainage events. The timing, magnitude, and geomorphic impact of these floods remain poorly constrained. The lower SCRV contains terraces, paleochannels, a prominent paleovalley, a bedrock gorge, fluvially sculpted landforms, and an enigmatic terrace-like landform containing pendant-bar-like features (Osceola Bench – OB). These landforms reside within proximity of the terminus of the Grantsburg Sublobe (GSL) and south of a lake GSL dammed – Glacial Lake Grantsburg (GLG). We compile prior data and add sediment descriptions from cores, geochronology (OSL), sediment geochemistry (XRF), interpretations of ground penetrating radar (GPR) imagery, and map landforms using LiDAR-derived DEMs. GPR profiles from OB and inset terraces display shallow, continuous horizontal reflections interpreted as straths. Overlying this are hyperbolic and subhorizontal to inclined reflections interpreted as boulders within cross-bedded sand and gravel. GPR interpretations are consistent with sediment cores containing large clasts in sand and gravel, conformably capped by sandy loam that fines upward. Landform mapping, GPR, and sedimentologic interpretations, coupled with prior surficial geologic maps suggests meltwater flowed through a spillway near St. Croix Falls, WI, and into a paleovalley consistent in elevation with GLG. Drainage rerouted to the lower OB, consistent with the orientation and morphology of sculpted gravel bars on the OB erosional surface. We hypothesize drainage of GLG began as a high-magnitude outflow initiated by retreat of GSL from its terminus near the St. Croix Falls. However, the mechanism (subaerially vs subglacial) by which meltwater arrived at this location is still enigmatic. This outflow, fed by melting ice to the north, accessed progressively lower outlets to the west as it followed retreat of GSL between 16.3-14.1 ka. Incision may have stabilized ~60 m lower than the original outlet ~12.9-12.1 ka. Later proglacial lake drainage in the SCRV was superimposed across the landscape resulting from GLG drainage and further incised the SCRV to its current form.