A Sedimentologic, Morphometric, and Geochronologic Investigation of Ambiguous Dune-like Landforms: An Indicator of Proglacial Lake Drainage in the Lake Superior Basin, USA

Preliminary observations of three trains of dune-like landforms, just south of the shore of Lake Superior, near Christmas, MI, USA, reveal the presence of large and imbricated boulder clasts on their surface and 20–33 m deep bedrock canyons in close proximity. These characteristics suggest an ambiguous episode of high-magnitude discharge across this landscape before the modern physical geography of the Lake Superior basin was established. Understanding the formation of these landforms is important in reconstructing regional deglacial chronology, meltwater routing history, and proglacial lake-level fluctuations within the Lake Superior basin. In addition, because these landforms are similar to other landscapes where catastrophic drainage occurred, like the Camas Prairie (Missoula Floods, Montana, USA), such comparisons further our understanding of the processes that occur during these high-magnitude events. Unfortunately, little data exists from this site that can elucidate the depositional chronology and genesis of these landforms, herein named the Christmas Dunes (CD).  We collected 20 ground penetrating radar (GPR) lines and measured 814 boulders (dimensions, strike and dip). Additionally, we collected 7 cosmogenic nuclide (CN) samples for 10Be exposure ages, 6 from imbricated sandstone boulders and 1 from a granitic boulder. Morphologic analysis was conducted using newly available LiDAR DEMs. The GPR data from a landform most proximal to a spillway contained inclined reflections that dip up-flow. It is possible dipping reflections are imbricated boulders buried within the dune because the ~23° reflection angle is similar to imbrication angles of surface boulders (21° - 59°), but no down-flow reflection indicating a potential buried boulder could be positively identified. Thus, we hypothesize these are antidune-like forms. The presence of antidunes suggests that the flows stopped abruptly because antidunes are commonly obliterated once a flow transitions from supercritical to subcritical. We hypothesize rapid lake draw-down caused abrupt spillway abandonment allowing the antidune forms to be preserved.  Dune-like landforms further from the spillways contain inclined GPR reflections interpreted as down-flow dipping sedimentary structures and suggest a transition in flow regime beyond the most spillway-proximal landforms. Boulder B-axis diameters (0.2 - 10.7 m) decrease with distance from the spillways, supporting the interpretation of a flow and transport-regime shift. Preliminary estimates of paleodischarge suggest flows may have been 0.22 Sv (0.106 – 0.33 Sv; Breckenridge and Fisher, 2021). Given similarities between CD and sites like Camas Prairie, we hypothesize that CD formed during rapid proglacial lake draw-down across the sandstone bedrock ridge into which the spillways are incised. CD also represents a well-preserved location indicative of internal basin evolution dynamics during the rapid draining of a proglacial lake basin – inadequately understood in overflow and outburst flood literature. This event likely occurred when the easternmost outlet of the Lake Superior basin opened, abruptly rerouting southward-flowing meltwater from the Au-Train/Whitefish spillway across the CD site prior to 10.5 ka.