AN INVESTIGATION OF MASSIVE DUNE-LIKE LANDFORMS CREATED BY A FAILURE OF AN ICE DAM AND PROGLACIAL LAKE DRAINAGE WITHIN THE LAKE SUPERIOR BASIN (MICHIGAN)

Many outburst floods from proglacial lakes along the southern Laurentide Ice Sheet (LIS) are inferred from spillways containing grossly underfit streams and from strandline-based, lake-level re-constructions that suggest abrupt drops in lake level. Understanding the history of outburst flooding is important in reconstructing regional deglacial chronology, meltwater routing history, and proglacial lake-level fluctuations. In the Lake Superior basin, near Christmas, Michigan, two sets of dunes (possibly antidunes) were revealed by lidar digital elevation models, which occur downflow from spillway canyons linked to one of many, stepped drainages of glacial Lake Duluth. To our knowledge, these bedforms are unique in the Great Lakes basin. However, this site may be an analog to landforms present in the Camas Prairie, Montana, associated with the drawdown of glacial Lake Missoula. Each set of dunes are roughly crescent-shaped, radiate outwards from each spillway, and are composed of 6 to 9 ridges. The largest ridge in each set is the first, up to 25 meters high, and has a wavelength with the second ridge of around 600 meters. The final ridge in each set has a height of only 1–2 meters and a wavelength of 100 meters. Fieldwork during the summer of 2023 mapped 803 boulders along the crests of the bedforms. Length, width, and height measurements, as well as strike and dip were taken on these boulders, most of which were sandstone, presumably eroded from sandstone canyons upflow. Boulder size decreases with distance from the spillways as the height and wavelength of the dunes decrease. The largest of the boulders measured 10.7 m along the longest dimension, while the smallest boulders on the most distal ridge have a typical long-axis of 0.5 meters. Many boulders dip upflow at a mean angle of 30 degrees and some are imbricated. Based on boulder characteristics, dune morphology, and initial ground penetrating radar data, we hypothesize the dunes formed during rapid proglacial lake draw-down into a proglacial lake, with a change in flow regime occurring with distance from the spillway as flows progressed into deeper water. Ongoing work includes ¹⁰Be exposure dating of boulders to establish timing and shoreline mapping to link this event to former proglacial lake levels.