TWO UNUSUAL SETS OF GIANT DUNES CREATED BY CATASTROPHIC ICE-DAM FAILURE AND GLACIAL LAKE DRAINAGE INTO A PROGLACIAL LAKE, UPPER PENINSULA OF MICHIGAN, USA

Many outburst floods from ice-dammed lakes rimming the southern Laurentide Ice Sheet (LIS) are inferred from spillways containing grossly underfit streams, and with strandline-based, lake-level reconstructions suggesting abrupt drops in lake level. Depositional landforms such as boulder lags, and expansion, pendant and eddy bars also occur but are less common. To our knowledge, giant current dunes like the ones described here have never been reported from the southern LIS region. The dunes are revealed by lidar digital elevation models (published in 2015) and are located northwest of Christmas (Michigan) near Lake Superior. The giant dunes (possibly antidunes) occur within two distinct sets, each located immediately down flow from a spillway canyon linked to one of many, stepped drainages of glacial Lake Duluth in the Lake Superior basin. The two spillways are only separated by 1-km, and the spillway floors have similar elevations, and may have operated simultaneously, or nearly so. The 25-m deep spillways are 2–2.5 kilometers long and 250–650 meters wide. Preserved erosional scarps at the head of each channel, and in the downstream basin, suggest a water level drop of about 12 meters associated with the flood. The two sets of giant dunes are crescent-shaped, radiate outwards from each spillway, and composed of 6 to 7 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 tops of the first ridges are covered by vehicle-size boulders of Munising Formation sandstone eroded from the spillway channel, and the ridge may be a hydraulic jump deposit. The period, height, wavelength, and elevation steadily decrease with each succeeding ridge, such that the final ridge in each set has a height of only 1–2 meters, and a wavelength of 100 meters. Diminution of the ridges downflow likely reflects deposition in a deepening ice-marginal lake. The preservation of these large bedforms likely required both abrupt spillway abandonment, and drawdown of the glacial lake in which the they formed soon after deposition. Based on a regional compilation of lake levels and radiocarbon dates, the dunes and spillways formed at around 9.4 ¹⁴C ka BP (10.7 cal. ka BP). This report is intended to draw attention to these amazing landforms, which rival the huge potholes of Interstate State Park (Minnesota) along the St. Croix River for their ability to communicate to the public the massive size and power of ice age floods.