GEOMORPHIC, SEDIMENTOLOGICAL AND MINERALOGICAL SIGNATURES OF EARLY HOLOCENE OUTBURSTS OF GLACIAL LAKE AGASSIZ IN EASTERN UPPER MICHIGAN

Glacial Lake Agassiz covered ~350,000 square kilometers of the isostatically depressed north central portion of North America during terminal retreat of the Laurentide Ice Sheet. Retreating ice eventually uncovered outlets that permitted huge amounts of fresh water to drain from Agassiz along several widely divergent paths to the Arctic Ocean, North Atlantic Ocean and the Gulf of Mexico at different points in time. These discharge events must have left significant geomorphic records along all drainage paths. However, no geomorphic signature has yet been linked to the history of Agassiz's eastern outlet to the Saint Lawrence Seaway (Nipigon Phase). We used ground penetrating radar (GPR) and optical (OSL) dating of quartz sand in dunes to test the hypothesis that the Lake Michigan-Lake Superior watershed divide just west of McMillan MI bears traces of large floods that stemmed from outbursts of Lake Agassiz into the Superior Basin. We established a single GPR transect, four kilometers long and perpendicular to the trend of the broad valley that surrounds McMillan. We obtained optical ages from three samples of quartz sand from dunes adjacent to the GPR transect. GPR reveals a gently south-sloping fan at ~15 m depth along the northern edge of the valley. The fan is buried by four apparent aggradational surfaces that span variable portions of the valley and also slope gently to the south. These inclined strata are capped by a horizontal set of reflections that display a braided pattern. The disappearance of radar reflections of the basal units along the southern third of the GPR transect is likely a result of silt deposition. Since dunes sampled for OSL cap the stratigraphic sequence, their optical ages (tightly clustered around ~8,500 cal BP) provide a minimum date for landscape stabilization after a through-flowing flood event. These results support the hypothesis that water from the Superior Basin (Lake Minong) coursed south and west across the present site of McMillan into the Michigan Basin (Lake Chippewa) during an outburst of Lake Agassiz prior to ~8,500 cal BP. The three buried aggradational surfaces revealed in GPR may represent evidence of earlier discharge events along the same path.