OPTICAL DATING OF THE DEMISE OF LAKE MINONG REVEALS AN EARLY HOLOCENE CONNECTION WITH LAKE CHIPPEWA ACROSS EASTERN UPPER MICHIGAN USA

Seventy-two optical (OSL) ages on quartz sand from eolian dunes broadly distributed across interior eastern Upper Michigan suggest that widespread dune building occurred between ~10.5 and 8.5 ka. Similar ages on sand from eight littoral lake terraces that underlie these dunes support the notion that the dunes were emplaced by deflation soon after levels of Glacial Lake Minong fell. Surprisingly, optical ages display similar patterns on both sides of the present Lake Superior-Lake Michigan watershed divide. Extensive field reconnaissance, ground penetrating radar (GPR) transects, NRCS soil survey maps, well logs, and a GIS-based reconstruction of rebound-adjusted paleotopography are combined to suggest a possible explanation. Adjacent to Nadoway Point, sand-dominated well logs hint that the original Nadoway Barrier consisted of a massive earthen dam of sand which, once overtopped, would have failed completely. We propose that, with the Nadoway Barrier intact, Lake Minong initially drained south to Lake Chippewa across the Niagara escarpment near Gulliver via a narrow arm occupying the present Tahquamenon and Manistique River Valleys and the drainage divide between them. Reflections of lacustrine sediments within this arm are interpreted from GPR profiles collected at Danaher and along highway M123 southwest of Upper Tahquamenon Falls. OSL ages suggest that the fall of Lake Minong occurred in two steps, the first between 10.5 and 9.9 ka and the second near 9.0 ka. These events are temporally correlated with peaks of meltwater discharge to the Superior Basin from the wasting Laurentide Ice Sheet inferred from varve sequences in cores from Lake Superior. We propose that the earlier event led to a westward shift in Lake Minong's outlet via the breaching of a debris dam northeast of Manistique to bedrock. The later event near 9.0 ka caused the breaching of the Nadoway Barrier, the final destruction of Lake Minong, and a shift of the southerly outlet of the Superior Basin northeastward to its modern position at Sault Sainte Marie. Active dune building on the newly exposed bed of Lake Minong apparently entrained loess that was subsequently deposited in fine-textured soil caps on uplands leeward of the former lake.