LATE-PLEISTOCENE FLUVIAL INCISION AND PERCHED-DUNE FORMATION ALONG THE LOWER AU SABLE RIVER IN NORTHEASTERN LOWER MICHIGAN, USA

We report on the age and evolution of five well-developed, inland parabolic dunes that are perched on high (~40 m) bluffs along the southern lip of the Au Sable River valley in the northeastern Lower Peninsula of Michigan. These dunes are located 20-25 km west from the modern Lake Huron shoreline. They formed on west-facing exposures and mantle sediments associated with the Jackpines delta, which developed when the glacial Au Sable River system deposited sands and gravels into glacial Lake Warren in the Lake Huron basin sometime between ~15.1 and 14.7 ka. Lake-level subsequently fell > 35 meters as the ice margin retreated northward and uncovered the Kirkfield outlet (Trent River valley, Ontario) during the Twocreekan interstadial. This regression resulted in abandonment of the Jackpines delta and incision of the associated surface by the ancestral Au Sable River. Eight samples were collected for OSL-age determination of the dunes and yielded ages that suggest dune formation occurred between ~15 and 13 ka. This result, coupled with the landscape position of the dunes (i.e., south side of river; west-facing bluffs) suggests that the dunes formed 1) when the river incised due to base level fall, or soon after fluvial incision ceased, 2) by west-northwesterly winds that transported eolian sands from exposed bluff faces to the adjacent valley edge, and 3) during or soon after ice advanced during the Greatlakean stadial, but probably before the Younger Dryas (cool) interval. This study contributes to the understanding of perched-dune formation in the Great Lakes region and further supports models indicating that westerly winds prevailed within 150 km of the ice margin during the late Pleistocene.