Paleoenvironmental and geomorphic significance of bluff-top dunes along the Au Sable River in Northeastern Lower Michigan, USA

 

Alan F. Arbogast, Michael D. Luehmann, G. William Monaghan, William Lovis and Hong Wang

 

          We report on the age and evolution of five well-developed parabolic dunes that are perched on high (~ 40 m) bluffs along the southern lip of the Au Sable River valley in northeastern Lower Michigan, U.S.A. These dunes are located on west/northwest-facing exposures and mantle the Jackpines Delta, which formed when the glacial Au Sable River system deposited sands and gravels into glacial Lake Warren in the Huron basin sometime between ~ 15.1 and 14.7 ka. Lake level subsequently fell > 35 m as the ice margin retreated northward 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. Lake level subsequently rose ~ 50 m during the Algonquin transgression, which likely promoted valley widening through channel migration. Eight samples were collected for OSL age determination of the dunes and yielded ages that suggest dune formation occurred mostly between ~ 13.5 ka and 13 ka. This result, coupled with the landscape position of the dunes (i.e., south side of river; west/northwest-facing bluffs) suggests that the dunes largely formed: 1) during the Algonquin transgression when bluffs were eroded by stream-channel migration, and 2) by west-northwesterly winds that transported eolian sands from exposed bluff faces to the adjacent valley edge. 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. It also demonstrates that interior dunes in this region formed in similar environmental settings to interior dunes in northern Europe and suggests that local factors, rather than specific climate intervals, should also be considered as they relate to dune evolution.
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