OSL Ages on Glaciofluvial Sediment in Northern Lower Michigan Constrain Expansion of the Laurentide Ice Sheet
Randall J. Schaetzl and Steven L. Forman
We report new ages on glaciofluvial (outwash) sediment from a large upland in northern Lower Michigan—the Grayling Fingers. The Fingers are cored with N150 m of outwash, which is often overlain by the (informal) Blue Lake till of marine isotope stage (MIS) 2. They are part of an even larger, interlobate upland comprised of sandy drift, known locally as the High Plains. The ages, determined using optically stimulated luminescence (OSL) methods, indicate that subaerial deposition of this outwash occurred between 25.7 and 29.0 ka, probably associated with a stable MIS 2 ice margin, with mean ages of ca. 27 ka. These dates establish a maximum-limiting age of ca. 27 ka for the MIS 2 (late Wisconsin) advance into central northern Lower Michigan. We suggest that widespread ice sheet stabilization at the margins of the northern Lower Peninsula, during this advance and later during its episodic retreat, partly explains the thick assemblages of coarse-textured drift there. Our work also supports the general assumption of a highly lobate ice margin during the MIS 2 advance in the Great Lakes region, with the Fingers, an interlobate
upland, remaining ice-free until ca. 27 ka.