LUMINESCENCE AGES ON A KAME DELTA CONSTRAIN THE GLACIAL RETREAT FROM CENTRAL LOWER MICHIGAN

In association with a freshman Honors Seminar at Michigan State University, we studied two small kame deltas in north-central Lower Michigan. Only recently identified, the deltas provide key evidence for a previously unknown proglacial lake (Glacial Lake Roscommon) in this broad, sandy, interlobate upland. Our goal was to document these deltas and characterize their geomorphology. Both deltas are composed of well-sorted sands with little gravel. Samples taken across the deltas, from depths of up to 1.5 m, show little variation in texture, aside from a general fining toward their outer margins. Gullies on the outer margins of both deltas probably postdate the formation of the deltas proper, formed instead by runoff during a permafrost period that followed lake drawdown. Because both deltas are tied to ice-contact ridges that mark the former position of the retreating ice margin within the lake, luminescence ages we obtained for one of the deltas constrain the timing of ice retreat in this portion of Michigan, for which no information currently exists. Six OSL ages were obtained on the deltaic sands, recovered from 2 m deep soils pits on one of the deltas. These dates help to constrain the retreat of the subaqueously grounded ice margin through this region, which is thought to have been glaciated later (27 ka) than other parts of Michigan or the upper midwest.