THE GLACIATION OF NORTHERN LOWER MICHIGAN, A DISTINCTLY PALIMPSEST LANDSCAPE

Parts of northern Lower Michigan were last glaciated by the recently identified Mackinac Lobe of the Laurentide Ice Sheet, which was not only “late-arriving” but was also advancing into a proglacial lake (Glacial Lake Roscommon) for much of its latter stages. The Mackinac Lobe was located between the Lake Michigan Lobe on its western flank and the Saginaw Lobe on its eastern flank. Its distinct and unique mix of cobble lithologies helped us to delineate the geographic limits of the Mackinac Lobe and infer its flow direction. Mackinac Lobe ice flowed south and uphill, onto the center of the northern Lower Peninsula, ultimately filling a preexisting basin (Houghton Lake Basin) that was inundated by Glacial Lake Roscommon at the time. The slowly advancing ice (2.7-6.8 km/kyr) paused at least eight times, forming a series of high, sandy, ice-marginal ridges. Many of these ridges formed kame deltas on their distal margins, within Glacial Lake Roscommon. After forming each ridge, the ice overtopped it and advanced further, modifying and even destroying parts of the ridge and its deltas. Nonetheless, most of the ridges remain behind as prominent palimpsest landforms. We report here, for the first time, on a large suite of OSL ages on these ridges and deltas, which constrain the activity of the Mackinac Lobe. The ice entered the Houghton Lake Basin at ca. 40 ka (MIS 3) and reached its maximum extent shortly after 18 ka. This chronology implies that the Mackinac Lobe continued to advance within the Houghton Lake Basin until well after the LGM. Retreat of the ice from the Basin was rapid, taking, at most, 1000 years.