THE MACKINAC LOBE OF THE LAURENTIDE ICE SHEET – A CONSPICUOUS PLAYER IN THE GLACIATION OF NORTHERN LOWER MICHIGAN

Most of Southern Michigan was glaciated by the Lake Michigan Lobe on its west, the Saginaw Lobe through its eastern-middle, and the Erie Lobe along its southeastern and southern margins. Recently, the concept of a fourth lobe – the Mackinac Lobe – was introduced for areas in northern Lower Michigan between the Saginaw and Lake Michigan Lobes. Much of north-central Lower Michigan owes it geomorphology to the Mackinac Lobe. As the Mackinac Lobe entered Michigan from the north, it climbed two precipitous bedrock ridges (cuestas) before reaching north-central Lower Michigan. At that point, it turned and flowed to the southwest, eventually abutting the Lake Michigan Lobe at the Cadillac Interlobate. In this talk, we re-introduce the Mackinac Lobe and provide the first ages for its advance into the region, focusing on the Houghton Lake Basin (HLB), home to the most impressive and clear-cut Mackinac Lobe deposits.

Flowlines of the Mackinac Lobe were first determined based on orientations of massive, ice-marginal ridges that cross large areas of the HLB. Cobble lithology data from across the region further delimited its flow direction and its regional extent. Mackinac Lobe sediment is lithologically distinct from that of the neighboring ice lobes, because it contained larger percentages of Precambrian Cobalt Group rocks such as Lorrain Quartzite, Gowganda Tillite and Jasper Conglomerate, and lower contents of carbonate rocks.

As the Mackinac Lobe advanced into the HLB, its margin was often subaqueously grounded in Glacial Lake Roscommon. At least five times, the ice margin stalled within the proglacial lake, subaqueously forming large ridges of sand and gravel, along with kame deltas. Each ridge was then overridden, as the ice advanced farther, downslope, into the lake/basin. OSL dates on both these types of deposits provide the first age control on glaciation for this region. Mackinac Lobe ice reached the HLB at ≈40 ka, and over the next 20 k yrs slowly filled the Basin, reaching the interlobate with the Lake Michigan Lobe at ≈19-18 ka. These data imply that, at the LGM, north-central Lower Michigan was ice-free, retaining open water in the form of Glacial Lake Roscommon. The Mackinac Lobe retreated from its interlobate position rapidly, with the ice front periodically pausing at the preexisting ridges that it had formerly overridden.