Evidence for an Early Saginaw Lobe Ice Retreat and Drainage Adjustments Across Southern Michigan, USA

The Saginaw lobe of the Laurentide Ice Sheet (LIS) flowed out of Saginaw Bay, across Lower Michigan, and into northern Indiana. The Saginaw lobe deposited the Sturgis moraine in extreme southwestern Lower Michigan as it retreated. Positioned to the west and east of the Saginaw lobe were the Lake Michigan and Huron-Erie lobes, respectively. All three of these lobes retreated across the region asynchronously, with the Saginaw lobe retreating first. The asynchronous behavior of the lobes allowed for the Lake Michigan and Huron-Erie lobes to overprint and alter landscapes once occupied by Saginaw lobe ice.Optically stimulated luminescence (OSL) dates from lacustrine sediments within the Sturgis moraine yielded ages of 27.9 1.7, 26.6 1.7, and 23.4 1.9 ka. An additional radiocarbon date (22.22 0.8 cal ka BP) was measured from bryophytes sampled near the base of an ice-walled lake plain atop the moraine. These dates indicate that deglaciation of the Saginaw lobe could have been underway in southwestern Michigan between 27 and 22 ka, while ice margins of the Huron-Erie and Lake Michigan lobes were further south in Indiana and Illinois. This contrasts with previous reconstructions of the LIS which show a continuous ice margin at 20.5 cal ka BP much farther south of the Sturgis moraine.The Kalamazoo moraine, an ice-marginal ridge north of the Kalamazoo River, is up-ice relative to the Sturgis moraine. To understand the distribution of glacial deposits in this area, surficial geology and bedrock topographic mapping were completed north of the Kalamazoo moraine. Longitudinal and terrace profiles were created along prevailing meltwater channels in the area north of the Kalamazoo moraine using LiDAR data. Three OSL dates from prevailing meltwater channels yielded ages of 13.5 1.4, 13.6 1.5 ka, and 17.0 1.4. Together, surficial sediments, meltwater profiles, and OSL data imply that meltwater originating from the SaginawHuron-Erie interlobate area drained westward across the Saginaw lobe terrain, after the Kalamazoo moraine had been deglaciated. This finding is consistent with results from previous investigations south of the Kalamazoo moraine. 2022, The Authors. All rights reserved.