NEW CONSTRAINTS ON ICE RETREAT AT THE BOUNDARY BETWEEN THE SAGINAW AND HURON-ERIE LOBES OF THE LAURENTIDE ICE SHEET IN SOUTHERN MICHIGAN AND NORTHERN INDIANA, USA

The Saginaw and Huron-Erie Lobes of the Laurentide Ice Sheet occupied south-central Michigan and northeastern Indiana at the apex of the Wisconsin Episode (MIS 2). Recent geochronological studies have clarified our understanding of the timing of retreat for the Saginaw Lobe from its maximum extent and the nature of its boundary with the Huron-Erie Lobe. The position of the margin, however, is still displayed with large uncertainty in regional analyses of the ice sheet retreat chronology since the maximum extent. Here we present new cosmogenic 10Be depth profile ages and optically stimulated luminescence (OSL) dating of ice-marginal fan sediments from the Sturgis and Shipshewana glacial margins of the Saginaw Lobe, and of ice-marginal Huron-Erie Lobe fans, directly to the east. We directly dated the deposition and the stabilization of the proximal fan at the Sturgis moraine complex at ~22.9 ka, and the stabilization of a distal fan for the Huron-Erie Lobe, immediately east, at ~26.4 ka. These ages are consistent with OSL ages in the same sections of 24±2 ka and 26±2 ka respectively.

While consistent with previous studies indicating an early retreat (~23 ka) of the Saginaw Lobe north to Michigan from the maximum extent reached in northern Indiana, our data points to a complex retreat history. An ice-free zone centered around Elkhart County in Indiana was established along the boundary between the Saginaw and Huron-Erie lobes, as early as ~26 ka, followed by ice-free conditions by 22 ka near Sturgis, Michigan. This happened while other portions of the Huron-Erie and Michigan Lobes advanced farther west and south, respectively, in northwestern Indiana. We also dated distal sediments from ice-marginal fans from the Huron-Erie Lobe that were deposited south and east of the Elkhart-Sturgis region, infilling inter-fan areas recently vacated by ice, between 22 and 17 ka. We discuss our new data in the context of previous dating control for this region and focus on the implications for the deglaciation style of this sector of the Laurentide Ice Sheet.