LUMINESCENCE AGES (OSL) OF MORPHOSEQUENCE AND INTERLOBATE ICE MARGINS IN NORTHWESTERN INDIANA AND SOUTHWESTERN MICHIGAN: WHAT DO THEY TELL US ABOUT THE SOURCES AND TIMING OF THE KANKAKEE TORRENT AND THE ERIE INTERSTADE?

The Kankakee Torrent (KT) was first noted in the 19^thcentury as a significant meltwater deluge in northeast Illinois, northwest Indiana, and southwest (SW) Michigan. The KT has long been linked to large collective discharges from interlobate margins of the Lake Michigan (LM), Saginaw, and Huron-Erie (HE) lobes in SW Michigan. KT flow has been traced from these and related ice margins in Illinois, through their outwash plains and the Kankakee valley, and into ephemeral lake basins (Wauponsee, and Watseka) in Illinois. Direct discharge from Lake Michigan has also been suggested. These lakes drained catastrophically into the Illinois River valley (IRV) through outlet channels (i.e., Marseilles, Oswego, Pontiac), forming terraces that provide stratigraphy and chronology to the KT.

The age of the KT, collectively given by ¹⁴C dates from flood deposits in the IRV and source lakes in Illinois, is ~19 ka. Dates are absent from upstream meltwater sources in Michigan and Indiana or from Lake Michigan. We hope to develop a chronology for major interlobate discharges from this area and determine how they fit into the KT and the dynamics of the Laurentide Ice Sheet (LIS). We will use optically stimulated luminescence (OSL) ages from proximal fans of morphosequences tied to moraines in Michigan and Indiana. The crosscutting, interlobate relationships of these morphosequences provide a relative chronology of discharges that, placed in Bayesian framework, can help tighten the inherently large errors of OSL.

To test our approach and the reliability of OSL ages of outwash, we sampled proximal fans near Niles, Michigan, and Leesburg, Indiana (LM and HE lobes, respectively). These fans are tied to moraines that have been proposed to mark the earliest LM and HE post-Erie Interstade ice margins. OSL ages indicate that LM lobe outwash was deposited 16.0±1.2 ka and HE lobe outwash was deposited 17.9±1.1 ka. Both ages are younger than the KT, but generally older than glacial lakes Chicago and Maumee. The ~2 ky difference in the OSL ages, however, is problematic and may relate to sediment heterogeneity, partial bleaching, or unknown variations in water content history. We will discuss sampling and interpretive problems related to OSL ages of outwash and the significance of our results for KT chronology and sources and LIS dynamics.