DETRITAL ZIRCON GEOCHRONOLOGY OF ILLINOIS-EPISODE GLACIAL DEPOSITS: CONSTRAINING THE DYNAMICS OF THE PENULTIMATE LAURENTIDE GLACIATION IN THE GREAT LAKES REGION

Pleistocene continental ice sheet behavior can be, in part, determined by reconstructing ice flow trajectories. Ice flow trajectories historically have been interpreted from glacial erratic types, moraine patterns, and from the orientation of striations and streamlined landforms such as drumlins. While these approaches are useful in Wisconsin Episode (last glacial) sediments in the Great Lakes region of North America, recognizable landforms are sparse in some regions with the older Illinois Episode (penultimate) glacial deposits or are buried. During the deposition of the Illinois Episode Glasford Formation (till), glaciers covered more than 90% of Illinois. Till composition, moraine shapes, and glacial striations all indicate that the Illinois Episode ice sheet in Illinois advanced southward through the Lake Michigan Basin before eventually spreading radially across Illinois (an ancestral version of the Lake Michigan Lobe). Here we present zircon geochronology results for the Glasford Formation granitic cobbles in Ogle (z=37), Rock Island (z=25), and Fulton (z=42) counties, Illinois. To mitigate fertility bias, each cobble was crushed individually, and one zircon per cobble was analyzed at the University of Arizona Laserchron Center. Our group has previously obtained zircon age spectra for the Wisconsin Episode Tiskilwa Formation. We here compare the ice flow trajectories of the southern Laurentide Ice Sheet (more specifically, the Lake Michigan Lobe) during the last two major glaciations. Each of the three Illinois Episode samples have unimodal age peaks in granite cobble zircon ages of about 2700 Ma, which are statistically indistinguishable from those of the Tiskilwa Formation. The Hudson Bay Terrane in eastern Canada is the likely provenance of both the Illinois and Wisconsin Episode granitic cobbles. The similarity of zircon age peaks across the two data sets can be explained either by sediment recycling during the Wisconsin Episode, or that the Lake Michigan Lobe and southern Laurentide Ice Sheet flow path was similar during the two glaciations, crossing the same area of eastern Canada, which we prefer.