ZIRCON GEOCHRONOLOGY FROM A LAKE MICHIGAN LOBE MORAINE: CONNECTING PROVENANCE, TRANSPORT, AND DEPOSITION

Using detrital zircon geochronology to investigate provenance and linkages between erosion and deposition of Laurentide Ice Sheet tills is a relatively uncommon but growing field. In this study, detrital zircon U-Th-Pb geochronology, processed and run by the Arizona Laserchron Center, was used to look at provenance and deposition of till samples from a recessional Lake Michigan lobe moraine in Kenosha County, WI.

Each of the samples (1-2 meter deep boreholes) was taken to be roughly along the crest of the moraine but at different elevations. While the boreholes might represent different stratigraphic levels, due to erosion and differential deposition, the stratigraphy cannot easily be correlated at this scale. Coarse clasts had previously been separated for lithologic analysis, and previous visual analysis of these large grains found around 10% of the coarse samples to be granitic clasts. Granitic bedrock, from the Canadian Shield and more recent plutons, is the likely origin of many detrital zircons for these samples.

From the sub-2 mm size fraction of the boreholes, around 300 zircons were analyzed in each. Compared to the few geochronological analyses of Laurentide tills, these samples show some unique features, particularly a distinctive and dominant Archean peak in the grain ages. This likely reflects a lack of mixing across the Lake Michigan lobe, as this material was eroded and transported along the western edge of the lobe. Since Michigan sandstones, both in the basin and the Upper Peninsula, generally contain a small fraction of Archean-age zircons, it is most likely that the zircons derived in this recessional moraine were eroded directly from Archean Canadian Shield rocks, though it is difficult to determine if the zircons reflect a single erosion and deposition cycle or might have been recycled from prior glacial deposits. This correlates with most models of Laurentide flowbands for the Lake Michigan lobe, where the lobe should erode and carry more Archean-age materials on the western side than on the eastern. However, due to transport processes, variance in zircon fertility, and recycling of material in prior glaciations and within sedimentary rocks, accurately calculating erosion rates and their spatial variation under the receding Lake Michigan Lobe is still a challenge.