TIMING OF THE LAST GLACIAL MAXIMUM IN THE GREAT LAKES REGION OF THE LAURENTIDE ICE SHEET

Leverett and Taylor established the basic geomorphic relationships for the Great Lakes Region. However the chronology of the furthest extent of the ice sheet continues to improve. Initial efforts (1950’s-1970’s) at radiocarbon dating forced many revisions of the timing, and subsequent efforts (1980’s-2010’s) have identified more details. For example in the 1970’s it was taken that the maximum ice sheet extent occurred about ~18 ka. Given that chronology studies were conducted over several decades, that some areas have received more attention than others, and that there are a wide number of individual analysis for various stratigraphic sections, synthesis of the timing of the maximum has a challenge.

Here we take a limiting cumulative probability approach to estimate the age of the drift of the last Laurentide Ice Sheet. First, we inspect the literature to identify sections that that have multiple (~4 or more) age analysis. Then we compile, for these sites, all the radiocarbon analysis into a cumulative probability curve after calibration via intcal13. This approach allows us to choose a level of confidence for temporal assignment. Here we pick the 95% confidence level.. Finally, we recognize these ages provide a maximum limit for these events, not necessarily the age of the event. With this approach we can suggest that the age of the drift overlying the radiocarbon samples has a 95% confidence of being younger than the calculated age.

To illustrate this approach we identified 20 sites at or near the maximum extent. They may time transgressive age. The ice sheet reached its furthest extent in the Lake Michigan lobe after 24.0 cal ka. The central portion of the Lake Erie lobe reached its furthest extent at 23.1cal ka. Whereas the eastern portion, and perhaps some of the margin in central Indiana, of the Lake Erie lobe reached its maximum extent after 21.1 cal ka.