Revised time-distance diagram for the Lake Michigan Lobe, Michigan Subepisode, Wisconsin Episode, Illinois, USA

Based on the interpretation of 893 finite radiocarbon ages, we have revised the time-distance diagram for the Lake Michigan Lobe of the Laurentide ice sheet in Illinois. The data set contains 507 reliable ages determined using standard benzene synthesis–liquid scintillation, including “legacy” ages determined in the 1950s and 1960s at the inception of the radiometric radiocarbon dating method. In addition, the data set includes 278 radiocarbon ages determined by accelerator mass spectrometry. We analyzed the data set based on context, precision, and accuracy to vet minimum or maximum age estimates of diachronic phases.

The last glaciation in Illinois is marked by a local maximum margin in northeastern Illinois during the Marengo Phase (modal probability 28,000 cal [calibrated] yr B.P.), and subsequent glacial maximum culminating during the Shelby Phase (24,200 cal yr B.P.). From about that point, the Lake Michigan Lobe entered an overall retreat mode, with significant advances at ~22,200 and 21,100 cal yr B.P. (the Marseilles and Minooka Subphases of the Livingston Phase) and at 20,500 cal yr B.P. (Woodstock Phase). The latter age is also the conservative estimate of the onset of the lacustrine Milwaukee Phase, with referent deposits located as far north as Milwaukee, Wisconsin. This phase ended as the Lake Michigan Lobe made its final advance into Illinois during the Crown Point Phase (18,490 to ca. 16,500 cal yr B.P.), interfingering with the proglacial lacustrine Glenwood Phase deposits (16,900–15,000 cal yr B.P.).