RECENT CHANGES TO THE TIME-DISTANCE DIAGRAM OF THE LAKE MICHIGAN LOBE (MICHIGAN SUBEPISODE, WISCONSIN EPISODE)

Time-distance diagrams of continental ice lobes indicate the relative position of active ice margins over time. They are useful for indicating periods of interaction with adjacent lobes, documenting minimum and maximum ages that constrain the relative age of ice margin fluctuations, and indicating relative rates of ice advance. Over the past 8 years, the diagram has been modified as new discoveries have been made, primarily from fossils archived in deposits of ice-walled lake plains (IWLPs), slackwater lakes, and other environments. Notable ages and lessons learned include:

1) A date on a wood fragment in till of 25,740 ± 110 cal yr BP from Sheridan Park, South Milwaukee, Wisconsin, indicates that Lake Michigan lobe (LML) retreated to as far north as 43° N between the Marengo and Shelby Phases. It remains the only temporal evidence of retreat between these phases.

2) A pooled mean age of 24,190 ± 100 cal yr BP (4 samples) on samples of terrestrial bryophytes, caddisfly nests, needles, and other plant macros preserved in proglacial colluvium firmly establish the minimum advance age of the Peoria Sublobe (LML) for the Shelby Phase at Farm Creek, Illinois.

3) A pooled mean age of 20,530 ± 80 cal yr BP (3 samples) from tundra plant fossils in IWLP deposits on the Woodstock Moraine near Harvard, Illinois, show that deposits and ice margins associated with the Woodstock Phase are likely too old to have been associated with the 18,930 +/- 50 yr BP Kankakee Torrent (KT). Rather, one calibrated age of 18,490 +/- 110 cal yr BP from an IWLP on the Valparaiso Morainic System suggests that glacier activity during the Crown Point Phase is related to the KT.

4) The latter ages also provide the temporal framework for Glacial Lake Milwaukee (roughly between 20,500 and 18,500 cal yr BP).

5) Interpretation of IWLP ages are valid for interpreting the age of only underlying diamicton deposits; they may or may not have any bearing on the age of adjacent moraines.