OSL CONSTRAINTS ON GLACIAL AND POST GLACIAL LANDFORMS OF TERRAIN PREVIOUSLY OCCUPIED BY THE SAGINAW LOBE IN NORTHERN INDIANA AND SOUTHERN MICHIGAN

The Saginaw Lobe (SL) is the least understood of the Great Lakes ice lobes. Any attempts to reconstruct isochrones for the SL rely heavily on only a few dated geomorphic events such as the Kankakee Torrent and a few moraines with minimum ages from deglacial lakes. Geomorphic and stratigraphic evidence consist of SL deposits overlain by Huron-Erie Lobe (HEL) and Lake Michigan Lobe (LML) deposits, and geomorphic evidence of early retreat including SL tunnel channels crosscut by HEL meltwater pathways and buried by HEL fans. Current data suggests either the SL was overtaken by, or retreated earlier than its neighboring lobes. A small number of published OSL ages, and even more unpublished, and seemingly anomalously old ages, suggest that the SL retreat could be 5000 years earlier than its neighboring lobes. To better quantify and characterize the retreat of the SL, small aliquot OSL (SA-OSL) was used to date a variety of landforms and the upper stratigraphy in the Elkhart 100 K quad and surrounding area. Results include: the Pigeon River Meltwater Channel (23.3 ± 5.8 ka), Brighton Fan (21.3 ± 2.9 ka), Sturgis Moraine fans (24.4 ± 2.2 ka, 18.9 ± 1.9 ka), Gilead Fan (26.5 ± 2.2 ka) Kalamazoo Moraine fans (25.6 ± 2.4 ka, 24.5 ± 2.9 ka) Dowagiac Tunnel Channel (30.9 ± 3.9 ka), Topeka Fan (in prep.) and Mongo Dunes (in prep.). These OSL ages suggest that a significant portion of northern Indiana and south central Michigan occupied by the SL was ice free as early as the Last Glacial Maximum (LGM). This ice free terrain would be nearly, or completely, surrounded by the LML and HEL as they extended into and met in central Indiana during the LGM. This “hole” would have experienced permafrost conditions with patches of stagnant ice—likely very inhospitable to plant life, which may account for the lack of older radiocarbon ages, and is illustrated by buried ice features including tunnel channel impressions crosscutting other depositional landforms.