ZIRCON GEOCHRONOLOGY OF INDIANA GLACIAL TILL AS AN INDICATOR OF PROVENANCE

Four Wisconsin-age glacial till samples in Indiana were analyzed using detrital zircon geochronology to determine whether each glacial lobe has a distinguishable U/Pb detrital zircon signature that may provide a reliable provenance indicator for Indiana glacial till. The samples include the Lake Michigan, Saginaw, and Erie lobe tills of the Laurentide ice sheet and one sample from a till of unknown origin. A total of 450 grains were analyzed via MC-ICPMS and ages of detrital zircons range from ~405-3110 Ma.

All 4 samples have broadly similar distributions with dominant Grenville-aged populations (1-1.3 Ga), and smaller Superior-aged peaks (2.55-2.8 Ga). The Saginaw till sample is unique with an L-shape distribution, showing a single dominant peak at 1.1-1.15 Ga and an even distribution from 1.15-1.6 Ga. The distribution of zircon ages from the Lake Michigan and unknown samples are the most similar, as they lack the large peaks at 1.1-1.15 Ga that are present in the Saginaw and Erie samples.

The most frequent ages of all samples fall between 1.0-1.55 Ga with varying distributions. Considering only these ages, the Michigan till sample has a relatively even distribution of grain ages (38% 1-1.2 Ga, 39% 1.2-1.4 Ga, 23% 1.4-1.55 Ga), whereas the Erie, Saginaw and unknown samples all contain more than 50% of ages from 1-1.2 Ga. The percentage of grains in the 1.2-1.4 Ga and 1.4-1.55 Ga age ranges vary between these three samples with the Saginaw containing the most ages in the 1.2-1.4 Ga range (33%) and the unknown sample containing the most ages in the 1.4-1.55 Ga age range (32%). The percent distribution within the Erie till for these two age ranges is relatively similar.

The K-S test indicates that the unknown, Erie, and Saginaw samples were derived from different populations, but that the unknown and Lake Michigan samples may have a similar source (95% confidence). Because there is a lack of a clear difference in the distribution of ages between the four samples and a simple explanation cannot be reached connecting the unknown sample to one of the known ice lobes, more work is needed to assess whether this methodology could be successfully used in the Midwest as an indicator of till provenance. Other factors such as eroded sedimentary rocks in the Michigan basin and reworking of older tills may need to be taken into consideration.