STRATIGRAPHY AND LITHOLOGY OF TILLS IN THE HEMLOCK CROSSING CORE, OTTAWA COUNTY, MICHIGAN: INSIGHTS ON TILL PROVENANCE AND LOBE HISTORY

We analyzed a ~39 m sediment core from Hemlock Crossing Park in order to define and differentiate diamictons in the subsurface of Ottawa County, Michigan. Previous work defines three tills in southwestern Michigan: the Saugatuck, Ganges, and Glenn Shores tills, based on stratigraphy, matrix texture, and kaolinite/illite ratios. In our study, matrix texture was determined by hydrometer and sieving; X-ray diffraction was used to estimate kaolinite/illite ratios for clay sized fraction. A single sample of the Coldwater Shale from the bottom of the core was analyzed for kaolinite/illite ratio.

Four diamicton units are present in the Hemlock Crossing core overlying Coldwater Shale. A ~5 m thick organic-bearing unit lies between the upper diamicton and the lower three diamictons and three radiocarbon analyses yield an age range of 41,920 to 45,340 cal. yr BP (Middle Wisconsin). The upper diamicton is variable with matrix textures of silty clay, clay loam, or silty clay loam. The middle diamicton is a silty clay loam or loam. The lowest two diamictons have sandy clay loam, sandy loam or loam textures. Kaolinite/illite ratios of the upper and middle diamictons range from 0.79 to 1.12, and the lower two diamictons range from 1.13 to 1.33. Our sample and other Coldwater Shale samples in the Lake Michigan Basin have a mean kaolinite/illite ratio of 0.71 ± 0.13.

We interpret all four diamictons as basal tills based on massiveness, density, and clast forms. The upper two tills are finer than the lowest two, and the lowest three tills contain more gravel than the upper till. Kaolinite/illite ratios have a bimodal distribution, with the upper and middle tills having a mean of 0.94 ± 0.11, and the lowest two tills have a mean of 1.17 ± 0.07. When compared to the local Coldwater Shale all four tills have higher mean kaolinite/illite ratios. These lithologic differences suggest that the upper two tills incorporated more Coldwater Shale and glaciolacustrine sediments and have a Lake Michigan provenance. The sandier texture and higher kaolinite/illite ratios in the lowest two tills suggest that they have a different provenance, perhaps as a westward or southwestward flowing Saginaw Lobe during a pre-late Wisconsin Glaciation.