EVIDENCE FOR DISTRIBUTION AND THICKNESS OF ATHENS SUB-EPISODE AND OLDER SEDIMENTS IN OTTAWA COUNTY, MICHIGAN

Previous researchers have mapped and provided multiple radiocarbon ages for buried organic matter below Michigan Sub-episode age glacial tills in Lower Michigan. A 43 meter long rotosonic core to bedrock was recovered in August 2012 by Western Michigan University’s Hydrogeology Field School at Hemlock Crossing County Park in Ottawa County, Michigan. The core contains organic wood fragments in sand lying below the Saugatuck/Ganges tills and lying above Glen Shores till, and older unnamed units. The isolated wood fragments yield an AMS radiocarbon age of 37,840 ± 400 C14 years BP (41,920 to 42,950 calendar years BP two sigma error, Beta-329000). This age is within the uncertainty of three finite ages previously obtained by researchers at the Glen Shores Section in Allegan County. This suggests an Athens Sub-episode age for the organic sand and a possible pre-Athens Sub-episode age for the Glenn Shores till and unnamed units.

Similar buried organic material in sand between till units are common in Ottawa County, occurring in two clusters as reported in water well records. More than 200 water-well-records report buried organic materials well below the land surface. Of these well records, 137 appear to correlate to the Athens Sub-episode age organics in the Hemlock Crossing County Park core. The largest cluster occurs over a broad area north of the Grand River, and a smaller cluster occurs south of Pigeon Creek. The average depth of the top of the organic layer is 28 ± 16 m (2 sigma) and at an average elevation of 167 ± 28 m (2 sigma) above mean sea level. This is about 10 meters below the mean lake level of Lake Michigan. The mean thickness of the organic sand is about 3 meters.

Continuing research examining the glacial till(s) of pre-Athens Sub-episode age in the Hemlock Crossing Core will attempt to correlate these units to known units. Other yet unnamed units may also be defined. These tills could be of any age and could correlate to MIS-4 (early Wisconsin Glaciation/Episode), MIS-6 (Illinois Glaciation/Episode), or older pre-Illinoian glaciations recorded in the marine oxygen isotope records from ocean sediments and ice cores.