Athens Subepisode (Wisconsin Episode) non-glacial and older glacial sediments in the subsurface of southwestern Michigan, USA

Abstract We describe the first complete sediment core to bedrock in southwestern Michigan of a radiocarbon defined sequence of Michigan Subepisode (Marine Oxygen Isotope Stage [MIS] 2) glacial sediments, Athens Subepisode (MIS 3) non-glacial sediments, and two older tills using sedimentological, lithological, and radiocarbon analyses. Organics from Athens Subepisode lacustrine and palustrine sediments yield radiocarbon ages of 41,920–42,950 and 43,630–45,340, and > 43,500 14 C yr BP. We propose the name Port Sheldon Formation for these organic-bearing sediments. We interpret the underlying diamictons as two basal tills separated by glaciolacustrine fines. The youngest till (Hemlock Crossing till) lying below the Port Sheldon Formation is a dark gray, gravel-poor clay loam to loam with a mean kaolinite–illite ratio of 0.98 ± 0.04. The oldest till (Glenn Shores till) is a dark grayish brown, gravel-rich, clay loam to sandy loam with mean kaolinite–illite ratio of 1.22 ± 0.08. About 130 water-well records demonstrate that organic sediments and underlying diamictons are common in the subsurface of Ottawa County. These tills are likely Illinois Episode (MIS 6) or older, but an Ontario Subepisode (MIS 4) age cannot be ruled out. Deep bedrock basins in Lower Michigan provide an untapped archive of pre-Michigan Subepisode history.