The elevation and architecture of the peak Nipissing shoreline along southern Lake Michigan

Status Start Date End Date Locations
completed Mar 1, 2018 Jun 30, 2019 Lake
Director: Todd Thompson
Other Researchers: Erin Argyilan, Henry Loope John W. Johnston, Ken Lepper, Harry Jol, Ethan Theuerkauf
Funding: Indiana University - Indiana Geological & Water Survey
Issue: The Cook County Forest Preserve (Wentworth Woods) along Michigan City Road contains more than 25 beach ridges that record the elevation and age of ancestral Lake Michigan during the rise to and the fall from the peak Nipissing phase. This area is one of the last remaining undisturbed sites along the southern shore of Lake Michigan and is the closest site to the Sag channel of the Chicago outlet. We propose to construct a paleohydrograph from these relict shorelines that will yield the elevation of the lake when the shoreline formed and age of each of the shorelines. This site will establish the elevation of the peak Nipissing phase and rates of rise and fall to it at the Chicago outlet of ancestral Lake Michigan.
Objective: Construct a paleohydrograph for the area.
Approach: The proposed project follows the techniques of Thompson et al. (2011). An onshore-offshore-oriented transect will be established, and vibracores will be collected along the lakeward margin of each beach ridge along the transect. A vibracorer is a minimally intrusive method of collecting a relatively undisturbed sediment sample to a depth of ~15 ft. The vibracorer is portable, assembled at the site, and the main disturbance is foot traffic at each site and about 5 minutes of an engine noise. We commonly core before ground vegetation has emerged or after it has gone into senescence to keep disturbance to a minimum. Our vibracorer has collected ~1600 cores in private, county, state, and national parks and forest preserves of Indiana, Michigan, and Wisconsin, and was originally built for research in the Indiana Dunes National Lakeshore. A vibracorer built on our plans was used by the Illinois State Geological Survey at Illinois Beach State Park in the late 1980s. All vibracores will be returned to the laboratory at the Indiana Geological Survey at Indiana University to be split, described, sampled, photographed, and peeled. Grain size analyses will be conducted on the sediment samples and the peels will be used to enhance the visibility of sedimentary structures and preserve the core that will eventually dry and fall apart. Basal foreshore (swash zone) depth will be determined from the assembled data. The elevation of the basal foreshore is a close approximation of the elevation of the lake when the ridge formed. Pits (~1.5 m2) will be dug of every ~5th ridge, and sediment samples will be collected in small tubes. These samples will be used for an age determination using a technique called optically stimulated luminescence (OSL). OSL determines the last time that the grains in the sample were exposed to sunlight and provides an accurate age for the sampled ridge. Similar to vibracoring, we limit our disturbance of the site by removing leaf litter, limbs, and sod from the smallest hole needed, placing excavated sand and sod on tarps, and replacing all sand, sod and ground liter. A former park manager at the site in Wisconsin commented that had we not pointed out the sampled area, he would not have noticed it. Samples for OSL age-determination will be submitted to the Optical Dating and Dosimetry Lab at North Dakota State University. OSL age-determinations will be used to create and age model along the transect, permitting an age to be assigned to un-sampled ridges. Coring and OSL sampling sites will be temporarily staked and marked with flagging to preserve the location. All sites will be surveyed using a transit and global positioning system, and at the end of the surveying the stakes and flags will be removed. A maximum of two weeks will be needed to collect the cores and OSL samples in the field. Laboratory work on the cores will take ~ 4 months and OSL age-determinations up to 1 year. The age model and basal foreshore elevations will be combined to construct a paleohydrograph. This hydrograph will be added to other datasets from the upper Great Lakes to better understand the timing of outlet use and lake-level change and long-term patterns of glacial isostatic adjustments throughout the region. An open-file study will be created at the Indiana Geological Survey to provide permanent access to the data. The data will also be published in articles combined with other sites in the upper Great Lakes.
Products: All findings will be submitted to the Cook County Forest Preserve as a final report and as an IGS Report. We anticipate that the research will be shared at professional scientific research conferences in the form of posters and oral presentations. Results will also be included in peer-reviewed publications in scientific journals. The results from this site are relevant to interpreting lake-level history across the Upper Great Lakes.
Benefits: Understanding of mid to late Holocene lake level in the upper Great Lakes