VARIATION AND PROVENANCE OF THE COARSE FRACTION WITHIN A LAKE BORDER MORAINE TILL OF THE LAKE MICHIGAN ICE LOBE

The material deposited at the edge of a retreating ice sheet records not only the processes active at the time of deposition but also links these processes to those of original erosion and sediment transport. Even across short distances over a single moraine of less than 100 m, deposited material can reflect very different original sources. The coarse sediment fraction of the deposited material provides accessible and easily identifiable samples to tie the source, its transport path, and the final sink together.

As it retreated from its Wisconsin glaciation maximum, the Lake Michigan lobe of the Laurentide Ice Sheet laid down multiple, parallel moraines, including the Lake Border Morainic sequence in southeastern Wisconsin and northeastern Illinois. In Wisconsin, the largest and one of the earliest moraines of this sequence is colloquially referred to as the Petrifying Springs Moraine. This moraine is primarily composed of the Oak Creek Formation. Like many of the Laurentide Ice Sheet tills, the sediment in the Oak Creek Formation reflects not only local bedrock sources but also those more distal.

Focusing on the Petrifying Springs moraine in Kenosha County, Wisconsin, initial analysis reveals sedimentologic variation over single boreholes and among boreholes less than 100 meters apart. Samples taken from hand-drilled boreholes less than 2 meters deep contain widespread, calcareous silts and clays, as expected from previous studies, along with less common and isolated sand inclusions and cobbles. That the pockets of sand and coarser material appear more localized than the more continuous clay likely reflects sediment deposition through ice melt out, as dropstones, or through subglacial fluvial processes. Focusing on the sand and coarser portions of these samples, we investigate their possible provenance and transport using their lithology and other sedimentologic properties. Initial results reveal cobbles of basalt, siltstone, and granite. By using existing work mapping the flowpaths of the Lake Michigan Lobe, these clasts can be tied to their potential erosional sources within the Canadian Shield.