PRELIMINARY INVESTIGATION INTO SPATIAL VARIATION OF SEDIMENTARY PROPERTIES AND DEPOSITIONAL PROCESSES FOR A RECESSIONAL MORAINE OF THE LAKE MICHIGAN LOBE

The edge of a retreating ice sheet is a dynamic place, due in great part to melt water availability, with material being transported and deposited through various means, from ice melt out to lacustrine deposition to subglacial fluvial processes. A recessional moraine should have significant heterogeneity along its length and over its thickness, recording not only ice sheet edge processes but also transport of material from its original source. By applying sedimentary analysis to multiple samples over a single moraine, the spatial variability of Pleistocene ice sheet transport and depositional processes is investigated.

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. The Petrifying Springs Moraine, primarily composed of the Oak Creek Formation, is one of the more topographically distinctive moraines of this sequence. Like many of the Laurentide Ice Sheet tills, the Oak Creek Formation has been broadly classified and interpreted in bulk; however, small-scale variations have not been as extensively studied.

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 4 meters deep contain widespread, calcareous silts and clays, as expected from previous studies, but siliceous clays, well-sorted sand inclusions, and igneous and metamorphic cobbles are not uncommon. The differences in the material properties and grain sizes not only show spatial variations in depositional energy but also link to the local dolomitic bedrock and distal sediment sources. Many of the clay- and silt-rich layers are spatially continuous, but pockets of sand and coarser material are more localized. Sub- or super glacial streams likely deposited the well-sorted sands, whereas localized coarse clasts would have been deposited by ice melt out or as dropstones. These preliminary findings highlight the variability of the processes operating toward the edge of a recessional ice sheet.