SPATIAL VARIATION OF SEDIMENTARY PROPERTIES AND DEPOSITIONAL PROCESSES ALONG A RECESSIONAL MORAINE OF THE LAKE MICHIGAN LOBE

The edge of a retreating ice sheet is a dynamic place integrating many depositional processes, from ice melt out to lacustrine and subglacial fluvial processes. These processes can be recorded along the lengths of recessional moraines through sediment heterogeneity. By applying sedimentary analysis to multiple samples over a small stretch of a single recessional moraine, the spatial variability of Pleistocene ice sheet transport and depositional processes is investigated.

The Oak Creek Formation, was deposited by the retreating Lake Michigan Lobe of the Laurentide Ice Sheet approximately 14,500 yrs. B.P. During the deposition of the Oak Creek Formation, the Lake Michigan Lobe was likely warm and wet based resulting in numerous ice advances and retreats from out of the present day Lake Michigan Basin. The latest ice advances deposited multiple parallel moraines, including the Lake Border Morainic sequence in southeast Wisconsin. The innermost moraine of the Lake Border Morainic sequence is known informally as the Petrifying Springs Moraine, which is is topographically distinct and represents a distinct retreat and readvance of the Lake Michigan Lobe.

Focusing on the Petrifying Springs moraine in Kenosha County, Wisconsin, hand-sampled boreholes were taken over the modern crest of the moraine at less than 100 meters apart. Sediment variability of the Petrifying Springs Moraine is quantified by using Boyoucose hydrometer and dry sieving methods. The grain size data collected betters our understanding of the Lake Border Morainic sequence by allowing for the correlations to be made between sediment depositional processes and ice advances of the Lake Michigan Lobe. Overall, the Oak Creek Formation, which has been broadly classified and interpreted in bulk, has been found to be a fine-grained till composed mostly of lacustrine silts and clays, and initial measurements reveal widespread, calcareous silts and clays as expected. However, siliceous clays, well-sorted sand inclusions, and igneous cobbles are not uncommon. The differences in the material properties and grain sizes show spatial variations in depositional energy and highlight the variability of the processes operating toward the edge of a recessional ice sheet.