EVIDENCE FOR LATE PLIESTOCENE PERIGLACIAL FEATURES IN THE MIDWEST UNITED STATES

Periglacial processes during the Late Pleistocene significantly impacted the modern landscape in the Midwest, especially in areas not covered by ice during the Late Wisconsin Episode. Recent mapping endeavors in eastern Iowa have discovered relict periglacial features on the Iowan Erosion Surface (IES). This landscape has a well-developed drainage network, unlike the recently glaciated region (formed by the Des Moines Lobe) to the west. Large areas of the IES are formed in thick (>100 m) clay-rich glacial diamicton while other regions have Paleozoic carbonate bedrock <2 m from the surface.

Features such as involutions, ice wedge casts, and sand wedges (up to 4 m deep by 3 m wide) provide evidence that multiple periglacial processes occurred on the IES. Several hillslope coring transects show active layer detachment, indicative of the development of thermokarst terrain. These profiles suggest that most of the weathering profile and associated geosol were removed from the upland, and that the adjacent side slopes have preserved plug-like flow layers derived from parts of that missing stratigraphy. Furthermore, regressive thaw slumps are common in areas of shallow bedrock and/or valleys that carried glacial meltwater. Some thermocirques are >1km² and have linked to form thermokarst badlands >25 km² in size. As a result, this thermokarst mass wasting caused a significant percentage of stream valleys on the IES to be underfit. Radiocarbon ages from plant material within these valley deposits suggest that periglacial erosion occurred 26-15 ka. Future efforts will seek to identify and describe additional periglacial features, improve the overall chronology using a combination of methods, and then integrate this data with a multi-proxy climate record. By doing so, this work on the IES may prove to be a useful analogue to modern permafrost thawing.