Late-Quaternary landscape evolution in southeastern Minnesota: Loess, eolian sand, and the periglacial environment

Linear sand features (up to 7 km long, 50 m wide, and 1 m high, and generally oriented NW to SE) are a common geomorphic feature in the periglacial region south of the Laurentide ice sheet in Iowa, Minnesota, and Illinois. I investigated these eolian sand stringers and their surroundings to determine landscape evolution on the Iowan Erosion Surface region. Based on these investigations, I suggest the following sequence of paleoclimatic and geomorphic events in the late Quaternary. (1){A0}Developing permafrost with water erosion changing to wind erosion before the Last Glacial Maximum (determinations of ventifact and coversand distribution and the absence of paleosols). (2){A0}Wind-erosion dominated extreme dry and cold conditions during the LGM (sand wedges found under the stringers and coversand deposits throughout the region). (3){A0}Numerous cryoturbation features (solifluction slumping, suggestions of patterned ground, involutions, injections, and diapirs) suggest warmer and wetter conditions after the LGM with silt inputs from an approaching glacier. (4){A0}Warming and drying with windier conditions with widespread sand transport between 15,000--11,000 yrs BP (based on the first thermoluminescence dates from Minnesota). Mean sand particle size is consistent throughout indicating it has a local source. Mean sediment particle size changes as fines increase/decrease influenced by the nearness of the Des Moines lobe. (5){A0}Climate amelioration; stringer sand grades into the thin loess blanket on the erosion surface. (6){A0}No evidence of reactivation of sand stringers during the mid-Holocene. Modeling of late Wisconsinan climatic conditions needed to produce sand activity in these landscapes suggests winds were above threshold more than 50% if the time, with precipitation 25% of present. Eolian sand and loess transport in southeastern Minnesota suggest open landscapes without spruce forest until the end of sand deposition. Bare surfaces with sparse tundra predominated. Consistent orientation of the deep Peoria Loess border and eolian sand indicates that they were transported and deposited under the same conditions, which persisted over this entire time period. Dominant winds were from the northwest throughout; there is no geomorphic evidence for easterly anticyclonic circulation along the edge of the ice sheet in this region.