GSA Connects 2021 in Portland, Oregon

Paper No. 81-9
Presentation Time: 10:05 AM


KERR, Phillip, Iowa Geological Survey, University of Iowa, 340 Trowbridge Hall, Iowa City, IA 52242; Department of Earth and Environmental Sciences, University of Iowa, 115 Trowbridge Hall, Iowa City, IA 52242, BLOCH, Daniel, Earth and Environmental Sciences, University of Iowa, 136 Trowbridge Hall, Iowa City, IA 52242 and KOHRT, Casey, Iowa Department of Natural Resources, 502 East 9th St, Des Moines, IA 50319

Late Pleistocene eolian sediments have been identified in the Midwestern United States for nearly a century. New technology and geospatial data have allowed for identification of features previously unrecognized. This study created a new inventory of sand dunes in a ~50,000 sq km area in Eastern Iowa, largely consisting of the Iowan Erosion Surface (IES). The inventory and topographic analysis were generated using geographical information software (GIS) utilizing a combination of NRSC SSURGO soil data, geologic maps, aerial photos, and 1 m LiDAR products (DEM, hillshade, and slope rasters). Over 2,000 individual linear dune features were mapped and were dominantly parabolic or linear in shape. Accuracy was field-checked in selected test areas. Other eolian features found in the GIS and field evaluations include sand sheets, ventifacts, sand ramps, and eolian sand-filled wedges.

The area evaluated, including the IES, was not covered by the Laurentide Ice Sheet during the Late Wisconsin Episode (Marine Isotope Stage 2 [MIS 2]) but were adjacent to the Des Moines Lobe. The IES also lacks loess deposits commonly found in areas around the Midwest not covered by Late Wisconsinan (MIS 2) ice. Outwash from the Des Moines Lobe provided the primary source for eolian sediment, as dunes are found extensively along rivers that carried glacial meltwater. Dunes and sand sheets are found predominantly on one side of source-valleys (generally the southeast) and shows that the Late Wisconsin (MIS 2) effective wind was from the northwest. The pre-Late Wisconsin [MIS 2] topography was formed from thick glacial deposits (>25 m) and dispersed, isolated zones of outcropping carbonate bedrock. The contrast in rheology between the till and carbonate caused significantly varied bluff/valley gradients across the landscape. River valleys with stretches of steep slopes, e.g. bedrock-lined valleys, acted as topographic barriers to sand transport. Broad, flat valleys with gentle side slopes, often formed in glacial till, allowed sand to be blown out and transported great distances. Generally, areas where sand could easily escape the source-river valley and saltate across the landscape have <0.5 m thick loess deposits downwind. Zones where sand transport is blocked upwind, especially in the source-river valley, can have loess deposits ranging from 3 to 15 m. This inverse relationship between dune/sand sheet location and loess deposits reinforces the interpretation that topography of the IES was, in part, formed by the transport surface model.