AREA, MORPHOLOGY, GRAIN SIZE, AND PALEOWIND DIRECTIONS OF INLAND DUNE FIELDS ON THE ABANDONED BED OF GLACIAL LAKE CHICAGO IN SOUTHWESTERN LOWER MICHIGAN, U.S.A

Significant areas of Lower Michigan are covered by small, vegetated relict dunes, inland from Lake Michigan. A previous study shows that some of these dunes formed 13.3 ± 1.1 to 11.6 ± 0.9 ka, after Glacial Lake Chicago drained. We hypothesize that most if not all of the inland dunes on the abandoned surface of Glacial Lake Chicago formed during this interval. Here, we test this hypothesis by comparing inland dunes in six counties to see if they have similar form, grain size, and paleowind directions.

We mapped inland dune fields in six counties in southwestern Lower Michigan to determine area, morphology, grain size, and paleowind directions. We created a database using digital elevation models and ArcMap 10.4 to map dune areas, dune crests, and determine paleowind directions perpendicular to dune crests. Sediment samples from inland dunes were analyzed for grain size distribution by sieving, to see if spatial variations exist.

The total area of inland dune fields is at least ~570 km², or ~5.5 % of the six county area. Inland dunes are most common in Ottawa County. Dune morphology ranges from simple to compound parabolic, exhibiting hemicyclic, lobate, rake-like, and digitate patterns. Local relief of dunes ranges from ~3 m to ~13 m. We found that the median grain size is spatially consistent and ~230 ± 30 microns (fine to medium sand). Paleowind directions vary from ~285° in the north to ~306° in south.

Our mapping demonstrates that inland dune fields are common on the abandoned bed of Glacial Lake Chicago, and all have similar form, grain size, and paleowind directions. This suggests they all were active and stabilized during the same interval. Inland dune fields are most common in western Ottawa County, the only county with LiDAR coverage available. We believe LiDAR data would show a greater area of inland dunes in the other five counties. Grain size results show that inland dunes are primarily fine-medium sand, which appears to be finer compared to late Holocene coastal dunes. Future mapping will incorporate LiDAR data as it becomes available and will sample more dunes for grain size and optically stimulated luminescence analyses.