MIDDLE HOLOCENE MOBILIZATION OF EOLIAN SAND ON AN OUTWASH PLAIN IN WESTERN UPPER MICHIGAN

It is commonly known that sand dunes are ubiquitous features along Great Lakes coastlines in Michigan. These dune fields have long been a major source of interest and geomorphic inquiry. There are also a number of dune fields that lie deep (> 50 km) within the interior of the state, but, in contrast to coastal dunes, have been largely ignored and are thus poorly understood. Most of these dune fields mantle sandy late Pleistocene lake beds and outwash plains and are assumed to have formed in a poorly vegetated and windy deglacial environment. Recent research has focused on dating these dune fields in order to learn more about post-glacial paleoenvironmental change and landscape evolution in the core of the Great Lakes region.

One such dune field is the Baraga, which mantles ~ 20km² of the Baraga outwash plain in the western part of Michigan's upper peninsula. This dune field contains a variety of parabolic and sub-parabolic dunes, with axes generally oriented W/WNW (~ 280^o), that range from ~ 2 to 8.5 m in height. To determine the age of the dune field, eolian sands were collected from the upper and lower deposits of 5 dunes from across the field, and subsequently dated via optically stimulated luminescence (OSL) of quartz. Results indicate that dunes last stabilized sometime between ~ 8 and 6 ka. This interval correlates very well with reconstructed dune histories in northeastern Minnesota and on the topographically low Algonquin lake plain in eastern upper Michigan. Mobilization of dunes in Minnesota is clearly linked to the more arid Altithermal/Hypsithermal interval when vegetation shifted significantly from trees to grass. In contrast, this arid interval was relatively subtle in Michigan, making a definitive dune/climate link difficult to correlate. Dune formation on the Algonquin lake plain may be partly related to groundwater fluctuations associated with isostatic lake level adjustments, but this relationship probably does not apply to the topographically higher Baraga dune field. Instead, formation of the Baraga dune field may be related to the density of stabilizing tree cover on the outwash plain, which, according to regional pollen records, was generally less during the middle Holocene than it is today.