LATE HOLOCENE DUNE DEVELOPMENT AND SHIFT IN PREVAILING WIND DIRECTION ALONG SOUTHERN LAKE MICHIGAN

The youngest episode of dune development along the southern coast of Lake Michigan began soon after the Nippissing II highstand in lake level (~4,500 years ago). In a sand belt of approximately 1 to 2 km wide, large parabolic dunes - lobate, hairpin, digitate, compound, and windrift – predominate in topography, while less common linear and blowout dunes also exist. The apices of the parabolic dunes point in an easterly direction (85-105⁰ azimuth) revealing prevailing westerly winds in their formation. The OSL ages of sand grains suggest that the parabolic dunes closest to the lake, as well as those furthest from the lake, began developing ~ 4,000 years ago and that some of them were stabilized ~3,500 years ago. However, the OSL ages of sand collected below the Holland Inceptisol suggest that the growth of some parabolic dunes continued under steady westerlies until ~1,200 years ago. Two buried entisols with OSL ages ~3,000 years ago suggest brief episodes of dune remobilization during the Algoma highstand in the lake level. Linear dunes, oriented E-W or WSW-ENE, are commonly closest to the lake and represent former arms of parabolic dunes that were modified by wave action and mass wasting processes. Linear dunes further inland represent windrift dunes - arms of breached parabolic dunes. The OSL ages of linear dunes coincide with the ages of parabolic dunes. The youngest dunes are blowouts oriented in a southeasterly direction (120-135⁰ azimuth) suggesting their formation by prevailing north-westerly winds. Blowouts occur closest to the lake, whether developed in linear dunes or in the left (northern) arms of parabolic dunes. In addition to the topographic discordance between parabolic and blowout dunes, there are several entisols that have a discordant relationship with the Holland Inceptisol, indicating the shift in the prevailing wind direction. Most of our OSL ages from blowouts cluster ~500-800 years ago, possibly suggesting changes in anticyclone or extratropical cyclone paths over the Great Lakes. The blowout development in the last 200 years is likely related to anthropogenic causes.