EVOLUTION OF COMPOUND COASTAL DUNE COMPLEXES ALONG THE SOUTHEASTERN SHORE OF LAKE MICHIGAN

The largest dune complexes along the southeastern shore of Lake Michigan contain up to 3 sub parallel, backdune ridges, followed westward by overlapping sets of parabolic dunes separated from the lake by a coastal dune ridge. We investigated the geomorphic history of complexes at Warren and Hoffmaster State Parks. Quartz sand collected from between 1.3 and 1.8 meters below the surface was analyzed by optical luminescence (single aliquot regeneration protocol) at the Sheffield Center for International Drylands Research. This gives the age of sand burial. For samples at the crest of dunes we assume this corresponds to the time of surface stabilization. Ages at 3 Hoffmaster (4800-3760, 4260-3300, 4190-3230 years 2σ) and 1 Warren site (4430-3270) indicate stabilization of the western backdune ridge during drop in lake levels from Nipissing II highs. OSL ages from eastern back dune ridges indicate older stabilization at Hoffmaster (8150-6190) and Warren (11930-9050). Relatively young ages (2750-1750, 3230-2430) from 2 backdune sites suggest local remobilization after dunefield stabilization. The eastern parabolic dunes are forested and stable. OSL ages from the crests at Hoffmaster (3340-2270, 3200-2500, 3160-2440, 3000-2320, 2520-1980, 2160-1440) and Warren (4570-3490, 2560-1940) show that most stabilization occurred in a 2,000 year period after the fall from Nipissing lake levels. Paleo-Inceptisols exposed within active western dunes indicate an earlier period of stability. An OSL age from below this paleosol at Hoffmaster suggests stabilization between 2230-1750 while radiocarbon dates from the paleosol indicate remobilization after 740 – 920 cal. YBP. This age is consistent with calibrated radiocarbon ages (788-955, 691-910) from similar paleosols at Warren. The limbs of currently active parabolic dunes merge with the coastal dune ridge suggesting that they began as blowouts. A calibrated age of 1880-1710 was obtained from a paleo-Inceptisol midway down the ridge. A series of higher paleo-Entisols suggest episodic growth after this time. Stabilization of back dune ridges occurred before and during the drop from Nipissing II lake levels with stabilization of the parabolic dunes in a 2000 year period after this and remobilization of the dunes nearest to the coast within the last 1000 years.