OPTICAL DATING OF SAND DUNES ALONG THE ST. CROIX RIVER, NORTHWEST WISCONSIN

Optical (OSL) dating was used to determine when dunes on the eastern edge of the St. Croix River Valley near Grantsburg Wisconsin were active. The dune field covers ~ 650 sq km, and consists mainly of transverse dunes with relief between 2 and 20 m. The dunes were formed on a high terrace of the St. Croix River, described as the Airport Surface by Clayton (1984). The terrace was formed by the rapid incision of the modern St. Croix River, likely resulting from the rapid release of water from Glacial Lake Duluth. Twenty optical age estimates show the bulk of dune activity occurred between ~ 10,500 and 8,000 cal. yrs ago. Other authors have suggested that these dunes were the result of mid Holocene aridity. However, although the climate was relatively drier during this time, it is unlikely that dune activation was the result of aridity alone. Age estimates obtained from alluvium below the dunes suggest that the alluvium was deposited between ~ 12,900 and 11,000 yrs ago. We suggest that the dunes formed immediately following terrace formation, based on the close agreement between the deposition of the alluvium and the beginning of dune formation. Further dating of the Upper St. Croix River alluvium and Glacial Lake Duluth deposits will help to confirm this hypothesis.