OSL AGES ON LOESS CONSTRAIN THE ADVANCE OF THE CHIPPEWA VALLEY LOBE IN WESTERN WISCONSIN, USA

The timing of the advance and recession of the Chippewa Valley Lobe in west-central Wisconsin is poorly constrained, mainly because of the lack of closely controlled radiocarbon dates. To that end, we present the first OSL ages on loess for west-central Wisconsin, which constrain the advance of the Laurentide Ice Sheet out of the Lake Superior basin and into this region during the last part of the Wisconsin glaciation. The Chippewa River flows south, perpendicular to the terminal moraine, and eventually becomes confluent with the Mississippi River. After the advancing glacier crossed the southern edge of the Lake Superior basin and reached a drainage divide in northwestern Wisconsin, meltwater flowed into the northern part of the Chippewa drainage basin, and continued to flow there until the ice margin retreated back, across the divide. Today, loess covers bedrock uplands that lie scattered on either side of the river, just beyond the terminal moraine. Spatial patterns of particle-size data on loess, from 125 upland sites throughout the Chippewa basin, clearly show that this loess was derived from the sandy valley trains of the Chippewa River and its tributaries - all of which drained the ice front. The loess exceeds 5 m in thickness at sites near the widest valley train areas, in areas only a few km beyond the moraine. Using deep cores recovered from five ridge-top sites, we dated 12 loess samples - solely from depths > 3 m - using MAR OSL methods. The oldest age for basal loess on bedrock was ca 24 ka, which constrains the timing of the advance of the southern margin of the Laurentide Ice Sheet out of the Superior basin, across the drainage divide, and into the drainage of the Chippewa River. The remaining OSL ages from the deep loess, taken only slightly higher in the stratigraphic column, range between 19.7 and 12.3 ka, suggesting that the Chippewa Valley was a loess source for several millennia, and that most of the loess was deposited during ice recession. OSL ages from loess within 3 to 3.5 m of the surface are abnormally young, presumably due to post-depositional mixing.