AGE, ORIGIN, AND PROVENANCE OF WISCONSIN LOESS IN NORTHEASTERN NEBRASKA AND ADJACENT IOWA

We present new data on the age and provenance of Peoria Loess and Gilman Canyon/Pisgah Fm. loess in northeastern Nebraska and adjacent Iowa. New OSL ages bracket accumulation of most of the Peoria Loess between about 25 ka and 16 ka and accumulation of loess of the underlying Gilman Canyon/Pisgah Fm. between about 60 and 26 ka. Using the sequence of OSL ages and geochemical distinctions between loess sourced from the Missouri Valley and that originating from nonglacigenic sources west of the Missouri Valley, we are able to document the history of competing loess sources in the region. The Missouri Valley source dominated the loess sedimentary system of northeastern Nebraska and areas east of the valley between 25 and 21 ka. After that time, until the end of significant loess accumulation around 16ka, loess blown from nonglacigenic western sources dominated the loess sedimentary system. These new data document the timing of dramatic shifts in last glacial dust-producing geomorphic systems of the North American midcontinent.