ACTIVITY OF THE LAURENTIDE ICE SHEET EVIDENT IN SEDIMENT ARCHIVES OF THE LOWER WABASH VALLEY

The Wabash Valley was an important pathway for drainage from the Lake Michigan, Saginaw, and Huron-Erie lobes of the Laurentide ice sheet throughout the Quaternary. In the lower Wabash Valley, proglacial terraced deposits and surfaces in tributaries and in the trunk valley are archives of a complicated glacial history. Where preserved, Illinois Episode strata are recognized by Sangamon Geosol development. From overlying glacifluvial and glacilacustrine deposits up to 20 m thick, we have accumulated a growing collection of 30-40 ka ages based on (calibrated) radiocarbon and OSL dating. These deposits may reflect discharge from a previously unrecognized early Wisconsin Episode ice advance. Differentiation of provenance of the strata by geochemical and mineralogical analyses is so far elusive, however. Preliminary results suggest affinities with Lake Michigan lobe sediment, although that lobe has not yet been shown active in the watershed during that period, but neither are the data distinctive of Huron-Erie or Saginaw Lobe sources. Also curious are fossil assemblages in slackwater lacustrine deposits that are consistent with cool but not cold climates. Ages from the uppermost sequence range from 29-22 ka. They demonstrate that the valley was largely filled with sediment by the last glacial maximum. Incised terraces have younger ages; deglacial meltwater events were largely erosional. We have yet to find depositional evidence of flooding from Glacial Lake Maumee outbursts, although floodwater elevations are suspected to have been high enough to flood into the filled tributary valleys.