MIDDLE TO LATE QUATERNARY BASE LEVEL FLUCTUATIONS IN THE KASKASKIA LOWLAND, SOUTHWESTERN ILLINOIS

The Kaskaskia River Basin, in southwestern Illinois, has a rich history of glacial-interglacial sedimentary environments during the middle to late Quaternary. Waterlain deposits in the Kaskaskia Lowland (< 450 ft. elevation asl) consist of up to 140 ft of sand and fine gravel outwash, fine-grained lacustrine sediments, and fine sand to clayey alluvium. Inundation of the lowland with fluvial, glaciofluvial and lacustrine deposits has been an interplay between rising base levels during glaciations, in response to Mississippi River aggradation, and increased sediment inputs from the middle and upper Kaskaskia River system. Sedimentation was most pronounced during peak glaciations of the pre-Illinois, Illinois, and Wisconsin Episodes of the past 500 ka. Conversely, the lowland was partially excavated by fluvial erosion during interglacials of the Yarmouth Episode, Sangamon Episode, and Holocene Epoch. Thus, the record contains a succession of cut-and-fill sequences with overall fluvial-lacustrine aggradation during glacial and partial incision, with fine-grained fluvial deposition, during interglacial times. The river likely converted from braided to meandering during glacial-interglacial transitions.

During the last glaciation, large volumes of meltwater and sediment aggraded the mid-Mississippi River valley system ~ 80 feet above its current level. Consequently, a large slackwater lake (glacial Lake Kaskaskia) formed in the Kaskaskia Lowland, resulting today in terraces at about 415 to 425 feet asl. Due to the exceedingly low gradient of the Kaskaskia Valley (~0.9 ft/mile), a highly dendritic slackwater lake once extended over 70 mi (110 km) in length. Radiocarbon ages indicate a maximum extent from 25 to 15 cal. ka BP. Some proglacial fluvial sediment may have been provided to the upper basin between 25 and 22 cal. ka BP, by melting glaciers in east-central Illinois. Based on core and outcrop studies, earlier forms of glacial Lake Kaskaskia also existed during the Illinois and pre-Illinois episodes. In the lower Kaskaskia Basin, as much as 100 feet of Illinois Episode and 55 feet of pre-Illinois Episode slackwater lake deposits are preserved below till deposits. Fossil mollusks substantiate shallow lacustrine environments with a tendency to shoaling and inundation with loess as glacial ice approached.