SEDIMENTOLOGY AND STRATIGRAPHY OF SLACKWATER LAKE DEPOSITS, LITTLE WABASH RIVER, ILLINOIS

The Little Wabash River is a 390 km long meandering river and a major tributary of the Wabash River in Illinois. During the Wisconsin Episode glaciation, a slackwater lake formed in its lower reaches when the mouth was dammed by outwash filling the Wabash River valley. The valley-fill sediments of the Little Wabash River show that the Wabash Valley has undergone a complex series of aggradational and degradational events. We studied a 28 meter long core extracted about 5 km upstream of the sediment dam. Observations included visual description, particle size analysis by laser diffraction methods, magnetic susceptibility, X-ray fluorescence (XRF) elemental analysis, X-ray diffraction (XRD) clay mineral analysis, and study of prefossil assemblages. The bottom 2m of the core is shale bedrock. Above this bedrock lies a 16m interval revealing several episodes of lacustrine sedimentation, which are characterized by laminated zones with massive interbeds of silt and fine sand, and zones of gastropod shells. The uppermost 12 m is a fining-upwards sequence of coarse sand to silty loam alluvial and possibly eolian sediment with weak paleosols separating aggradational intervals, with the modern soil developed at the top. The magnetic susceptibility is generally low throughout the core, except for a large peak at 9 m depth, which coincides with an XRF spectrum of relatively high Cr, Fe and Rb elemental contents. The coincidence of MS and elemental spikes is not yet understood. The XRD spectra correlate with the weathering profile. The upper 6 m is rich in expandable clay, but transitions to a uniform assemblage of illite, chlorite and kaolinite throughout the lower 22 m. These results are the basis for establishing a sedimentary history of Glacial Lake Carmi, including aggradational events and sediment provenance.