ENVIRONMENT AND PROVENANCE OF SLACKWATER LAKE SEDIMENTS IN TERRACES PROXIMAL TO THE MISSISSIPPI RIVER VALLEY NEAR ST. LOUIS

As a response to glacial aggradation of the Mississippi River, tributary valleys in the St. Louis region have been cyclically inundated by slackwater lakes, resulting in stratified silty clay to fine sand deposits. Such lake deposits of the last glaciation (known as Equality Fm.) are up to 100 feet thick; are reddish-brown, tan or gray; are fossiliferous; and occur in terraces (at about 470 to 485 feet asl) near the mouth of the tributary valleys. Radiocarbon ages on gastropods and plant macrofossils indicate deposition between about 45 ka and 16 ka. Sedimentological and paleontological evidence suggests a dominant lacustrine environment with intervening periods of alluvial or littoral deposition. Downcore mineralogical and elemental zonations in one representative core (MNK-3 from near Caseyville, IL) are interpreted to reflect changing proportions of sediment derived from three compositionally distinct glacial source regions: (1) the Superior Lobe (abundant feldspar, kaolinite, Ti, Cr, Cu, Fe, Ni, Ga; high magnetic susceptibility; red color), (2) Des Moines Lobe (abundant Se, Mo, expandable clay minerals; tan to gray color), and (3) Lake Michigan/Green Bay Lobes (abundant dolomite, illite, C, MgO, CaO; tan to gray color). These statistically distinct glacial source regions (based on over 33 till sample analyses) supplied much sediment to the St. Louis region via the Mississippi, Missouri, and/or Illinois (Ancestral Mississippi) Rivers. A mathematical mixing model combined with principal component analyses can be used to estimate hypothetical contributions of these source areas. The data obtained can help to reconstruct glacial history, sediment transport, and fluvial history of the upper Mississippi River watershed during the mid to late Wisconsin Episode.