A NEW "CLASSIC" CORE FROM NEAR COLLINSVILLE, ILLINOIS, CONTAINING A FOUR-FOLD SUCCESSION OF GLACIAL DEPOSITS AND INTERGLACIAL SOILS

A recently acquired core from the Metro East St. Louis area contains an exceedingly important record of pre-Wisconsin deposits in this region, and will be a key reference site for stratigraphic and paleoenvironmental studies. The Robbins core, sampled to bedrock (174-foot depth), contains a succession of interglacial paleosols and fossiliferous silts preserved in a deep bedrock valley, tributary to the Mississippi Valley. The intertonguing of till units with lake deposits and paleosols, here near the southwestern terminus of Illinois and pre-Illinois episodes ice sheets (Labradoran Center), helped to preserve this impressive record of Quaternary climatic oscillations. The succession of deposits includes 40 feet of Wisconsin Episode loess (Peoria and Roxana Silts); 58 feet of Illinois Episode till, loess, and lacustrine silt (Glasford Formation and Petersburg Silt); 60 feet of late pre-Illinois episode till, loess, and lacustrine silt (Banner Formation and Harkness Silt); and 16 feet of early pre-Illinois episode alluvium and colluvium (nonglacial). Well-expressed modern, Sangamon, Yarmouth, and Bridgeton soils, respectively, are developed into and serve to separate the four main intervals of sediment deposition. Unweathered Glasford till, a gray, loam to silt loam diamicton, is sandier than unweathered Banner till, a gray, silty clay loam diamicton containing numerous local shale fragments. The Petersburg and Harkness Silts, comprised of loessal, organic, and lacustrine silt, immediately underlie the diamicton units and contain large fragments of conifer wood (probably spruce) and a rich assemblage of aquatic and terrestrial gastropods. Pomatiopsis, potentially useful for regional stratigraphic correlations, is a common genus in the Petersburg Silt and also in the early pre-Illinois alluvium. Amino acid ratios, determined on several genera, support a significantly older age for the pre-Illinois compared to Illinois episode deposits. Magnetic susceptibility (MS) values are high in the loessal silts (50 to 110 x 10^-5 SI) of Mississippi Valley source and low in diamicton and associated sorted sediments of Lake Michigan source. MS is also low in fine-grained alluvium, lake deposits, and poorly drained soil profiles, where anaerobic conditions have contributed to magnetite dissolution.