QUATERNARY GEOLOGIC MAPPING IN THE AMERICAN BOTTOMS, ILLINOIS, AND THE STRATIGRAPHIC IMPORTANCE OF RED CLAY BEDS

A complex sedimentary package was mapped on three quadrangles (Granite City, Monks Mound, and Wood River) in the American Bottoms on the east side of the Mississippi River, directly east of St. Louis, Missouri. Within this area, the Mississippi River floodplain ranges from eight to nine miles wide. Several abandoned meander channels of the Mississippi River are present in the floodplain. On the eastern edge of the mapping area, glaciated uplands are covered by up to 90 feet of loess. Red, silty clay beds occur only in easternmost areas within the American Bottoms. Typically, the beds are 0.3 to 2 feet in thickness and occur at 395 to 403 feet above sea level. The red clay layers occur at the base of postglacial silty clay (Cahokia Formation) and immediately above outwash sand (Henry Formation). Therefore, the red clay beds are used as marker beds for separating the Henry and Cahokia Formations. Radiocarbon ages indicate the red clay layers were deposited between approximately 9,900 to 9,500 C14 years ago (Hajic, 1993). The color of the beds is attributed to a glacial sediment source in the Lake Superior region. After deposition of the red clay layer, glacial influences in the Mississippi Rive basin declined and the river evolved from a braided to a meandering system. Over most of the American Bottoms, postglacial meandering of the Mississippi River has incised into and eroded the areas containing these red clay bed. Thus, their occurrence is restricted to a 0.5 to 3 mile wide geomorphic surface on the eastern side of the valley. Field and laboratory studies, soil survey maps, and previous mapping in the region provide a basis for understanding the local geology. Studies of core samples, well data, digital orthographic photos, and geophysical data has helped to reconstruct the surficial geology from the ground surface to bedrock. GIS methods were used to digitally create and compare data.