MAPPING OF QUATERNARY DEPOSITS AND BEDROCK TOPOGRAPHY IN THE KASKASKIA VALLEY REGION, SOUTHWEST ILLINOIS

Mapping of the surficial geology of the New Athens East Quadrangle (St. Clair County, Illinois) is being conducted as part of the STATEMAP program at a 1:24,000 scale. The region, about 30 miles southeast of St. Louis, Missouri, features till and outwash deposits resulting from glaciation during the pre-Illinois and Illinois episodes as well as loess and lacustrine deposits associated with Wisconsin Episode glaciation in the upper Midwest. Holocene alluvial deposits are found in the broad Kaskaskia River valley and its various tributaries. Exposures of bedrock have not been found and drift thickness ranges from about 6 to 140 feet in the quadrangle.

Data from nearly 700 engineering, coal, water well and stratigraphic test borings have helped to establish the subsurface stratigraphy of surficial deposits, as well as to create a bedrock topography map (from 350 of the borings). A buried bedrock valley within Pennsylvanian strata was found to generally follow the modern Kaskaskia River valley. Also, a distinct NW-SE trending bedrock ridge formed of sandstone (a preglacial cuesta) occurs in the middle of the quadrangle parallel to the edge of the Illinois Basin. Strip mining for coal occurred in the mid-20^th century along the trend and immediately southwest of the bedrock ridge.

Illinois Episode till, and in places pre-Illinois Episode deposits, occur below loess, lake sediments or outwash and are typically less than 50 feet thick. Illinois Episode outwash is commonly found filling the bedrock valley northeast of the bedrock ridge. A significant number of wells utilize groundwater in this outwash for water supply, whereas wells located in areas of thinner drift on or south of the bedrock ridge draw groundwater from Paleozoic sandstone or limestone. Wisconsin Episode slackwater lake deposits occur in terraces at elevations of 410 to 420 feet asl. Several outcrops studied along creek banks included lacustrine deposits ranging from 6 to 37 feet thick and with zones containing fossil gastropods (Fossaria spp. and Valvata tricaranata) common to shallow lakes. Wisconsin Episode loess varies in thickness from around 3 to 4 feet over last glacial lake terraces to around 6 to 10 feet on the older upland surfaces.