SURFICIAL GEOLOGIC MAPPING OF THE BEAVER CREEK QUADRANGLE IN SOUTHERN ILLINOIS: HIGHLIGHTING WELL-PRESERVED ILLINOIS EPISODE (MIS 6) ICE-CONTACT LANDFORMS AND DEPOSITS

Surficial geologic mapping in south-central Illinois entailed landform analysis, characterization of Quaternary deposits from multiple glacial episodes (interspersed with paleosols), and laboratory analyses. From a societal perspective, mapping in the Beaver Creek Quadrangle (1:24,000 scale) is providing an important geologic framework for land and ground­water use, sand and gravel resources, engineering concerns, and seismic hazards. From a Quaternary geologic perspective, this mapping highlights glacial landforms from the penultimate glaciation (Illinois Episode, MIS 6), when the Laurentide Ice Sheet reached southern Illinois in its southernmost extent of the Pleistocene. Because the study area is beyond the last glacial (Wisconsin Episode) limit, Illinois Episode glacial landforms and deposits are generally well preserved. Illinois Episode deposits contain Sangamon Geosol (interglacial) development in upper portions, separating them from last glacial loess. Pre-Illinois Episode glacial deposits are preserved in bedrock topographic lows and ancestral valleys. As much as 60 m of unconsolidated Quaternary sediments (mainly glacial) overlie Pennsylvanian sedimentary bedrock.

In the northwestern part of the quadrangle, a several km long esker or ice-walled channel (SW oriented), contains up to 35 m of gravelly sand and terminates in a partially eroded distributary system, leading into a proglacial outwash plain. Subsurface materials in the esker, draped by last glacial loess, are substantiated from stratigraphic test cores, water well boring logs, engineering boring logs, and two electrical resistivity transects. Regionally, this feature may occur in an interlobate area, where an eastern flank of the Springfield sublobe may have overridden the western side of the Kaskaskia Sublobe. Both sublobes are divisions within a proto-Lake Michigan Lobe (Illinois Episode). Two Illinois Episode glacial till units were delineated, the Vandalia Member (upper) and Smithboro Member (lower) of the Glasford Formation. The more stiff Vandalia till has a higher content of sand, dolomite, Mg, and illite, whereas the softer Smithboro till has more silt and less carbonate, having overridden proglacial fine-grained lacustrine sediment, the Yarmouth Geosol, and pre-Illinois Episode sediments.