GEOLOGIC MAPPING ALONG THE MIDDLE ILLINOIS RIVER VALLEY, ILLINOIS

Over the last 12 years, the Illinois State Geological Survey has mapped the Quaternary geology along the middle Illinois River valley from east of the big bend of the river to south of Chillicothe. Surface and subsurface mapping covers >500 mi², including 9, 1:24,000-scale USGS 7.5’ quadrangles and the southern half of 3 others. A set of 7 maps (including basic and derivative) have been developed – soil parent materials, surficial geology, bedrock topography, elevation of the basal sand and gravel, thickness of the basal sand and gravel, drift thickness, and aquifer sensitivity – based on 621 logs of water wells, engineering borings, and coal test borings, 21 continuously cored exploratory borings, several miles of seismic reflection profiles, and many field-described outcrops.

The Ancient Mississippi River (AMR) occupied the course of the now middle and lower Illinois River valleys until 24,770 ± 250 calendar years BP when a glacial advance blocked the river and diverted it to its present Mississippi River course. During the Illinois Episode and continuing throughout the late Wisconsin Episode, 5 separate glacial advances crossed the valley, and after each retreat, the river re-established its central Illinois course. Therefore, Quaternary deposits reflect both complex glacial erosional and depositional events coupled with glacial and interglacial fluvial events. To sort out complexities, 40 samples were obtained for age determination by optically stimulated luminescence of fluvial quartz sand. Dates reveal that the oldest sand and gravel was deposited interglacially during latest MIS 7 at ~193,000 years ago. Repeated incision and aggradation persisted through the Illinois Episode, which ended ~128,000 years ago. Definitive dates indicate interglacial fluvial sand deposition during the Sangamon Interglacial Episode when the AMR flowed unimpeded down the valley. During the final glacial retreat, thick cobble gravel and sand was deposited by large floods on terraces beginning ~19,000 years ago. The lowermost complex of valley-filling sand and gravel of various ages constitutes a major regional drinking water resource. This mapping provides insight into its thickness and significant lithologic and hydraulic variability.