GEOLOGIC MAPS OF THE MIDDLE ILLINOIS RIVER VALLEY, ILLINOIS

Throughout the Pleistocene, the Ancient Mississippi River (AMR) drained much of the mid-continent of North America through the present-day Illinois River valley to the Gulf of Mexico. This ancient river valley experienced numerous episodes of erosion and deposition as it was reoccupied before and after several glacial advances and retreats during the last several hundred thousand years until being blocked by a glacier that diverted it to its present Mississippi River course 20,780+- 140 14C yr BP (24,520-25,020 cal yr BP). This complex history is reflected in four geological maps of the region – surficial geology, bedrock topography, drift thickness, and top elevation of the lowermost thick sand and gravel.

Mapping the glacial and post-glacial deposits of the middle Illinois River valley region was conducted by describing more than 250 outcrops and using records from over 700 water wells and engineering borings, as well as 49 new boreholes and augerholes, and several miles of seismic profiling. The surficial geology map shows the distribution of glacial, glaciofluvial, and modern fluvial deposits at land surface, deep dissection of the modern Illinois River into the adjacent uplands with buried diamictons and fluvial deposits exposed particularly on the eastern valley wall, and exposures of bedrock in the middle portion of the western valley wall. The bedrock topography map shows multiple subtle channels (including a bifurcated main channel) carved in the bedrock valley floor, the 15-mile width of the bedrock valley, and 3 prominent bedrock valleys tributary to the main channel. The drift thickness map portrays thick glacial and fluvial deposits within the bedrock valley. The thick glacial sediments above the eastern side of the bedrock in this region have been found to be the oldest sediments (fluvial quartz sand below Wisconsin and Illinois Episode diamictons at a depth of ~330 feet dated by optically stimulated luminescence at 185−190ka) preserved in the bedrock valley. Other parts of the bedrock surface appear to have been eroded more recently, explaining why deposits older than the Illinois Episode have not been found. Finally, the map of top elevation of the lowermost thick sand and gravel shows a fluvial system deeply buried by younger diamictons and fluvial deposits. An early Wisconsin Episode channel and Illinois Episode terraces are visible.