DELINEATION OF KARST REGIONS IN DRIFT MANTLED ILLINOIS

Very little exposed karst (karst without sediment cover) occurs within the state, which is blanketed by Quaternary sediment (drift). Most of the karst consists of a combination of mantled (or covered) karst and fluviokarst. Karst features occur throughout the state, but most are clustered in five karst regions that comprise about 10% of the state by area. These regions are defined on the basis of occurrences of the three common karst features (caves, sinkholes, and springs) in areas where the bedrock is carbonate rock. Four of the regions occur within or encompass the four areas of the state where the bedrock is exposed or covered mainly by loess.

The delineation of the boundaries of the southernmost karst regions of the state (Shawnee Hills, Salem Plateau) is relatively straightforward. Both regions largely are demarcated by noncarbonated-dominated bedrock (mostly Pennsylvanian and some Cretaceous) or wide river valleys. In addition, the drift in these regions either is absent or consists of loess or loess over thin till.

Two karst regions, the Driftless and north-central (“sinkhole alley”) regions occur in the northern one-third of the state, where bedrock is carbonate-dominated, mostly consisting of dolomite. Except for the western boundary of the Driftless region, the boundaries of these karst regions are difficult to delineate. Karst features (sinkholes in particular) in these regions are less common and smaller than their southern counterparts, probably because of a combination of the dolomitic bedrock and the thicker drift. Springs are numerous but may be discharging from either bedrock or drift aquifers. These boundaries currently are indefinite and will migrate as more data are gathered from road cuts, drilling records, and geophysical studies.

The karst region in western Illinois is transitional between the southernmost and northernmost regions. The western and southern boundaries of the region are demarcated by the Mississippi River valley. The southern part is demarcated to the east by noncarbonate-dominated Pennsylvanian bedrock, and the drift within either is absent or consists of loess or loess over thin till. The northern part, like the regions to the north, cannot be demarcated by a distinct change in bedrock lithology and the drift within consists of loess over thin to thick till. Larger karst features occur along the Mississippi River valley.