Charleston Monocline (New)
Location
From southern Lawrence County (H-8) to southwestern Champaign County (F-7)
References
None
Description
The name Charleston Monocline is introduced here for the great monocline that marks the west edge of the southern part of the La Salle Anticlinorium. It is named after the city of Charleston, county seat of Coles County, which lies along the flexure. The Charleston Monocline terminates at the north end of the Tuscola Anticline in T18N, R7E, Champaign County. A northward extension, offset several miles to the west at this point, is the Osman Monocline.
The Charleston Monocline is slightly sinuous; it strikes N20°W and faces to the west-southwest. Several named domes and anticlines are aligned along the upper limb of the flexure. From south to north, these include the Lawrenceville Dome;the Bridgeport and Hardinville Anticlines; the Oblong, Martinsville, and Westfield Domes; and the Tuscola Anticline. The Charleston Monocline is flanked on the west by the Bogota-Rinard Syncline and other unnamed, shallow, south-plunging troughs. Maximum relief, measured on top of the Galena Group from the crest of domes along the monocline to the troughs of adjacent synclines, is as great as 2,500 feet (750 m) in several places (Bristol and Buschbach 1973). This is the greatest structural relief on any fold in the Illinois Basin.
The Charleston Monocline probably overlies a fault in Precambrian crystalline basement. Available evidence does not, however, establish the presence of faulting at Galena or higher levels. Delineation of possible faults is difficult because little drilling has taken place along the steep flank of the flexure. A seismic reflection profile (fig. 23) shows no detectable offset of reflectors down to the base of the Knox Group (Upper Cambrian). The quality of reflection is poor below this level. Faulting rather than folding in basement is assumed because the lack of metamorphism of Paleozoic rocks indicates that temperatures and pressures were insufficient to produce ductile deformation of crystalline Precambrian rocks.
