Edgar Monocline (New)

Part of the La Salle Anticlinorium

Location

From southwestern Vermilion through western Edgar and Clark to northwestern Crawford County (E-8 to G-8)

References

Clegg 1965b, Stevenson et al. 1981

Description

A large monocline defines the east margin of the [[La Salle Anticlinorium]] in east-central Illinois and separates the anticlinorium from the Marshall-Sidell Syncline on the east. The monocline is about 60 miles (95 km) in length and has vertical relief ranging from about 200 to more than 800 feet (60-240 m). The name Edgar Monocline, taken from Edgar County, is introduced for this flexure.

Detailed structure contour maps of the Pennsylvanian Colchester, Herrin, and Danville Coal Members show the Edgar Monocline in Edgar and Clark Counties (Clegg 1965b). As mapped on the coals, the monocline trends due north and faces east. It is sinuous along strike, and has several salients and reentrants. A series of domes or short ovate anticlines occurs along the upper limb of the flexure. These include, from north to south, the Hume Anticline and Brocton, Dudley, and Grandview Domes. The axes of these features are aligned at various angles to the trend of the Edgar Monocline; they are neither parallel nor en echelon.

As mapped on the Pennsylvanian coals by Clegg, the dipping flank of the monocline is 1.5 to 4 miles (2.5-6.5 km) wide and relief ranges from 225 to 450 feet (69-137 m). The amount of relief increases downward by 15% to 25% from the Danville to the Colchester Coal. At the base of the New Albany Group (Upper Devonian), the structural relief of the Edgar Monocline is 700 to 800 feet (210-240 m), which is twice the relief mapped on the coals (Stevenson et al. 1981). This pattern of relief increasing with depth is typical of the entire La Salle Anticlinorium and reflects the progressive growth of the structure during the late Mississippian and Pennsylvanian Periods.