Gravity measurements, extensive field mapping, and drill hole data have been used to determine the structural relations between the Cottage Grove and Shawneetown fault systems of southeastern Illinois.

The Cottage Grove structure is an east-west-trending, right-lateral wrench fault which extends across southern Illinois. The Shawneetown fault is the western extension of the Rough Creek fault system of Kentucky and consists of several high-angle reverse and normal faults. Both fault systems are part of Heyl's 38th Parallel lineament; however, the relation between them has been obscured by Pleistocene lacustrine and fluvial sedimentation.

Over 400 new gravity stations, with approximate 0.5 km (0.3 mi) spacing, were occupied in eastern Saline and western Gallatin Counties. The data were reduced to the Bouguer values, contoured, and selected profiles were two-dimensionally modeled. The new data, in conjunction with coal mine, drill hole and field mapping data, suggest an eastward extension and bifurcation of the Cottage Grove system. The northern segment continues eastward, dying out just west of the north-south Wabash Valley fault system. The southern segment trends southeastward, possibly merging with the Shawneetown fault system in a complexly faulted zone.

The use of gravity data in conjunction with other structural information may provide a useful tool for defining structure obscured by unconsolidated deposits in southern Illinois and other parts of the Illinois basin.

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