Breccia and small-scale Lower Pennsylvanian overthrusting in southern Illinois

Two unusual exposures of Pennsylvanian sediments occur adjacent to a tunnel along the Illinois Central Railroad in northwestern Pope County, Illinois. At the north end of the tunnel is an 1,800-foot exposure of a breccia probably Caseyville in age. At the south end, are two small imbricate overthrusts of Caseyville age. Midway between the pene-contemporaneous breccia and overthrusts is the McCormick fault zone-anticlinal trend. Both breccia and imbricate overthrusts are believed to be free gravity slides related to Caseyville activity of the McCormick fault zone-anticlinal trend. These exposures show that slide structures of appreciable magnitude are not limited to argillaceous sandstone sequences of major geosynclines but can also occur locally in the relatively clean sandstone sequences of intracratonic basins.