Abstract The Middle Ordovician High Bridge Group and Kentucky River fault system are exposed along the new part of U.S. 27 just north and south of the Kentucky River near the village of Camp Nelson, Kentucky, in the southwestern quarter of the southwestern section of the Little Hickman 7½-minute quadrangle (Fig. 1). There is ample room to safely park along roadcuts here, but caution is strongly urged because of the common occurrence of heavy traffic. Stop 4 is located on the east side of “Old U.S. 27” along an unnamed road just north of the river and below stop 1.

Abstract Drilling and geophysical data demonstrate that the Mississippi Valley graben, the Rough Creek graben, and the Rome trough are fault-bounded graben structures filled with as much as 27,000 feet of Cambrian sediments. Data including stratigraphic tops from 1,764 wells, 106 seismic profiles, aeromagnetic and gravity surveys, and mapped surface geology at a 1:24,000 scale have been used to study seven stratigraphic packages resolvable in both wells and seismic profiles across parts of Kentucky, Ohio, Indiana, Illinois, Missouri, and Tennessee. Detailed analyses of the thickness patterns of these stratigraphic packages have been used to interpret the locations and timing of movements along major faults systems in the study area. Active rifting of the Precambrian crystalline bedrock began by the Early Cambrian, and resulted in thick, sand-rich deposits of the Reelfoot Arkose in the Mississippi Valley graben and Rough Creek graben, and the Rome Formation in the Rome trough. Subsidence continued in these grabens during the Middle to Late Cambrian, leading to an alternating succession of shales and carbonates being deposited (Eau Claire Formation of the Illinois basin and Conasauga Group of the Appalachian basin) on top of the coarse clastic Reelfoot Arkose and Rome Formation. Although the tectonic extension that formed these features ended by the Late Cambrian, fault zone reactivation during the Taconic, Acadian, and Alleghenian Orogenies altered fault block orientations and produced areas of basin inversion, creating the possibility of numerous deep structural traps for hydrocarbons sourced by the Cambrian shales of the Eau Claire Formation and Conasauga Group.