Rough Creek Graben
Location
See text below and figure 6
References
(selected) Soderberg and Keller 1981, Schwalb 1982, Nelson and Lumm 1987, Bertagne and Leising 1991, Kolata and Nelson 1991a, Nelson 1991
Description
The idea of a deep graben in western Kentucky was suggested by several researchers (Rudman et al. 1965, Avila 1971, Schwalb 1979) before Soderberg and Keller (1981) delineated and named the Rough Creek Graben. Unpublished seismic data constitute the main source of evidence for this buried structure, which is partially expressed by faulting at the surface.
The Rough Creek Graben is an eastward extension or branch of the Reelfoot Rift. Its north boundary is marked by the subsurface portion of the Rough Creek-Shawneetown Fault System, whereas the south border is along the parallel Pennyrile Fault System of southwestern Kentucky. The Reelfoot Rift and Rough Creek Graben intersect in southeastern Illinois. The junction of the northern borders probably underlies the sharp turn in the Shawneetown Fault Zone in southeastern Saline County.
Seismic reflection profiles from Bertagne and Leising (1991) and various unpublished sources indicate pre-Eau Claire Formation (Upper Cambrian) growth faulting along both the Rough Creek-Shawneetown and Pennyrile Fault Systems. Displacements reach 8,000 feet (2,500 m) on the former and are considerably smaller on the latter (Bertagne and Leising 1991). Thus, the graben is asymmetrical, with its floor tilted down to the north. Some seismic profiles show deep faulting within the graben as well. Downfaulted areas contain thick pre-Mt. Simon Sandstone layered rocks. As in the Reelfoot Rift, a few deep wells have penetrated dark shales and arkoses in the upper part of this sequence. The Exxon No. 1 Bell well in Webster County, Kentucky, encountered both felsic and basaltic igneous rock near total depth. As indicated by radiometric dating, the felsic rock is tentatively considered to be Permian and the basaltic rock to be early Cambrian (James A. Drahovzal, Kentucky Geological Survey, personal communication 1995).
The main period of graben faulting apparently ended by Late Cambrian, but the area continued to subside rapidly and contains overthickened Mt. Simon Sandstone through Knox Group sections. The western part of the Rough Creek Graben probably contains the thickest and most complete Paleozoic succession in the Illinois Basin (see Structural History).
Post-Pennsylvanian stresses reactivated faults in the Rough Creek Graben and created the surficial Rough Creek-Shawneetown, Pennyrile, and related fault systems. It is uncertain when this faulting died out. The area is seismically quiet today and no indication of Quaternary deformation has been reported (Nelson and Lumm 1987).
