Centralia Fault Zone
Location
Along the west edge of Marion and Jefferson Counties (H, I-5)
References
Bell 1927, Brownfield 1954, Bristol and Buschbach 1973, Keys and Nelson 1980, Nelson 1981
Description
The Centralia Fault Zone is known from exposures in underground coal mines (now abandoned), well records, and seismic profiles. The zone consists of high-angle normal faults that strike north to south and follow the east-dipping flank of the Du Quoin Monocline. The largest fault was traced through mines for 10 miles (16 km) northward from the Marion-Jefferson county line. It is downthrown to the west and has a maximum displacement of 160 to 200 feet (51-61 m). Smaller faults strike parallel to the main one. They are downthrown to the east and west, and produce horsts and grabens. Maximum width of the fault zone, as indicated from mining records, is about 1,250 feet (380 m; Brownfield 1954).
Borehole data indicate that the Centralia Fault Zone continues southward along the west edge of Jefferson County. Keys and Nelson (1980) mapped a fault displacing the Shoal Creek Limestone (upper Pennsylvanian) as much as 200 feet ( 60 m) down to the west in T3S, RlE. Recent remapping, using additional well data, indicates that the displacement is only 50 to 100 feet (15-30 m) here. Several well logs show missing sections or strata, indicative of normal faulting.
The Centralia Fault Zone is directly in line with the Dowell Fault Zone of southeastern Perry County. These fault zones are alike in structural style and relation to the Du Quoin Monocline. In all probability, they are part of a continuous fracture zone along the limb of the monocline.
Two east-west, seismic reflection profiles that cross the Centralia Fault Zone in Jefferson County indicate a normal fault that has downthrow to the west. Dip of the fault plane was calculated to be 70° to 75° and displacement is 100 to 150 feet (30-45 m). The fault displaces all reflectors down to the St. Peter Sandstone (Ordovician), the deepest coherent reflector on these profiles.
The fact that the Centralia Fault Zone follows the Du Quoin Monocline, but has an opposite direction of throw, led Brownfield (1954) to propose two periods of deformation here. He theorized that the monocline developed during late Mississippian through Pennsylvanian time. Uplift on the west, succeeded by post-Pennsylvanian uplift on the east, produced the faulting.
Brownfield's hypothesis of two episodes of deformation is plausible. Folding of the Du Quoin Monocline was accomplished mainly during the late Mississippian to middle Pennsylvanian Periods, as shown by thinning of strata of these ages on the upthrown limb of the monocline. The Du Quoin, similar to monoclines in the Rocky Mountain region, probably was produced by compression and overlies a reverse fault in basement. The Centralia Fault Zone is younger than the main stage of folding (faults displace upper Pennsylvanian rocks). I infer that post-Pennsylvanian extension induced normal movement on the fault and propagated itself to the surface. Sedimentary rocks fracture readily under extension, whereas they tend to fold under compression.
