Wabash Valley Fault System

From ILSTRUC

Location

Southeastern Illinois, southwestern Indiana, and adjacent part of Kentucky (I, J-7, 8)
Not one map.jpg

References

Cady et al. 1939, Clark and Royds 1948, Pullen 1951, Heyl et al. 1965, Bristol 1975, Sullivan et al. 1979, Bristol and Treworgy 1979, Ault et al. 1980, Tanner et al. 1980a-c, Tanner et al. 1981a-c, Sexton et al. 1986, Nelson and Lumm 1987, Treworgy 1988b

Description

The Wabash Valley Fault System is composed of high-angle normal faults, most of which trend north-northeast, in the lower Wabash River valley of southeastern Illinois, southwestern Indiana, and a small part of Kentucky. The system extends about 55 miles (88 km) northward from the Rough Creek-Shawneetown Fault System and is as wide as 30 miles (48 km). Structure of the Wabash Valley Fault System is known from records of thousands of oil test holes, including many that penetrate fault surfaces. Additional details are provided by exposures in underground coal mines and by seismic reflection profiles.

The Wabash Valley Fault System contains more than one dozen named faults and fault zones, many of which contain parallel faults that overlap one another end to end. Faults along the west edge of the Wabash Valley Fault System trend north to south, whereas those farther east strike more northeast. The faults outline elongated gently arched or tilted horsts and grabens. Overall, the axial portion of the Wabash Valley Fault System is downfaulted relative to the margins.

Most individual faults are simple normal shears dipping 60° or steeper (figs. 43, 56, 57). Fault surfaces are generally planar. Drag is minimal. Faults exposed in mines have fairly narrow zones of gouge and breccia and numerous antithetic fractures are present. Slickensides indicate dip-slip movement, except in a few cases where horizontal striations indicate a minor component of strike-slip movement on part of a fault zone. The greatest recorded vertical displacement on a single fault is 480 feet (145 m) on the Inman East Fault. A few cross faults, normal faults that strike east to west and connect the overlapping ends of northeast-trending faults, have been mapped.

Well data indicate that the faults of the Wabash Valley Fault System tend to splinter upward. Thus, more faults are mapped on Pennsylvanian than on Mississippian horizons (Bristol and Treworgy 1979, Tanner et al. 1980a-c, Tanner et al. 1981a-c). Upward splitting of faults is confirmed by seismic reflection profiles of Sexton et al. (1986) and also by proprietary seismic profiles. On these seismic profiles, faults also appear to lose displacement downward. Most of them appear to die out within the Knox Group; only a few visibly offset the prominent basal Knox (Cambrian) reflector.

The time of structural movement in the Wabash Valley Fault System cannot be defined more precisely than post-late Pennsylvanian, pre-Pleistocene. There is no evidence of faulting contemporaneous with Paleozoic sedimentation.

The Wabash Valley Fault System is clearly a product of horizontal extension, but its regional significance is obscure. Contrary to Heyl et al. (1965), the Wabash Valley Fault System is not simply a northeast extension of the Fluorspar Area Fault Complex. The Wabash Valley and Fluorspar Area faults are separated by the east-trending Rough Creek-Shawneetown Fault System (Nelson and Lumm 1987). Moreover, faults of the Wabash Valley Fault System trend more northward than faults of the Fluorspar Complex.

Bristol and Treworgy (1979) and Treworgy (1988b) suggested that the faults in the Wabash Valley Fault System extend down into basement and may be reactivated basement faults. Sexton et al. (1986) proposed that the Wabash Valley Fault System formed when faults of a Precambrian rift zone, the Southern Indiana Arm, were reactivated. They based their theory on interpretation of their own seismic reflection profiles. Some of the Wabash Valley faults do not line up with inferred deeper faults of the Precambrian rift on the seismic profiles. Also, most of the Wabash Valley faults seen on seismic lines, die out downward in Ordovician and Cambrian strata. If these faults were reactivated basement faults, their displacement should increase with depth. Moreover, reactivation of basement faults should have produced monoclinal folds in sedimentary cover (Steams 1978). No such folds are present in the Wabash Valley Fault System.

An alternate explanation of the Wabash Valley Fault System is that the crust was arched gently along a north-northeast-trending axis, probably in late Paleozoic time. Arching stretched the sedimentary layers and created a series of axial tension fractures. The tension would have been greatest at the surface where the arc of folding was longest and would have diminished with depth. Thus, the tension fractures died out downward. This style of faulting was demonstrated by Sanford (1959) in laboratory experiments (fig. 68).

References

Figure(s)

  • Figure 56   A segment of the New Harmony Fault Zone in an underground coal mine, Wabash County. Nearly horizontal mudstone on the footwall (right) is displaced against steeply dipping and sheared sandstone and dark shale on the hanging wall. Total throw on this fault is approximately 120 feet (37 m) down to the northwest (left).

  • Figure 57   Drill core penetrating a segment of the New Harmony Fault Zone, Wabash County. Pennsylvanian siltstone at top of core is in the hanging wall and displays nearly horizontal laminations. Remainder of the core consists of gouge and breccia. Note steep angle of fault plane (compare with fig. 56). Scale is graduated in tenths of feet.

  • Figure 68   A fracture pattern resulting from arching in laboratory experiments by Sanford (1959, fig. 15). The pattern shown here is similar to that of the Wabash Valley Fault System.

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