Cottage Grove Fault System

Location

Western Gallatin, Saline, Williamson, southern Franklin and Perry, northern Jackson Counties; possibly westward into Randolph County (J-7 to J-4)

References

(selected)Butts 1925, Cady 1925, Fisher 1925, Cady et al.1938, 1939, Clark and Royds 1948, Heyl and Brock 1961, Heyl et al. 1965, Heyl 1972, Wilcox et al. 1973, Nelson and Krausse 1981, Nelson and Lumm 1987

Description

The Cottage Grove Fault System is a principal tectonic feature of southern Illinois. It is known in more detail than most fault systems of Illinois as a result of extensive exposures in underground coal mines. The structural pattern is further defined by hundreds of coal and oil test borings and by seismic profiles. Because of covering glacial drift, surface exposures are rare and the fault system has slight topographic expression. Thus, the faulting was unknown until widespread coal mining began in the early 20th century.

The Cottage Grove presents a classic pattern of right-lateral, strike-slip faulting. Clark and Royds (1948) were the first to postulate wrench faulting; other geologists since have refined this hypothesis. Horizontal displacement probably is on the order of several hundred to a few thousand feet (100- 1,000 m). Mapping of two Pennsylvanian paleochannels that cross the fault zone limits maximum horizontal offset to less than 1 mile (1.6 km) (Nelson and Krausse 1981).

The fault system consists of (1) a master fault zone, (2) a series of en echelon or pinnate extensional faults flanking both sides of the master fault, and (3) a belt of anticlines along the master fault (fig. 26). The master fault zone trends slightly north of west and is approximately 70 miles (113 km) long. It is several hundred feet wide in most places and comprises high-angle faults outlining narrow horsts and grabens (fig. 27). Maximum dip-slip displacements are about 200 feet (60 m) in Pennsylvanian and Chesterian strata. In some places, the major displacements are down to the north; elsewhere, they are down to the south. Drag and slickensides indicate strike-slip motion, but no direct measurements of offset have yet been obtained. Proprietary seismic reflection profiles indicate that the master fault zone in Williamson and Jackson Counties is almost vertical and penetrates the.entire sedimentary column. The seismic data show that several reverse faults diverging upward from the main vertical strand produce a "positive flower structure," which is an earmark of strike-slip faulting (Harding 1985).

Northwest-trending extensional faults occur en echelon both north and south of the master fault zone throughout its length. Some extensional faults are several miles long and reach at least 7 miles (11 km) away from the master fault. Most extensional faults are high-angle normal faults that have displacements ranging from less than 1 inch to about 50 feet (15 m). High-angle reverse, oblique-slip, and strike-slip faults also have been observed in mines. Some faults give evidence of two or more episodes of movement. Ultramafic dikes follow many of the faults in the eastern part of the fault system (fig. 28).

Named folds along the Cottage Grove Fault System include, from east to west, the Cottage, Brushy (fig. 21), Pittsburg (fig. 26), Vergennes, and Campbell Hill Anticlines (fig. 22). Additional, small, unnamed anticlines have also been mapped (fig. 26). Anticlinal axes either strike parallel with the master fault or lie in right-handed en echelon arrangement. The limbs adjacent to the master fault are short, steep, and truncated by the fault; the opposite limbs are wide and the slopes are gentle. This style of folding is characteristic of strike-slip faults (Harding 1973).

The eastern terminus of the Cottage Grove master fault is accurately placed by using coal mine data from Section 17, T9S, R8E, Gallatin County. Here the fault lies about 2.5 miles (4 km) north of and strikes parallel to the Shawneetown Fault Zone. No connection between the Cottage Grove and Shawneetown is evident in near-surface strata (Nelson and Lumm 1987). The western terminus of the Cottage Grove is poorly defined because of the lack of suitable data. The westernmost mapped faults lie on the north flank of the Campbell Hill Anticline in northwestern Jackson County. The Wine Hill Dome and Bremen Anticline, in Randolph County, are in line with and en echelon to the master fault, and may express westward continuation of the fault system. Heyl (1972) suggested that the Cottage Grove continues into Missouri and links with the Ste. Genevieve Fault Zone, but no geologic evidence has been presented to support this idea.

The time of faulting is well indicated. The faults displace Missourian (late Pennsylvanian) and older strata. Peridotite intrusions along extensional faults in Saline County have been radiometrically dated as Early Permian (Nelson and Lumm 1987). Only a few of the intrusions that have been observed in mines are faulted (Clegg 1955). Therefore, most faulting probably was post-Missourian, pre-Early Permian, and only minor displacements occurred later.

Geologists have speculated about whether the Cottage Grove Fault System existed prior to the Pennsylvanian Period. Heyl (1972) included the Cottage Grove in his 38th Parallel Lineament, which also contains the Rough Creek-Shawneetown Fault System and Ste. Genevieve Fault Zone. Heyl proposed that the lineament represents a Precambrian suture or shear zone of continental proportions, and he suggested that it may have undergone several tens of miles of right-lateral displacement in Precambrian time. This hypothesis cannot be verified at present. Schwalb (1982) thought that the Cottage Grove was part of the northern boundary of the Rough Creek Graben. The strong magnetic gradient that roughly follows the Cottage Grove Fault System across southern Illinois suggests either a fault or contrasting basement lithologies. Proprietary seismic reflection profiles indicate no growth faulting at the Cambrian level across the Cottage Grove Fault System. The graben boundary instead appears to swing toward the southwest in Saline and Pope Counties and follows the Lusk Creek Fault Zone into the Mississippi Embayment.

Nelson and Lumm (1987) speculated that the Cottage Grove Fault System may have acted as a transform fault separating extensional faulting in the Fluorspar Area Fault Complex and Wabash Valley Fault System.

This scenario seems unlikely because the Early Permian was a time of compressional rather than extensional tectonics. Right-lateral movement on the Cottage Grove, and reverse faulting in the Rough Creek-Shawneetown Fault System and Lusk Creek Fault Zone, are all consistent with a compressive force from the southeast, associated with the Alleghenian Orogeny. A Precambrian zone of weakness, as proposed by Heyl (1972), might have guided the strike-slip faulting.

Figure(s)

Figure 22   Campbell Hill Anticline and related structures (after Root 1928). Note northwest-trending faults along the steep north flank of the fold near the Campbell Hill Anticline. The west-northwest-trending fault near the Wine Hill Dome is inferred from drill hole data . Contours are on the" Ava Shale," a local marker bed about 100 feet (30 m) below the Colchester (No. 2) Coal Member. Contour interval is 20 feet. Figure 26   Structure (ft) of the base of the Herrin Coal Member in eastern Williamson County. Data are from mine surveys and closely spaced coal test boreholes. Note en echelon anticlines and synclines in central part of the area. Contour interval is 10 feet. Figure 27   Exposure of the southern branch of the master fault of the Cottage Grove Fault System on the high wall of a strip mine in the SW, SE, Section 9, T9S, RSE, Saline County. This feature has been covered by mine reclamation. Four parallel east-striking faults are present. Faults A and Bare nearly vertical but sinuous in the dip direction, and bear prominent horizontal slickensides and mullion. Further evidence that the major motion was strike-slip is shown by difference in thickness of the Anna Shale on opposite sides of fault A, and of the Brereton Limestone across fault B. Dip-slip movement is primarily indicated by orientation of drag and slickensides of faults C and D. For location see figure 21. Figure 28   The Absher Dike, peridotite intrusion into the Springfield Coal Member in a now abandoned strip mine in southeastern Williamson County. This is one of many such dikes found along northwest-trending subsidiary faults in the Cottage Grove Fault System. Coal alongside the dike is coked (photo byK.E. Clegg 1955).