Crown Fault
Location
T12N, R5 and 6W, Macoupin and Montgomery Counties (G-4)
References
Nelson and Nance 1980, Nelson 1981 (p. 28), Nelson 1987b, Ledvina 1988
Description
Detailed geologic mapping in the Crown II underground mine of Freeman United Coal Mining Company disclosed a left-lateral fault in the Herrin Coal Member (Pennsylvanian) and adjacent strata. The fault trends east to west and to date has been traced 17,000 feet (5,200 m) without either end being found . Unpublished field notes (ISGS open files) suggest that the fault also was encountered in the northern workings of the abandoned Crown I Mine, east of Crown II. This would extend the length of the fault to at least 7 miles (11 km). The fault was named the Crown Fault by Ledvina (1988) for the Crown Mines.
Left-lateral displacement is demonstrated by offset of lenticular rock bodies in the mine roof. Measured horizontal offset varies from about 15 to 70 feet (4.5-21 m); observed vertical slip is up to 4 feet (1.2 m). The fault branches and splits in several places. Northeast-trending vertical open joints and fissures occur along the fault and reflect the tensional component of the shearing stress. The vertical extent of the fracture zone is unknown, but it is thought to be considerable because freshwater enters the mine along the fault from above and oil and gas enter from below. The Crown Fault offsets and therefore is younger than one of the Girard Faults, which are normal faults (fig. 29).
The Crown Fault occurs in a region of the Western Shelf noted for its lack of tectonic structure. Nevertheless, it is difficult to conceive of a nontectonic origin for this fault. Because the displacement is so small, a feature like the Crown Fault would be nearly impossible to detect by ordinary subsurface mapping techniques. See also GIRARD and SICILY FAULTS.
