Johnsonville Dome (New)

Location

T1N, R6E, Wayne County (I-6)

References

Cassin 1949, Du Bois and Siever 1955, Cluff et al. 1981, Treworgy 1981

Description

Previously unnamed, this dome was listed as a "significant unnamed structure" by Treworgy (1981). It is now named Johnsonville Dome, for the nearby town and Johnsonville Consolidated Oil Field. As mapped on the top of the Ste. Genevieve Limestone and Beech Creek ("Barlow") Limestones, the Johnsonville Dome is roughly circular and is 2.5 to 3 miles (4-5 km) in diameter (Cassin 1949). Closure on the Ste. Genevieve and Beech Creek is about 120 feet (36 m). Closure decreases upward to less than 20 feet on the Herrin Coal and Carthage (Shoal Creek) Limestone of Pennsylvanian age (Cassin 1949, Du Bois and Siever 1955). Upward loss of structural relief is accompanied by thinning of Chesterian and Pennsylvanian strata across the crest of the dome.

The Johnsonville Dome is situated on a terrace northwest of the structurally lowest point of the Fairfield Basin. Several smaller domes are located on the same terrace. The east flank of the terrace is rather sharp and linear.

A deep well drilled near the apex of the Johnsonville Dome disclosed two unusual features. The Ullin Formation (lower Valmeyeran Series) is thicker than normal for the area and is composed of pure, white, crinoidal, bioclastic limestone, in contrast to the finer grained argillaceous or silty limestone of neighboring wells. Cassin (1949) hypothesized that this represents a reef. In the same well (Texas Co. No. 1 Greathouse in Section 27, T1N, R6E.), approximately 130 feet (40 m) of strata in the Devonian-Mississippian New Albany Group appears to be repeated by a reverse fault. Below the fault, regional mapping of the base of the New Albany Group indicates that no dome is present (Cluff et al. 1981).

A COCORP seismic profile that traversed the Johnsonville Dome indicates a probable fault east of the dome, along the east edge of the structural terrace. The fault is normal, dips steeply, and penetrates Precambrian basement (Whitaker and Treworgy 1990). Whitaker speculated that Johnsonville Dome is situated on a horst, which favored the local development of reefs in early Mississippian time. According to this interpretation, Johnsonville Dome is a reef-drape structure analogous to those associated with numerous Silurian pinnacle reefs in Illinois.