Pittsfield Anticline

Location

Central Pike County (F-1,2)

References

Coryell 1919, Krey 1924, Bell 1926d, Collingwood 1933, J. Weller and Bell 1937, Workman and Bell 1948, Bristol and Buschbach 1973

Description

The Pittsfield Anticline, also called the Pittsfield-Hadley Anticline, is the largest anticline in western Illinois north of the Cap Au Grès Faulted Flexure. It is a highly elongated anticline and has a northwest-trending axis. The highest point is located in the western part of T5S, R4W, where Krey (1924) reported 175 feet (53 m) of structural closure on the base of the Mississippian Burlington Limestone. Bristol and Buschbach (1973) showed more than 200 feet (60 m) of closure on top of the Galena Group (Ordovician). Nosing of contours extends well beyond the area of closure at both ends of the fold axis. Some authors have traced nosing as far southeast as Greene County, Illinois, and northwestward into Missouri.

One well near the crest of the Pittsfield Anticline was drilled to Precambrian granite. The top of Precambrian is about 800 feet (240 m) higher than in another well 8 miles (13 km) northwest and off the structure. The Mt. Simon Sandstone is absent in the well on the anticline; the Eau Claire Formation rests directly on the granite. Workman and Bell (1948) inferred that a Precambrian hill underlies the highest point of the Pittsfield Anticline. That inference appears well founded; however, the anticline is much too large to attribute solely to compaction over a buried hill. The parallelism of the Pittsfield to other large folds and fault zones in western Illinois and eastern Missouri suggests an origin by reactivation of faults in the crystalline basement.

In 1886, gas was discovered in Silurian dolomite on the Pittsfield Anticline at a depth of 265 feet (81 m). The Pittsfield Gas Field was subsequently developed, but was abandoned by 1930.