Cap Au Grès Faulted Flexure

Location

Southern Jersey and Calhoun Counties, Illinois (H-2, 3), and Lincoln and Pike Counties, Missouri

References

Worthen 1870, Keyes 1894, 1898, 1917, Krey 1924, Rubey 1952, Collinson et al. 1954, Mateker 1958, Douthit 1959, Cole 1961, Tikrity 1968, McCracken 1971, Treworgy 1979a, Nelson and Lumm 1985

Description

The Cap au Grès structure has been called a fold, a fault, and a fault zone, but the term faulted flexure best describes it. Worthen (1870), during the first geological survey of Illinois, observed the deformed zone, but Keyes (1894) named it. The name Cap au Grès means sandstone point and was taken from a headland along the Mississippi River where St. Peter Sandstone is exposed on the high side of the structure.

In general, the name Cap au Grès has been applied only to the steeply dipping and faulted southwest limb of the Lincoln Anticline in Illinois and the adjacent part of Missouri. The name Cap au Grès is, therefore, redundant and a case could be made for eliminating Cap au Grès in favor of Lincoln Anticline. The name Cap au Grès, however, is so firmly established in the literature that any attempt to discontinue using this name would be futile.

The flexure is about 60 miles (100 km) long. It trends generally east-southeast in Missouri and runs east to west in Illinois. The north side has been raised as much as 1,200 feet (360 m) relative to the south side. Steepest dips range from about 65° south through vertical to 65° north (overturned). The zone of dips steeper than 5° is only 1,000 to 1,500 feet (300-450 m) wide.

Interpretation of the Cap au Grès structure is hampered by incomplete exposures. Most geologists originally believed folding to be the product of drag along a large fault. Rubey (1952) took a more cautious approach and mapped faults only where data required them. By means of careful calculations and geometric constructions, he demonstrated that no more than one-third of the structural relief need be attributed to faulting. Rubey mapped a series of high-angle, discontinuous faults parallel to and slightly north of the line of maximum flexure along the Cap au Grès. A few small faults transverse to the fold also were recognized. Rubey concluded that the drag hypothesis was not tenable. Instead, he favored an origin by horizontal compression that came from the north and produced a deep-seated reverse fault and a sharp, locally fractured fold near the surface. Rubey suggested an alternative- uplift of the northern basement block along a steep reverse fault with forced folding of sedimentary strata above.

Cole (1961) proposed alternatively that the Cap au Grès is a left-lateral strike-slip fault. He regarded the Waterloo-Dupo Anticline in Illinois as the offset continuation of the Lincoln Anticline in Missouri. If true, it would imply approximately 30 miles (48 km) of horizontal displacement. Cole's theory is untenable because there is no way to account for such a huge displacement at the two ends of the structure. Moreover, no indications, such as en echelon folds, have been reported along the Cap au Grès.

Mateker (1958) conducted a gravity survey and Douthit (1959) conducted a magnetic survey of the Cap au Grès Faulted Flexure. Both concluded that Precambrian basement is cut by a continuous fault along which the north side has been uplifted about 1,000 feet (300 m). Tikrity (1968) postulated uplift of the northern block of the Cap au Grès along a high-angle reverse fault in the Precambrian basement and forced folding of the Paleozoic sedimentary cover. Nelson and Lumm (1985) agreed with Tikrity and compared the Cap au Grès flexure with Laramide monoclines in the Rocky Mountains and Colorado Plateau, where folds in the sedimentary cover overlie faults in Precambrian crystalline basement.

The Cap au Grès structure has undergone recurrent movement. Initial uplift of the structure occurred in Devonian and early Mississippian time (Rubey 1952, Tikrity 1968). Major displacements took place in late Mississippian to early Pennsylvanian time, as shown by angular unconformity along the flexure (Rubey 1952). Later movements tilted and displaced Pennsylvanian rocks. Still more recent uplift is indicated by apparent displacement of the Grover Gravel (Pliocene to early Pleistocene) and its underlying peneplain. The gravel and erosional surface lie about 150 feet (45 m) lower, south of the flexure, as compared with the north (Treworgy 1979a). This is one of the few places where Tertiary tectonic activity has been documented in Illinois.

See also LINCOLN ANTICLINE.