Peru Monocline (New)
Part of the La Salle Anticlinorium
Location
From western Livingston to Lee County (D-6 to B-5)
References
None
Description
The name Peru Monocline is proposed for the northernmost major segment of the La Salle Anticlinorium. It is named for Peru, the sister city of La Salle. This was the first part of the La Salle Anticlinorium to be recognized, based on exposures along the Illinois River (fig. 45fig. 45). The structure originally was named the La Salle Anticline. The name was changed to the La Salle Anticlinal Belt when the compound nature of the zone was recognized and no individual name was assigned to its northernmost element.
The Peru Monocline, as mapped on top of the Galena (Trenton) Group by Bristol and Buschbach (1973), is about 65 miles (100 km) long. It arises in western Livingston County, north of the Downs Anticline, striking northwest and facing southwest. The monocline is broad and gentle on the south, but the dips steepen sharply northward; relief exceeds 1,300 feet (390 m) near the west end of the Ancona Anticline in southern La Salle County. The trend of the monocline in this area changes to nearly north and then curves back toward the northwest as the flexure continues across western La Salle and eastern Bureau Counties into Lee County. The Galena Group is missing east of the monocline in La Salle and Lee Counties, where basal Pennsylvanian rocks lie unconformably on strata as old as St. Peter Sandstone (Middle Ordovician) and are considerably less deformed than pre-Pennsylvanian rocks. Structural relief on Pennsylvanian beds is less than half of that on pre-Pennsylvanian units.
The Kolata et al. (1983) structure map of the top of the Cambrian Franconia Formation (fig. 17) shows as much as 1,600 feet (480 m) of relief across the Peru Monocline in western La Salle County. Relief decreases farther north and the monocline gradually loses its identity. The northern part of the flexure is markedly sinuous.
Coal mining activities provided some details of structure on the flank of the Peru Monocline. In the Black Hollow mine near Oglesby, a set of headings was driven down the flank of the fold in the Colchester Coal Member, which is near the base of the Pennsylvanian System in this area. The dip of the coal increased steadily down the flank to a maximum of 45°. The coal was reported to be harder and more brittle than usual; both the coal and the overlying shale were severely fractured. Low-angle normal faults, having displacements of several feet, displaced the dipping coal. A low-angle thrust fault and bedding-plane faults within the coal (flexural slip?) were encountered in the Oglesby mine that operated on the lower limb of the monocline near the foot of the steep flank (Cady 1915, 1919). Younger Pennsylvanian strata that crop out on the flank of the monoclines dip less steeply than the coal and are not faulted (Cady 1919). The latter observation probably reflects the progressive growth of the fold during Pennsylvanian sedimentation.
Figure(s)
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