Plum River Fault Zone
Location
Northern Ogle and Carroll Counties, Illinois,
westward to Linn County, Iowa (A-3,4)
References
Kolata and Buschbach 1976, Ludvigson et al. 1978, Heyl and West 1982, Trapp and Fenster 1982, Heyl 1983, Kolata et al. 1983, Bunker et al. 1985, Ludvigson 1985
Description
Structural disturbance was recognized early in this area, but it was interpreted to be an anticline rather than a fault zone (Cady 1920). The geologic map of Illinois (Willman et al. 1967) indicates a fault 4 miles long at Savanna, Illinois, on the Mississippi River. Kolata and Buschbach (1976) named the Plum River Fault Zone and traced it from T25N, R10E, Ogle County, Illinois, to the vicinity of Maquoketa, Iowa. Bunker et al. (1985) used geophysical surveys, core drillings, and detailed outcrop studies to extend the fault zone as far west as Cedar Rapids, Iowa.
As presently mapped, the Plum River Fault Zone is 112 miles (180 km) long and trends slightly north of east. Its width varies from a few hundred feet to about 3,900 feet (1,170 m). The zone is composed of high-angle faults that are subparallel or possibly braided in map view. The cumulative displacement is 100 to 400 feet (30-120 m) down to the north. At several places in Iowa, the fault zone contains a central graben in which downfaulted slices of Devonian rocks are preserved between Silurian and Upper Ordovician strata outside the fault zone. Vertical offsets of individual faults are as great as 500 feet (150 m) in near-surface rocks and may reach 1,100 feet (330 m) at the Precambrian basement surface (Bunker et al. 1985).
The fault zone is generally bordered by anticlines along the south side and synclines along the north (fig. 37). Prior to recognition of the fault zone, the southern anticlinal belt was called the Savanna-Sabula Anticline and it included the Leaf River Anticline and Forreston Dome in Illinois. The northern syncline in Illinois is called the Uptons Cave Syncline.
Sedimentary rocks along the Plum River Fault Zone generally dip gently, except in immediate proximity of faults, where dips reach or exceed 45°. Dolomite along faults has undergone severe cataclastic deformation. Mineralization with lead and zinc has been reported at several localities (Heyl and West 1982).
Bunker et al. (1985) inferred, on the basis of detailed stratigraphic studies, that the north side of the Plum River Fault Zone may have begun to subside near the middle of the Silurian Period. Movement early in the Middle Devonian Epoch is strongly indicated; formations of this age are present north of the fault zone but absent south of it in Iowa. The main movements evidently were post-Devonian, pre-Pennsylvanian. Small outliers of Pennsylvanian rocks are found at similar elevations on opposite sides of the fault zone. Structural relationships of these outliers could allow at most about 30 feet (9 m) of post-Pennsylvanian movement (Bunker et al. 1985, Ludvigson 1985).
The stress field responsible for the Plum River Fault Zone has not been defined. Several geologists (Ludvigson et al. 1978, Heyl and West 1982, Trapp and Fenster 1982, Heyl 1983) have suggested that right-lateral faulting has taken place. Following comprehensive study, Bunker et al. (1985, p. 65) concluded, "While the interpreted structural geometries described in this report are not incompatible with strike-slip faulting, this hypothesis is untestable with the current data set."
