Des Plaines Disturbance
Location
Centered in T41N, R12E, Cook County (A-8)
References
Thwaites 1927, Longwell et al. 1944, Bays et al. 1945, Pemberton 1954, Emrich and Bergstrom 1962, Buschbach 1964, Beck 1965, Willman 1971, Buschbach and Heim 1972, Buschbach et al. 1982, McHone et al. 1986a, b
Description
Anomalous geologic conditions in the northwest Chicago suburb of Des Plaines were noted by water well drillers as early as the 1890s. Thwaites (1927) first suggested faulting in the area. On their tectonic map of the United States, Longwell et al. (1944) identified the area of faulting as the "Des Plaines Disturbance," which has generally been used since. The disturbance was labeled a "cryptovolcanic structure" by Bays et al. (1945).
The Des Plaines Disturbance is roughly circular and about 5 miles (8 km) in diameter. It lies on the east flank of the Wisconsin Arch and is surrounded by Silurian bedrock that dips gently eastward. Within the disturbance, rocks ranging from Champlainian to Pennsylvanian subcrop beneath Pleistocene glacial deposits. The oldest rock at the bedrock surface, St. Peter Sandstone, is found at the center of the disturbance and is more than 800 feet (240 m) above its expected position. The sandstone is pervasively shattered and contains abundant silt-sized quartz shards. Sand-sized grains contain prominent strain lamellae and sets of crystallographically oriented planar fractures (McHone et al. 1986a, b). Outward from the center, younger rocks are downdropped within a mosaic of fault blocks (Emrich and Bergstrom 1962, Buschbach and Heim 1972). Ordovician rocks in cores south of the central uplift are steeply tilted, intensively brecciated, and cut by reverse faults (McHone et al. 1986a, b). Mississippian and Pennsylvanian rocks have been identified in wells north and east of the central uplift. These include bluish gray to black shale with beds of coal (Pennsylvanian), dolomite of the Keokuk-Burlington (Valmeyeran), and at least 500 feet (150 m) of shale and siltstone thought to be of Kinderhookian and early Valmeyeran age (Emrich and Bergstrom 1962). The next nearest outliers of Pennsylvanian and Mississippian strata are about 40 and 90 miles (65 and 145 km) south of the Des Plaines Disturbance.
McHone et al. (1986b) found shatter cones (fig. 30) in cores of dolomitic beds within the Maquoketa Group. The presence of shatter cones confirms the theory that the Des Plaines Disturbance was caused by impact of an extraterrestrial body (Dietz 1959).
The time of deformation cannot be more accurately determined than post-Pennsylvanian, pre-Pleistocene (Emrich and Bergstrom 1962, p. 962).
See also GLASFORD STRUCTURE, HICKS DOME, OMAHA DOME.
Figure(s)
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