Cook County Faults (New)

Location

Cook County (greater Chicago area) and vicinity (A, B-8)

References

Buschbach and Heim 1972, Graese et al. 1988, Harza with ISGS 1988

Description

Faults mapped by Buschbach and Heim (1972) in the greater Chicago area were listed as "significant unnamed structures" by Treworgy (1981). Because these faults form no clearly defined zones or systems, the term "Cook County Faults" is applied as an informal name of convenience.

The faults were interpreted on the basis of detailed seismic reflection surveys made in connection with the rock-tunneling project of the Metropolitan Water Reclamation District of Greater Chicago. The faults were indicated by small offsets of the prominent reflection that marks the contact of the Maquoketa Group (shale) above and Galena Group (carbonates) below. Inferred displacements of the faults range from 10 to 55 feet (3-17 m). Subsequent tunneling confirmed the existence of some faults, but showed that other predicted faults are folds. Observed faults have vertical offsets ranging from a few inches to 50 feet (15 m) and also bear evidence of strike-slip displacement (Graese et al. 1988, Harza with ISGS 1988).

Defining patterns of these faults is difficult because the seismic traverses covered a limited area (most of Cook County, except for the northwestern and southwestern corners, plus a very small part of Du Page County). Some of the faults mapped in northern Cook County may be related to the Des Plaines Disturbance. These faults are roughly tangential to the intensely disturbed area. Southward, the dominant trend of faults is northwest to west-northwest. The nearest major structure, the Sandwich Fault Zone, has a similar trend. Paul C. Heigold (ISGS, personal communication 1990) has examined some of the seismic profiles upon which fault interpretations were based, however, and considers interpretation of the faults to be questionable.