Unnamed Fault Zone

Location

From central Cook County to southeastern Winnebago County (B-8 to A-6)

References

McGinnis 1966, Sargent and Buschbach 1985, Graese et al. 1988, Heigold 1991

Description

McGinnis (1966) inferred, on the basis of geophysical and borehole data, that a basement fault zone extended west-northwest from Chicago to Rockford, Illinois. Graese et al. (1988) also discussed this fault zone, which was not named in either report. This fault zone will not be named herein because its existence is doubtful.

McGinnis stated that the Precambrian basement surface is displaced more than 1,000 feet down to the southwest along the unnamed fault zone. According to his interpretation, the unnamed fault zone and the Sandwich Fault Zone outline a graben. Little or no displacement of Paleozoic rocks has occurred along McGinnis's fault zone. Structure on the Ironton Sandstone (Upper Cambrian) sags to the south near the inferred fault zone, whereas the Glenwood Formation (Middle Ordovician) apparently is not deformed (McGinnis 1966). From these data McGinnis inferred that principal displacements along the fault zone occurred before deposition of the Ironton. Graese et al. (1988), using additional subsurface control not available to McGinnis, mapped minor folding of the Glenwood Formation and the younger Galena Group in the vicinity of McGinnis's fault zone. They suggested minor reactivation of the fault zone during the Ordovician Period.

Other newly available data appear, however, to disprove significant offsets of the Precambrian surface in the area indicated by McGinnis. New wells drilled to basement in northeastern Illinois indicate that the Precambrian surface is unfaulted and slopes gently southeastward in this area. One of the new wells is in northern Du Page County a few miles south of McGinnis's fault zone and along his seismic line. The actual elevation of the top of Precambrian in this well is 800 feet higher than the elevation contoured by McGinnis. Recontouring the area using the new well data eliminates the fault zone (Sargent and Buschbach 1985). Also, a high-resolution seismic reflection profile was acquired for Du Page County across the trend of the McGinnis fault zone. The new seismic profile, which has reflection quality much better than that of McGinnis's seismic profiles, indicates no offset of the Precambrian surface (Heigold 1991).