SEISMIC REFLECTION INVESTIGATION OF THE INMAN EAST FAULT IN THE WABASH VALLEY FAULT ZONE

The Wabash Valley Fault Zone (WVFZ), located in the Illinois Basin in Southeastern Illinois and Southwestern Indiana, is characterized by north-northeast trending normal faults. This zone of faulting is just north of the Reelfoot Rift and northwest of the Rough Creek Graben, two major regions of faulting associated with a Late Pre-Cambrian failed rift in the mid-continent. The WVFZ is thought by some to be a northeastern extension of this rift complex. Many faults have been previously identified and mapped in the WVFZ by use of drill hole and geophysical data. The previously mapped Inman East Fault extends from Southeastern Indiana into Southern Illinois, where the fault forks and terminates just north of the east-west trending Shawneetown-Rough Creek Fault.

In August 2005, high-resolution, shallow seismic reflection data totaling over 22.3 km was collected along four lines in Gallatin County, Illinois, to investigate the nature of faulting associated with the Inman East Fault. Interpretation of this seismic data is presented, showing the Inman East Fault along with many small offset and flexural faults. Many of these interpreted northeast-trending faults are observed on two seismic profiles.

The Springfield (No. 5) coal is a strong reflector located approximately 185 meters below the surface in the study area. Faulting along this coal seam served as the primary focus of this study. The coal seam dips to the north-northwest in the study area and was correlated to the seismic reflection data using a synthetic seismogram derived from a sonic log from a drill hole adjacent to one of the seismic lines. The maximum offset of the Springfield coal, estimated to be approximately 10 meters, occurs where the Inman East Fault cuts across the northernmost seismic line. The shallow seismic data show that the offset along the Inman East Fault decreases to the Southwest. The flexural and small offset faults that may be related to the Inman East Fault result in coal seam dip changes that are often significant enough to require additional planning for mining of the coal seam.