A geological and geophysical study of the Maunie fault

The Wabash Valley Fault System is located near the eastern edge of the Illinois Basin in southeastern Illinois and southwestern Indiana. It extends in a north-northeasterly direction from the Cottage Grove Fault Zone and Rough Creek Graben, in southeastern Illinois and western Kentucky (Ault et al., 1979), into southwestern Indiana where it terminates (Sullivan et al., 1979). Within this fault system, there are 12 named faults and 3 named grabens in Illinois (Bristol and Treworgy, 1979; Sexton et al., 1986) with an additional 6 faults and 1 graben in Indiana (Sullivan et al., 1979). Recent earthquakes in the region show that the Wabash Valley Fault System is currently seismically active.; Because earthquakes in the eastern and central United States typically do not rupture the surface, studies pertaining to faults in this area are significantly enhanced by using subsurface imaging techniques. Several different data types have been used for this study in order to get a more complete picture of the geology in this region. Data utilized for this study include: oil-well completion cards, petrophysical well logs, seismic reflection, and ground penetrating radar (GPR) profiles. These data have combined to show the increase of fault complexity as it approaches the surface.; Bodziak (1999) interpreted two episodes of fault reactivation associated with Wabash Valley Faults. One episode of reactivation occurred post Pennsylvanian and the other episode occurred post Cretaceous and possibly Holocene. Both of which are seen in the data used in this study. The post Pennsylvanian reactivation is seen on cross section W-3 between wells 354 and 359. Here the fault shows both normal and reverse offsets. The change in the offset direction occurs between the Herrin #6 and Harrisburg #5 coal seams. Supporting data for the statement of post Cretaceous and potentially Holocene reactivation comes from the radar profiles collected for this study. These radar profiles show many small offset reverse faults. In conjunction with these are many small normal faults.; Analysis of industry seismic data (seismic lines S-1 and S-2) shows that the Maunie fault has a maximum apparent displacement of 168 feet (51 meters) on the Cambrian Eau Claire formation decreasing to 80 feet (24 meters) on the lower Mississippian/upper Devonian New Albany formation. Continuing upwards and analyzing the borehole data, the maximum apparent displacement is 95 feet (29 meters) on the Pennsylvanian West Franklin formation located between wells 251 and 210 on cross section W-2. Using the ground penetrating radar data we see that the faults come close to surface, within 3 feet (1 meter) on profiles R-2 and R-5. They have less apparent displacement, a maximum of 4 feet (1.2 meters) on profile R-7. Also, there are occurrences where both normal and reverse faulting is imaged on radar profiles R-2, R-4, and R-5. These shallow faults occur within the Quaternary/Holocene sediments and represent evidence for geologically young fault activation.