Geophysical studies of the Herald-Phillipstown fault and other shallow structures near Carmi, Illinois

Four seismic reflection profiles from the Wabash Valley area of southeastern Illinois successfully imaged the Herald-Phillipstown fault. From the reflection profiles and geophysical well logs, it was interpreted that the Herald-Phillipstown fault is an inverted flower structure. The main fault of the group is a steeply dipping normal fault with associated splay faults that are reverse in character. All the faults of this inverted flower structure are basement penetrating. Structural contour maps as well as maps of various reflector surfaces are prepared using two way travel time as well as depth. These maps show that the reflectors are undulating in character. Also distinct east-west rotational movements of the reflectors are observed along these faults and the main fault shows a curved geometry from south to north. The combination of the rotational movement, the curved nature of the fault plane and the undulating reflectors resulted in variation of offsets along these faults. The nature of faults of the inverted flower structure as observed on the seismic reflection profiles also indicates that there has been a change in the stress regime from extensional to compressional. Structural cross sections were generated using geophysical well logs from drill holes located along these seismic lines to analyze the nature of these faults at shallower depths. From these cross sections numerous shallow faults were observed and many of them can be correlated with those observed on seismic records indicating upward migration of basement penetrating faults into shallow younger sediments. Many of these faults, including the Herald-Phillipstown fault show evidences of reactivation.