Geologic and high-resolution seismic reflection studies of Pennsylvanian coal deposits have provided new insights concerning the Cottage Grove Fault System (CGFS), a major east-west oriented wrench fault system in southern Illinois. The age of faulting has previously been estimated to be late Pennsylvanian to early Permian, based upon K-Ar dating of igneous intrusive rocks associated with subsidiary faults. Although these dates are accurate for intrusions and faulting at certain locations, the duration of Cottage Grove master fault zone or subsidiary faulting is still unclear. High-resolution seismic reflection profiles were collected approximately 6.4 km north of the master fault zone in the vicinity of mapped subsidiary faulting to study coal deposits. Several small-offset, high-angle normal and reverse faults were interpreted from the seismic records. Two small-offset fault zones observed on the seismic records, interpreted to be subsidiary faults of the CGFS, bound a gentle asymmetric anticline. Several small sandstone channels located on the down-thrown sides of these faults are interpreted from the seismic data. Vertical stacking of these sandstone channels and other sands within the coal measures is also observable in the correlated well data. Location of channel deposits suggests depositional control by these faults, indicating that subsidiary faulting was contemporaneous with sand deposition. Thus, subsidiary faulting of the CGFS occurred as early as middle Pennsylvanian time.