AGE ESTIMATION OF THE COTTAGE GROVE FAULT SYSTEM (ILLINOIS, USA) FROMHIGH-RESOLUTION SEISMIC-REFLECTION STUDIES OF COAL MEASURES

Geologic and high-resolution seismic reflection studies of Pennsylvani an coal deposits have provided new insights concerning the Cottage Gro ve Fault System (CGFS), a major east-west oriented wrench fault system in southern Illinois. The age of faulting has previously been estimat ed to be late Pennsylvanian to early Permian, based upon K-Ar dating o f igneous intrusive rocks associated with subsidiary faults. Although these dates are accurate for intrusions and faulting at certain locati ons, the duration of Cottage Grove master fault zone or subsidiary fau lting is still unclear. High-resolution seismic reflection profiles we re collected approximately 6.4 km north of the master fault zone in th e vicinity of mapped subsidiary faulting to study coal deposits. Sever al small-offset, high-angle normal and reverse faults were interpreted from the seismic records. Two small-offset fault zones observed on th e seismic records, interpreted to be subsidiary faults of the CGFS, bo und a gentle asymmetric anticline. Several small sandstone channels lo cated on the down-thrown sides of these faults are interpreted from th e seismic data. Vertical stacking of these sandstone channels and othe r 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 contemporaneo us with sand deposition. Thus, subsidiary faulting of the CGFS occurre d as early as middle Pennsylvanian time.