SEISMIC REFLECTION INVESTIGATION OF THE COTTAGE GROVE FAULT SYSTEM, SOUTHERN ILLINOIS BASIN

Mapping and reprocessing of industry seismic reflection data, recently released to the Illinois State Geologic Survey, provide new images of deformation structure of the Cottage Grove fault system (CGFS) in the southern Illinois Basin. Two seismic stratigraphic horizons, the New Albany Shale (upper Devonian) and the Knox Formation (Cambro-Ordovician), have been used to create regional isotraveltime structural contour maps. This mapping illustrates areas of distinct monoclines, broad anticlines, and synclines that express Paleozoic-age deformation associated with strike-slip along the fault system. Prominent near-vertical faults can elsewhere be delineated throughout the entire Paleozoic section associated with flower structures, which are characteristic of strike-slip fault systems. Previous studies of the CGFS interpret a dextral master strike-slip fault with multiple step-overs trending southeast implying releasing bends or zones of local extension. However, significant reverse faults, monoclines, and anticlines in the vicinity of the fault and at the ends of the step-overs suggest compression in contrast to extension, as would be expected from purely dextral motion. Reconstructive vectorization and seismic migration of the reflection profiles allow us to characterize the near-vertical faulting, flower structures, and other compressive structures more exactly by collapsing diffractions and correctly positioning dipping reflections on the time sections. Well-log data are used to identify stratigraphic horizons on the seismic sections as well as confirm previous observations from coal-mine mapping. Gravity and magnetic studies indicate anomalous gradients across the CGFS showing evidence of structural disruption or intrusion in Precambrian basement. Patterns of dipping reflections and diffractions deep within the basement may illustrate possible local heterogeneity or deformation from a past tectonic event. Through reprocessing and mapping the seismic data, and then comparing the results with independently derived kinematic models of strike-slip step-overs and bends, we can more accurately constrain our knowledge of the tectonic history and evolution of the CGFS.