PRELIMINARY RESULTS OF A GEOPHYSICAL INVESTIGATION OF A POSSIBLE PROTEROZOIC TERRANE BOUNDARY: COTTAGE GROVE FAULT SYSTEM, SOUTHERN ILLINOIS BASIN

The Cottage Grove fault system (CGFS) is a 113-km long right-lateral wrench zone that cuts across the southern Illinois basin and that has previously been proposed to mark a major Proterozoic terrane boundary. One of the most enigmatic structures of the basin, the CGFS shows en echelon, pull-apart, and other subsidiary structures typical of a textbook strike-slip fault system. A major oil company has recently released several hundred kilometers of seismic reflection lines to the Illinois State Geological Survey gathered in the early 1980’s across and in the vicinity of the CGFS. This high-quality 30-fold reflection data set was acquired using mostly a vibroseis source with a broadband sweep (e.g., 10-100 Hz) recording to approximately 3 s (or roughly 9 km depth). The stacked sections show well-imaged fault displacements that affect the entire Paleozoic section and disrupt the top of Precambrian basement. Significant intra-basement reflectivity also appears, showing dipping reflections and/or diffractions beneath the CGFS, which suggest a major crustal boundary. Vertical faulting is evident in the lower Paleozoic section and uppermost basement and propagates upwards into flower structures that would be expected of a major strike-slip fault system. The results of experimental 2-D filtering of Bouguer gravity and especially magnetic intensity data indicate that a major magnetized intra-basement source is located at the CGFS. Outcomes of the gravity and magnetic data show a stair-step or en echelon pattern closely following the trend of the CGFS. This pattern is believed to represent ultramafic intrusions Permian in age that occurred approximately coeval with fault zone development. Filtering the potential field data and interpreting the 2-D structural mapping of the seismic lines will provide more insight into the possibility of a crustal boundary being located near this fault zone.