New 2D/3D Seismic Reflection Data Guide Geophysical Interpretation of Central Illinois Basin

Acquisition and processing of over 200 km of new 2D seismic reflection data (augmented by a 3D seismic data set) in central Illinois provides fresh constraints for interpreting Paleozoic and Precambrian structure and stratigraphy for the Illinois Basin. A long east-west profile extends across a portion of central Illinois north of Decatur (Illinois River to ~9 km north of Atwood) in an area of the basin where little or no high-resolution geophysical data are publicly available. The new data thus allow the northward extension of geophysical observations from the better-studied southern portion of the Illinois Basin. The 2D data were collected and processed at high resolution to 5 seconds traveltime with a common mid-point spacing of 20 ft (~6.1 m) and are tied into a 3D seismic data set using north-south and east-west profiles. Seismic attribute analysis of key horizons aided our understanding of the lateral variation in physical rock properties and general data quality along the profile. Interpretation of the seismic data is constrained locally by new deep drill-hole data, which reveal a rhyolitic Precambrian "basement." Wedge-shaped zones of reflectivity are well-resolved in Precambrian "basement" just beneath the Cambrian Mt. Simon Sandstone, interpretable as "seismic stratigraphic" sequences nearly identical to Precambrian sequences previously mapped further south in the basin. Paleozoic structure and deformation can also be well-resolved with unprecedented detail, relative to other publicly available seismic data in the basin. For example, possible syn-depositional faults appear to affect the lower part of the Paleozoic stratigraphic section, extending into Precambrian "basement." These faults are localized along the up-dip extent of a wedge-shaped Precambrian sequence, which may have provided a "zone of weakness" that facilitated deformation and/or reactivation. Both the fault zone and the Precambrian seismic sequence spatially correlate to a north-south elongated zone of thickened Mt. Simon Sandstone, interpreted as a possible zone of early Paleozoic rifting. This correspondence of Paleozoic faulting, thickened early Paleozoic sedimentation, and Precambrian sequences is tentatively interpreted as evidence of a rifting event associated with the break-up of the Supercontinent of Rodinia that was influenced by pre-existing Precambrian structure.