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New Regional and 3-D Seismic Data Give Insights Into Deep Structure of the Illinois Basin


The intersection of regional seismic profiles with a high-resolution 3D seismic volume in the central Illinois Basin provides a unique opportunity to expand our understanding of deep Paleozoic and Precambrian structure related to basin development. The new data, acquired in Decatur, Illinois and extending east and west across the basin, allow the northward continuation of geophysical observations from the better-studied southern portion of the Illinois Basin. The geophysical interpretation of both profiles and volume data was guided by application of a suite of seismic attribute calculations, including reflection heterogeneity and cosine of phase angle. Geological interpretation is constrained locally by new deep drill-hole data, which reveal a rhyolitic Precambrian “basement.” The seismic data volume provides a detailed 3D view of internal basement reflectivity beneath the basin (i.e., below the base of the Cambrian Mt. Simon Sandstone), expressed locally as a very strong dipping reflector. This strong reflector could be evidence of volcani-clastic material associated with rhyolitic volcanism; or part of a sequence of basaltic sills intruded into a known granitic-composition basement, as hypothesized from geophysical data further south in the Illinois Basin. The basement reflector may be overlain by a possible “wedge” of relatively horizontal reflectivity, interpretable as a “seismic stratigraphic” sequence, similar to Precambrian sequences mapped further south in the basin; however, we also investigate the possible influence of complex-path multiple reflections generated in the lower Paleozoic portion of the seismic data volume. Lower Paleozoic and upper basement structure can also be well-resolved on the new regional 2D seismic data. For example, possible syn-depositional faults appear to affect the lower part of the Paleozoic stratigraphic section, possibly extending into Precambrian basement. Both, these faults and the Precambrian reflectivity, spatially correlate to a regional north-south elongated zone of thickened Mt. Simon Sandstone. The thickened zone of Mt. Simon can be interpreted as a zone of early Paleozoic rifting, possibly associated with the break-up of the Supercontinent of Rodinia.