3D VISUALIZATION OF DEPOSITIONAL SEQUENCES BENEATH ILLINOIS BASIN PALEOZOIC: CLUES TO ORIGIN OF PROTEROZOIC CRUST IN USA MID-CONTINENT

Although the Illinois Basin is one of the world’s most-studied sedimentary basins, the composition and origins of its underlying Proterozoic crust remain poorly understood. Numerous geophysical studies have helped to constrain interpretations of the “basement” below the Paleozoic section; however, high-resolution 3D images have not been available. Newly acquired 3D seismic reflection data from the central Illinois Basin provide the first detailed view of internal basement reflectivity beneath the basin. Interpretation of the seismic data is constrained by three deep drillholes in the data volume that penetrate the Paleozoic section and extend into presumed shallow basement rocks, which are found to be rhyolitic in composition and texture. Using state-of-the-art seismic mapping and visualization, we have delineated a seismic stratigraphic sequence within “basement” (i.e., significantly beneath the base of the Paleozoic sedimentary section) that shows clear depositional features such as, onlap, pinch-outs and angular unconformities. Isochron (i.e., isochore, in traveltime) maps constructed from the seismic data reveal a wedge-shaped package, similar to those mapped in sedimentary depositional settings. The relatively steep dip of the base reflector (~15°, assuming a simple conversion velocity of 6 km/s) contrasts with the nearly flat base of the overlying Paleozoic section. In order to better visualize the internal and boundary surfaces of the sequence, we have applied seismic attribute analysis such as dip azimuth (emphasizes subtle structure trends) and cosine of phase angle (emphasizes bedding). The base of the depositional package ranges from about 1000 to 1800 meters (for 6 km/s) beneath the base of the Paleozoic section (locally, Mt. Simon Sandstone). The structure of the base reflector is complex and consists of more or less north-dipping reflector with a pronounced ridge and cut by significant faults. In the absence of direct drill-hole confirmation for the interior of the sequence, we entertain three possible explanations for the depositional package: a previously unknown sedimentary succession bordered or surrounded by igneous rocks; volcani-clastic material associated with rhyolitic volcanism; a sequence of basaltic sills intruded into a granitic composition Proterozoic basement.