A "NEW" PRECAMBRIAN GEOLOGIC PROVINCE BENEATH THE ILLINOIS BASIN, USA

Surprisingly little is known of the deeply buried Precambrian rocks of the Illinois Basin, despite the fact that it is one of the world’s most intensively studied intracratonic basins. Since the discovery of petroleum here in 1886, the Illinois Basin has produced over 4 billion barrels of oil and an estimated 4 trillion cubic feet of associated dissolved natural gas. Production shot up rapidly beginning in 1937 with the use of seismic reflection. Until now, little of the resulting reflection data have been available to researchers. A spectacular first-order result from released and reprocessed seismic profiles is that the reflectivity of Precambrian upper crust is richly coherent and widespread. Long regional profiles reveal vertically stacked, broad basinal “seismic stratigraphic” sequences beneath the Paleozoic basin. The internal structure of the sequences is marked by dipping and offset reflectors, and by extensive apparent angular unconformities, all of which give the impression of a sedimentary (or volcani-clastic) succession. The regional structure of the sequences is well developed with discrete boundaries along which the sequences “pinch out” beneath the base of the basin’s Paleozoic sediments. For many years we have known that an areally extensive granite-rhyolite terrane (~ 1.5 Ga) lies beneath much of the Paleozoic strata of the basin based on a few scattered drill holes that just penetrate the top of the Precambrian. This terrane, however, is thought to only represent a thin veneer (a “few kilometers thick”) or isolated igneous intrusions—what lies beneath, or within, the granite-rhyolite rocks has remained a mystery but might correspond, at least in part, to the sub-basin reflection sequences. The newly mapped “basement” reflectivity could represent extensive remnants of Proterozoic continental rifting that have not yet been drilled. This reflectivity therefore indicates a new geologic province beneath the Illinois Basin whose economic potential remains to be tested. Perhaps we are seeing an older analogy of the Keweenawan rift-related volcanic and sedimentary rocks deposited during Proterozoic rifting elsewhere in the central USA.