THE ENTERPRISE SEQUENCE: EVIDENCE FOR A THICK SEDIMENTARY SUCCESSION BELOW THE LATE CAMBRIAN MT. SIMON SANDSTONE IN THE ILLINOIS BASIN?

The Enterprise sequence, interpreted from regional seismic reflection profiles from the central part of the Illinois Basin, is a succession of stacked seismic facies that occur immediately beneath the Late Cambrian Mt. Simon Sandstone and overlie a more diffractive and less well-layered reflectivity section presumed to be part of the Proterozoic eastern granite-rhyolite province. The seismic facies, which are situated in east-central Illinois and west-central Indiana, are bounded by strong, laterally continuous reflectors that are mappable over distances in excess of 200 km and are expressed as broad "basinal" packages that become areally more restricted with depth. The profiles indicate the presence of at least four highly coherent, layered, unconformity-bounded sequences that could total 3750 m in thickness (as estimated from the observed 1.5 s traveltime interval and assuming 5000 m/s rock velocity). The geometry of reflections in many places strongly suggests the presence of stratiform unconformity-bounded deposits. The types of stratal discordance and lateral terminations observed include onlap, downlap, toplap, and truncation like that observed for known seismic sequences of clastic sedimentary rocks, although somewhat similar forms have also been observed in layered volcanic rocks. The reflection terminations and configurations bear a strong resemblance to the prograding clinoforms typically imaged along many passive continental margins. Iso-traveltime "structure" and "isopach" maps of the Enterprise show that its overall thickness and distribution mimics closely that of the overlying Mt. Simon (Late Cambrian) clastic sediments and indicates that an episode of localized subsidence was underway before deposition of the post-Cambrian Illinois Basin stratigraphic succession, which is centered farther south over the New Madrid rift system. Although drill holes into “basement” rocks in and around the basin have encountered Proterozoic granite and rhyolite, the Enterprise sequence itself has not been tested and should be considered for future deep exploration for hydrocarbons.