McBRIDE, JOHN H. , Illinois State Geological Survey, University of Illinois, at Urbana-Champaign, 615 East Peabody Drive, Champaign, IL 61820

Abstract: Precambrian "Seismic Stratigraphy" Beneath the Illinois Basin

Reprocessing of regional seismic reflection profiles reveals multiple Precambrian "stratigraphic" sequences beneath the Paleozoic Illinois Basin. Three-dimensional mapping of this reflectivity using an open network of profiles indicates a broad basinal sequence ("Centralia sequence") that reaches a maximum depth of about 10 km and a thickness of 6 km (from generalized travel time-to-depth conversion). The internal structure of this sequence is marked by dipping and offset reflectors, and widespread apparent unconformities. The regional structure of the sequence is well developed with discrete western and southern boundaries along which the sequence pinches out beneath the base of the Paleozoic section. Two additional Precambrian sequences are present beneath the Centralia sequence and both appear to be areally more restricted. The upper poorly reflective sequence (maximum depth, about 17 km) is underscored by an extremely bright reflection; a second reflective package, structurally beneath the former, is marked by abrupt offsets that may be related to an ancient syn-rift sequence. At this stage of our understanding, more than one hypothesis is possible for the origin of the Precambrian reflectivity, including basaltic flows or sills interlayered with clastic sediments and/or emplaced within the basement granite-rhyolite terrane. These sequences may represent older Keweenawan-type riftrelated volcanism and sedimentation, which cut through the northern Midwest and Lake Superior. 

AAPG Search and Discovery Article #90926©1999 AAPG Eastern Section Meeting, Indianapolis, Indiana