Abstract: Precambrian/Cambrian Rifting in the Southern Illinois Basin: A Potential New Hydrocarbon Province

BEAR, GLENN W., ALBERT J. RUDMAN, JOHN A. RUPP, and CARL STEFFENSEN

Two pre-Mount Simon Sandstone (Cambrian) rift-fill units have been mapped underlying portions of Illinois, Indiana and Kentucky using well and geophysical data. The lower unit, here referred to as the Centralia sequence, is layered and underlies most of the Illinois Basin. Layering may represent Proterozoic sedimentary rocks, perhaps interbedded with volcanic and/or intrusive igneous rocks, in excess of 20,000 feet thick.

In south-central Indiana and northern Kentucky, a pronounced east-dipping angular unconformity separates deformed Centralia rocks from the overlying undeformned rocks of the upper unit. Because the unconformity is tilted and truncates underlying reflectors, it probably postdates the Grenville orogeny and may represent the base of an asymetric late-Precambrian-early Cambrian rift. Potential fields indicate the upper unit is less dense and less magnetic than the underlying Centralia. This interpretation, in combination with half-graben asymmetry and a stratigraphic architecture imaged on seismic data, indicates that the upper unit is composed of sedimentary rocks onlapping the east-dipping unconformity. This unit was probably deposited following post-Grenville erosion during rifting that culminated approximately 600 MYBP, and may correlate with interbedded clastics, limestones, and shales encountered in deep tests of the Rough Creek Graben. Suubsequent lower Paleozoic cratonic stability ensued, leading to the deposition of the typical Illinois Basin stratigraphic sequence. Late Precambrian-early Cambrian sediments in the Illinois Basin could have reached the oil generation window in early Cambrian times. The presence of multiple unexplored superimposed basins and the identification of world-class stratigraphic and structural traps provides and enticing new target for hydrocarbon exploration.

AAPG Search and Discovery Article #90939©1997 AAPG Eastern Section and TSOP, Lexington, Kentucky