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