BEAR, GLENN W. and RUDMAN, ALBERT J., Department of Geological Sciences, Indiana University, Bloomington, IN; RUPP, JOHN A., Indiana Geological Survey, Bloomington, IN; and STEFFENSEN, CARL, Vastar Resources, Inc., Houston TX
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 asymmetric 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.
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