ABSTRACT: Sequence Stratigraphy of the Lower Pope Group (Chesterian) in Illinois

WHITAKER, STEPHEN T., Illinois State Geological Survey, Champaign, IL

Application of sequence stratigraphy to the lower part of the Pope Group (Chesterian) indicates that many stratigraphic units represent tongues of siliciclastics and carbonates that prograded into the basin or shifted shoreward depending on eustatic sea level fluctuations. Subdivision of these units into parasequences and sequences makes interpretation of the continuous transition of facies represented by the rock units easier than by using conventional correlations. The evolution of the Illinois basin during Chesterian time can then be reconstructed with a relatively high degree of predictability.

The basin was dominated by carbonate deposition through most of the early Mississippian. Siliciclastic influx began relatively abruptly with the progradation of the Rosiclare and Aux Vases sandstones from the north and northwest. During the remainder of the Mississippian, sandstones, shales, and limestones were deposited in a cyclic pattern that may have been due to glacio-eustatic sea level fluctuations analogous to those that occurred during the Pleistocene. Because of the extremely low depositional slopes in the intracratonic Illinois basin, these minor sea level fluctuations caused dramatic shifts in environments of deposition.

Many hydrocarbon plays in the Illinois basin have been overlooked because of limited understanding of basin stratigraphy. Hydrocarbon migration and entrapment was dependent on the geometries of the intertonguing strata. Predictions of new play types and traps can emerge from studies such as this and enable improved exploration strategies for the basin.

AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)