ABSTRACT: Regional Intertonguing of Lithologic Intervals, Pope (Chester) Group, Illinois Basin

NELSON, W. JOHN, and ROBERT D. COLE, Illinois State Geological Survey, Champaign, IL

Large-scale (basinal) intertonguing of limestones and siliciclastic intervals in the Valmeyeran-Chesterian Pope Group has been recognized for more than 40 years, yet geologists continue to describe and classify these rocks in layer-cake fashion. Siliciclastics derived from multiple sources north of the basin are intercalated with contemporaneous carbonates which formed on a broad, low-gradient, shallow marine shelf Depositional systems previously interpreted as fluvial-deltaic were actually shallow marine, and contain such depositional facies as subtidal shelf sandstones, offshore bars, tidal and interbar channel fills, tidal deltas, tidal flats, and possible lagoonal/estuarine fills. Many of these facies contain body and/or trace fossils with marine affinities, which indicate that li estones and siliciclastics were deposited laterally adjacent to each other in similar depositional environments.

A proposed reclassification of the lower half of the Pope Group is presented in a series of cross-sections. The number of formations is reduced from as many as 14 to six, and many units previously referred to as formations or members are revised to tongues and lentils. The revised formations are the major limestone and clastic intervals that are of uniform lithology in some parts of the basin and split into tongues in others parts. The revised limestone formations are the Ste. Genevieve and the Okaw; the revised siliciclastic formations are the Aux Vases, West Baden, Big Clifty, and Hardinsburg.

This understanding of stratigraphic relationships and depositional processes may have a profound effect on hydrocarbon exploration and the continued development of existing reserves. The Aux Vases Formation and the Bethel and Cypress tongues of the West Baden Formation are among the most significant hydrocarbon producers in the Illinois basin.

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