--> Abstract: The Illinois Basin in Cross Section, by J. D. Treworgy, S. T. Whitaker, M. L. Sargent, and M. C. Noger; #91013 (1992).

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ABSTRACT: The Illinois Basin in Cross Section

TREWORGY, JANIS D., STEPHEN T. WHITAKER, and MICHAEL L. SARGENT, Illinois State Geological Survey, Champaign, IL, and MARTIN C. NOGER, Kentucky Geological Survey, Lexington, KY

The structural and stratigraphic framework of the Illinois basin is portrayed in a network of regional cross sections being prepared by the Illinois basin Consortium (Illinois, Indiana, and Kentucky Geological Surveys). The sections consist of wireline logs of the deepest wells and show formation boundaries and gross lithofacies of the entire stratigraphic column as well as possible structural and stratigraphic hydrocarbon plays. Available seismic data, sample and core descriptions, and existing structure, isopach, and facies maps are also being used in preparing the sections. The vertical scale is 1 in. = 400 ft (1:4800); the horizontal scale is 1 in. = 4 mi (1:250,000).

Several stratigraphic and structural relations seen in the cross sections modify previous concepts of the basin's evolution. Recognition of a previously unreported stratigraphic unit below the lower Ordovician Everton suggests that there may have been continuous deposition across the Sauk-Tippecanoe Sequence boundary in the southernmost part of the Illinois basin. Local absence of this unit and significant thinning of the underlying Shakopee Dolomite (Sauk Sequence) in parts of the Rough Creek graben suggests uplift and erosion of horst blocks in that area prior to deposition of the Everton

Formation (Tippecanoe Sequence). Thinning of Mississippian Valmeyeran units along the La Salle anticlinorium suggests that compressive forces began acting there earlier than previously thought.

Many units that have been considered discrete and sequentially stacked in one part of the basin are apparently in facies relation with one another in other areas. For example, the upper part of the Everton Dolomite and the lower part of the St. Peter Sandstone appear to be in facies relation. The same may be true of the overlying Dutchtown Formation, the upper part of which may be in facies relation with the lower part of the overlying Joachim Formation. Relatively pure Silurian carbonate bank deposits of the northern reaches of the basin are shown to be laterally equivalent to shaly carbonate basinal facies. The Mississippian Borden Siltstone is interpreted here as being in facies relation with the Ft. Payne cherty limestone, and likewise the Ft. Payne with the Ullin Limestone. These and other facies relations indicated on the cross sections should stimulate further research.

Potential structural and stratigraphic hydrocarbon plays are apparent in many areas of the basin and at a number of horizons, including several that have received minimal exploration to date. The structural plays include anticlinal and fault entrapments and fracturing of organic-rich shales and of carbonate rocks. The stratigraphic plays include pinchouts of porous siliciclastic and carbonate rocks, sand bars within incised paleovalleys, reefs, onlap and pinchouts of reservoir rocks above unconformities and on paleostructures, and truncation of reservoir rocks below unconformities.

 

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