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Silurian of Illinois Basin--A Carbonate Ramp

Gary W. Coburn

The Silurian of the Illinois basin has classically been defined as a shelf-basin sequence. According to the shelf-basin model, the Illinois basin is a deep-water basin in the extreme southern part (southern Illinois-Tennessee), with a slope in the south (Illinois-Indiana) and a shelf extending from central Illinois and Indiana northeast to the Michigan basin. Reef buildups are in a continuous trend along the shelf break.

However, I propose that the Silurian of the Illinois basin represents a carbonate ramp. The down-ramp position is located in southern Illinois and grades into deeper water environments south of Illinois. In this environment, reef buildups would form in the late Alexandrian of early St. Clair, and would begin in the down-ramp position. Therefore, using the new model, reef buildups are expected throughout the basin, rather than being confined to an imaginary shelf break. This model would facilitate exploration in southern Illinois, Indiana, and western Kentucky for reefal hydrocarbon deposits.

A ramp model is indicated for the Illinois basin because: (1) the basin lacks a shelf-slope break; (2) the facies sequence is compatible with a ramp environment and incompatible with a shelf-slope environment; (3) discontinuous reef trends are typical of a ramp environment; and (4) facies changes and slope are gradual, extending over hundreds of miles as expected in a ramp environment.

Modern carbonate models border on ocean basins. However, the Illinois basin is a cratonic basin, which may have affected the depositional environments. How much that environment differed from present-day models is unknown.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.