Depositional Aspects of Golconda Group (Chesterian) Oolite Bodies, Southwestern Illinois Basin

Clayton D. Harris, George H. Fraunfelter

The Golconda Group (Chesterian), southwestern Illinois basin, comprises three formations: Beech Creek Limestone, Fraileys Shale (Big Clifty Sandstone Member), and Haney Limestone. The Golconda was deposited during the early to middle Chesterian, a time of transition in the Mississippian from when conditions favored ooid formation (Valmeyeran) to when conditions were unfavorable (late Chesterian). As a result, the Golconda Group is one of the youngest Chesterian carbonate units in the Illinois basin that contains abundant ooids.

Lateral variations in environmental conditions throughout the duration of Golconda deposition resulted in considerable variability in facies type, thickness, continuity, and distribution, especially in oolitic zones. Conditions appear to have favored ooid formation in the western and southwestern areas of the basin, as evidenced by the abundance of oolitic bodies there. Along the southern and eastern margins of the basin, the Golconda Group interval also contains a considerable volume of oolitic limestone. Elsewhere, oolitic deposits are more sporadic and thin.

The presence of a positive area southwest of the Illinois basin induced conditions appropriate to ooid formation during Golconda Group deposition. Limited siliciclastic input from this feature along with the remoteness of this area from the primary source of siliciclastics for the basin also were important factors. Evidence of storm influence (e.g., hummocky beds and intraclasts) is preserved in several oolitic bodies in southwestern Illinois.

AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.