Lithofacies and sequence framework of the Sauk II-III transition interval in the Illinois Basin

The Sauk II-III transition interval in Illinois includes the Galesville and Ironton Sandstones that cover the northern half of the state and grades southward into a dominantly carbonate lithofacies. The Ironton and the underlying Galesville Sandstone are classified in the Cambrian Knox Group and consist of mature sandstone and sandy dolomite. This study focuses on lithofacies analysis, stratigraphy and lateral distribution of the succession using available subsurface data. The Galesville and Ironton Sandstones occur in the Middle-Upper Cambrian boundary interval. They are over 200 feet thick in northern Illinois but their thickness decreases toward the south. These units generally consist of fine- to medium grained, quartz sandstone interlayered with dolomite cemented sandstone or sandy dolomite and can be easily distinguished in the subsurface. The Galesville (up to 100 feet thick) underlies, with a gradational contact, the dolomitic Ironton Sandstone and overlies, with a sharp contact, the Eau Clair Formation. It is a white, porous, and commonly friable, fine-grained, mature quartzose sandstone. The Ironton Sandstone (over 100 feet thick) is fine to coarse-grained, commonly fossiliferous, porous quartzose sandstone that is interbedded with dense dolomitic sandstone or sandy dolomite. It overlies the Galesville Sandstone and underlies, with a sharp contact, the glauconitic sandstone of the Franconia Formation. The Ironton and Galesville Sandstones thin southwestward and grade to dolomite and sandy dolomite of the upper part of the Bonneterre Formation. Deposition of Bonneterre carbonates in the southern part of the Illinois Basin occurred when sea level rise during late Middle Cambrian resulted in the development of a vast carbonate platform and confined terrigenous sedimentation to the northern part of the basin. The platform developed along the northwest margin of the Reelfoot rift and the adjacent craton; the platform margin was facing the deep and rapidly subsiding Reelfoot-Rough Creek rift basin.