LITHOFACIES AND SEQUENCE FRAMEWORK OF THE SAUK II-III TRANSITION INTERVAL IN THE ILLINOIS BASIN
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.