The Upper Cambrian (Furongian) Galesville Sandstone and the overlying Ironton Sandstone cover the northern half of Illinois and are important aquifer in northern part of the state. They are a part of the Knox Group and constitute the upper part of the Sauk II subsequence. The Galesville and Ironton are over 200 feet thick in northern Illinois but their thickness decreases toward the south where they grade into a dominantly carbonate lithofacies. This study focuses on lithofacies analysis, depositional setting, stratigraphy, and lateral distribution of the succession using available subsurface data. In northern Illinois, the Galesville (up to 100 feet thick) conformably overlies the Eau Claire Formation and underlies, with a gradational contact, the dolomitic Ironton Sandstone. It is a white, very porous, and fine-grained mature quartzose sandstone. The Ironton Sandstone (up to 100 feet thick) is fine to coarse-grained, white, and porous quartzose sandstone that is interbedded with dense dolomitic sandstone or sandy dolomite containing relicts of ooids and bioclasts. Its contact with the overlying glauconitic sandstone of the Franconia Formation appears to be unconformable. Numerous erosive based dolomitic sandstone storm beds composed of quartz sand, shale clasts, and carbonate grains are present in the Ironton Sandstone. Exceptional maturity and coarsening-upward cycles of these units indicate deposition in a shorface setting under fair to storm weather conditions. The upper part of the Eau Claire, the Galesville, and Ironton constitute a depositional sequence in which the gray glauconitic shale and sandstones in the upper parts of Eau Claire represent the transgressive package and the succeeding coarsening upward shoreface deposits of the Galesville-Ironton represent the highstand package. The Ironton and Galesville Sandstones thin southwestward and grade into 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 rose during early Late Cambrian resulting in the development of a vast carbonate platform and confined terrigenous sedimentation to the northern part of the basin.
|Original language||English (US)|
|State||Published - 2016|