HIGHSTAND STORM DOMINATED MIXED SILICICLASTIC-CARBONATE SYSTEM ACROSS A RAMP PLATFORM, THE CAMBRIAN GALESVILLE-IRONTON SUCCESSION IN THE ILLINOIS BASIN

The Cambrian Galesville and Ironton Sandstones constitute the uppermost part of Sauk II subsequence and consist mainly of quartz sandstone. This study focuses on facies variability and stratigraphic architecture of the succession deposited across a ramp profile in a storm dominated mixed siliciclastic-carbonate system. Galesville and Ironton Sandstones are over 200 feet (61 m) thick in northern Illinois but their thickness decreases toward the south. In south-central Illinois, they grade into a dominantly carbonate lithofacies in the upper part of Eau Claire/Bonneterre Formation. Galesville primarily consists of white, very porous, and fine-grained mature quartzarenite and conformably overlies Eau Claire Formation. The overlying Ironton Sandstone consists of white, fine to coarse-grained, porous quartzarenite interbedded with dense dolomitic sandstone or sandy dolomite and overlies Galesville with a gradational contact. The upper contact of Galesville-Ironton succession and their coeval carbonate deposits with glauconitic sandstone of the Franconia Formation is unconformable.

Galesville and Ironton Sandstones encompass numerous erosive-based storm deposits, which may include sandy dolomite containing relicts of ooids and bioclasts or dolomitic sandstone showing various percentage of dolomite matrix. Exceptional maturity and coarsening-upward cycles within the sandstone intervals suggest deposition in a shoreface setting. The Galesville and Ironton Sandstones and their basinward carbonate equivalent, the upper part of Eau Claire/Bonneterre Formation, constitute the highstand package of a depositional sequence. Intertonguing relationship with the upper part of Eau Claire/Bonneterre carbonates and intercalations of sandy dolomite/dolomitic sandstone storm beds suggest deposition in a storm dominated mixed siliciclastic-carbonate depositional system. The mixed system developed north of a vast carbonate ramp platform in the northwest margin of the Reelfoot rift during sea level highstand when terrigenous sedimentation was confined to the northern part of the basin.