Mississippian (Chesterian) brachiopods of the Illinois Basin: A paleoecologic and paleoenvironmental analysis of the Clore Formation in the Illinois Basin

During the late Mississippian (Chesterian Series, Elviran Stage) most of southern Illinois was covered by an epeiric sea. During this time carbonates were being formed on a shelf environment and siliciclastic sediment was transported from highlands in the northeast by ancient river systems. As the basin subsided from sediment loading, the shoreline and deltaic environments migrated in and out of the Illinois Basin, resulting in the cyclic deposition of sandstone and limestone units (Schuberth 1986, Nelson 1995). The focus of this study is the Clore Formation (Chesterian) throughout the southern part of the Illinois Basin. The Clore Formation is comprised of three members: the Ford Station, the Tygett and the Cora. Brachiopod specimens were collected at seventeen localities across the southern portion of the Illinois Basin. Seventeen genera of brachiopod were identified in the Clore Formation, notably Diaphragmus, Composita and Anthracospirifer. Statistical analysis using PRIMER v5.0, resulted in three water depth controlled brachiopod biofacies: the Diaphragmus biofacies, the Composita biofacies and the Composita-Anthracospirifer biofacies. These biofacies are gradational and represent small changes in water depth in a shallow, low-energy epeiric platform environment. The Tygett Member represents a period of delta lobe migration during this time that temporarily halted carbonate production.