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Evolution of the Mississippian Carbonate Platform in the Illinois Basin As Seen From the Eastern Shelf in South-Central Indiana


Four major carbonate platforms formed across eastern North America during the Paleozoic in the Late Cambrian to Early Ordovician, Late Ordovician, Silurian to Middle Devonian, and Middle Mississippian. These platforms formed at times of global high sea level and warm climate associated with periods of quiescence between plate collisions along the eastern margin of North America. An extensive coring program in Mississippian rocks in south-central Indiana provides a robust data set to examine the evolution of the youngest Paleozoic carbonate platform.

The Mississippian carbonate sequence begins with the Ramp Creek (Muldraugh) Formation, a complex unit deposited on top of the waning Borden Group delta. As sea level lowered, a shallow carbonate platform dominated by skeletal grainstone shoals of the Harrodsburg (Ullin) and Salem Limestones developed. The lower St. Louis Limestone on top of the Salem was then deposited in very shallow to supratidal conditions, producing carbonate mudstones lacking normal marine fossils and local evaporite deposits and numerous subaerial exposure horizons. A marine flooding event in the middle of the St. Louis correlates with a worldwide sea-level rise. The upper St. Louis is a shallow normal marine limestone overlain by the shallow water deposition of ooid grainstone shoals of the Ste. Genevieve Limestone. Within the Ste. Genevieve another flooding event occurred, which may also correlate with a worldwide sea-level rise. The Mississippian carbonate platform ended with subaerial exposure on top of the Paoli Limestone. Many of these carbonate rocks contain significant petroleum reservoirs in the subsurface of the Illinois Basin.