THE INDIAN CREEK MEMBER: A NEW UNIT IN THE MISSISSIPPIAN (CHESTERIAN) STE. GENEVIEVE LIMESTONE, INDIANA

The Middle Mississippian Ste. Genevieve Limestone (Ste. Gen) is a shallow water to non-marine carbonate in the Illinois Basin (IB). Composed of oolitic and skeletal grainstones with a smaller variety of other carbonate and siliciclastic rocks, the Ste. Gen has been of great interest within the IB for its economic potential as an oil and gas reservoir, aggregate, and aquifer. The Indiana Geological and Water Survey (IGWS) recognizes two distinct members of the Ste. Gen in Indiana, the lower Fredonia Member and the upper Karnak Member (Droste and Carpenter, 1990), both representing shallow marine shoaling deposits. With much of the Ste. Gen composed of grain-supported limestones, the presence of a consistent lime mudstone unit ranging from 4 ft (1.2 m) to nearly 40 ft (12 m) in thickness should be apparent yet has historically failed to be recognized for its importance. In recent years, the IWGS has become aware of this mudstone, known informally as the Indian Creek Limestone Beds (IC), as a stratigraphically correlative unit for structure mapping (Keith and Thompson, 2020). While region-wide studies on the IB have broadly characterized this unit, no detailed study on the IC in Indiana has been conducted. This study formally characterizes the IC by using outcrops and cores to describe the lithology and analyzes stable light isotopes (δ13C and δ18O) from core samples. Using the stratigraphy and geochemical analyses to determine the depositional environment and history of the IC, this study plans to recommend the Indian Creek be elevated to Member status in the Ste. Genevieve Limestone.