APPLICATION OF CARBON ISOTOPIC STRATIGRAPHY OF THE MIDDLE TO UPPER DEVONIAN (GIVETIAN-FRASNIAN) CEDAR VALLEY GROUP IN THE IOWA BASIN – NEW DATA FROM A CORE IN NORTH-CENTRAL IOWA

The Middle Devonian age Cedar Valley Group was deposited on the marine epeiric carbonate platform of the Iowa Basin. These carbonates recorded several transgressive-regressive cycles throughout the Givetian (upper Middle Devonian). Four major T-R cycles within the Cedar Valley Group are well-documented. In this study, undergraduate students from the Biogeochemical Evolution of The Atmosphere (BETA) Project at the University of Northern Iowa sampled successions of the Cedar Valley Group, including the Peterson Core (W-11749) for d¹³C_carb. The Peterson Core was drilled in 1960 into a known dome structure located in northeastern Webster County, Iowa to investigate its potential for natural gas storage. The 2,185 feet deep core contains approximately 580 feet of Devonian strata, has been lithostratigraphically described and biostratigraphically constrained in previous studies, and is housed at the Iowa Geological Survey’s Core Facility. Samples from the Gizzard Creek and Iowa City members of the Coralville Formation and the Osage Springs and Idlewild members of the Lithograph City Formation were drilled and analyzed for d¹³C_carb. Our preliminary results indicate that the d¹³C_carb document a general decrease from -2.7‰ to -4.7‰ within the Coralville Formation and an increase from -2.7‰ to -0.6‰ in the Lithograph City Formation. The approximately 2 ‰ increase between the formations is consistent with the unconformable contact at the sequence boundary. Future work will correlate d¹³C_carb trends in core and quarry sections from the Cedar Valley Group across north-central and northeastern Iowa, with a focus on the presence of minor unconformities occurring within single T-R cycles and provide additional characterization of sea level change within the Devonian Iowa Basin.