Date of Graduation
Master of Science in Geography (MS)
Glenn R Sharman
Second Committee Member
Phillip D Hays
Carbon isotope;Chemostratigraphy;Cloverly Formation;Mean Annual Precipitation;Paleoclimate;Pedogenic carbonates
The Early – Late Cretaceous transition in Western North America recorded a period of rapid climatic and tectonic change in Earth’s history. Major climate events associated with large igneous province eruptions caused several instances of ocean anoxic events (OAE) and perturbations to the global carbon (C) – cycle. These perturbations to the global C-cycle are observed in the bulk organic C record of both marine and terrestrial deposits and can be used to correlate units across major depositional basins. Major efforts are being made to generate time-constrained palaeontologic and paleoclimate information from the North American Cordilleran foreland basin and C-isotope chemostratigraphy can aid in making these correlations. This study uses the isotopic composition of bulk organic carbon from the Lower Cretaceous Cloverly Formation to constrain the age of the Cloverly Formation within the Bighorn Basin, Wyoming. The study also investigates paleoclimate proxies such as mean annual precipitation and atmospheric CO2 concentrations. C-isotope chemostratigraphy of bulk organic carbon results range between -21‰ and -30.1‰ and average at -23.97 ‰. Two pedogenic carbonates nodules from the Little Sheep Member (CCC-12 and CCC-24) were analyzed for δ13Corg, δ13Ccarb, and δ18Ocarb. The Little Sheep Mudstone Member carbonate nodules have average δ13Ccarb values of -6.72 ‰ at CCC-12 and -7.25 ‰ at CCC-24. The mudstone organic C isotopic values of the carbonate nodules are -26.27 ‰ for CCC-12, and -25.9 ‰ for CCC-24. The S(z) values (soil CO2 concentration) were estimated to be between 1000 – 3000 ppm, typical of micro-high vertic soils. Atmospheric pCO2 concentrations are calculated using the paleosol CO2 paleobarometer of Ekart et al. (1999) and are reported at S(z) = 2000 ± 1000 ppm. Atmospheric C-isotopic composition of pCO2 is estimated using the δ13C of bulk organic C in mudstones and the relationship between plant matter and pCO2 outlined in Arens et al. (2000). Atmospheric pCO2 values at 4 m above the Pryor Conglomerate, 14.5 m above the Cloverly-Morrison lithologic boundary are 352 ± 176 ppm and increase to 931 ± 465 ppm at 18.5 m. Mean annual precipitation (MAP) was calculated from weathering indices of metal oxides in soils using CALMAG and CIA-K equations. MAP increases up section from a minimum of 697- 984 mm/yr in the LSM to a maximum of 1291-1705 mm/yr in the Upper Himes Member. Given the maximum depositional age of the base of the Little Sheep Mudstone Member (129.4 ± 3.4 Ma) from D’Emic et al. (2019), a maximum depositional age for the lower Himes Member at Crooked Creek of 103.6 ± 1.3 Ma (D’Emic et al., 2019), the maximum depositional age of 112.09 ± 0.34 Ma (Carrano et al., 2021) from other Little Sheep Mudstone outcrops, and the high resolution C-isotope chemostratigraphic trend of the Crooked Creek Section (this study) constrains the positive C-isotope trend and decreasing (negative CIE) trend to the C-10 to C-11 C-isotope excursions associated with the CIE of Bralower et al. (1999) that occurs in the Late Aptian to Early Albian.
Kalu, Q. A. (2023). Chemostratigraphic and Paleoclimatic Studies of Cloverly Formation, Northern Wyoming, U.S.A. Graduate Theses and Dissertations Retrieved from https://scholarworks.uark.edu/etd/4915