SEDIMENTOLOGY AND CARBON ISOTOPES OF THE JEFFERSON FORMATION (LATE DEVONIAN), BRIDGER MOUNTAINS, MONTANA

The Devonian Period (419.2-358.9 million years ago) was an interval of significant change in the Earth’s history with the proliferation of land plants and a shift to icehouse conditions. Many of these global changes impacted the carbon cycle. One primary method for identifying perturbations to the global carbon cycle is by using carbon isotopes. However, these records can potentially be influenced by local environmental factors and/or altered by diagenetic processes. In Wyoming and Montana, the Jefferson Formation (Frasnian) was significantly dolomitized. The goal of this study is to better understand the relationship between dolomitization, depositional environments, and carbon isotopes in the Jefferson Formation. These results will have implications for our ability to distinguish between primary carbon isotopic trends and those that have been influenced by dolomitization.

To accomplish this task, we focused on the Jefferson Formation exposed in the Bridger Mountains of Montana. We measured and described 185.1 meters of rock exposed on the flank of Sacagawea Peak. Samples were collected for thin sections, polished slabs, and geochemical analysis. Based on these observations, we have identified seven facies associations ranging from supratidal to basinal. We present 189 new carbon isotopic values that range from -4.44‰ to 3.68‰ with an average -0.12‰. These values are similar to those seen in other basins for the Late Devonian. Values generally decrease at the base of the section (0-20 m), followed by an ~2‰ increase in average values from 35 – 75 m in the section. There are no discernible trends and significant scatter from 75-115 m. Values show much less scatter near the top of the section (115-185.1 m) and record an ~1‰ decrease. With these identifiable trends, we explore the relationship between carbon isotopes, facies associations, and dolomitization.