Measuring Carbon Isotope Variations and Their Relationship to Dolomitization in the Devonian Jefferson Formation (2025)

The Jefferson Formation in western Montana is composed of layered dolomite that formed in repeating shallowing cycles on a shallow platform during the Late Devonian. Past studies proposed hypotheses about how dolomitization influenced the chemistry of the rock. This study tests two of these hypotheses: first, that dolomitization and geochemical alteration increase to the west; and second, that alteration increases near the tops of the depositional cycles. To test these ideas, we studied four sites across western Montana, combining observations of rock features with carbon isotopic measurements. Initial results do not support the first hypothesis—rocks farther west do not record different carbon isotopic signals than those in the east. The second hypothesis about changes at cycle tops is also not supported by our results.