MEASURING CARBON ISOTOPIC VARIATIONS AND THEIR RELATIONSHIP TO DOLOMITIZATION IN THE JEFFERSON FORMATION (LATE DEVONIAN)

The Jefferson Formation (Late Devonian) of western Montana is an approximately 170 m thick package of carbonates that represents cyclic deposition along a shallow carbonate platform during the Frasnian (382.7 – 372.2 Ma). Variations and patterns in the extent of dolomitization have long been recognized in the Jefferson Formation but no recent studies have explored the relationship between dolomitization, cyclicity, depositional environment, and recorded stable carbon isotopic values. Here, we test two hypotheses: 1) that dolomitization and carbon isotopic alteration increase to the west in more basinward sections and 2) that dolomitization and carbon isotopic alteration increase toward cycle tops. We test these hypotheses by focusing on four sections in western Montana: Baker Mountain (easternmost), Sacagawea Peak, Cottonwood Canyon Road, and Gibson Reservoir (westernmost). We present 475 new carbon isotopic values paired with detailed sedimentological and petrographic observations. Carbon isotopic values range from -4.44‰ to 3.68‰ with an average of 0.36‰ and fall within the range of normal values for the Late Devonian. We found that carbon isotopic values from the study sections are statistically distinct. These initial results are consistent with hypothesis one, meriting further exploration. The relationship between dolomitization, carbon isotopes, and cyclicity is more complex than that indicated in hypothesis two.