CARBON ISOTOPES AND SEA LEVEL HISTORY OF THE MADISON GROUP CARBONATES FROM MILLIGAN CANYON, MONTANA

Carbon isotopic trends are routinely interpreted to reflect major perturbations in the global carbon cycle. However, δ¹³C values from carbonates record a combination of local, regional, and global processes. This is especially true of the shallow epicontinental carbonate platforms and ramps that dominated the Paleozoic. In these settings, local and regional environmental influences (e.g. relative base level, proximity to shore, carbonate mineralogy) are often superimposed on global trends. As these shallow carbonate settings represent a significant portion of the available carbon isotopic record, it is important to understand the relationship between local processes and recorded δ¹³C values. To achieve this, we document the relationship between sea level and carbon isotopes in Early Mississippian carbonates from Milligan Canyon, Montana. The Madison Group carbonates are ideal for this study because they record significant lithological changes thought to be related to fluctuations in eustatic sea level. Additionally, they span the Kinderhookian-Osagean boundary, an interval for which there is a globally recognized positive carbon isotope excursion. This study allows us to examine the relationship between depositional environments, sea level, and carbon cycling.

We measured and described 107 m of exposure of the Lodgepole Formation and applied a sequence stratigraphic framework. We identified at least 9 parasequences and a sequence boundary at the top of the formation marked by a 2-m-thick breccia. A preliminary set of bulk carbon isotopic analyses were performed on 25 samples. Measured δ¹³C values range from 1.9‰ to 5.4‰ with and average value of 3.7‰. We found no evidence of the globally recognized Kinderhookian-Osagean positive carbon isotopic excursion. Instead carbon isotopic values decrease by ~2.5‰ in the lower 40 m of section. This decrease occurs across a single parasequence. These results suggest that local carbon cycling, likely influenced by sea level change, played a significant role in recorded δ¹³C values at Milligan Canyon. Future research will focus on extending the carbon isotopic record into the Mission Canyon Formation and producing a high resolution carbon isotopic record to document any systematic relationships between lithofacies and δ¹³C values.