SEQUENCE STRATIGRAPHY AND CARBON ISOTOPES FROM BENBOW MINE ROAD IN MONTANA: DID SEA LEVEL CHANGE INFLUENCE CARBON ISOTOPIC TRENDS IN THE MADISON SHELF?

The Early Mississippian Lodgepole Formation records a series of carbon isotopic excursions that have been linked to sea level fluctuations on the Madison shelf. One of these excursions is the ~7‰ globally recognized Tournasian Isotope Carbon excursion (TICE). The possibility that one of the largest carbon isotopic excursions in the Phanerozoic was influenced by organic carbon burial associated with rising sea level has significant implications about the role sea level played in the global carbon cycle and Earth’s climate history. However, the relationship between sea level and carbon isotopic trends in the Madison shelf is not without complications. Perhaps most important is the fact that there is debate about whether there are two or three sequences in the Lodgepole Formation. This discrepancy regarding the identification and placement of sequence stratigraphic surfaces complicates interpretations of the relationship between sea level and carbon isotopes. To address these issues, this study focused on the Benbow Mine Road section in southern Montana. The Benbow Mine Road section is ideal for this study because in previous studies it provides one of the best examples of correlation between carbon isotope and sequence stratigraphic framework, it is also one of the locations where the three-sequence model for the Lodgepole was first proposed, and despite appearing in multiple studies, a detailed stratigraphic analysis has not been conducted since the early 90’s.

We measured and described 141 meters of exposure at Benbow Mine Road. We paired these field observations with targeted petrographic work to assign facies associations and establish the sequence stratigraphic framework. Our results are consistent with a three-sequence model for the Lodgepole Formation. With this framework established we used Spearman Rank Correlation to test for correlation between carbon isotopic trends and sequence stratigraphy. Our results demonstrate that there is no statistically significant relation, suggesting that sea level change was not the primary driver of carbon isotopic trends in the Madison shelf.