DEEP WATER PARASEQUENCES AND CARBON ISOTOPES IN THE LODGEPOLE FORMATION OF SOUTHERN MONTANA

The Early Mississippian Lodgepole Formation records a series of carbon isotopic excursions driven by changes in sea level within the Madison shelf. These interpretations are based on the idea that carbon isotopic values rise and fall in accordance with sea level, reflecting a combination of organic carbon burial, freshwater input, and terrestrial organic carbon input. Here we examine the relationship between sea level and carbon isotopes at smaller scales of sea level change (e.g. parasequence level) from deeper water settings where we would expect the relationship to be weakest due to distance from the shoreline and mixing with the open ocean. To accomplish this goal, we addressed three research questions: 1) can we identify parasequences in deep water environments of the Madison shelf, 2) can we see systematic differences in average carbon isotopic values with depth, and 3) can we see any consistent carbon isotopic trends within parasequences? To investigate these questions, we focused on three deep water sections from southern Montana: Baker Mountain, Sappington, and Sacagawea Peak. For each section detailed sedimentological descriptions were paired with petrographic work to assign facies associations and develop a sequence stratigraphic framework. Paired carbon isotopic analyses allow us to directly tie our geochemical trends to sequence stratigraphic boundaries and therefore changes in relative sea level.

Our results suggest that sea level was not the primary control on carbon isotopic trends in deep water settings in the Madison shelf. We were able to confidently identify several parasequences in all three sections. There is evidence for consistently lower carbon isotopic values in the most proximal study location (Baker Mountain) relative to the most distal study location (Sacagawea Peak). These results are consistent with findings in modern carbonate platforms where near shore environments tend to have 2‰ lower carbon isotopic values than distal settings. Despite this evidence that position relative to shoreline did influence carbon isotopic values, we found no evidence of consistent trends at the parasequence level.