THE EARLY MISSISSIPPIAN δ13C & δ18O RECORD IN SOUTHWEST MISSOURI AND IMPLICATIONS FOR REGIONAL TECTONISM

Carbon and oxygen isotope values were measured from the Kinderhookian-Osagean Stage boundary (early Mississippian) in southwest Missouri. Forty bulk carbonate samples were collected at 0.3 meter intervals through a section exposed along U.S. Highway 65 at the Chestnut Ridge roadcut, ~30 miles south of Springfield, MO. The Chestnut Ridge section has been interpreted to represent a relatively stable portion of the shelf during the Mississippian. A second set of 40 samples was collected through a section exposed along U.S. Highway 71 near Jane, MO. The Jane section has been interpreted to represent the shelf margin. Stratigraphic units exposed at these sections include the Bachelor, Compton, Northview, Pierson, and Reeds Spring Formations.

The goal of this project is to establish a high resolution carbon isotope stratigraphy for the early Mississippian throughout southwest Missouri in order to better understand the region’s tectonic history. Powdered micrite milled from each sample was analyzed by GC-IRMS at the University of Missouri and show a comparable positive δ¹³C excursion to previously measured sections farther west, while recording a significantly reduced magnitude. Lower Mississippian sections in Nevada, Utah, Iowa, and Belgium record a positive δ¹³C excursion of 6-8‰ across the K-O boundary. In southwest Missouri the δ¹³C excursion has a peak magnitude of approximately 4‰. When combined with stratigraphic features such as low-angle truncation surfaces, fault-aligned allocthonous mounds, and significant regional variations in formational thickness (such as the locally quite thin Northview Formation), these characteristics suggest an incomplete chemostratigraphic record in the region even though it lacks an obvious erosional hiatus. One explanation is that the size of the δ¹³C excursion in SW Missouri reflects the removal or non-deposition of sediment at the K-O boundary due to the onset of tectonism during the Ouachita orogeny, indicating an earlier initiation than previously thought. An alternative explanation would be a destabilization of the shelf margin due to glacioeustatic sea level drop. In order to distinguish between these two processes conodont samples were collected to establish a more reliable δ¹⁸O curve, which will help discriminate between the two possible underlying processes.