Carbon isotope stratigraphy and correlation of plant megafossil localities in the Hell Creek Formation of eastern Montana, USA

Questions of biotic and environmental change during deposition of the Upper Maastrichtian Hell Creek Formation require a robust and replicable system for intra-formational correlation of fossil localities. In this paper, we present a carbon isotope chemostratigraphic curve based on terrestrial organic carbon. Data were taken from a complete measured section spanning the full 93 m of the Hell Creek Formation at our study site. Sedimentary beds were described at the centimeter scale, and samples for carbon isotope analysis were taken at approximately 10 cm intervals. Each sedimentary bed was analyzed in thin section, and grain-size data were assembled based on petrographic point counts. The well-documented Cretaceous-Paleogene boundary negative carbon isotope excursion, six negative carbon isotope excursions, and four tentative positive carbon isotope excursions provide chronostratigraphic tie points within the Hell Creek Formation. We used this curve to precisely correlate 12 additional fossil-bearing localities from throughout the Hell Creek Formation across its type area. These correlations revealed significant local variation in sediment accumulation rates, confirming that simple stratigraphic position relative to the diachronous base and top of the Hell Creek Formation introduces significant error in correlation.