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Integrated Sequence Stratigraphic Framework of the Late Cretaceous Western Interior, Denver Basin, CO, USA


Recent hydrocarbon exploration targeting the unconventional Niobrara Formation has renewed interest in Late Cretaceous mudstones of the Western Interior. A robust sequence stratigraphic framework of this mixed siliciclastic-carbonate system from Cenomanian – Campanian time will have the potential to improve exploration strategies and provide new insight into the stratigraphic relationships across two third order sequences. This study builds an integrated sequence stratigraphic framework based on sedimentology, high-resolution carbon and oxygen stable isotope chemostratigraphy, elemental chemostratigraphy, and borehole image analysis of the Greenhorn, Carlile, and Niobrara Formations (GCN) in the Denver Basin, CO, U.S.A. Sedimentological analysis was conducted using core, thin sections, and QEMSCAN to document the primary facies of the GCN. High-resolution (6-inch to 3-foot sample spacing) carbon and oxygen stable isotope measurements were taken from eight cores in the Denver Basin, CO, U.S.A. Previous studies linking carbon isotope chemostratigraphy to biostratigraphy and radiometric age dating provide a robust foundation from which carbon isotope chemostratigraphy can be used as a chronostratigraphic tool in the Western Interior. Borehole image analysis was conducted using an automated bed thickness measurement technique. Results show new high-resolution carbon isotope chemostratigraphic correlations across the Denver Basin, CO and their relationship to systems tracts. Oxygen isotope chemostratigraphy shows variable absolute values, but consistent trends among the eight cores. Chronostratigraphic application of carbon isotope chemostratigraphy shows that lithostratigraphy is consistent with chronostratigraphy in the Denver Basin, CO. Carbon isotope chemostratigraphy is observed to make gradual excursions across complete sections. Excursions that are abrupt are evidence of missing section or depositional hiatus. A relationship between carbon isotope chemostratigraphy and systems tracts can also be seen.