292-15 Nonmarine Facies Analysis and Refined Chronostratigraphy for Campanian-Maastrichtian Formations in Central Wyoming and Implications for Paleobiogeography and Biostratigraphy of Laramidia

Abstract:

The Upper Cretaceous of the Western Interior of North America (Laramidia) is among the most fossiliferous and well-studied stratigraphic sequences in the world. During the Campanian and Maastrichtian, Laramidia contained diverse northern and southern biomes, but few studies have examined the transition zone between biotas in central Laramidia. Numerous fossiliferous nonmarine formations occur within Wyoming during this critical period and region. This study examines three questions: 1) What is the depositional system?, 2) What is the age of these units?, and 3) What is the potential for significant paleontological advances? Over 5,000 meters of lithostratigraphic sections were measured in the Mesaverde, Meeteetse, and Lance Formations with new fossil collections made throughout the sequence.

The typical depositional sequence throughout the Upper Cretaceous is offshore marine shale transitioning to deltaic and/or shoreface deposits, followed by paralic or delta plain carbonaceous shale, with coastal plain to fluvial plain channel sandstone and overbank mudstone deposits capping the sequence. Using ammonite biostratigraphy, the middle member of the Mesaverde Formation was constrained to the Baculites reduncus biochron (77.2 – 76.5 Ma). Peak extent of the Meeteetse Formation occurred in the Baculites baculus biochron (72.3 – 71.5 Ma). The Lance Formation likely represents four million years of deposition (66.9 – 66.0 Ma) in the Powder River Basin, based on Baculties clinolobatus occurring near the base with the K/Pg boundary at the top. Vertebrate fossils were relatively common within both the Mesaverde and Lance Formations with numerous new localities discovered. The Campanian and Maastrichtian deposits have significant potential to contain key fossils for advancing paleobiogeographic and stratigraphic studies of Laramidia. In conclusion, Wyoming has significant potential for paleontological and stratigraphic work to improve our understanding of Late Cretaceous ecosystems during the period of peak dinosaurian diversity and leading up to the K/Pg mass extinction.

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