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AAPG Rocky Mountain Section Meeting

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New Perspectives on the End of the Cretaceous from the Southern Western Interior


Among W.A. Cobban’s most enduring legacies are the reconstruction of ancient coastlines for the Upper Cretaceous of the Western Interior and the high-resolution biostratigraphic zonation of marine strata leading up to the end of the Cretaceous. Recent investigations of the Cretaceous-Paleogene (K-Pg) boundary strata of the Raton Basin in northeastern New Mexico and southcentral Colorado add to this wealth of knowledge and alter perspectives on the end of the Cretaceous. Studies of marine and non-marine strata reveal a significant, westward shift in the position of the western shoreline of the Western Interior Seaway (WIS) in northeastern New Mexico and south-central Colorado during the Maastrichtian and indicate that traditional pollen markers for the Campanian-Maastrichtian stage boundary, such as the first appearance datum (FAD) of either Wodehouseia or Kurtzipites, do not mark the boundary effectively. Evidence from land plants suggests the WIS closed to the north in the early late Maastrichtian, whereas evidence from marine invertebrates suggests the WIS remained open to the Gulf of Mexico through the end of the Cretaceous—a perspective supported by non-marine stratigraphy in the type area of the Raton Formation and geochemical analysis of coal in this region— despite repeated reactivation of the Las Animas arch during the Upper Cretaceous. Such a paleogeography could have permitted tsunamis from the Chicxulub impact to propagate to North Dakota. Associations between fern foliage and spores collected from the Raton Basin suggest that the monotaxial global fern-spore spike at the K-Pg boundary may better reflect a nearly decade-long impact winter rather than the general proliferation of ferns in the weeks or months after the impact event, as has traditionally been assumed based on comparison with other “disaster floras.” The millennial timescale for fern dominance in the earliest Danian in the Raton Basin is reminiscent of fern dominance in similar habitats during the Maastrichtian (e.g., Meeteetse flora), and the dominant dicots above and below the K-Pg boundary in the Raton Basin probably left no palynologic record. Although it has been argued that extinction across the K-Pg boundary was stochastic, the mass-extinction event appears to have been selective due to global deforestation at the K-Pg boundary