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Microfossil Record of the Paleoenvironment of the Late Cretaceous Niobrara Formation, Western Interior U.S.


The Upper Cretaceous Niobrara Formation of the western U.S. has been the target of oil and gas drilling for decades, but recent technological improvements have spurred an exponential growth in exploration and production. The Niobrara extends from NM to MT, and NE to CO (with equivalent strata further west, e.g., Mancos Shale), and is quite variable in terms of chalk-marlstone development and total organic content. The Niobrara was deposited during a time of generally enhanced organic matter production and preservation associated with Oceanic Anoxic Event 3 (OAE3). Reconstructing local variations in oceanographic conditions can help explain and predict variations in organic content across the Western Interior Sea (WIS). Understanding the conditions that lead to the development of anoxic conditions in the WIS also has broad implications for processes that drive the formation of anoxia in restricted seaways around the world. We present data on foraminiferal paleoecology and biostratigraphy from a transect of sites across the central part of the seaway in CO, KS, and NM to quantify paleoenvironmental variability across the Niobrara Formation and understand how circulation or runoff/precipitation changes that may have lead increased organic matter preservation in the WIS. We also compare the development of OAE3 in the Western Interior to the development of the Cenomanian-Turonian OAE2 (which is related to, for example, the Eagle Ford Shale) to use the differences and similarities between the two to understand the underlying mechanisms that drive organic carbon preservation in the Western Interior. The lithology of the Niobrara Formation varies strongly both east-west and north-south, with the purest chalks deposited in a belt from west TX to central KS. Foraminiferal paleoecology varies greatly across the Niobrara as well. Chalk units in the Fort Hays Limestone Member have a higher percentage and diversity of benthic foraminifera than do the thin interbedded dark gray shales; the chalks also have a higher percentage of biserial planktic foraminifera, a generalist group that typically dominates low-diversity assemblages, suggesting high productivity conditions. The OAE3 interval of the Smoky Hill Chalk Member is generally dominated by a low diversity planktic assemblage with very few to no benthic foraminifera. This is coincident with increased organic matter content, and suggests dysoxic to anoxic conditions on the seafloor created by increased stratification.