The Hell Creek Formation (HCF), in the northern Great Plains of the United States, is the most studied source for understanding the changes in the terrestrial biota across the Cretaceous-Paleogene boundary, because preserves an extraordinary record comprised of fossil flora, vertebrates, invertebrates, microfossils, a range of trace fossils, and critical geochemical markers such as multiple iridium anomalies associated with the Chicxulub impact event. The HCF is a fine-grained, fluvially derived, siliciclastic unit, that occupies part of the western Williston Basin, and overlies the Fox Hills Formation (Clemens and Hartman, 2014).
The history of research focused on the Hell Creek Formation and its biota started in October 1901, when William T. Hornaday, director of the New York Zoological Society, travelled to northeastern Montana and discovered three fragments of the nasal horn of a Triceratops in the valley of Hell Creek. He showed the fossils to Henry Fairfield Osborn who decided to include the valley of Hell Creek on the list of areas to be prospected by Barnum Brown the following year.
In July 1902, B. Brown arrived to Hell Creek. His field crew included Dr. Richard Swann Lull, and Phillip Brooks. Brown recounted that after their arrival, he found the partial skeleton that would become the type specimen of Tyrannosaurus rex. In 1904, William H. Utterback, preparator and collector for the Carnegie Museum of Natural History, collected a fragment of a jaw of Tyrannosaurus and two skulls of Triceratops. In the summer of 1906, B. Brown returned to Montana, and a year later he published a complete manuscript about the valley of Hell Creek. The field expeditions of 1908 and 1909 were crowned by the discovery of another skeleton of T-rex. Between 1902 and 1910, Osborn, Brown, and Lull published the analysis of some of the fossil vertebrates discovered in the Hell Creek Formation, including Tyrannosaurus rex, Triceratops, and Ankylosaurus.
Plants are represented by fossil leaves, seeds and cones. Fossil wood is also commonly found in the HCF as permineralized fragments. The Hell Creek macroflora is largely dominated by angiosperms including palms, associated with several ferns, conifers, and single species of cycads and Ginkgo. The study of pollen and spores has played a very important role in the identification of the K/Pg boundary in the HCF. Palynologists were the first scientists to recognize that a major, abrupt change occurred at the end of the Cretaceous. Unlike the Permian-Triassic and Triassic-Jurassic boundaries, the palynologically defined K/Pg boundary is based on the extinction of Cretaceous taxa rather than the appearance of Paleocene taxa. Intimately associated with the K/Pg boundary globally, is the so-called “fern spike”, occurring exclusively at localities where the iridium anomaly is present. (Fastovsky and Bercovici, 2015; Vajda & Bercovici, 2014.)
Fastovsky, D. E., & Bercovici, A., The Hell Creek Formation and its contribution to the CretaceousePaleogene extinction: A short primer, Cretaceous Research (2015), http://dx.doi.org/10.1016/j.cretres.2015.07.007
Clemens, W. A., Jr., & Hartman, J. H. (2014). From Tyrannosaurus rex to asteroid impact: early studies (1901- 1980) of the Hell Creek Formation in its type area. In J. Hartman, K. R. Johnson, & D. J. Nichols (Eds.), Geological society of America special paper: 361. The Hell Creek Formation and the Cretaceous-tertiary boundary in the northern great plains (pp. 217-245).
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Johnson, K. R., Nichols, D. J., & Hartman, J. H. (2002). Hell Creek Formation: A 2001 synthesis. The Hell Creek Formation and the Cretaceous-Tertiary Boundary in the northern Great Plains: Geological Society of America Special Paper, 361, 503-510.