Over a century has passed since 1901 when W.T. Hornaday showed a fragment of a horn of Triceratops found in the valley of Hell Creek to H.F. Osborn at the American Museum of Natural History. The following year Osborn's assistant, Barnum Brown, was dispatched to eastern Montana and began investigations of its geology and paleontology. By 1929, Brown had published a geological analysis of the rocks exposed in the southern tributaries of the Missouri River, named the Hell Creek Formation, and published studies of some of the dinosaurs discovered there. Parts of his collections of fossil mollusks, plants, and vertebrates contributed to research by others, particularly members of the U.S. Geological Survey. From 1930 to 1959, fieldwork was slowed by the Great Depression and World War II, but both the continuing search for coal, oil, and gas as well as collections of fossils made during construction of Fort Peck Dam set the stage for later research. Field parties from several museums collected dinosaurian skeletons in the area between 1960 and 1971. In 1962, concentrations of microvertebrates were rediscovered in McCone County by field parties from the University of Minnesota. Ten years later, field parties from the University of California Museum of Paleontology began collecting microvertebrates from exposures in the valley of Hell Creek and its tributaries. The research based on this field research provided detailed geological and paleontological analyses of the Hell Creek Formation and its biota. In turn, these contributed to studies of evolutionary patterns and the processes that produced the changes in the terrestrial biota across the Cretaceous-Paleogene boundary.

In 1907, Barnum Brown named the Hell Creek beds (Formation) for the strata exposed in the Hell Creek Valley and other downstream tributaries of the Missouri River. In the absence of a stratotype section, a lectostratotype is herein proposed for the Hell Creek Formation based on 84.2-m-thick exposures at Flag Butte (local name) in Ried Coulee (archaic use; East Fork of Hell Creek) and East Ried Coulee, tributaries of Hell Creek, Garfield County, Montana (sec. 29, T. 21 N., R. 38 W., NAD27 CONUS; base 47.55931°N, 106.88111°W; top 47.55533°N, 106.86810°W). The formation is underlain with general conformity by sandstone beds of the Fox Hills Formation (as characteristically known, the Colgate Member is absent) and is for the most part conformably overlain locally by the Tullock Member of the Fort Union Formation. The upper contact at Flag Butte is demarcated at the base of the IrZ lignite bed (above an iridium anomaly). The boundary has been demonstrated to be somewhat unconformable in areas to the west. The IrZ bed is also missing at Bug Creek in McCone County. In its type section, the Hell Creek Formation is subdivided (simply and informally) into Ried Coulee (lower Hell Creek), East Ried Coulee (middle Hell Creek), and Flag Butte (upper Hell Creek) units, each containing a sandstone and a mudstone lithofacies. Formational thickness varies with local depositional and erosional history of various coastal-deltaic environments across the Williston Basin and a trend of overall thinning to the east and northeast.

The interval spanning the uppermost Hell Creek Formation to the overlying lowermost Fort Union Formation in north-central Montana encompasses a marked paleoenvironmental change (associated with the formational contact), the Chicxulub impact event, and the Cretaceous-Paleogene boundary. We have examined the record of this transition at the Hell Creek Formation lectostratotype to determine the placement of these events using a series of lithological, geochemical, palynological, and 40Ar/39Ar geochronological analyses. The claystone derived from the Chicxulub impact is identified based on lithological criteria, enrichment of iridium and osmium, and osmium isotope ratios. The impact claystone also contains a Cyathidites fern spike. The first continuous lignite horizon in the section immediately overlies this claystone and represents the Hell Creek–Fort Union formational contact. A tuff ~3 m above the impact layer is dated to 66.024 ± 0.059 Ma. Given this evidence, at the lectostratotype the Cretaceous-Paleogene boundary is coincident with the impact claystone and therefore with the formational contact. Due to poor preservation and apparent reworking of palynomorphs surrounding the formational contact, the Cretaceous-Paleogene boundary is difficult to identify based on biostratigraphically significant taxa. The presence of marine dinoflagellates is suggestive of reworking of older marine sediments during the deposition of the Cretaceous-Paleogene boundary interval.

Magnetostratigraphic evaluation of a well-exposed stratigraphic section in northeastern Montana has been undertaken to expand upon and better understand the timing of the Hell Creek and Fort Union Formations and the biological processes recorded within them. Characteristic remanent magnetizations show clear magnetostratigraphic patterning of chrons C28n, C28r, C29n, C29r, C30n, and possibly C30r. Differentially corrected global positioning system coordinates, including elevation, were recorded at each sample site, allowing the magnetostratigraphic framework to be precisely relocated in the field and traced laterally across the landscape. In this way, important chronologic boundaries such as the Cretaceous-Paleogene boundary can be projected into or above topography of the surrounding region. Localities in Montana that have been sampled for fossil studies were mapped and correlated to the same stratigraphic sections as the magnetostratigraphy, and so they can be compared directly to the geomagnetic polarity time scale. The new magnetostratigraphy can also be used to relate to other basins of Cretaceous and Paleogene age using information independent from biostratigraphic zonation, making it possible to directly compare the composition of coeval faunas from significantly different latitudes.

Questions of biotic and environmental change during deposition of the Upper Maastrichtian Hell Creek Formation require a robust and replicable system for intra-formational correlation of fossil localities. In this paper, we present a carbon isotope chemostratigraphic curve based on terrestrial organic carbon. Data were taken from a complete measured section spanning the full 93 m of the Hell Creek Formation at our study site. Sedimentary beds were described at the centimeter scale, and samples for carbon isotope analysis were taken at ~10 cm intervals. Each sedimentary bed was analyzed in thin section, and grain-size data were assembled based on petrographic point counts. The well-documented Cretaceous-Paleogene boundary negative carbon isotope excursion, six negative carbon isotope excursions, and four tentative positive carbon isotope excursions provide chronostratigraphic tie points within the Hell Creek Formation. We used this curve to precisely correlate 12 additional fossil-bearing localities from throughout the Hell Creek Formation across its type area. These correlations revealed significant local variation in sediment accumulation rates, confirming that simple stratigraphic position relative to the diachronous base and top of the Hell Creek Formation introduces significant error in correlation.