Stratigraphy and depositional environments of the upper Fox Hills and lower Hell Creek Formations at the Concordia hadrosaur site in northwestern South Dakota

Many of the dinosaur-bearing bone beds in the Hell Creek Formation of the Dakotas and Montana involve multiple species preserved in the upper Hell Creek Formation. In contrast, the Concordia Hadrosaur Site is mono-specific with respect to dinosaurian taxa and is situated in the lower Hell Creek Formation in a lithostratigraphic unit we associate with the Little Beaver Creek Member. This member consists of organic-rich sandstones, siltstones, and claystones that are distinctive within the Hell Creek Formation based on their uniformly fine grain size, purplish color, and presence of highly lignitic shale rather than coal. Similar lignitic deposits occur at other marine-terrestrial boundaries of the Fox Hills-Hell Creek Formations in the Little Missouri and Missouri River valleys. The bone bed at the Concordia Hadrosaur Site (CHS) is associated with an extensive coastal swamp rather than a localized fluvial subenvironment such as river channel, floodplain, or abandoned channel. The bone bed itself lies at the transition from an extensive swamp (represented by highly organic mudstones) to a more fluvially dominated, distributary environment characterized by variegated mudstones, siltstones, and channel sandstones. The thirty meters of exposed section at the Concordia site include the top of the Fox Hills Formation and lower parts of the Hell Creek Formation. We identify marine silts, muds, and sands, coastal dune sands, coastal swamp muds and silts, and fluvial sands and silts. The sediments are indicative of the marine-terrestrial transition from upper shore-face and foreshore environments to a complex system of coastal dunes, swamps, and distributary channels that formed during the progradation of the Hell Creek sediments into the Cretaceous Fox Hills seaway. Locally, grain size and organic fraction varied due to differences in the proximity to distributary channels, supply of organic material, and water depth. Despite the concentration of bones dominated by a single species in the CHS bone bed, the high clay fraction of the bone bed matrix, combined with the fact that the lowest part of the bone bed has the greatest clay fraction, indicates that the bones were not introduced by way of a high-energy, catastrophic event, such as a flood. Rather, the bones accumulated in an area of quiet standing water. Although preliminary examination of the bones is consistent with this depositional interpretation, it does not necessarily provide direct support for it.