Biostratigraphy of selected K/T boundary sections in southwestern North Dakota, USA: toward a refinement of palynological identification criteria

The Cretaceous/Tertiary (K/T) boundary corresponds to one of the very few unique events in the geological record discovered to date, representing a single traceable timeline across the world. This timeline, coincident with the geochemical and mineralogical singularities caused by the impact of a large extraterrestrial body, is also coincident with the end-Cretaceous extinction event in North America. This precise timeline gives an ideal context for comparing the trends recorded by the different groups of the terrestrial fossil record during the K/T extinction event. However, in southwestern North Dakota, numerous studies conducted on excellent exposures of the K/T boundary showed that the geochemical and mineralogical criteria associated with the impact are rarely preserved. For that reason, palynology is preferred as a simple and efficient way for identifying the K/T boundary. In this context, a previously undescribed outcrop section from southwestern North Dakota was found to preserve an extensive record of the continental K/T boundary with miospores and plant megafossils, as well as microvertebrates and large vertebrate elements. Preliminary studies on the fossil associations recovered from this site showed an inconsistency between the Paleocene age given by the vertebrate and megafloral component and the Cretaceous age given by the palynological record. This issue is investigated in this paper, with a major emphasis on the description and analysis of the palynological record associated with sedimentological and paleoenvironmental data. Results shows that palynologically, the K/T boundary is placed more accurately using relative abundance variations of selected taxa.