Five microfossil groups are herein described from the Upper Cretaceous Smoking Hills and Mason River formations in the Horton River area (Northwest Territories). Microfossil assemblages from the Smoking Hills Formation are dominated by radiolarians and foraminifera whereas the Mason River Formation mainly contains diatoms, silicoflagellates, and sponge spicules. These microfossil groups have been reported before in these units and age-equivalent strata from the Canadian Arctic except for radiolarians. The radiolarian assemblage described in this study represents one of the most diverse and abundant assemblages reported in Campanian-Maastrichtian rocks in North America and can be used to reconstruct the climatic, paleoceanographic, and paleobiogeographic conditions that took place at the end of the Cretaceous Period. This Open File documents the stratigraphic occurrence of the microfossil types.

A radiolarian assemblage containing 11 species of both nasellarians and spumellarians was recovered from the Upper Cretaceous Niobrara Formation in southwestern Saskatchewan, Canada. This assemblage represents the first report of Coniacian radiolarians in the entire Western Interior Basin and one of the few reports for the Upper Cretaceous in North America. The presence of radiolarians and the partial disappearance of foraminifera in the only bentonitic interval in this formation suggest that high silica concentrations supplied by volcanic events favored ecological conditions for radiolarians to thrive and or enhanced their preservation before and after deposition. Correlation of this assemblage with other Upper Cretaceous radiolarian assemblages in North America shows a close affinity with the microfauna recovered in the Sverdrup Basin (Canadian Arctic).