The middle Cambrian Stephen Formation of the western Canadian Rocky Mountains might be best known for including the famous Burgess Shale with its exceptionally preserved fossils. Likewise remarkable is, however, its depositional setting, which was characterized by the presence of a submarine cliff that divided the environment into a deeper, basinal and a shallower part. This division, caused by the so called Cathedral Escarpment, resulted in the development of two distinct regional expressions of the formation, a thick, i.e., basinal, and a thin, i.e., shallower, one. Whereas previous paleontological studies of the Stephen Formation mainly focused on the exceptional preserved fauna and its macroscopic fossils, we present here the first systematic study of the microfossil content of the limestone horizons occurring throughout the formation. Five sections of the thin and thick Stephen Formation have been measured and sampled in Yoho and Kootenay National Park. In total, samples from over 130 horizons have been dissolved in diluted formic acid revealing a variable fauna of dominantly phosphatic shelled brachiopods (mainly Acrothyra and Paterina, but also lingulids and zhanatellids) and trilobites (mainly ptychoparids and Pagetia; other trilobites and agnostids are rare). Locally recrystallized echinoderm ossicles, which can be referred to ctenocystoids and edrioasteroids, and a variety of siliceous sponge spicules are common. Molluscs, such as helcionellids, stenothecoids, or hyoliths, are present, but typically form only an insignificant part of the associations. Bradoriids as well as protoconodonts are rare and have only been found in individual horizons. The evaluation of the distribution of the fauna in the individual sections aims towards a detailed correlation of the sections, a more exhaustive biostratigraphy, and a better understanding of the regional variability of the formation.
Uppsala, 2011. 22-22 p.