Trilobites and biostratigraphy of the Kuonamka Formation, northern Siberian Platform (Olenek River)

A study of triobite distribution allowed a biostratigraphic subdivision of the Kuonamka Formation section exposed in the middle reaches of the Olenek River (18 km downstream of the village of Olenek). Through detailed description of the section, a biostratigraphic subdivision down to the level of biozones and faunal beds was performed. These are beds with Calodiscus–Triangulaspis annio, Delgadella ultima–Bergeroniellus expansus Zone, Lermontovia dzevanovskii–Neopagetina orbiculata Zone, Anabaraspis Zone, Cheiruroides arcticus– Pagetia horrida Zone, Kounamkites Zone, Triplagnostus gibbus Zone, and beds with Tomagnostus fissus–Paradoxides sacheri. These biostratigraphic subdivisions were correlated with regional zones of the Siberian Platform. The study also provides analysis of trilobite assemblages from the Lower/Middle Cambrian boundary interval, which was identified in the studied section and other previously described sections (Nekekit, Molodo, Boroulakh Rivers). In the studied section, the base of the Middle Cambrian is defined by the FAD of Cheiruroides arcticus, i.e., at the base of the Cheiruroides arcticus–Pagetia horrida Zone. This level correlates with a presumed lower boundary of Stage 5 of a new International Stratigraphic Chart for the Cambrian System.