New biostratigraphical constraints on the Lower and lower Middle Cambrian of the Kharaulakh Mountains (northeastern Siberian Platform, Chekurovka anticline)

Abstract

The Cambrian section of the western limb of the Chekurovka anticline was studied, with layer-by-layer collection of animal fossils. This permitted us to refine the biostratigraphic division of this section. Four biostratigraphic units (beds) with small shelly fossils were identified: Lapworthella tortuosa, Lapworthella bella, Mobergella radiolata, and Rhombocorniculum cancellatum. They characterize most of the Tommotian and Atdabanian Stages of the Lower Cambrian. The Lower Tyuser Subformation and the Sekten Formation were constrained by small shelly fossils for the first time. Numerous trilobite finds helped to reveal intervals correlating with the Delgadella anabara–Nevadella and Judomia–Uktaspis (Prouktaspis) Regional Zones in the upper Atdabanian.The upper Sekten Formation (black shales) forms the Kuonamka Formation. Within this interval, the following trilobites were found: Triplagnostus gibbus, Tomagnostus sibiricus, Pseudanomocarina sp., Eodiscus oelandicus, Paradoxides sacheri, and Ptychagnostus aff. atavus. They are widespread in the vast territory of the eastern Siberian Platform, within the Kuonamka Formation.Also, Korobovia khorbosuonica, Charaulaspis postprimus, and Hebediscus convexus trilobites were observed, which had been found earlier in the Khorbosuonka River section (Olenek uplift). This permits a detailed correlation between these localities.Similarity was observed between Triangulaspis annio trilobites and young Charaulaspis. This casts doubt upon the fact that the first Triangulaspis annio finds in the northeastern Siberian Platform belonged to the upper Atdabanian.