The Jurassic-Cretaceous boundary in Boreal Russia : radiolarian and calcareous dinoflagellate potential biomarkers

The International Berriasian Working Group (ISCS) suggested primary and secondary marker “datums” to fix the basal Berriasian boundary and thus to detine the Jurassic-Cretaceous boundary (Wimbledon et al., 2011, 2013). Two primary markers Calpionella, as well as calcareous nannoplankton, are practically unknown in the Boreal Realm. Testing and calibration of these markers, as well as of fossils of radiolarians and other signals, in the most complete sections, were declared as an important task for the near future. In the Tethys, the Jurassic-Cretaceous boundary based on radiolarians falls inside zone UAZ 13 of Baumgartner et al. (1995), whereas in the palaeo-Pacific it corresponds to the boundary between zones 4 and 5 of Pessagno et al. (2009), and in boreal Siberia it probably falls between the biohorizons of Parvicingula haeckeli and P. khabakovi. The radiolarian events at the Jurassic-Cretaceous boundary in the boreal successions of Russia can be proposed to be used as an additional biomarker to help develop new integrated boundary criteria. Thus, as the first appearance of the zonal species Calpionella alpina, which defines the Jurassic and Cretaceous boundary, coincides with the first occurrence of the calcareous dinocyst zonal species Stomiosphaerina proxima (Reháková, 2000), it is logical to propose a calcareous dinoflagellate, widely represented in the Upper Jurassic-Lower Cretaceous Bazhenovo Formation of Siberia, as a secondary marker.