Radiolarian Biostratigraphy of the Bazhenovo Regional Substage (Upper Jurassic-Lower Cretaceous) of Western Siberia

On the basis of published and new data, the features of the history of study of radiolarians and their stratigraphic distribution in the Upper Jurassic-Lower Cretaceous Bazhenovo Regional Substage (Tithonian-Berriasian) of the West Siberian depositional basin are summarized and analyzed. A general geological characteristic of the Bazhenovo Formation is given. The previous regional schemes are briefly characterized. For the first time, radiolarians were studied using SEM. It is proposed to consider five radiolarian-based units: (1) Parvicingula antoshkinae-P. blowi Zone, the lower Volgian and the lower part of the middle Volgian (lower Tithonian-lower part of the middle Tithonian); (2) Parvicingula jonesi-P. excelsa Zone, the middle Volgian (the middle Tithonian-the lowermost upper Tithonian; (3) Parvicingula rotunda-P. alata Zone, the upper Volgian (the uppermost Tithonian, possibly including the lower part of the Berriasian); (4) Parvicingula khabakovi-Williriedellum salymicum Zone, the uppermost Volgian-Ryazanian (the Berriasian); (5) Williriedellum Bed, the uppermost Ryazanian-Valanginian (the Berriasian-Valanginian). The age of all units is confirmed by rare ammonite occurrences. However, the stratigraphic position of two units (Parvicingula antoshkinae-P. blowi Zone and Williriedellum Bed) still requires clarification and additional justification.