Volcanic Tuffs and Tuffites in Jurassic–Cretaceous (Volgian–Ryazanian) Boundary Rocks of Western Siberia

Altered ash tuff and tuffite layers in the Jurassic–Cretaceous boundary rocks (Bazhenovo Formation) were studied in detail. These layers represent an interval ranging from the upper part of the middle Volgian substage to the uppermost Ryazanian and cover a huge area (more than 400 000 km2). Despite small thickness (10–3–10–2 m), these layers are prominent in borehole cores and correlated confidently in interborehole spaces. Therefore, they can be used as marker horizons and isochronous stratigraphic levels in the Bazhenovo Formation. The composition of ash layers is quite consistent and marked by lateral zoning, probably, related to fractionation of the primary pyroclastic material during the eolian and hydrodynamic transport and deposition. The petrographic, mineralogical, and geochemical data suggest that the original ash corresponds to basaltic andesite in composition and is related to arc (suprasubduction) volcanism beyond the West Siberian Plate. The probable source of ash material was located most likely southwest of the study region in the Transcaucasus. The studied tuff layers are very important for the regional stratigraphy and global correlation, since they can potentially be traced in both Boreal and Tethyan successions, and zircons found in tuffs can be used for the U–Pb dating.