Provenance and the U-Pb age constraints on the tuff beds of the Late Jurassic-Early Cretaceous Bazhenovo Formation, West Siberian Basin

Thin tuff beds of the Late Jurassic-Early Cretaceous Bazhenovo Formation are laterally widespread in the central part of the West Siberian Basin (ca 0.5 million km(2)). However, the source of the tuff beds remains unclear. The stratigraphy, geochemistry and geochronology of the tuff beds were investigated to reveal their magmatic origin and potential source region. Most of the tuff beds are recorded in member 4 and can be correlated through the Bazhenovo sequence. Thin-section petrography and X-ray diffraction indicate that the tuffs mostly comprise clay minerals and K-feldspars. Less common minerals are plagioclase, quartz and pyrite. The geochemical composition of the Bazhenovo tuff beds suggests a parental magma origin of andesite/basalt, which came from volcanic arc-related settings. Considering the results of geochemical studies along with LA-ICP-MS zircon U-Pb dating (141.3 +/- 0.3/2.8 Ma), the palaeovolcanoes of the Caucasus region or south-east Mongolia-North-East China are one of the potential source regions of the tuffs. The record of these tuffs indicates the intensive volcanic eruption during the Volgian-Ryazanian, accompanied by a very low-sedimentation rate and preservation in a reducing environment. The tuff beds have broad implications as an isochronous marker horizon and constraints for the numerical age of the Jurassic-Cretaceous boundary in the Boreal Realm.