Reference section for the Upper Jurassic and Callovian in northern West Siberia

The section of Zapadno-Purpeiskaya BH-710, drilled in the Pur basin, is one of the reference sections of the Upper Jurassic in northern West Siberia. This section is represented by a succession of formations most typical of this range in West Siberia: Vasyugan, Georgiev, and Bazhenov. The rock lithology was examined in detail. The section was paleontologically characterized using a complete range of fossils: ammonites, bivalves, microfauna, dinocysts, spores, and pollen. On this basis the section was subdivided biostratigraphically. Analysis of logging data has led to new stratigraphic subdivisions. The middle part of the Callovian (uppermost Lower Callovian, Middle Callovian and, partly, Upper Callovian) in the studied section is severely reduced to absent. The ammonites Indosphinctes (Elatmites) mokschaensis (Sas.) have been found in West Siberia for the first time. This species is widespread in the Middle (and partly Upper) Callovian of the Russian Platform. The Middle-Upper Volgian part of the Bazhenov Formation is much reduced in thickness. It has been established that the largest (upper) part (about two thirds) of the Bazhenov Formation in this section is of Berriasian age. The interval between the ammonite-based uppermost beds of the Lower Berriasian and the lowermost beds of the Lower Valanginian is about 12 m. About a half of this interval is the upper part of the Bazhenov Formation overlapped by the Sortym Formation.