The Jurassic Abalak Formation in West Siberia (paleontological characteristics, litho- and biostratigraphy)

The paleontological characteristics and bio- and lithostratigraphic feature s of the uppermost Middle Jurassic and Upper Jurassic of the western part o f West Siberia have been established. The uppermost Upper Bathonian lowermo st Lower Volgian are the clay Abalak Formation overlain by formations of th e Bazhenovka Horizon. Complete paleontological characteristics (ammonites, bivalves, gastropods, and foraminifers) of the studied sections are given. A number of biostratons of the regional;Jurassic scale were established by bivalves (b-zones) and foraminifers (f-zones) earlier unknown in the studie d area. The photos and brief characteristics of the most important foramini fer species are presented. Analysis of macro- and microfossils has allowed refining the stratigraphic volume of biostratons of the regional Jurassic s cale of West Siberia. The general regularities of distribution of productiv e sand beds in the Abalak Formation were estimated. It is proved that the b asal bed of the Abalak Formation has a complex polyfacial structure. This b ed was formed in shallow-marine to subcontinental environments. The lower b oundary of the basal bed lies in the upper part of the Upper Bathonian, and the upper boundary has different positions - from Lower to Upper Callovian . The sand beds (productive P-beds) occur on the periphery of the Abalak Fo rmation. Analysis of the vertical distribution and stratigraphic volume of P-beds shows their conformity to the productive beds U-2(0), U-1(3), U-1(2) , and U-1(1) of the Vasyugan Horizon in the southeast of West Siberia. Acco rding to the received paleontological and biostratigraphic data, alternatio n of major sand and clay beds in the Abalak Formation is well correlated wi th the minima and maxima of eustatic curve for West Siberia, which was cons tructed in study of Jurassic sections in its southeastern regions. The quas i-isochronous development of sand beds on different Banks of West Siberia w as most probably governed by eustatic fluctuations. The sections with the m ost fully described assemblages of macro- and microfossils are proposed as hypostratotypes of the Abalak Formation.