A condensed succession at the Jurassic/Cretaceous transition in a shallowing basin on the eastern Russian Platform

Svetlana O. Zorina; Dmitry A. Ruban; A.J (Tom) Van Loon

doi:10.2298/GABP0970001Z

Svetlana O. Zorina Central Research Institute of Geology of Industrial Minerals, Zinin Street 4, Kazan, Republic of Tatarstan, 420097, Russia
Dmitry A. Ruban Department of Geology, University of Pretoria, Pretoria 0002, South Africa. contact address: a/jashik7333, Rostov-na-Donu, 344056, Russia
A.J (Tom) Van Loon Institute of Geology, Adam Mickiewicz University, Maków Polnych 16, 61-606 Poznan, Poland

DOI: https://doi.org/10.2298/GABP0970001Z

Keywords: condensed succession, shallowing, ammonite zones, Jurassic, Cretaceous, Russian Platform

Abstract

The eastern periphery of the Russian Platform houses an exceptional record of the Jurassic/Cretaceous transition, which is represented by very thin sandstone beds. The presence of glaucony grains, phosphorite concretions and shark teeth indicates that the transitional sediments constitute a condensed succession, although the allochtonous origin of the glaucony grains in itself is not a reliable indicator. The combination with thin ammonite zones and a strongly diminished sedimentation rate, as low as ~0.05 cm/ka are, however, convincing evidence. The Jurassic/Cretaceous transitional deposits accumulated in a basin the depth of which decreased simultaneously with a global eustatic sea-level fall. This coincidence suggests that condensed successions may form in shallowing environments, which contradicts the sequence-stratigraphic concept. Considering the character of the sediments under study, it appears that both stratigraphic and taphonomic condensation patterns occur in this part of the eastern Russian Platform.