A condensed succession at the Jurassic/Cretaceous transition in a shallowing basin on the eastern 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 con- cretions 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, convinc- ing 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 charac- ter of the sediments under study, it appears that both stratigraphic and taphonomic condensation patterns occur in this part of the eastern Russian Platform. Apstrakt. Isto~ni obod Ruske Platforme pokazuje izuzetan podatak o jursko-krednom prelazu, koji je predstavqen veoma tankim slojevima pe{~ara. Prisustvo glaukonitskih zrna, fosfornih konkrecija i zubi ajkula ukazuju da prelazni sedimenti izgra|uju kondenzovanu sukcesiju i ako alohtono poreklo glaukonitskih zrna nije uvek pouzdan pokazateq. Kombinacija uskih amonitskih zona i jako smawewe brzine sedimentacije od ~0.05 cm/ka su svakako ubedqiv dokaz. Prelazni jursko-kredni sedimenti depono- vani su u basen ~ija se dubina istovremeno pove}ava sa padom eustati~kog nivoa svetskog mora. Ova koin- cidencija ukazuje da se kondenzovane sukcesije mogu formirati u plitkim sredinama, a koja je u suprotnos- ti sa konceptom sekventne stratigrafije. Razmatraju}i prou~avani karakter sedimenata, proizilazi da su se i stratigrafska i tafonomska kondenzacija de{avale u ovom delu isto~ne Ruske Platforme.