PALEOCENE BIOTA OF THE WEST SIBERIAN PLAIN

Abstract

In the Paleocene, most of the present West Siberian plain was occupied by a vast epicontinental sea, which reached from the present Arctic and Atlantic oceans in the North to the Aral Basin in the South. Climate and ocean circulation fluctuated and affected marine and continental biota. Benthic foraminifera that occurred over a large part of the sea throughout the Paleocene were used as representatives of the marine biota, and spores and pollen of terrestrial plants as representatives of the continental biota, in order to study these environmental and biotic changes. Foraminifera and palynomorphs were studied in sedimentary sequences of Danian, Selandian, and Thanetian age. These sequences have regional names, and encompass the upper part of the Gankinskaya Stage, the whole Talitskaya Stage, and the lower part of the Lyulinvorskaya Stage. In the southwest part of the basin (the Omsk depression), upper Gankinskaya sediments (Maastrichtian to lowermost Danian) contain calcareous benthic foraminifera. Benthic foraminiferal assemblages suggest that sea level fell during the early Paleocene, so that the West Siberian basin shallowed significantly and decreased in size. During the earliest Danian, the basin in the Ust-Tym depression also became shallower, and "primitive" agglutinated foraminifera were dominant (e.g., Bathysiphon, Glomospira, Ammodiscus). Sea level rose during the Selandian boreal transgression (Talitskaya Stage), and agglutinated foraminifera occurred over large areas. In the Thanetian (represented by the uppermost Talitskaya and the base of the Lyulinvorskaya Stage) the basin again shallowed, and foraminifera became rare. The analysis of palynomorphs (spores and pollen) in allochthonous assemblages in marine sediments documents that an alluvial plain with lakes and bogs was situated on the southeast coast of the Paleocene West Siberian marine basin. The climate became more arid during the Danian and late Thanetian, with a pronounced phase of drying at the boundary between the Talitskaya and Lyulinvorskaya Stages (early Thanetian).