Sequence-stratigraphic subdivision of the Upper Cretaceous - Eocene of the south-east of Western Siberia

Paleontological remains play a significant role in theallocation of sequences of the southeast of Western Siberia. The analysis of microfauna foraminifers from sections of wells In Western Siberiawas carried out: composition, structure, form and relationship of sea organisms with the changing environment, their reaction to eustaticchanges. The morphological analysis of higher plants, which are found in the wells and outcrops of the extreme southeast of Western Siberia,was also performed. In the Upper Cretaceous section the context of the study area can be divided into three sequences: the Cenomanian-Turonian, Coniacian-Santonian and Campanian-Maastrichtian ones. In the Lower Eocene section there are two sequences: the Selandian-Early Thanetian and Upper Thanetian-Bartonian ones. Tracts of low sea level stands in Western Siberia were formed in the conditions of thesea level decrease, its shallowing. They have depleted foraminifers complexes, spread all over the territory of Western Siberia as quartzsiliceousand lime benthos. The Late Cretaceous floristic complexes are characterized by small leaves and lamina integrity, which is connectedwith the arid climate in the plains remote from the sea coast. Transgressive tracts were formed with the rapid increase of the sealevel.The foraminifers fauna is characterised by its diversity of species and abundance of quantitative composition. The shells are smallandmedium-grained, of grey colour. The southeast of the sea palaeobasin is characterised by lime benthos, and the western and central partsare relatively deep, mainly quartz-siliceous. Floristic complexes, connected with the formation of the transgressive tract in the Turonian andEocene, are characterized by a variety of species composition, large leaves and lamina pinnation. Tracts of high sea level stands wereformed in the conditions of a slower marine transgression growth, followed by slow regression. The lower part of the tracts reflects the foraminiferscolour. The upper part characterises the regressive phase of the palaeobasin development, associated with the penetration of limebenthos in the central and western parts of the basin, as well as an increase in the content of plankton foraminifers. Thus, the lithologic featuresof rocks along with the palaeoecology characteristics of the fossil fauna and flora allow allocating sequences in the Upper Cretaceousand the Palaeocene-Eocene sediments of the southeast of Western Siberia, reflecting eustatic changes of the palaeobasin. Sequences reflectthe evolution of the basin of sedimentation and explain the behaviour of sedimentary minerals.

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