Predominance of primitive foraminifers during the recession of transgressions in the Mesozoicand Cenozoic of Western Siberia

Inthe Mesozoic and Cenozoic of Western Siberia the assemblages dominated by primitively arranged forms are assigned to the final stagesof transgressions. These assemblages are commonly met in shallow and probably desalted basins, thus indicating the ascending tectonicmovements in this region. In the Late Volgian the emergence of the Ammodiscus veteranus - Evolutinella volossatovi is noted, which isdominated by planispiral forms of the Ammodiscus Genus and more complicated Evolutinella. These foraminifers confined to the top ofthe Bazhenovskian Horizon succeeded the Middle Volgian Dorothia tortuosa - Spiroplectammina vicinalis Assemblage of a morediverse systematic composition and of a more complicated arrangement of tests. In the uppermost beds of the Bazhenovskian Horizon,in the Jurassic/Cretaceous transition, the systematic composition of the benthic foraminifers is significantly changed. The Trochamminarosaceaformis Assemblage is most often distinguished and occurring in the bottom of the Berriassian. Here trochamminas are abundant,Western Siberia and the Yenisey-Khatanga depression this assemblage is not traced, but the alternative Gaudryinopsis gerkeiAssemblage is known, which is also characterized by the predominance of morphologically complicated tests. The upper boundarybetween the Jurassic and Cretaceous passes within almost monotonous clayey rocks of the top Bazhenovskaya Suite and other coevalsuites. This boundary is not established by the lithological characteristics and from the logs. It may be only traced by the disappearanceof the Late Volgian foraminifers of the Ammodiscus veteranus - Evolutinella volossatovi Zone and the emergence of the Berriassianforms. In the Late Cretaceous epoch the shallowing of the basin is observed in the beginning of the Campanian Age from theaccumulation of the top of the Slavgorodskaya Suite. Here the Late Santonian Cribrostomoides exploratus - Ammomarginulina crispaAssemblage composed mostly of agglutinated quartz-siliceous foraminifers is followed by the Early Campanian Bathysiphon vitta,Recurvoides magnificus Assemblage predominated by the primitive forms. If compared with the Campanian foraminiferal zones of theEast-European Platform, in Western Siberia the omission of a number of middle zones from the section is noted in the boundarybetween the lowermost and uppermost beds of the Campanian. A similar phenomenon is known from the Upper Eocene top of theLyulinvorskaya Suite. The Reophax subfusiformis - R. dentaliniformis Assemblage with the impoverished foraminiferal compositionwas distinguished from the more complete sections (Podobina, 1998). When comparing with the Paleogene benthic foraminifers scale ofthe East-European Platform and Central Asia, the analogues of the Upper Eocene zones were not discovered in Western Siberia. Thechange in the transgression direction resulted in the omission of a number of foraminiferal zones corresponding to substages and someother subunits of the general stratigraphic scheme in the Cretaceous and Paleogene.

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