The interrelation between the aspect, systematic composition of the late Cretaceous foraminiferal assemblages and the lithology of host rocks of Western Siberia

The investigation of the late Cretaceous Turonian-Campanian foraminiferal assemblages of Western Siberia made it possible to reveal a significant dependence of their aspect and systematic composition on the lithological characteristics of the host deposits. The main factors influencing these changes of foraminifera are: the depth of the basin, lithological peculiarities of the ground, chemistry, temperature conditions and the mobility of water environment. All these features are reflected in the upper Cretaceous tests and host rocks studied by the present author. The dwelling conditions of foraminifera could be inferred by the mineral composition of the agglutinated material, and, besides, paleogeographic reconstructions can be conducted within Western Siberia. The calcareous secreted and agglutinated tests with calcite grains are commonly confined to the carbonate clays, whereas foraminifera agglutinating mainly quartz grains, chalcedony to a lesser degree, sponge siliceous spicules are encountered in the non-carbonate clays and siltstones. The chemical composition and test wall microstructure in representatives of separate genera and, especially, species are relatively constant, which demonstrates a certain elective ability of foraminifera in relation to dimensions and composition of minerals adsorbed from the basin bottom and used for constructing test walls. The grain dimensions and mineral composition of the agglutinated material are indicative of the foraminifera dwelling conditions along with the sedimentation processes in marine basins, which is of certain interest as complementary data for paleogeographic reconstructions. The Turonian foraminiferal assemblages differ significantly from the superposed association in their aspect and systematical composition. When analyzing the early Turonian assemblages from the central district of Western Siberia (Tara, Novy Vasyugan, Surgut), it is possible to note the relative fine granularity of agglutinate, the sufficient rate of integrity and the grey colour of tests. The Coniacian assemblages are sporadically encountered and differ significantly from each other. Grey silica clays compose the Sedelnikovskaya Suite correlatable with the Ipatovskaya Suite occurring eastward. Almost on the whole territory of Western Siberia, the Santonian foraminiferal assemblages are composed by agglutinated quartz tests and confined to the Slavgorodskian Suite. The early Campanian Bathysiphon vitta - Recurvoides magnificus Assemblage is represented by heterogeneous test accumulations. In the assemblage from the central district of the studied territory (Novy Vasyugan, Surgut), there are numerous genera of the secreted calcareous forms, along with the primitive fine-grained sugary-white tests of the genera Hyperammina, Bathysiphon, Glomospira and others. The late Campanian Cibicidoides primus Assemblage is confined to the upper part of the Kolpashevo iron-ore horizon (siltstones, sandstones) or its analogues, and also to the bottom of the Gankinskian suite. The assemblage is dominated by the calcareous secreted forms, which are widely distributed.

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