Biostratigraphy of the Slavgorodskian Horizon (Santonian - Lower Campanian) of the southeastof Western Siberia

The paper considersrecent data of the investigation on foraminifera and biostratigraphy of the Slavgorodskian Horizon dated to the Santonian - Early Campanian.Eighteen core samples were analysed from the borehole ZN-4 section (the vicinity of town Seversk) in the southeast of WesternSiberia. The foraminiferal assemblages were encountered at three stratigraphic levels: the Lower Santonian beds with the Haplophragmoidestumidus Assemblage (the Ammobaculites dignus - Pseudoclavulina admota Zone); the Upper Santonian beds with the Ammomarginulinacrispa Assemblage (the Cribrostomoides exploratus - Ammomarginulina crispa Zone) and the Lower Campanian bedswith the Recurvoides magnificus Assemblage (the Bathysiphon vitta - Recurvoides magnificus Zone). Three palaeontological platesillustrate the main species of agglutinated quartz-siliceous and secreted-calcareous foraminifera. The Early Santonian assemblage encounteredhere is dominated by species of the Haplophragmoididae Family, of which index-species Haplophragmoides tumidus wasestablished for these beds. In the middle beds of the Slavgorodskian Horizon, the assemblage is dominated by coarse-grained agglutinatedtests and the index-species Ammomarginulina crispa among them. In the central district this index-species was established in theUpper Santonian beds along with the other characteristic species Cribrostomoides exploratus Podobina that is missing in the boreholeZN-4 section. In the upper beds of the Slavgorodskian Horizon, the systematic composition of the assemblage is significantly renewed,and the Recurvoides magnificus Podobina Species known for the Lower Campanian appears along with the species of agglutinated testsmet previously. The species of the secreted-calcareous tests also occur, that are characteristic for this part of the section and migratedhere from the southern (Kazakhstan) province. The systematic composition of large agglutinated coarse-grained tests is similar to thatfrom the central district, where the Arctic Regions influenced the relatively cold-water basin of Western Siberia. The significant similarityof these West-Siberian foraminifera is also observed with the forms from Canada and Northern Alaska assigned to the Arctic PalaeozoogeographicRealm. Southern immigrants entering the assemblage commonly as pseudomorphs and rare secreted-agglutinated testsare similar in taxonomy to the species from Kazakhstan and East-European provinces of the Boreal-Atlantic Realm. The mixed taxonomiccomposition of foraminifera from various provinces enabled the Santonian - Early Campanian age of the Slavgorodskian Horizonto be established from these two ways (northern and southern). The shallow-water coastal environmental conditions for the studied foraminiferaltests left their imprints on their aspects and systematic composition. In the borehole ZN-4 section, the Santonian - LowerCampanian beds containing varied index-species are correlatable with the widespread foraminiferal zones. The lithologo-microfaunalcharacterization of the studied section beds and their correlatability with the established foraminiferal zones is presented in the table.

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