Foraminifera and biostratigraphy of Irbitskian Horizon (Middle Eocene) on south-east of Western Siberia

The issue relating to the age of the middle subsuite of the Lulinvorskaya suite of just the same Horizon has been debatable up to now. S.B. Shatsky (1989) established the independent Irbitskian horizon in this part of the section. According to the latest data, the author regards the age of these subsuite or suite (Irbitskian Horizon) as the middle Eocene one M.A. Akhmetjev and the others (2004) determine the age of the middle Lulinvorskaya subsuite as the early Eocene mainly with regarding to the dinocysts and spore-pollen complexes. The age of the upper layers of the upper Lulinvorskaya subsuite is defined as the Lyutet by these authors. Earlier, the conclusions about the early Eocene age of the middle and upper Lulinvorskaya subsuite (Irbitskaya and Njurolskaya suites) had been reflected in the latest regional stratigraphic scheme (2001). For many years the author of the paper has been presenting the data on the foraminiferal studies specifically from the middle Lulinvorskaya subsuite in favour of the middle Eocene. Later, based on the new factual evidence from the sections of the south-east boreholes of Western Siberia (the envions of Seversk), the age of this part of the section was identified as the Lyutet-Barton one. Earlier the author obtained the assemblages of agglutinated quartz-siliceous foraminifera on the basis of the long studies of the kern samples of the Lulinvorskaya suite from the sections of the boreholes of predominantly the West Siberian central and eastern districts. The rocks with these assemblages from the middle and upper Lulinvorskaya subsuites are established by the author as foraminiferal layers with Bolivinopsis spectabilis and Gaudryinopsis subbotinae zone (Lulinvorskaya subsuite or Irbitskaya suite). Upper Labrospira honesta zone (upper Lulinvorskaya subsuite or Njurolskaja suite) in view of the age is dated by the author to the upper Eocene. In favour of the determination of the age of the early Oligocene or Ryupelian the finds of plankton and bethos foraminifera from the upper lying Tavdinskaya suite are used. The age of these foraminifera is clarified with regard to the correlation with those from the other European sections and especially with the foraminifera from the stratotype of the Ryupelian layer of Belgium. The author has written about these comparisons in a number of previously published works. Recently the age of the middle Lulinvorskaya subsuite (Irbitskaya suite) has been more definitely dated to the Lyutet-Barton due to the finds of characteristic calcareous secretic foraminifera together with agglutinated ones in the Western Siberian south-east. Calcareous foraminiferal assemblages of the Lyutet and Barton from the typical section of borehole T-29 accepted in the south-east are presented. In the composition of the assemblage at the depth of 217,5 m (borehole T-29) the following species: Alabamina aff. wilcoxensis Toulman, Gyroidinella aff. magna Le Calvez, Eponides aff. polygonus Le Calvez, E. candidulus (Schwager), Cibicides carinatus Ter-quem, С. aff. productus (Terquem), C. tenellus (Reuss), Anomalinoides aff. auris (Le Calvez), Bolivina cf. crenulata Cushman, Nonionella aff. wemmelensis Kaasschieter, Rotalia aff. discoides Orb. are determined. The foraminiferal assemblage is characteristic of the Northern European Lyutet stage - England the Paris basin, the Netherlands. The established stage is characterized by the index-species such as Eponides candidulus (Schwager), Cibicides tenellus (Reuss). Besides, Cibicides carinatus Terquem, Rotalia discoides Orb., Bolivina cf. crenulata Cushman are the most distinctive ones of the Lyutet age. In this case the latter species is characteristic of only the middle Lyutet. The species Eponides candidulus (Schwager) is discovered in the rocks corresponding to the lower subzone Planulina, Cibicides of the middle-upper Eocene zone Vaginulinopsis decorate, Pseu-dohastigerina micra from the Netherlands. The species Rotalia discoides Orb. is known from the Lyutet stage of the northern France. The above-lying deposits of the section of borehole T-29 (int., depth 214,6-206.0 m) consist of dark-gray, aleurite, slightly opoka-like, stratus clays. Foraminifera and pseudomorphoses approminately of a similar specific composition are found at the depths of 214,6; 211,3; 211,15; 208,2; 208,0 in five samples. Much more various assemblage in determined at the depth of 211,5 m including the following species: Reophax sp. indet, Haplophragmoides cf. deplexus Podobina, Ammomarginulina aff. deflexa (Grzybowski), Miliola aff. pseudocarinata Le Calvez, Quinqueloculina aff. impressa Reuss, Leticulina cultrata (Montfort), Cibicides cf. pigmeus (Hantken), С. aff. lobatulus (Walker et Jacob), C. cf. fortunatus Martin, Cibicidoides ungerianus (Orb.), Pararotalia cf. spinigera (Le Calvez), P. aff. andouini (Orb.), Asterigerina aff. bartoniana (ten Dam), Protoelphidium aff. roemeri (Cushman). Except for the identified forms numerous pseudomorphoses are present. Cibicidoides ungerianus (Orb.) and Pararotalia cf. spinigera (Le Calvez) species are the most distinguished ones in the assemblage. These species are suggested by us as index-species identified in the list are characteristic of the Barton stage. The entire assemblage of the enumerated characteristic foraminiferal species suggests the Bartonian age of the content sediments that are the greater part of the middle Lulinvorskaya subsuite (the upper layers of Irbitskaya suite). Angular chamber-like forms of the Pararotalia cf. spinigera (Le Calvez) are characteristic of the Barton and catch the eye. Two tables illustrate the schemes of the layers correlation with the assemblages from the section of borehole T-29 with those of Bolivinopsis spectabilis and the Gaudryinopsis subbotinae zone from the Western Siberian central district. In addition to it, separate agglutinated shells along with the zonal species occur in association with characteristic species of calcareous forms. The most distinctive species of the Lyutet and Barton are presented in the pictures down from life in the accompanying foraminiferal tables (I-V). Most quartz bearing pseudomorphoses are found in the above-lying upper Lulinvorskaya subsuite (Njurolskaya suite) and Tavdinskaya suite in the south-east in more sandy rocks as well as the specific section T-29. According to the published works, a similarity of separate species of the south-east of Western Siberia with those of the Lyutet and Barton of Northern Europe known from their stratotypes is determined. In the late Eocene within Western Europe as well as Western Siberia agglutinated foraminiferal species are isolated and coarse-grained. Isolated but quartz bearing pseudomorphoses are discovered in the south-east. It is safe to assume the origin of positive tectonic movements in the Arctic during the late Eocene which led to some rise of the adjacent Northern European, Western Siberian areas and the charge of the transgressional direction. This influenced on the changes of the foraminiferal assemblages not only in the upper Lulinvorskaya subsuite (Njurolskaya suite) but the above-lying Tavdinskaya suite. The occurrence of characteristic early Oligocenian plankton and calcareous benthos forms suggests the transgressional direction from the Turgay strait and probably through the strait in the Urals from the European basin. Thanks to it in the coastal or shallow-water and warm areas as, for instance, in the south-east of Western Siberia there appeared distinctive Lyutet-Barton species of calcareous forms which earlier had been well-known in Northern Europe.

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