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The foraminiferal assemblages of the uppermost Kimmeridgian-Volgian succession of Western Siberia and their correlation potential 157 INTRODUCTION During the last few decades, the stratigraphy of the uppermost Jurassic-lowermost Cretaceous (scope, sequen

Podobina V., Gabysheva E., Tatyanin G.

Volumina Jurassica

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2009

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Vol. 7, no. 1

157-172

EN

The present study summarizes the data on the continuous stratigraphic sequence of foraminiferal assemblages from the Volgian deposits of the south-east of Western Siberia. Microfaunal analyses of 650 core samples from 18 boreholes of Volgian deposits were performed. Eighty two foraminiferal species, identified from the samples, were combined into the Volgian assemblages. On the basis of these assemblages, four biostratigraphic units were established in the range of foraminiferal zones and foraminiferal beds with characteristic species. These biostratigraphic units correspond to the upper part of the Kimmeridgian/ lower part of the Volgian, the Middle Volgian (lowermost and uppermost beds), and the Upper Volgian. A detailed biostratigraphic analysis of the Volgian beds was carried out, which enabled a correlation scheme to be developed for major regions of Russia. West-Siberian foraminiferal assemblages were correlated with those of Europe and North America within the Panboreal Superrealm. The species composition of foraminiferal assemblages was analyzed, and correlatable species ere established. This provided an opportunity to correlate coeval strata from Western Siberia, East European (Russian) Platform, northern territories of Siberia and Arctic Islands, Canada, Spitsbergen and England. The possibility was established for correlating Volgian-Tithonian deposits of the Panboreal and Tethys-Panthalassa Superrealms through the Dnieper-Donets Basin where faunas of both southern and Boreal-Arctic types are met.

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The problem of establishing the Tithonian Stage in Western Siberia

Podobina V., Gabysheva E., Tatyanin G.

Volumina Jurassica

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2006

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Vol. 4, no. 1

205-206

EN

In 1996 the Volgian Stage was withdrawn from the General Stratigraphic Scale and replaced with the Tithonian Stage by the Resolution of the Interdepartmental Stratigraphical Committee of Russia. However, this did not solve the problem of correlation between the Volgian and Tithonian stages. The problem of identifying and mapping the Tithonian Stage within the Boreal deposits becomes unresolved. On this basis the Volgian Stage was retained in the West Siberian Stratigraphical Scale as the terminal Jurassic Stage, owing to the impossibility of the application of the new standard directly to the Boreal sections. The difficulties in comparison between the Regional Scale and the Standard one arise not only for the ammonite zones, but also in correlating the West Siberian foraminiferal zones. Numerous boreholes penetrating Western Siberia and combined studies on core samples enable tracing the continuous sequence of the Volgian foraminiferal zones and beds. The foraminiferal sequence traced through the Nyarginskian type section in the southeast of Western Siberia may serve as an example. There, in the Upper Kimmeridgian – Lower Volgian deposits, the f-beds with the Pseudolamarkina sp. Assemblage were established. The Middle Volgian Spiroplectammina vicinalis-Dorothia tortuosa F-zone was divided into two subzones: the lower Spiroplectammina vicinalis-Saracenaria pravoslavlevi subzone and the upper Dorothia tortuosa subzone The Upper Volgian deposits enclose the Ammodiscus veteranus-Evolutinella volossatovi F-zone. At the Jurassic/Cretaceous boundary, the taxonomy of benthic Foraminifera is essentially different. The phylogenetic succession of numerous foraminiferal genera during the Volgian Age indicates the integrity of the Volgian Stage. A number of genera are characteristic only for the Volgian. Benthic Foraminifera forming the basis for establishing the Upper Jurassic assemblages, and the Volgian ones in particular, showed a wide distribution in the Late Jurassic marine basins of the Boreal belt: Northern Europe (Russia and Poland), Northeast Asia, Northern America (Northern and Northwest Canada, Northern Alaska), islands of the Arctic Region and shelves of northern seas. This wide distribution of the Volgian Foraminifera resulted from the Late Jurassic transgression. The Boreal and Tethyan foraminiferal assemblages differ essentially each other in their taxonomy, which result in significant difficulties in correlation between the Volgian and Tithonian deposits of Boreal and Tethyan belts, respectively. Besides, the distinctive feature of all provinces of the Boreal belt is the absence of planktonic foraminifers or their insignificant development. The direct correlation of the Volgian assemblages is possible only within the Boreal belt, basing on the taxonomical analysis of foraminiferal assemblages and the presence of the species of correlation value. The correlation of the Volgian-Tithonian deposits of the Boreal and Tethyan belts is feasible only at the level of Lower-Middle Volgian substages and the Tithonian. We suppose that at present it is impossible to find direct counterparts of the Upper Volgian Substage.

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Problems on the Middle Oxfordian biostratigraphy of Western Siberia

Podobina V., Polkovnikova E., Tatyanin G.

Volumina Jurassica

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2006

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Vol. 4, no. 1

206-207

EN

The knowledge of biostratigraphy of the Middle Oxfordian part of the section of the Verkhnevasyuganskiy Subhorizon although quite good is insufficient for solving all the problems of dating, detailed correlation, and identification of sandy beds of the substantial oil-and-gas bearing deposits complex in the new borehole sections. Another cluster of problems includes the detailed subdivision of two Middle Oxfordian ammonite zones (a-zones) of Western Siberia and their correlation with the coeval Plicatilis Zone of the Submediterranean zonal scheme. At present, two a-zones are distinguished in the Middle Oxfordian of Western Siberia: the lower zone with the Cardioceras densiplicatum and the upper zone with Cardioceras tenuiserratum. Two parts are distinguished in the Cardioceras densiplicatum Zone corresponding to the Cardioceras vertebrale and Cardioceras maltonense subzones. The upper zone is not subdivided, and it is based on the appearance of ammonites of the subgenus Cawtoniceras. Foraminifera are of exceptional biostratigraphical importance when the material from cores is considered where ammonoid finds are generally rare. The Middle Oxfordian sedimentary complex of Western Siberia contains foraminiferal assemblages of two f-zones: Ammodiscus thomsi-Tolypammina svetlanae and Trochammina oxfordiana. The former includes several isochronous beds recognized in different areas of the region. In the circum-polar Urals, L. G. Dain distinguished beds with Ammodiscus thomsi, Tolypammina svetlanae. In western areas, V. I. Levina distinguished beds with Ammodiscus cheradospira (=Ammodiscus aff. pseudoinfimus (=Ammodiscus thomsi) and Eomarssonella paraconica. In the southern area, V. F. Kozyreva recognized beds with Ammodiscus ex gr. pseudoinfimus (=Ammodiscus thomsi) and Trochammina oxfordiana. In the central and southern districts, beds with Trochammina oxfordiana and with Glomospirella galinae were established by G. M. Tatyanin. The geographical range of the Ammodiscus thomsi-Tolypammina svetlanae Zone covers: Western Siberia, the Preuralian, Yamal-Tyumen, Kazym-Konda, Froly-Tambey, Pur-Vasyugan facies districts (the Danilovskoye, Abalak, Vasyugan, Maurynya formations). In the Yenisey-Khatanga depression N. V. Sharovskaya distinguished beds with Trochammina oxfordiana, Lenticulina memorabilissima, now this is the Trochammina oxfordiana Zone (its top correspond to the Ammodiscus thomsi - Tolypammina svetlanae Zone). The range of the Trochammina oxfordiana Zone covers: Western Siberia, the Khatanga and Taz-Kheta facies districts, Central Siberia, the Paksa facies district (Sigovoe Formation). The analysis on the available findings of the Middle Oxfordian ammonites in the south-east of Western Siberia may offer a key to reconciling the combined macro- and microfauna finds with the ammonite scale, and refining the lower and upper boundaries of the Middle Oxfordian foraminiferal associations. This could make possible tracing and comparing the changes in the systematic composition of the foraminiferal assemblages within two Middle Oxfordian ammonite zones which include the sections of the oil-and-gas bearing horizons.