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1 2006

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Jurassic/Cretaceous boundary in the north of Middle Siberia: micropalaeontological and palynological data

Nikitenko B., Pestchevitskaya E., Lebedeva K., Ilyina V.

Volumina Jurassica

2006

Vol. 4, no. 1

266-267

Palynological and micropalaeontological analysis of Middle Volgian - Lower Valanginian section from Nordvik Peninsular, a key section of Jurassic/Cretaceous boundary in Boreal regions, allows its detailed zonation (Fig. 1). The changes of taxonomical composition of the assemblages and stratigraphical position of foraminiferal and dinocyst zones have been analyzed taking into consideration two versions (A and B) of Jurassic/Cretaceous boundary. The level between Chetae-Sibiricus ammonite zones traditionally considered as Jurassic/Cretaceous boundary for Arctic regions (version A) is characterized by the lack of considerable taxonomical changes of foraminiferal assemblages. Dominant shifts and appearance of new taxa have been revealed slightly upward the section within the first ammonite zone of Boreal Berriasian. Some of foraminiferal zones are characterized by wide geographical distribution being traced over Siberia as well as Barents Sea region. They may be used for circum-Arctic correlations. Microphytoplankton assemblages also include a wide number of characteristic dinocysts taxa providing long distance correlation in some levels. The characteristic feature of Subzone Pareodinioideae, Cassiculasphaeridia reticulata, Batioladinium varigranosum is the first appearance of index species. It is also observed in the middle of Berriasian in Polar Ural Mountains, Moscow Syneclise, Arctic Canada. Paragonyaulacysta borealis(?) - Dingodinium spinosum(?) Zone is a reliable marker level for circum-Arctic correlations as the acme of Paragonyaulacysta borealis(?) in Upper Volgian-Berriasian, and the extinction of this species in lowermost Valanginian well traced over northern regions of Siberia, Barents Sea shelf, Arctic Canada and Greenland. First appearance of Dingodinium spinosum(?) in the lower part of Upper Volgian is observed in many regions of Western Europe, where it is regarded as the index of Dingodinium spinosum(?) Zone. In general the taxonomical changes of microphytoplankton assemblages are rather continuous with no considerable extinctions or diversity peaks. The most important taxonomical shift is observed near the base of Paragonyaulacysta borealis(?) - Dingodinium spinosum(?) Zone (version B). Palaeofacies analysis evidence that the richest foraminiferal assemblages are associated with the middle sublittoral, while the communities of lower sublittoral are less abundant and diverse. Sharp decrease of taxonomical diversity (2-3 species) and wide distribution of opportunist species Ammodiscus veteranus are typical for dysaerobic palaeoenvironments. The characteristic feature of Siberian microphytoplankton assemblages is low abundance and diversity of chorate and proximochorate dinocysts. The microphytoplankton associations strongly dominated by dinocysts with numerous gonyaulacaceans evidence rather deepwater and well aerated palaeoenvironments of lower sublittoral. Dysaerobic and poor aerated conditions of the same zone are characterized by the assemblages almost completely composed of prasinophytes.