Dinocyst Biostratigraphy of the Lower Cretaceous in North Siberia E

ISSN 0869-5938, Stratigraphy and Geological Correlation, 2007, Vol. 15, No. 6, pp. 577–609. © Pleiades Publishing, Ltd., 2007. Original Russian Text © E.B. Pestchevitskaya, 2007, published in Stratigrafiya. Geologicheskaya Korrelyatsiya, 2007, Vol. 15, No. 6, pp. 28–61. Dinocyst Biostratigraphy of the Lower Cretaceous in North Siberia E. B. Pestchevitskaya Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia Received June 28, 2006; in final form, March 6, 2007 Abstract—The described scheme of the Lower Cretaceous (Berriasian–Barremian) stratigraphic subdivisions is elaborated based on palynological study of sections in the Khatanga depression, Ust-Yenisei region, Pur–Taz interfluve, and around the Ob River latitudinal segment, the North Siberia. Stratigraphic distribution of microphytoplankton studied in detail is used to distinguish 10 biostratigraphic units in the rank of dinocysts zones. Stratigraphic position of the zones is determined with confidence using data on the Lower Cretaceous reference sections in the Khatanga depression, which were principal ones by constructing the Boreal standard zonation. In majority, boundaries of the dinocysts beds are of a high correlation potential and can be regarded as reliable stratigraphic markers, as they are recognizable not only in Siberia, but also in northern Europe and America. DOI: 10.1134/S0869593807060020 Key words: Lower Cretaceous, stratigraphy, dinocysts, Siberia, correlation, northern regions of Eurasia and America. INTRODUCTION Natural outcrops of the Lower Cretaceous in the Khatanga depression are of special importance for Organic-walled microphytoplankton is widespread elaboration of the Siberian dinocyst scale. The respec- in the Lower Cretaceous deposits of western and cen- tive sections bearing diverse paleontological remains tral Siberia, as experts in palynology noted repeatedly. facilitated construction of the Boreal standard zona- This group of very diverse microfossils includes tions (Zakharov et al., 1997), and the distinguished dinoflagellate cysts (dinocysts), acritarchs, Chloro- palynostratigraphic units have been correlated with phyceae and Zignemataceae remains. A considerable zonal subdivisions of macro- and microfauna. In the stratigraphic potential of dinocysts well studied in tax- Berriasian, Valanginian and Hauterivian intervals of onomic aspect is confirmed by a great amount of data sections in the Nordvik Peninsula, the Anabar Bay east- on their stratigraphic and lateral distribution in Europe, ern coast, their stratigraphic position has been con- Asia and America. At present, dinocysts are broadly in trolled by data on ammonites, belemnites, bivalves and use for a high-resolution subdivision of the Lower Cre- foraminifers (The reference Section…, 1981; Bogo- taceous in West Europe and Canada. In contrast, spe- molov et al., 1983; Zakharov et al., 1983; Bogomolov, cialized investigations of dinocysts from the Lower 1989; Shenfil’, 1992; Marinov and Zakharov, 2001; Cretaceous of Siberia with parallel assessment of strati- Nikitenko et al., 2004). Foraminifers and ammonites graphic value of separate taxa are comparatively rare. have been also identified in borehole section Severo- Hence, new data in this sphere of biostratigraphy are of Vologochanskaya 18 (Nikitenko et al., 2004), while ammonites, foraminifers and bivalves have been a great interest. detected in the lower Valanginian of borehole section Presented in this work are the results of palynologi- Romanovskaya 140 (Zakharov et al., 1999). In bore- cal study of the Lower Cretaceous sections in the north hole section Gorshkovskaya 1017, age of deposits is of western and central Siberia (Fig. 1) and the analysis defined by palynological data only, as faunal remains of stratigraphic distribution of Early Cretaceous have not been found here. dinocysts in northern areas of Eurasia. The results of palynological analysis are used to subdivide in detail MATERIAL AND INVESTIGATION METHODS the respective North Siberian sections, to establish general succession of the Lower Cretaceous dinocysts This study is based on palynological analysis of the zones, and to define correlation levels recognizable far Lower Cretaceous marine sections extending one beyond Siberia in northern Europe, Greenland and another in vertical direction with partial overlap (Fig. 2). Canada. In total, the analyzed sections span stratigraphic inter- 577 578 PESTCHEVITSKAYA 50° 60° 70° 80° 90° 100° 110° 75° Nordvik section Anabar R. Anabar KARA SEA section Khatanga 70° Borehole Severo-Vologochanskaya 18 Dudunka Taz R. 65° Nadym R. Pur R. Yenisei R. Borehole Romanovskaya 140 Ob R. Borehole Gorshkovskaya 1017 Surgut 60° Nizhnevartovsk 0 400 km Fig. 1. Geographic localities of the studied sections. val from the Berriasian up to the lower Barremian are represented by dark gray shales and sandstones inclusive. In the Khatanga depression, the most com- (Fig. 2). plete succession has been studied in sections of the Nordvik Peninsula, where the Berriasian and Valangin- The lit-par-lit palynological analysis is performed ian deposits are represented by gray clays and aleurites for the majority of sections (Fig. 2). Core samples have (Fig. 2). Sands prevailing in the Hauterivian Stage are been selected for analysis with interval of 0.5 to 2 m intercalated with subordinate aleurite and clay interlay- depending on composition of the rocks: preference was given to clayey and silty sediments usually containing ers of lesser thickness. In section of the Anabar Bay most representative assemblages of palynomorphs. eastern coast, there is exposed only a lower part of the Before palynological analysis, rock samples have been Valanginian. Silty calcareous clays prevailing in this treated in hydrochloric and nitric acids, and in sodium section are intercalated with sandstone interlayers. In pyrophosphate solution. The cadmium liquid (CdI + the Ust-Yenisei region, the upper Berriasian–lower KI) with specific gravity 2.25 was used to separate Hauterivian succession studied in borehole section heavy fraction of sediments. Not less than 200 grains Severo-Vologochanskaya 18 is composed of gray aleu- were counted in each slide to determine percentage of rites with thin clay and sand interlayers (Fig. 3), and individual taxa relative to total amount of microphyto- there is a general tendency of sandy material increase fossils (spores, gymnosperm pollen, microphytoplank- upward in the section. In the northwestern Siberia, the ton). Boundaries of palynostratigraphic units are sub- Lower Cretaceous successions are studied in borehole stantiated using the following criteria: first and last sections Romanovskaya 140 (Berriasian–Valanginian) occurrences of species and genera, the grown diversity and Gorshkovskaya 1017 (Hauterivian–basal Barre- of certain genera and families, and increasing percent- mian). The first of these sections is composed of dark- age of particular taxa (as additional indication). A spe- colored shales in its lower part and of alternating silt- cial attention was paid to stratigraphic distribution of stones and sandstones in the upper one (Fig. 4). The taxa most important for determination of biostrati- Hauterivian–Barremian deposits of the second section graphic boundaries. First or last occurrence of such a STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 15 No. 6 2007 DINOCYST BIOSTRATIGRAPHY OF THE LOWER CRETACEOUS IN NORTH SIBERIA 579 taxon at the boundary of biostratigraphic unit corre- Zakharov et al., 1999), and around the latitudinal seg- sponds respectively to its oldest or latest datum level ment of the Ob River (Pestchevitskaya, 2005a). not only in North Siberia, but also in other regions. Comprehensive study of the dinocysts’ morphology To detect taxa of this kind, distribution of nearly was used to clarify characterization of certain taxa, to 300 dinocyst species have been analyzed in Lower Cre- analyze their facies and paleogeographic distribution, taceous sections of Siberia (data on the studied sec- and to describe new species (Pestchevitskaya, 2001, tions), northern Europe and America (published data). 2003, 2006a, 2006b). Stages of dinocysts’ development Species of greatest interest for the Lower Cretaceous in the Cretaceous seas of Siberia were identified, lateral stratigraphy of Siberia are listed in Fig. 5 and figured in radiation of microphytoplankton assemblages and Plates I and II. Consequently, boundaries of distin- diversity dynamics of some taxonomic and morpholog- guished dinocysts zones are in most cases the important ical groups depending on evolution of environments in stratigraphic markers of a great correlation potential. the Siberian paleobasin were detected (Lebedeva and Pestchevitskaya, 1997; Pestchevitskaya, 2002, 2003). INVESTIGATION HISTORY OF LOWER The succession of the Berriasian–Barremian dinocyst CRETACEOUS DINOCYSTS IN SIBERIA zones has been considered earlier (Pestchevitskaya, 2005a, 2006). Description of these zones is presented Systematic investigations of Lower Cretaceous below for the first time. microphytofossils from Siberia commenced in the ter- minal 1980s. Il’ina (1988) was first to study microphytoplankton from the Jurassic–Cretaceous boundary DINOCYST ZONES OF THE LOWER deposits in the Nordvik Peninsula, and Fedorova et al. CRETACEOUS IN NORTH SIBERIA (1993) examined the Berriasian dinocysts of the Boyarka River section. Dinocyst assemblages from Pareodinioideae–Batioladinium varigranosum– particular ammonite zones have been

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