Biofacies of Upper Jurassic and Lower Cretaceous Sediments of Central West Siberia V

ISSN 0869-5938, Stratigraphy and Geological Correlation, 2006, Vol. 14, No. 4, pp. 418Ð432. © åÄIä “Nauka /Interperiodica” (Russia), 2006. Original Russian Text © V.A. Marinov, S.V. Meledina, O.S. Dzyuba, O.S. Urman, O.V. Yazikova, V.A. Luchinina, A.G. Zamirailova, A.N. Fomin, 2006, published in Stratigraﬁya. Geologicheskaya Korrelyatsiya, 2006, Vol. 14, No. 4, pp. 81Ð96.

Biofacies of Upper Jurassic and Lower Cretaceous Sediments of Central West Siberia V. A. Marinov, S. V. Meledina, O. S. Dzyuba, O. S. Urman, O. V. Yazikova†, V. A. Luchinina, A. G. Zamirailova, and A. N. Fomin Institute of Petroleum Geology, Siberian Division, Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090 Russia e-mail: [email protected] Received December 16, 2004; in ﬁnal form, October 12, 2005

Abstract—Paleontological study of Upper Jurassic and Lower Cretaceous sediments recovered by boreholes in the AganÐVakh and NadymÐVengapur interfluves clarified environments of their deposition. As is shown, influx of siliciclastic material to central areas of the West Siberian sea basin varied through time. Taxonomic composition and ecological structure of nektonic and benthic fossil assemblages are analyzed and considered in terms of environmental factors such as hydrodynamics, aeration, temperature, and salinity of seawater. DOI: 10.1134/S0869593806040058

Key words: Biofacies analysis, Upper Jurassic, Lower Cretaceous, West Siberia.

INTRODUCTION ing quantity of recent publications devoted to problems of their genesis. Genesis and structure of Lower Creta- The Upper Jurassic and Lower Cretaceous marine ceous sediments are of particular interest. Some sediments of West Siberia attract attention of geologists researchers believe that their structure reflects a lateral primarily because of their hydrocarbon potential. After filling of paleobasin with siliciclastic material, and sed- a long-term study, depositional setting of these unique iments form therefore a clinoform composed of cross- facies are known better than for over- and underlying bedded layers (Naumov, 1977; Gurari, 2003; Karogo- stratigraphic intervals of the MesozoicÐCenozoic sedi- din et al., 2000, Grishkevich, 2005; and many others), mentary cover. Many works are dedicated to various while other scientists argue for a horizontally bedded aspects of Jurassic and Early Cretaceous history of the structure of Neocomian sediments (Onishchenko, region, e.g., to paleogeography (Gol’bert et al., 1968; 1994; Dankov, 1996; and others). Accumulation envi- Atlas…, 1976; Yasamanov, 1976; Bulynnikova et al., ronments of the unique Bazhenovo Formation of black 1978; Zakharov et al., 1983; and others), sedimentation shales, one of the main hydrocarbon sources in West and sequence-stratigraphy modeling of the JurassicÐ Siberia, are also interpreted controversially. A most Lower Cretaceous structural stage of the West Siberian plausible explanation is that the elevated content of dis- plate (Chernavskikh, 1994; Shurygin et al., 1999; persed organic matter in these sediments reflects spe- Ershov et al., 2001; Polyakova et al., 2002; Yan, 2003; cific paleogeographic, hydrologic, and hydrodynamic Grishkevich, 2005; and others), lithology (Gurova and conditions, primarily an extremely low sedimentation Kazarinov, 1962; Atlas…, 1976; Zakharov et al., 1983; rate in a relatively deep basin with restricted water aer- Yan et al., 2001; an others), and geochemistry (Kontor- ation, rather than a high biological productivity in the ovich et al., 1975, 2000; Ushatinskii and Zaripov, 1978; Bazhenovo sea (Zakharov and Saks, 1983; Braduchan and others), the isotope analysis included (Berlin et al., et al., 1986; and others). Depth in particular areas of the 1970, Kontorovich, 1985; Zakharov et al., 2005). In the JurassicÐCretaceous West Siberian basin, their position other works, there were published geophysical charac- relative to the shore, hydrodynamics, water mineraliza- teristics (Nesterov and Vysotskii, 1985; Mkrtchan et al., tion, temperature, and aeration are variably assessed as 1987; and others) and data on paleogeomorphology, well. hydrogeology, and paleoecology of the paleobasin (Naumov, 1977; Bulynnikova et al., 1978; Braduchan To evaluate variations in some environmental et al., 1986; Bochkarev, 1999; and others). Neverthe- parameters of the West Siberian sea during the Jurassic less, interest to origin of Mesozoic hydrocarbon-bear- and Cretaceous, we analyzed in relevant aspects main ing sediments remains vivid, as it is evident from grow- fossil groups. Gekker (1957, p. 5) justly noted: “fossil organisms are more adequate indicators of habitat and † Deceased. sedimentation environments than sediments.” Many

418

BIOFACIES OF UPPER JURASSIC AND LOWER CRETACEOUS SEDIMENTS 419

Boundaries of the West Kazym R. Siberian Levaya Kheta R. Nadym R. 4

Lowland Pur R. 1 2 10 12 Dudinka 5 3 11 8 9 6 14 7 13 Pyakupur R. Vengapur R.

Noyabr’sk 15 16 Ob R. Ob

Aivasedapur R. Surgut Kogalym

Ob R. Surgut Irtysh R. Agan R. Kolpashevo 23 26 Ob R. Tyumen 24 18 21 19 25 22 17 20 29 27 28 30 Nizhnevartovsk Vakh R. Sabun R. 0 200 km 0 50 km Lar’yak AB

Fig. 1. Location of study region in West Siberia (A) and studied drilling areas (B): (1) Nasel’skaya; (2) Pyakuta; (3) Malaya Pyakuta; (4) Malaya Kheta; (5) Nyudeyakh; (6) Verkhnii Nadym; (7) Yuznyi Inuchin; (8) Velitoi; (9) Vostochnaya Pyakuta; (10) Sugmut; (11) Romanov; (12) Umei; (13) Yuzhnaya Pyakuta; (14) Sutormin; (15) Ort’yagun; (16) Zapadnyi Novogodnyaya; (17) Biryuzovaya; (18) Vatin; (19) Zapadnyi Samotlor; (20) Samotlor; (21) Rubinovaya; (22) Zapadnyi Soromin; (23) Vanegan; (24) Tyumen; (25) Enitor; (26) Srednii Kul’egan; (27) Kolik’egan; (28) Labaznaya; (29) Khokhryakov; (30) Permyakov. researchers demonstrated efficiency of paleoecological tool for a high-resolution biostratigraphic subdivision approach to reconstruction of depositional environ- of drilled sections, chronostratigraphic positioning of ments for Jurassic and Lower Cretaceous sediments in formations and subformations, and for their spatial cor- the region under consideration (Bulynnikova et al., relation. Biostratigraphic zonations of a high resolution 1978; Komissarenko and Tylkina, 1981; Nezhdanov represent a strict framework that can be used to estab- and Ostanina, 1981; Belousova et al., 1981; Zakharov lish relationships between separate lithostratigraphic and Saks, 1983; Braduchan et al., 1986; and others). units of sedimentary succession, to define peculiarities of sedimentation, and to reconstruct geological history During the last 15 years, geologists from the Insti- of the region. In this work, that framework is used in tute of Petroleum Geology of the Siberian Division biofacies analysis aimed at reconstruction of fauna hab- RAS sampled a unique collection of macrofossils from itats and deposition environments in the Upper Jurassic the Upper Jurassic and Lower Cretaceous sections of and Lower Cretaceous basins of the study region. central West Siberia (AganÐVakh and NadymÐVen- gapur interfluves), which are represented by many hun- dreds specimens. In addition, microfossils have been INVESTIGATION METHODS studied in nearly 500 samples from more than 50 bore- Prior to biofacies analysis, we assessed secular holes drilled in 30 areas (Fig. 1). Data on taxonomic changes in siliciclastic material influx to central areas composition of studied fossils, their distribution in sed- of the West Siberian basin. Correlating local bio- and imentary sections, and stratigraphic inferences are dis- lithostratigraphic units with regional biostratigraphic cussed in several works (Zakharov et al., 1999; Shury- zones of West Siberia, Boreal standard, and General gin et al., 2000, Marinov et al., 2003, 2005; Dzyuba, stratigraphic scale (Resolution…, 2004, 2005), we 2004; and others). Paleontologic and lithologic charac- assessed ranges and accumulation rates of individual teristics of separate intervals penetrated by boreholes formations, subformations, and sequences. Chronolog- have been discussed as well (Marinov et al., 2005). The ical intervals of zonal units are calibrated relative to the main result of mentioned works is a composite bios- International geochronological time scale (Gradstein et tratigraphic scheme that includes the parallel ammo- al. 2004) as in the work by Haq et al. (1988). Sedimen- nite, bivalve, and foraminiferal zonations, which are tation rates are calculated by dividing the lithostrati- correlated between each other. The scheme is a reliable graphic unit thickness to its formation time. A possible

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420 MARINOV et al. effect of diagenetic sediment compacting has been dis- of core material specifics, it is difficult to perform bio- carded, since our purpose was to establish general facies analysis with details inferable from natural out- trends of sedimentation rate variations but not the abso- crops. Examining core samples, we cannot accomplish lute values. The established variations of sediment full taphonomic and paleoecologic observations in the influx through time are so significant that the compact- layer proper and define the whole composition of the ing effect could hardly influence the relevant general relevant bivalve assemblage in the section. The biofa- trend of changes in sedimentation rates. As the entire cies analysis is performed therefore for stratigraphic sedimentary succession composed of siliciclastic intervals wider than a layer, and results are naturally largely pelitic and silty rocks is of a uniform lithologi- generalized to some extent. cal structure, it is possible to suggest that ratio of a unit Based on available data on ecological specialization thickness to its deposition time approximates the sedi- of molluscan genera established in drill cores, we ment influx to the deposition basin in the relevant defined the following groups of taxa adaptable to cer- period of time. tain environmental parameters: We paid particular attention to analysis of fossils, (1) Rheophobic infauna includes genera dwelling on since they record sedimentation environments of a rock muddyÐclayey substrate and prosperous under oxygen unit (Logvinenko, 1967). For example, presence of deficiency (Malletia, Nuculana); ammonites and belemnite rostra in sediments is indica- (2) Euryoxybiont infauna living on soft substrate tive of their deposition in a sea basin. Ammonites and under conditions of low-energy hydrodynamics belemnites are remains of stenohaline organisms, and (Astarte, Nuculoma, Thracia, Paleoneilo, Dacryomya); their occurrence in sections outlines existence period of a sea basin with normal salinity. Morphological charac- (3) Euryoxybiont infauna preferring low-energy teristics of ammonite shells and belemnite rostra are hydrodynamics (Buchia, Praebuchia, Inoceramus, also informative. Hydrodynamic and hydrostatic prop- Oxytoma, Aequipecten, Limea, Limatula, Camp- erties of particular cephalopod groups, which dwelt at tonectes); certain depth in different hydrological zones of the (4) Oxyphilic infauna and epifauna of habitats with basin, depended on the shell shapes, their ornamenta- medium-energy near-bottom hydrodynamics (Melea- tion, and structure of septae (Reyment, 1958; Saks and grinella, Grammatodon, Chlamys, Protocardia, Pleu- Nal’nyaeva, 1970; Gustomesov, 1976; Westerman, romya, Pinna); 1990, 1993; Dzyuba, 2000). (5) Oxyphilic and rheophilic infauna and epifauna Changes in taxonomic compositions of families and populating zones with high-energy hydrodynamics genera through succession of stratigraphic units are (Liostrea (Praeexogyra), Entolium, Isocyprina, Tan- determined by both evolutionary and migratory pro- credia, Mclearnia, Pronoella, Arctica, Rolierella, Pro- cesses, i.e., by invasion of taxa untypical of the basin. veniella, Lima, Striatomodiolus); Par example, genera Rasenia, Zonovia, Aulacostepha- (6) Nektonic fauna of free-floating stenohaline nus (Aulacostephanidae), Ringsteadia, and Aspi- dwellers of pelagic zone: ammonites, belemnites, doceras migrated from the west and became wide- teuthids (represented in sediments by onychites only). spread in the West Siberian basin in the terminal Oxfor- Categories “rare,” “common,” and “abundant” are dian and Kimmeridgian. Invasion of immigrants was used for quantitative assessment of particular ecologi- determined by paleogeographic reorganization accom- cal groupings in the core. panied by changes in current directions and by temper- The biofacies analysis has been applied to benthic ature increase in the basin. Leveling or differentiation foraminifers as well. Ecological specialization of indi- of taxonomic composition in assemblages occupying vidual foraminiferal taxa is considered in many works different parts of the basin and widening or, in contrast, (Kipriyanova et al., 1975; Basov et al., 1975; Bulynni- contraction of arctic taxa distribution areas, which kova and Gol’bert, 1980; Kipriyanova, 1981; Basov, reflect migration of hydrological fronts, provide valu- 1991; Marinov and Zakharov, 2001; and others). Pres- able information as well. ence and diversity of foraminiferal ecological group- Benthic organisms are important for reconstructing ings are informative with respect to abiotic factors. Of physicochemical conditions in the bottom zone of the particular significance among these groupings are basin. Of particular significance are bivalves, the main oxyphilic taxa (nodosariids, polymorphinids, and cera- components of macrofossil assemblages found in drill tobuliminids), euryhaline genera Globulina and Gut- cores. They are reliable indicators of various environ- tulina, thermophilic immigrants from the BorealÐ mental parameters, e.g., of salinity, temperature, sub- Atlantic biogeographic region (genera Epistomina, Val- strate, hydrodynamics, bottom waters aeration and, to anginella, Ceratolamarckina, Pseudolamarckina), and some extent, of the basin bathymetry. The available representatives of rheophilic taxa (genus Ammodiscus, paleoecological classifications (Zakharov and Yudov- species with spheroid tests of the genus Cribrosto- nyi, 1974; Zakharov and Shurygin, 1978; Zakharov moides) dwelling at the bottom regularly affected by et al., 1979) depict relationships between taxonomic wave activity, i.e., at the depth of 10 m (Zakharov et al., composition and abiotic environmental factors. Because 2005). Analysis of variations in abundance of indicative

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BIOFACIES OF UPPER JURASSIC AND LOWER CRETACEOUS SEDIMENTS 421 foraminiferal taxa in the Upper Jurassic and Lower NadymÐVengapur and AganÐVakh areas is illustrated Cretaceous sediments is useful for defining periods of in Figs. 3 and 4. The lower Vasyugan Subformation the warm water influence, strengthened/weakened composed largely of clays and argillites with subordi- hydrodynamics, changes in concentration of dissolved nate interbeds of siltyÐsandy sediments encloses rare oxygen, unstable salinity, and bathimetric changes in remains of nektonic and benthic mollusks. The Callov- particular areas of the basin. We performed such an ian and early Oxfordian ammonites and belemnites analysis for the KimmeridgianÐHauterivian interval belong to Boreal families Cardioceratidae and Cylin- based on sections with most representative foramin- droteuthidae. Taxonomic composition of ammonite iferal assemblages, which are recovered in the Pyakuta genera Longaeviceras, Quenstedtoceras (Soaniceras), (NadymÐVengapur interfluve), Kolik’egan and Khokh- and Cardioceras (Scaburgiceras), and belemnites of ryakovo areas (AganÐVakh interfluve). the genus Longibelus (Holcobeloides) suggest that the Taxonomic diversity of foraminiferal assemblages West Siberian sea, the NadymÐVengapur and AganÐ is determined based on the Simpson’s index: Vakh areas included, was closely connected in the Call- ovian and early Oxfordian with Arctic seas washing the Σ 2 D = 1/ pi ,where pi is the occurrence frequency of Siberian platform and with the East European basin. the i-species. Widespread remains of nektonic cephalopods imply a The diversity is classed as low, medium, and high, normal-salinity of open sea basin. Bivalve mollusks in when D is 1Ð3, 4Ð6, and >6, respectively. Lagoonal for- siltyÐsandy sediments are represented by species of aminiferal communities are of low diversity. Maximal genera Meleagrinella, Protocardia, Pleuromya, Ento- diversity is peculiar of assemblages from shallow and lium, Mclearnia, Arctica, Proveniella, Lima, and of the modestly deep settings. Deepwater assemblages are family Buchiidae. The prevailing oxyphilic and rheo- again of the low taxonomic diversity. philic forms of bivalves are indicative of shallow-water sedimentation settings, good water aeration, and high- energy hydrodynamics near the bottom, i.e., of condi- RESULTS tions favorable for many bivalve genera. These bivalves Data on thickness of lithostratigraphic units (forma- are particularly abundant in the AganÐVakh interfluve, tions, subformations, and individual sequences) and where they are accompanied by thermophilic Mclear- time spans of their deposition, which are coordinated nia species. Other taxa could exist in a wide tempera- with the high-resolution biostratigraphic scale, substan- ture range. tiate the qualitative assessment of sedimentation rate The upper Vasyugan Subformation composed dynamics in the study region. Earlier estimates of this largely of sandstones and siltstones with argillite inter- kind (Ushatinskii and Zaripov, 1978; Nesterov and beds hosts remains of nektonic and benthic organisms Vysotskii, 1985; Chernavskikh, 1994) were based on (epifauna and infauna). The ammonite assemblage is substantially less detailed biostratigraphic scale (stage still dominated here by representatives of Arctic genera level only), being obtained mostly for the formation- Cardioceras and Amoebeoceras. Sandstones and silt- rank units. The average deposition rate of the JurassicÐ stones yield diverse bivalve genera Malletia, Nuculana, Neocomian succession in the study region was highly Astarte, Nuculoma, Thracia, Paleoneilo, Oxytoma, variable through time (Fig. 2). In the Vasyugan period Aequipecten, Camptonectes, Meleagrinella, Entolium, (CallovianÐOxfordian), sedimentation rates were low, Tancredia, Pronoella, Rolierella, and Striatomodiolus. with minimal values corresponding to the Georgievka Thus, benthic macrofossils correspond in this case to (terminal OzfordianÐKimmeridgian) and Bazhenovo both the rheophobic and rheophilic forms (Fig. 3) that (VolgianÐearly Boreal Berriasian) times. In the second can be explained by unstable aeration and hydrody- half of the Boreal Berriasian, sedimentation rate namic in bottom waters. It is likely that zones with slightly increased to reach maximal values in the termi- high-energy hydrodynamics and sufficient aeration of nal BerriasianÐinitial Valanginian. This maximum is bottom waters alternated in the basin with areas of a best expressed in the AganÐVakh interfluve, being low-energy hydrodynamics. In the NadymÐVengapur slightly lower in the NadymÐVengapur area. The onset area, siltstones yield eurybiont and rheophlic bivalve of avalanche sedimentation corresponds to formation of genera Thracia, Praebuchia, Inoceramus, Limatula, the Achimovka siltstoneÐsandstone sequence. In the Entolium, and Pseudolimea, but rheophobic forms have terminal ValanginianÐinitial Hauterivian, influx of not been encountered. siliciclastic material decreased, though remained sev- In the AganÐVakh area, the molluscan assemblage eral times higher than in the Late Jurassic. Since the from the Georgievka Formation of substantially clayey upper Vanden Subformation (upper ValanginianÐBarre- sediments includes representatives of all ecological mian) is lacking marine fossils, we failed to establish its bivalve groupings (Malletia, Paleoneilo, Buchia, formation time and to calculate sedimentation rates for Meleagrinella, Chlamys, Isocyprina), which indicate separate members. variable hydrodynamic and aeration environments and Distribution of ecological groupings of mollusks diverse substrate types in their biotopes. Presence of and foraminifers in different stratigraphic intervals of stenohaline ammonites, teuthids, and abundant belem- the Upper JurassicÐLower Cretaceous section in the nites implies that salinity in the basin was normal.

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422 MARINOV et al. and more 150–180 150 125 115–150 of the Sortym Formation 100 Average sedimentation rate of the Achimovka Sequence and upper part Average sedimentation rate, mm/k.y. 75 70–110 ? 50 3–15 Average sedimentation rate 25 of the Podachimovka Sequence 5.5–13.5 1–10 1–4.5 0.1–1.4

upper lower upper lower Subformation

einVanden Megion Bazhenovo Georgievka Vasyugan Formation AB 100 65–130 Average sedimentation rate of the Achimovka Sequence 75 and upper part of the Sortym Formation 60–100 45–95 50 Average sedimentation rate 25 of the Podachimovka Sequence 3–9 Average sedimentation rate, mm/k.y. 4–9 5.5–8 0.1–2 2.5–4

upper lower Subformation

Formation Ust’-Balyk Sortym Bazhenovo Georgievka Vasyugan

, - Kitchini – . spp. Scar spp. spp. s. str. ( ) spp. Ex gr. spp Regulare narium Maynci ensis Jason , Subordi- Tolli Klimovski- 2005) Quadrifidus Kochi

Okensis Magnum

Astieriptychus Vogulicus Analogus Pectinatus Versicolor Insolutus Ë and beds of Keyserlingi Ramulosus Subcrassum ex gr. Bojarkensis

West Siberia somites Dichotomites

Densiplicatum Gloenlandicus Milaschevici Tenuiserratum Mesezhnikowi Cadoceratinae spp. Ë Homol- Sigaloceras Syzranicus burgiceras Autissiodorensis

Involuta Ammonite zones Sibiricus berti Eudoluta cordata Dividuum Lam- Amoeboceras Cardioceras Pseudo-

Cardioceras 5–6 1–4 (Resolution..., 2004;

(ISS, 2004) (ISS, Age, Ma Age,

136.4±2.0 140.2±3.0 145.5±4.0 150.8±4.0 155.7±4.0 161.2±4.0 164.7±4.0 1988)

(Haq et al., et (Haq Age, Ma Age,

121 122 124 125 127 128 130 131 138 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 Kitchini Scarb. , Tolli Calyx Jason Kochi 1997) Athleta Nikitini Boreal Klimovi Fulgens Panderi Nodiger Apertum Sokolovi Eudoxus Eudoxus Virgatus Subditus Sibiricus Regulare Glosense Lamberti Kotschetkovi Serratum Analogus standard Bidichotomus Cordatum Versicolor Oppressus Acanticum Sosvaensis Gloriosum

Coronatum Calloviense Bojarkensis Quadrifidus Triplodiptychus Ramulicosta Ramulicosta

Percaelatum Rosenkrantzi Mesezhnikowi Bidichotomus Astieriptychus Densiplicatum Tenuiserratum Klimovskiensis Nordenskjoeldi Pseudoscythica Involuta

Borealis Autissiodorensis

(Zakharov et al.,

Obliteratum Substage l. upper lower upper oe middle lower upper lower upper middle lower upper mid. lower Berriasian

Boreal Stage Hau. Valanginian Volgian Kimmeridgian Oxfordian Callovian

Lower Upper Middle Series

System Cretaceous Jurassic

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BIOFACIES OF UPPER JURASSIC AND LOWER CRETACEOUS SEDIMENTS 423 System Series Stage Substage Formation Formation Subformation Subformation Hauter. upper Ust’-Balyk u. Vanden lower Lower Cretaceous lower Valanginian Achimovka Sequence Sortym

Podachimovka Megion Sequence Boreal Berriasian upper lower Bazhenovo Bazhenovo Volgian l. middle Upper Kimmer. Georgievka Georgievka Jurassic m. upper upper Oxford. l. l. u. Vasyugan Vasyugan lower lower Middle Callov. l. m. u. u. AB

a 12 3 45 6 7 8910 b

Fig. 3. Stratigraphic distribution of ecological molluscan groupings in the CallovianÐlower Hauterivian section of the NadymÐVen- gapur (A) and AganÐVakh (B) interﬂuve areas; (1Ð3) occurrence frequency (1) rare, (2) common, and (3) abundant; (4Ð9) ecologic groupings of bivalves and cephalopods: (4) rheophobic infauna, (5) euryoxybiont infauna preferring low-energy hydrodynamics, (6) euryoxybiont epifauna preferring low-energy hydrodynamics, (7) oxyphilic infauna and epifauna preferring moderate-energy hydrodynamics, (8) oxyphilic and rheophilic infauna and epifauna; (9) nektonic (a) ammonites, (b) belemnites and teithids; (10) sediments; (Callov.) Callovian; (Oxford.) Oxfordian; (Kimmer.) Kimmeridgian; (Hauter.) Hauterivian; (l.) lower; (m.) middle; (u.) upper.

Belemnites are represented by species of subgenera the NadymÐVengapur area are represented by rare Pachyteuthis (Pachyteuthis, P. (Boreioteuthis), Simo- remains of euryoxybiont and oxyphilic genera Oxy- belus (Simobelus), and Lagonibelus (Lagonibelus) of toma, Meleagrinella, and Protocardia. the family Cylindroteuthide, which suggest shallow or Abundant bivalves found in the lower argillaceous relatively deep sedimentation settings. Foraminiferal part of the Bazhenovo Formation (the uppermost lower assemblages are very diverse (Marinov et al., 2005), VolgianÐmiddle Volgian) of the AganÐVakh area repre- typical of the same environments. Bivalve mollusks in sent byssiferous euryoxybiont epifauna of biotopes

Fig. 2. Variations of sedimentation rate in the CallovianÐearly Hauterivian, the NadymÐVengapur (A) and AganÐVakh (B) interﬂuve areas, (1Ð6) ammonite zones and beds: (1) Lariensis, (2) Ilovaiskii, (3) Maximus, (4) Crendonites spp., (5) Taimyrensis, (6) Mau- rynijensis, Pulcher; (Scarb.) Scarburgense; (Hau) Hauterivian; (l.) lower; (m.) middle; (u.) upper.

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424 MARINOV et al.

in bottom sediments. In the NadymÐVengapur area, remains of benthic organisms in the lower part of Bazhenovo Formation are scarce, represented only by Subformation System Series Stage Substage Formation Formation buchiids, while prevailing ammonites (Pavlovia, Stra- jevskya, Dorsoplanites, Laugeites, Epilaugeites) occur Thermophilic Oxyphilic Rheophilic Euryhaline in association with belemnites (Cylindroteuthis, Pachy- teuthis) and onychites. Liostrea plastica valves are Oxyphilic Rheophilic Euryhaline

Hauterivian present as well. In opinion of Zakharov, buchiids could Ust’-Balyk

upper survive a slight but not high oxygen deﬁciency (Zakharov and Saks, 1983). Prevalence of hemipelagic u. nekton over benthos suggests however that the oxygen-

Vanden deﬁcient zone in the Nadym–Vengapur area involved a part of bottom waters that is a common consequence of stagnation. lower In the middle part of the Bazhenovo Formation lower lower (upper Volgian Substage) in the AganÐVakh area, dom- Valanginian Sortym Lower inant onychites and ammonites Craspedites (Cras- Cretaceous pedites), C. (Taimyroceras), Kachpurites (family Cras- peditidae), and Virgatosphinctes are accompanied by scarce bivalves (Inoceramus). A low proportion of mol-

Boreal lusks in the upper Volgian sediments of the AganÐVakh Berriasian interﬂuve might be explained by a slight anoxia in bottom waters. In the NadymÐVengapur area, coeval sediments contain, along with Craspeditidae, Vir- upper gatosphinctes, and onychia, abundant bivalves of genera Buchia and Inoceramus, and aeration of bottom Bazhenovo Megion Bazhenovo waters in this part of the basin was therefore sufﬁcient

Volgian for epifauna existence. In both the NadymÐVengapur and AganÐVakh 1 u. l. Upper areas, molluscan assemblages are of low diversity in the Jurassic 2 upper part of the Bazhenovo Formation (lower Boreal l. middle Kimmer. Georgievka Georgievka 3 Berriasian, Chetaites sibiricus Zone and lower part of the Hectoceras kochi Zone). Like in underlying sedi- ABments of the formation, benthic organisms are represented only by euryoxybiont byssiferous epifauna Fig. 4. Stratigraphic distribution of some ecological foraminiferal groupings (A) in the KimmeridgianÐlower Hau- (Buchia, Inoceramus). Ammonites Praetollia, Shulg- terivian of the Pyakuta area (NadymÐVengapur interfluve); inites, Subraspedites, and Hectocera, fish skeletons, (B) in the KimmeridgianÐlower Valanginian of the and onychites are more numerous. Paleontological data Kolik’egan amd Khokhryakov areas (AganÐVakh inter- allow an assumption that in the terminal Bazhenovo fluve); occurrence frequency (1) single, (2) common, time benthic organisms could dwell only on the bottom (3) dominant (other symbols as in Fig. 3). surface, since below there were anoxic environments. Density of benthic populations became substantially with low-energy hydrodynamics (Buchia, Oxytoma, lower at that time, as it is evident from their decreased Inoceramus). Rare foraminifers represent an impover- abundance in core samples. ished assemblage with dominant Trochammina septen- Highly carbonaceous argillites of the Bazhenovo trionalis. Abundant remains of nektonic organisms Formation penetrated by borehole Kolik’eganskaya- belong to ammonites (Dorsoplanites, Laugeites, Epil- 148 (AganÐVakh interfluve) contain non-mineralized, augeites), belemnites (Pachyteuthis), and teuthids partly coalified platy and band-like algae remains (onychia). In addition, there were found scarce (Plate). In their appearance, macro- and microscopic pseudoplanktonic Ostrea species Liostrea plastica phytoleims (remains of altered organic matter) are (Trd.), the bivalves dwelling attached to shells of living characterized by multicellular anatomic structure. ammonites and migrated together with them in the Many fragments are pyritized. Abundance of algae and basin (Zakharov and Saks, 1983). As infaunal organ- their good preservation imply environments favorable isms are missing from bivalve assemblages, conditions for accumulation of organic matter in the basin. It can below the sedimentÐwater interface were likely unfa- be assumed that branching benthic algae comparable vorable for them. Disappearance of infauna from with present-day Phaeophyceae (Laminaria and Sar- benthic community at the beginning of the Bazhenovo gasso alga) formed large communities in coastal zones. time is explained by the onset of anoxic environments During their development, algae could be torn off the

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Plate

1 5 3

2 12

10 11 6

Plate. Algal remains from rocks of the Bazhenovo Formation recovered by borehole Kolik’eganskaya-148 (photos in transmitted light (thin section), magnification ×40, except for fig. 1, magnification ×10). (1) general view of the rock in the thin section with alternating thin sediment laminae and fragmentary remains of filamentous algae, Sample 148-43, interval 2377.0Ð2382.0 m, depth 2380.5 m; (2, 3) fragments of organic-walled filamentous algae, Sample 148-22, interval 2377.0Ð2382.0 m, depth 2380.5 m; (4) fragment of a band-like thallus at the bifurcating area, Sample 148-20, interval 2388.5Ð2395.0 m, depth 2391.5 m; (5) a thallus fragment with signs of transverse cellular structure, Sample 148-20, interval 2388.5Ð2395.0 m, depth 2382.5 m; (6Ð12) fragments of organic-walled filamentous algae: (6) Sample 148-11, interval 2395.0Ð 2402.0 m, depth 2396.0 m, (7) Sample 148-45, interval 2377.0Ð2382.0 m, depth 2380.5 m, (8) Sample 148-52, interval 2377.0Ð 2382.0 m, depth 2377.2 m, (9) Sample 148-12, interval 2395.0Ð2402.0 m, depth 2396.5 m, (10) Sample 148-22, interval 2388.5Ð 2395.0 m, depth 2391.5 m, (11) Sample 148-26, interval 2388.5Ð2395.0 m, depth 2380.5 m, (12) Sample 148-38, interval 2388.5Ð 2395.0 m, depth 2388.5 m. substrate to float freely at the surface through the basin. the AganÐVakh area, benthic macrofossils are repre- As is thought (Braduchan et al., 1986), algae and sented by rare buchiids. Assemblages of benthic fora- microphytoplankton were main sources of organic mat- minifers are dominated by species of the genus Tro- ter in the Bazhenovo Formation. chammina. There are also rare shells of unidentifiable The fauna from higher layers of the Boreal Berria- ammonites, abundant onychites, and fish remains. sian is found in the Podachimovka clayey sequence. In In the NadymÐVengapur area, the assemblage of scarce

STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 14 No. 4 2006 426 MARINOV et al.

Terminal Valanginian – initial Hauterivian lowermost Valanginian), the AganÐVakh area, ammonites (Tollia, Neotollia) and buchiids are equally abundant. In examined boreholes of the NadymÐVengapur area, the Achimovka Member of the Sortym Formation yielded only rare fragments of unidentifiable molluscan shells. Mid-early Valanginian The interval of the Euryptychites quadrifidus– Euryptychites astierptychus ammonite zones in the lower Vanden Subformation is composed in the western part of the AganÐVakh area of mudstone members alternating with sandstoneÐsiltstone beds; the rocks contain only representatives of the ammonite genus Neotollia and bivalves genera Buchia and Limatula. Terminal Boreal Berriasian – initial Valanginian The foraminiferal assemblage is of a modest taxonomic diversity, dominated by Recurvoides forms. The fauna composition indicates normal salinity of the basin in the mid-early Valanginian, aeration of bottom waters sufficient for the existence of epibenthic bivalves, and low-energy hydrodynamics. The eastern part of the basin (Kolik’egan area) was populated at that time by diverse benthic communities more tolerant to environmental factors; these communities included both the Volgian – early Boreal Berriasian oxyphilic bivalve genera (Protocardia, Tancredia, Mclearnia, Pronoella, Arctica) preferring well-aerated mobile waters and euryoxybionts (genera Buchia, Astarte, Nuculoma, Dacryomya). Stenohaline genera and species were dominants. In addition to eurythermal taxa, bivalve communities included thermophilic forms (genus Mclearnia). Infauna was also diverse, represented by genera Protocardia, Tancredia, Astarte, Nuculoma, and Dacryomya. In the NadymÐVengapur area, the middle siltyÐ clayey member of the Sortym Formation corresponding to the interval of the Euryptychites quadrifidus–Euryp- Callovian – Kimmeridgian tychites astierptychus ammonite zones encloses the molluscan assemblage, which is close in terms of ecological groupings to assemblage from easterly sections of the AganÐVakh area. The assemblage includes nektonic ammonite genera Neotollia and Euryptychites (Propolyptychites) associated with bivalve epifauna (genera Buchia, Oxytoma, Limatula, and Entolium) and infauna (genera Astarte, Thracia, Striatomodiolus, and 123456Pinna). The foraminiferal assemblage is here of a high taxonomic diversity. Assemblages of macro- and Fig. 5. Biotopes and habitat environments of the Callovian, Late Jurassic, and Neocomian marine fauna in the central microfossils suggest favorable habitat conditions in a part of the west Siberian basin: (1) water column; (2) sedi- shallow well-aerated basin (oxyphilic genera Pinna, ments; (3) anoxic environments; (4) hydrodynamics in the Entolium, Striatomodiolus are diverse) with alternating bottom water layer (arrow size is proportional to hydrody- zones of low- and high-energy hydrodynamics. Judging namic intensity); (5) belemnites; (6) teuthids (other symbols as in Fig. 3). from presence of stenohaline groupings, salinity in the basin was normal. It should be noted that development of infauna and oxyphilic forms, which are absent in the benthic foraminifers consists of specimens belonging to Volgian and Berriasian stages, commenced in the the genus Recurvoides. It is conceivable that this part of NadymÐVengapur area earlier than in AganÐVakh area. basin was characterized in the second half of the Boreal Numerous representatives of Astarte (infauna), Berriasian by environment unfavorable for the benthic (?)Pleuromya, and Entolium (oxyphilic genera) are fauna because of oxygen deficiency in bottom waters. identified in the uppermost part of the Neotollia klimovskiensis Zone in the Sortym Formation. In general, In the Achimovka siltstoneÐsandstone sequence of the bivalve assemblage from sediments under consider- the Megion Formation (uppermost Boreal BerriasianÐ ation consists of genera Astarte, Buchia, Inoceramus,

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(?)Limea, (?)Pleuromya, and Entolium. The foramin- fossils are found mainly in siltyÐsandy rocks the Vasyu- iferal assemblage is of a medium taxonomic diversity. gan Formation, which have been deposited in shallow- Rostra of belemnite genus Pachyteuthis (Acroteuthis) water settings (Fig. 5). In the terminal Bathonian, Call- are also present. The listed bivalve genera preferred ovian, and initial Oxfordian (the early Vasyugan time), mostly the low-energy hydrodynamic conditions and benthic communities were dominated by oxyphilic and soft substrate, although some of them are rheophilic rheophilic bivalves (epifauna and infauna) indicative of ((?)Pleuromya, Entolium). well-aerated bottom environments with high-energy In the NadymÐVengapur area, the bivalve assem- hydrodynamics. During the remaining interval of the blage from the uppermost Sortym Formation and over- Oxfordian (late Vasyugan time), benthic communities lying Ust’-Balyk Formation of sandy, silty and clayey populated biotopes with different aeration and hydro- sediments (uppermost lower Valanginian, upper Val- dynamics in the bottom layer. It can be assumed that anginian, and lowermost Hauterivian) includes repre- areas of high-energy hydrodynamics and good aeration sentatives of rheophobic infauna (Malletia) and associ- alternated at that time in the basin with more stagnant ates with oxyphilic and rheophilic epifauna typical of zones of low-energy hydrodynamics. In the AganÐVakh biotopes with high-energy hydrodynamics: genera area, even rheophobic bivalve forms appeared in some Entolium and Liostrea (Praeexogyra). The last genus is periods. thermophilic, in addition. Such a combination of eco- A high diversity of biotopes was retained in the ter- logical groupings suggests contrasting environments. minal Oxfordian, Kimmeridgian, and initial Volgian, Diversity of foraminifers in the Ust’-Balyk Formation although bottom water were slightly less aerated and is lower than in the Sortym Formation; they are domi- calmer, as it is evident from decreased abundance of nated by representatives of the genus Cribrostomoides oxyphilic and rheophilic genera. A relatively deep zone whose spheroid tests are typical of sea shoals with high- of the West Siberian sea, the areas under consideration energy hydrodynamics (Mariniov and Zakharov, 2001). included, extended at that time (Zakharov et al., 1983), Ammonites are scarce, belonging to genera Siberites which is reflected in very slow accumulation of fine (uppermost Sortym Formation) and Simbirskites (Ust’- clays (Georgievka Formation) during a long period (8 Balyk Formation). m.y.). Abundance of belemnites, distant relatives of present-day squids, implies the water temperature close to that characteristic of the modern tropical zone DISCUSSION (Gol’bert et al., 1968). The paleotemperature analysis During the early Callovian through mid-Kimmerid- supports this inference (Zakharov et al., 2005). gian, the West Siberian basin developed against back- While in the CallovianÐinitial Oxfordian, the West ground of tectonic subsidence gradually progressing Siberian basin was populated predominantly by Arctic (Belozerov, 1989; Kontorovich et al., 2001). Character- families and genera, the ammonite, belemnite, bivalve, istic of that time was gradual eustatic sea-level rise and foraminiferal communities of the terminal Oxford- (Haq et al., 1988; Sahagian et al., 1996). The relevant ianÐKimmeridgian developed under influence of a period of Mesozoic history corresponded to a large warm current, which promoted invasion of thermo- transgression in the West Siberian sedimentation basin philic ammonites (perisphinctids) and other inverte- that terminated by the middle of the Boreal Berriasian. brate groups into the West Siberian basin. In the Callovian and Oxfordian, West Siberia was cov- ered by a spacious epicontinental sallow sea with well- During the VolgianÐearly Boreal Berriasian aerated water column (Zakharov et al., 1983; Kontorov- (Bazhenovo) time, sedimentation in the basin pro- ich et al., 2000). Several transgressiveÐregressive gressed under influence of significant regional subsid- cycles are recorded in accumulated sediments (Shury- ence, the onset of which corresponded with the general gin et al., 1999). Regression phases particularly notable eustatic sea-level rise (Kontorovich et al., 1975; Beloz- in the Oxfordian were characterized by influx of erov, 1989; Shurygin et al., 1999). There is an opinion coarser clastic material the from the south and east to that this subsidence took place in a relatively short the basin central part, where sandyÐsilty and sandy period (not more then 1 m.y.) in the early Volgian time sequences accumulated (Yan et al., 2001; Yan, 2003). as a consequence of eclogitization and compaction of The greater part of the Callovian and some periods of mafic rocks at the crust base (Artyushkov, 1993). What- the Oxfordian corresponded to accumulation period of ever the reason, the Bazhenovo sea was undoubtedly siltyÐclayey sediments. In general, the CallovianÐ deepest in the entire Mesozoic history of the West Sibe- Oxfordian interval is represented in the basin areas by rian basin. Many researchers presume development of alternating lithofacies deposited in coastalÐmarine and a pseudoabyssal depression in its interior part, where shallow-marine settings, as it is evident from the min- depth ranged from approximately 400 to 800Ð900 m in eralogical composition, structural features, boundary local areas (Gol’bert et al., 1968; Bulynnikova et al., patterns, distribution of trace fossils, and their character 1978; Zakharov and Saks, 1983; Gurari et al., 1983; (Vakulenko and Yan, 2001; Yan et al., 2001; Yan, 2003). Braduchan et al., 1986; and others). In opinion of men- Influx of siliciclastic material was insignificant (Fig. 2). tioned researchers, presence of that depression nearby In the NadymÐVangepur and AganÐVakh areas, benthic the western shore of the basin is well consistent with

STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 14 No. 4 2006 428 MARINOV et al. distribution of biota and concentration of organic mat- with occurrence of ichno-fossils in black shales of the ter. According to calculations (Ushatinskii and Zaripov, Bazhenovo Formation (Zakharov et al., 1998; Eder 1978), this was a starved sedimentation basin: the aver- et al., 2003), which needed some quantity of dissolved age subsidence rate of 0.012Ð0.015 mm/year versus the oxygen in interstitial waters for their life activity, average sedimentation rate of 0.002Ð0.003 mm/year. undoubtedly for the Chondrites activity. The biota as a We estimated similar sedimentation rate for the whole confirms this conclusion. NadymÐVengapur and AganÐVakh areas (Fig. 2). Temperature, illumination, and water circulation in Nektonic molluscan groups in the Bazhenovo sea coastal zone of the West Siberian sea were favorable for were represented by ammonites, belemnites, and development of multicellular branching algae whose teuthids. Belemnites in the relevant core samples are remains have been found in the study areas. Algae pre- rare, having small rostra, especially in the Bazhenovo served in a form of isolated phytoleims have been likely Formation. They are interpreted as younger specimens transported from their original biotope and could repre- from deepest zone of the basin, while adult specimens sent an important source of organic matter buried in preferred shallow-water settings (Saks and Nal’nyaeva, sediments of the Bazhenovo Formation. 1979). According to other viewpoints, these belemnites The Neocomian stage in development of the West represent separate small species. It is conceivable that Siberian sedimentary basin differed substantially from young and, consequently, feeble specimens could be the preceding one. It was a time of intensified tectonic transported by currents or storms to open sea, where activity in the basin and surrounding land areas; char- they died. Conditions were substantially more favor- acteristic of the basin was regressive trend in general able for teuthids, which had no solid skeleton (carbon- and increased sedimentation rate (Gurova and ate or chitinous), being represented in the Bazhenovo Kazarinov, 1962; Kontorovich et al., 1994; and others). Formation by abundant onychites (Braduchan et al., 1986). During the second half of the Boreal Berriasian (pre-Achimovka time), accumulation of clayey sedi- Benthic communities in areas under consideration ments was in progress in the study areas, although were practically lacking representatives of infauna influx of siliciclastic material slightly increased (Fig. 2), forms that is well consistent with development of and content of organic matter in sediments declined. anoxic environments in bottom waters (Fig. 5). The rel- Foraminifers appeared in benthic communities, which evant basin areas were populated by epibenthic buchi- remained practically unchanged in structure of macro- ids and inoceramids, which are tolerant to a wide spec- fauna assemblages. The pelagic zone was populated as trum of environmental factors: hydrodynamics, before by teuthids, which dominated among nektonic bathymetry, and oxygen concentration in waters. At the mollusks. In the terminal BerriasianÐinitial Valanginian same time, they preferred relatively deep (moderately (Achimovka time), at the commencement of avalanche cold) settings with low-energy hydrodynamic environ- sedimentation, largely siltyÐsandy material was sup- ments, but could not survive a considerable oxygen plied into the basin, but benthic communities with deficiency (Zakharov and Saks, 1983). In opinion of increased abundance of bivalve mollusks experienced Zakharov, populations of buchiids and inoceramids no transformation in structure. As is assumed by many were of a high density in the deepest part of the Bazhen- researchers, the West Siberian basin was shoaling at ovo sea, being concentrated here in the shallowest bot- that time, remaining relatively deep, however, compa- tom areas. Inasmuch as these bivalves need well-aer- rable in depth with the lower subtidal zone: 100Ð150 m ated environments, the oxygen deficiency or even con- deep in the NadymÐVengapur area (Bochkarev, 1999) tamination of bottom waters by hydrogen sulfide, and 70Ð100 (Bulynnikova et al., 1978) to 150Ð200 m which was postulated by many researchers, is admissi- deep (Nesterov and Vysotskii, 1985) in the AganÐVakh ble only for deepest seafloor areas and/or for epochs area. Judging from the studied fossil assemblages, between existence periods of bivalves (Zakharov and hydrodynamics and water aeration did not differ from Saks, 1983; Braduchan et al., 1986). The content of dis- conditions of the Bazhenovo time (Fig. 5); otherwise solved oxygen in bottom water of the basin was con- benthic communities just had no time for reorganiza- trolled by intensity of water exchange with the Arctic tion. The granulometric analysis of the Achimovka basin, i.e., by shoaling or deepening of seaways for deposits in the Ob River middle reaches (Nesterov and oxygen-saturated Arctic waters (Braduchan et al., Vysotskii, 1985) suggests the low-energy environments 1986). Nevertheless, the idea of bottom waters contam- of sedimentation. Deposition of high-carbonaceous ination by hydrogen sulfide in the deep part of the sediments probably ceased under influence of several Bazhenovo sea is still popular among some researchers factors: an increased influx of siliciclastic material, the (Kontorovich et al., 1994, 2000; and others). basin shoaling, and aeration of bottom waters, although We share the standpoint that anoxic conditions in the last factor was less influential. bottom waters and at the sedimentÐwater interface in The mid-early Valanginian (Euryptychites quadrifi- the central deep part of the Bazhenovo sea were discon- dus and E. astieriptychus phases) was a time of more tinuous and probably disappeared in some periods intense aeration in bottom sediment of the West Sibe- (Braduchan et al., 1986). The inference is consistent rian sea. The intensified hydrodynamics in the study

STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 14 No. 4 2006 BIOFACIES OF UPPER JURASSIC AND LOWER CRETACEOUS SEDIMENTS 429 areas stimulated development of infauna represented by philic, oxyphilic, and thermophilic taxa disappeared rheophilic and oxyphilic benthic organisms. Simulta- from benthic communities. In the Volgian and Berria- neously, thermophilic Mclearnia and Pinna species sian ages, the basin bottom was colonized by bivalves appeared here in addition to eurythermal bivalve gen- and foraminifers characteristic of low-energy hydrody- era. The assumed changes in hydrodynamic environ- namic environments and relatively cold waters. In the ments are consistent with granulometric characteristics Neocomian, the West Siberian basin was in a regressive of the lower Valanginian sediments (Nesterov and stage of its development and experienced shoaling. At Vysotskii, 1985). Based on biofacies analysis, Bulynni- the BerriasianÐValanginian transition, sedimentation kova et al. (1978) suggested that the basin in the Agan- rates sharply increased, although hydrodynamics and Vakh area was 60Ð100 m deep with reducing environ- water aeration in the central part of the basin appeared ments in the upper sedimentary layer. Corresponding to be inherited from the preceding stage. The distinct paleontological data for the NadymÐVengapur area intensification of aeration and influence of high-energy were unavailable at that time. It seems that rheophilic, hydrodynamics were characteristic of the mid-early oxyphilic and thermophilic benthic forms point to a Valanginian time only, when the basin experienced a substantial shoaling of the basin in both areas, and the substantial shoaling. Rheophilic, oxyphilic, and ther- oxyphilic infauna found in many areas points to a good mophilic taxa appeared again in benthic communities. aeration and favorable geochemical conditions in both Almost throughout the Late Jurassic and Neocomian, the bottom water and upper sedimentary layers. normal marine environments existed in the study areas. In the Valanginian and early Hauterivian, regression The decrease of water salinity, intermittent in the in the West Siberian sea and northwestward displace- NadymÐVengapur area and continuous in AganÐVakh ment of its deep zone led to expansion of coastal and area, is established for the terminal Valanginian and ini- lagoonal settings with relevant sedimentation in south- tial Hauterivian. The salinity decrease was responsible ern, central, and eastern areas of the basin (Bulynnik- for quantitative and qualitative impoverishment of typ- ova et al., 1978; and others). In the terminal Valangin- ical marine fauna in the former area and for disappear- ian, this zone involved the Agan-Vakh area. At the same ance of marine taxa in the latter. During the time span time, the basin shoaling resulted in a sharp differentia- under consideration, the central part of the basin was tion of molluscan biotopes. In the NadymÐVengapur largely populated by high-Boreal (Arctic) families and area, there was a spectrum of biotopes, from stable genera. On the other hand, the local fauna experienced calm stagnant habitatas of rheophobic genera, which also the influence of low-Boreal waters. In the terminal flourished in anoxic environments, to high-energy well- OxfordianÐKimmeridgian, for example, thermophilic aerated and warmer settings populated by rheophilic ammonites and other groups of invertebrates immi- and thermophilic benthic communities. The contrasting grated together with a warm current into the West Sibe- environments remained unchanged until the early Hau- rian sea. terivian. In the late ValanginianÐHauterivian, stenohaline nektonic forms became scarce in the NadymÐVen- ACKNOWLEDGMENTS gapur area, and benthos was represented here by marine taxa associated with species resistant to freshening and We are grateful to L.G. Vakulenko, A.I. Voznesen- indicative, therefore, of intermittent salinity deviations skii, V.A. Zakharov, B.N. Shurygin, and P.A. Yan for from the standard marine values. In the AganÐVakh consulting and valuable comments used to improve the area, typical marine fauna is missing at all. manuscript. This work was supported by the Russian Foundation for Basic Research (project no. 03-05- 64780), United Institute of the Geology, Geophysics, CONCLUSIONS and Mineralogy of the Siberian Division RAS (project Data considered in this work elucidate time bound- VMTK No. 1737), and by the Program of Leading Sci- aries between stages of sedimentation in the central part entific Schools of the Russian Federation (grant of the West Siberian basin. An irregular trend of NSH-1569.2003.5). changes in sedimentation rate is established based on thickness of lithostratigraphic units and estimated peri- Reviewers A.I. Voznesenskii and V.A. Zakharov ods of their accumulation. Using the biofacies analysis, we detected some abiotic factors responsible for environmental changes. The results obtained have been REFERENCES interpreted with due account for geological history of 1. E. V. Artyushkov, Physical Tectonics (Nedra, Moscow, the study areas. 1993) [in Russian]. 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