INTRODUCTION

The Jurassic–Cretaceous boundary is currently one of the most intensively studied stratigraphic intervals of the Mesozoic. This is due, on the one hand, to the fact that this boundary is the only boundary between systems that has not been yet adopted at the international level (Granier et al., 2020; Wimbledon et al., 2020), and its stratigraphic position is debatable even at the level of stages (Énay, 2020). On the other hand, black shale oil source strata (Bazhenov Formation and its analogs; Braduchan et al., 1989; Leith et al., 1992; Trabucho-Alexandre et al., 2012; Rogov et al., 2020) are confined to the Jurassic–Cretaceous boundary deposits in the high latitudes of the Northern Hemisphere. Finally, sufficiently large-scale paleoclimatic and paleoceanographic turnovers occurred at the Jurassic–Cretaceous boundary (Adatte et al., 1996; Tremolada et al., 2006; Zakharov et al., 2014), with no evidence of major biotic rearrangements (Rogov, 2013).

In Western Siberia, the stratigraphic interval under consideration has been intensively studied in recent decades, primarily in the central regions that house the largest oil fields (Khafizov et al., 2022) and the oil-bearing deposits of the Bazhenovo Formation. Over the last years, there has been growing interest in the geology of eastern parts of the West Siberian Basin and adjacent areas of the Yenisei–Khatanga regional trough (YeKhRT) (Rozbaeva et al., 2022, 2023).

At the same time, the stratigraphic division into formations and structural-facies zonation of this part of the YeKhRT are still debatable (Baldin et al., 2021), not least due to unsufficient age determinations of available core materials. The Yanov Stan Formation is a key interval that is still understudied. In general, it is similar in material composition to the Bazhenovo Formation, being, however, much thicker (up to ~450 m and more; Reshenie…, 2004). It contains more silt-to sand-sized clastic material and is characterized by a relatively low Corg content. Since the discovery of the formation (Baibarodskikh et al., 1968, 1969; Bylinnikova et al., 1970), only a small number of molluscan remains applicable for age dating have been found in the Yanov Stan Formation. Most of them were depicted in the classic monograph by V.I. Bodylevsky and N.I. Shulgina (1958). Later, there were only single finds of fossil macrofauna in the Yanov Stan Formation (Atlas…, 1990; Alifirov and Igolnikov, 2007; Rozbaeva et al., 2022, 2023). As well, there are very few published data on the lithological features of the formation.

Therefore, the data obtained from the Novoyakimovskaya-1 parametric well (projected drilling depth of 5000 m) are extremely important. This well was drilled in the northwestern part of the YeKhRT (Krasnoyarsk Krai, Taimyr (Dolgano-Nenets) Municipal District, the 1 : 200 000 geological map, sheet R-46-I, coordinates 71°45′12.5′′ N, 90°54′04.3′′ E; Fig. 1a). The drilling data processing and interpretation were performed at the All-Russian Research Geological Petroleum Institute (VNIGNI), in cooperation with the Geological Institute of the Russian Academy of Sciences (GIN RAS). Unlike the majority of wells drilled in the region, where core material is usually available only for narrow intervals of the Yanov Stan Formation, the Novoyakimovskaya-1 well penetrated a 290-m thick section of the Yanov Stan Formation. The core contains abundant macrofaunal fossil remains, primarily bivalves (buchiids and inoceramids) and ammonites, which made it possible to divide and date relatively narrow intervals of the section. Preliminary results were reported by the authors at the XI All-Russian Conference “Cretaceous System of Russia and Neighboring Countries: Problems of Stratigraphy and Paleogeography (Tomsk, September 19–24, 2022; Rogov et al., 2022). The present paper considers the results in more detail. Core samples from Novoyakimovskaya-1 well, as well as the studied fossils are stored in the Aprelevka Branch of VNIGNI (Aprelevka, Moscow oblast).

Fig. 1.
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The geological scheme of Jurassic–Cretaceous boundary deposits of Western Siberia and Yenisei-Khatanga regional trough (modified after Shurygin and Dzyuba, 2015) and the location of wells, mentioned in the text: (1) Novoyakimovskaya-1; (2) Sukhodudinskaya 1-Р; (3) Dolganskaya 1-Р; (4) Khalmerpayutinskaya-2099; (5) Turukhanskaya 1-Р.

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THE HISTORY OF STUDYING THE YANOV STAN FORMATION, ITS AGE AND BOUNDARIES

The Yanov Stan Formation was first described by Baibarodskikh et al. (1968), without specifying the type section. The authors provided a brief description of the formation, illustrated by logs from the Sukhodudinskaya 1-P well. They stated that the formation “is composed of black, black–gray or greenish-black clays, usually argillite-like, and siltstones. The rocks are often thinly laminated, foliated, with a specific jointing, with abundant and diverse fossils, pyritized plant fossils, and pyrite nodules” (Baibarodskikh et al., 1968, p. 17). It was also pointed out that the formation is clearly distinguished on logs by its spontaneous polarization potential (SP). A year later, the same authors (Baibarodskikh et al., 1969) in the “Decisions and Proceedings of the Interdepartmental Meeting on Updating of Unified and Correlation Stratigraphic Charts of the West-Siberian Lowland” (the meeting was held in November 1967, and the decisions were approved in January 1968) specified the stratotype of the formation in a depth range of 2260–2022 m of the Turukhanskaya reference well 1-P and provided its age (Kimmeridgian–Berriasian). Slightly earlier, Baibarodskikh (1962) noted finds of Volgian ammonites in this borehole, but without specifying depths. Bulynnikova et al. (1970, N.I. Baibarodskikh was among the co-authors of this paper) soon specified the stratotype of the Yanov Stan Formation “in the Dolganskaya 1-R borehole section (depth int. 1880–1330 m).” It should be noted that the manuscript of the article was submitted to the editorial office in June 1967, before the interdepartmental meeting was held and before the publication of other papers describing this formation. It is probable that the authors initially had no consensus on which section should be considered as the reference one. The numerous definitions of Yanov Stan ammonites from wells drilled within the Malaya Kheta anticline, described and depicted by V.I. Bodylevsky and N.I. Shulgina (1958), were cited in (Bulynnikova et al., 1970). Then, the stratotype of the Yanov Stan Formation was specified in the “Resolutions of the 3rd Interdepartmental Regional Stratigraphic Conferences on the Mesozoic and Cenozoic of Central Siberia” (Resheniya..., 1981) as “the formation in the Malaya Kheta River basin” without mentioning specific wells. The core material from both wells proposed as reference ones for the Yanov Stan Formation has been lost. Only a few fossils, including only one ammonite, are depicted in the publications (Atlas…, 1990), providing little to substantiate the age. Both proposed “stratotypes” of the Yanov Stan Formation appear to be unsuccessful and do not allow a sufficiently reasonable substantiation of either the age or the material composition and boundaries of the formation.

In the early 1970s, the age of the Yanov Stan Formation in the type region (the western part of the YeKhRT) was assumed to be from the Late Oxfordian–Early Kimmeridgian to the terminal part of the Berriasian (Ryazanian) (Kartseva et al., 1971, 1974). At the same time, the age of the formation seems to be based on definitions by V.I. Bodylevsky (Bodylevsky and Shulgina, 1958) and it is not excluded that a part of the Sigovskaya Formation was also referred to the Yanov Stan Formation. However, even later, there were finds of Late Oxfordian ammonites from the Yanov Stan Formation (Atlas…, 1990, pl. 48, fig. 1; Sredneyarovskaya well 3). Due to this, it cannot be excluded that the lower boundary of the formation in some areas may fall into the Oxfordian. The most complete mollusk-based biostratigraphic zonation of the Yanov Stan Formation, which allows us to substantiate its age boundaries (upper part of the lower Kimmeridgian–mid-Ryazanian), was developed for the Malaya Kheta anticline. Interesting data have also been obtained from the Khalmerpayutinskaya-2099 well (Shurygin et al., 2007), in which the formation reaches a considerable thickness (~470 m). The datings of the upper boundary of the formation, which is confined to the Surites subanalogus Zone of the lower Upper Ryazanian are the most important (Alifirov and Igolnikov, 2007). The section of this well was suggested as a reference one for the Yanov Stan Formation of the Taz–Kheta structural-facies area (Alifirov and Igolnikov, 2007). It should be noted, however, that only few relatively narrow intervals of the well were characterized by core samples.

THE BRIEF LITHOLOGICAL CHARACTERISTICS OF THE SECTION

The studied section of the Yanov Stan Formation (core depth int. 3810.00–4100.00 m, corrected to 3822.5-4111 m taking the log data into account; Figs. 2, 3) is divided into three units, taking into account a small (13.9 m, depth int. 4051–4064.9 m) gap in core sampling between two of them and the slightly different lithological compositions of rocks (Figs. 2, 3). These are the upper (3822.5–3972.70 m), middle (3972.70–4051.00), and lower (4064.9–4111 m) units, respectively.

Fig. 2.
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The section of the lower part of the Yanov Stan Formation, Novoyakimovskaya-1 well. See legend in Fig. 3. Abbreviations of ammonite zones: P.lid., Paravirgatites lideri; Pavl., Pavlovia; D.i., Dorsoplanites ilovaiskii; Dors., Dorsoplanites; D.s., Dorsoplanites sachsi; C.o., Craspedites okensis.

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Fig. 3.
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The section of the upper part of the Yanov Stan Formation, Novoyakimovskaya-1 well. Abbreviations: P.may., Praetollia maynci.

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The section of the lower (first) unit of the Yanov Stan Formation (beds 376–403) is composed predominantly of dark gray to almost black silty mudstones, aleurite mudstones, and siltstones, which alternate with interlayers and lenses of brownish-gray mixed clayey-silty-carbonate rocks (3 mm to 13 cm thick). Siltstones locally have calcite cement. The rocks are irregularly bioturbated to varying degrees, with concretions, pyrite, and siderite, as well as rare shell debris. Glendonites were found in the lower and upper parts of the unit. X-ray phase analysis (XRF) was used to study mudstones and mixed clayey-silty-carbonate rocks. The composition of mudstones is relatively homogeneous throughout the section. The following carbonate minerals are present: siderite (1.1% on average) and calcite (0.9%). Carbonate minerals in the rocks of mixed composition are calcite and siderite to a greater extent (19.7 and 36.1% on average, respectively). Pyrite is present in the rocks (1.2% on average); some interbeds may be enriched with pyrite. According to total carbonate content, the entire lower unit of the Yanov Stan Formation is weakly carbonatized; carbonate minerals are mainly represented by siderite (XRF data). The rocks of the lower unit of the Yanov Stan Formation, as well as those of the middle and upper ones, show insignificant values of reservoir porosity and permeability. According to petrographic studies, the porosity values are less than 1%. Higher porosity values (9–14%) are due to the presence of micro-scale porosity in the clay matrix of the rocks. Permeability in all lithotypes does not exceed 1 md.

The middle (second) unit of the Yanov Stan Formation (beds 348–375) is composed of silty mudstones, aleurite mudstones with interlayers of mixed silty-clay-carbonate rocks, as well as silty-clay limestones and siderites varying in thickness from 6 cm to half a meter. Siderite dominates among carbonate minerals of the middle unit. There are also some carbonate concretions, pyrite inclusions, shell debris throughout the section; an interval with abundant glendonites is in the lower part of the section (Fig. 4). X-ray phase analysis confirms the higher clay content in deposits of the middle unit compared to those of the upper one. The middle unit is characterized by the presence of a large number of carbonate interbeds and a more homogeneous composition of silty-clayey deposits. According to petrographic studies, porosity does not exceed 1–1.5%. A higher porosity (up to 8–13%) is mainly due to the presence of micro-scale porosity in the clay matrix.

Fig. 4.
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Glendonites from the Yanov Stan Formation, Novoyakimovskaya-1 well. Scale bar 2 cm. (1) Specimen M180/NYa-1, depth 3883.49 m, Lower Ryazanian, Kochi Zone; (2) specimen 262/NYa-1, depth 4038.84 m, Middle Volgian; (3) specimen 381/NYa-1, depth 4066.16 m, Middle Volgian, Ilovaiskii Zone; (4) specimen 332/NYa-1, depth 4104.55 m, Lower Volgian, Lideri Zone.

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The upper (third) unit consists of irregular, often indistinct alternation of gray to dark gray clayey siltstones and silty mudstones, with rare interbeds of mixed clayey-silty-carbonate rocks. Some carbonate nodules, rare shell debris, and glendonites occur in the lower and middle parts of the unit. Siltstones are fine- to coarse-grained and coarse- to fine-grained, clayey and sandy to different degrees. Mudstones are silty, aleurite, and sandy-silty. The rocks are irregularly bioturbated layer-by-layer. In this interval of the section, sandstones do not form separate layers, occurring only in rare thin interlayers and lenses. According to X-ray fluorescence analysis, the average mineral compositions of mudstones and siltstones are similar. Carbonate minerals (calcite, dolomite, and siderite; first percent on average) occur in mudstones, siltstones, and their interbedded zones. Dolomite and calcite dominate in the carbonate rocks of the unit. The rocks have low porosity and low permeability. According to the petrographic studies, the porosity does not exceed 2%. Higher porosity (up to 15%) is mainly due to the presence of micro-scale porosity in the clay matrix.

All three units show increased organic carbon content. Total organic carbon (TOC) content varies from ~1 to 4.45% (in single cases little less than 1%); the maximum values were recorded in the upper Middle Volgian part of the section.

The presence of glendonites (Fig. 4) in all three units of the Yanov Stan Formation attracts attention. Finds of these calcite pseudomorphs after ikaite are confined to stratigraphic intervals in which they occur rarely (the Variabilis Zone of the Middle Volgian and Kochi Zone of the Lower Ryazanian) and to intervals, where they have not been recorded so far (upper Lower Volgian and lower Middle Volgian) (Rogov et al., 2021).

THE MACROPALEONTOLOGICAL CHARACTERISTICS OF THE SECTION AND ITS MOLLUSK-BASED ZONATION

The studied core material from the Yanov Stan Formation is unusually well characterized by finds of well-preserved macrofossils. In total, 207 samples with molluscan fossil remains, mostly ammonites and bivalves, were collected from the core samples. The bivalve remains of the genus Buchia are the most common. They often form clusters, as well as thin coquina lenses and interlayers up to several cm thick. Ammonites occur rarely. Other groups of organisms include inoceramid bivalves, which are common in some intervals of the section, and rare coleoids represented by belemnite rostra and arm hooks (onychites). The following stratigraphic subdivisions can be distinguished in the section of the Yanov Stan Formation (from bottom to top) (Figs. 2, 3).

Ammonite-Based Stratigraphic Subdivisions

Lower Volgian

The Paravirgatites lideri Zone (depth int. 4103.65–4099.44 m). The terminal part of the Lower Volgian in Western Siberia, characterized by the ammonites of the genus Paravirgatites, has recently been proposed (Rogov, 2021) to refer to the Paravirgatites lideri Zone, established by M.S. Mesezhnikov (Zakharov and Mesezhnikov, 1974). This zone has not been distinguished in the Yanov Stan Formation so far. A single find of Pectinatites rotor (Bodylevsky), a species from the lower Pectinatites fedorovi Zone, is known from the Lower Volgian (Malokhetskaya 12-P borehole) (Bodylevsky and Shulgina, 1958, pl. VIII, fig. 1; Rogov, 2021, pl. XCIII, fig. 4). The Lideri Zone in Novoyakimovskaya-1 well was established based on numerous finds of Paravirgatites. Despite the fact that these ammonites are poorly preserved and usually cannot be identified to species level (pl. I, figs. 3, 4), and, nevertheless, their finds in a depth interval of 4099.64–4103.65 m allow one to substantiate the presence of this zone. The stratigraphically lowest find of Paravirgatites is of greatest interest for interregional correlation. This is a small, relatively coarse-rubbed ammonite, close to P. infrequens Buckman (pl. I, fig. 1), an index species of the biohorizon in East Greenland (Rogov, 2021, pl. XIII, figs. 1, 2).

Middle Volgian

The Pavlovia iatriensis Zone (depth int. 4086.11–4066.64 m). The lower boundary of the Middle Volgian in the Artic, as well as in Boreal regions, is determined by the first occurrence of ammonites of the genera Pavlovia (as a rule, sharply prevailing in the number of finds) and Dorsoplanites (very rare fossils east of the Urals at this level). The studied section of the Novoyakimovskaya-1 well is unique, besides other, because a small ammonite was found here at the base of the Middle Volgian (pl. I, fig. 9). Despite the fact that this ammonite is different from the known species of this age, it undoubtedly belongs to the genus Dorsoplanites. Stratigraphically higher of the level of this find, several ammonites were found that can be assigned to Pavlovia iatriensis (Ilov.) (pl. I, figs. 10, 12–14, 15 (cf.)).

This zone was not established eastward, in the Eastern Taimyr, the Kheta River basin, and on the Nordvik Peninsula. In the case of the presence of both the Lower and Middle Volgian strata in the sections, there is a hiatus between them (Rogov, 2021).

The Dorsoplanites ilovaiskii Zone (depth int. 4065.68 m). The Ilovaiskii Zone in the section of the studied well was established conditionally to a certain degree because it is substantiated by a single find of the ammonite Dorsoplanites cf. antiquus Spath (pl. I, fig. 16). However, based on the presence of this species in the Ilovaiskii Zone in Spitsbergen (Rogov, 2010), Western Siberia (Braduchan et al., 1986; Atlas…, 1990), Taimyr and Subpolar Urals (Mesezhnikov, 1984), it is possible to outline the presence of this zone. Despite the fact that rare, undepicted finds of D. ilovaiskii from the Subpolar Urals were also reported from the Strajevskyi Zone and more numerous ones, from the two overlying Ilovaiskii and Maximus zones (Zakharov and Mesezhnikov, 1974), a slightly older or younger age of this find cannot be excluded.

The Dorsoplanites maximus Zone (depth int. 4064.92–4048.09 m). Traditionally, the Maximus Zone in sections of the Subpolar Urals and Siberia is established by the presence of large Dorsoplanites (D. maximus Spath, D. laevis Rogov) and Epipallasiceras (Zakharov and Mesezhnikov, 1974; Mesezhnikov, 1984; Rogov, 2021).

At the same time, the criteria for determining the boundaries of this zone have not been previously discussed in detail. The “stratotype” of the zone indicated by M.S. Mesezhnikov (“in outcrops of Milne Land (East Greenland)”; Zakharov and Mesezhnikov, 1974, p. 60) is not reliably specified because there are dozens of sections with sediments of the corresponding age in the Milne Land, and, accordingly, is unsuccessful. In Greenland, few finds of the index species are known (Callomon and Birkelund, 1982) and the Maximus Zone has not been distinguished there. The outcrops in the Subpolar Urals or Eastern Taimyr should be chosen as a type section, but these outcrops require reexamination at the present level of knowledge. Despite the uncertainty in establishing the boundaries of the zone, its assemblage, defined by finds of Epipallasiceras and large Dorsoplanites, is well traced over a vast territory from Spitsbergen in the west to northeastern Russia in the east (Rogov, 2021). In the Novoyakimovskaya-1 well section, the zone can be outlined by finds of Epipallasiceras sp. (pl. I, fig. 17), Dorsoplanites cf. maximus Spath (pl. I, fig. 18), and Dorsoplanites gracilis Spath (pl. II, fig. 1).

The Dorsoplanites sachsi Zone (depth int. 4044.25-4044.13 m). This zone was originally established (as a subzone) by N.P. Mikhailov (1966) in the lower reaches of the Lena River. Subsequently, its distribution was traced to other regions of the Arctic (Rogov, 2010, 2021), but it has not been previously established in Western Siberia and the YeKhRT. The zone is characterized by the presence of specific Dorsoplanites with a high branching ratio, as well as the first Taimyrosphinctes and Praechetaites. In the studied well, this zone was established in a relatively narrow interval by the presence of Dorsoplanites sachsi Michlv. (pl. II, fig. 4) and Dorsoplanites aff. sachsi Michlv. (pl. II, fig. 5). Stratigraphically higher, there is a significant interval (about 17.5 m) where no ammonites have been found.

Fig. 5.
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The zonation of the Volgian and Ryazanian of Western Siberia (Rogov, 2021; Panchenko et al., 2022), north of Central Siberia (Igolnikov, 2006; Rogov, 2021) and the western part of the YeKhRT (proposed herein, taking into account data published in Rozbaeva et al., 2022, 2023). The intervals with finds of ammonites in the section of Novoyakimovskaya-1 well are shown in gray. C. (Taim.), Craspedites (Taimyroceras).

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The Epivirgatites variabilis Zone (depth int. 4026.68–4003.15 m). This unit has so far been identified only in the eastern part of the YeKhRT (Rogov, 2021). Although the index species also occurs in Western Siberia and European Russia (in Vogulicus and Nikitini zones, respectively), the assemblage of the zone is specific and differs from the same-age assemblages of other regions. Small Epivirgatites (E.) variabilis Schulg., as well as Epilaugeites and Laugeites, are most common here. The thickness of the zone in the Novoyakimovskaya-1 well section is more than 20 m. The zone is clearly distinguished by the finds of Epivirgatites sp., Epivirgatites (E.) variabilis Schulgina (pl. II, fig. 7), Laugeites cf. groenlandicus (Spath) (Plate II, fig. 8), Epilaugeites (pl. II, fig. 9).

figure a

Plate I . Early-Middle Volgian mollusks. Scale bar here and in Plates II–VI is 1 cm; before taking images, mollusks were coated with ammonium chloride. (1) Paravirgatites cf. inflatus Buckm., specimen 333/NYa1, depth 4103.65 m, Lower Volgian, Lideri Zone; (2) Arcticeramus cf. arcticus (Kosh.), specimen 339/NYa1, depth 4100.46 m, Lower Volgian, Lideri Zone; (3, 4) Paravirgatites sp. indet., Lower Volgian, Lideri Zone: (3) specimen. 340/NYa1, depth 4100.24 m; (4) specimen 342/NYa1, depth 4099.44 m; (5, 6, 8, 11) Buchia mosquensis (Buch), Lower Volgian–Middle Volgian, Mosquensis Buchiazone: (5) specimen 344/NYa1, depth 4098.29 m; (6) specimen 349/NYa1, depth 4097.03 m; (8) specimen 350/NYa1, depth 4096.92 m; (11) specimen 361/NYa1, depth 4080.49 m; (7) Cylindroteuthis sp., specimen 348/NYa1, depth 4097.1 m, the transition interval between Lower- and Middle Volgian: (a) ventral view, (b) lateral view; (c) cross-section at the posterior end of the preserved fragment; (9) Dorsoplanites sp. nov., specimen 358/NYa1, depth 4086.11 m, Middle Volgian, Iatriensis Zone; (10, 12–14) Pavlovia iatriensis (Ilov.), Middle Volgian, Iatriensis Zone: (10) specimen 359/NYa1, depth 4073.9 m; (12) specimen 365/NYa1, depth 4074.04, with Argutostrea roemeri (Quenst.) shell; (13) specimen 368a/NYa1, depth 4072.02 m; (14) specimen 368b/NYa1, depth 4072.02; (15) Pavlovia cf. iatriensis (Ilov.), Middle Volgian, Iatriensis Zone, specimen 377/NYa1, depth 4066.64 m, with Argutostrea roemeri (Quenst.) shell, Middle Volgian, Iatriensis Zone; (16) Dorsoplanites cf. antiquus Spath, specimen 378/NYa1, depth 4065.68 m; (17) Epipallasiceras sp., specimen 379/NYa1, depth 4064.92 m, Middle Volgian, Maximus Zone; (18) Dorsoplanites cf. maximus Spath, specimen 384/NYa1, depth 4048.92 m, Middle Volgian, Maximus Zone.

Plate II.
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Middle Volgian mollusks. (1) Dorsoplanites gracilis Spath, specimen 385/NYa-1, depth 4048.09 m, Middle Volgian, Maximus Zone; (2) Buchia fischeriana (d’Orb.), specimen 385/NYa-1, depth 4047.42 m, Middle Volgian; (3) Dorsoplanites sp., specimen 387/NYa-1, depth 4046.27 m, Middle Volgian; (4) Dorsoplanites sachsi Michlv., s-pecimen 261/NYa-1, depth 4044.25 m, Middle Volgian, Sachsi Zone; (5) Dorsoplanites aff. sachsi Michlv., specimen 260a/NYa-1, depth 4044.13 m, Middle Volgian, Sachsi Zone; (6) Arctoteuthis sitnikovi (Sachs et Nalnyaeva), specimen 263/NYa-1, depth 4040.64 m, Middle Volgian: (a) ventral view, (b) left view, (c) cross section near the anterior end of the preserved fragment; (7) Epivirgatites (E.) variabilis Schulgina, specimen 274/NYa-1, depth 4011.63 m, Middle Volgian, Variabilis Zone; (8) Laugeites cf. groenlandicus (Spath), specimen 275/NYa-1, depth 4007.93 m, Middle Volgian, Variabilis Zone; (9) Epilaugeites sp. indet., specimen 279/NYa-1, depth 4003.9 m, Middle Volgian, Variabilis Zone; (10) Laugeites sp., specimen 278/NYa-1, depth 4003.15 m, Middle Volgian, Variabilis–Exoticus Zone; (11–13) Praechetaites rudicostatus (Schulgina), Middle Volgian, Exoticus Zone: (11) specimen 277/NYa-1, depth 4001.34 m; (12) specimen 280/NYa-1, depth 4001.32 m; (13) specimen 284/NYa-1 depth 4000.78 m.

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Plate III.
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Middle Volgian mollusks of Praechetaites exoticus Zone. (1) Praechetaites schulginae Rogov, specimen 281/NYa-1, depth 4001.05 m; (2, 5–7) Praechetaites cf. schulginae Rogov: (2) specimen 282/NYa-1, depth 4001.04 m; (5) specimen 288/NYa-1, depth 3994.34 m; (6) specimen 287/NYa-1, depth 3994.93 m; (7) specimen 290/NYa-1, depth 3993.55 m; (3) Praechetaites rudicostatus (Schulgina), specimen 283/NYa-1, depth 4000.94 m; (4) Praechetaites cf. confusus Rogov, specimen 286/NYa-1, depth 3999.14 m; (8) Praechetaites sp. juv., specimen 289/NYa-1, depth 3993.84 m; (9) Praestriaptychus sp., specimen 291/NYa-1, depth 3991.52 m; (10) Buchia fischeriana (d’Orb.), specimen 293/NYa-1, depth 3988.64 m.

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Plate IV.
figure 8

Middle Volgian–Early Ryazanian mollusks. (1) Laugeites mesezhnikowi Kiselev et Rogov, specimen 300/NYa-1, depth 3984.9 m, Middle Volgian, Exoticus Zone; (2) Praechetaites aff. bicostatus (Schulg.), specimen 302/NYa-1, depth 3983.54 m, Middle Volgian, Exoticus Zone; (3) Laugeites aff. mesezhnikowi Kiselev et Rogov, specimen 303/NYa-1, depth 3981.53 m, Middle Volgian, Exoticus Zone; (4) Craspedites (C.) cf. schulgnae Alifirov, specimen 304/NYa-1, depth 3979.43 m, Upper Volgian, Okensis Zone; (5) Buchia aff. okensis (Pavlow), specimen 305/NYa-1, depth 3977.87 m, Upper Volgian (?); (6) Praesurites sp. juv., specimen М161/NYa-1, depth 3974.44 m, Lower Ryazanian, Maynci Zone; (7) Pachypraetollia sp., specimen M162/NYa-1, depth 3971.99 m, Lower Ryazanian, Maynci Zone; (8) Praetollia maynci Spah, specimen M163/NYa-1, depth 3971.14 m, Lower Ryazanian, Maynci Zone; (9) Praetollia sp., specimen М164/NYa-1, depth 3967.84 m, Lower Ryazanian, Maynci Zone (?); (10) Inoceramus subplanus Zakharov et Turbina, specimen M166/NYa-1, depth 3966.99 m, Lower Ryazanian; (11) Hectoroceras kochi Spath, specimen M174/NYa-1, depth 3943.64 m, Lower Ryazanian, Kochi Zone; (12) Boreophylloceras cf. densicostatum Igolnikov, specimen M178/NYa-1, depth 3934.18 m, Lower Ryazanian, Kochi Zone; (13) Boreophylloceras densicostatum Igolnikov, specimen M179/NYa-1, depth 3931.39 m, Lower Ryazanian, Kochi Zone.

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Plate V.
figure 9

Early Ryazanian mollusks. (1) Buchia unschensis (Pavlow), specimen M172/NYa-1, depth 3960.94 m, Unschensis Buchiazone; (2, 6, 7, 10, 11, 14) Hectoroceras kochi Spath, Kochi Zone: (2) specimen. 106/NVK-1, depth 3919.17 m; (6) specimen 72/NYa-1, depth 3905.65 m; (7) specimen 71/NYa-1, depth 3905.99 m; (10) specimen 74/NYa-1, depth 3898.03 m; (11) specimen 119a/НВК-1, depth 3897.95 m; (14) specimen 137/NVK-1, depth 3879.04 m: (a) impression, (b) counterimpression; (3, 4) Onychites sp., specimen 67/NYa-1, depth 3918.42 m, Kochi Zone; (5) Borephylloceras densicostatum Igoln., specimen 68b/NYa-1, depth 3913.22 m, Kochi Zone; (8) Buchia volgensis (Lahusen), specimen 114/NVK-1, depth 3901.71 m, Okensis Buchiazone; (9, 13) Buchia okensis (Pavlow), Okensis Buchiazone, (9) specimen 117/NVK-1, depth 3901.31 m; (13) specimen 152/NYa-1, depth 3887.89 m; (12) Buchia ex gr. okensis (Pavlow), specimen 151/NYa-1, depth 3887.74 m, Okensis Buchiazone.

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Plate VI.
figure 10

Ryazanian mollusks. (1, 8) Buchia volgensis (Lahusen), Volgensis Buchiazone: (1) specimen 140/NVK-1, depth 3873.46 m; (8) specimen 24/NYa-1, depth 3822.74 m; (2) Surites sp. indet., specimen NYa-1, depth 3866.29 m, Kochi Zone; (3) Borealites (Borealites) cf. schulginae Igolnikov, specimen 93/NVK-1, depth 3861.95 m, Kochi Zone, (a) core with partially removed outer whorl; (b) impression; (4) Borealites (Borealites) cf. antiquus (Jeletzky) sensu Igolnikov, 2019, specimen 93a/NVK-1, depth 3861.79 m, Kochi Zone; (5) Hectoroceras kochi Spath, specimen 151/NVK-1, depth 3861.38 m, Kochi Zone; (6) Buchia cf. volgensis (Lahusen), specimen 77/NYA-1, depth 3858.11 m, Volgensis Buchiazone; (7) Surites ex gr. subanalogus Schulgina, specimen 164/NVK-1, depth 3829.17 m, Subanalogus Zone.

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The Praechetaites exoticus Zone (depth int. 4001.34–3981.53 m). The terminal Exoticus Zone of the Middle Volgian is well traced in the Arctic, from East Greenland and Spitsbergen in the west to lower reaches of the Lena River in the east (Rogov, 2021). Its lower boundary is clearly defined by the first occurrence of large Praechetaites ex gr. exoticus (Rogov and Zakharov, 2011). Epivirgatites, which have not been found in the Exoticus Zone, disappear at approximately the same level. In the studied section the Praechetaites exoticus Zone has a relatively large thickness (somewhat less than 20 m) and is unusually rich in ammonite shells, which form accumulations in some intervals.

Praechetaites rudicostatus (Schulgina) and P. schulginae/cf. schulginae Rogov (pl. III, figs. 1, 2, 5–7) in the Exoticus Zone of the studied section are the most numerous (pl. II, figs. 11–13, pl. III, fig. 3); other species are less common (pl. II, figs. 4, 8). In addition, Laugeites mesezhnikowi Kiselev et Rogov/aff. mesezhnikowi (pl. IV, figs. 1, 3) and large Praechetaites aff. bicostatus (Schulgina) (pl. IV, fig. 2) occur in the upper part of the zone. A small Praestriaptychus sp. aptychus (pl. III, fig. 9), similar to the previously described Laugeites aptychus (Rogov and Mironenko, 2016, figs. 2j, 2k, 2s), was also found together with Praechetaites species.

Upper Volgian

The Craspedites okensis Zone (depth 3979.43 m). The Upper Volgian has a sharply reduced thickness (the interval between the uppermost occurrence of Middle Volgian ammonites and the lowermost occurrence of Ryazanian ones is about 6 m). However, there are no signs of hiatus or condensation in the section, and fossil remains occur here sporadically.

A single ammonite Craspedites (C.) cf. schulgnae Alifirov (pl. IV, fig. 4), which is widespread in the Upper Volgian of Siberia (Alifirov, 2009), was found. In our opinion, finds of this species in the Middle Volgian of the Khalmerpayutinskaya 2099 well (Alifirov, 2009) are incorrectly dated. The same situation occurs with small ammonites, similar to Praetollia. They occur stratigraphically higher and were originally recognized as Laugeites (Alifirov and Igolnikov, 2007, pl., fig. 8). These specimens have a very narrow umbilicus that is completely untypical of Laugeites. They are very similar to juvenile Praetollia from East Greenland (Spath, 1952, pl. II, fig. 1; Surlyk, 1978, pl. 5, figs. 1–4). It is most likely that the Upper Volgian is biostratigraphically incomplete in the Novoyakimovskaya-1 well section. Compared to the Middle Volgian and Lower Ryazanian substages, the Upper Volgian has a small thickness. This is typical of many sections of Western Siberia (Panchenko et al., 2015, 2021; Eder et al., 2022), but these differences are not as great as in the studied well. The Upper Volgian in the YeKhRT sections located eastward sometimes reaches a significant thickness (more than 40 m) comparable to that of the Lower Ryazanian interval (Opornyi…, 1969).

Lower Ryazanian

The Praetollia maynci Zone (depth int. 3974.44–3948.99 m). The lower boundary of the Ryazanian is clearly established in the entire Panboreal Superrealm by the last occurrence of Volgidiscus and Subcraspedites and the first occurrence of Praetollia. Despite the fact that Chetaites and Craspedites (Taimyroceras) cross this boundary, it is, as a rule, clearly fixed both in sections and wells by the first occurrence of Praetollia, which are sharply different from more ancient Craspeditidae in morphology. The lower boundary of the Ryazanian in the Novoyakimovskaya-1 well is determined by the first occurrence of coarsely sculptured Praesurites (?) (pl. IV, fig. 6) and Pachypraetollia (pl. IV, fig. 7). Igolnikov (2010, 2019) believes that Praesurites can be considered a synonym of Praetollia. In our opinion, however, Praesurites differs from Praetollia s. str. by rarer and coarser sculpture, as well as by the peculiarities of ribbing on inner whorls. The genus Praesurites is similar to Pachypraetollia, but there is not enough paleontological material to judge the possible isolation of these taxa or to consider them synonyms. Praetollia, including P. maynci, occur slightly higher (pl. IV, fig. 8). Praetollia shells (pl. IV, fig. 9) found stratigraphically higher are also conditionally assigned to the Maynci Zone. However, it cannot be excluded that they may already belong to the Kochi Zone.

The Hectoroceras kochi Zone (depth int. 3943.64–3861.38 m). The Kochi Zone is one of the most reliably traced wide intervals of the Ryazanian in the Panboreal Superrealm. The boundaries of the zone are determined by the distribution of the genus Hectoroceras. Other ammonite genera of this zone also occur in the underlying and overlying deposits. The thickness of the zone in the studied well is more than 80 m. The index species Hectoroceras kochi, one of the most easily recognizable taxon of Ryazanian ammonites even by fragments, is the most typical of the Hectoroceras kochi Zone (pl. IV, fig. 11; pl. V, figs. 2, 6, 7, 10, 11, 14, 15; pl. VI, fig. 5). The boreal phylloceratides Boreophylloceras densicostatum/cf. densicostatum Igolnikov (pl. IV, figs. 12, 13; pl. V, fig. 5), which were previously known only from the eastern part of the YeKhRT (Igolnikov, 2007, 2019) and the lower reaches of the Lena River (Rogov et al., 2011), are also found in the lower part of the zone. Borealites (pl. VI, figs. 3, 4) and Surites (pl. VI, fig. 2) appear in the upper part of the zone.

Upper Ryazanian

The Surites subanalogus Zone (a depth int. 3829.17 m). The presence of the Upper Ryazanian and Subanalogus Zone (suggested instead of Analogus Zone in Panchenko et al., 2022) in the studied well section was established with a certain degree of conditionality by the finding of Surites ex gr. subanalogus Schulgina (pl. VI, fig. 7) in the upper part of the Yanov Stan Formation above the finds of Borealites and Hectoroceras.

Higher in the well section, ammonites are found only after a significant break in core samples, in the deposits of the Sukhodudinskaya Formation.

Bivalve-Based Stratigraphic Units

Lower Volgian–Middle Volgian

The Buchia mosquensis Buchiazone (depth int. 4110.05–3981 m). Despite the fact that bivalve shells occur in the core of the studied well, their diversity in the Yanov Stan Formation is low. High-level filter feeders, buchiids, and inoceramids, are in predominance here. The Lower–Middle Volgian interval is characterized mainly by the occurrence of Buchia mosquensis (Buch) (Lower Volgian–Iatriensis Zone of the Middle Volgian; pl. I, figs. 5, 6, 8, 11); higher in the succession, B. fischeriana (d’Orb.) occurs (pl. II, fig. 2). The lower boundary of the buchiazone (not characterized by the core) was established by the appearance of monospecific accumulations of B. mosquensis (Zakharov, 1981). It has been recently established in East Greenland that B. mosquensis appears at the base of the Pectinatus ammonite Zone (Kelly et al., 2015), i.e., a zone earlier than the core-characterized interval in the lower part of the Yanov Stan Formation. Apart from buchiids, oysters attached to ammonite shells were also found in deposits of the lower Middle Volgian (pl. I, figs. 10, 15). Such oyster-ammonite assemblages are typical of the Lower Volgian and lower Middle Volgian interval of the Subpolar Urals, Western Siberia, and the eastern part of the YeKhRT (sections in the Kheta, Boyarka, and Dyabaka-Tari river basins; Kosenko, 2017). They have also been found to the west, in sections of European Russia (Gerasimov et al., 1995), East Greenland (Fürsich, 1982), and England (Cope, 1968). Arcticeramus inoceramids also occur in the boundary interval between the Lower and Middle Volgian (pl. I, fig. 2). In the upper Middle Volgian, only Buchia fischeriana (d’Orb.) was identified (pl. III, fig. 10); no other species of the genus Buchia were found together with the latter.

Upper Volgian–Lower Ryazanian

The Buchia unschensis Buchiazone (int. 3977.29–3901.31 m). This zone covers the Jurassic–Cretaceous boundary interval from the upper part of the Okensis ammonite Zone to the lower part of Kochi Zone (Zakharov and Rogov, 2020). The lower boundary of the zone is fixed by the first occurrence of an index species (Zakharov, 1981), and the upper boundary by the first occurrence of Buchia okensis (Pavl.). In the studied section, bivalve assemblages of the lower and upper parts of the zone differ noticeably. The assemblage in the lower part of the zone (Upper Volgian and Maynci Zone of the Lower Ryazanian) includes B. unschensis (Pavl.)/cf. unschensis (Pavl.) (pl. V, fig. 1), Buchia aff. okensis (Pavlow) (pl. IV, fig. 5), and abundant Inoceramus subplanus Zakharov et Turbina (pl. IV, fig. 10). The level of mass appearance of inoceramids seems to correspond to the base of the “inoceramid unit” of Western Siberia, which is also characterized by the presence of numerous inoceramids (Panchenko et al., 2015). The upper part of the zone is characterized exclusively by the presence of B. volgensis (Lahusen), which also occurs higher in the succession up to the upper part of the Yanov Stan Formation. It is not improbable that the interval with B. volgensis should be attributed to the overlying zone since this species occurs at the level of the first occurrence of B. okensis (Pavl.) (see Zakharov, 1987).

The Lower Ryazanian

Buchia okensis Buchiazone (depth int. 3901.31–3886.79 m). The lower boundary of the buchiazone is determined by the first occurrence of B. okensis (Pavl.), which is widespread in the Arctic and which is an easily identifiable index species. The abundant B. volgensis (Lahusen) and B. fischeriana (d’Orb.) are also found in the zone. The finds of Ryazanian B. fischeriana (d’Orb.) are separated from the Volgian ones by the interval without B. fischeriana (d’Orb.). We have previously suggested that the name B. rjasanensis (D. Sok.) could be used for these late B. fischeriana (Rogov et al., 2022). However, D. N. Sokolov (1908, p. 64) attributed one of two syntypes of the species B. trigonoides (Lahusen) depicted by I. Laguzen (1888, pl. II, figs. 23, 24) to B. rjasanensis. Both specimens depicted by Laguzen (Laguzen, 1888, Table II, figs. 21–24) are from the same locality (Glebovo, Yaroslavl oblast) and are of the same age (Virgatus Zone of the Middle Volgian). At the same time, Laguzen (1888, p. 14) noted that “the distribution of A. trigonoides is mainly observed in sands with Hoplites rjasanensis,” i.e., in the boundary interval between the Lower and Upper Ryazanian. Therefore, if it is possible to substantiate the attribution of B. fischeriana from Middle Volgian and Ryazanian deposits to different taxa, the name B. trigonoides (Lahusen) can be used for the younger of them. The Okensis Zone in the studied well section is characterized by the occurrence of a mixed Buchia assemblage, consisting of the index species Buchia okensis (Pavlow) (pl. V, figs. 9, 12, 13), Buchia volgensis (Lahusen) (pl. V, fig. 8), and late B. fischeriana (d’Orb.).

The Lower–Upper Ryazanian

The Buchia jasikovi Buchiazone (a depth int. 3881.94 m). The zone is distinguished rather conditionally by the occurrence of the index species B. jasikovi (Pavlow) found above the last B. okensis (Pavlow). This index species was identified in open nomenclature. In Siberia, the lower boundary of the zone is determined by the first occurrence of this index species. It is also found higher in the succession (Zakharov, 1981, 1987).

The Buchia volgensis Buchiazone (a depth int. 3884.49–3822.74 m). The long-ranging B. volgensis Zone can also be distinguished in the studied well section along with the ?Jasikovi Buchiazone. Such a unit in the rank of zone or beds with Buchia assemblage was distinguished above the Okensis Buchiazone in Eastern Greenland, in the lower reaches of the Lena River, in northeastern Siberia, the Russian Far East, and northeastern China (Rogov et al., 2011; Zakharov and Rogov, 2020). B. volgensis (Lahusen) dominates in the assemblage (pl. VI, figs. 1, 6), along with it B. fischeriana (d’Orb.) occurs.

Belemnite rostra and other coleoid fossils: Features of distribution

The finds of coleoid fossil remains in the Novoyakimovskaya-1 well are relatively rare. These are mainly fragments of belemnite rostra of the genus Cylindroteuthis from the Volgian part of the section (Table I, Fig. 6), which are not identifiable to species level to make any detailed conclusions about their age. It should be noted that half of such finds is confined to a narrow depth interval of 4038.13–4038.60 m, which corresponds to the boundary interval between the Sachsi and Variabilis ammonite zones. In addition, the only specimen of common Early–Middle Volgian (Dzyuba, 2004) species Arctoteuthis sitnikovi (Sachs et Nalnyaeva) was found at a depth of 4040.64 m, approximately at the same level (pl. II, fig. 6). No belemnite rostra were found in the Ryazanian part of the section. However, large arm hooks of coleoids–megaonychites Onychites sp. (pl. V, figs. 3, 4)-were found in the lower part of the stage (depth int. 3918.42 m). These are very typical fossils of the Kimmeridgian–Ryazanian interval of the Arctic (Hammer et al., 2013; Rogov et al., 2017), which often occur in Western and Eastern Siberia.

DISCUSSION

OM-rich strata are unusually widespread in the Jurassic–Cretaceous boundary interval at the high latitudes of the Northern Hemisphere. One such black shale strata is the Yanov Stan Formation, which consists of black, black-gray, or greenish-black clays, usually mudstone-like, as well as siltstones (Baibarodskikh et al., 1968). In the section of the Novoyakimovskaya-1 well, this formation is of typical composition. It begins with mudstones interbedded with siltstones and mixed clayey-silty-carbonate rocks. Siderite predominates among carbonates. These rocks are enriched with organic matter (up to 4.5% TOC in units 1 and 2). Higher in the section, in the third unit, the section is dominated by gray to dark gray siltstones; mudstones and sandstones are also present. There are carbonate concretions made mainly of dolomite and calcite. Glendonites in the Yanov Stan Formation were noted for the first time, although they have often been reported in publications and reports since the 1960s in the more ancient Jurassic strata from the Aalenian to the Kimmeridgian in the western part of the YeKhRT (Baibarodskikh et al., 1968). At the same time, very few finds of glendonites are known so far in Western Siberia in the Callovian–Lower Hauterivian (?) interval, including the Bazhenovo Formation. In the eastern part of the YeKhRT and further to the east, in outcrops of the Laptev Sea coast, glendonites are often found in the Upper Pliensbachian and Middle Jurassic deposits, but their Upper Jurassic occurrences are extremely rare (Rogov et al., 2021, 2023). In the Volgian and Ryazanian deposits, very few finds of glendonites have been known so far. At high latitudes of the Northern Hemisphere the Volgian was one of the warmest ages of the Late Jurassic–Early Cretaceous interval (Price and Rogov, 2009; Zakharov et al., 2014; Dzyuba et al., 2018; Rogov et al., 2019). Therefore, the presence of abundant glendonites in Volgian and Ryazanian of the Yanov Stan Formation of the Novoyakimovskaya-1 well is most likely related to local paleogeographic features and the presence of paleocurrents, due to which the bottom waters could be at about 0°C for rather long time intervals. Taking into account the fact that, according to the core data, glendonites are also abundant in the overlying and underlying intervals (Sukhodudinskaya, Sigovskaya, and Tochinskaya formations), one can assume that the abundance of these pseudomorphs could be controlled by long-term regional factors.

The interval of Lideri–Sachsi zones (Fig. 5) is extremely important for clarifying the evolution of ammonite faunas in the YeKhRT, because this interval is practically absent to the east (sections of the Kheta, Boyarka, and Nordvik rivers). The Middle Volgian begins with the Variabilis Zone, while older species are found to be redeposited. Somewhat more complete sections of the Middle Volgian are known in Taimyr (the sections of Dyabaka-Tari and Leningradskaya rivers), where they begin with the Ilovaiskii and Maximus zones, respectively (Mesezhnikov, 1984). The ammonite and bivalve assemblages from the lower zones of the Middle Volgian in the studied well are similar to coeval assemblages of Western Siberia. Buchia species dominate among bivalves; inoceramids and oysters attached to ammonite shells occur. Ammonites are represented by the same or similar species as in Western Siberia. The situation changes starting from the upper Middle Volgian: both ammonite assemblages and the distinguished zones are closer to those from the central and eastern parts of the YeKhRT than to those from Western Siberia. Despite the fact that the same ammonite genera occur in the upper Middle Volgian of Western Siberia and in the YeKhRT, their quantitative ratios are sharply different. In Western Siberia and the Subpolar Urals, Laugeites and Epivirgatites dominate in Groenlandicus and Vogulicus zones, while Epivirgaites shells occur rarely. The Variabilis Zone (Fig. 5), dominated by the ammonites of genus Epivirgatites, is distinguished in this interval in the studied well and eastward.

A single Exoticus Zone was established in the terminal part of the Middle Volgian of Western Siberia and the YeKhRT. Laugeites are still abundant in this zone in Western Siberia; to the east this zone is mainly characterized by the predominance of large Praechetaites ex gr. exoticus. The Upper Volgian in the studied well has an extremely reduced thickness. In this respect, it is closer to the sections of Western Siberia (Panchenko et al., 2021, 2022), because in the central and eastern parts of the YeKhRT the thickness of the Upper Volgian exceeds that of the Middle Volgian interval (Opornyi…, 1969; Zakharov et al., 2014; Rogov, 2021).

In contrast to the Middle and Late Volgian ammonite assemblages, which are essentially different in regions of the Pan-Boreal Superrealm, the ammonite assemblages of the Ryazanian from different boreal regions are similar, and most genera of boreal ammonites are distributed in a circumpolar manner. Accordingly, the Siberian zonation can be used for the Ryazanian of practically the entire Panboreal Superrealm. Nevertheless, there are differences between the Ryazanian ammonite assemblages of the Novoyakimovskaya-1 well and wells of Western Siberia. Several representatives of Boreal phylloceratids of the genus Boreophylloceras were found in the lower part of the Ryazanian of the studied well. This genus is common in the Upper Volgian–Lower Valanginian of the Kheta River basin, Nordvik Peninsula, lower reaches of the Lena River (Igolnikov, 2007; Rogov et al., 2011; Rogov, 2021). However, it has never been reported so far either from the western parts of the YeKhRT or from Western Siberia. Bivalves of the Ryazanian age are geographically widespread species of the genus Buchia. However, the studied assemblages are also similar to those from the eastern part of the YeKhRT in the very large size of the shells (in some cases exceeding 10 cm along the long axis), whereas in Western Siberia the same species of the genus Buchia are usually represented by smaller specimens, and large shells occur rarely.

Such peculiarities of assemblages of Middle Volgian–Ryazanian mollusks of the studied section allow us to state that despite the existence of a strait that connected the basin of the YeKhRT with the West Siberian Sea, some environmental factors (climatic, hydrological, or paleogeographic) prevented typical Arctic assemblages of both benthic and nektonic organisms from penetrating westward through this strait.

CONCLUSIONS

The section of the Yanov Stan Formation penetrated by the parametric Novoyakimovskaya-1 well is the thickest and richest in fossil finds among all the sections described earlier in publications or reports. Based on finds of numerous ammonite shells, we were able to divide the formation into zones, to establish the features of ammonite assemblages, and to show the similarity of ammonite faunas of the studied section with those from the eastern part of the YeKhRT. Eleven ammonite zones and four buchiid zones were established. Finds of glendonites, which have not been previously noted in the Yanov Stan Formation, are exceptionally important. Along with the peculiarities of molluskan assemblages, this allows us to assume that there was a specific paleoclimatic and hydrological regime in the western part of the YeKhRT during the Jurassic/Cretaceous transition time. This regime was significantly different from the one that existed westward, in the West Siberian Basin.