Provenance and the U–Pb age constraints on the tuff beds of the Late Jurassic–Early Cretaceous Bazhenovo Formation, West Siberian Basin

Thin tuff beds of the Late Jurassic–Early Cretaceous Bazhenovo Formation are laterally widespread in the central part of the West Siberian Basin (ca 0.5 million km2). However, the source of the tuff beds remains unclear. The stratigraphy, geochemistry and geochronology of the tuff beds were investigated to reveal their magmatic origin and potential source region. Most of the tuff beds are recorded in member 4 and can be correlated through the Bazhenovo sequence. Thin‐section petrography and X‐ray diffraction indicate that the tuffs mostly comprise clay minerals and K‐feldspars. Less common minerals are plagioclase, quartz and pyrite. The geochemical composition of the Bazhenovo tuff beds suggests a parental magma origin of andesite/basalt, which came from volcanic arc‐related settings. Considering the results of geochemical studies along with LA‐ICP‐MS zircon U–Pb dating (141.3 ± 0.3/2.8 Ma), the palaeovolcanoes of the Caucasus region or south‐east Mongolia–North‐East China are one of the potential source regions of the tuffs. The record of these tuffs indicates the intensive volcanic eruption during the Volgian–Ryazanian, accompanied by a very low‐sedimentation rate and preservation in a reducing environment. The tuff beds have broad implications as an isochronous marker horizon and constraints for the numerical age of the Jurassic–Cretaceous boundary in the Boreal Realm.

silica in water.Nutrient-rich pyroclastic materials stimulate primary productivity, which promotes the organic matter enrichment of sediments (Hamme et al., 2010;Tian et al., 2020;Li et al., 2021).In marine basins, pyroclastic rocks undergo various types of alteration, including the transformation of volcanic glasses into clay minerals, which complicates the identification of volcanic sedimentary rocks.Geological records often indicate the coexistence of volcanic ash beds and organic-rich black shales in basins of various ages and locations (Smith et al., 2003;Sonnenfeld et al., 2015;Yuan et al., 2019;Acosta et al., 2021).
The study history of the Late Jurassic-Early Cretaceous Bazhenovo Formation of the West Siberian Basin is over 50 years old.The Bazhenovo organic-rich rocks have attracted the attention of geoscientists for many decades, not only as an unconventional reservoir but also as excellent archives for palaeoenvironmental studies.The reports of pyroclastic traces in the Jurassic successions of the West Siberian Basin have been mentioned previously (Sarkisyan & Protsvetalova, 1964; Van & Kazansky, 1985).Some intervals in the Bazhenovo Formation are characterised by the presence of tuff beds (Karnyshina, 2003;Shaldybin et al., 2019;Kondrashova, 2020;Bulatov et al., 2021;Panchenko et al., 2021).All observations suggest explosive volcanic activity during the Late Jurassic-Early Cretaceous.However, the source of pyroclastic rocks in the Bazhenovo epicontinental basin has not yet been fully understood.According to previous studies, the probable source of volcanic material was located in the Lesser Caucasus (Panchenko et al., 2022;Rogov et al., 2023).
Determining the numerical age and sources of volcanic material in the Bazhenovo Formation is of both fundamental and applied importance.The dating and correlation of the tuff beds as a widespread isochronous marker are valuable in establishing the Jurassic-Cretaceous (J-K) boundary and well correlation through the Bazhenovo sequence.A chronostratigraphic framework of the Bazhenovo Formation will help improve the depositional model.A fundamental understanding of these aspects can help guide future hydrocarbon explorations.This paper presents new data on the zircon U-Pb dating of the tuff beds in the Bazhenovo Formation.The findings from the current study add to the previous studies on the numerical age of the tuffs and their provenance (Panchenko et al., 2021;Rogov et al., 2023).

| GEOLOGICAL SETTINGS
The West Siberian Basin is a large intracratonic basin located between the Ural Mountains and the Yenisey River and covers an area of about 2.2 million km 2 (Peterson & Clarke, 1991;Vyssotski et al., 2006).The basin consists of the Mesozoic-Cenozoic Sag that overlies the Early Triassic rift system.The basement of the basin is highly heterogeneous and consists of Hercynian-accreted terranes.The sedimentary succession consists of Triassic through Quaternary rocks (Ulmishek, 2003).The majority of oil deposits are Mesozoic, including the Jurassic and Cretaceous (Figure 1).The West Siberian Basin has a long research history and is now the focus of a regional petroleum prospective assessment of the Bazhenovo Formation.It is poised to become the next major Russian unconventional shale oil province.
The Bazhenovo source rocks and stratigraphic analogues (the Tutleim (lower), Golchikha, Maryanovka, Yanov Stan formations and others) deposited during the Volgian to early Valanginian are characterised by dark, mostly laminated organic-rich shales (Figure 1).These shales were deposited mainly under anoxic bottom water/ sediment conditions in an epicontinental palaeosea (Kontorovich et al., 2013(Kontorovich et al., , 2019;;Burnaz et al., 2022).The total organic carbon (TOC) content is up to 30 wt%, with an average of 8-10 wt%.The precursors of this organic matter were predominantly algae and bacteria.The organic matter in the Bazhenovo Formation is commonly classified as type II kerogen with a minor contribution from type I kerogen (Bulatov et al., 2022;Goncharov et al., 2021;Spasennykh et al., 2021).
The Bazhenovo Formation is commonly 20-30 m thick and subdivided into six members (Panchenko et al., 2016).Member 1 is comprised of mainly organiclean siliceous shale.Member 2 consists mainly of dark grey siliceous shales with interlayers of radiolarites (Khotylev et al., 2019).A radiolarite is a siliceous sedimentary rock formed by accumulating abundant skeletons of radiolarians.Subsequently, radiolarians may undergo recrystallisation and be replaced by calcite or dolomite.Member 3 consists mainly of radiolarites and siliceous shales.The first three members constitute the lower part of the Bazhenovo Formation, characterised by lower TOC and gamma-ray values.Member 4 is comprised of mainly dark grey to dark brown fine-grained organic-rich siliceous shales.The light brownish, thin tuff beds occur in this member.Member 5 consists mainly of dark brown calcareous organic-rich shale with coccolithophore remains.Member 6 is comprised of dark grey organic-rich shale with pyrite lenses and nodules.Members 4-6 comprise the upper part of the Bazhenovo Formation, characterised by higher TOC and gamma-ray values.The overlying strata (the Froly, Tutleim (upper), Achimovo or Sortym formations) consist of dark grey clayey-siliceous organic-lean shales interbedded with sandstones.The underlying Abalak Formation or its stratigraphic analogue, the Georgievka Formation, is comprised of grey shales with low TOC values.The studied tuff beds in member 4 are assigned to the latest middle Volgian Praechetaites exoticus Zone (Rogov & Zakharov 2011;Rogov et al., 2023).Recent publications indicate the presence of a series of tuff layers in members 3 and 5 of the Bazhenovo Formation (Panchenko et al., 2021(Panchenko et al., , 2022)).However, due to their extreme thinness (1-3 mm), they are very difficult to select for analysis without contaminating the sample with host rock and therefore were not used in this study.

| Samples
Tuff beds were sampled and studied in detail from 30 wells drilled in the central part of the West Siberian Basin over a depth range from 2429 to 3057 m (Figure 2).All of the tuff samples were selected based on their purity to avoid the potential for considerable interference from the host organic-rich shales.These tuffs represent T1 tuff based on the classification by Panchenko et al. (2021).Due to bed thinness (<1 cm) and the small amount of available pyroclastic materials, not all samples were analysed.The 16 samples were selected for petrographic and geochemical analyses.Zircons for geochronological analyses were extracted from five samples (D541, S5, T709, M14 and Y66).

| Optical microscopy
Micro-scale petrographic descriptions were conducted using rock thin sections analysed under an Axioscop 5 polarising microscope (Zeiss, Germany) in both plainpolarised (PPL) and cross-polarised (XPL) light.Thinsection images were taken with an Axiocam 506 colour digital camera.Analyses were performed at the Centre for Petroleum Science and Engineering, Skolkovo Institute of Science and Technology.
F I G U R E 2 Map of the West Siberian Basin with the location of wells (modified after Zanin et al., 2005).

| X-ray diffraction analysis
The mineral compositions were performed at the V.I.Shpilman Research and Analytical Centre for the Rational Use of the Subsoil using X-ray diffraction (XRD) using an ARL X'TRA X-ray diffractometer (Thermo, Switzerland) with CuKα radiation and analysed over the 2-60° 2θ angular range at a scanning speed of 2° per minute with a step size of 0.02°.The voltage and current of the X-ray tube were operated at 40 kV and 40 mA, respectively.The whole-rock samples were ground to a fine powder.X-ray diffractograms of the samples were subjected to quantitative mineralogical analysis using the Rietveldbased Siroquant V5 software package (Bish & Post, 1993).

| X-ray fluorescence analysis
The major element content (SiO 2 , Al 2 O 3 , MgO, Fe 2 O 3 , TiO 2 , CaO, P 2 O 5 , MnO, Na 2 O, BaO and S tot ) of the bulk samples was analysed using an ARL Perform'X X-ray fluorescence (XRF) spectrometer (Thermo, Switzerland) at the V.I.Shpilman Research and Analytical Centre for the Rational Use of the Subsoil.The samples were prepared as pressed powder pellets on a boric acid substrate.The fundamental parameter method was applied for calculation.The analytical precision and accuracy for major elements were generally better than 5%.

| Inductively coupled plasma mass spectrometry
The analysis of trace elements was performed at the Institute of Geology and Petroleum Technologies of Kazan Federal University using iCAP Qc ICP-MS (inductively coupled plasma mass spectrometry; Thermo Fisher Scientific, Germany).The 100 mg of powdered samples were preliminarily decomposed in autoclaves in a mixture of purified hydrochloric (3 mL, 38% HCl), hydrofluoric (1.5 mL, 38% HF) and nitric (1.5 mL, 68% HNO 3 ) acids at 210°C for 30 min using an ETHOS UP Microwave Digestion System (Milestone).Boric acid (15 mL, 4.5% H 3 BO 3 ) was added to the autoclaves to transfer rare earth element fluorides into the solution, and then a further digestion at 170°C for 30 min was performed.The resulting solutions were quantitatively transferred to 50 mL volumetric flasks (Corning) and brought to 50 mL with deionised water.An aliquot of 500 μL of the resulting solutions was taken and diluted with deionised water in 10 mL volumetric flasks (Corning) to 10 mL with the addition of an internal indium standard (Inorganic ventures) with a final concentration of 5 ppb.Hydrochloric acid was added to bring the final content of all acids in the solution equal to 2%.Calibration was provided using a multi-element standard solution, with concentrations of each element ranging from 1 to 100 ppb.Interference removal was performed by the kinetic energy discrimination (KED) system.The obtained values were normalised to the initial concentration, taking into account the blank sample, sample mass and degree of dilution.Analytical precision was better than 5% for the majority of trace elements.
An algorithm was written that determines the azimuth with the best correlation in chemical composition to investigate the regional heterogeneity of tuff composition.The Python code with comments is available in Supplemental Materials S1.

| Zircon U-Pb geochronology
The tuff samples were crushed in a mortar.Then zircon grains were separated by a standard magnetic separator and heavy liquid, hand-picked under a binocular and mounted and polished in epoxy for the cathodoluminescence (CL) imaging.The isotopic analysis positions were selected considering the inner structure of the crystal.The U-Pb ages of zircon grains were measured by the LA-ICP-MS method using the iCAP Qc quadrupole ICP-MS coupled to an Analyte Excite 193 nm excimer-based laser ablation system.The sample analysis was performed according to the following scheme: three and two measurements of two standards (external [Plešovice] and control [91500]), respectively, were performed at the beginning and the end of the measurement session.Then, one measurement of the external and control standards was taken every 10 measurements.Mass spectrometry data processing, correction accounting, selection of the optimal signal area and calculation of isotope ratios ( 207 Pb/ 206 Pb, 206 Pb/ 238 U, 207 Pb/ 235 U, 208 Pb/ 232 Th) and corresponding ages were performed using the Iolite 3.65 program integrated in Igor Pro 7 (Paton et al., 2010).Weighted mean age calculations and the construction of concordia diagrams were made using the IsoplotR software package (Vermeesch, 2018).A systematic uncertainty (S sys ) of the LA-ICP-MS U-Pb method was also included in the results after standard deviation uncertainty (Horstwood et al., 2016).All uncertainties are reported at the 2σ level.Detailed metadata for the U-Pb analysis methodology is presented in Table S1.

| Visual description
The organic-rich shale of member 4 is greyish-black to dark black and mostly laminated.The studied tuff beds are beige to brownish or grey in colour and consist of a friable cineritic (ash) matrix.Tuffs vary from 1 to 10 mm thick and have sharp contact with underlying organic-rich siliceous shales of the Bazhenovo Formation (Figure S1).The tops of the tuff beds usually have gradational contacts with overlying host rocks over several millimetres.
The studied tuff beds are characterised by bright orange to yellow fluorescence under UV light, which facilitated their recognition (Figure 3).The nature of the intense fluorescence of the tuff under UV light may be related to the content of moderately crystalline kaolin-group minerals due to radiation-induced defects brought about by their relatively high uranium and thorium contents (Shaldybin et al., 2019).According to Panchenko et al. (2021), luminescence can also be associated with the presence of baryte in the tuffs.
Since the thin tuff beds are found in shales, this indicates that they were deposited in situ in marine environments with an instant sedimentation rate and probably were not reworked by higher-energy currents.The majority of the tuff beds are too thin to produce a characteristic petrophysical response sufficient for recognition on the wireline logs.

| Petrographic description and XRD analysis
Petrographic examination reveals the tuffs are slightly compacted, poorly sorted and absent of any structure.
They predominantly consist of angular, thorn-shaped to cuspate quartz splinters (in width from 10 to 50 μm) and feldspar crystal fragments supported in a light yellowbrownish, devitrified (altered) volcanic groundmass consisting of clay minerals (Figure 4A through F).The tuff beds display normal grading.Some quartz grains show slightly undulatory extinction of the grey interference colours under XPL.Kaolinite occurs as single vermicular crystals or as pellets (Figure 4C,D).Most of the feldspars were altered to kaolinite or hydrated halloysite.Several grains of plagioclase with polysynthetic twinning are present (Figure 4F).Pyrite is present in the form of framboids.Pyrite in the tuff beds was probably formed as a result of sulphate reduction during diagenesis.
The XRD analysis of the tuff beds confirms the macro-scale and petrographic observations.Mineral compositions from XRD analysis are listed in Table 1.An example of the XRD pattern of the bulk tuff sample is presented in Figure S2.The tuffs are predominantly comprised of clay minerals (up to 86.5 wt%) and K-feldspars (up to 31.4 wt%).The clay minerals include mixed-layer illite/smectite and kaolinite.Minor constituents include plagioclase (up to 22.5 wt%), quartz (up to 10.7 wt%) and pyrite (up to 3.8 wt%).Three samples contain dolomite (up to 29 wt%).Some mineralogical variations of the tuffs can be caused by differences in the depositional environment, the lithology of host rocks or diagenetic alteration.Trace constituents unobtainable by XRD analyses but observed in thin-sections include plagioclase and zircon.Table 1 also  for the average Bazhenovo organic-rich shale within member 4. The mineralogy of the tuffs differs from the Bazhenovo shale.The quartz content is significantly higher in the shale than in the tuffs.
The angular shape and morphology of the clasts are typical of ashfall deposits (McPhie et al., 1993).Volcanic ash is relatively unstable and can easily transform into clay minerals.The primary glassy matrix of volcanic ash has been altered to clay minerals as a result of deposition in the marine environment.Fisher and Schmincke (1984) examined the alteration of volcanic glass, while Bohor and Triplehorn (1993) offer additional insights into the rates of alteration.The texture suggests that clay minerals can form quickly, although factors like ash composition, pore solution chemistry, depositional environment and sediment permeability influence the alteration rates.
The Bazhenovo tuffs are similar in composition and texture to tonsteins that commonly occur in coal-bearing strata (Arbuzov et al., 2016;Dai et al., 2017).Tonsteins are volcanic ashfall deposits that have been altered to clay minerals and have stratigraphic value in coalfield exploration.

| Inorganic geochemistry
The most abundant major oxides in all tuff beds are SiO 2 and Al 2 O 3 (Table 2).This finding is consistent with the analysis of the mineral compositions of the samples.Clay minerals and feldspars dominate in all samples.The SiO 2 content varies from 48.15 to 58.52 wt%.The Al 2 O 3 content varies from 28.17 to 33.63 wt%, which is higher than the general composition of the Bazhenovo black shale (Table 2).Relatively high concentrations of CaO and MgO are only observed in three samples (K33, Y66 and Y213), which agrees with the presence of dolomite according to XRD data and thin-section petrography observations.The Fe 2 O 3 (0.83-4.02 wt%) and S tot (0.68-3.19 wt%) bulk content is related to the presence of pyrite.The K 2 O and Na 2 O values reached 5.06 and 3.11 wt%, respectively.The TiO 2 concentrations vary from 0.29 to 0.84 wt%.The relatively high P 2 O 5 value in one sample (K33) probably indicates the presence of apatite.However, XRD did not confirm the presence of apatite.The clay mineralogy of the tuffs is reflected in the increased concentrations of many elements.The concentration of total rare earth elements (REE) varies from 85.2 to 345.94 ppm, with an average value of 202.87 ppm (Table 3).The ratio of total light REE (LREE) to total heavy REE (HREE) varies from 6.34 to 22.50, with an average value of 15.49.The Bazhenovo rocks have a lower concentration of REE than in the tuff beds.However, Ni, Cu and Zn values are higher in the Bazhenovo shale than in the tuffs, probably due to their association with organic matter, which is significantly higher in the shale.
The tuff beds were plotted on a chondrite-normalised (Sun & McDonough, 1989) REE diagram showing negative sloping curves with an overall enrichment of LREE and depletion of HREE (La N /Yb N from 4.58 to 36.73; Figure 5A).Slight but consistently negative Eu anomalies are present in the samples (Eu/Eu* values vary from 0.74 to 0.81 with an average of 0.77).The primitive mantle-normalised (Sun & McDonough, 1989) diagram for the tuffs demonstrates similar patterns and indicates negative anomalies of Nb and Zr, along with positive anomalies of U and Pb (Figure 5B).

| Zircon CL imaging and U-Pb geochronology
Due to the limited availability of tuff material, zircons were extracted only from five samples (D541, S5, T709, M14 and Y66).The CL images of representative zircons are presented in Figure S3.The CL images reveal that the zircon crystals are euhedral and subhedral.Most of them are short prismatic, with an average length of 80-120 μm and an aspect ratio from 1:1 to 1:3.In CL images, zircons exhibit varied core types and typical external magmatogenic oscillatory zoning.The centres of some zircons show diffuse zonation patterns, but most are characterised by small planar growth.Several crystals have luminescent homogenous tips, which truncate the oscillatory-zoned zircon and may be formed before eruption as a healing process of broken tips (Schmitz & Bowring, 2001).A total of 91 spot analyses of zircon grains for five samples of the tuffs from the Bazhenovo sequence were performed.All LA-ICP-MS data of samples and standard zircons are presented in Table S2.

| Tectono-magmatic setting
The tuff beds were plotted on a total alkali-silica (TAS) diagram in order to identify their sources (Figure 7A).The samples mostly plot in the basaltic andesite field, with some scattered in the basalt, andesite and basaltic trachy-andesite fields.Based on relatively stable elements, the Zr/TiO 2 versus Nb/Y diagram suggests the tuffs are released from andesitic and basalt to andesitic eruptions (Figure 7B).However, the original compositions may have been partially altered due to the devitrification of the primary glass, framboidal pyrite and secondary dolomite formation (which would have caused a decrease in SiO 2 ).The mobility of niobium and zirconium is more significant during the alteration of ash materials than that of titanium and yttrium (Möller, 2000;Spears & Arbuzov et al., 2019).These elements must be used cautiously to interpret thin tuff beds (<1 cm).Considering the different mobility of elements during the ash transformation and secondary alteration, the parent magmas of the tuff beds may have been silicic in composition.
The inorganic geochemistry not only provides evidence for the volcanic origin but also enables tectonomagmatic conclusions to be made for areas external to the sedimentary basin.The chemical composition of the tuff beds is mainly controlled by the parent rock composition and its depositional environments.Relatively immobile REEs are widely used to identify the former tectono-magmatic setting and to constrain the provenance composition.Since the tuffs were predominantly classified as mafic to intermediate, tectonic discrimination diagrams of basalts were used to determine the tectono-magmatic settings.The tuffs plot exclusively in the field of arc basalts on the ternary (Th-Hf-Nb) diagram (Figure 8A).The discrimination diagram based on Zr, Ti and Y shows that the tuffs plot predominantly in the field of calc-alkaline volcanic arc basalts (Figure 8B).According to the Ta/Yb versus Th/Yb diagram, the studied tuffs are mostly related to oceanic arcs (Figure S5).Considering that the tuff beds in the Bazhenovo Formation may have a felsic magma origin, discrimination diagrams of granites were used.The diagram based on La/Th and Hf content indicates magmatic composition.The tuff beds have low La/Th ratios (0.24-1.13) and intermediate Hf values (4.12-6.83ppm), compatible with an acidic arc source (Figure 8C).The Rb versus Y + Nb plot shows that the tuffs mostly plot in the field of volcanic arc granites (Figure 8D).Therefore, chemical analysis of the tuffs as a whole is compatible with a felsic or mafic volcanic arc source.Based on the similar distribution of trace elements, it can be assumed T A B L E 3 (Continued) that the source of the pyroclastic material for the studied tuff beds from different wells was the same.This assumption is consistent with the petrographic data, mineral composition and chemical composition, except for several samples (K33, Y66 and Y213) with increased carbonate content, which can be explained by diagenetic alteration.

| Origin of the Bazhenovo tuff beds
Identifying the potential source of ash material is quite a challenging task.Considering the lack of evidence for volcanic eruptions occurring within the Bazhenovo Basin during the period of ash deposition, it is probable that the volcanic vents were situated outside the basin.The range of transport and the size of deposited ash particles depend on several factors, including the velocity and volume of erupted ash material, duration of the eruption, the height of the eruption column, wind pattern, settling velocity and sea water depth at the deposition site (Ledbetter & Sparks, 1979).
Various models estimate the range of transport of pyroclastic material, considering various factors.For example, Mastin et al. (2014) demonstrated a model of the Yellowstone volcanic eruption, which estimated that the eruption could form ash deposits of 10 mm in thickness at a distance of 2000-2500 km.Ninkovich et al. (1978) analysed the products of the Toba volcanic eruption, in which a tephra layer 30 cm thick and a grain size of 130 μm was formed 2500 km from the source.The ash clouds of the Eyjafjallajökull eruption in 2010, which had significantly lower power than the examples above, transported particles up to 100 μm in size only for a distance of about 1000 km (Beckett et al., 2015).Therefore, depending on the conditions of eruption and transport of ash material, considering the size of zircon grains, possible volcanic arcs responsible for the wide spreading of ashfalls during the deposition of the Late Jurassic-Early Cretaceous organicrich shales of the West Siberian Basin may be localised to a distance of 2000-3000 km.
A closer examination of the tuff composition in the Bazhenovo Formation reveals the zonation of the studied area.Changes in the composition of tuffs may be caused by the interaction of pyroclastic particles with sea water and secondary alteration (Van & Kazansky, 1985).However, the main factor causing compositional heterogeneity is a fractionation of pyroclastic material during transport by air.This factor may play a major role given the significant distance from the eruption source (Panchenko et al., 2021).
The direction of the tuff heterogeneity varies depending on the compositions being considered.Together with the data on the composition of the T1 tuffs from Panchenko et al. (2021), we can obtain more reliable trends in regional heterogeneity.Elements of the alkaline group have a distinct areal correlation in a north-east direction.The Na 2 O concentration reciprocally decreases with increasing K 2 O content (Figure 9A,B).This trend may be explained by the early post-emplacement ion F I G U R E 6 Concordia and 206 Pb/ 238 U weighted mean age plots for all zircons from the Bazhenovo Formation tuff beds.Age uncertainties are presented in 'without/with propagation for S sys ' format.Bas et al., 1986) and (B) Zr/TiO 2 versus Nb/Y diagram (Winchester & Floyd, 1977) for the tuff beds.
exchange (Fisher & Schmincke, 1984).Among trace elements, the increase in Cs and Rb content is evident in this direction (Figure 9C,D).Another trend observed in the south-east direction is associated with trace elements such as Nb, Th, Ta, Zr, Hf, Y and HREE, as well as TiO 2 .They are not subject to secondary changes in the tuffs and are associated with high-density accessory minerals and can be used to determine the source of the pyroclastic rocks (Kiipli et al., 2017).The most evident trends are shown for Nb, Th, Ta and Zr + Hf (Figure 10).
The lack of information on the Late Jurassic-Early Cretaceous volcanism in the region and the distance between the source and study region limit the identification of pyroclastic sources.The U-Pb dating of zircons from the Volgian-Valanginian andesites and sandstones indicate the existence of suprasubduction volcanism on the Chukotka margin between 150 and 140 Ma (Vatrushkina et al., 2019).The Early Cretaceous Crimean Mountains may be a source of pyroclastic material.The 40 Ar/ 39 Ar dating of the volcanic rocks from the Karadag volcanic complex (Eastern Crimea) reveals an age range of between 151 and 142 Ma (Meijers et al., 2010).However, Karadag volcanics are problematic as a potential source because of the conflict of isotopic ages with the biostratigraphy of the region, in addition to the more felsic composition of magmatic rocks.Magmatic processes took place on the territory of the Greater Caucasus during the Jurassic period.The K-Ar isotopic dating of basaltic andesites, gabbros and diorites varies from 190 ± 10 to 142.0 or 140.5 Ma (Kasumzade et al., 2002).Evidence of the possible presence of pyroclastic material was also indicated in the Middle Volgian succession of the Russian Platform (Rengarten & Kuznetsova, 1967;Zorina et al., 2020;Nikashin & Zorina, 2021).Therefore, the palaeovolcanoes F I G U R E 8 Discrimination diagrams of the tuffs for distinguishing tectonic settings based on (A) Th-Hf-Nb compositions (Wood, 1980); (B) Zr-Ti-Y compositions (Pearce & Cann, 1973); (C) La/Th-Hf contents (Floyd & Leveridge, 1987); (D) Rb-(Y + Nb) content (Pearce et al., 1984).CAB, calc-alkaline volcanic arc basalts; E-MORB, enriched MORB; IAB, island arc tholeiites; MORB, mid-ocean ridge basalts; N-MORB, normal MORB; ORG, ocean ridge granites; Syn-COLG, syn-collisional granites; VAG, volcanic arc granites; WPB, within-plate basalts; WPG, within-plate granites.
of the Caucasus region as the source of pyroclastic material are in line with the geographical distribution and variability of the chemical composition of the tuff beds in the West Siberian Basin (Van & Kazansky, 1985;Panchenko et al., 2021Panchenko et al., , 2022)).
However, considering the finding of the chemical heterogeneity of the studied tuffs in a south-east direction, a potential source of pyroclastic material could also have been the extensive Mesozoic volcanic activity that was observed in the south-east Mongolia-North-East China-Great Xin'an Range (Wang et al., 2006).The Tamulan Formation volcanics, composed mainly of basalts and basaltic andesites, yielded ages between 147 and 139.7 Ma.The tuff bed of the Tuchengzi Formation from northern Hebei to western Liaoning has a U-Pb LA-ICP-MS age of 141.6 Ma.The 1-10 m thickness of the tuff beds and their widespread occurrence implies strong volcanic activity (Zhang et al., 2009).Figure 11 shows the location of the considered sources of pyroclastic rocks in relation to the presence of the tuff beds.

Cretaceous boundary
Currently, the J-K boundary is based on the Tethys Ocean stratigraphy.However, despite active discussion, researchers still need to improve qualitative markers when correlating the continuation of the Tithonian-Berriasian boundary of the Tethys in isolated basins.In some cases, this problem has been solved using magnetostratigraphy (Wimbledon et al., 2020).
The J-K boundary in the Boreal Realm is also controversial.Biostratigraphic data from ammonites, belemnites, radiolarians and palynomorphs, as well as stable isotope data, are used to determine the J-K boundary within the Bazhenovo Formation of the West Siberian Basin (Braduchan et al., 1986;Dzyuba et al., 2013;Vishnevskaya, 2017;Amon et al., 2021).However, the J-K interval usually lacks biostratigraphic and chemostratigraphic markers that help fix the boundary.Highresolution magnetostratigraphic data from the Bazhenovo sections are rare and debatable (Manikin et al., 2019).The fact that the Late Jurassic and Early Cretaceous were periods when significant differences existed between the Boreal and Tethyan marine biota caused a problem with the Boreal-Tethyan correlation of the J-K boundary (Cecca et al., 2005;Houša et al., 2007).Therefore, the J-K boundary is still not defined at the global level and lies between a wide interval of the upper Tithonian and middle Berriasian.
A study by Pálfy et al. (2000), based on a database of U-Pb isotopic ages and magnetostratigraphic data from the North American Cordillera, proposed an approximate age for the Tithonian-Berriasian boundary of 141.8 Ma.Zircons from ash interbeds in southern Tibet, confined to the Berriasian ammonites and assemblages of calcareous nannofossils, yielded SIMS U-Pb ages ranging from 140.0 ± 1.3 to 141.8 ± 1.2 Ma (Liu et al., 2013).The U-Pb age of zircons from the Pimienta Formation (central-eastern Mexico) bentonite layer overlying the late Tithonian Crassicollaria Zone and underlying the early Berriasian Calpionella Zone was determined as 139.1 ± 2 Ma (Lopez-Martínez et al., 2015).
In the Andes, the J-K boundary is well studied along the extension of the Vaca Muerta Formation from the Neuquén Basin.The shales of this formation contain ash tuffs associated with ammonites and calpionellids, a specific marker of the J-K boundary.The CA-ID-TIMS U-Pb ages of zircons from ashfall tuffs confined to the Noduliferum Zone of the Berriasian are 139.55 ± 0.18 Ma (Vennari et al., 2014).The study by Lena et al. (2019) of this formation has extended the U-Pb CA-ID-TIMS isotopic data of this region.Zircons from tuffs deposited close to the Virgatosphinctes andesensis Zone correspond to 147.112 ± 0.078 Ma and are consistent with the early Tithonian.The first occurrence of the late Tithonian Rhagodiscus asper and Umbria granulosa nanofossils, closest to the J-K boundary, was also dated using zircons from ash interbeds with calculated ages of 140.6 ± 0.4 and 141.31 ± 0.56 Ma, respectively.Tuff interlayers from the Mazatepec section (Mexico) confined to the early Berriasian Elliptica Subzone correspond to an age of 140.512 ± 0.031 Ma.This extensive study has suggested a revision of the J-K boundary, reducing its age to between 140.7 and 140.9 Ma.Based on these radioisotopic data, a link between the J-K boundary and the Steen River impact structure in Canada has also been suggested, with an LA-ICP-MS U-Pb age of 141 ± 4 Ma for apatites (McGregor et al., 2020).The LA-ICP-MS U-Pb dating of zircons from the fossil-bearing Late Jurassic to Early Cretaceous volcaniclastic sandstones of the Murihiku Terrane (New Zealand) has provided significant components in the age range 140-143 Ma, possibly straddling the J-K boundary (Browne et al., 2020).This shows disagreement with the age interpretation of its palynofloras, indicating only the latest Jurassic age.Adams and Campbell (2020) suggest that this is resolved if the age of the J-K boundary is lowered to ca 140 Ma.All these considered studies gave coherent results indicating a difference in the sections studied with the official limits of the Tithonian-Berriasian boundary between 140 Ma and 141 Ma.
Closer in biostratigraphic correlation to the Boreal Realm are sediments containing Buchia piochii found in Mexico (Pessagno et al., 2009), whose distribution zone covers the Praechetaites exoticus ammonite Zone (Zakharov & Rogov, 2020).The U-Pb age of zircons from pillow lavas in this zone is 143.734 ± 0.060 Ma.In addition, the dated Berriasian-Valanginian Buchia uncitoides Zone in northern California corresponds to an absolute age of 135.1 Ma and magnetochron mid CM16-CM16n, which suggested the J-K boundary age of 141.1 Ma (Bralower et al., 1990).
Studies devoted to the calibration of age boundaries of the Jurassic-Cretaceous stages note that U-Pb dating of zircons is systematically 3 or 4 Myr younger than the estimated 40 Ar/ 39 Ar age of the basalts used in the M-sequence model (Ogg, 2012;Wimbledon et al., 2020).However, a recent study of the Late Jurassic-Early Cretaceous ages revised U-Pb age estimates by proposing a basement age for the Berriasian of 144.77Ma (Scott, 2022), and the International Commission on Stratigraphy (ICS) still kept the age of the J-K boundary in the International Chronostratigraphic Chart as ca 145 Ma (Cohen et al., 2013).Recognition of these isotopic U-Pb ages may significantly affect the accepted duration of ammonite zones and the sediment accumulation rate, and therefore, it is still under consideration.
In another chronostratigraphic chart by the Geological Society of America, the Geologic Time Scale, published in 2012, the numerical age of the J-K boundary was also accepted as 145.0 Ma (Gradstein et al., 2012).A recent revision of the Geologic Time Scale corrected the age of the base of the Berriasian stage to 143.1 Ma (Gale et al., 2020).However, this age is still not ratified, despite the U-Pb isotope data of zircons from different regions demonstrating a younger age of the J-K boundary.
The discovered tuff beds in the Bazhenovo Formation represent a useful correlation tool as an isochronous marker horizon due to their widespread distribution over a large area of the West Siberian Basin (ca 0.5 million km 2 ) and the distinct characteristics that facilitate their recognition.The studied tuffs are situated in member 4 of the Bazhenovo Formation (Figure 12).Radiometric ages obtained from tuffs are useful in sedimentary strata lacking other volcanogenic minerals.The ages help improve the stratigraphic timescale and lead to the possibility of inter-basinal correlations.
Based on the biostratigraphy of the Boreal Realm in Western Siberia, the J-K boundary (Volgian-Ryazanian boundary) is blurry and located between the Chetaites chetae Zone and Praetollia maynci Zone, which does not coincide with the Tethyan J-K boundary.The tuffs are located within the Praechetaites exoticus ammonite Zone (Panchenko et al., 2021), corresponding with the uppermost middle Volgian and the weighted average age of 141.3 ± 0.3/2.8Ma, which corresponds to the late Tithonian age determined in the Andes (Lena et al., 2019), and fit with magnetostratigraphically based Boreal-Tethyan correlation.Therefore, the U-Pb data from the tuff bed supports the revision of the J-K boundary age currently accepted in chronostratigraphic charts.
The studied tuff beds occur in the Late Jurassic-Early Cretaceous Bazhenovo Formation.The tuffs are composed predominantly of mixed-layer illite/smectite, kaolinite and K-feldspar minerals.The tuff beds represent the traces of the extremely intensive explosive volcanic eruption during the deposition of the Bazhenovo black shales.Based on the geochemical characteristics of the tuffs, the proposed magma source parent rocks are mafic to intermediate rocks that belong to the volcanic arc.The most suitable source of pyroclastic materials could have been palaeovolcanoes from the Caucasus region or south-east of the Mongolia-North-East China region.
The zircon U-Pb age for the tuffs shows their isochronous deposition in the Bazhenovo palaeosea during Volgian-Ryazanian times, with the weighted mean age of 141.3 ± 0.3/2.8Ma for all studied samples.The tuff beds, as an independent precise marker, may assist in refining the stratigraphic framework of the Bazhenovo sequence.New U-Pb dates for the tuffs provide evidence for a younger numerical age of the J-K boundary in the West Siberian Basin.
shows the XRD data F I G U R E 3 Photographs of core showing bright fluorescence of the tuff beds (marked in red) under UV light.

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I G U R E 5 (A) Chondrite-normalised REE diagram and (B) primitive mantlenormalised trace-element distribution patterns for the tuff beds and the typical Bazhenovo black shale.

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Diagram of the tuff composition projection on the azimuth with the optimal correlation coefficient.Black dots represent data of T1 tuffs from Panchenko et al., 2021.(A) Na 2 O concentration with the best correlation along the azimuth 20.69°.(B) K 2 O with the best correlation by azimuth 30.78°.(C) Cs content in ppm with the best correlation by azimuth 50.06°.(D) Rb content in ppm with the best correlation by azimuth 34.14°.

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Diagram of the tuff composition projection on the azimuth with the optimal correlation coefficient.Black dots represent data of T1 tuffs from Panchenko et al., 2021.(A-С) Nb, Th and Ta content in ppm with the best correlations by azimuths 123.02°, 161.22° and 137.58°, respectively.(D) Zr + Hf content in ppm with the best correlation by azimuth 147.78°.

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Early Cretaceous (ca 140 Ma) palaeogeographic map showing the potential sources of pyroclastic material (white ellipses) and its distances to the location of the studied tuffs (red circle).The red dot indicates a found Russian Platform Middle Volgian pyroclastic material (modified from Scotese, 2021).
Mineral composition of the tuff beds and the typical Bazhenovo black shale determined by XRD analysis (wt%).
Rogov et al. (2023)ge (n = 23) mineral composition of the Bazhenovo black shale within member 4.zircon morphology may also indicate a single-source eruption event for all studied samples.The age of the samples is in line with the CA-ID-TIMS U-Pb age for the eight zircons (141.11±0.25 Ma) extracted from the single Bazhenovo T1 tuff bed obtained byRogov et al. (2023).
Major element compositions of the tuff beds and the typical Bazhenovo black shale determined by XRF analysis (wt%).
T A B L E 2 a Average (n = 23) major element composition of the Bazhenovo black shale within member 4. T A B L E 3 Trace elements content of the tuff beds and the typical Bazhenovo black shale determined by ICP-MS (ppm).a Average (n = 23) trace elements content of the Bazhenovo black shale within member 4. b Eu/Eu*-Eu N /(Sm N × Gd c LREE-La, Ce, Pr, Nd, Sm and Eu.d HREE-Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu.