INTRODUCTION

The feature of the transgressions in the epicontinental areas of the Peri-Tethys in the Paleogene is the presence of terrigenous-siliceous facies in the marginal parts of marine basins (Radionova et al., 2003). One of the most common stratigraphic intervals rich in remains of siliceous plankton is the Paleocene–Eocene transition interval. In this interval, sequences of diatom and silicoflagellate assemblages were traced in the Middle Volga region (Aleksandrova and Radionova, 2006; Glezer, 1995; Oreshkina and Aleksandrova, 2007, 2017; Oreshkina and Oberhansli, 2003; Oreshkina and Radionova, 2014; Strelnikova, 1992), Voronezh anteclise (Strelnikova, 1992), Kaliningrad oblast (Strelnikova et al., 1978), Polar Cis-Urals (Iakovleva et al., 2000; Oreshkina, 2000, 2012; Oreshkina et al., 1998), Trans-Ural region (Aleksandrova et al., 2012; Oreshkina et al., 2004, 2008), and Turgay Depression (Radionova et al., 2001). In the south of the Russian Plate in the Cis-Donets structural-facies zone (SFZ) in the eastern frame of the Donets basin (Akhmetiev and Beniamovsky, 2003; Postanovleniya…, 2001), the Upper Paleocene–Lower Eocene cycle of sedimentation is represented by the Buzinovka, Veshenskaya, Surovikino, and Osinovo beds (Aleksandrova et al., 2020; Beniamovsky, 2016). These layers were first distinguished by Leonov (1961) and then transferred to the status of formations (except for the Osinovo Beds) by Semenov (1965). The Buzinovka Formation is represented by phosphorite-pebble deposits bearing rolled mollusc nuclei in the basal part. Higher in the section, they are overlain by the strata of green glauconite sands (up to 20 m thick). The Veshenskaya Formation is composed of opoka- or tripoli-like sandy clays (up to 16 m thick), which show facies replacement by heterogeneous loose clayey quartz-glauconite sands, sometimes with interlayers and lenses of siliceous sandstones. The Surovikino Formation comprises light fine-grained quartz, low-glauconite sands with interlayers of concretionary siliceous, and massive sandstones (thickness, up to 25 m). Locally, this formation is separated from the underlying deposits by a layer of massive coarse-grained sandstones containing silica clay pebbles. The monotonous stratum of quartz sands and sandstones of up to 25 m thick is attributed to the Osinovo Beds, which rest erosively on underlying strata.

The integrated lithological and biostratigraphic study of the stratigraphic interval in the reference sections of the Cis-Donets SFZ did not reveal the presence of microfossils in most of the sections sampled. The only exception is borehole 1238, drilled near the village of Bazkovskaya (Sholokhov district, Rostov oblast), where Thanetian–lower Ypresian dinocyst assemblages were recognized (Aleksandrova et al., 2020; Iakovleva and Aleksandrova, 2021). No silicofossils were found in borehole 1238. The opposite situation is observed in borehole P-321 drilled in the same structural-facies zone. Here, palynomorphs are absent; representative diatom and silicoflagellate assemblages were recognized in the Buzinovka and Veshenskaya formations.

The present paper aims to consider the taxonomic composition and the stratigraphic position of this assemblage and correlate it with silicofossils from other localities of the marginal basins of the Northern Peri-Tethys.

MATERIAL AND METHODS

Borehole Р-321 was drilled 2.2 km to the west of the settlement of Grigor’evskii of Uryupinsky district, Volgograd oblast (50°52′ N, 41°37′ E), with an elevation of the well head of 185 m (Fig. 1). Core samples and the description of the borehole section were provided by V.M. Ryborak, an employee of the South Voronezh geological field party, who supervised the 1 : 200 000 geological, hydrogeological, and engineering-geological additional appraisal within the Sheet M-37-XII (Novokhopersk) (Ryborak and Shokurov, 1999). The borehole section (Fig. 2) is as follows (from bottom to top):

Fig. 1.
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(a) Paleogeographic diagram of Northern Eurasia (after Paleogeograficheskii…, 1998) for the Paleocene–Eocene boundary interval and (b) localities of borehole Р-321 (this work) and other sections mentioned in the text.

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Fig. 2.
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Distribution of marker species of the diatom Moisseevia uralensis Zone in the borehole Р-321 section and the ratio of the dominant diatom assemblages. (1) Sands; (2) sandstones; (3) silty clays; (4) “ironstone”; (5) conglomerate.

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Upper Cretaceous (Danian?).

Layer 1, interval of 18.45–15.95 m. Sand oxide yellow, fine-grained, silty, feldspar-quartz.

Buzinovka Formation.

Layer 2, interval of 15.95–15.45 m. “Ironstone.”

Layer 3, interval of 15.45–15.3 m. Conglomerate.

Layer 4, interval of 15.3–14.8 m. Opoka-like sandstone.

Veshenskaya Formation

Layer 5, interval of 14.8–6.6 m. Clay greenish gray, silty.

Oboyan Formation.

Layers 6–13, interval of 6.6–0.6 m. Sands and sandstones fine- to medium-grained, quartz.

Samples were processed using the standard technique of selecting biosiliceous microfossils and organic-walled palynomorphs accepted in Laboratories of Paleofloristics and Micropaleontology of the Geological Institute, Russian Academy of Sciences (Aleksandrova et al., 2012). No marine palynomorphs were found in the studied samples. Semiquantitative assessment of the total content of diatoms and individual species was conducted according to a point system at the magnification ×400: A (Abundant) = 1 or more specimens in each field of view (FV); C (Common) = 1 specimen in 2 FVs; F (Few) = 1–2 specimens in each row of the cover glass; R (Rare) = a few specimens per sample; B (Barren) = complete absence of diatoms or their fragments; T (Trace) = rare fragments of valves. The preservation of diatoms is evaluated as follows: G (Good) = minimum amount of fragments; M (moderate) = about the same amount of fragments and complete valves; P (poor) = complete valves occur rarely. The microimages were taken on a Motic BA310 light microscope and a Tescan Vega MV-2300 scanning microscope. For the taxonomic identifications of diatoms, changes in the names of genera recognized as preoccupied were taken into account (Blanco and Wetzel, 2016), as well as other taxonomic novations.

For the biostratigraphic subdivision, the modified diatom-based zonal scale developed for the Paleogene extratropical realm of the Northern Hemisphere (Strelnikova, 1992) and correlated with the dinocyst zones is used (Iakovleva et al., 2000; Oreshkina, 2000, 2012; Oreshkina and Aleksandrova, 2007, 2017; Oreshkina et al., 2004). The stratigraphic distribution, taxonomic composition of diatoms, and silicoflagellates and their images are given in Fig. 2, Table 1, and Plates I–III.

figure a

Plate I . Diatoms from the Paleogene deposits (borehole Р-321). Scale bar, 20 µm for figs. 1–3 and 10 µm for figs. 4–24. (1, 2, 5) Eupyxidicula moelleri (Grunow) Blanco et Wetzel: (1, 2) specimen 14; (5) specimen 10; (3) Costopyxis broschii (Grunow) Strelnikova et Nikolaev, specimen 14; (4) Moisseevia uralensis (Jousé) Strelnikova, specimen 10; (6) Hemialus febriatus (Grunow) Fenner, specimen 10; (7) Hemiaulus aff. incurvus Schibkova, specimen 3; (8) Hemiaulus frigidus (Grunow) Fenner, specimen 14; (9) Hemiaulus sp., specimen 14; (10, 11) Hemiaulus morsianus (Grunow) Fenner: (10) specimen 14; (11) specimens 10; (12) Hemiaulus curvatulus Strelnikova, specimen 14; (13) Hemiaulus arcticus var. bornholmensis Cleve-Euler, specimen 12; (14, 23, 24) Coscinodiscus argus Ehrenberg, specimen 10; (15) Grunowiella sp., specimen 13; (16) Grunowiella gemmata (Grunow) Van Hearck, specimen 14; (17–19, 22) Cymatosira spp.: (17–19) specimen 13; (22) specimens 14; (20) Jousea elliptica (Jousé) Gleser, specimen 13; (21) Soleum exsculptum Heiberg, specimen 13.

Table 1. Distribution of diatoms and silicoflagellates through the Р-321 borehole
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Plate II.
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Diatoms from the Paleogene deposits (borehole Р-321). Scale bar, 20 µm for fig. 1 and 10 µm for figs. 2–24. (1, 2, 7) Paralia grunowii Gleser: (1) specimen 14; (2) specimen 10; (7) specimen 8; (3) Paralia selecta (A. Schmidt) Gleser, specimen 10; (4) Anuloplicata concentrica (A. Schmidt) Gleser, specimen 14; (5) Paralia crenulata (Grunow) Gleser, specimen 8; (6) Anuloplicata ornata (Grunow) Gleser, specimen 8; (8) Actinoptychus pericavatus Brun, specimen 10; (9) Actinoptychus simbiskianua A. Schmidt, specimen 8; (10) Radiaplicata clavigera (Grunow) Gleser, specimen 14; (11) Hyalodiscus radiatus (O’Meara) Grunow, specimen 10; (12, 13) Actinoptychus sp., specimen 14; (14) Actinoptychus seductilis A. Schmidt, specimen 14; (15) Moisseevia probabilis (A. Schmidt) Strelnikova, specimen 12; (16) Biddulphia tuomeyi (Bailey) Roper, specimen 12; (17) Rhaphoneis lancetulla Grunow, specimen 10; (18) Stephanogonia polygona Ehrenberg, specimen 3; (19) Pterotheca spada Tempère et Peragallo, specimen 10; (20) Biddulphia sp., specimen 9; (21) Eunotogramma variabile Grunow, specimen 12; (22) Goniothecium wittianus Pantocsek, specimen 14; (23) Diploneis sp., specimen 13; (24) Porotheca danica (Grunow) Fenner, specimen 14.

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Plate III.
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Diatoms and silicoflagellates from the Paleogene deposits (borehole Р-321). Scale bar, 20 µm for figs. 1, 23, 24 and 10 µm for figs. 2–22. (1) Sheshukovia sp., specimen 14; (2, 6) Sheshukovia flos (Ehrenberg) Fenner, specimen 13; (3, 4, 8, 9) Sheshukovia fenestra (Witt) Fenner, specimen 10; (5, 7) Trinacria excavata Heiberg, specimen 10; (10) Naviculopsis foliacea Deflandre, specimen 10; (11) Naviculopsis eobiapiculata Bukry, specimen 12; (12) Corbisema hastata globulata Bukry, specimen 13; (13) Corbisema gleserae Bukry, specimen 13; (14) Dictyocha precarentis Bukry, specimen 10; (15, 20) Dictyocha deflandrei Frenguelli ex Glezer: (15) specimen 14; (20) specimen 13; (16, 17) Dictyocha brevispina (Lemmermann) Bukry: (16) specimen 10; (17) specimen 12; (18, 19) Naviculopsis minor (Schulz) Frenguelli: (18) specimen 10; (19) specimen 14; (21) Rattrayella sp., specimen 14; (22) Stellarima microtrias (Ehrenberg) Hasle et Sims, specimen 3; (23) Pseudopodosira pileiformis Jousé, specimen 14; (24) Rhizosolenia clavigera (Grunow) Homann, specimen 9.

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RESULTS

When studying core samples of ferruginated sandstones from the “ironstone” layer in the lower part of the section, attributed to the Buzinovka Formation, the abundant occurrence of diatom valves with signs of ferrugination was established. The main dominant is the group of tychopelagic genera Paralia/Anuloplicata.

Higher in the Veshenskaya Formation sequence, the group Paralia/Anuloplicata retains the dominant position, varying in a range of 82–96% (Fig. 2). The intervals of abundant occurrence of diatoms (specimens 3, 8, 10, 12, 14, 15) alternate with intervals with a lower content and impoverished taxonomic composition (Samples 9, 11, 13), which is typical of the unstable marginal environment of the sea basin. The diatom assemblage is represented by several diatom groups (Plates I–III). The first group of the tychopelagic and benthic genera includes, in addition to the genus Paralia, the genera Anuloplicata, Radiaplicata, Pseudopodosira, Hyalodiscus, Actinoptychus, Rhaphoneis, Eunotogramma, and Diploneis. There occur rare species of the genus Actinoptychus—A. seductilis, A. simbiskiana, and A. pericavatus. The second group, so-called spore genera Сhaetoceros, Goniothecium, Pterotheca, Stephanogonia, and Jousea are also confined to coastal environments. Neritic species with a wide range of stratigraphic distribution are represented by Eupyxidicula turris, Stellarima microtrias, Trinacria excavata, T. subcoronata, Sheshukovia fenestra, S. flos, and Coscinodiscus argus. The genus Rattrayella, represented by small valves, not identified to the species level, can be attributed to alien species. Also, the occurrence of several morphotypes of the genus Cymatosira, which are reported at present from the deposits of the Norwegian Sea, beginning from the Late Eocene, should be noted (Schrader and Fenner, 1976).

The stratigraphically important taxa, such as Moisseevia uralensis, Soleum exsculptum, Eupyxidicula moelleri, Grunowiella gemmata, Hemialus febriatus, H. frigidus, H. arcticus var. bornholmensis, H. curvatulus, H. morsianus, Costopyxis broschii, C. reticulata, Biddulphia tuomeyi, and Coscinodiscus decrescenoides are typical of the Lower Eocene Moisseevia uralensis Zone of the diatom scale for the Extratropical Realm. It appears that a regional feature of the distinguished assemblages is the occurrence of Jousea elliptica, Rhaphoneis lancetulla, Cymatosira spp., Proboscia spp., and the species of the genus Actinoptychus mentioned above. On the basis of the data available on the Polar Cis-Urals (Iakovleva et al., 2000; Oreshkina, 2000, 2012) and Western Siberia (Aleksandrova et al., 2012; Oreshkina et al., 2004, 2008), the Moisseevia uralensis Zone corresponds to the Deflandrea oebisfeldensis and Dracodinium (=Wetzeliella) astra dinocyst zones, which, in turn, correspond to the nanoplankton NP10 Zone and dated at 55.6–54.8 Ma (Iakovleva, 2017). This zone is characterized by the decrease in taxonomic diversity in comparison with the assemblages of the Trinacria mirabile (upper Thanetian) and Hemiaulus proteus (lower Ypresian) diatom zones of the Volga region and the Trans-Urals region (Oreshkina and Aleksandrova, 2007, 2017).

Three genera represent the silicoflagellate assemblage (Plate III): genus Corbisema, known from the Cretaceous, as well as genera Dictyocha and Naviculopsis, appearing in the Middle and Late Paleocene, respectively (McCartney et al., 2020; Perch-Nielsen, 1976, 1985). There occur Corbisema gleserae, C. hastata globulata, D. deflandrei, and D. precarentis. N. foliacea and N. aff. minor appear in the composition of the genus Naviculopsis. The last taxon differs from the type species in smaller sizes, the rhomboidal shape of the basal part of the skeleton, and a narrow apical bridge. The occurrence of zonal index species Naviculopsis foliacea, which is reported, according to various data, from the beginning of the Eocene (McCartney et al., 2020; Perch-Nielsen, 1976, 1985), is an additional argument in favor of the Early Eocene age of the Veshenskaya Formation.

DISCUSSION

The data obtained on silicofossils from the borehole Р-321 section and those on dinocysts from boreholes 1238 and 5/93 (Monastyrshchina) of the same SFZ of the Cis-Donets monocline (Iakovleva and Aleksandrova, 2021; Oreshkina et al., 2021) make it possible to clarify the stratigraphic volume of the Buzinovka and Veshenskaya formations.

The following zones were distinguished in the borehole 1238 section: the lower Thanetian dinocyst Alisocysta margarita Zone in the Buzinovka Formation and the upper Thanetian–lower Ypresian Apectodinium hyperacanthum and Axiodinium augustum zones in the Veshenskaya Formation. It should be noted that the dinocyst Axiodinium augustum Zone is a marker of a global climatic event at the Paleocene–Eocene boundary (РЕТМ; ~55.8–55.6 Ma). In borehole 5/93 (Monastyrshchina), the dinocyst assemblage occurs only in the upper part of the Veshenskaya Formation. This assemblage was attributed to the Stenodinium meckelfeldensis Zone, which is correlated with the upper part of the nanoplankton NP10 Zone (Oreshkina et al., 2021). In deposits of borehole Р-321, attributed to the Buzinovka and Veshenskaya formations at the primary description of the core, the diatom Moisseevia uralensis Zone was recognized. On the basis of the data available, this zone is correlated with the Deflandrea oebisfeldensis and Dracodinium (=Wetzeliella) astra (NP 10) dinocyst zones (Fig. 3). According to the micropaleontological data available, “ironstone,” conglomerate, and opoka-like sandstone layers (total thickness of 1.15 m) should be attributed to the Veshenskaya Formation but not the Buzinovka Formation. The dynamics of silicofossil assemblages and the lithological structure of the section reflect two stages of development of a shallow-water basin: (1) relatively stagnant conditions with signs of ferrugination of bottom sediments (Layers 2–4); (2) development of transgression with the occurrence of abundant sponge spicules at the beginning of the transgression stage and the appearance of individual open-sea species as marine transgression increases (Layer 5). The mass occurrence of tychopelagic species through the borehole Р-321 section indicates that the basin was shallow-water. Thus, the Veshenskaya Formation in the studied sections of the Cis-Donets SFZ has different stratigraphic volumes and varies in the lithological composition.

Fig. 3.
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Correlation of sections of the Cis-Donets and Ulyanovsk–Syzran structural-facies zones (SFZ) with international and regional stratigraphic charts and with diatom and dinocyst zonal scales. Abbreviations: dinocyst zones: Och.rom./S.chl.—Ochetodinium romanum/Samlandia chlamydophora, E.—Eatonicysta, Ch.—Charlesdownea, Dr.—Dracodinium, D.—Deflandrea, A.—Axioidinium; diatom zones: M.—Moisseevia, H.—Hemiaulus.

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On the basis of data from Strelnikova (1992), taxonomically similar diatom assemblages were distinguished in the sections of boreholes 169 (near the Novokhopersk station) and 306 (Novokhopersk area) (Figs. 1, 3). N.I. Strelnikova assigned these assemblages to beds with Aulacodiscus tener (Witt) Hustedt and considered them transitional between the Paleocene and Eocene. They include the same taxa as in the assemblage from borehole Р-321, including the background assemblage with the predominance of the group Paralia/Anuloplicata and marker species Coscinodiscus (=Moisseevia) uralensis, Jousea elliptica, Pyxidicula (=Eupyxidicula) moelleri, Coscinodiscus decrescenoides, C. argus, and Actinoptychus pericavatus.

The assemblage of silicofossils with the similar composition of diatoms and silicoflagellates was described from the Kalinin Formation in the borehole S-29 section drilled near the village of Karanino, Ulyanovsk oblast (Figs. 1, 3), to the northwest of the town of Sengilei in the Attsy River upper reaches (Glezer, 1995). The Kalinin deposits are underlined by the diatomite stratum of the Sengiley (“Granoe Ukho”) section, attributed to the Karanino stratum. A continuous sequence of diatom zones was established in the Paleocene–Eocene transition interval of this stratum: the Trinacria mirabile Zone with two subzones (upper Thanetian) and the Hemiaulus proteus Zone (lower Ypresian) (Oreshkina and Aleksandrova, 2017; Unifitsirovannaya…, 2015). The diatom assemblage of the Kalinin Formation demonstrates the impoverishment of the taxonomic composition compared to the assemblage of the Hemiaulus proteus Zone. However, individual taxa, such as Hemiaulus proteus, H. polymorphus var. morsianus, H. febriatus, Soleum exsculptum, and Grunowiella gemmata, continue to exist. Diatoms Pseudotriceratium exornatum (Meinster) Gleser and Actinopthychus pericavatus and silicoflagellates Naviculopsis foliacea appear.

The intervals rich in remains of siliceous microfossils at the Paleocene–Eocene boundary are widespread in the Paleogene basins of Northern Europe and bottom sediments of the North Sea (Figs. 1, 4). Dark gray clays and siltstones of the Sambia Formation, Kaliningrad oblast (Pionerskaya-2 borehole; depth interval of 36.6–33.3 m), contain the diatom assemblage with the zonal index species Hemiaulus proteus (lower Ypresian) and typical representatives of the Late Paleocene diatomaceous flora: Triceratium sundbyense Hustedt, Trinacria ventriculosa (A. Schmidt) Gleser, Anaulus weyprechtii Grunow, and Pseudopodosira anissimovae (Gleser et Rubina) Strelnikova (Strelnikova et al., 1978; Strelnikova, 1992). The composition of this assemblage is typical of the Hemiaulus proteus Zone. The younger age of the Sambia Formation was obtained from dinocysts from the nearby borehole Yantarny-1 (Aleksandrova and Zaporozhets, 2008; Kasinski et al., 2020), where the lower Ypresian Deflandrea oebisfeldensis Zone and its analogs were established. It can be assumed that the difference in the age estimates may be related to the varying volume of the Sambia Formation in the studied sections or the impact of erosion and redeposition characteristic of the glacial dislocation zone.

Fig. 4.
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Correlation of diatom assemblages from the Paleogene of the North Sea in the Paleocene–Eocene boundary interval.

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Diatom and silicoflagellate assemblages similar in the taxonomic composition from Paleocene–Eocene boundary deposits are known from numerous publications on the North Sea (Fig. 5), as noted by Strelnikova (1992), Glezer (1995), and others. Among the most well-known terrestrial localities are the Fur Formation of North Denmark (Benda, 1972; Grunow, 1866; Heiberg, 1863; Homann, 1991; Kitton, 1870; Sims, 1989; Tsutsui et al., 2018) and the sections of Northern Germany (Benda, 1965; Schulz, 1927), which were studied mainly in terms of taxonomy. The scheme of silicoflagellate-based zonal subdivision of the Fur Formation, including five zones (Fig. 4), was proposed by Perch-Nielsen (1976). The distinguished zones were tied to the tephrochronological scale with a correlation potential covering practically all of Northern Europe and the adjacent areas of the North Atlantic. Later, Mitlehner (1996) identified the biotic event at the level of the –21 ash layer when studying a limited set of samples from the Fur Formation. This event is expressed in the increase in a number of open-marine species, indicating an enhancement of links with the North Atlantic and Tethys. The data from Willumsen (2004) also indicate a significant restructuring of dinocyst assemblages at this level in the Fur Formation. It is significant that a monotypic taxon with ultrashort stratigraphic range and atypical morphology described (Mitlehner, 1995) as the new genus Cylindrospira appears at this level. Strelnikova and Nikolaev (1995) identified and described this taxon as the new genus Gyrocylindrus in the upper part of the Sengiley section of Ulyanovsk oblast almost simultaneously. Currently, this interval of the section belongs to the Karanino strata and the Hemiaulus proteus Zone, which corresponds to the dinocyst Apectodinium augustum Zone (Oreshkina and Aleksandrova, 2007, 2017).

The lower Ypresian diatom assemblages similar in the taxonomic composition have been described from the Harre borehole drilled in Limfjord Bay, about 20 km from the coastal outcrops of the Fur Formation (Fenner, 1994). The interval between ash layers from –34 to +130 (~25 m thick) is dominated by meroplanktonic diatom species and chrysophytes. Freshwater and benthic diatom species play a subordinate role, suggesting shallow shelf sedimentation with a slight impact of fluvial and aeolian factors. To determine the age of the identified assemblages, Fenner (1994) analyzed the stratigraphic distribution of diatoms in oceanic sediments based on the Deep Sea Drilling Project data and joint data on tephrochronology and carbonate plankton in the North Atlantic. On the basis of these correlations, the interval between ash layers from –30 to +130 in Harre borehole is assigned to the nanoplankton NP10 (=CP9a) Zone.

Paleoenvironmental reconstructions (Mitlehner, 1996) made for the Fur Formation suggest depths of no more than 50 m. The proximity of the coastline is confirmed by the abundance of fossil remains of insects, terrestrial plants, and birds. The mechanism of diatomite accumulation is associated with local upwelling caused by seasonal offshore winds. Findings of glendonites confirm relatively low near-bottom temperatures (below 5°C; Vickers et al., 2020).

The intervals containing pyritized diatom valves, the presence of which are associated with anoxic conditions in the benthic layers, were established in the bottom sediments of the North Sea (Malm et al., 1984; Mitlehner, 1995; Richardt and Sheldon, 2014; Thomsen and Danielsen, 1995) and in some terrestrial sections of the Paris and Belgian basins. Starting with the article devoted to the London clays (Shrubsole and Kitton, 1881), scientists tried to use pyritized diatom assemblages as references for stratigraphic subdivisions and correlation of the North Sea bottom sediments. Mitlehner (1996) developed a detailed microplankton-based zonal scheme for the deposits of the North Sea (King, 1983), relying on the change of dominant species assemblages in pyritized diatom assemblages (Fig. 4), which, in his opinion, marked the Paleocene–Eocene boundary. Later, it was shown in a series of works on North Sea basins (Van Eetvelde, 2005; Van Eetvelde and Cornet, 2002; Van Eetvelde et al., 2004) that the Paleocene–Eocene boundary (the base of the isotopic event of the CIE) is below the level of the abundance peak of diatom Fenestrella antiqua (Grunow) Swatman and close to the acme of Coscinodiscus morsianus var. morsianus (Sims) Mitlehner. Given the historical differences in the diatom nomenclature for Northern Europe presented in the Russian and foreign publications, the lists of species given in these works are practically identical to the composition of lower Ypresian diatoms from the Cis-Donets and Ulyanovsk–Syzran SFZs. Nevertheless, the correlation of the successive change of the diatom assemblages in the Paleogene of the North Sea Basin and the epicontinental basins of the East European Platform is complicated by the absence of a unified approach to the zonal biostratigraphic subdivision and direct correlations with other microplankton groups, in particular, with dinocysts.

CONCLUSIONS

The terrigenous-siliceous facies in the Cis-Donets SFZ are confined to the upper part of the Veshenskaya Formation, developed in the central part of the region under consideration. The diatom assemblage is attributed to the diatom Moisseevia uralensis Zone, corresponding to the Deflandrea oebisfeldensis and Dracodinium (=Wetzeliella) astra dinocyst zones (Oreshkina, 2012; Oreshkina and Aleksandrova, 2007; Oreshkina et al., 2004), which, in turn, are correlated with the nanoplankton NP10 Zone (55.6–54.8 Ma) (Iakovleva, 2017). The silicoflagellate assemblage was attributed to the lower Ypresian Naviculopsis foliacea Zone (McCartney et al., 2020; Perch-Nielsen, 1976, 1985). Thus, the development of biosiliceous facies in the Cis-Donets SFZ was confined to the lower Ypresian and the interval covering from the end of the Lutetian to the beginning of the Priabonian (Oreshkina et al., 2021).

The diatom assemblage of the Moisseevia uralensis Zone with the predominance of the Paralia/Anuloplicata group and a sporadic occurrence of a zonal index species Moisseevia uralensis, as well as species Jousea elliptica, Eupyxidicula moelleri, Coscinodiscus decrescenoides, C. argus, Soleum exsculptum, Grunowiella gemmata, Rhaphoneis lancetulla, Actinopthychus pericavatus, A. seductilis, and A. simbirskiana, reflects unstable paleoenvironments of a shallow-water basin, proximity of a coastline, and possible low salinity. The lower part of the Veshenskaya Formation (1.15 m thick) is represented by an “ironstone” layer, a conglomerate interlayer, and an opoka-like sandstone layer, reflecting an ingressive stage in a stagnant basin with low hydrodynamics and reduced oxygen content in the bottom-dwelling beds. The subsequent phase of the developing transgression in an unstable coastal environment (siliceous clay stratum of 8.2 m thick) is characterized by taxonomically more diverse diatom assemblages not bearing signs of pyritization.

The studied interval of the Veshenskaya Formation, enriched in siliceous nanoplankton, is a part of the regional event of the biogenic silica accumulation at the end of the Paleocene–the beginning of the Early Eocene recorded in the Middle Volga region, the Baltic Sea coast, and the North Sea basins. The similar taxonomic composition of diatoms and silicoflagellates from the above regions supposes stable water exchange and migration links between these regions of the Northern Peri-Tethys.