INTRODUCTION

The stratigraphic codes of the USSR and Russia of all editions (Stratigraficheskii…, 1977, 1992, 2006, 2019) prescribe the elaboration and approval of regional, interregional, and local stratigraphic schemes. In addition to this, the events which occurred in Northeast Asia can now be termed as an attempt at elaboration of a stratigraphic scale of nonmarine Cretaceous deposits, which will be parallel to the General Stratigraphic Chart and reliably tied to it. As we know, the General Stratigraphic Chart is mainly based on data of the evolution of marine invertebrates. For nonmarine deposits, their role can be played only by remains of terrestrial plants. The microfossils are the most promising: a huge amount of pollen and spores is produced by parent plants and is widely distributed by the air. However, they are rarely preserved in Cretaceous deposits (mostly, in weakly lithified rocks) and their extraction is difficult. At the same time, numerous burials of macrofossils (impressions of vegetative, less commonly reproductive, parts of plants) were found in the region almost from the beginning of its geological study.

The Middle Cretaceous was characterized by a radical transformation of the planetary plant world: a transition from Mesophyte to Cenophyte. The composition of plant communities changed relatively fast, and the ancient species of gymnosperms and ferns gave way to young forms: angiosperms and accompanying plant groups. Knowing the initial and final composition of plant communities, we can extrapolate the entire process of temporal evolution of their compositions. In other words, we can refer to the change in a systematic composition of plant communities rather than to the evolution of plants. This successive series of paleofloras was composed and proposed to the scientific community in the 1970s. This scientific concept became popular in geological practice, because it provided a possibility of precise determination of the age and the correlation of the various facies nonmarine deposits of Northeast Asia. No other methods to do this were known at that time. Owing to real and popular practical significance, the representatives both of academic and industrial science and practical geologists during geological survey and thematic works actively sought and collected plant macrofossils for several decades.

Recently, Shczepetov (2022a) tried to compose a full list of paleofloristic objects of the nonmarine Cretaceous of Northeast Asia, which were introduced to scientific practice over the last half-century. This list contains 71 objects, and only 15 of them are known by the lists of their composing species. The other objects are described in monographs or atlases of images of plant imprints and indications of collection numbers and storage places. At present, the last known paleofloristic and phytostratigraphic object was introduced into practice (Shczepetov and Herman, in press), and no new materials are predicted in the foreseeable future. This allows us to review the results of long-term studies in general: what was at the beginning and what is the result.

Below, we will use the terms “paleofloristic assemblage” or “taphoflora”: these are elementary paleofloristic units, which are an assemblage of fossil plants from one or several localities close in area and stratigraphy. They are characterized by similar features (typical combination of taxa, qualitative and quantitative amount of plant groups, etc.) and are considered the fossil flora or paleoflora characterizing a certain evolution stage of the plant world of a significant territory (e.g., the North Okhotsk region). Many authors give another meaning to these terms. In these cases, they will be quoted. The mentioned flora types are characterized by features of stratofloras according to Samylina (1974; see below) or “stage floras” of Herman (2011), but without identification of a territorial location.

CONCEPT OF STRATOFLORAS

V.A. Samylina made a report at the Botanical Institute, USSR Academy of Sciences (Leningrad), on October 16, 1972. Later, it was published as a preprint “The Early Cretaceous Floras of Northeast USSR: To a Problem of the Origination of Cenophyte Floras” (Samylina, 1974). This small book became a bestseller for specialists. Her main idea can be expressed as follows: “The analysis of floristic assemblages from various stratigraphic levels allows us to outline eight large stages of the evolution of flora of Northeast USSR over the Late Jurassic–Early Cretaceous–Cenomanian–Turonian, which are the successive series of paleofloras. In my opinion, the stratoflora is the flora corresponding to each of these stages. The stratoflora is a general systematic composition of plants from coeval deposits, which occur on a restricted but significant territory with a common evolution of geology and the organic world” (Samylina, 1974, p. 7) (Figs. 1, 2).

Fig. 1.
figure 1

Main areas of occurrence of deposits with Late Jurassic, Early Cretaceous, Cenomanian, and Turonian stratoflora, modified after (Samylina, 1974, p. 9, Fig. 2).

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Fig. 2.
figure 2

Correlation scheme of some continental Cretaceous deposits of the Northeast USSR, modified after (Samylina, 1974, pp. 32, 33, Table 3).

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The stratofloras were named Pezhenka, Ozhogina, Silyap, Buor-Kemyus, Toptan, Arman, Arkagala, and Grebenka. In this succession, the age of the Pezhenka Stratoflora is reliably determined by the proportion between the plant-bearing beds and marine deposits as Volgian (the end of the Late Jurassic). The beds with typical assemblages of the Grebenka Stratoflora are overlapped by marine deposits with remains of the Turonian mollusks. One more age clue is known for the Buor-Kemyus Stratoflora: a few floristic assemblages (non-typical) assigned to it derive from the beds occurring directly above the marine deposits with remains of Aptian mollusks. The age of other taphofloras was determined conditionally by extrapolation and comparison of their systematic composition with floras of other regions. According to Samylina (1974), the age of the Arman Stratoflora was debatable: “the other taphofloras successively replaced each other over the Early Cretaceous, but it is difficult to identify whether their boundaries coincide with those of stages of the General Stratigraphic Chart because of the lack of objective data” (Samylina, 1974, p. 8).

It follows from (Samylina, 1974) that her concept is based on two postulates: (i) in the Cretaceous, there was the transformation of the plant world, with the systematic composition of these plants in the region in the beginning and the end of the process is, in general, known, and (ii) changes of the systematic composition of the ancient flora were approximately simultaneous and similar for the entire region. The third postulate cannot directly be deduced from this work, but it was announced by Valentina Alekseevna Samylina in one of the discussions: all plant burials are multiplicative and, in case of careful investigation, each could contain almost all plants which grew during that period. An extremely important conclusion follows from these initial assumptions, which are accepted without proof: in the case of full collections, the proportion between young and ancient plant forms (= evolutionary advancement) reflects its age.

An individual scale with eight horizons, which were recognized on the basis of paleofloristic features, as deposits with fossil remains of one of the stratofloras, was accepted at the 2nd Interdisciplinary Regional Stratigraphic Meeting (IRSM), which was held in Magadan in 1974–1975 (Resheniya…, 1978). The sequence of stratofloras of V.A. Samylina was superstructed by the Tylpegyrgynai Stage (Horizon) (Fig. 3).

Fig. 3.
figure 3

Correlation and age of regional stratons on the nonmarine Cretaceous of Northeast Asia, modified after (Resheniya…, 2009, figure on p. 233). The decisions of the 2nd and 3rd IRSMs are shown in bold. The coeval stratons are shaded (Shczepetov and Herman, 2019).

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This was a brilliant success of the Cretaceous regional stratigraphy: over several years, a purely academic concept was involved in geological practice and even became a law or a practical paradigm for the geologists of the huge Northeastern Territorial Geological Department. Of course, it was supported by expert and authoritative geologist Vassilii Feofanovich Belyi, who was an apologist of the concept of stratofloras and was highly convincing in scientific discussions. In fact, the history of the origination of the Cenophyte floras in Northeast Asia was accepted (discovered), and from that moment, it could only be refined, specified, and applied in geological practice. Heated discussions over many years were dedicated to the age of stratified geological bodies with plant remains rather than to the history of the origination of Cenophyte floras in the region. It was forgotten that the possible dating of certain floristic assemblages was considered by V.A. Samylina a “by-product” of her concept.

PRACTICAL VERIFICATION OF THE CONCEPT OF STRATOFLORAS

New paleobotanical and stratigraphic data were contributed for the entire half-century after the publishing of the concept. Below, they will be described stratigraphically (from bottom to top) from the Pezhenka Stratoflora to the youngest one.

To characterize each stratoflora except for the Toptan one, Samylina took the data on the two or three most fully floristically characterized sections. For example, the data on sections of the Pezhenka Formation on the Pezhenka River and the lower part of the section of the Ozhogina Formation on the Silyap River were used for the Pezhenka Stratoflora (Samylina, 1974, p. 13). On the Pezhenka River (the left tributary of the Bolshoi Anyui River), the plant remains are hosted in beds which have contact with marine deposits. This allows the reliable dating of plant-bearing deposits to the Volgian Stage (Paraketsov, 1970). The fate of these collections of plant remains, however, is unknown: most likely, they were lost.

On the Silyap River (the right tributary of the Ozhogina River), the lower part of the plant-bearing section of the Ozhogina Formation was ascribed to the Upper Jurassic. According to Samylina (1967, p. 163), this was mainly based on the finding of fern Raphaelia diamensis Sew. This material stored at the Botanical Institute, Russian Academy of Sciences (BIN RAS), however, is poorly preserved and this fossil fern could be ascribed to the present-day genus Osmunda, the remains of which occur in younger deposits (Shczepetov, 2020). Later, Samylina (1970) did not mention it among the plants of the Ozhogina Formation.

The Ozhogina Stratoflora was characterized by the paleofloristic data from the upper part of the section of the Ozhogina Formation on the Silyap River and the section of the Ozhogina Formation on the Indigirka River (Samylina, 1974, p. 13). As far as we know, the flora of this formation on the Indigirka River has not been described. As was shown above, on the Silyap River, there are no serious grounds to divide part of the plants from the deposits of the formation into another assemblage or older flora. If we consider the flora of the Ozhogina Formation in general and in a stratigraphic context, it has no dating except for paleofloristic. The comparison of the systematic composition with that of the assemblage of the Pezhenka Formation is impossible, because it is known only from the published list of species. In the monographs by Samylina (1964, 1974), the images of plants of the Ozhogina, Silyap, and Buor-Kemyus floras are provided together. The Ozhogina plants from BIN RAS are individually presented by Shczepetov (2022a). We believe that the paleoflora from the Ozhogina Formation can be ascribed to the Buor-Kemyus type. Note that, in the 50 years of studies, no occurrence of the Ozhogina Stratoflora has been found beyond the Zyryanka coal basin.

The Silyap Stratoflora is characterized on the basis of paleofloristic data from the section of the Silyap Formation on the Silyap and Indigirka rivers and the section of the lower subformation of the Omsukchan Formation of the eponymous coaliferous area (Samylina, 1974, p. 13). As was already mentioned, the material from the Indigirka River includes only a list of species. The plant remains of the Lower Omsukchan Subformation of the Omsukchan region known at that time were classified by Samylina (1974, 1976) as possible occurrences of the Silyap Stratoflora. In 1983, Shczepetov collected this material and later, under the leadership of Samylina, was able to find the “Silyap” species, thus passing the proficiency test as a young specialist (Shczepetov, 1991a). The main material of the Silyap Stratoflora, which was studied and described in monographs, originated from the Silyap River (Samylina, 1964, 1967). It was collected in 1957 by Samylina together with G.G. Popov. Samylina (1967) noted the high water level of the river and difficult access to outcrops. The studied sections along the river were accepted as stratotypical for the Silyap Horizon (Resheniya…, 1978, 2009). From 1976 to 1981, a geological survey on a scale of 1 : 200 000 was conducted under the leadership of V.N. Bobrov on the territory including the Silyap River basin. The plant remans were again collected from the stratotypical outcrops of the Silyap Horizon. The Moscow geologists probably did not know that these are typical sections. The plant remains were studied by the famous paleobotanist N.D. Vassilevskaya, who surely and reasonably classified them as Buor-Kemyus Stratoflora. She was guided by works of Samylina (1964, 1967), but probably also did not know that these remains are typical of the Silyap Stratoflora. This episode remains unknown in geological society, and the corresponding sheets of the geological map have not been published yet in contrast to the adjacent sheets. Shczepetov (2020) found this information by chance only several years ago.

The Buor-Kemyus Stratoflora was characterized on the basis of sections of the Buor-Kemyus Formation on the Zyryanka River and two upper subformations of the Omsukchan Formation of the eponymous coaliferous area (Samylina, 1974, p. 13). The paleofloras from these sections were studied and described in monographs (Samylina, 1964, 1967, 1976). The occurrences of the Buor-Kemyus Stratoflora are most abundant in Northeast Asia. The deposits including Buor-Kemyus assemblages of plant remains are traditionally dated to the early–middle Albian or simply Albian. This tradition began from the report by Samylina (1974, p. 24): “The age of the Buor-Kemyus and Toptan stratofloras undoubtedly ranges within the Early Cretaceous and is conditionally considered Albian by me, probably, excluding the late Albian.” Recently, Shczepetov (2020) analyzed all available materials on Northeast Asia that are directly or indirectly related to the problem of the age of the Buor-Kemyus flora. It is hard to say when this flora originated (including the Ozhogina) because of the absence of materials for the comparison. It is likely that it was absent in the end of the Jurassic. It is reliably known that it already existed just after the Aptian Stage. In area of active continental volcanism of the Okhotsk–Chukotka Volcanic Belt, it perished at the beginning of the Coniacian Stage (Shczepetov et al., 2020), and it is not excluded that it disappeared even later to the west and northwest of this structure toward the continent. In addition, there were several floras rather than one constant flora, but we cannot distinguish them from the available paleontological material (Shczepetov, 2020).

The Toptan Stratoflora was characterized by the data from the section of the Toptan Formation of the Omsukchan coaliferous area (Samylina, 1974, p. 13). Our recent investigation showed that most localities of plant remains of the “flora of the Toptan Formation” indicated by Samylina (1976) occur outside the outcrops of the Toptan Formation and derive from deposits of the underlying Aigur Formation, which was previously distinguished as the upper subformation of the Omsukchan Formation. In addition to the “Toptan” assemblages, the Aigur Formation includes many floristic assemblages considered typical of the Buor-Kemyus Stratoflora (Samylina, 1974, 1976). It remains enigmatic how this could have occurred (Golovneva et al., 2017; Shczepetov et al., 2019).

The Arman Stratoflora was characterized by sections of the Arman Formation on the Arman River and its analogs on the Khasyn and Nelkandzha rivers (Samylina, 1974, p. 13). The plant remains from these sections are described by Herman et al. (2016). In addition, the taphofloras of the Zarya and Parninskaya formations identified in the southern part of the Balygychan–Sugoi interfluve and adjacent part of the Okhotsk–Kolyma drainage basin were ascribed to the occurrences of the Arman Stratoflora (Samlyina, 1988; Shczepetov, 1991a). It is strange, but the age of the typical flora of the Arman Stratoflora has precisely been established: “The isotopic dating of plant-bearing rocks of the Arman and Narauli formations and the determination of the age of the Arman flora by its composition are thus consistent and allow us to consider its age the Turonian–Coniacian” (Herman, 2011, p. 231).

The Arkagala Stratoflora is characterized on the basis of “sections of the Arkagala Formation of the eponymous coaliferous basin and Tumany Formation in the basin of the Amguema River” (Samylina, 1974, p. 13). The monograph by Samylina (1988) “The Arkagala Stratoflora of Northeast Asia” was published later, presenting the elaborated concept of stratofloras (Figs. 4, 5). She described together the plant remains of the Arkagala and Dolgii formations of the Arkagala Basin, as well as the Ola, Pervomaisk, and Mygdykit formations of the Okhotsk–Kolyma drainage basin within the North Okhotsk region. The images of plant remains from three last formations, as well as collections of 1993 and 2014, were also published in the form of an atlas in (Shczepetov et al., 2019).

Fig. 4.
figure 4

Main regions of occurrence of deposits with taphofloras of the middle of the Cretaceous (Aptian–Coniacian), modified after (Samylina, 1988, p. 100, Fig. 21).

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Fig. 5.
figure 5

Correlation of the most important mid-Cretaceous continental deposits of the Northeast USSR, modified from (Samylina, 1988, pp. 108, 109, Fig. 7).

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Samylina (1974) primarily ascribed the Arkagala Stratoflora to the Cenomanian and further to the early Cenomanian (Samylina, 1988). At present, the age of floras of the Arkagala, Dolgii, Ola, Pervomaisk, and Mygdykit formations is accepted as the Santonian–Campanian. This age is supported by palynological data (Markevich, 1989, 1990, 1995), the results of paleomagnetic and isotopic dating of the Ola and Mygdykit formations (Minyuk et al., 1998; Ivanov and Raikevich, 1999; Akinin et al., 2000; Akinin and Hourigan, 2002; Hourigan and Akinin, 2004), and collection of data on systematic composition (Herman, 2011). No common opinion was elaborated on the age of beds hosting the remains of the Arkagala plants at the 3rd IRSM (Resheniya…, 2009), although the Cenomanian age was not discussed.

The Grebenka Stratoflora was characterized on the basis of data on the sections of the Grebenka Formation along the Grebenka and Ubienka rivers (a basin of the Anadyr River) (Samylina, 1974, p. 13). In this work, the formation with plant remains of the Grebenka flora was erroneously named: it is the Krivaya Reka (Krivorechenskaya) Formation (Samylina, 1988). The Grebenka flora (Samylina, 1974) was dated to the Turonian or late Cenomanian–early Turonian (Samylina, 1988). By the amount of plant-bearing beds with marine deposits, which host the remains of invertebrates, as well as the isotopic data, the current age of the Grebenka flora is estimated as the end of the Albian–the beginning of the Turonian (Shczepetov et al., 1992; Spicer et al., 2002; Herman, 2011; Shczepetov and Herman, 2019).

Filippova (1975, 2006) and Filippova and Abramova (1993) considered that the Arman and Grebenka floras are coeval. It is important to take into account that, Galina Grigorievna Filippova was a paleontologist of the Northeast Territorial Geological Department and her duties included the determination of plant remains collected by geologists and conclusions on their age according to the IRSM decisions. Herman (2002) had the same opinion. Later, however, together with Shczepetov, he concluded that the systematic composition of the Arman flora is most similar to that of the Turonian–Coniacian Penzhina and Kaivayam floras of Northwestern Kamchatka (Herman, 2004а, 2005, 2011; Herman et al., 2016). In other words, the Arman flora is younger than the Grebenka flora.

In 1988, Samylina introduced one more stratoflora, Valizhgen Stratoflora, which is “an assemblage of taphofloras from sections of the Valizhgen Formation of Northwestern Kamchatka in the area of Cape Konglomeratovyi and on the Yelistratov Peninsula (type taphofloras) … as well as taphofloras from the Poperechnaya and Tylpegyrgynai formations in the northern part of the Pekulnei Range … and three taphofloras of the North Okhotsk region: from deposits of the Chingandzha Formation of the Chingandzha and Kananyga rivers and from the section which was mapped as the Tavatum Formation on the Tap River” (Samylina, 1988, p. 104). The proportion between the beds with some taphofloras and the marine deposits “allows dating the Valizhgen Stratoflora generally to the late Turonian–Coniacian” (Samylina, 1988, p. 104).

In 1982, Belyi organized the field works in the southern part of the Balygychan–Sugoi interfluve and the adjacent part of the Okhotsk–Kolyma drainage basin. A young trainee researcher Shczepetov took part in these works and further continued the studies independently. As a result, it was shown that the deposits of the Chingandzha Formation with plant remains of the Valizhgen Stratoflora occur at the same stratigraphic level as the deposits of the Zarya and Parninskaya formations, which include the taphofloras of the Arman Stratoflora (Shczepetov, 1995). All these taphofloras were identified by Samylina, but she could not believe their similar age. Finally, Belyi accepted the similar age, but ascribed all taphofloras to the late Albian (Resheniya…, 2009) (Fig. 3).

As one can see, the concept of stratofloras contains several errors. Because it rapidly became a paradigm, these errors affected the geological practice. For example, Belyi (1977) collected in 1972 a small assemblage of plant remains from a volcanic sequence on the Enmyvaam River in Chukotka, which, according the Samylina, could not be younger than the Cenomanian (the occurrence of the Arkagala Stratoflora). The marine deposits with remains of Cenomanian mollusks, however, occur a few tens of kilometers to the south stratigraphically below the plant-bearing sequence. It was concluded that these are other sequences and the section above the marine Cenomanian should crown the section with “Cenomanian” plant remains. These ideas were accepted by decisions of the 2nd IRSM (Resheniya…, 1978), which became the basis for the approved new legend of the Anadyr series of sheets of the State Geological Map on a scale of 1 : 200 000 with related maps.

In the middle of the 1970s, the geological survey in the Ul’ya–Urak interfluve (in the Ul’ya Trough) was conducted by specialists from the organization Aerogeologiya. The plant remains were collected by paleobotanist E.L. Lebedev. He gathered more than three dozen collections in various localities from one sequence of felsic volcanic rocks and volcano-sedimentary rocks (Golovneva, 2013; Shczepetov and Herman, in press). By systematic composition, he distinguished six “assemblages,” positioned them in a continuous range by the age from the Albian to Turonian, and considered that they are coeval to the Toptan, Arman, Arkagala, and Grebenka stratofloras (Lebeved, 1987). Samylina (1988, p. 107, 108) was unfortunately wrong when she wrote: “My suggested succession of the Middle Cretaceous paleofloras … is supported by long-term stratigraphic and paleobotanical studies of Lebedev (1979, 1983; Gromov et al., 1980) in the Ul’ya Trough.” The careful long-term analysis of Lebedev’s materials shows that he did not observe the sequence of “assemblages” of the Ul’ya Trough in the section, but artificially positioned them according to the sequence of Samylina’s stratofloras, which was considered an indisputable dogma. This was inconsistent with observed geological reality, but Lebedev convinced the Moscow geologists to radically change the current regional stratigraphic scheme of the Cretaceous deposits (Gromov and Lebedev, 1978; Gromov et al., 1980; Lebedev, 1987). Four sheets of the State Geological Map on a scale of 1 : 200 000 were composed and published according to a new phantom stratigraphic scheme in the southwestern Okhotsk region (Shczepetov and Herman, in press).

MAIN RESULTS

The floristic-paleogeographical division of Northeast Asia for the Albian–Paleocene was proposed by Herman (1993). The Verkhoyansk–Chukotka, Okhotsk–Chukotka, and Anadyr–Koryak subregions were distinguished (Fig. 6), which was consistent with features of geological evolution. For example, nonmarine sedimentation in the Verkhoyansk–Chukotka subregion occurred in isolated intracontinental sedimentation basins. The Okhotsk–Chukotka subregion was an area of active continental volcanism, which formed the Okhotsk–Chukotka Volcanic Belt. The Anadyr–Koryak subregion comprised seashore lowlands and plains. Shczepetov supported this idea in a 1995 review. Later, this division was accepted by the decisions of the 3rd IRSM (Resheniya…, 2009) with another ranking and names of the domains: the Okhotsk–Chukotka subregion preserved its name and became a lithotectonic area of the Verkhoyansk–Okhotsk–Chukotka region, whereas the Anadyr–Koryak subregion became the Penzhina–Anadyr–Koryak region (Fig. 6b). The evolution of the plant world of Northeast Asia was different in each subregion. This is important for our aim of the work (the attempt at elaboration of the regional scale of the nonmarine Cretaceous), because it constrains the division of common regional stages of the Cretaceous evolution of the plant world. This therefore means that at least three stratigraphic scales must be presented.

Fig. 6.
figure 6

Regional division of Northeast Asia for the Cretaceous after (Herman, 1993) (a) and (Resheniya…, 2009) (b).

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At present, just the monographs on paleobotanics, paleofloristics, phytostratigraphy, and stratigraphy of the nonmarine Cretaceous of Northeast Asia could make a small library (Samylina, 1964, 1967, 1974, 1976, 1988; Bely, 1977, 1994; Lebedev, 1987; Herman and Lebedev, 1991; Shczepetov, 1991a, 1991b, 1995, 2020, 2021; Shczepetov et al., 1992, 2019; Golovneva et al., 1994, 2018; Pokhialainen, 1994; Belyi and Belaya, 1998; Herman, 1999, 2004b, 2011; Herman et al., 2016). The evolution of the plant world of the region is presented below according to all available data (Fig. 7).

Fig. 7.
figure 7

Existence time of types of Cretaceous paleofloras in various subregions of Northeast Asia.

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The Early Cretaceous until the middle of the Albian inclusive in Northeast Asia was characterized by ubiquitous diverse Mesophytic Buor-Kemyus flora. It is still unclear when it originated, because there are no representative and independent datings of the Cretaceous pre-Buor-Kemyus taphofloras. The end of the Albian Stage in the northwestern margin of the Anadyr–Koryak subregion on seashore lowlands of the Penzhina Trough yielded the Grebenka flora, which shows all features of evolved Cenophyte, first of all, numerous and diverse angiosperm plants. This flora was not a product of the evolution of the Buor-Kemyus plant communities: no succession of systematic compositions is observed. The Grebenka flora was probably adapted to the conditions of seashore lowland and did not invade the remote continental regions of Asia, where the Buor-Kemyus flora continued to flourish.

The landscape changed in the middle of the Turonian Stage or slightly earlier, which resulted in the formation of the Okhotsk–Chukotka subregion as an area of active terrestrial volcanism and contrasting topography. There were vast zones of disturbed habitats promoting the fast settlement of angiosperms and accompanying plant groups. The plants of the Penhzina–Kaivayam broad-leaved flora formed in the Anadyr–Koryak subregion. Because the blockade of seashore lowlands was violated, this flora actively penetrated to the continent, where it was assimilated by local plants forming the intriguing combinations of young and ancient plant forms in the burials. There is only one case where the Penzhina–Kaivayam flora upon invasion could have preserved its composition unchanged and formed the burials of the Coniacian Chingandzha flora (Shczepetov and Yudova, 2020).

The phytostratigraphic method became mostly demanded for dating and correlation of volcanic sequences of the Okhotsk–Chukotka Volcanogenic Belt, which is located within the eponymous subregion. Our field observations, as well as the results of the analysis of almost all published data and mapping materials of this structure, allow us to assert the presence of at least three flora types in the Turonian–Coniacian: the Buor-Kemyus in the regions which were still free from invasion of the Cenophyte floras with abundant angiosperms, the Penzhina–Kaivayam with various traces of local plants, and Aliki, which is registered only in volcanic deposits. The latter is characterized by a variegated systematic composition even of obviously coeval floristic assemblages. By analogy with the processes of recovery of plant cover in the area of present-day Kamchatka volcanism, we suggest that the composition of plants in each volcanic burial was affected by so many factors that it can be taken as random (Shczepetov and Neshataeva, 2019, 2021; Shczepetov et al., 2019). The possibility and the precision of the determination of the age of a volcanic sequence by plant remains are therefore limited. For example, the presence of Quereuxia or Metasequoia in the burial indicates that the age of host deposits is no older than Turonian, whereas the absence of these forms means nothing.

In the Turonian–Coniacian, the Buor-Kemyus flora in the Verkhoyansk–Chukotka subregion was replaced by the Penzhina–Kaivayam flora. The phytostratigraphic information here is poor, but there are grounds to believe that this replacement was not momentary (on a geological scale) on the entire territory (Shczepetov, 2020). As in the adjacent subregion, the appearance of young forms in the burials only indicates the age of no older than the Turonian (Herman and Shczepetov, 2021).

It is likely that the Ola and Barykov floras were coeval: this is supported by serious arguments with no contradictory data. In most cases, the floristic assemblages from floras of these types are surely recognized by systematic composition with some exceptions. For example, Golovneva (2018) in her work about the “Chaun flora” of Central Chukotka included it in the composition of several floristic assemblages from the Ola stratigraphic level. It is evident that they were similar by systematic composition to older “Chaun” ones. The authors could not find a common opinion whether the Ayanka “flora” of the Penzhina–Anadyr interfluve belongs to either the Baryk or the Ola type (Moiseeva et al., 2022; Shczepetov, 2022b). It is difficult to judge whether the precise moment of appearance of these floras in the geological record corresponds to the Coniacian–Santonian boundary. We believe that the time of existence of the Aliki and Penzhina–Kaivayam floras could have included the very beginning of the Santonian Stage.

The Cretaceous evolution of the plant world of Northeast Asia is completed by the Maastrichtian Koryak flora of the Anadyr–Koryak subregion (Herman, 2011; Moiseeva, 2012). Its age is reliably determined by the proportions of host rocks with marine beds. It is considered that the Rarytkin flora was synchronous with the Koryak flora in the subregion (Golovneva, 1994; Herman, 2011), but this seems unlikely to us (Shczepetov, 2021) or, in any case, this requires serious evidence.

CONCLUSIONS

If we consider that the paleofloristic, phytostratigraphic, and stratigraphic studies of nonmarine Cretaceous deposits of Northeast Asia for the last half-century are an attempt at the elaboration of their regional stratigraphic scale (not scheme), then it is finished. The result can be formulated as follows: it is convincingly shown that, if the elaboration of this scale is possible, then it will be less detailed than the stage marine scale, at least, at current methods of knowledge of inorganic nature. Is it possible to make a sequence of events of evolution of the plant world with duration of less than a stage in a chronological sequence for significant continental areas? The experience of studies on Northeast Asia shows that the answer is probably no. The composition of plant communities on land rapidly changes and depends on many factors, which cannot be registered in a geological record. The problem of the reliable determination of the age of the plant-bearing rocks that have no contacts with marine beds is an insuperable difficulty. This could probably be solved using modern methods of isotopic dating, but it seems they are far from success.

Summing up, we should state that the desired result is still in progress, but our knowledge on the plant world has significantly been expanded and, finally, this is the aim of academic science. Abundant paleofloristic material has been collected and introduced into scientific practice, and it will be studied by paleobotanists and paleoflorists for a long time.