Introduction

There are four highly siliceous and organic-rich suites in Russia, which are unique and of great economic importance due to their high content of organic matter. The rocks of the Domanik (Upper Devonian), Bazhenovo (Upper Jurassic and Lower Cretaceous), Kuma (Paleogene) and Maikop (Neogene) suites are rich in siliceous tests of Radiolaria and sponge spicules. Studies of radiolarians of Bazhenovo suite have allowed the compilation of provincial zonal schemes and correlation with other regions. A good example and illustration of the use of radiolarians as a tool in evaluating stratigraphical and palaeoenvironmental aspects of hydrocarbon-rich sedimentary basins, is a special issue of Micropaleontology entitled “Radiolaria of giant and subgiant fields in Asia” which was published in 1993. In that volume, the emphasis was on the Asian part of the Eurasian continent; in consequence, the main oil and gas provinces of northern Europe were not included. Moreover, the generalised map of selected Eurasian basins (Blueford and Gonzales 1993) did not show any of the giant or subgiant fields of the Russian Arctic, exclusive of the western Siberian sedimentary basin, which is predominantly located in Siberia, but not in the Arctic. The North Sea, Norwegian Sea, and Barents Sea areas were not represented either.

Radiolarian biostratigraphy is vital for hydrocarbon exploration in these regions, because these biota often are the only fossils present. Because radiolarian assemblages are abundant and diverse, they can easily be used to constrain the age of core samples from drill sites in these hydrocarbon-rich successions. Recently, the Upper Jurassic and Lower Cretaceous Bazhenov oil-producing sequence of western Siberia and the Kimmeridgian and Volgian bituminous beds of northern Russia have attracted special attention (Hantzpergue et al. 1998; Zakharov 2006). Similar highly bituminous deposits are known along the Barents Sea margin and in the Volga-pre-Ural Basin, from Kara Sea, the Laptevs Sea margin and also from the Norwegian and North Seas. The origin of the Volgian siliceous combustible shaly sequence is of great importance, so as is subject of constant discussions. Typically, these deposits, rich in organic matter, are non-calcareous, hydrophobic and distinguished by higher radioactivity among country rocks. Previously it has been shown that the Volgian combustible shaly sequences of the Bazhenov suite of western Siberia and the Norwegian continental shelf of the Barents Sea are essentially enriched relative to common clay rocks by organophilic elements which accompany sapropelic organic matter: V ten times higher than normal, Ni six times, Cu and Zn two to three times as much as the average of Recent oceans; 60% of U, Mo, As, Sb of their quantity in present-day oceans (Gavshin and Zakharov 1991). It has also been noted that deposits of that kind occur at different stratigraphic levels in the major oil and gas basins; within the Persian Gulf in the Callovian and Oxfordian, in the North Sea in the Kimmeridgian (Galimov 1986). For this reason, it is important to determine the chronostratigraphic position of these deposits as precisely as possible within the Volgian in the Boreal Realm, as well as to try to locate its equivalents in the Tethyan Realm. Radiolarian biostratigraphy offers the best means to make such important chronostratigraphic correlations, because these biota are common in these oil shale sequences. In addition, radiolarian biostratigraphy is well established for this time interval in the Tethyan Realm (Baumgartner et al. 1995; De Wever et al. 2001) as well as in the present day California (Pessagno 1977; Hull 1997; De Wever et al. 2001).

Fig. 1.

Location of some radiolarian-bearing source rocks in the Russian Arctic and along the Pacific margins; localities A and B correspond to Fig. 2. 1, Mezen Basin, Pesha section. 2, southeastern Barents-Pechora Basin: 2, Narjan-Mar, borehole 5; 2A, Kolguev, borehole 140. 3. Volga-Pre-Ural Basin: 3, Shilovka section; 3A, Gorodicshe section, Uljanovsk region. 4, Northern and western Siberian basins: 4, Polar Ural, borehole 22; 4A, Upper Salym, borehole 17. 5, western Kamchatka and Chukotka: 5, Palana section; 5A, Omgon; 5B, Semiglawaya Mountains. 6, Sakhalin.

Here we consider two siliceous intervals: the Volgian and the Campanian. Because siliceous bituminous rocks of the Russian Arctic and along the Ural margin are part of the concept of the Volgian Stage (i.e., the Bazhenovo productive horizon and others), the stratigraphic correlation of the Boreal Volgian Stage with its counterpart in the Tethyan province needs to be considered. Based on the views accepted by the Interdepartmental Stratigraphic Committee of Russia (Zhamoida and Prozorovskaya 1997), the Upper Volgian Substage corresponds to the lower Berriasian (Lower Cretaceous), while the Middle Volgian Substage equates with the Tithonian (Upper Jurassic). Thus, the Volgian siliceous interval is the highest interest with regard to the position of the Jurassic—Cretaceous boundary, which is situated between the Middle and Upper Volgian.

Institutional abbreviations.—GIN, Geological Institute, Moscow, Russia; VNIGRI, All-Russia Petroleum Research Exploration Institute, Sankt-Peterburg, Russia.

Historical background

The Mesozoic radiolarians of the Russian Arctic Margin were first studied by Kozlova and Gorbovetz (1966) and Kozlova (1971, 1983, 1994b). Three different radiolarian assemblages were introduced for the Jurassic (Lower Kimmeridgian, Middle Volgian and Upper Volgian) of the Timan-Pechora region (Kozlova 1971, 1994b) and the Middle Volgian, Middle/Upper Volgian, and Upper Volgian of Siberia (Kozlova 1983), as well as the Lower and Upper Campanian of Siberia (Kozlova and Gorbovetz 1966). All radiolarians of the Timan-Pechora region studied (Kozlova 1971, 1994b) were collected from soft clays and illustrated exclusively by line drawings (Kozlova 1971, 1983). Only the 1994a paper by Kozlova contains scanning electron micrographs, but in turn the descriptions are missing. The Late Jurassic to Early Cretaceous radiolarians of Siberia were studied in thin sections and no images of species are available (Kozlova 1983; Lipnizkaya 2006). The Late Cretaceous radiolarians of Siberia were illustrated in line drawings (Kozlova and Gorbovetz 1966). Moreover, the 1994b paper by Kozlova and two abstracts of papers presented at international conferences (Kozlova 1994a, c), and some other key contributions (Braduchan et al. 1984; Repin et al. 1999) listed several names of new genera and species, among them Colgus (Kozlova 1994c), Pseudocrolanium (Kozlova 1994a), Quasicrolanium (Repin et al. 1999), Excingula, Spinicingula, Parvicingula alata, Parvicingula papulata, Parvicingula simplicima (Kozlova 1994b) and others, which, to this date, were never formally introduced remaining merely nomina nuda. A new family was erected by Bragin (2009) for material from Arctic Siberia. Some new radiolarian species were also described from the Pechora Basin (Vishnevskaya 1998) and the Polar Ural (Vishnevskaya 2011). The Mesozoic radiolarians of the Russian Pacific margin have been studied both in thin sections by Lipman and Zhamoida (Vishnevskaya 2001) and by means of the SEM (Vishnevskaya et al. 2005).

Fig. 2.

Correlation of Upper Jurassic and Cretaceous sequences from the Barents-Pechora region of the Arctic to Sakhalin in the far east of the Pacific Margin. Localities: 1, Mezen Basin, Pesha section. 2, southeastern Barents-Pechora Basin: 2, Narjan-Mar, borehole 5; 2A, Kolguev, borehole 140. 3, Volga-Pre-Ural Basin: 3, Shilovka section; 3A, Gorodicshe section, Uljanovsk region. 4, Northern and western Siberian basins: 4, Polar Ural, borehole 22; 4A, Upper Salym, borehole 17.5, western Kamchatka and Chukotka: 5, Palana section; 5A, Omgon; 5B, Semiglawaya Mountains. 6, Sakhalin. Abbreviations for Jurassic: k, Callovian; km, Kimmeridgian; ox, Oxfordian; tt, Tithonian; v, Volgian regional stage. Abbreviations for Cretaceous: al, Albian; ap, Aptian; br, Barremian; bs, Berriasian; cn, Coniacian; cp, Campanian stage; h, Hauterivian; st, Santonian; t, Turonian; v, Valanginian.

The lack of proper descriptions of the above-mentioned genera and species hampers correlations. Therefore, the objectives of the present paper are threefold. First, to erect formally some endemic genera and species from organic-rich shales along the Russian Arctic Margin; secondly, to re-examine the radiolarian assemblages of the Volgian, Berriasian/Valanginian and Campanian intervals of the Russian Arctic and Pacific Margin and to supply images and descriptions of the characteristic species; and lastly, to compare and correlate the Tithonian—Berriasian and Santonian—Campanian boundaries of the Russian Boreal province on the basis of radiolarians.

Fig. 3.

Types of microfaunas from Kamchatka. A–J. Palana Section: Campanian spumellarians (A–D), Santonian foraminifera (E–G), Santonian radiolarians (H–J). K–R. Omgon section: Tithonian parvicingulids (K–N), Jurassic nassellarians (O–R). A–C. Lithomespilus mendosa (Krasheninnikov, 1960). A. GIN 76a. B. GIN 76b. C. GIN 76c. D. Amphisphaera goruna (Sanfilippo and Riedel, 1973), GIN 76/v. E. Archeoglobigerina bosqiensis Pessagno, 1967, GIN N 173/99/F1. F. Hedbergella holmdelensis Olsson, 1964, GIN N 173/99/F2. G. Hedbergella delrioensis (Carsey, 1926), GIN N 159/99. H. Pseudoaulophacus venadoensis Pessagno, 1976, GIN N 202/01/1. I, J. Pseudoaulophacus floresensis Pessagno, 1963. I. GIN N 202/01/2. J. GIN N 202/ 01/3. K–N. Parvicingula omgoniensis Vishnevskaya, 1998. K. GIN N 604/3k. L. GIN N 604/31. M. GIN N 604/3m. N. GIN N 604/3n. O–R. Parvicingula sp. O. GIN N 603/5/7. P. GIN N 603/5/2. Q. GIN N 603/5/5. R. GIN N 603/5/1.

Geological setting

In the section below, radiolarian events are reviewed for six areas of the Russian Arctic: Mezen, southeastern Barents-Pechora, Volga-pre-Ural, northern and western Siberia, western Kamchatka, and Chukotka, and Sakhalin (Fig. 1, Table 1). A provisional calibration of the various radiolarian Boreal zonations for these areas with the Mesozoic time scale and other zonations is shown in Tables 1 and 2.

The Mezen Basin.—This basin is the northwestern segment of the East European Craton. Its rift origin was documented by deep seismic, wide-angle reflection, refraction, and profiling studies carried out by the GEON Centre in 1985 and by Spetsgeofizica in 2001–2002 (Kostyuchenko et al. 2006). The seismic data from these two basins have been used to identify several tectonic units within the basement, related to rifting and a weakly reflected sedimentary cover. Within the Middle—Upper Volgian sequences, highly bituminous shale is widely distributed in this area.

Abundant Middle Volgian radiolarians were recovered from borehole 234 (Fig. 2: locality 1) in the central part of the Pesha Depression in the Chekh-Gulf of Barents Sea (Fig. 1). The Volgian radiolarian-bearing deposits are represented by 20 m of bituminous shale and clay. The emphasis was on the Middle Volgian (= Late Tithonian) Parvicingula haeckeli Zone (Fig. 4). The coeval radiolarian association is widely distributed, having been documented also from borehole of 61 on the left bank of the Pesha River (Sysola River Basin) and borehole 6406/6-1 in the Norwegian Sea (Kozlova 1994b).

The Middle Volgian radiolarian assemblage of sample 234 from borehole 234 (Fig. 2: locality 1) is represented by Orbiculiforma iniqua Blome, 1984; Orbiculiforma mclaughlini Pessagno, 1977; Orbiculiforma retuza (Kozlova, 1971); Hagiastrum cf. plenum Rüst, 1885; Pentalastrum sp. 1; Tetraditruma aff. emilei Hull, 1997; Caneta blomei (Yang, 1993); Pseudoeucyrtis aff. paskentaensis Pessagno, 1977; Stichomitra sp. A sensu Kiessling, 1999; Parvicingula alata Kozlova and Vishnevskaya sp. nov.; Parvicingula grantensis Pessagno and Whalen, 1982; Parvicingula haeckeli (Pantanelli, 1880); Parvicingula cf. jonesi Pessagno, 1977; Parvicingula cf. blowi Pessagno, 1977; Parvicingula cf. obstinata Hull, 1995; Parvicingula cf. rothwelli Pessagno, 1977; Praeparvicingula aff. sencilla Hull, 1995; Praeparvicingula cf. rotunda Hull, 1995; Praeparvicingula holdsworthi (Yang, 1993); and Zhamoidellum boehmi Kiessling, 1999 (Fig. 4).

The radiolarian faunas are characterised by low diversity, but high abundance of Parvicingula and spongy spumellarians and nassellarians, and the complete absence of Tethyan species and genera (Baumgartner et al. 1995), such as Tritrabs, Andromeda, Mirifusus, Podobursa, and Tethysetta. Foraminifera co-occur with radiolarians in some samples, including the benthic species Lenticulina ponderosa Mjatluk, 1939; Saracenaria pravoslavlevi Fursenko and Polenova, 1950; Geinitzinita nodulosa (Fursenko and Polenova, 1950); Kutsevella labythnangensis (Dain, 1972); and others (Fig. 5). They belong to the Lenticulina ponderosa assemblage, which corresponds to the Dorsoplanites panderi—Virgatitus virgatus ammonite zones (Lyrov and Vishnevskaya 2000). Samples also contain numerous sponge spicules, ostracods, and algal debris (Fig. 5).

This discovery of assemblages in eastern Europe that are dominated by Parvicingula sensu lato, which is indicative of the Northern Boreal Province, is very important in light of the good age control from other fossils. Majority of the previously described Parvicingula-rich faunas are derived from Pacific Rim sequences (Chukotka, western Kamchatka) in which tectonic positions are uncertain (Vishnevskaya 1993, 1997; Filatova and Vishnevskaya 1997; Vishnevskaya and Filatova 1996, 2008; Vishnevskaya and Murchey 2002), as well as the North American Pacific Rim (Hull 1997; Pessagno et al. 2000).

Fig. 4.

Scanning electron micrographs of Late Jurassic radiolarians from siliceous clay rocks in northern Russia, Barents offshore. Sample 234, Middle Volgian (Dorsoplanites panderi Ammonite Zone) of the Pesha River Basin, borehole 234 (A–C, E–G, I–T). Sample G2, Late Volgian (Craspedites sub-ditus Ammonite Zone) of the Uljanovsk Volga Basin, species from siliceous clay rocks in northern Russia, sectionGorogische (D,H).A. Pentalastrum sp., GIN N 234-R-15. B. Praeconocaryomma hexagona (Rüst, 1898), GIN N 234-R10. C. Stylartus sp., GIN N 234-R12. D. Stichocapsa devorata arctica Vishnevskaya and Murchey, 2002, GIN N G-2-R1-3. E, F. Hagiastridae. E. GIN N 234-R11a. F. GIN N 234-R11b. G. Orbiculiforma? retuza (Kozlova, 1971), GIN N 234-R13. H. Spinicingula ceratina Kozlova and Vishnev-skaya sp. nov., GIN N G-2-R1-5b. I. Praeparvicingula aff. sencilla Hull, 1995, GIN N 234-R21. J,M. Parvicingula cf. jonesi Pessagno, 1977. J. GIN N 234-R18b. M. GIN N 234-R18c. K. Parvicingula jonesi Pessagno, 1977, GINN234-R18a. L. Parvicingula blowi Pessagno, 1977, GINN234-R16.N. Parvicingula cf. grantensis Pessagno and Whalen, 1982, GIN N 234-R19b. O. Praeparvicingula holdsworthi (Yang, 1993), GIN N 234-R17. P. Parvicingula cf. obstinata Hull, 1995, GIN N 234-R23b. Q. Stichocapsa sp., GIN N 234-R27. R. Parvicingula rothwelli Pessago, 1977, GIN N 234-R24. S. Praeparvicingula rotunda Hull, 1995, GIN N 234-R25. T. Parvicingula alata Kozlova and Vishnevskaya sp. nov., GIN N 234-R1-2.

The southeastern Barents-Pechora Basin.—This basin, and the Volga-pre-Ural Basin, are located in the foreland of the northern parts of the Ural Orogen. The southeastern BarentsPechora Basin is situated in the northeastern segment of the East European Craton known as the Pechora-Kolva Aulacogen. It is the axial suture of the Barents-Pechora Basin and the spreading rift zone is located in the northern part of the Timan-Pechora Basin. Measured in a regional deep seismic survey throughout the southeastern Barents Basin, the Moho reaches 36–40 km in its western part; 38–42 km in its central portion and 34–36 km in its northeastern part (Kostyuchenko 1993). The Kimmeridgian—Volgian phase of active rifting of this sedimentary basin (Table 1) has been traced to the north into the Barents, Norwegian, and North Seas (Dyer and Copestake 1989) and to the south into the Volga-Ural or Volgapre-Ural basins (Vishnevskaya and Baraboshkin 2001; Vishnevskaya 2001).

The tectonic setting which governs the distribution of the Volgian—Lower Cretaceous bituminous formations within the southeastern Barents-Pechora and Volga-Ural basins (Fig. 2: localities 2, 2A, 3A) is similar to that in the North Sea, where rifting probably caused rapid subsidence, which outpaced sedimentation to create basinal troughs or grabens responsible for the formation of oil (Dyer and Copestake 1989).

The Volgian sequences in the southeastern BarentsPechora Basin (Fig. 2: localities 2, 2A) clearly show a trans-gressive depositional system starting with Middle Jurassic sands and deepening upwards to the accumulation of the higher-grade source rocks in the Volgian and Early Cretaceous. The Jurassic sequences, starting in the Kimmeridgian and continuing to the Volgian, include highly bituminous horizons which occasionally contain well-preserved radiolarians (Kozlova 1971, 1994b), among them Parvicingula papulata Kozlova and Vishnevskaya sp. nov. (Fig. 6). It is very important to emphasise that the early Kimmeridgian radiolarian assemblages of the Pechora Basin (Ukhta Section) are similar to the ones from borehole 7018/5-4 in the Norwegian Sea (Kozlova 1994b), but contain Tethyan elements, including Pantanelliidae (Pantanellium tierrablankaense Pessagno and McLeod, 1987; Pantanellium lanceolata [Parana, 1890]; and Vallupus sp.). Late Jurassic representatives of the family Pantanellidae were also recorded by Bragin (1997) from the Moscow Basin. The presence of pantanelliids and some ammonites indicate a Tethyan influence, which is probably related to the input of warm water. This is confirmed by palaeotemperature data (Riboulleau et al. 1998). The Middle Volgian Parvicingula haeckeli Assemblage Zone from the Pechora River Basin (Vishnevskaya and Murchey 2002) also includes Parvicingula alata Kozlova and Vishnevskaya sp. nov., Parvicingula papulata Kozlova and Vishnevskaya sp. nov., and other species (Fig. 6). The Middle Volgian (= Late Tithonian) age is supported by the supplementary marker taxon Zhamoidellum boehmi (Yang 1993; Kiessling 1999). Other fossils include ammonites of the Dorsoplanites panderi and Virgatites virgatus zones, the buchiid bivalve Buchia mosquensis (von Buch, 1818) and the benthic foram Doratia tortosa Dain and Komissarenko, 1972 (Kozlova 1994b; Vishnevskaya 2001).

Table 1.

The main characteristics of Russian Arctic and Pacific Margin radiolarian-bearing basins.

The Middle Volgian radiolarians assemblages are very similar in taxonomic composition to coeval ones from the Pesha sequences (Fig. 4). Analyses of radiolarian biodiversity have shown that radiolarian endemism increased between the Kimmeridgian and Middle/Late Volgian, in a period when the radiolarian diversity decreased.

Abundant endemic (or typical of the Boreal province) Parvicingulidae with external cephalic spines and apophyses (Fig. 6) first appear (e.g., sample 5 of borehole Narjan-Mar) in the uppermost Middle Volgian and lowest Upper Volgian (Fig. 2: locality 2). This type of parvicingulid often makes up 50%, or more, of the Late Volgian radiolarian fauna.

The Late Volgian Stichocapsa devorata arctica assemblage (Vishnevskaya and Murchey 2002) of the Pechora River Basin (sample 5 of borehole Narjan-Mar) and Kolguev Island (sample 140 of borehole Kolguev) contains (see Fig. 6) the index-species, Spinicingula ceratina Kozlova and Vishnevskaya sp. nov. and the first representatives of Quasicrolanium planocephala (Kozlova, 1976). The Late Volgian (= Early Berriasian) age is indicated by the first appearance of Quasicrolanium planocephala and the last occurrence of Spinicingula ceratina sp. nov. in the uppermost Upper Volgian (Kozlova 1994b). Macrofaunas of the same organic-rich clay strata include the ammonite Craspedites cf. ocensis (d'Orbigny, 1845) and the buchiid Buchia unschensis (Pavlow, 1907) (Kozlova 1994b; Vishnevskaya 2001). The Quasicrolanium planocephala acme event occurs near the top of the Upper Volgian (Fig. 6A). It is distinguished by trihedral-pyramidal forms with longitudinal ribs. Quasicrolanium appears to be characteristic of the Boreal Realm. The latest Middle Volgian representatives of Parvicingula papulata Kozlova and Vishnevskaya sp. nov. from depths between 227 and 234 m (Kozlova 1994b) have visible external circumferential ridges, while the Late Volgian Spinicingula ceratina Kozlova and Vishnevskaya sp. nov. from depths between 218 and 223 m (borehole 5, Narjan-Mar, sample 5) and the Volga Basin lack ridges (Fig. 6).

The Late Jurassic in the northern East European Platform was a critical period; following a prolonged continental regime, marine sedimentation started. The sequences studied are shown in Figs. 1, 2. Several hypotheses have been proposed to explain the co-existence within these deposits of ammonites and members of other faunal groups that belong to exotic palaeozoogeographic provinces. The most plausible hypotheses are those which postulate the existence of “sea straits” between the Boreal and Tethyan realms or the existence of “cold streams” between western and eastern provinces (Gavshin and Zakharov 1991).

The Jurassic marine deposits in the northern part of the East European Platform reach thicknesses between 10–30 and 80–150 m. The lowest values are reported for Central Russia (0–50 m), whilst the thickest sequences are found to the northeast, in the Pechora Basin (>150 m).

The late Berriasian—Valanginian Parvicingula khabakovi assemblage (Vishnevskaya and Murchey 2002), containing small individuals of Parvicingula aff. boesii (Parona, 1890), Parvicingula khabakovi (Zhamoida, 1963), and Williriedellum salymicum (Kozlova, 1983), was traced in the Pechora and Siberia basins (Kozlova 1983). It differs from the Late Volgian Stichocapsa devorata arctica assemblage in yielding abundant cryptocephalic representatives of the genus Williriedellum. The Berriasian age is confirmed by the co-occurring ammonite Bojarkia mesezhnikovi Schulgina, 1969 in the Izhma River Basin (Kozlova 1994c). Thus, two new genera, Spinicingula and Quasicrolanium, make their first appearance near or just above the Jurassic—Cretaceous boundary.

Volga-pre-Ural Basin.—This basin is located to the west of the Urals and is considered to be an ancient passive margin with foredeep slope, where Kimmeridgian—Volgian hydrocarbon-rich facies (Fig. 2: locality 3A) contain numerous organic shale beds yielding radiolarians (Vishnevskaya 1998). Radiolaria from Kimmeridgian—Volgian organic shale horizons have been described previously by Vishnevskaya (1998) and Vishnevskaya and Baraboshkin (2001). Data from those studies are used herein for comparative analysis.

The most complete section of Kimmeridgian—Volgian strata in the Volga-Ural basins is exposed 10 km upstream from Gorodishce, where the lectostratotype of the Volgian Stage has been established (Fig. 2: locality 3A). These Kimmeridgian strata also bear Tethyan elements in the ammonite assemblage. However, the overlying Volgian succession is distinguished by its Boreal assemblage (Vishnevskaya and Baraboshkin 2001). The radiolarian assemblages of the Early Volgian Parvicingula jonesi Zone in the Gorodishce section are equivalent to the Ilowaiskya klimovi Ammonite Zone and the Middle Volgian Parvicingula haeckeli Zone is coeval with the Dorsoplanites panderi Ammonite Zone. Both assemblages show a predominance of Parvicingula sensu lato. A wide range of morphotypes is represented, with most specimens possessing regular hexagonal frame pore frames. Moreover, Tethyan genera such as Andromeda, Tethysetta, Bernoullius, Mirifusus, and Podobursa are altogether absent. Only several individuals of Pantanellium (P. tierrablankaense Pessagno and MacLeod, 1987) have been recorded.

Kiessling (1999) documented the pantanelliid abundance in Antarctica and noted that the main characteristics of the North Boreal Province were low diversity, and a marked predominance of Parvicingula sensu lato, as based on personal observations of Vishnevskaya's materials. Antarctic radiolarian faunas described by Kiessling (1999) are characterised by an abundant, albeit poorly diversified, pantanelliid assemblage (including Vallupinae). Bragin (1997) showed that Pantanelliidae occurred and were represented by several species in the southern part of the Boreal Province. The same holds true for material from Scotland and the North Sea (John Gregory, personal communication 1997).

Fig. 5.

Scanning electron micrographs of Volgian foraminifera and ostracods from siliceous clay rocks in northern Russia, Barents offshore; sample 234, Middle Volgian (Dorsoplanites panderi Zone) of Pesha River Basin, borehole 234. A, B. Astacolus? suspectus Basov, 1967. A. GIN N 234-F1-1. B. GIN N 234-F2. C. Nodosaria tubifera Reuss, 1863, GIN N 234-F3. D. Citharina? angustissima (Reuss), 1863, GIN N 234-F4-2. E. Marginulinita cf. pyramidalis (Koch, 1851), GIN N 234-F5-3. F–H. Ramulina nodosarioides Dain, 1972. F. GIN N 234-F6-1. G. GIN N 234-F6-2. H. GIN N 234-F6-3. I. Pseudonodosaria? multicostata (Bornemann, 1854), GIN N 234-F7-2. J. Ammodiscus veteranus Kosyreva, 1972, GIN N 234-F8. K, L. Lenticulina sp.? K. GIN N 234-F9-1. L. GIN N 234-F9-2. M. Indeterminate ostracod, GIN N 234-F10.

Middle Volgian specimens of Parvicingulidae from the Dorsoplanites panderi Ammonite Zone and Kimmeridgian Parvicingula display well-developed circumferential ridges with three rows of very large pores (Fig. 6H), while Late Volgian ones have weakly developed circumferential ridges, irregular hexagonal frameworks with small pores and a spindle-like test form; the proximal part of the tests tends to be bulbous (Fig. 6D, E). Characteristic species of the Parvicingula haeckeli assemblage Zone of the Middle Volgian are Parvicingula alata Kozlova and Vishnevskaya sp. nov. and Parvicingula papulata Kozlova and Vishnevskaya sp. nov. (sample G–I). Species of Parvicingula are dominant.

Table 2.

Correlation of radiolarian zonations proposed for the Russian Arctic Margin.

Specimens of Parvicingula from the Late Volgian (= Berriasian) Stichocapsa devorata arctica Assemblage Zone (sample G-2) are very small in size, possess small pore frames and have post-abdominal chambers with weakly developed circumferential ridges or almost none (Fig. 6). The degree of endemism is high, on account of the great abundance of Parvicingula-like morphotypes with external cephalic spines (Fig. 6) and apophyses (Kozlova 1994b; Vishnevskaya 1998, 2001). Also, a decrease in the number of chambers was documented in Late Volgian representatives of the genera Parvicingula and Stichocapsa above the Gorodishce section. The commonest taxa within the Craspedites subditus Ammonite Zone (sample G-2) are Stichocapsa devorata arctica Vishnevskaya and Murchey, 2002; Parvicingula alata Kozlova and Vishnevskaya sp. nov.; and Spinicingula gen. nov. (Fig. 6). Associated foraminifera include Ammodiscus veteranus Kosyreva, 1972; Kutsevella labythnangensis (Dain, 1972); Lenticulina pseudoarctica Ivanova, 1970; Marginulina transmutata Basov, 1967; Marginulina glabroides Gerke (Basov, 1967); Recurvoides obskiensis Romanova, 1960; and Bullopora vivejae Jkovleva, 1974. Stichocapsa devorata arctica Vishnevskaya and Murchey, 2002 (Fig. 6) is a highly characteristic species of Berriasian assemblages of the North Arctic basins of Russia (Vishnevskaya and Murchey 2002). Thus, a change from the Parvicingula-rich assemblage to the Stichocapsa- and Spinicingula-rich one occurs at the Jurassic—Cretaceous boundary. This shift was probably caused by sea level changes and cooling (Kozlova 1994b; Riboulleau et al. 1998).

Assessments of the biodiversity of fossil radiolarian assemblages have made it possible to trace different evolutionary rates of siliceous microfossils (Vishnevskaya 1993, 1997, 2009) and to define the intervals of minimum biodiversity. Low diversities in Phanerozoic radiolarians and relatively small numbers have commonly been recorded for intervals associated with anoxic events, which are linked with the occurrence of endemic species in the Boreal realm. In view of the fact that the Volgian contains abundant organic-rich intervals and extinction horizons, the lectostratotype of the Volgian Stage (i.e., the Gorodishce section) has been studied in detail. Middle Volgian faunal assemblages were collected from the Dorsoplanites panderi Ammonite Zone, the lithology of which suggests anoxic sedimentary conditions. This zone spans an interval represented by rhythmically bedded, alternating carbonate clays and non-calcareous, organic-rich shale. A horizon of reworking and dissolution was recorded at the base, whereas black bitumen shales occur in the upper part, and the amount of organic matter increases from 1–1,5 % at the base to 22% in the upper shales. The bituminous beds contain an abundance of small juvenile forms of the benthic foraminifera Loropes fischerianus and Scurria maeotis and non-pionic young ammonites which suggest a strong anoxia during shale formation. Only a few benthic foraminiferal species (Evolutinella emeljancevi [Schliefer, 1966]; Kutsevella labythnangensis [Dain, 1972]; Dorothia tortuosa Dain and Komisarenko, 1972; Lenticulina infravolgaensis Fursenko and Polenova, 1950; Marginulina robusta Reuss, 1863; Marginulina striatocostata Reuss, 1863; and Pseudolamarckina zatonica Mjatluk, 1939) have been recorded.

A comparison of data on diversity dynamics of radiolarians and ammonites in the Late Jurassic shows that episodes of significant decrease in taxonomic diversity in both groups (i.e., the Late Volgian Crisis) were synchronous. This crisis coincided with significant changes in ammonoid morphotypes and radiolarian skeletons. Mass explosions of radiolarians are correlated with anoxic episodes, whereas no such correlation is established for ammonites. Wide-ranging extinction of radiolarians and ammonites at the end of the Jurassic began in the Volgian, and most likely resulted from a marine regression and climatic cooling. This was confirmed by a predominance of cold-water representatives of the genus Parvicingula in radiolarian associations and the Boreal ammonite family Craspeditidae at that time in the Central Russian, Timan-Petchora and western Siberian seas (Mitta and Vishnevskaya 2006). The rapid evolution of Radiolaria and a bloom of morphological diversity of Parvicingula with the development of numerous abnormal skeletons may have been caused by stressed conditions. Probably, only the more generalist and primitive forms of Parvicingula and Stichocapsa survived, in order to give rise to new evolutionary trends.

In the Upper Cretaceous of the Volga Basin (Uljanovsk region, Shilovka section; Fig. 2: locality 3; sample 7–1), the siliceous horizon separated into the Prunobrachium articulatum Beds containing many representatives of the genus Prunobrachium (Fig. 7), together with Afens liriodes Riedel and Sanfilippo, 1974 and Amphymenium sibiricum Lipman, 1960 correspond to the Campanian interval. The interval of deposits with Prunobrachium articulatum is well recognised in sections of the Russian platform, western Siberia and the Subpolar Urals (Fig. 2: locality 4), being a perfect biostratigraphic marker for the Campanian due to an acme of index species (Hollis 1997).

Northern and western Siberian basins.—These basins extend beneath the Kara and Laptevs seas; they form the largest sedimentary basin, and represent a system of intracontinental rift basins. The well-known Volgian—Berriasian or Volgian— Valanginian Bazhenovo Formation bears radiolarians (Fig. 2: locality 4A) and contains the richest productive horizon (Braduchan et al. 1984). A second siliceous Radiolaria-bearing interval (Fig. 2: locality 4) is Campanian in age.

The Bazhenovo Formation is developed over a huge area of more than 1 million square kilometres with an average thickness of about 30 m. In comparison, this formation is 20 m thick in the western and central parts of the western Siberian Basin to 160 m in the southeastern part. The shallow-water (= pseudo-abyssal) palaeo-area has been located in northwestern and Recent Kara Sea territories (Zakharov 2006).

Autochthonous planktonic organic matter accumulated in the marine basin (normal salinity) during Volgian time and continued into the Berriasian. The organic-rich bazhenovites are about 30 m thick and comprise, from the bottom to the top, twelve ammonite zones, which correspond to 10–12 myr. Slow warping of the basin floor was not compensated under the conditions of minimum supply of terrigenous material, and palaeodepth could reach 500–700 m (Gavshin and Zakharov 1991).

The rocks predominating this formation are very typical by their detritus. Originally, they were described as black and brownish-black mudstone, often platy, bituminous, with lots of fish remains, crushed buchiid shells, ammonites and belemnite rostra. However, it became clear later that the name “mudstone” was not at all adequate to describe their composition which varied within wide limits, due to a varying content of three basic components: clayey material, sapropelic organic matter and biogenic silica. The term “bazhenovites” has been proposed for these rocks; workers abroad would undoubtedly refer to them as “oil shale” or “organic-rich shale” (Gavshin and Zakharov 1991).

Fig. 6.

Scanning electron photomicrographs of Late Jurassic radiolarians from siliceous clay rocks in northern Russia, Barents Sea region. A–F from Kozlova (1994b). A. Quasicrolanium planocephala (Kozlova, 1976), up- permost Volgian, Kolguev offshore, borehole 140, sample 140, VNIGRI N 140-667/41a. B–E. Spinicingula ceratina Kozlova and Vishnevskaya sp. nov. B, C. From Narjan-Mar, borehole 5, Upper Volgian, sample 5. B. Holotype, VNIGRI N 667/66. C. VNIGRI N 667/66-1. D, E. From Upper Volgian, Gorodishce section (Craspedites subditus ammonite Zone) of Uljanovsk Volga Basin, sample G2. D. GIN N G-R1-1. E. GIN N G-R1-2. F, G. Parvicingula alata Kozlova and Vishnevskaya sp. nov., Middle Volgian, Gorodishce section (Dorsoplanites panderi Ammonite Zone) of Uljanovsk Volga Basin, sample G1. F. Holotype, GIN N G-1-2Ka. G. GIN N G-1-3K. H. Parvicingula papulata Kozlova and Vishnevskaya sp. nov., holotype, GIN N P-1K. Pechora Basin, Ukhta section, sample P, Lower Kimmeridgian. Scale bars 50 µm.

The Volgian—Berriasian radiolarians of the Bazhenovo Formation and Campanian taxa from siliceous intervals have previously been documented exclusively by drawings (Kozlova and Gorbovetz 1966; Braduchan et al. 1984). Here, we supply scanning electron micrographs (Fig. 7) and descriptions of new Campanian radiolarians (compare Vishnevskaya and Alekseev 2008; Vishnevskaya 2011). The Santonian radiolarian fauna from western Siberia is very poor and dominated by discoidal spongy spumellarians; an assemblage with Discoidea and single Prunoidea (Kozlova and Gorbovetz 1966). Just above the Santonian—Campanian boundary, a siliceous interval with abundant radiolarians has been recognised (Fig. 2: locality 4). The Campanian radiolarian assemblages (sample 57) of the Siberian Arctic (Vishnevskaya 2011) are dominated by several genera, such as Orbiculiforma (4 species), Prunobrachium (6 species), Pseudobrachium (2 species), Spinibrachium (1 species), Spongurus (S. arcticus Kozlova and Vishnevskaya sp. nov.), Dictyomitra (2 species), Lithostrobus (2 species), and Amphipyndax (3 species). Total diversity is low, but the predominance of Prunobrachium and Lithostrobus (L. rostovzevi Lipman, 1960; L. borealis Kozlova and Vishnevskaya sp. nov.) can be seen in most samples (Fig. 7). Upsection, diversity is higher (Sarkisova 2007).

Fig. 7.

Scanning electron micrographs of Late Cretaceous (Early Campanian) radiolarians from siliceous rocks of the Russian Arctic Margin (western Siberia, borehole 22, Ust-Manja, sample 57), except for D which is from the Volga Basin, Uljanovsk region, Shilovka section, sample 7-1. A–C. Spongurus arcticus Kozlova and Vishnevskaya sp. nov. A. Holotype, GIN N K22-2-57. B. GIN N K22-2a-57. C. GIN N K22-2b-57. D–F. Prunobrachium articulatum (Lipman, 1952). D. GIN N K22-15a-57. E. GIN N K22-15b-57. F. GIN N K22-15c-57. G, K–N. Lithostrobus ex gr. rostovzevi Lipman, 1960. G. GIN N K22-1a-57/1.K. GIN N K22-1b-57/1. L. GIN N K22-1c-57/2.M. GIN N K22-1c-57. N. GIN N K22-1d-57. H–J. Lithostrobus borealis Kozlova and Vishnevskaya sp. nov. H. GIN N K22-1a-57. I. GIN N K22-1b-57. J. Holotype, GIN N K22-1-57. O. Lithostrobus longus Grigorieva, 1975; GIN N K22-11-57. P–R. Immersothorax marinae (Gorbovets, 1966). P. GIN N K22-12-57. Q. GIN N K22-12a-57. R. GIN N K22-12b-57. S. Amphipyndax stocki (Campbell and Clark, 1944), GIN N K22-13-57.

A comparative analysis of Jurassic radiolarian assemblages from bituminous sediments, characterised as immature potential source rocks, in the Barents Sea region (Kozlova 1994b; Vishnevskaya 2001; Vishnevskaya and Murchey 2002), the North Sea (Dyer and Copestake 1989), as well as the Norwegian Sea (Bob Goll, personal communication 1991) has demonstrated the presence of similar radiolarian events (Table 2) within the coeval latest Jurassic—earliest Cretaceous radiolarian associations of the Laptevs Sea shore north of Siberia (Vishnevskaya and Malinovsky 1995; Bragin 2009) and western Siberia (Braduchan et al. 1984; Lipnizkaya 2006). There are four radiolarian assemblages in the Bazhenovo Formation (Braduchan et al. 1984), as based on borehole 17 of the Upper Salym (Fig. 2: locality 4A). The Middle Volgian Parvicingula cf. multipora Beds (interval 2901–2912 m) (Braduchan et al. 1984; Kozlova 1994b) or Zone (Lipnizkaya 2006) include an assemblage with Parvicingula jonesi Pessagno, 1977; Parvicingula cf. multipora (Khudyaev, 1931); Parvicingula papulata Kozlova and Vishnevskaya sp. nov.; and P. santabarbarensis Pessagno, 1977. The age is confirmed by Dorsoplanites maximus and other ammonites (Braduchan et al. 1984). This interval is approximately equivalent to the Parvicingula jonesi assemblage Zone. The Middle to Late Volgian Parvicingula cf. seria Beds (interval 2896–2901 m) (Braduchan et al. 1984; Kozlova 1994b) or Zone (Lipnizkaya 2006) include an assemblage with Parvicingula crassitestata (Rüst, 1885), Parvicingula rostrata (Khabakov, 1937), Parvicingula conica (Khabakov, 1937), and Parvicingula haeckeli (Pantanelli, 1880). The Volgian age is supported by the ammonite Dorsoplanites groenlandicus (Braduchan et al. 1984). These strata can be correlated with the Parvicingula haeckeli Zone on the basis of the consistent presence of index species, Parvicingula haeckeli (Vishnevskaya 2001). The Late Volgian Quasicrolanium planocephala Beds (interval 2891–2895 m) (Braduchan et al. 1984; Kozlova 1994b) or Zone (Lipnizkaya 2006) contain an assemblage with Spinicingula ceratina Kozlova and Vishnevskaya sp. nov., Quasicrolanium planocephala (Kozlova, 1976), and Stichocapsa devorata arctica Vishnevskaya and Murchey, 2002. The Late Volgian age is corroborated by the first appearance of the ammonite Craspedites okensis (d'Orbigny, 1845) (see Kozlova 1994b). These strata are coeval with the Stichocapsa devorata arctica Zone, on account of the co-occurrence of the characteristic species Quasicrolanium planocephala and Stichocapsa devorata arctica.

The late Berriasian—Valanginian Williriedellum salymicum Beds (interval 2880–2890 m) (Braduchan et al. 1984; Kozlova 1994b) or Zone (Lipnizkaya 2006) include an assemblage with Parvicingula gracilis (Khabakov, 1937), Parvicingula khabakovi (Zhamoida, 1963), and Williriedellum salymicum (Kozlova, 1983). These upper Berriasian—Valanginian strata correspond to the Hectoroceras kochi and Bojarkia mesezhnikovi ammonite zones (Braduchan et al. 1984).Due to the presence of the primary marker taxon, Parvicingula khabakovi, these strata can be correlated with the Parvicingula khabakovi Zone (Vishnevskaya 2001). All assemblages of the Bazhenovo Formation also exhibit reduced diversity, a predominance of Parvicingula and a low abundance. Thus, these radiolarian zonations are similar to those proposed for the Arctic region of Russia (Vishnevskaya 2001; Vishnevskaya and Murchey 2002) and possible correlation is re-examined here. Based on the high diversity of Parvicingula, it is possible to equate the Parvicingula cf. multipora beds with the upper portion of the Parvicingula jonesi Zone, the Parvicingula cf. seria beds with the Parvicingula haeckeli Zone, the Stichocapsa dolium beds with the Stichocapsa devorata arctica Zone, and the Williriedelum salymicum beds with the Parvicingula khabakovi Zone (Tables 1, 2). A radiolarian event, with the onset of a new family and two new genera just above, and one new genus near the Jurassic-Cretaceous boundary, has also been recorded from the locality of Nordvik in Arctic Siberia (Bragin 2009).

Fig. 8.

Scanning electron micrographs of Late Cretaceous (Early Campanian) radiolarians from siliceous rocks of the Russian Pacific Rim (western Kamchatka, locality Palana, sample134/01). A. Phaseliforma subcarinata Pessagno, 1975, GIN N 134/01-R3. B, C. Lithomespilus aff. coronatus Squinabol, 1904. B. GIN N 134/01-R5a. C. GIN N 134/01-R5b. D, E, J. Protoxiphotractus perplexus Pessagno, 1973. D. GIN N 134/01-R-8a. E. GIN N 134/01-R8b. J. GIN N 134/01-R5c. F. Protoxiphotractus? sp., GIN N 134/01-R8. G. Stylosphaera hastata (Campbell and Clark, 1944); GIN N 134/01-R4. H. Protoxiphotractus kirbui Pessagno, 1973; GIN N 134/01- R6. I, K. Cornutella californica Campbell and Clark, 1944. I. GIN N 134/01-R11a. K. GIN N 134/01-R11b. L. Coniforma antiochensis Pessagno, 1969, GIN N 134/01-R-10. M. Stichomitra livermorensis (Campbell and Clark, 1944), GIN N 134/01-R15. N. Amphipyndax stocki (Campbell and Clark, 1944), GIN N 134/01-R17. O. Theocapsomma sp., GIN N 134/01-R12.

Western Kamchatka and Chukotka.—The former area extends beneath the Okhotsk Sea, the latter beneath the Bering Sea (Fig. 1). New data on age, composition and relationships of Mesozoic complexes of western Kamchatka and Chukotka were obtained during detailed studies carried out between 1998 and 2010 (Vishnevskaya et al. 1999, 2005; Vishnevskaya and Filatova 2008). These new data provide the basis for estimating the hydrocarbon potential of Mesozoic formations developed along the Okhotsk Sea shoreline. Some Jurassic to Cretaceous radiolarians from western Kamchatka were described in detail by Vishnevskaya et al. (2005). The late Tithonian and Berriasian Parvicingula khabakovi assemblage (sample 603–5, 6) is characterised by a predominance of nassellarians (Fig. 3), especially Parvicingula (Vishnevskaya et al. 2005: pl. 41), which is typical of high palaeolatitudes, as shown in the Arctic samples. Unfortunately, all radiolarian finds derived from tectonostrati-graphic sections (Fig. 2: localities 5, 5A, 5B). Jurassic and Cretaceous (sample 4–1) sites did not yield other fossils; only at one locality in Chukotka (Semiglawaya Mountain) Buchia is present (Fig. 2: locality 5B), together with radiolarians (Vishnevskaya and Filatova 2008). Thorough palaeonto-logical studies of volcanogenic-siliceous sections conducted in western Kamchatka between 2002 and 2005 have demonstrated that both inoceramid remains and numerous identifiable calcareous foraminifera locally accompanied abundant Santonian—Campanian radiolarians.

A description of one Campanian radiolarian assemblage (Fig. 8) from the locality Palana (Fig. 2: locality 5; sample 134/01) is presented here for comparison with coeval faunas from the western Siberian Basin. In contrast to the Siberian assemblage, the Prunobruchium crassum assemblage (Kozlova and Gorbovetz 1966; Amon 2000), the P. crassum assemblage of Kamchatka (Fig. 8) is very diverse and includes Protoxiphotractus perplexus Pessagno, 1973; Protoxiphotractus kirbyi Pessagno, 1973; Stylosphaera hastata (Campbell and Clark, 1944); Heliodiscus borealis Vishnevskaya, 2002; Spongasteriscus rozanovi Vishnevskaya, 2002; Prunopyle stansilavi Vishnevskaya, 2002; Cornutella californica Campbell and Clark, 1944; Coniforma antiochensis Pessagno, 1969; Stichomitra livermorensis (Campbell and Clark, 1944); Amphipyndax stocki (Campbell and Clark, 1944); and other species (Vishnevskaya et al. 2005), but there is no Prunobrachium acme here. In some samples, radiolarians were found together with numerous cold-water benthic foraminifera. Only a single planktonic foraminiferal species co-occurs with the Early Campanian radiolarian assemblage from western Kamchatka, whereas the warmer-water, Late Santonian Pseudoaulophacus floresensis assemblage (Fig. 3) is accompanied by several planktonic foraminiferal species, namely Archaeoglobigerina bosquensis Pessagno, 1967; Hedbergella delrioensis (Carsey, 1926); Hedbergella holmdelensis Olsson,1964; Heterohelix globulosa (Ehrenberg, 1840); Heterohelix reussi (Cushman, 1938); and Globigerinelloides ultramicra (Subbotina, 1949). The underlying Santonian radiolarian Pseudoaulophacus floresensis assemblage of western Kamchatka includes many Californian species (Vishnevskaya et al. 2005).

Maximum erosional activity in the Pacific Ocean Basin, caused by tectonic activity, rearrangement of lithospheric plates, locally accompanied by a new volcanism phase (Basov and Vishnevskaya 1991) was recorded during the Late Cretaceous, with peaks at the Cenomanian—Turonian and Santonian—Campanian boundaries. The presence of numerous representatives of genera Theocapsomma and Cryptamphorella with submerged cephalis and Excentrosphae-rella with an eccentric inner microsphere at these crisis intervals was recorded; this can probably be explained by good adaptation of these skeletal types to incisive changes in the water column (depth, oxygen content, etc.).

Sakhalin Basin.—Sakhalin Island is the northern continuation of the Japan island arc system and is subdivided tectonically into two parts: western and eastern Sakhalin. The eastern Sakhalin Basin extends beneath the Okhotsk Sea (Fig. 1); the onshore part of it is considered the major source rock for oil and gas in the area.

Up to the late 1990s, Jurassic—Cretaceous radiolarians from eastern Sakhalin were studied exclusively in thin sections. For the present paper the samples were analysed using new techniques for sample processing; SEM was used for illustration of Tithonian and Berriasian radiolarian assemblages extracted using the HF method of Pessagno (1977) (Fig. 9). Prior to the present study, only faunal lists were ever published for eastern Sakhalin; previous work (Vysotskii et al. 1998) did not include any scanning electronmicrographs. The Tithonian assemblage was collected from tuffaceous chert of the Rocky Ridge tectonostratigraphic section in the eastern Sakhalin Basin (Fig. 2: locality 6; sample 114); it includes Orbiculiforma lawreyensis Pessagno, 1977; Triactoma mexicana Pessagno and Yang, 1989; Podobursa tricola Foreman, 1973; and Triversus tsunoensis (Aita, 1987). The late Tithonian age was determined by the first appearance of Orbiculiforma lawreyensis (De Wever et al. 2001). The Berriasian—Early Valanginian radiolarian assemblage (sample 102) originates from radio-larian cherts in the Samokhino tectonostratigraphic section (Fig. 2: locality 6) of the Aleksandrovsk area, eastern Sakhalin. It includes the following species: Acaeniotyle diaphorogona Foreman, 1973; Pantanellium aff. masirahense Dumitrica, 1997; Archaeodictyomitra excellens (Tan, 1927); Archaeodictyomitra leptocostata Wu and Li, 1982; Archaeodictyomitra tumandae Dumitrica, 1997; Mirifusus mediodilatata (Rüst, 1887); Mirifusus appeninicum Jud, 1994; Mirifusus chenodes (Renz, 1974); Podobursa tythopora (Fore- man, 1973); Pseudodictyomitra depressa Baumgartner, 1984; Sethocapsa kitoi Jud, 1994; Sethocapsa pseudouterculus Aita, 1987; and Tethysetta usotanensis (Tumanda, 1989). The Ber- riasian—Early Valanginian age is based on the co-occurrence of Sethocapsa kitoi (UAZ 13–16) and Mirifusus appeninicum (UAZ 14–20) (DeWever et al. 2001). The Valanginian assemblage (sample 61) stems from siliceous tuffs of the Pilenga tectonostratigraphic section and includes Cenodiscaella nummulitica Aliev, 1965; Ditrabs sansalvadorensis (Pessagno, 1971); Godia cf. coronata (Tumanda, 1989); Thanarla conica (Aliev, 1965); Thanarla aff. brouweri (Tan, 1927); Sethocapsa cetia Foreman, 1973; Sethocapsa cf. polyedra Steiger, 1992; Pseudodictyomitra aff. leptoconica (Foreman, 1973); and Xitus cf. robustum Wu, 1993. The age is defined by the first appearance of Thanarla conica and the last occurrence of Sethocapsa cetia in the Valanginian (De Wever et al. 2001; Kurilov and Vishnevskaya 2011).

The typical Tethyan genera Pantanellum, Tethysetta, Miri- fusus, and Podobursa are widely distributed here. Only rarely has Parvicingula been noted in this assemblage (Vishnevskaya et al. 2005). The co-existence of Tethyan and Pacific species illustrates their ecotone nature and will make them useful for both correlations on a regional scale and palaeogeographic interpretations.

Conclusions

Methods

The clay samples were boiled in H₂O₂ and treated with NaOH. The chert samples were treated with hydrofluoric (1–3 %) acid, the siliceous limestone samples with acetic (10%) and hydrofluoric (1–5%) acids. The resulting residues yielded well-preserved faunas that were studied for taxonomic and biostratigraphic purposes. Here, we summarise data supplied by Kozlova (1994b) and add our own, inclusive of SEM images of radiolarian assemblages (SEM ISI-160, GIN RAN, Moscow).

Systematic palaeontology

All species considered characteristic of the Russian Arctic and Pacific Rim are here illustrated (Figs. 4, 6–9). Some of them have never been provided with a complete description, because the original account was published in the Russian fond issue of VNIGRI, while others are planned to be published in a book on the Russian Arctic (BP Exploration Operating Company), or are impossible or difficult to locate to date, and should thus be considered nomina nuda. Unfortunately, some data cannot be accessed for reasons of confidentiality involving the Petroleum Company and its partners. All new radiolarian taxa illustrated in the present study are formally described below, inclusive of synonymies.

Fig. 9.

Scanning electron micrographs of Berriasian (A–S) (sample 102-1, Tymov area, Veba River) and Tithonian radiolarian (T—AB) assemblages from the eastern Sakhalin Mountains (sample 114, locality Rocky Ridge). A. Acaeniotyle sp., GIN N 102-1-1. B. Praeconocaryomma haeckeli (Aliev, 1965), GIN N 102-1-3. C. Archaeodictyomitra tumandae Dumitrica, 1997, GIN N 102-1-20. D. Archaeodictyomitra leptocostata (Wu and Li, 1982), GIN N 102-1-22. E. Tethysetta usotanensis (Tumanda, 1989), GIN N 102-01-24. F. Cenodiscaella numulitica (Aliev, 1965), GIN N 102-1-5. G, O, P. Syringocapsa lucifer Baumgartner, 1984. G. GIN N 102-1-31. O. GIN N 102-1-33. P. GIN N 102-1-34. H. Stichocapsa altiforamina Tumanda, 1989, GIN N 102-1-35. I. Mirifusus chenodes (Renz, 1974), GIN N 102-1-36. J. Godia sp., GIN N 102-1-39. K, L. Xitus cf. robustum Wu, 1993. K. GIN N 102-1-41. L. GIN N 102-1-42. M. Mirifusus appeninicus Jud, 1994, GIN N 102-1-45. N, S. Sethocapsa pseudouterculus Aita, 1987. N. GIN N 102-1-46. S. GIN N 102-1-47. Q. Sethocapsa zweili Jud, 1994, GIN N 102-1-48. R. Sethocapsa kitoi Jud, 1994, GIN N 102-1-49. T. Hexinastrum? sp., GIN N 114-R1. U. Triactoma mexicana Pessagno and Yang, 1993, GIN N 114-R2. V. Tritrabs sp., GIN N 114-R5. W. Orbiculiforma lowreyensis Pessagno, 1977, GIN N 114-R6. X. Zhamoidellum? ovum Dumitrica, 1970, GIN N 114-R7a. Y, Z. Podobursa tricola Foreman, 1973, Y. GIN N 114-R8. Z. GIN N 114-R9. AA. Stichomitra cf. tairai Aita, 1987, GIN N 114-R10a. AB. Triversus tsunoensis (Aita, 1987), GIN N 114-R13.