Volgian and Santonian—Campanian Radiolarian Events of the Russian Arctic and Pacific Rim

Radiolarians are widely distributed in two siliceous intervals that coincide with the Tithonian—Berriasian and Santonian— Campanian boundaries in the Mesozoic of the Russian Arctic and Pacific Rim. The first level is rich in organic matter and typical of Jurassic—Cretaceous boundary strata from the Russian North European Margin (Barents-Pechora, Volga-Urals, and Siberian hydrocarbon provinces, as well as western Kamchatka). Abundant and diverse representatives of the family Parvicingulidae provide a basis for establishing the new genus Spinicingula (uppermost Middle Volgian—Lower Berriasian); another new genus, Quasicrolanium (Upper Volgian—Upper Berriasian) is also described. A Santonian— Campanian siliceous interval with radiolarians is documented from the margins of northern Asia (eastern Polar Ural, Kara Basin, Kamchatka). The Boreal genus Prunobrachium makes its first appearance at the Santonian—Campanian boundary and reaches an acme in Campanian strata. Radiolarian data can be used for basin biostratigraphy and correlation, as well as palaeogeographical interpretation of these hydrocarbon-rich facies. The Arctic and northern Pacific rims are well correlated on the basis of parvicingulids, while in Sakhalin these are absent and calibrations are based on Unitary Associations zones of the Tethys. In addition to the two new genera noted above, five new species (Parvicingula alata, Parvicingula papulata, Spinicingula ceratina, Lithostrobus borealis, and Spongurus arcticus) are erected, while 60 radiolarian species typical of the Russian Arctic and Pacific rims are illustrated.


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 il− lustration of the use of radiolarians as a tool in evaluating stratigraphical and palaeoenvironmental aspects of hydro− carbon−rich sedimentary basins, is a special issue of Micro− paleontology 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 Eu− rope were not included. Moreover, the generalised map of se− lected 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 ex− ploration 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 Creta− ceous Bazhenov oil−producing sequence of western Siberia and the Kimmeridgian and Volgian bituminous beds of northern Russia have attracted special attention (Hantzper− gue 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 mar− gin 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−cal− careous, hydrophobic and distinguished by higher radioac− tivity among country rocks. Previously it has been shown that the Volgian combustible shaly sequences of the Bazhe− nov 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 accom− pany sapropelic organic matter: V ten times higher than nor− mal, 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 bas− ins; 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 chronostrati− graphic position of these deposits as precisely as possible within the Volgian in the Boreal Realm, as well as to try to lo− cate its equivalents in the Tethyan Realm. Radiolarian bio− stratigraphy 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).
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 con− cept of the Volgian Stage (i.e., the Bazhenovo productive ho− rizon 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 (Zha− moida and Prozorovskaya 1997), the Upper Volgian Sub− stage corresponds to the lower Berriasian (Lower Creta− ceous), while the Middle Volgian Substage equates with the Tithonian (Upper Jurassic). Thus, the Volgian siliceous in− terval is the highest interest with regard to the position of the Jurassic-Cretaceous boundary, which is situated between the Middle and Upper Volgian.

Historical background
The Mesozoic radiolarians of the Russian Arctic Margin were first studied by Kozlova and Gorbovetz (1966) and Kozlova (1971Kozlova ( , 1983Kozlova ( , 1994b. Three different radiolarian as− semblages were introduced for the Jurassic (Lower Kimme− ridgian, Middle Volgian and Upper Volgian) of the Timan− Pechora region (Kozlova 1971(Kozlova , 1994b  gian, 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(Kozlova , 1994b were collected from soft clays and illustrated exclusively by line drawings (Kozlova 1971(Kozlova , 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 sec− tions and no images of species are available (Kozlova 1983;Lipnizkaya 2006). The Late Cretaceous radiolarians of Sibe− ria were illustrated in line drawings (Kozlova and Gorbovetz 1966). Moreover, the 1994b paper by Kozlova and two ab− stracts of papers presented at international conferences (Koz− lova 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 Sibe− ria. Some new radiolarian species were also described from the Pechora Basin (Vishnevskaya 1998) and the Polar Ural . The Mesozoic radiolarians of the Rus− sian 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).
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 Barents− Pechora Basin is situated in the northeastern segment of the East European Craton known as the Pechora−Kolva Aula− cogen. 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 ) is similar to that in the North Sea, where rifting probably caused rapid subsidence, which outpaced sed− imentation to create basinal troughs or grabens responsible for the formation of oil (Dyer and Copestake 1989).
The Volgian sequences in the southeastern Barents− Pechora 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 Creta− ceous. The Jurassic sequences, starting in the Kimmeridgian and continuing to the Volgian, include highly bituminous ho− rizons which occasionally contain well−preserved radiolarians (Kozlova 1971(Kozlova , 1994b, among them Parvicingula papulata Kozlova and Vishnevskaya sp. nov. (Fig. 6). It is very impor− tant to emphasise that the early Kimmeridgian radiolarian as− semblages of the Pechora Basin (Ukhta Section) are similar to the ones from borehole 7018/5−4 in the Norwegian Sea (Koz− lova 1994b), but contain Tethyan elements, including Pan− tanelliidae (Pantanellium tierrablankaense Pessagno and McLeod, 1987;Pantanellium lanceolata [Parona, 1890]; and Vallupus sp.). Late Jurassic representatives of the family Pan− tanellidae were also recorded by Bragin (1997) from the Mos− cow Basin. The presence of pantanelliids and some ammo− nites indicate a Tethyan influence, which is probably related to the input of warm water. This is confirmed by palaeo− temperature data (Riboulleau et al. 1998) O. Praeparvicingula holdsworthi (Yang, 1993), GIN N 234−R17. P. Parvi− cingula cf. obstinata Hull, 1995 (Yang 1993;Kiessling 1999). Other fossils in− clude ammonites of the Dorsoplanites panderi and Virgatites virgatus zones, the buchiid bivalve Buchia mosquensis (von Buch, 1818) and the benthic foram Doratia tortosa Dain andKomissarenko, 1972 (Kozlova 1994b;Vishnevskaya 2001). 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 Jurassic in the northern East European Platform was a critical period; following a prolonged continental re− gime, marine sedimentation started. The sequences studied are shown in Figs. 1, 2. Several hypotheses have been pro− posed to explain the co−existence within these deposits of ammonites and members of other faunal groups that belong 778 ACTA PALAEONTOLOGICA POLONICA 57 (4), 2012 to exotic palaeozoogeographic provinces. The most plausi− ble 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).
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 hydro− carbon−rich facies ( Fig. 2: locality 3A) contain numerous or− ganic shale beds yielding radiolarians (Vishnevskaya 1998). Radiolaria from Kimmeridgian-Volgian organic shale hori− zons 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 Kim− meridgian 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 as− semblages show a predominance of Parvicingula sensu lato. A wide range of morphotypes is represented, with most spec− imens 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. tierrablanka− ense 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 pre− dominance of Parvicingula sensu lato, as based on personal observations of Vishnevskaya's materials. Antarctic radio− larian faunas described by Kiessling (1999) are characterised by an abundant, albeit poorly diversified, pantanelliid assem− blage (including Vallupinae). Bragin (1997) showed that Pantanelliidae occurred and were represented by several spe− cies 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).
Assessments of the biodiversity of fossil radiolarian as− semblages have made it possible to trace different evolution− ary rates of siliceous microfossils (Vishnevskaya 1993(Vishnevskaya , 1997(Vishnevskaya , 2009 and to define the intervals of minimum biodiversity. Low diversities in Phanerozoic radiolarians and relatively small numbers have commonly been recorded for intervals as− sociated with anoxic events, which are linked with the occur− rence 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, alter− nating 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 foramini− fera Loropes fischerianus and Scurria maeotis and non−pionic young ammonites which suggest a strong anoxia during shale formation. Only a few benthic foraminiferal species (Evolu− tinella emeljancevi [Schliefer, 1966]; Kutsevella labythnan− gensis [Dain, 1972] A comparison of data on diversity dynamics of radiolar− ians 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 morpho− types and radiolarian skeletons. Mass explosions of radiolar− ians are correlated with anoxic episodes, whereas no such correlation is established for ammonites. Wide−ranging ex− tinction of radiolarians and ammonites at the end of the Ju− rassic 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 ge− nus Parvicingula in radiolarian associations and the Boreal 780 ACTA PALAEONTOLOGICA POLONICA 57 (4), 2012 Age Pechora Basin (Kozlova 1994) Siberia (Kozlova 1983) Boreal province (Repin et al. 1999) Siberia (Lipnizkaya 2006)  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 Sticho− capsa 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 articu− latum Beds containing many representatives of the genus Prunobrachium (Fig. 7), together with Afens liriodes Riedel andSanfilippo, 1974 andAmphymenium 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 biostrati− graphic marker for the Campanian due to an acme of index species (Hollis 1997).
Northern and western Siberian basins.-These basins ex− tend 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−bear− ing 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 Sibe− rian Basin to 160 m in the southeastern part. The shallow−wa− ter (= pseudo−abyssal) palaeo−area has been located in north− western 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 ma− terial, 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 bel− emnite rostra. However, it became clear later that the name "mudstone" was not at all adequate to describe their compo− sition which varied within wide limits, due to a varying con− tent of three basic components: clayey material, sapropelic organic matter and biogenic silica. The term "bazhenovites" has been proposed for these rocks; workers abroad would un− doubtedly refer to them as "oil shale" or "organic−rich shale" (Gavshin and Zakharov 1991).
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 nu− merous representatives of genera Theocapsomma and Cryp− tamphorella with submerged cephalis and Excentrosphae− rella with an eccentric inner microsphere at these crisis inter− vals 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 continua− tion of the Japan island arc system and is subdivided tectoni− cally into two parts: western and eastern Sakhalin. The east− ern 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.
The typical Tethyan genera Pantanellum, Tethysetta, Miri− fusus, and Podobursa are widely distributed here. Only rarely has Parvicingula been noted in this assemblage (Vishnev− skaya 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 palaeo− geographic interpretations.

Methods
The clay samples were boiled in H 2 O 2 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 im− ages of radiolarian assemblages (SEM ISI−160, GIN RAN, Moscow).
Description.-Test conical with five (or more?) post−abdomi− nal segments. Cephalis and thorax together pyramidal−domed, cephalis without perforation, thorax irregularly perforated, with coarse to thorny surface. Apical horn massive, wide coni− cal, with apical pore; spines 2, L and D short, pyramidal or papula shaped, situated at centre of cephalo−thorax. Distinct complete circumferential ridge beginning at the base of the first post−abdominal segments. Thorax and following seg− ments approximately equal in height, with three rows of uni− form hexagonal pore frames; pores circular in outer rows, cir− cular to elliptical in middle row.
Measurements ( Diagnosis.-Parvicingulid−like skeleton without or with weakly developed circumferential ridges, bearing 1 apical and 4-6 additional spines. Description.-Test subconical with 4 (or more?) post−abdom− inal segments, slightly undulating in outline. Cephalis together with thorax dome shaped, with short apical horn and outer spines. Segments slightly increasing in width and being con− stant in height. All segments separated internally by planiform ring−shaped portions. Test wall thick, subpolygonal to oval pore frames arranged in transverse rows in three rows per seg− ment. Circumferential external ridges weakly developed or not developed. Pores relatively equal in size. Abdomen and following segments trapezoidal or X−shaped in cross section. Spine A conical, often very thick, spine V thinner, between A and V one large pore. Spines D and L extend outwards from near base of thorax (D) or near base of cephalis (L) as tooth− shaped wings, short, smooth, oval in cross section. Spines L short and rather small. Two short, cone−shaped smooth thorns in the middle portion of the cephalis, spines "l".
Remarks.-Differing from species of Parvicingula in having weakly developed circumferential ridges (or altogether ab− sent) and in possessing spines next to or surrounding the massive apical horn. Diagnosis.-Test multisegmented, trihedral−pyramidal in form, with three ribs extending from top of thorax (rib "D"), or of abdomen (ribs "L") to distal portion. Cephalis hemi− spherical, with short conical spines "A", "V" and 2, 1'; re− maining segments with concave wall as in Parvicingulidae; last segment free of ribs may be circular in cross section and turn into a long tube. Abdomen and post−abdominal seg− ments separated externally by concentric circumferential ridges, not circular, but crooked triangle−shaped. Test net− work regularly hexagonal with four to six transverse rows of small round pores at each segment.
Remarks.-The new genus is closely related to Crolanium Pessagno, 1977 andPseudocrolanium Jud, 1994, having outer ribs in common. The distinguishing feature is the pres− ence of longitudinal ribs on the entire test segments, with ex− ception of the last one; segments and ribs, bearing net on the test wall, resemble to landing−net. It is doubtful if this new genus can be attributed to the same family as Crolanium and Pseudocrolanium. Quasicrolanium gen. nov. differs from members of the Stichocapsidae by hav− ing concave, not convex, segments and transverse circumfer− ential ridges at chamber boundaries. From parvicingulid gen− era it can be distinguished by possessing three longitudinal ribs and fewer pores at each segment: 4-6 rows (predomi− nantly 6). Geographic and stratigraphic range.-Late Volgian in height latitudes of Northern Hemisphere; Timan−Pechore Basin and eastern Ural Slope, Russia.