Ammonite stratigraphy

Based on the occurrence of ammonites (Lynnia and common Surites), and Buchia volgensis, a Ryazanian age can be assigned to the studied core sections. A more specific age is based on the occurrence of Lynnia icenii indicating the Icenii Zone, and thus marking the base of the late Ryazanian. The Icenii Zone was introduced by Casey (Reference Casey, Casey and Rawson1973) based on ammonite findings in temporary outcrops near ‘The Wash’, an inlet at the border between Norfolk and Lincolnshire at the east coast of the UK (Image 1). The Icenii Zone is best known from exposures in the vicinity of North Runcton and King’s Lynn (Casey & Gallois, Reference Casey and Gallois1973; Casey, Reference Casey, Casey and Rawson1973), where it consists of only a few decimetres of glauconitic sands and clay with clay ironstone and seams of phosphatic nodules (Image 3).

Lynnia is the characteristic ammonite genus of this zone. Surites and Bojarkia were also reported but were not figured and are generally less common (Casey, Reference Casey, Casey and Rawson1973, p. 210). The co-occurrence of Lynnia and Bojarkia has been reported from a condensed 5 cm thick nodule bed only (Casey, Reference Casey, Casey and Rawson1973, p. 200). Subsequently, Casey et al. (Reference Casey, Mesezhnikov and Shul’gina1987, Reference Casey, Mesezhnikov and Shul’gina1988) indicated the co-occurrence of Lynnia, Surites and Bojarkia throughout the Icenii Zone. However, in the remainder of the Panboreal Superrealm Surites and Bojarkia do not co-occur and are only known from different, succeeding ammonite zones (Shul’gina, Reference Shul’gina1985, p. 21–22; Baraboshkin, Reference Baraboshkin2004, p. 53). Hence, the ammonite assemblage of the Icenii Zone in its type area remains controversial. There are three options, regarding the range and occurrence of Lynnia: (1) Lynnia is restricted to the Icenii Zone. (2) Lynnia occurs in the Icenii Zone and also at the base of the Stenomphala Zone. (3) Lynnia is represented as reworked phosphatic casts at the base of the succeeding Stenomphala Zone. Casey (Reference Casey, Casey and Rawson1973, p. 200, 210) mentions Bojarkia sp. from a 5 cm thick level (bed 6 at King’s Lynn Bypass) with black rolled phosphatic nodules, resting on an irregular surface of the bed below, a condensed bed (bed 5) of 10 cm with pale, sandy, clay ironstones with semi-phosphatised knobs on the upper surface yielding Lynnia sp. nov. From this, it is clear that these Bojarkia co-occurred with Lynnia in the condensed level. It is well possible that bed 6 yields taxa characteristic for both the Icenii Zone and the succeeding Stenomphala Zone, which would explain the presence of Bojarkia in this bed. Actually, Casey (Reference Casey, Casey and Rawson1973, p. 200) mentions almost 5 m of strata above bed 6 without ammonites. The two condensed beds are part of the Upper Spilsby Sandstone (Mid-Spilsby Nodule Bed) (see Casey, Reference Casey, Casey and Rawson1973, p. 201). Casey (Reference Casey, Casey and Rawson1973, p. 254) further noted that ‘This subgenus is represented by a number of species in the interval between the Kochi and Stenomphala Zones in England’. However, to date only the type species Surites (Lynnia) icenii is figured and described.

Casey, Reference Casey, Casey and Rawson1973 and Casey et al. (Reference Casey, Mesezhnikov and Shul’gina1987, fig. 4; Reference Casey, Mesezhnikov and Shul’gina1988, p. 81) correlate the Icenii Zone with the Analogus Zone, taking into account its stratigraphic position (above the Kochi Zone, which is widely traced throughout the Boreal regions) and findings of unfigured Surites in this zone, and we follow the same correlation.

In summary, well B18-02 yielded an exceptionally rich but monospecific buchiid fauna (B. volgensis), combined with frequent Boreal ammonites, mainly Surites, possibly Praetollia, and few endemic Lynnia, but very few belemnites. In northern Siberia, similar abundances of B. volgensis occur above the Kochi Zone, at the base of the Analogus Zone (Zakharov, Reference Zakharov1981, fig. 79; Zakharov, Reference Zakharov1990, figs 2–3). Surites occurs in abundance in northern and western Siberia (e.g. Shul’gina, Reference Shul’gina1975, Reference Shul’gina1985; Vyachkileva et al., Reference Vyachkileva, Klimova, Turbina, Braduchan, Zakharov, Meledina and Aleinikov1990; Igolnikov, Reference Igolnikov2019), the Russian Platform (e.g. Sazonova, Reference Sazonova1977; Mitta, Reference Mitta2017), East Greenland (Alsen, Reference Alsen2006), Spitsbergen (Ershova, Reference Ershova1983) and Franz-Josef Land (Dibner & Shulgina, Reference Dibner and Shulgina1998).

Recapitulation of the ammonite zonal succession of the Volgian/Ryazanian boundary and the Ryazanian of NW Europe and its Panboreal correlation

Some comments should be provided concerning ammonite zonal succession applied in this paper for the Ryazanian, and its correlation with other Boreal key successions yielding ammonites (Image 5), primarily with those of European Russia, which is the type area of the Ryazanian stage, and of Northern Siberia, where a continuous succession of Boreal ammonite stages occurs. The Ryazanian Stage was initially proposed by Sazonov (Reference Sazonov1951) without detailed characterisation, but a few years later Sazonov (Reference Sazonov1953) provided details concerning the faunal composition and geographic distribution including the type region, the Pericaspian area and the Northern Caucasus. Outside Russia this stage name was first applied by Casey (Reference Casey1962) as ‘Riasan Beds’ and by Casey (Reference Casey1963) as Ryazanian for a sedimentary succession in Norfolk and Lincolnshire (UK). Casey considered the Ryazanian as the first stage of the Cretaceous, and, in Lincolnshire, drew its base at the top of Lower Spilsby Sandstone. The Ryazanian Stage became widely used for Boreal regions after the publication of the seminal paper by Casey (Reference Casey, Casey and Rawson1973) comprising the description of ammonites and zonal succession of the upper Volgian and Ryazanian deposits of eastern England, as well as its correlation with other Boreal successions. Unfortunately, several important ammonite taxa mentioned by Casey remain unfigured, and later only minor additions concerning the English succession were published in cooperation with Soviet scientists (Casey et al., Reference Casey, Mesezhnikov and Shul’gina1977, Reference Casey, Mesezhnikov and Shul’gina1987, Reference Casey, Mesezhnikov and Shul’gina1988).

Image 5. Extension of cores investigated and mentioned herein versus Panboreal correlation of late Volgian-Ryazanian (Jurassic-Cretaceous) ammonite zones. Modified after: Rogov et al., Reference Rogov, Baraboshkin, Guzhikov, Efimov, Kiselev, Morov and Gusev2015, Reference Rogov, Shchepetova and Zakharov2020; Kiselev et al., Reference Kiselev, Rogov and Zakharov2018. Note: asterisk indicated pre-Rjasanensis beds containing a Buchia–Shulginites association and possibly being the source of the ammonite Chetaites chetae.

The Lamplughi Zone is the uppermost unit of the Volgian Stage (Images 3–5) introduced by Casey (Reference Casey, Casey and Rawson1973) and is characterised by a very specific ammonite assemblage that consists of Volgidiscus and Subcraspedites, genera that disappear at the Volgian/Ryazanian boundary. The genus Volgidiscus occurs in coeval strata of the Russian Platform, Subpolar Urals and Northern Siberia (Kiselev et al., Reference Kiselev, Rogov and Zakharov2018; Rogov, Reference Rogov2020, Reference Rogov2021). Furthermore, on the Russian Platform and in the Subpolar Urals other Boreal taxa, such as Shulginites and Chetaites chetae (especially common east of the Subpolar Urals) occur, providing direct correlation of the English Lamplughi Zone with the Russian Singularis and Chetae zones.

Subsequently, Casey et al. (Reference Casey, Mesezhnikov and Shul’gina1987, Reference Casey, Mesezhnikov and Shul’gina1988) and Hoedemaeker & Herngreen (Reference Hoedemaeker and Herngreen2003) introduced one more unit above the Lamplughi Zone but below the base of the Ryazanian in NW Europe. The first authors proposed the ‘beds with Subcraspedites claxbiensis’ on the base of the occurrence of their index species in re-deposited pebbles from the basal Albian. However, Casey (Reference Casey, Casey and Rawson1973) and later Casey et al. (Reference Casey, Mesezhnikov and Shul’gina1988) also mentioned this species from the Preplicomphalus Zone and a younger age (occurrence in the Lamplughi Zone) of this taxon cannot be negated. Hoedemaeker & Herngreen (Reference Hoedemaeker and Herngreen2003) indicated an assemblage with ‘Subcraspedites spp.’ above the Lamplughi Zone in the North Sea region, following Casey et al. (Reference Casey, Mesezhnikov and Shul’gina1987) and additionally based on the ammonite data published by Abbink et al. (Reference Abbink, Callomon, Riding, Williams and Wolfard2001b; but see reinterpretation of several species by Kiselev et al., Reference Kiselev, Rogov and Zakharov2018, p. 220).

The base of the Ryazanian was defined by Casey (Reference Casey, Casey and Rawson1973) as to coincide with the transition from beds with Volgidiscus to Praetollia (Runctonia), that is, at the boundary of the Lamplughi and Runctoni zones. Casey regarded these genera as members of a single lineage Swinnertonia – Subcraspedites – Volgidiscus – Runctonia – Hectoroceras, providing a solid background for both the subdivision and the correlation of the late Volgian and early Ryazanian throughout the Boreal areas. At first, Casey designated Runctonia as a separate genus, but later, based on the opinion of Shul’gina, considered it a subgenus of Praetollia (Casey et al., Reference Casey, Mesezhnikov and Shul’gina1977), and additionally figured Praetollia (Runctonia) from the Subpolar Urals (loc. cit., pl. II, fig. 3). Here, P. (Runctonia) co-occurs with Chetaites sibiricus, a typical lowermost Ryazanian species. The ammonite assemblage of the Runctoni Zone of NW Europe also includes Praesurites (considered a junior synonym of Praetollia by Igolnikov, Reference Igolnikov2019), an ammonite genus whose first occurrence marks the base of the Ryazanian in the Subpolar Urals (Casey et al., Reference Casey, Mesezhnikov and Shul’gina1988). The lower and upper boundaries of the Runctoni Zone can be traced throughout the Boreal areas, defined by first and last occurrences of distinct common and short-ranging ammonite genera. In addition to the last occurrences of Volgidiscus and Subcraspedites, the lower boundary of this zone is marked by the first occurrence of Praetollia, while the upper boundary coincides with the appearance of Hectoroceras.

The genus Shulginites first appears in the latest Volgian and typically occurs in the lowermost zone of the Ryazanian in Western Siberia, in the Subpolar Urals and on the Russian Platform (cf. Mitta, Reference Mitta2017, p. 143–144; the upper part, bed 5b, of section A along the Unzha river were it yields Shulginites and Praesurites, herein considered as a typical earliest Ryazanian taxa). However, in the Subpolar Urals, the uppermost occurrence of this genus is in the lower part of the overlying Kochi Zone (Mesezhnikov et al., Reference Mesezhnikov, Alekseev, Klimova, Shul’gina, Gyul’khadzhan, Zakharov and Nal’nyaeva1983). Shulginites appears to be restricted to the uppermost upper Volgian, lower part of the Rjasanensis Zone and the Kochi Zone, while Hectoroceras seems to be confined to the Kochi and the Transfigurabilis zones (sensu Baraboshkin, Reference Baraboshkin2004). Mesezhnikov et al. (Reference Mesezhnikov, Alekseev, Klimova, Shul’gina, Gyul’khadzhan, Zakharov and Nal’nyaeva1983, pl. VI, fig. 4) figured a typical Shulginites characterised by weak ribbing from the lower (but not basal) part of the Ryazanian. The underlying bed contains Riasanites and Garniericeras. However, this bed is thought to contain a reworked and re-deposited assemblage (Sasonova & Sasonov, Reference Sasonova and Sasonov1984). Based on this record and the re-interpretation of Hectoroceras from the Subpolar Urals figured in Mesezhnikov et al. (Reference Mesezhnikov, Alekseev, Klimova, Shul’gina, Gyul’khadzhan, Zakharov and Nal’nyaeva1983, pl. V, fig. 3) as Shulginites (see Mitta, Reference Mitta2019), Mitta considers Shulginites as synonym of Hectoroceras. At present, we consider Shulginites and Hectoroceras as separate genera, but transitional morphologies from one genus to another as well as a potential co-occurrence of the two genera should be further investigated. Both on the Russian Platform and in the Subpolar Urals, the range of Shulginites seems to be similar. The oldest findings of this genus co-occur with Volgidiscus (Kiselev et al., Reference Kiselev, Rogov and Zakharov2018, pl. II, fig. 7). Above Shulginites is recorded from the lowermost Ryazanian pre-Rjasanensis beds (Kiselev et al., Reference Kiselev, Rogov and Zakharov2018, pl. VI, fig. 8), as well as from the Rjasanensis Zone in association with Hectoroceras (Mesezhnikov et al., Reference Mesezhnikov, Gol’bert, Zakharov, Klimova, Kravets, Saks, Shul’gina, Alekseev, Bulynnikova, Kuzina and Yakovleva1979, pl. I, fig. 5).

The Kochi Zone is defined as the range zone of the genus Hectoroceras s.str. and is among the best correlated intervals within the Ryazanian. Hectoroceras is characterised by a wide geographical range and its remarkable morphologic features permit to determine even fragmentary and/or crushed specimens. The additional components of the ammonite assemblages in this zone vary and include Borealites in NW Europe, Surites, Pseudocraspedites and Runctonia in Northern Siberia, and Submediterranean taxa in the corresponding interval on the Russian Platform, which is traditionally referred to as the Rjasanensis Zone (Mitta, Reference Mitta2019, Reference Mitta2021).

The index species of the overlying Icenii Zone in NW Europe is an endemic species, which previously was only known from its type locality and a few adjacent sections, and L. icenii is the only ammonite species figured from this zone. As a result, the correlation of this zone with other regions was mainly based on the positions of the underlying and overlying units. Casey et al. (Reference Casey, Mesezhnikov and Shul’gina1977, Reference Casey, Mesezhnikov and Shul’gina1987, Reference Casey, Mesezhnikov and Shul’gina1988) mentioned also ‘Surites ex gr. spasskensis’ and Bojarkia from this zone. However, as has been emphasised above, Bojarkia was only mentioned from a condensed level at the boundary with the overlying zone from Lincolnshire, and the in situ co-existence of Surites and Bojarkia still cannot be confirmed. By its relative position, this zone corresponds to the ‘Analogus’ Zone of Northern Siberia as well as the Spasskensis and the lower part of Tzikwinianus zones of the Russian Platform.

The Siberian ‘Analogus Zone’ is mentioned in quotes, as the index species is taxonomically instable and needs revision. At least some ammonites referred by Shul’gina to S. analogus were later described as the new species S. subanalogus (Shul’gina, Reference Shul’gina1972), and few specimens were figured as ‘S. ex gr. analogus’. Also, it should be noted that S. analogus is an insufficiently known species as after its first description by Bogoslowsky (Reference Bogoslowsky1896) it was not figured again from the type region. Moreover each specimen figured by him was proposed as lectotype subsequently by Sazonova (Reference Sazonova1971, p. 45 – Bogoslowsky, Reference Bogoslowsky1896, pl. III, fig. 6, indicated as a variety loc. cit. p. 67) and Shul’gina (Reference Shul’gina1972, p. 153 – Bogoslowsky, Reference Bogoslowsky1896, pl. III, fig. 5), while another specimen figured by Bogoslowsky was referred to S. subanalogus.

The ‘Analogus Zone’ of northern Siberia is at its base defined by the highest appearance datum of Hectoroceras and in its top by the first appearance datum of Bojarkia and disappearance of Surites (Alekseev, Reference Alekseev1984; Baraboshkin, Reference Baraboshkin2004). It is well-traceable throughout the Panboreal Superrealm. This zone has been introduced by Shul’gina (in Sachs & Shul’gina, Reference Saks and Shul’gina1962) as ‘Paracraspedites analogus subzone’. Subsequently, she provided some information about the ammonite assemblage of this unit (in Saks and Shul’gina Reference Saks and Shul’gina1962), which included various ‘Paracraspedites’ (i.e. Surites), among others P. analogus subsp. nov. Subsequently, this taxon was described as Surites subanalogus sp. nov. (Shul’gina, Reference Shul’gina1972), while S. analogus was mentioned and described in open nomenclature only (i.e. Surites ex gr. analogus, loc. cit., pl. XIII, figs 1–2; Igolnikov, Reference Igolnikov2019, pl. XIII, fig. 2) and any evidence for the presence of S. analogus outside the Russian Platform is still missing. Moreover, to emphasise the difference between S. analogus from the Russian Platform and S. ex gr. analogus we added ‘Shul’gina, Reference Shul’gina1972’ as an author of the latter binomen, though strictly speaking it was not introduced as a new species (group).

The Stenomphala Zone of Casey (Reference Casey, Casey and Rawson1973) is characterised by a mix of ammonites which includes both Bojarkia and Surites (at least in the lower part). Casey et al. (Reference Casey, Mesezhnikov and Shul’gina1977, p. 23) re-assigned ‘Bojarkia’ tealli (Casey) to the genus Surites (Casey, Reference Casey, Casey and Rawson1973 considered Bojarkia as a subgenus of Surites) and additionally mentioned a finding of an unfigured Surites aff. caseyi (Sazonova) close to the top of this zone (bed 12, Stenomphala Zone of Mintlyn Wood section). Later, the same authors (Casey et al., Reference Casey, Mesezhnikov and Shul’gina1988) restricted the ranges of Surites s.str. and Lynnia to the Icenii Zone, while only Bojarkia was shown from the Stenomphala Zone. Thus, the co-existence of Surites and Bojarkia still should be further clarified and cannot be proven. Roughly, this interval corresponds at least partially to the Tzikwinianus Zone of the Russian Platform (a condensed sequence). As a result, the precise ranges of the ammonites are unclear in this zone, but it yields Surites, Bojarkia and Peregrinoceras (in its upper part only). Because it is based on the range of the genus Bojarkia, this zone corresponds to the lower part of the expanded Mesezhnikowi Zone of Northern Siberia.

The assemblage of the Albidum Zone, also defined by Casey (Reference Casey, Casey and Rawson1973), contains Peregrinoceras spp. with the addition of Bojarkia in the lower part of this zone. This can be concluded from data presented in Casey, Reference Casey, Casey and Rawson1973, although later Casey et al. (Reference Casey, Mesezhnikov and Shul’gina1987, Reference Casey, Mesezhnikov and Shul’gina1988) show the range of Bojarkia covering the whole of that zone. Such co-occurrence of Peregrinoceras and Bojarkia was also mentioned from the Tzikwinianus Zone of the Russian Platform. Eventually, the occurrence of P. aff. albidum (Casey et al., Reference Casey, Mesezhnikov and Shul’gina1977, p. 32, pl. I, fig. 1) in the latter region leads to the establishment of the ‘P. albidum beds’ (Casey et al., Reference Casey, Mesezhnikov and Shul’gina1988) and P. albidum Zone (Baraboshkin in Rogov et al., Reference Rogov, Baraboshkin, Guzhikov, Efimov, Kiselev, Morov and Gusev2015). However, although the Peregrinoceras-bearing interval can be recognised in the topmost part of the Russian Platform, its direct comparison with the Albidum Zone of England remains doubtful and needs further clarification. Apparently, the total range of the genus Peregrinoceras in the Russian Platform exceeds its range in NW Europe. On the Russian Platform, the oldest findings of Peregrinoceras co-occur with Surites and Riasanites (Mesezhnikov, Reference Mesezhnikov and Menner1984; Casey et al., Reference Casey, Mesezhnikov and Shul’gina1988). Currently, the occurrence of an assemblage consisting of Peregrinoceras or of Peregrinoceras with additionally Bojarkia is confirmed for condensed sections of the Middle Volga area only (Rogov et al., Reference Rogov, Baraboshkin, Guzhikov, Efimov, Kiselev, Morov and Gusev2015). But information concerning the ammonite ranges in the uppermost Ryazanian is limited. Hence, correlation of the Albidum Zone with other Boreal successions is only possible through indirect evidence based on finding of Bojarkia in its lower part.

The overlying part of the Lower Cretaceous succession in England was referred by Casey to the Paratollia beds or Paratollia horizon (Casey, Reference Casey, Casey and Rawson1973; Casey et al., Reference Casey, Mesezhnikov and Shul’gina1987, Reference Casey, Mesezhnikov and Shul’gina1988) but is separated by a minor fault in the Lincolnshire area. This unit is characterised by the occurrence of Paratollia, Delphinites (= Pseudogarnieria (‘Proleopoldia’) of Casey, Reference Casey, Casey and Rawson1973), Propolyptychites and Menjaites (Casey, Reference Casey, Casey and Rawson1973; Casey et al., Reference Casey, Mesezhnikov and Shul’gina1977). The presence of Peregrinoceras in this zone has been shown by Casey et al. (Reference Casey, Mesezhnikov and Shul’gina1988, fig. 4) but was not mentioned elsewhere, and there is no evidence to corroborate. Approximately this unit corresponds to the Undulatoplicatilis Zone of the Russian Platform, which also yields a Delphinites – Menjaites assemblage. In Siberia, it can be correlated with the Tolli Zone, as a rich assemblage with Tollia has been found in the Undulatoplicatilis Zone of the Russian Platform (Bogomolov et al., Reference Bogomolov, Mitta and Starodubtseva2011). However, the age of the top of the Paratollia unit and its correlation with other Boreal succession is unclear.

Depositional setting

The bituminous mudrocks (Clay Deep Member) of the Lutine Formation were likely deposited under the influence of ‘temporarily stagnated circulation, at least in the deeper parts of the graben, which resulted in partially dysaerobic basin-floor conditions. In the relatively shallower settings, mixing prevailed with normal basin-floor ventilation (Schill Grund Member)’ (Munsterman et al., Reference Munsterman, Verreussel, Mijnlief, Witmans, Kerstholt-Boegehold and Abbink2012, p. 585–586).

The core B18-02 yields benthic-nektonic cephalopods, fish remains and their waste products (pyritised coprolite; Image 2). Buchia bivalves occur throughout the core (Image 2), sometimes in abundance. Buchiids lived as epifauna, byssally attached suspension feeders (Zakharov, Reference Zakharov1981, fig. 78; Fürsich, Reference Fürsich1984, fig. 26; Hryniewicz et al., Reference Hryniewicz, Little and Nakrem2014). The single limid bivalve recorded, Pseudolimea cf. arctica (Zakharov, Reference Zakharov1966) (Pl. 1, Fig. 14) was also a byssally attached suspension feeder. All these fossils indicate oxic-marine waters, eventually above an ephemeral oxygen-restricted zone just above the water/sediment interface but never in or reaching the photic zone as vegetative cysts of prasinophytes are very few only. Buchiids, like all non-chemosymbiotic bivalve taxa, required some oxygen to thrive, thus their (abundant) occurrence excludes anoxic conditions. However, anoxic or dysoxic conditions prevailed in the sediment as the presence of infauna, indicated by bioturbation, appears to be absent but for some minor perpendicular structures between 2248.20 and 2250.18 m (Image 2).

Paleo(bio)geography

The investigated and cited boreholes were all located in an area south of 40°N paleo-latitude (Mutterlose et al., Reference Mutterlose, Brumsack, Flögel, Hay, Klein, Langrock, Lipinski, Ricken, Söding, Stein and Swientek2003). The North Sea area was connected through the southern Viking Graben to the Greenland-Norwegian seaway (Image 6). From the middle Volgian towards the late Ryazanian, alternating dysoxic to anoxic bottom conditions prevailed in the southern Viking Graben as suggested by trace element concentrations and foraminiferal assemblages (Mutterlose et al., Reference Mutterlose, Brumsack, Flögel, Hay, Klein, Langrock, Lipinski, Ricken, Söding, Stein and Swientek2003). North of the southern Viking Graben and certainly in the basins bordering the Arctic Ocean buchiids were diverse and abundant, but they were apparently rare in the North Sea area and totally absent during the late Volgian (Image 4). Very few Ryazanian occurrences of Buchia are recorded in NW Europe. More common and diverse, buchiid faunas are encountered along the Norwegian coast (Lofoten Islands, Birkelund et al., Reference Birkelund, Thusu and Vigran1978; Zakharov et al., Reference Zakharov, Surlyk and Dalland1981; Århus et al., Reference Århus, Kelly, Collins and Sandy1990), the Barents Sea (Smelror et al., Reference Smelror, Kelly, Dypvik, Mørk, Nagy and Tsikalas2001, fig. 5), the east coast of Greenland (Håkansson et al., Reference Håkansson, Birkelund, Piasecki and Zakharov1981; Surlyk & Zakharov, Reference Surlyk and Zakharov1982), Svalbard (Frebold, Reference Frebold1930; Sokolov & Bodylevski, Reference Sokolov and Bodylevski1931; Ershova, Reference Ershova1983) and on the Russian Platform (part of the East European Province) (Sazonova, Reference Sazonova1975; Zakharov, Reference Zakharov1987; Urman et al., Reference Urman, Shurygin and Dzyuba2019; Fig. 4). Only in the uppermost Volgian Singularis Zone buchiids are missing on the Russian Platform (Kiselev et al., Reference Kiselev, Rogov and Zakharov2018, p. 231), as well as in contemporary strata of the Subpolar Urals (Dzyuba et al., Reference Dzyuba, Pestchevitskaya, Urman, Shurygin, Alifirov, Igolnikov and Kosenko2018). All in all, compared to the before mentioned areas and especially northern Siberia and Canada (see Zakharov, Reference Zakharov1987, p. 144–145; Grey, Reference Grey2009, fig. 5.2), the West European Province or Boreal-Atlantic Province appears to lack any or any diversified buchiid fauna.

Image 6. Paleogeographic reconstruction of North Sea area modified after Abbink et al. (Reference Abbink, Targarona, Brinkhuis and Visscher2001a, fig. 15). Left part early – early late Ryazanian, right figure latest Ryazanian (- earliest Valanginian?). Legenda: northern Viking Strait = Greenland Norwegian Seaway, IM = Irish Massif, RBM = Rhenisch-Bohemian Massif, RHB = Rockall-Hatton Bank, and SM = Scottish Massif. Red arrows indicate warm (Tethyan influenced) water currents, blue arrows indicate relative cold water currents; the thicker the arrow, the bigger the influence.

The absence of a diverse buchiid fauna in this region is in contrast with the similarity between the ammonite faunas of West European Province and those of other Boreal areas. Since the latest late Volgian and throughout the Ryazanian, Boreal ammonites were mainly represented by the same genera and closely allied species throughout the Panboreal Superrealm. Although in the West European Province some endemic ammonite taxa are known (e.g. Lynnia), the other ammonites here are belonging to genera and species which occur in other Boreal regions (e.g. Hectoroceras kochi, Peregrinoceras spp., Surites spp., and Bojarkia spp.). This faunal homogeneity appears at least since the latest Volgian, when Volgidiscus lamplughi and V. pulcher dispersed through this huge area, from NW Europe to the Barents Sea shelf, the Russian Platform, the Subpolar Urals and Northern Siberia (Rogov, Reference Rogov2020).

The low diversity of buchiid bivalves and their overall rarity in the West European Province during the Ryazanian cannot be explained by paleogeographic restrictions alone. In contrast to ammonites, bivalves strongly depend on the configuration of the paleo-currents and the presence of a proper substrate for juvenile settling. Taking into account the common presence of TOC-enriched oxygen-depleted bottom settings throughout the Greenland-Norwegian seaway (northern Viking Graben; Image 6) during the Volgian and Ryazanian (Mutterlose et al., Reference Mutterlose, Brumsack, Flögel, Hay, Klein, Langrock, Lipinski, Ricken, Söding, Stein and Swientek2003), southward migrations of bivalves were recurrently hampered, in the southern Viking Graben. Especially during the late(st) Volgian, when oxygen depletion was highest.

The abundant occurrence of B. volgensis in the late Ryazanian of the region under discussion points to higher tolerance of this species to normally unfavourable environments, an opportunistic taxon, which is further evidenced by its wide geographic range outside the Panboreal Superrealm, as it is known from Submediterranean regions, like the Crimea, Mangyshlak and Mexico (see Zakharov, Reference Zakharov2015).

During the latest Jurassic-earliest Cretaceous, the (low)lands bordering the mid-latitudinal West European Province (marine realm) were characterised by a warm and arid climate (Hallam et al., Reference Hallam, Grose and Ruffell1991; Abbink, Reference Abbink1998; Turner, Reference Turner2018). The relatively shallow basins that occurred at high to mid northern latitudes were all connected to the Arctic area while some east-west connections, having been closed during latest Volgian to early Ryazanian at mid-latitudes, started to open in the late(st) Ryazanian (Image 6). These basins share the accumulation of organic-rich mudstones, for example, the hot and warm shales of the North Sea area, the Bazhenov facies of the Siberian Basin, with onset of deposition in the Kimmeridgian, accumulating rapidly in the Volgian and waning in the Ryazanian (Rogov et al., Reference Rogov, Shchepetova and Zakharov2020, fig. 2).

According to Sinclair (Reference Sinclair1994, p. 195), the approximate top (at least Icenii age) of the highly organic, condensed shales of the ‘hot shale’ facies can be considered the uppermost maximum flooding surface which developed prior to a subsequent increase in clastic input. This level is based on the last occurrence of abundant amorphous organic matter (Stenomphala Zone, according to Fraser et al., Reference Fraser, Robinson, Johnson, Underhill, Kadolsky, Connel, Johannessen, Ravnås, Evans, Graham, Armour and Bathorst2003, p. 165), but more recently placed in the Albidum Zone (Verreussel et al., Reference Verreussel, Bouroullec, Munsterman, Dybkjær, Geel, Houben, Johannessen and Kerstholt-Boeghold2018, figs 2, 15), that is, latest Ryazanian and possibly into the earliest Valanginian. It is regarded as the first indication of the occurrence of micro-fossils with Cretaceous (i.e. Valanginian) affinity, hence ‘Cretaceous flooding’.

It coincides with the influx of Tethyan taxa at high latitudes, indicated in the latest Ryazanian and earliest Valanginian (Möller et al., Reference Möller, Mutterlose and Alsen2015). Previously, this event was believed to have happened earlier in the Ryazanian (Mutterlose et al., Reference Mutterlose, Brumsack, Flögel, Hay, Klein, Langrock, Lipinski, Ricken, Söding, Stein and Swientek2003; Alsen & Mutterlose, Reference Alsen and Mutterlose2009; Pauly et al., Reference Pauly, Mutterlose and Alsen2012, Reference Pauly, Mutterlose and Alsen2013). From that time on, more calcareous, low organic, sediments occur temporarily.

In the late Ryazanian, the North Sea area was bordered to the south, stretching into parts of France, Belgium, the Netherlands, Germany and Poland, by continental to very shallow marine Wealden facies, restricting eventual connections to the south, south-west and east (Image 6). Marine influence on the continental Wealden facies increased and became dominant in the latest Ryazanian to earliest Valanginian, as large parts of the Wealden deposits were overlain by fully marine sediments, especially in the Lower Saxony Basin to the south-east and the Vomb Trough, the western most extension of the Polish Strait, to the east. Farther to the east, fully marine conditions were already established in Kochi Zone-time or slightly earlier, when Tethyan ammonites occur on the Russian Platform (Mitta, Reference Mitta2017; Grabowski et al., Reference Grabowski, Chmielewski, Ploch, Rogov, Smoleń, Wójcik-Tabol, Leszczyński and Maj-Szeliga2021). Contemporaneous sediments were then still marginally marine to continental towards the North West European Province, with only occasional flooding surfaces evidencing marine influence (Schneider et al., Reference Schneider, Heimhofer, Heunisch and Mutterlose2018, fig. 2).

Connections to the southeast along the Polish Strait, lows superimposed on the Sorgenfrei-Tornquist Zone (see e.g. maps envisaged by Kelly, Reference Kelly1990, p. 144), connecting either the Russian Plain and/or the Tethys with the West European province, were established around the transition from the early to the late Valanginian, perhaps as early as the latest Ryazanian-earliest Valanginian. A further possible connection towards the south is south of the Market Weighton High (Image 1) during the earliest Ryazanian (see Abbink, Reference Abbink1998, p. 136; Abbink et al., Reference Abbink, Targarona, Brinkhuis and Visscher2001a, fig. 15) but was definitely closed during the late Ryazanian when Wealden facies was deposited.

Marginal or proximal marine facies often include abundant continentally derived organic matter, in part as pollen and spores. A climate shift was postulated based on a sudden decline in the abundance of Classopollis (Abbink, Reference Abbink1998; Abbink et al., Reference Abbink, Targarona, Brinkhuis and Visscher2001a, Reference Abbink, Mijnlieff, Munsterman and Verreussel2006, fig. 6). This so-called Kochi climate shift occurs in the Kochi Zone just below J76. It is also called the Kochi flooding of Partington et al. (Reference Partington, Copestake, Mitchener and Underhill1993) and marks the transition from a warm and arid climate (cf. Hallam et al., Reference Hallam, Grose and Ruffell1991; Turner, Reference Turner2018) to a tropical wet climate. The sudden decrease in Classopollis in favour of Gleicheniaceae is well recorded on the Russian Platform (Fedorova & Gryazeva, Reference Fedorova and Gryazeva1984, tab. 1, p. 154) as well as in the Laptev Sea (northern Siberia) (Kashirtsev et al., Reference Kashirtsev, Nikitenko, Peshchevitskaya and Fursenko2018, fig. 2; approximately between their members 12 and 13). In the Ryazanian sediments of the Laptev Sea, though Classopollis is overall sparsely represented, but fern spores indicating tropical wet conditions increase in abundance. Nevertheless, Lindström & Ekström (Reference Lindström and Ekström2011) and Schneider et al. (Reference Schneider, Heimhofer, Heunisch and Mutterlose2018, fig. 2) indicate Classopollis to diminish already in the top of the Lamplughi Zone in the Cherty Freshwater Beds. Hoedemaeker (Reference Hoedemaker1999) and Hoedemaeker & Herngreen (Reference Hoedemaeker and Herngreen2003, Reference Hoedemaker and Herngreen2004) date these Purbeckian cherty beds between the Lamplughi and Runctoni flooding surfaces though.

Based on paleogeographic reconstructions, a marine passage has been inferred between the Shetland area and the south of Greenland (Laughton, Reference Laughton1975, p. 179, fig. 3; Ziegler, Reference Ziegler1988; Roberts et al., Reference Roberts, Thompson, Mitchener, Hossack, Carmichael and Bjørnseth1999, figs 14–15). However, recent research does not provide strong evidence for this NE Atlantic rift system, at least not before the Early Cretaceous (Stoker et al., Reference Stoker, Stewart, Shannon, Bjerager, Nielsen, Blischke, Hjelstuen, Gaina, McDermott and Ólavsdóttir2017, p. 57–58). The latter age is demonstrated, as throughout the Viking Graben, up to the Norwegian-Greenland seaway Tethyan calcareous microfossils (Nannoconus) have been observed. They occur regularly in the late(st) Ryazanian to the earliest Valanginian at high latitudes, 55ºN palaeolatitude (Århus et al., Reference Århus, Verdenius and Birkelund1986; Jeremiah, Reference Jeremiah2001; Pauly et al., Reference Pauly, Mutterlose and Alsen2013; Möller et al., Reference Möller, Mutterlose and Alsen2015) but probably not before the Albidum Chron (Jeremiah, Reference Jeremiah2001; Möller et al., Reference Möller, Mutterlose and Alsen2015, fig. 3).

In the latter context, it should be noted that for instance in North-East Greenland the underlying organic-rich mudstones of the Bernbjerg Formation are generally indicated to be Volgian or older (Alsgaard et al., Reference Alsgaard, Felt, Vosgerau and Surlyk2003; Piasecki et al., Reference Piasecki, Callomon and Stemmerik2004). However, these beds were considered to be of Late Jurassic-earliest Cretaceous age by Pauly et al. (Reference Pauly, Mutterlose and Alsen2012, Reference Pauly, Mutterlose and Alsen2013) and are coeval to the Upper Jurassic source rock of the mid-Norwegian shelf and the Barents Sea (Alsgaard et al., Reference Alsgaard, Felt, Vosgerau and Surlyk2003). Apparently, the top of the Bernbjerg Formation can be strongly diachronous partially being as young as the Ryazanian. Recent biostratigraphical research (Piasecki et al., Reference Piasecki, Bojesen-Koefoed and Alsen2020) revealed two different formations, that is, the Bernbjerg Formation of Oxfordian-Kimmeridgian age and the Stratumbjerg Formation of latest Hauterivian-Albian age, which are virtually impossible to separate from a lithological point of view. However, the latter formation is much younger as compared to the strata dealt with herein.

Thus, either there is a significant hiatus, or Nannoconus are not preserved in the organic-rich facies, or indeed not present. If so, it is likely to assume that the (sudden) Nannoconus influx arrived with what is commonly referred to as the ‘Stenomphala flooding’ (K10 of Partington et al. (Reference Partington, Copestake, Mitchener and Underhill1993) and or ‘Paratollia flooding’ (K15 of Partington et al. (Reference Partington, Copestake, Mitchener and Underhill1993), that is, that is after the Icenii Chron. Along the northeast coast of Greenland, the Ryazanian-Valanginian deposits yield additionally Tethyan cephalopods (belemnites like Pseudobelus and Duvalia; Alsen & Mutterlose, Reference Alsen and Mutterlose2009; Mutterlose et al., Reference Mutterlose, Alsen, Iba and Schneider2020) and brachiopods (e.g. Pygope; Ager & Walley, Reference Ager and Walley1977; Sandy, Reference Sandy1991; Harper et al., Reference Harper, Alsen, Owen and Sandy2005), along with relatively frequent ammonites of oceanic type, such as Lytoceras and Ptychophylloceras (Alsen, Reference Alsen2006), which invaded this area from the south (Mutterlose et al., Reference Mutterlose, Alsen, Iba and Schneider2020). All in all, the arrival of these typical Tethyan micro- and macrofossils can only be explained by a connection between the Tethys and the Boreal via the so-called Atlantic route (Image 6).

Swientek (Reference Swientek2002), Mutterlose et al. (Reference Mutterlose, Brumsack, Flögel, Hay, Klein, Langrock, Lipinski, Ricken, Söding, Stein and Swientek2003), Alsen (Reference Alsen2006), Pauly et al. (Reference Pauly, Mutterlose and Alsen2012, Reference Pauly, Mutterlose and Alsen2013) and Möller et al. (Reference Möller, Mutterlose and Alsen2015) argued that high meridional temperature gradients and cool-cold climatic conditions in high latitudes caused the formation of (cold) deep water in the South Anyui Gulf (Arctic) in the late Ryazanian-Valanginian and palaeoceanographic changes, reflected in a counter-balanced ocean current system in the Greenland-Norwegian Seaway and allowed Tethyan biota to spread as far north as North-East Greenland during the late Ryazanian and early Valanginian. Simultaneously, with incoming Tethyan waters, the accumulation of calcareous sediments at high latitudes relates to an important influx of calcareous nannofossils into the Greenland-Norwegian Seaway, exhibiting an influx of Tethyan and low-to-mid latitudinal taxa, synchronous with observed influxes of Tethyan ammonite and belemnite species (Pauly et al., Reference Pauly, Mutterlose and Alsen2013). However, the northward extension of this warm-water fauna was mainly restricted to East Greenland and adjacent areas of the Greenland-Norwegian seaway. North of this area only a single record of Lytoceras is known to date from the Valanginian of Spitsbergen (Frebold & Stoll, Reference Frebold and Stoll1937, p. 52). During the (late) Valanginian, the Tethyan influence became stronger and caused the occurrence of a specific ‘warm-water foraminiferal assemblage’ in the Russian part of the Barents Sea shelf (Basov & Vasilenko, Reference Basov and Vasilenko1999).

These influxes suggest the occurrence of northward flowing warmer, less saline surface currents in the Norwegian-Greenland seaway (northern Viking Graben), which allowed Tethyan nekton and plankton to spread even as far north as North-East Greenland at 55°N palaeolatitude (Pauly et al., Reference Pauly, Mutterlose and Alsen2013; Möller et al., Reference Möller, Mutterlose and Alsen2015; Mutterlose et al., Reference Mutterlose, Alsen, Iba and Schneider2020) and further north-east, but also suggest these surface currents to flow south-ward through the (southern extension of the) Viking Graben, to approximately 35–40°N, ‘piggy backing’ the normal colder north-south currents, enabled – to some extent – the exchange of cephalopods and bivalves (Image 6). Perhaps connections with a strong north-south component (Image 6) existed already temporarily, because of the general similarity between some ammonite assemblages described from the West European Province and other Boreal provinces. However, during these times, the southern connections, as envisaged by Kelly (Reference Kelly1990, p. 144), were seriously hampered.