The Obnazhennaya pipe belongs to the Kuoika kimberlite field of the Yakutsk diamondiferous province. The pipe is attributed to the Olenek Uplift in the northeastern part of the Siberian Platform, exposed in a coastal rocky outcrop on the Kuoika River (a left tributary of the Olenek River) 3.5 km from the mouth (Fig. 1). Despite the fact that this pipe is not diamondiferous, the outcrop of kimberlite formations, exposed directly on the surface (outcrop height is up to 15 m), attracts attention by its uniqueness for the Siberian Platform, as well as by the abundance of deep and crustal xenoliths. The Obnazhennaya pipe is relatively close to the large suture zone of the crust, which separates the Birekte block and the Eyekit orogen. It breaks through the rocks of the Birekte granite–greenstone terrane and the sedimentary cover overlying them, coming to the day surface in the middle of the Upper Vendian–Lower Cambrian rock field [1]. Kimberlites contain xenoliths of mantle eclogites, peridotites, and Vendian and Cambrian sedimentary, predominantly carbonate, rocks, as well as xenoliths of the Permian terrigenous-sedimentary rocks with poorly preserved plant remains, Triassic traps, and fragments of belemnite rostra [2].

Fig. 1.
figure 1

Location of the Obnazhennaya pipe, from the kimberlite of which a new belemnite record occurs.

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The age of kimberlites of the Kuoika field, determined by the U–Pb method using zircon, varies from 157 Ma (D’yanga pipe) to 147.7 Ma (Slyudyanka pipe) [3, 4]. According to SHRIMP U–Pb dating of perovskites of the Montichellitovyi stock and Velikan dyke, the age was established in the range of 170–128 Ma [5]. Later, the age of the breakthrough of a number of kimberlite pipes in the Kuoika field was determined by U–Pb dating of perovskites in the range of 171–156 Ma, which is believed to correspond to the last (Middle–Late Jurassic) episode of kimberlite magmatism in the Yakutsk diamondiferous province [6], which is associated with the subduction processes that accompanied the accretion of the Omolon, Kolyma, and surrounding terranes [7]. It is noteworthy that the U–Pb age (“perovskite”) of the Slyudyanka pipe (161.8 ± 5.6 Ma) is 14.1 ± 5.6 Ma older than the U–Pb age (“zircon”). The age of 147 ± 6 Ma was obtained for the same pipe using the U-track method [8], although this approach cannot exclude a “rejuvenation” result due to the destruction of track traces under the influence of high-temperature magmatic or hydrothermal-metasomatic effects. No zircons have been found in the Obnazhennaya pipe, and no U–Pb dating is available, so its age remains a subject of numerous discussions.

The Rb–Sr dating of 161 Ma and a number of K–Ar datings were obtained for kimberlites of the Obnazhennaya pipe: 418 ± 14 Ma, 288 ± 10 Ma, 205 ± 10 Ma, and 185 ± 10 Ma [1, 9]. According to the paleomagnetic data, one of the obtained values of the pipe age corresponds to 168 ± 11 Ma and the other, to 151 ± 14 Ma [7]. The dating of mica from deformed phlogopite–amphibole rock by the 40Ar/39Ar-method was close to the first value of 167 Ma [10]. Belemnites of the Late Jurassic–Early Cretaceous age in kimberlites of this pipe have also been recognized [2, 11–13], which is significant when discussing the time of the breakthrough, but is in contradiction with the available geochronological dating. The new belemnite record changes the existing concepts about the paleontological evidence of the age of the Obnazhennaya pipe and raises the question of the need to revise the available data on belemnites.

During studies of the xenolith in kimberlites of the Obnazhennaya pipe in 2022, M.G. Oshchepkova found the upper part of the belemnite rostrum directly in kimberlite (Fig. 2). This specimen, including the alveolus, was cut from the rock and determined by O.S. Dzyuba as Arcobelus cf. krimholzi (Sachs, 1970), a representative of the Early Megateuthididae. The family affiliation is indicated by the well-defined conical shape of the rostrum of the initial stages of ontogenesis, which is visible in the longitudinal section (Figs. 2d, 2e). The rather large size of the rostrum (dorsal–abdominal diameter of about 26 mm) suggests that it belongs to the pre-Bajocian representatives of Megateuthididae, because, in Arctic sections, the late Megateuthididae (Bajocian–Bathonian) are represented exclusively by small- and medium-sized Paramegateuthis [14].

Fig. 2.
figure 2

Belemnite Arcobelus cf. krimholzi (Sachs, 1970) from the Obnazhennaya pipe, Eq. GEOCHRON, No. 2119/1: (a) fragment of the upper part of the rostrum in kimberlite rock before dissection; (b) cross section near the anterior edge; (c)–(e) longitudinal splits and enlarged fragment of one of them.

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Among the early Megateuthididae, the remains of which are found in northern Russia, rare species, namely, the Toarcian–Early Aalenian A. krimholzi, Rarobelus gigantoides (Pavlow, 1914), and Toarcian R. obscurus (Nalnjaeva, 1970), had a large rostrum significantly compressed laterally (lateral diameter <85% of the dorsal–abdominal diameter). However, the representatives of the genus Rarobelus, are characterized by a cross section at the dorsal edge, which is wider than the ventral one due to the development of ventral–lateral flattening [15]. This is not observed in the studied specimen. Instead of this, there are signs of an oval shape of the cross section to slightly widened at the ventral edge (Fig. 2b), which is quite typical of A. krimholzi ([16], Table II, Fig. 7). The species is widely distributed in the upper Lower Toarcian (starting from the ammonite zone of Harpoceras falciferum)–Lower Aalenian in northern Siberia (including the Olenek River basin) and the Okhotsk coast [16]. Due to the incomplete preservation of the studied belemnite, other distinguishing characters of Arcobelus and Rarobelus (the degree of elongation of the rostrum and the size of the alveolar angle in the dorsal–abdominal plane) cannot be discussed in the present study. For the same reason, the belemnite is defined in the open nomenclature.

For the first time, a belemnite (postalveolar part of the rostrum) was discovered in the kimberlite breccia from the Obnazhennaya pipe by V.A. Milashev and determined as Pachyteuthis (?) sp. by N.I. Shul’gina in consultation with V.I. Bodylevskii and G.Y. Krymholtz ([11], Figs. 1, 2). The age of the genus Pachyteuthis (family Cylindroteuthididae) at that time was considered Late Jurassic–Early Cretaceous. It is now well known that the first appearance of representatives of this genus occurred in the middle Bajocian (Middle Jurassic), including in the northern part of Siberia ([14] and others). In particular, the specimen considered has no principal differences from the Late Bajocian–Early Cretaceous species P. optima Sachs et Nalnjaeva, 1966 and may well belong to it taking into consideration its image and the described characteristics of the size, shape, and elongation of the preserved part of the rostrum, cross section, ventral furrow, and axial line.

A subtrapezoidal form of the cross section of the rostrum of this species, well manifested in the alveolar part, acquires rounded-rectangular features in the postalveolar part ([12], Table II, Fig. 4b; and others). It is likely that it was the shape of the cross section that served as the main basis for the species interpretation of the belemnite. Thus, it has been suggested to refer it to the Early Cretaceous species P. subrectangulata (Blüthgen, 1936) [13], from which, however, it is well distinguished by the subconical shape of the rostrum. Saks and Nal’nyaeva [12] assigned the belemnite to Simobelus cf. insignis (= Pachyteuthis (Simobelus) cf. insignis sp. nov. auct.) and, thus, considered it to be Volzhskii (Late Jurassic). However, the species S. insignis (Sachs et Nalnjaeva, 1966) is noticeably shorter. Considering that the full length of the postalveolar part remains unknown, the question of whether the belemnite belongs to the more elongated forms of cylindroteuthidids of the genus Lagonibelus (to its largest representatives) deserves attention. Thus, the very large rostrum of the Kimeridgian species L. sarygulensis (Krimholz, 1929) is characterized by a considerably elongated apical part of the subconical form. However, this species is not known east of the Yenisei–Khatanga trough and its rostrum is more rounded in cross section [17]. From the analysis, we can conclude that the belemnite is rather confidently diagnosed to the genus and is closest to the species Pachyteuthis optima. In the present work, taking into account the incomplete preservation of the rostrum, it is redefined as P. cf. optima.

Three fragments of different rostra were found in kimberlites of the Obnazhennaya pipe by B.A. Malkov [2, 13]. One of the specimens was determined by V.A. Gustomesov [13] as a representative of the Late Jurassic–Early Cretaceous genus Arctoteuthis (=Cylindroteuthis (Arctoteuthis) auct.). Their belonging to Arctoteuthis was proved by the rounded (not compressed from the sides) cross section of the rostrum of the mature stage of ontogenesis and by the strongly elongated spindle-shaped form of the rostrum of the initial stage. Later, the image of this specimen was published ([2], photo, Figs. 1b, 1c). It is obvious that it is a fragment of the postalveolar part of the rostrum (the specimen length does not exceed 20 mm) with a diameter of about 10 mm. Due to the absence of the alveolar part of the rostrum and its posterior edge, it was unclear which interval of the post-alveolar part was occurred in the kimberlite. The elongation of the initial stage rostrum on the available segment of the longitudinal split is hardly indicative for the establishment of the genus Cylindroteuthis, if we exclude the fact that representatives of the subfamily Simobelinae also had a spindle-shaped initial, but shorter, rostrum [18]. The cross section, close to the rounded form, falls within the limits of variability of the genera Cylindroteuthis, Pachyteuthis, Lagonibelus, and Communicobelus, the first two of which first appear in the Bajocian [14]. Therefore, we can judge with certainty only about the belonging of the considered specimen to the family Cylindroteuthididae. It is important that all the rostrum fragments encountered in kimberlites have been revealed directly in kimberlite rock. On this basis, it was concluded that the kimberlites were embedded in poorly lithified and watered sediment, so that the rostra were easily separated from the host rock [13].

The range of existence of belemnites of the genus Arcobelus (Toarcian–Early Aalenian), a representative of which was first found in kimberlites of the Obnazhennaya pipe, corresponds to the stage of kimberlite magmatism at 177 ± 1.5 Ma (Late Toarcian) in the northeastern part of the Siberian craton, recently established on the basis of U–Pb dating of zircons during the study of diamondiferous placers of the Ebelyakh River basin (eastern slope of the Anabar Shield) [19]. Meanwhile, the supposed Late Bajocian–Early Bathonian age of the representative of the genus Pachyteuthis, found earlier in the Obnazhennaya pipe and defined in the present work as P. cf. optima, agrees with one of the datings based on paleomagnetic data (168 ± 11 Ma [7]) and with the 40Ar/39Ar-dating (167 Ma [10]) of kimberlites in the Obnazhennaya pipe, most likely indicating the Early Bathonian age of the kimberlite formation.

Due to the fact that the Obnazhennaya pipe is exposed on the surface surrounded by Upper Vendian–Lower Cambrian rocks, the zones of marine paleo-water areas have not extended to the Kuoika kimberlite fields on most paleogeographic schemes of the Toarcian, Aalenian, and Bajocian paleo-basins of Siberia. Only, in some cases, was the inner part of the shelf hypothetically extended to this area on the paleogeographic schemes of the East Siberian sedimentary basin. However, in general, for the Toarcian–Early Bathonian time, the zone of the unstable shoreline was assumed here [20]. This hypothesis was based on indirect signs, that is, the sandiness isolines, extrapolated to the territory of the Kuoika field according to the analysis of the distribution of sand facies in the Jurassic paleo-delta sections of small watercourses on the northwestern side of the paleo-Vilyui Bay. Taking into consideration the records discussed above, the existing paleogeographic schemes of the Toarcian–Early Bathonian interval of the northeastern Siberian Platform should be corrected (Fig. 3). The investigated occurrence is important evidence of the immensity of the disappearance of traces of marine ingression in the geological record.

Fig. 3.
figure 3

Paleogeographic scheme of the Toarcian–Early Bathonian of the East Siberian sedimentary basin according to [20] with modifications. (1) External shelf (sea, relatively deep-water part) (ExtSh); (2) internal shelf (sea, shallow-water part) (IntSh); (3) alluvial–lacustrine–boggy settings; (4) drift areas (supplying provinces: WSP, Western; SSP, Southern; ESP, Eastern; NSP, Northern); (5) locations of modern outcrops of the same-type clayey and sandy–clayey marine formations of the Toarcian–Lower Aalenian of the Vilyui paleo-basin (Suntar Formation and similar sequences) along the eastern and southern contours of the zone of the distribution of such outcrops; (6) location of the Obnazhennaya pipe; (7) the main directions of clastic material transit within the outline of the supplying provinces.

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