The work is conducted jointly by scientists from the Diamond and Precious Metal Geology Institute, Siberian Branch, Russian Academy of Sciences (DPMGI, Yakutsk) and the Trofimuk Institute of Petroleum Geology and Geophysics SB RAS (Novosibirsk).
The Obnazhennaya kimberlite pipe attributed to the Kuoika kimberlite field, Yakut diamondiferous province, is located in the northeast portion of the Siberian platform, within the Olenek uplift (Sakha Republic (Yakutia)). The size of the pipe is 20 × 30 m. Despite the fact that this pipe is not diamond-bearing, a particular attention attracted to it from science is explained by its being exposed up to 15 m in a coastal rocky outcrop on the Kuoika River (a left tributary of the Olenek River), which is a rare case within the bounds of the Siberian craton, as well by abundance of deep and crustal xenoliths (fragments of rock trapped by magma).
In the course of research into xenoliths in kimberlites of the Obnazhennaya kimberlite pipe conducted by lead specialist from DPMGI SB RAS M.G. Oshchepkova, a specimen of the upper part of the belemnite rostrum was found directly in the kimberlite rock. O.S. Dzyuba, Head of the Laboratory of Mesozoic and Cenozoic Paleontology and Stratigraphy, IPGG SB RAS, DrSci (geol.-mineral.), determined it as Arcobelus cf. krimholzi, a representative of the Toarcian-Early Aalenian (late Early–Middle Jurassic) belemnites.
Belemnite Arcobelus cf. krimholzi from the Obnazhennaya kimberlite pipe, specimen from the GEOCHRON collection, 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 an enlarged fragment of one of them
The range of records of belemnites of the genus Arcobelus accounts for the kimberlite magmatism stage that occurred about 177 million years ago in the northeast of the Siberian craton. Experts have shown that previously known Belemnite finds from the kimberlites of the Obnazhennaya pipe whose age was determined as Late Jurassic or Early Cretaceous according to U–Pb geochronological data. The estimated Late Bajocian–Early Bathonian age of the representative of the genus Pachyteuthis, previously found in kimberlites of the Obnazhennaya pipe, is consistent with the 40Ar/39Ar age (167 Ma), as well as with the age based on some paleomagnetic data (168 ± 11 Ma) reported for the pipe.
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. In addition, scientists were able to draw new conclusions on paleogeography. According to them, since the Obnazhennaya kimberlite pipe outcrops are surrounded by rocks of very ancient (Vendian-Cambrian) age, belemnites belonging to the extinct order of cephalopods were inhabitants of Mesozoic seas. The investigated occurrence is important evidence of the immensity of the disappearance of traces of marine ingression in the geological record.
The remains of the Toarcian-Early Aalenian belemnite, described as the genus Arcobelus cf. krimholzi (Sachs, 1970), are exposed in the kimberlite of the Obnazhennaya pipe. A, B, D: a – kimberlite, b – fragment of belemnite rostrum; C: a – longitudinal splits of a fragment of belemnite rostrum, b – kimberlite rock filling the alveolus of belemnite rostrum, c – alveolus.
– The obtained results provide insights into a very intriguing aspect of geological research related to the issues of the deep structure and sedimentation conditions in the East Siberian Boreal sedimentary paleobasin, the researchers noted. – This is important for more accurate predictions of occurrence and formation of minerals in the area of the Olenek Uplift located on the northeastern border of the Siberian platform.
The work was carried out according to the state assignment of DPMGI SB RAS (projects No. FUEM-2019-0001, FUEM-2019-0003, 2024-0005; 2024-0007) and IPGG SB RAS (project no. FWZZ-2022-0004) and funded by the Ministry of Education and Science of the Russian Federation. The complex analysis of paleontological, geochronological, and paleogeographic data was carried out at the expense of the Russian Science Foundation, grant no. 22-17-00228, https://rscf.ru/project/22-17-00228/, on the basis of Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch, Russian Academy of Sciences.
Published by IPGG press Service
Photos courtesy of the researchers