An Erratum to this paper has been published: https://doi.org/10.1134/S1028334X23060077

The results of the determination of the age of zircon grains from tuffites of the Leskovo Unit of the Unda–Daya Basin, Eastern Transbaikalia, are presented for the first time. The age of the youngest population of zircon grains is 145.8 ± 3.8 Ma, approximately corresponding to the Jurassic–Cretaceous boundary and indicating the Early Cretaceous age of most of the Leskovo Unit. Given that the similar taxonomic composition of ostracods from the middle part of the Leskovo Unit and the Valanginian–Lower Hauterivian Dabeigou Formation of northeastern China, our U–Pb age from the lower part of the Leskovo Unit allows confident correlations of these lithostratons.

Some taxa of belemnites, bivalves, gastropods, and brachiopods, previously unknown in the Russian Far East (except for Eastern Siberia and Northeast Russia), of both Tethyan and Boreal Arctic origin, have been recorded in the Middle Jurassic of the Soloni River (Bureya River basin). In particular, bivalve assemblages were found to increase in abundance throughout most of the Bajocian section at the Soloni River, similar to what is observed in Siberian sections. However, the first appearance of some species of the genus Grammatodon and trigoniids was recorded in this area much earlier than in Siberia. This, combined with a revision of the taxonomic composition of previously recorded fossils, has made it possible to refine the biostratigraphic subdivisions of not only the studied section, but also the uppermost Lower–Middle Jurassic marine deposits of the Bureya Trough as a whole. The age ranges of the local lithostratigraphic units have been refined to a greater degree. It has been established that the Sinkal’tu Formation corresponds to the Upper Toarcian–lowermost Lower Bajocian, the Epikan Formation to the uppermost Lower Bajocian, the El’ga Formation to the Upper Bajocian, and the Chagany Formation to the Bathonian–Callovian. Data on the palynological assemblages identified in these formations are provided. The recent studies of cephalopods and bivalves from Siberia, the Russian Far East, and southern Alaska have led to the development of a refined interregional correlation scheme for Bajocian deposits in these regions.

Some taxa of belemnites, bivalves, gastropods and brachiopods, previously unknown in the Russian Far East (excluding eastern Siberia and North-East Russia), of both Tethyan and Boreal Arctic origin, were recorded in the Middle Jurassic of the Soloni River (Bureya River basin). In particular, it was revealed that bivalve assemblages increase in number up-section throughout most of the Bajocian at the Soloni River, much as they do in Siberian sections, but the first appearance of some species of the genus Grammatodon and trigoniids was recorded here much earlier than in Siberia. All this, combined with the revision of the taxonomic composition of fossils recorded earlier, made it possible to refine the biostratigraphic subdivision of not only the studied section, but also of the uppermost Lower to Middle Jurassic marine deposits of the Bureya Trough as a whole. Age ranges of the local lithostratigraphic units have been brought to a higher degree of refinement. It was established that the Sinkal’tu Formation corresponds to the Upper Toarcian–lowermost Lower Bajocian, the Epikan Formation to the uppermost Lower Bajocian, the El’ga Formation to the Upper Bajocian, and the Chagany Formation to the Bathonian–Callovian. The data on the spore-pollen complexes identified in these formations is provided. The most recent findings of the studies on cephalopods and bivalves from Siberia, the Russian Far East, and southern Alaska are summarized to refine the interregional correlation scheme of Bajocian deposits of these regions.

In this article, we discuss the age range for the formation of the Obnazhennaya kimberlite pipe (Kuoyka field, north-eatern Siberian platform) considering the recent unique discovery of a representative of the Toarcian-Early Aalenian (end of the Early–beginning of the Middle Jurassic) belemnites along with the revision of previously known belemnites of the Upper Jurassic–Lower Cretaceous age. We investigated the specifics and general genesis of the facies in the complex vertical structure of the sections. In addition, we assessed the role of the ancient Kimmeriyan, Dunlap, and New Cimmerian phases of Cimmerian tectono-magmatic activation in the stabilization of the Kuoika and Khorbusuon kimberlite fields. Moreover, a new model of the eruptive system of Mesozoic kimberlite fields was considered. The existing paleogeographic schemes of the Toarcian-Early Bathonian interval northeast of the Siberian Platform have been corrected. The study location provides important evidence of the disappearance scale of marine ingression traces in the geological record.

The first records of dinocysts and foraminifers in the Middle Jurassic deposits of the Russian Far East and their taxonomic affiliation are reported. The use of foraminiferal and ostracod assemblages as a tool for biostratigraphic subdivision and correlation of the Middle Jurassic sections of this region is very difficult due to the poor preservation and limited occurrence of these microfossils. Findings of the dinocysts Endoscrinium galeritum,Tubotuberella cf. apatela,Wanaea fimbriata, and Meiourogonyaulax cf. caytonensis raise the question of correcting the stratigraphic range of the lithological formations recognized in the Middle Jurassic.

Middle Jurassic sedimentary strata of Siberia remain poorly studied chemostratigraphically. We contribute to the knowledge with pioneering C, O, and Sr isotopic data for carbonate material of belemnites from the Yuryung-Tumus Peninsula and the lower reaches of the Lena River, as a basis for comprehensive description of the north Siberian Bajocian and lower Bathonian. The obtained chemostratigraphic constraints, with new 87Sr/86Sr ratios and previous delta 13C and delta 18O estimates, also include data for the lower Bathonian in the Sokur section (Central Russia). Despite the limited amount of material, chemostratigraphy, along with the available biostratigraphic data, allows reliable correlation of the Boreal sections with the primary standard of Northwest Europe, which is impossible for these strata with any of the two methods alone. The delta 13C, delta 18O, and 87Sr/86Sr patterns correlate with the records of eustatic, climatic, tectonic, and paleogeographic events. The new delta 18O data and the inferred paleotemperatures for the latest early Bajocian and the Bajocian/Bathonian boundary reveal two excursions of notable seawater warming near the Siberian Arctic coast, which were synchronous with episodes of global sealevel rise. It was presumably during the eustatic events that the N-S Komi Strait (first naming) opened twice in the territory of the Russian Plate. The strait connected the Boreal and Tethyan seas and thus changed the oceanic circulation patterns. Specifically, it opened a gateway for a warm current from the south to northern Siberia responsible for the high seawater temperatures recorded in the delta 18O patterns of belemnites.

Среднеюрские отложения Сибири практически не изучены хемостратиграфическим методом. На основе исследования карбонатного материала ростров белемнитов, происходящих с п-ова Юрюнг-Тумус и низовьев р. Лена, впервые получена комплексная С-, О- и Sr-изотопная характеристика большей части байоса и нижнего бата севера Сибири. В работе также обобщены δ13C и δ18О характеристики и новые 87Sr/86Sr данные, которые стали хемостратиграфической основой нижнего бата в разрезе Сокур (Центральная Россия). Вкупе с имеющимися биостратиграфическими данными хемостратиграфия, несмотря на ограниченное количество материала, позволила надежно сопоставить бореальные разрезы с северо-западно-европейским первичным стандартом, что недостижимо в исследуемом интервале при использовании только одного из этих методов. Охарактеризованы взаимосвязи между изменениями δ13C, δ18О, 87Sr/86Sr, эвстатики, климата, тектоническими и палеогеографическими событиями. По δ18O данным и рассчитанным по ним палеотемпературам для двух временных интервалов (конец раннего байоса, рубеж байос/бат) регистрируются кратковременные значительные повышения температуры морской воды у арктического побережья Сибири, что приходится на эпизоды глобального подъема уровня моря. С эвстатическими событиями предположительно связывается двукратное открытие на территории Русской плиты меридионального морского пролива Коми (наименование дано впервые), временно соединявшего северные и южные моря, меняя схему циркуляции водных масс. Как предполагается, раскрытие пролива могло приводить к периодическому поступлению теплого течения, идущего с юга на север, которое достигало северосибирских акваторий, неся тепло и влияя на изотопный состав кислорода в рострах белемнитов. The Middle Jurassic sedimentary strata of Siberia remain poorly studied chemostratigraphically. We contribute to the knowledge with pioneering C, O, and Sr isotopic data for carbonate material of belemnites from the Yuryung-Tumus Peninsula and the lower reaches of the Lena River, as a basis for comprehensive description of the north Siberian Bajocian and lower Bathonian. The obtained chemostratigraphic constraints, with new 87Sr/86Sr ratios and previous δ13C and δ18O estimates, also include data for the lower Bathonian in the Sokur section (Central Russia). Despite the limited amount of material, chemostratigraphy, along with the available biostratigraphic data, allows reliable correlation of the Boreal sections with the primary standard of Northwest Europe, which is impossible for these strata with any of the two methods alone. The δ13C, δ18О, and 87Sr/86Sr patterns correlate with the records of eustatic, climatic, tectonic, and paleogeographic events. The new δ18O data and the inferred paleotemperatures for the latest early Bajocian and the Bajocian/Bathonian boundary reveal two excursions of notable seawater warming near the Siberian Arctic coast, which were synchronous with episodes of global sealevel rise. It was presumably during the eustatic events that the N-S Komi Strait (first naming) opened twice in the territory of the Russian Plate. The strait connected the Boreal and Tethyan seas and thus changed the oceanic circulation patterns. Specifically, it opened a gateway for a warm current from the south to northern Siberia responsible for the high seawater temperatures recorded in the δ18О patterns of belemnites.

In the Izhma River basin, the Upper Bajocian–Lower Bathonian sections crop out at several localities along the Dreshchanka River. A recently published opinion (Ippolitov, A.P., Kiselev, D.N. “Geological Features of the Bajocian–Bathonian in the Reference Section of the Izhma River Basin (North of European Russia) and the Succession of Ammonites of the Subfamily Arctocephalitinae Meledina,” Stratigraphy and Geological Correlation, 2021, vol. 29, pp. 742–755) on the correlation of these sections, as well as the comments made in the same paper on the results of our research, are critically reviewed. The correlation model constructed using the strato-isohypses of the top of the Arcticoceras-bearing sandstone horizon, constructed by Ippolitov and Kiselev relative to four marks of the water level, turned out to be factually unfounded and methodologically unsubstantiated. Arguments against this model are as follows: the low efficiency of the interpolation method on a small sample size when constructing the riverbed dip profile under conditions of intensive meandering of the Dreshchanka River on the floodplain terrace; ignoring the geomorphological situation as a whole; the irrelevance of the topographic map of the 1999 edition; the underestimation of the variability of the low-water position of the water level in different years due to changes in the amount of precipitation.

Upper Mesozoic (Upper Jurassic and Cretaceous) rocks are widely distributed in Sikhote-Alin (Russian Far East) and northeastern China. In Sikhote-Alin, the Upper Jurassic and Lower Cretaceous rocks are represented mainly by marine deposits, whereas the Upper Cretaceous rocks are mainly volcanic and non-marine sedimentary-volcanic. In northeastern China, the upper Mesozoic rocks are, on the contrary, mainly non-marine. Fully marine Upper Jurassic–Valanginian deposits are restricted to the northeastern Heilongjiang Province near the border with Russia. Barremian–Albian non-marine deposits alternating with marine ones also occur in this region. They contain marine and non-marine fauna and, therefore, represent an important object for non-marine and marine correlation. On the rest of the territory of northeastern China, upper Mesozoic rocks are represented by non-marine sedimentary and volcanogenic deposits. The complex geological structure of the region as well as different types of sedimentary basins cause difficulties in correlation between the upper Mesozoic rocks of Sikhote-Alin and northeastern China. Despite the long history of investigations, their correlation schemes are still rare. Creation of a correlation scheme between Sikhote-Alin and northeastern China should be based on revision of published stratigraphic data on both regions. This article is the first in a series of articles devoted to the upper Mesozoic stratigraphy of Sikhote-Alin and northeastern China. In the present article the Upper Jurassic–Hauterivian stratigraphy of Sikhote-Alin and northeastern China is reviewed. The Upper Jurassic–Hauterivian deposits of Sikhote-Alin and northeastern China contain abundant Buchia and rarer ammonites. Based on stratigraphic distribution of buchiid assemblages, the correlation scheme for Kimmeridgian–Valanginian (possibly lowermost Hauterivian) strata of northeastern China and Sikhote-Alin is proposed. The succession of Jurassic–Cretaceous buchiid assemblages of Sikhote-Alin and northeastern China is similar to those of the other regions, e.g., Northern Siberia (Arctic Realm) and California (North Pacific Realm).

On the basis of the results of studying the faunal collections of bivalves and foraminifera collected in the period of 1976–2014 in the Cape Tsvetkov region in Eastern Taimyr, the knowledge of the Pliensbachian–Aalenian part of the section of the Tsvetkov geological region is expanded. The bedding in the section is described, and a comprehensive paleontological characteristic is given. For the first time for this section, the following taxa are included in the composition of characteristic bivalve assemblages: Praemeleagrinella deleta, Siungiudella cf. parvula, Mytiloceramus (Lenoceramus) vilujensis, Mytiloceramus (Pseudomytiloides) oviformis, Oxytoma ex gr. kirinae, Arctotis (Praearctotis) similis, and Arctotis (Arctotis) tabagensis. On the basis of the use of auxiliary biostratigraphic subdivisions—beds with bivalves and F-zones based on foraminifera—the section is correlated with the standard boreal ammonite scale. The age of the local stratigraphic units of the Eastern Taimyr structural-facies zone is refined. The Kiterbyutskaya Formation is considered within the Tiltoniceras antiquum–Dactylioceras commune zones, the Korotkinskaya Formation is considered within the Dactylioceras commune–Pseudolioceras falcodiscus zones, and the Aprelevskaya Formation is considered within the Pseudolioceras maclintocki Zone. For the first time, images of the leading species of bivalves are presented.

The present paper is the second article in a series of publications dedicated to the upper Mesozoic stratigraphy of Sikhote-Alin (Russian Far East) and northeastern China. The Barremian–Albian stratigraphy of Sikhote-Alin and northeastern China is reviewed and a correlation scheme between these regions is proposed. Barremian–Albian rocks in Sikhote-Alin are mainly represented by marine deposits. Non-marine deposits occur in the south of Sikhote-Alin in the Partizansk and Razdolny basins. In northeastern China, in contrast, the Barremian–Albian rocks are mainly non-marine often intercalated with volcanic rocks. Non-marine deposits alternating with marine ones occur in northeastern Heilongjiang near the border with Russia. They fill a system of sedimentary basins restricted to the Dunhua-Mishan Fault Zone and linked in the east with the Alchan Basin of Sikhote-Alin. The deposits of these basins, represented by the Jixi, Longzhaogou, Dajiashan and Huashan groups, and the Assikaevka and Alchan formations, are of high importance for non-marine and marine correlation because they contain non-marine fauna and flora assemblages together with marine molluscs. The range of the Jixi and Longzhaogou groups is clarified based on the correlation with the Assikaevka Formation containing marine index fossils and is considered here as Barremian (possibly extending to Hauterivian)–middle Albian. The Jixi Group yielding elements of the Jehol Biota corresponds to the Jehol Group, and, therefore, the range of the Jehol Group is also considered as Barremian–middle Albian.

—We study the morphology and sedimentary facies of trace fossils formerly identified as Arctichnus found at the base of the Jurassic section near Cape Airkat (northern Siberia). They are most often found in shoreface silty sand and are similar to Rosselia socialis ichnospecies in morphology, taphonomy, and depositional environments. On the basis of this similarity, the Airkat trace fossils should be identified as Rosselia socialis Dahmer, 1937. Analysis of the type collection of Arctichnus arcticus has revealed new morphological features of the taxon.

—We propose a facies-stratigraphic zoning of the Vasyugan and Georgievka horizons in the sedimentary cover of the West Siberian Basin, including the southern part of the Kara Sea. Based on the typification of well sections and taking into account the paleogeography of the Callovian–Kimmeridgian deposits, we have recognized 12 regions with different structures of the Vasyugan and Georgievka horizons. The Purpei–Vasyugan area is divided into three subareas according to the structure of the Vasyugan Formation including the petroliferous horizon Yu1. Transition zones have been recognized between the Vasyugan Formation and the bordering Abalak, Tatar, and Naunak formations along the western, southern, and eastern boundaries of the area, respectively. The results of zoning of the Callovian–Kimmeridgian deposits, including the petroliferous horizon Yu1, can be used on planning of exploration work, for selecting standard facies models, and for predicting the petrophysical properties of a reservoir.

New paleontological and lithological data characterizing the upper boundary of the Chekurovskii Formation are considered for the stratoregion located in the lower reaches of the Lena River, northern Siberia. We propose to accept this boundary at the bottom of a sandy bed with pebbles, considering the latter to be basal sandstones of an overlying transgressive sequence. Taking into account ammonite records, we can conclude that the boundary corresponds to a level within the Upper Bathonian Catacadoceras barnstoni Zone. The predominantly silt–sized sediments lying above (uppermost Bathonian? – Lower Callovian) can be treated an analogue of a lower part of the Innokent’evka Formation recognized in adjacent facial region.

During the field works in the summer of 2018, in the Lena River delta on the Sardakh-Sisse Island, prints of leaf flora belonging to species of broad and small-leaved deciduous trees were found. The prints found by us are confined to the layer of ferruginous sandstones at the base of the section. The following taxa established: Platanus sp., Alnus sp., Fagus sp., Salix sp. In addition, a fossilized fruit belonging to Magnolia sp. The findings allow to conclude that there are mixed forests in the early-middle Miocene in this region, consisting of coniferous, small- and broad-leaved deciduous species of trees and shrubs.

In the sections of Late Pleistocene deposits of the Kurai Depression of Gornyi Altai studied, an ostracod assemblage, including Leucocythere sp. 1, L. sp. 2, Leucocythere dorsotuberosa Huang and Leucocytherella sinensis Huang, was identified. These species were previously found in deposits (from the Late Pleistocene up to the present day) of high-altitude glacial lakes on the Plateau of Tibet and were ascribed to endemic fauna. Our study results show that the habitat area of Tibetian species is much wider. Based on the ecological data available for the species studied, the development conditions of a Late Pleistocene glacier-dammed lake in the Kurai Depression were reconstructed.