International Commission On Stratigraphy Research Articles

First U-Pb (CA-ID-TIMS) Dating of the Uppermost Permian Coal Interval in the Minusinsk Coal Basin (Siberia, Russia) Using Zircon Grains from Volcanic Ashfalls

This study presents the first U-Pb (CA-ID-TIMS) radioisotopic dating of zircon grains extracted from tonsteins within the uppermost Permian coal interval of the Minusinsk Coal Basin (Siberia, Russia). Petrographic, structural, and mineralogical analyses confirm the volcanic ash origin of the tonsteins. The parent pyroclastic materials are identified as rhyolite–pantellerite for tonstein I-22 and dacite–rhyodacite for tonstein I-12. Morphological analysis of zircon crystals, along with cathodoluminescence and melt inclusion studies, confirms their volcanic origin and crystallisation temperatures of 700–900 °C. New radioisotopic dates of 261.4 ± 0.7 Ma and 261.3 ± 0.4 Ma clarify the age of the Izykh Formation, enabling its direct correlation with the Capitanian Stage of the International Chronostratigraphic Chart. The results emphasise the possible discontinuity of the coal-bearing succession of Siberian palaeocontinent and highlight the potential for further stratigraphic refinement through continued radioisotopic dating of tonsteins.

The Global Humanization Event (GHE)

The recent rejection of the “Anthropocene Series/Epoch,” as proposed by the Anthropocene Working Group (AWG), by the International Commission on Stratigraphy (ICS) and the International Union of Geological Sciences (IUGS), has reopened discussion about the Anthropocene as a geological event. This event has been referred to as the “Anthropocene Event” (AE) or the “Anthropogenic Modification Episode” (AME). This essay briefly discusses these terms and concepts, and proposed to use new terms such as “Earth System Humanization Event” (ESHE) or “Global Humanization Event” (GHE), which are more specific and terminologically more consistent with the already defined geological events.

The Anthropocene Is More Than a Time Interval

AbstractFollowing the recent rejection of a formal Anthropocene series/epoch by the Subcommission on Quaternary Stratigraphy (SQS) of the International Commission on Stratigraphy (ICS), and its subsequent confirmation by the International Union of Geological Sciences (IUGS), the opportunity arises to reset the definition of the Anthropocene. The case for informally recognizing the Anthropocene to be a major planetary event of Earth system transformation offers a promising way forward, but this has been criticized by proponents of an Anthropocene series/epoch. In order to move on from the assumption that it must be a time interval, and to foster a more transdisciplinary and inclusive approach, the main points of the critique must be directly addressed.

Dating the Dawn of the Anthropocene

Abstract This article provides indepth insight into the debate (based on stratigraphic findings) as to the exact date that Earth entered the Anthropocene Epoch. Generally, in Human and Social Sciences, the date of the boundary is paid less attention than the systemic shifts resulting from it. (There have been a number of publications on our surpassing of the planet’s limits, and passing of the point of no return.) However, it is important to understand this debate based on stratigraphic data, because sooner or later, the International Union of Geological Sciences will amend the geological timescale to include the Anthropocene – even if, to date, the process of formal definition has been interrupted by the negative vote on 5 March 2024 of the Subcommission on Quaternary Stratigraphy (SQS) of the International Commission on Stratigraphy (ICS) after a positive vote in 2023 by the Anthropocene Working Group (AWG) proposing to date the onset of the Anthropocene in 1950 (with a “golden nail” in the Crawford lake beds). In future, almost all the world’s students will learn the date when we entered it. This article explores each of the possible dates when stratigraphic data suggest the Anthropocene may have started. This should hand tomorrow’s teachers the keys to understand what may sometimes seem an obscure expert debate.

A contribution to the knowledge of Cretaceous to Neogene Palynology in the Colombian Caribbean.

The palynological study of a composite section of 14 cores drilled from the Sinu-San Jacinto Basin in the Colombian Caribbean (NW South America) provides for the first time a Cenozoic palynostratigraphic zonation scheme for this region. In addition, the study of calcareous nannofossils and planktonic foraminifera allowed an independent calibration of the palynomorph assemblages to the international chronostratigraphic chart. The palynological results were compared with other studies for the northern part of South America and Colombia. Twelve Late Paleocene-Pliocene palynological zones are proposed for the Caribbean, some of which differ from those proposed for the Llanos and Llanos Foothills basins. A Paleocene palynological assemblage is reported for this part of the basin. Variations in the biostratigraphic ranges of some palynomorphs and new species with stratigraphic value are described. High diversity and abundance of dinoflagellate cysts in some stratigraphic intervals require more detailed study. Evidence of reworking in some stratigraphic intervals (e.g., Cretaceous and Oligocene in Miocene sediments), makes it necessary to be careful about the last appearance record of microfossils in exploration and geologic mapping.

Late Kungurian Radioisotope Age of the Boundary Between the Balakhonka and Kolchugino Groups in the Kuznetsk Basin (Western Siberia, Russia) – Additional Evidence for the Validity of the Ufimian Stage of the East European Permian

This article discusses the Late Kungurian radioisotopic age (276.9 ± 0.4 Ma) of the middle part of the Starokuznetsk Formation (Kuznetsk Subgroup, Kolchugino Group) of the Kuznetsk Basin determined by chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS). The analysis of the biostratigraphic data confirms that the layer dated belongs to the interval in which the Balakhonka Flora (cordaitoid) was replaced by the Kolchugino Flora (fern-pteridosperm-cordaitoid). This indicates that the change from the Balakhonka Flora to the Kolchugino Flora in the low latitudes of Angaraland took place during the Late Kungurian. The data obtained were used for direct correlation of the lower part of the Kolchugino Group with the Upper Kungurian of the International Chronostratigraphic Chart. Similar sequences of non-marine bivalve assemblages in the Permian successions of Angaraland (giant Prokopievskia, Khosedaella-Redikorella-Palaeomutela, and dominant Palaeomutela) and Eastern Europe (giant Sinomya, Palaeomutela-Khosedaella-Redikorella, and dominant Palaeomutela) further support the validity of the correlation of the Kuznetsk Subgroup with the Ufimian Stage. The placement of the lower boundary of the Kolchugino Group in the upper part of the Kungurian Stage of the International Chronostratigraphic Scale raises the question of the continuation of the Ufimian Stage as an independent straton in the East European Stratigraphic Scale. Its lower boundary coincides with the beginning of the change in the Balakhonka and Kolchugino Flora in the low latitudes of Angaraland, as well as with the faunal exchanges between the Euramerican and Angarian non-marine bivalve assemblages.

U-Pb Dating of the Kolchugino Group Basement (Kuznetsk Coal Basin, Siberia): Was the Change in Early–Middle Permian Floras Simultaneous at Different Latitudes in Angaraland?

The Kuznetsk Basin (Kuzbass) is one of the largest coal basins in Siberia and a reference area for the ancient Angaraland continent. The proximity of the Kuzbass and Siberian Platform caused their biotic similarities in the Late Palaeozoic. However, due to biota endemism, the Kuzbass Upper Palaeozoic does not correlate directly with the International Chronostratigraphic Chart (ICC). This paper discusses radioisotopic (CA-ID-TIMS) dating of zircons from a volcanic tuff located in the Starokuznetsk Formation (Fm). This level matches the interval of the Balakhonka/Kolchugino (B/K) floral change in Kuzbass, i.e., the gradual replacement of cordaitoid-dominated wet forests (Balakhonka flora) with more arid fern–pteridosperm–cordaitoid assemblages (Kolchugino flora). New age (276.9 ± 0.4 Ma) directly correlates the Starokuznetsk Fm with the Upper Kungurian of the ICC. We compared the Kuzbass data with data of the Western Verkhoyanie, where Middle Permian ammonoids (Sverdrupites assemblage) occur in strata recording the B/K floral change. The available (ICC) and new datings indicate the lag between the B/K floral change in low (Kuzbass) and high (Verkhoyanie) latitudes of Angaraland. The B/K floral change in the Kuzbass began in the early Late Kungurian and was completed by the end of this age. In contrast, the B/K floral change in Verkhoyanie began at the end of the Late Kungurian and was completed in the Late Wordian. The delay in the floral changes at different latitudes of Angaraland suggests that existing interregional correlations need further improvement.

УСОВЕРШЕНСТВОВАНИЕ ЗОНАЛЬНОЙ ДИАТОМОВОЙ ШКАЛЫ НЕОГЕНА ПРИМОРЬЯ (РОССИЯ)

Comprehensive study of diatoms from some Neogene stratotype sections of southern Primorye (Sineutesovskaya and Novokachalinskaya formations, Shufansky horizon) using light and scanning electron microscopy methods allowed adjustments to be made to the Zonal Diatom Scale of the Neogene in Primorye. Studies have shown the absence of some zone index species in the sediments: Alveolophora bifaria, A. jouseana, A. areolata. As a result, the corresponding subdivisions of the Scale received new names: Alveolophora hachiyaensis-Aulacoseira elliptica zone, Alveolophora khankaica zone, Alveolophora khursevichiae zone. The position of the Shufansky horizon zones has changed: the Aulacoseira praeislandica zone now corresponds to the Early Pliocene, and the Alveolophora tscheremissinovae zone to the Upper Pliocene. According to the International Stratigraphic Chart of the Cenozoic, the age of the upper boundary of the Alveolophora tscheremissinovae zone was revised to 2.58 Ma ago.

Emphasizing Lithostratigraphic Rules In Distinction Of Formal Units: Sinjar Vs Khurmala Formations, Case Study

Recognition, identification, and correlation of formal lithostratigraphic units should always follow the rules of the definition of lithostratigraphic units established in the Stratigraphic Code of Nomenclature, set by the International Stratigraphic Commission. This case study is attempting at elaborating on this argument by discussing the identification and differentiation between the Sinjar and Khurmala formations in outcrop and well sections. Stratigraphic data for this study were compiled basin-wide, with nearly fifty localities of both formations from the Zagros foreland basin of Kurdistan in NE Iraq. Examinations include a review of microfacies documentation, Facies reinterpretation as well as sequence analysis. The Sinjar and Khurmala formations are carbonate units developed during the final filling stage of the Zagros foreland basin of northeast Iraq. The Sinjar Formation is characterized by shallow marine coralgal-foraminiferal limestone of reefal, lagoonal, shoal, and overall ramp setting. Other lithologies such as dolomitic limestone or marly limestone are minor and secondary. The Khurmala Formation, on the other hand, is dominated by thick beds of dolostone with secondary lithologies including recrystallized limestone and dolomitic limestone and marly limestone. Microfacies analysis of the Khurmala Formation reveals that it represents a lagoonal lithofacies equivalent to the lagoonal facies of the Sinjar Formation but intensively dolomitized at specific areas of the basin. This close relationship between the two formations, leading in some cases to mixing between both lithologies or rather confusion and misidentification. The best way of differentiating between the two units is by adhering to the lithostratigraphic rules and principles of the formal stratigraphic description, which emphasize the formal lithostratigraphic description of each formation.

A geological perspective of plastic pollution

The Anthropocene, the most recent geologic time division, marks humanity's profound impact on Earth. Amidst debates, the Anthropocene Working Group recommended its inclusion in the International Chronostratigraphic Chart (ICC). This period is characterized by the mid-20th century Great Acceleration Event Array (GAEA), which includes widespread presence of pollutants such as radionuclides, organochlorine pesticides, PCBs, and plastic production. The Anthropocene concept should raise public awareness of these threats, with plastic pollution being a primary concern. Plastics are now pervasive and serve as a marker for the Anthropocene Epoch or Age. Understanding their entry into the geological record requires considering the “Plastic Geological Cycle,” which encompasses extraction, production, use, disposal, degradation, fragmentation, accumulation, and lithification. This cycle reveals the transformation of plastics into new forms of pollution characteristic of the Anthropocene. With 91 % of discarded plastics never recycled, they accumulate in the environment and enter the geological record through processes like photodegradation, thermal stress, and biodegradation. The proposed Plasticene stage within the Anthropocene is defined by the post-World War II surge in plastic production and their incorporation into sedimentary processes and rocks. The study of plastics in the geologic record serves as a warning of their negative impacts and highlights the urgency of addressing plastic pollution for a sustainable future.

The first CA-ID-TIMS U-Pb dating of the Tithonian/Berriasian boundary beds in a Boreal succession

The Tithonian/Berriasian (T/B) boundary has attracted increasing attention in recent decades, but there are currently only a few numerical ages for this boundary, nearly all of which are derived from sections located in Central and South Americas. Here we present the first reliable geochronological data immediately below the T/B boundary from the West Siberia sedimentary basin, based on the dating of zircon grains derived from a single tuff bed sampled from three different drill cores. This represents the first high precision dates from the latest Jurassic succession of the Panboreal Superrealm. The ages of the studied comagmatic zircon grains from the topmost middle Volgian (equivalent to the middle part of the upper Tithonian) tuffs, based on material from 3 wells, range between 141.83 and 141.11 Ma, where the youngest zircon is interpreted to represent the depositional age of the volcanic horizon. Based on detailed biostratigraphy, our studied succession can be correlated with the middle part of the M20n magnetochron, allowing correlation with coeval J/K boundary successions worldwide. Our data support a re-consideration of the T/B boundary age currently accepted in the international chronostratigraphic chart and are supported by previously published U-Pb ages from Argentina and Mexico.

Precambrian–Cambrian Transition at the Igarka Uplift (Northwestern Siberian Platform)

Abstract —The Precambrian–Cambrian transition is one of the most fundamental evolutionary turnovers in the Earth’s history. However, in many cases precise identification and correlation of this crucial boundary in lithologically contrasting sections is complicated, even if the whole spectrum of biostratigraphic and chemostratigraphic methods is applied. The Precambrian–Cambrian transitional strata of the Igarka Uplift (Sukharikha Formation) perfectly illustrate this problem. This unit has arguably one of the most detailed carbonate carbon isotope curves for this time interval (at least within the Siberian Platform). However, an extremely poor paleontological record previously reported from the Sukharikha Formation makes identification of major stratigraphic boundaries highly debatable. We present a detailed study of the Sukharikha and overlying Krasnyi Porog formations in three sections (Sukharikha River, Kulyumbe River, and Khantaisko-Sukhotungusskaya-1 well). Our chemostratigraphic and biostratigraphic data provide a correlation basis for these sections and identify the Cambrian boundary in terms of both the International Chronostratigraphic Chart and General Stratigraphic Scale of Russia. We show that lithologic boundaries and levels of the local first appearance of Tommotian small skeletal fossils are diachronous within the Igarka paleobasin. The latter is putatively caused by paleoecology of the early Cambrian biota and by taphonomic factors. Our data specify the location of the Tommotian Stage (sensu lato)/Cambrian Stage 2 boundary in other key Precambrian–Cambrian transitional sections of the Siberian Platform.

The Ordovician of Central Asia (Kyrgyzstan, Uzbekistan and Tajikistan)

Abstract The region of Kyrgyzstan, Tajikistan and Uzbekistan includes five first-order tectonic units with an Early Paleozsoic sedimentary record, comprising North Tien Shan, Karatau Naryn, Turkestan–Alai, Zeravshan–Hissar and the Central Pamirs. Available palaeobiogeographical and palaeomagnetic data suggest that these were widely dispersed in the Ordovician. North Tien Shan, Karatau Naryn, Turkestan–Alai were separate microcontinents located in the low southern latitudes throughout the Ordovician in relative proximity to the western Gondwana margin. Zeravshan–Hissar and the Central Pamirs were also parts of the Gondwana supercontinent but were located in temperate latitudes. The geological literature on the Ordovician of the region is assessed to provide an updated palaeontological record, outline of lithostratigraphy and biostratigraphic correlation based on the International Chronostratigraphic Chart. The Ordovician biostratigraphy of Central Asia is mainly graptolite-based; however, that record is discontinuous, and the absence of detailed faunal logs and lack of monographic studies causes difficulty in precisely locating system and stage boundaries. Although an extensive faunal record has been documented, often it is based on preliminary taxonomical identifications which are not reliable for high-resolution biostratigraphy and tracing biodiversity patterns.

The Ordovician of the Middle East (Iran, Afghanistan, Pakistan)

Abstract Ordovician studies in Iran have shown significant progress since the beginning of the century. A number of individual faunas have been documented and a biostratigraphical framework based on conodonts, chitinozoans, acritarchs and trilobites developed. Correlation of Ordovician successions with the International Chronostratigraphic Chart has been significantly improved, and the position of the series and stage boundaries can be recognized with greater precision. While geographical proximity to temperate latitude Gondwana is apparent for most Iranian terranes, biogeographical links of Alborz and Kopet-Dagh with South China prevailed through the Early–Middle Ordovician. In Pakistan, Ordovician deposits have a restricted distribution in the Karakorum block (Chitral). Here they are represented by the Yarkhun and Vidiakot formations with Floian–Darriwilian acritarchs, chitinozoans and early Darriwilian conodonts. In Peshawar District of the North-West Frontier Province, an Early–Middle Ordovician age is likely for the Misri Banda Quartzite with Cruziana rugosa trace fossils. It is overlain conformably by carbonates of the Panjpir Formation, which has an inferred Middle Ordovician–Silurian age. Presently available information on the Ordovician of Afghanistan is mostly based on reconnaissance studies performed almost half a century ago, and a few monographed Early and Late Ordovician faunas.

Ophiolite association of Cape Fiolent (western part of the Mountainous Crimea) – the upper age constraint according to the U-Pb isotope dating of plagiorhyolites (Monakh Cliff)

The article presents the results of U-Pb isotope dating (SHRIMP-II, VSEGEI, Saint Petersburg) of zircon crystals extracted from plagiorhyolites of the Monakh Cliff in the area of Cape Fiolent in the western part of the Mountainous Crimea (southern suburb of Sevastopol). a concordant age estimate of 168.3±1.3 Ma was obtained from 20 zircon crystals. It exactly corresponds to the Bajocian/Bathonian boundary of the Middle Jurassic according to the International Chronostratigraphic Chart (February 2022 version). The available results of isotope dating of igneous rocks from the Mountainous Crimea, as well as their geochemical typification are synthesised. The plagiorhyolites of the Monakh Cliff in the area of Cape Fiolent are spatially, and most likely paragenetically, associated with the wallrock (Cape Vinogradny) and ore (Heraclea Plateau on the cognominal peninsula) massive sulphide formations, as well as pillow basalts, gabbroids, and serpentinized hyperbasites, combined into the ophiolite association of Cape Fiolent. The obtained dating is the upper age limit for the entire ophiolite association of Cape Fiolent.

High-Resolution Triassic Biostratigraphy of the Kotelny Island (New Siberian Islands, Arctic Siberia)

Abstract The study of Triassic paleontology and stratigraphy of various regions of northeastern Russia and adjacent Arctic shelf is essential not only for improving and refining zonal biostratigraphic schemes, interregional and global correlation of Triassic deposits, and resolving problems of stratigraphic boundaries but also for developing and substantiating a new generation of Triassic stratigraphic schemes, which could serve as the stratigraphic basis for different regional and detailed geological investigations of the Arctic. The results of the study were used to improve existing zonal scales based on various groups of fauna and palynomorphs, develop a more detailed biostratigraphic subdivision of the Triassic, and characterize individual horizons using both terrestrial and marine palynomorphs. The zonal scales are calibrated to each other and to the regional zonal scale of the Triassic of Siberia and northeastern Russia, which provides the subsequent correlation with the International Chronostratigraphic Chart of the Triassic System. The set of coeval zonal scales for the Triassic of Kotelny Island sections based on ammonoids, nautiloids, coleoids, bivalves, brachiopods, and foraminifers and the analysis of microphytoplankton and terrestrial palynomorph assemblages are a useful tool for detailed subdivision and correlation of the eastern part of the Laptev Sea shelf and adjacent regions of northeastern Russia.

Attempting discussion of the International Chronostratigraphic Chart after the Global Standard Boundary Stratotype Section and Point

<p indent=0mm>To date, approximately two-thirds of the Phanerozoic chronostratigraphic boundaries have been defined in terms of the Global Standard Boundary Stratotype Section and Point (GSSP) concept in the International Chronostratigraphic Chart. However, in practice, almost all definitions and correlations of these boundaries are controversial, and some of the definitions are indeed questionable. Consequently, the definitions of some boundaries for the standard of correlation become almost meaningless, and it is therefore necessary to reexamine the guiding ideology and research methods for determining stratigraphic boundaries. In addition, the current International Chronostratigraphic Chart is no longer able to meet the need for accuracy of the temporal and spatial framework for Earth system science. Therefore, it is necessary to establish a new generation of the International Chronostratigraphic Chart after the GSSP. Given the problems encountered in the establishment and practice of the International Chronostratigraphic Chart, this paper proposes that the International Chronostratigraphic Chart after the GSSP should be developed to solve the following issues: (1) Improving the time resolution; (2) improving the identifiability of stratigraphic boundaries; (3) improving the isochronism of stratigraphic boundaries; (4) establishing a standard for chronostratigraphic boundaries that is suitable for accurate correlation of different sedimentary facies in different regions; (5) establishing objective, repeatable, and verifiable criteria for the definition and selection of GSSPs; and (6) ensuring that the chronostratigraphic boundaries reflect the historical stages of the evolution of Earth. The present paper attempts to combine biostratigraphy and event stratigraphy to define chronostratigraphic boundaries, and takes the geological event surface as an important criterion for defining, selecting, and identifying GSSPs. In the specific approach, the speciation event of the first widely distributed species above the top surface of the event stratigraphic unit, that is, the base of the first widely distributed biozone above the top surface of the event stratigraphical unit, should be selected as the biostratigraphic definition of a chronostratigraphic boundary (the first criterion). The section with the shortest distance between the first appearance datum (FAD) of the index fossil selected to define a chronostratigraphic boundary and the top surface of the event stratum should be chosen as the boundary stratotype section (the second criterion), and the FAD of the index fossil in the section is the GSSP. The top surface of the event stratum below the index fossil, as the biostratigraphic definition of a chronostratigraphic boundary, is an important reference criterion for identification and correlation of chronostratigraphic boundaries. Within the framework of biostratigraphy, the records of all types of geologic events and other strictly isochronous natural boundaries are used for further division and more accurate correlation of deposits in detail, which may improve the resolution of the stratigraphic division and the identifiability of stratigraphic boundaries. This would allow a new generation of the International Chronostratigraphic Chart to reach the same time scale as the Quaternary (tens to hundreds of thousands of years) and improve the identifiability of chronostratigraphic boundaries in the field to benefit geological mapping.

Establishing the Lower-Middle Jurassic Boundary in the UNESCO Global Geopark Molina & Alto Tajo (Central Spain)

The Global Boundary Stratotype Section and Point (GSSP) of the Aalenian Stage (base of the Middle Jurassic), was established in Fuentelsaz (Central Spain) by the International Commission on Stratigraphy (ICS) of the International Union of Geological Sciences (IUGS) in 2000. This stratotype is one of the most important geosites of the Molina-Alto Tajo UNESCO Global Geopark. Since the declaration of the Geopark in 2015, several conservation and geotouristic initiatives have been developed for the GSSP, including its protection as Natural Monument.

The Pridoli (Silurian) Lithostratigraphy and Biostratigraphy of Gorny Altai

Abstract —The paper presents new data on the upper Silurian litho- and biostratigraphy of the Gorny Altai area. Sediments within this interval store a succession of taxonomically representative middle–upper Ludfordian, lower Pridoli, and Lower Devonian (Lochkovian–Pragian) conodont assemblages. The new fauna constraints made a basis for updated correlations of the local and regional stratigraphic units at the Silurian/Devonian boundary of Gorny Altai with the stages of the International Stratigraphic Chart. The correlation results reveal a mismatch between the boundaries of the local and regional Silurian units and the respective boundaries of stages in the International Stratigraphic Chart.

Neoproterozoic Deposits of the Bashkir Mega-Anticlinorium (Southern Urals): State of the Art in Regional Stratigraphy

Abstract—This paper presents an alternative variant of stratigraphic subdivision of the upper Riphean–Vendian deposits (approximately corresponding to the Neoproterozoic Erathem of the International Chronostratigraphic Chart (ICC)) of the Bashkir Mega-Anticlinorium (BMA), based on the modern concepts of changes in the biosphere, atmosphere, and hydrosphere (and, accordingly, in the climate and the course of sedimentary processes) in the period ~1000–540 Ma, with regard to the results of recent studies and the proposals for improving the Russian General Stratigraphic Scale (GSS) of the Precambrian. The strict necessity of subdividing chronostratigraphic and lithostratigraphic units is shown. The presented data give grounds to regard the Zil’merdak Formation as a Group and assign three of its lower formations to the Middle Riphean (Yurmatinian). It is shown that the period of accumulation of the Karatau Group (comprising the Katav, Inzer, Min’yar, and Uk formations) was significantly shorter than the duration of the Late Riphean (Karatavian). Special attention is focused on the age of the Uk Formation (analysis of the existing data made it possible to refine its stratigraphic position): It formed, most likely, in the period 780–740 Ma. A high uncertainty of the stratigraphic position of the overlying geologic bodies is demonstrated. The Bakeevo, Tolparovo, Suirovo, and Kurgashla formations must be regarded as part of the Terminal Riphean (Arshinian). The Arsha Group, comprising the Bainas, Makhmutovo, Igonino, and Shum formations in the Tirlyan trough and represented by the Krivaya Luka Formation in the Krivaya Luka syncline, should not be totally assigned to the Terminal Riphean. It is proposed to exclude the Bakeevo Formation and the Tolparovo–Suirovo sequence from the Asha Group, because these deposits are, most likely, a modification of the Arsha Group located on the western flank of the BMA. The Uryuk Formation is probably of pre-Vendian age. The current contradictory data on the stratigraphy of the supra-Uryuk unit of the Asha Group are interpreted. The ages of the geologic bodies composing the Group and its stratigraphic interval still call for a thorough study.