Home – Home – Jornals – Russian Geology and Geophysics 2015 number 4

The history of the development of the General Stratigraphic Scale (GSS), its significance, purpose, and difference from the International Stratigraphic Scale (ISS) are briefly reported. The most global achievements in the GSS refinement based on the Proceedings of the All-Russian conference at the Geological Institute of the Russian Academy of Sciences (May 2013) are considered. The main problems related to the GSS improvement are formulated: the structure of the Precambrian and Quaternary stratigraphic scales, the location of the Vendian System in the GSS, formal substantiation of the “Russian” stages of the Cambrian, Carboniferous, and Permian Systems, and the significance of the regional limitotypes of the ISS subdivisions.

The International Stratigraphic Chart of the Phanerozoic is constructed by drawing lower boundaries of stratigraphic units with reference to Global Boundary Stratotype Section and Point . Some systems retain former, usually European, names of stages, whereas new sequences of stages are proposed for others, such as Cambrian and Ordovician. The boundaries of 64 stages of 100 had been approved by the beginning of 2014. Fifty-three of these boundaries were drawn with regard to biostratigraphic criteria; one, ichnological; six, magnetostratigraphic; two, chemostratigraphic; one, impact; and one, climatic criteria. Their type sections are distributed over continents as follows: thirty-nine in Europe, twelve in Asia, nine in North America, and four in Africa. They are located in 19 countries: ten in China, nine in the United Kingdom, nine in Italy, seven in the United States, six in France, five in Spain, three in Czech, two in Morocco, two in Sweden, and one in each of several more countries. Although none is in Russia, candidates are proposed for stages of the Cambrian, Carboniferous, Permian, Triassic, and Jurassic Systems. Type sections for the Sakmarian and Artinskian have already been agreed. With the present work speed, it will take two or three decades to complete the scale construction. The General Stratigraphic Scale of Russia and the International Chart gradually converge. Their similarity at the stage level is 84%. Major disagreements exist only in the Cambrian and Permian Systems.

Some contradictory positions and concepts of the theory and practice of stratigraphy regarded as “paradoxes” are discussed. Attention is paid to the fundamental impossibility to associate the wide geographic distribution of a group of organisms with a high rate of speciation of these organisms. However, it is this feature that is commonly used as the basis for determining orthostratigraphic groups of fossil organisms. The dualism of time in stratigraphy, which led to the emergence of chronostratigraphy and the GSSP concept, is considered. The incompatibility of chronological and chronometric datings in stratigraphy is proved. Finally, the paradox of the smallest chronological unit is examined, which is related to the existence of not one but several smallest units (zones) in zonal chronology.

Glaciations took place in five long intervals of the geologic history, called glacioeras: Kaapvaal (Late Archean), Huronian (Early Proterozoic), African (Late Proterozoic), Gondwanan (Paleozoic), and unfinished Antarctic (Late Cenozoic). The glacioeras were similar in structure, duration, and dynamics of evolution. They consisted of three to six glacioperiods including several discrete glacio-epochs. The glacioeras lasted ~200 Myr. They started with small regional glaciations, which expanded, reached intercontinental sizes, and then quickly degraded. There were serious differences between the Precambrian and Phanerozoic glacioeras. A series of ecologic crises related to numerous glacial events led first to abiotic and then to biotic factors. Glaciations caused extinction and stagnation of the Earth’s biota, the appearance of crucial bionovations and new biota, and acceleration of evolution processes. Thus, the glacioeras were the turning intervals of the biosphere evolution.

The Vendian was proposed by B.S. Sokolov as a stratigraphic subdivision comprising the last of the Proterozoic glacial periods (the Laplandian Glaciation) and the overlying strata delineated by a full stratigraphic range of fossil soft-bodied organisms. For over three decades the Vendian had been an informal part of the Standard Global Chronostratigraphic Chart, until 2004, when it gave place to the new Ediacaran System. Further research has shown that the Ediacaran System significantly exceeds the stratigraphic range of the Vendian in Sokolov’s definition and includes stratigraphic analogs of the Laplandian Glaciation in sections across North America, Australia, Newfoundland, Scotland, Ireland, Chinese Tien Shan Range, and Tasmanian microcontinent. Carbon isotope variations in carbonates provide criteria for subdivision of the Ediacaran into two series. If a relationship between the Laplandian Glacial Period (600-580 Ma) and the negative excursions EN2 and EN3 on the δ ¹³С curve for the Doushantuo Formation of China is established, the Vendian might take its place in the Standard Global Chronostratigraphic Chart as a formal upper series of the Ediacaran System. The Vendian Series, in turn, might be further subdivided into the Laplandian, Redkinian, Belomorian, and Kotlinian stages typified by regional stages of the Vendian of the East European Platform.

Fossiliferous Upper Vendian strata are discovered in the Upper Proterozoic to Lower Paleozoic Fore–Yenisei sedimentary basin under a thick Mesozoic–Cenozoic cover in the southeastern West Siberia. Two depositional systems are recognized based on sedimentological features: (1) wave– and current–agitated shoreface–forereef–biohermal reef system (Vostok-3 Borehole section) and (2) tidal–flat–evaporite basin (Averinskaya–150 Borehole section). The forereef facies yielded fossilized tubular calcareous skeletons of reef–building metazoans Cloudina riemkeae, Cloudina hartmannae, and Cloudina carinata, phosphatized Namacalathus–like fossils, and a diversity of tubular phosphatized and agglutinated tubular fossils. The fossil assemblage can be interpreted as the evidence of ecological complexity of the reef system. Paleontological characteristics suggest correlation of the Vendian strata with the lowermost Purella antiqua Assemblage Zone and the boundary interval with the underlying Anabarites trisulcatus Assemblage Zone of the Siberian Platform. Therefore, at least in the late Proterozoic, the Fore–Yenisei sedimentary basin was part of a larger pericratonic depositional system on the western margin of the Siberian Platform.

The first biozone (Anabarites trisulcatus Assemblage Zone) in the Siberian hypostratotype of the Vendian (northwestern slope of the Olenek Uplift) is represented by the Turkut Formation of the Khorbusuonka Group and most of the Syhargalakh Formation of the Kessyusa Group. The lower part of the Kessyusa Group in some of the sections includes stratiform breccia coeval with the middle part of the Syhargalakh Formation. The breccia is shown to be the alteration product of tuff breccia which is widely distributed in the region and occurs as diatremes. A U–Pb zircon date of 543.9 ± 0.24 Ma for tuff breccia provides the best constraint on the age of the boundary between the Anabarites trisulcatus and Purella antiqua Assemblage Zones. The first appearance of small skeletal fossils Cambrotubulus decurvatus (which define the base of the Anabarites trisulcatus Assemblage Zone) is 1.4 m above the lower boundary of the Turkut Formation. Ichnofabric in the underlying Khatyspyt Formation is globally distributed in the 553–551 strata Ma, always predating the first appearance of small skeletal fossils of the Anabarites trisulcatus Assemblage Zone. The base of the Anabarites trisulcatus Assemblage Zone is therefore younger than 553–551 Ma, whereas the duration of the assemblage zone does not exceed six million years.

Based on the study of the litho- and biofacies of the Vendian Nepa Horizon in the central area of the Siberian Platform inland, a paleoecological model for the Vendian microbiota has been developed. The sedimentation environments of the Katanga saddle have been reconstructed, and three sedimentary systems have been recognized: (1) lower continental, formed by the deposits of proluvial fans and riverbeds of temporary streams; (2) middle transgressive, made up of littoral sand facies in the lower part and of fine-clastic shelf strata in the upper part; and (3) upper, of sea highstand, composed of alternating sand bank facies and fine-clastic lagoon deposits. Four biofacies have been recognized in the fine-terrigenous deposits of the Nepa Horizon: (1) Appendisphaera, represented by a Doushantuo-Pertatataka acanthomorph assemblage; (2) Transitional, with a great diversity of plankton and benthic (including complex) taxa; (3) Vanavarataenia, dominated by Vanavarataenia complex benthic algae; and (4) Oscillatoriopsis, represented by taxonomically poor biotas with morphologically simple (mainly prokaryotic) remains. These biofacies are confined to the following sedimentation environments: Appendisphaera is widespread in the distal open-sea areas; the Transitional biofacies is localized in the distal settings of the semi-isolated inner basin; Vanavarataenia occurs in the proximal areas; and Oscillatoriopsis is typical of the shallow-water environments, both extended (corresponding to the highstand period) and local.

We summarize data on the biostratigraphic units of the Siberian Ordovician deposits based on pelagic groups of fauna: graptolites, conodonts, and chitinozoans. It is shown that graptolite and conodont zones and beds have a high potential for correlation. We have determined the precise zonal position of most of the lower boundaries of the Ordovician stages and informal Ordovician substages of the International Stratigraphic Chart in the Lower Paleozoic key sections of the Taimyr Peninsula, Siberian Platform, and Altai-Sayan Folded Area.

Based on the inventory, revision, and analysis of stratigraphic, paleontological, and lithofacies data on the area of Ordovician deposits in the southern Siberian Platform (Irkut amphitheater), the refined and detailed schemes of biofacies zonation of this paleobasin are substantiated. Schemes of zonation have been compiled for the Nyaian, Ugorian, Kimaian, Mukteian, Volginian, Kirenskian-Kudrinian, Chertovskian, and Baksanian Horizons of the regional Ordovician stratigraphic chart of the Siberian Platform. The schemes present biofacies different in lithologic composition, spread of dominant groups of fauna, and hydrochemical regime (normal-marine salinity, freshwater, or high salinity). It is shown that contrasting changes in paleogeographic environments and the spread of faunal communities under changing environmental conditions were influenced by the transgression-regression cyclicity of the evolution of the Siberian Platform paleobasin and by the proximity of the land. Specific groups of fauna localized in particular facies are described. These groups are regarded as communities that were the first to occupy the littoral zones of epicontinental sea basins in the Ordovician.

It is suggested to perform chronostratigraphic division using type sections instead of boundary stratotypes, which are rather virtual units devoid of material content representing the respective deposition events. In this respect, global chronozones are more preferable for subdivision of stages than biozones poorly constrained in space and time. The stratotype-based approach to division of stages is advantageous, being associated with the deposition of chronostratigraphic units recorded in holo- and hypostratotypes.

There is no international consensus regarding the GSSP for the Berriasian, the basal stage of the Cretaceous System. Any of the events discussed by the international expert community can be regarded as a marker of the Jurassic/Cretaceous boundary: a phylogenetic change of taxa, paleomagnetic reversal, or isotopic excursion. However, the problem of identification of this level in Boreal sections can be solved only using a combination of data obtained by paleontological and nonpaleontological methods of stratigraphy (bio-, chemo-, magnetostratigraphy, etc.). With any of the accepted markers, the Jurassic/Cretaceous boundary in Siberian sections will be within the upper part of the regional Bazhenovo Horizon. The least interval of the uncertainty of the position of this boundary in Siberian sections will be ensured by the selection of one of two markers: biostratigraphic (base of the Pseudosubplanites grandis Subzone) or magnetostratigraphic (base of the M18r magnetozone).

Jurassic strata are widespread through Arctic Siberia and host oil and gas fields. However, in most cases, the geology of such vast areas still remains unexplored, and study of the Jurassic stratigraphy and reconstructions of geologic history are possible only through analysis of sediment cores. In this connection, there is a clear need for detailed studies of microfaunas (foraminifera, ostracods) and palynomorphs (dinocysts, spores, and pollen). The studied reference section of the Upper Jurassic and Lower Cretaceous is located on the left side of Anabar Bay of the Laptev Sea (Nordvik Peninsula, Cape Urdyuk-Khaya). An uninterrupted and continuous section from the Upper Oxfordian to the Lower Valanginian is exposed in coastal cliffs and consists mainly of silty clay deposits with abundant macro- and microfossils. Integrated field studies (biostratigraphy, lithostratigraphy, sedimentology) allow a more detailed characterization of the regional geologic framework. A detailed subdivision of the section is based on the systematic composition of ammonites from Upper Oxfordian and Kimmeridgian deposits. Several foraminiferal zones of the Upper Oxfordian and Lower Volgian are defined, and some of them are denfined for the first time. The distribution of ostracods in the section is analyzed for the first time. The section is also studied using palynological analysis, that results in its detailed subdivision on palynological data and recognition of two sequences of palynostratigraphic units. The integrated stratigraphy is used to establish the precise position of stage and substage boundaries. The continuity of the section is defined based on micropaleontological and palynological data.

The Quaternary System/Period represents the past 2.58 million years and is officially subdivided into the Pleistocene and Holocene series/epochs, with the base of the Holocene assigned an age of 11,700 calendar years before AD 2000. The lowest two stages (ages) of the Pleistocene, the Gelasian (base 2.58 Ma) and the Calabrian (base 1.80 Ma), have been officially approved and constitute the Lower Pleistocene Subseries/Subepoch. The Middle and Upper Pleistocene have yet to be formally defined, which is a crucial future challenge, along with the subdivision of the Holocene, consideration of the Anthropocene, and fine-scale subdivision of the Quaternary.