Redistribution of Au in volcanic ashes in the course of their postsedimentary alterations

The redistribution of Au in Late Precambrian volcanic ashes from the Vendian section of the East European Platform and the Sinian section of the South China Platform is characterized by high Au concentration (up to >20 ng/g) in Fe³⁺-rich samples. The Au content decreases to ～2 ng/g in Fe²⁺-rich volcanic ashes from the Vendian section of the East European Platform. This distribution pattern suggests the mobility of Au-bearing compounds in postsedimentary processes.     

Alkali earth elements in Vendian-Cambrian clayey rocks, the East European Platform

Low Sr and Ca contents in fine-grained terrigenous rocks of the Vendian sedimentary cover in the East European Platform may be related to the following reasons: (1) the absence of authigenic carbonates in Vendian mudstones owing to desalination of epiplatformal sedimentary basins; (2) significant contribution of the chemically weathered rocks in the formation of the Vendian cover of the East European Platform.     

Variations of Eu/Eu* ratio in Vendian shales of the Volga-Ural region and the boundary traced between the Redkino and Kotlin Horizons

Variations of Eu/Eu* ratio in shales from the upper Vendian Kairovo and Shkapovo groups of the Shkapovo-Shikhan depression, the Volga-Ural region, are considered. Shales in the lower part of the Karlin Formation are established to be similar in geochemistry to the shale member with abnormally low Eu/Eu* ratios at the base of the Upper Vendian Kotlin Horizon of the East European Platform. According to these data, the studied formation appears to be a lithostratigraphic unit of the Kotlin Horizon, while the Baikibashevo, Staropetrovo and Salikhovo formations belong to the Redkino Horizon of the upper Vendian.     

Rb-Sr and K-Ar indicators of the transformation of precambrian clayey rocks in the East European Platform

Rb-Sr and K-Ar characteristics of Vendian and Upper Riphean sections in various structural units of the East European Platform are studied. It is shown that Neoproterozoic clayey rocks of the platform underwent postsedimentary transformations, primarily owing to processes of K accumulation (illitization). Their intensity decreases with depth and isotopic signatures of provenance rocks are partly retained in Riphean rocks. Stages of the most active transformations approximately 400 and 600 Ma ago are manifested in the Vendian clayey rocks. Events of approximately 1000 Ma ago are recorded in Riphean rocks of the East European Platform. The least altered rocks of the Pachelma aulacogen can retain ancient hydrocarbon pools.     

Volcanic Tuffs,Red- and Usual-Colored Clayey Rocks in Upper Riphean–Vendian Deposits in the Middle and Southern Urals: Comparison of Lithochemical Characteristics

Lithology and Mineral Resources - In the lower part of Upper Vendian deposits on the East European Platform, packs of chocolate-brown clayey rocks/tuff-argillites containing intercalations of...     

Late Vendian Kotlinian Crisis on the East European Platform: Lithogeochemical Indicators of Depositional Environment

Lithology and Mineral Resources - Sharp changes in the biodiversity of fossil organisms in the Upper Vendian of the East European Platform are considered as the manifestation of global crisis...     

Redkinian Biota of Macroscopic Fossils from the Northwestern East European Platform (South Ladoga Region)

The stratigraphic distribution of microfossils and macroscopic fossil biota in Vendian deposits of the South Ladoga region (northwestern East European Platform) is analyzed. In the sequence of the Shotkusa- 1 well, three taxonomically heterogeneous microfossil assemblages are distinguished: two of them refer to the Redkinian age (Starorusskaya Fm.) and one to the Kotlinian age (Vasileostrovskaya Fm.). Deposits of the Starorusskaya Fm. contain Redkinian biota of macroscopic fossils, of which the most characteristic representatives are Chuaria circularis, Doushantuophyton lineare, Morania zinkovi, Orbisiana simplex, and Redkinia spinosa. These new findings expand the paleontological characteristics of Upper Vendian deposits, also providing additional criteria for distinguishing the Redkinian horizon in the northwestern East European Platform.     

The Darriwilian acritarch assemblage from Ordovician deposits of the Arkhangelsk Oblast, the northern Russian Plate

Borehole 2506 drilled in the northern area of the Arkhangelsk Oblast penetrated through the Paleozoic sedimentary block isolated in the Vendian thick sequence. A diverse acritarch assemblage has been established within the depth interval of 119.9–217.5 m. The assemblage comprises more than 70 taxa, including species characteristic of the boundary interval between the Volkhov and Kunda horizons of the East European Platform (the graptolite Didymograptus hirundo Zone). Stratigraphic position of host deposits was established within the Darriwilian Stage of the Middle Ordovician. The described assemblage of microphytofossils is similar to coeval assemblages from NW Russia, Baltic region, and Scandinavia, being typical of the Baltic phytoplankton province of temperate latitudes. A great number of species in common suggests that the assemblage under consideration is correlative with coeval assemblages of southern China thus offering a possibility of remote correlation.     

The Yudomian of Siberia,Vendian and Ediacaran systems of the International stratigraphic scale

In Russia, the terminal Neoproterozoic formally includes the Vendian of western part of the East European platform and the concurrent Yudoma Group of Siberia. As is shown in this work, the designated subdivisions correspond in the stratotypes only to the upper, Yudomian Series of the Vendian. In the Siberian platform, the Ust-Yudoma and Aim horizons of the Yudomian are tightly interrelated. The lower of them, bearing remains of Ediacaran Fauna, represents the Ediacarian Stage, whereas the upper one containing small-shelled fossils (SSF) corresponds to the Nemakit-Daldynian Stage divided into the trisulcatus and antiqua superregional zones. In more complete sections of the platform periphery, sediments of these subdivisions conformably rest on siliciclastic succession that should be ranked as basal subdivision of the Yudomian. The succession is concurrent to the Laplandian Stage of the East European platform. According to geochronological dates obtained recently, the Yudomian Series spans interval of 600–540 Ma. In the East European platform, the Upper Vendian (Yudomian) begins with the Laplandian basal tillites of synonymous stage. In the west of the platform, tillites are dated at 600 Ma like the Upper Vendian base in Siberia. The next Ediacarian Stage of the East European platform is stratigraphic equivalent of the Redkino Horizon, while summary range of the Kotlin and Rovno horizons is concurrent to that of the Nemakit-Daldynian Stage. The Vendian of Russia is conformably overlain by the Tommotian Stage of the Lower Cambrian. Intense pre-Vendian events constrained distribution areas of the Lower Vendian sediments in Russia. The Lower Vendian deposits of the East European platform are most representative and well studied in the central Urals, where they are attributed to the Serebryanka Group. In Siberia, separate subdivisions representing the Lower Vendian are the Maastakh Formation of the Olenek Uplift, two lower members of the Ushakovka Formation in the Baikal region, and the Taseeva Group of the Yenisei Range. Chronological interval of the Lower Vendian corresponds to 650–600 Ma. The Marinoan Glaciation dated in Australia at 650–635 Ma is concurrent to basal part of the pre-Yudomian interval of the Vendian in Russia, whereas the Laplandian Tillite and Gaskiers Glaciation (600–580 Ma) correspond to onset of the Yudomian Epoch. The new Ediacaran System (Knoll et al., 2004) legalized in the International Neoproterozoic scale is close in range to the entire Vendian (635–544 Ma), although without basal beds (Marinoan Tillite) it deprives the terminal Neoproterozoic of its original sense. Inferiority of the system consists also in its indivisibility into stages. Hence, it is clear that the Vendian System subdivided in detail in Russia should be retained in the rank of terminal system of the Precambrian, one of the basic in general scale of the Neoproterozoic.     

Lithostratigraphy and geochemistry of Upper Vendian‒Lower Cambrian deposits in the northeastern Baltic monocline

The results of investigations of Upper Vendian?Lower Cambrian deposits in the northeastern part of the Baltic monocline specify views on the evolution of depositional environments of sedimentary successions constituting the basal part of the sedimentary cover in inner areas of the northwestern East European Platform. It is shown that the Late Vendian and initial Cambrian were characterized by the consecutive influx of relatively mature terrigenous detrital material that originated from both the weathering crust of the Baltic Shield and new sources. Its deposition was interrupted by notable, although likely asynchronous, hiatuses, which are registered at the base of the Upper Vendian Vasileostrovskaya and Voronkovo formations and Lower Cambrian Lomonosov Formation. In the Late Vendian, sedimentary material was transported from the Baltic Shield, while beginning from the initial Early Cambrian the additional contribution to the formation of the sedimentary cover of the Baltic monocline was provided by coarse-grained sedimentary material from the Timan margin of the Baltica as follows from U?Pb isotopic ages obtained for detrital zircons. At the same time, lithogeochemical parameters of fine-grained rocks experienced no substantial changes.     

New data on Late Vendian problematic fossils from the genus Harlaniella

The morphology and stratigraphic distribution of Harlaniella podolica Sokolov, 1972 and H. ingriana Ivantsov, sp. nov. were investigated using the collection of impressions sampled recently in Upper Vendian sections of Russia (southeastern White Sea region) and Ukraine (Podoliya). These fossils, which were previously considered as representing coprolites or grazing or locomotion traces, are interpreted as internal casts and impressions of fragments of tubes of initially organic composition. Streptichnus narbonnei Jensen et Runnegar, 2005 from Vendian-Cambrian boundary strata of Namibia are also attributed to this group. It is assumed that the tubes are similar to remains of the algal genera Vendotaenia Gnilovskaya, 1971 from Vendian deposits of the East European Platform and Liulingjitaenia Chen et Xiao, 1992 from Sinian sections of China.     

Sedimentation Conditions at the Central East European Platform in Riphean and Early Vendian

Recent lithological and geophysical studies of Riphean and Lower Vendian sedimentary rocks in the Kresttsy and Mid-Russian (Soligalich) aulacogens resulted in the recognition of four stages in the evolution of the East European Platform in the Late Proterozoic: (1) late Early Proterozoic–Early Riphean stage (formation of the protoplatform cover); (2) Middle Riphean stage (rifting only at platform margins); (3) late Middle Riphean–initial late Riphean stage (formation of the paleoplatform cover related to the existence of the epi-Grenville Rodinia supercontinent, which united all continents of the Earth at that time); (4) latest Riphean–early Vendian stage (rifting and origination of the Central Russian aulacogen system during the breakup of Rodinia and Cadomian orogeny.     

The room for the Vendian in the International Chronostratigraphic Chart

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 5¹³C 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.     

Lower Vendian microfossil assemblages of East Siberia: Significance for solving regional stratigraphic problems

Both published and original data are used for the analysis of the stratigraphic position and taxonomic characteristics for acanthomorphic microfossils of the Pertatataka type in Lower Vendian sediments of the central and southeastern Siberian Platform and its surrounding structures. Four taxonomically different microbiotas are distinguished in sections of the Nepa-Botuoba, Fore-Patom; Syugdzher, Anabar, Zhuya-Lena, and Berezovo lithotectonic zones: two diverse (Nepa and Ura) and two impoverished (Otradnino and Torga) each including diagnostic Early Vendian taxa. Beyond the Siberian Platform, genera and species characterizing these microbiotas occur in single assemblages and represent diagnostic taxa of the second (Tc-Sr-Vl) and third (Ti-Cg-Mp) zones of the acritarch scale proposed by Australian geologists as a biostratigraphic basis for subdividing the Ediacarian of the Standard stratigraphic scale (SSS). In the Russian General stratigraphic scale, the distribution of Pertatataka acritarchs corresponds to the Lower Vendian. The appearance and mass development of acanthomorphic microorganisms represents a global biological event, which should be taken into consideration as a biostratigraphic criterion for defining the Vendian System. For substantiating age, the following species among diagnostic Lower Vendian forms of East Siberia should be used: Appendisphaera grandis, “Appendisphaera” tabifica, A. tenuis, Ceratosphaeridium glaberosum, Dicrospinosphaera virgata, Multifronsphaeridium pelorium, “Polygonium” cratum, Tanarium conoideum, Variomargosphaeridium litoschum, and Talakania obscura. These taxa are readily recognizable, morphologically stable, and characterized by their wide lateral distribution and relatively narrow stratigraphic range. Their occurrence in the upper part of the Dal’nyaya Taiga Group of the Baikal-Patom region indicates that host sediments should be attributed to the Lower Vendian, not the Upper Riphean as is currently accepted.     

First results of U–Pb dating of detrital zircons from the Upper Ordovician sandstones of the Bashkir uplift (Southern Urals)

The first results of U–Pb dating of detrital zircons from Upper Ordovician sandstones of the Bashkir uplift in the Southern Urals and U–Pb isotopic ages available for detrital zircons from six stratigraphic levels of the Riphean–Paleozoic section of this region are discussed. It is established that the long (approximately 1.5 Ga) depositional history of sedimentary sequences of the Bashkir uplift includes a peculiar period lasting from the Late Vendian to the Emsian Age of the Early Devonian (0.55–0.41 Ga). This period is characterized by the following features: (1) prevalence of material from eroded Mesoproterozoic and Early Neoproterozoic crystalline complexes among clastics with ages atypical of the Volga–Urals segment of the East European Platform basement; (2) similarity of age spectra obtained for detrital zircons from different rocks of the period: Upper Vendian–Lower Cambrian lithic sandstones and Middle Ordovician substantially quartzose sandstones.     

Episodes of development of some major structural elements of Eurasia in connection with distribution of oil-gas provinces (basins) and their resources

The area of Eurasia and the shelves of the marginal and internal seas have been sufficiently investigated by geological, geophysical, and drilling means to formulate a number of regional conclusions about the association between various types of tectonic structures and distribution of the petroliferous provinces (basins). This problem has been considered against the background of platforms of different age, the ancient pre-Riphean (East European and Siberian) and the epi-Paleozoic types (West Siberian and West European, and also the South Caspian intrageosynclinal basin). There are significant differences in their geological evolution, which exerted a direct effect on the distribution of the petroliferous complexes. Within the Siberian Platform, the fundamental stages in downwarping were associated with the deposition of Vendian and lower Paleozoic sediments. The principal stages in the formation of the sedimentary cover of the East European Platform were associated with the Devonian, Carboniferous, and Permian periods.

For all types of structural zones under consideration, an association has heen established between distribution of oil and gas reserves and episodes of development of major structural elements. They define the overall stratigraphic range of the petroleum occurrences within the young and old platform and the fold regions, and also the concentration of the reserves of hydrocarbons in the complexes corresponding to the principal stages of downwarping of the basins.—Authors     

Paleoclimatic Environments of the Formation of Upper Vendian Rocks on the Belomorian-Kuloi Plateau,Southeastern White Sea Region

Reconstruction of Vendian climatic conditions on the East European Craton is of principle importance for elucidation of the Ediacarian biota habitat. However, paleogeographic reconstructions for this time are largely based on fragmentary and controversial paleomagnetic data. The degree of rock maturity deduced from lithochemical indicators allowed us to identify two stages of Late Vendian sedimentation on the Belomorian-Kuloi Plateau. The first (Lyamitsa-Verkhov) stage was characterized by the delivery of immature clastic material to the basin from a provenance with arid climate. The second (Erga-Padun) stage was marked by the input of relatively mature aluminosilicate clastics from a provenance with mild humid climate. The sedimentation stages approximately coincide with replacement of the shallow-water marine environment by the fluvioalluvial environment marked by steady and intense perennial river drainage from a highland in the northeast. In the Late Vendian (since 555 Ma ago), the northeastern area of the East European Craton was influenced by humid climate.__________Translated from Litologiya i Poleznye Iskopaemye, No. 3, 2005, pp. 267–280.Original Russian Text Copyright © 2005 by Grazhdankin, Podkovyrov, Maslov.     

四川万州中侏罗统上沙溪庙组粘土岩中火山灰的岩矿和地球化学证据

四川盆地东北缘万州地区中侏罗世上沙溪庙组产出稳定层状浅色粘土岩。利用X射线、X荧光光谱、等离子光谱（ICP）等方法对矿物成分、常量、微量和稀土元素进行了测试分析,粘土岩主要矿物为蒙脱石、伊利石和绿泥石。通过与澳大利亚后太古代平均页岩（PAAS）、北美平均页岩（NASC）、地壳粘土岩背景值以及其它具有代表性岩石的对比发现,本区粘土岩微量元素具有明显的低V,低Ti和低铁族元素的特点,高Al2O3／TiO2比值和低Ti／Th比值特征显著,某些层位具有明显的Eu负异常。推断本区粘土岩沉积物源具有火山灰和陆源碎屑的二元混合成因,各层位成分的差异显示了不同沉积时间内二者的混合比例发生变化。区域对比和地球化学资料显示火山灰可能来自于西部600km远的特提斯洋岛弧火山。     

Geology and geodynamic evolution of the high-P/low-T Maksyutov Complex, southern Urals, Russia

The high-pressure/low-temperature Maksyutov Complex is situated in the southern Urals between the Silurian/Devonian Magnitogorsk island arc and the East European Platform. The elongated N-S-trending complex is made up of two contrasting tectono-metamorphic units. Unit 1 consists of a thick pile of Proterozoic clastic sediments suggested to represent the passive margin of the East European Platform. The overlying unit 2, composed of Paleozoic sediments, volcanic rocks, and a serpentinite mélange with rodingites, is interpreted as a remnant of the Uralian Paleo-ocean. Devonian eastward subduction of oceanic crust beneath the Magnitogorsk island arc resulted in an incipient blueschist-facies metamorphism of unit 2 indicated by lawsonite pseudomorphs in the rodingites. While unit 2 was accreted to the upper plate, subduction of the continental passive margin caused the high-pressure metamorphism of unit 1. Buoyancy-driven exhumation of unit 1 into the forearc region led to its juxtaposition with unit 2 along a retrograde top-to-the-ENE shear zone. Further exhumation of the Maksyutov Complex into its present tectonic position was accomplished by later shear zones that were active as normal faults and are exposed along the margins of the complex. At the western margin a top-to-the-west shear zone juxtaposed a low-grade remnant of a Paleozoic accretionary prism (Suvanyak Complex) above the Maksyutov Complex. Along the eastern margin a top-to-the-east shear zone and the brittle Main Uralian Normal Fault emplaced the Maksyutov Complex against the Magnitogorsk island arc in the hanging wall.     

Vendian tectono-magmatic events in mantle ophiolitic complexes of the polar Urals: U-Pb dating of zircon from chromitite

Geological and isotopic evidence of Late Vendian magmatic events in restitic ultramafic mantle rocks of the Voikar-Syn’ya ophiolitic massif are considered and correlated with events at the eastern margin of the East European Platform. The geological and isotopic data show that the ophiolitic complexes of the Polar Urals were formed during several stages. The percolation of melts through peridotites was recorded in the newly formed mineral assemblages, for example, olivine + chromite ± zircon. Zircon crystallized from the residual fraction of the evolved basic melt that impregnated peridotite. The active interaction of hot restitic harzburgite with the migrating melt resumed repeatedly and could have led to the formation of several generations of chromite-bearing dunite. An important geological inference can be made from this suggestion: There is a high probability that isotopic markers of different age have been retained in restitic mantle complexes of ophiolites. The U-Pb dating of zircons with a SHRIMP-2 ion microprobe has shown that the isotopic age of seven grains is 585.3 ± 6 Ma (MSWD is 0.036 and the probability of concordance is 0.85). The obtained age of zircon from chromitite marks a Vendian tectonomagmatic event that occurred in the upper mantle of the transitional zone between the East European Plate and the oceanic basin. The island-arc complexes of the Polar Urals developed on the tectonically juxtaposed fragments of the Early Paleozoic and pre-Paleozoic oceanic crust. These crustal rocks were reworked during younger magmatic events related to the origin of the Middle Paleozoic island arcs. As a result, the rocks that formed in different geological epochs were locally retained in the restitic mantle complexes of a spatially indivisible ophiolitic association.