Lithogeochemistry of the fine-grained siliciclastic rocks of the Vendian Serebryanka Group of the Central Urals

Sedimentation environments were reconstructed for the Early Vendian successions of the western slope of the Central Urals, which comprises one of the most complete sections of the terminal Precambrian system in northern Eurasia. It was shown that, despite the presence of several diamictite levels in the sections of the Serebryanka Group, mature and multiply recycled fine-grained siliciclastic materials (CIA = 65–77) were delivered into the sedimentation basin over the whole Early Vendian. Based on the lithochemical characteristics of shales, the climate of Serebryanka time can be estimated as semiarid-semihumid, similar to that dominating in Late Vendian paleocatchments. Based on relatively high Mo/Mn values (0.011–0.024), it was suggested that anoxic or similar conditions existed in the basin of Buton time, whereas other sedimentary complexes of the Serebryanka Group were formed in well aerated environments. The systematics of Sr, Ba, Zr, Cu, and V in fine-grained siliciclastic rocks and Sr isotopic data for carbonate rocks indicate that the sediments were accumulated in a fresh-water basin. The values of trace-element indicator ratios, e.g., Th/Sc, La/Sc, Th/Cr and others, in the shales of the Serebryanka Group and Nd model age estimates indicate that a variety of mainly Early Proterozoic complexes, ranging from granitoids to basic rocks, occurred in the Early Vendian paleocatchments. The basic rocks were eroded most extensively probably in the end of Serebryanka time. Based on the Ce/Ce* values of shales, it was concluded that submarine volcanism had no significant influence on sedimentation processes in the Early Vendian. An exception is Koiva and Kernos time, when hematite-bearing shales were accumulated in association with pillow basalts in some zones of the basin. The distribution of the compositions of shales from various formations of the Serebryanka Group in discrimination diagrams suggests that the Early Vendian sedimentary sequences were formed in passive geodynamic settings.     

Vendian Suprasubduction Magmatism in the Southern Urals

New data on the Vendian age of the volcanogenic–sedimentary sequence of the Uraltau zone (Southern Urals) were obtained. The U/Pb (SHRIMP-II) isotope age obtained for zircons from rhyolite tuffs is 591.5 ± 3.5 Ma. The sequence under consideration is intruded by Vendian granites and overlain unconformably by Ordovician terrigenous and volcanogenic deposits. The composition of Vendian volcanics and granites testifies that they were formed in a suprasubduction setting at the continental margin, complexes of which are known in the Middle and Northern Urals.     

Arumberia-type structures in the Upper Vendian of the Urals

Problematic arumberia-type structures are widespread in Upper Proterozoic sedimentary rocks deposited in depositional environments subject to periodic desiccation and fluctuating salinity. Here we report two new occurrences of arumberia-type structures in the Krutikha Member of the Upper Vendian Chernyi Kamen Formation on the western slope of the Central Urals (Us??va River) and the Zigan Formation of the Asha Group in the South Urals (new road Sterlitamak-Beloretsk). The results of the study of these structures significantly expand paleontological characteristics of the Upper Vendian non-marine rocks and allow us to re-examine the paleontological importance of the hastily forgotten group of arumberia-type organisms. We distinguished six varieties of arumberia-type structures that for the sake of convenience were given formal Latin names in binominal nomenclature although they are not biological species. Two species??Arumberia banski and A. vindhyanensis??were erected by previous researchers, whereas the A. beckeri, A. ollii, A. usvaensis, and A. multykensis varieties are recognized for the first time. Despite similarity to erosional structures on the bottom of turbidite beds, erosion does not explain the genesis of the entire spectrum of arumberia-type structures including those preserved on ripple marks, with evidence of soft-body deformation, or with ribbon and filamentous fabric replaced by clay minerals. The diversity of arumberia could be only a manifestation of living systems. The species of Arumberia are distinguished by their different biostratinomy, the distance between the main structural units, the preservation of the main structural units, and the presence of additional structures between the main structural units. Most likely, the diversity of arumberia reflects not only taphonomic, but also ontogenetic and phenotypic variation. The level of morphological complexity and biostratinomic features of arumberia apparently exceed the organization of microbial mats and represent a non-actualistic type of microbial communities. The occurrence in extremely shallow-water depositional setting suggests that arumberia had evolved an adaption to periodic desiccation and that arumberia-type organisms, along with microbial mats, had been an important factor of sediment biostabilization in the Late Vendian non-marine environments.     

Upper Riphean and Vendian sandstones of the Bashkirian anticlinorium

The analysis of lithogeochemical data on the Upper Riphean and Vendian sandstones from the Bashkirian anticlinorium showed that sandstone associations formed in a passive sluggish tectonic regime in the middle Late Vendian were replaced by associations accumulated in the more active tectonic settings. This is well seen in the SiO₂-K₂O/Na₂O and (Fe₂O₃^* + MgO)-TiO₂ diagrams reflecting the particular and median compositions of psammites. The lithochemical characteristics of sandstones were examined to determine the compositional variation of rock complexes eroded on paleodrainage areas. Quartz-rich sedimentary, metasedimentary, and metamorphic rocks, as well as felsic igneous rocks prevailed in the paleodrainage areas throughout the entire Late Riphean and Early Vendian, while the main sources of clastic material in the Late Vendian were igneous intermediate and basic rocks. With allowance made for the previous comparative-lithological data and some other materials, significant similarity in the position and orientation of compositional fields of psammites from the middle and upper levels of the Asha Group (Bashkirian anticlinorium) with fields of psammites from different syncollisional (flysch and molasse) basins in the SiO₂-K₂O/Na₂O, K₂O/Na₂O-SiO₂/Al₂O₃, F1–F2 and other diagrams suggests that the middle Late Vendian (beginning from the Basa level) was marked by a variation in tectonic/geodynamic settings of sandstone accumulation and in composition of the eroded paleodrainage systems. The revealed trend agrees well with concept of the existence of the Late Riphean-Vendian Pechora paleocean.     

Lithological,Lithochemical, and Geochemical Indicators of Paleoclimate: Evidence from Riphean of the Southern Urals

Complex lithological, lithochemical, and geochemical data are commonly considered necessary for the reliable reconstruction of Late Precambrian climatic parameters. However, correlation of results deduced from different methods is becoming a more and more topical task. In this paper, we compare paleoclimatic data obtained with various lithological, lithochemical, and geochemical methods for the Riphean stratotype section on the western slope of the southern Urals. The results obtained turned out to be generally convergent, but, paleoclimatic reconstructions based on different methods are directly opposite to each other at some stratigraphic levels. Most likely, the controversy is probably caused by underestimation of recycling effects and incorrect extrapolation of conclusions related to the first-cycle sediments over the rocks that underwent multifold redeposition. Criteria for the recognition of recycled sedimentary units show that more than half of the Riphean stratotype section is composed of recycled materials. Hence, application of standard litho- and geochemical methods (CIA index and others) to these units yields virtual models only remotely resembling the reality rather than actual paleoclimatic indicators. Elaboration of new methods, which make it possible to avoid such errors, is a problem of in the near future.     

First results of U/Pb dating of detrital zircons from middle Riphean sandstones of the Zigalga Formation,South Urals

The results of U/Pb dating of detrital zircons from sandstones of the Zigalga Formation, which is the base level of the Middle Yurmatu Group of the Bashkir uplift, southwestern Urals, are presented. The U/Pb ages of detrital zircons from sandstones of the Lower, Middle, and Upper Riphean are compared.     

Belogubite,a New Mineral of the Chalcanthite Group from the Gaiskoe Deposit,South Urals,Russia

Geology of Ore Deposits - Belogubite CuZn(SO4)2 ⋅ 10H2O, a new mineral of the chalcanthite group, is found in the Gai (Gaiskoe) massive sulfide deposit, South Urals, Russia. This mineral...     

Some Specific Features of Early Vendian Sedimentation in the Southern and Middle Urals

The existence of the pre-Early Vendian hiatus in the Bashkir Meganticlinorium is supported by large incised valleys filled with Early Vendian sediments both on the western and eastern limbs of the meganticlinorium. It is suggested that a great glacioeustatic lowering of the sealevel occurred in the sedimentation basin. Taking into account the reconstruction of similar Early Vendian events for many provinces elsewhere, we can suppose that Early Vendian sedimentation basins in the southern and middle Urals were connected with the World Ocean. The comparative study of Lower Vendian sedimentary sequences in the Urals and Norway demonstrates similarities in rock associations. The correlation of Lower Vendian sections in the Urals with modern facies models of glacial sedimentation shows that glacimarine sequences of the southern Urals fit the shelf sedimentary associations proper. The character of section at the Serebryanka level in the middle Urals and the spatial distribution of mixtites therein correspond to sedimentation on the basin shelf and slope.     

Origin of vitriol Lake Gay,South Urals

This paper can be considered a case history in which geology, geophysics, and more particularly geohydrology and geochemistry, when applied to the problem of origin of a vitriol lake, led to discovery of a copper prospect. After a review of various postulated origins for the vitriol lake and previous interpretation of the local structure, new evidence based on borehole and geophysical data demonstrated that the structure consists of two anticlines and two fault systems. Study of the geohydrologic conditions as related to the fault systems and the geochemistry of their water indicate the lake is the result of mineralized water whose circulation is controlled by the fault zone. Source of the copper in these waters may be some hidden copper deposit in a direction away from the known ore bodies. -- J. Lemish.     

Zirconology of miaskites from the Ilmeny Mountains,South Urals

It is shown that the replacement and long evolution of miaskitic zircons led to the formation of two main age groups: 420–380 Ma (I) and 260–240 Ma (II). The age of miaskites is estimated at 440–445 Ma. Zircons I bear traces of fragmentation, dissolution, and replacement; they have “flat” REE patterns typical of metasomatic (hydrothermal) types, which is caused by allochthonous nature of the studied miaskites. Zircons II with differentiated REE patterns are similar to magmatic varieties, but have metamorphic origin. Mineralogical–geochemical and age characteristics of zircons in combination with structural–compositional features of miaskites define their metasomatic nature. The origin of the early zircon generations was related to the Ordovician rifting, while late generations were formed during shear deformations at the final stage of the evolution of the Uralian orogen.     

Palaeopascichnids from the Upper Vendian Chernyi Kamen Formation of the Middle Urals (Perm Region)

Doklady Earth Sciences - In 2020–2021 the palaeopascichnid fossils were found in the Krasnaya Gorka Site (Shirokovsky Reservoir, Kosva River area, Perm Region), a type Vendian fossil locality...