Distribution of Microfossils in the Vendian Deposits of the Orsha Depression of the East European Platform, Belarus

Abstract— The complete sections of the Lower and Upper Vendian of the East European Platform were opened in the boreholes of Belarus. This allows us to consider this region as a stratotype area. In order to update the paleontological characteristics of the Vendian, organic-walled microfossils from the Bogushevsk-1, Bogushevsk-2 (Liozno), and Lepel-1 reference boreholes drilled in the north of the Orsha Depression were studied. The transitive assemblage I with Leiosphaeridia minutissima–Leiosphaeridia tenuissima was recognized in the Lower Vendian Liozno Formation of the Volyn Series and the Upper Vendian Kotlin Formation. The Redkino microfossil assemblages were identified at three stratigraphic levels in the Nizy, Selyava, and Chernitsa formations: assemblage II with Morania zinkovi, assemblage III with Morania zinkovi–Tynnia precambrica, and assemblage IV with Morania zinkovi–Tynnia precambrica–Striatella coriacea, respectively. The Kotlin assemblage V with Vendotaenia antiqua–Primoflagella speciosa was distinguished in the upper part of the Kotlin Formation (Bogushevsk-1 borehole). On the basis of the taxonomic composition, assemblages III and IV are correlated with the first Redkino biota of the Starorusskaya Formation; assemblage V is correlated with the third Kotlin biota of the Vasileostrovskaya Formation of the northwestern part of Russia. Our data show a high biostratigraphic potential of organic-walled microfossils, which should be used to substantiate regional stratigraphic units of the Upper Vendian in the updated stratigraphic scheme of the East European Platform.

LITHOLOGICAL AND FACIAL FEATURES, COMPOSITION, AND SEDIMENTATION CONDITIONS OF TERRIGENOUS-CARBONATE ROCKS OF THE MOTY GROUP (SHAMAN CLIFF, IRKUTSK REGION)

We have studied terrigenous-carbonate rocks in the area near the Sayan mountains in the Irkutsk Region (Russia), specifically at the Shaman Cliff, being the stratotype area of rocks that belong to the Moty group. The cliff’s lower part is composed of sandstones, which fragments gradually decrease in size upward the cross-section. The middle and upper parts are composed of sandy dolomites and dolomites, respectively. In terms of material characteristics, the terrigenous rocks correspond to arkoses. According to the genetic typification, the arkoses are composed of destructed primary igneous rocks. The terrigenous-carbonate rocks contain a carbonate component that gradually increases in the upper part of the cross-section. In the Shaman Cliff cross-section, we distinguish 32 lithological units and eight lithologicalgenetic types of deposits. Paleogeodynamic conditions are reconstructed for the formation of the sedimentation basin. Our study of the Shaman formation reveals specific features of the lithological facies, which suggest that these rocks accumulated in a coastal environment affected by tides. In the study area, clastic materials were mainly removed from an orogen that formed due to the Vendian accretion-collision events in the southern folded frame of the Siberian platform. Dolomites composing the upper part of the cliff are attributed to the Irkutsk formation of the Moty group. Their lithological features give evidence of shallow-marine conditions of their formation, without any supply of clastic material, which contributed to mass dispersal of the Cambrian biota described in [Marusin et al., 2021]. It is our first initiative to draw a boundary between the Shaman and Irkutsk formations of the Moty Group along the base of the carbonate eluvial breccia unit that marks the stratigraphic break. In the cross-section, this boundary represents the border between the Upper Vendian and Lower Cambrian.Our conclusions are generally consistent with the ideas of most researchers about the Late Vendian evolution of the southern margin of the Siberian platform. The results of our study can be used in further investigation of this area and provide a basis for correlating the studied strata with the same-age reference cross-sections of other regions in Siberia.

Provenance reconstructions. Article 1. Mineralogical and petrographic approaches and methods

Research subject. This is the first in the series of four articles, which aims to review existing provenance reconstruction tools.Methods and materials. The data obtained during evaluation of different sandstone allothigenous components was used as a basis for analysis. The data was collected from the Upper Riphean Zilmerdak Formation Bir’yan Member and the Upper Vendian Bassa Formation (Southern Urals); the Upper Devonian Kodinka and Ust’kodinka formations (Middle Urals); the Lower Cretaceous Sortym Formation (Western Siberian sedimentary megabasin, Bolshekhetskaya depression).Results. The sandstone composition patterns of the lithostratigraphic units under study allowed the most common provenance features to be established by means of comparing data obtained by various diagrams.Conclusions. It is established that only the Kodinka, Ust’kodinka and Sortym sandstone compositions can be used as a source for rock reconstruction analysis, while the Bir’yan and Basa sandstone compositions have undergone massive transformations.

Late Vendian kotlinian crisis on the East European Platform: lithogeochemical indicators of depositional environment

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 immediately prior to the “Cambrian Explosion.” However, they could be caused by local environmental perturbations. Variations of some lithogeochemical indicators of depositional environment (indicators of paleoclimate, exhalation activity, redox settings, and paleobioproductivity) were analyzed in order to establish the possible influence of sedimentary systems on evolutionary processes in the Late Vendian and at the boundary with the Cambrian. The applied algorithm of lithogeochemical studies revealed no significant perturbations in physical properties of the environment on a scale of sedimentary basins. The obtained data suggest that local factors did not affect the evolution of Ediacaran biota on the East European Platform and confirm the global nature of transitions between the Redkinian, Belomorian, and Kotlinian biotas.