Variations of Th/U values in Vendian clayey rocks of north-east East-European Platform and Middle and Southern Urals as reflection redox status of sedimentary basins

Earlier, we considered the features of the redox changes in the near-bottom layer of water in the sedimentation basins that existed during the Late Riphean and Vendian in the eastern and northeastern (in modern coordinates) margins of the Baltic. It is concluded that fairly pronounced variations in the values of a number of redox environment indicators in the Upper Riphean deposits suggested that accumulation of sediments in some regions occurred in dysoksis or close to anoxic conditions. Opposite in the Vendian oxidation conditions predominated in the near-bottom waters. In the present publication, these representations are verified by data on variations in Vendian clay rocks of the of the Th/U values. Analytic data obtained by ICP-MS in IGG UB RAS (Ekaterinburg) were used to analyze the features of the change in this parameter. A total of ≈380 samples of shales and mudstones were analyzed. The average Th content in these samples is 12.0 ± 5.4 ppm, in average post-Achaean Australian shale (PAAS) and upper continental crust (UCC) are 14.6 and 10.5 ppm, respectively. For U, the same parameters are 2.1 ± 1.1, 3.1 and 2.7 ppm. The value of Th/Uav for the Vendian shales of the east, northeast and north of the East European platform is equal to 5.8 ± 2.0 (in PAAS and UCC - 4.7 and 3.9 respectively). In the clay rocks of the Vendian of the Southeast White Sea, Th/Uav varies from below upward along the cross section from 5.8 ± 3.4 to 6.9 ± 2.3 without any definite regularity. The Vendian shales and mudstone of the southern part of the Vychegda trough have the Th/Uav value from 3.5 ± 1.1 to ≈5.0 ± 1.7. Dark-colored low carbonaceous shales of the Buton Formation of the Lower Vendian Serebryanka Group (the Middle Urals), which we considered earlier as deposits formed in a basin with distinctly oxygen-free sedimentation conditions, have the Th/Uav 5.9 ± 1.2, and, most likely, do not refer to sediments of stagnant environments. The new data on the distribution of Th/U values in the clayey rocks of the Valdai, Serebryanka, Sylvitsa, Asha, Kairovo and Shkapovo groups correspond to the idea of the accumulation of these sedimentary formations under conditions of dominance of oxidation environments in very shallow sedimentary basins, some of which may not were actually marine. They emphasize the complex/non-linear nature of oxygenation of the ocean, which began at the end of the Late Riphean and continued in the Vendian, with the inversion of “oxygen environments” in some large sedimentary basins or parts of them in the Early Vendian, to the Vendian and Cambrian boundary, and actually in the Cambrian. Obviously, not only global, but also local factors had a significant impact on this process

Upper Vendian in the east, northeast and north of East European Platform: Depositional processes and bio­tic evolution

Subject. Analysis of lithogeochemical proxies in the Upper Vendian mudstones reveals little if any variation in depositional environment for the Redkinian, Belomorian and Kotlinian regional stages in the east, northeast and north of East European Platform. The coeval macrobiota, in contrast, demonstrates significant macroevolutionary and macroecological transformations. Thus, the Avalon-type ecological association consisting of frondomorphs and vendobionts evolved in low-energy inner shelf during the Redkinian, the Belomorian Stage is characterised by diversification of frondomorphs, migration of vendobionts into relatively high-energy depositional settings (shoreface and prodelta), and emergence of tribrachiomorphs and bilateralomorphs, where as the Kotlinian Stage is marked by a sharp decline in taxonomic diversity of soft-bodied organisms (the Kotlinian Crisis). We don’t know to what degree, if at all, depositional parameters as palaeogeodynamics, palaeoclimate, sediment composition, volcanic activity influenced the Ediacaran biota, but these agents were not responsible for the above mentioned biotic transformations. Materials and methods. We suggest that intrinsic factors such as ecological interactions could be the primary trigger of the Kotlinian crisis. This conclusion has been reached based on the study of composition of major rock-forming oxides, rare- and trace elements in fine-grained aluminosiliciclastic rocks (argillites, shales and silt-rich mudstones). Geological samples were collected in outcrops of the Asha Group of South Urals and Sylvitsa Group of Central Urals, as well as from the drill core of the Keltma-1 (Vychegda Trough) and Tuchkino-1000 (Southeast White Sea area) boreholes. We also used the data on chemical composition of mudstones from the Staraya Russa and Vasil’evsky Ostrov formations form the southern slope of the Baltic Shield. Results. With this information in hand we could assess, with varying degree of confidence, such parameters as a degree of recycling of the material supplied into the late Vendian Mezen Basin; sediment provenance; composition of the substrate that microbial mats and soft-bodied organisms lived on in different parts of the basin; and palaeogeodynamic environment at the time when different groups of soft-bodies organisms were emerging