Литосфера (Dec 2017)
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
Abstract
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.
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