The article presents the results of the analysis of the content and values of enrichment factors (EF) of redox-sensitive elements (Mo, U, V and Zn) in Riphean and Vendian clay rocks of the eastern part of the East European Platform (Kama–Belaya and Sernovodsk–Abdulino aulacogens, Shkapovo–Shikhan depression), as well as their FeT/Al values, one of the indicators widely used for the reconstruction of redox environments of sedimentation. It is shown that the EFMo, EFU, EFZn and EFV values for the overwhelming majority of analyzed samples are lower than those in the PAAS and the Wedepohl’s average shale. The FeT/Alaverage value (0.50 ± 0.16) calculated for the Upper Precambrian clay rocks of the Volga–Urals region is comparable, taking into account the errors, with that (0.46 ± 0.04) calculated for the clay and carbonate-clay rocks of the Lower Riphean Arlan Subformation of the Kaltasy Formation. Together with a number of other data, this formed the basis for the conclusion regarding the presence of free O2 in the “Arlan paleobasin” and its oligotrophic nature. It is concluded that the accumulation of Upper Precambrian sedimentary sequences in the eastern part of the East European Platform occurred for more than 1.2 billion years in fairly well-aerated environments with low bioproductivity (i.e., oligotrophic environments) in the presence of free O2 in the water. Apparently, the sedimentation basins in the area under consideration did not have a noticeable water exchange with the open ocean, which prevented the penetration of deep anoxic water into them for a long time.
A. V. Maslov (2026) studied this question.