Geochemical Proxies of Endo- and Exospheric Processes in Sedimentary Sequences and the Upper Precambrian Section in the South Urals

Abstract

The article characterizes some geochemical proxies of endo- and exospheric processes in sedimentary sequences and analyzes their average values in Upper Precambrian clayey rocks in the South Urals. It is shown that a gradual decrease of the Th/Scaver value in Lower Riphean (Mesoproterozoic) ~1750–1300 Ma ago rocks correlates to a certain extent with the assembly and disintegration of the Columbia/Nuna supercontinent. The assembly of Rodinia corresponds to two peaks (Biryan Member and Min’yar Formation) of relatively high Th/Scaver values. However, they are separated by rocks with Th/Scaver values comparable with those typical of the fine-grained clastic rocks of the Mashak plume episode (~1380 Ma). Variations of Th/Craver in the Lower–Middle Riphean clayey rocks also correlate quite well with the assembly of Columbia/Nuna, whereas the Upper Riphean–Vendian (Neoproterozoic) interval does not show any significant variation in Th/Craver values. Hence, the assembly and disintegration of Rodinia and the associated changes in rock composition in the provenance did not have any notable effect on the fine-grained aluminosilicate rocks of the Riphean stratotype. The Middle–Upper Riphean sequences were also formed at relatively constant values of TDM and eNd(T). Hence, the Grenvillian events did not make significant contribution to the formation of rocks of the Yurmatau and Karatau groups. Average values of indicators of the redox parameters of water in the Late Precambrian basins, which existed in the eastern part (in modern coordinates) of the East European Platform, indicate that their sedimentary infilling was deposited predominantly under oxidizing or similar conditions. This conclusion contrasts markedly with the notion of the prevalence of ferruginous and euxinic environments in the World Ocean during the Late Precambrian. Comparison of variations in the mean EFNi and EFZn values in clayey rocks of the South Urals, as well as other paleoproductivity proxies, does not reveal any definite relationship of their excursions with the Late Precambrian global and subglobal, endo- and exospheric events. In the Early–Middle Riphean succession, the P2O5 values, slightly higher than in PAAS, correspond approximately to the period of Columbia/Nuna supercontinent existence. Lower values are typical for the clayey rocks formed during its disintegration, although the volume of mafic igneous rocks (main sources of phosphorus) in the provenances at that time should be larger. During the existence of Rodinia, the Upper Riphean clayey rocks were characterized by P2O5aver values both higher and lower than the PAAS. The same is true for its disintegration period. These facts suggest that the global and subglobal, endo- and exospheric processes did not significantly affect the formation of the Riphean–Vendian sedimentary sequences in the South Urals. More important here is the role of local factors.