SOME FEATURES OF THE EVOLUTION OF THE BULK CHEMICAL COMPOSITION OF CLAY ROCKS OF THE UPPER PRECAMBRIAN SEDIMENTATION SERIES (ON THE EXAMPLE OF THE SOUTHERN URALS)

The article presents the results of a study of some general features of the evolution of the bulk chemical composition of clay rocks of the Upper Precambrian sedimentary series of the Southern Urals, allowing us to judge the composition of the source rocks of the aluminosilicoclastics that compose them (ln(Ni/Co) and ln(Th/Sc) values) and the paleogeodynamic settings of formation (DF1 and DF2 discriminant functions). The resulting conclusions are compared with ideas about the formation of the named series, based on general geological data and an analysis of the formational nature of volcanics known at a number of levels of the Riphean stratotype. In the last few decades, most experts believe that the Yurmatau and Karatau or Burzyan and Yurmatau groups are rift-depression/platform series. This suggests that the formation of the sedimentary units that compose them was subject to similar patterns and should be reflected in the lithogeochemical characteristics of the latter. We have shown that in terms of “lithochemistry of fine-grained elastic rocks–paleogeodynamics”, the architecture of the Upper Precambrian sedimentary series of the Southern Urals appears to be largely diverse. The observed trends in the change in the bulk chemical composition of clay rocks within each series in the DF1–DF2 diagram are in most cases not comparable with the model trend “sedimentary formations of rift settings → the same of platform settings”. In the ln(Ni/Co)—ln(Th/Sc) diagram, the clay rocks of the sedimentary series are characterized by trends in the change in the bulk chemical composition of two types. Moreover, each sedimentary series is unique in the above parameters. Thus, the features of the bulk chemical composition of clay rocks determined by the composition of the provenances do not have the same trends in changing from the bottom up along the sedimentary series. Apparently, all of the above indicates a significant role in the formation of sedimentary filling of the Early, Middle and Late Riphean, as well as Vendian, sedimentation basins of the Southern Urals not only of paleogeodynamics and the composition of the provenances, but also of many (?) local factors.

sedimentary series, Riphean, Vendian, bulk chemical composition, clay rocks