Riphean of the Kamsko-Belsk aulacogen: Hiatuses and lithogeochemistry of clastic rocks

Abstract

Research subject. Sandstones and clay rocks of the Lower, Middle, and Upper Riphean (the Kyrpino, Serafimovo, and Abdulino groups, respectively) of the Kamsko-Belsk aulacogen. Changes in the characteristic values of the ratios of a number of oxides and some trace elements that are the indicators of rock composition in paleowatersheds are considered, taking the hiatuses of different nature and duration into account. Methods. An analysis of the average values of  K2O/Al2O3 etc., Th/Sc, La/Co and (La/Yb)N, characteristic of sandstones and fine-grained clastic rocks (shales, fine-grained clayey siltstones and mudstones) of the main lithostratigraphic units of the Riphean section, as well as an analysis of their changes given the position below and above the break surface. Results. Throughout the entire Riphean, regardless of breaks in sedimentation associated with certain rearrangements in the recharge and sedimentation areas, the lithogeochemical characteristics of both sandstones and fine-grained clastic rocks (K2O/Al2O3, TiO2/Al2O3, Th/Sc, La/Co, etc.) were found to experience no signi ficant changes. Conclusions. The hiatuses presented in the Riphean section of the Kamsko-Belsk aulacogene were unlikely to have any significant effect on the average values of a number of ratios–indicators of rock composition in paleowatersheds (K2O/Al2O3, SiO2/Al2O3, TiO2/Al2O3, Th/Sc, La/Co, and (La/Yb)N) in sandstones and clay rocks occurring below and above the hiatus surfaces. This suggests that there were no cardinal changes in the composition of rocks in the paleowatersheds surrounding such a large negative structure as the Kamsko-Belsk aulacogene during the Riphean. Similarly, no fundamental changes occurred in the pathways of clastics entering the sedimentation area associated with the named structure.