The Middle-Upper Devonian sandstones of the West Magnitogorsk zone of the Southern Urals

Abstract

To date, the composition of the Devonian sandstones of the West Magnitogorsk zone has been known from only a few samples. However, given the large thickness of the sections (up to the first kilometers), their number should be increased, up to several tens for every formation in each section. The deposits of the Irendeyk, Ulutau, Biyagoda, and Zilair Formations were studied using 169 samples from a continuous section at the latitude of Magnitogorsk. In this paper, the results of the petrographic des­criptions and analysis of the contents of rock-forming elements, chromium, cobalt, and nickel are presented and discussed. According to the ratio of clastic grains in the Irendyk Formation, two groups of rocks were distinguished – graywackes and feldspar graywackes; the two-membered structure of the lower subformation of the Biyagoda Formation and the Zilair Formation was established. Based on the petrographic data, it was revealed that the clastic rocks belong to different genetic types. Sandstones of the Irendyk, Ulutau, and Zilair Formations are tephroids, i.e., their accumulation is caused by tephra removal. Sandstones of the bottom part of the lower subformation of the Biyagoda Formation are volcanic-terrigenous, i.e., they consist of grains of lithified volcanics. Sandstones of the upper member of the lower subformation of the Biyagoda Formation are terrigenous, i.e., they are composed of fragments of volcanic, sedimentary, and metamorphic rocks. The diagrams constructed as described by F. Pettijohn, M. Bhatia, B. Maynard, and other researchers demonstrated that the studied sandstones are similar in their geochemical parameters and can be identified as volcaniclastic graywackes formed in the waters of an oceanic island arc. The sediments underwent no intensive chemical weathering. The graywackes were compared and it was shown that they differ in the contents of Cr, Co, Ni, as well as in the values of TiO2/Al2O3, (FeO + Fe2O3 + MnO)/(TiO2 + Al2O3), (Na2O + K2O)/Al2O3, which can be used to justify the age of sediments that are not characterized by fauna. The accumulation of the upper part of the lower subformation of the Biyagodya Formation is related to the collision and the global Upper Kellwasser Event. The results obtained confirm the feasibility of sampling a large number of samples in individual sections.

Keywords

• tephrogenic
• volcanic-terrigenous
• tefroturbidit
• graywackes
• irendyk formation
• ulutau formation
• biyagoda formation
• zilair formations
• island arc
• upper kellwasser