Sources of material drift into the Ural foredeep at the beginning of collision (Southern Urals)

Introduction. This work is devoted to the study of the sources of   drift material during the formation of Late Paleozoic deposits of the southern part of the Pre-Ural trough. Sample for the study was taken in a quarry near the Urgala region, Bashkortostan area. The section is represented by conglomerates with a sand matrix. These deposits belong to Ural forland basin. The age of this conglomerate formation – Moscovian (Middle Carboniferous).

Materials and methods. The most reliable determination of sources is possible due to U-Pb zircon dating. We also analyzed some thin sections for detailed studying of sandstone composition.

Results and discussion. Zircon grains vary greatly in shape and size. In some grains, the core and edges are clearly visible; others are full of inclusions, cracks, and zones of metamict decay. The size of the crystals varies from 60 to 400 microns. Most of the ages obtained fall in the interval from the Ordovician to the Devonian, less on the Lower and Middle Riphean. Single grains are of Cambrian, Vendian and Late Riphean age. Early Proterozoic and Archean grains are absent in the sample.

The most difficult interval is from the Cambrian to the Devonian, it accounts for the majority of the ages (410-430 Ma). Within the studied territory, the volcanic rocks closest to the sampling site are located in Nyazepetrovsk and Bardym allochthons, as well as in the Tagil arc. In addition, Devonian granitoids are found within the Ufalei anticlinorium. The largest number of Precambrian dates falls on the Middle Riphean. The source of zircons during the middle Riphean could be the Mashak formation, whose age is 1350-1346 Ma, however, there are no grains with the age of the Mashak formation in the sample.

A relatively large number of grains have the early Riphean age of 1650-1500 Ma, which correlates perfectly with the age of the Ai formation. However, almost all Riphean formations, including the Ai formation, contain zircons with the peak at 2050 Ma (the age of migmatization in the Taratash block),but  the studied sample contains no zircons of 2050 million years age or older. This means that the Taratash block and the surrounding Riphean formations were not exposed at that time.

Also, the largest number of lithoclasts in the studied sandstones are represented by siliceous rocks. The similar rocks compose the Ordovician-Devonian section of the Mayaktau Allochthon, which is located closely to the sampling site. Also, the thickness Aziam  formation   increases towards Mayaktau Allochthon. In addition to the sources described above, there is jne more source – Asha series (Vendian), because there are quite a large number of Middle-Riphean dates in the sample, which are typical for the rocks of the Asha series.

Financial support. The research has been funded by RFBR and CNF as a part of the research project № 19-55-26009 Czechia_a