Asha Group Research Articles

Upper Riphean and Vendian sandstones of the Bashkirian anticlinorium

The analysis of lithogeochemical data on the Upper Riphean and Vendian sandstones from the Bashkirian anticlinorium showed that sandstone associations formed in a passive sluggish tectonic regime in the middle Late Vendian were replaced by associations accumulated in the more active tectonic settings. This is well seen in the SiO2-K2O/Na2O and (Fe2 + MgO)-TiO2 diagrams reflecting the particular and median compositions of psammites. The lithochemical characteristics of sandstones were examined to deter� mine the compositional variation of rock complexes eroded on paleodrainage areas. Quartzrich sedimentary, metasedimentary, and metamorphic rocks, as well as felsic igneous rocks prevailed in the paleodrainage areas throughout the entire Late Riphean and Early Vendian, while the main sources of clastic material in the Late Vendian were igneous intermediate and basic rocks. With allowance made for the previous comparativelitho� logical data and some other materials, significant similarity in the position and orientation of compositional fields of psammites from the middle and upper levels of the Asha Group (Bashkirian anticlinorium) with fields of psammites from different syncollisional (flysch and molasse) basins in the SiO2-K2O/Na2O, K2 O/Na 2 O-SiO 2 /Al 2 O 3 , F1-F2 and other diagrams suggests that the middle Late Vendian (beginning from the Basa level) was marked by a variation in tectonic/geodynamic settings of sandstone accumulation and in composition of the eroded paleodrainage systems. The revealed trend agrees well with concept of the exist� ence of the Late Riphean-Vendian Pechora paleocean.

Late Precambrian rifting in the geological history of the western slope of the South Urals

The rift-related geodynamic setting of the Late Precambrian geological evolution on the western slope of the South Urals is reconstructed on the basis of localization of lithotectonic complexes of this age, their formation conditions, and the geochemistry of rocks. The Early Riphean stage comprises accumulation of coarse-clastic rocks intercalating with alkaline volcanic rocks of the Navysh Complex, which is a constituent of the Ai Formation, and emplacement of doleritic and picritic intrusions of the Shuida Complex and melanocratic dolerite and gabbrodolerite of the Yusha Complex. The Middle Riphean stage is characterized by wide-spread coarse-clastic terrigenous rocks of the Mashak Formation that intercalate with volcanic rocks of the bimodal basalt-rhyolite association, the Berdyaush pluton of rapakivi granite, the Kusa-Kopan layered intrusive complex, the Lapyshta Complex of dolerites and picrites, and numerous occurrences of gabbrodolerites. The terrigenous rocks of the Vendian stage include conglomerate, gravelstone, and sandstone of the Asha Group, while igneous rocks comprise alkaline volcanics of the Arsha Complex, alkali gabbroids of the Miseli Complex, and melanocratic syenite of the Avashla Complex. The geological evolution of the region is distinguished by local (failed or aborted) rifting. The occurrence of lithotectonic complexes is controlled by dynamic conditions of rifting. A certain inheritance in the evolution may be traced for the Early and Middle Riphean and partly for the Late Riphean and Vendian.

REE systematics and Nd model age of Upper Vendian argillites of the Shkapovo-Shikhan Depression

The Shakapovo-Shikhan Depression—one of the largest in the eastern part of the East European Plat- form—is filled with Upper Vendian sedimentary rocks of the Bizhbulyak Complex about 1.6-1.8 km in total thickness. The depression is framed by the Orenburg, Tatar, and Komi-Permyak arches in the southwest, west, and northwest, respectively. These arches are composed of the Archean (~80% of total area of ancient drainage basin (2)) and Paleoproterozoic crystalline rocks. According to the seismostratigraphic data (3), the Upper Vendian sedimentary rocks give way east- ward to the coeval Asha Group of the Bashkir Anticli- norium. The transition occurs via a system of fold- thrust dislocations. Different aspects of the formation of the Upper Ven- The systematics of rare earth elements (REE) and estimation of the Nd model age for the fine-grained ter- rigenous rocks of the Bizhbulyak Complex allowed us to get principally new information on the composition of rocks, their relationships in the paleodrainage basin, and the contribution of the western and eastern prove- nances to the formation of the sedimentary sequence of the Shkapovo-Shikhan Depression in the Late Vendian. The Bizhbulyak Complex comprises (from bottom to top) the Baikibashevo, Staropetrovo, Salikhovo, and Karlin formations (1). The Baikibashevo Formation (10-85 m thick) is composed of sandstone, gravelstone, and fine-pebble conglomerate, as well as silty sand- stone and silty argillite mainly occurring in its upper portion. The Staropetrovo Formation (85-320 m) con- sists largely of siltstone and argillite with a variable amount of sandy material as separate interbeds and members or dissemination. The Salikhovo Formation (100-450 m) comprises sandstone and siltstone with subordinate argillite interbeds. The Karlin Formation (0-600 m) is made up of argillite with interbeds and packets of lighter colored siltstone and sporadic sand- stone. The K-Ar age of glauconite from silty sandstone of the Staropetrovo Formation is ~595 Ma. Glauconite from sandstone and siltstone of the Basa Formation was dated with the same method at 600-557 Ma (7). In the unified stratigraphic chart of Vendian sequences in the Volga-Ural province, the Baikibashevo Formation is correlated with the Uryuk Formation; the Staropetrovo Formation, with the Basa Formation; the Salikhovo Formation, with the Kukkarauk Formation; and the Karlin Formation, with the Zigan Formation of the Bashkir Anticlinorium (1).