Abstract

In the Devonian Period the eastern slope of the inner part of the Ural geosyncline was separated from miogeosynclinal zones to the east and west by abyssal fractures.

In the beginning of the Early Devonian, the Ural eugeosyncline was complexly differentiated and delimited by fractures into regions of uplift and subsidence. The zones of subsidence are characterized by the appearance of active volcanism. Within complex differentiated zones of subsidence, uplifts are formed characterized by slight volcanism, carbonate and terrigenous sedimentation and the formation of reefs. The zones of uplifts are characterized by the accumulation of shallow water and continental sediments, by the development of intrusive magmatism and the presence of insular land.

At the end of the Early Devonian and in the beginning of the Eifelian there took place a reconstruction of the structural plan. The Middle Devonian is characterized by the accumulation of volcanogenic formations and by a wide development of reefs.

In pre-Frasnian time further structural reconstruction took place. The Frasnian is characterized by the accumulation of volcanogenic formations. The spreading of uplifts and wide distribution of flysch deposition is typical for the Famennian. The lower boundary of the Devonian System is drawn at the bottom of deposits containing the remains of a Late Silurian and Early Devonian fauna. The upper boundary is drawn at the top of the Clymenia laevigata Zone.

In the Lower Devonian, lower and upper stages (Gedinnian and Coblenzian) are distinguished which probably correlate with the Upper Gedinnian, Siegenian and Lower Emsian of the Rhine region.

The Eifelian Stage may be correlated with the Upper Emsian and Couvinian of Western Europe.

The nonconformity of stage divisions of the Lower, and lower parts of the Middle Devonian of the Urals and other regions of the USSR with stage divisions of Western European type sections, and also the inferiority of type sections, forced the Soviet geologists to question the introduction of new stage names and definitions in their volumes.