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Biostratigraphic subdivision of the Devonian in the Far East and Transbaikal region, USSR
The Devonian in the eastern portion of Siberia is represented by its three series and forms, with the Silurian and Lower Carboniferous, a single sedimentary cycle. The study of the included organic remains has enabled the subdivision of the Devonian into local zones.
In lithologically uniform deposits one may distinguish a lower Devonian with Favosites porfirievi var. oldoica Dubat. and Phacops logani asiaticus Z. Max. and an upper Lower Devonian with Pleurodictyum mongolicum Tchern., Rhitistrophia beckii Hall, Delthyris perlamellosus Hall and Phacops cristata Hall.
Most widely distributed are the Middle Devonian deposits, particularly the Givetian, along with the Early Frasnian. The lower half of the Middle Devonian (corresponding to the Onondaga of eastern North America) is characterized by reef limestones with corals, bryozoa and crinoids. They form a favosites halanerisis Dubat—Stenophyllum spinulosum Shosh. assemblage-zone. Synchronous terrigenous deposits contain the remains of brachiopods, trilobites and some other organisms. Most notable as guide fossils are Fimbrispirifer divaricatus Hall and Delthyris missouriensis Tansey.
It is difficult to distinguish the Givetian from Early Frasnian deposits on a lithological basis. They are considered, therefore, as a single subdivision, though, on the basis of rich assemblages of organic remains, they may be subdivided into a lower unit with Spinocyrtia martianofi Stuck., Mucrospirifer mucronatus Conr., Euryspirifer ex gr. cheehiel Kon. and an upper unit with Cyrtospirifer achmet Nal. and Spinocyrtia ali Nal.
The Upper Devonian comprises three units—lower beds with Cyrtospirifer achmet Nal.—middle beds with Atrypa, Cyrtospirifer ex gr. verneuili Mureh., Mucrospirifer posterus H. et Cl., and upper beds with Cyrtospirifer sulcifer, H. et Cl., C. ex gr. disjunctus Sow., and Platycrinus; they embrace the Frasnian and Famennian Stages and are equivalent to the Seneca and Chautauqua series of North America (New York State).
The Devonian fauna of the Far East and the Transbaikal region, apart from endemic features peculiar to Mongolo-Okhotsk province, possesses some of the peculiarities of the Dzungaro-Balk hash and Altai-Sayan provinces and belongs to the Pacific Zoogeographie region.
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