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Alla Ragozina, Paleontological Institute RAS (Russian Federation)
Dorj Dorjnamjaa, Paleontological center MAS (Mongolia)
Alexey Krayushkin, Paleontological Institute RAS (Russian Federation)
Ekaterina Serezhnikova, Paleontological Institute RAS (Russian Federation)
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The Vendian-Cambrian boundary is one of the most important event in the geological history. The recognition in 1950th by B.S. Sokolov of the Vendian System below the Cambrian sediments on the Eastern-European platform with the glacial deposits at the base of the System and with its unique Ediacara-type fauna strongly stimulated further investigations in this direction. In spite of the tremendous progress in the Vendian-Cambrian stratigraphy and paleontology the problem of the Vendian-Cambrian boundary and stratigraphic subdivisions of both systems remains under hot discussion.
The Vendian-Cambrian siliciclastic-carbonate sections of the South-Western Mongolia contain both the ichnofossils and SSF that provides a better possibility to determine the age of the fossiliferous strata. Due to our field study of the Vendian-Cambrian deposits of the Dzabkhan Zone, it's possible to conclude that the Vendian-Cambrian boundary can not recognized there by the first appearance of T. pedum trace fossils. The associated ichnofossils from the layer where T. pedum is distributed form the complex typical for the younger ichnozones namely Upper Tommotian - Lower Atdaban ichnozones III (according to Crimes, 1987) and Rusophycus avalonensis ichnozone from Newfoundland. This fact casts doubt in rightfulness of accepted international practice of establishing of boundary between Precambrian and Cambrian using ichnofossils only (Rozanov et al, 1997, 2008). The Vendian-Cambrian transitional layers comprise the problematic fossils of the Vendian (Ediacaran) appearance, microfossils, the remains of algae and cyanobacteria. Problematic cyclic organisms from the Tsaganolom Formation can be conventionally referred to the common Ediacaran genera Beltanelloides, Beltanelliformis or Nemiana by the morphological features as the poor preservation of the material does not permit to identify them more strictly. The algal remains are represented by the Rhodophyta, possibly genus Thallophycoides Zhang Y. et Yuan, 1992. Similar forms were described from the upper part of the Doushantuo Formation from South China. The Vendian-Cambrian boundary can be established more precisely at the base of the Tommotian Stage by the presence of the SSF associations, trace fossils, microfossils and biogerm-forming algae. Definition of the boundaries between the high rank stratigraphic subdivisions on the basis of the trace fossils is extremely risky and ineffective because of considerable control of the ichnofossil shape (and taxonomy!) by the organism's behavior, the sediment type and degree of its aeration etc. In the sections of Dzabkhan zone of Western Mongolia, where the Vendian-Cambrian deposits are represented by both silicoclastic and carbonate rocks, the SSF and T. pedum traces are found on different stratigraphical levels and in different sedimentary facies. The observations confirm the suggestion of a strong environmental control on the distribution of T. pedum (Grotzinger et al., 2006).
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