SIBERIA

The Cambrian of the Siberian Platform and adjacent areas are among the best studied Cambrian sequences. Particularly the Lower Cambrian reaches at times the status of a global standard, and the traditional Siberian Early Cambrian stages (Tommotian, Atdabanian, Botoman, Toyonian) are often used for global comparisons although no formal decision has ever been made. The book on "The Tommotian Stages and the Cambrian lower boundary problem" (Rozanov et al., 1969) with the publication of small shelly fossils (SSFs) as a new type of faunal assemblages was certainly the start into a new chapter of Cambrian research. Sections on the Aldan River were in discussion for several years as the GSSP for the Precambrian-Cambrian Boundary.

	. . Boundary between Yudoma Formation (dolomites, bed 14, with earliest Tommotian small shelly fossils) and Pestrotsvet Formation (glauconitic limestones, bed 15, with abundant Tommotian small shelly fossils) Dvortsy section, Aldan River, Siberia Copyright (C) G. Geyer, 1997
	. . Proterozoic to Cambrian transition with Yudoma Formation (white cliffs), Pestrotsvet Formation (red) and Tumuldur Formation (top). Dvortsy section, Aldan River, Siberia. Copyright (C) G. Geyer, 1997

The Lower Cambrian on the Siberian Platform can be subdivided into a southwestern Turukhansk-Irkutsk-Olekma facies belt, characterized by lagoonal deposits (particularly dolomites); a northeastern Yudoma-Olenek facies belt with deeper shelf bituminous limestones and black shales; and a transitional Anabar-Sinsk facies belt in between characterized by patch reefs and bioherms composed of archaeocyaths and skeletal algae. Rocks at the base of the Lower Cambrian in the Yudoma-Olenek facies belt, the Tommotian stage, are dominated by red limestones and include characteristic fauna mainly composed of SSFs. Late Early Cambrian rocks of the Yudoma-Olenek and the Anabar-Sinsk facies belts include a wealth of well-preserved trilobites, which are especially well studied from the localities along the middle Lena River.

Steep cliffs formed by upper Lower Cambrian Kutorgina Formation.

Labaya, Lena River, Siberia.

Lower-Middle Cambrian transition,

Highly fossiliferous limestones

of the Elanka Formation.

Elanka, Lena River, Siberia

Lermontovia grandis, cranidium.

Late Early Cambrian (Toyonian) L. grandis Zone

Olenek Uplift, banks of Arga Sala river, northern Siberia

The Middle and Upper Cambrian in the Turukhansk-Irkutsk-Olekma facies region is dominated by a thick carbonate-shale unit with prevailing of red-colored sulphate-rich rocks and a large number of endemic trilobites. A thick pile of limestones and subordinate shales constitutes the Middle and Upper Cambrian of the Yudoma-Olenek facies region. It forms more-or-less a non-saliniferous outer belt to the Turukhansk-Irkutsk-Olekma facies region. The transitional Anabar-Sinsk region shows a complicated facies pattern with lagoonal and normal marine sediments accompanied by distinct changes in thickness. Among endemic trilobites are some taxa with Pacific affinities.

Further reading

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