Ordovician Sequences and Megasequences of Siberian Platform (revised)

Ordovician geological history of the Siberian platform is surprisingly similar to that of Luurentia. The megasequences Sauk and Tippecanoe established for the North America could be easily recognized in Siberia. In the Tungus epicontinental basin the boundary between these two megasequences coincides with the regional erosional unconformity at the base of the Baykit Sandstone (Darriwilian Stage). Baykit Sandstone (up to 80 m thick) rests with ungular unconformity on Precambrian, Cambrian, Lower Ordovician and lower part of the Middle Ordovician (Dapingian) deposits. It separates two Ordovician carbonate seccessions of contrast lithology: 1) Tremadocian-Dapingian warm-water tropical carbonates, analog of the Great American Carbonate Bank (up to 420 m thick) and 2) Darriwilian-Katian cool-water temperate climate carbonates (up to 225 m thick). Destruction of the tropical “Great Siberian Carbonate Bank” connected with uplift of the western (in present day coordinates) margin of the Siberian palaeocontinent and its transformation from passive into active one in the process of Taconic orogeny. Numerous K-bentonite beds in the Upper Ordovician of Siberia reflects active explosive vulcanism along this margin. Ordovician succession of the Tungus basin was previously subdivided into nine depositional sequences. Resent investigations allow introducing some corrections into this scheme. In the Ordovician part of the Sauk megasequence in Siberia seven 3-rd order depositional sequences could be recognized: Nya-1, Nya-2, Nya-3, Nya-4, Nya-5, Ugor and Kimai. The former Nya sequence have been subdivided into 5 depositional sequences based on the new data from the Moyero River section. For the Ordovician part of the Tippecanoe megasequence the following corrections are introduced. 1) The former Baykit sequence that corresponded to the Baykitian and Mukteian regional stages subdivided into Baykit s.stricto and Muktey sequences. 2) The former Kirensk-Kudrino sequence turned out to be compiled of two depositional cycles separated by unconformity: Kirensk and Kudrino sequences. 3) The former Mangazea sequence subdivided into Chertovka and Mangazea s.stricto sequences which correspond to the Chertovskian and Baksian regional stages. 4) The former Kety sequence also subdivided into Nirunda and Bur sequences corresponding to the Nirundian and Burian regional stages. Revised sequence stratigraphic scheme of the Darrivilian, Sandbian and Katian of the Siberian platform consists of ten depositional sequences: (1) Baykit, (2) Muktey, (3) Volgino, (4) Kirensk, (5) Kudrino, (6) Chertovka, (7) Mangazea, (8) Dolbor, (9) Nirunda, (10) Bur. Hirnantian deposits are not known on the Siberian platform.