Vendian not synchronous carbonate sedimentation in Paleo-Asian Ocean

Vishnevskaya I.A.

V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia

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The Vendian and Cambrian of Paleoasian Ocean is represented by thick sedimentary successions. The Baikal Group in particular characterizes passive continental margin settings of the Siberian Craton associ­ated with opening of the Paleoasian Ocean [1]. The coeval carbonate sequences on the shelf of microcontinents have very poor stratigraphic constraints on account of insufficient paleontological (despite rare occurrences of Ediacaran fossils, archaeocyathids and trilobites) and geochronological information. Sr isotopic chemostratigraphy appears to be the only approach to establish the relative age of sedimentation of these carbonate sequences. We studied Sr isotope composition of Vendian and Cambrian carbon­ates in the south of Siberian Craton (Baikal Gr), as well as Tuva-Mongolia (Boxon and Khubsugul groups and Muren formations of eastern Sayan and northern Mongolia), Dzabkhan (Tsaganolom and Bayangol formations of Western Mongolia) and Batenev (Eniseyan formation, Alatau Kuznetsk) terranes (Fig.). Based on petrographic and geochemical analyses we identified the least altered rock samples; these samples are thought to have primary Sr isotope composition reflecting that of the paleocean. We used samples with the lowest amount of terrigenous component and meeting the following criteria: Mg/Ca<0.024, Fe/Sr<5.0, Mn/Sr<0.2 for limestones and Mg/Ca<0.6, Fe/Sr<3.0, Mn/Sr<1.2 for dolomites with the lowest degree of postsedimentary alteration [2]. Samples from the Baikal Gr have ⁸⁷Sr/⁸⁶Sr values of 0.7084-0.7087 [1]; from the Tsaganolom Fm, 0.7072-0.7079; and from the Bayangol Fm, 0.7084-0.7086. The least altered carbonates of the Tuva-Mongolia terrane have Sr isotope values of 0.7073-0.7086. The lowest ⁸⁷Sr/⁸⁶Sr ratios are

Fig. Sr-isotope chemostratigraphy of Vendian-Cambrian carbonates of the eastern segment of Paleoasian Ocean (not to scale).

Legend: 1 - limestones; 2 - dolostones; 3 - silicified limestones; 4 - sandstones; 5 - stromatolites; 6 - diamictites; 7 - granites of Primor'e Complex; 8 - shales and siltstones; 9 - phosphorites; 10 - Lower Cambrian trilobites; samples: 11 - limestones meeting the geochemical criteria (Fe/Sr<5.0, Mn/Sr<0.2), 12 - altered limestones, 13 - dolostones.

obtained for carbonates of the Zabitsk (Uha-Gol River) and Muren formations [3]. The Yenisei Gr is characterized by ⁸⁷Sr/⁸⁶Sr rations of 0.7075-0.7085 [4-6]. The following age constrains are established by methods of isotope chemostratigraphy. Maximum age of the Baikal Gr in the south of Siberian Craton is late Vendian (550-520 Ma). Carbonates of the Tsaganolom Fm formed during the Vendian (590-560 Ma), and the Bayangol Fm, mostly in the early Cambrian (550-520 Ma). Carbonate sedimentation in various parts of the Tuva-Mongolia terrane was diachronous and started between 600-580 My ago and continued into middle Cambrian. The Yenisei Gr was deposited during Vendian and early Cambrian (580-517 Ma). The commence of carbonate sedimentation in the Vendian was not synchronous in different parts of the Paleoasian Ocean and cannot be used for lithological correlation of the carbonate successions.

The work was supported by the Russian Foundation of Basic Research (projects nos. 09-05-01030, 10-05-00971, 12-05-00569) and the Presidium of the Siberian Branch of Russian Academy of Sciences (project no. 19, 68).

References:

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