Paleomagnetism of Vendian rocks in the southwest of the Siberian Platform

Objects of Study: Geology and Age

[9] During the field work done in 2001-2003 we collected Late Precambrian rock samples in the area of the Yenisey Ridge and also in the Biryusa and Central Sayan areas. Some reconnaissance work was done in the Yenisey Ridge to study the rocks of the Lower Cambrian Kliminskaya Formation.

Figure 1

Figure 2

[10] In the Yenisey Ridge (Figures 1 and 2), following the Angara, Irkineeva, and Taseeva rivers, we studied the variegated terrigenous and terrigenous-carbonate rock sequences of the Taseeva Series (Alesha, Chistyakova, and Moshakova formations) of Late Riphean (?) to Vendian age, the Redkolesnaya and Ostrovnaya formations of Vendian age, and the Kliminskaya Formation of Lower Cambrian (Atdabanian) age, and the Kliminskaya Formation of Lower Cambrian (Atdabanian) age [Rozanov et al., 1992].

[11] The Alesha Formation consists of red and cherry-red polymictic sandstones. Samples were collected in the stratotype rock sequence at the right bank of the Taseeva R., below the Usolka R., from the Antoshka horst.

[12] The Chistyakova Formation is represented by gray and greenish gray polymictic sandstones with scarce dolomite interlayers. Samples of the green rocks were collected along the Angara R.: (1) at its left bank below the Man'zya Settlement; (2) at the right bank above the Greben Rock (Shalyga brachyanticline), and (3) at the right bank of the Taseeva R. below the Usolka R.

[13] The Moshakova Formation consists of brick-red and cherry-red sandstones and siltstones of quartz and feldspar-quartz composition. Samples were collected along the Angara R.: (1) at its left bank below the Man'zya Settlement and (2) along its right bank about 2 km below the Gremyachiy Creek to the Greben Rock.

[14] The Redkolesnaya Formation is composed mainly of brick-red, inequigranular, quartz-feldspar sandstone, some of the layers including reddish and greenish siltstone interbeds. Samples of these rocks were collected at the right bank of the Angara R. below the Gremyachiy Creek and in two outcrops at the left and right banks of the Irikineeva River (Irkineeva High) ~30 km and 27 km above the river mouth, respectively.

[15] The Ostrovnaya Formation is composed mainly of light-color dolomite alternating with layers of red dolomite marl and dolomite-bearing siltstone, this rock sequence being completed by brownish dolomitic sandstone. Samples of the red rocks were collected at the right bank of the Angara R., above the Gremyachiy Creek.

[16] The Kliminskaya Formation is composed mostly of dolomite and dolomitized limestone with the scarce interbeds of reddish-brown quartz sandstone. The red rock samples were collected at the right bank of the Taseeva R. at the base of the Dyrovatyi Cliff (Upper Taseeva anticline). The total number of the oriented samples collected from the Precambrian-Cambrian rocks of the Yenisey Range was about 450.

[17] The study objects belong to the eastern zone of the Yenisey Range and are represented by the deformed sedimentary cover of the Siberian Craton. As follows from the data available [Orlov, 2002], the folded structure of the Yenisey Range was shaped during several time periods ranging from the Precambrian to the Mesozoic. The modern structural pattern of the range was formed as late as the Early Jurassic [Makarenko, 1971]. The frequent rearrangements of the structural style are recorded by frequent erosion scarps and angular unconformities [Orlov, 2002]. The platform sedimentary cover of the Yenisey Range includes several structural stages: Late Riphean-Lower Cambrian, Middle-Late Cambrian-Early Ordovician, Carboniferous-Permian, Early Triassic, Early Jurassic, and Cenozoic.

Figure 3

[18] Studied in the Biryusa area of the Sayan region were the variegated terrigenous and terrigenous-carbonate rocks of the Aisa (Late Riphean (?)-Vendian) and Ust-Tagul (Vendian) formations in a series of outcrops along the Biryusa and Tagul rivers in the areas of the Serebrovo and Georgievka villages up to the area where these rivers flow together (see Figures 1 and 3).

[19] The Aisa Formation (the upper member of the Oselkova Series) is represented by the alternation of variegated polymictic sandstones, siltstones, argillites.

[20] The Ust-Tagul Formation rests with erosion and without visible unconformity on the rocks of the Aisa Formation and consists of two subformations. The lower subformation is composed mainly of red terrigenous rocks, ranging in particle size from conglomerates to argillite; the upper subformation is represented by terrigenous-carbonate rocks with interbeds of red sandstone and siltstone. About 180 oriented samples were collected from the Biryusa area of the Sayan region.

[21] The structural features investigated during this study are deformed monoclines of the NW-SE strike (from sim 120^o to 160^o ) dipping to the northeast at the angles of 7^o to 35^o. The modern fold structure of the Biryusa Sayan region was not formed during some single act event: the platform cover includes seven structural units: Late Riphean, Late Riphean-Vendian, Late Vendian-Early Cambrian, Middle-Late Cambrian, Ordovician-Silurian, Early Devonian, and Jurassic. On the whole, the region has a "general" monoclinal structure with the steady growth of the rock sequence to the northeast (that is, toward the center of the platform) [Nalivkin, 1967].

[22] Studied in the Central Sayan area (Figure 1) were the Vendian variegated terrigenous and terrigenous-carbonate rocks of the Mota (Shaman) and Irkutsk formations in two outcrops at the left bank of the Urik River, higher than the Shankhar Settlement, at a distance of about 5 km from each other.

[23] The Mota (Shaman) Formation is represented in the outcropping part of its sequence in the Krasnaya Mt. by a thick (~190 m) sequence of red fine-grained sandstones and siltstones. This outcrop is a gentle monocline with an average azimuth of 330^o and a dip angle of 5^o.

[24] The rocks of the Irkutsk Formation rest conformably on the rocks of the Mota Formation, have a terrigenous-carbonate composition, and are about 160 m thick. Samples were collected from the red and green rocks in the middle of the Seraya Mountain rock sequence (from Member 4, after [Khomentovskiy et al., 1972]) and from the Urik variegated rock member crowning the Irkutsk Formation rock sequence in this region. The azimuth dip of the Urik rocks is 80^o and dip angle 14^o. 107 oriented samples were collected from the outcrops in the Urik R. outcrops.

Figure 4

[25] According to the modern views, the Late Precambrian rocks of the Sayan area and Yenisey Range area showed a fairly good correlation (Figure 4). However, because of the highly scarce fauna remains, the regional correlation was based mainly on the known historic events [Khomentovskiy and Postnikov, 2001; Khomentovskiy et al., 1972]. Although the upper boundary of the Vendian rocks (traced along the top of the Nemakit-Daldynian rocks) in the southwest of the Siberian Platform is now fairly well supported by stratigraphic data. For instance, in the Yenisey Range the basis of the Tommotian Stage was placed in the vicinity of the basis of the Usolie (Irkineeva) Formation [Khomentovskiy et al., 1998]. Yet, the boundaries of the "intra-Vendian" rock units and the position of the Vendian bottom are still the matter of hot discussion (see Figure 4). Recently, Yu. K. Sovetov discovered diamictite boulders, similar in lithology to tillite [Sovetov, 2002a, 2002b, 2002c]. Similar rocks were found by him in a number of other stratigraphic analogs of the Oselkova rock series (in particular, in the Taseeva rock series. This allowed Sokolov [1997] to combine these rocks into one tillite unit, corresponding to the Varangerian glaciation in terms of its Lower Vendian volume. Later, Sovetov [2002b] included the Aisa Formation into the Kotlin Horizon of the Upper Vendian rocks of the East European Platform [Sovetov, 2002b], that is, to the Late Ediacarian level in terms of Siberia.

[26] These views contradict the conventional views concerning the Vendian stratigraphy in the southwest of Siberia [Khomentovskiy et al., 1972; Kochnev, 2002], according to which the Vendian basement is placed in the regions discussed at the bases of the Ust-Tagul and Redkolesnaya formations, and the underlying rocks of the Taseeva and Oselkova rock series are ranked as Baikalian (Late Riphean). At the same time, the recent isotopic and geochemical studies ( d¹³ C and ⁸⁷ Sr/ ⁸⁶ Sr) of the rocks of the Baikalian Series in the stratotype area, compared with the standard ones, suggested the Vendian (pre-Nemakit-Daldynian) age of these rock sequences [Letnikova et al., 2004]. Moreover, Kochnev [2002] discovered the traces of living organism activity at the base of the lower subformation of the Ust-Tagul Formation. These organisms were identified by D. V. Grazhdankin (Academic Institute of Paleontology) as Treptichnus pedium, which is known to be typical of the Nemakit-Daldynian Stage basement.

[27] In other words, these data suggest the Vendian age of the Taseeva and Oselkova rock series. Summing up the data deposited by the present time for the Vendian rocks of the study areas, we can suggest the following reference ages for the Vendian rocks (Figure 4): (1) the basement of the Vendian rocks correlates with the basement of the Oselkova rock series in the Biryusa area of the Sayan region; (2) the basement of the Nemakit-Daldynian Stage is located in the vicinity of the bottom of the Ust-Tagul Formation in the Biryusa area of the Sayan region, which suggests that the underlying rocks of the Aisa Formation of the Oselkova Series can be dated Ediacarian or some of its part; (3) the top of the Vendian (the boundary between the Nemakit-Daldynian and Tommotian stages) corresponds to the top of the island rock sequence of the Yenisey Range; (4) the Redkolesnaya Formation of the Yenisey Range may include the rocks of the Nemakit-Daldynian and Ediacarian (Vendian) rocks of the Vendian age, as has been proved by the finding of the Cyclomedusa ex. gr. Davidi fauna remains [Chechel, 1976].

[28] Up to now, no confident chemo- or biostratigraphic and geochronological data have been obtained to prove the Vendian age of the rocks of the Mota and Irkutsk formations in the Central Sayan (Urik R.). Therefore, in this paper we use the stratigraphic correlation version offered by Kochnev [2002]. According to this version, the stratigraphic units identified agree with the Ust-Tagul Formation of the Biryusa area of the Sayan region and can be attributed to the Nemakit-Daldynian Stage of the Vendian.