RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES6003, doi:10.2205/2005ES000183, 2005

Age of the Sediments and the Late Proterozoic Marine Sedimentation in the Sayany-Baikal Fold System

[6]  According to tectonic structure, the SBFS territory is divided into outer and inner belts (Figure 1). The structural zones within both belts differ in rock composition and degree of tectonic deformations. The geologic structures of the outer and inner belts are separated by a system of sutures and uplifts. The Early Proterozoic granitoids and sedimentary volcanogenic rocks are recovered on the uplifts. The outer belt is characterized by the most complete and well-studied sections of the Late Proterozoic and Lower Paleozoic sedimentary sequences with the established superposition. They include the Medvezhevka, Ballaganakh, Dal'netaiga, Zhuya, and Judoma regional horizons (Resolutions of the All-Union Stratigraphic Conference on the Precambrian, Paleozoic, and Quaternary of Central Siberia, SNIIGGiMS, unpublished material, 1983) that represent greater sedimentation cycles (Figure 2).

[7]  The inner belt is characterized by a complicated set of fault blocks and thrust sheets of heterogeneous composition. The rocks of different age from Precambrian to Lower Paleozoic are recovered there. The Late Proterozoic is represented by igneous and sedimentary volcanogenic sequences. Among them the riftogenic, subduction, and subplatform lithocomplexes are recorded. The age of metasediments in the inner zones has long been a matter of discussion and was estimated in the range from Early to Late Proterozoic [Fedorovskii, 1985; Perevalov et al., 1983; Salop, 1964; Stanevich and Faizulina, 1992]. The complex results of geological prospecting, radioisotopic and biostratigraphic research, and formational correlation allowed the recognition of Riphean and Vendian regional horizons in the sections of the Baikal-Muya zone (Figure 2) [Buldygerov et al., 1988; Dol'nik et al., 1980; Konnikov et al., 1994; Nemerov and Stanevich, 2001; Rytsk et al., 1999, 2000, 2001; Stanevich and Faizulina, 1992]. The sediments assigned to the Vendian (Yudomian) in the sections of both outer and inner zones are overlain by formationally similar carbonate sequences bearing the Lower Cambrian algae and fauna [Salop, 1964].

[8]  The absence of radiochronologic records and ambiguity of correlative importance of the phytolith and microfossil interregional forms leave the question on the range of the Upper Riphean in reference sections of the outer zones open to discussion [Stanevich and Faizulina, 1992]. The extreme age estimates of basal layers of the Dal'netaiga Horizon [Dol'nik, 2000; Khomentovsky et al., 1985, 1998] differ by over 300 million years. Most of microfossils known from the SBFS are rare in the Proterozoic sediments of other regions [Stanevich et al., 1999], however, the findings of specimens with a complicated morphology [Faizullin, 1998; Pyatiletov, 1983] most likely indicate the Late Riphean age of the Dal'netaiga Horizon. The first occurrence of major taxa of this "microbiota" was recorded in the Neoproterozoic sediments in Australia, China, Spitsbergen, and Uchur-Maiskii region of Siberia [Khomentovsky et al., 1998].

[9]  As regards the Middle-Late Riphean boundary in the SBFS sections, there are two approaches to the problem. According to a traditional one, the boundary is defined by means of interregional correlation based on phytoliths and microfossils or using the evidence of great tectonic transformations. Despite a great amount of records used for these correlations, the final results are ambiguous. A comprehensive interregional correlation can be supplemented with the data of intraregional correlations and with the recently obtained reliable isotopic datings of igneous rocks from the Baikal-Muya zone.

[10]  The Dal'netaiga and Zhuya regional horizons are recorded in both outer and inner [Nemerov and Stanevich, 2001; Stanevich and Faizulina, 1992] belts of the SBFS (Figure 2). The reliable isotopic datings from sedimentary volcanogenic sequences referred to these horizons fall in the interval from 650 to 850 Ma [Buldygerov et al., 1988; Rytsk et al., 1999, 2000, 2001; Sklyarov et al., 2003]. Thus among all variants of interregional correlation the above-mentioned records to a greater extent than others [Dol'nik, 2000; Stanevich and Faizulina, 1992] confirm the assignment of the Dal'netaiga and Zhuya sediments to the upper half of the Late Riphean [Khomentovsky et al., 1985, 1998; Shenfil', 1991]. The biostratigraphic data substantiating the age of the SBFS regional horizons in the unified scheme do not contradict with the Late Riphean position of the Dal'netaiga Horizon.

[11]  The continental rift, volcanic arc, backarc basin, and orogenic belt geodynamic environments were reconstructed for different periods of the Late Precambrian history of the SBFS [Dobretsov and Bulgatov, 1991; Konnikov et al., 1994; Nemerov and Stanevich, 2001; Stanevich and Perelyaev, 1997]. The Riphean-Vendian sediments represent three major stages of the region evolution, namely, the Medvezhevka-Ballaganakh, Dal'netaiga-Zhuya, and Judoma (Figure 2).

[12]  At the Medvezhevka-Ballaganakh stage the sediment deposition occurred in the riftogenic marine basin [Dobretsov and Bulgatov, 1991; Korobeinikov, 1990; Nemerov and Stanevich, 2001]. Initially (Figure 2) the riftogenic troughs were filled with coarse detritus. Basalt effusion occurred along the faults that confined troughs. The inherited downwarping resulted in the extension of the basin. The environments favorable for deposition of fine-grained carbonaceous sediments appeared in its distant and deep parts. Finally, as the trough became filled with terrigenous sediments, the carbonate shallow deposits were deposited throughout the basin. Concurrently the deposition of carbonate terrigenous sediments of the Oktolokit, Stoibishchnaya, and other formations occurred in the Baikal-Muya zone. The terminal stage of the riftogenic development is represented by laterally continuous uppermost beds of the Ballaganakh Horizon, which are of regional correlative importance and underlie the sediments referred to the Dal'netaiga Horizon in the sections of the outer and inner zones.

[13]  The sediment deposition of the Dal'netaiga-Zhuya stage (Figure 2) occurred in the basin characterized by marginal marine environments, mainly by island-arc complexes inferred for the Baikal-Muya zone. The stage began with the sea transgression towards the craton margin. The sedimentation was of cyclic character. During an active downwarping the heterogranular clastic rocks were deposited, then followed by clayey carbonaceous and carbonate sediments. The analysis of facies composition and relationships permits to reconstruct of transverse profiles of that part of the basin where shelf areas (Baikal and Patomskoe zones), back-arc basin depression (Bodaibo zone), and island-arc system have been established (Baikal-Muya zone) (Figure 2). The position of island arcs is confirmed by interpretation of fragments of ophiolitic suites as derivatives of the back-arc basin spreading [Dobretsov and Bulgatov, 1991; Stanevich and Perelyaev, 1997].

[14]  Sediments of the outer, near-platform zones of the Dal'netaiga period (Figure 1) represent different types of shelf sedimentation. In the Baikal area a combination of phytolithic constructions and beds and lenses of finely terrigenous sediments of the Uluntuiskaya Formation is characteristic of littoral and inner sublittoral environments with contrasting facies relationships in the bioherm areas. In the northern Patomskoe zone the prodelta of a large stream occurred, which resulted in a heterogeneous facies composition of the Valyukhta, Urin, and other formations composed of quartz sandstones and chemobiogenic carbonates likely formed in the offshore area at a depth down to 50 m. In southeastward direction (the Zhuya River) the carbonates grade into silty pelitic cyclic sediments with finely ribbon banding, formed in the distal part of the shelf at depths over 100 m. Further southwards, in the Bodaibo zone the level corresponding to the Valyukhta Formation is represented by carbonaceous, terrigenous sediments of the Khomolkha Formation. These flyschoid deposits with graded bedding and traces of turbidity currents are characteristic of the continental slope and continental rise environments formed at possible depths of 400 m to 2000 m [Kennett, 1987]. The overlying carbonaceous quartz sandstones and silty pelites of the Zhuya Horizon (Aunakit and Vacha formations) were formed in stagnation conditions in the most downwarped part of the paleobasin. In the Baikal-Muya zone the Valyukhta sedimentogenesis is represented by fragments of sedimentary volcanogenic sections (Dzhalagun, Ondoko, and other formations, Figure 2) bearing products of aerial volcanism. The volcanic activity significantly affected the geochemical properties of sediments of the Valyukhta and other formations from the northern areas [Nemerov, 1988; Nemerov and Stanevich, 2001].

[15]  The sediments of the Zhuya Horizon in the Patomskoe zone (Figures 1 and 2) were deposited in conditions of clayey carbonate sedimentation on a prograding shallow shelf platform with following wide distribution of stromatolite biostromes in the late Zhuya period. In the Bodaibo zone (Figure 2) the variegated marls of the Imnyakh Formation were deposited in a distal part of the shelf platform, which sharp downwarping in the upper Zhuya period resulted in deposition of highly carbonaceous, sandy and silty pelitic deposits of the Vacha Formation. Turbiditic textures indicate large depths and stagnation in this part of the paleobasin. The maximum values of carbonaceous component in the Vacha sediments correlated with the biochemogenic carbonates of the Patomskoe zone [Nemerov, 1988; Nemerov and Stanevich, 2001], indicate a high biological productivity of deposits. In the Baikal-Muya zone (Figure 2) the carbonaceous silty pelites are associated with lavas of contrasting composition and with psammitic tuffs and tuffites of andesite-dacite series (Asektamur, Ust'-Kelyana, and other formations/sequences). In the Baikal zone the late Kachergat sedimentation was characterized by stagnant conditions in deep parts of the middle and distal shelf. Against the background of stagnant depositional conditions the Kachergat Formation is marked by the appearance of polymictic psammitic material. These records define the pre-Judoma conditions of sedimentation in all zones as stagnant and preorogenic, indicating the alteration from a back-arc to foreland basin environment [Nemerov and Stanevich, 2001].

[16]  The sediments of the Judoma stage are represented by two formation types. The platform type, characteristic of proximal and middle shelf areas, is recorded in the sections of the Patomskoe zone and eastwards. It is represented by quartz sandstones, silty pelites, and chemobiogenic carbonates (Zherba Formation, Figure 2). The Siberian platform was the source of the material. Towards the Baikal zone the detrital composition of terrigenous facies grades from quartz through arkosic to polymictic and fragments' dimensions increase up to pebble size. These molassoid sediments of the Judoma Horizon are the most characteristic of the SBFS inner zones. The thickness of the horizon reaches there 7 km and the lower parts of the sections are commonly represented by graywacke, coarsely grained tuffites, and hybrid turbidites (Ushakova, Anangra, Dogalda, Kholodnaya, and other formations, Figure 2). The local uplifts separating troughs of sublatitudinal extension served as a source of detritus.

[17]  The second half of the Yudomian in different zones of the SBFS is characterized by a similar type of shallow carbonate terrigenous sediments. Compensation sedimentation occurred in the relict depressions of the basin. Its rapid filling and flattening of topography resulted in the formation of a relatively shallow but extended basin. In the Lower Cambrian its carbonate deposition expanded throughout the Siberian platform and adjacent SBFS areas [Khomentovsky et al., 1972; Salop, 1964].


RJES

Citation: Stanevich, A. M., V. K. Nemerov, Yu. K. Sovetov, E. N. Chatta, A. M. Mazukabzov, V. I.  Perelyaev, and T. A. Kornilova (2005), Precambrian microfossil-characterized biotopes from the southern margin of the Siberian craton, Russ. J. Earth Sci., 7, ES6003, doi:10.2205/2005ES000183.

Copyright 2005 by the Russian Journal of Earth Sciences

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