The structure and depositional environments of Mesoproterozoic petroliferous carbonate complexes in the western Siberian craton

Russian Geology and Geophysics 52 (2011) 923–944 www.elsevier.com/locate/rgg

The structure and depositional environments of Mesoproterozoic petroliferous carbonate complexes in the western Siberian craton E.M. Khabarov *, I.V. Varaksina A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia Received 28 March 2011

Abstract On the basis of available sedimentological, isotope-geochemical, and geophysical data, we have refined the intrabasinal correlation and the structure of Mesoproterozoic deposits of the Baikit anteclise. A pre-Vendian section was stripped within the Baikit anteclise, which can be correlated with the section of the Mesoproterozoic Teya and Sukhoi Pit (pre-Pogoryui) Groups of the Yenisei Ridge. Most of the overlying deposits of the Tungusik Group were destroyed during the tectonic events in the Early Neoproterozoic (~860 Ma) and in pre-Vendian time. Using the results obtained, we have refined the variations in the sedimentation environments of Mesoproterozoic deposits in the Baikit anteclise and Yenisei Ridge and constructed paleogeographic schemes for particular time intervals in the modern frame of references. The predominance of peritidal complexes and numerous signs of subaerial exposition indicate that the carbonate accumulation was periodically interrupted. In the periods (sometimes, long) when this process stopped, fine silicoclastic material transited from the shelf edge into a deep basin localized southwest of the Baikit anteclise. The longest transition periods were in Early Yurubchen, Dolgokta, and Late Vingol’da time, though the main flow supplying silicoclastic material to the Yenisei Ridge passed south of the Baikit carbonate shelf. In the Mesoproterozoic, the basin with predominantly carbonate sedimentation occupied a vast area in the western Siberian Platform, extending to the western margin of the Anabar Shield, where the Mesoproterozoic deposits were similar in structure and isotope-geochronological and isotope-geochemical characteristics to the pre-Vendian deposits of the Baikit anteclise. Most part of the area with carbonate sedimentation in the Baikit anteclise was remote from the shelf edge. The Mesoproterozoic shelf was much wider than its fragments in the recent structure on the western margin of the Siberian craton. © 2011, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: petroliferous complexes; sedimentology; carbon and strontium isotopes; Meso-Neoproterozoic; Baikit anteclise; Yenisei Ridge; Katanga saddle; Siberian craton

Introduction In the early 1960s, Academician A.A. Trofimuk substantiated the petroleum potential of the Precambrian deposits in the Siberian Platform (Trofimuk, 1960). Later on, his hypothesis was confirmed by drilling, and many oil and gas fields were discovered within the platform. One of the petroliferous objects is the Baikit anteclise localized in the west of the Siberian Platform, where Meso- and Neoproterozoic deposits are petroleum-productive (Kharakhinov et al., 2000; Kontorovich et al., 1996; Koshchuk et al., 1998; Trofimuk, 1992).

* Corresponding author. E-mail address: [email protected] (E.M. Khabarov)

A review of the available data on the pre-Vendian deposits of the Baikit anteclise shows that the sedimentary complex is of superintricate structure because of the abundant disjunctions and plications of different ranks. Therefore, the intrabasinal correlation of the deposits is still debatable. The age and correlation between the Mesoproterozoic complexes of the Baikit anteclise and the Neoproterozoic sediments of the neighboring regions, e.g., the Yenisei Ridge, have also being discussed. Many researchers assign the pre-Vendian carbonate deposits mainly to a Neoproterozoic unit (Khomentovsky and Nagovitsin, 1998; Mel’nikov et al., 2005; Surkov et al., 1996) and correlate them with the essentially carbonate complexes of the Neoproterozoic Tungusik and Oslyanka Groups of the Yenisei Ridge, and other geologists refer them to a preRiphean unit (Vinogradov et al., 1998). According to the

1068-7971/$ - see front matter D 201 1, V . S. S o bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2011.07+.014

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results of our sedimentological and isotope-geochemical studies, Mesoproterozoic deposits are predominant in the pre-Vendian Baikit section (Khabarov et al., 1998, 2002; Ponomarchuk and Khabarov, 2001). In recent years, the Slavneft’ Enterprise drilled new wells and sampled the cores. The results of their study made it possible to refine the regional and interregional correlation of the pre-Vendian sections, reconstruct the sedimentation environments, and reveal the rebuilding of the basin structure within the Baikit anteclise and adjacent areas. Note that the Baikit anteclise is one of a few objects permitting the elucidation of the specific carbonate sedimentation in Late Precambrian intracratonal basins. In this paper we present the results of study of the structure, intra- and interbasinal correlation, and evolution of the sedimentation environments of the carbonate complexes in the Baikit anteclise and adjacent areas.

Stratigraphy and geologic setting The Baikit anteclise is localized in the west of the Siberian Platform (Fig. 1) and is the largest petroliferous structure with productive Riphean (Meso- and Neoproterozoic) deposits (Kontorovich et al., 1996; Trofimuk, 1992). Beneath the Vendian sedimentary cover, there are several blocks (the most famous ones are the southwestern Yurubchen–Tokhomo and northeastern Kuyumba) characterized by different degrees of dislocation and separated by abundant faults. The blocks are grouped into two systems of NE and NW strikes, often with the amplitude of vertical block dislocation of more than 1 km. The dislocations led to the exposure of deposits of different stratigraphic horizons to the pre-Vendian erosive surface. The pre-Vendian deposits occur mainly subhorizontally, but in the near-fault zones the bed slope reaches 30–50º and even 90º.

There are also reverse faults; as a result, the young deposits in the sections of some wells are overlapped by older ones. This is observed, e.g., in the well Yu-66, where the granitegneisses of the Archean–Lower Proterozoic basement are underlain by Mesoproterozoic carbonate deposits. Several strata have been recognized in the pre-Vendian section (from bottom to top) (Fig. 2): Thickness, m (1) Zelendukon—quartz and feldspar-quartz sandstones and, more seldom, gravelstones, conglomerates, and siltstones . . . . . . . . . . . . . . . . . . . . . . . . . .

200

(2) Vedreshe—greenish-gray and dark gray argillites with packets of siltstones and, seldom, limestones and dolomites . . . . . . . . . . . . . . . . . . . . . . . . . . up to 300 (3) Madra—dark gray argillites, often silty, with fine beds and packets of clayey dolomites (seldom, limestones) and siltstones . . . . . . . . . . . . . . . . . . . . . . . . . .

~300

(4) Yurubchen—gray and dark gray stromatolite and peloid-intraclastic dolomites with beds and packets of quartz sandstones and sandy dolomites, especially in the lower part . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500–550 (5) Dolgokta—argillites and siltstones with partings and packets of dolomites and siliciclastic sandstones . . . . .

100–120

(6) Kuyumba—greenish-gray and dark gray stromatolitic and peloid-pisolitic-intraclastic dolomites (sometimes, limestones) with rare partings and packets of silty sandstones and argillites . . . . . . . . . . . . . . . . . . . . . . . . 500–600 (7) Kopchera—gray, dark gray, and greenish-gray argillites and dolomites . . . . . . . . . . . . . . . . . . . . . . . . 110–120 (8) Yukta—light gray, with a greenish and creamy tint, stromatolitic and peloid-pisolitic-intraclastic dolomites .

400–420

(9) Rassolka—oolitic-pisolitic-intraclastic and stromatolitic dolomites with packets of argillites, siltstones, and, more seldom, sandstones . . . . . . . . . . . . . . . . . . . . . 120–180

Fig. 1. The structure-formational zoning and main studied sections of the Meso-Neoproterozoic deposits of the Yenisei Ridge (A) and the localization of wells in the central part of the Baikit anteclise (B). 1, 2, boundaries: 1, between zones, 2, of Yenisei Ridge; 3, main studied sections; 4, localization of wells.

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(10) Vingol’da—oolitic-pisolitic-intraclastic and stromatolitic dolomites with packets of siliciclastic rocks in the basement and in the upper part of the strata . . . . . . . . . up to 600 (11) Tokura—greenish-gray argillites with dolomite partings . . . . . . . . . . . . . . . . . . . . . . . . . . . (12) Iremeken—stromatolitic dolomites with argillite horizons . . . . . . . . . . . . . . . . . . . . . . . . . . . .

120–130 >200

The thickness and composition of lithologic complexes vary laterally. In the sections of some wells, the Madra unit is presented not in full stratigraphic volume, which might be due to its partial erosion in pre-Yurubchen time. Our variant of correlation of wells is based on the results of sedimentological, geophysical, and isotope-geochemical (δ13C) studies. The methods of correlation of the pre-Vendian deposits of the Baikit anteclise were considered in more detail earlier (Khabarov et al., 2002). The Yenisei Ridge is a complex fold-thrust structure formed mainly during the Early Neoproterozoic collision (~860 Ma) and complicated during the later geologic events (Khabarov et al., 1996; Khain et al., 1993; Kheraskova, 1999; Khomentovsky, 1996, 2007; Kontorovich et al., 1996; Nozhkin et al., 1999; Postel’nikov, 1980; Semikhatov, 1962; Surkov et al., 1996; Vernikovsky et al., 2009; Volobuev, 1993; Votakh, 1968). Several types of sections of Meso-Neoproterozoic deposits are recognized in the Yenisei Ridge area, which reflect the paleotectonic and sedimentation zoning of sedimentary basins and correspond to large structure-formational zones: Kamenka, Gorbilok, Kait’ba, Glushikha, and Isakovka (Figs. 1 and 3). Though the boundaries of these zones not always coincide with the modern boundaries of the main dips and strikes of the Yenisei Ridge, there is a particular concordance in the position of modern and paleotectonic zones. The sedimentary complex of the Yenisei Ridge is thrust over the Siberian Platform along the Vel’ma regional fault. The Meso-Neoproterozoic sedimentary complex is formed by laterally varying siliciclastic, carbonate, and volcanosedimentary deposits (10–12 km thick). Volcanosedimentary strata are present mainly in the Glushikha and Isakovka zones. The degree of rock metamorphism generally increases in the western direction. Several groups are recognized in the Meso-Neoproterozoic unit (from bottom to top): Teya, Sukhoi Pit, Tungusik, and Oslyanka, which are locally overlain, with a sharp angular discordance, by Late Neoproterozoic (Baikalian and Vendian) strata. The Mesoproterozoic age of the Teya Group is still debatable (Postel’nikov, 1980, 1990; Shenfil’, 1991; Volobuev et al., 1976). According to our data for the Gorbilok zone (Chirimba River) (Khabarov, 1994), the upper part of the Penchenga Formation of the Teya Group is intimately related to the overlying Korda Formation of the Sukhoi Pit Group and forms a single thick sedimentary complex with the above-lying Mesoproterozoic deposits. A detailed description of the Meso-Neoproterozoic deposits of the Yenisei Ridge was made earlier (Khabarov, 1994; Mel’nikov et al., 2005).

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Results and discussion Correlation between the Riphean sections of the Baikit anteclise and the Yenisei Ridge. The age of the pre-Vendian section of the Baikit anteclise is under discussion. General geological data evidence that these deposits formed before the Early Neoproterozoic collision, which is markedly manifested in the neighboring Yenisei Ridge area, i.e., no later than 860 Ma, but the time of the origin of the Baikit basin is still unclear. According to the present-day concepts, the Baikit anteclise lacks deposits older than 1.0–1.1 Ga (Khomentovsky, 1996; Mel’nikov et al., 2005; Surkov et al., 1996), though some researchers (Khabarov et al., 1998, 2002; Kontorovich et al., 1996; Shenfil and Primachok, 1996) admitted a more ancient age of the deposits. Most researchers support the concept that it is impossible to make correlation between the mainly carbonate deposits of the Baikit anteclise and the mainly fine terrigenous deposits of the Teya and Sukhoi Pit Groups of the Yenisei Ridge. Therefore, a direct correlation is made between the carbonate deposits of the lower pre-Vendian beds of the Baikit anteclise and the lower thick carbonate strata (Kartochki and Alad’ina Formations) of the Yenisei Ridge. This correlation seems more logic, the more so that the basement of the pre-Vendian sediments of the Baikit anteclise is made up of the feldspar-quartz and quartz sandstones of the Zelendukon strata, which come to the depth level of the Pogoryui sandstones in the Yenisei Ridge section. Since the age of the latter is no older than 1.1 Ga, it is considered to be the lower age bound for the basement of the pre-Vendian Baikit deposits. This correlation scheme is far from unambiguous in terms of both geology and sedimentology. The Riphean (pre-Baikalian) section in the eastern zones of the Yenisei Ridge is subdivided into several groups: Teya, Sukhoi Pit, Tungusik, and Oslyanka. These deposits are more than 12 km thick; 6 km of the thickness falls on the pre-Alad’ina deposits. Following the most widely used correlation scheme, we should admit the ENE-directed sedimentation wedging-out of the 6 km thick strata formed mainly by the deep-water deposits of the lower parts of the Sukhoi Pit and Teya Groups, which are traceable in the eastern Yenisei Ridge. Probably, there was intense erosion that destroyed the thick strata before the accumulation of the sandstones of the Zelendukon and Pogoryui Formations, but in this case is it unclear why there are no at least fragments of more ancient Mesoproterozoic deposits anywhere. On the other hand, the geological data on the Irkineeva and Chadobets uplifts (Votakh, 1968) unambiguously evidence intense erosion of the Lower Neoproterozoic deposits. Therefore, it seems more logic that the section of the Baikit anteclise has preserved, first of all, the most ancient Mesoproterozoic deposits. Note that carbonate deposits occur at the Yenisei Ridge not only in the Oslyanka, Tungusik, and upper Sukhoi Pit Groups but also in the Penchenga Formation of the Teya Group and in the basement (Korda Formation) and middle beds (Uderei

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Fig. 2 (to be continued).

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Fig. 2 (to be continued).

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Fig. 2. Correlation between the sections of wells drilled in the Baikit anteclise. 1–4, dolomites: 1, stromatolitic, from columnar Conophytonide (a), columnar branched (b), and laminated (algal-mat laminites) (c) stromatolites; 2, oolite-peloid-pisolitic (a), oolite-peloid-intraclastic (b); 3, siltite-micritic, partly recrystallized (a), argillaceous (b); 4, sandy (a), silty (b); 5–8, limestones: 5, stromatolitic, 6, peloid-intraclastic, 7, siltite-micritic (a), argillaceous (b), 8, sandy (a), silty (b); 9, argillites (a) and silty argillites (b); 10, sandstones (a) and siltstones (b); 11, dolerites; 12, large gaps. Strata: zl, Zelendukon; vdr, Vedreshe; mdr, Madra; yrb, Yurubchen; dl, Dolgokta; km, Kuyumba; kp, Kopchera; ykt, Yukta; rsl, Rassolka; vng, Vingol’da; tkr, Tokur; irm, Iremeken.

Formation and upper part of the Pogoryui Formation) of the Sukhoi Pit Group (Khabarov, 1994). The directions of the material transition in the clay-carbonate turbidites and deposits of carbonate debris flows in the eastern sections of the Uderei Formation of the Yenisei Ridge evidence the existence of a source of carbonate material east and northeast of the formation. This source was a carbonate shelf, across which part of siliciclastic material that arrived at the Yenisei Ridge area periodically transited during the sea level lowstand. The main flow supplying this material to the ridge passed south of the Baikit anteclise. Therefore, we suggest that a significant part of the carbonate deposits of the Baikit anteclise was replaced mainly by the fine siliciclastic deposits of the Yenisei Ridge with preservation of some “tongues” of the Baikit carbonates. This is clearly observed within the Yenisei Ridge area. For example, in the transition from Kamenka to Gorbilok zone (i.e., at a distance of several tens of kilometers), the carbonate rocks of the Alad’ina and Kartochki Formations decrease in thickness from 700 to 200–350 m and are finally replaced by the Sosnovka Formation. The thickness of the overlying terrigenous Krasnaya Gora Formation (the thickness of the sediments filling the depression localized ahead of the carbonate platform) increases in the same direction from 40–100 to 800–900 m. A similar mutual replacement is observed for the Dzhur carbonate platform in the transition from Kamenka to

Gorbilok zone (Khabarov, 1994; Khabarov and Tanygin, 1993). Among the stromatolites composing most of the pre-Vendian carbonate section of the Baikit anteclise, stromatolitic laminites and ministromatolites with a typical fibrous structure are widespread. Such rocks are observed in all carbonate and terrigenous-carbonate strata, except for the uppermost ones— Tokura and Iremeken (Khabarov et al., 2002; Varaksina and Khabarov, 2007). Stromatolites with such structures are typical of Paleo- and Mesoproterozoic deposits. On the Siberian Platform, stromatolites with a fibrous structure occur mainly in the lower Mesoproterozoic beds—in the Kyutingda Formation in the Olenek uplift and in the Omakhta Formation in the Uchur–Maya region (Knoll and Semikhatov, 1998). These data again confirm that the most widely used variant of correlation between the sections of the Yenisei Ridge and Baikit anteclise (Khomentovsky, 1996; Mel’nikov et al., 2005; Surkov et al., 1996) is not by far the best. Thus, additional information was necessary to confirm or refute the predominant viewpoint. This problem was solved, first of all, by the determination of the absolute age of the deposits by isotopegeochemical methods. In the Baikit anteclise area, a sill of dolerites was revealed in the well Yu-30, where they occur among the sandstones of the Zelindukon Formation. In the well K-5, similar dolerites

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Fig. 3. Schematic correlation between the Yenisei Ridge sections. 1–6, limestones: 1, micrite-siltitic, recrystallized to different degrees, 2, stromatolitic, 3, coarse- (a) and fine-intraclastic (b), 4, oolite-peloid-pisolitic, 5, argillaceous (a) and argillaceous carbonaceous (b), 6, sandy; 7–12, dolomites: 7, micrite-siltitic, recrystallized to different degrees, 8, stromatolitic, 9, coarse- (a) and fine-intraclastic (b), 10, oolite-peloid-pisolitic, 11, argillaceous, 12, sandy; 13–15, argillites: 13, lowly carbonaceous (a) and carbonaceous (b), 14, calcareous (a) and calcareous carbonaceous (b), 15, silty (a) and silty carbonaceous (b), 16, siltstones, 17, sandstones, 18, volcanosedimentary deposits. Formations: krp, Karpinsky Ridge; pn, Penchenga; kd, Korda; gb, Gorbilok; ud, Uderei; pg, Pogoryui; ss, Sosnovka; kr, Kartochki; al, Alad’ina; pt1, Lower Potoskui; krg, Krasnaya Gora; pt2, Upper Potoskui; dg, Dzhur; sn, Shuntara; udr, Udoronga; br, Borema; sk, Seryi Klyuch; rb, Rybinsk; mk, Mokraya; dd, Dadykta; na, Nizhneangarsk; tk, Tokma; ds, Dashka; shb, Sukhoi Ridge; cn, Chineul.

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were found in granite-gneisses, in the zone near their contact with Mesoproterozoic deposits. The 40Ar/39Ar dating showed that the dolerites were intruded at ~1.5 Ga (Khabarov et al., 2002; Ponomarchuk and Khabarov, 2001). Hence, the sandstones of the Zelendukon Formation cannot be analogs of the sandstones of the Pogoryui Formation of the Yenisei Ridge and are surely older. The variations in the carbon and strontium isotope compositions of carbonate complexes of the Yenisei Ridge, Baikit anteclise, and Katanga saddle were studied and partly reported earlier (Khabarov et al., 1999, 2002). Therefore, we will present data only on the strontium isotope composition of the Yenisei Ridge and Baikit anteclise. The strontium isotope composition was studied in the soluble part of carbonates (after the solution evaporation and drying of the solid residue) by the chromatographic separation of Sr and Rb isotopes. The Sr and Rb isotope compositions were measured on an MI 1201-T mass spectrometer, using Re ribbons. The average 87Sr/86Sr value normalized to 86Sr/88Sr = 0.1194 in the standard SrCO3 sample SRM-987 (USA) was 0.71026 ± 0.00009 (the error at the 2σ level for three analytical runs) and that in the standard SrCO3 sample VNIIM (Russia) was 0.70805 ± 0.00005 (2σ, fifteen analytical runs). The internal error of determination of 87Sr/86Sr did not exceed 0.00014 and averaged 0.00006 (2σ). The carbon and oxygen isotope composition was measured on a Finnigan-delta mass spectrometer (the reproducibility of the measured values was no more than 0.1‰ for C and 0.15‰ for O for the OSO KN-2 (Germany) and MSA (Russia) standard samples), and the contents of Ca, Mg, Fe, and Mn in the soluble part of carbonates were determined by the atomic-absorption method on an SP9 PI UNIKAM setup (the error was no higher than 5%). In this study we accepted that the dolomite samples with the most reliable Sr isotope ratios must meet the following geochemical criteria: Mn/Sr < 2.5, Fe/Sr < 60, and Rb/Sr < 0.005. Moreover, we took into account the contents of Mn (<200 ppm), Fe (<2000 ppm), and Rb (mainly <0.5 ppm). For the scarce limestone samples, we accepted more severe restrictions for element ratios: Mn/Sr < 0.5, Fe/Sr < 5.0, and Rb/Sr < 0.001. The geologic ocean history shows a trend of increasing Sr isotope ratios, which is complicated by variations reflecting a change in the balance of mantle and continental strontium with different isotope compositions in sea water. It was established that the 87Sr/86Sr values below 0.705 are not specific for Neoproterozoic and younger carbonate deposits (Asmerom et al., 1991; Brasier and Lindsay, 1998; Gorokhov et al., 1995; Hall and Veizer, 1996; Kei et al., 2007; Kuznetsov et al., 2006; Mirota and Veizer, 1994; Pokrovskii and Vinogradov, 1991; Semikhatov et al., 2002; Vinogradov et al., 1998). These data indicate that if the rocks have low Sr isotope ratios, the corresponding section interval is hardly Neoproterozoic. The least altered carbonate rocks of the Baikit anteclise are characterized by low (<0.705) 87Sr/86Sr values clearly pointing to their pre-Neoproterozoic age (Fig. 4, Table 1). Probably, the section also lacks rocks younger than 1100 Ma, since a

positive anomaly of this age specific for the 87Sr/86Sr evolution curve of the World Ocean is not observed. Note that low 87Sr/86Sr values (<0.75) were detected in the Baikit anteclise section in the earlier investigations (Khabarov et al., 1998; Vinogradov et al., 1998). They were also established in the Siberian Platform—in the pre-Vendian deposits of the Olenek uplift (Gorokhov et al., 1995), Anabar massif (Pokrovskii and Vinogradov, 1991), and Katanga saddle (Vinogradov et al., 1994). The results of isotope-geochronological studies (Gorokhov et al., 1995, 2006; Kraevskii and Pustyl’nikov, 1991; Vinogradov et al., 1994) show that the main part of the sections of the pre-Vendian deposits in these regions formed in the Mesoproterozoic. In composition and formation sequence these deposits (sandstones in the basement, which give way to argillites and, then, to thick carbonate beds with occasional packets of siliciclastic deposits) are similar to those in the Baikit anteclise, which also suggests their similar stratigraphic position. For example, a sedimentological analysis of the pre-Vendian deposits from the Katanga saddle showed that the Dolgokta strata (1250–1270 Ma) (Khabarov et al., 2002) in the Baikit anteclise are well correlated with the Dzhelindukon essentially siliciclastic strata in the lower part of the pre-Vendian section of the Katanga saddle. The K/Ar age of the well-preserved glauconite from the Dzhelindukon strata is 1240–1260 Ma (Kraevskii and Pustyl’nikov, 1991). The Riphean carbonate rocks of the Yenisei Ridge are characterized by a wide spread in 87Sr/86Sr values (0.7050– 0.7095 and higher). However, the petrographic, geochemical, and isotope-geochemical studies showed that the high 87 86 Sr/ Sr values are mainly due to the significant postsedimentational alteration of rocks (e.g., the samples from the upper part of the Penchenga Formation and from the basement of the Korda Formation). The samples of the Neoproterozoic Tungusik Group (Dzhur and Seryi Klyuch Formations) and Oslyanka Group (Dashka Formation) with the best preserved initial Sr isotope composition show 87Sr/86Sr = 0.70502– 0.70515 (Fig. 5, Table 2). At the same time, the weakly altered limestones of the Seryi Klyuch and Dadykta Formations show rather high 87Sr/86Sr values (0.7060–0.7069); however, a positive anomaly is unlikely. High Sr isotope ratios were also revealed in the altered rocks from the upper beds of the Mesoproterozoic Sukhoi Pit Group. Probably, the higher ratios in the Alad’ina strata coincide with the positive anomaly in the rocks dated at ~1.1 Ga. In general, the 87Sr/86Sr curve for the Lower Neoproterozoic part of the Yenisei Ridge section corresponds to a fragment of the 87Sr/86Sr evolution curve for the Lower Neoproterozoic deposits and points to a higher stratigraphic position of the essentially carbonate complexes of the Yenisei Ridge relative to the carbonate deposits of the Baikit anteclise. According to the recent data on the evolution of the carbon isotope composition of the Late Precambrian ocean (Bartley et al., 2001; Derry et al., 1992; Halverson et al., 2005; Kah et al., 1999; Kaufman, 1997; Khabarov and Ponomarchuk, 2005; Knoll et al., 1995; Lindsay and Brasier, 2002; McKirdy et al., 2001; Pokrovskii et al., 2006; Walter et al., 2000), the

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Fig. 4. Carbonate carbon and strontium isotope curves for the pre-Vendian deposits of the Baikit anteclise. 1, stromatolitic limestones; 2–5, dolomites: 2, stromatolitic, from columnar Conophytonide (a), columnar branched (b), and laminated (algal-mat laminites) (c) stromatolites; 3, oolite-peloid-intraclastic (a) and oolite-peloidpisolitic (b); 4, siltite-micritic, partly recrystallized (a), argillaceous (b); 5, sandy (a); silty (b); 6, argillites (a) and silty argillites (b); 7, sandstones (a) and siltstones (b); 8, dolerites; 9, granite-gneisses of the basement; 10, stratigraphic gaps; 11–13, dolomites: 11, least altered, 12, moderately altered, 13, altered; 14–16, limestones: 14, least altered, 15, moderately altered, 16, altered. Strata indices are given in Fig. 2.

transition of δ13C values from nearly zero to variations from –2.0 to 2.0–3.0‰ takes place in the deposits dated at ~1.3 Ga. The Neoproterozoic carbonate rocks with an age of 1000–800 (or 750) Ma are characterized by much wider δ13C variations (from –2.0 to –3.0‰ to 4.0–6.0‰), and in the younger Neoproterozoic rocks the amplitude of these variations is still greater (from –5 to –10‰ to 8–12‰). In the latest Neoproterozoic, the amplitude of variations again slightly decreased. It was established that the δ13C variations are controlled not only by global but also by regional and local processes. Therefore, isotope studies are usually carried out along with analysis of the specific evolution of basins and possible postsedimentational disturbance of primary isotope systems.

As seen from the integral δ13C variation curve for the pre-Vendian deposits of the Baikit anteclise (Fig. 4), relatively low positive δ13C values, 1.5–2.0‰, prevail in the section, increasing to 2.5–2.8‰ in its upper beds. The positive anomalies are periodically changed by negative ones. The greatest negative shift (to –2.0‰) has been established for Late Yurubchen–Dolgokta time. According to the δ13C values of carbonates, the pre-Vendian deposits of the Baikit anteclise are within the Mesoproterozoic unit. Comparison of the absolute δ13C values and δ13C trends for the Yenisei Ridge and Baikit anteclise (Figs. 4 and 5) shows their significant difference. The δ13C values for the Yenisei Ridge are 3.5–4.5‰, reaching 5.6‰, and are much higher than those for the Baikit anteclise (Khabarov et al.,

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E.M. Khabarov and I.V. Varaksina / Russian Geology and Geophysics 52 (2011) 923–944

Table 1. The chemical and isotopic composition of pre-Vendian carbonate rocks of the Baikit anteclise 87

Rb/86Sr

87

Sr/86Sr meas.

(87Sr/86Sr)0

δ18O‰, PDB

0.70417

0.70404

–6.0

0.70459

0.70448

–7.2

Sample

Strata

Mn, ppm Fe, ppm

Mg/Ca

Mn/Sr

Fe/Sr Rb/Sr

Rb, ppm Sr, ppm

1.30.92

vdr

888

427

0.014

10.1

4.8

0.0030

0.277

88.2

0.0090

1.29.92

vdr

244

717

0.016

1.7

4.9

0.0025

0.369

146.1

0.0073

1.85.91

mdr

72

2756

0.620

3.6

134.0 0.0055

0.108

19.5

0.0159

0.70468

0.70445

–3.6

1.22.92

jrb

88

415

0.610

2.2

10.4

0.0009

0.038

39.7

0.0027

0.70440

0.70436

–4.1

12.22.92

jrb

104

1920

0.512

2.2

41.3

0.0009

0.044

46.4

0.0026

0.70475

0.70474

–2.1

12.21.92

jrb

120

2610

0.522

2.3

50.0

0.0009

0.048

52.1

0.0027

0.70422

0.70421

–2.5

1.79.91

jrb

93

845

0.590

3.1

27.9

0.0034

0.112

30.2

0.0109

0.70468

0.70452

–2.9

1.13.92

jrb

91

2061

0.580

2.9

66.7

0.0010

0.051

30.9

0.0133

0.70503

0.70484

–4.2

12.5.92

jrb

132

2541

0.515

3.5

68.5

0.0013

0.050

37.1

0.0035

0.70491

0.70490

–2.6

12.1.92

jrb

100

2430

0.531

2.2

52.9

0.0039

0.182

45.9

0.0114

0.70516

0.70510

–2.5

1.10.92

jrb

81

533

0.670

4.5

30.7

0.0050

0.094

18.0

0.0015

0.70433

0.70430

–5.2

B1.14.97

dl

122

1180

0.403

4.0

36.2

0.0060

0.219

32.6

0.0206

0.70510

0.70507

–2.1

K12.129

kmb

47

2222

0.098

0.3

12.2

0.0013

0.300

181.7

0.0047

0.70443

0.70442

–2.3

K12.126

kmb

69

1408

0.062

0.4

8.7

0.0017

0.280

160.7

0.0050

0.70461

0.70460

–2.3

K12.111

kmb

64

1041

0.069

0.2

3.7

0.0025

0.720

281.8

0.0074

0.70489

0.70488

–2.6

21.17.92

kmb

23

889

0.009

0.1

4.0

0.0009

0.216

218.8

0.0028

0.70453

0.70452

–5.5

K12.101

kmb

62

1679

0.055

0.4

12.3

0.0016

0.230

136.7

0.0047

0.70471

0.70470

–2.9

21.16.92

kmb

118

830

0.012

0.8

5.6

0.0017

0.251

146.9

0.0049

0.70461

0.70460

–3.9

1.62.91

kmb

67

661

0.620

1.4

14.4

0.0035

0.182

48.4

0.0103

0.70554

0.70539

–2.7

2.43.91

jkt

100

800

0.570

2.8

22.4

0.0009

0.034

35.6

0.0029

0.70573

0.70571

–4.4

3.11.92

vng

70

3160

0.518

2.0

92.4

0.0059

0.203

34.2

0.0171

0.70601

0.70576

–2.1

1.49.91

jkt

27

384

0.570

1.4

20.0

0.0020

0.042

19.2

0.0065

0.70518

0.70509

–4.2

1.43.91

jkt

54

382

0.540

1.8

12.7

0.0005

0.016

30.1

0.0014

0.70460

0.70458

–2.8

1.29.91

jkt

52

509

0.530

2.1

21.0

0.0030

0.076

24.2

0.0087

0.70553

0.70521

–3.2

1.22.91

jkt

63

885

0.570

1.0

14.0

0.0007

0.094

63.0

0.0044

0.70478

0.70472

–4.2

1.17.91

jkt

75

1085

0.610

2.5

36.1

0.0010

0.038

30.0

0.0035

0.70468

0.70463

–3.9

1.8.91

jkt

100

1370

0.540

2.0

27.5

0.0010

0.067

49.8

0.0064

0.70570

0.70561

–2.2

2.8.91

irm

330

2771

0.500

6.5

54.8

0.0040

0.241

50.6

0.0137

0.70523

0.70503

–5.9

2.5.91

irm

401

2450

0.420

8.3

50.8

0.0030

0.174

48.2

0.0100

0.70535

0.70520

–4.1

Note. Strata: vdr, Vedreshe; mdr, Madra; jrb, Yurubchen; dl, Dolgokta; kmb, Kuyumba; jrt, Yukta; rsl, Rassolka; vng, Vingol’da; irm, Iremeken. Data are given only for the least and moderately altered samples (excluding the Vedreshe, Madra, and Iremeken strata). The wells from which the samples were taken were reported by Khabarov et al. (2002).

1999, 2002). This is indirect evidence for the different ages of the essentially carbonate deposits of the Baikit anteclise and carbonate deposits of the Tungusik and Oslyanka Groups of the Yenisei Ridge. The former are localized at a lower stratigraphic level. Two large subunits are recognized in the section of the pre-Vendian deposits of the Katanga saddle: lower, terrigenous-carbonate (Ognevka Group), and upper, essentially carbonate (Kamo Group) (Mel’nikov et al., 2005). The lower subunit is characterized by δ13C variations from –2 to 2.2‰, and the δ13C curve is similar to that for the lower part of the pre-Vendian deposits of the Baikit anteclise. The Dzhelindukon negative shift (to –2‰) is particularly distinct in the deposits of the Katanga saddle (analog of the Late YurubchenDolgokta shift with an age of 1250–1270 Ma) (Fig. 4).

Thus, the complex of geological, sedimentological, isotopegeochemical, and isotope-geochronological data shows that the pre-Vendian deposits of the Baikit anteclise formed mainly in the Mesoproterozoic. In recent time, the Mesoproterozoic age of these deposits has also been accepted by some other researchers (Nagovitsin et al., 2010). The proposed correlation between the Riphean deposits of the eastern Yenisei Ridge and the Baikit anteclise is demonstrated in Fig. 6. The Zelendukon, Vedreshe, and Madra strata seem to correspond to the Lower Mesoproterozoic Teya Group or its main part and to the lowermost part of the Korda Formation. The above-lying strata are correlated with the pre-Pogoryui part of the Sukhoi Pit Group section (we accepted this correlation variant here). It is also possible to correlate the Vingol’da strata with the Alad’ina Formation,

E.M. Khabarov and I.V. Varaksina / Russian Geology and Geophysics 52 (2011) 923–944

933

Fig. 5. Carbonate carbon and strontium isotope curves for the Riphean deposits of the Yenisei Ridge. For the carbonate curve: 1, 2, limestones: 1, unaltered, 2, altered; 3, 4, dolomites: 3, unaltered, 4, altered; for the strontium curve: samples: 1, least altered, 2, moderately altered, 3, altered. Other designations follow Fig. 3.

and the Tokur and Iremeken strata can be considered analogs of the Krasnaya Gora Formation and lower part of the Dzhur Formation of the Tungusik Group. Within the Baikit anteclise, the above-lying deposits of the Tungusik Group were removed mostly in Early Neoproterozoic and pre-Vendian time. The main types of the Baikit anteclise deposits and sedimentation environments. The pre-Vendian deposits of the Baikit anteclise are mainly carbonate rocks, first of all, synsedimentational early-diagenetic dolomites significantly altered during late diagenesis. The dolomites are of several varieties: stromatolitic, grainstones, wackestones, packstones, micritic. The stromatolitic dolomites are dominated by laminites (stratiform-stromatolitic varieties). They usually alternate with peloid-intraclastic, peloid-pisolite-intraclastic, and intraclastic dolomites (the alternating layers can be several millimeters, centimeters, or decimeters in thickness); sometimes, they form

individual packets several meters thick. These rocks are often of fenestrated and tepee structures, with regular synsedimentational destruction of micropartings and, sometimes, shrinkage cracks. In places, laminites form associations with ministromatolites (fine columns 0.5–1.0 cm in size). The laminites and ministromatolites have micropartings with a typical fibrous structure, which points to the primary aragonitic composition of the sediment (Grotzinger and Read, 1983). All this evidences that laminites formed within the carbonate shelf, mainly in tidal and supratidal environments with a moderate hydrodynamic regime periodically disturbed by storms. Dolomites composed of unbranched columnar stromatolites (Conophytonides) are much scarcer. These are large subcylindrical subvertical columns, often more than 1 m in height, with a synoptic relief reaching 0.8 m. The intercolumnar space is usually filled with dolmicrite. Conophytonides formed within the middle shelf (between the fair-weather wave base

934

E.M. Khabarov and I.V. Varaksina / Russian Geology and Geophysics 52 (2011) 923–944

Table 2. Chemical and isotopic composition of Riphean carbonate rocks from the eastern Yenisei Ridge Sample

Height from formation basement, m

Mn, ppm Fe, ppm Mg/Ca Mn/Sr Fe/Sr

Rb/Sr

Rb, ppm Sr, ppm

87

Rb/86Sr

87 Sr/86Sr meas.

(87Sr/86Sr)0

δ18O‰, PDB

ss-1

60

1710

46,900

0.034

3.2

86.9

0.0085

4.640

540.0

0.0278

0.71483

0.71425

–11.9

ss-2

150

473

1820

0.019

0.6

4.5

0.0002

0.086

399.1

0.0074

0.70739

0.70728

–9.9

ss-3

250

541

1473

0.028

1.1

3.0

0.0017

0.836

482.8

0.0034

0.70627

0.70622

–11.0

ss-4

270

319

1496

0.003

1.5

7.0

0.0001

0.033

212.2

0.0007

0.70663

0.70662

–11.7

ss-5

300

1080

10.370

0.042

4.6

43.9

0.0373

8.820

236.0

0.0549

0.71835

0.71757

–11.5

al-3.12.7

310

90

2700

0.297

2.9

88.4

0.0011

0.035

30.6

0.0035

0.70558

0.70553

–3.6

dg-8

78

127

751

0.005

0.4

2.7

0.0014

0.398

276.7

0.0033

0.70598

0.70592

–6.4

dg-10

137

77

875

0.007

0.5

5.7

0.0003

0.043

153.4

0.0031

0.70547

0.70543

–7.8

dg-11

148

118

366

0.034

0.3

1.0

0.0014

0.536

359.4

0.0043

0.70514

0.70508

–7.1

dg-14

320

152

2060

0.020

0.3

4.8

0.0010

0.504

430.0

0.0051

0.70518

0.70511

–5.0

sn-4

380

914

874

0.004

1.0

0.9

0.0006

0.596

912.0

0.0015

0.70675

0.70673

–6.6

dg-2.1b.7

90

164

1610

0.011

0.3

2.6

0.0014

0.932

632.8

0.0042

0.70550

0.70546

–4.5

dg-2.1.7

110

290

2040

0.012

0.4

3.2

0.0012

0.825

635.6

0.0047

0.70509

0.70505

–2.9

dg-16a

140

131

461

0.011

0.3

0.9

0.0018

0.890

486.5

0.0016

0.70507

0.70505

–2.9

dg-11

219

43

642

0.002

0.3

4.4

0.0002

0.026

145.6

0.0004

0.70656

0.70655

–6.5

dg-13

345

60

345

0.011

0.3

1.9

0.0002

0.043

182.1

0.0009

0.70696

0.70695

–6.5

rb-2

440

175

3610

0.006

0.2

3.6

0.0008

0.804

996.8

0.0008

0.70611

0.70604

–10.1

rb-3

445

177

2870

0.005

0.2

2.7

0.0002

0.214

1070.0

0.0043

0.70627

0.70621

–10.0

dg-1

100

298

6670

0.051

0.2

3.8

0.0003

0.590

1750.0

0.0008

0.70514

0.70513

–7.7

dg-6

225

374

5640

0.037

0.4

5.0

0.0009

0.924

944.1

0.0023

0.70530

0.70527

–7.2

dg-8

305

644

1420

0.008

1.4

3.1

0.0004

0.196

458.0

0.0009

0.70539

0.70538

–6.8

dg-9

350

205

3570

0.108

0.5

8.1

0.0009

0.440

440.8

0.0042

0.70671

0.70666

–3.7

dg-11

515

42

820

0.016

0.3

5.4

0.0005

0.078

152.0

0.0011

0.70651

0.70650

–6.3

dg-18

625

63

530

0.009

0.4

3.9

0.0009

0.123

137.7

0.0018

0.70586

0.70584

–4.7

dg-24

835

42

957

0.032

0.2

4.6

0.0010

0.212

206.2

0.0024

0.70502

0.70502

–2.9

dg-25

850

48

1430

0.011

0.2

7.6

0.0009

0.187

189.3

0.0022

0.70515

0.70512

–3.1

dg-27

877

36

670

0.007

0.2

4.7

0.0002

0.036

142.6

0.0006

0.70559

0.70558

–3.6

dg-28

905

64

539

0.008

0.4

3.1

0.0004

0.065

172.5

0.0008

0.70561

0.70560

–6.2

dg-31

961

183

1050

0.007

0.5

2.8

0.0002

0.062

383.3

0.0003

0.70564

0.70563

–6.1

dg-33

1075

228

504

0.007

0.5

1.2

0.0002

0.086

435.9

0.0005

0.70640

0.70639

–3.0

dg-35

1110

56

877

0.024

0.3

4.6

0.0012

0.226

191.1

0.0025

0.70669

0.70666

–8.6

dg-37

1148

51

355

0.025

0.4

3.0

0.0005

0.061

132.5

0.0012

0.70669

0.70667

–4.3

dg-39

1235

32

681

0.003

0.04

0.9

0.0001

0.092

708.9

0.0003

0.70547

0.70546

–5.4

dg-40

1275

62

1760

0.005

0.1

2.9

0.0001

0.064

602.6

0.0002

0.70598

0.70597

–5.2

Note. Formations: ss, Sosnovka; al, Alad’ina; dg, Dzhur; sn, Shuntara; sk, Seryi Klyuch (dg-16a; dg-2.(a,b).7, samples from the Angara section); rb, Rybnaya (Dadykta); ds, Dashka. The samples were taken from the formation basements. Also, data for the least and moderately altered samples are given (excluding the samples from the Sosnovka and Alad’ina Formations). The section correlation and distribution of samples were considered by Khabarov et al. (1999).

and storm wave base). The branched columnar stromatolites are dominated by low- to medium-column (up to 0.15 m high) varieties. The intercolumnar space is filled mainly with intraclastic or peloid-intraclastic material. Grainstones and packstones are characterized by different combinations of grains (pelloids, oolites, pisolites, intraclasts). An exclusion is intraclastic dolomites, in which intraclasts amount to 90% of all grains. Intraclasts are often fragments of stromatolitic laminas—stromatoclasts. There are also dolo-

mites with coarse composite grains having a common microbially coated grains formed at the back of oolite-intraclastic bars. In places, flat-pebble conglomerates are observed. Grainstones and packstones (often with abundant quartz clastics) are widespread and form thick (tens of meters) bodies of well-sorted grainstones. They also occur in association with stromatolitic dolomites. Micritic (microcrystalline) (<10% of grains) and micriticwackestones, and rare packstone (10–50% of grains), often,

E.M. Khabarov and I.V. Varaksina / Russian Geology and Geophysics 52 (2011) 923–944

935

Fig. 6. Stratigraphic correlations between the Riphean sedimentary complexes of the Yenisei Ridge and Baikit anteclise. Formation indices are given in Figs. 2 and 3.

clayey dolomites (sometimes, limestones) occur in most strata, though are scarce. Most frequently they are observed in the Madra, Dolgokta, Kopchera, and Tokura strata. These rocks formed both within the shallow, sometimes, isolated shelf (in association with stromatolitic laminites) and within the deep shelf, below the storm wave basis (together with argillites), where they usually compose the upper parts of the thin layers of carbonate storm turbidites. Some varieties of micritesparitic dolomites resulted from the fine recrystallization of primary stromatolitic and granular rocks. Limestones are much scarcer than dolomites. In the well Yu-30 they form a member of variegated stromatolitic limestones in the basement of the Vedreshe strata (Fig. 2). These limestones composed of branched columnar stromatolites are overlain by a thin (~1 m) bed of oolitic grainstones. In the well-K-12, the lower part of the Kuyumba strata (~200 m) is formed mainly by thin-layered peloid-intraclastic sandy and micritic, often, clayey limestones alternating with calcareous argillites. These rocks often compose thin (<15 cm) beds, with the particle size decreasing up the bed section (storm tempes-

tites and turbidites deposited from turbid flows initiated by storms). They formed above and below the storm wave basis. Siliciclastic rocks are subordinate in the pre-Vendian sedimentary complex of the Baikit anteclise. In predominantly carbonate strata they occur as beds and packets, where both sandstones and clay-silty deposits are present. Sometimes they finely alternate with dolomites. There are often signs of fine ripple and shrinkage cracks on the bedding surfaces. The sandstones contain both quartz clastics and dolomite intraclasts, sometimes large. Siliciclastic rocks are most widespread in the lower part of the pre-Vendian section. The sandstones here are mainly coarse-grained massive, sometimes with indistinct cross bedding and conglomerate lenses, and are of quartz and feldspar-quartz composition. Argillites usually occur as partings (0.1–3.0 cm thick) or, more seldom, packets (up to 1–2 m) among carbonate rocks but sometimes form thick (tens of meters) members. In the Vedreshe strata, dark gray argillites enriched in OM alternate with greenish-gray silty argillites. The silty argillites are regarded as bottom flow deposits, and the dark gray argillites,

936

E.M. Khabarov and I.V. Varaksina / Russian Geology and Geophysics 52 (2011) 923–944

Fig. 7. Sedimentation environments in the Yenisei–Baikit region in the Early Mesoproterozoic (Vedreshe–Early Madra (Penchenga) time). 1, shallow and middle shelf with terrigenous-carbonate sedimentation; 2, slope and basin environments with terrigenous-carbonate sedimentation, sometimes in euxinic conditions (rift slops); 3, differentiated-rift environments with volcanogenic and volcanoclastic deposits (axial rift zone); 4, shallow shelf environments with siliciclastic deposits around islands; 5, deep shelf environments with accumulated silt-clayey and carbonate-clayey hemipelagites (sometimes, carbonaceous) and turbidites; 6, modern eastern boundary of the Yenisei Ridge.

as hemipelagites. There are widespread rocks with a clay-silty groundmass and dispersed plane large (up to 7 cm long) and small clastics of dark gray carbonaceous argillites. The presence of these rocks, which were earlier found in the Vedreshe strata of the Baikit anteclise and in the Ognevka Group of the Katanga saddle, points to a periodic increase in the power of bottom flows and the destruction of hemipelagite aprtings. There are also thin (1–5 cm) intercalates of storm turbidites of clay-silty composition. By these features the thick argillites members are interpreted as deep-shelf deposits (below the storm wave basis). Paleogeographic zoning and basin evolution. The sedimentological data and results of geophysical studies showed the existence of six second-rank sequences in the section (Zelendukon–Vedreshe, Yurubchen, Dolgokta–Kuyumba, Kopchera–Yukta, Rassolka–Vingol’da, and Upper Vingol’da– Iremeken), which, in turn, include three to five third-rank sequences. Sedimentary systems have been revealed within the sequences, which formed during the sea level lowstand, transgression, and highstand. The recognition of sequences of different ranks was based on analysis of layering, revealing of subaerial-exposition surfaces and gaps, and interpretation of siliciclastic-deposit genesis. The section is composed mainly of peritidal deposits, including those of the uppermost sublittoral zone, tidal and supratidal plains, and(or) shallowwater lagoon-shelf environments (peritidal carbonates, after Pratt et al. (1992)), with abundant microbial laminites and associated peloid-pisolite-intraclastic rocks. Therefore, the complexes formed during the sea level lowstand were the most difficult to recognize. They are usually reduced, because siliciclastic material transited across the carbonate shelf in

these periods. In addition, these deposits were reworked during transgressions (Khabarov et al., 2002). In the analysis of the sedimentation environments in the Yenisei–Baikit region, we used the data not only on the Yenisei Ridge and Baikit anteclise but also on other regions of the Siberian Platform. In particular, we invoked data on the Chadobets uplift, where Meso-Neoproterozoic deposits were stripped. These deposits include stratigraphic analogs of the Teya, Sukhoi Pit, and Tungusik Groups of the Yenisei Ridge (Votakh, 1968). In addition, data on the Katanga saddle were used. Stratigraphic analogs (Ognevka and, partly, Kamo Groups) of the lower section of the Baikit anteclise (from Vedreshe to Kuyumba strata) were distinctly recognized in the Precambrian section of the saddle from the lithological and isotope-geochemical data. We also took into account the information on the structure of the sections of pre-Vendian deposits on the margins of the Anabar and Olenek uplifts (Gorokhov et al., 1995, 2006; Knoll et al., 1995; Pokrovskii and Vinogradov, 1991), where Mesoproterozoic complexes are distinguished. The pre-Vendian deposits of the Baikit anteclise and the margins of the Anabar and Olenek uplifts form a single vast basin, which is confirmed by the results of seismic studies. In paleogeographic schemes, the arrangement of sedimentation zones is shown in the modern frame of references without palinspastic reconstructions; therefore, it somewhat incorrectly depicts the paleogeographic situation that existed during the sedimentation. The scheme omits the sites lacking deposit complexes. In the basement of the Mesoproterozoic deposits of the Baikit anteclise, in the Zelendukon strata, there is a thick

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Fig. 8. Sedimentation environments in the Yenisei–Baikit region in the late Early Mesoproterozoic (Yurubchen (Early Korda) time). 1, shallow shelf with predominant laminites and grainstone-packstones; 2, high-energy shallow shelf with predominant grainstones; 3, shallow shelf with abundant sandstones in the basement of the Yurubchen strata; 4, deep shelf (intrashelf depressions) with accumulated thin-layered clay-carbonate deposits (often, storm turbidites); 5, middle shelf environments with accumulated silt-clayey and carbonate deposits, sometimes, with stromatolites; 6, predominantly shallow carbonate-siliciclastic shelf; 7, middle shelf with siliciclastic sedimentation; 8, deep shelf with predominantly siliciclastic sedimentation; 9, slope environments with predominant gravitites; 10, basin plain with fine siliciclastic sedimentation; 11, basin plain with accumulated volcanosedimentary deposits.

sandstone member with conglomerate lenses, which rests upon the basement granite-gneisses. In the lowermost part of the member stripped by the well K-204 (Fig. 2), there are 1 m thick cycles of deposits with the grain size decreasing from bottom to top of the cycle, which are similar to the alluvial cycles. Upsection, the alluvial and littoral-marine sediments are changed by deep-water silt-clayey ones. The transgressive trend ends above the member of the Vedreshe stromatolitic limestones (well Yu-30, Fig. 2). The maximum-flooding surface is observed among the argillites of the Vedreshe strata with the minimum number of storm turbidite interbeds. Upsection, silty argillites with interbeds and packets of deep-shelf storm turbidites are predominant. They formed during the sea level highstand. During the initial rifting, the siliciclastic, including alluvial, sediments of the Karpinsky Ridge Formation accumulated in the Yenisei Ridge area. Later on, in Vedreshe–Early Madra (Penchenga) time (Fig. 7), a NW-striking depression formed in the north of the Baikit anteclise, where silt-clayey and carbonate-clayey hemipelagites (sometimes, carbonaceous) and storm turbidites accumulated in the deep shelf. The wide spread of the Madra deep-water clay-carbonate complexes evidences the presence of carbonate shelves, which served as a source of carbonate material. An essentially carbonate shelf is well traceable in the eastern sections (Gorbilok River) of the Yenisei Ridge. Fragments of this shelf are also observed on the Chadobets uplift (Chadobets Formation). The shelf is also distinguished in the transitional zone between the Baikit anteclise and the Yenisei Ridge; the seismic data show that carbonate complexes in this zone occur near its boundary with the basement.

In the Gorbilok zone of the Yenisei Ridge, a rift structure (Panimba–Rybnaya Rift) formed with mainly carbonate gravitational complexes of different types in association with clay-carbonate and clayey hemipelagites (upper part of the Penchenga Formation on the Chirimba River) on its slopes. West of the recent Yenisei Ridge, the rifting probably led to the splitting of the sialic crust. At the same time, dolerite sills formed within the Baikit anteclise (wells Yu-30 and K-5). In pre-Yurubchen time, when the rift stage was changed by the passive-margin stage, the uplifting of the Baikit anteclise area took place, which was accompanied by the erosion of Mesoproterozoic deposits. The most serious erosion occurred within the western Yurubchen–Tokhomo block of the Baikit anteclise. During this process, the transition of siliciclastic material with the destruction and redeposition of underlying carbonate rocks took place, as well as the deposition of this material by gravitational flows in the slope and basin environments in the eastern zones of the Yenisei Ridge (lower part of the Korda Formation). Within the Baikit anteclise, traces of siliciclastics transition were detected as a member of quartz sandstones and sandy dolarenites in the basement of the Yurubchen strata. Somewhat later, the situation in the basin became stable, and carbonate sedimentation became predominant in the Baikit anteclise area. It existed mainly in the peritidal environments, except for the northern part of the region, where an intrashelf depression is observed (Fig. 8). Between the peritidal shelf and the central part of the depression, there are zones of high-energy shallow-water shelf (oolite-intraclastic grainstones) and middle shelf with silt-clayey and carbonate sediments, sometimes with

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Fig. 9. Sedimentation environments in the Yenisei–Baikit region in the early Late Mesoproterozoic (Dolgokta (Late Korda) time). 1, peritidal shelf with terrigenous-carbonate sedimentation; 2, probable local land sites (prolonged shelf exposition); 3, intrashelf depression with accumulated fine-layered carbonate-siliciclastic deposits; 4, shallow and middle shelf with carbonate-siliciclastic sedimentation; 5, predominantly shallow (before the subaerial conditions) shelf with siliciclastic sedimentation; 6, deep-shelf environments with mainly fine siliciclastic deposits; 7, slope environments with siliciclastic hemipelagites and turbidites; 8, basin plain with hemipelagites and fine flow siliciclastic deposits.

stromatolites. Fine-layered clay-carbonate deposits (often with storm turbidites) accumulated in the deep shelf in the central part of the depression. The northern slope of the depression was steeper, as evidenced from the slump structures observed among the turbidites in the well Yu-116 (Figs. 1 and 2). Toward the Yenisei Ridge, the shelf with carbonate sedimentation was changed by a shelf with fine carbonate-siliciclastic sedimentation, which, most likely, also occupied the southeastern part of the Baikit anteclise; in the south, there was a shelf with siliciclastic sedimentation. Farther westward, within the Kamenka and Gorbilok zones of the Yenisei Ridge, the deep shelf gave way to slope and basin plain environments. In Dolgokta (Late Korda) time, the amount of siliciclastics drastically increased in the section. This points to a significant reduction in carbonate accumulation in the basin as a result of the progradation of carbonate-siliciclastic tidal plains across the carbonate shelf during the sea level lowstand. The siliciclastic rocks (argillites, clayey siltstones, sandstones) alternate with peloid-intraclastic sandy dolomites and, more seldom, laminites with admixtures of brown and greenish-gray argillites with shrinkage cracks. The carbonate rocks abound in fenestras and fibrous structures. Also, lenses of flat-pebble conglomerates were found in the tidal channels. There are two zones with terrigenous-carbonate sedimentation in the peritidal shelf within the Baikit anteclise, which are separated by a depression with accumulated fine-layered carbonate-siliciclastic deposits (Fig. 9). The peritidal shelf with predominantly carbonate sedimentation was changed laterally by shallow and middle shelves with prevailing siliciclastics. Father west- and southwestward, the middle shelf passed into a deep shelf, which was changed by a slope and a basin plain within the

Yenisei Ridge. East- and southeastward, within the Katanga saddle and, probably, Chadobets uplift, siliciclastites accumulated on the shallow-water shelf (sometimes in subaerial conditions). In Early Kuyumba (Early Gorbilok) time, the sea level rise led to the expansion of the area of predominantly carbonate sedimentation. Most of the Baikit anteclise area had a shallow-water shelf, where migrating oolite-intraclastic bars, sometimes with columnar-stromatolite buildings, formed. East of this area, within the Katanga saddle, stromatolitic buildings were predominant. Here, an intrashelf depression with mainly carbonate storm turbidites and hemipelagites has been preserved, as well as sites of a peritidal shelf with predominant tidal-plain complexes. West and southeast of the area, the carbonate shelf was changed by middle and deep shelves with carbonate-siliciclastic and mainly siliciclastic sedimentation. Hemipelagites, turbidites, and, seldom, thin debrites accumulated in the slope environments (Fig. 10). Hemipelagites were predominant within the basin plain. In Middle Kuyumba (Late Gorbilok) time, the transgressive trend of the basin evolution ended; after the maximum flooding, sea level highstand sediments began to form. In the Baikit anteclise area, oolite-peloid-intraclastic (often, sandy) dolomites with laminitic stromatolites are changed upsection by clayey rocks and stromatolites, including deep-water columnar ones. During the maximum shelf flooding, Conophytonides in association with argillites and clay-limestonedolomite deposits formed buildings (Fig. 11). The lower part of the tract of the sea level highstand sediments consists mainly of aggradation shallowing cycles. Upsection, there are progradation cycles with predominant oolite-peloid-intraclastic

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Fig. 10. Sedimentation environments in the Yenisei–Baikit region in the Late Mesoproterozoic (Early Kuyumba (Early Gorbilok) time). 1, peritidal shelf with predominant complexes of tidal plains, sometimes with shallow depressions; 2, shallow shelf with predominant migrating oolite-intraclastic bars, sometimes with columnar-stromatolite buildings; 3, shallow shelf with columnar-stromatolite buildings; 4, intrashelf depression with storm turbidites; 5, middle and deep shelf with carbonate-siliciclastic sedimentation; 6, middle and deep shelf with predominantly siliciclastic sedimentation; 7, slope sedimentation with hemipelagites, turbidites, and, seldom, debris flows; 8, basin plain with predominant hemipelagites.

(packstones and grainstones) and laminitic dolomites. This general basin evolution trend was complicated by decameterthick cycles marking the periodical basin deepening with a subsequent bar progradation. The intense lateral migration of sedimentation environments resulted in the indistinct boundaries between their complexes. In Kopchera (Early Uderei) time, the rapid transgression lead to the flooding of the shelf, reduction of carbonate accumulation, and sedimentation of clayey and, more seldom, carbonate-clayey material in the deep shelf. In the Yenisei Ridge, mainly hemipelagites enriched in organic carbon accumulated in deep-water environments with oxygen deficit in the bottom waters. In Yukta (Middle Uderei) time, the carbonate shelf significantly expanded and occupied most of the recent Baikit anteclise area (Fig. 12). The sedimentation basin has a peritidal shelf with a predominant complex of tidal plains, sometimes with bars, which laterally passes into a shallow (sometimes, middle) shelf with predominant columnar-stromatolite buildings. Southwest and southeast of the basin, there is mainly a deep shelf with carbonate-siliciclastic sedimentation. Hemipelagites, turbidites, and debris flow deposits, including carbonate ones, formed in the slope environments. The slope gravitational carbonate deposits show that carbonate sedimentation episodically took place in the shelf edge zones, which is somewhat atypical of the Yenisei–Baikit basin. The Rassolka strata are dominated by peloid-intraclastic sandy or stromatolitic (laminitic) dolomites with mudstone, siltstone, and sandstone interbeds. The deposits have fenestras and shrinkage cracks, contain lenses of flat-pebble conglomerates, and show early-diagenetic silicification with preservation of synsedimentation microstructures. In some sections,

there are beds with columnar stromatolites in this interval. These data suggest that the complex formed during the relative lowering of sea level and at the initial transgression phase. On the paleogeographic scheme constructed for Middle Rassolka time (Fig. 13), a peritidal shelf is recognized within the Baikit anteclise, with a predominance of tidal-plain complexes, sometimes with bars and shallow-water depressions. The shelf outlines a probable temporal land site (rather prolonged shelf exposition). The peritidal shelf is changed by a shallow shelf with a predominance of bars, which then passes into a zone of stromatolite buildings (columnar stromatolites associated with intraclastites). Conophytonide buildings developing in more quiet conditions are observed in the middle and, partly, deep shelves. The area of intense carbonate sedimentation is changed by deep-shelf environment with carbonate-siliciclastic sedimentation and then by continental slope and basin plain environments. The available data are insufficient to explain the sedimentation zoning in the Baikit anteclise in Vingol’da and later time. We can only note that in Early Vingol’da time, carbonate sedimentation sometimes ceased, and siliciclastic material transited across the carbonate shelf. In Middle Vingol’da time, the predominantly peritidal shelf and shallow- and middleshelf environments alternated, and columnar-stromatolite buildings formed. In Late Vingol’da time (before the formation of the Upper Vingol’da–Iremeken sequence), a large carbonate structure composed mainly of Conophytonides formed (Fig. 14). North of this building, a peritidal shelf with a predominance of tidal-plain and migrating-bar complexes was localized, and southwest and southeast of the building, carbonate-siliciclastic sedimentation took place on the deep shelf. The subsynchro-

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Fig. 11. Lateral correlations between the shelf complexes in the middle part of the Kuyumba strata. 1, bar systems; 2, Conophytonide buildings; 3, cis-bar systems of middle shelf; 4, progradation wedge; LST, lowstand systems; mfs, maximum-flooding surface; TST, transgression systems; HST, highstand systems.

Fig. 12. Sedimentation environments in the Yenisei–Baikit region in the Late Mesoproterozoic (Yukta (Middle Uderei) time). 1, peritidal shelf with predominant complexes of tidal plains, sometimes with bars; 2, shallow (sometimes, middle) shelf with predominant columnar-stromatolite buildings; 3, middle and shallow shelf with carbonate-siliciclastic sedimentation; 4, predominantly deep shelf with carbonate-siliciclastic sedimentation; 5, slope environments with hemipellagites, turbidites, and debris flow deposits, including carbonate; 6, basin plain with predominantly siliciclastic hemipelagites and flow deposits.

nously formed slope deposits, sometimes with carbonate gravitites, show that the carbonate accumulation area periodically reached the shelf margin. The basement of the Upper Vingol’da–Iremeken sequence contains a member of intraclastic sandy and, more seldom, stromatolitic (laminitic) dolomites with interbeds of sandstones and silty sandstones; in places, argillites with sandstone and

siltstone partings are present. Shrinkage cracks and clastics of underlying rocks are observed. These deposits formed during the sea level lowstand and(or) initial transgression. The above-lying deposits were generated during the prolonged transgression. These are laterally variable complexes of clayey and carbonate rocks. Analysis of gamma-log curves and scarce core material shows the formation of large stromatolitic

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Fig. 13. Sedimentation environments in the Baikit–Yenisei region in the Late Mesoproterozoic (Middle Rassolka (Middle Uderei) time). 1, peritidal shelf with predominant complexes of tidal plains, sometimes with bars and depressions; 2, shallow shelf with predominant bars; 3, shallow shelf with columnar stromatolites and intraclastites; 4, middle or, sometimes, deep shelf with Conophytonide buildings; 5, deep shelf with carbonate-siliciclastic sedimentation; 6, middle and shallow shelf with predominantly siliciclastic sedimentation; 7, slope environments with hemipelagites and turbidites; 8, basin plain with predominantly siliciclastic hemipelagites and flow deposits; 9, probable temporal land (prolonged shelf exposition).

Fig. 14. Sedimentation environments in the Baikit–Yenisei region in the Late Mesoproterozoic (Late Vingol’da (Middle Uderei) time). 1, mainly peritidal shallow-depth shelf with predominant complexes of tidal plains and migrating bars; 2, middle shelf with Conophytonide buildings; 3, deep shelf with carbonate-siliciclastic sedimentation; 4, medium-depth shelf with predominantly siliciclastic sedimentation; 5, slope environments with hemipelagites, turbidites, and debris deposits, including carbonate; 6, basin plain with predominant siliciclastic hemipelagites, sometimes, carbonaceous.

buildings (Conophytonides and branched stromatolites with dolmicrite filling of the intercolumnar space) in the basin, which were laterally changed by fine-layered clayey and clay-carbonate deposits (Fig. 15). The lateral analogs of the upper part of the Sukhoi Pit, Tungusik, and Oslyanka Groups in the Baikit anteclise area

were, most likely, eroded during the Early Neoproterozoic and pre-Vendian events. But in the zones intermediate between the Irkineeva and Chadobets uplifts, part of these stratigraphic subunits has been probably preserved noneroded. Note that in Sosnovka–Alad’ina time, the carbonate shelf also occupied the Baikit anteclise area, whereas in the

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Fig. 15. Lateral correlations between the shelf complexes in the lower part of the Iremeken strata. 1, large stromatolitic buildings; 2, 3, deposits in depressions: 2, fine-intraclastic and clayey dolomites, 3, clayey dolomites and argillites.

Neoproterozoic (Tungusik and Oslyanka time), the rebuilding of the system of basins on the western margin of the Siberian craton hampered the development of the carbonate shelf throughout the anteclise area. The reason is that the carbonate sedimentation transferred to the Yenisei Ridge area, and the sedimentological data suggest the replacement of some carbonate complexes of the ridge by siliciclastic or carbonatesiliciclastic ones in the Baikit anteclise area. Moreover, the present-day geologic structure of the region does not correspond to its paleostructure, in particular, the area of the recent Baikit anteclise and Yenisei Ridge were localized much farther from each other in the Mesoproterozoic than today.

Conclusions Based on the sedimentological, isotope-geochemical, and geophysical data, the intrabasinal correlation and, correspondingly, the structure of the Mesoproterozoic deposits of the Baikit anteclise have been refined. It is shown that these deposits are correlated with the Teya and Sukhoi Pit (pre-Pogoryui part) Groups of the Yenisei Ridge. But correlation is also possible between the Vingol’da strata and the Alad’ina Formation, and the Tokur and Iremeken strata can be correlated with the Krasnaya Gora and Dzhur Formations of the Tungusik Group. The above-lying deposits of the Tungusik Group within the Baikit anteclise, except for its southeastern part, were mostly destroyed in Early Neoproterozoic and pre-Vendian time. We have established siliciclastic and carbonate sedimentary systems of different ranks and their sequences and have reconstructed predominant processes and sedimentation environments: deep shelf (below the storm wave basis) with a predominance of hemipelagites and storm turbidites, middle shelf (between the fair-weather wave base and storm wave base) with a predominance of various storm deposits, shallow shelf (above the fair-weather wave base) with deposits of high-energy environments, partly isolated shelf with wide-

spread peritidal plains, etc. The new and earlier published data (Khabarov, 1994; Mel’nikov et al., 2005; Varaksina and Khabarov, 2007) helped to refine the variations in sedimentation environments for the Mesoproterozoic deposits of the Yenisei Ridge and Baikit anteclise and to construct paleogeographic schemes for particular time intervals in the modern frame of references. The predominance of peritidal complexes in the Riphean section of the Baikit anteclise and numerous indications of subaerial exposition show that the carbonate accumulation was periodically interrupted and that in these periods (sometimes rather long) the transition of mainly fine siliciclastic material from the shelf edge to the deep water basin (located southwest of the shelf) took place. The longest transition periods were in Early Yurubchen, Dolgokta, and Late Vingol’da time, though the main flow supplying siliciclastic material to the Yenisei Ridge area passed south of the Baikit carbonate shelf. Obviously, the rate of sagging of the Mesoproterozoic basin bed in the Baikit anteclise area was extremely low and, most likely, rather stable for a long time. The rate of the expansion of the accommodation space was also low; therefore, even during the prolonged transgressions, episodic carbonate sedimentation could totally compensate the sea level rise, and shallow-water carbonate complexes accumulated on the shelf. Under these conditions, the sedimentation on the Baikit shelf was controlled by sea level fluctuations (mainly low-amplitude). In the Mesoproterozoic, the basin with predominantly carbonate sedimentation occupied the western, northwestern, and northern parts of the Siberian Platform and, according to the geophysical data, extended as a broad band to the western margin of the Anabar Shield, where the Mesoproterozoic deposits are similar in structure and isotope-geochronological and isotope-geochemical characteristics (Knoll et al., 1995) to the Mesoproterozoic deposits of the Baikit anteclise. The carbonate sedimentation area in the Baikit anteclise was mostly remote from the shelf edge. The Mesoproterozoic shelf was much wider than its fragments observed on the presentday western margin of the Siberian craton.

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