C and Sr isotope chemostratigraphy of Vendian-Lower Cambrian carbonate sequences in the central Siberian Platform

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C and Sr isotope chemostratigraphy of Vendian–Lower Cambrian carbonate sequences in the central Siberian Platform B.B. Kochnev a,b,*, B.G. Pokrovsky c, A.B. Kuznetsov d, V.V. Marusin a,b a

A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia b Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia c Geological Institute, Russian Academy of Sciences, Pyzhevsky per. 7, Moscow, 119017, Russia d Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, nab. Makarova 2, St. Petersburg, 199034, Russia Received 16 March 2017; received in revised form 13 June 2017; accepted 11 July 2017

Abstract We propose a detailed δ13C curve for the Vendian and Lower Cambrian (Tommotian) strata of the central Siberian Platform. Two positive carbon isotope excursions identified near the base of the Yuryakh Formation (up to 5.5‰) and in the lower Bilir Formation (up to 5‰) are assigned to the lowermost and middle Tommotian, respectively. This correlation is supported by paleontological data, specific 87Sr/86Sr values (0.70845–0.70856), and similar C isotope record in coeval Early Cambrian basins. The documented minor vertical oscillations (a few meters) of these isotope excursions relative to the formation boundaries in remote boreholes is presumably caused by the spatiotemporal migration of facies. A high-amplitude negative δ13C excursion (–8 to –11‰) in the upper Nepa Regional Stage putatively corresponds to the global Shuram–Wonoka negative carbon isotope excursion (Middle Ediacaran). Carbonates of the lower Nepa Regional Stage (Besyuryakh Formation) demonstrate positive δ13C values (up to 5‰) and minimum 87Sr/86Sr ratios of 0.70796–0.70832. The C and Sr isotope record of the Nepa Regional Stage provides its robust correlation with the Dal’nyaya Taiga and Zhuya Groups of the Patom Foredeep. Micropaleontological data herein reported and glacial diamictites documented at the base of the Vendian sedimentary cover both in the central Patom Foredeep and on its periphery suggest a full stratigraphic volume of the Ediacaran System in the most stratigraphically complete sections of the central Siberian Platform. © 2018, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: Vendian; Lower Cambrian; Ediacaran; carbon isotopes; strontium isotopes; carbonate deposits; isotope chemostratigraphy; central Siberian Platform

Introduction The first δ13C and 87Sr/86Sr chemostratigraphic data for Vendian–Early Cambrian successions of carbonate sediments in the Siberian Platform were obtained in the late 1980s in its southeastern part (Aldan Shield and Uchur–Maya area) which was then well documented in terms of lithology and paleontology (Magaritz et al., 1986). The studies were further extended to other areas (Brasier et al., 1993, 1994; Kouchinsky et al., 2005; Semikhatov et al., 2003, 2004): Olenek Uplift (Knoll et al., 1995; Pelechaty et al., 1996), Anabar Shield (Kaufman et al., 1996; Kouchinsky et al., 2001; Pokrovsky and Missarzhevsky, 1993; Pokrovsky and Vinogradov, 1991), Kharaulakh Uplift (Khabarov and Izokh, 2014), Turukhansk and Igarka Uplifts (Bartley et al., 1998; Kouchinsky et al., * Corresponding author. E-mail address: [email protected] (B.B. Kochnev)

2007), Yenisei Ridge (Khomentovsky et al., 1998a; Pokrovsky et al., 2012), Pribaikalia (Khabarov and Ponomarchuk, 2005; Kuznetsov et al., 2013) and Patom Upland (Khomentovsky et al., 2004; Melezhik et al., 2009; Pelechaty, 1998; Pokrovsky and Buyakaite, 2015; Pokrovsky et al., 2006). The C and Sr isotope compositions in the Siberian Vendian–Early Cambrian sections show quite large ranges (δ13C = –12 to +8‰ and 87Sr/86Sr = 0.7076 to 0.7088), and most of the excursions correlate with events in the global Late Precambrian δ13C and 87Sr/86Sr chemostratigraphy (Halverson et al., 2010; Kuznetsov et al., 2014; Saltzman and Thomas, 2012). However, most of Siberian sections miss large intervals and change markedly in structure and lithology near craton margins, which complicates intrabasinal and regional correlations based on isotope data (Khomentovsky et al., 2004; Kouchinsky et al., 2005; Pelechaty, 1998; Semikhatov et al., 2004).

1068-7971/$ - see front matter D 201 8, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.201 + 8.05.001

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B.B. Kochnev et al. / Russian Geology and Geophysics 59 (2018) 585–605

The Vendian–Cambrian transitional carbonate successions in the craton interior are more continuous laterally than those on the craton margins. Their detailed regional stratigraphy stems from records in thousands of deep petroleum exploration boreholes (Lebedev et al., 2014; Melnikov, 2005; Resolutions, 1989). However, it has been impossible to trace all δ13C variations in sediments from the craton interior because of sporadic and discontinuous core sampling and hindered access to some core material for different reasons. A few examples of such studies are known for southwestern craton areas (Katanga Saddle, Irkut basin), and only a few tens of isotope analyses of Vendian carbonates are available (Vinogradov et al., 1994, 2006). We document C, O, and Sr isotope compositions of Vendian and lowermost Cambrian carbonates from the central Siberian Platform. The results provide compilation and interpretation of δ13C and 87Sr/86Sr curves, which age-calibrate the regional stratiraphic units and correlate them with the well documented sections of the craton margin.

Stratigraphy The study region is located mainly within the Syugdzhera Saddle between the Nepa–Botuobiya and the southern slope of the Anabar Shield (Fig. 1). Its stratigraphy has been studied using data from boreholes in the southeastern uplift slope which grades further southward into the Baikal–Patom pericratonic basin (foredeep). The sampled sections span the lower part of the 2–5 km-thick Vendian–Lower Paleozoic succession. The stratigraphic boundaries and sediment lithologies were determined with reference to well-log data as backup for missing core intervals (Resolutions, 1989). The Vendian–Lower Paleozoic section comprises several regional stages (“horizons”). It begins with clastic sediments of the Vilyuchan, which overlap glacial deposits (conglomerate and dolostone sequences) of sporadic occurrence (Kochnev et al., 2015) found mostly in the south and southeast of the region (Fig. 2). The Nepa Regional Stage unconformably overlays the crystal basement or, less often, the Vilyuchan Regional Stage. It embraces terrigenous coarse-grained Talakh Formation and transgressively overlaying mixed carbonatesiliciclastic Besyuryakh, Ynakh, and Kharystan Formations (southeast) and Parshino Formation (southwest) (Fig. 2). Towards the central Nepa–Botuobiya Uplift and Anabar Shield, the Nepa Regional Stage demonstrates appearance of the regional hiatus in its middle part and gradually thins out (Lebedev and Chernova, 1996). Within the Nepa Regional Stage, carbonates occur locally and comprise silty dolostones and marlstones in the Besyuryuakh, Ynakh, Kharystan, and Parshino Formations (Resolutions..., 1989), and algal bioclastic dolostones of the Chaika Regional Stage in the middle Parshino Formation, exposed by the Ch-279 borehole (Shemin et al., 2011). The Tira Regional Stage (Byuk Formation) is composed of dolostones with abundant anhydrite lenses and layers and a thick salt member in the southern part of the region, which

were deposited during the sea-level lowstand. The Bilir Formation (Usol’e Regional Stage), together with the Yuryakh, Kudulakh, and Uspun Formations (Danilovo Regional Stage), compose the upper part of the Vendian–Lower Cambrian section (Fig. 2). In the southwest, they are mainly dolostones, often with anhydrite and sometimes with minor amounts of salt; limestone/dolostone ratio increases northward. In the Danilovo Regional Stage, silt and clay compound increases downward the section. The Danilovo carbonates are of two main facies: (i) extended buildups of shallow-water algal and stromatolite limestones, often dolomitic, associated with fine clastic microphytolitic and brecciated limestones (mainly of the Yuryakh and Bilir Formations) and (ii) deeper-water laminated or massive, often biolaminite, claycarbonate deposits of wide lagoons and shallow shelf (Gurova and Chernova, 1988). The Uspun Formation and its stratigraphic equivalents lie, with a large regional gap, over partly eroded strata (Resolutions..., 1989). The available age constraints of the sediments are restricted to relatively rare fossils and a few isotope dates. Detrital zircons from glacial diamictite conglomerates in the southeastern Syugdzhera Saddle have the youngest ages around 700 Ma (Kochnev et al., 2015). The Nepa succession stores a rich assemblage of organic-walled microfossils, with some forms being common to the Lower–Middle Ediacaran of China and Australia (Golubkova et al., 2010; Moczydlowska, 2005; Nagovitsyn and Kochnev, 2015). The Uspun Formation contains skeletal fossils similar to Namacalathus (Shemin et al., 2011), while its equivalents in the southwestern Siberian Platform bear macrofossils of vendotaenian algae and small skeletal fossils typical of the sediments younger than 550 Ma (Kochnev and Karlova, 2010). Small shelly fossils of the Purella antiqua Assemblage Zone found in the upper Kudulakh Formation belong to Upper Vendian assemblages (Khomentovsky et al., 1998b; Melnikov et al., 2005), and those found in the lowermost Yuryakh Formation mark the base of the Tommotian Stage in the General Stratigraphic Scale of Russia (Kochnev et al., 2014). The upper Bilir Formation correlates with the Osa reservoir, where trilobite fragments were found in the upper part, and thus belong to the lower Atdabanian Stage of the Lower Cambrian (Resolutions..., 1989; Sukhov et al., 2016). Correlation of Precambrian sediments of the central Siberian Platform with their counterparts from its southern periphery has been a subject of discussions because of notable difference in sediment thickness and lithology, as well as poor age constraints. Thick sections in the Baikal–Patom Trough were presumed to be much thicker than the Vendian units in the platform center (Khomentovsky et al., 2004). However, this assumption is inconsistent with the similarity of microfossils in the Nepa and Dal’nyaya Taiga (Ura Formation) successions (Vorob’eva et al., 2008). New δ13C, δ18O, and 87 86 Sr/ Sr chemostratigraphic constraints can help reconcile the contradiction.

B.B. Kochnev et al. / Russian Geology and Geophysics 59 (2018) 585–605

587

Fig. 1. Location map of study area in the Siberian Platform. 1, box contours area of Figs. 3 and 4; 2, sampled boreholes (A-703, Aikhalskaya-703; ZB-362-0, Zapadno-Botuobinskaya-362-0; M-223-0, Meikskaya-223-0; O-286-1, Onkuchakhskaya-286-1; O-252-0, Onkhoidokhskaya-252-0; S-706, Sokhsolokhskaya-706; SM-225-0, Srednemarkhinskaya-225-0; Kh-322-0, Khanninskaya-322-0; Ch-279, 367, Chaikinskaya-279 and 367; Sh-471, Sheinskaya-471; E-343-0, Eyikskaya343-0); 3, Vendian sections characterized previously by chemostratigraphic data and mentioned in the text (Arabic numerals in circles): Igarka Uplift (1), Turuhansk Uplift (2), northern Yenisei Ridge, Teya–Chapa basin (3), southern Yenisei Ridge (4), Irkutsk Amphitheater basin (5), Pribaikalia (6), western Patom basin (7), eastern Patom basin (8), Aldan Shield (9), Yudoma–Maya basin (10), western slope of Anabar Uplift (11), eastern slope of Anabar Uplift (12), Olenek Uplift (13), Kharaulakh Uplift (14). AlSh, Aldan Shield; AnSh, Anabar Shield; NBU, Nepa–Botuobiya Uplift.

Materials and methods The bulk chemistry and carbon, oxygen and strontium isotope compositions were studied in 280 samples from 12 deep boreholes which span a stratigraphic range from the lower Bilir Formation to the oldest carbonate rocks of the Nepa Regional Stage (Table 1). The borehole sections were subdivided and correlated accordingly to available preliminary drilling reports. Then stratigraphic boundaries were updated for some boreholes using well logs and core data. Specifically, the boundary between the Yuryakh and Kudulakh Formations in BH E-343-0 was placed 80 m below the previous position (Khomentovsky et al., 1998b), which provided better intrabasinal correlation of the underlying formations (Fig. 3). Samples with the finest grain sizes, the lowest clay contents, and the most homogeneous appearance were selected from the original collection, at every 3–4 m, for analysis. Limestones rather than

dolostones were preferably selected from the upper section part. Powders for analysis were drilled out of the visually less-altered segments of sawed core samples. The major-element compositions were analyzed by the XRF method in 70 samples (Table 2) on a Bruker S4 Pioneer spectrometer at the Geological Institute (GIN, Moscow), in 20 g aliquots of powdered bulk samples. The contents of Ca, Mg, Fe, Mn, Rb and Sr in the carbonate fraction of other 31 samples (Table 3) were measured by ICP-MS at the Institute of Geology and Mineralogy (IGM, Novosibirsk). The preconditioning of samples for the ICP-MS analysis included digestion of 100 mg aliquots of the powdered samples in a 1 M solution of HCl for 3 days at room temperature, centrifuging and weighing the residue, and subsequent 1:1000 dilution. The measurements were run on an Element II multicollector mass spectrometer with the analytical errors laying below <5%.

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Fig. 2. Vendian stratigraphy of the study area, complemented after (Lebedev et al., 2014; Melnikov, 2005; Resolution..., 1989). Fortun., Fortunian Stage; Atd., Atdabanian Stage; N.-Dal., Nemakit–Daldynian Stage; Tira, Tira Regional Stage.

The carbon and oxygen isotope compositions in the carbonate samples were studied on Thermoelectron Delta V Advantage (GIN, Moscow) and Finnigan MAT-253 (IGM, Novosibirsk) mass spectrometers equipped with a Gas Bench II system. The samples and the standards (KH-2, C-O-1 and NBS-19) were digested in H3PO4 at 50 °C. The δ18O and δ13C estimates were accurate to ±0.2 and ±0.1‰, respectively. The δ13C and δ18O values are quoted in ‰ relative to the V-PDB and V-SMOW standards, respectively. The Sr isotope composition in carbonates and anhydrites was analyzed on a Triton TI multicollector mass spectrometer at the Institute of Precambrian Geology and Geochronology (St. Petersburg). For analysis of the Rb–Sr isotope systematics, 50 mg aliquots of the carbonate samples were enriched by leaching (Kuznetsov et al., 2005); magnesite was treated with a 2.5 M solution of HCl at T = 60 °C (Kuznetsov et al., 2007). Anhydrite samples (20—30 mg) were digested in concentrated HCl at T = 90 °C, and SO2− 4 anions were removed using ion exchange resin (ARA-8 anionite); then Sr was extracted from the residual solution in the same way as in the carbonate samples (Kuznetsov et al., 2008). The average 87Sr/86Sr ratios in the standard samples SRM 987 and EN-1, normalized to

86

Sr/88Sr = 0.1194, were, respectively, 0.710282 ± 0.000006 (2σav., n = 35) and 0.709209 ± 0.000008 (2σav., n = 27).

Results Elemental composition About 60% of all samples are dolostones (Mg/Ca = 0.6–0.7) with the amount of noncarbonate lithologies varying from 1 to 20–25%, or less often to 43% (Tables 2 and 3). Limestones are present at all stratigraphic levels except for the Tira Regional Stage. Carbonates with the clastic component less then 2–3% are rare. The contents of TiO2, Al2O3, and FeO + Fe2O3 correlate with SiO2 and record the amount of the aluminosilicate component (Table 2). The Fe content ranges from 210 ppm in pure dolostones to 49,300 ppm in dolomitic mudstones, and from 252 ppm in pure limestones to 60,700 in silty marlstones. The Mn concentration (50 to 3100 ppm) poorly correlates with both Mg/Ca and Fe. The concentrations of Rb vary from 0.07 to 82 ppm and are proportional to clay contents; strontium ranges from 130 to 1329 ppm in limestones and 51 to 2566–3479 ppm in

589

B.B. Kochnev et al. / Russian Geology and Geophysics 59 (2018) 585–605 Table 1. δ13C and δ18O variations in Vendian–Cambrian carbonate rocks from the central Siberian Platform No.

Depth, m

Lithology

δ13C, PDB

δ18O, SMOW

No.

Depth, m

Lithology

‰ 1

2

3

Bilir Formation BH Zapadno-Botuobinskaya-362-0 1* 1709.0 LD 2 1711.6 LD 3 1715.5 LD 4 1719.8 D 5 1723.3 D 6* 1727.3 CD 7 1730.0 CD 8 1736.5 CD 9 1741.6 CD 10* 1744.7 CD BH Chaikinskaya-367 11* 860.3 L 12 864.0 L 13* 892.0 L 14 897.0 CL 15* 902.3 LD 16 906.3 SD 17* 911.0 AD 18 917.4 CD BH Eyikskaya-343-0 19 1652.3 D 20 1655.6 D 21 1657.0 D Danilovo Regional Stage Yuryakh Formation BH Aikhalskaya-703 22* 1962.0 L 23 1964.0 L 24 1970.3 L 25* 1974.0 DL 26 1976.0 DL 27* 1979.0 DL 28 1980.5 DL 29 1982.8 DL 30* 1986.2 DL 31 1988.2 DL 32 1992.6 DL 33 1995.5 DL 34 1998.6 DL 35* 2000.8 DL BH Zapadno-Botuobinskaya-362-0 36 1748.3 CD 37* 1751.0 CD 38 1754.8 CD 39 1758.4 CD 40* 1762.3 CD 41 1765.9 CD 42 1770.1 CD 43* 1774.1 CD BH Onkhoidokhskaya-252-0 44 2020.3 CL

4

δ13C, PDB

δ18O, SMOW

‰ 5

–1.2 –1.3 –0.9 –0.2 0.4 1.4 3.0 3.5 3.5 4.5

23.8 23.9 22.7 23.0 22.9 22.7 25.0 24.0 24.3 24.5

–1.0 –1.2 0.7 1.0 2.7 1.9 4.0 4.6

22.7 21.7 22.5 22.2 24.0 24.3 25.1 25.0

1.6 2.0 2.0

28.9 28.4 28.6

1.3 1.5 1.4 0.0 0.6 1.8 4.6 2.4 4.1 5.1 2.9 3.2 4.9 5.5

26.8 26.4 25.7 24.9 26.6 27.5 26.6 26.2 28.3 28.4 25.6 28.1 28.7 29.2

3.2 –0.1 0.2 –0.6 –0.3 0.5 0.9 0.7

24.4 24.9 24.4 24.6 25.0 24.1 24.9 24.5

1.4

26.8

1

2

45 2022.5 46 2025.0 47 2028.5 48 2031.6 49 2036.8 50 2038.9 51 2040.6 52 2044.0 53 2073.9 54* 2077.8 55 2080.6 56 2083.8 57 2088.0 58* 2089.4 BH Meikskaya-223-0 59** 3892.2 60 3893.3 61 3894.4 62 3895.4 63 3896.2 64 3898.7 65 3900.4 BH Khanninskaya-322-0 66** 2478.5 67** 2480.1 68** 2482.4 69** 2483.2 70** 2484.0 71** 2484.8 72** 2486.0 73** 2488.5 74** 2490.0 75** 2491.5 BH Chaikinskaya-367 76* 922.0 77 927.7 78* 933.5 79 938.4 80 943.1 81* 948.2 82 953.1 83** 958.1 84 963.2 85 968.0 86* 973.6 87 979.3 88 985.6 89* 990.0 90 993.9 91 996.2 92 999.3 93* 1003.5

3

4

5

CL CD CD CD CDL CL CD CL L L L L L L

1.4 1.0 1.2 0.1 1.9 1.4 2.0 1.7 0.7 0.2 0.9 1.2 1.0 –0.9

24.9 26.8 27.2 28.2 27.8 23.4 26.2 27.6 24.2 23.6 25.5 25.6 24.7 27.3

CDL CDL CL CDL CD CDL CD

2.1 4.0 4.0 4.0 2.2 4.0 3.5

26.9 26.1 27.6 27.4 28.0 26.6 28.1

CL CL CL LD CL LD CL CL CL CL

0.6 –0.5 –0.7 –5.7 –0.2 –5.9 1.0 0.9 1.2 1.1

26.0 27.1 25.5 27.2 26.2 27.1 25.4 25.3 25.1 25.3

DM AD CLD CD CD CD CD AD CD DM DM CD CD CD AD D D CD

4.0 –1.8 –2.2 –0.4 –0.1 –0.1 1.0 0.9 0.1 0.3 1.8 –0.1 –0.1 –0.1 –0.3 –0.3 0.0 0.8

26.7 26.4 25.9 24.5 25.0 25.0 25.9 26.0 25.4 26.0 26.4 26.4 24.7 24.7 24.6 24.6 24.8 25.5

(continued on next page)

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Table 1 (continued) No.

Depth, m

Lithology

δ13C, PDB

δ18O, SMOW

No.

Depth, m

Lithology

‰ 1

2

BH Eyikskaya-343-0 94 1659.0 95 1661.3 96 1663.0 97 1664.4 98 1666.4 99 1720.3 100 1722.2 101 1724.5 102 1725.8 103 1727.9 Kudulakh Formation BH Onkuchakhskaya-286-1 104* 1792.2 105 1794.9 106 1799.0 107* 1803.2 108 1806.3 BH Onkhoidokhskaya-252-0 109 2092.8 110 2095.6 111* 2100.1 112* 2141.3 113 2142.4 114 2145.5 115* 2190.7 116 2194.4 117 2197.7 118* 2203.3 119 2208.0 120* 2215.3 BH Sokhsolokhskaya-706 121 2650.0 122* 2654.0 123 2656.0 124 2659.5 125 2663.5 126 2666.0 127* 2669.0 BH Khanninskaya-322-0 128 2536.0 129* 2538.0 130 2542.0 131 2582.0 132* 2587.0 133 2592.0 134* 2595.0 135 2597.0 136 2600.0 137* 2604.0 BH Eyikskaya-343-0 138 1732.6 139 1734.8

3

4

δ13C, PDB

δ18O, SMOW

‰ 5

D D D D D L L L L CL

3.1 2.6 2.9 4.9 4.2 5.0 5.1 5.0 4.8 3.7

26.1 25.1 25.2 29.1 28.2 27 25.8 25.6 27.5 27.3

CDL CDL CDL CDL CDL

–2.7 –2.0 –1.4 –2.9 –4.7

24.5 24.8 25.2 26.9 26.7

L L L D D CD L L LD CL CL D

–0.8 –1.6 0.7 –3.0 –1.8 –1.7 –1.4 –0.6 –1.0 1.7 2.1 –5.2

24.5 24.4 23.8 26.1 27.0 26.0 26.3 24.2 26.5 23.9 25.6 25.9

D CD D D CD CD CD

–1.7 –1.7 –2.3 –1.5 –2.1 –1.5 –2.1

25.0 26.9 26.7 26.9 26.4 26.7 26.6

D DL DL CL CDL CDL CDL CDL CDL CDL

0.2 0.4 –2.1 0.5 –1.8 –7.2 –7.1 –7.4 –7.4 –6.5

27.8 23.0 23.6 24.0 26.9 26.4 26.2 26.0 26.1 25.5

L L

3.7 4.1

26.7 26.1

1

2

3

140 1736.8 L 141 1739.8 L 142 1877.5 CDL 143 1880.0 CDL Uspun Formation BH Zapadno-Botuobinskaya-362-0 144* 1940.6 CD 145 1944.3 CD 146 1946.8 CD 147 1951.6 CD 148* 1955.4 CD 149 1958.8 CD 150 1962.0 CD 151 1968.0 CD 152 1971.5 CD BH Sokhsolokhskaya-706 153* 2852.0 CDL 154 2862.5 CDL 155 2866.0 CDL 156 2871.0 CL 157* 2875.0 CL 158 2879.0 D 159 2887.5 D 160 2890.0 D 161* 2893.0 D BH Srednemarkhinskaya-225-0 162* 3490.3 CD 163 3493.6 CD 164 3495.3 CD 165* 3541.3 CL 166 3547.5 M BH Khanninskaya-322-0 167 2654 CL 168* 2657 CL Tira Regional Stage Byuk Formation BH Aikhal’skaya-703 169 2273.0 D 170* 2280.0 D BH Zapadno-Botuobinskaya-362-0 171* 1974.7 CD 172 1978.9 CD 173 1985.9 CD 174 1989.8 CD 175* 1993.8 CD 176 1998.2 CD 177 2002.2 CD 178 2006.0 CD 179* 2010.8 ACD 180 2016.3 ACD 181 2020.0 ACD 182 2023.1 ACD 183* 2025.4 ACD

4

5

3.6 3.0 –1.3 –2.4

27.9 26. 25.0 25.0

–6.1 –4.3 –5.0 –3.2 –3.3 –1.8 –0.1 1.3 –0.4

24.5 24.6 24.5 24.8 24.7 24.4 24.5 24.3 25.4

–6.2 –6.0 –6.7 –4.8 –4.7 -4.0 0.6 0.9 0.1

25.8 24.4 23.8 24.1 25.6 25.7 25.6 27.2 27.

–6.6 –7.2 –6.8 –2.8 –1.2

25.9 25.7 25.6 24.8 24.2

–6.7 –6.1

23.7 24.6

–2.0 –1.0

29.7 27.6

–1.4 –3.7 0.7 1.6 –0.5 0.5 1.0 1.1 –1.0 –0.2 –1.1 0.7 –0.8

26.3 25.9 28.4 28.9 29.0 30.9 29.2 29.7 27.3 26.4 25.4 27.0 24.7 (continued on next page)

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B.B. Kochnev et al. / Russian Geology and Geophysics 59 (2018) 585–605 Table1 (continued) No.

Depth, m

Lithology

δ13C, PDB

δ18O, SMOW

No.

Depth, m

Lithology

‰ 1

2

BH Onkuchakhskaya-286-1 184* 2032.5 185 2036.8 186** 2046.3 187 2048.5 BH Sokhsolokhskaya-706 188 2944.2 189* 2950.0 190 2960.3 191* 2964.0 192* 3031.0 193 3040.8 194* 3043.5 195 3050.0 196* 3070.5 197 3075.6 198* 3079.4 199 3083.4 200 3088.0 BH Khanninskaya-322-0 201 2820.0 202* 2829.0 203 2961.0 BH Sheinskaya-471 204** 2705.0 205 2718.0 BH Chaikinskaya-367 206* 1278.6 207 1282.6 208 1286.1 209* 1289.7 Nepa Regional Stage Kharystan Formation BH Khanninskaya-322-0 210* 2988.0 211 2988.2 212 2988.5 213 2989.5 214 2993.0 215 3010.1 216 3010.2 217* 3011.6 218 3011.8 219 3012.3 220 3013.2 221 3014.8 222* 3020.0 BH Sheinskaya-471 223 2723.0

δ13C, PDB

δ18O, SMOW

‰

3

4

5

CD ACD ACD CD

–0.8 1.1 –0.9 –1.0

30.6 31.3 30.3 30.6

CD CD CD CD CD DS CD CD AD ACD CD AD CD

–0.4 –1.1 –1.6 –4.0 0.0 –3.0 –6.3 –5.8 0.8 1.3 0.8 1.6 –2.0

28.5 30.0 29.0 28.6 30.4 29.4 31.8 29.6 29.6 30.7 29.2 29.6 24.5

D CD Mg

–3.3 –5.0 –2.1

28.6 31.5 32.0

CD DS

–0.5 –0.8

30.2 29.1

CD CD CD DM

–3.4 –5.7 –3.4 –9.5

27.2 27.1 25.3 22.5

CD CD CD CD DMS DM DM DM DM DM DM DM DM

–7.3 –7.4 –7.4 –7.3 –0.5 1.0 1.1 0.3 0.9 0.8 0.5 0.8 0.3

25.7 26.1 25.8 26.0 26.5 28.3 28.1 26.0 27.4 26.7 26.0 28.7 25.8

DMS

–7.0

23.3

1

2

Parshino Formation BH Chaikinskaya-279 224* 1621.4 225 1625.8 226 1629.1 227 1633.2 228 1645.2 229* 1646.7 230 1677.4 231* 1704.0 BH Chaikinskaya-367 232 1294.7 233* 1299.1 234 1302.5 235 1306.5 236* 1308.6 237 1449.8 238* 1452.3 Ynakh Formation BH Sheinskaya-471 239** 2754.2 240 2755.6 241 2756.9 242 2758.9 243 2759.7 Besyuryakh Formation BH Meikskaya-223-0 244** 4610.6 245 4614.3 246** 4620.3 247 4624.7 248 4625.6 249 4626.5 250 4627.8 251** 4629.3 252** 4630.8 253 4666.0 254** 4666.8 255 4669.1 256** 4671.5 257 4672.3 258** 4675.7‘‘ 259 4677.5 260 4681.2 BH Sheinskaya-471 261 2804.4 262 2805.3 263** 2806.5 264 2807.5 265 2808.6 266 2809.9 267 2810.9

3

4

5

CD CD CD CD DMS DM DMS DM

1.0 1.9 –0.1 –0.4 –3.0 –3.0 –4.0 –7.7

24.9 25.0

DM M M M M DM CD

–10.4 –10.7 –10.6 –11.0 –11.1 –0.8 –1.8

25.8 22.4 22.3 21.7 21.6 26.2 26.2

CD CD CD DM DMS

1.9 –1.3 1.3 –0.3 1.2

30.8 29.0 30.7 29.8 29.2

CDL CL CDL CDL CL CL CL CL CL CL CL CL CL CL CL CLD DM

0.0 –0.2 –0.2 4.3 4.6 4.3 5.1 4.4 4.7 5.7 5.6 5.7 4.4 4.2 3.8 4.4 3.1

25.6 27.0 25.4 22.9 23.8 22.7 23.1 23.5 23.5 24.5 25.5 24.7 25.5 23.5 24.8 27.3 25.4

M M M M CL CL CD

3.5 3.5 4.3 5.1 2.6 3.1 0.7

22.2 21.4 23.5 22.7 23.8 24.4 23.5

25.2 25.0 24.6 25.0 18.6

Note. Abbreviations for macrosocpically identified lithology: L, limestone; CL, clayey limestone; DL, dolomitic limestone; CDL, clayey dolomitic limestone; D, dolostone; LD, limy dolostone; M, marl; CD, clayey dolostone; A, anhydrite; AD, anhydrite dolostone; ACD, anhydrite clayey dolostone; CLD, clayey limy dolostone; SD, salt-bearing dolostone; DM, dolomitic marl; DS, dolomitic sandstone; DMS, dolomitic mudstone; Mg, magnesite. * Samples analyzed by XRF (Table 2). ** Samples analyzed by ICP-MS (Table 3).

Fig. 3. Correlation of Vendian and Lower Cambrian sections in gamma-logged boreholes. 1, limestone; 2, dolostone; 3, clayey limestone; 4, clayey dolostone; 5, marl; 6, dolomitic marl; 7, anhydrite; 8, salt; 9, gravelstone and conglomerate; 10, sandstone; 11, siltstone; 12, mudstone; 13, diamictite; 14, crystal basement; 15, abbreviated formation names: bl, Bilir; jur, Yuryakh; kd, Kudulakh; usp, Uspun; bk, Byuk; pr, Parshino; kr, Kursovsky; hrs, Kharystan; in, Ynakh; bs, Besyuryakh; tl, Talakh; hr, Khoronokh; bt, Botuobiya; tlk, Talakan; hn, Khanny (Khanni); kg, conglomerate bed.

592 B.B. Kochnev et al. / Russian Geology and Geophysics 59 (2018) 585–605

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B.B. Kochnev et al. / Russian Geology and Geophysics 59 (2018) 585–605 Table 2. Representative XRF analyses Borehole

ZB-362-0

Ch-367

A-703

ZB -362-0

O-252-0 Ch-367

O-252-0

O-286-1 S-706 Kh-322-0

ZB-362-0 SM-225-0 S-706

Kh-322-0 A-703 ZB-362-0

O-286-1 S-706

Core Formation Lithology SiO2 depth, m %

TiO2

1709.0 1727.3 1744.7 860.3 892.0 902.3 911.0 1962.0 1974.0 1979.0 1986.2 2000.8 1751.0 1762.3 1774.1 2077.8 2089.4 922.0 933.5 948.2 958.1 973.6 990.0 1003.5 2100.1 2141.3 2190.7 2203.3 2215.3 1792.2 1803.2 2654.0 2669.0 2538.0 2587.0 2595.0 2604.0 1940.6 1955.4 3490.3 3541.3 2852.0 2875.0 2893.0 2657.0 2280.0 1974.7 1993.8 2010.8 2032.5 2046.3 2950.0 2964.0 3031.0

0.02 0.02 0.08 0.01 0.02 0.01 0.02 0.06 0.02 0.10 0.09 0.11 0.01 0.01 0.02 0.01 0.01 0.41 0.39 0.01 0.01 0.27 0.01 0.02 0.01 0.10 0.04 0.04 0.03 0.09 0.08 0.27 0.02 0.01 0.04 0.06 0.14 0.24 0.04 0.16 0.01 0.19 0.20 0.09 0.11 0.02 0.08 0.02 0.01 0.02 0.01 0.01 0.01 0.02

bl bl bl bl bl bl bl jur jur jur jur jur jur jur jur jur jur jur jur jur jur jur jur jur kd kd kd kd kd kd kd kd kd kd kd kd kd usp usp usp usp usp usp usp usp bk bk bk bk bk bk bk bk bk

D D CD L L CD AD DL LD LD LD CDL D CD CD L L DM CLD CD AD DM D CD L CD DL CL CD CDL CDL CD CD L CDL CDL CDL CD CD CD CL CDL CDL CD CDL D CD CD ACD CD ACD D D CD

1.07 1.36 7.33 1.35 1.59 9.89 4.60 3.68 1.18 6.10 5.13 8.36 0.59 5.53 1.67 0.86 1.70 29.0 28.4 1.40 2.03 10.1 0.79 1.79 0.92 8.25 6.82 3.17 12.0 10.1 15.0 25.6 1.31 0.93 4.13 21.7 18.2 20.9 3.21 17.1 3.96 28.7 28.9 4.38 24.6 0.85 7.90 4.45 1.03 1.27 1.38 0.56 0.49 2.71

Al2O3 FeO+Fe2O3 MnO

MgO

CaO

Rb

Sr

LOI, %

Mg/Ca Fe/Sr

Mn/Sr

74 63 64 121 91 76 211 318 115 287 3479 281 78 78 60 719 267 99 100 88 1329 154 65 203 158 213 217 255 129 243 312 154 176 174 208 107 157 79 103 95 437 161 186 61 263 58 93 477 269 65 340 67 85 278

46.6 46.5 42.7 43.1 43.4 41.5 40.2 43.8 46.2 42.5 42.7 41.4 46.4 41.5 45.9 43.8 47.1 27.2 28.9 45.9 24.0 23.3 46.5 42.5 41.9 43.5 42.1 42.5 40.8 39.4 38.6 31.3 45.7 40.3 44.1 36.7 36.9 33.3 44.7 36.7 42.7 31.4 30.7 43.4 31.9 46.6 41.9 25.6 29.6 46.3 39.1 46.2 45.9 41.1

0.61 0.59 0.59 0.06 0.07 0.52 0.42 0.37 0.52 0.52 0.47 0.51 0.57 0.48 0.57 0.04 0.50 1.01 0.77 0.59 0.16 0.33 0.59 0.47 0.05 0.60 0.42 0.06 0.55 0.25 0.62 0.63 0.55 0.02 0.53 0.59 0.62 0.84 0.58 0.62 0.05 0.64 0.62 0.57 0.22 0.59 0.64 0.39 0.27 0.60 0.41 0.58 0.57 0.48

<1.4 1.7 <1.6 <0.83 <1.1 <1.3 <0.47 <0.31 <0.87 <0.35 <0.03 0.28 <1.3 <0.60 <1.7 <0.14 1.3 2.8 2.3 <1.1 <0.08 1.5 1.3 <0.49 <0.63 0.34 <0.46 <0.39 <0.78 <0.41 0.17 0.96 0.69 <0.57 0.64 1.6 1.1 3.9 4.0 2.9 <0.23 2.2 1.6 6.0 0.45 1.7 9.6 0.27 1.7 <1.5 <0.29 <1.5 <1.2 <0.36

ppm 0.46 0.39 1.61 0.39 0.49 0.33 0.46 1.29 0.43 2.10 1.78 2.42 0.19 0.29 0.39 0.21 0.30 6.00 5.22 0.39 0.31 2.91 0.39 0.49 0.20 2.06 1.15 0.81 0.84 2.15 1.50 3.30 0.34 0.20 1.05 1.20 2.65 4.20 0.92 2.90 0.28 3.33 3.42 1.59 2.25 0.44 1.63 0.66 0.20 0.55 0.41 0.21 0.16 0.58

0.24 0.41 0.60 0.17 0.26 0.25 0.27 0.45 0.39 0.77 0.67 0.87 0.26 0.42 0.31 0.15 0.32 3.86 2.43 0.21 0.23 2.69 0.34 0.51 0.24 0.74 0.47 0.33 0.50 0.68 0.65 1.18 0.44 0.15 0.43 0.70 1.11 2.06 0.62 1.32 0.24 1.69 1.69 0.61 0.98 0.24 0.79 0.34 0.30 0.22 0.21 0.08 0.21 0.24

<0.01 0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.01 <0.01 <0.01 <0.01 <0.01 0.04 0.03 0.02 <0.01 <0.01 0.02 0.01 <0.01 <0.01 0.01 <0.01 <0.01 <0.01 <0.01 0.01 0.01 0.01 <0.01 0.01 0.02 0.02 0.03 0.04 0.03 <0.01 0.04 0.03 0.04 0.01 0.01 0.09 0.01 0.05 <0.01 <0.01 <0.01 <0.01 <0.01

21.7 21.2 19.7 3.43 4.19 18.5 18.1 15.3 19.8 18.2 17.9 17.5 21.4 19.1 21.1 2.71 18.8 16.8 15.0 21.6 11.5 2.03 21.5 19.7 3.27 18.6 16.5 3.36 18.2 10.9 18.7 15.2 20.7 1.88 19.5 16.1 16.9 18.8 20.7 17.2 2.81 13.8 14.0 20.0 8.16 21.5 20.5 21.9 16.8 21.5 19.3 21.8 21.6 20.3

29.8 29.9 27.6 51.4 49.9 29.4 36.2 35.0 31.9 29.4 31.2 28.5 30.9 33.1 30.5 52.2 31.5 13.8 16.2 30.4 61.8 5.08 30.4 34.8 53.4 25.8 32.5 49.6 27.4 36.1 24.8 19.9 31.3 56.6 30.3 22.8 22.9 18.6 29.5 22.9 50.0 18.0 18.7 29.2 29.9 30.1 26.7 46.6 52.0 29.9 39.4 31.0 31.5 34.8

2.3 <1.5 9.2 <1.5 <1.5 <1.5 <1.5 7.6 1.7 15 7.5 16 <1.5 <1.5 <1.5 <1.5 4.7 51 50 <1.5 <1.5 82 <1.5 1.6 <1.5 16 5.9 4.8 4.1 11.6 8.7 22 <1.5 <1.5 4.9 5 13 29 4.2 17.9 <1.5 19 18 8.6 15 2.0 8.5 3.1 <1.5 2.3 2.0 0.3 <1.5 3.4

32.2 64.2 92.7 14.3 28.7 33.4 12.9 14.2 34.2 26.8 1.9 31.0 33.5 25.0 52.4 2.1 12.1 389.7 243.7 24.2 1.69 175.4 52.7 25.1 14.9 35.0 21.8 12.8 38.8 28.1 20.9 76.4 25.2 8.3 20.7 65.4 70.9 260.7 59.8 139.5 5.4 105.2 90.6 101.3 37.4 41.4 84.9 7.0 11.4 33.9 6.2 11.9 25.1 8.7

(continued on next page)

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B.B. Kochnev et al. / Russian Geology and Geophysics 59 (2018) 585–605

Table 2 (continued) Borehole

S-706

Kh-322-0 Ch-367 ZB-362-0 Kh-322-0

Ch-279

Ch-367

Core depth

Formation Lithology SiO2

3043.5 3070.5 3079.4 2829.0 1278.6 1289.7 2025.4 2988.0 3011.6 3020.0 1621.4 1646.7 1704.0 1299.1 1308.6 1452.3

bk bk bk bk bk bk kr in in in pr pr pr pr pr pr

TiO2

Al2O3 FeO+Fe2O3 MnO

MgO

CaO

% CD AD CD CD CD DM CD CD DM DM CD DM DM M M DM

19.0 2.77 6.09 24.6 21.2 42.3 1.18 10.0 59.2 53.9 9.91 46.4 40.4 40.0 39.7 37.9

Rb

Sr

LOI, %

Mg/Ca Fe/Sr

Mn/Sr

43 111 195 54 282 282 444 254 241 311 51 86 298 122 136 79

42.8 47.3 39.7 43.6 33.9 13.8 0.86 41.2 10.3 13.3 40.7 17.3 16.1 18.2 17.9 24.2

2.71 0.73 0.61 3.72 0.75 0.33 0.64 0.53 0.92 0.65 0.57 0.72 0.17 0.27 0.22 0.61

20.1 2.1 7.0 2.1 1.9 2.2 5.4 0.78 12.5 15.4 51.8 53.3 26.4 7.5 7.2 74.2

ppm 0.18 0.01 0.13 0.12 0.08 0.70 0.01 0.05 0.59 0.56 0.07 0.65 0.46 0.64 0.64 0.51

1.52 0.10 2.47 1.05 1.63 12.47 0.29 0.89 9.97 8.67 1.97 8.10 8.85 10.91 11.44 7.08

2.23 0.14 1.45 0.86 0.58 3.32 1.15 0.78 3.79 3.28 1.94 4.93 6.07 4.46 4.48 4.52

0.09 0.02 0.14 0.01 0.06 0.06 0.24 0.02 0.30 0.48 0.26 0.46 0.79 0.09 0.10 0.59

22.5 23.1 20.8 20.7 19.8 6.88 14.2 18.1 5.74 6.79 18.1 9.07 4.37 5.42 4.58 9.93

6.92 26.2 28.4 4.65 22.0 17.3 22.0 28.5 5.22 8.69 26.4 10.6 21.3 16.5 17.1 13.6

25 <1.5 13.4 14.5 3.9 55 <1.5 3.2 62 48 9.8 30 33 58 62 33

517.6 13.0 74.4 158.0 20.7 117.8 25.9 30.9 156.9 105.3 379.4 571.4 203.8 364.3 328.7 568.6

Note. Abbreviations in small letters stand for formation names: bl, Bilir; jur, Yuryakh; kd, Kudulakh; usp, Uspun; bk, Byuk; pr, Parshino; kr, Kursovsky; hrs, Kharystan; in, Ynakh; bs, Besyuryakh; abbreviations in capital letters are as in Table 1.

dolostones. There are few samples with high Sr in dolostones associated with the presence of Sr carbonate and sulfate. Strontium concentrations in limestones are commonly 3–5 times as high as those in dolostones within the same stratigraphic levels. The Fe/Sr ratios are from 0.08 to 360 in limestone and 0.97 to 571 in dolostones. The Mn/Sr ranges are 0.1 to 9.8 in limestone and 0.23 to 74 in dolostones. The Fe and Mn contents and the respective Fe/Sr and Mn/Sr ratios are the highest in the Nepa carbonate-clay samples and the lowest in the Yuryakh and Bilir pure limestones or less often in the Tira and Danilovo dolostones. Carbon and oxygen isotope compositions The carbon isotope composition is especially representative for the Bilir and Yuryakh carbonates (Fig. 4), where two major positive δ13C excursions (to 5–5.5‰) are recognized within the succession with values around zero. The upper excursion appears near the base of the Bilir Formation in all three boreholes that stripped the respective depth interval, while the other excursion in the lower Yuryakh Formation is restricted to boreholes in the northeastern Syugdzhera Saddle (Figs. 1 and 4) but is absent from other sections. Furthermore, the BH Khanninskaya-322-0 core from this level includes samples with negative δ13C (–5‰) associated with dolostones, while higher values correspond to limestone beds. Higher δ13C values at this stratigraphic level are close to the primary ratio in the paleobasin seawater while δ13C decrease may result from subsequent dolomitization. Note that the assumed stratigraphic boundaries are not perfectly consistent with δ13C variations (Fig. 4). The δ13C values in the Kudulakh Formation are moderately negative (–1 to –3‰), but are more depleted (–5 to –7‰) above a small positive excursion in the lower part of the

formation (Fig. 4). These variations appear in three boreholes (O-286-1, O-252-0 and X-322-0) irrespective of limestone or dolostone lithology. Most of the Uspun Formation samples show negative δ13C values (–6 to –7‰) that grade to slightly positive values (to 2‰) near the formation base. The Byuk Formation (Tira) samples mostly have slightly negative δ13C, with slightly positive values near the unit base (Fig. 5). The δ13C values are more depleted (–5 to –7‰) within intervals with high siliciclastic compound and reach –7 to –8‰ in the upper Nepa Regional Stage in the eastern part of the region (Kharystan Formation) or even –11‰ in the west (uppermost Parshino Formation). However, they again approach zero near the Kharystan Formation base, as well as in the Ynakh Formation (to –1.3‰). The Besyuryakh Formation carbonates, the oldest in the Nepa Regional Stage, show positive δ13C excursion to 5.5‰ (Fig. 5). The oxygen isotope composition in the analyzed samples ranges from 18.6 to 32.0‰ V-SMOW (–12 to +1‰ V-PDB). The δ18O values in the Bilir Formation and the Danilovo Regional Stage are within 23–28‰ and are never fall below 22‰ V-SMOW. These values are as high as 30–32‰ in the Byuk Formation with abundant evaporates. The δ18O values considerably vary in samples from the Nepa Regional Stage: 21–22‰ (as low as 18.6‰ in one sample) in carbonates with high silt contents to 29–30‰ in pure carbonates, especially in the upper part of the regional stage (Ynakh and Kharystan Formations). The δ13C and δ18O variations in the sampled Vendian and Lower Cambrian carbonates were summarized taking into account proportional thickness variations in some formations and erosion of the upper Byuk Formation in the central Nepa–Botuobiya Uplift (BH ZB-362-0). The composite δ13C curve for the craton interior (Fig. 6) was composed using the

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B.B. Kochnev et al. / Russian Geology and Geophysics 59 (2018) 585–605 Table 3. Sr isotope composition and ICP-MS analyses of samples digested in HCl Borehole

Kh-322-0

M-223-0 O-252-0 Ch-367 Kh-322-0 SM-225-0 O-286-1 Kh-322-0 Kh-322-0 Sh-471 Kh-322-0 Ch-279 Sh-471 M-223-0

Core depth, m

Formation Lithology Sil.

2478.5 2480.1 2482.4 2483.2 2484 2484.8 2486 2488.5 2490 2491.5 3892.2 3897.5 2077.8 958.1 2538 3541.3 2046.3 2958 2961 2705 3013 1621.4 1629.1 2754.2 2806.5 4610.6 4617.5 4618.9 4620.3 4625.5 4629.3 4630.8 4666.8 4667.8 4668.3 4671.5 4674.2 4675.7 4677.5 4678.5

jur jur jur jur jur jur jur jur jur jur jur jur jur jur kd usp bk bk bk bk hrs pr pr In bs bs bs bs bs bs bs bs bs bs bs bs bs bs bs bs

Ca,

Mg

% CL CL L LD CL LD L L L L CL A L AD L L AD A MC D A CD CD CD M CDL CL A CDL CL CL CL L CL CL CL CL CL A CL

3.9 3.2 2.7 14.0 5.1 21.6 2.0 1.6 0.7 2.2 22.0 – 0.7 3.0 1.2 6.0 1.6 – 1.7 1.0 – 12.0 15.9 18.0 42.8 22.1 17.4 – 13.4 – 28.3 15.9 5.4 8.0 16.0 3.6 18.0 25.7 – 19.0

Fe

Mn

Sr

Fe/Sr

Mn/Sr

87

Rb/86Sr

Sr/86Sr 87Sr/86Sr measured initial*

103 95 52 174 53 200 43 49 35 40 291 – 87 44 94 101 87 – 6954 506 – 1400 3100 2742 954 644 321 – 609 – 460 484 536 – – 1840 – 758 – –

160 203 471 146 647 139 245 228 263 255 882 – 869 164 216 129 384 – 129 2566 – 51 73 144 447 496 786 – 217 – 592 897 1160 – – 190 – 344 – –

8.9 7.9 1.0 32.8 1.8 34.7 2.6 2.7 0.97 2.2 5.0 – 0.80 5.4 2.6 9.1 1.6 – 182.4 0.08 – 209.8 316.4 87.6 41.2 11.2 5.0 – 20.3 – 10.6 3.8 2.3 – – 57.0 – 15.4 – –

0.64 0.47 0.11 1.19 0.08 1.44 0.18 0.21 0.13 0.16 0.33 – 0.10 0.27 0.43 0.78 0.22 – 53.9 0.20 – 27.4 42.4 19.0 2.13 1.30 0.41 – 2.81 – 0.77 0.54 0.46 – – 9.68 – 2.20 – –

– – 0.0022 – – – 0.0074 – 0.0019 0.0057 – – – – 0.0039 0.0052 – – 0.0052 – – 0.0918 0.0561 – – – 0.0051 – – 0.0032 – – 0.0011 0.0032 0.0059 – 0.0080 – – 0.0037

– – 0.70849 – – – 0.70856 – 0.70847 0.70846 – 0.70828 – – 0.70846 0.70845 – 0.70915 0.71542 – 0.70892 0.70976 0.71002 – – – 0.70818 0.70832 – 0.70822 – – 0.70796 0.70801 0.70818 – 0.70886 – 0.70964 0.70856

ppm 33.8 31.0 44.4 27.2 40.1 26.6 45.8 44.8 49.8 48.4 26.0 – 45.8 32.2 50.0 65.5 28.2 – 3.4 10.9 – 19.8 19.0 26.6 36.3 33.6 51.6 – 31.9 – 36.7 38.7 45.2 – – 44.2 – 34.9 – –

8.25 10.5 2.7 12.6 5.37 12.4 2.10 2.49 0.53 0.82 6.7 – 2.4 6.7 1.3 2.4 10.8 – 23.6 28.6 – 11.8 11.4 11.9 6.7 7.8 2.5 – 11.0 – 2.4 2.6 1.7 – – 11.4 – 3.5 – –

1433 1594 491 4795 1146 4828 647 622 254 549 4444 – 694 883 571 1177 634 – 23526 210 – 10700 23100 12618 18411 5455 3955 – 4405 – 6274 3378 2675 – – 5130 – 5289 – –

87

– – 0.70847 – – – 0.70850 – 0.70845 0.70841 – 0.70828 – – 0.70843 0.70841 – 0.70915 0.71538 – 0.70892 0.70900 0.70956 – – – 0.70814 0.70832 – 0.70819 – – 0.70795 0.70798 0.70813 – 0.70879 – 0.70964 0.70853

Note. Abbreviations of lithologies and formation names are same as in Tables 1 and 2. Sil, Siliciclastic content (%). En-dash means not defined. *Initial 87Sr/86Sr ratios in carbonate samples are calculated assuming an average age of 550 Ma. Data for anhydrite samples are not corrected because of high Sr contents.

maximum δ13C values for each stratigraphic level, in samples with the least altered isotopic systems. Sr isotope composition The Sr isotope composition was studied in 21 core samples from four boreholes (M-223-0, SM-225-0, Kh-322-0 and Ch-279), including 15 samples of carbonate sediments from several stratigraphic levels, five anhydrite samples from the Besyuryakh, Kharystan, Byuk, and Yuryakh Formations (Table 3), and one sample from a magnesite lens in the lower

Byuk Formation. The 87Sr/86Sr ratios range within 0.70841 to 0.70850 in six limestone samples from the upper section part (Danilovo Regional Stage). These values fit the range known for transitional Vendian–Cambrian successions in basins on the craton margin (Kaufman et al., 1996; Kuznetsov et al., 2014; Semikhatov et al., 2003). The 87Sr/86Sr ratio is 0.70828 in one anhydrite sample from the Yuryakh Formation (close to the Yuryakh limestones) and reaches 0.70915 in another sample of anhydrite from the lower Byuk Formation (Tira Regional Stage), where the clay component reaches 20–25% and causes 87Sr contamination of anhydrite. Sr isotope com-

Fig. 4. δ13C and δ18O (‰) variations in the studied wells and correlation of Bilir Formation with Danilovo Regional Stage. 1, biolaminite and stromatolite carbonate rocks; 2, δ13C (a) and δ18O (b) ratios. Open squares are δ13C for samples with low carbonate content (within 10%). Other symbols as in Fig. 3.

596 B.B. Kochnev et al. / Russian Geology and Geophysics 59 (2018) 585–605

Fig. 5. δ13C and δ18O (‰) variations in the studied wells and correlation of Tira and Nepa Regional Stages. Abbreviations stand for names of stratigraphic units: bt, Botuobiya Member, dt, Dolostone Member. Other symbols as in Figs. 3 and 4.

B.B. Kochnev et al. / Russian Geology and Geophysics 59 (2018) 585–605 597

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B.B. Kochnev et al. / Russian Geology and Geophysics 59 (2018) 585–605

Fig. 6. Composite δ13C and δ18O (‰) curve of the Venidan–Lower Cambrian carbonates in the central Siberian Platform and its proposed correlation with the global δ13C curve (Saltzman and Thomas, 2012). Values right of the curve are minimum 87Sr/86Sr ratios in carbonate and anhydrite (anh). 1–6, fossils occurrences: 1, Anabarites trisulcatus Assemblage Zone, 2, Purella antique Assemblage Zone, 3, Tommotian Stage, Lower Cambrian, 4, Vendotaenia algae, 5, Namacalathus sp., 6, Ediacaran acantomorphic microfossils. Other symbols as in Figs. 3–5.

B.B. Kochnev et al. / Russian Geology and Geophysics 59 (2018) 585–605

position of magnesite from the Byuk Formation (0.71542) is much higher than in the Vendian–Cambrian marine carbonates. It may reflect the magnesite being either epigenic (Kuznetsov et al., 2007, 2008), or deposited in an isolated basin with anomalous sea-water composition. The 87Sr/86Sr ranges in samples from the Nepa Regional Stage (10 limestones, 2 dolomstones, 1 anhydrite, Table 3), are 0.70795–070832 in pure limestones and in anhydrite; higher (0.70856–0.70964) in limestones with 10–25% siliciclastic component; and the highest (0.70976–0.71002) in clayey dolostones. Thus, the presence of aluminosilicates distorts the 87Sr/86Sr ratios of the primary carbonate sediments with 87Sr inputs. The 87Sr/86Sr values are the lowest (0.70796– 0.70798) in the Besyuryakh Formation limestones from BH M-223-1.

Discussion Preservation of isotopic systems The utility of isotope data for stratigraphic applications depends on preservation of the initial isotope systems, which

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was checked on the basis of δ18O, δ13C values (Fig. 7), Fe/Sr and Mn/Sr (Fig. 8) ratios and their correlations. Relatively high δ18O in most of the analyzed samples (above 21–22‰ V-SMOW or –9.6 to –8.6‰ V-PDB) is evidence of minor postdepositional alteration (Kaufman and Knoll, 1995; Podkovyrov et al., 1998; Semikhatov et al., 2004). The δ13C vs. δ18O correlation indices vary greatly between the formations. Namely, it is quite poor (R = 0.35) in the Lower Cambrian Yuryakh and Bilir Formations (Fig. 7a), with two groups of samples, one with δ13C = –2 to +2‰ and the other with positive values (+4 to +6‰), δ13C in the dolostones being generally lower than in the limestones. The δ13C–δ18O correlation is even lower (R = 0.14) in the Uspun and Kudulakh Formations (Fig. 7b) and is positive only in the Byuk Formation samples with the highest δ18O (Fig. 7c). In the Nepa Regional Stage, positive δ13C–δ18O correlation at R = 0.42 is restricted to dolostones (Fig. 7d), while limestones and marlstones split into three groups with positive (~4‰), nearly zero, and negative (–10 to –11‰) δ13C values. The Fe/Sr and Mn/Sr ratios are mostly quite high, except for few limestone samples from the Yuryakh and Kudulakh Formations (Table 3, Fig. 8), and exceed the boundary values of 5.0 and 0.2, respectively, assumed as criteria of least altered

Fig. 7. δ13C vs. δ18O (‰) in carbonate sediments of Bilir and Yuryakh Formations (a), Kudulakh and Uspun Formations (b), Byuk Formation (c), and Nepa Regional Stage (d). Open and bold circles correspond to dolostone and limestone lithologies, respectively.

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Fig. 8. δ13C and δ18O (‰) vs. Mn/Sr and Fe/Sr ratios. 1, Bilir and Yuryakh Formations; 2, Kudulakh and Uspun Formations; 3, Byuk Formation; 4, Nepa Regional Stage. Open and bold symbols correspond to dolostone and limestone lithologies, respectively. Light and dark dashed lines show Mn/Sr and Fe/Sr ratios for dolostones and limestone, respectively, which separate rocks affected and not affected by postdepositional alteration.

Rb–Sr isotope systems in sedimentary carbonates (Kuznetsov et al., 2005, 2008). Other samples were likely contaminated with 87Sr in the postdeposition history. On the other hand, the Yuryakh algal and detrital limestones with the lowest silicate component were more strongly altered than the clayey varieties because of high initial porosity and permeability. The preservation criteria for the C and O isotope systems are less rigorous: Mn/Sr < 5 and Fe/Sr < 10 for limestones and Mn/Sr < 10 and Fe/Sr < 40 for dolostones (Kaufman et al., 1996; Khabarov and Izokh, 2014; Podkovyrov et al., 1998; Semikhatov et al., 2004). Hence, there are quite many samples with δ13C close to the initial values (Fig. 8), while the Fe/Sr ratios exceed the critical values more often than Mn/Sr. However, high iron contents may be mainly caused by the clastic component, as they positively correlate with SiO2, Al2O3, and TiO2 in bulk samples (Table 2). Thus, relatively high SiO2 in

some samples do not affect much the initial carbon and oxygen isotope compositions in the carbonate phase. Thin sections of the samples from different stratigraphic levels show more prominent recrystallization of primary carbonate grains in many organic-detrital limestones of the Yuryakh and Bilir Formations. Selective dolomitization appears within the same stratigraphic level, on different scales from millimeter to tens of meters. Intense recrystallization is common in the Byuk Formation anhydritic dolostones. The postdepositional growth of carbonate mineral grains reduces with increasing amount of silt and clay, but remains notable in the presence of anhydrite. The fact that clayey carbonate rocks are less prone to postdepositional alteration and the ensuing δ13C distortion than pure carbonates was also reported earlier (Khabarov and Ponomarchuk, 2005; Podkovyrov et al., 1998; and others).

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The geochemical criteria of the preservation of C and O isotope systems are poorly applicable to many samples with high siliciclastic content and, correspondingly, with Fe and Mn higher than in pure carbonates. In this case, similar δ13C patterns within the same or neighbor stratigraphic levels in remote boreholes can be an alternative criterion (Figs. 4, 5). Postdepositional alteration may be responsible only for some of the Yuryakh dolomitic limestones, exhibiting δ13C values ~5‰ lower. These samples are restricted to a narrow stratigraphic interval and never occur in other wells. Taking into account high δ18O in these samples, the 13C depletion may be attributed to diagenetic interactions with oxidized organic matter. This assumption is implicitly supported by the abundance of bituminous carbonate rocks at this stratigraphic level. Major δ13C events in Vendian–Lower Cambrian successions of the central Siberian Platform The composite Vendian–Lower Cambrian δ13C curve for the central Siberian Platform comprises several major positive and negative excursions (Fig. 6), which correlate with global chemostratigraphic events occurred in the Late Ediacaran and Lower Cambrian (Halverson et al., 2010; Maloof et al., 2010a; Saltzman and Thomas, 2012). Two positive peaks (up to 5.5‰) in the lower Yuryakh and Bilir Formations (the lower one near the Tommotian base) correlate with similar excursions in the sections of Marocco (Maloof et al., 2005, 2010b) which have U–Pb ages of 525–527 Ma and about 530–533 Ma (Maloof et al., 2010a). Representative Tommotian skeletal fauna appears in the Kessyusa Group (Olenek Uplift, northeastern Siberian Platform) slightly below a large positive excursion of δ13C up to 5‰ (Marusin, 2016) and is well age-calibrated with a U–Pb zircon age of 529.56 ± 0.24 Ma of volcanic tuff layer (Kaufman et al., 2012). Thus, the paleontological and geochronological constraints independently confirm that the Tommotian base (currently assumed to be at 530 Ma) corresponds to the base of the Yuryakh Formation in the Siberian sections. The observed misfit within 7–8 m in the positions of both lower and upper Tommotian δ13C excursions versus the bases of the Bilir and Yuryakh Formations in different boreholes most likely corresponds to facies migration rather than to asynchronicity in the global carbon cycle and isotope anomalies. The EN4 negative δ13C excursion near the base of the Fortunian placed at ~541 Ma in the International Time Scale (Zhu and Xiao, 2007) is another major event in the pre-Tommotian interval of the composite global δ13C curve, which follows several smaller excursions. This level presumably corresponds to origin and initial diversification of small skeletal fauna (Maloof et al., 2010a; Zhu and Xiao, 2007; and others). However, the oldest representatives of this group, Cambrotubulus, were found in sediments older than 544 Ma in the Olenek Uplift (Rogov et al., 2015) and in the Katanga Formation of the craton interior, which is a stratigraphic equivalent of the Uspun Formation (Kochnev and Karlova, 2010). The negative excursion in the lower Kudulakh Formation putatively correlates with EN4 (the negative isotope event

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in the underlying Uspun Formation is considerably more extensive than EN4), but this correlation has no proof yet as no skeletal taxa of the Purella antiqua Assemblage Zone have been found in the interval. An excursion of ~4‰ δ13C, which appears in the lower Staraya Rechka Formation of the western Anabar area (Kaufman et al., 1996), in the lower Khatyspyt Formation in the Olenek Uplift (Knoll et al., 1995), and in the Ust’-Yudoma Formation of the Uchur–Maya area (Semikhatov et al., 2004) may be equivalent to the relatively short positive excursion 1p (to 5–6‰) observed in Namibia, Oman, and China at 548–551 Ma (Fike et al., 2006). In the southern Siberian Platform, it may correspond to the lower Zherba Formation in the western Patom Upland; however, this stratigraphic level with high δ13C (5–6‰) may pinch out (Pokrovsky and Buyakaite, 2015) or be affected by postdepositional alteration in the east of the region. The suggested composite δ13C curve (Fig. 5) for the craton interior lacks such high δ13C values in the Upper Vendian interval. A positive δ13C excursion (4–5‰) occurs in the lower Tira Regional Stage (as well as in the lower Danilovo one), but δ13C are mostly negative and do not exceed 1–2‰ in the sediments above, i.e., are much lower than in the lower Zherba Formation in the western Patom Upland. The origin of a prominent negative δ13C event (–8 to –11‰) in the upper Nepa Regional Stage remains debatable. The greenish-gray color and pyrite inclusions in sediments indicating deposition in anoxic environments would suggest early diagenetic processes and interactions with weathered organic matter to be main causes of the depleted C isotope composition. However, the preservation of isotopic systems cannot be judged from geochemical criteria because of abundant siliciclastic lithologies at this stratigraphic level, and the ensuing high Fe and Mn contents. On the other hand, the upper Nepa negative excursion has the same amplitude, thickness, lithology, and stratigraphic position as the DOUNCE event in the upper Doushantuo Formation in South China (Lu et al., 2013) which is well traced in many Ediacaran carbonate basins worldwide and correlates with the Middle Ediacaran Shuram–Wonoka event (580–551 Ma). Both the DOUNCE negative excursion and the upper Nepa event span a few tens of meters, judging by nearly zero δ13C in the middle Parshino and lower Kharystan Formations (Figs. 5, 6). The exact age, duration, and origin of the Shuram–Wonoka event are subjects of discussions which lay beyond consideration of our study. However, it may have equivalents in several units from the southern Siberian Platform where carbonates have depleted δ13C compositions (–8 to –12‰): Yukanda Formation in the Yudoma-Maya basin and the Zhuya Group in the Patom Foredeep, as well as their equivalents of different facies from shallow-marine stromatolite and oolite limestones to distal-shelf carbonates and shales (Melezhik et al., 2009; Pokrovsky et al., 2006; Pokrovsky and Buyakaite, 2015; Semikhatov et al., 2004). The lower Nepa sediments (Besyuryakh Formation) with positive δ13C reaching 5.5‰ (Figs. 5, 6) may fully or partly correspond to quite a long early Ediacaran (580–635 Ma)

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interval of the global δ13C curve. There is another negative excursion, EN2, above this interval (Zhu and Xiao, 2007), but it does not appear in our data, except for a single available δ13C value (–7.7‰) from BH Ch-279. This curve part is separated from the negative δ13C excursion at the top of the Nepa Regional Stage by quite a long interval of δ13C values around zero (–2 to +1‰) observed in the lower Kharystan and middle Parshino Formations, as well as in the Ynakh Formation. Thus, the composite Lower Vendian–Lower Cambrian (Tommotian) section of the central Siberian Platform contains many globally recognized carbon-isotopic events. They are especially the large positive excursions in the lowermost and middle Tommotian, as well as the negative excursion similar to the Shuram–Wonoka event and the positive excursion below it. Other Ediacaran δ13C events are less prominent in the Siberian sections, either because some stratigraphic levels pinch out or because their correlation still has poor paleontological and geochronological support. Furthermore, δ13C values may vary within the basins under the effects of paleodepths, dissolved gases, and benthic microbial communities on carbon isotope fractionation near the water-sediment interface. The contributions of these factors were discussed for the case of South China in a number of publications (Cui et al., 2015; Zhu et al., 2013; and others).

from diamictites of the Conglomerate Member (Kochnev et al., 2015). The age of the Dal’nyaya Taiga base in the Patom Foredeep has no explicit constraints, but is older than 600 Ma judging by the lowest 87Sr/86Sr ratios of 0.7073 in the Barakun Formation (Pokrovsky et al., 2006). Applying a correlation of the Bolshoy Patom tillites with the Marinoan glaciation (635–650 Ma) (Chumakov, 2015), the base of the Dal’nyaya Taiga Group may be slightly older than 635 Ma age assumed for the Ediacaran base in the International Chronostratigraphic Chart (Kuznetsov et al., 2014). That was one of arguments for an older age of the Vendian base (640 Ma) in the General Stratigraphic Scale of Russia (Semikhatov et al., 2015). Irrespective of whether the age of the Vendian base will be revised in future or the 600 Ma date obtained for the Vendian base in the Central Urals (Grazhdankin and Maslov, 2015) will remain in the Russian General Stratigraphic Scale as the Riphean/Vendian boundary, the inference of correlation between the Patom sediments in the southern periphery of the craton and in its central regions has important practical implications. Specifically, it provides a tool for more precise correlations of Late Precambrian oil and gas bearing sediments north and east of the Patom Foredeep.

Regional correlations based on chemostratigraphy

The reported isotope geochemical studies have made basis for the first representative δ13C curve for Vendian and Tommotian (Lower Cambrian) carbonate successions in the central Siberian Platform. High (>22‰ V-SMOW) δ18O values, major δ13C excursions observed at the same stratigraphic level in different sections, and chemical criteria complied by many samples indicate well preserved isotope systems of carbonate sedimentary rocks over the largest part of the interval. Sediments with their C isotope composition affected by postdepositional alteration occur mainly in the upper section among highly permeable saline shallow-marine facies in the craton southwest. The δ13C values within the Vendian–Tommotian interval vary from –11 to +6‰ V-PDB, with two large positive excursions in the lower Bilir and Yuryakh Formations. The upper one (to 5‰), at the base of the Bilir Formation, is more laterally continuous, and the lower event (to 5.5‰) near the Yuryakh base is restricted to the northeastern part of the region, where postdepositional effects are less prominent. Fossil assemblages typical of the Tommotian stage first appear near the base of the lower excursion, which provide correlation of this interval with the “5p” event at ~530 Ma of the global δ13C curve. Localization of the upper Ediacaran boundary in the studied sections remains debatable due to lack of other age constraints. The δ13C values in the middle and lower Danilovo Regional Stage and in the Byuk Formation generally stay around zero or negative (–5 to –7‰) and increase to slightly positive (1–2‰) near the base of the Uspun and Byuk Formations. The large negative δ13C excursion (–8 to –11‰) extending over a few tens of meters in the upper Nepa Regional Stage

The craton interior sections we studied positively correlate with sections from the Patom Foredeep on the southeastern craton margin, which was suggested as reference for the Venidan of southern Siberia (Chumakov et al., 2013). The isotope systems in the carbonate sediments of the upper Patom section (Tinnaya and Zherba Formations) were possibly disturbed by postdepositional processes (Pelechaty, 1998), hence the small shelly fossils (Khomentovsky et al., 2004) acquires special importance therein (Fig. 9). The hiatus at the base of the Tinnaya Formation correlates with a similar hiatus at the base of the Danilovo Regional Stage. The regressive Zherba Formation is an equivalent of the Byuk Formation in the Tira Regional Stage. In this case, the major negative δ13C excursion the Zhuya Group corresponds to the upper Nepa Regional Stage, which is consistent with our data and is further supported by the lithological and chemostratigrapic correlation of the upper Torgo Formation (boreholes of the Berezovyi Trough) with the Nepa Regional Stage in the craton interior (Melnikov, 2005) and with the Zhuya Group in the Patom Foredeep (Pokrovsky et al., 2006). The lower Nepa Regional Stage, with a positive δ13C excursion, correlates with either the whole Dal’nyaya Taiga Group or its part. In addition to isotope data, this correlation is confirmed by the presence of typically Ediacaran ornamented organic-walled microfossils at the same level (Golubkova et al., 2010; and others). The Bolshoy Patom diamictites can be tentatively correlated (Fig. 9) with glacial deposits at the base of the craton interior sections, which is consistent with the youngest ages of detrital zircons from the Zhuya Group (Chumakov et al., 2011) and

Conclusions

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Fig. 9. Chemostratigraphic correlation of Vendian successions from the Ura Uplift (Patom basin, southern craton margin) (Chumakov et al., 2013; Pelechaty, 1998; Pokrovsky et al., 2006) with those of the central Siberian Platform. 1, isotope ages in Ma, according to different data; 2, youngest ages of detrital zircons, Ma. Abbreviations of stratigraphic units: mr, Mariinsk Formation; bp, Bolshoy Patom Formation; br, Barakun Formation; vl, Valukhta Formation; nk, Nikolsk Formation; cn, Chenchin Formation; gr, Zherba Formation; tn, Tinnaya Formation; nh, Nokhtuisk Formation; N-D, Nemakit–Daldynian Stage; Tm, Tommotian Stage. Sh-W, Shuram–Wonoka δ13C event. Other symbols as in Figs. 3–5.

correlates with the Shuram–Wonoka negative event of the global δ13C curve (EN3, DOUNCE). Below this excursion, δ13C is around zero and then increases to 5.5‰ in the Besyuryakh Formation of clay and carbonate lithologies (lower Nepa Regional Stage), which is typical of the early Ediacaran (older than 580 Ma). The 87Sr/86Sr ratios in the Danilovo carbonates and anhydrites are 0.70845 to 0.70856, which corresponds to the range obtained previously for the Vendian–Cambrian transition. The 87Sr/86Sr ratios for the Tira and Nepa successions are apparently much higher than the normal sea water values, mainly because the section lacks pure carbonates and the sediments are contaminated with 87Sr from the clay component. The obtained composite δ13C curve is applicable to correlations of sediments both within and beyond the Siberian

Platform. Of special importance is the correlation of the Nepa Regional Stage and sediments below it with the Dal’nyaya Taiga and Zhuya Groups of the Patom Foredeep in the southern craton margin. This correlation is supported by the isotopic data, as well as by the presence of typical Ediacaran microfossils at the same or close stratigraphic levels and glacial deposits downsection. Thus, the most continuous sections of the craton interior contain the whole Venidan (Russian General Stratigraphic Scale)/Ediacaran (International Chronostratigraphic Time Scale) succession. The study was performed as part of a basic research program of the Russian Academy of Sciences (Project IX.126.1.1 and Assignments 0153-2015-0011 and 0135-20160017). Core sampling and geochemical studying, including Sr isotope composition, and manuscript preparation were sup-

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ported by grants 14-05-00274 and 17-05-00418 from the Russian Foundation for Basic Research. Carbon and oxygen isotope compositions and chemistry of sediments were analyzed under a support of grants 16-05-00487 from the Russian Foundation for Basic Research and 17-17-01241 from the Russian Science Foundation. We wish to thank V.P. Zhernovsky and D.A. Petrov from Yakutskgeologiya (Yakutsk), and A.M. Fomin, M.Yu. Skuzovatov, and D.A. Tokarev from the Institute of Petroleum Geology and Geophysics (Novosibirsk) for assistance in core studies.

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Editorial responsibility: D.V. Grazhdankin