The Cambrian sediments of the Cis-Yenisei basin (West Siberia): stratigraphy and correlation

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ScienceDirect Russian Geology and Geophysics 55 (2014) 704–715 www.elsevier.com/locate/rgg

The Cambrian sediments of the Cis-Yenisei basin (West Siberia): stratigraphy and correlation Yu.F. Filippov a,*, S.V. Saraev a, I.V. Korovnikov 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 Received 14 November 2013; accepted 26 November 2013

Abstract A scheme is proposed for the stratigraphic division and correlation of the Cambrian sediments of the Upper Proterozoic–Paleozoic Cis-Yenisei basin. It is based on data from the drilling of parametric wells (Lemok-1, Averinskaya-150, Tyiskaya-1, Vostok-1, Vostok-3, Vostok-4, and others). Two structure-facies zones are recognized in the study area: Kas zone (Lemok-1, Averinskaya-150, Tyiskaya-1, Vostok-4, and Eloguiskaya-1 wells), in which the sedimentary complexes accumulated in a salt subbasin, and Ket’ zone (Vostok-1 and Vostok-3 wells) with open-sea-basin sedimentation. The boundary between the structure-facies zones passes along the reconstructed zone of a barrier reef stretching in the N–S direction. The Vostok-4 well is localized in the western Kas structure-facies zone, at the salt subbasin/barrier reef boundary. Local stratigraphic units (formations) are described and compared with the adjacent Turukhan–Irkut–Olekma facies region of the Siberian Platform. Keywords: West Siberian geosyneclise; Cis-Yenisei sedimentary basin; parametric wells; Cambrian; stratigraphic chart

Introduction The Paleozoic sediments in the Cis-Yenisei part of the West Siberian geosyneclise were first stripped by the Kasskaya and Eloguiskaya key wells, along with wells in the Kyksa field, in the 1950s (Bulynnikova et al., 1968, 1973; Dragunov et al., 1967). The lithology of the rocks and rare finds of trilobites and brachiopods (Eloguiskaya-1R well) permitted a comparison between the sediments stripped beneath the Mesozoic cover and known Middle–Late Cambrian sections in the adjacent areas of the Yenisei Ridge and Siberian Platform (Litvintsevo, Letnyaya, Verkholensk, and Evenki Formations and their analogs). Extensive CDP seismic surveying was carried out on the left bank of the Yenisei River from the middle 1980s to the early 1990s (in the northeastern Tomsk Region, from the late 1970s). Analysis of the seismic data showed that a thick unit of Upper Proterozoic–Lower Paleozoic subplatformian sediments, analogous to the sections of the Siberian Platform, was present in the region beneath the Mesozoic sedimentary cover. At the same time, a comprehensive analysis of geophysical

* Corresponding author. E-mail address: [email protected] (Yu.F. Filippov)

data was used in some works (Benenson et al., 1987; Dashkevich and Kashtanov, 1990; Kashtanov and Filippov, 1994; and others) to predict a salt-saturated Lower Cambrian section on the left bank of the Yenisei River. According to these authors, the Lower Cambrian complex experienced facies changes, and its thickness decreased considerably from east to west. For a more profound study of the ancient sedimentary basin and an estimation of its petroleum potential, deep wells were drilled in the study area (Fig. 1): Tyiskaya-1 (Kashtanov et al., 1995), Vezdekhodnaya-4 (Kontorovich et al., 1999; Yolkin et al., 2000), and Lemok-1 (Krinin, 1998; Yolkin et al., 2001) in the 1990s and Averinskaya-150 in 2000 (Saraev et al., 2004). A fragment of a Lower Cambrian carbonate section with an age reliably constrained by trilobites was stripped in the Tyiskaya-1 well. A thick terrigenous-volcanic section with predominant basaltoids, overlain by a dolomite unit of uncertain age (originally, Late Devonian), was exposed in the Vezdekhodnaya-4 well. The volcanogenic sediments and granodiorites from the Vezdekhodnaya-4 well belong to the Cambrian (Ar–Ar dating). A complete Cambrian section, from the saliferous Lower Cambrian sediments of the Usolka unit to the red-colored and variegated carbonate terrigenous complexes (analogs of the Middle–Upper Cambrian Evenki For-

1068-7971/$ - see front matter D 201 4, 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 + 4.05.013

Yu.F. Filippov et al. / Russian Geology and Geophysics 55 (2014) 704–715

mation), was stripped and described from the Lemok-1 and Averinskaya-150 wells. Upper Vendian sediments in these regions were first exposed by the Averinskaya-150 well in the lower part of the section. In 2004, the Program for Regional Geological and Geophysical Studies of the Eastern West Siberian Geosyneclise (“Vostok”) was developed at the Trofimuk Institute of Petroleum Geology and Geophysics (with the participation of SNIIGGiMS and regional resource management authorities) under the leadership of A.E. Kontorovich by the order of the Ministry of Natural Resources of Russia. Under the program, two wells (Vostok-1 and Vostok-3) were drilled in the Tomsk Region (Kontorovich et al., 2008a,b) and one (Vostok-4) in the left-bank part of the Krasnoyarsk Territory (Kontorovich et al., 2012) (Fig. 1). Integrated analysis of core samples, GIS results, and seismic data showed that the sections stripped by the wells include different facies of Vendian and Lower–Upper Cambrian sediments, which demonstrate a succession of sedimentation conditions from a salt subbasin (Kas structurefacies zone (SFZ)) (Fig. 1) in the east (Averinskaya-150 and Lemok-1 wells) to the outer (prereef) zone (Ket’ SFZ) in the west (Vostok-1 and Vostok-3 wells) and the separating system of barrier reefs (Vostok-4 well). According to the seismic data, a N–S pinchout of Lower Cambrian evaporite sediments is observed between the halmeic and marginal (backarc) parts of the basin. The wave pattern on the seismic sections at the western periphery of this zone, along with the drilling data on the Vostok-4 well in this area, suggests that the salt subbasin is bounded in the west by a linear system of barrier reef structures, which separates it from the open sea (Fig. 2). This barrier reef, which supplied an enormous quantity of carbonate debris to the sedimentary basin, might have been a continuation of the global Cambrian rift system on the margins of the Siberian Platform (Astashkin et al., 1984; Kontorovich and Savitskii, 1970; Mel’nikov et al., 1989; Pisarchik et al., 1975; and others). Note the dramatically increased reliability of the reference ages in the recently drilled wells owing to the presence of well-defined faunal remnants at different levels of the sections. The paleontological materials from the wells were defined by members of the Trofimuk Institute of Petroleum Geology and Geophysics: small-shelled fauna was defined by N.V. Novozhilova and G.A. Karlova; trilobites, by I.V. Korovnikov and A.V. Timokhin; algae, by V.A. Luchinina and A.A. Terleev; and archeocyaths, by D.A. Tokarev. Also, members of SNIIGGiMS (Yu.Ya. Shabanov and T.V. Pegel’) took part in the study of the fossil fauna. On the basis of the integrated studies, stratigraphic divisions in the above-mentioned sections have been considerably refined at the Trofimuk Institute of Petroleum Geology and Geophysics. Besides that, the stratigraphic chart of the Vendian–Cambrian sediments of the Cis-Yenisei sedimentary basin has been updated, and schemes of correlation with coeval sediments in the adjacent regions have been developed.

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Fig. 1. Position of wells and Cambrian structure-facies zones in the Cis-Yenisei sedimentary basin. 1, basin boundaries; 2, wells; 3, line of the Vostok-10 regional seismic profile; 4, zonal boundary.

Stratigraphy and correlation The succession of Cambrian formations and series in the Cis-Yenisei part of West Siberia is shown on refined stratigraphic and correlation schemes of the well logs (Figs. 3, 4). Some of the formations have retained the form described in publications on this and adjacent areas, whereas others have partly changed their extent and names. On one hand, the stratigraphy of the eastern Cis-Yenisei basin, which is recognized as a salt subbasin and a large bay of the single East Siberian salt basin (Saraev et al., 2013), is close to the stratigraphy of the latter. On the other hand, the western Cis-Yenisei sedimentary basin is located outside the East Siberian salt basin, so that the coeval sediments have a totally different character. The stratigraphic divisions were refined and, sometimes, changed with the help of detailed lithological studies, paleontologic finds, and GIS data from the new wells, drilled under the “Vostok” project. All this permitted the detection of considerable facies differences and a more detailed correlation of coeval units. The Usolka Formation is detected in two wells: Lemok-1 (interval 3665–4298 m) and Averinskaya-150 (2825–4429 m). According to the GIS data, the formation shows an alternation of decreased and increased gamma-ray log responses, which is typical of saliferous carbonate sediments. Note that these values are abnormally low in the Lemok-1 well. They rarely exceed 3 µR/h (in the upper part, up to 4–5), suggesting the greater role of “pure” salts in the section compared to the Averinskaya-150 well, where the gamma-ray log responses vary from 1–2 to 10 µR/h. The Usolka Formation is best described by core studies in the Averinskaya-150 well (Saraev et al., 2004). The lower part of the section consists of

Fig. 2. Seismic section along the Vostok-10 regional profile. Formations and series: us, Usolka; oks, Oksym; t, Tyya; av, Averin; kl, Kol’chum; el, Elogui; ch, Churbiga; pd, Paidugina; pud, Pudzhelga; pod, Podelga; kn, Kondes; sh, Shedelga; pn, Pyzhina; ev, Evenki; bl, Belaya.

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interbedded (~200 m) clayey dolomites, rock salt, and evaporite dolomites showing graded bedding. These sediments are overlain by an interval (~1000 m) of thick interbeds of rock salt, sulfate–carbonate rocks, and dolosiltites. This interval is followed by a series (220 m) of thin interbeds of calcisiltites, calcarenites, and dolosiltites with anhydrite intercalates. Interbeds of stromatolitic limestones and red-colored clay–silt–sulfate–dolomite rocks are observed near this level. The section ends with thick interbeds of rock salt and bituminous calcarenites as well as dolomites interbedded with stromatolitic dolomites and storm intraclastic breccias. Rare finds of small-shelled fauna (Saraev et al., 2004), the rock composition, and correlation with the sections of the Siberian Platform permit a confident correlation of the Usolka Formation with the Usolka sediments of the Tommotian and lower Atdabanian Stages (Lower Cambrian). The formation of this sedimentary complex took place within a large bay of the East Siberian salt basin. The saliferous rocks of the formation are replaced to the west by the facies of evaporite (often, redeposited) carbonate–sulfate sediments. This type of section is represented by the Oksym Formation, exposed in the Vostok-4 well. Farther west (west of the barrier reef zone), the composition of the complex becomes calcareous or clay–calcareous and the total thickness of the section decreases considerably. The coeval sediments of the Usolka and Oksym Formations are here correlated with the lower Churbiga Formation (Vostok-1 and Vostok-3 wells). This interval in the western Yenisei Ridge is correlated with the upper part of the lower subformation of the Lebyazh’e Formation, which consists of sandstones and gravelstones 60 to 250 m thick in the lower part and of dolomites ~100 m thick (or more) in the upper one. In the northwestern Siberian Platform, it corresponds to the lower part of the Krasnoporog Formation, which is observed in this region. The Oksym Formation is detected in the Vostok-4 well (4570–5105 m) (Kontorovich et al., 2012). Judging by the GIS data, the formation shows an alternation of decreased and increased gamma-ray log responses (from 2–3 to 10–12 µR/h). In the lower part of the formation, there is a 150-m interval of decreased gamma-ray log responses (1–2 µR/h). The formation, which is a limestone–dolomite and anhydrite–limestone–dolomite complex, formed in the rear part of a reefogenic ridge at its boundary with the salt basin in the Early Cambrian. It is divided into two subformations. The lower one (4953–5105 m) consists of sulfatized and silicified fine- and very fine-clastic and stromatolitic dolomites interbedded with dolomite–calcareous–anhydrite rocks. The upper one (5570– 5953 m) is composed of interbedded fine- and very fine-clastic dolomites, limestones, and anhydrites, differently enriched in clay material with intercalates of stromatolitic limestones, dolomites, intraclastic breccias, and stacks of red-colored clay–carbonate rocks. Finds of faunal remnants (Kontorovich et al., 2012; Korovnikov et al., 2010) permit assigning the formation to the Tommotian and correlating it with the Usolka Formation (Lemok-1 and Averinskaya-150 wells) as a salt-free analog of the latter. Hyolithelminths Torellella cf. biconvexa were found at the bottom of the Vostok-4 well.

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The age correlation with the western Siberian Platform and western Yenisei Ridge is analogous to that of the Usolka Formation. The Churbiga Formation is observed in the Vostok-1 (Kontorovich et al., 2008a) (4945–5010 m) and Vostok-3 (Kontorovich et al., 2008b) (3660–3870 m) wells. According to the GIS data, the formation is characterized by low and medium gamma-ray log responses: on average, 2–3 µR/h in the lower part (less often, up to 5–6 µR/h) and 1–2 µR/h in the upper one. In the stratotype section of the Vostok-3 well, the Churbiga Formation is divided into two subformations. The lower one (3786–3870 m) consists of interbedded fineclastic pelitomorphic limestones and clay–carbonate rocks with small intercalates of pyrite-bearing carbonaceous mudstones. The upper one (3660–3786 m) is composed of peculiar monomictic breccias (conglomerate-like limestones). Abundant faunal remnants in the lower part of the formation (Kontorovich et al., 2008a,b) permit correlating this part with the Tommotian and Atdabanian Stages (Lower Cambrian). The upper part of the formation in the Vostok-1 well contains mollusks Aldanella sp., hyoliths Conotheca circumflexa, gastropods Aegides sp., and sponge spicules. In general, the Churbiga Formation in the western prereef part of the Cis-Yenisei basin is correlated with the Tommotian–Botomian section (Lower Cambrian), stripped in the salt subbasin. In the northwestern Siberian Platform, it corresponds to the Krasnoporog and lower Shumnaya Formations. In the western Yenisei Ridge, this interval is occupied by the Lebyazh’e Formation without its upper and lower parts. The Lower Cambrian section in the southwestern Siberian Platform includes the Usolka, Klimino, and lower Agalevo Formations in the Lower Angara zone and the Usolka, Belaya, and Bulai Formations in the Baikit zone. The Belaya Formation is recognized in the Averinskaya150 (1020–2825 m) and Tyiskaya-1 (697–1286 m) wells by analogy with the coeval Belaya sediments of the Siberian Platform (Fig. 3). The formation section in the Averinskaya150 well is complicated by a fault at a depth of 1930 m, along which the formation is displaced with secondary alterations in the overthrust (interval 1020–1930 m) part of the section (dissolution of sulfate–salt sediments) (Saraev et al., 2004). Judging by the GIS data, the formation interval below the fault (1930–2825 m) is characterized by an alternation of decreased and increased gamma-ray log responses. The lower part shows a uniform combination of decrease (2–3 µR/h) and increase (6–9 µR/h) peaks, whereas the upper one is characterized by rare peaks of increased radioactivity (6–10 µR/h) and predominant decreased values (0–2 µR/h). The saliferous section of this formation is subdivided into two members, the lower one of which is composed of very fine-clastic and evaporite limestones, dolomites, and rare anhydrites, whereas the upper one is composed of thick interbeds of rock salt, dolomites, limestones, and thinly laminated red-colored rocks of mixed clay–silt–anhydrite–dolomite composition. The section in the Tyiskaya-1 well consists of calcareous–dolomite carbonate sediments admixed with clay material (Kashtanov et al., 1995). Abundant intervals of brecciated rocks might be related to the

Fig. 3. Stratigraphic chart of the Cambrian sediments of the Cis-Yenisei basin.

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secondary alterations of the sulfate–salt sediments. Similarly to the Usolka complex, this sedimentary complex formed in a salt subbasin. The remnants of small-shelled organisms (Saraev et al., 2004) and trilobites Bulaiaspis (Kashtanov et al., 1995) detected in the Tyiskaya-1 well (depth 1047 m) indicate the Atdabanian age of the formation. The Belaya Formation in the Averinskaya-150 well is correlated with the Tyya Formation, exposed in the Vostok-4, Lemok-1, and Tyiskaya-1 wells. In the outer (prereef) zone of the CisYenisei basin, it is correlated with the middle Churbiga Formation (in the Vostok-1 and Vostok-3 wells). This interval in the northwestern Siberian Platform corresponds to the upper Krasnoporog Formation. The Belaya Formation in the western Yenisei Ridge is correlated with the lower part of the Middle Lebyazh’e Subformation, whose outcrops consist of dolomites 300–400 m thick, which are sometimes sandy and slightly siliceous. The Klimino Formation is detected at this level in the Lower Angara zone of the Siberian Platform (Resolutions, 1983). The Tyya Formation is observed in the Vostok-4 (3863– 4570 m) and Lemok-1 (2963–3665 m) wells. In an earlier paper (Kontorovich et al., 2012), the upper part of the formation was assigned to the lower part of the Averin Formation, which was rejected because of its complicated correlation with coeval sections of the Siberian Platform. According to the lithological data, the Tyya Formation is divided into two subformations. The lower one (4215–4570 m in the Vostok-4 well and 3270–3665 m in the Lemok-1 well) in the most representative section of the Vostok-4 well (western boundary of the salt subbasin) consists of very fine-clastic clayey dolomite–anhydrite (redeposited) sediments, stromatolitic limestones, intraclastic breccias, and evaporite carbonate and anhydrite rocks (often, with signs of salinization). The lower part of the upper subformation (3863–4215 m in the Vostok-4 well and 2963–3270 m in the Lemok-1 well) consists of karst anhydrite–limestone–dolomite breccias (Vostok-4 and Averinskaya-150). The upper part of the subformation is composed of stacks of interbedded very fine- and fine-clastic anhydrite clayey dolomites with more rare interbeds of evaporite carbonate and sulfate rocks and stromatolitic limestones. The Tyya Formation in the Vostok-4 well shows the same behavior of gamma-ray logs and neutron gamma-ray logs as the Upper Oksym Subformation. The formation in the Lemok1 well shows similar behavior of the curves with less differentiated values. Note that the gamma-ray log responses decrease in the lower parts of the upper subformation in both wells, particularly in the section of the Lemok-1 well. Trilobites Bulaiaspis cf. sajanica were found at the top of the formation in the Vostok-4 well (3864.6 m). This genus is mostly typical of the upper Atdabanian, except the species Bulaiaspis sajanica, which occurs in the lower Botomian. As this species is poorly preserved and defined in open nomenclature, the authors assigned the interval to the upper Atdabanian based on the maximum abundance of specimens of this genus. Also, small-shelled fauna, algae, and trilobite fragments

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in the lower and upper subformations indicate Atdabanian age (Kontorovich et al., 2012). The sedimentary complex formed in the Early Cambrian in a salt bay of increased salinity. It is coeval with the Belaya and Klimino Formations in the inner Siberian Platform. The complex is correlated with the middle Churbiga Formation in the Vostok-1 and Vostok-3 wells. This interval in the northwestern Siberian Platform corresponds to the upper Krasnoporog Formation. The Tyya Formation in the western Yenisei Ridge is correlated with the lower part of the Middle Lebyazh’e Subformation. The Averin Formation is detected in the Vostok-4 (3715–3863 m), Lemok-1 (2780–2963 m), and Averinskaya150 wells (880–1020 m). The earliest (working) versions of demarcating the Averin Formation in the Vostok-4 and Lemok-1 wells (Kontorovich et al., 2012; Yolkin et al., 2001) were, to a considerable extent, arbitrary and based mainly on the differentiation of the section by the GIS curve. In the present paper, its extent is modified. The age of the formation is constrained by Botomian fauna (Lower Cambrian). Trilobite remnants are particularly abundant in the Vostok-4 well (Kontorovich et al., 2012; Korovnikov et al., 2010). Trilobites from a depth of 3831 m are defined as Micmaccopsis? sp., Bathyuriscellus sp., Astenaspis cf. tenius, Binodaspis sp., Termierella sp., and Tungusella manica. Trilobites Tungusella manica are found near the formation bottom, at a depth of 3852.3 m. In the western Siberian Platform, these trilobites are typical of the lower Botomian. Trilobites Binodaspis paula, which are also typical of the second half of the Lower Cambrian, occur in the Lemok-1 well, within the interval 2804.4–2805.9 m. According to the GIS data, the formation shows low gamma-ray log responses (0–2 µR/h) with a gradual increase (to 5–6 µR/h) toward the upper and lower boundaries. The formation consists of fine- and very fine-clastic dolomites and limestones with shell detritus containing stacks of interbedded limestones and marls (sometimes, with signs of sedimentation boudinage) and intercalates of sedimentary anhydrites. This complex formed in the second half of the Early Cambrian in waters of increased salinity (Lemok-1 and Averinskaya-150 wells) and rear reefogenic clastic facies (Vostok-4 well). The Averin Formation is correlated with the upper Churbiga Formation in the prereef zone of the basin (Vostok-1 and Vostok-3 wells). It corresponds to the lower Agalevo Formation in the Lower Angara zone of the Siberian Platform and to the Bulai Formation in the Baikit zone. The Averin Formation in the western Yenisei Ridge corresponds to the upper part of the Middle Lebyazh’e Subformation and the lower part of the Upper Lebyazh’e Subformation, consisting of dolomites with interbeds of variegated silt–clayey rocks 400–600 m thick. The Kol’chum Formation is detected in the Vostok-4 (3360–3715 m), Averinskaya-150 (770–880 m), and Lemok-1 (2450–2780 m) wells. According to the GIS data, the thin (70–110 m) lower part is characterized by increased and more differentiated gamma-ray log responses (5–9 µR/h), whereas the upper part is characterized by decreased (1–3 µR/h) and

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Fig. 4. Correlation of Vendian–Cambrian sediments in the wells of the Cis-Yenisei basin. 1–5, limestones: 1, limestones; 2, oolitic limestones; 3, lumpy limestones; 4, stromatolitic limestones: a, bedded; b, columnar; 5, clayey limestones; 6, dolomites; 7, calcareous dolomites; 8, sandy dolomites and limestones; 9, limestones with shell detritus; 10, silty dolomites and limestones; 11, dolomites and limestones with detrital biotite; 12, intraclastic dolomitic and calcareous breccias; 13, anhydrite dolomites; 14, mudstones; 15, calcareous mudstones; 16, siltstones; 17, calcareous siltstones; 18, carbonaceous silt–clayey rocks; 19, carbonaceous carbonate–clayey rocks; 20, carbonaceous clayey–siliceous rocks; 21, sandstones; 22, gravelstones; 23, conglomerates; 24, anhydrites; 25, dolomite–anhydrite–clay–silty rocks; 26, rock salts; 27, karst breccias; 28, sedimentation faulting; 29, sites of faunal finds.

more consistent gamma-ray log responses. The formation is divided into two subformations by lithology and the shape of the gamma-ray log. The Lower Kol’chum Subformation in the Vostok-4 and Lemok-1 wells (3600–3715 and 2710–2780 m, respectively) is not constrained by core studies. Judging by the GIS data and the composition of sludge, it is composed of limestones, dolomites, and carbonate–clayey rocks (in some places, they might be carbonaceous). The Upper Kol’chum Subformation (3360–3600 and 2450–2710 m, respectively) is constrained by core samples only in the Vostok-4 well. It consists of calcareous–dolomite rocks, oolite-lumpy limestones, and stacks of interbedded carbonate and marly rocks with signs of sedimentation boudinage (conglomerate-like limestones). The formation accumulated within the preexisting Early Cambrian salt subbasin. In (Saraev et al., 2004), the Zeledeevo Formation was recognized in the Averinskaya-150 well within the interval of the present Kol’chum Formation, but it was decided to retain the local name because of the distinctive features of this formation. This level in the northwestern Siberian Platform corresponds to most of the Shumnaya Formation. In the inner Siberian Platform, it includes the lower Zeledeevo and upper Agalevo Formations (Lower Angara zone) as well as the Litvintsevo and Angara Formations (Baikit zone). The Kol’chum Formation is correlated with the Paidugina Formation in the Vostok-1 and Vostok-3 wells. The Paidugina Formation is observed in the Vostok-1 (4825–4945 m) and Vostok-3 (3635–3660 m) wells (Kontorovich et al., 2008a,b). According to the GIS data, the formation shows abnormally high gamma-ray log responses (up to 20–25 µR/h), particularly in the lower part, which is a good criterion for its detection, identification, and correlation. The formation has a complete section in the Vostok-1 well. It is composed of carbonaceous carbonate–clayey rocks with thin partings of sedimentary pyrite and felsic tephroids containing anthraxolite segregations. Along with the rocks described above, the less complete section in the Vostok-3 well contains siliceous rocks. The carbonaceous sediments formed in the latest Early and early Middle Cambrian in the outer (prereef) zone under deep-water anoxic conditions. Amgan trilobites Tomagnostus sibiricus, Ptychagnostus contortus, and Ptychagnostus gibbus? are defined in the middle part of the formation (Vostok-1 well, 4872–4884 m) (Kontorovich et al., 2008b). The thickness of the Paidugina Formation in the Vostok-3 well is 25 m, but this might be underestimated value because of the uncertain present position of its upper boundary. No faunal remnants are observed. Correlation with analogous sediments in the Vostok-1 well is based on characteristic log and lithological data. The formation in the well log is overlain

by the thick (>240 m) Maloe Omutli Series of homogeneous composition and structure, which is of uncertain age. The Paidugina Formation in the northwestern Siberian Platform corresponds to most of the Shumnaya Formation (its upper part). This level in the inner Siberian Platform corresponds to the lower Zeledeevo and upper Agalevo Formations (Lower Angara zone) as well as the Litvintsevo and Angara Formations (Baikit zone). The Elogui Series is distinguished in the Vostok-4 (3025– 3360 m), Lemok-1 (2100–2450 m), and Eloguiskaya-1 (1680– 1884 m) wells. The series was first introduced into the stratigraphic chart (Resolutions, 1999) and recognized as a formation in the Lemok-1 well with slight modifications in extent (Yolkin et al., 2001). During the subsequent stratigraphic division of the Vostok-4 well section (Kontorovich et al., 2012), the sediments of this interval were included into the Evenki Formation in the working version owing to their lithologic similarity to the Early Evenkian sediments of the Podelga, Kondes, and Shedelga Series in the Vostok-1 well section. After the age of this stratigraphic level had been specified and discussions had been held with SNIIGGiMS experts (Yu.Ya. Shabanov and S.S. Sukhov), it was decided to recognize the series as an independent stratigraphic unit (Elogui Series) of pre-Evenkian age (Late Amgan–Early Mayan). The GIS and lithological data indicate that the series is subdivided into two parts. According to the gamma-ray log data, they show slightly differentiated decreased gamma-ray log responses (1–3 µR/h) with a characteristic jumpy increase in gamma-ray log responses (to 6–9 µR/h) near the bottom. The lower part in the Vostok-4 well section (3215–3360 m) is composed of microlumpy and pelitomorphic limestones with rare partings of bedded stromatolitic limestones. The subformation in the Lemok-1 well (2364–2450 m) is not constrained by core samples. The bottom of the upper part (3025–3215 m, Vostok-4 well) is an alternation of stacks of fine- and very fine-grained limestones and clayey dolomites showing sedimentation boudinage. The top consists of interbedded oolitic and lumpy dolomitized limestones. The upper part of the series in the Lemok-1 well (2100–2364 m) is composed of dolomites and dolomite limestones with fragments of trilobites, gastropods, hyoliths, and microphytolites and traces of bioturbation. The faunal remnants (trilobites and brachiopods) found in the Vostok-4 well permit assigning the interval to the second half of the Middle Cambrian and the first half of the Late Cambrian. Middle Cambrian trilobites Anomocaridae (Pseudanomocarina? sp. and Chondranomocare sp.), Corynexochidae (Acontheus?), and Agnostidae (Peronopsis ex gr. fallax (Linnar.)) were defined in the Eloguiskaya-1 well. Also, brachiopods of the genera Acrothele, Linnarssonia (?), and

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Micromitra are defined in this interval (Dragunov et al., 1967). Elements of exoskeletons of trilobites Deltocephalus? sp. are observed in the Vostok-4 well at a depth of 3352.5–3352.3 m from lower Amgan (Middle Cambrian) to lower Mayan Stages. Poorly preserved cranidia of trilobites Proasaphiscidae (genus Pseudanomocarina) are found higher in the section, at depths of 3101.6 and 3102.4 m. This genus is widespread in the upper Amgan Stage and the first half of the Mayan Stage (Middle Cambrian). The Elogui Series corresponds to the Pudzhelga Series in the Vostok-1 well and to the Ust’-Brus Formation in the northwestern Siberian Platform. This level in the inner Siberian Platform and the Yenisei Ridge corresponds to the Olenchimo and upper Zeledeevo Formations. The Pudzhelga Series is recognized as a formation in the Vostok-1 well (Kontorovich et al., 2008a) (4250–4825 m). According to the GIS data, the series is characterized by medium gamma-ray log responses with medium differentiation (3–6 µR/h; rarely, up to 8). The series consists of alternating stacks of interbedded pelitomorphic limestones and variegated carbonate–clayey rocks, more rare limestone breccias, quartz– feldspar sandstones, and calcarenites showing bedding with and without sedimentary dislocation. Pygidium of trilobite Kootenia amgensis and rare traces of deposit feeders are found at the base of the series. The species occurs throughout the Siberian Platform in the Amgan and lower Mayan Stages (Middle Cambrian). Therefore, the series is assigned to the Middle Cambrian (Amgan–Mayan), taking into account its stratigraphic position. The Pudzhelga Series is correlated with the Elogui Series and lower Evenki Formation (Eloguiskaya-1, Vostok-4, and Lemok-1 wells). The Evenki Formation was first detected by G.I. Kirichenko (1950) in the basin of the Stony (Podkamennaya) Tunguska River and in the Yenisei Ridge. It is divided into three subformations in the stratotype locality. Within the Cis-Yenisei basin, it is observed in the Lemok-1 (750– 2100 m), Vostok-4 (2262–3025 m), and Eloguiskaya-1 (1467– 1680 m) wells. The formation shows considerably differentiated increased (6–10 µR/h) gamma-ray log responses. It is composed of cherry red and variegated mixed silt–anhydrite– clay–dolomite and dolomite–clayey rocks with numerous interbeds of dolosiltites, anhydrite dolosiltites, carbonaceous mudstones, and storm intraclastic breccias. Stromatolitic and oolitic limestones, as well as anhydrites and sandstones, occur as rare thin (few centimeters) interbeds. In the Vostok-1 well, the section coeval with the Evenki Formation becomes thicker and its lower (more carbonate) part differs in the GIS data (lower and less differentiated gammaray log responses) and smaller quantity of red-colored rocks. The previous division of this thick section (>1.5 km) in the Vostok-1 well into independent formations (Kontorovich et al., 2008b) is now rejected, and they are regarded as series. In this interpretation, the Evenki Formation in the Vostok-4 and Lemok-1 wells corresponds to the Podelga, Kondes, Shedelga, and Pyzhina Series, whose age (Middle–Late Cambrian) is reliably confirmed by paleontological data (Kontorovich et al., 2008b). In the areas of the Yenisei Ridge

adjacent to the Cis-Yenisei basin, the Evenki Formation (Group) consists of three subformations (Kirichenko, 1950) or formations (Olenchimo, Chernyi Ostrov, and Vel’mo) (Kachevskii et al., 1998). The Evenki Formation in the inner Siberian Platform (Baikit zone) is divided into two subformations (Mel’nikov, 2009). The Evenki Formation in the northwestern Siberian Platform is correlated with the upper Ust’Brus, Labaz, Orakta, Kulyumbe, and Uigur Formations. The Podelga Series is detected in the Vostok-1 well (4005–4250 m). According to the GIS data, it shows slightly differentiated medium gamma-ray log responses (4–9 µR/h). The series is predominantly of dolomitic and anhydrite–dolomite composition. It is subdivided into two parts. The lower one (4090–4250 m) consists of cerise sandy dolarenites interbedded with volcanomictic sandstones, whereas the upper one (4005–4090 m) consists of dark gray anhydrite dolomites interbedded with cerise oolitic limestones, calcarenites, and volcanomictic sandstones with anhydrite nodules. The sediments formed in the outer (prereef) zone of open shelf. The position of the series corresponds to the upper Mayan Stage (Middle Cambrian). In the Eloguiskaya-1, Vostok-4, and Lemok-1 wells, it corresponds to the lower Evenki Formation. In the northwestern Siberian Platform, the series is correlated with the Labaz Formation. This interval corresponds to the lower Evenki Formation in the inner Siberian Platform and to the Chernyi Ostrov Formation (Evenki Group) in the Yenisei Ridge. The Kondes Series is observed in the Vostok-1 well (3600–4005 m). According to the GIS data, the series is characterized by medium and low gamma-ray log responses (3–6 µR/h; sometimes, up to 8 µR/h). It is composed of variegated and red-colored calcarenites, oolitic limestones, and calcisiltites with less widespread interbeds of pelitomorphic limestones enriched in clay material, dolomite, and, to a small extent, anhydrite. The sediments formed in the outer (prereef) zone of open shelf. The series contains trilobite remnants of the Nganasanian and Tavgian Horizons (Late Cambrian), which permit its correlation with the Ayusokkanian and lower Sakian Stages (Upper Cambrian). Trilobites Kuraspis obscura, Kuraspis similis, Kuraspis spinata, Kuraspis similis ex gr. vera, Kuraspis similis ex gr. deflexa, and Letniites sp. occur within the interval 3659–3759 m. This assemblage corresponds to the Tavgian Horizon of the Ayusokkanian Stage (Upper Cambrian). Trilobites Bolaspidina insignis, Parakoldinia sp., and Kuraspis similis from the Nganasanian Horizon of the Ayusokkanian Stage (Upper Cambrian) are found within the interval 3902.6–3953.5 m. The series is correlated with the lower–middle Evenki Formation in the Eloguiskaya-1, Vostok-4, and Lemok-1 wells. In the inner Siberian Platform, this interval also corresponds to the lower–middle Evenki Formation. In the Yenisei Ridge, this interval corresponds to the lower Vel’mo Formation. In the northwestern Siberian Platform, the Kondes Formation is correlated with the Orakta Formation. The Shedelga Series is detected in the Vostok-1 well (3210–3600 m). According to the GIS data, the series shows low (2–6 µR/h) gamma-ray log responses and is predomi-

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nantly calcareous. The Shedelga Series is subdivided into two parts. The lower one (3438–3600 m) is composed of variegated and red-colored oolitic limestones, sandy calcarenites, and calcisiltites interbedded with carbonate–anhydrite–clayey rocks. The upper one (3210–3438 m) is dominated by interbedded pelitomorphic limestones and carbonate–clayey rocks of coarse-clastic structure, both undisturbed and showing sedimentary dislocation (conglomerate-like limestones), with interbeds of oolitic limestones and sandy calcarenites. The sediments formed within shallow-water open shelf. Trilobite remnants of the Maduian and Entsian Horizons (Late Cambrian) are found in the series, permitting its correlation with the upper Sakian Stage (Upper Cambrian). The interval 3241–3400 m contains trilobites (Parakoldinia salairica, Pseudagnostus sp., Parakoldinia striata, Koldinia pusilla, Komaspidella rara, Hadragnostus sp., Homagnostus sp., Bolaspidellus sp., Parakoldinia kureiskaya, Plethopeltoides lepidus, Amorphella sp., and Pesaiella sp.) and brachiopods (Billingsella sp., Eoorthis sp., and Lingulella sp.). This assemblage is typical of the Entsian Horizon of the Sakian Stage (Upper Cambrian). The interval 3465–3471 m contains the following taxa: Idahoia cf. composita, Raashellina paula, Bolaspidina sp., Pesaiella sp., Saonella cf. saonica, Ammagnostus simplexiformis, Bolaspidina cf. insignis, Schoriecare sp., Parakoldinia sp., Komaspidella rara, Nordia aff. lepida, Verkholenoides sp., and Parakoldinia striata. This assemblage is typical of the Maduian Horizon of the Sakian Stage (Upper Cambrian). The Shedelga Formation is correlated with the middle Evenki Formation in the Eloguiskaya-1, Vostok-4, and Lemok-1 wells. This interval corresponds to the middle Evenki Formation in the inner part of the platform; to the middle Vel’mo Formation, in the Yenisei Ridge; and to the lower Kulyumbe Formation, in the northwestern Siberian Platform. The Pyzhina Series is recognized in the Vostok-1 well (2766–3210 m). According to the GIS data, it differs greatly from the underlying sediments, showing increased and more differentiated gamma-ray log responses (5–10 µR/h). The series is of mixed sand–silt–clay–calcareous composition with slightly predominant silt- or sand-sized calcareous debris. The series has an obvious variegated character owing to the contrast of cherry red, green, and gray. The sediments formed in open shelf in the Late Cambrian. In the Vostok-1 well, rare trilobites Monosulcatina laeve in the upper part of the stripped series interval (depth 2772 m) permit a correlation with the Ketyian Horizon of the Aksaian Stage (Upper Cambrian). Reliable correlation of the Pyzhina Series with the sections of the Evenki Formation in the Lemok-1 and Vostok-4 wells is complicated by different circumstances. First, this is the difference in the lithologic composition of the lower parts of the Evenki interval (increased carbonate content in the western parts of the Cis-Yenisei basin and the lack of large carbonate interbeds in its eastern parts). Second, the upper fragments of this interval in different wells show different degrees of integrity owing to pre-Mesozoic erosion. This interval in the inner Siberian Platform might correspond to the upper Evenki Formation. In the Yenisei Ridge, it is tentatively correlated

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with the upper Vel’mo Formation. In the northwestern Siberian Platform, the Pyzhina Series corresponds to the upper Kulyumbe Formation. The sediments tentatively correlated with the Evenki Formation also include the lower part (2200–2505 m) of the section of the Kasskaya-1 well, in which thin interbeds of pinkish and greenish gray marls and limestones occur within terrigenous rocks. The rocks are predominantly brownish red; sometimes, with greenish spots; and, less often, with greenish and bluish interbeds and members. The terrigenous rocks are highly calcareous and contain a large amount of mica. In the sandstones, it is concentrated on the bedding planes. The age of this section is not constrained by any paleontological data.

Disputable issues of stratigraphy and correlation The proposed correlation is not the only possible version. The small number of wells (localized in different facies zones) and rare faunal finds permit accepting different age determinations for some section fragments and their stratigraphic correlation. First and foremost, this applies to the dating of the Elogui and Kol’chum Formations in the Vostok-4 and Lemok-1 wells and to their correlation with the corresponding interval in the Vostok-1 well. Also, note that the wells drilled into the Cis-Yenisei basin in the 1950s–1970s exposed pre-Jurassic sedimentary complexes belonging to different lithofacies. Their age was not constrained by any paleontological data, and they were tentatively assigned to the Devonian System based on their similarity to the Devonian Minusa sections. The latter include – the Kas Series of red-colored terrigenous (sandstones, conglomerates, and siltstones) and sand–calcareous rocks in the Kasskaya-1 well (interval 1625–2200 m); – the Vanzhil’ Series of predominantly terrigenous rocks (red-colored and gray sandstones, mudstones, and siltstones) in the Vanzhil’skaya-1 well (1945–3100 m); and – the Dunaeva Series of effusive–sedimentary rocks: basalt porphyrites as well as their tuffs and tuffites (the stratotype is localized in the Nyarginskaya-1 well within the interval 2667–2783 m, and analogs are stripped in some other areas). Also, a series of variegated terrigenous and terrigenous carbonate rocks of uncertain age is stripped in the SeveroLymbel’skaya-1 well (interval 2187–3041 m). A small interval of these rocks (2820–2970 m) is recognized as the Lymbel Series of tentatively Silurian age (Krasnov et al., 1993; Resolutions, 1999; Surkov and Zhero, 1981). Sediments similar to the Dunaeva Series are exposed in the adjacent Elan’ and Korbyl’ areas and farther south, in the Yar, Karbino, and Chachana areas, whereas those similar to the Vanzhil’ Series are exposed in the nearby Western area. The new drilling data (Vezdekhodnaya-4, Vostok-1, Vostok-3, Vostok-4, Lemok-1, and Averinskaya-150 wells) and the interpretation of the new seismic data obtained in 2000– 2012 show that the Nyarga, Elan’, Korbyl’, Vanzhil’, and Western areas are localized in the zone of regional uplifting

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of Upper Proterozoic–Cambrian sedimentary complexes (Raiga–Azharma Ridge). Judging by the seismic wave pattern, the base of the overlying Jurassic rocks should be here underlain by pre-Late Paleozoic complexes (most likely, Vendian or Lower Cambrian). Along the sides of the ridge and farther east, the section contains younger Cambrian sediments. Upper Cambrian terrigenous sediments of predominantly variegated character, represented by the Evenki Formation (Group), outcrop here at the base of the Jurassic complexes in most of the basin territory. With regard to these new data, the sediments of the Kas and Vanzhil’ Series (tentatively assigned to the Devonian before) should be assigned to the sediments of the Evenki Group, which have similar appearance and must belong to the same seismic complex. Apparently, the sediments exposed in the Severo-Lymbel’skaya area should also be partly assigned to this unit. We propose that the volcanic sediments of the Dunaeva Series be regarded as analogs of the Lisitsa Series. The latter was stripped by the Vezdekhodnaya-4 well in the neighboring area (interval 3541–4800 m) and dated at the Early Cambrian by the Ar–Ar method. An alternative correlation of the series is its consideration as an independent unit of Late Ordovician age, which is suggested by some early (1970s) absolute age determinations (Surkov and Zhero, 1981). Unfortunately, the dating method is not specified in this publication, which does not permit assessing the reliability of the presented estimates. Either version might be correct for different areas, but there is no evidence for the Devonian age of the series in any case. Along with the above-mentioned units, the Middle Cambrian sediments might include the Maloe Omutli Series in the Vostok-3 well (interval 3393–3635 m). It overlies the Paidugina Formation, lacks organic debris, and consists of interbedded calcareous sandstones and dark gray calcareous siltstones and mudstones with interbeds of intraclastic breccias. This series differs from the underlying sediments only in the presence of larger (up to 0.7 m) graded rhythms and, consequently, higher differentiation of the carbonate terrigenous sediments. It is possible that the lower part of this formation should be included into the Paidugina Formation.

Conclusions The present studies permitted the development of the Cambrian stratigraphic chart of the Cis-Yenisei sedimentary basin. The series stripped by the wells were compared with coeval sections of the western and northwestern Siberian Platform as well as the Cambrian sedimentary complexes of the western Yenisei Ridge. It was found that the formation of the Early and early Middle Cambrian sediments stripped by the Vostok-1 and Vostok-3 wells took place in a predominantly open sea basin (outer prereef zone). On the contrary, the Early Cambrian sediments in the wells localized in the eastern Cis-Yenisei basin accumulated under conditions similar to those of the East Siberian salt basin.

Judging by lithofacies characteristics and the composition of trilobite assemblages, the sediments of the upper half of the Middle Cambrian and the Upper Cambrian formed in the Vostok-1 well under conditions similar to those in the northwestern Siberian Platform (Kotui–Igarka facies region of the Upper Cambrian of the Siberian Platform), namely, at the outer edge of a carbonate platform. Two SFZs are recognized in the study area: Kas SFZ (the areas of the Lemok-1, Averinskaya-150, Tyiskaya-1, Vostok4, and Eloguiskaya-1 wells) and Ket’ SFZ (the areas of the Vostok-1 and Vostok-3 wells). The boundary between these SFZs passes along the reconstructed zone of a barrier reef stretching from north to south. The Vostok-4 well is localized in the western Kas SFZ, at the salt subbasin/barrier reef boundary. The study was supported by the Siberian Branch of the Russian Academy of Sciences and the Uralian Branch of the Russian Academy of Sciences (integration project no. 50), the Russian Foundation for Basic Research (no. 13-05-00334), project 28.1 “Biotic Events and Crises in Siberian Paleozoic Basins (Chronostratigraphic Position, Relationship with Sedimentation Events, and Regional Manifestations).”

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