Blackbourn Reports: The Palaeozoic of Western Siberia Tectonic Development of Western Siberia during the Palaeozoic
Graham Blackbourn: Blackbourn Geoconsulting
The crystalline basement and Palaeozoic deposits within the West Siberian Basin have been penetrated by numerous wells, and detailed geophysical studies have been undertaken over recent decades. However, although the overall lithology and the distribution of different types of basement are quite well known, the absence of any outcrop within the basin itself precludes the type of field study which might enable structural relationships to be investigated directly. Wells in central parts of the basin, and the Kara Sea, have penetrated only the uppermost sections of the thick Palaeozoic intervals where they exist. The evolution of the basement to the WSB, from the Precambrian to the Triassic, is therefore largely inferred from that of surrounding regions where detailed outcrop studies have been possible.
To the west and east of the WSB are the cratonic blocks of the East European Platform (west of the Urals) and the Siberian Platform respectively. However, the evolution of the WSB appears also to be closely related to that of the tectonic belts to the south and southeast within Kazakhstan, other parts of Central Asia, and eastwards from there towards Mongolia and China. Indeed, it is now generally agreed that most of the central and southern parts of the WSB, including parts of the areas which accommodate the majority of the known hydrocarbons, are floored by crust with affinities to that of Kazakhstan.
The late Precambrian and early Palaeozoic history of the area remains largely conjectural, and interpretations of various authors differ significantly in detail, although a general consensus is emerging regarding the overall pattern. Sengör et al. (1994) speculate that Vendian to Cambrian rifting along the margin of a combined East European–Siberian supercontinent initiated the separation of these two major continental blocks, together with the creation of the Kipchak arc, which drifted away from the continental blocks, separating them from the Turkestan ocean. The oceanic basin which opened up between the continents and the Kipchak arc (the “Khanty-Mansi ocean”) began to close again by the mid-Silurian by subduction beneath the Uralian margin of the Eastern European block. The Kipchak arc broke up into a series of segments, one corresponding to the future “Kazakhstan” crustal block. Meanwhile, Siberia and Eastern Europe began to rotate in relation to each other, clockwise and anti-clockwise respectively, drawing the various arc fragments into the space between them and stretching them out, rather in the manner or an old-fashioned laundry mangle pulling clothing to be dried into the space between the rollers. This resulted in large-scale strike-slip faulting. Buslov et al (2003) and other authors interpret a broadly similar tectonic setting at this time (Fig. I.2.1 & I.2.2), which continued during closure of the Uralian and South Mongol (or Mongol- Okhotsk) oceans. The resulting triangular wedge of crustal fragments lying between the two cratonic blocks finally jammed the system before the end of the Palaeozoic (pre-Late Permian). This mosaic of crustal blocks was subsequently to form the basement to the WSB (Enclosure 2).
Enclosures 3 and 4 are interpreted deep crustal geotraverses across the WSB, taken from Egerov & Chistyakov (2003). Note that the age range of some sedimentary successions given by these authors – notably the Triassic rift-fills – differs from that given elsewhere in this study and by most authors, although the overall structural pattern appears sound. Enclosure 5 is a cross section from the central WSB (Surgut Arch) to the Yenisei Ridge on the eastern margin, depicting the Triassic and Palaeozoic beneath the Jurassic, from Kirda (2005). The stratigraphy and structure of the Palaeozoic section are shown in a very simplified form.
Description of the Main
Basement Blocks and Palaeozoic Deposition
The basement of the West Siberian Basin has been broadly divided into five main genetic blocks (Enclosures 2 & 4 (Egorov & Chistyakov, 2003; Surkov & Smirnov, 2003)). These are the Urals Fold Belt, the Central West Siberian Fold System, the Kazakhstan Fold Belt (including the Salym and Irtysh zones), the Altai-Sayan Fold Belt and the Yenisei Fold Belt. These are considered briefly in turn below. It should be emphasised that, owing to the absence of outcrop, and the considerable depth of the Palaeozoic and older deposits in parts of the WSB, knowledge of these lower horizons is limited.
Urals Fold Belt
Gravimetric and magnetic data, together with limited drilling data, suggest that this is essentially a continuation of the Urals fold system exposed to the south and west. The NS- trending Tagil-Magnitogorsk zone (Enclosures 2 & 4) can be traced beneath the Mezozoic and Cenozoic cover, where it includes early and middle Palaeozoic belts of spilite-keratophyre series rocks, porphyritic dolerites and carbonates. These are associated with numerous basic and ultrabasic intrusives. To its east lies the Sherkala zone (including the Talitsk zone) which curves around in the north to a NE-SW orientation, and marks the eastern limit of the Urals basement block (Enclosure 2). The Sherkala zone is similar according to gravimag data to the Magnitogorsk zone, and it appears to be a northward continuation of the East Urals synclinorium. Between the Tagil-Magnitogorsk and the Sherkala zones is the East Urals Block (or East Urals anticlinal zone), a northward continuation of the Mugodzhar anticlinorium, and this passes in turn into the Isetsk-Saldinsk anticlinal zone. It includes the Sartyn’insk and North Sos’va anticlinoria. These structures have Precambrian gneiss complexes at their core, intruded by late Palaeozoic granitic plutons, and with metamorphosed Ordovician to Silurian volcanics and sediments around their flanks (Enclosure 4).
Central West Siberian Fold System
This basement block underlies the central part of the West Siberian Basin (Enclosures 2 & 4). In the south of this block the late Palaeozoic Kalba-Narym and Tom’-Kolyvan fold belts outcrop. The central part of the block comprises a series of sub-parallel anticlinal zones (Novosibirsk, Pyl’-Karaminsk, Taz and others), separated by basins infilled by late Palaeozoic sediments (the Upper Tol’kinsk and Chekov troughs and the Yugan-Pokur and Nyurol’ basins). The Nyurol’ Basin hosts the only significant oil-producing Palaeozoic reservoirs known in the WSB (Section II.2.1.1). The Novosibirsk and other anticlinal zones, like those of Kalba-Narym and Tom’- Kolyvan, resulted from inversion of sedimentary basins infilled from the early Devonian. The Devonian to Early Carboniferous deposits, mixed clastics and carbonates, were intruded by late Palaeozoic granites during contemporary collisions. The Novosibirsk zone is therefore cored by granite, within a sequence of metamorphosed Devonian and Early Carboniferous sands and shales. Comparable basement structures can be traced northwards through the Central West Siberian Fold Belt as far as the “North Yamal inversionary anticlinorium” on the Yamal peninsula. Further specific detail on individual basement blocks within the Central West Siberian Fold System is provided by Surkov and Smirnov (2003).
Kazakhstan Fold Belt (including the Salym and Irtysh zones)
This basement block is approximately triangular in plan beneath the West Siberian Basin, with its apex in the north, and lies between the Pre-Urals block in the west and the Central West Siberian Block in the east (Enclosure 2). The Kazakhstan Fold Belt is composed, on the basis of drilling and geophysical data, of a series of anticlinoria and superimposed basins filled with Devonian to Carboniferous (and possibly Early Permian) deposits.
The Ural-Kazakh Trough runs along the boundary between the Kazakhstan Fold Belt and the Pre-Urals block to the west (not developed in the line of section depicted in Enclosure 4, which crosses the northern apex of the Kazakhstan block). The Urals-Kazakh Trough, which forms a positive gravity and magnetic zone, is filled with Devonian and Early Carboniferous volcanics and mixed clastic and carbonate deposits. It is divided into internal (to the west) and external zones, separated by the Kustanai deep fault (thought to be a major strike-slip suture; Enclosure 2) associated with hyperbyssal diorites. The internal zone, on the Uralian side, comprises mainly deformed and metamorphosed Carboniferous volcanics and sedimentary deposits; the external zone contains a less deformed sedimentary succession.
In the northern apex area of the Kazakhstan Fold Belt lies the Khanty-Mansi block (Enclosures 2 & 4). It is thought to be an ancient continental fragment, and in the south comprises mainly gneisses of possibly early Proterozoic age. Much of the block is composed of highly deformed schists (quartz-mica, quartz-graphite, and other greenschist facies lithologies) with numerous granitic intrusions. In plan the block appears to be composed of a complex mosaic of individual structural blocks, in which a number of basinal areas are filled with a variety of mid-Palaeozoic carbonate, clastic and volcanic deposits.
The approximately NW-SE-trending series of structures between and including the Irtysh and Salym structural zones (Enclosure 2) are thought to be a northwestern extension of the Chingiz-Tarbagatai fold system which outcrops in the Kazakh uplands to the south. By analogy with the latter it is interpreted as comprising a series of structures thrust over one another towards the Kokchetav block in the southwest. Where more uplifted blocks have been penetrated by drilling they comprise volcanic rocks metamorphosed to greenschist facies, with granitic intrusions; these structures have been traced in the subsurface as linear gravity and magnetic anomalies. The NW-SE trending Tara- Muromtsev trough, lying between two of these linear structures, is filled by mid- to late Palaeozoic deposits, and locally with highly metamorphosed early Palaeozoic lithologies.
Altai-Sayan Fold Belt
This block, in the southeast of the region (Enclosure 2), is largely outcropping, although its northwestern apex dips beneath the Mesozoic cover. It is thought essentially to comprise a series of Palaeozoic basement ridges, which can be traced as positive linear gravitational and magnetic anomalies, with intervening basins filled with up to 2 km of later Palaeozoic deposits. Wells have penetrated Early and Middle Devonian volcanics, and clastic sediments of Carboniferous and, possibly, Permian age. Red-bed deposits of possibly Middle Devonian age have also been encountered.
Yenisei Fold Belt
The Yenisei Fold Belt lies in the east, alongside the Siberian Platform, and is characterised by Baikalian (late Precambrian) crustal structures (Starosel’tsev et al., 2003). The pre-Mesozoic surface of the Fold Belt dips quite uniformly westwards from the Siberian Platform towards the basin (Enclosure 3).
Seismic coverage of the Yenisei Fold Belt is sparse, and its structure is known largely from gravity and magnetic surveys. Several ancient “massifs” have been recognised, including the Upper Keta massif in the southern part of the Fold Belt, and the Nadoyakh massif in the north. There are also several elongate thrust-bounded basins, filled with supposed Palaeozoic deposits; the deep Kassk survey well penetrated Devonian deposits similar to those which outcrop in the west of the Siberian Platform. The Lemok-1 stratigraphic well, drilled beside the River Syma, penetrated (oil-bearing) Cambrian deposits at depths between 1200-4200 m, comprising an upper red-bed section of siltstones and claystones, and a lower carbonate section similar to Early Cambrian deposits in the South Tungusska area (Surkov and Smirnov, 2003; Zadoenko et al., 2004). Note that Permian (and possibly Permian) deposits have rarely been encountered within the WSB. This was probably a result at least in part of orogenic uplift caused by collisions of various continental plates, not least that associated with the final closure of the Ural Ocean (Fig. I.2.2). However, regional uplift may also have been associated with the development beneath the northern WSB of a major mantle plume, which caused rifting and massive trap volcanism at around the Permian-Triassic boundary (Section I.3.1). Subsequent thermal subsidence during the Mesozoic and Tertiary was probably the main control on the development of the West Siberian Basin.