Wednesday, May 25th, 2011
I.3.3.2 Aptian, Albian, and Cenomanian
Sedimentary rocks of this age are up to 1500 m or more thick in the northern part of the WSB, with the thickest sediments now being deposited in the region of the Yamal Peninsula, although the deepest-water facies continued to accumulate in the KhantyMansi region (Fig. I.3.17). The laterally equivalent stratigraphic units are 800-1000 m thick in central parts of the basin, thinning quite steadily to zero around the basin margins. Early Aptian marine deposition was restricted to the west-central part of the basin as the late Neocomian regression reached its greatest extent. A major transgression began during the mid- to late Aptian and continued into the Albian. During this transgression a regional shale unit, the Alym Suite, was deposited across the basin, followed by the widespread marine and continental sands and shales of the Aptian to Cenomanian Pokur Suite. The general pattern of sedimentation was broadly similar to that of the Neocomian, with dark shales dominating in the Khanty-Mansi Trough, and a broad zone of coastal-plain and shallow-shelf sandstones, siltstones, and shales occupying central regions of the basin. Continental sandstones, shales, and red bed facies are widely developed along the eastern and southern basin margins. There appears to have been no repeat of the Neocomian clinoformal depositional
system, however.
Regression occurred again during the Late Albian and continued into the Cenomanian, when continental, lacustrine, and coastal plain sandstone and shale facies shifted westward to blanket the eastern, central, and southern parts of the basin, depositing the widespread upper part of the Pokur Suite, which forms a very important gas reservoir over a large part of the northern West Siberian Basin. The proportion of sandstone reaches 70 – 80% over this interval in eastern parts of the basin, falling to no more than 20 – 30% to the west of the central basin.
I.3.3.3 Late Cretaceous (post-Cenomanian)
These deposits are up to 1000 m or more thick in the northern basin, and 200-400 m over most of the central basin. A major transgression from the north occurred during the Turonian (Fig. I.3.20), and by mid-Late Cretaceous, the boreal marine West Siberian Basin had expanded to its greatest extent. The Late Cretaceous sea was also connected with the Tethyan Ocean to the south at this time through the narrow Turgai Trough in the southwest of the basin (Fig. I.3.22), to the west of the Kazakhstan high. The Turonian Kuznetsov Suite consists largely of grey and dark-grey bituminous argillaceous deposits.
The Late Cretaceous section as a whole is dominated by shales and claystones. Coarse clastic influx was greatly reduced at this time; the total proportion of sandstone barely exceeds 10%. Continental red beds were deposited in southern and southeastern parts of the basin, although they were less widespread than earlier in the Cretaceous. Regression occurred again from the late Campanian to the Danian, and land masses appeared along the eastern margin of the basin and in the vicinity of the Taimyr uplift (Figs. I.3.22). The basin retained its connection with the Arctic Ocean through a narrowing corridor west of Urengoi. Argillaceous limestones and marls became widespread in central and southern parts of the basin in the Maastrichtian. Continental and lacustrine deposits in the Pre-Urals region were interbedded with marine shales and glauconitic sandstones.
The connection with the Arctic Ocean was temporarily cut at the end of the Cretaceous, probably by the North Siberian Sill, but was restored again during the Palaeocene. At the same time the basin margins in the east and south became emergent.
I.3.4 Cenozoic
I.3.4.1 Palaeocene and Eocene
Palaeogene to Early Miocene sediments are 600 m or more thick in parts of the central and northern basin, reducing to less than 400 m over most of the remainder of the basin. Sedimentary environments during the Palaeocene and Eocene were dominated by shallow-marine shelf clay-dominated facies over most of the basin area (e.g. Bakieva, 2003). Coastal plain and continental sandstones were deposited on the eastern and southern basin margins (Fig. I.3.24).
The Palaeocene is known by some authors as the Tibeisansk Suite, whereas others include it in the overlying Lyulinvorsk Suite (e.g. Akhmet’ev et al., 2004).
The southwestern connection with the Tethys Ocean through the Turgai Trough was restored during the Eocene, although the connection with the Arctic Ocean to the north had finally been cut off. The Lower Eocene is known as the Lyulinvorsk Suite. A unit of argillaceous diatomites within this suite (which is dominated by sandstones and claystones) is noted for its diapiric behaviour, especially in the northern part of the Urengoi field. A large area to the south of 64° N began to subside gently, so that a shallow-marine basin (the Tavda Basin) persisted within the centre of the West Siberian plain. To the north of the basin was a lowland plain with localised areas of deposition and erosion, while marine deposits of the Tavda Suite accumulated within the basin (Fig.I.3.26). The Tavda Suite has been divided into two sub-suites: the lower sub-suite is represented by greenish and grey-green laminated siltstones and montmorillonite clays with occasional sands and silts. The succession contains pyrite, shell fragments, fish remains and coaly detritus. It is 20-70 m thick.
The Upper Tavda Suite is similar in composition to the lower, although illite is present in addition to montmorillonite. The top of the Tavda Suite is irregular, apparently resulting from a stepped regression of the Tavda Sea. Though palaeontologically sparse, palynological and other studies suggest that the Tavda Suite dates from the Middle and Late Eocene (Volkova & Kil’kova, 1996; Akhmet’ev et al., 2004). It had previously been dated as Late Eocene to Early Oligocene.
I.3.4.2 Oligocene
The Oligocene and Lower Miocene are widely developed within the West Siberian Basin. They are usually blanketed by a thick cover of Late Neogene and Quaternary, although subaerially deposited sediments of this age outcrop within river valleys in the lower reaches of the River Irtysh, the River Ob north of Khanty-Mansi, and the middle reaches of the River Tavda.
There was a fundamental change in the palaeogeography of Western Siberia at the beginning of the Early Oligocene. The whole of northern Asia experienced irregular uplift, leading to a gradual marine regression and the onset of continental deposition. Eastern and northern parts of the basin were uplifted during the early Oligocene, with an east-west-trending arch which developed across the basin north of a latitude of about 64° N. This gentle folding was probably a far-field effect associated with the collision of the Indian continent with Eurasia. By the mid-Oligocene the basin was completely cut off from the world ocean and had become a continental interior basin. Localised zones of subsidence became a series of lacustrine basins fed by streams emerging from uplands on the eastern, southern, and western sides of the basin. Lacustrine deposits were particularly prevalent in the area of the Khanty-Mansi Trough and in the western part of the Middle Ob area.
Fluvio-lacustrine deposits began to be deposited, comprising the Atlym Suite. This suite is typically composed, both in wells and outcrop, of fine-grained white quartzose sandstones, interbedded in many sections with lenses and beds of poorly sorted quartzose clastics, up to very-coarse grained and often with coaly material, and with lenses and beds of illite and kaolinite clays. The Atlym Suite is 5-50 m thick, and it often infills an irregular top-Tavda topography. The base of the Suite is sharp, commonly with a bed of granule or pebble conglomerate. The upper boundary of the Suite is less sharp, and is marked by the gradual appearance of more common but laterally impersistent beds of clay and marl characteristic of the overlying Novomikhailovsk Suite. The transition marks a change from dominantly fluvial to lacustrine and swamp conditions. The Atlym and Novomikhailovsk suites together are dated as Early and Middle Oligocene. Towards the top of the Novomikhailovsk Suite are beds of brown coal, 5-10 m thick. They are particularly abundant towards the eastern flanks of the basin. Tectonic movements at the end of the Middle Oligocene led to the formation of a single very extensive lake surrounded by an alluvial plain, occupying the entire central region of Western Siberia.
Further tectonic movements at the beginning of the Late Oligocene led to the formation of a large enclosed lake, in which the Turtass Suite (or Zhuravsk Suite) accumulated, represented largely by finely laminated greenish-grey siltstones and thin sands. The Turtass Suite is dated on the basis of quite an abundant lacustrine flora and fauna as Late Oligocene. The upper parts of this suite, 50-75 m thick, outcrop in some river valleys and the cores of anticlines in various parts of the West Siberian Plain. The Turtass Suite, unlike the unconformably underlying Novomikhailovsk Suite (and the overlying Abrosimovka Suite), contains no coals.
The term Nekrasovsk Suite is used in places for the Oligocene of the WSB. At the end of the Late Oligocene the extensive Turtass depositional basin began to contract.
I.3.4.3 Neogene.
During the Early Miocene the lacustrine environment gave way gradually to swamps, with the accumulation of peat and coal. The resulting lacustrine clays interbedded with brown coal comprise the Abrosimovka Suite (sometimes called the Upper Turtass Suite). Although broadly of Early Miocene age, sections of the suite in different locations appear to differ in their precise age, although most are Aquitanian (Volkova et. al, 2002).
There is a clear break between the deposits of the Abrosimovka Suite and the overlying Middle Miocene Beshcheul’sk Suite, corresponding to a tectonic rejuvenation which resulted in renewed deposition of fluvial deposits. The Beshcheul’sk Suite is represented by interbeds of yellow and white quartz sands of varying grain size, and with brownish yellow argillaceous intercalations. A marked reduction in biodiversity is thought to have corresponded with a cooling of the climate.
Younger Miocene and Pliocene deposits are quite widespread within Western Siberia. They are all continental formations, up to several tens of metres thick, and very variable in facies (Volkova et al., 2002; Muratov & Nevesskaya, 1986).
I.3.4.4 Quaternary
Pleistocene glacial deposits form a very widespread, though generally thin, cover throughout the West Siberian Basin. The Middle Pleistocene Salekhardsk Suite and the Late Pleistocene Kazantsev and Zyryansk suites are recognised. Their great variability precludes any useful discussion here.
Graham Blackbourn: Blackbourn Geoconsulting
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Friday, December 10th, 2010
Graham Blackbourn: Blackbourn Geoconsulting
The Lower Jurassic rests on Palaeozoic or Precambrian rocks over most of the West Siberian Platform area, except where Triassic graben-fill deposits are present. In the northern basin region, however, Early Jurassic sediments of mainly continental (including lacustrine) origin overlie widespread Late Triassic deposits (Tampei Series) (Figs. I.3.1 & I.3.2). During the Early Jurassic, and most notably during the Pliensbachian, the WSB as a whole began to subside, although a tendency remained for a while for the greatest subsidence to occur in areas underlain by Triassic rifts. Early and Middle Jurassic marine transgressions proceeded from the north along the lines of the grabens, and spread outward from them to cover most of the basin floor, progressively burying the earlier structural and erosional topography. During this period the Mansi Trough, which had been an area of late Palaeozoic uplift, became a major area of subsidence. It formed a semi-starved basin to the west of the Khanty regional high (Fig. I.3.5). Later in the Jurassic, the Khanty High also began to subside, and from the Late Jurassic the combined Khanty-Mansi Trough had become the deepest-water zone within the WSB. These structures appear to have been partly responsible, especially during the Late Jurassic and Cretaceous, for determining the position of marine—non-marine facies transitions, and the position of the slope lying between areas of shallow- and deeper-marine shelf deposits. During periods of low sea level and continental sedimentation, the more rapidly subsiding areas above the sites of Triassic grabens commonly accommodated stream systems, which deposited fluvial and deltaic facies, whereas erosion sometimes occurred on the less rapidly subsiding areas. In the present-day basin the late Palaeozoic basement blocks, in addition to the former rift zones, are characterized by higher heat flow (Surkov et al., 1982).
Early and Middle Jurassic sediments are 2 km or more thick in the northern part of the basin, and 500 m thick or less in central and southern parts. During deposition of these sediments the WSB comprised a continental interior or marginal basin, connected in the north with the Arctic basin over the North Siberian Sill and through the Khatanga trough (Enclosure 1). During the late Mesozoic the southwestern part of the basin was also connected through the narrow Turgai Trough with the Tethyan sea to the south, although for most of the time this corridor lay above sea level and accumulated continental sediments. By the end of the Middle Jurassic, the structural configuration of the basin was essentially the same as that of the present day.
Owing to the great extent of the WSB and the level of facies variation, stratigraphic terminology, especially in the Jurassic and the earlier Cretaceous, is complex. A few stratigraphic units are recognised over much of the Basin, whereas others are only locally developed. Figure I.3.3 is an attempt to illustrate the main stratigraphic nomenclature used in different parts of the basin along a NW-SE section. Figure 1.3.4 is a similar chart, with the same vertical scale, over a SW-NE section further north in the basin, covering just the Jurassic section, within which local variations are greatest.
The earliest Jurassic deposits occur only in structural and erosional lows on the basin floor, whereas later deposits also cover the uplifts and onlap the basin margins (Figs. I.3.3 and I.3.5). The Early Jurassic is largely characterised by alluvial and lacustrine deposits, with coals in places. The area was characterised at this time by successive and regular, but quite abrupt changes in palaeogeography, caused by subsidence of the platform and eustatic sea-level fluctuations. A large part of the area was represented by hill and mountain topography, plateaux and erosional plains, with a complex dissected relief. The area of deposition was very restricted, and occurred in the deepest parts of the basins and several linear troughs of erosional and tectonic origin. The main clastic depocentres lay in the north, the site of a marine basin at this time. Sediments were deposited on a fluviolacustrine plain, over which sediments were transported by rivers.
Further subsidence of the region and an increase in sea level during the Middle Pliensbachian led to a widening of the depositional area and the deposition of claystones and siltstones above bed Ju12, associated with the first major marine incursion within the basin which extended as far south as approximately 64° N (Fig. I.3.5). These clays, up to 50 m thick, are regionally distributed and form a reliable seal to bed Ju12.
Deposits south of this latitude are mainly coastal plain and fluvial sandstones and shales on the eastern and southeastern flanks of the basin. The main sediment source areas at this time lay to the southeast, the south (the Kazakhstan and Altai-Sayan uplifts), the northeast (the Taimyr uplift) and possibly along the Yenisei-Khatanga Trough, and to a lesser extent to the west (the Urals).
During the later Pliensbachian and Early Toarcian bed Ju11 (the Sherkala Suite) and its seal, the Togur Member (Fig. I.3.3), were deposited. Further regional subsidence occurred at this time. The sedimentary basin widened, and the area of erosion was reduced. This occurred in steps, from the most basinal areas towards the various basement uplifts, and also towards the western and southern margins of the basin, which at this time were represented by erosional, gently sloping plains, hills and mountains. A substantial southern portion of the basin up to about 63° N formed a low-lying fluviolacustrine plain with an extensive fluvial network, which carried large volumes of clastic material into the marine basin. Some levelling of the topography and a rise in sea level led to the formation of a transitional group of facies on the coastal plain, occasionally inundated by the sea. These facies lay adjacent to the marine basin in the north, and passed southwards into a fluvio-lacustrine plain bounded in the south by the northern slopes of major uplifts such as the Surgut, Nizhnevartovsk, Aleksandrovsk arches and others. Most of the clastic material was deposited on the coastal plain, periodically inundated by the sea, and in the marine basin. The thickest and most homogeneous reservoir sandstones accumulated on the submarine and subaerial parts of deltas, and alsoalong palaeo-channels. Extensive areas between river valleys were occupied by lakes and swamps in which clays and silts accumulated. Alluvial fan and scree sediments accumulated on the slopes of uplifts. Sediment sources were along the southern and locally along the western margins of the basin, and also local basement steps. Further subsidence of the region occurred during the Early Toarcian, with a reduction in the topography. The area of marine deposition extended further southwards, leading to periodic penetration of sea water into central areas of Western Siberia, as demonstrated by the occurrence of microfauna and microphytoplankton in well sections drilled in the Sherkala, Khanty-Mansi, Emangal’sk, Maloagansk, Poikinsk, Yugansk and other areas. The argillaceous and silty sediments of the Togur Member were deposited at this time. They were widespread but pinched out on the slopes of uplifts.
During the Middle and Late Toarcian, bed Ju10 (Gorelaya/Khudoseevsk Suite) and its argillaceous cap rock (the Radomsk Sub-Suite) were deposited during further regional subsidence (Fig. I.3.3; I.3.6). The area of erosion was sharply reduced. Several remnant erosional “islands” remained in central parts of the region: the Verkhnelyaminsk, Gorshkovsk, Konitlorsk and other areas. The areas of erosion within the Krasnoleninsk and Surgut arches were considerably reduced, as they were over the Nizhnevartovsk, Aleksandrovsk, Parabel’sk and other palaeo-highs on which erosional processes had earlier prevailed. The depositional area extended considerably to the west and south. Depositional environments altered quite abruptly, following the further marine transgression, causing another southward spread of the area of marine deposition, and also a widespread development of the transitional coastal plain facies. Deposition occurred on a fluvio-lacustrine plain, a lowland depositional plain with a varying sedimentary environment, a coastal plain occasionally inundated by the sea and a marine basin. Floral and spore-and-pollen analyses indicate that the climate throughout the Early Jurassic was warm and humid. Pine forests grew in upland areas, with a variety of ferns in the lowlands. A humid climate is also indicated by the dominantly kaolinitic nature of clays. During the Late Toarcian (during formation of the Radomsk Sub-Suite), further regional subsidence, with blanketing of topography, occurred. Renewed marine transgression caused periodic penetration of the sea not only into central but also into southern parts of Western Siberia, as demonstrated by the occurrence of microfauna and microphytoplankton in well sections. The argillaceous and silty deposits of the Radomsk Sub-Suite covered a wide area but pinched out on the slopes of palaeo-highs. This unit forms a reliable seal to bed Ju10.
The end of the Late Toarcian and the beginning of the Aalenian was characterised by a reactivation of tectonic activity and a retreat of the sea. A subaerial regime became established across a considerable part of the basin, and persisted throughout the whole Aalenian.
Beds Ju7-Ju9 were deposited during the Aalenian. The sedimentary basin became a broad fluvio-lacustrine plain with an extensive fluvial network and numerous lakes and swamps (Fig. 1.3.7). River channels migrated across wide valleys. The positions of the main water courses were the same as those established during the Early Jurassic. Sediment thicknesses increased towards the channels, as did their sand and silt content. Extensive areas beyond the river valleys were occupied by lakes and swamps in which argillaceous and silty sediments accumulated, together with peat, as demonstrated by the numerous quite thick (1-3 m) coal interbeds. A series of erosional remnants of older rocks protruded through the deposits of the fluvio-lacustrine plain.
The most widespread stratigraphic unit dating from the Early and Middle Jurassic is the continental coal-bearing Tyumen Suite. Its base is diachronous, most commonly lying within the Toarcian or Aalenian, although some authors take it down to a basal unconformity as early as the Hettangian, where such sediments exist. The top of the Tyumen Suite lies around the Bathonian/Callovian boundary (Fig. I.3.3). The Bajocian (when the Ju5-Ju6 group of beds was deposited) was characterised by a significant shift in the depositional environment, with the expansion once more of marine conditions and a widespread development of transitional coastal plain facies. The number of basement uplifts protruding through the alluvial plain was reduced, and isolated basement steps are known only from the Kaimysov, Surgut, Nizhnevartosk Aleksandrovsk and Shaim arches. The margins of the sedimentary basin expanded considerably towards the west and south. Much of the southern part of the basin was represented by a lowland depositional and coastal plain, occasionally inundated by the sea, and across which a system of delta channels developed, together with islands, sand-banks and uplifted parts of the lowland depositional plain.
The sands and silts of beds Ju5-6 were deposited in this environment. Within the deeper depressions and adjacent troughs, sediments accumulated in isolated salt-water basins with periodic connections to the sea. Tectonic activity in the sediment source areas was muted, the topography mild and the climate humid. Clastic material was mostly sourced from the southern and western margins of the basin, and the role of local sources was sharply reduced.
The Bathonian saw the deposition of the Ju2-Ju4 group of beds. The palaeogeography during the deposition of these beds was more complex (Fig. I.3.8), as sea-water penetrated into the interior of ancient uplifted areas along erosional channels. A shallowmarine zone developed around a considerable number of palaeo-highs and adjacent areas, with the development typically of erosional stacks, islands, sand banks, delta channels, bays and lagoons. Clastic sediments continued to be transported from southern and western parts of the basin, with local sources of little significance. By the late Bathonian, marine conditions were well-established, interfingering with continental deposits within the central part of the WSB. The main marine transgression from the north occurred, however, during the mid-Callovian (Yan, 2003; Fig. I.3.9), and established marine conditions across the basin. The Callovian in much of the Russian-language literature is grouped with the Late Jurassic, and will be considered in the next issue.
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Monday, August 30th, 2010
Graham Blackbourn: Blackbourn Geoconsulting
Triassic
Beginning at some point during the late Permian, and continuing through the Triassic, dominantly north-south and northeast-southwest-oriented rifting occurred within the area of the West Siberian Basin, apparently in part reactivating Palaeozoic lineaments (Enclosure 2). This followed a period of Permian uplift across much of the WSB, where Permian deposits are now scarce (Chapter 2). The main rift runs N-S through the northern WSB, passing below the Urengoi gas field, and another parallel rift runs to the east through the Yenisei Fold Belt. These are the Urengoi and Khudosei rifts respectively. The Khudosei rift joins at its northern end with a NE-SW-trending rift that runs along the Yenisei-Khatanga Trough. To the south, within the central WSB, the two major rifts split up into a number of smaller rifts with more variable orientations. The Urengoi rift is in fact just the northern portion of a more extensive rift system, the Koltogor-Urengoi graben, which extends for approximately 1800 km in an approximately north-south direction from Omsk in the south to the southern Kara Sea in the north. Indeed this graben aligns in turn with the Saint Ann Trough in the Arctic Ocean, which opens into the deepwater Nansen Trench, although it is uncertain whether there is any genetic relationship between the two. The width of this graben increases from several kilometres in the south to 80 km in the north.
The rifts were associated with, and filled by, up to at least 2 km of latest Permian to Early Triassic basic volcanics. The origin of the rifting and volcanism is debated; many Russian authors have related them to a “superplume” beneath the WSB. This model has been strongly supported by Saunders et al. (2005), based on a study of a substantial amount of seismic data from the Northern WSB, together with well records. Saunders et al. have calculated that crustal extension (ß-factors) associated with the rifting may have been as high as 1.6 across the Urengoi rift in the north, reducing to about 1.1 in the central WSB (Surgut area). They conclude therefore that the plume was located directly beneath the area of the Urengoi and Khudosei rifts in the northern WSB. These authors consider that the co-eval Siberian traps, which outcrop over a huge area of the Siberian Platform adjacent to the eastern margin of the WSB, were generated by the same episode of magma formation, and that the trap basalts on the Siberian Platform flowed there either across the surface, or along subsurface dykes or sills.
The Urengoi rift was penetrated to a depth of about 7500 m by the Tyumen superdeep well, SG-6, the stratigraphy of which is illustrated schematically in Fig. I.3.1. The deep crustal cross section illustrated in Enclosure 3 also passes through the location of the Tyumen SG-6 well. Igneous activity associated with the superplume is thought to have begun around 250-253 Ma in the form of alkali to ultrabasic activity in the Maimecha-Kotui region, but the greatest volume of traps formed around the Permo-Triassic boundary from 249-250 Ma. Medvedev et al. (2003) obtained Ar/Ar dates confirming this age for basalts obtained from wells in the the north of the WSB. It has been postulated that the huge outpouring of volcanic material and gases was responsible for the major extinction event which defines the Permo-Triassic stratigraphic boundary. Traps were forming at about the same time within rift basins in the WSB and surrounding areas, and also within the Kuznetsk coal basin during its final stages of formation. The igneous petrology of the Permo-Triassic volcanics of Western Siberia has been considered in detail by Medvedev et al. (2003).
The western limit of the Triassic volcanism occurs at Chelyabinsk and other coal-bearing grabens on the western slopes of the Urals; there are no traps here, but Early Triassic basite dykes. More common within the Urals are Late Permian to Early Triassic granitic rocks and bimodal volcanics, considered as late-collisional. They are not thought to be associated with the trap formation, although they are of a similar in age. The most well-defined link between the trap formation and sub-alkaline granitic intrusions has been established on the Taimyr Peninsula (Fig. I.1.1). The Taimyr traps are a continuation of those on the Siberian Platform, although probably slightly younger (220-230 Ma). Saunders et al. (2005) consider that following the main period of continental flood-basalt volcanism in the WSB, the locus of magmatism (i.e. the plume) migrated northwards relative to the overlying crust, to the Taimyr region, before migrating further onto the Barents shelf. Like the Kara Sea basalts, some of the trap intrusives here are highly differentiated, containing monzonites and sub-alkaline granitic rocks.
The depth as well as the width of the Triassic grabens increases to the north, where in addition to volcanics they may contain as much as 5 km of Triassic sedimentary rocks. Within the grabens, variegated conglomerates and sandstones are interbedded with volcanic rocks, which predominate in the Lower and Middle Triassic deposits. The upper parts of the rift-fill mostly lack volcanics, and coals beds are common. North of approximately 64° N, the basin contains a sequence of mixed continental and marine sandstones, siltstones, and shales of Triassic age (Tampei Series; Fig. I.3.2), up to 3 km or more thick, possibly including basal Jurassic deposits. The sea is thought to have penetrated the basin from the north, over the West Siberian Sill or possibly along the Yenisei-Khatanga Trough, and spread at first along the rift basins, but extended in time over the intervening platformal area (Fig. I.3.1). The Tampei Series sediments are broadly similar to those of the overlying Jurassic deposits, and represent the initial cycle of Mesozoic platformal marine sedimentation in the basin. Seismic data indicate that these deposits may be more than 6 km thick in some troughs in the northern basin region. In the Khatanga region, up to 3 km or more of Triassic clastics occur, sourced from the Taimyr uplift.
There appears, however, to have been some delay between the ending of trap volcanism in the WSB and the onset of significant thermal subsidence (Saunders, 2005), which corresponds with the start of the main phase of Jurassic deposition, in about the Pliensbachian. However, once begun, thermal subsidence continued until at least the Oligocene, with an almost complete stratigraphic sequence broken only by short-lived discontinuities resulting primarily from eustatic effects.
The lengthy period prior to deposition of the earliest Jurassic sediments was one of weathering and erosion over much of the West Siberian Basin. Brecciation, leaching and
chemical transformation of the pre-Jurassic surface in many areas created a porous network which was later to host numerous, though largely small, sub-unconformity oil and gas accumulations (Section II.2.1).
Jurassic
The post-rift Mesozoic-Cenozoic sedimentary cover of the West Siberian basin, beginning with the Lower Jurassic, is up to 8-10 km thick in the northern part of the basin, and averages about 3-4 km over the remainder of the basin, thinning to zero around the basin margins (Enclosure 6). The sediments were mostly deposited in an extensive shallow inland sea, with coastal plain and continental environments around the margins. The sea was generally deeper in the west and north owing to the main source provenances lying to the east and south.
The sediments are almost entirely clastic (sandstones, siltstones, and shales), apart from some quite extensive argillaceous limestones towards the top of the Cretaceous (Maastrichtian), and a few locally developed limestones elsewhere. Deposition in the deeper parts of the basin was virtually continuous from the Early Jurassic to at least the mid-Miocene, although unconformities of variable extent are present at the base of or within the Callovian, Kimmeridgian, Hauterivian, Barremian, Aptian, Turonian, Palaeocene, Middle Oligocene, and Miocene. These result mostly from eustatic rather than tectonic events. The Jurassic deposits have undergone only mild tectonic disturbance since deposition.
As noted above, the major sediment-source areas during the Mesozoic lay to the east and southeast of the basin. The Ural, Novaya Zemlya and Taimyr uplifts formed subordinate but still significant sources. The western side of the Siberian plateau to the east appears however not to have been a major sediment source; it was covered with Triassic trap basalts and Late Proterozoic to early Palaeozoic clastic sediments, whereas the sedimentary fill of the WSB is dominantly arkosic, derived from a granitic terrane. However, it is possible that during the Jurassic the precursor to the Lena River, which now drains the eastern side of the Siberian Platform and flows northwards into the Laptev Sea, flowed along the Yenisei-Khatanga Trough from east to west and transported sediment into the northern WSB. Local uplifts within the basin also acted as minor sediment sources during the Jurassic, before they were blanketed by sediments. The Jurassic to Recent evolution of the WSB, in simple terms, comprises the passive infill of a (structurally) remarkably symmetrical, gently subsiding basin, and the simplest model for this subsidence is one of thermal sag which followed doming associated with high heat flow in the Basin during the Triassic, and which was in turn associated with the contemporary volcanism (Section I.3.1).
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