Blackbourn Reports: Development of the West Siberian Basin during the Mesozoic and Tertiary: Palaeogeography & Stratigraphy

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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