Blackbourn Reports: Development of the West Siberian Basin during the Mesozoic and Tertiary: Early and Middle Jurassic

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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