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