Structure and Stratification of Triassic-Jurassic Formations in the Northern Part of Western Siberia
A.A. Nezhdanov, V.V. Ogibenin, M.V. Melnikova, A.S. Smirnov: LLC “TyumenNIIgiprogaz”
The analysis of deep and ultradeep drilling data, regional and areal CMP seismic data carried out with increased study depth over the recent years at Gydan peninsula and Yenisei-Khatanga regional trough (YKRT) territory, made it possible to establish that troughs up to 15 km deep in the north-east of Western Siberia are not filled with Paleozoic subhorizontally stratified (platform-type) deposits, but with effusive and effusive-sedimentary Permian-Triassic formations, i.e. they pertain to rifting type. Earlier, based on the results of seismic-stratigraphic interpretation of regional CMP data [1], wide distribution of upper Paleozoic, including carbonate, formations was assumed not only for Gydan peninsula and Yenisei-Khatanga trough, but further south, in Nadym-Taz interfluve area. This point of view substantiated locating ultradeep wells SG-6 and SG-7, which, as it’s known, instead of Paleozoic, uncapped Triassic basalts over 1,0 – 1,5 km thick, respectively and were shut in these rocks. Nonetheless, this version of stratification for the lower horizons of sedimentary mantle is used at the present time for the territories of Gydan peninsula and YKRT [2 and others], although most researchers accepted rifting model of the basin with primarily Triassic age of multi-kilometer thickness at its foundation. This part of Western Siberia, directly bordering Yenisei-Khatanga Through features the highest degree of downwarping, minimum thickness of Earth’s crust and was related by S.V. Aplonov to the Triassic Ob paleo-ocean [3].
With a purpose of updating the stratification for the lower parts of sedimentary mantle, the following data was used: Tyumenskaya and Yenyakhinskaya ultradeep wells SG-6 and SG-7, data for Tota-Yakhinskaya well 25 that was drilled in Gydan peninsula and uncapped Toarcian deposits; information on other deep wells in Yamalo-Nenets AO and YKRT, as well as regional and areal seismic data of SRM CMP works carried out in Gydan peninsula in the area adjacent to the territory of Yenisei-Khatanga regional trough and near ultradeep wells SG-6 and SG-7.
The marker horizons for the structures described are the Tithonian-Berriasian bituminous clays which in this territory are united into Bazhenov and Golchikhin suites and are controlled with the marker reflecting horizon Б; the top of Tyumen (for Taz-Gydan basin – Malyshev) suite (Bathonian), related to MH T, traced based on the well data and the Toarcian Togur member (or Kiterbyut horizon) corresponding to MH T4, that features marker horizon properties in the north-east of Western Siberia. In Nadym-Taz region, this MH does not feature dynamically strong evidence, however its correlation does not present a problem. Below this MH, the stratification of Jurassic, Triassic and bottomset (possibly Paleozoic) formations was almost entirely hypothetical until SG-6 and SG-7 wells were drilled, however L.S. Girshgorn and others [1] separated and correlated marker horizons Iа, Iб, Iв, IIа, IIб, IIв below MH T4, in subregional view, and assigned them stratigraphic importance: MH Iа – Jurassic bottom, Iб – upper Triassic, Iв – mid Triassic, Iв – lower Triassic, IIа, IIб – Permian, IIв – Carbonic. Such indexing system is also used for geophysical research in Krasnoyarsk Kray, but only for Cretaceous and Jurassic MH. It is apparent that using similar indices to mark MH of different ages almost within the same basin, is not reasonable. A different indexation system should be established for deep marker horizons (Triassic and Paleozoic). Even the effort itself of tracing recurrent and extended MH in pre-Jurassic part of the section, and moreover, assigning them a definite stratigraphic position, should be doomed wrongful.
Data obtained as a result of drilling ultradeep wells SG-6 and SG-7 evidences wide distribution of Triassic basalts in the northern part of Western Siberia; however the detailed stratification of lower Jurassic as well as the position of boundary between Triassic and Jurassic in these wells remains disputable. Thus, in relation to the boundary between Triassic and Jurassic in well SG-6, a number of viewpoints exist, and depth variation between proposed boundaries reaches 424 m:
Y.A. Yekhlakov et all (1995) – 5588 m; N.K. Mogucheva (2001)- 5592 m; V.S. Bochkarev (1995) – 5655 m; A.I. Sidorenkov (1995) – 5603 m; V.S. Surkov et all – 6012 m [4]. Also disputable is the issue of stratification for pre-Jurassic formations, and in particular the presence of Permian deposits in SG-6 well section. V.S. Bochkarev, S.I. Purtova, Y.A. Ekhlakov, A.N. Ugryumov, A.F. Fradkina insist that SG-6 uncapped volcanites of the Permian system. The specialists from Novosibirsk and Tomsk (A.M. Kazakov, N.K. Mogucheva, O.V. Serebrennikova, T.Y. Filippova) believe that in SG-6 well, the lower part of Triassic volcanites is erroneously labeled as pertaining to the Permian system. V.V. Lipatova, Y.A. Volozh, N.V. Miletenko, M.B. Keller, M.P. Antipov, N.V. Ilyina T.F. Bukina and others [4] deny the presence of Permian deposits in this well.
The most distinct lithological boundary where a drastic change is observed in lithological-mineralogical associations, rock appearance, cyclic recurrence for the section in SG-6 well is found at 6012 m depth [5]; below it are sandstones enriched with dark-colored minerals and “confluent” type, almost black argillites with appearance of tuffargillites; above it are light grey sandstones and banded argillites that are typical for Jurassic. At the same time, there are no real reasons to consider that these lithological features along with others described by M.B. Keller, V.V. Lipatova and others [5] for SG-6 well section have any stratigraphic significance and could be used as a basis for Triassic-Jurassic boundary. However, drawing the Triassic-Jurassic boundary at 5592 m depth (N.K. Mogucheva [4]) based on paleofloric data is even less reliable.
M.B. Keller and others [5], when describing validity of paleofloric dating for lower horizons of SG-6, emphasized that during a technical meeting in Yaroslavl, N.K. Mogucheva argued against drawing the Triassic-Jurassic boundary in SG-6 section at 6012 m depth based on the fact that higher up in the section, to 5620 m depth, in three intervals, she found numerous remnants of three types of late Triassic equisetums species: Neocalamites carrerei, Equisetites cf. conicus, Cladophlebis shensiensis Pan. The species diversity that is typical for Pursk suite is absent here. Could these three species surmount Triassic and Jurassic period? The authors [5] quote data from I.A. Dobruskin (1976), who wrote that three stages are established for Triassic flora, with the third stage covering the very end of Triassic and beginning of early Jurassic. Equisetums continued to exist after Triassic as well, but no longer played such a role in vegetation. Thus, the presence of individual late Triassic equisetum species in lower Jurassic system as found in SG-6 section is a widely distributed occurrence.
Figure 1 shows the distribution chart for equisetum flora remnants in well sections SG-6 and SG-7 (as per N.K. Mogucheva). Based on this chart, it may be concluded that the most vivid boundary in change of paleoflora is still the boundary found in SG-6 well at about 6012 m depth. In SG-7 section, this boundary is distinguished (based on available data) at about 6060 m depth. Directly below this boundary, a “flash” of genus and species diversity for equisetums is observed, while above it only isolated equisetum forms are found. In principle, this confirms the validity of drawing Triassic-Jurassic boundary in SG-6 section at 6012 m depth substantiated on geological grounds as well as fairness of I.A. Dobruskin’s viewpoint described above; however it is impossible to appraise whether the variations in quantity of equisetum taxons are due to alterations of age or facies conditions. At the same time, drawing the Triassic-Jurassic boundary based on this data directly above the latest equisetum findings while completely ignoring geological data seems to us as a profanation of biostratigraphy.
More reliable in stratigraphic regard are the apparent geological principles that logically follow accepting the rifting model of West Siberian petroleum basin. The version of stratification for 27 wells uncapping pre-Jurassic foundation at YNAO testifies the existence of direct correlation between the thickness of lower Jurassic and overall thickness of lower-mid Jurassic. SG-6 and SG-7 (fig.2) wells also match this correlation, whereas relating Triassic-Jurassic boundary in sections of these wells to smaller depths as viewed by V.S. Bochkarev [2, 4], i.e. going for smaller thickness of lower Jurassic, upsets this correlation drastically, which in our opinion, confirms incorrectness of such stratification.
Tota-Yakhinskaya well 25, as noted above, had not uncapped the bottom of Jurassic, therefore the data for ultradeep wells was used to calculate the sedimentation rate in Jurassic and Triassic and to determine the possible position of Jurassic bottom and Triassic thickness at the point of this well (table).
The thickness of Toarcian-Bathonian deposits in Tota-Yakhinskaya well 25 is significantly greater than in SG-6 and SG-7 (1470 m against 990 m in well SG-6 and 1089 m in SG-7), that is why sedimentation rates in those are higher. Following this and based on determination of sedimentation rates for Bathonian-Toarcian and average rates of sedimentation for Hettangium-Bathonian for these wells, using simple calculations, we estimated the thickness of Pliensbach-Hettangium at the point of Tota-Yakhinskaya well 25 at 2660 m (table).
It should be noted that sedimentation rates in mid and early Jurassic for the ultradeep wells are quite close, and their highest values are typical not for platforms, but for geosynclinal areas. In Tota-Yakhinskaya well 25, the sedimentation rates in Jurassic are even higher, which testifies active downwarping of TGB. Figure 3 shows a relation graph for thicknesses of mid and lower Jurassic for 27 deep wells drilled in Yamalo-Nenets autonomous okrug. This graph confirms strong inherited features of warping in Jurassic in the north of Western Siberia and the validity of establishing the thickness of lower Jurassic (1260 m) in Tota-Yakhinskaya well 25 based on data from wells SG-6 and SG-7.
Based on seismic data, through to the top of the acoustic foundation (MH A) below the Jurassic bottom in the north of Western Siberia, no distinct angular discordances are found that could be related to non-depositional hiatuses. On the contrary, the troughs’ morphology (their amplitudes increase down the section) suggests increasing rates of downwarping and these troughs being filled with sedimentation in the Triassic. We shall illustrate this occurrence below, but for now we should note that the calculation of sedimentation rates and Triassic thicknesses were made based on the sedimentation rates in lower and mid-lower Jurassic (see table). Based on this data, Triassic thickness at the point of Tota-Yakhinskaya well 25 exceeds 4000 m.
Literature
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5. Keller M.B., Lipatova V.V. et all. Stratigraphics of pre-Jurassic unit in ultradeep Tyumenskaya well SG-6 // Triassic in Western Siberia (materials for stratigraphic meeting on Mesozoic of West-Siberian plate): collection of scientific works. Novosibirsk: SRIGGMR, 2001. P. 16-21.
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