The example of the Pokurskaya suite of the south Varieganskian bar

Afonin I.V.

Tomsk State University, Tomsk, Russia

[e-mail]

The Nizhnevartovsk oil-and-gas bearing region has been investigated during 30 years. The southern part of the Varieganskian bar is considered as the most prospective object, of which the Cretaceous deposits enclose over 44 oil and gas pools. These deposits are stratigraphically confined to the Pokurskaya Suite characterized by the complicated structure and the inconsistent lithological composition.

At present, there exist several models of composing this structure, however they do not enable the precise correlation to be performed and prognoses to be made. Hence, the problem of subdividing these formations is pressing at the moment. The seismo-stratigraphic method was basic in studying these deposits, however in view of the complexity in the data interpretation, it does not always allow the exact correlation of sequences to be carried out. So, methods of litho-geochemistry have been proposed for searching reference indicators.

Rock mass PK_1-2 was selected as an object for studying; argillites and siltstones dividing beds of sandstones were picked out from this rock mass. Geochemical characteristics of these rocks were studied on the base of 112 determinations of petrogenic oxides content with the RFA and microelements composition using ICP-MS. The studies were carried out in the Centre of Shared Access (CSA) "Analytical centre of the geochemistry of nature systems" of Tomsk State University (TSU). For the lithogeochemical typification of these rocks, the following techniques were applied: hydrolyzate module (HM), titanium module (ТM), femic module (FM), normalized alkalinity (NKM), geochemical coefficients (La/Yb, Fe/Mn, Sr/Ba, Ce/Ce*, Eu/Eu*), as well as a number of binary diagrams Sr/Ba- Fe/Mn, La/Yb-∑TR.

Relying on the performed investigation, the studied samples were divided into five geochemical groups. The first group includes rocks characterized by a low level of the accumulation of the rare-earth elements (43.6-63.3 g/t) on the background of elevated values of ratios La/Yb and Sr/Ba being respectively 12.98-15.83 and 0.7-0.83, thus suggesting the formation the studied deposits in lagoonal conditions.

In rocks of the second group, the progressive increase was marked in the accumulation of rare-earth elements (111.1-155.2) in combination with values 9.9-12.4 for ratio La/Yb, 0.44-0.54 for Sr/Ba, 93-163 for Fe/Mn. This indicates to the maritime conditions of sedimentation (maybe, conditions of the shallow-water gulf).

The third group includes samples characterized by variations in the rare-earth elements content in the range of 108-198, which, if combined with other indices (La/Yb 9.8-16.4; Sr/Ba 0.13 – 0.37; Fe/Mn 27-99) are indicative of forming the rock mass in the weakly salinized shallow sea basin.

The fourth group is characterized by the abrupt jumps of the lanthanoids content from 41.1 to 325 g/t, while the symbasic changes in the ratios La/Yb and Fe/Mn were respectively 7.1-17.7 and 36-193. These results point to the fact that this rock body was forming in the middle part of a shelf zone, which experienced periodical submergences. As a consequence, the facies conditions of sedimentation changed abruptly, that is much pronounced by geochemical coefficients.

The fifth group consists of cyclic sediments distinguished on the base of the CaO behaviour. They are marked for their wide variations of the geochemical indices. However, it should be noted that a potential reference indicator has been found, making possible the relative correlation of these sediments. The abnormal clayey horizon serves the purpose, whose ratio Се/Се* changes as a rule in the range 1.25-1.47 (rarely reaching the value of 1.7).

Hence, the performed investigation has allowed sediments to be divided into several geochemical groups making possible the correlation between boreholes. Besides, conditions of forming the rock bodies have been established, which are substantiated by the petrographic characteristics of rocks. The stable geochemical reference indicator has been distinguished for the cyclic sediments, providing a means for a relatively clear-cut determination of the strata boundary.

References

1. Balashov Yu.V. Geochemistry of rare-earth elements. M.: Nauka, 1981. 278 pp.

2. Taylor S.R., McLennan S.M. The continental crust: its structure and evolution. M., 1984. 384 pp.

3. Maslov A.V. Sedimental rocks: methods of studies and interpretation of obtained data. The manual. – Yekaterinburg: The UGSU Publishing House, 2005. 289 pp.

4. Shatrov V.A., Voitsekhovskiy G.V. Microelements in the Devonian sedimental rocks of the Kuboiskaya Suite as sedimentation indicators in the settings of passive margins. – Vestnik VSU. Geology. 2008. No.1. P. 20-28.

5. Yudovich Ya.E., Ketris M.P. Fundamentals of lithogeochemistry. – S. Petersburg: Nauka, 2000. 479 pp.

Comparison of sedimentation on passive continental margins versus island arcs (by the example of the Ulutau Formation in the South Urals)

Fazliakhmetov A.M.

Institute of Geology USC RAS, Ufa, Russia

[e-mail]

The Magnitogorsk Megazone of the South Urals that corresponds to the Devonian volcanic arc terranes is typified by widespread volcanoclastic deposits. They have a variegated, mainly volcanimictic composition and belong to different genetic types. The deposits of the Ulutau Formation (Givetian to Lower Frasnian) can be taken as an example [7]. They are represented by granulometrically different clastoliths and siliceous rocks. Clastic material is dominated by andesites, andesi-basalts and plagioclases, with less common rhyolites, dacites, quartz and limestones.

It has been determined that the accumulation of the deposits in the Ulutau Formation took place by force of gravity currents under deep-sea conditions [5]. Their sources were the volcanic terranes of the Magnitogorsk Island Arc System [8]. There were several deep-sea fans replacing each other in both vertical and horizontal directions [3]. Such an intricate structure is the result of varied and in many respects unclear settings not only as regards sedimentation, but also as regards mobilization of volcaniclastic material. Their reconstruction needs a comprehensive analysis of all available factual data, including the investigation results on recent sediment genesis within the sea areas adjacent to island arcs. Equally important are the data for sedimentation processes on passive continental margins where the mechanism of gravitational sedimentary movements (turbidity currents, etc.) has been most thoroughly investigated. In all probability, gravitites of exactly these zones are now understood in the greatest detail. They can be used as the standard when reconstructing mobilization and sedimentation in other geodynamic settings, but one should take into account the specific features of the terranes under investigation (front/inner island-arc slope, ensymatic/ensialic arc, etc.)

A correlation between conditions and processes of clastic mobilization, transport and accumulation on passive continental margins and island arcs (Table) made it possible to reveal the following general features of the volcanisedimentary genesis relevant in reconstructing the geologic history of the South Urals during the Devonian.

Table. Comparison of sedimentation on passive continental margins versus island arcs.

Mobilization of the main volcaniclastic part within the island-arc terranes occurs through explosions and coastal abrasion (if the volcanoes protrude above the water level) [1,9]. Active tectonic and volcanic processes cause the formation of rugged and rapidly changing topography. This is the reason for the occurrence of multiple sedimentary systems (in Lisitsin’s terminology [6]) replacing each other along the strike of the island-arc slope. The sedimentary cycle, including mobilization to deposition of volcaniclastic material, is of short duration, sometimes at an avalanche speed [6,3]. Combined with the juvenile character of the sediments, it presents problems in recognizing rock genetic types both during the field description and when lithochemical methods are applied.

References:

1. Carey, S., Sigurdsson, H. (1987) A model of volcanogenic sedimentation in marginal basins. In: Kokelaar B.P., Howells M.F. (eds) Marginal Basins Geology. Moscow. Mir Publ. pp. 65-101. (In Russian).

2. Fazliakhmetov, A.M. (2011₁) On the causes for avalanche sedimentation of the Ulutau Formation in the West Magnitogorsk Zone of the South Urals. Herald Inst. Geol. Komi Sci. Centre 193:19-21. (In Russian).

3. Fazliakhmetov, A.M. (2011₂) Sedimentation conditions of the Ulutau Formation in the West Magnitogorsk Zone of the South Urals. Lithosphere 2:42-52. (In Russian).

4. Frolov, V.T. (1992) Lithology. Moscow Univ. Vol. 1. 336 pp. (In Russian).

5. Khvorova, I.V., Eliseeva, G.G. (1965) Volcanogenic clastic (psammitic) rocks of the Ulutau Formation. Lithology and Mineral Resources 1:53-69. (In Russian).

6. Lisitsin, A.P. (1988) Avalanche sedimentation and sedimentation gaps in seas and oceans. Moscow. Nauka Publ. 309 pp. (In Russian).

7. Maslov, V.A., Artyushkova, O.V. (2010) Stratigraphy and correlation of Devonian deposits in the Magnitogorsk Megazone of the South Urals. Ufa. DesignPolygraphService Publ. 288 pp. (In Russian).

8. Maslov, V.A., Artyushkova, O.V., Baryshev, V.N. (1984) Stratigraphy of Devonian ore-bearing deposits of the Sibai District. Ufa. USSR AS Bashkir Branch. 100 pp. (In Russian).

9. Murdmaa, I.O. (1961) Recent marine sediments in the Kuril’s volcanic zone. In: Strakhov, N.M. (ed) Recent Sediments in Seas and Oceans. Moscow. USSR AS Publ. pp. 403-418. (In Russian).

10. Oceanic sedimentation and magmatism (1979) Bezrukov, P.L. (ed) Nauka Publ. Moscow. 1979. 416 pp. (In Russian).

11. Pavlidis, Yu.A., Nikiforov, S.L. (2007) Morpholithogenetic settings in the coastal zone of the World Ocean. Moscow. Nauka Publ. 455 pp. (In Russian).

12. Strakhov, N.M. (1963) Lithogenetic types and their evolution in the Earth’s history. Moscow. Gostoptekhizdat Publ. 299 pp. (In Russian).

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