The Triassic is one of the main periods in the evolution of the Western Siberian oilandgas field [1–15], which is the most important geologicaleconomicregion of Russia. By the Triassic, Late Paleozoic compression and granitization had ended and the Paleozoic complexes in the territory of the future megafieldwere consolidated [5–7]. According to the ideas ofmost researchers dealing with Western Siberia, in theTriassic, compression within this region changed tosublatitudinal tension accompanied by the formationof rift and graben systems [11]. This idea was developed by some leading scientists [10, 14], but the problem is not yet solved. For example, based on the magnetic data interpretation, it was supposed [1] that theWestern Siberia basement contains oceanic elements.This hypothesis was not supported by most researcherswho argued that, in particular, Triassic deposits of theregion are of continental origin. The ideas about a Triassic superplume [3], which existed beneath Westernand East Siberia and was manifested mainly in theform of basaltoid magmatism, are also well known.According to [2], Triassic basalts of Western Siberiarefer to the same geodynamic type—socalled synorogenic—which is related to the main folding in theadjacent orogens. As a probable cause of grabens formation, mantle pulses were suggested [13]: they propagate in the form of a geodynamic wave that leads tothe appearance of shift structural parageneses. Thisbrief overview of ideas about the geodynamic nature ofTriassic units in the region shows that the problem,whose solution directly affects understanding the regularities of the formation and structure of WesternSiberian oilandgas megafield, is not completelystudied.Based on complex geological–geophysical studiesand mapping the large segments of the territory, quitedetailed geological maps of PreJurassic basement ofthe west Western Siberian Plate were compiled [5, 8,15]; in addition, it has been shown that the Paleozoicgeodynamic history of the region ended with a collision accompanied by tectonic piling, metamorphism,granitic intrusions, and the appearance of a newformed crust of the continental type. The new resultsallow us essentially to specify the evolution of theWestern Siberian megafield basement. It has beenshown [5–7] that anticlinorium “cores” are not Precambrian blocks (middle massif, rises of the ancientProterozoic protocrust), but Paleozoic deep igneousand metamorphic complexes (i.e., units from thelower and middle crust). They reached the surface (orupper crust level) when Early Triassic riftogenesis andextension of Western Siberia took place, i.e., in theperiod when the Western Siberian oilandgas megafield proper was forming.The Triassic units have been studied by manyresearchers [2–6, 8–15]. The most widespread arebasaltic and rhyolitebasaltic formations (Tura Seriesand its analogs), mainly filling some grabens. In theShaim oilandgas area, west Western Siberia, theseare Danilovskii and Polovinkinskii grabens, and someother structures. The spatial orientation of the twomentioned grabens suggests they formed duriungabsolutely different crustal motions (in different stressfields): the Danilovskii graben was produced by sublatitudinal tension and the Polovinkinskii graben, by sublatitudinal compression (Fig. 1). This provides anopportunity to date these tectonic processes. This wasdone by the Ar/Ar method at the Institute of Geologyand Mineralogy, Siberian Branch, Russian Academyof Sciences, under the leadership of A.V. Travin. Basedon the extracted pyroxene monofractions from our samples collected from the Symor’yakhskaya 10249 borehole (from a depth of 2138 m, Danilovskii graben), theobtained age was 249.4 ± 4.5 Ma. Pyroxenes frombasalts collected in the Kruglaya 1r borehole (from adepth of 1900 m, Polovinkinskii graben) yielded theage of 275.7 ± 10.4 Ma (Fig. 2). Below are some briefcharacteristics of the discussed geological structures.
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Feb 2, 2016
Paulo Galvão + 4
Open Access
