Georesursy (May 2017)
Сonditions of sedimentation and paleogeographic zoning of the Bazhenov horizon (Tithon-Lower Berrias) in West Siberia
Abstract
In the global Upper Jurassic-Neocomian (Oxford-Valanginian) black-shale interval, the Bazhenov Formation occupies only the Tithonian – Lower Berriasian. The main regional reasons for this localization were the tectonic regime, the morphology of the basin’s bottom and hydrodynamics. Under the conditions of sublatitudinal transtension southward of the active Priuralsko-Khatanga left shift on the western and eastern slopes of the basin, sharp increased the deflections that began in the Late Oxford. The central part of the West Siberian basin became isolated as a shallow (up to 100 m) hilled plateau, surrounded by deep (up to 500 m) troughs. Along the north-western Nadym-Karaminsky fault a zone of relative uplifts was formed, dividing the basin into two sub-basins – Ob and Pur-Taz. The Taz palaeodepression is limited to meridional downthrow, it served as a traprock for terrigenous material that was carried down from Siberian platform and Taimyrsky Island. Beginning from Tithonian, a large debrise cone of interbasin drainage flow was formed in the Taz trough. The dischange current followed the Kheta channel, filled the deep-water trough with cold arctic waters and pycnocline was formed in the basin. Contour currents were formed on the Purskoy stage, which is situated on the western slope of the shallow. The fluent of these currents and the upwelling from the Taz trough provided the nutrients to the surface. The high primary bioproductivity that resulted in the accumulation of black shales in the West Siberian basin is a result of genetically related sequence of environments and facies: the fluvial facies of the runoff stream of the Bukaty Formation – the fan of outflow of runoff currents of the Yanovstanskaya formation – mainly the biochemogenic Bazhenov formation of upwelling. Rapid immersion of the shallow hilled plateau in the Berriasian time led to stop the accumulation of black shales almost throughout all West Siberian basin, the bottom of which was now actively aerated by the stock flow. In the Berezovo-Tobolsk zone, which remained a shallow relic of the former underwater landscape, black shales of the Tutleem and Muliminsky formations continued to form.
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