Internet Geology News Letter No. 139, March 4, 2002

Even in the early stage of study of the geology of West Siberia it was supposed that the platform cover had a wider stratigraphic range in the north than in the south, and that it could include Triassic and Paleozoic sediments. This concept has now been confirned by geological and geophysical studies. As of 1988 about 150,000 km of common depth point seismic surveys had been shot in the north of West Siberia. Refracting horizons at depths of 10-12 km have been recorded.

A very large break in deposition is found in the south and central regions of West Siberia from Middle Carboniferous to Late Permian and in many regions to the Triassic and Early Jurassic. This hiatus is not present to the east in the northwest of the Siberian craton and in the Yenisey-Khatanga downwarp. The Yenisey-Khatanga downwarp is a continuation of the West Siberian basin, and within its sedimentary fill are deposits of Middle and Upper Carboniferous, Permian, and all three divisions of the Triassic. These Carboniferous to Triassic sediments can be expected to extend on westward into the northeast of West Siberia. This area in the northeast of the West Siberian oil-gas province conicides in general with the Gyda Peninsula and is designated as the Pur-Gyda basin.

In the northeast parts of West Siberia reflecting horizons are systematically traced that are deeper than regionally expressed Horizon T-4, which is interpreted to be a transgressive member of Early Jurassic age. Its base is the Triassic-Jurassic boundary, beneath which is a Triassic complex 1.5-2.0 km thick that is conformable with the overlying Jurassic rocks. Zones of pinchout of these Triassic rocks (Rhaetian-Olenekian) are well expressed along the borders of the Pur-Gyda basin. This Triassic basin opened out to the east into the Yenisey-Khatanga downwarp. Individual tongues of Triassic sediments extend to the south beyond the limits of the Pur-Gyda basin. This basin had no connection with the sea on the north.

The next important stratigraphic and structural break in the seismic section carries the index A. In the Pur-Gyda basin it is traced at depths of 6-10 km and is at the level of Horizon A of the rest of the West Siberian platform where it is identified as the top of the Lower-Middle Paleozoic clastic-carbonate complex or ancient intrusives. Between this A boundary and the base of the Rhaetian-Olenekian complex in the Pur-Gyda basin is an Upper Paleozoic-Induan complex 2-5 km thick. Deposits of this age had not been recognized earlier in the north of West Siberia.

On a basis of structural features that combine the Pur-Gyda and Yenisey-Khatanga zones of subsidence, the clastic and clastic-volcanic deposits of the Tunguska Series of the Siberian craton can be regarded as an analog of the Upper Paleozoic of the Pur-Gyda basin of northeast West Siberia.
Continued in News Letter 140
Taken from Girshgorn, 1988; digested in Petroleum Geology, vol. 30, no. 3, 1996. one seismic section and one tectonic map.
Copyright 2002 James Clarke. You are encouraged to print out this News Letter and to forward it to others. Earlier News Letters are available at: http://geocities.com/internetgeology/
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