In the Early Triassic, graben structures filled with clastic rocks and basalts formed in the Urals.. The bulk of the Palaeozoic orogenic structures in Russia lie under the West Siberian
Trang 1Palaeozoic pre-granitic terranes strike obliquely
rela-tive to its western and eastern fault limits These are
two Precambrain metamorphic terranes and one
Palaeozoic island arc terrane inbetween The latter is
principally similar to the Magnitogorsk Terrane, but
it is offset sinistrally for 300 km This suggests
that the boundary faults are likely to be strike-slip
faults Seismic studies revealed that they dip to the
west, producing an apparent divergent structure of
the orogen
Further to the east is the Trans-Uralian Terrane,
consisting of Ordovician to Devonian accretionary
wedge rocks The easternmost structure in the
south-ern Urals is the poorly exposed Carboniferous
Valer-ianovka Volcanic Arc It has been considered as an
accreted arc, but regional airborne magnetic maps
suggest that it can be traced to the Kurama Arc of
Uzbekistan This indicates that the Uralides can be
interpreted as the result of arc-arc-continent collision
This collision was accompanied by the formation
of flysch in the Famennian to Early Carboniferous,
which continued on into the Middle Carboniferous
This collision progressed northward, because in the
northern Urals the accumulation of flysch and thrust
deformations started in the Early Visean and ended
by Kungurian time, when accumulation of salt and
molasse took place The collisional deformations
terminated at the end of the Permian In the Early
Triassic, graben structures filled with clastic rocks
and basalts formed in the Urals They were deformed
in Early Jurassic times
The northern continuation of the Urals is a
long-lasting debate Some researchers suggested that
Pai-Khoi and Novaya Zemlya represent its ‘degraded
continuation’ However, airborne magnetic data
clearly suggest that magmatic arcs in the Polar Urals
turn to the south-east under Mesozoic-Cenozoic
sediments and might also be exposed in eastern
Kazakhstan, forming a giant orocline In the core of
this orocline are Early and Middle Palaeozoic
accre-tionary, magmatic arc and 1.0 Ga metamorphic
terranes, identified on the basis of geophysical
and drilling data They are well exposed in the
Kazakh uplands The bulk of the Palaeozoic orogenic
structures in Russia lie under the West Siberian
sedimentary basin
Mesozoic-Cenozoic Sedimentary
Basins of Western Siberia
The West Siberian Basin is one of the three world’s
largest supergiant oil and gas reservoirs It occurs
be-tween the Urals Orogen and the Siberian Craton
(Figure 1) Its basement consists of various
Neoproter-ozoic to PalaeNeoproter-ozoic terranes that were amalgamated
in the Permian It has steep eastern and western borders and a flat bottom that dips to the north
At the Permo-Triassic transition, all western Siberia experienced orogenic collapse which was followed
by Triassic rifting and voluminous basalt magmatism
of the Siberian Superplume Rifts formed between the Urals and the Siberian Craton and in the western Kara Sea Basin Urengoi is the largest rift, which was drilled down to a depth of more than 7 km At the Triassic-Jurassic transition was a weak inversion and uplift, followed by post-rift subsidence for 4–7 km in the Jurassic-Cenozoic
In Early-Middle Jurassic times, continental sedi-mentation dominated in the south, whereas there was
a shallow-water marine environment in the north The shoreline frequently migrated north or south The dominant rocks are clastic alluvial, limnic, and marine sediments In the Late Jurassic, the proportion
of marine sandy and clayey sediments increased
At the Jurassic-Cretaceous transition, West Siberia subsided for 500 m, accompanied by the accumula-tion of the Bazhenovo oil-bearing facies which are enriched in organic matter This subsidence was pos-sibly facilitated by synchronous dextral strike-slip faulting along the western flank of the Siberian Craton
In the Neocomian it became a relatively deep basin, which was quickly filled with sand-clay clinoforms The main flux of clastic rocks was from the Siberian Craton and the Altai-Sayan Orogen In the Aptian-Albian, the sedimentation environment became shal-low-water In the Late Cretaceous, Paleocene, and Eocene, the typical rocks are clay and sand with some cherty sediments At the end of the Eocene, the northern part of the basin was uplifted and its link with the Arctic Seas was terminated The Oligocene-Neogene in the central and southern parts of the basin
is represented by shallow-water marine, limnic, and alluvial facies Quaternary sediments are widely dis-tributed These are various sandy-clay facies which accumulated during several episodes of glacial and interglacial events and development of limnic-alluvial systems
Circum-Pacific Orogenic Collages
The Circum-Pacific orogenic collages started to form
in the Palaeozoic and their growth has continued until the present time They occupy the Russian North-east and Far East In the Russian North-east is the Verkhoyansk-Chukotka Orogenic Collage, bordered
by the Nipponides
Verkhoyansk-Chukotka Orogenic Collage The Ver-khoyansk-Chukotka Orogenic Collage (Figure 10)
468 RUSSIA