The Junggar Basin (), also known as the Dzungarian Basin or Zungarian Basin, is one of the largest sedimentary basins in Northwest China. It is located in Dzungaria in northern Xinjiang, and enclosed by the Tarbagatai Mountains of Kazakhstan in the northwest, the Altai Mountains of Mongolia in the northeast, and the Heavenly Mountains (Tian Shan) in the south.
The geology of Junggar Basin mainly consists of sedimentary rocks underlain by igneous and metamorphic basement rocks. The basement of the basin was largely formed during the development of the Pangea supercontinent during complex tectonic events from Precambrian to late Paleozoic time. The basin developed as a series of foreland basins – in other words, basins developing immediately in front of growing mountain ranges – from Permian time to the Quaternary period. The basin's preserved sedimentary records show that the climate during the Mesozoic era was marked by a transition from humid to arid conditions as monsoonal climatic effects waned.
The Junggar basin is rich in geological resources (e.g. petroleum, coal and ore deposits) due to effects of volcanism and sedimentary deposition. According to Guinness World Records it is a land location remotest from open sea with great-circle distance of 2,648 km (1,645 miles) from the nearest open sea at .
Regional tectonic setting
The major structural components of the Junggar Basin divided into six parts:
Wulungu Depression were formed by faulting and flat depression. There was about 2,000 – 4,000 m thick sedimentary layers that deposited from Permian to the present. Luliang uplift (Sangequan uplift) was surrounded by narrow but steeply dipping at the north and wide but gently dipping at the south. There were about 1,100 – 4,000 m thick sedimentary layers and the complete layer from Permian to the present can be found in the southern part. Also, the plunging fold was found in this area. Central Depression was formed by three major lowland plains where are in Manas, Central, and Wucaiwan. There were 5,000 m thick sedimentary layers from Carboniferous to Quaternary. West Uplift consists of Chepaizi-Paotai uplift and Urho-Karamay monocline. Chepaizi-Paotai uplift formed by eastward plunging fold with faulting. The footwall includes Jurassic-Quaternary sedimentary layers while the hanging wall consists of post-Carboniferous sedimentary layers. Urho-Karamay monocline was formed with thrust fault along the west-northwest boundary of the basin. The Indo-Australian plate collision during Neogene resulted in uplift of the northern Junggar basin. This also resulted in re-activation of Permian thrust faults, produced faults on basement rocks and rifting on basin margin to form Karamay-Urho monocline. This area concentrated abundant hydrocarbons on the anticline part. East Uplift (Zhangpenggou-Qitai uplift) was formed by deformations in several times. The formation of NE-trend plunging fold in this area activated the faulting of basement rocks. Tian Shan Foredeep formed during lower-middle Triassic since the Tian Shan has uplifted continuously. During Cretaceous, the basin sank again and thus water depth became shallow due to tectonic deformations. In Paleogene, the size of the lake kept reducing and the eastern basin become a landmass. Also, there was further subsidence of Tian Shan Foredeep because of the formation of Himalayan in Paleogene.
Geology
Basement rock of Junggar Basin In Precambrian section was made up of felsic-intermediate granite with the inclusion of greenstones and ophiolites, where the Paleozoic section consists of mainly potassium- and sodium-deficient extrusive rocks. The basalts in the basement which indicated trapped late Paleozoic oceanic crust that came from the mantle.
Sedimentary stratigraphy The sedimentary facies started to be dominant in Permian. The layers continuously deposited fluvial and lacustrine facies until the present day, mostly containing conglomerate, sandstone, siltstone, and mudstone.
Major stratigraphic units in the Junggar basin from Carboniferous are shown in ascending order in the following table:
TimePeriodNorthwestSouthEastRock unit (Distinct geological features)Rock unit (Distinct geological features)Rock unit (Distinct geological features)QuaternaryHoloceneTerrestrial sediments with regional mud volcanoes and evaporitePleistoceneglacial tills and eolian loess & gravelsTertiaryNeogeneChangjihe Group
(Dark-brown mudstone, siltstone & sandstone with thin conglomerate & limestone)Dushanzi Formation
(Thickly-bed shale interbedded with sandstone & calcite grains)Suosuoquan Formation
(Mudstone, sandstone)Taxihe Formation
(Shale with ostracods, dolomitic sandstone)Shawan Formation
(Orange-red sandy mudstone)PaleogeneUlungurhe Formation
(quartzose sandstone & mudstone)Anjihaihe Formation
(Green shale with marls)Honglishan Formation
(medium-grained sandstone & mudstone)Ziniquanzi Formation
(Orange-red sandy shale)CretaceousUpperAilika Formation
(Mudstone at upper unit but conglomerate at lower unit)Donggou Formation
(Sandy shale, siltstone, sandstone & conglomerate, some calcite nodules)LowerKalaza megasequence (99-154 Ma)
(Sandstone with marls, but mudstone and shale dominated in southern part. Conglomerate with cross-bedding structure at lowest unit. Presence of gypsum and fossil fish.)JurassicUpperShishugou megasequence (154-169 Ma)
(Sandy mudstone to sandstone, with calcite materials and dinosaur fossils.)MiddleSangonghe megasequence (169-195 Ma)
(Presence of mudcracks in lower unit, coal red beds and dinosaur tracks at the top unit. Petrified woods were preserved.)LowerBadaowan megasequence (195-206 Ma)
(Conglomerates in southern and eastern part. Massive mudstone beds and soft-sediment deformation in northwest. Widespread coals were present. Petrified woods and plant fossils were preserved.)TriassicUpperHaojiagou Formation
(Yellow silty shale with some coal)Xiaoquangou Group
(Yellow conglomerate, sandstone, mudstone & shale)Huangshanjie Formation
(Greyish-green sandstone & mudstone)MiddleKelamayi Formation
(Conglomerate, sandstone with graded bedding)LowerBaikouquan Formation
(Red conglomerate, sandstone & mudstone)Shangcangfanggou Group
(orange-red conglomerate with red mudstone)PermianUpperUrho Group
(siltstone, sandstone & conglomerate)Xiacanfanggou Group
(Greyish-green mudstone with sandstone, with some purple-red conglomerate and plant fossils)Pingdiquan Group
(orange-red alluvial sandstone & conglomerate, with shale)Shangjijicaozi Group
(Greyish-green feldspathic sandstone & mudstone, some fossiliferous limestone and black oil-shale)LowerXiazijie Group
(orange-red clastic sedimentary rocks)Xiajijicaozi Group
(limestone with the presence of stromatolite)Chidi Group
(Grey mudstone, sandstone, conglomerate)CarboniferousUpperJiamuhe Group
(Organe-red conglomerate, and fossil plant-bearing sandstone & volcanic flows)Bashan Group
(pyroclastic turbidite, locally thinly-bedded limestone)Shiqiantan Group
(Conglomerate, sandstone, calcareous shale)LowerDishuiquan Group
(Grey tuff with regional limestone included brachiopod fossils, and pillow lava)Precambrian to DevonianBasement rocks (various plutonic and volcanic rocks, ophiolites, turbidites tuffaceous and metasedimentary rocks)
Paleoclimate and environment Throughout Mesozoic, Junggar Basin was mainly in the fluvial and lake depositional environment.
The climate in the late Permian showed the fluctuation between dry- or wet-dominated climate. The pieces of evidence included the presence of both organic beds and red beds. In the early Triassic, reddish sedimentary rocks formed that indicated the dominance of semi-arid climate.
During Late Triassic-Early Jurassic, the Junggar basin was in a warm and wet climate due to the effect of continental monsoonal climate. From middle to late Jurassic, the climate shifted as a seasonal arid climate that initiated from the northeast and then widespread to the whole basin. This is because the Pangea started to break apart that halted the effect from the mega-monsoon system. Therefore, the basin became affected by westerlies. The westerlies contained the lesser moisture that has come from the northwest since the marine areas gradually minimized to the recent Caspian Sea. With the continuous uplift along the Tian Shan, the orographic effect around the basin intensifies the rain-shadow effect. The prominent rain-shadow effect results in a warmer seasonal arid climate in the basin. At the same time, the lakes in the basin had higher salinity and lower sedimentation influx.
Tectonic evolution
Pre-Permian (before 290 Ma): basement rock evolution Xinjiang paleocraton was pulled apart for a continental rifting episode to form extensional basins in Late Cambrian. The continuous divergence of the continental crust during late Cambrian to Ordovician shaped the West Junggar Ocean. The West Junggar Ocean presented as the present Tangbale and Honguleleng ophiolites from intra-plate volcanism, and then this volcanism shut in mid-upper Ordovician. The Ordovician first ocean basin indicated that eastern Junggar was over passive margin. Another rifting event established the Mayilashan ocean basin and back-arc basin in east Junggar during Silurian. However, the compressional environment restricted the two landforms so that they ended up shut and folded in the late Silurian. This eventually led to the convergence of Tarim, Kazakhstan and Siberian paleo-plates. They were from the original Xinjiang paleocraton that puzzled each other again.
Junggar Ocean and Kulumudi Ocean were produced from the third rifting event during lower-mid Devonian. Eventually, the Junggar ocean and Kulumudi Ocean moved towards the north and undergone the subduction from upper Devonian to Late-Carboniferous. At the same time, several volcanic arcs were developed during subduction. Three plates (Tarim, Kazakhstan, and Siberian) converged together to form a trapped ocean that surrounded volcanic arc and orogens in Mid-Carboniferous. Alkali-rich granites with gold deposits intruded the converged plates. This revealed the partial melting of the oceanic crust. This also marked as the last subduction event following the post-collisional stage in Late-Carboniferous. Besides, Such intrusive rocks demonstrated that this was the last melting episode of oceanic crust. As part of the Eurasian plate started to combine continental crusts for the three plates, to consolidate them as another modern stable continental block.
Underplating events The mafic-ultramafic igneous rocks formed due to underplating with crustal stretching during Carboniferous to Permian. The magma underplating during Carboniferous to Permian (330-250 Ma) period heated up the lower crust and thus the crust got hotter. The following cooling crustal episode led to part of the mountain belt sink by thermal subsidence, which ended up forming the Junggar Basin. Another magma underplating event occurred in the Mesozoic era. This was forming heterogenic silica-rich igneous rocks due to the partial melting of numerous oceanic crusts contaminated with mantle wedge.
Permian to Present (From 290 Ma): Junggar Basin evolution