Junggar Basin

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:

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

With the influence of Variscan orogeny, Early Permian marine facies changed into the terrestrial environment during Late Permian. This is because orogenic compression and crustal thickening resulted in higher sedimentation and withdrawn of the sea. At that time, widespread uplift occurred with subsidence formed a graben at first. Then, the area gradually became a mountain-trapped peripheral foreland basin due to high-temperature and relaxing subsidence from the regional extension. Some also suggested this landform caused by the combined effect of shearing and extension or thrusting from crustal shortening. Starting from Permian, Junggar Basin was formed to initiate the foreland basin cycle. There presented extensional shearing and continuous deposition of non-marine foreland basin-fill till Triassic. Since the level of the trapped lake in the basin was rising, finer deposits covered widespread the basin with denudation. This also marked as the end of the foreland basin cycle. From Jurassic to Palaeogene, the Junggar Basin undergone intra-continental depression. There was covered in braided delta with few lakes and increasing subsidence rate towards the south from 20 to 120 m/Ma during Jurassic. The collision of the Lhasa block from the south resulted that the delta formed along the margin of the basin. Also, the deeper lake was at the basin centre during Lower Cretaceous. Afterward, the southward lake depression leading the basin centre shift to the south in the Upper Cretaceous period. In Paleogene, braid-delta developed over the basin where sediments entered from the mountains of the surrounding basin. Starting from Neogene, the thrust fault in the Junggar Basin was reactivated. At the same time, there was rapid uplift of Tian Shan since Himalayan orogeny formed by the collision between Indo-plate and Eurasian Plate. This developed an alluvial-rich delta system around shallow lakes with the clastic sediments influx from uplifted Tian Shan and the northern part of the basin.

Geological resources

Petroleum system

Junggar Basin contains the third-largest petroleum reservoirs in China. About two-thirds of oil can be found in the Karamay-Urho monocline area. There was formed in Carboniferous deep-sea sedimentary rocks and lake sedimentary layers from Permian to Tertiary. For Carboniferous oil and gas deposits in this area, they were moved upward and accumulated in the Permian period and Mesozoic era sandstones. Then, the layers altered as the structural trap locations by tectonic activities in the later stage. Petroleum is dominant in Karamay, Baikouquan, Urho, Dushanzi, and Qigu. The oil and gas fields can be found on Tertiary Dushanzi sandstone. Besides, gas fields are found in the Karamay as well as the inland region of the basin.

Besides, Tian Shan Foredeep in the southern Junggar Basin (including Urumqi) is also available for the petroleum resources. The petroleum there were formed due to rapid subsidence, regional ductile with mobile intrusion, and cross-cutting on anticlines by orogenic activity (probably in Neogene) from the Tian Shan. Part of the oil-bearing sedimentary rocks was deposited in the salty oxygen-deficient lake environment during Permian. The crude oil in this sedimentary rocks formed by remains of algae and humus.

Coal

Bituminous coal was found in Tian Shan Foredeep. It was deposited in the lake or swamp environment in the Early to Middle Jurassic periods. For example, Badaowan, Sangonghe, and Xishanyao Formation. About 18 gigatonnes of coal can be recovered in Tian Shan Foredeep. Apart from Tian Shan Foredeep, the coal can be found in the base of alluvial fan and nearby the margin of lakes in the eastern and western margin of Junggar Basin.

Ore deposits

Ore deposits in the Junggar Basin were mainly formed in the Paleozoic era which was related to tectonic development. The followings are the available ore deposits in Junggar Basin:

• Porphyry copper-gold deposits found in the west and northeast of the Junggar Basin.
• Iron deposits found in the eastern part of the basin due to early subduction events during Lower Carboniferous.
• During post-collision extensional events in Permian, gold deposits found on the west side and tin deposits found on the east side.