The oldest stratum in this division is the Lower Carboniferous, and the Upper Triassic is only distributed in Changdu community. Middle Jurassic and Holocene are widely distributed, with a large area, and early Cretaceous-Paleogene strata are scattered.
Table 2-5 Lithostratigraphic Sequence in Qiangtang-Changdu Stratigraphic Area
1. Late Paleozoic strata
The Carboniferous strata confirmed by fossil identification belong to Carboniferous heteropoly group, and the lower part is mainly gray and dark gray medium-thick silty slate and timely sandstone, with carbonaceous slate, siliceous siltstone, fine sandstone and medium-thick sandy limestone, and occasionally gypsum thin layer and coal seam, with the thickness greater than113 m, and the sandstone develops parallel bedding and plate shape. The upper part is mainly gray-dark gray thick layer to dense massive micrite limestone, amphibolite bioclastic limestone, gravelly limestone, oolitic limestone, granular limestone, marl and fine-grained timely sandstone, with a thickness of188 ~1005m. This group is rich in brachiopods, corals, ammonites, gastropods, reptiles and non-foraminifera fossils, and its age is the early Carboniferous Weixian period. The lower part of heteropoly group is coastal-lagoon facies deposit, and the upper part is formed in shallow sea carbonate platform-shoal environment with sufficient light and clear water. Kaixinling Group is a parallel unconformity on the heteropoly group, and the sedimentary characteristics on both sides of the Dangqu-Muta fault zone (F 1) are different. The bottom of the south side is dominated by carbonate rocks; The middle and lower parts are grayish green-purplish red radiolarian siliceous rocks; The middle part is gray medium-coarse grained lithic feldspathic sandstone and medium-fine grained tuffaceous lithic feldspathic sandstone, which is composed of several retrogressive cycles tapering upwards, and there are fan-shaped channel conglomerate and seismic breccia limestone locally, with the thickness of1947 m; ; The upper part is gray, gray-black thick bedded micrite limestone, bioclastic limestone and sandy limestone, and the metamorphic rocks in the central uplift zone overlap. On the north side of the fault zone, only the middle and upper strata are seen, that is, grayish purple and grayish green medium-grained lithic feldspathic sandstone and feldspathic quartz sandstone, with argillaceous siltstone, micrite biolimestone, multi-layer oceanic island andesite basalt and basalt lens, with the thickness of 16 17 m, the debris is bright in color, coarse in particle size, small in thickness and many volcanic rock interlayers.
Figure 2-6 Tectonic-Stratigraphic Zoning of Qinghai-Tibet Plateau
Figure 2-7 Plate-like cross bedding in heteropoly sandstone
The biological fossils of Kaixinling Group include coral, brachiopod, bivalve fossil fragments, warm-warm water type, ammonites, foraminifera and radiolarians, especially the radiolarian fossils in siliceous rocks are very rich, and the geological age is Maokou period of Middle Permian.
Late Paleozoic strata were strongly deformed by folds, and the overall structural form was moderate-closed folds (Figure 2-8). The core of syncline is Kaixinling Group, and the heteropoly groups are asymmetrically distributed on the two wings. The core of anticline is coal-bearing clastic rock of heteropoly group, and its wings are composed of carbonate rock of this group and Kaixinling Group.
Figure 2-8 Structural Stratigraphic Profile of Zaduo Group in Danrong Township, Zaduo County, Qinghai Province (south wing of syncline)
2. Mesozoic strata
There is no Triassic section with continuous outcrop and complete top and bottom in Qiangtang area so far. The Middle-Lower Triassic is only found in the west of the central uplift belt near Rezi Shaka and Shuanghu Lake. The sedimentary environment of the Late Triassic has been greatly differentiated, and the rock assemblage, paleontological fossils and contact characteristics with the underlying strata in different blocks have their own characteristics. The Middle Jurassic is the main structural entity in the basin, and the Upper Cretaceous is scattered.
The lower part of Lower Triassic is composed of river conglomerate and gravelly coarse sandstone. The upper part is delta sandstone and siltstone, and gradually transits upward to tidal flat carbonate mixed with siltstone and mudstone, which is rich in biological fossils and unconformity with Upper Paleozoic, and the thickness is greater than1000 m.
The lower part of Middle Triassic is gentle slope carbonate mixed with mudstone, and the upper part is extremely thick carbonate.
The Upper Triassic in the northern Qiangtang area is a continental marine carbonate rock continuously deposited on the Middle Triassic, with a small amount of clastic strata overlapping with the central uplift belt. The lower part of Upper Triassic in Changdu Group is red clastic rock of Jiapila Formation; The middle part is bluish gray, dark gray micrite limestone and light pink dolomite limestone of Bolila Formation. The upper part is the coal-bearing clastic rock of Bagong Formation, which is rich in biological fossils and stable in lithofacies and stratum thickness.
The Lower Jurassic is a set of rhyolite-related coarse andesite volcanic rocks, mixed with fluvial fine tuffaceous lithic sandstone and argillaceous siltstone, which are not integrated on the underlying stratum and are 163 ~ 640 m thick. Isotopic chronology and magnetostratigraphy show that the formation age of this group is early Jurassic [100].
Middle Jurassic Yanshiping Group is divided into five groups. The Xiakemokou Formation is mainly composed of purplish red variegated composite conglomerate, gravelly coarse sandstone, medium-fine clastic timely sandstone, grayish green siltstone mixed with a small amount of marl and sandy limestone, and locally mixed with gypsum layer. The transverse thickness varies greatly (498 ~ 1983 m), showing a geometric shape of thick in the west, thin in the north and thin in the south. The formation was in contact with the Lower Jurassic as a whole, or directly superimposed on the Upper Triassic and Upper Paleozoic, and was formed in the coastal environment in the early Balu-Batumian period. Buqu Formation is composed of dark biomineral limestone, oolitic limestone and siltstone, with a thickness of 142 ~ 1446 m, rich in bivalves and brachiopods, accompanied by a small amount of sea urchins, ammonites and nautilus, and its age is Batong period. The distribution of Li Xia Formation is relatively limited, consisting of purple, gray and gray-green feldspar chronological sandstone, argillaceous siltstone mixed with yellow-gray marl and dark gray bioclastic oolitic limestone, with occasional calcareous nodules or gypsum layers. Low-angle cross bedding, bidirectional cross bedding, wedge cross bedding and low-angle flushing cross bedding and wave marks are widely developed in sandstone, and round and oval sand balls and sand pillows are developed and deposited in delta-coastal environment. This group contains relatively complete fossils of bivalves, ostracods, gastropods, ammonites and echinoderms, and some plant fragments can be seen, which were formed in the Baton-Kalovian period [76].
The Upper Jurassic Suowa Formation is mainly distributed in the middle of the basin, and it is composed of organic carbonaceous biolimestones and biolimestones with dark gray and medium-thick layers. The lithology is monotonous, the distribution is stable and the types of biofossils are diverse. Among them, reef-building organisms include corals, algae and stromatolites. There are abundant fossils of ammonites, brachiopods and bivalves, dating from Oxford to Kimori [77 ~ 83]. Xueshan Formation is the highest Jurassic stratum in Qiangtang area, which is composed of conglomerate, sandstone, siltstone mixed with gray-green marl and microcrystalline limestone, with variegated or purplish red transition phase. The fossils of ammonites, brachiopods and bivalves are very rich, and the age is Tiaotang period.
The Upper Jurassic-Lower Cretaceous is distributed in the northern side of the Wulanlula Lake-Dangqu-Muta-North Lancang River fault zone, which is composed of basic volcanic rocks, but not integrated in the pre-Jurassic.
In Indosinian period, it developed into a closed positive anticline with a northwest axis and a steep axial plane. Jurassic-Early Cretaceous folds are a series of large anticlinorium and synclines. Among them, the syncline is wide and gentle, the anticline is closed and vertically inclined, and the structural combination form is barrier type, which embodies the basic characteristics of shallow (superficial) thin skin detachment structure.
Cretaceous is a set of variegated clastic rocks with limestone, gypsum-bearing mudstone (shale) and copper-bearing sandstone in fluvial and lacustrine facies, and some high-K-Ca alkaline volcanic rocks are not integrated in the pre-Jurassic strata, resulting in freshwater bivalves, ostracods, plants, sporopollen and other fossils.
3. Cenozoic strata
Paleocene is brick red, purple red and yellowish brown composite conglomerate, which contains gravelly coarse sandstone, sandstone and siltstone, with mudstone and limestone locally. The upward thinning sedimentary cycle developed, resulting in ostracods, charophytes and sporopollen fossils. The occurrence is relatively flat, the thickness varies greatly from place to place, and it is not integrated with the underlying stratum.
The Oligocene Yaxicuo Formation is only distributed in Changdu community. The rock assemblage is mainly gray-white and light-gray carbonate rocks and purplish red sandstone, with gypsum beds and gypsum-bearing mudstone, which were continuously deposited in Paleocene, producing ostracods, Charophytes, Gastropods and sporopollen fossils. The basic sequence and sedimentary sequence reflect the mixed genesis of fluvial and lacustrine facies and alluvial fans.
Miocene is a set of volcanic rocks containing continental eruptive potassium, which is in unconformity contact with the underlying strata. Lithology and lithofacies are distributed in an annular-semi-annular shape around the eruption center, and the occurrence and thickness of rocks are controlled by volcanic mechanism.
Pliocene is composed of purplish red-orange conglomerate, gravelly sandstone, grayish purple-grayish white sandstone and siltstone, with various sedimentary rhythms, yielding ostracods, charophytes, gastropods and sporopollen, which are alluvial fan-river deposits in the faulted basin.
Pleistocene series is scattered, less preserved and semi-consolidated, and the genetic types are moraine, ice water accumulation, chemical accumulation and alluvial, which are not integrated with the underlying strata.
Holocene has a wide distribution area, including alluvial deposits, swamp deposits, glacial deposits, glacial deposits, chemical deposits, lake deposits and aeolian deposits with a single cause, and alluvial deposits and diluvial deposits with complex causes.
(2) Stratigraphic division of Qiannan-Baoshan
The distribution and development of strata in this sub-region are quite different, with 1 12 Daoban-Benta fault zone as the boundary, and the pre-Cretaceous sedimentary formations on the east and west sides are quite different.
1. Pre-Carboniferous metamorphic rock series-Tangji Group
This rock formation is intermittently exposed in NW-SE direction in the south of Tanggula Mountain uplift belt, and its name comes from the name of Lipu Jinshajiang rock series [13 1] (Table 2-6). According to the different rock types and their combinations, metamorphism and structural deformation, it can be divided into Enda Formation and Youxi Formation, bounded by ductile shear zone.
Table 2-6 Summary of Division and Comparative Evolution of Tangji Group
( 1) Enda Formation (AnCe。 )
It is composed of eyeball banded biotite plagioclase (II) gneiss, migmatite, garnet (II) plagioclase (II) plagioclase granulite, garnet (II) plagioclase granulite, garnet (II) quartz schist mixed with plagioclase amphibole and quartzite, which is equivalent to the so-called "A Mu Gang Formation" in the west of Qiangtang.
The middle Jurassic enda formation was covered by the Quemocuo formation unconformity or invaded by Indosinian rock mass. The original rock is argillaceous sandstone dominated by basic volcanic rocks in rift environment [62, 64], which was subjected to shear and high greenschist facies dynamic thermal metamorphism in the Late Triassic, and developed small rootless shear folds and rheological folds.
(2) Youxi Formation. )
The core of anticline, which is located in the south of Enda Formation and often exposed in relatively low-lying places or deep valleys, is an important part of the "shallow metamorphic basement" of Tangji Formation [109]. The rock assemblage is mainly composed of (graphite) synbiotic schist containing siliceous muscovite, biotite plagioclase gneiss, biotite synbiotic schist and garnet siliceous muscovite albite schist, with quartzite and andesite mylonite locally, which is equivalent to the "Gomuri Formation" in the west of Qiangtang. The protolith consists of argillaceous sedimentary rocks and basic volcanic rocks. It experienced the regional dynamic heat flow metamorphism of high greenschist facies-low amphibolite facies, and the ductile shear deformation superposition of Indosinian low temperature and high pressure blue schist facies and greenschist facies retrogradation, which became layered disordered structural schist with small closed equidistant rootless back and directional structure development. There are S-type, η-type and Ω-type folds.
Tangji rock group has obvious structural integration, that is, it contains ordered and disordered rock units in different periods of pre-Carboniferous, and was covered by the unconformity of early Carboniferous Kagong Group after strong structural hybridity, ductile shear metasomatism and metamorphic homogenization.
2. Carboniferous-Permian strata
The early Carboniferous strata of Suoxian-Zuogong Group are only distributed in Tangji and Kagong areas of Chaya County, Tibet, and are called Kagong Group. The rock assemblage is mainly composed of gray slate mixed with crystalline limestone, with occasional conglomerate, thin coal line, purplish red slate and tuff, including early Carboniferous foraminifera, chitin fossils and sporopollen. , belonging to shallow sea facies-marine facies. Carboniferous-Permian strata in the Doma community are only found in the Doma-Chabu area in the west. Among them, Carboniferous is Gondwana facies stratum [23], which is rich in typical cold-water brachiopod, bivalve and small single coral fossils such as Ambikella and Eurydesma, with a thickness of 3254 m. The middle Permian Lugu Formation is composed of alkaline basalt, terrigenous clastic turbidites, slope carbonate rocks and clastic rocks, with huge thickness, including cladocera, brachiopod, coral and bivalve fossils.
3. Mesozoic strata
Triassic deposits in Qiannan-Baoshan strata are very active. The Lower-Middle Triassic is only developed in the western part of Doma community, with local distribution. The Euler Formation is mainly dolomitic limestone, and the lower part is mudstone, trachyte and radiolarian siliceous rock, with a thickness of more than 600 m, which is integrated on the Longge Formation. Rigan mismatch group is composed of limestone, sandstone, shale and basalt, and its thickness varies greatly. It is a basic volcanic rock in open platform facies-deep water slope flysch turbidite mixed with rift environment, with radiolarian siliceous rocks in deep water environment in some areas, and the rocks are generally shallow metamorphic rocks [4, 6, 66, 75]. Suoxian-Zuogong Formation is only developed in Upper Triassic, while Dongdacun Formation is a set of coarse clastic rocks and sandstone mixed with crystalline limestone, and the angle is not integrated on Youxi Formation. The rock assemblages and paleontological fossils of Jiapila Formation, Bolila Formation and Bagong Formation are basically the same as those of Changdu Community in the northern Qiangtang-Changdu-Lanping stratigraphic division, and can be roughly compared. The two stratigraphic divisions are adjacent, so no new group names are given in this book for the time being.
The Lower Jurassic in Duoma area is continuously deposited on the Upper Triassic, which is a set of deep-water dark fine clastic rocks sandwiched with bioclastic limestone, and shallow-sea carbonate rocks sandwiched with clastic rocks are continuously deposited on it, which is rich in biological fossils. The middle-upper Jurassic is a cyclic deposit of carbonate rocks and fine clastic rocks. Suoxian-Zuogong community is short of lower Jurassic deposits, and the rock assemblage characteristics, paleontological characteristics and contact relationship of Yanshiping Group in the middle Jurassic are consistent with the corresponding groups in the North Qiangtang-Changdu stratigraphic division, indicating that they were connected at that time and accepted the deposits together, and the seawater retreated early at the end of Kaloway, which led to the lack of late Jurassic deposits in this division.
The Lower Cretaceous is developed in the Bangonghu-Nujiang junction zone and its northern margin, and the Dongqiao Formation is widely distributed. The lower part is dominated by coarse clastic rocks, and the upper part is a single carbonate stratum with a thickness of 1422 m, and the angle is unconformity on Bangonghu-Nujiang ophiolite melange and Bangai Formation.
The upper Cretaceous is in unconformity contact with the underlying strata. Tielongtan Formation in the west of Duoma Community is a rhythmic combination of purple-gray clastic rocks and variegated limestone along the coast and shallow sea, with a thickness greater than 160 m, while Abushan Formation in the east is a purple-red continental molasse deposit, whose distribution and development are controlled by faults.
4. Cenozoic strata
Cenozoic strata in Qiangtang area are widely distributed, but outcrops are scattered. The Paleocene Niu Bao Formation is a set of fluvial red coarse clastic rock molasses deposits. The Miocene Kangtuo Formation is a variegated conglomerate, sandwiched with lenticular purplish red sandstone and silty mudstone, with occasional basic volcanic rocks, which is in unconformity contact with Niu Bao Formation and its lower strata. Pleistocene-Holocene strata are mainly distributed in modern riverbeds and terraces on both sides of Nujiang River Basin and Suoqu River Basin on the southern slope of Tanggula Mountain, and are alluvial gravel layers. Among them, the Pleistocene strata are semi-consolidated and located on the high pedestal terrace; Holocene strata are loose deposits, distributed in modern river beds or some low-order accumulation terraces [84].