Indosinian period is a unified period of China ancient continent formation and continental dynamic mechanism transformation, with transitional characteristics of sedimentary formation and structural deformation. The Triassic coal-bearing basins developed in Chinese mainland are concentrated in the Late Triassic, and they are transitional. The late Triassic coal-bearing basins are all developed on the stable plots (or plots) where the Carboniferous and Permian coal-bearing basins are located, which has obvious inheritance. Indosinian period is the completion period of ancient continental convergence in China and the transition period of Mesozoic continental activation. The structural types of Triassic coal-bearing basins developed on relatively stable plots have certain inheritance. The Triassic coal-bearing basin located in the transitional zone of the continental corridor in North China, Tazhong and North China is dominated by depressions, which inherited the basic characteristics of Paleozoic large-scale sedimentary depressions, but was napped by the thrust nappe belt at the edge of the basin, forming a foreland depression basin. Triassic coal-bearing basins developed in the southern continent, mainly developed on the edge of the land block due to the retreat of the continental surface sea, forming a foreland sedimentary depression, with a narrow range of marine deposits in the early stage and a wide range of continental deposits in the later stage. The coal-bearing basins distributed in the more active South China active belt are different. In the early stage, it was an intermountain continental basin, and in the late stage, transgression formed a unified marine and land-sea alternating sedimentary basin. Carboniferous and Permian coal-bearing basins in North China, based on stable landmasses, formed marine and marine sedimentary sequences in the open offshore basin environment. The northern Late Triassic coal-bearing basin inherited the coal-bearing series of continental clastic rocks, while the southern Late Triassic coal-bearing basin still inherited the marine sedimentary characteristics of previous basins, forming a coal-bearing sedimentary sequence of marine facies, alternating marine facies and continental facies in the coastal and bay environment of offshore regression.

Triassic coal-bearing basins in northern Chinese mainland are mainly distributed in the transitional zone of North China block, Tarim block and corridor. The North China Basin (prototype), located in the North China landmass, ended the mutual deposition of marine and continental facies in the late Late Permian, and the coal accumulation was interrupted. Early and Middle Triassic basins are still in a state of overall subsidence with high in the north, low in the south and slow in the south, and clastic rocks in inland rivers and lakes are developed. In the late Indosinian period, the tectonic framework of the basin changed due to the influence of regional stress field. The uplift and dip in the northeast of the basin are in a denudation state, and the late Triassic sediments are missing. The southwestern part of the basin has a relatively large subsidence, and the subsidence center has moved to Tongchuan-Jiyang area, with a deposition thickness of 2300 m. In the Late Triassic, with the paleoclimate changing into a semi-humid and warm environment, lush vegetation was formed, which extended the later period (Wayaobao period) and formed lacustrine coal-bearing clastic rock deposits in inland rivers. The Triassic coal-bearing basin located in Tarim block also inherited the characteristics of Permian intracontinental sedimentary depression. The western margin of the basin was uplifted in the early and middle Triassic, and the sedimentary basins were mainly distributed in the middle of Tarim block (including Kuqa depression), forming a set of continental lacustrine sandstone and mudstone deposits, and the coal-bearing clastic rock deposits of Tariq Formation were formed in the middle and late Triassic. On the basis of the Carboniferous-Permian coal-bearing basins in the Kunlun-Qinling active belt (northern belt), the north and south of the Central Qilian block and the corridor transition zone, there are coal-bearing measures in the Late Triassic, and the Nanyinger Group in the corridor transition zone is deposited by coal-bearing clastic rocks in inland rivers and lakes. Triassic coal-bearing basin in northern Chinese mainland inherited the characteristics of late Paleozoic craton basin, with open depression as the main feature. Because the craton basin ended the deposition of continental marine facies and land-sea interaction in the late Permian, the Triassic coal-bearing basins were all deposited by coal-bearing clastic rocks in inland rivers and lakes. Triassic coal-bearing basin is generally weak in coal accumulation and poor in coal-bearing property because of its ancient climate, ancient vegetation and short coal accumulation period.

Triassic coal-bearing basins in southern Chinese mainland are mainly distributed in Yangtze block and South China active zone of South China plate, Qiangbei-Changdu-Simao (micro) block and Qiangzhongnan-Tanggula-Baoshan block of South Tibet plate. The Paleozoic craton basin of South China plate developed for a long time, and the epicontinental marine deposits did not end until the end of Middle Triassic. Late Permian Changxing period experienced extensive transgression, covering almost the whole southern continent. In the eastern part of South China Plate, except for the ancient land in the southern margin of North China Block in the northern boundary, the ancient land in Kangdian in the western boundary and the ancient land in Huaxia in the eastern boundary, Jiangnan and Yunkai ancient lands in the land area bounded by ancient lands are all submerged into islands by seawater. During Indosinian period, the South China plate was subjected to multi-directional stress and its structural deformation was complicated. However, the Yangtze block and South China active belt still show regional ups and downs, and the ancient land around the Early and Middle Triassic changed, and the sea area advanced and retreated. At the end of the Middle Triassic, the strong Indosinian movement caused regional uplift and large-scale low-grade fold in the central and western regions (Luzhou-Kaijiang ancient uplift), and strong fold in the east until the late Triassic Jiangnan and Yunkai ancient land. The unified Yangtze Paleozoic craton basin was decomposed into two late Triassic coal-bearing basins (prototype) in Indosinian: Sichuan (generalized) and Nanhua (including lower Yangtze). The Triassic coal-bearing basins of Qianbei-Changdu-Simao (micro-) landmass located in the western margin of South China plate and Qianzhongnan-Tanggula-Baoshan landmass located in the northeastern margin of Tibet-Yunnan plate also have certain inheritance, mainly developed in Carboniferous and Permian coal-bearing basins formed on the basis of Changdu and Tanggula landmasses. The late Triassic coal-bearing basin is a shallow-sea sedimentary environment, and TuPamela Formation (Naqu) is composed of shallow-sea black deposits, land-sea coal-bearing clastic deposits and continental sandy argillaceous deposits from bottom to top. Bagong Formation (Changdu) is composed of marine sediments, coal-bearing clastic sediments and continental clastic sediments. The two coal-bearing sedimentary sequences consist of marine facies, marine facies and continental facies alternately. Coal-bearing sediments are all developed in alternating marine phases, and most of them are covered by shallow water, forming a peat swamp coal-accumulating environment in coastal facies belt during a short regression period. Generally speaking, the thickness of coal seam is small and the sulfur content is high.

Late Triassic coal-bearing basins in Sichuan (broad sense) include Sichuan (narrow sense), Xichang and Yunnan medium-sized basins. At the end of Middle Triassic, Indosinian movement of South China plate lifted Kangdian ancient land, Motianling ancient land and upper Yangtze block. In the early Late Triassic, only Chengdu Bay kept in touch with Guang Hai in the west. By the Xiaotangzi period, seawater had spread all over the western Sichuan basin, and the depression spread eastward and southward until the early and late Xujiahe.

In the early and middle Triassic, Sichuan Basin inherited the marine sedimentary characteristics of the late Permian, and the land sea area continued to expand. Marcand ancient land retreated to the north, and the western part is still Kangdian ancient land, forming an occluded carbonate platform deposit and developing gypsum and salt layers. In the late Middle Triassic, the whole basin uplifted to the land, and at the same time slightly folded and deformed, forming the Luzhou-Kaijiang uplift belt in Northeast China. After weathering and denudation, the core of the uplift is the Lower Triassic. The early stable basement of craton basin is the paleotectonic condition for the formation of coal-bearing basin in late Triassic.

In the early Late Triassic, due to lateral compression, a foreland depression was formed in front of Longmen Mountain in the northwest of the basin, and the Tethys Sea in the west of the basin continued to expand. Seawater invaded the foreland depression through narrow channels, forming Chengdu Bay, and clastic rocks and carbonate rocks were deposited in the shallow sea along the coast of Kuhongdong Formation. In the middle of Late Triassic, the basin structure was stable and maintained the early paleogeographic pattern. The eastern part of the basin was denuded and leveled, and the coastline moved eastward to Huaying Mountain, where the coal-bearing measures of Xiaotangzi Formation were deposited. In the late Late Triassic, Longmen Mountain rose rapidly, and the whole basin rose to land, and seawater withdrew from the basin. The Sichuan Basin, bounded by Longmenshan, Micangshan, Dabashan and Jiangnan ancient land, is integrated with Yunnan, Guizhou and Hubei regions, and a wide range of river and lake coal-bearing strata have been deposited in Xujiahe Formation. At the end of Late Triassic, the Indosinian movement folded Longmen Mountain into a mountain, ending the marine sedimentary history of the basin.

The filling sequence of Late Triassic sedimentary strata in Sichuan Basin consists of four lithologic combinations: Maantang Formation (Kuhongdong Formation), Xiaotangzi Formation, Lower Subgroup of Xujiahe Formation and Upper Subgroup of Xujiahe Formation. Maantang Formation is a bay filling sequence, distributed in the foreland depression zone in front of Longmenshan Mountain, composed of shallow-sea argillaceous rocks and carbonate rocks, and rich in marine animal fossils. Xiaotangzi Formation consists of barrier-delta filling sequence. Barrier coast sequence is located in the northwest of the basin, delta sequence is located in the southeast of the basin, and coal seams are developed in the parts where each sequence tapers upward. The lower sub-group of Xujiahe is composed of filling sequence of rivers, lakes and lakeside deltas. The river sequence is located in the northern margin of the basin, and the lake-lakeside delta sequence is located in the middle of the basin, which is characterized by continental coal-bearing debris. Xujiahe sub-group consists of alluvial fan-river and lake-lakeside delta filling sequence, and its distribution characteristics are the same as those of the lower sub-group. The alluvial fan filling sequence appears in the front of Longmen Mountain and Daba Mountain. The lake filling sequence in the middle of the basin has developed into coal-bearing debris filling in the inland basin. The above-mentioned filling sequence reflects the sedimentary evolution of the basin, with the slow subsidence of the basin basement, the gradual westward withdrawal of seawater, the continuous enhancement of intracontinental sedimentation, and the gradual expansion of the basin sedimentary range, from the early gulf basin to the middle offshore basin and then to the late inland basin, forming the late Triassic regressive overlapping stratigraphic sequence.

The late Triassic coal-bearing basins in Sichuan include alluvial fan-river sedimentary assemblage, lake-lakeside delta sedimentary assemblage and barrier delta sedimentary assemblage. Alluvial fan-river sedimentary assemblage is distributed in the near edge of the basin and developed in Xujiahe Formation, with a thickness of ten to several hundred meters. The framework of sand body is composed of thick fan conglomerate and alluvial channel body. Fan conglomerate thins or pinches out to the lake area, and alluvial channel body becomes distributary channel body to the lake area, but it extends far laterally. Lake-lakeside delta sedimentary assemblage is distributed in Xujiahe Formation in the central and western part of the basin. Lake sediments are siltstone and mudstone with well-developed horizontal bedding, containing freshwater animal fossils and occasionally plant fossils. In the middle of the basin, lake sediments are thin and often coexist with lakeside delta sediments. The lacustrine facies in the southwest of the basin are mostly thick mudstone and siltstone, with shallow lacustrine facies and thin medium-fine sandstone. Lakeside delta deposits are composed of bottom layer, front layer and top layer.

The foreland basin formed on the east side of Longmenshan nappe belt in Late Triassic has obvious asymmetry, the west of the basin is a deep belt, and the uplift belts of Sichuan, Hubei, Hunan and Guizhou formed on the east side of the basin are denuded areas. Two concealed faults, Longquan Mountain and Huaying Mountain, also control the sedimentary evolution of the basin. The foreland depression in the west of Longquanshan fault has a large subsidence, and the thickness of coal-bearing measures is huge, reaching 2000 ~ 3000 m. It is a littoral-delta deposit and residual brackish water deep lake facies fine clastic rock deposit, with many coal seams and small single layer thickness. The sedimentary range east of Huayingshan fault is small, the thickness of coal-bearing rock series is less than 500 m, mainly alluvial sandstone, and the coal seam is undeveloped. The subsidence between the two faults is moderate, and the coal-bearing series is 500~700 m thick, which is a lake delta deposit with good coal-bearing property, few coal seams and large single layer thickness. The paleogeographic environment of the late Triassic coal accumulation period in Sichuan Basin is mainly coastal plain, coastal lake delta and coastal alluvial plain, among which the coastal lake delta plain type has the best coal content, followed by coastal plain type and coastal alluvial plain type has the worst coal content. Different paleogeographic environments of coal accumulation have different genetic units of coal accumulation. The lakeside delta plain has the best coal content, followed by the dam back plain, and the river alluvial plain has the worst coal content.

Three coal-rich belts were formed in the Late Triassic in Sichuan Basin, the Dayi-Ya 'an coal-rich belt in the northeast parallel to the coastline in the west, the Daxian-Zigong-Emei coal-rich belt in the northeast in the middle and Guangyuan coal-rich belt in the north. The thickness of minable seams in coal-rich zones is all above 1 m, only 1 ~ 2 m in the east, 1 ~ 5 m in the west, 1 ~ 3 m in the middle and 5 ~ 10 m in some areas. The subsidence of the western foreland depression belt is large, and the seawater advances and retreats frequently, which is not conducive to the formation of a stable peat swamp environment, and multiple thin coal seams can be formed in the plain behind the dam. Dayi-Ya 'an area is rich in coal. Due to the low terrain, peat bogs are mostly low-level bogs with stable water level, forming semi-bright coal with medium ash content and ultra-low sulfur content. In the middle area between the two faults, the settlement rate and deposition rate are close to balance, and a large area of stable minable coal seam is formed in a large area of flat delta plain. There are fewer coal seams in the coal-rich belt, but the single layer thickness is large. Peat marshes are mostly low-grade swamps with stable water cover, mainly with medium ash and low sulfur semi-bright coal. The eastern Huayingshan fault and the northern margin of the basin have slow subsidence speed and strong river action, which is not conducive to the formation of a large-scale stable peat swamp environment, with few coal seams, thin thickness, poor coal quality and instability. Due to the unstable coverage of peat swamp water, vitrinite and filament carbonization often alternate, and debris seeps into peat accumulation, and coal accumulation is interrupted. Most of them are semi-bright and semi-dark gray low sulfur coal, and the coal seam structure is complex.

Xichang basin is located on the east side of Kangdian ancient land. The fault basin formed in the early Late Triassic is non-coal deposit, and the Baiguowan Formation in the middle and late period (Daqiaodian period) is lacustrine sandstone, siltstone, mudstone and coal seam. Huili Basin (Baoding and Ni Hong) belongs to Mianning-Yongren fault depression on the west side of Kangdian structural belt, and coal-bearing clastic rocks with a thickness of 3000 meters are deposited. Xiadaqiao bottom formation is the main coal-bearing stratum and belongs to braided river deposit. Its sedimentary sequence shows that the basin was in an overcompensated state during the sedimentary period, and the marginal facies of the basin continued to advance to the center of the basin. The upper part of Baoding Formation is deposited by rivers and lakes, with thin coal seam and poor coal-bearing property. Xichang and Huili basins belong to intermontane basins, where near-source alluvial fans, rivers and lakes are developed, and there are marine interlayer deposits during the main coal accumulation period because they are connected with the outside sea. During the period of strong basement subsidence, the debris supply in Huili basin was sufficient, and alluvial fan-braided river deposits developed. Alluvial fan system is not easy to lateral transfer, mainly longitudinal accretion. In the stage of moderate basement subsidence, the front edge of alluvial fan skirt and the groundwater drainage zone in front of fan, or the flat floodplain swamp behind the river bank, are beneficial to the formation of peat flat. Lacustrine environment appears in Xichang basin due to insufficient supply of debris, and peat swamp environment conducive to coal accumulation can continue to appear in the lake swamp area filled with lake basin siltation. Due to the periodic alternation of sedimentary environment, extremely thick coal seams or multi-layer coal seams are formed, forming good covering and preservation conditions.

Chuxiong (central Yunnan) basin is located on the west side of Kangdian ancient land, with faults on the west side of structural belt as the east boundary, Ailaoshan fault uplift belt as the southwest boundary, Sichuan-Yunnan Plateau in the northeast and shallow sea in the southwest, which is a northwest-oriented coastal basin. In the early Late Triassic, the eastern part of the basin was a uplift and denudation area, and the western part of the basin was a coastal-shallow carbonate rock, containing mudstone, siltstone and coal lines, with a thickness of1800 m. In the middle period (Nori period), the fluvial coarse clastic rocks in the intermountain basin were deposited by a small basin in the eastern part of the basin, and the upper part was developed with minable coal seams. Luojiashan Formation in the west of the basin is composed of pyroclastic rocks and fine clastic rocks sandwiched with coal seams, and contains marine and brackish bivalve fossils. In the late regressive period, coal-bearing deposits were formed in the coastal tidal flat environment. In the late stage, the seawater in the western part of the basin has retreated, and the Baitutian Formation is deposited by coarse clastic rocks of fluvial facies, with poor coal-bearing property. Ganhaizi Formation and Shezi Formation in the east of the basin are coastal intermontane lake basins with a thickness of more than 2,000 meters, and the sedimentary range is expanded. There are alluvial fans at the edge of the basin, and the marine interlayer may be connected with the outside sea. Multi-layered unstable coal seams were formed in the middle and late Late Triassic in Chuxiong Basin. There are as many as 27 ~ 40 layers of coal in the west of the basin, with the maximum thickness of 52 ~ 54 m, and the eastern part of the basin contains coal 13 ~ 27 layers, with the maximum thickness of18 ~ 56 m. The eastern part of Yipinglang has the best coal, with a mining capacity of 2 m, which is a lakeside delta environment. The coal-bearing measures in the eastern Chuxiong Basin were formed in the intermountain basin environment, mainly in the near-source inland deposits, with undeveloped marine and transitional facies deposits, which are similar to Huili Basin and belong to the coastal intermountain plain type. The coal-bearing strata in the western Chuxiong basin were formed in the coastal tidal flat environment, and the coal seam was formed in the regressive period. The sedimentary characteristics are similar to those of Sichuan foreland depression, belonging to coastal plain type. Compared with Sichuan basin and Xichang basin, the coal accumulation in Chuxiong basin was obviously weakened in the late Triassic.

The late Triassic coal-bearing basin (prototype) in South China is located in the east of South China Plate, and its basement is a part of Cathaysian old land and lower Yangtze block. Coal-bearing rock series are mainly developed in the lower Yangtze, Jiangxi, Hunan, Guangdong and three depression zones of Zhejiang, Fujian and Guangdong. At the end of the Middle Triassic, the Indosinian movement in the eastern part of South China Plate was very strong, which was not only manifested by the large-scale uplift of Cathaysian ancient land and its connection with Yunkai ancient land, but also by strong folds, which made the coal-bearing measures in the Late Triassic unconformity with the underlying folded strata and covered the strata in different periods. Therefore, the paleogeographic features before the coal-accumulating period were complicated, and a series of narrow depressions in the northeast were formed on the unreleveled basement. Therefore, the early coal-accumulating basins were small in scope and strongly separated. Post-filling filling, connection extension overlap. The sedimentary types are mainly marine alternation, mostly the interaction between bay-lagoon deposits and continental deposits.

Late Triassic coal-bearing series in Nanhua Basin can be divided into alternating facies and continental facies. Alternating marine facies include: marine facies in the middle of Anyuan, Pingxiang, marine facies in the upper part of Zhangpingkeng and marine facies in Shaowujiaokeng. The continental sedimentary types include: fine clastic rocks of Wuzao Lake facies in Yiwu and coarse clastic rocks of Lingshan Tanxu River facies. According to the sedimentary sequence and diagenetic indicators of coal-bearing rock series, it can be divided into alluvial fan facies, braided river facies, meandering river facies, lake facies, estuary facies, lagoon-tidal flat facies and swamp-peat swamp facies. Alluvial fan facies is distributed in the margin of ancient land and consists of thick layered conglomerate, glutenite, gravelly sandstone mixed with thin sandstone and mudstone. The surface of alluvial fan is dominated by debris flow or piedmont braided river deposits, followed by alluvial fan overflow and lake swamp deposits. Braided fluvial facies often coexist with alluvial fans, mainly composed of medium-coarse sandstone and gravel sandstone. The lower part of each meandering fluvial sequence is medium-thick sandstone in river channel and beach, and the upper part is fine clastic deposits in natural dikes and floodplains. The lacustrine facies is dominated by mudstone and siltstone, with horizontal bedding and gentle wave bedding. The estuary facies is characterized by frequent rhythmic layers and consists of medium-fine sandstone, siltstone and mudstone. The lithologic combination characteristics of lagoon-tidal flat facies are similar to those of lakes, but marine bivalve fossils contain brackish water and brackish water, interbedded with limestone and marl thin layers and tidal bedding. The swamp-peat swamp facies is mainly composed of black carbonaceous mudstone and carbonaceous mudstone, with a small amount of siltstone, a large number of plant fossils, coal seams or coal lines. Pyrite nodules or interlayers are common, with small sedimentary thickness and wide distribution.

The formation of late Triassic coal-bearing series in Nanhua Basin is closely related to the advance and retreat of seawater, with a regressive sequence in the early and middle period and a regressive sequence in the middle and late period. In the early Late Triassic, the terrain differentiation was obvious, and coal-bearing deposits developed in unconnected small depressions. From the accumulation of foothills and alluvial glutenite deposits to lakes and swamps, fine clastic coal-bearing deposits are formed. Large-scale transgression occurred in the middle of late Triassic, and seawater from the ancient Pacific Ocean connected most sedimentary basins. A long and narrow depression zone that intrudes into northern Fujian from the west of Huaxia ancient land and southwest Fujian and distributes in northern Fujian and eastern Guangdong; The other branch passes through eastern Guangdong to northern Guangdong and reaches Liuyang, Pingxiang and Pingle depression belts in eastern Hunan and western Jiangxi. In the middle of Late Triassic, a set of coal-bearing measures was formed, mainly argillaceous rocks with poor coal-bearing properties, and river and lake facies deposits were formed in inland or intermountain basins where seawater could not reach. Regression occurred in the late Late Triassic, and river sediments developed widely, forming a set of coal-bearing strata with thin coal seams. From the analysis of coal-accumulating environment in Nanhua Basin, it is found that the basement of the Gulf Basin slowly sinks, and the best coal-accumulating effect is achieved under the condition of balance with the supply efficiency of clastic materials. On the contrary, in the deep-water coverage area of the Gulf Basin, the basement subsidence is too fast, the supply of clastic materials is insufficient, and the coal-forming conditions are poor. The coal-bearing paleogeographic environment in marina bay is the best, and the recoverable coal seams are distributed in a large area, with a thickness of 2 ~ 5 m, and the maximum thickness exceeds10 m. The coal-gathering effect is strongest in the area from southeast Hunan to Pingxiang in west Jiangxi. Secondly, it is the paleogeographic environment of coal accumulation in lagoon-Hekou Bay. The recoverable coal seams are distributed in a large area, with a thickness of 1 ~ 3 m, and the coal-bearing property of mountain-lake basin and mountain-basin type is poor.