| Yanyuan area,located between Qinghai-Tibet Tethys tectonic domain and Yangtze plate tectonic domain,is in the southwestern margin of Sichuan Basin.During the formation of Sichuan foreland basin at the end of Triassic,Yanyuan area was a set of coal-bearing strata of marine-continental transitional facies.It is,however,an important coal-accumulating basin in Western Sichuan by intracontinental orogeny after the collision of sub-Indian plate in Himalayan period.Based on the study of sedimentary and sequence of coal-bearing strata and combined with the study of coal-accumulating regularity of formation mechanism of Yanyuan basin,it can not only enrich the coal-accumulating theory system in the Sichuan Basin,but also guide the coal exploration work in Western Sichuan Basin.Major scientific issues,such as the formation process of Sichuan basin and the collision of sub-printed plates,also have great academic significance.Based on the previous research results and combined with the actual coal exploration and development,and guided by some basic theories,i.e.sedimentary geology,sequence stratigraphy,coal-bearing strata sedimentology,structural geology,this paper studies the sedimentary and sequence in late Triassic coal-bearing strata in Yanyuan area and analyses the coal accumulation regularity in combination with the formation mechanism of Yanyuan basin.Through the systematic and detailed observation of field profiles and the comprehensive analysis of drilling,logging and regional data,and on the basis of the analysis of rock types,sedimentary structures and sedimentary geochemical characteristics,the sedimentary systems of the late Triassic Donggualing Formation and Boda Formation in the study area are systematically studied.A total of five types of sedimentary facies,including meandering River,braided river,delta,lake and tidal flat,are identified,whose sedimentary facies characteristics were discussed one by one in detail.What's more,the developmental characteristics of sedimentary facies in each coal-bearing prospect area are studied according to the sedimentary facies of single profile and contrast profile.For the first time,it is systematically clarified that the coal-accumulating environment of the Late Triassic coal-bearing strata are meandering river,delta and tidal flat.Among them,the coal seam in the tidal flat coal-enriched environment is developed in the peat bog in the upper tide zone,and the slow transgression process makes the swamping of the shore zone continuous,which is conducive to the formation of thick coal seams;the others,coal seams in meandering river coal-accumulating environment develop in floodplain and in delta develop in the depression between the diversions,respectively.They all belong to the continental coal-accumulating environment,and the thick coal seams are mainly formed in the water recession.Based on the study of the characteristics of sequence interface and the genetic types of the interface,a sequence division scheme of late Triassic coal-bearing strata in Yanyuan area is established by using the method of Vail classical sequence stratigraphy.It can be divided into four third-order sequences.Sequence I corresponds to the Boda Formation,Sequence II corresponds to the lower part of the Donggualing Formation,Sequence III corresponds to the middle part of the Donggualing Formation,and Sequence IV corresponds to the upper part of the Donggualing Formation.Each of these four third-order sequences has a corresponding system and a detailed discussion of the sequence characteristics of different levels.In addition,the sequence development characteristics at different levels in each prospective coal-bearing area are discussed in detail under the guidance of the sequence framework of single profile and contrast profile.The research of coal accumulation law studies the foundation of coal seam formation,the change of coal accumulation environment,the distribution characteristics of coal seam and the occurrence state from the aspects of paleoclimate characteristics,paleogeographic pattern evolution,sequence system domain and later tectonic movement.The changes in the coal-collecting environment of the Donggualing Formation(meandering river,delta)and Boda Formation(tidal flat)are the sedimentary responses of the basin-mountain pattern evolution during the formation of the Sichuan Basin.In the tidal flat coal-enriched environment,the coal seam develops in the peat swamp in the upper tide zone.After a slow transgression process,the coastal zone is continuously swamped,which is conducive to the formation of thick coal seams.The coal seams in the meandering coal-enriched environment and the delta-coalized coal environment develop in the floodplain and the diversion depression respectively,which belong to continental coal-accumulating environment,and the thick coal seams are mainly formed in the water recession.The study shows that the coal formation in the transgressive system tract is the best,followed by high level system region.The coal-controlling structure of fold faults is the result of intracontinental orogenic compression during the Himalayan Asian-Indian collision.The coal-controlling structure of single-slope fault block is the tensile stress manifestation of lateral extrusion during the tectonic escape of Sichuan-Yunnan block.Finally,the coal accumulation model of late Triassic coal-bearing strata in Yanyuan area is established. |
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