The Sedimentary Characteristics And Formation Mechanism Of Black Shale In The Southern Longmenshan Foreland Basin In The Late Triassic

This thesis focuses on the black shale of the Xujiahe Formation in the southern part of the Longmenshan foreland basin in the Late Triassic.The sedimentary characteristics analysis,rock composition analysis and geochemical analysis of fine-grained sedimentary rocks are used.Follow the main line of the Longmenshan foreland basin and its peripheral sedimentary-tectonic evolution in the Late Triassic,research the black shale Sedimentary characteristics,seismic and gravity flow Sedimentary characteristics in the black shale interbeds,and the debris and oganic source and sedimentary environment of black shale.Then research the relationship between the black shale formation conditions and the evolution of the foreland basin.According to the theory of the foreland basin system and the principle of“the theory of the present”,through answer to the five basic questions of black shale?the source of mud,how was it transported,redox conditions,deposition rate,organic matter enrichment conditions?,comprehensive analysis the formation mechanism of marine and continental black shale in different development stages of the Late Triassic Longmenshan foreland basin was proposed.This paper has achieved the following results and understanding:1.The characteristics of the black shale in the Xujiahe Formation in the southern Longmenshan foreland basin are closely related to the evolution of the foreland basin.Longmenshan is still under water when the black shale of the Xujiahe Formation 1^stt section deposit,the sedimentation rate is slow,and the black shale thickness is small.After the Longmenshan uplift,the debris and organic matter for the black shale deposits of the Xujiahe Formation 3^rd and 5^th section are provided,the deposition rate is fast,and the black shale thickness is increased.The black shale of the Xujiahe Formation 1^st section is marine sedimentary,with high initial productivity,sufficient organic matter and relatively high TOC.The black shale of the Longmenshan bulge is converted to terrestrial deposits during the sedimentary period of the Xujiahe Formation 3^rd and 5^th section,and the terrigenous clastic composition increases,the TOC has been reduced.The maturity of organic matter of the Xujiahe Formation 1^stt black shale is controlled by the tectonic activities and thermal events of the Longmenshan fault zone.Due to the distribution of the black shale and the black shale deposit of the Xujiahe Formation 3^rd and 5^th section transfer to the basin centre,and the maturity of organic matter is influenced by the Daliangshan fold belt in Himalayan,which is higher in the area close to the DaLiangshan fold belt.2.A large number of Soft-sediment Deformation Structures?SSDS?and gravity flow sediments were identified in the interbeds of the black shale of the Xujiahe Formation.Judging from the sedimentary characteristics and sedimentary composition of the SSDS,most of these deformation structures are earthquake genesis,which begin from the lower part of the Xujiahe Formation 1^st section and concentrated in the Xujiahe Formation 4^th and 5^th section.Therefore,it is believed that the Longmenshan fault has collided in Carnian and tectonic activity was strong in Rhaetian.According to the distribution and overlay relationship of gravity flow sedimentation and seismite in the strata,it is concluded that the gravity flow in the Late Triassic Longmenshan foreland basin is mostly induced by earthquake or flood,and it is widely deposited in the delta front to the deep lake,and brings a lot of organic matter and nutrients to the basin,which has the characteristics of rapid deposition.Gravity flow has an important impact on the formation of black shale in the Xujiahe Formation in the foreland basin.3.Through the analysis of sandstone composition of 401 samples of Xujiahe Formation and the geochemical characteristics of fine-grained sedimentary rocks,it is believed that the source of the Lower Member of the Xujiahe Formation in the front of the Longmen Mountains comes from the clastic rocks of the re-spinning orogenic belt after Norian.The trace elements and REE of the fine-grained clastic rocks in the lower part of the Xujiahe Formation,the standardized distribution pattern of REE chondrite and the age distribution pattern of zircon are similar to the Upper Triassic clastic rocks in the Songpan-Ganzi fold belt.It is indicated that from the Norian period,the source of the black shale of the Xujiahe Formation in the Longmenshan foreland basin is mainly from the Songpan Ganzi fold belt,and a small amount comes from the western margin of the Yangtze plate.According to the ratio of trace and rare earth element and tectonic background discrimination diagram,this paper suggests that the Songpan Ganzi is wrinkled and thrusts over the western margin of the Yangtze plate in Carnian.The formation of the Longmenshan ancient island arc began in Norian,and the thrust fault was the Mao-Wen fault.4.Through the black shale geochemical analysis of the sensitive elements in the sedimentary environment,combined with the characteristics of the combination of paleontology and minerals composition,this paper considers that the 2^nd section of Xujiahe Formation must be gradually converted to terrestrial deposition,and the paleo-salinity of the sedimentary water is significantly reduced,which is brackish water to Fresh water,which is generally weakly oxidized,has a warm and humid tropical-subtropical climate with prosperous vegetation,high initial productivity and rapid deposition rate.The sedimentary environment of the Longmenshan foreland basin in Norian-Rhaetian period is partially similar to the current Caspian Sea.5.The black shale(Xujiahe Formation 1^st section)deopsit in the Longmenshan foreland basin in Carnian is a marine sediment.The paleomagnetic analysis shows that the paleogeography of the Longmenshan area in Carnian is similar to that of the west coast of India now.The monsoon is prevalent,and the upwelling caused by the monsoon brings in a lot of organic matter and nutrients,and enhances the reducibility of water.The upwelling pattern is the main cause of the formation of marine black shale.In Norian-Rhaetian period,black shale(Xujiahe Formation 3^rd and 5^th section)were converted to terrestrial deposits,and the sedimentary waters were weakly oxidized.Black shale was formed under sufficient organic matter supply and rapid deposition.In addition,the gravity flow brings in a lot of debris,nutrients and organic matter to the sedimentary basin,and also accelerates the burial and preservation of the underlying lake deposit black shale,and reduces the oxidation of organic matter.