Study On Sedimentary System And Filling Sequence Of The Late Triassic Flysch In Litang

The Yajiang residual basin is characterized by a thick,monotonous flysch of Triassic Xikang Group,which is located in the eastern section of the Tethyan structural area at the junction of the Eurasian plate and the Indian plate at the site of the tectonic settin.The region is extremely important strategic metallogenic belt in China,and is currently a hotspot region for studying the Sanjiang orogenic belt in the southwest of China.The research on the“Xikang Group”flysch has always been highly valued.The Sedimentary stratum of flysch in the study area are extremely developed,and in the regional geological survey,exploration of geology and mineral resources,geophysical and geochemical exploration,remote sensing geology survey and comprehensive geological scientific research has carried out the long-term,a systematic work,but previous studies on sedimentary systems and seqenceu-filling sequences in this area have been rare.The unique geotectonic locations,complex sedimentary basins,abundant sequence stratigraphic types,and unique and complex geological evolution of the study area have achieved the“natural laboratory”and ideal research base for the analysis and study of sedimentary systems and sequence-filling sequences in the region.In this paper,on the basis of a large number of collection and collation of data from the study area,the full absorption of previous research results and the development of a comprehensive survey of 1:50000 geological minerals.Taking sedimentary strata and filling sequence of the flysch of the Late Triassic Yajiang residual basin in Litang-Xinlong area,Sichuan Province as research target.Applying the theory of modern stratigraphy and paleontology,sedimentology,geochemistry and structural geology,it mainly adopts field geological surveys,outcrop sedimentology,outcrop sequence stratigraphy,sedimentary geochemistry,indoor rock,mineral identification and test analysis,comprehensive map compilation,comprehensive analysis and other methods,focusing on the investigation and analysis of the sedimentary systems in the study area,sedimentary(sequence)filling sequence,sedimentary(parasequence set)filling type,filling combinations and other aspects.The following recognition and progress have been made:(1)Through the survey and comprehensive study of large-scale measured profiles and high-resolution sedimentary(sequence)strata in the field outcrops,14sets of lithologic associations with genetic linkages including basic combinations,basic rhythms,complex combinations and complex rhythms have been identified.(2)Based on the systematic analysis of the lateral variation of sedimentary bedding thickness and its occurrence,the sedimentary combinations and rhythm changes,the sand-mud ratio,the combination(or rhythm)scale in the measured profiles and collection profiles.The structure diagram of the measured sedimentary stratigraphic profiles was compiled,the sedimentary system and major lithofacies,combinations have been summarized,and the facies of Xikang group submarine turbidite fan in the study area have been systematically identified and identified;Dividing and identifying systematically the 4 subfacies of the inner fan,middle fan,outer fan and deep sea plain,16 microfacies paleogeographic units,7 basic lithofacies,16 lithofacies combinations and corresponding filling structures.(3)According to the study,the“flysch”submarine turbidite fan in the Late Triassic Yajiang residual basin in the study area was generally developed on the deep sea-basin background,but the sedimentary basin background was different in different periods.Sedimentation and filling indicate that the one section of the Lianghekou Group(T₃lh¹)and the two section of the Lianghekou Group(T₃lh²)are submarine fan(SF)with progradational type filling under the background of deep-water slope basin-deep sea basin(LP-Ba);The three section of the Lianghekou Group(T₃lh³)is a complex submarine turbidite fan(MSF)with retrogradational–progradational type filling under the background of slope-shelf(SL-Sh);The one section of the Yajiang Group(T₃y¹)is a complex Submarine fan(MSF)with progradational type filling under the background of the tedge of continental shelf(Sh).(4)In sequence stratigraphic analysis areas or intervals,using sequence stratigraphy analysis methods and tools,the basic filling sequence and its combination,lithologic lithofacies and its combinations,sedimentary microfacies and its combinations,etc,the filling structure of the filling unit are systematically identified and described.Three basic filling structures of UC,UF and UA,three basic filling types of PG,TG,AG and deformation patterns were identified;The sedimentary facies and facies groups of each group segment,the filling patterns and filling sequences of each sequence group were systematically analyzed;On the basis of horizontal comparative analysis,the filling cycles and period were divided and identified;A total of 1 level 2 cycle(sequence),2 level 3 cycles(or period,or sequence),and 17 levels of 4-5 cycles are identified,laying a solid foundation for the classification and identification of filling combinations.(5)The understanding of the geochemical process of flysch includes three aspects:“three-dimensional spatial representation of materialization+environmental control elements+environmental control elements”.Therefore,the expression form of the flysch mapping unit may be a set of geological entities consisting of“facies combination”+“sedimentary facies”+“(tectonic-sequential)filling type”with environmental genetic links.Through the above investigations and researches on the sedimentary system and its filling in the study area,five types of 16 kinds of phase-phase(RF-SF)filling combinations and corresponding filling patterns of the submarine turbidite fan in the study area are summarized and can be used as a flysch mapping unit refinement unit.