Study On The Type And Sea Level Changes Of The Late Triassic Yajiang Subsea Fan

The research area(Ganzi-litang metallogenic prospect area in Sichuan)is located in the tectonic position of the tethys tectonic region at the junction of Eurasian plate and India plate.although the research area has been working for a long time,due to natural geography and human social factors,the analysis and research degree of sedimentary filling and sequence stratigraphy are low.at the same time,beacuse of the superposition transformation of multi-stage tectonic metamorphism and thermal history evolution,it is very difficult to identify the sedimentary strata and the sequence identification of the flysch,which is one of the bottlenecks in the study.The research work of the dissertation is supported by the mentor and the twelve gold detachment of the armed police force.the research on the sedimentary types and their tectonic background of the yakjiang flysch are emphasized.a number of stratigraphic sections with good outcrops,rich geological phenomena,continuous strata and clear boundary have been found,which lay a foundation for identifying the types of submarine fans,the analysis of sea level fan and sea level changes in the Litang-Xinlong area of Yajiang stratigraphic district.The research is based on outcrop sedimentology.through field geological survey,6 sedimentary stratigraphic sections,such as the township of Heping County and Xinlong County in Li Tang,have been selected and measured.the characteristics of lithological lithology and its assemblage,sedimentary structure,sedimentary structure and vertical transverse changes,paleontology,sedimentary assemblage and rhythm change,sand mud ratio,combination(or prosody)scale,sequence basic type,filling structure,superposition style,paleo direction and so on are systematically observed and measured and described.the lithology of its assemblage,turbidite facies,their combination,sedimentary(micro)facies and sedimentary filling sequences(such as filling structure,superposition style,filling cycle and deformation,combination,detailed characteristics,longitudinal characteristics and longitudinal characteristics)are carried out in this area.the characteristics of transverse distribution are analyzed and compared,comprehensive maps,such as comprehensive histogram,section structure,filling structure and filling style profile are compiled.,the tectonic background of submarine fan is analyzed and the types of submarine fans are preliminarily judged.the following understanding and progress have been made in the research work,which based on the sedimentary characteristics and sedimentary geochemical analysis(1)14 sets of lithologic associations including genetic association,basic prosody,complex combination and complex prosody are identified and summarized.(2)The system division and identification of the seabed turbidite fan facies in the Xikang Group in the study area are systematically divided and identified:the main fan distributary channel,the tongue shape body and the floodplain system,the middle fan distributary channel-the bank combination system,the middle fan tongue form and the outer lingual body system.(3)The filling structure of the filling units,such as the basic filling sequence and its combination,lithologic facies,lithologic facies,sedimentary microfacies and their combination,are systematically identified and described by using sequence stratigraphic analysis methods and tools,three basic filling structures of UC,UF and UA and the deformation types such as LO?MN?RA?RH and so on on are identified the region or layer with better sequence stratigraphic analysis,(4)Three basic filling types and their deformation patterns of PG?TG and AG were identified,the filling patterns and filling sequences of each group,sedimentary facies and phase groups,sequence groups were analyzed systematically.(5)1 of 2 level cycles(sequence),2 of 3 cycle(or stage,or sequence)and 17 0f4-5 level cycles are divided and identified on the basis of horizontal comparative analysis(6)It is considered that the type of seabed fan is a subsea composite fan with an active basin edge filling type,a leafy body,a multi stage development,and an overall progradation type,and presents diversity with the development stage of the sedimentary basin.(7)According to the preliminary analysis of sea level changes,we believe that the submarine fan in the working area has undergone four basic stages of sea-level changes.the early period(T₃lh¹-T₃lh² early):the main form of the high water level normal regressive stage is the slow decline of the sea level.the middle period(T₃lh² in the middle and late stages):the main stage is the rapid sea-level rapid decline stage of forced regressive low water level normal regression stage.the middle and late period(T₃lh³)mainly formed sea-level rise in a short period of transgression-the sea level pulsatile descending period of longer-term high water level regressive period.the late period(T₃y¹)enters the forced sea retreat-the rapid decline of the sea level during the normal low water regressive stage.