Tmultiple Stratigraphic Division And Correlation And Depositional System Analysis Of Middle Late Triassic In West Of Sichuan

Stratigraphic Classification (stratigraphic division) is the integration of stratigraphy theory and practice. Multiple stratigraphic classification theory is the representatives. of contemporary stratigraphic classification and concept. Depositional system is combinations of three-dimensional lithofacies in the depositional environment and sedimentary processes. Tts basic unit of depositional system is sedimentary facies. The study of depositional system is the basis and prerequisite of other studiesIn this paper, the studied area is located in the east-south edge of Longmenshan structure belt in west of Sichuan (north-west foreland basin of west Sichuan ). Triassic strata of the studied area has clear sequence and abundant fossils. Several section of Middle to Upper Triassic have been measured and described in detail in the studied area. On the basis of studying stratigraphic division, according to the characteristics of the rock composition, the Middle to Upper Triassic is divided into four formal lithostratigraphic units which are Leikoupo Formation, Tianjingshan Formation, Maantang Formation and Xujiahe Formation. Xujiahe Formation is further divided into six lithologic member. The characteristics of lithologic association and lateral variation of all Formations and members are discussed. Based on previous study about biostratigraphy, according to the characteristics of fossils association and stratigraphic distribution, biostratigraphic units of Middle-Upper Triassic are established. They are five bivalve biological Assemblage Zones which are bivalve Eumorphotis (Asoella) illyrica-Myophoria (Costatoria) goldfussi Assemblage Zone, Entolium tenuistriatu- m-Myophoria (Myophoria) vulgaris Assemblage Zone, Halobia pluriradiata-Halobia convexa Assemblage Zone, Burmesialirata-Myophoria (Costatria) napengensis Assem- blage Zone, Permophoms emeiensis-WeiyuaneLla rhomboidalis Assemblage Zone and a plant Pterophyllum ptilum-Podozamites schenki-Neocalamites curcinoides Assemb- lage Zone. The ownership of chronostratigraphy of each Formation was discussed on the basises of fossil Assemblage Zone. Facies marks of Formation of Middle-Upper Triassic are recongnised from lithologic features, compositional maturity, sedimentary structure and characteristics of paleontology. Combining with the sedimentary background of the studied area, the sedimentary facies and subfacies and micro facies of measured sections have been divided and analyzed. Four depositional system are recognized which are delta depositional system, carbonate platform depositional system, shoreline depositional system, shallow-marine shelf depositional system from Leikoupo Formation to Xuliahe Formation in the western Sichuan region of Middle-Late Triassic. The spatial distribution of depositional systema are also analyzed. And two sedimental modeles are put forward witch are the carbonate platform sedimentary model from Middle Triassic to early Late Triassic and the delta sedimentary model of Middle to Late period of Late Triassic.