Sequence Stratigraphic Framework Of Upper Triassic Formations In The Western Sichuan Basin

The emergence of sequence stratigraphy had provided a effective means for the classification and correlation of sedimentary rocks in chronostratigraphic framework.Using sequence stratigraphy in fluvial strata is originated from the sequence stratigraphy research of the Exxon school.When the geologists wanted to explain the relationship between the sea-level changes and the fluvial sedimentary strata in the coastal area,the extended the application of sequence stratigraphy from marine sedimentary environment to terrestrial sedimentary environment.This means the conception of sequence can be used in the completely continental environment as a method to divide the strata.The Chinese geologists always focus on the fluvial sequence stratigraphy,and always have the new results of research in this area until today,from the four division of the system tracts to high frequency sequence stratigraphy.This paper focuses on the Upper Triassic sequence stratigraphy in the Western Sichuan basin,includes Xiaotangzi formation and Xujiahe formation.With the use of sequence stratigraphy methods and well logging and rock outcrop,We analyze the changing of sedimentary environment from carbonate depositional environment to Continental Molasse environment in the most part of Sichuan basin when the seawater exited the upper Yangtze region from the middle Triassic to the late Triassic.The evidence of these huge change events are recorded in the sedimentary rock,from Middle Triassic Leikoupo carbonate deposited formation to Upper Triassic Xujiahe terrestrial deposited formation.We interpret the facies of sedimentary rock and find the changes of depositional trends.To establish the sequence stratigraphic framework,and through the research of sequence stratigraphic framework to find the material provenance of the foreland basin and to understand the background of the tectonic.By identification the third order sequence stratum boundaries,Four third-order sequence have been divided from the sedimentary filling sequence of the Upper Triassic Xiaotangzi formation to Xujiahe formation,the third sequence includes S-1-1-1?S-1-1-2?S-1-2-1?S-1-2-2.But some subareas are impacted of the Indosinian Movement,the erosion of the formation are large and the sequence develops incomplete.The Late Triassic sedimentary system includes Shelf and Marine Delta and Braided river and Meandering river and Alluvial Fan and Fan-Delta system.The research and the sequence framework have indicated the source of the terrestrial sedimentary strata in the Sichuan Basin,the depositional record was controlled by the northern QinLing orogenic belt,and the clastic sediment are from the north area.The source area of the northwestern terrestrial sedimentary sequence is also the north area.The western sequence stratigraphic framework shows that the sediment is not from the east and the west.But the more to the north the thicker of the strata.This also illustrates that the filling of the Sichuan Basin is obviously controlled by the northern QinLing orogenic belt rather than the LongMenShan uplifting or the SongPan-GanZi region which was envisaged previously.The filling sequence of the Sichuan Basin is different with the classic filling patterns of the foreland basin.