Sedimentary Environment And Paleogeographic Analysis Of The Middle - Upper Triassic Boundary In The North Of Sichuan Basin

Based on the principles of sedimentology, the trace of the sediment provenance,the construction of the evolutionary model and the division of the sedimentary system are always the core contents in the sedimentary study. They are also the foundation of the reconstruction of the paleogeographical environment, the playback of the tectonical, climatological, biological and other geological events during the geological period. The sedimentary basin is a negative terrain which has had been filled with continuous deposition during a long geological time. It is the perfect profile to conduct the sedimengtary study.Sichuan Basin, located in the southwestern China(28~32°N,103~108°E), is a typical sedimengtary basin in our country. Triassic(252~201Ma) is the crucial period in the evolutionary history of the Sichuan Basin. Influenced by the Indosinian Movement, the sea water exited Upper Yangtze Region westward and the rudiment of the basin was preliminary formed during the Middle-Late Triassic. The sedimengtary environment changed from marine to continental, while the Middle-Upper Triassic strata transferred from marine to transitional and terrestrial. The marine and transitional and terrestrial sequences from Anisian Age(Middle Triassic) to Rhaetian Age(Late Triassic), including several stratotype sections, are well developed and outcropped in northern Sichuan Basin, which yielding abundant animal, plant and sporopollen fossils and the boundaries between different Ages of Middle-Late Triassic are recognizable. The systematic study on a number of stratotype sections and outcrops shows the conclutions below.(I) The succession of the Middle-Upper Triassic strata in northern Sichuan Basin is composed of the marine carbonate dolomite, the marine clastic rocks and the terrestrial sandstone. The marine carbonate dolomite, including the Leikoupo Formation(Middle Trissic), the Tianjingshan Formation(Middle Triassic), and the lower Ma’antang Formation(Upper Triassic) is inferred a carbonate platform facies.The marine clastic rock, composed of the Upper Ma’antang Formation(Upper Triassic) and the 1st member of the Xujiahe Formation(Upper Triassic, also known as the Xiaotangzi Formation) is the typical delta facies. And the Xujiahe Formation without the 1st member should deposit in a terrestrial riverine environment. The process of the sedimentary facies from carbonate platform to delta and riverine is aprogradation sedimentation and the records of the uplift of the Upper Yangtze Rigion.Moreover the further division of the microfacies of the strata can reflected the paleogeographic environmental conditions and the transgression and regression during the Middle-Late Triassic.(II) The alternation between sea and land of the Sichuan Basin happened during the Middle-Late Triassic is the most important tectonical deformation event in its evolutionary history. In Ladinian Age(Middle Triassic), the Upper Yangtze Platform uplifted to land and weathered, the whole platform was open to the west as a shape of dustpan. The Longmenshan Chains began to rise from the Late Ladinian Age to Early Carnian Age, while the prototype of a basin preliminary formed as well. Because of the continuing rise of the Longmenshan Chains and the relative subsidence of the Upper Yangtze Platform, the continual transgression started in the Carnian Age did arrive at the southeastern Sichuan Basin in the Late Norian Age(Late Triassic), the closed sea basin was similar to the current basin. At the end of the Norian Age(Late Triassic), another regression event happened in the Upper Yangtze Platform.Therefore the Sichuan Baisin became a relatively complete geological unit at the very beginning of the Rhaetian Age(Late Triassic).(III) The marine fossils data shows a good eco-environment of the western residual sea basin during the Carnian Age(Late Triassic), a warmer weather, and a stable salinity ensured by the supply of the fresh water. Plant and sporopollen fossils indicate that most of the vegetation species are ferns in the Carnian Age(Late Triassic), and conifers as well as cycads began to appear at the Norian Age and Rhaetian Age(Late Triassic). The plant community represents a subtropical and tropical climate condition and a subaquatic environment.(IV) This study based on the analysis of sedimentary facies and supplemented by the fossil data, especially the sporopollen fossil data, can be comparative in promoting the research work of deeptime strata and the reconstruction of the paleogeographical environment.