Continental Facies Molasse Foreland Basin Filling Sequences For Middle Yangtze Area From Late Triassic To Mid Jurassic

To establish 3^rd-order sequence stratum framework fully and systematically, and to make clear sedimentological issues about sedimentary facies distribution and migration, palaegeographical background and evolvement, and the basin attribution for Middle Yangtze and its adjacent areas from Late Triassic to Mid Jurassic, some studies have been determined and worked on. The results are useful for comprehensive understanding of Middle Yangtze Basin from Late Indosinian to Early Yanshanian, and provide important sedimentological clues for further research in basin-orogen evolvement of Yangtze and Qinling-Dabieshan system, and enrich geological theory such as Tectonics, Sedimentology, Basin Analysis and Hydrocarbon Reservoir Forming, meanwhile, the results are guidance for Middle Yangtze hydrocarbon prospecting. On the base of information from outcrops, well drilling, well logging and paleontology and under the guidance of principles from Modern Sedimentology and Continental Sequence Stratigraphy, 3^rd-order sequence strata are divided and correlated, and the sedimentary facies(sub-facies), provenance, sediment-tectonic paleogeography and basin attribution are analyzed and studied for Middle Yangtze and its adjacent from Late Triassic to Mid Jurassic. The conclusions and results are showed as following:â‘ By identification the third-order sequence stratum boundaries, the Middle Yangtze Basin sedimentary filling sequence from Late Triassic and Early Jurassic can be totally divided into 12 third-order sequence stratum units, in which the 3 of T3S1, T3S2 and T3S3 belongs to Upper Triassic, and the 3 of J₁S₁, J₁S₂ , J₁S₃ and the 6 of J₂S₁, J₂S₂, J₂S₃, J₂S₄, J₂S₅, J₂S₆ belong to Lower Jurassic and Mid Jurassic respectively, but some subareas in Middle Yangtze and its adjacent get less 3^rd-order sequence stratum units because of stratigraphic lacunae. The sequence sedimentary features are different from one to one and from subarea to subarea;â‘¡The Late Triassic sedimentary system is characteristic by the sedimentary facies assemblage of Anastomosing River + Meandering River and their Delta + Residue Lake, while the Lower and Mid Jurassic is characteristic by the assemblage of Braided River +Meandering River and their Delta + Newly Lake, which the Braided River system is dominant, and Alluvial system or Fan-Delta system develops partly;â‘¢On the influence of flexuring subsidence resulted from subduction to north of Middle Yangtze North Edge, the Middle Yangtze Foreland Basin and its adjacent in Late Triassic keeps a stable paleogeographic background and the center of subsidence and sediment locates in the north edge such as Zigui Basin and Jingdang Basin, and also west edge. The physiognomy is characteristic by"higher east and lower west, higher south and lower north"and the sedimentary system is dominated by Meandering River and its Delta + Residue Lake, while Anastomosing River system distributes in Edong Basin and Jingdang Basin only. Except the impact of flexuring subsidence which has the same mechanism with that in Late Triassic, the Middle Yangtze Area in Early Indosinian is controlled by inland strong compression and deforming, so it has some different centers of subsidence and sediment and has diversiform and complicated physiognomy. During this period, the foreland basin sedimentary system is dominated by assemblage of Braided River and its Delta + Newly Lake, while Meandering assemblage distributes in Edong Basin and Adjacent of West-Edge, and Alluvial system and Fan-Delta system exists in the South Edge;â‘£By analysis Sedimentology and Tectonic features, Middle Yangtze Basin from Late Triassic to Mid Jurassic is a Continental Molasse Foreland Sedimentary Basin probably.