Eastern Edge Of The Tibetan Plateau Langmusi Sedimentary Basin Filling Process And Tectonic Significance

A great deal of studies on Cretaceous strata in Songpan area have been done. But because of the sparce Cretaceous strata, its sedimentary filling process hasn’t been concretely studied, which lead to a lack of studies on sedimentary characteristics, provenance and regional sedimentary background in this strata. The location that Langmusi Basin, which is one of the areas best developed and reserved Cretaceous strata on the northeastern border of Tibetan Plateau, is situated in the shift conversion part of Songpan- Ganzi fold belt, west Qinling and Monodiexodina developing the Cretaceous continental clastic sedimentary formation, which provides favorable conditions for the sedimentary filling process and its tectonic significance. Thus, concrete studies on the Mesozoic Cretaceous clastic sediments of this basin have great significance in understanding the stratigraphic evolution and regional tectonics.The previous investigations of the oil and gas resources in Zoige plateau in Songpan area ignored the Cretaceous Period in Zoige plateau and merely put an eye on the Trias of the fold zone, lacking of concrete studies on th esedimentary filling process in the late Mesozoic Cretaceous basin and interior structural and sedimentary records of this plateau, which made it quite difficult to understand the provenance characteristics,sedimentary filling process and tectonic pattern of the Cretaceous Langmusi basin. Based on the specific measurement of the geological profile, the design of a complete sedimentary sequence of the Cretaceous Heat Dam group and the cooperating consideration of the field paleo-current analysis, the gravels composition statistics, the heavy minerals analysis and the under-microscope fragmental rocks composition statistics in this essay, a concrete analysis of the materials origin, lithofacies, sedimentary environment and the evolution environment is conducted. Combined with the previous studies and based on the sedimentology and stratigraphy, this essay makes a division and comparison of the late Mesozoic strata of Langmusi Basin. Sandstone grits in the gravels with red mud, argillaceous siltstoue, silty mudstone with gypsum layer on the top, whose major gravel components are carbonate rocks and sandstones, and then flint, vein quartz and metasandstone, and then metasandstone and volcanic rock in the minimum, combined with the composition of sandstone fragments show that the main lithological characters of the provenance are carbonate rocks and sandstones. The sedimentary sequence of the late Cretaceous heating dam group whose bottom margin and multiple sets of old strata have an unconformable contact is made up of two big sedimentary cycles from down to up and the fragmental deposits distinctly represent a change from coarse grained sandstone conglomerate, fine grained argillaceous siltstone to mudstone. Combined with sedimentary structures, the analysis is that the main materials origin is upper Triassic series in Songpan- Ganzi area of Ruoergai in southeastern part of Langmusi Basin and its sedimentary settings have undergone a constant transformation from Braided river facies, continental alluvial fan facies to Saline Lake facies. Alluvial fans can be further divided into fan genya phase, middle fan and fan subfacies. Langmusi Basin in the late Mesozoic strata has undergone a evolvement from volcano rock basin, intermountain fault basin to river basin; the sediments in Redangba group with its thick sediments in wide range reveal that the basins in the late period of Yanshan reached its biggest dimensions; carbonate rock formations in southern upper Triassic series are the main stratas of material origins, much less or hardly none from the early Triassic strata; a small part of fragmental deposits come from west Qinling; while the underlying pre Triassic strata appearing around basins come out pre Nakagami Shi EC strata, the boundary as the discordant contact of the northern basin and the Silurian System. The southern basin bordered by the Reerlang Mountain and Diebu Wudu fault mainly appear the pre Triassic strata, which shows that when the late Cretaceous Redangba group deposits, the periphery of Triassic strata hasn’t appeared t suffer corrosion and the time relationship of the formation of White Long River uplift later than the sedimentation of Redangba Group. At the same time, this conclusion corresponds to the tectonic setting revealed by the thermochronologic data in Songpan area.Further combined with the program of the essay sources, tectonic deformation of the adjacent area and other related materials, this essay concludes that Langmusi Basin belongs to the strike slip extension basin, the volcano rocks zircon age laser shows that the opening- Langmusi fracture can start to activate in middle Triassic, when the basin started to take into forms, which developed into its maximum range in late Cretaceous, and the main basins took on the linear opening- Langmusi sinistral strike slip fault, shifting from southeast to northwest and because of the left lateral strike slip fault, the secondary sedimentary center was formed in Redangba group. At the same time, the sedimentary evolution characteristics in Langmusi Basin respond to the extensional tectonic environment under the function of the late orogenic uplifting in Yan Mountain in Songpan- Ganzi area of Ruoergai.