Brief Study On Tectonic-Palaeogeography And Its Evolution Of Jurassic Qiangtang Basin In Tibetan Plateau, West China

In this thesis, the focal point is the main structural system which regulates the formation and evolution of the Jurassic Qiangtang basin. We analyze the structure of in-basin sedimentary section plane and the character of sequence stratigraphic, by means of using the data of regional geological surveying as basis, geological basic theories which include sedimentology, tectonics, sequence stratigraphy, geochemistry as guidance, and in association with stratigraphic analysis of rock stratigraphic unit, biostratum, chrono stratigraphic unit, sequence stratigraphic, event stratigraphic, sedimentary geochemistry analysis and isotope geochemistry analysis. We also initially establish the evolution model of Jurassic Qiangtang basin by means of analysis of basin-regulation structure, especially analyzing the suture zone's character of litho-structure.Qiangtang basin, which is sandwiched between Hoh Xil-Jinsha River suture zone and Bangong-Nu suture zone, is situated in the northern Tibetan Plateau. Basin shape and geological features are regulated by North-South border fault system.Lungmuco-Shuanghu suture zone is the dividing line between South Qiangtang stratigraphic region and North Qiangtang stratigraphic region. Based on comprehensive analysis of existing geologic setting of ridge type ophiolite-ophiolitic mélange and intraoceanic arc-oceanic island-seamount accretionary wedge mélange, and the isotope chronologic data, the geochemistry features, and the confirmation of deep sea Radiolarla silicalite, we deduce that the Lungmuco-Shuanghu suture zone, South Qiangtang Palaeozoic accretionary wedge and Bangong-Nu suture zone constitute the giant suture zone (or Thetys suture zone) which represents the final demise of Tibetan Plateau Proto-Thetys ocean.In this thesis, we conduct the division and correlation of the Jurassic strata of Qiangtang basin objectively, by using the previous regional geological survey reports of Qiangtang and numerous research results about basin, sedimentation, sedimentary facies and palaeogeographic, and based on the systematical lithostratigraphic division and biostratum, and in association with analysis and recognition of the Qiangtang Jurassic formation boundary. Jurassic strata are thick and widely distributed in Qiangtang region. According to the strata-lithologic association, we divide Qiangtang structural-stratigraphic area into two stratigraphic regions called North Qiangtang region and South Qiangtang region respectively. Jurassic strata develop completely in North Qiangtang region. And it is obviously that two limestone layers are contained within three sandstone layers. Fossils of benthic brachiopod and bivalvia are abundant. The Jurassic strata in North Qiangtang region, from bottom to top, could be divided into six formations. They are called Upper Triassic-Lower Jurassic Nadigangri Formation(T3J1n), Middle Jurassic Quemocuo Formation(J2q), Buqu Formation(J2b), Xiali Formation(J2x), and Upper Jurassic Suowa Formation(J3s), Xueshan Formation(J3x), respectively. The Jurassic strata of South Qiangtang region is marine facies carbonate, mudstone and shale as a whole, and does not have the character that two limestone layers are contained within three sandstone layers. It could be divided into six formations. They are called Lower Jurassic Quse Formation(J1q), Middle Jurassic Sewa Formation(J2s), Buqu Formation(J2b), Xiali Formation(J2x), and Upper Jurassic Suowa Formation(J3s), Xueshan Formation(J3x), respectively.Via identification of the unconformity interface and division and correlation of the depositional sequence, we study and analyze the lithofacies and palaeogeographic spatial distribution, which could represent the different evolution stage of the Jurassic basin, and then identify the filled deposit's feature in different evolution stage. Most Qiangtang Jurassic layers form in the symbiotic environment including clastic rock and carbonate rock. Few are pure carbonate rock. We identify 16 facies types and 37 subfacies types in the entire basin, based on the facies classification including terrestrial facies environment, transitional facies environment, clastic marine facies environment, carbonate marine facies environment and volcanogenic sedimentation environment. We also identify 2 second order sequence and 16 third order sequence in sequence stratigraphic research of Jurassic Qiangtang basin. In 16 third order sequence, 1 is Type I sequence, other 15 are Type II sequence.By comprehensive analysis of the tectonic elements which regulate the basin and the tectonic evolution of Tibetan Plateau and its adjacent areas, and by generalization of the information of the Jurassic Qiangtang basin's generation and evolution, we recognize that the North Qiangtang basin is differ from the Southern part in basin characteristic. The Jurassic North Qiangtang basin is a back arc foreland basin, which evolved from the Late Triassic foreland basin. The magmatic arc is located in Nadigangri-Geladandong, southern margin of the North Qiangtang. From Early Jurassic Toarcian stage to Middle Jurassic Bajocian stage, the South Qiangtang basin totally inherited from the features of Late Triassic residual basin, was a monocline basin that had shallow northern and deep southern, morphologically. In Late Jurassic, with the northward subduction of Bangong-Nu Ocean, South Qiangtang region gradually uplifted in land. Bangong-Nu oceanic basin had been transformed divergence plate boundary into convergence plate boundary. Sea water had transferred to the Southern part. Thick and wide Jurassic strata had been deposited at South Qiangtang basin near Bangong-Nu oceanic basin.Through investigation of the relationship between tectonic descents, sediment fill, eustatic sea level change of sedimentary basin and tectonic uplift, cessation of deposition, eustatic sea level change of sedimentary basin, and in association with tectonic-magmatic event analysis, evolutionary model and form pattern of Jurassic Qiangtang basin are established. In Early Jurassic, pyroclast-terrigenous detrital offshore-shallow sea facies sedimentary areas were widely distributed in North Qiangtang back arc foreland basin. And few delta facies and fluvial facies deposit existed. There is a set of offshore transitional facies deposit developed in the Dirangbicuo-Tumen-Suoxian area, Northeastern margin of the North Qiangtang basin. The central uplift belt mainly manifested the feature of island chain, and was partly underwater uplift. And few marine facies carbonate platform developed at the underwater uplift areas. Because of its southern Bangong-Nu oceanic basin's constantly augmentation, the South Qiangtang basin inherited from the features of Late Triassic residual basin, and developed from epicontinental clastic shoreland facies sandstone to shallow sea facies and hemipelagic deposit in succession. Shoreland facies developed along the southern central uplift belt. Fluvial facies deposit in South Qiangtang basin was less than in the Northern part. This shows that the topography of South Qiangtang basin is smoother than North Qiangtang basin. Epeiric sea-hemipelagic deposit were formed by terrigenous clastic mudstone-shale, and gradually transited southward to Bangong-Nu oceanic basin. The provenances of North Qiangtang basin were the Muzitage-Bayanhar orogenic denuded zone and the Qiangtang central uplift belt. The provenance of South Qiangtang basin was mainly the Qiangtang central uplift belt. In Middle Jurassic, Marine transgression expanded gradually. Middle Jurassic deposit was widely distributed, and was made up of terrigenous clastic rocks and carbonate rocks. These two types of rocks formed a cycle of sedimentation including the basal clastic rock combination, the lower part carbonate rock combination, the middle part clastic rock combination and the upper part carbonate rock combination. There were also some layers of evaporate held within, and well developed bioherm. In Middle-Late Triassic, because of the formation of the meso-Tethys, after the Riganpeicuo Group which is restricted shallow sea environment, the entire South Qiangtang region entered a new marine transgressing stage in Early Jurassic, and was in the normal epicontinental marginal sea environment. But the Duoma area had become a broad epicontinental sea environment, and mainly formed by shallow sea carbonate deposit. Duoma area, in Middle Jurassic, was the same environment and deposit as in Early Jurassic. Late Jurassic Strata in Qiangtang area were mainly distributed in North Qiangtang. The volcanic rock strata, Wujianfeng Formation, developed in Ritu, southwestern edge of the South Qiangtang. The features of some strata developed at north Ritu, near the Bangong-Nu suture zone, are same as North Qiangtang. Late Jurassic strata include Suowa Formation and Xueshan Formation.Jurassic Qiangtang basin has experienced four stages of development. The first stage is continental margin chasmic stage. In early-middle stage of Early Jurassic, the basin entered into the stage of gestation and generation. The second stage is rapid expansion of basin depression stage. In late stage of Early Jurassic, North Qiangtang area entered into the stage of rapid expansion of basin depression, the previous continental margin faulted depression depressed rapidly, and finally evolved into the North Qiangtang basin. South Qiangtang basin was still a residual basin. The third stage is continued basin subsidence and filling stage. From middle stage of the Middle Jurassic to early stage of the Late Jurassic, Qiangtang basin manifested the relative stable and persistent depression and fill. This stage was the developing stage of the basin. The forth stage is basin atrophy stage. From late stage of the Late Jurassic to early stage of the Early Crataceous, South Qiangtang basin uplifted and underwent the last extensive marine regression. At the same time, North Qiangtang basin deposited a set of sediment which represented the process of sea level descent, and reflected the basin was gradually atrophied.