Studying On Paleogene Sedimentary Records Of The South Depression Of Nyima Basin In Tibet

As the"roof of the world"and the"third pole", Qinghai-Tibet Plateau, with its unique geological, geomorphological features and forming, evolutionary history,become one of research focus in domestic and foreign geological field. Cenozoic collision of Indian plate with the Asian plate led to uplift of the Qinghai-Tibet Plateau and make significant impact on global climate and environment,the Qinghai-Tibet Plateau become the ideal place for global environmental change and lithospheric evolution.This Article , based on"oil and gas resources strategical investigation and selection of the Qinghai-Tibet Plateau Basin"project,make a research on the sedimentary record of south depression of Nyima basin at the middle of Bangong -Nujiang suture zone in order to explore the forming and evolutionary mechanism of Cenozoic continental basins in hinterland of Qinghai-Tibet Plateau,and Mainly research on stratigraphic age, tectonic setting, sedimentary facies, sedimental supply and comprehensive analysis of tectonic-sedimentary evolution. Accomplishments are introduced as follows:1. By comprehensive stratigraphic correlation with the measured sections of Wang Boming, Kapp, DeCelles and Lunpola well data, we redetermine the Stratigraphic era of sections in the south depression of Nyima basin. Stratigraphic age of the south depression is of Eocene - Oligocene, age of section P01 and P02 is of Oligocene and their strata should be Dingqing Hu formation; Stratigraphic age of the lower strata of P03 is of Paleogene Eocene and corresponding strata is assigned to Niubao Formation; the upper strata P03 is of Paleogene Oligocene and Oligocene strata is assigned to Dingqing Hu Formation.2. According to lithology, sedimentary structure, color, fossils, grain size analysis, palaeocurrents and comprehensive analysis of the sedimentary assemblage, we identify two kinds of sedimentary facies:fan delta facies and lacustrine facies of the Paleogene strata in the south depression of Nyima basin. Fan delta can be further divided into fan delta plain (can be divided into braided channels, and interdistributary areas two sedimentary microfacies), fan delta front (can be divided into the underwater distributary channels and undrerwater interdistributary areas two sedimentary microfacies), and Pre-fan delta three subfacies. lacustrine facies can be divided into the shore Lake, shallow lake, semideep-deep lake three subfacies.3. Using ripple index and the asymmetry index,ripples of south depression can be divided into current and wave ripple marks. Paleocurrent rose diagrams show that the main source of sedimentary supply of south depression from south and north, a small amount of source supply from east and west, because formation and evolution of the south depression of Nyima basin was controlled by the east-west stretching boundary faults.4.Sandstone composition analysis show that the Dingqing Hu Formation from P01 to P02 in western of south depression , stable particles as single-crystal quartz and total quartz particles were significantly declining, rather than nonsiliceous debris, debris and volcanic rocks debris, metamorphic lithic, feldspar, carbonate debris and other unstable components were significantly increased; and there were similar changes form Niubao to Dingqing Hu Formation in the east of south depression.The changes of reduction of stable components and the increasing of unstable debris are inseparable with late Oligocene tectonic uplift. Qm-F-Lt, Qt-FL triangular diagram show that sandstone componental particles mainly falling into recycling orogenic zone, a very small amount into recycling of orogenic belts and volcanic arc mixing zone, indicating that there were overall more intensive tectonic activities of south depression in the Paleogene.5. In the south depression, heavy minerals stemming from acid, intermediate magma accounted for 14.42%, basic, heavy minerals of ultrabasic rocks source accounted for 23.62% in the total heavy minerals, source of sedimentary rocks accounted for the highest of 60.96%, source of metamorphic accounted for 1%. Paleocurrent data and heavy mineral assemblages show the sediment major source of the South depression were south and North, plus a small amount of source from other direction. Andesitic debris came from the andsite plus dacite rock assemblage of Qushen La Formation, and the dramatic increasing of zircon of acidic sources in section P01 was in connection with the Early Cretaceous granite in north of Nyima basin. Based on the vertical and horizontal variation of all kinds of heavy minerals, ZTR index, GZi index,curve of heavy minerals can be divided into three distinct phases: unstable segment I, most stable segment II, unstable segment III.6. According to sedimentary facies and variation of heavy minerals, tectonic and sedimentary evolution in the south depression of Nyima basin can be divided into four phases: the stable tectonic stage (Early Eocene), the strong tectonic stage (late Eocene), the highly stable stage (early Oligocene), the highly active tectonic stage (late Oligocene).