Study Of Paleogene Sedimentary Record In The Northern Depression Of Nima Basin, Tibet

The history of Cenozoic uplift of Qinghai-Tibet Plateau has always been considering as one of the important issue of fundamental geological research of this plateau. Predecessors have made some achievements of Cenozoic uplift of Qinghai-Tibet Plateau by studying on its surrounding and internal basins.Nima Basin inside of the Bangong Co-Nujiang junction zone locates on the westen part of the Lunpola Basin, the development of Paleogene strata in the Nima area demonstrates that a Cenozoic sedimentary basin still occurs in the area. The stratigraphy, sedimentology and provenance of the northern depression of Nima Basin are studied thoroughly for the purpose to solve Paleogene sedimentary evolution and formation mechanics of the basin, and we hope this thesis can provide some materials to reveal the evolution of the Qinghai-Tibet Plateau.The redefinition of regional geological data show basement of Cenozoic Nima Basin compose of Mesozonic metamorphosed marine siliclastic rocks and carbonate rocks, including tectonic melange and a little Early Cretaceous volcanic rocks. By means of conscientiously analysis of all sections and regional correlation, we reconstruct stratigraphic structure of northern depression of Nima Basin, and uphold that this strata is equivalent to Niubao Formation.By studying sediementary of sections, combining with grain distribution analysis, ripplemarks analysis, five facies including alluvial fan, fan-delta, acustrine, delta, fluvial can be identified, we also divided the sedimentary subfacies and sedimentary microfacies of the sections.According to longitudinal variation of facies, we argue that the water depth of lake had the variation of low-deep-low. Combining with the study of stratigraphy and sedimentary characteristics, the the northern part of the depression proves to be the quick subsidence center.Base on the analysis of the palaeocurrent directions, conglomerate, sandstone framework minerals, cuttings, Dickinson triplot of clastic component, the heavy mineral assemblage, and comparing to the regional geological data, this thesis argue that the provenances of the basin were from the south and the north, source area had changed during different stages. Early source of sediments primaryly came from the northern Jurassic marine siliclastic rocks and carbonate rocks, Early Cretaceous granitic rocks,and the sources of the sediments were predominantly granites, and some metamorphic rocks and sedimentary rocks, tectonic nature of the source area was block, Late source of sediments primaryly came from the sourthern tectonic melange consist of Mugagangri groups, limestone, ophiolites, and the sources of the sediments were predominantly sedimentary rocks, basic rocks, radiolarian chert, and some ultrabasic rocks, metamorphic rocks, tectonic nature of the source area was Orogenic belt. By the composition analysis of gravels and heavy mineral, Early Cretaceous granites in the north of the northern depression had been stripped at the beginning of the Niubao Formation development.After researching rock color of section, we find rock color of Niubao Formation had the variation of red-black-red, combining with the development of Gypsum layer, we believe Paleocene-Eocene paleoclimate in the study area had the variation of dry and hot-warm and humid-dry and hot.Longitudinal changes of sedimentary characteristics and detrital indicate that north depression can be divided into four evolutionary stage of embryonic extending, strong extending, reduced extending and basin shrinkage, tectonic activity had variation of weak-strong-weak, and the near-source sediment filled the northern depression of Nima Basin, source of sediments is the primary factor in determining the composition of light and heavy mineral of rocks.Combining with Predecessors'achievements, it was the strike-slip pull-apart in the Bangong Co-Nujiang junction zone produced from Collision of the Indian plate and the Eurasian plate that caused formation, evolution of Paleogene Nima basin.