Later Triassic-Middle Jurassic Basin History Of Qiangtang, Northern Tibet, China

With the closure of KoKo Xili -Jinshajiang Ocean and Longmucuo -Shuanghu rift belt during the ending of Late Paleozoic, North Qiangtang Basin (NQB) come into the evolution phase of remnant epicontinental sea (RES), till the early stage of Late Triassic - the ending of Xiaochaka depositional stage, the RES was over. NQB was in the developing phase of rift basin from the middle stage of Late Triassic to Jurassic Period.This dissertation was focused on the research about the formation and evolution of NQB from the Late Triassic to the early middle Epoch of Jurassic Period, including integrated analyzing the characteristic of the key stratigraphic surfaces, the time of the key stratigraphy, regional rock straigraphy, biostratigraphy and chronostratigraphy, sedimentary system and depositional facies. On basis of these analysis, a tectonic evolution model have been established for NQB during from the Late Triassic to the early middle Epoch of Jurassic Period.1. After the systematic surface analysis about main stratigraphy units, we found that: (1) the bottom of Xiaochaka formation was comformity with the middle Triassic in the basin, and in the north margin it was deposited and overlapped on the different period stratigraphy before Triassic. (2) The top surface of Xiaochaka formation developed regionally karst and paleo-karst breccia, and was uncomformity with Nadigangri Formation, these things suggest that NQB in Xiaochaka depositing stage was developed from middle Triassic basin, and then an extensive regression occurred, the deposition was over and erosion come into being. (3) The lithologic assemblage, geochemistry, mixed fossils, and stratigraphy overlapping characteristic of Quemocuo Formation in early middle Jurassic indicted that the uncomformity surface was an extension-unconformity.2. Integrated analysis about stratigraphy division and correlation indicted that: (1) the paleontologic assemblage of Xiaochaka formation show its time was Carian -Norian stage of late Triassic. According to it was overlapped by late Triassic Nadigangri Formation, so Xiaochaka formation should be Carian - early Norian stage. (2) the SHRIMP Zircon U-Pb age of Nadigangri formation volcanic rocks wear vary form 216 to 205 Ma, so its time should be Norian, but maybe extended into Rhaetian Stage. (3) The age of its bottom volcanic rocks, the fossil assemblages of its middle and upper members and overlapped by lower Jurassic, indicted the age of Riganpeicuo Formation was Norian - Rhaetian stage. And the same time, the volcanic rocks geochemistry of Nadigangri and Riganpeicuo formation both reflected that they wear formed in rift tectonic settings. The lower lithologic and surface characteristic, fossil assemblages and its upper maybe was conformity with lower Jurassic, all reflected that the age of Tumengela Formation in Tumen area should be Norian - Rhaetian stage. Therefore, Nadigangri formation in NQB and Riganpeicuo formation in South Qiangtang Basin should be the relationship of "same stage and difficult facies".3. The stratigraphy of late Triassic -early middle Jurassic Peroid in Qiangtang basin could be divided 6 sedimentary systems and 14 depositional facies. Xiaochaka formation developed Fan delta facies, delta facies, and carbonate platform facies, which basically continued the paleography of early and middle Triassic, but with intensive progress in early stage and quickly regress in late stage, companied by depositional overlapping in early stage and weathering erosion in late stage respectively.Nadigangri formation mainly consisted of volcanic deposition, including continental effusive rocks, marine effusive rocks, and volcanc clastic deposition, which reflected the early paleography of rift basin. Riganpeicuo formation developed marine effusive facies, delta facies, carbonate platform facies and shelf facies, which suggest the paleography of South Qiangtang Basin (SQB) in late Triassic experienced the process of rifting, sea level rising continuously, and then developing carbonate platform and shelf facies. Lower Jurassic in NQB could be near-shore lake deposition, in SQB was shallow shelf deposition, which followed the paleography of late Triassic. During early middle Jurassic period, Quemocuo formation in NQB was offshore lake deposition with marine feature, in SQB was shelf deposition, which indicted that an intense rifting and sea level rising with more large range, and consequently, eroding area was shrunk and deposition area was expanded.4. According to the result of stratigraphic surface features, division and correlation, and the paleographic characteristic, Qiangtang Basin during late Triassic -early middle Jurassic period could be divided into two basin developing stages with difficult tectonic nature, remnent epicontinental basin stage in Carian -early Norian of late Triassic and rifting basin stage in middle Norian of late Triassic - early middle Jurassic period respectively. The latter can be divided into three tectonic evolution phases. The first is intensively extending between middle Norian to Rhaetion, and in this period the Qiangtang basin has been a tectonic paleogeography of "two depressed holder one blowups". The second that the tectonic move is gentleness and the basin floor is been depressing in the early Jurassic period. The third that the tectonic might another time become intensively extending.