| This dissertation draw a conclusion that the deposition of rift basin in early Neoproterozoic in eastern Guizhou consisted of three lithostratigraphic units in Xiang-Qian-Gui rift basin, not only developed typical Banxi Group, but also deposited grayish green Xiajiang Group with flysch suite in eastern Guizhou, and Danzhou group like in Northern Guangxi, by studying depositional sequences, sedimentary systems, depositional lithofacies and paleo-geography, sequence stratigraphy, event stratigraphy (analyzing the stages of volcanic events) and the stratigraphic correlation between Xiajiang group and the other Group in adjacent region and so on, on the basis of predecessors' data and achievement in scientific research. The three Groups corresponding to shore-shallow shelf facies, shelf-slope facies and deep-water basin facies respectively, and each Group has its distinguishing depositional feature. Therefore, Eastern Guizhou is an ideal area carrying out the research on rift basin evolution in Neoproterozoic.Analyzing from the sedimentary sequence, the deposition of Neoproterozoic rift system in Eastern Guizhou is a relatively independent stratigraphic unit. The unit bottom was the unconformity developed by Wuling Movement, and the unit top was overlapped by glacial deposition of Nanhua System. What's more, an unconformity can be found in Northern Guizhou and North-western Hunan. At the same time, this boundary was an important transformation between different deposition types: Banxi Group was rift basin deposition with flysch suite, and bearing a vast amount of pyroclasts, and this suggests that there was strong activity of crust. But the overlapping glacial deposition stood for steady deposition with penecaprock characteristic, and under unique climate environment. For this reason, this thesis thinks that Xuefeng movement exist, its time was before the Sturtian glacial stage (about 750Ma). The movement not only had tectonic significance, but also had climate significance.This thesis pointed out that its each evolution stage of Xiang-Qian-Gui rift basin in Neoproterozoic accompanied by magmatism on the basis of summing up high precision isotope timing data at present: the onset of rift basin was related intimately to bimodal magma intrusion at 820ma or so; the expansion of rift basin was related to marine basal volcanic eruption at 790~780Ma or so; the quickly filling and withering out of rift basin was related closely to basal magma intrusion and acid volcanic eruption at 760Ma or so. From then on, the evolution of East-southern Yangtze marine basin came into being a corresponding peaceful period, so the deposition of rift basin in Neoproterozoic was a foundation for marine basin evolution from Nanhua to Paleozoic in South China.General speaking, sequence's developing is closely related to sea level change, but to rift basin in Neoproterozoic in Eastern Guizhou, sea level change had not evident effect on sequence development, the key effect factor was the deposition—tectonic evolution stage, this was determined by early evolution characteristic of rift basin. The rift basin evolution in Neoproterozoic in Eastern Guizhou can be divided into two stages: Gradually Expanding Stage and Quickly Filling Stage.Gradually Expanding Stage: namely the depositional stage of Jialu Fm and Wuye Fm, during this time, the depositional rate was low, sedimentation and development of depositional sequence were directly related to sea level change, and Paleogeographic distribution was clear. Two guide layer were developed in the stage: calcareous rocks in upper member of Jialu Fm and carbonaceous rocks in upper member of Wuye Fm, they both were related to low depositional rate.Quickly Filling Stage: namely the depositional stage of Fanshao Fm, Qingshuijiang Fm, Pinglue Fm and Longli Fm, during this time, the basin was quickly filled, the depositional rate was high, the stratigraphic thickness was great, sedimentation and development of depositional sequence were nearly no directly related to sea level change, but obviously controlled by syngenetic faults, and Paleogeographic distribution was obscure.Generally, gradual expanding stage was a continual transgression, and developed retrogradation sequences, and at last (during largest expansion) developed condensed section—carbonaceous slate (shale). Quick filling stage was a continual regression, and developed progradation sequences, and finally basin was filled completely, and deposition was over. These two stages composed integrated basin evolution, built up complete super-sequence.At the same time, On the basis of analyzing the depositional ages for rift basin in Eastern Guizhou in early Neoproterozoic, and the new data above the Sturtian glacial epoch, the bottom of Nanhuan System, and the geochronology of Qingbaikouan System, author propose a concept of "Banxi System", which is representative for a set of wedge-shaped strata, such as Banxi Group, Xiajiang Group, Danzhou Group, Gaojian Group, Dengshan Group, Likou Group and so on in South China.Depositional age of "Banxi System" was 740～820Ma, which was no-glacial deposition, and was the filling deposition under the condition of Rodinia fragmentation before Nanhuan glaciation. But Qingbaikouan System should be the deposition of depression basin under the mechanism of plate collision related to Rodinia formation, and Nanhuan System was the glacial deposition corresponding to Cryogenian, and was the first sedimentary overlay of Neoproterozoic rift basin in South China. Therefore, they both are not reasonable whether Banxi Group, Xiajiang Group and corresponding deposition was labeled as Nanhuan or Qingbaikouan. The proposal of "Banxi System" will not only benefit to strata correlation of Nanhuan, which exists for a long-time, but also benefit to the research about the relationship between Neoproterozoic rift basin evolution and Rodinia fragmentation, glacial epoch formatiom, and the probe into corresponding important problems about climate and environment tremendous changes.
|
PDF Full Text Request