| Slope area of northern South China Sea is an important geological tectonic units of the South China Sea basin, recording abundant geological information including continental splitting and basin expansion. Besides, the study area is one of the key area of offshore oil and gas exploration in our country. This thesis analyzes the grain size distribution of sediments from DLW3101 and DLW3102 drilling in slope area of northern South China Sea, to ascertain the evolution of East Asia Monsoon and sedimentary environment in the study area.Grain-size is one of the most important feature of deposit, and it could be regarded as a quantitative indicator about the classification of deposit. The main impact factors of grain-size distribution are material source, hydrodynamic conditions, transportation medium and trends. Grain-size can be used to infer sedimentary environment and determine the sedimentary models, estimate hydrodynamic conditions and speculate grain-size trends. There are two basic diagrams that can describe the grain-size:probability plot and frequency diagram. And the most basic statistics is named grain-size parameters. Grain-size parameters include mean particle size, sorting coefficient, skewness and kurtosis. According to the analysis, the sediments of the two drilling in the study area can be divided into several sections, and grain size distribution, hydrodynamic condition and sedimentary environment distinguish among differing sections.It is difficult to further study the ancient-environment influenced by different sources and deposit dynamics by sediment grain properties of total samples. Environmental sensitive factors by grade-standard deviation method and end-member modeling and their ancient-environment significance are compared based on sediment grain properties of two drilling.The major conclusions are as follows:1. The grain size of DLW3101 samples is a bit larger than DLW3102, corresponding to stronger hydrodynamic conditions. The transportation of its sediments was affected by the monsoon-controlled current, and the mutation of particle size may be related to the rapid lifting of sea level or climate quick changes. While, the deposition in the near area of DLW3102 had a more sensitive response to short-scale interglacial cycle, and was affected by submarine canyon to some extent.2. With the grade-standard deviation method, two environmentally sensitive grain size components can be easily identified according to variations in the sediment series, the ratio of which can use to indicate Asia monsoon changes.3. EMI and EM2 retrieved from the end-member modeling method reflect the hydrodynamic conditions (ancient rivers or modern current included) change of half-pelagic sediment. The EM3 mainly related with the turbidity current. Moreover, EM2/(EM1+EM2) can be used as the proxy of east Asia winter monsoon, its change rule is in accordance with the ratio.4. Turbidity current has different influence on the deposition processing and sediment characteristics in the study area. The near area of DLW3101 drilling is the turbidity current deposits prone zone, shows that the hemi-pelagic and turbidity setting control the sedimentary process in this area. DLW3102 was less influenced the turbidity current, mainly showing hemi-pelagic characteristics.5. To study the sedimentary environment evolution in certain area, it is important to establish age framework, determine the characteristics of sedimentary facies and restore the ancient climate through geophysical data, lithology, fossil, clay minerals and carbonate test. The analysis of the grain size in this paper provides the corresponding evidence for the further research. |
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