Sedimentary Characteristics And Tectonic Significance Of The Silurian Mayilashan Formation Of Kuokehada Area, Western Junggar

This dissertation is object on the sedimentary stratum of Silurian System in the Kuokehada area of Western Junggar, Xinjiang. By studying the rock assemblages, biological fossils, primary structure, grain size analysis, basic stratigraphy specifically, we gained the achievements as follows:1. Based on the lithologic characters, rock assemblages and marked layers, the Silurian Mayilashan formation is divided into four sections. The first section (S2-3m1) is marine facies which is mainly composed of gray-green tuff fine-grained sandstone and muddy siltstone, the second section (S2-3m2) is marine face which is composed mostly by pebbly rock-fragments sandstone, with the gray-green and dark purple tuff- and muddy-siltstone occur interbedded or as sandwich layers. Some microcrystal limestone distributed on the north of the research area, the third section (S2-3m3) is littoral facies which is characterized by tuff-silt fine sandstone and muddy siltstone, and the fourth part (S2-3m4) is neritic facies which is featured by mostly chert and siliceous siltstone.2. There is a set of slumped accumulation facies at the upper layer of silurian Mayilashan formation in qia da area, west Junggar, which is redefinition as in-situ slumped accumulation after carefully analyzed the lithologic features, distribution horizon, tectonic deformation and space combination relationship. And the identification could provide reliable evidences to rebuilt the silurian sedimentary environment and paleogeography evolution of the west Junggar, which also offers important evidence for the further analysis of silurian basin evolution.3. Based on sedimentary facies analysis, we conclude that the sedimentary environments of silurian Mayilashan formation are shoreface deposit-foreshore deposit- shoreface deposit- shallow deposit from bottom to top. It presents characters of frequent change and water unrest on the whole. By contrasting two rock stratas exposed in south and north area, we find that the content of pyroclastic gradually decreased from the north to the south, and we also find a set of proximal slump accumulation rock developed only in the up third lithologic section of north area, but this symbol layer doesn't develop in the south area. So we can infer that paleogeography pattern in Middle-Late Silurian should be ocean in the north and mainland in the south.4. In our research area, the neutral volcanic rocks and basic volcanic rocks are not common, and the stratum is relatively thick (over 3,000 meters), fossils are relatively rare. Stratums exposed in our research area are poor in sedimentation continuity, and strongly deformed. Apparently, they should be products of active tectonic system. This conclusion provide important scientific basis and reference value in terms of correctly establish the sequence of stratums in our research area, and restore the sedimentary environment and paleogeography pattern in west Junggar basin.