Sedimentary Characteristics And Paleogeography Of Mingxinsi Formation Of Cambrian In Northern Guizhou,China

The North Guizhou area is located in the Southeast margin of the middle-upper Yangtze plate.The relationship is extremely close between the sedimentary evolution characteristics in study area and the tectonic evolution of the middle-upper Yangtze plate..Sedimentary environment analysis is an important part on paleogeographic reconstruction,The study on sedimentary environment and sedimentary evolution characteristics of the Mingxinsi Formation of Cambrian Qiandongian in North Guizhou is unsystematically,which lead to various understanging in the palaeogeographic pattern of the Mingxinsi sedimentary period in the middle-upper Yangtze region.Therefore,this paper focuses on the analysis of the sedimentary characteristics and facies of the Mingxinsi Formation in Northern Guizhou,Based on emphasis analysis the distribution scope of the shore facies of the upper Mingxinsi Formation,it provides new data and basis for paleogeographic reconstruction of Cambrian Mingxinsi sedimentary period in the middle-upper Yangtze region.On the premise of three-level sequence division,with extensive reading,field section observation,indoor section identification,particle size analysis and trace element geochemical analysis,and regional geological data,the Mingxinsi Formation is divided into upper and lower sections.First of all,we identified sedimentary facies according to the lithology,sedimentary structure and thin section authentication of the Cambrian Mingxinsi formation in Northern Guizhou,and we studied the sedimentary evolution sequence of typical profile.Then,we compiled lithofacies paleogeographic maps,analyzed the sedimentary evolution model of the study area on the basis of sedimentary facies research,and then drawn the sedimentary evolution pattern of the lower and upper sections of the Mingxinsi Formation.Finally,we discuss the paleogeographic significance of the shorefront facies of the upper Mingxinsi Formation in Northern Guizhou.Through the above research,it shows that:(1)The rock types of the lower member of the Mingxinsi Formation in Northern Guizhou are mainly gray-green,gray-yellow argillaceous siltstone,silty mudstone and gray sparry granular limestone,dark gray micrite.The developing horizontal bedding in rocks is mainly shallow-water shelf deposition;And the upper part of it widely developed shore facies deposition.The rock types are mainly grayish-green-light gray and livid thin-medium-thick layer quartz siltstone,the quartz particles are better rounded and the rock composition is more mature under the microscope.Low Angle flush cross-bedding,plate cross bedding and ripple marks are developed in rocks.(2)The upper and lower sections of the Mingxinsi Formation were used as mapping units to compile lithofacies paleogeographic maps.The lower part of the Mingxinsi Formation is mainly composed of shallow water shelf facies,the deep-water shelf facies developed in the southeast corner,and the development of shore facies is small.In the upper part of the Mingxinsi Formation,the shore facies sedimentary area is widely extended,and the shelf facies sedimentary area is greatly reduced in the study area.The sedimentary period of the Mingxinsi Formation in the study area is characterized by the progressive regressive sedimentation.(3)We analyzed the ratio of characteristic trace elements of quartz sandstone in Songlin section of Zunyi,which shows that Sr/Ba<1,the value of V/(VNi)was between 0.03?0.43(mean 0.13),the value of V/Cr was between 0.72?3.03(mean 1.53),and the value of Ni/Co was between 1.35 and 8.37(mean 3.57).The above results indicate that the sedimentary environment of Mingxinsi formation is shallow water and oxygen-rich.The La-Sc-Th?Th-Sc-Zr/10 and Ti/Zr-La/Sc diagrams show that the source areas of the debris have tectonic backgrounds of continental island arc,active continental margin and passive continental margin.Combined with previous studies,it is speculated that the tectonic background of the source region may be related to the Neoproterozoic magmatic activity.(4)The shore facies of the upper part of the Mingxinsi Formation in the Northern Guizhou area indicates that the paleogeographic distribution of the Cambrian Mingxinsi formation in the middle and upper Yangtze area may not be a simple pattern that gradually deepens from Northwest to Southeast,which also provides a new direction for studing the tectonic setting of the Cambrian Qiandongian in the middle and upper Yangtze region.