Sedimentary Evolution Characteristics Of The Middle Proterozoic Luanshigou Formation-Taizi Formation In Shennongjia Area

The research takes the Mesoproterozoic Luanshigou-Taizi Formation in the Shennongding section of Shennongjia area as the research object.It analyzes the stratigraphic and sedimentary structural characteristics of Luanshigou-Taizi Formation by multiple field investigations.In addition,basis on the methods of thin section analysis,cathodoluminescence analysis,size analysis,trace element analysis and isotopic analysis and so on,the deposition backgrounds,the deposition environments,the paleoclimates and the evolution of sedimentary facies of Luanshigou,Dawokeng,Kuangshishan Formations were systematically studied,togethering with the sedimentary evolution model of the Luanshigou-Taizi Formations have also been established in this research.The research is of great scientific significance for the restoration and reconstruction of the mesoproterozoic paleocontinent,and provides the basis and scientific basis for the selection or establishment of the late expected mesoproterozoic mesoproterozoic stratigraphic section.The Shennongding section during the sedimentary period of Luanshigou-Taizi Formation,adjacent the ancient supercontinent,is situated on a passive continental margin,and its sedimentation is affected by terrigenous clast and volcanic activity at the same time.A set of carbonate rocks,mixed sedimentary rocks and clastic rocks are mainly developed in the study area.Among them,the mixed sedimentary rocks are mainly in the upper part of the Luanshigou Formation and the lower part of the Dawokeng Formation,while the clastic rocks are mainly developed in the Taizi Formation.The Luanshigou Formation of the study area is mainly a set of the mixed tidal flat of the epeiric sea,of which the early stage of sedimentation was dominated with intertidal zone and subtidal zone,whereas the late sedimentary period was dominated with supralittoral zone and intertidal zone,along with the sea levels dropping,intermittent exposure and a relative weakness of hydrodynamic condition.The Dawokeng Formation as a whole preserves the palaeogeography features of Luanshigou Formation,but it is mainly characteristic of intertidal zone and subtidal zone,as well as a relative strength of hydrodynamic condition.During the sedimentary period of Kuangshishan Formation,the scale of stromatolites opens up,and gradually evolves into organic reef indicating that a bigger submarine landform,a stronger wave action and a rudiment of carbonate platform have already occurred in this studied area featured with the platform-edge reef.The relative sea level of Taizi Formation continues to rise during the sedimentary period,and mainly develops a series of shelf facies.Notably,the “quartz sandstones” is actually the combined results of re-transportation of the early shore facies sediment and the accumulation of foreign sandy debris flow.During the sedimentary period of the Mesoproterozoic Luanshigou-Taizi Formation,the paleoclimate of Shennongjia area has changed dramatically from dry and hot to warm and wet.Meanwhile,the lithofacies paleogeography in the research area went through an evolutionary process with “epeiric sea-carbonate platformshelf”.Moreover,the extension of the Yangtze platform in Mesoproterozoic is the major cause of the striking palaeoclimatic change in the studied area.