Carbonate Microfacies And Paleoenvironmental Significance Of The Upper Ordovician Sambiian-Kaidian Transition Period On The Upper Yangtze Platform

In this paper,the upper Ordovician Sandbian-Katian stage of Qijiang-Anwen and Leshan-Xuekoushan in the Yangtze Platform is the research site,Comprehensive use of petrology,sedimentology,paleontology and other methods,the use of microfacies as a basic research method,the combination of field outcrops,hand specimens,indoor identification analysis,etc.The petrological characteristics,sedimentological characteristics and paleontological characteristics of the Upper Yangtze platform and the Upper Ordovician Sandbian-Katian stage of the Qijiang and Leshan were studied in detail.And explain the paleoenvironmental significance of the same period through the carbonate microfacies.The upper Ordovician Sandbian-Katian stage transitional carbonate rocks of the Upper Yangtze platform are a set of stratigraphic mixtures of clastic rocks and carbonate rocks.Not only does the sedimentary sequence show the mutual deposition of clastic rocks and carbonate rocksBut also a mixture of terrigenous clastic-carbonate on the microstructure.The classification of limestone by the Dunham classification method,the use of the standard microfacies type described by the Flügel classification method,and the use of the Mount classification method were used to analyze the origin of different mixed rocks in the study area.Using the sequence stratigraphic division method of Fan Ru et al.,the Upper Ordovician is divided into the Meitan Formation,the Shihtzupu Formation,and the Pagoda Formation,and the distribution of the mixed rock in the sequence stratigraphy and the causes of the mixing are studied.This dissertation aims to study the carbonate sedimentary microfacies and the paleoenvironmental significance of the Late Ordovician.The main research results obtained are as follows:1.The Upper Ordovician strata are relatively stable in the upper Yangtze plate,and are mainly carbonate rocks,rich in argillaceous sandy terrigenous debris,and rich in bioclastic.According to the type of rock particles and the rock structure,the limestone is divided into Lime mudstone,Wackestone,Packstone,Grainstone.2.The sedimentary facies of the Meitan Formation are mixed mesa facies;the Shihtzupu Formation are mixed mesa facies;the Pagoda Formation are deep-water facies.Among them,the Meitan Formation is in contact with the Shihtzupu Formation;the Shihtzupu Formation and the Pagoda Formation are in parallel unconformity.3.The Upper Ordovician microfacies in the study area was divided into the open-marine bioclastic wackestone,bioclastic packstone,bioclastic grainstone,reef bioclastic grain-packstone,Limestones with shell concentrations,open-marine bioclastic packstone.4.The paleo-environment of the Upper Ordovician indicates that there was a large-scale continuous transgression incident in the early stage of the Pagoda Formation,and the corresponding age of the strata is the Sandbian-Katian stage.