Study Of The Sedimentary Facies Characteristics And Evolution For Wufeng Formation-longmaxi Formation In Northeast Of Chongqing

On the premise of fully collecting predecessor's information and research production,this paper summarized the basic geological characteristics of stratigraphy,structure,and sedimentary evolution.The close survey of stratigraphic profile with a large-scale as 1:200 and 1:500,as well as the work of the record that related,the sample collection,and photoing,was carried out for Wufeng-Longmaxi formation in northeast of Chongqing.On the basis of field geological work,the stratigraphic characteristics,petrologic characteristics,mineralogical characteristics,and sedimentology characteristics was study deeply for Wufeng-Longmaxi formation in northeast of Chongqing,with the tests and experiments of x-ray diffraction,as well as thin section observation and synthetical analysis.On this condition,the sedimentary facies was refined divided for Wufeng-Longmaxi formation in the research area,and also,the lateral and vertical evolution characteristics of sedimentary facies was discussed,finally,the sedimentary model was summarized for Wufeng-Longmaxi formation in northeast of Chongqing.The black rock series of Wufeng-Longmaxi formation was generally developed in northeast of Chongqing.The conclusion that outcrop located at midwest and east as band in the research area was drawn on the basis of regional geological features.The black rock series was characterized by the rich organic matter,a large number of graptolite fossil,with the single sedimentary structure of horizontal bedding and laminated structure.By the test and analysis of x-ray diffraction,as well as thin section observation under the microscope,a conclusion was drawn that mudstone was the main rock type of Wufeng-Longmaxi formation in the research area,including carbonaceous mudstone,siliceous mudstone,silty mudstone,calcilutite and the mudstone of their transiens and mixed type,intercalated with siltstone,dolomite,silicalite,marlstone,and tuff.The mineral type included quartz,clay mineral,potash feldspar,plagioclase,calcite,dolomite,pyrite,barite,siderite,hematite,gypsum,anhydrite,anatase,analcime,and marcasite.Thereinto,the percentage composition of brittle mineral(quartz+feldspar+calcite+dolomite)was 72.2%,the percentage composition of clay mineral was 22.1%,the rest of the mineral content was very few.The black rock series of Wufeng-Longmaxi formation in the research area was divided into deep-water shelf subfacies and shallow shelf subfacies by the facies marks including colour,fabric,sedimentary structure,sedimentary succession and ancient fossils.And by the facies mark of microphysiography,deep-water muddy shelf microfacies,deep-water silicious and muddy shelf microfacies,deep-water sandy and muddy shelf microfacies,deep-water calcareous and muddy shelf microfacies,deep-water dolomitic and muddy shelf microfacies was recognized in deep-water shelf subfacies,shallow sandy and muddy shelf microfacies was recognized in shallow shelf subfacies.Based on the analysis of sedimentary facies,reaching a conclution that Wufeng formation was deposition of deep-water silicious and muddy shelf microfacies,intercalated with deposition of deep-water calcareous and muddy shelf microfacies;It was only in Tianba area that Guanyinqiao member was shallow sandy and muddy shelf microfacies,however,Guanyinqiao member was deep-water muddy,calcareous and muddy,dolomitic and muddy shelf microfacies in other areas;Longmaxi formation was deposition of deep-water muddy shelf microfacies,intercalated with deposition of deep-water sandy and muddy,dolomitic and muddy shelf microfacies.The northeast of Chongqing was deep-water shelf sedimentary environment,where the northwest was higher than the southeast,with accidented topography and individual small uplift under the water at the stage of Wufeng-Longmaxi.