Carbonate Microfacies Analysis And Evolutions Of The Middle Jurassic Buqu Formation In The Amdo Area,Northern Tibet

The Jurassic is a very important period in geological history.However,due to the influence of the Indosinian orogeny,many areas in my country lack marine Jurassic strata.The Qiangtang area in northern Tibet,as an important part of the global Tethys region,has developed a relatively complete Jurassic marine stratigraphic deposit,which provides a typical profile for the study of Jurassic strata in this area,and is also the formation of Jurassic in northern Tibet.Among them,the Buqu Formation is characterized by the development of a large set of shallow sea carbonate deposits.The paleontological fossils are extremely rich,including paleotopography,sedimentary water environment and sea level changes.It is the most ideal stratum for the development of carbonate sedimentary microfacies.This article systematically organizes the biological strata,rock strata,and chronostratigraphy of the Qiangtang Basin.Based on field surveys and previous data,this paper uses a combination of field exploration,profile measurement,sedimentary facies analysis,and indoor thin-film identification In this paper,we systematically studied the regional location,geotectonics,paleontological distribution,petrology,sedimentary characteristics and lithofacies palaeogeographic characteristics of the Buqu Formation in the study area.The following results were obtained initially;(1)The rock types of the Buqu Formation in the study area are mainly carbonate rock,clastic rock and gypsum.Among them,limestone is the most common rock type,and it is also the key research object of the thesis.Clastic rocks and gypsum are concentrated in the Dazhuoma section;(2)The matrix type of limestone in the Buqu Formation is dominated by micrite,with abundant and diverse types of particles,mainly bioclastic,as well as a considerable number of spherulites,oolitic particles,sand debris,and nucleated stones,among which Biological debris includes bivalves,brachiopods,echinoderms,ostracods,foraminifera,bryozoans,gastropods,sponges,and calcium algae.There are differences,in addition,a small amount of terrestrial detritus and authigenic minerals,such as pyrophyllite,pyrite,etc.can be seen;(3)According to the matrix type,particle type and content of limestone(especially bioclastic type),the limestone of the Buqu Formation in the study area is divided into 17 types of rock microfacies,the most common of which is the MF-1 micrite limestone.Facies,MF-3 contains bioclastic micrite limestone microfacies;the more common is mainly MF-2 bioclastic micrite limestone microfacies,MF-4 contains bivalve bioclastic micrite limestone.Microfacies,MF-6 containing bivalve-bryozoan bioclastic micrite limestone microfacies,MF-7 containing echinoderm bioclastic micrite limestone microfacies,MF-10 containing brachiopod biodegradation The microfacies of clastic micrite limestone and MF-12 contain bivalve-gastropod bioclastic micrite limestone microfacies;the type of microfacies with relatively low content is MF-9 microclastic micrite limestone microfacies Facies,MF-15 brilliant oolitic limestone microfacies,MF-16 micrite spherulite limestone microfacies and MF-17 microclastic nucleated limestone microfacies;(4)Through a comprehensive analysis of sedimentary characteristics,it is considered that the limestone in the profile of the Buqu Formation mainly forms a carbonate-type gentle slope sedimentary environment,which can be further subdivided into three subfacies: inner gentle slope,middle gentle slope,and outer gentle slope,and 5 types of microfacies such as tidal flats,lagoons and shoals,and established a depositional environment distribution model of carbonate microfacies of the Buqu Formation in the Anduo area of northern Tibet;(5)Based on the composition characteristics of various types of microclastics and the distribution of vertical types of biodetritus,the vertical evolution characteristics of sedimentary water bodies during the deposition period of the Buqu Formation in Yanshiping and Dazhuoma were analyzed.It is considered that the development process of the Buqu Formation in the Yanshiping section is divided into five stages,with three shallow waters interspersed with two deep waters.In the development of the Buqu Formation on the Dazhuoma section,the environment of gentle slopes and shallow gentle slopes is generally stable in sedimentary environment.As far as the nature of the sedimentary environment is concerned,it can be divided into shallow saline water in the early stage,shallow water in the middle stage,and shallow water in the late stage.In this stage,two sedimentary cycles were identified in the late vertical evolution sequence,forming a regressive sequence that gradually decreased relative to sea level;(6)Based on a comprehensive analysis of the depositional model of the Buqu Formation in the Qiangtang Basin and the spatial distribution characteristics of dolomites,we believe that the shoals with type ? dolomites are mostly point shoals,and the spatial distribution is discontinuous,making oil and gas exploration more difficult;The tidal flat lagoon facies on the inner gentle slope are mostly distributed in continuous bands,and their distribution range is mainly concentrated in the Dazhuoma-Nadigangri area,which can be used as the main target for the next oil and gas exploration in the Qiangtang Basin.