| This article mainly studies the age and sedimentary facies of Bang'ai Formation in Bang'ai Township,Anduo County,Nagqu City,Tibet,which is of great significance for further exploring the evolution of Ban-nu ocean.Through field surveying profiles,abundant bivalve fossils were obtained in the micrite limestone interlayer of the Bang'ai Formation,which redefined the age of the Bang'ai Formation.Research under the indoor microscope revealed that the carbonate rocks of the Bang'ai Formation are doped with a large amount of terrigenous clastic materials,which have obvious mixed sedimentary characteristics.The main results and understandings of the thesis are as follows:Bivalve fossils collected in Bang'ai Township,Amdo County,Tibet,were identified in detail indoors,and contained 5 genera and 8 species(including unidentified species),and the Camptonectes auritus-Protocardia lamellosa(CP)fossil combination was established.Based on the discussion of the domestic and foreign stratigraphic distribution of each genera and species in the assemblage and the analysis of the assemblage,this paper determines the age of the Bang'ai Formation in the study area as Middle Jurassic to Late Jurassic.The Bang'ai Formation is a set of clastic rocks mainly intercalated with limestone.The terrigenous clastic rocks mainly include fine conglomerate,gravel-bearing coarse sandstone,finegrained quartz sandstone,siltstone,argillaceous siltstone and calcareous mudstone.Carbonate rock mainly includes micrite limestone,bioclastic limestone,oolitic limestone,bioclastic micrite limestone,marl and so on.According to the mixed sedimentary relationship of terrigenous clastic rocks and carbonate rocks of the Bang'ai Formation,there are three types of mixed strata,including:(1)The alternate deposition of terrigenous clastic rocks and mixed rocks: gray-yellow thin siltstone and ash Red thin calcareous mudstone rhythmic interbedded layer,mixed with light gray-yellow thin layer of marlstone;(2)Alternative deposition of carbonate rock and mixed rock: gray middle-layer fine sand micrite limestone and light yellow middle-thin layer mud mixed deposition of interbedded limestone;(3)Alternative deposition of different types of mixed rocks themselves: gray-red thin calcareous siltstone and light gray-yellow thin argillaceous limestone rhythmically interbedded,interbedded with gray-black medium-thin layer mixed sediments of bioclastic fine sand micrite limestone.According to the characteristics of rock assemblage,sedimentary structure and fossil paleoecology of Bang'ai Formation,Bang'ai Formation is divided into tidal flat-mixed ground shed sedimentary system.And the tidal flat facies and mixed ground shed facies are divided into clastic tidal flat subfacies,mixed tidal flat subfacies and shallow water ground shed subfacies,as well as subtidal flats,intertidal flats,upper tidal flats,and limited ground shed,shallows and open ground shed microfacies.According to the combination law and appearance sequence of these sediments and subfacies,the profile is divided into three complete sedimentary cycles from bottom to top.From the perspective of the overall profile,the sand and mud content of the rock rises.Therefore,although the water depth changes rhythmically,the overall trend is regression.Because the Bang'ai Formation is covered by the Lower Cretaceous Dongqiao Formation in an angular unconformity,the Bang'ai Formation is the highest marine horizon in the late Ban-nu ocean closure of the study area. |
PDF Full Text Request