Study On Limestone Microfacies And Cyclostratigraphy Of Huanglong Formation In Hushan Area Of Nanjing

The Huanglong Formation in the Hushan area of Nanjing is a typical section of the lower Yangtze plate,which has a complete outcrop,obvious sedimentary marks and abundant fossils.On the basis of detailed section measurement,this paper systematically collects samples and makes thin sections of rock,and systematically observes and describes the microscopic characteristics of the thin sections of rock under the microscope,including particle type,microstructure,diagenesis,etc.,especially for the detailed identification and analysis of the types and characteristics of bioclastic,biological combination and microfacies.Based on these analysis the limestong microfacis and cycls stratigraphy are studyed.The main rock type of Huanglong Formation is micrite grainstone,followed by bright grainstone,and a small amount of micrite limestone.The particle type is mainly bioclastic,a small amount of sand chips,pellets,etc.The main types of bioclasts are foraminifera,fusulinoid,crinoid stems,calcareous algae,brachiopods,gastropods,bryozoans and a small number of corals,ostracods,calcium spheres,etc.,which can be divided into normal marine bioclasts and abnormal salinity bioclasts.The diagenesis types of Huanglong Formation are mainly compaction,cementation and recrystallization.The diagenetic environment is mainly sea water diagenetic environment,atmospheric fresh water diagenetic environment and burial diagenetic environment,among which the most important are atmospheric fresh water diagenetic environment and sea water diagenetic environment.The microfacies of Huanglong Formation limestone can be divided into five categories and 23 types of microfacies.Five microfacies sequence combinations can be divided by analyzing the causes and conditions of the formation of various microfacies types and integrating the vertical change rules of various microfacies types.The main microfacies reflect that the main sedimentary environment of Huanglong Formation is open platform,limited platform and shoal environment on platform.According to the petrological characteristics and microfacies combination sequence of Huanglong Formation,25 meter level cycles are divided in Huanglong Formation,corresponding to six level cycles,all of which belong to upward shallowing sedimentary sequence,according to the vertical stacking sequence,the fifth,fourth and third-order cycles are further divided.The formation mechanism of the cyclic strata is mainly related to the sea level change caused by the glacial activity of Milankovitch cycle.