Study On The Graptolite Biostratigraphy At The Turn Of The Ordovician-Silurian In Nanba,Yiyang,Hunan Province

The turn of Ordovician-Silurian period as an important time node in geological history,is an important period of global environmental evolution,Paleozoic fauna evolution and fossil energy formation.The Ordovician Branch of the International Commission on Stratigraphy hopes to establish a high-precision chronostratigraphic ruler through the combination of multiple disciplines to provide help for the study of the "Ordovician Earth System".At the turn of the Ordovician-Silurian period,graptolite fossils are extremely rich and have important stratigraphic correlation significance.However,in recent years,the names of some graptolite species have been changed,the original characteristics have been revised,and the classification system of graptolites has changed,which may affect the based on high-precision biostratigraphic correlations.The marine sediments from Late Ordovician to Early Silurian in central Hunan are well developed,which can provide good information on the evolution of graptolite fauna and biostratigraphy in this period.In order to increase the data diversity of graptolite fauna and biostratigraphy at the turn of Ordovician Silurian in South China,this paper selects Nanba area in Yiyang,Hunan Province,which is located in the Jiangnan Slope Belt,measures the outcrop profile in the study area,and collects graptolite fossils layer by layer.The main results of this paper are as follows:(1)The collected graptolite specimens were identified one by one,and a total of 16 genera,31 species and 2subspecies were found,and Based on the systematic description of 9 genera and 12 species of graptolites with important biostratigraphic significance,the systematic paleontology of graptolite fauna in Nanba area is analyzed and discussed.(2)Based on the obtained graptolite data,the graptolite biostratigraphy of Nanba section is divided,and the graptolite belt sequence is established and determined.Finally,the Metabolograptus persculptus biozone,Akidograptus ascensus biozone and Parakidograptus acuminatus biozone are identified from bottom to top,and the geological age of the study section is determined to be from Late Ordovician Hirnantian to Early Silurian Rhuddanian;Combined with the contemporaneous stratigraphic data at home and abroad,the study area and other regions of the world are systematically combed and summarized.(3)The Ordovician Silurian boundary is temporarily set at YYNF33-1 according to the first appearance of graptolite fossils and the lithostratigraphic data in the study area.