Study On Forming Environment And Genesis Of Devonian Oncoids In Ganxi, Beichuan, Sichuan Basin

As an important particle type of microbial rock,oncoids has become a research hotspot in recent years.The Devonian oncoids in Ganxi Section of Beichuan are exposed in Tuqiaozi and Jinbaoshi Formation.The oncoids exposed in Tuqiaozi Formation have never been studied.And there is still controversy about the oncoids genesis of the Jinbaoshi Formation.Base on above,the oncoids in this area have been studies systematically in this paper.Through observations of field profiles and thin slices,combined with geochemical characteristics,it can be understood the role of microorganisms in the informing process of oncoids,then the paper analyzes its genetic mechanism systematically.According to the measrured field profile and sedimentary characteristics,the sedimentary facies developed in the Jinbaoshi Formation mainly consisted of carbonates with gentle slopes,secondly the mixed continental shelf,estuarine facies,littoral facies,and incised valley in exposed shelf.There are three main types of sedimentary facies in the Tuqiaozi Formation concluding carbonate terrace,carbonate gentle slope and basin facies.The oncoidal limestone could be divided into 3sedimentary cycles vertically in both formation.Then the every cycle was composed of oncoidal limestone and micrite.Base on detailed observation under the microscope,the core of oncoids is dominated by bioclastics,then the laminations are dominated by muddy laminations and Girvanella laminations.Girvanella filaments can be seen in both the core and cortex of oncoids,playing an important role in the forming of oncoid.Combined with the type of laminations,morphology of Girvanella filaments,and the relationship between infiltration sand and laminae,it can be concluded that the oncoids are formed in shoals and lagoon in the reef back.There are two type infiltration sand,respectively consisting in intragranular and intergranular,subjected to leaching of meteoric water after exposure after the oncoids sedimented.Combined with the analysis of carbon and oxygen isotopes,the?¹³C?PDB?of oncoids in Jinbaoshi Formation was significantly negative,reflecting organic activity in the metabolism of Girvanella was extremely high.The?¹⁸O?PDB?were all negative,reflecting the oncoids was indeed exposed to the leaching of meteoric water.By observing the thin slices of oncoids in Tuqiaozi Formation,it can be concluded that the core of oncoids is dominated by bioclastics,and the laminations are dominated by muddy laminations and algae laminations.A small amount of Girvanella and Laminaria can be seen in the laminations.Combined with the type of laminations,morphology of Girvanella filaments and the appearance of Laminaria,it is believed that the oncoids in Tuqiaozi Formation was formed in the shoals.Comparing the oncoids in the Jinbaoshi and Tuqiaozi Formation,observing thin-slice combined with analysis of main controlling factors,there are several reasons for the formation of giant oncoids in the Tuqiaozi Formation.The first is the difference in size of the oncoids core,the second is the abundance of algae,and the third is the changes in energy of water body.