1 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,Chengdu University of Technology,Chengdu 610059,Sichuan; 2 Institute of Sedimentary Geology,Chengdu University of Technology,Chengdu 610059,Sichuan
Abstract：A set of oncoidal limestone was developed in the top of Jinbaoshi Formation of Middle Devonian in Longmenshan area,Sichuan Province. Based on field profile survey and detailed observation under the microscope,the oncoidal limestone could be divided into three sedimentary cycles vertically. Each cycle was composed of oncoidal limestone and micrite. Girvanella filaments can be seen in both the core and cortex of oncoids,and they played an important role in the formation of oncoid. Combined with the types of laminae,morphology of Girvanella filaments,and the relationship between infiltration sand and laminae,it can be concluded that the oncoids were formed in bank and lagoon in the reef back. There are two types of infiltration sands, intragranular and intergranule sands,subjected to leaching of meteoric water during the exposure after the oncoids deposition. By analysis of the formation environment of oncoids,it can be concluded that the hydrodynamic condition gradually diminished from bottom to top,reflecting a gradual rise of the relative sea level at that time.
Cheng Xiao-Yu,Li Feng-Jie,Jing Xi-Gui et al. Petrologic characteristics and genesis analysis of oncoids from the Middle Devonian Jinbaoshi Formation in Longmenshan area,Sichuan Province[J]. JOPC, 2018, 20(2): 207-218.
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