| The southwestern part of Sichuan Basin is located on the southwestern margin of the Yangtze platform. The Dengying formation in local area developed a set of microbial carbonate sediments. On the basis of the former studies, field profile, core observation and microscopic identification, after researching stratigraphic features and petrologic features in Xianfeng profile and Jinshi No.1 well, the Dengying formation can be divided into 53 layers(including layer 0): the member 1 of Dengying formation was divided into 18 layers, dominated with dolomicrite and crystal powder dolomite. The member 2 of Dengying formation was divided into 21 layers, including stromatolite dolomite and leopard porphyritic clotted dolomite. The member 3 of Dengying formation was divided into 14 layers, mainly consist of dolomite with lamellar stone and leopard porphyritic clotted dolomite. According to the stratigraphic classification, the Dengying formation in study area contained two types of sediments, named little-non microbial dolomite and microbial dolomite, and little-non microbial dolomite can be further divided into particle dolomite and crystal grain dolomite.According to composition, structure and construction characteristics, the number of microbial content, the difference on ecological characteristics of thees micro-organisms, and all the sediments they provided, microbial dolomite can be further divided into 10 types, including horizontal-shaped stromatolite dolomite, wavy-bedding stromatolite dolomite, mound-shaped stromatolite dolomite, columnar stromatolite dolomite, coniform stromatolite dolomite, irregular stromatolite dolomite, dolomite with lamellar stone, leopard porphyritic clotted dolomite, broken-shaped clotted dolomite or arena microbial dolomite and oncolite dolomite.Through the analysis of C, O isotope and the absolute content of trace elements Mn2+ and Fe2+ as well as the relative content reflected by cathode luminescence, the depositional environment can be easily conjectured. In this paper, the diagram of δ13C-Z was the key approach to indicate the depositional environment. The microbial carbonates in study area share the value of δ13C-Z with diagenetic environment in sea water with higher δ13C and Z. The Z was between 127.25 and 134.75, obviously higher than 120, which indicated marine deposition(tidal deposition) where is under high salinity. Cathode luminescence showed orange to orange-red light, which suggested higher Mn2+ with lower Fe2+, corresponded to the absolute content of Mn2+ and Fe2+. It reflected the depositional environment has little atmospheric water to supply Fe2+ and it should belong to the marine deposition(tidal deposition).The microbial dolomite in Dengying formation mainly developed stromatolithic structure, clotted texture, microbioclastic structure and oncolite structure, which can be the key mark to characterize the depositional environment of microbial dolomite. According to geochemical characteristics, cathode luminescence and the analysis of microfacies, the microbial dolomite depositions of Dengying formation in study area can be identified six kinds of microfacies, including the supratidal zone, the upper intertidal zone with low-energy, the lower intertidal zone with high-energy, the subtidal lagoon, subtidal barrier beach and subtidal low-energy zone. These six kinds of sedimentary microfacies were in an orderly combination by time and space, constituting the tidal flat subfacie depositions of Dengying formation in restricted platform. According to the changes of sedimentary facies in local area, the sedimentary model of microbial carbonates in Dengying formation can be established. The evolution of sedimentary facies model was well-controlled by the cycle of sedimentation, where the relative sea level was shown as double regressive depositon to transgressive deposition. It provided the key information for geological research and the exploration of oil and gas. |
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