Microfacies And Sedimentary Environment Analysis Of The Ordovician Carbonates In The Dashetai Area, Nouthern Margin Of The Ordos Basin

The tectonics position of the northern margin of the Ordos Basin is located at the border of the North China plate and old Asia ocean orogenic belts. Dashetai area is located in the south side of Yinshan?s western section, and in the north of Wulate banner in Inner Mongolia. In the study area Ordovician System exposed completely, from down to up is divided into: Shanheila Formation in Lower Ordovician Series, Erhagong Formation, Wulahudong Formation in Middle Ordovician Series, and Baiyanhua Formation in Upper Ordovician series. The Dashetai town in Wulate banner in Inner Mongolia is located in most northern part of the Ordos Basin(north China platform), campared with the research degree of Ordovician System in the east, south, and west margin of the basin, the research degree of north margin is quit low.In this paper, through the method of measured 2 field profiles- Dashetai town Shanheila profile and Baiyanhua profile in Wulate banner in Inner Mongolia, and indoor microscopic observation and microfacies analysis, to explore the evolution in Ordovician of sedimentary environments and tectonic environment of Dashetai town.Through the research on ordovician carbonate rocks microfacies in the studied area, we identified 20 different microfacies in total(MF-1~MF-20). In which the Shanheila profile is divided into 14 kinds of microfacies; while the Baiyanhua profile is divided into 14 kinds of microfacies. Combined with field observations and microfacies division, and compared and divide with the Wilson Standard microfacies(SMF), we divided the sedimentary environment in the study area into six microfacies of evaporative platform facies FZ-6, restricted platform facies FZ-5, open platform facies FZ-4, platform edge shallow facies FZ-3, clinothem facies FZ-2 and basin margin facies FZ-1, and sumed up the sedimentary characteristics of every sedimentary environment.The change of depositional environment, mainly reflected in the nature of the sediment, which is the change of composition and fabric. Therefore, by comparing the sedimentary facies belt association of these 2 profiles, we think that the study area?s sedimentary environment in Heilashan period is platform facies, change from the bottom up appeared as the open platform to the restricted platform deposits, in which the open platform developed for development of oolitic limestone and calcarenite as characteristics, showing strong hydrodynamic environment; The depositional environment of Erhagong Formation is platform facies with clinothem facies, from the bottom up appeared as the open platform facies, platform edge shallow facies and evaporative platform deposits; The depositional environment of Wulahudong period is platform facies and clinothem facies, from the bottom up appeared as evaporative platform, open platform facies, restricted platform and gradual slope deposit; Baiyanhua period from the bottom up appeared as deep water basin and gradual slope deposit environment.