Microfacies And Facies Analysis Of The Ordovician Carbonates In The South Margin Of The Ordos Basin

The study area is located in the southern margin of the Ordos Basin, includePingliang-Jingyang-YaoXian region， which across the Shaanxi and Gansuprovinces.That is located in the east of Liupanshan moutain， north of Qinlin.Itsevolution is mainly effected by the evolution of Qin Ling Ocean.Ordovician carbonate rocks are well-developed and various depositional typescan be recognized in the Pingliang-Jingyang-Yaoxian area,which is located in thesouth margin of the Ordos Basin. Based on the observation at the outcrop and indoormicrofacies analysis,combined the theory of sedimentology, petrology, etc.10microfacies can be divided, as Pingliang has9species,Jingyang has9species andYaoxian has6species. According to these microfacies’features of the vertical andhorizontal combination and reference Wilson’s (1975) carbonate sedimentary faciesmodel,5sedimentary facies belts can be summarized according to the microfaciescombinate as restricted platform, open platform, slope facies, sloped footing and deepshelf. The combination characteristics of the sedimentary facies belts indicate thatthere are several transgression to regression occurred during the Ordovician. Thecomparison of sedimentary facies of Ordovician System in the study area indicate thatsea shallowed from west to east, which can be infer that Majiagou Formationtransgression direction from south to the north. In the deposition of the PingliangFormation, regression occurred the whole Ordos Basin,but the water in the southmargin of the Ordos Basin had suddenly deepened. And regression in this memberoccured at deposition of Beiguoshan Formation. That’s may be related to QinLingyang’s evolution.Which is means in the6thMajiagou Formation,influenced by theCaledonian movement, Yangtze plate subduction to the north and the southern marginof the ordos basin,Southern margin of the basin area from the passive continentalmargin at the early stage of the early ordovician turned to a typical groove-arc basinsystem of active continental margin.