Microfacies Response to the Drowning Process of the Carbonate Platform: A Case Study on the Ordovician Succession at the Taoqupo Section, Tongchuan City, Shannxi Province, North China Block
详细信息
- 责任者：Liu Cai (School of Earth-Sciences and Resources ; China University of Geosciences ; Beijing 100083 ; China) ; Qin Song
- 刊名：Geological Survey and Research
- 出版年：2013
- 卷期：36(1)
- 页码：p.23-38
- issnNo：ISSN1672-4135
- isbnNo：CN12-1353/P
摘要
During the Late Ordovician Period,the carbonate platform at the south margin of the Ordos region,North China Block,had been drowned.This drowning caused the earlier rimmed-carbonate platform into the later distal steepened-ramp ones.As the most representative section at the south margin of this platform,the Taoqupo section in Tongchuan City,Shaanxi Province,entirely recorded this drowning process.Based on the observation at the outcrop and the microfacies analysis under the thin-sections,three phases of the drowning process of the carbonate platform can be distinguished,including the pre-drowning phase,the drowning phase and the post-drowning phase.At this section,the drowning surface is the litho-stratigraphical boundary between the Jinhe Formation and the Jinsushan Formation,which indicates an abrupt change of the depositional environment from the typical reef-shoal-dominated platform into the much deeper water facies around the offshore shelf area.The pre-drowning phase is the upper part of the Jinhe Formation,which is characterized by the typical platform-marginal biohermal facies with the bio-sparite microfacies,the pelmicsparite microfacies,and the coated bioclastic grainstone microfacies,as well as the intra-spararenite microfacies;The drowning phase is a condensed depositional interval at the base of the Jinsushan Formation,mainly represented by the micrite and fossiliferous micrite in microfacies,which suggests a deeper water offshore shelf environment;The post-drowning phase is the middle and upper part of the Jinsushan Formation and the lower of the Taoqupo Formation,which was shown by the offshore shelf to the slope of the platform-margin facies with the fossiliferous micrite,the sparse biomicrite and the spiculite marlstone in microfacies.Combining with the regional data of the stratigraphic and the tectonic research,it can be concluded that this drowning event should be triggered by the ladder-shape faults developed at the margin of the Ordos carbonate platform,caused by the subduction of the Yangtze Block to the North China Block along the Qinling Orogen during the Late Ordovician Period.All the microfacies associations at the Taoqupo section,from the upper of the Jinhe Formation to the lower of the Taoqupo Formation,correspond to the complete process of this specific macroscopic drowning event in the vivid microscopic angle.