New lithological cognition of Guanyinqiao bed at Guanyinqiao section in Qijiang area,Chongqing,Southwest China
SHI Zhiqiang1,2, PENG Shenyuan1,2, ZHAO Ankun3, GE Yu1, LI Changhao1,2
1 Institute of Sedimentary Geology,Chengdu University of Technology,Chengdu 610059,China; 2 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,Chengdu University of Technology,Chengdu 610059,China; 3 Chengdu Geological Survey Center,China Geological Survey,Chengdu 610081,China
Abstract：The Upper Ordovician Guanyinqiao bed was established based on the Guanyinqiao section located in Qijiang area,Chongqing. In this section,the successive sequence of the Upper Ordovician and Lower Silurian was well developed and widely studied. The Guanyinqiao bed belongs to the uppermost part of the Ordovician strata,with plenty of typical Hirnantia fossils developed,reflecting the decline of sea level in the Hirnantian glaciation. Previous studies believed that the lithology of Guanyinqiao bed is limestone. However,the faint bubbles occur after dripping the diluted hydrochloric acid on rocks. Thin sections,scanning electron microscope(SEM),and cathode luminescence tests show that the content of dolomite in the Guanyinqiao bed is larger than 50%,while the content of calcite is less than 15%. The dolomite shows the microcrystalline-powder crystal structure,euhedral to subhedral crystal. Therefore,the lithology of Guanyinqiao bed should be revised to lime dolostone. The dolomite is nonluminous under cathode luminescence,and calcite presents the strong red light. Most of the dolomite should be ferruginous dolomite according to the SEM tests. The observation of thin sections reveals that dedolomitization occurs for some dolomites,indicating that these dolomites were replaced by calcites. The carbon isotopes in dolomites have the larger negative anomaly compared with that of brachiopod fossils and calcite veins. Furthermore,the carbon and oxygen isotope in the brachiopod fossils is inconsistent with the sea during the same period. The above studies implied that strong dolomitization occurred for the carbonates of Guanyinqiao bed during the diagenetic stage. The results from this paper is inconsistent with the previous studies on the lithology of Guanyinqiao bed in Qijiang area. The identification of dolomite is helpful in understanding the evolution of environment and geological events during the transition interval between the Ordovician and Silurian.
SHI Zhiqiang,PENG Shenyuan,ZHAO Ankun et al. New lithological cognition of Guanyinqiao bed at Guanyinqiao section in Qijiang area,Chongqing,Southwest China[J]. JOPC, 2023, 25(1): 43-55.
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