Some “calcareous concretion” were found in the oil shale of Chang 7 section of Yanchang Formation in the outcrops of the Tongchuan area, the southern part of the Ordos Basin. These “calcareous concretion” were layered developed in the oil shale of the Chang 7 section in the outcrops, in the form of “lenticular”,“ellipsoid”,“discoid”, about 10～70 cm in diameter. In order to identify the rock type and main rock components, the petrological characteristics of these concretion were analyzed by microscope observation of section, electron microprobe experiment and EDS analysis. On this basis, the origin of these “calcareous concretion” were preliminarily discussed. The “calcareous concretion” in Bawangzhuang and Taotan field section of Tongchuan area are mainly composed of recrystallised calcite and crystalline calcite, with “globular” structure, and its rock type is limestone. The rock type of “calcareous concretion” in Niejiahe field section are silt-micrite dolostone. The results show that these “calcareous concretion” in the field occurence and the composition of the rocks meet the characteristics of hydrothermal exhalative deposition of carbonate rocks and hydrothermal activities. And the Tongchuan area had the condition of hydrothermal activity in the Chang 7 period. It is tentatively believed that these “calcareous concretion” are lacustrine carbonate rocks that contribute to the hydrothermal activity.
Ma Yao Li Wenhou Liu Jiangbin Li Zhichao. A preliminary discussion on the origin of “calcareous concretion” in oil shale of Yanchang Formation in Tongchuan area, southern Ordos Basin[J]. Chinese Journal of Geology, 2018, 53(2): 458-469.