The Lower Carboniferous Formed in the Special Shallow-Water Environment at the Northeast Margin of the Dianqiangui Basin: A Case Study from the Mengdong Section of the Huanjiang

In order to explore the characteristics of the Lower Carboniferous formed between the deep-water environment and the shallow-water environment in the northeast margin of the Dianqiangui Basin, taking the section of Mengdong in Huanjiang as an example. According to the sedimentary characteristics and stratigraphic cyclic features of the Lower Carboniferous at the Mengdong section, the sequence division and sedimentary facies identification of the Lower Carboniferous at the Mengdong section of the Huanjiang section were carried out. In addition, through the comparison with the deep-water and shallow-water environments, it is found that the of the Lower Carboniferous Mengdong section is located in the transition area between the deep-water environment and the shallow water environment, and its sedimentary environment and sequence stratigraphy have particularities. Findings: (1) The Lower Carboniferous of the Mengdong section is composed of a special sedimentary sequence of “Yaoyunling Formation + Yingtang Formation + Huangjin Formation + Simen Formation + Luocheng Formation”, which can be divided into three tertiary sequences SQ1, SQ2 and SQ3. (2) Two types of sedimentary facies can be identified in the Mengdong section of Huanjiang section: delta facies and shelf facies. Among them, delta facies can be divided into three facies: pro-delta facies, delta front facies, and delta plain facies. (3) The Lower Carboniferous at the Mengdong section is different from the Lower Carboniferous dominated by dark shale in the deep-water environment because it contains large sets of carbonate rock Strata stratum such as Yaoyunling Formation and Luocheng Formation. It is also distinguished from the carbonate-dominated Lower Carboniferous Strata stratum by the presence of deep-water sediments of the Yingtang Formation and the sea-land transition facies sediments of the Simen Formation, which are found in the shallow-water environments. The Lower Carboniferous at the Mengdong section of Huanjiang is formed in the transition area between deep-and shallow- water environments, and this special sedimentary environment and sequence stratigraphy can provide new materials and important clues for the accurate construction of the sequence stratigraphic framework of the Lower Carboniferous in the Yunnan-Guizhou-Guangxi area.