Characteristics of rare earth elements in Lower Cambrian Qiongzhusi Formation in northeastern Sichuan Basin and its geological implications: a case study of Yangba section, Nanjiang

The Qiongzhusi Formation is an important shale gas production layer in the Sichuan Basin. A case study was made in the Yangba section of the northeastern Sichuan Basin. Samples were collected for rare earth and trace element analyses from the Lower Cambrian Qiongzhusi Formation in the study area so as to reveal the tectonic settings, provenance features and sedimentary environment when favorable source rocks were deposited. The distribution pattern and parameter characteristics of rare earth elements indicated that the Qiongzhusi Formation had a relatively consistent material source, mainly a passive continental margin. The parent rocks were mainly composed of sedimentary rocks and granites. When carbonaceous shale was deposited at the bottom of the Qiongzhusi Formation, the content of clay minerals increased in land sourced debris.The δCe and V/Cr parameters indicated that the bottom of the Qiongzhusi Formation was deposited in an anoxic reducing environment, while the middle and upper parts were deposited in a dysoxic-suboxic environment.