| Citation: |
Jun-ping Liu, Si-cun Song, Wei Wang, Feng Tang, Jing Li, Xiang-dong Duan, Xiao-hu Wang, Bai-dong Sun, Sai-ying Yu, Shao-bin Hu, Wen-ting Duan, 2020. Protoconodont fossils for refining the Cambrian bottom and the contribution to shale gas formation along the southwest margin of Yangtze Block, China Geology, 3, 558-566. doi: 10.31035/cg2020063
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Protoconodont fossils for refining the Cambrian bottom and the contribution to shale gas formation along the southwest margin of Yangtze Block
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Abstract
It has been an intense debate on the exact boundary between Ediacaran and Cambrian in the southwest Yangtze Block. The calibration of this critical boundary has a remarkable influence on the further investigation of the break-up of the Rodinia Supercontinent, the early life evolution, and the mechanism of the phosphorite deposit. Ediacaran and Cambrian strata and fossils are widely distributed in Anning, Yunnan Province in China. In recent years, the Xiaowaitoushan Member from the Lower Yuhucun Formation has been studied. Through this interval with continuous collections, the first appearance datums (FADs) of the protoconodont (Fomitchella cf. inchoate Yang et He, Protohertzina cf. anabarica Missarzhevsky) and globular embryos fossil (Olivooides sp.) earlier than these in the Lower Cambrian strata of the Meishucun Formation were discovered. This discovery indicates that the Xiaowaitoushan Member has included more FADs than the previously discovered single FAD of Anabarites primitivus Qian et Jiang, and the Ediacaran–Cambrian boundary in southwest China should be replaced below the Point “A” of the Meishucun Formation in Yunnan Province. The Point “B” of the Meishucun Formation is younger than the suggested age 541 Ma of the Ediacaran–Cambrian boundary and can no longer reference the Global Boundary Stratotype Section and Point (GSSP) correlation in southwest China. It can be suggested based on the previous stratigraphy and palaeontology studies from northern Sichuan and southern Shaanxi and the FAD of the globular embryos fossils that the Ediacaran‒Cambrian boundary in the southwest Yangtze Block should be placed at the base of the Xiaowaitoushan Member; other phosphorite strata refer to Xiaowaitoushan Member. The discovery of the FADs of the shelly fossils in the Xiaowaitoushan Member provides new evidence for the global correlation of the Ediacaran–Cambrian boundary in the southwest Yangtze Block. The conodont discoloration index (CAI) of the specimens in Anning is between 2 and 3, which indicates that the organic matter in Xiaowaitoushan Member is matured and has high potential to form a shale gas reservoir.
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References
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Jun-ping Liu, Si-cun Song, Wei Wang, Feng Tang, Jing Li, Xiang-dong Duan, Xiao-hu Wang, Bai-dong Sun, Sai-ying Yu, Shao-bin Hu, Wen-ting Duan, 2020. Protoconodont fossils for refining the Cambrian bottom and the contribution to shale gas formation along the southwest margin of Yangtze Block, China Geology, 3, 558-566. doi: 10.31035/cg2020063
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Figure 1.
Tectonic division sketch of the south China continent (a), geological sketch map of central Yunnan area (b) and geological sketch map and sampling locations (c) of the Anning area, central Yunnan.
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Figure 2.
Stratigraphic division of the Damaidi section in Anning area, central Yunnan (modified from Luo HL et al., 1984).
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Figure 3.
Protoconodonts and globular embryo fossils from Anning area, Yunnan Province. A1–A4–Fomitchella inchoate Yang et He, 1984. A1–lateral view; A2–front view; A3–posterior view; A4–posterior view. B1–B2–Protohertzina cf. anabarica Missarzhevsky, 1973. B1–B2–side view. C1–C2, D1–D2, the radial structure on the surface of Olivooides. E, Olivooides individuals with tubular bodies. All specimens are collected from the Xiaowaitoushan Member of the Yuhucun Formation in Anning area, Yunnan Province.