Sedimentary Environment and Organic Accumulation of the Ordovician–Silurian Black Shale in Weiyuan, Sichuan Basin, China
Abstract
:1. Introduction
2. Geological Setting
3. Materials and Methodology
4. Experimental Results
4.1. Sedimentological and Petrological Characteristics
4.2. Total Organic Carbon and Sulfur Content
4.3. Major Element
4.4. Trace Element
5. Discussion and Implications
5.1. Redox Condition
5.2. Paleo-Productivity
5.3. Degree of Water Restriction in Basin
5.4. Tectonic Environment Analysis
5.5. Detritus Input
5.6. Organic Enrichment Characteristics and Controlling Factors
5.7. Organic-Rich Black Shale Development Model
- During the early sedimentation of the Wufeng Formation, the Cathaysia and Yangtze plates had not fully amalgamated. A vast ocean exited at the southeastern edge of the Yangtze Plate, connecting the Yangtze continental shelf and the open ocean. Therefore, the reducing conditions were weak, and the abundance of organic matter was low.
- As the plate amalgamation movement intensified in the later stages of the Wufeng Formation sedimentation, the water body had poorer connectivity with the open ocean. The sediments were predominantly muddy and devoid of biological disturbances, forming an anoxic sedimentary environment. Furthermore, several volcanic activities provided abundant nutrients for plankton, elevating the paleo-productivity level. Consequently, the mudstone of the later Wufeng Formation sedimentation had a higher organic matter abundance.
- During the Guanyinqiao period, global sea levels fell, leading to the proliferation of cold-water animal groups like bivalves and brachiopods, which resulted in an oxidizing water environment. Although the paleo-productivity level was high during this period, the oxidizing environment was inconducive to organic matter preservation.
- After the end of the Hirnantian glaciation, sea levels rapidly rose again. The tectonic pattern of the Yangtze Plate was still similar to that of the late Wufeng Formation, an enclosed and restricted sea basin. The high sea level and the restricted tectonic background of the sea basin provided abundant organic matter sources, creating large-scale anoxic environments on the seafloor, which provided favorable conditions for effective organic matter preservation. Intense tectonic activities increased the terrigenous detritus input, bringing terrestrial organic matter into the seabed and increasing the organic matter abundance in the sediments. In the early stage of the Longmaxi Formation sedimentation, high paleo-productivity and strong reducing conditions promoted organic matter enrichment.
- In the later stages of Longmaxi Formation sedimentation, as terrigenous detritus input continued to increase, the sedimentary water continuously shallowed. The depositional environment of the study area gradually transitioned from deep-water shelf deposition to shallow-water shelf deposition, forming more siltstone and claystone shales. The hypoxic/anoxic environment of the sediments was disrupted. The low level of paleo-productivity also affected organic matter accumulation, resulting in a low abundance of organic matter.
6. Conclusions
- The value of P/Al indicators reveal that the black shales of the Wufeng–Longmaxi formations were deposited under high paleo-productivity levels. High organic carbon fluxes provided the material basis for organic matter development and enrichment in the shales.
- The values of V/(V + Ni), V/Cr, and Ni/Co indicated that the shales of the Wufeng–Longmaxi Formations were primarily formed in an anoxic sedimentary environment. The anoxic conditions correlate well with the organic carbon content, making it crucial for controlling organic matter enrichment.
- The content of Al and Ti indicated that terrigenous detritus has some impact on organic matter enrichment. Detritus noticeably increased in the late Longmaxi period, reducing organic matter concentration.
- Tectonic activity and glaciers melting due to global warming are crucial for black shale development. Tectonic activity played a dominant role in controlling the initial paleo-productivity and redox conditions of shale formations. The Longmaxi Formation was also influenced by widespread marine transgression after the glaciation, which lasted longer and had a greater sediment thickness.
Author Contributions
Funding
Data Availability Statement
Conflicts of Interest
References
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XEF | VEF | NiEF | MoEF | UEF | CoEF | |||||
---|---|---|---|---|---|---|---|---|---|---|
Range | Average | Range | Average | Range | Average | Range | Average | Range | Average | |
Wufeng Formation | 1.5–7.2 | 4.7 | 1.5–7.9 | 4.6 | 3.2–188.5 | 78.2 | 3.08–23.8 | 12.7 | 0.5–2.1 | 1.1 |
Longmaxi Formation | 0.7–3.9 | 2.7 | 1.5–3.6 | 2.6 | 15.3–69.4 | 39.5 | 4.3–20.8 | 8.5 | 0.8–1.3 | 1.0 |
Sedimentary Environment Discrimination Indicators | Anoxic Environment | Oxidizing Environment | |
---|---|---|---|
Anaerobic | Hypoxic | ||
V/(V + Ni) | >0.6 | 0.45–0.6 | <0.45 |
V/Cr | >4.25 | 2–4.25 | <2 |
Ni/Co | >7 | 5–7 | <5 |
U/Th | >1.25 | 0.75–1.25 | <0.75 |
S/C | high, >0.36 | low, <0.36 |
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Fu, W.; Hu, W.; Cai, Q.; Wei, S.; She, J.; Wang, X.; Liu, X. Sedimentary Environment and Organic Accumulation of the Ordovician–Silurian Black Shale in Weiyuan, Sichuan Basin, China. Minerals 2023, 13, 1161. https://doi.org/10.3390/min13091161
Fu W, Hu W, Cai Q, Wei S, She J, Wang X, Liu X. Sedimentary Environment and Organic Accumulation of the Ordovician–Silurian Black Shale in Weiyuan, Sichuan Basin, China. Minerals. 2023; 13(9):1161. https://doi.org/10.3390/min13091161
Chicago/Turabian StyleFu, Wei, Wangshui Hu, Quansheng Cai, Sile Wei, Jiachao She, Xiaochen Wang, and Xiaodong Liu. 2023. "Sedimentary Environment and Organic Accumulation of the Ordovician–Silurian Black Shale in Weiyuan, Sichuan Basin, China" Minerals 13, no. 9: 1161. https://doi.org/10.3390/min13091161
APA StyleFu, W., Hu, W., Cai, Q., Wei, S., She, J., Wang, X., & Liu, X. (2023). Sedimentary Environment and Organic Accumulation of the Ordovician–Silurian Black Shale in Weiyuan, Sichuan Basin, China. Minerals, 13(9), 1161. https://doi.org/10.3390/min13091161