Geochemical Characteristics of Organic-Enriched Shales in the Upper Ordovician–Lower Silurian in Southeast Chongqing
Abstract
1. Introduction
2. Regional Geological Conditions
- First subsegment: Predominantly gray–black carbonaceous graptolite shale exhibiting an abundant distribution of graptolite fossils along layers. From bottom to top, the graptolite content progressively declines. Occasional radiolarians and a small amount of sponge bone needle fossils are visible alongside various types of pyrite. Horizontal bedding in shale is prominent.
- Second subsection: This section is composed mainly of black–gray carbon-containing silty mudstone with tiny areas of dark-gray carbon-containing argillaceous siltstone. Silty and argillaceous stripes are distributed within the shale matrix. This subsection contains a certain amount of graptolites and a smaller quantity than the first subsegment. Pyrite mainly occurs in laminae.
- Third subsection: The lower lithology consists primarily of gray–black graptolite carbonaceous shale that transitions to the upper part, which is characterized by black–gray silt-bearing mudstone with a less developed horizontal bedding structure. Graptolites are present throughout this section along with a small amount of radiolarians; however, powdery pyrite only exists in the lower portion.
3. Samples and Test Methods
4. Results
4.1. Characteristics of the Mineral Content
4.2. Major Element Geochemistry
4.3. Trace Element Geochemical Data
5. Discussion
5.1. Provenance Analysis
5.1.1. Ancient Weathering and Sedimentary Recycling
5.1.2. Type of Source Rock
5.2. Paleoenvironment
5.2.1. Paleoclimate
5.2.2. Paleo–Redox Conditions
5.2.3. Ancient Productivity Level
5.2.4. Water Retention Environment
5.3. Enrichment Mechanism of Organic Matter
6. Conclusions
- (1)
- Geochemical indicators revealed a weak degree of weathering and negligible sedimentary recycling. The provenance of the Wufeng–Longmaxi Formations primarily consisted of felsic acidic rocks, with minor contributions from ancient sedimentary rocks.
- (2)
- A suite of paleoclimate proxies, including Sr, Sr/Cu, and CIA, clarified the transition from a moist to a dry climate in the Wufeng–Longmaxi phase. Moreover, the geochemical characteristics imply that the deposition of the Wufeng–Longmaxi Formations occurred in an oxygen-deprived paleoenvironment with moderate–high ancient productivity and a moderately–strongly restricted setting.
- (3)
- The organic enrichment mechanism of the Wufeng–Longmaxi Formation is different. In the case of the Wufeng Formation, the accumulation of organic matter is attributed primarily to strongly restricted conditions, resulting in significant anoxic environments. Furthermore, the correlation between paleoredox conditions and TOC further reinforces the idea that these conditions are the crucial factor influencing the organic accumulation in the Longmaxi Formation.
Author Contributions
Funding
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Position | Sample Number | Mineral Contents (w,%) | ||||||||
---|---|---|---|---|---|---|---|---|---|---|
Quartz | K-Feldspar | Plagioclase | Calcite | Dolomite | Siderite | Pyrite | Gypsum | TCCM | ||
O3w | YY-1 | 45 | 9 | 8 | 3 | 2 | 0 | 3 | 0 | 30 |
YY-2 | 55 | 8 | 7 | 0 | 4 | 0 | 2 | 0 | 24 | |
YY-3 | 69 | 4 | 5 | 0 | 5 | 0 | 3 | 0 | 14 | |
O3g | YY-4 | 50 | 4 | 6 | 23 | 5 | 0 | 2 | 0 | 10 |
S1l | YY-5 | 52 | 6 | 8 | 13 | 5 | 0 | 2 | 0 | 14 |
YY-6 | 51 | 8 | 12 | 8 | 5 | 0 | 2 | 0 | 14 | |
YY-7 | 50 | 7 | 11 | 8 | 6 | 0 | 2 | 0 | 16 | |
YY-8 | 49 | 8 | 11 | 11 | 5 | 0 | 2 | 2 | 12 | |
YY-9 | 37 | 9 | 25 | 9 | 5 | 0 | 1 | 5 | 9 | |
YY-10 | 45 | 6 | 20 | 5 | 3 | 0 | 2 | 2 | 17 | |
YY-11 | 44 | 8 | 15 | 4 | 2 | 0 | 0 | 2 | 25 | |
YY-12 | 52 | 7 | 13 | 10 | 4 | 0 | 0 | 0 | 14 | |
O3w | PS-1 | 36 | 6 | 7 | 8 | 3 | 11 | 0 | 0 | 29 |
PS-2 | 58 | 6 | 8 | 0 | 3 | 0 | 2 | 0 | 23 | |
PS-3 | 65 | 3 | 4 | 4 | 5 | 0 | 2 | 3 | 14 | |
O3g | PS-4 | 54 | 6 | 7 | 6 | 3 | 0 | 5 | 2 | 17 |
S1l | PS-5 | 50 | 8 | 9 | 6 | 6 | 0 | 4 | 0 | 17 |
PS-6 | 46 | 8 | 14 | 6 | 5 | 0 | 3 | 0 | 18 | |
PS-7 | 47 | 8 | 17 | 5 | 4 | 0 | 2 | 0 | 17 | |
PS-8 | 46 | 10 | 21 | 5 | 3 | 0 | 1 | 0 | 14 | |
PS-9 | 47 | 8 | 21 | 4 | 2 | 0 | 1 | 0 | 17 | |
PS-10 | 43 | 10 | 23 | 6 | 3 | 0 | 1 | 0 | 14 | |
PS-11 | 43 | 7 | 17 | 4 | 0 | 0 | 2 | 0 | 27 | |
O3w | XS-2 | 55 | 6 | 14 | 0 | 4 | 0 | 0 | 0 | 21 |
O3g | XS-3 | 46 | 0 | 10 | 20 | 11 | 0 | 1 | 0 | 12 |
S1l | XS-4 | 46 | 6 | 15 | 12 | 5 | 0 | 2 | 0 | 14 |
XS-5 | 44 | 11 | 21 | 9 | 4 | 0 | 1 | 0 | 10 | |
XS-6 | 46 | 10 | 22 | 10 | 3 | 0 | 0 | 0 | 9 | |
O3w | QL-2 | 57 | 5 | 14 | 8 | 3 | 0 | 0 | 0 | 13 |
QL-3 | 71 | 2 | 8 | 5 | 2 | 0 | 1 | 0 | 11 | |
O3g | QL-4 | 39 | 0 | 5 | 0 | 48 | 0 | 1 | 0 | 7 |
S1l | QL-5 | 49 | 3 | 7 | 0 | 30 | 0 | 3 | 0 | 8 |
QL-6 | 51 | 4 | 18 | 0 | 6 | 0 | 0 | 0 | 21 | |
QL-7 | 52 | 0 | 18 | 0 | 3 | 0 | 0 | 0 | 27 |
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Fu, C.; Feng, Z.; Xu, C.; Zhao, X.; Du, Y. Geochemical Characteristics of Organic-Enriched Shales in the Upper Ordovician–Lower Silurian in Southeast Chongqing. Minerals 2025, 15, 447. https://doi.org/10.3390/min15050447
Fu C, Feng Z, Xu C, Zhao X, Du Y. Geochemical Characteristics of Organic-Enriched Shales in the Upper Ordovician–Lower Silurian in Southeast Chongqing. Minerals. 2025; 15(5):447. https://doi.org/10.3390/min15050447
Chicago/Turabian StyleFu, Changqing, Zixiang Feng, Chang Xu, Xiaochen Zhao, and Yi Du. 2025. "Geochemical Characteristics of Organic-Enriched Shales in the Upper Ordovician–Lower Silurian in Southeast Chongqing" Minerals 15, no. 5: 447. https://doi.org/10.3390/min15050447
APA StyleFu, C., Feng, Z., Xu, C., Zhao, X., & Du, Y. (2025). Geochemical Characteristics of Organic-Enriched Shales in the Upper Ordovician–Lower Silurian in Southeast Chongqing. Minerals, 15(5), 447. https://doi.org/10.3390/min15050447