# Re-Understanding the Sedimentary Environment of the Wufeng-Longmaxi Shales in the Sichuan Basin
## 3 The Sedimentary Environment of the WLS in the Sichuan Basin
### Enclosure and Salinity of Water Bodies  
Abundant geochemical evidence suggests that the sedimentary environment of Wufeng-Longmaxi shales is a closed lagoon. [PERSON] (1991) [28] utilized the C/S paleosalinity method to analyze the black ggrptolite shales of the Wufeng-Longmaxi Formation in the Northern Sichuan Basin, concluding that this period represented a desalination sea. Some sampling points are indicated by the five-pointed-star symbol in Figure 5. The black ggrptolite shales of the Wufeng-Longmaxi Formation in the upper Yangtze region are characterized as semi-closed sedimentary products formed in an anoxic environment within a shallow continental water stagnant sea basin [26; 27; 28]. Barnett shales exhibit similarities to Longmaxi shales, as both were developed in closed basins [82]. Additionally, [PERSON] et al. (2018) [83] demonstrated that the shales of the Wufeng Formation were deposited in a semi-enclosed, large bay-like stagnant basin environment and proposed that at this time, the \"Yichang area was undergoing tectonic uplift\".  
The degree of sealing in sedimentary environments can be reflected by geochemical indicators observed through vertical changes. [PERSON] et al. (1993) [38] discovered that the Sr/Ba ratio of ggrptolite-bearing shales at the base of the Daduhe Formation and Longmaxi Formation at Jiaodingshan in Hanyuan County is less than 0.15, indicating a low-salinity lagoon environment. On a broader scale, the Sr/Ba values of the middle Ordovician Shizipu Formation, Upper Ordovician Baota Formation, and Linxiang Formation in the Guanyinqiao section of Qijiang, Chongqing, range from 2.0 to 5.75, from 2.12 to 5.62, and from 0.78 to 1.37, respectively [84]. These values gradually decrease to 0.15 in the Wufeng Formation, indicating a historical environmental transition characterized by offshore regression, moving from distant to proximal areas and from deep to shallow waters, as depicted in Figure 8 with square blue lines. This trend aligns with the fluctuations in sea level illustrated in Figure 6. In this study, field sampling and a trace element analysis of Linxiang Formation limestone and Wufeng-Longmaxi shales were conducted in the Huangying Township, Wulong, Chongqing, as indicated by the triangle marks in Figure 8 and Table 1.  
As can be seen from Table 1, the Sr/Ba of the shale segment is less than 0.5, indicating a low-salinity water environment. The lower limestone has normal salinity, Sr/Ba > 0.5. U/Th > 1.25 and V/Cr > 4.25 generally indicate hypoxia conditions. Half of the former index did not meet this requirement, and three samples of the latter did not meet the hypoxia conditions. Therefore, an anoxic environment is not necessary for the development of black shales.  
The final conclusion is that the limestone of the Linxiang Formation developed in an environment with a slightly normal salinity, whereas ggrptolite-bearing shales developed in a more enclosed and freshwater-influenced marine environment.  
Based on the previous discussion, this study proposes that the sedimentary environment rich in organic matter during the shale deposition period exhibits four key characteristics: it is shallow, continental, closed, and cold. Specifically, the water body is shallow, located close to the land, possesses effective sealing properties, and is characterized by a cold and wet climate.