The redox level is a key factor in the preservation of organic matter. Redox-sensitive elements Mo, U, V, Cr, Ni, and Co are commonly used for the reconstruction of bottom water redox environments (Tribovillard et al., 2006). These elements are prone to precipitate in sediment under reducing conditions, while they are prone to dissolve in water under oxidizing conditions (Tribovillard et al., 2012). In order to weaken the dilution effect of terrestrial debris, the ratio of element content to Al content was used as an indicator in this study. Figure 4 shows that the reducibility of bottom water in both Eastern and Southern Sichuan increases rapidly during the TST1 period, and reaches its maximum in the early TST2 period before slowly decreasing.

In order to quantify redox levels in different system tracts and areas, the cross-plots of Th/U-Ni/Co, V/Cr-V/(V + Ni), and Mo_EF-U_EF were applied (Figures 5A–C). In the Southern Sichuan Basin, the data points of TST1 reflect a gradual transition from suboxic to anoxic, sample distribution area and data points exhibit stronger reduction of anoxic bottom water in the early TST2 with some intervals even reaching the euxinic condition, while gradually decrease to weak anoxic or suboxic conditions during the late TST2 and HST2 period. The TST1 period in the Eastern Sichuan Basin is mainly in a anoxic environment, and the bottom water reduction in the TST2 period is also reach its maximum, manifested by some data points being close to the euxinic zone. In addition, a small number of data points in the late TST2 period near the suboxic zone may be related to a decrease in the degree of late reduction. The reducing conditions in HST2 continued slowly decreasing after a slight increase, and it remained mainly in a weak anoxic environment in the early period.

Mo/TOC can be used to determine the degree of limitation of water bodies (Algeo and Lyons, 2006; Algeo and Tribovillard, 2009). The data points of TST1 in the Southern Sichuan Basin are relatively scattered (Figure 5D), suggesting an unstable degree of water limitation. Considering the continuous uplift of the surrounding paleo-uplift and the anoxic bottom water environment, the Mo/TOC ratio is mostly less than 4.5, suggesting that the water is strongly restricted in the middle and later periods of TST1. The Mo/TOC enclosed area in the TST2 period is widely distributed, but mainly in a moderately enclosed environment. Its changing characteristics also indicate that as the degree of transgression increases, the degree of restriction gradually strengthens. During the HST2 period, the reducibility of water decreased, but the degree of restriction was significantly reduced in the early suboxic-anoxic environment. The TST1 data points in the Eastern Sichuan are relatively scattered, with Mo/TOC values mostly greater than 4.5, indicating moderate restriction. The limitations of TST2 period have changed significantly, with some data points having Mo/TOC ratios less than 4.5, indicating a significant increase in restriction during certain intervals, but still in a moderately restricted environment. The degree of restriction in HST2 period decreases with the decrease of sea-level.

The content of Ba, Cu, Ni, and Zn is closely related to the life activities of marine organisms, Ba_bio and (Cu + Ni + Zn)/Al can serve as reliable indicators for evaluating productivity. The Ba_bio in Figure 6 reflects that the paleo-productivity of the Eastern Sichuan showed an overall increasing trend during the TST1-HST2 period, while showed an increasing trend during the TST1-TST2 period and a decreasing trend in HST2 in Southern Sichuan. The (Cu + Ni + Zn)/Al indicates that the paleo-productivity of these two regions all showed a unimodal distribution, increasing during the TST1 period and reaching its peak in the early TST2 period before slowly decreasing.

The Si_excess in the black shale of the Wufeng-Longmaxi Formation is related to biogenic origin, so this study also used Si_excess to evaluate paleo-productivity (Cai et al., 2022). There is a negative correlation between Si_excess and Al (Figure 5E), ruling out the possibility of terrestrial input and clay mineral transformation to form Si_excess. The positive correlation between K₂O-Rb (Figure 5F) indicates that the source of Si_excess is not related to magmatic activity or provenances (Floyd and Leveridge, 1987; Huang et al., 2023). The Al-Fe-Mn data (Figure 5G) points all fall into the non hydrothermal zone, indicating that the formation of Si_excess is independent of hydrothermal activity (Xie et al., 2021). The positive correlation between Si_excess and TOC (Figure 5H) confirms its correlation with biological sources. The Si_excess in the Eastern Sichuan generally maintains a trend of first increasing and then decreasing from TST1 to HST2, with the highest paleo-productivity values distributed in the early period of TST2. The peak of the total amount of Si_excess in Southern Sichuan also appeared in the early period of TST2 and gradually decreased thereafter.

Due to the fact that Ba_bio is not suitable for oxygen-deficient environment, while the mass fraction of Cu, Zn and Ni is closely related to the sedimentation amount of organic matter, and suitable for reducing environment. As a result, there is no consistency in these indicators, and the reasons for this can be fully explained. As for Ba_bio, sulfate reduction reactions are common on the surface and/or bottom of sediments in anoxic and euxinic environments during TST1 and HST2, barium sulfate is a latent source of sulfate, and partial dissolve when sulfate supply is insufficient, resulting in a lower content of barium and estimated productivity. On account of the eastern and southern Sichuan are mainly in a relatively reducing environment, (Cu + Ni + Zn)/Al is basically consistent with the change trend of Si_excess, and that also verified the accuracy of the our results. In conclusion, the paleo-productivity during the depositional process of TST1 and TST2 in Eastern and Southern Sichuan is stronger than that of HST2. In addition, the Si_excess and (Cu + Ni + Zn)/Al in Eastern Sichuan is generally higher than that in Southern Sichuan, reflecting the strong paleo-productivity of the former.

Al and Zr elements are not easily migrated during transportation and can be used as effective indicators to characterize the intensity of terrestrial inputs. Al is mainly came from fine-grained aluminosilicate clay minerals, while Zr existed in clay minerals and coarse-grained minerals (quartz, zircon, etc.). Therefore, Zr/Al ratio is considered an effective indicator for quantifying the content of coarse-grained debris in terrestrial inputs. Figure 7 indicates that the changes of Zr and Al curves in the Eastern and Southern Sichuan is basically consistent, showing a gradually decreasing trend during the TST1 and HST1 periods, as well as a fluctuating trend during the TST2 period and a gradually increasing trend during the HST2 period. The variation of Al contents suggest that the total terrestrial input amount of fine-grained clay minerals are similar in this two areas. However, the overall Zr content in the Eastern Sichuan Basin is lower than that in the Southern Sichuan Basin, especially during the TST1 period with rapid drop. This indicates that the difference in terrestrial input during the TST1-HST2 sedimentary period between the Eastern and the Southern Sichuan is relatively small, but the input of coarse-grained debris is lower during the TST1-HST2 period in the eastern Sichuan.

Zr/Al in the Eastern Sichuan showed a slow decreasing trend during TST1-TST2, suggesting a significant slowdown in the input rate of coarse-grained debris, while a slow increase during HST2 indicates a faster input of coarse-grained debris. However, the input of coarse-grained debris in the Southern Sichuan Basin remained relatively stable during the TST1-TST2 period, while it also increased during the HST2 period. In addition, the Zr/Al values in the Southern Sichuan were higher than those in the Eastern Sichuan at all periods. This indicates that the input rate of coarse-grained debris in the sediment composition of the Eastern Sichuan Basin is generally lower than that of the Southern Sichuan, especially during the HST2 deposition period. During this period, the development of more mixed shale in the Southern Sichuan can be confirmed. In the late period of TST2, the Al content gradually increased, but the Zr/Al and Zr contents showed a slow decreasing trend in fluctuation. It is speculated that the rapid increase of fine-grained clay minerals diluted coarse-grained minerals, resulting in an opposite trend of Al, Zr, and Zr/Al.

The Chemical Index of Alteration (CIA) is often used to indicate the climate and physical and chemical weathering intensity of provenance region. CIA values of 50–65, 65–80, and 80–100 indicate weak, moderate, and strong weathering degrees, respectively, and correspond to cold-dry, warm-humid, and hot-humid conditions, respectively. In the actual application process, in order to exclude the influence of CaO content that is bound to non-silicates, the corrected CIA (CIA *) was calculated in this study (Figure 5I; Figure 7). The lowest values of CIA * in both the Eastern and Southern Sichuan appear at HST1, indicating a global temperature drop during the Hirantanian glaciation. The CIA * value in the Southern Sichuan ranges from 60 to 75. During the TST1-HST2 period, the CIA * gradually decreases and undergoes an evolutionary process from a hot warm climate to a cold dry climate. The cooling event caused by the Hirnantian glaciation is still accompanied by the early deposition of TST2. The CIA * value in the Eastern Sichuan ranges from 55 to 70 during the TST1 to early TST2 period and a CIA * value below 65, indicating a cold-dry environment. In the late period of TST2, the CIA * was greater than 65, and the climate gradually became mild. However, in the early period of HST2, it experienced a short-term cold-dry climate, and in the late period, it eventually transformed into a warm-humid climate. Compared with the input indicators of terrestrial debris, it is found that the low degree of chemical weathering in the cold climate during the TST tract is an important reason for the low input of terrestrial debris.

The differences in provenances not only affect the mineral compositions, but also control the types of terrestrial nutrient inputs. The Al₂O₃/TiO₂ ratio of fine-grained sediments can effectively identify the type of source rocks. Due to the low solubility of Al and Ti oxides at low temperatures, their proportion in sedimentary rocks is very close to that of the source rock. The distribution of Al₂O₃/TiO₂ ratio in<8, 8–21, and>21 indicates basic, neutral, and acidic igneous rocks, respectively. The provenance in the southern Sichuan region is mainly a mixture of neutral and acidic igneous rocks (Figure 8A). The provenance in the Eastern Sichuan is a mixture of neutral and acidic igneous rocks in TST1, while in TST2 and HST2 are neutral igneous rocks. The weathering trend of the A-CN-K diagram (Figure 5I) further confirm that the provenance in Southern Sichuan during TST1-TST2 tends to be neutral igneous rocks, and HST2 tends to be acidic igneous rocks. While in the Eastern Sichuan during the TST1-HST2 tends to have neutral granodiorite sources.

Due to the higher content of compatible element Sc and lower content of incompatible elements Zr and Th in basic rocks compared to acidic rocks, and the relatively constant abundance of Sc, Zr, and Th during weathering, the Zr/Sc-Th/Sc ratio can be used for source rock type analysis. Figure 8B shows that the distribution plots in the Southern Sichuan is more towards the upper right than that in the Eastern Sichuan, also suggesting that there are more acidic igneous components in provenance. In addition, in the provenance composition of the Eastern Sichuan, TST1 period is more acidic and HST2 period is more neutral.

Roser and Korsch (1986) proposed that the K₂O/Na₂O-SiO₂ cross-plot can be effectively used to distinguish the tectonic environment of fine-grained sedimentary rocks. The results indicate that the Eastern Sichuan is in an active continental margin environment during the early Wufeng period. The data points of the Longmaxi period in the Southern Sichuan are close to the active continental margin, but the data points of the Wufeng period are closer to the passive continental margin environment (Figure 8C). Stable rare earth and trace element combinations (such as La, Th, Sc, Zr, etc.) can also be used to analyze and determine tectonic backgrounds. The Th-Sc-Zr/10 and Th-Co-Zr/10 diagrams established by Bhatia and Crook (1986) were applied for background analysis (Figures 8D, E). The analysis reveals that the majority of data points fall within or proximate to the active continental margin zone, while a minimal proportion is dispersed in the continental island arc zone. Compared to Eastern Sichuan, the Southern Sichuan exhibits a stronger correlation to the active continental margin region. The sedimentary tectonic context in both regions remains steady from Late Ordovician to Early Silurian, characterized predominantly by an active continental margin setting.

Under the influence of the Caledonian Movement, the Xuefeng paleo-uplift and the Central Sichuan underwater uplift rapidly uplifted. In the Kaitian stage, relative sea-level rise led to a transformation of the sedimentary environment from an early open platform to a restricted basin, initiating the deposition of black shale in the Wufeng Formation.

In the early TST 1 period, intense tectonic uplift enhanced the intensity of detrital input. The separation effect of paleo-uplift and relative sea-level rise leads to an increasing reduction and restriction of bottom water, but also accompanied by intermittent relative oxic conditions. Active tectonic activity also promotes volcanic eruptions, and the volcanic ash formed by it is transported and deposited in the ocean, providing rich nutrients for surface seawater. Under this setting, plankton such as graptolites have proliferated on a large scale, and the productivity level of the water surface is constantly improving. These plankton can be preserved as organic matter under anoxic conditions after death. In the late TST 1 period, tectonic activity tended to stabilize, but the early large-scale volcanic activity had led to a gradual cooling of the global climate and changes in graptolite biodiversity, triggering the Late Ordovician extinction. In cold climates, the weathering intensity is weak, and the amount of terrestrial input is significantly reduced. A large number of aquatic organisms die, providing organic matter supply and enhancing the reducibility of bottom water through degradation of organic matter. During this period, organic-rich siliceous shale is mainly formed.

During TST 1 period, the trend of TOC variation in the Eastern Sichuan and Southern Sichuan is consistent, but the average value of the former is significantly higher than that of the latter. It is speculated that this phenomenon is related to the high paleo-productivity in the Eastern Sichuan. The high excess Si and a large amount of siliceous organisms confirm the enrichment of organic matter, manifested by a higher quartz content in Eastern Sichuan than in Southern Sichuan (Table 1). The detrital input in the Eastern and Southern Sichuan is basically the same, and both provenances tend to be acidic magmatic rocks. The high content of feldspar in the Eastern Sichuan is related to the relatively cold-arid climate background, where feldspar is weakly affected by weathering during transportation. Due to being closer to the Central Guizhou-Xuefeng paleo-uplift, there is more input of coarse-grained carbonate debris in the Southern Sichuan Basin, which is characterized by high carbonate mineral content, especially in the late TST 1 period. This has also been confirmed by the Zr/Al ratio.

In the mid to late Hirnantian stage, the formation of glaciers on the Gondwana continent led to a significant decrease in global sea-level (Bertrand et al., 1996), resulting in sharp shallowness of water mass and unfavorable organic matter preservation under the oxic bottom water condition. A thin layer of carbonate-rich argillaceous limestone was deposited in a shallow water environment.

The Hirnantian glaciation has ended, and global temperatures are rapidly rising. Glacial melting has caused widespread transgression. Volcanic activity and tectonic movements tend to stabilize. In the early TST 2 period, the separation of paleo-uplift and sea-level rise kept the bottom water in a restricted and anoxic condition. Climate warming has led to an increase in weathering intensity and enhanced detrital influx. The glacial melting results in a large amount of cold fresh water flowing into the ocean, which not only brings nutrients but also causes stratification of fresh and saline water, exacerbating anoxic conditions. Planktonic organisms, including radiolarians and graptolites, have once again flourished on a large scale, increasing productivity levels. The continuously sinking biological remains continue to consume dissolved oxygen in the bottom water, maintaining anoxic environments, even euxinic environments. During early TST 2 period, less detrital influx helps to weaken the dilution effect and form organic-rich siliceous shale. During middle TST 2 period, the sea-level continues to rise, increasing the circulation between the restricted basin and the ocean, and reducing the restriction degree of bottom water. However, under moderate to strong paleo-productivity, the consumption of oxygen by the remains of organisms still ensures that the bottom water is in an suboxic-anoxic environment. In the late TST 2 period, the fluctuating sea-level drop results in limited surface water productivity. The increase in dissolved oxygen in bottom water is not conducive to organic matter preservation. The humid climate leads to an increase in the intensity of paleo-weathering, while the increase in terrestrial input disrupts the enrichment of organic matter. Compared to the middle TST 2 period, the organic carbon content is significantly reduced, and the shale lithology is argillaceous-rich siliceous shale or mixed siliceous shale.

During early-middle TST 2 period, the high productivity level in the Eastern Sichuan leads to a higher average TOC content than in the Southern Sichuan. In the late TST 2 period, the paleo-productivity in the Eastern Sichuan is still at a high level, but the rapidly increasing weathering intensity has led to a significantly higher detrital influx than in the Southern Sichuan. The dilution effect of terrestrial debris reduces the accumulation of organic matter in the Eastern Sichuan. The mineral composition indicates that the quartz content in the Eastern Sichuan was higher in the early TST 2 period (Figure 9), which is related to the deposition of a large amount of biogenic quartz, while the late TST 2 period was lower than that in the Southern Sichuan (Table 1). The clay content in the Eastern Sichuan is higher than that in the Southern Sichuan, while the carbonate content is lower than that in the Southern Sichuan. This is related to the large amount of terrestrial debris input and the high proportion of clay minerals. From the perspective of provenance differences, the Eastern Sichuan Basin is mainly composed of neutral igneous rocks, while the Southern Sichuan Basin is mainly composed of acidic igneous rocks. The former has a higher proportion of plagioclase feldspar in sediment supply, while the latter has a higher proportion of potassium feldspar. However, potassium feldspar is prone to kaolinization during transportation, so the mineral composition in eastern Sichuan has a significantly higher content of feldspar, mainly plagioclase feldspar.

The climate in the late Rhuddanian stage continued to warm, with a slow decrease in sea-level, but there was short-term transgression. The tectonic uplift intensifies the weathering process in the provenance region, and the amount of terrestrial input continues to increase. The productivity level has undergone significant changes (significantly higher in Eastern Sichuan than in Southern Sichuan), but the bottom water conditions are not conducive to the preservation of organic matter. Diluted by terrestrial debris, organic matter is dispersed in sediments. More and more terrestrial quartz and clay minerals have replaced biogenic quartz as the main component of minerals, and the shale lithofacies are gradually transitioning to argillaceous-siliceous mixed shale.

The low productivity, low preservation conditions, and high terrestrial input in the Southern Sichuan Basin are reflected in a rapid decrease in TOC. The TOC in the Eastern Sichuan increased in the early stages and gradually decreased in the later stages, which is speculated to be related to higher paleoproductivity and a high proportion of clay in terrestrial debris. The high productivity in the Eastern Sichuan not only brings organic matter input, but also consumes the oxygen content of the bottom water during deposition, which to some extent increases the reducibility of the bottom water. In addition, high clay content is conducive to the adsorption and rapid settling of organic matter, reducing consumption during the sinking process, and facilitating the aggregation and preservation of organic matter.

The difference in the detrital influx between the Eastern and Southern Sichuan is small, but the quartz mineral content in Southern Sichuan is higher than that in Eastern Sichuan (Table 1), while the clay content is lower than that in Eastern Sichuan. This is related to the slightly stronger weathering intensity and coarse-grained debris input (silt-size quartz) in Southern Sichuan. The provenance sources in the Eastern Sichuan tend to be neutral magmatic rocks, while those in the Southern Sichuan tend to be acidic magmatic rocks. Moreover, the climate in the Eastern Sichuan Basin is cold-dry, with low weathering intensity. Therefore, the mineral composition of plagioclase in the Eastern Sichuan Basin is relatively high, while the content of quartz and potassium feldspar in the Southern Sichuan is higher (Table 1). The content of carbonate rocks in Southern Sichuan is higher than that in Eastern Sichuan, especially in calcite, which is related to the high input of coarse-grained carbonate debris. The average value of pyrite in the Eastern Sichuan is 37.5% higher than that in the Southern Sichuan, which indirectly confirms that the oxygen content in the bottom water of the Eastern Sichuan is lower, which is conducive to the formation of pyrite.