In order to evaluate the likelihood of an event, this study utilizes significance tests from statistical theory to calculate the p-value among sedimentary geochemical parameters, as depicted in Figure 3. The RSEs, such as V, Mo, U, Ni and Cr, demonstrate variations in sedimentary rocks in response to different redox conditions. These elements are frequently utilized as indicators of paleoredox conditions within the sedimentary environment (Tribovillard et al., 2006). Multiple parameters are necessary to collectively constrain paleoenvironmental conditions and mitigate potential analytical errors in the determination of single-element concentrations (Qiu et al., 2021).

In this study, paleoredox conditions, represented by V/Cr, Ni/Co, and U/Th ratios, were utilized to confirm the correlations among sedimentary geochemical parameters. The resulting correlation coefficients were 0.96, 0.94, and 0.35, respectively. Furthermore, consistent explanatory outcomes observed across diverse paleoredox conditions in Section 5.3.1 offer additional support for the reliability of the data.

When performing a quantitative analysis of sedimentary geochemical characteristics in a paleoenvironment, it is crucial to recognize the complex composition of sediments in the source area, as well as the effects of potassium metasomatism and recycling during diagenesis on these characteristics (Gaillardet et al., 1999; Garzanti et al., 2013; Abedini and Calagari, 2017a; Perri, 2018).

The characteristics of REE content in clastic sedimentary rocks can be used as parameters to assess diagenetic intensity. Stronger diagenesis is associated with a stronger correlation between δCe and ΣREE, as well as Dy_S/Sm_S (Shields and Stille, 2001). The δCe of O₃w-S₁l₁ black shales shows a weak correlation with Dy_S/Sm_S and ΣREE, indicating a limited influence of diagenesis on these elements (Figures 4A,B). Additionally, the comparison of the correlation between CIA, K-co CIA, and CIW suggests that potassium metasomatism has had a negligible impact on the O₃w-S₁l₁ black shales (Nesbitt and Young, 1984; Fedo et al., 1995; Zhang X. S. et al., 2017) (Figures 4C,D). The majority of black shale samples have ICV values greater than 1, ranging from 0.87 to 2.51, with an average value of 1.41, suggesting limited recycled sedimentation and low maturity of the clastic rock composition (Van De Kamp and Leake, 1985; Cox et al., 1995). These samples can be used to assess the paleoenvironmental in the study area.

Paleoredox conditions refer to the equilibrium between oxygen and reducing agents in the sedimentary environment, which significantly influence the type and preservation of OM. An elevated oxidant content accelerates the degradation of OM, while a high reducing agent content inhibits it (Zhang et al., 2016). Marine sedimentary environments are commonly reconstructed using RSEs in marine deposits, such as V, Cr, Th, U, and Mo, owing to their distinct behaviours under seawater oxidation-reduction conditions (Tribovillard et al., 2006; Algeo and Rowe, 2012; Scott and Lyons, 2012; Abedini et al., 2019). V is typically taken up by OM and becomes enriched in reducing environments. Sediments deposited in a low-oxygen environment exhibit high U values (Abedini and Calagari, 2017b).

The redox classifications (O₂ concentration in bottom waters, ml O₂/H₂O) in this research adhere to the standards established by Tyson and Pearson (1991), which include oxic: [O₂]>2, suboxic: 2>[O₂]>0.2, anoxic(no free H₂S in the water column): [O₂]<0.2, and euxinic(free H₂S present in the water column): [O₂]=0. Hatch and Leventhal (1992), Jones and Manning (1994), and Wignall and Twitchett (1996) identified that V/Cr<2, Ni/Co<5, and U/Th<0.75 indicate an oxygen-rich environment, while V/Cr indicate 2-4.25, 5 <Ni/Co<7, and 0.75 <U/Th<1.25 indicate an oxygen-depleted suboxic-anoxic environment. Finally, V/Cr>4.25, Ni/Co>7, and U/Th>1.25 show a euxinic environment. Based on the results presented in Figures 6A–C, the O₃w black shales indicate an oxic-anoxic environment, while the S₁l1 1 black shales exhibit a strongly reducing sedimentary environment with a decreasing reduction trend vertically.

The retention of seawater in a basin has a significant impact on the enrichment and preservation of OM. OM in water is more likely to be enriched and preserved under conditions of slower water exchange and renewal (Algeo and Rowe, 2012). U_EF-Mo_EF and TOC-Mo are effective indicators of the oxygen conditions and the extent of sediment retention in the environment (Algeo and Tribovillard, 2009; Tribovillard et al., 2012). Mo/TOC can determine seawater retention degree under reducing paleoenvironmental conditions. Under euxinic environmental conditions, the degree of seawater retention can be determined using Mo/TOC, whereas, in oxic environments, the values of Mo and Mo/TOC are determined by the redox environment rather than the degree of seawater retention (Zheng et al., 2002; Algeo and Rowe, 2012; Li et al., 2015). The low TOC and Mo content in the O₃w black shales make it unsuitable for using Mo/TOC to determine its seawater retention degree. In contrast, the S₁l₁ black shales were deposited in an anoxic-euxinic environment with high seawater retention (Figures 5E,F).

Upwelling sediments tend to have higher enrichment of Cd and Mo, while Co and Mn are relatively depleted. Co×Mn<0.4, Co_EF×Mn_EF<0.5, and Cd/Mo>0.1 are characteristic indicators of upwelling sediments. Stronger upwelling activity corresponds to lower values of Co×Mn or Co_EF×Mn_EF in the sediment and higher Cd/Mo ratios. Sediments in seasonal upwelling environments typically exhibit Cd/Mo ratios ranging from 0.1 to 0.6, while sediments in perennial upwelling environments have Cd/Mo ratios >0.6 (Crusius et al., 1996; Algeo and Tribovillard, 2009; Nakagawa et al., 2012; Scott and Lyons, 2012; Sweere et al., 2016). Figures 5G–I and Figure 6D illustrate the enhanced retention of O₃w black shales during its sedimentation period, while S₁l₁ black shales were subjected to seasonal upwelling influences.

During the Hirnantian stage (late Ordovician), the cold climate and expanding ice cover may have led to the emergence of high paleo-topography (Zigong uplift) above sea level. As a result, the accommodation space and relative paleoseawater depth of the Weiyuan area may have been affected. The periodic closure of the environment resulted in interrupted exchange between the basin and seawater, leading to a weaker upwelling intensity as indicated by the geochemical elements (Wang et al., 2018; Liang et al., 2019; Qiu et al., 2020b). In contrast, the relatively high sea level during the Rhuddanian stage (early Silurian) promoted seawater exchange and nutrient supply between the basin and the vast oceanic currents (Haq and Schutter, 2008; Cai et al., 2022a).

Paleoproductivity refers to the primary productivity of paleoseawater ecosystems during a geological period, encompassing plant and phytoplanktonic primary productivity, among others (Tribovillard et al., 2006; Wei, H., 2012). Barium (Ba) is a well-established element utilized as an indicator of paleoproductivity (Wedepohl, 1971). The average Ba_bio content in the O₃w-S₁l₁ black shales of the Yangtze foreland basin is 497.79 μg/g and 683.40 μg/g, respectively. Notably, the Ba_bio content of high TOC samples in the S₁l₁ black shales are significantly lower than that of other samples, suggesting reduced overall productivity (Figure 6F). This is due to the decomposition of barite (the primary occurrence form of Ba by sulfur-reducing bacteria and other microorganisms through bacterial sulfate reduction (BSR) in a relatively restricted environment, leading to a relatively low Ba_bio content in organic-rich black shales. Consequently, employing Ba as an indicator of productivity, may produce distorted results, and relatively low Ba_bio content does not necessarily signify low levels of paleoproductivity during the deposition of the O₃w-S₁l₁ black shales (Dymond et al., 1992; Murray and Leinen, 1993; Yan et al., 2015). Previous studies have demonstrated that the biogenic component Si_bio (Si_xs) is a valuable parameter for estimating paleoseawater productivity (Tribovillard et al., 2006; Khan et al., 2019). According to Chapter 5.2, the siliceous content in the O₃w-S₁l₁ black shales are primarily biogenic, which can be used to evaluate paleoproductivity. The vertical variation of Si_xs content in the O₃w-S₁l₁ black shales suggests a higher paleoproductivity level for the S₁l₁ shales than the O₃w shales.

Wang et al. (2002) reported that the Hirnantia fauna thrived as a result of increased seawater oxygen content in the Yangtze Block. Rong and Huang, 2019 proposed that the Yangtze Block was located in the equatorial-tropical zone during the Late Ordovician and experienced minimal impact from the Gondwana glaciation. As a result, although the biological extinction rate was high, the ecological impact was minimal, and the level of paleoproductivity remained stable. The Zigong uplift in the Weiyuan area restricted the nutrient supply from upwelling. Additionally, the decline in global sea levels during the Hirnantian glacial epoch led to an elevation in oxygen content within the stratified water column. These changes resulted in an unfavourable sedimentary environment for preserving OM during the Late Ordovician (Wang et al., 2018). In the Early Silurian, the Yangtze foreland basin obtained nutrient supply from the open ocean, such as the Qinling Ocean to the north (Wang et al., 2017). Seasonal upwelling had a significant impact on seawater exchange and nutrient supply, consequently affecting paleoproductivity. Planktonic organisms, such as radiolarians and sponge spicules, exhibited their highest paleoproductivity levels during the Early Silurian (Figures 6E–G). The paleoproductivity levels of the O₃w-S₁l₁ black shales in the Weiyuan area exhibit substantial variations, primarily attributed to the influence of the regional oceanic environment and nutrient supply (Guo, 2017).

Paleoclimate and paleosalinity conditions exert a significant influence on the provenance, abundance, and type of OM. Humid climate conditions may increase the deposition of OM, while high salinity water bodies can preserve it (Jin et al., 2008). The abundance of specific major and trace elements in shale, such as the CIA, C-value, and Fe/Mn ratios, can serve as indicators of paleoclimate conditions (Nesbitt and Young, 1982; Wang et al., 1997; Getaneh, 2002). Elevated Fe/Mn ratios in sediment are indicative of warm and humid climatic conditions, whereas low ratios suggest a hot and arid environment (Wang et al., 1997).

The CIA and K-co CIA values suggest that the O₃w-S₁l₁ black shales in the Yangtze foreland basin were deposited during warm and humid paleoclimatic conditions (Figures 5J,K; Figue 6J). The C-value and Fe/Mn trends suggest that the O₃w-S₁l₁ black shales experienced a shift in climatic conditions from humid to dry and then back to humid during the deposition process (Figures 6K,L). The paleosalinity indicators, Ca/(Fe+Ca) and Rb/K in Figures 6H–I, indicate that seawater salinity increased during the late Ordovician because of global climate change and the stagnant water habitat in the basin. In contrast, during the early Silurian, water circulation increased as the glacial epoch ended, reducing seawater salinity. Dong et al. (2022) states that this change is attributed to regional climate conditions and the oceanic environment in the Yangtze foreland basin during the early Silurian.

During the Early to Late Ordovician, the South China Plate migrated northward from the subtropical climate zone to the tropical climate zone near the equator, resembling the modern-day Galápagos Islands in Ecuador(Figure 7) (Torsvik and Cocks, 2013). This northward migration was expected to lead to higher paleoseawater temperature. However, evidence from stable oxygen isotope data of conodonts and apatites indicates that the South China Plate underwent a transition from warm to cold water during the Early to Late Ordovician, contradicting the results of paleogeographic reconstructions (Jin et al., 2018). The Sr element estimation of paleoseawater temperature in the Yangtze foreland basin during the O-S interval ranged from 10.64°C to 26.08°C (17.92°C). This value is considerably lower than the average equatorial temperature of 27°C recorded during that period (Figure 6M). The substantial global temperature decrease of approximately 9°C observed during the same period could account for the apparent contradiction and imply that the Hirnantian glacial event might have impacted the marine environment of the Yangtze foreland basin (Melchin et al., 2013).

Terrigenous detrital input particularly impacts sedimentary environments, as it can enrich sediment with nutrients and dilute the abundance of OM. Al and Ti serve as reliable indicators for assessing the intensity of terrigenous detrital input, as they are minimally affected by weathering and diagenesis, thereby preserving the signature of the source region (Nesbitt et al., 1996). A strong correlation between Al and Ti, Th, Nb, Zr, or Hf indicates that Al is derived from typical silicate detrital input (Tribovillard et al., 2006; Tribovillard et al., 2012). Ti/Al and Zr/Al ratios provide insights into the intensity of terrigenous detrital input.

The elevated Ti/Al and Zr/Al ratios observed during the S₁l₁ ¹ in comparison to the O₃w (Figures 6O, P) indicate a relatively more significant input of terrigenous detritus in the Yangtze foreland basin during this period. This can be attributed to the intensified tectonic activity (Guangxi movement), which supplied a substantial detrital input starting from the late O₃w. Furthermore, the sea level decrease during the Hennantian glaciation led to the exposure and weathering of certain portions of the original seafloor material. Subsequently, these materials were transported to the seabed after the glaciation, resulting in a comparatively higher input of detrital material during this period. Following the sea level rise to a certain threshold, the water environment stabilizes, leading to a gradual decline in the terrigenous detritus (Ti/Al) content, albeit the change is not significant in its entirety. During the S₁l₁ ², a global decrease in sea level occurred, resulting in an upward trend in Ti/Al ratios, signifying an augmentation in the input of terrigenous detritus.

Moreover, the variation in shale lithofacies indicates alterations in the intensity of terrigenous detritus input. Chapter 4.3 demonstrates that the lowermost portion in S₁l₁ comprises siliceous shale, which gradually transitions vertically to mixed shale and clay shale with a higher clay mineral content. This vertical change signifies an augmentation in the intensity of terrigenous detrital input.

The sedimentation rate (SR) notably impacts the preservation duration and type of OM (Ding et al., 2021; Khaled et al., 2022). In marine sedimentary environments, sediment rich in OM tends to accumulate in areas with low oxygen levels and slow SR (Wignall and Twitchett, 1996; Schulte et al., 2000; Chen et al., 2006). According to Ibach (1982), an increase in SR can enhance the burial efficiency of OM when the SR of black shale falls below 3.2 m/Myr. Differential sediment particle SR induces REE fractionation (Murray et al., 1991). Under slow SR, REE are adsorbed by clay, complexed with OM, and subjected to pertinent chemical reactions. This process leads to the depletion (or enrichment) of light (or heavy) REE, resulting in significant shift in the (La/Yb)_N (with N representing the chondrite-normalized parameter). An approaching (La/Yb)_N of 1 indicates minimal or weak fractionation of REE, implying a higher SR. Conversely, substantial deviation from 1 signifies notable REE fractionation, suggesting reduced SR (Elderfield and Greaves, 1982). The (La/Yb)_N for the O₃w-S₁l₁ black shales are S₁l₁ (4.48–11.33, avg. 9.47)>O₃w (7.13–9.32, avg. 8.23), suggesting SR is lower for S₁l₁ than O₃w (Table 4; Figure 6Q). Guan (2020) calculated average SR in the graptolite zone in the Yangtze foreland basin: LM4 was 6.78 m/Myr; LM1-LM3 was 1.5 m/Myr; WF2-WF4 is 2.67 m/Myr, which is similar to the result of (La/Yb)_N indication in this paper.

Variations in SR are closely linked to the paleogeographic conditions of the era, with a stable tectonic setting being a crucial factor for the low SR development (Zhang et al., 2005; Li et al., 2021). From the late Katian to Early Rhuddanian, the Yangtze foreland ramp experienced relatively minor and later tectonic influence from the southeastern margin of the Yangtze Block (Cathaysia orogenic belt). More substantial influence emerged during the Telychian (Llandovery, Silurian) due to heightened orogenic activity in the southeast (He, 2022; Zhao et al., 2023). The foreland ramp, distanced from uplifting and erosional regions, has slow SR, favoring OM enrichment and preservation. The black shales with high TOC content at the base of S₁l₁ was deposited in an environment with relatively high SR (Sample S-1, (La/Yb)_N=4.48, TOC=8.21%). OM may have been quickly buried after early diagenesis, efficiently bypassing regions with intense degradation during sedimentation. This likely contributed to a higher proportion of preserved OM (Zhang et al., 2023).

The δCe values in the black shales of O₃w-S₁l₁ range from 0.78 to 1.17. The presence of a substantial negative δCe anomaly in the black shales at the bottom of S₁l₁ indicates a significant sea level drop during this period, reaching its lowest point compared to the reference plane. This drop corresponds to the Hirnantian glacial event that took place during the O-S (Figure 6R). The development of the Antarctic Gondwana ice sheet resulted in a swift and substantial global sea level decrease of approximately 100 m over a brief interval (Rong, 1984; Chen et al., 2001; Zhu et al., 2021). After the glacial period ended, the global sea level experienced frequent fluctuations. Nevertheless, the δCe value gradually increased and maintained stability within the range of 0.78–0.91. This pattern indicates a contemporaneous rise in sea level that aligns with trend depicted by the previously established global sea level curve (Liu et al., 2017). However, in the early Silurian, the regional Guangxi movement restricted the Yangtze Sea, impacting the water depth of the Yangtze Block due to the influence of surrounding uplifts. The magnitude of sea level rise was smaller than the uplift magnitude of the Zigong underwater highlands, resulting in a gradual or delayed increase in relative sea level (Li et al., 2023). The rise in relative sea level decreases the mixing efficiency in the water column, ultimately promoting the accumulation and preservation of OM in oxygen-depleted bottom water conditions. Additionally, this phenomenon creates more space for the burial of OM (Peng, 2021).

Considering the influence of lithofacies types on TOC, the TOC content is classified into five groups based on the respective reference values: lean carbon (<1%), low carbon (1%–2%), medium carbon (2%–3%), high carbon (3%–4%), and rich carbon (>4%) (Wu et al., 2020). Organic-rich shale with TOC content exceeding 2% consists primarily composed of mixed shale (M-4, M-5, M-7) and siliceous shale (S-3, S-4, S-8) lithofacies (Figure 2C). These two shale lithofacies dominate primary gas-bearing interval of O₃w-S₁l₁, spanning from O-1 to S-12. The lithofacies thickness ratio between them is approximately 67.99% (41.17 m).

Commonly used proxies, such as quartz, QFM, SiO₂, and Si_xs (Si_bio), reflect silicon content characteristics in shale (Cai et al., 2020; Zhang et al., 2022). However, the correlation between silica content and TOC is nonlinear (Qiu et al., 2020a). Previous studies have demonstrated that shale reaches its highest TOC content when a specific balance between terrigenous silicon and biogenic silicon (Si_bio, Si_xs) is achieved. However, exceeding a threshold for Si_xs content leads to dilution and disaggregation effects that reduce TOC conten (Qiu et al., 2020a; Cai et al., 2020). Based on findings presented in Figure 8, the highest TOC content is reached when the quartz, QFM, and SiO₂ content around 55%, or when the proportion of Si_xs is approximately 30%.

Mechanisms influencing OME in marine sediments encompass diverse factors, classified into productivity and preservation mechanisms (Demaison, 1980; Pedersen. and Calvert, 1990; Murphy et al., 2000). The productivity mechanism underscores the pivotal role of primary productivity, proposing that heightened primary productivity and sufficient nutrient availability elevate oxygen consumption in the water column. Consequently, this fosters anoxic conditions, culminating in OME. Stated differently, anoxic bottom water conditions ensue as an outcome, not a cause, of OME (Pedersen. and Calvert, 1990; Sageman et al., 2003; Gallego Torres et al., 2007). The preservation mechanism indicates that anoxic water bodies formed in restricted basins can preserve OM.

TOC serves as a representative parameter to assess the extent of OME (Ma et al., 2018; Qiu et al., 2020a). We explore the correlation between TOC and eight sedimentary paleoenvironmental conditions, encompassing paleoredox conditions, paleoproductivity, paleoclimate, paleosalinity, paleowater temperature, terrigenous detrital input, sedimentation rate, and sea level change. Correlation analysis reveals the most substantial impact on OME by paleoredox conditions (e.g., V/Cr, Ni/Co) and paleoproductivity indicators (e.g., P/Al, Ba_bio), demonstrating positive correlations with TOC. Close behind are the paleosalinity indicators (e.g., Rb/K). Conversely, paleoclimatic conditions (e.g., CIA, C -value), the degree of terrigenous detrital input (e.g., Si_xs, Ti/Al), and sedimentation rate ((La/Yb)_N) show negative correlations with OME. In essence, a heightened OME concentration corresponds to reduced humid climate, terrigenous detrital input, and a relatively high SR (Figure 3). Overall, the OME observed in O₃w-S₁l₁ black shales from the Yangtze foreland basin reflects the influence of numerous factors. Thus, relying solely on individual mechanisms such as the productivity mechanism or preservation mechanism proves insufficient in comprehensively elucidating the enrichment and preservation of OM in black shales (Khaled et al., 2022; Li et al., 2022; Hu et al., 2023; Xu et al., 2023).

The tectonic-sedimentary evolution of the early Paleozoic Yangtze foreland basin divided in two stages, each marked by subordinate tectonic loading: the Katian stage and the Rhuddanian stage (Su et al., 2007; Zhang et al., 2020). O₃w and S₁l₁, representative of two distinct sets of black shales, emerge as products of disparate tectonic movements, revealing varying paleogeographic origins (Ge et al., 2021). By assimilating the aforementioned paleoenvironmental geochemical indicators and regional geological context, we can summarize an OME mechanism for O₃w and S₁l₁ black shales within the Yangtze foreland basin (Figure 9).

During the late Katian, the development of the Gondwana ice sheet triggered a worldwide decline in sea levels, isolating the Yangtze foreland basin from the Yangtze Sea and impeding upwelling (Delabroye and Vecoli, 2010; Elrick et al., 2013; Algeo et al., 2016; Lu et al., 2019). Amidst the interplay of global sea level fluctuations and regional tectonic shifts (Guangxi movement), the Yangtze foreland basin transformed, fostering a seawater setting characterized by retention and high oxygen content (oxic) (Yan et al., 2009). Despite the flourishing of the Hirnantian fauna within the global cold water environment (Wang et al., 2002; Rong and Huang, 2019), the preservation of OM encountered hindrances within the oxic paleoenvironment.

During the Early Rhuddanian, a shift from a cold and arid to a warm and humid climate triggered the rapid melting of glaciers. The glacial meltwater substantially surged sea levels and relatively high SR (Haq and Schutter, 2008; Munnecke et al., 2010). Nevertheless, the continued compression associated with the Guangxi movement brought about a delayed or gradual relative sea-level rise for the Yangtze foreland basin (Li et al., 2023). These rising sea levels fostered euxinic conditions conducive to OM preservation. Furthermore, the Yangtze foreland basin experienced reduced terrestrial detrital input and diminished OM dilution compared to the source region (Cathaysia orogenic belt), augmenting OME (Zhao et al., 2023). Consequently, the OME observed in S₁l₁ black shales of the Yangtze foreland basin is rooted in regional tectonic dynamics and climatic factors. Subsequently, during the late Rhuddanian, the ongoing uplift of the forebulge brought about shallowing of the seawater on the foreland ramp. This period had a substantial volume of terrigenous coarse debris, disrupting the anoxic bottom environment. Consequently, the preservation conditions essential for OME deteriorated, leading to the dilution of OM abundance and a decline in TOC content.