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Article

Linking Volcanism, Hydrothermal Venting, and Ordovician/Silurian Marine Organic-Rich Sediments in the Eastern Sichuan Basin, Southwest China

by
Shaojie Li
1,2,†,
Zhou Zhu
2,†,
Qilin Xiao
2,
Suyang Cai
2,* and
Huan Li
1,2
1
Key Laboratory of Exploration Technologies for Oil and Gas Resources, Ministry of Education, Yangtze University, Wuhan 430100, China
2
College of Resources and Environment, Yangtze University, Wuhan 430100, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
J. Mar. Sci. Eng. 2025, 13(3), 483; https://doi.org/10.3390/jmse13030483
Submission received: 12 February 2025 / Revised: 25 February 2025 / Accepted: 26 February 2025 / Published: 28 February 2025
(This article belongs to the Special Issue Recent Developments and Advances in Geological Oceanography and Ocean Observation in the Pacific Ocean and Its Marginal Basins—2nd Edition)
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Abstract

:
The Ordovician/Silurian boundary (Wufeng/Longmaxi formations) in the Shizhu region, eastern Sichuan Basin, China hosts organic-rich black shales which are frequently interbedded with bentonite and hydrothermal minerals (e.g., pyrite). This study investigated the mineralogical, total organic carbon (TOC), total sulfur (TS), and major and trace element compositions of organic-rich samples. Non-visible volcanic input is identified to influence organic matter accumulation, as shown by the correlations between TOC and proxies, including Zr and Hf contents and the Cr/Al2O3, V/Al2O3, Ni/Al2O3, and SiO2/Al2O3 ratios. Redox indicators (V/Cr, v/v + Ni, degree of pyritization (DOP), U/Th, and Mo contents) display positive correlations with TOC values, suggesting that an oxygen-depleted environment is necessary for organic matter (OM) preservation. The TOC values exhibit better regression coefficients (R2) against redox indicators, including DOP (0.43), U/Th (0.70), and Mo contents (0.62), than V/Cr (0.16) and v/v + Ni (0.21). This may because some V, Cr, and Ni is hosted in non-volcanic ashes within shales but not inherited from contemporaneous water columns. The greater scatter in TOC-DOP and TOC-Mo relative to TOC-U/Th relations may result from hydrothermal venting in shales, evidenced by the coexistence of framboid and euhedral pyrite and the previous finding of hydrothermally altered dolomites in the studied sections. There is no systematic relation between TOC and Ni/Co ratios, and this means that portions of Ni are contributed by non-visible volcanic ashes and Ni and Co are redistributed during the precipitation of hydrothermal pyrites due to their strong chalcophile affinities. Such a feature may further suggest that most pyrites are precipitated during hydrothermal venting. The DOP displays broad correlations with non-visible volcanic indicators, supporting that hydrothermal venting may be triggered by volcanic activities. The outcomes of this study highlight that caution is necessary when evaluating the sedimentary facies features of volcanism-affected organic-rich black shales with the used metallic proxies.

1. Introduction

Volcanism is a crucial process linking Earth’s interior and surface domains [1,2,3,4,5,6]. It may have an influence on contemporaneous environments, releasing toxic substances like hydrogen sulfide, carbon monoxide, and mercury [7,8,9,10,11,12,13,14,15,16,17]. Volcanism may bring some nutrient elements to contemporaneous seawater (e.g., Ni, Co, and P), providing favorable living conditions for microorganisms [11,12,13,14]. Mantle-derived materials can also be deposited as volcanic ashes in sedimentary basins [13,18,19,20,21] (e.g., bentonite), or occur as non-visible volcanic ashes in sediments [10] (i.e., cryptotephra).
Hydrothermal fluid flows are active during volcanism and deep-originated minerals may also be transported by fluids [22,23,24,25,26]. Hydrothermal fluid may provide nutrient elements to contemporaneous seawater and facilitate the organic matter accumulation and the occurrence of hydrocarbon source rocks [14,27,28,29]. Additionally, hydrothermal fluid flows can modify hydrocarbon source rocks or reservoir rocks in multiple mechanisms, such as altering reservoir pore connectivity through mineral dissolution or precipitation [30,31,32,33], accelerating hydrocarbon generation with abnormal heating stress [34], driving hydrocarbon migration [35,36], and altering physicochemical properties of reservoired hydrocarbons [37,38,39]. Similar to volcanic activity, which is often associated with hydrothermal fluid flows, the influx of external metallic elements from hydrothermal fluids into pre-existing sedimentary rocks means that caution may be required when recognizing sedimentary features with metal compositions (e.g., transition metals).
Organic-rich black shales are important petroleum source rocks and unconventional reservoirs (i.e., shale gas and shale oil). During the transition from the Ordovician to the Silurian period, significant changes occurred in the Earth’s paleo-climate, paleo-ocean physical/chemical features, and biological evolution [10,40,41,42]. Black shales are commonly found across the Ordovician to the Silurian boundary, as proven by many regions around the world, such as the Yangtze Platform in South China [43], Holy Cross Mountains in central Poland [44], Parahio Valley in north India [45], and Anticosti Island in Canada [46]. There is a typical Ordovician/Silurian boundary in the Yangtze Platform, South China which comprises the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation [47,48]. This sedimentary succession (namely, the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation in the Yangtze Platform) mainly consists of black shales (with portions of carbonate rocks), providing an ideal opportunity to reconstruct the paleo-ocean environment and paleo-ecosystem during the Ordovician to Silurian transition [42]. Additionally, abundant gaseous hydrocarbons with industrial significance have been found within the black shales of the Wufeng and Longmaxi formations, e.g., the Fuling shale gas field in the eastern Sichuan Basin [49]. In the past decade, extensive research efforts have been carried out on the Wufeng and Longmaxi formations to characterize the paleoenvironment, volcanism, organic matter accumulation, fluid flow history, unconventional reservoir properties, and natural gas genesis [50,51,52,53,54,55].
A well-preserved the Ordovician/Silurian boundary outcrop was found in the Shizhu region of the eastern Sichuan Basin, southwest China (Figure 1A,B) [56], and black shales across the Ordovician/Silurian boundary are interbedded with bentonite and hydrothermal minerals (such as pyrite and calcite), suggesting that volcanism and hydrothermal fluids have affected this section [57]. In this study, the mineralogical compositions and geochemical features of forty-five Ordovician/Silurian organic-rich black shale and carbonate samples were analyzed, including major and trace metal contents, sulfur content, and total organic carbon content. These geochemical data would provide robust insights to constrain the potential linkage among volcanism, hydrothermal fluid flows, and organic matter preservation.

2. Geological Background

The study area is located on the eastern boundary of the Sichuan Basin, a large hydrocarbon-bearing basin in southwest China [58,59] (Figure 1B). The basin is surrounded by the Micang–Dabashan, Hubei–Hunan–Guizhou, Emeishan–Liangshan, and Longmenshan fold belts [60]. Six sub-units are identified within the Sichuan Basin, including the North Zone, East Zone, South Zone, Southwest Zone, West Zone, and Central Zone [60,61,62,63]. Neoproterozoic to Cenozoic sedimentary rocks occur in the eastern Sichuan Basin [64]. The sedimentary facies in the eastern Sichuan Basin, China vary through the Paleozoic eras and carbonate rocks and shales are major sedimentary rock types of this period. An open carbonate platform formed in this area between the Neoproterozoic and Ordovician [47]. The eastern Sichuan was a deep-water continental shelf during the Late Ordovician to Early Silurian [65]. During this period, the sedimentary facies gradually transitioned from a shallow-water continental shelf to a deep-water continental shelf, forming the transitional facies [65]. A shallow-water continental shelf slope formed from the Silurian to the Carboniferous [66]. Thereafter, an open carbonate platform environment formed during the Permian [67]. The Dongwu movement, Yunnan movement, and Gugangxi movement in the eastern, southern, and western parts of the Yangtze platform had a significant impact on the geological structure and sedimentary facies of the region [68,69].
Over-mature, organic-rich black shales occur across the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation in the eastern Sichuan Basin [70,71,72]. In this study, the Ordovician/Silurian boundary outcrop section is located in Shizhu County, eastern Sichuan Basin, China [56] (Figure 1B). There are three sub-units across the Ordovician/Silurian boundary, including the Wufeng Formation, Guanyinqiao Bed, and Longmaxi Formation. The Wufeng and Longmaxi formations are mainly composed of organic-rich black shales, whereas the Guanyinqiao Bed is a set of organic-rich dolomite rocks [56].

3. Samples and Analytical Methods

3.1. Black Shale and Carbonate Rock Samples from the Ordovician/Silurian Boundary

In this study, forty-five samples were taken from the Ordovician/Silurian boundary across the outcrop in the Shizhu region, including twenty-seven black shale samples from the Wufeng Formation, sixteen black shale samples from the Longmaxi Formation, and two dolomite samples from the Guanyinqiao Bed (Table S1). Thin sections of ~30 μm thick were prepared for scanning electron microscope (SEM) observation. The mineralogical compositions, major element contents, trace element contents, total organic carbon (TOC) contents, and total sulfur (TS) contents of samples were determined with X-ray diffraction (XRD), X-ray fluorescence spectrometry (XRF), inductively coupled plasma mass spectrometry (ICP-MS), and C-S elemental analysis, respectively.

3.2. Methodology

3.2.1. Total Organic Carbon and Total Sulfur Content Analysis

Fresh black shale and carbonate rock samples were ground into ~200 mesh powders and dried completely for geochemical analysis. The total organic carbon (TOC) and total sulfur (TS) contents of black shale samples were analyzed with the LECO CS230 CS elemental analyzer, which is manufactured by LECO Corporation in the St. Joseph, MI, USA. Firstly, approximately 100 mg powders of each sample were reacted with 10% HCl at ~80 °C to remove carbonate species in samples and acquire the TOC within the silicate species. Then, the sample remnants were washed with pure water, dried, and combusted to determine TOC and TS values [73].

3.2.2. Mineralogical Composition and Major Element Analysis

The mineralogical compositions of samples were analyzed by X-ray diffraction (XRD). Finely ground powders of each sample were scanned from 5° to 75° at the rate of 2°/min using an X’Pert PRO MPD X-ray diffractometer, which is made by Malern Panalytical in Almelo, The Netherlands. The major element contents of samples were determined using a specific model (PANalytical Axios of Malvern Panalytical in Almelo, The Netherlands) of X-ray fluorescence (XRF) spectrometer. Approximately 1 g of powder of each sample was dried completely at 105 °C for 12 h. To measure the loss on ignition (LOI) value of each sample, sample powers were initially heated at 1000 °C for 2 h in a muddle furnace and were cooled down to 400 °C to determine the weight of the remaining portion of powders. After that, sample powders were mixed with Li2B4O7 eight times and fused into glass beads, which were then analyzed with the XRF [73]. The X-ray fluorescence (XRF) analysis was conducted using standards provided by the National Institute of Standards and Technology (NIST) to ensure the accuracy of the measurements. We conducted normalization on the samples. The weight percentages of the major elements were calculated based on the total weight of the sample, with the loss on ignition (LOI) correction taken into account. The LOI correction was determined by subtracting the weight loss due to heating from the initial dried weight of the sample.

3.2.3. ICP-MS Trace Element Analysis

The trace element contents of samples were determined using an ICP-MS instrument (a laser ablation (LA-ICP-MS) Thermal X series 2, made by Thermo Scientific in the Waltham, MA, USA). Approximately 100 mg powders of each sample were pressed into a pellet or embedded in a resin mount. The mounted samples were then polished to ensure a flat and clean surface for laser ablation. The laser ablation system was used to directly ablate the solid samples. The ablated sample material was then carried by a carrier gas (usually argon) into the ICP–MS instrument. Before trace element analysis, internal spikes of 10ppb 61Ni, 6ppb Rh, In, and Re were added to the sample solutions to correct for instrumental drift and mass bias. USGS standard W-2a was used as the reference standard and cross-checked with BHVO-2, SGR-1b, and SBC-1. Instrument drift mass bias was corrected with internal spikes and external monitors. Acids used during the trace element analysis were double-distilled [73]. The data acquisition was performed in a suitable mode (such as peak-hopping or scan mode) to measure the trace element contents. The acquired data were processed using appropriate software to calculate the concentrations of the trace elements in the samples.

4. Results

4.1. Mineralogical Compositions, TOC and TS Contents

The mineralogical compositions of black shale samples are presented in Table S1 and Figure 2, as determined by XRD analysis. These samples from the Wufeng and Longmaxi formations mainly contain quartz, clays, feldspar, and carbonate minerals (calcite, dolomite, and ankerite), Additionally, pyrites are also identified in the samples (Table S1 and Figure 2). Samples from the Guanyinqiao Bed mainly consist of dolomite, quartz, clays, feldspar, calcite, and pyrite, suggesting that these samples are dolomites.
The samples from the Wufeng Formation (a larger stratigraphic unit), Longmaxi Formation, and Guanyinqiao Bed (a thinner, more specific layer within a formation) are marked by their substantial organic matter content. Total organic carbon (TOC) contents assessing the organic richness are presented in Table S2. The total sulfur (TS) values of the samples from these formations and bed are shown in Table S2 as well.

4.2. Major Element Composition of Samples

The major metal contents of samples are listed in Table S2 and shown in Figure 3. Black shale samples from the Wufeng and Longmaxi formations have intermediate SiO2 contents (40–76 wt.% and 51–73 wt.%, respectively). Dolomite samples from the Guanyinqiao Bed have lower contents of SiO2 (15–20 wt.%). The K2O/Na2O ratios of samples from the Wufeng Formation, Longmaxi Formation, and Guanyinqiao Bed are distributed in the ranges of 1.5–5.2, 1.1–1.3, and 1.1–5.3, respectively. The Fe2O3 + MgO contents of samples from the Wufeng Formation, Longmaxi Formation, and Guanyinqiao Bed are distributed in the ranges of 2.1–16, 2.9–11, and 16–16.1, respectively. Shales from the Wufeng and Longmaxi formations have lower contents of CaO (0.39–9.7 wt.% and 0.24–5.6 wt.%, respectively) compared to samples from the Guanyinqiao Bed (22–23 wt.%). The Al2O3/(Na2O + CaO) ratios of samples from the Wufeng Formation (0.45–9.3) and Longmaxi Formation (1.27–9.1) are higher compared to those of samples from the Guanyinqiao Bed (0.11–0.12). The K2O/Al2O3 ratios of samples from the Wufeng Formation, Longmaxi Formation, and Guanyinqiao Bed are distributed in the ranges of 0.26–0.36, 0.22–0.29, and 0.20–0.21, respectively, with an average value of 0.28, similar to the range of clay mineral values (0–0.3).

4.3. Trace Element Composition of Samples

Trace element contents and elemental ratios are listed in the Tables S3–S5 and shown in the Figure 4 and Figure 5. Samples from the Wufeng Formation, Longmaxi Formation, and Guanyinqiao Bed have uniform contents of Rb, Cs, and Sr. They also have high Ba contents. There are variations in the contents of Zr, Hf, Nb, Ta, Y, Th, and U. The Zr/Hf ratios and Nb/Ta ratios of the samples are also variable. The transitional trace metal contents, such as Co, Cr, Ni, Sc, and V, are variable, and so are the transition trace metal ratios like V/Cr and Ni/Co. The transitional trace metal contents, such as Co, Cr, Ni, Sc, and V, are variable and so are the transition trace metal ratios like V/Cr and Ni/Co.
The total rare earth element (REE) contents of the samples from the Wufeng Formation, Longmaxi Formation, and Guanyinqiao Bed show certain differences. The PAAS-normalized (Post-Archean Australian Shale) REE distribution patterns of all samples are uniform (Figure 5). There are insignificant Eu anomalies and Ce anomalies for samples from the Wufeng Formation, Longmaxi Formation, and Guanyinqiao Bed.

5. Discussion

5.1. Recognizing Non-Visible Volcanic Input Within Ordovician/Silurian Black Shales in the Shizhu Area

Bentonite layers frequently occur across the Ordovician/Silurian boundary, suggesting the widespread volcanic activities throughout the globe during this transition period [19,20,74,75,76,77,78,79,80]. Previous studies have also identified bentonite layers interbedded with organic-rich black shales in the Ordovician/Silurian boundary outcrop in the Yangtze Platform [13,19,21], including the Shizhu region [56]. This implies that volcanism may have played an important role in facilitating organic matter accumulation in the study area. In addition to bentonites, non-visible volcanic ashes may also occur within sediments during volcanic activities [2]. Revealing the proportions and features of non-visible volcanic ashes within organic-rich shales, coupled with TOC data, will provide direct evidence for recognizing the linkage between organic matter accumulation and volcanism. To identify the non-visible volcanic ashes across the Ordovician/Silurian boundary in the Yangtze Platform, Yang et al. [10] compared the geochemical features of bentonites and adjacent shales and found that bentonites have higher contents of Zr and Hf, as well as lower Cr/Al2O3, V/Al2O3, Ni/Al2O3, and SiO2/Al2O3 ratios, relative to shales. In this study, the proxies in samples from the Wufeng Formation, Longmaxi Formation, and Guayinqiao Bed are compared to recognize the differential influence of volcanism on the non–volcanic rocks and the linkage between volcanism and organic matter accumulation.
Non-visible volcanic ash contains high contents of zircons generated during volcanic activities (e.g., magmatic zircons), where zircon is the dominant host of Zr and Hf. Yang et al. [10] found that black shale samples with Zr and Hf contents above 160 ppm and 2.8 ppm, respectively, likely received volcanic input. In this study, black shale samples from the Longmaxi Formation generally contain higher contents of Zr and Hf (average is 160.4 ppm and 3.99 ppm, respectively) compared to black shales from the Wufeng Formation (average is 109 ppm and 2.44 ppm, respectively) and dolomites from the Guanyinqiao Bed (average is 37.0 ppm and 0.928 ppm, respectively; Figure 6A,B). Such features suggest that non-visible volcanic input may have higher contributions to shales within the Longmaxi Formation than those within the Wufeng Formation and carbonates within the Guanyinqiao Bed [10]. The black shales with higher non-visible volcanic input are relatively depleted in silicon due to devitrification [10]. Black shale samples from the Longmaxi Formation generally have lower SiO2/Al2O3 ratios (average is 6.83) than those from the Wufeng Formation (average is 10.8) and comparable ratios to those of dolomites from the Guanyinqiao Bed (average is 6.60; Figure 6C). This supports the hypothesis that the non-visible volcanic input has a higher contribution to the Longmaxi Formation than Wufeng Formation, and the lower SiO2/Al2O3 ratios in samples from the Guayinqiao Bed are attributed to the lithology of the samples, as dolomites are generally depleted in silicon [81]. In addition to providing metal budgets of Zr, Hf, and Si, non-visible volcanic input may have important contributions to redox-sensitive elements in the study area because samples from the Wufeng Formation, Longmaxi Formation, and Guanyinqiao Bed all have Cr/Al2O3 (10), V/Al2O3 (10), and Ni/Al2O3 (20) ratios within the respective threshold values for shales containing non-visible volcanic ashes as proposed by Yang et al. [10]. Non-visible volcanic ashes generally have higher sedimentation rates compared to shales, thus redox-sensitive elements including Cr, V, and Ni are depleted in non-visible volcanic ashes [10]. Therefore, organic-rich black shales and carbonates within the studied the Ordovician/Silurian section were affected by non-visible volcanic input according to major and trace metal data of the studied samples.
As shown in Figure 6A–F, the TOC values of samples display broad positive correlations with Zr and Hf contents, whereas broad negative correlations with SiO2/Al2O3, Cr/Al2O3, V/Al2O3, and, Ni/Al2O3 ratios, suggesting that non-visible volcanic input may have promoted organic matter accumulation in the study area. It is noted that some samples deviate from the main trends between TOC and non-visible volcanic ash proxies, and such deviations may be related to the geochemical behavior of relevant elements. For example, portions of Zr, Hf, and Si in sediments are derived from continental detritus, such as detrital zircon (Zr and Hf) and quartz [82] (Si), and the mixing of detritus-derived and volcanic-derived elements possibly resulted in the deviations in the scatter plots of TOC vs. Zr, Hf, and SiO2/Al2O3 (Figure 6A–C). Elements including Cr, Ni, and V can be drawn down from seawater via redox reactions [83]; therefore, the mixing of seawater-derived and volcanic-derived Cr, Ni, and V possibly resulted in the deviations in the scatter plots of TOC vs. Cr/Al2O3, Ni/Al2O3, and V/Al2O3 (Figure 6A–C).

5.2. Influence of Non-Visible Volcanic Input on Metal Compositions of Black Shales

Specific metal ratios can provide useful information about the sedimentary environment, such as the paleoproductivity and redox conditions of seawater. Several proxies based on nutrient elements are widely applied for evaluating seawater nutritious conditions, such as biogenic barium (Babio, ppm), organic phosphorus (Porg, %), and biogenic silicon [84,85,86,87] (Sibio, %). As discussed in the preceding section, there are higher degrees of non-visible volcanic input in shales from the Longmaxi Formation compared to those from the Wufeng Formation and Guanyinqiao Bed, and thus it was expected that the Sibio, Babio, and Porg values of samples from the Longmaxi Formation were higher than those from the Wufeng Formation and Guanyinqiao Bed. However, most samples from the Longmaxi Formation do not have remarkably higher Babio, Porg, and Sibio values relative to those from the Wufeng Formation and Guanyinqiao Bed (Figure 7A–C): the Babio averages of samples from the Wufeng Formation, Longmaxi Formation, and Guanyinqiao Bed are 1520 ppm, 1700 ppm, and 587 ppm, respectively; the Porg averages of samples from the Wufeng Formation, Longmaxi Formation, and Guanyinqiao Bed are 0.21%, 0.20%, and 0.71%, respectively; the Sibio averages of samples from the Wufeng Formation, Longmaxi Formation, and Guanyinqiao Bed are 21%, 15%, and 4%, respectively. In addition, the correlations between TOC and Sibio, Babio, and Porg values are weak (Figure 7A–C), suggesting that these proxies may not be useful for evaluating paleoproductivity conditions in the study area. Indeed, Babio, Porg, and Sibio proxies are established with elements that can be contributed by non-visible volcanic ashes (e.g., Si and Al) and elements hosted by non-visible volcanic ashes may not be equilibrated with those in contemporaneous seawater due to higher sedimentation rates of volcanic ashes compared to fine-grained sediments [56]. Thus, it may bring biased results when using these proxies for evaluating the paleoproductivity of seawater in the study area.
Redox-sensitive elements such as V, Cr, and Ni, in samples, are also affected by non-visible volcanic input in the study area, and paleoredox proxies based on these elements, e.g., V/Cr, Ni/Co, and v/v + Ni, do not display rigorous correlations with TOC values (Figure 7D–F). However, paleoredox proxies that are negligibly affected by volcanism, such as degree of pyritization (DOP), U/Th, and Mo contents, display appreciable positive correlations with TOC values (Figure 7G–I). This indicates that the oxygen-depleted environments are favorable for the accumulation of organic matter in the study area because OM can be prevented from oxidative degradation under reducing conditions [17,83,88,89,90]. Although paleoredox proxies such as V/Cr, Ni/Co, and v/v + Ni are affected by volcanism and exhibit weak correlations with TOC values, empirical diagrams based on these proxies, such as Moef vs. Uef, V/Cr vs. Ni/Co, v/v + Ni vs. Ni/Co, and Mo vs. Ni/Co, still suggest that studied samples were deposited in oxygen-depleted environments (Figure 8A–D). These features indicate that contemporaneous seawater still contributes portions of redox-sensitive elements to sedimentary rocks in the study area [83,88,89,91].

5.3. Redox Variation Controls Organic Matter Accumulation in Ordovician/Silurian Black Shales

The TOC values of samples from the Wufeng Formation, Longmaxi Formation, and Guanyinqiao Bed are generally higher than 2.00%, with average values of 3.90%, 6.26%, and 6.27%, respectively (Table S2). Such high organic matter contents suggest that these samples are excellent hydrocarbon source rocks and may have generated large quantities of hydrocarbons with industrial significance [92]. Revealing the controlling factors of organic matter accumulation may assist in understanding the features of the paleo-environment and paleo-ecosystem, as well as the distribution patterns of hydrocarbon resources [41,93] (e.g., shale gas). Previous studies have shown that organic matter accumulation may be mainly controlled by primary productivity, redox conditions, or a combination of both [94,95,96].
The primary productivity of contemporaneous seawater is the direct factor that affects the formation and enrichment of depositional organic matter [9,97,98]. Multiple geological processes may enhance the primary productivity of seawater, such as the increased input of volcanic ash to seawater [10,99], intensified oxidative erosion of the continental crust [69,100], shifting from a felsic to a mafic provenance of sediments [101,102], climate warming [103,104], and sedimentation rate, etc. [105,106].
Volcanism may facilitate organic matter accumulation by enhancing the paleoproductivity of seawater because large amounts of nutrient elements can be brought into seawater [7]. However, it seems difficult to determine whether the volcanism-facilitated paleoproductivity has a genetic relationship with organic matter accumulation in the study area because the paleoproductivity metal indicators may have been biased by the input of non-visible volcanic ash, as discussed in the preceding paragraph. The increased paleoproductivity may further result in the flourishing of seawater organisms, which may consume oxygens dissolved in seawater [107]. Such an oxygen-depleted situation may be intensified with the input of reducing gases with the volcanic eruption [108] (e.g., SO2). In this study, redox-sensitive elements in the samples are mainly contributed by contemporaneous seawater; thus, plotting TOC against redox metal indicators may provide useful details on whether organic matter accumulation is controlled by seawater redox conditions. As shown in the binary plot of TOC vs. redox indicators, there are broad positive correlations between TOC and commonly used redox-sensitive indicators such as V/Cr (Figure 7D), v/v + Ni (Figure 7F), and U/Th (Figure 7H). These features suggest that organic matter accumulation and preservation in the study area is likely mainly controlled by the redox conditions of seawater.

5.4. Divergent Paleoredox Metal Proxies and Evidence of Hydrothermal Venting

Although TOC displays similar broad positive correlations against multiple redox-sensitive indicators, the regression coefficients vary among different TOC–redox indicator relations (Figure 7D–I), suggesting that portions of redox-sensitive metals may not be sequestered from seawater via redox reactions. For example, the positive correlations between TOC values and the V/Cr of v/v + Ni ratios (Figure 7D,F) are much weaker compared to those between TOC and DOP, U/Th, or Mo contents (Figure 7G–I). Such inconsistency may be attributed to non-visible volcanic input to sediments, as portions of V, Cr, and Ni in samples were possibly contributed by non-volcanic ashes (Figure 6D–F). Further, there is an interesting finding that although U/Th, Mo, and DOP display appreciable positive correlations with TOC values, the regression between U/Th and TOC values yields a better fit-coefficient (R2 = 0.70) than those of Mo-TOC (R2 = 0.62) and DOP-TOC (R2 = 0.43; Figure 7H,I). Such differences suggest that U and Th in samples are likely largely derived from seawater during the deposition of sediments, and the U/Th ratios of samples may provide more reliable details about the redox conditions of contemporaneous seawater [91,99]. In contrast, significant portions of Mo and pyrite budgets in samples may have been formed in post-depositional hydrothermal infiltration. The variation of Mo contents in sediments may be readily controlled by the precipitation/dissolution of pyrites because Mo is very compatible with pyrites [109]. Indeed, there is a good positive correlation between DOP and Mo contents (R2 = 0.74), suggesting that the Mo budget is mainly hosted by pyrite in the studied samples (Figure 9A).
Additionally, the positive correlation between DOP and U/Th values is also appreciable (R2 = 0.49; Figure 9B), and this indicates that large quantities of pyrites were still generated under reducing conditions during the deposition of sediments [110,111]. Therefore, pyrites in the studied section can be formed by two processes, according to bulk geochemical data, including sedimentary pyrites precipitated directly from contemporaneous seawater and pyrites formed by hydrothermal venting. The binary genesis of pyrites in the study area is also supported by photomicrographs of shale samples (Figure 10A–D), which show the coexistence of pyrite framboids (i.e., sedimentary pyrites) and euhedral pyrites [33,112] (i.e., pyrites formed in post-depositional hydrothermal fluid infiltration). In addition, we observed that the pyrite framboids in Figure 10A,D exhibit well-preserved spherical shapes, which may suggest an association with bioclast cavity infilling [110,111,112]. Pyrite is an important host for chalcophile elements in shales, such as Ni and Co, and these two elements were found to be enriched in pyrites precipitated in both sedimentary and hydrothermal settings [31,33,109]. Further, the Ni/Co ratio of shales is a widely used redox indicator, yet the correlation between the TOC and Ni/Co of samples is negligible in this study (Figure 7E). This supports that significant portions of the Ni and Co in samples may not be directly derived from seawater via redox processes but may have been precipitated from and/or redistributed by hydrothermal fluids [112,113]. The presence of hydrothermal fluid flows in the study area is also supported by the previous study [112,113]. With the application of stable isotopic analysis, Hu et al. [56] found that dolomites within the Guanyinqiao Bed were formed by hydrothermal alteration of pre-existing sedimentary limestones in the Shizhu area and the fluids are derived from clay transformation in black shales of the Wufeng and Longmaxi formations.

5.5. Volcanism–Hydrothermal–OM Interactions and Their Impact on Metal Indicators of Source Rocks

The outcomes of this study suggest the potential occurrence of non-visible volcanic input and hydrothermal venting across the Ordovician/Silurian boundary in the Shizhu area. Previous studies show that hydrothermal fluid flows were active during volcanism, suggesting a close relationship between these two processes [14,114,115,116]. Pyrite is a ubiquitous disulfide mineral precipitated from hydrothermal fluids [31]. Both geochemical data and photomicrographs show a binary genesis of pyrites in the studied shale samples, i.e., sedimentary pyrites and hydrothermal pyrites. The sedimentary pyrites are characterized by euhedral morphology, while the hydrothermal pyrites are subhedral to anhedral and rich in Co and Ni (Table S5 and Figure 10), and hydrothermal pyrite contributes significantly to the entire pyrite budget in the studied samples (Table S5 and Figure 10). Thus, the DOP values of shales may be used to approximate the degree of hydrothermal venting in the study area. The DOP values of the studied samples display broad correlations with metal indicators of non-visible volcanic input, such as Zr contents, Hf contents, SiO2/Al2O3, Cr/Al2O3, Ni/Al2O3, and V/Al2O3 ratios (Figure 11A–F). These correlations support that hydrothermal venting in the study area may be triggered by volcanic activities when faulting and fracturing processes are active, facilitating the migration of hydrothermal fluid flows [8,117,118,119,120,121,122].
Volcanism and associated hydrothermal venting influence hydrocarbon source rocks in sedimentary basins (Figure 12). Namely, volcanic ash deposits form bentonite and crypto-volcanic ash layers, which cover organic-rich shale, affecting its preservation and maturation. The migration and eruption of hydrothermal fluids bring metals and minerals, promoting the transformation of organic matter and the generation of hydrocarbons (Figure 12). In addition to providing nutrient elements to contemporaneous seawater and establishing a reducing environment that preserves organic matter from oxidative degradation [14,27,28,107], volcanism and associated hydrothermal activities may facilitate the thermal maturation of source rocks and hydrocarbon generation. In general, the temperature of black shales in sedimentary basins is mainly controlled by the burial-temperature process, and organic matter in black shales is thermally degraded into liquid and gaseous hydrocarbons with the increase of burial depth [51,123]. The hydrothermal fluid flows commonly have temperatures higher compared to black shales in sedimentary settings and they can increase thermal maturity levels of source rocks and trigger hydrocarbon generation instantaneously [37]. The high temperatures induced by volcanism and/or hydrothermal activities can also accelerate the clay transformation in black shales, for example, loss of interlayer water molecules of smectite [72], and these processes can further facilitate fluid flows within the shales. Additionally, hydrothermal fluid flows may facilitate hydrocarbon generation by reducing the activation energy barrier for thermal degradation of organic matter and providing metallic catalysts for petroleum generation [124,125,126].
Another useful insight obtained from this study is that non-visible volcanic input and hydrothermal alteration are two important processes that may affect the primitive metallic signatures of organic-rich black shales. Traditional paleoproductivity proxies such as Sibio, Babio, and Porg may not be useful for evaluating whether the organic matter accumulation in volcanically-affected black shales is mainly controlled by nutrient conditions of seawater, as these elements may be hosted by non-visible volcanic ashes in black shales. In addition, the precipitation of authigenic minerals (e.g., pyrite) during hydrothermal fluid flows may also affect trace metal distribution in shales, such as by inheriting metals from hydrothermal fluids, redistributing metals via in-situ dissolution–reprecipitation processes [31,127]. Thus, redox proxies based on metals with strong chalcophile affinity, such as Ni and Co, may rather reflect hydrothermally-affected micro-environmental redox features instead of those of contemporaneous seawater. Proxies based on elements that display more conservative behavior during hydrothermal infiltrations may provide reliable information about redox features of seawater (e.g., U/Th) in the study area and possibly other hydrothermally-affected sections elsewhere in the world.

6. Conclusions

This study investigated the mineralogical, total organic carbon (TOC), and major and trace element compositions of organic-rich samples from a volcanic/hydrothermally affected Ordovician/Silurian (Wufeng/Longmaxi formations) section in the Shizhu region, eastern Sichuan Basin. Non-visible volcanic input is identified to influence organic matter accumulation according to the broad correlations between TOC and proxies including Zr and Hf contents, Cr/Al2O3, V/Al2O3, Ni/Al2O3, and SiO2/Al2O3 ratios. Redox indicators, such as V/Cr, v/v + Ni, degree of pyritization (DOP), U/Th, and Mo contents, display positive correlations with TOC values, suggesting that an oxygen-depleted environment is necessary for organic matter preservation. The TOC values exhibit better regression coefficients (R2) against redox indicators, including DOP (0.43), U/Th (0.70), and Mo contents (0.62), compared to V/Cr (0.16) and v/v + Ni (0.21). This may be attributed to the fact that portions of V, Cr, and Ni are hosted in non-visible volcanic ashes in shales but not inherited from water columns. The higher degree of scatter in TOC-DOP and TOC-Mo relative to TOC-U/Th relations may be caused by hydrothermal venting, as evidenced by the coexistence of framboid and euhedral pyrites, as well as the previous finding of hydrothermally altered dolomites in the studied section. Mo is a chalcophile element and can be redistributed during the precipitation of hydrothermal pyrites. In addition, there is no systematic relationship between TOC and Ni/Co ratios, and this may be caused by portions of Ni being contributed by non-visible volcanic ashes and Ni and Co also being redistributed during the hydrothermal venting due to their strong chalcophile affinities. Such a feature may further suggest that large portions of pyrites are precipitated during hydrothermal venting. The DOP displays broad correlations with non-visible volcanic indicators, supporting the idea that hydrothermal venting may be triggered by volcanic activities.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/jmse13030483/s1, Table S1. Mineralogical compositional variation from XRD analysis across the Ordovician/Silurian boundary in the Shizhu outcrop. Table S2. Total organic carbon (TOC), total sulfur (TS), and major element content variations across the Ordovician/Silurian boundary in the Shizhu outcrop. Table S3. Trace metal content variations (in ppm) across the Ordovician/Silurian boundary in the Shizhu outcrop. *F.m. denotes formation. LMX, GYQ, and WF denote the Longmaxi Formation, Guanyinqiao Bed, and Wufeng Formation, respectively. Table S4. Rare earth element content variations (in ppm) across the Ordovician/Silurian boundary in the Shizhu outcrop. *F.m. denotes formation. LMX, GYQ, and WF denote the Longmaxi Formation, Guanyinqiao Bed, and Wufeng Formation, respectively. Total REE denotes total rare earth element content. Table S5. Metal ratio variations across the Ordovician/Silurian boundary in the Shizhu outcrop. *F.m. denotes formation. LMX, GYQ, and WF denote the Longmaxi Formation, Guanyinqiao Bed, and Wufeng Formation, respectively. DOP denotes Degree of Oxidation Parameter. δCe = [Ce{measured}/(La {measured} + Pr {measured})]/[Ce {PAAS}/(La {PAAS} + Pr {PAAS})]. δEu = [Eu {measured}/(Sm {measured} + Gd {measured})]/[Eu {PAAS}/(Sm {PAAS} + Gd {PAAS})]. Ba bio denotes biological barium, Si bio denotes biological silicon, Porg denotes organic phosphorus, and EF denotes enrichment factors.

Author Contributions

S.L., Conceptualization, Writing—original draft, and Methodology; Z.Z., Validation, Data curation, and Writing—original draft; Q.X., Validation, Writing—review and editing, and Supervision; S.C., Formal analysis, Validation, Data curation, and Writing—review and editing; H.L., Resources, Visualization, and Supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Open Fund of the SINOPEC Key Laboratory of Geology and Resources in Deep Stratum (grant number 33550000-24-ZC0699-0020), Postdoctoral Fellowship Program of CPSF under Grant Number GZC20232240, the National Natural Science Foundation of China (grant numbers U20B6001, 92255302), China Postdoctoral Science Foundation (grant number 2024M762776), and Post-doctoral Project of Hubei Province under Grant Number 2024HBBHCXB082. The APC was funded by the National Natural Science Foundation of China (grant number 42073066, 42030803).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original data presented in the study are openly available in Kdocs at https://kdocs.cn/l/csM6hhpO0KCE.

Acknowledgments

This work was supported by the Open Fund of the SINOPEC Key Laboratory of Geology and Resources in Deep Stratum (33550000-24-ZC0699-0020), Postdoctoral Fellowship Program of CPSF under Grant Number GZC20232240, the National Natural Science Foundation of China (No. 42030803, U20B6001, 92255302), China Postdoctoral Science Foundation (2024M762776), and Postdoctoral Project of Hubei Province under Grant Number 2024HBBHCXB082.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (A) Location of Sichuan Basin in China (modified after Hu et al. [56]); (B) The simplified strata column of the studied area (modified after Hu et al. [56]).
Figure 1. (A) Location of Sichuan Basin in China (modified after Hu et al. [56]); (B) The simplified strata column of the studied area (modified after Hu et al. [56]).
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Figure 2. Variation in mineralogical compositions and occurrence of coarse pyrites across the Ordovician/Silurian boundary in the Shizhu area with burial depth. “*”denotes “Note”.
Figure 2. Variation in mineralogical compositions and occurrence of coarse pyrites across the Ordovician/Silurian boundary in the Shizhu area with burial depth. “*”denotes “Note”.
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Figure 3. Variations in contents of SiO2 (A), Al2O3 (B), CaO (C), MgO (D), Fe2O3 (E), K2O (F), N2O (G), MnO (H), TiO2 (I), P2O5 (J), TOC (K), and S (L) and S/Fe ratios (M), degree of pyritization (DOP, (N)), and Ti/Al ratios (O) across the Ordovician/Silurian boundary in the Shizhu area.
Figure 3. Variations in contents of SiO2 (A), Al2O3 (B), CaO (C), MgO (D), Fe2O3 (E), K2O (F), N2O (G), MnO (H), TiO2 (I), P2O5 (J), TOC (K), and S (L) and S/Fe ratios (M), degree of pyritization (DOP, (N)), and Ti/Al ratios (O) across the Ordovician/Silurian boundary in the Shizhu area.
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Figure 4. Variations in V/Cr (A), v/v + Ni (B), V/Sc (C), Ni/Co (D), U/Th (E), δU (F), Mo/S (G), Mo/TOC (H), δCe (I), δEu (J), total REE contents (K), La/Sc (L), Nb/Ta (M), Th/La (N), Zr/Hf (O), Babio (P), Sibio (Q), and Porg (R) across the Ordovician/Silurian boundary in the Shizhu area.
Figure 4. Variations in V/Cr (A), v/v + Ni (B), V/Sc (C), Ni/Co (D), U/Th (E), δU (F), Mo/S (G), Mo/TOC (H), δCe (I), δEu (J), total REE contents (K), La/Sc (L), Nb/Ta (M), Th/La (N), Zr/Hf (O), Babio (P), Sibio (Q), and Porg (R) across the Ordovician/Silurian boundary in the Shizhu area.
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Figure 5. REE distribution patterns of organic-rich samples from the Longmaxi Formation (A) and Wufeng Formation (B).
Figure 5. REE distribution patterns of organic-rich samples from the Longmaxi Formation (A) and Wufeng Formation (B).
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Figure 6. Binary plots of element contents and ratios for identifying non-visible volcanic input to organic-rich samples: (A) TOC vs. Zr; (B) TOC vs. Hf; (C) TOC vs. SiO2/Al2O3; (D) TOC vs. Cr/Al2O3; (E) TOC vs. Ni/Al2O3; and (F) TOC vs. V/Al2O3. The threshold values are derived from (Yang et al. [10]). The red dotted lines in the figure indicate the threshold values for each ratio.
Figure 6. Binary plots of element contents and ratios for identifying non-visible volcanic input to organic-rich samples: (A) TOC vs. Zr; (B) TOC vs. Hf; (C) TOC vs. SiO2/Al2O3; (D) TOC vs. Cr/Al2O3; (E) TOC vs. Ni/Al2O3; and (F) TOC vs. V/Al2O3. The threshold values are derived from (Yang et al. [10]). The red dotted lines in the figure indicate the threshold values for each ratio.
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Figure 7. Binary plots of TOC against proxies based on metallic elements in organic-rich samples: (A) TOC vs. Babio; (B) TOC vs. Porg; (C) TOC vs. SiObio; (D) TOC vs. V/Cr; (E) TOC vs. Ni/Co; (F) TOC vs. v/v + Ni; (G) TOC vs. DOP; (H) TOC vs. U/Th; and (I) TOC vs. Mo.
Figure 7. Binary plots of TOC against proxies based on metallic elements in organic-rich samples: (A) TOC vs. Babio; (B) TOC vs. Porg; (C) TOC vs. SiObio; (D) TOC vs. V/Cr; (E) TOC vs. Ni/Co; (F) TOC vs. v/v + Ni; (G) TOC vs. DOP; (H) TOC vs. U/Th; and (I) TOC vs. Mo.
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Figure 8. Binary plots for identifying redox conditions of contemporaneous seawater for organic-rich samples: (A) MoEF vs. UEF; (B) V/Cr vs. Ni/Co; (C) v/v + Ni vs. Ni/Co; and (D) Mo vs. Ni/Co. The threshold values are derived from Rimmer [83] and Tribovillard et al. [15].
Figure 8. Binary plots for identifying redox conditions of contemporaneous seawater for organic-rich samples: (A) MoEF vs. UEF; (B) V/Cr vs. Ni/Co; (C) v/v + Ni vs. Ni/Co; and (D) Mo vs. Ni/Co. The threshold values are derived from Rimmer [83] and Tribovillard et al. [15].
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Figure 9. Binary plots of the degree of pyritization (DOP) vs. redox proxies Mo contents (A) and U/Th (B).
Figure 9. Binary plots of the degree of pyritization (DOP) vs. redox proxies Mo contents (A) and U/Th (B).
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Figure 10. Typical SEM (scanning electron microscope) photomicrographs of organic-rich shales showing the coexistence of pyrite framboids and euhedral pyrite grains in the study area: (A) Longmaxi Formation, 0.82 m; (B) Longmaxi Formation, 1.12 m; (C) Wufeng Formation, 5.26 m; and (D) Wufeng Formation, 5.75 m. Py = pyrite; OM = organic matter.
Figure 10. Typical SEM (scanning electron microscope) photomicrographs of organic-rich shales showing the coexistence of pyrite framboids and euhedral pyrite grains in the study area: (A) Longmaxi Formation, 0.82 m; (B) Longmaxi Formation, 1.12 m; (C) Wufeng Formation, 5.26 m; and (D) Wufeng Formation, 5.75 m. Py = pyrite; OM = organic matter.
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Figure 11. Binary diagram of DOP against Zr content (A), Hf content (B), SiO2/Al2O3 (C), Cr/Al2O3 (D), Ni/Al2O3 (E), and V/Al2O3 (F). The red dotted lines in the figure indicate the threshold values for each ratio.
Figure 11. Binary diagram of DOP against Zr content (A), Hf content (B), SiO2/Al2O3 (C), Cr/Al2O3 (D), Ni/Al2O3 (E), and V/Al2O3 (F). The red dotted lines in the figure indicate the threshold values for each ratio.
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Figure 12. Illustrative diagram showing the interplay among volcanism, hydrothermal fluid venting, and organic matter accumulation in the study area.
Figure 12. Illustrative diagram showing the interplay among volcanism, hydrothermal fluid venting, and organic matter accumulation in the study area.
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Li, S.; Zhu, Z.; Xiao, Q.; Cai, S.; Li, H. Linking Volcanism, Hydrothermal Venting, and Ordovician/Silurian Marine Organic-Rich Sediments in the Eastern Sichuan Basin, Southwest China. J. Mar. Sci. Eng. 2025, 13, 483. https://doi.org/10.3390/jmse13030483

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Li S, Zhu Z, Xiao Q, Cai S, Li H. Linking Volcanism, Hydrothermal Venting, and Ordovician/Silurian Marine Organic-Rich Sediments in the Eastern Sichuan Basin, Southwest China. Journal of Marine Science and Engineering. 2025; 13(3):483. https://doi.org/10.3390/jmse13030483

Chicago/Turabian Style

Li, Shaojie, Zhou Zhu, Qilin Xiao, Suyang Cai, and Huan Li. 2025. "Linking Volcanism, Hydrothermal Venting, and Ordovician/Silurian Marine Organic-Rich Sediments in the Eastern Sichuan Basin, Southwest China" Journal of Marine Science and Engineering 13, no. 3: 483. https://doi.org/10.3390/jmse13030483

APA Style

Li, S., Zhu, Z., Xiao, Q., Cai, S., & Li, H. (2025). Linking Volcanism, Hydrothermal Venting, and Ordovician/Silurian Marine Organic-Rich Sediments in the Eastern Sichuan Basin, Southwest China. Journal of Marine Science and Engineering, 13(3), 483. https://doi.org/10.3390/jmse13030483

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