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Article

Multiple Controls on Organic Matter Accumulation in the Intraplatform Basin of the Early Cambrian Yangtze Platform, South China

by
Qiyang Zhang
1,
Entao Liu
1,*,
Songqi Pan
2,
Hua Wang
1,*,
Zhenhua Jing
2,
Zhengfu Zhao
2 and
Ruiyue Zhu
1
1
Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences, Wuhan 430074, China
2
Sinopec Petroleum Exploration and Production Research Institute, Beijing 100083, China
*
Authors to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2023, 11(10), 1907; https://doi.org/10.3390/jmse11101907
Submission received: 4 September 2023 / Revised: 28 September 2023 / Accepted: 29 September 2023 / Published: 2 October 2023
(This article belongs to the Special Issue High-Efficient Exploration and Development of Oil & Gas from Ocean)
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Abstract

:
Studying the accumulation rules of organic matter (OM) in paleo-ocean sediments can not only enhance our understanding of how OM becomes enriched in ancient oceans but also provide guidance for the exploration of shale gas in unconventional shale strata. A breakthrough has been made in shale gas exploration in the early Cambrian Qiongzhusi Formation in South China. However, less attention has been paid to the intraplatform basin of the Yangtze Platform, and the factors controlling organic matter enrichment in this special region remain unclear. This study focuses on a continuous drilling core across the full well section of the Qiongzhusi Formation in the intraplatform basin of the Yangtze Platform. Through the comprehensive analysis of total organic carbon (TOC), major and trace elements, and Mo isotopes, this study investigates the controlling factors for OM enrichment with δ98/95Mo ratios utilized to identify the existence of euxinic bottom water. The examined 240 m long core can be divided into four units, where the TOC values of the lower Units 1 and 2 (0.2–5.0 wt.%) average higher than the upper Units 3 and 4 (0.2–2.5 wt.%). Redox indicators (U/Th, Ni/Co, EF(Mo)—EF(U)) indicate an increasing oxidation of bottom waters from the bottom upwards. δ98/95Mo data further confirm the presence of weakly euxinic conditions in Units 1 and 2, addressing the ongoing controversy surrounding bottom water redox environments. Primary productivity indicators (Ni/Al, Cu/Al) suggest a relatively low average productivity level within the intraplatform basin. The upwelling indicators EF(Co) * EF(Mn) of different profiles in the Yangtze Platform suggest that low productivity within the intraplatform basin can be mainly attributed to the absence of upwelling. Consequently, this study proposes an organic matter enrichment mechanism for the Qiongzhusi Formation in the intraplatform basin, which emphasizes the significance of the redox environment in the formation of high-quality hydrocarbon source rocks in restricted environments that lack upwelling, setting it apart from the deep ocean. These findings have the potential to provide valuable insights for the exploration of high-quality hydrocarbon source rocks in other similar regions.

1. Introduction

Significant biological, oceanographic, and geochemical changes occurred during the Ediacaran–Cambrian (E–C) transition [1,2,3,4,5,6]. Serving as a key record of this interval, lower Cambrian black shales contain valuable information about the marine environment and the evolution of life [7,8,9]. There are two main modes of organic matter (OM) accumulation: a preservation mode primarily controlled by redox conditions [10,11], with the modern Black Sea being a typical example, and a productivity mode primarily influenced by high productivity [12], such as the modern Arabian Sea. The organic matter enrichment of marine shale is affected by high palaeoproductivity and anoxic environment [13,14,15,16]. Additionally, some studies emphasize that OM enrichment is also influenced by factors like water mass restrictions, circulation pattern changes, and terrigenous input [11,17,18,19,20,21]. Recently, an increasing number of studies suggest that the enrichment of OM in shale is likely controlled by multiple factors, including productivity and redox conditions, with potentially different controlling processes in different environmental types [15,20,21,22,23].
During the Early Cambrian, widespread deposition of lower Cambrian Qiongzhusi Formation black shale occurred on the Yangtze Block under global sea level rise and transgression [24,25]. During this interval, the Yangtze Block developed various depositional environments, including deep-water slope and basinal facies, transitional facies, carbonate platform facies, and an intraplatform basin [7,26,27] (Figure 1a). Previous studies based on profile sampling and drilling analysis have revealed sea level fluctuations and upwelling events occurring in deep-water slope and basinal facies and carbonate platform facies. These events further influenced the OM enrichment through modifying water column redox conditions and primary productivity within the region [20,28,29,30,31,32]. However, compared to other well-investigated regions of the Yangtze Block [23,33,34], whether the upwelling existed in the region of the intraplatform basin is still unclear.
The facies of the intraplatform basin represent a favorable accumulation zone for high-quality hydrocarbon source rocks during the Early Cambrian [20], with the Anyue and Weiyuan Gas Fields discovered on the two sides [37] (Figure 1a), and possess high hydrocarbon recovery [38]. Compared to the other structural units (e.g., slope to deep ocean and carbonate platform) that have been widely investigated in previous studies (such as in References [23,33,34]), less attention has been paid to the intraplatform basin [20]. Moreover, there exist different points of view on the bottom water redox conditions and OM accumulation mechanisms in the western part of the intraplatform basin [20,39,40,41]. Previous studies, based on the absence of Ni-Mo sulfide deposits in shale cores and sedimentary iron speciation analysis, suggest that Qiongzhusi Formation shale in the western part of the basin was deposited in an anoxic environment without euxinic bottom waters [39,40] (Figure 1a). Conversely, some studies (such as in References [41,42]) indicate the presence of euxinic bottom waters in the western part of the intraplatform basin (Figure 1a). Consequently, the primary controlling factors for OM enrichment in the lowermost high-total organic carbon (TOC) zone and the mechanism of OM enrichment during the deposition of Qiongzhusi Formation in the western part of intraplatform basin remain unclear.
In this study, we conducted a comprehensive geochemical analysis of the continuous well W207 located in the western part of the intraplatform basin of the Yangtze Platform, South China. This analysis included major elements, trace elements, TOC contents, and Mo isotopes, aiming to reconstruct the bottom water redox conditions and primary productivity conditions during the deposition of lower Cambrian Qiongzhusi Formation shale in the western part of the intraplatform basin, as well as to clarify the primary controlling factors and enrichment mechanisms of OM accumulation at different units.

2. Geological Setting

Around 850–820 Ma [23,43,44], the South China Block gradually separated into the Yangtze Block and the Cathaysia Block under intense rifting processes (Figure 1b). During the E-C transition, the Yangtze Block, evolving from a rift basin into a passive continental margin basin, formed a carbonate platform dominated by carbonate deposits, a narrow marginal transitional zone dominated by black chert and shale series (outer shelf), and a deep-water slope and basinal zone dominated by Liuchapo Formation siliceous rocks [9,20,24,45,46] (Figure 1a), with increasing water depth across the three facies belts.
Recent studies have discovered the presence of an intraplatform basin within the carbonate platform, which developed from the Late Ediacaran to Early Cambrian [47,48] (Figure 1a), with the slopes on both sides of the intraplatform basin showing a steeper gradient in the east and a gentler gradient in the west. The distribution of black shales formed during transgressions may be controlled by the intraplatform basin [20,40,48,49,50]. A newly drilled evaluation well (W207) near the western side of the intraplatform basin provides an opportunity for this study. In the lower Cambrian, the Maidiping Formation, Qiongzhusi Formation, and Canglangpu Formation developed from bottom to top. However, with the uplift and erosion of Tongwan Movement, the Maidiping Formation only exists in some areas in the middle of Yangtze Platform [51,52] and is missing in the W207 well. The Qiongzhusi Formation unconformably overlies the dolomite of the Dengying Formation (Figure 1c).
Stratigraphic correlation of the Qiongzhusi Formation has been carried out through typical marker beds, biostratigraphy, U-Pb ages, cyclostratigraphy, and geochemistry [36,53]. The black shale at the base of the Shiyantou Formation in the Xiaotan section represents the onset of the last appearance of a small shell fossil assemblage (SSFA-3), which can serve as the boundary between the Maidiping and Qiongzhusi Formation [36]. The CA-ID-TIMS zircon U-Pb ages of 526.86 ± 0.16 Ma in the Meishucun section reported by Yang [54] provide support for the age of the base of the Qiongzhusi Formation. Lower Cambrian Ni-Mo ore layers discovered at the bottom of the GK1 and Z4 wells can be used as a boundary between the second and third stages of the Cambrian, with a Re—Os isochron age of 521 ± 5 Ma identified within the layers [53].
Based on biostratigraphic division, Liang et al. [55] found that the appearance of Tsunyidiscus niutitangensis in the W207 well corresponds to the extreme values of the GR curve at the boundary of Units 3 and 4. This biozone of trilobite may be associated with Tsunyidiscus acutus-Tsunyidiscus yanjiazhaiensis at the GR extreme values (3295.89 m) in the JY1 well, and the age of this interval in the JY1 well has been defined at approximately 519.43 Ma [36,56]. Additionally, existing research has shown that the Tsunyidiscus acutus-Tsunyidiscus yanjiazhaiensis found at 3295.89 m in the JY1 well can be fully matched with the fossil assemblage of Tsunyidiscus niutitangensis that appears in the Jinsha section, possibly representing the first appearance of trilobites [36,56]. The boundary between the Qiongzhusi and Canglangpu Formation in the Xiaotan section is determined as 515 Ma [31], which may reveal the boundary between the Qiongzhusi and Canglangpu Formation in wells such as ZY1 and GK1 [20].
The Qiongzhusi Formation can be divided into four third-order sequences (SQ1, SQ2, SQ3, SQ4) from bottom to top [20,36,57,58]. Combining the previously defined sequence boundaries with the geochemical data in this study, the Qiongzhusi Formation is divided into four units: Unit 1 (3245–3203 m) primarily consists of black shales; Unit 2 (3203–3111 m) mainly comprises interbedded black shales and dark gray siltstone; Unit 3 (3111–3045 m) is primarily composed of lower black shales and upper dark gray siltstone; and Unit 4 (3045–3000 m) mainly consists of black shales with a top layer of dark gray siltstone (Figure 1c).

3. Samples and Methods

According to the lithology and thickness of the target interval, uniform sampling was carried out for the Qiongzhusi Formation in the W207 well. A total of 90 samples were measured for TOC analysis, major and trace element analysis, and 30 samples for Mo isotope analysis. The location and depth of sample collection are shown in Figure 1 and Table S1. Of course, before all geochemical analysis, samples with veins, nodules, and phosphatic were removed to ensure the authenticity of the data. Then, the remaining samples were ground to 200 mesh size through hand grinding with agate mortar.

3.1. TOC

First, the carbonate in the powder samples (200 mesh size) was removed through adding hydrochloric acid (10%). Then, the residual hydrochloric acid in the sample was removed through washing with distilled water. After the samples were dried overnight, they were analyzed using the CHNOS Elemental Analyzer (Vario EL III) in the Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan).

3.2. Major and Trace Elements

The analysis of major elements in the samples was performed using the wavelength-dispersive X-ray fluorescence (XRF) spectrometer AXIOSmAX. Mix the processed powder sample, flux (Li2B4O7, LiBO2, LiF) and oxidant (NH3NO3) in a ratio of 1:10:3. After being heated in a furnace at 1150 °C for 14 min, glass slices were prepared for the XRF test.
Trace elements were analyzed using an Agilent 7700 inductively coupled plasma mass spectrometer (ICP-MS). The volatiles in the samples were removed via burning them in a muffle furnace at 600 °C for 12 h. Then, at a temperature of 190 °C, hydrofluoric acid (HF) and nitric acid (HNO3) were added into the Teflon beaker containing the powder sample for dissolution treatment. The final solution was then analyzed via ICP-MS with a final analysis accuracy better than ±5%.
The enrichment factors can be calculated as follows:
XEF = [(X/Al) sample/(X/Al) PAAS],
where the “X” is the selected target trace element. The subscript “sample” refers to the weight ratio of element X to AL in this study sample [59], and “PAAS” denotes the Post-Archean Australian Shale (PAAS) standard [60].

3.3. Mo Isotopic Compositions (δ98/95Mo)

Mo isotopic compositions were analyzed via Thermo-Fisher Scientific Neptune Plus (MC-ICP-MS) in the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China. According to the double-spike method [61], an appropriate amount of 97Mo–100Mo double spikes was added to the sample according to the Mo concentration of the samples. Then, at 130 °C, the sample was dissolved and purified with HNO3, HF, and HCI, and then column separation was performed. The purified Mo was determined via MC–ICP–MS by means of ion-exchange chromatography using BPHA resin [61]. The double-spike and normalizing standard (NIST SRM 3134) was corrected through following the procedure of Siebert et al. [62] and Zhao et al. [63]. At the same time, during the analysis process, the δ98/95Mo of NIST SRM 3134 standard solution and IAPSO seawater was repeatedly measured. The measured values of δ98/95Mo of the two are 0.00 ± 0.05‰ (2SD, n = 15) and 2.06 ± 0.06‰, respectively, which are consistent with the certified values and literature reports [63,64,65]. Meanwhile, the Mo procedural blank value (<0.1 ng) is much smaller than the total amount of Mo in the samples. Finally, in order to facilitate international comparison and analysis, the δ98/95Mo values in the samples were recalculated based on the NIST SRM 3134 standard solution (0.25‰).

4. Results

As shown in Figure 2 and Figure 3 and Table 1, Unit 1 (3245–3203 m) exhibits relatively high TOC contents (average of 2.5 wt.%), ranging from 0.7 to 4.6 wt.%, with a decreasing trend from bottom to top. The δ98/95Mo values vary between −0.25‰ and 2.23‰, displaying a significant fluctuation range. The ratios of U/Th (0.6–4.4) and Ni/Co (2.6–11.3) (Figure 2), as well as the enrichment levels of EF(U) (3.2–22) and EF(Mo) (9.5–204.2), show relatively high values and exhibit similar variations (Figure 3). Additionally, Ni/Al (6.0 × 10−4 to 28.0 × 10−4) and Cu/Al (4.1 × 10−4 to 10.0 × 10−4) ratios are also relatively high (Figure 2).
In Unit 2 (3203–3111 m), the TOC content varies from 0.2 to 5.0 wt.%, with an average of 1.9 wt.%. The δ98/95Mo values range from −0.54‰ to 2.37‰, gradually increasing from the bottom to the middle part of the unit (3158.59 m), and then decreasing. The ratios of U/Th (0.3–5.3) and Ni/Co (2.3–10.0) (Figure 2), as well as the enrichment levels of EF(U) (1.5–23.7) and EF(Mo) (1.9–200.1), show relatively consistent values compared to Unit 1 (Figure 3). The ratios of Ni/Al (4.5 × 10−4 to 27.4 × 10−4) and Cu/Al (3.0 × 10−4 to 9.2 × 10−4) exhibit similar trends (Figure 2), with higher values at the bottom and lower values in the middle part of Unit 2.
In Unit 3 (3111–3045 m), the TOC content ranges from 0.2 to 1.5 wt.%, with an average of 0.7 wt.%, relatively lower compared to Unit 1 and Unit 2. It gradually decreases in the vertical direction. The δ98/95Mo values are relatively stable, fluctuating between 0.92‰ and 1.26‰, with a decreasing trend in the vertical direction (Figure 3). The ratios of U/Th (0.4–1.7) and Ni/Co (3.0–6.5), as well as the enrichment levels of EF(U) (2.4–8.3) and EF(Mo) (4.3–33.2), are relatively low and show small variations. The ratios of Ni/Al (4.6 × 10−4–13.3 × 10−4) and Cu/Al (3.0 × 10−4–6.7 × 10−4) gradually decreased from the bottom and then rebounded at the top of the interval, lower than those of Unit 1 and Unit 2 (Figure 2).
In Unit 4 (3045–3000 m), the TOC content varies between 0.2 and 2.5 wt.% (average of 0.9 wt.%), obviously higher than that of Unit 3. The δ98/95Mo values continued to decrease from 0.69‰ to −0.10‰ (Figure 3). The enrichment degree of U/Th (0.5–2.5) and Ni/Co (2.8–4.1) ratios, as well as EF(U) (2.4–12.6) and EF(Mo) (4.6–46.0), was relatively low within this interval. The Ni/Al (6.8 × 10−4–17.6 × 10−4) and Cu/Al (3.7 × 10−4–7.7 × 10−4) ratios showed a noticeable increase compared to the third stage, steadily rising upwards (Figure 2).

5. Discussion

5.1. The Degree of Oxygenation in the Bottom Water Environment Continues to Increase

The ratios of U/Th and Ni/Co have been widely used to reconstruct paleoceanic redox environments [17,70,71,72,73,74]. The U/Th and Ni/Co ratios in Unit 1 (average of 2.10 and 6.41, respectively) and Unit 2 (average of 1.32 and 5.13, respectively) were relatively high, reflecting an anoxic environment. The relatively low values of these ratios in Unit 3 (average of 0.73 and 4.06) and Unit 4 (average of 0.77 and 3.71) reflect a relatively stable oxic–suboxic environment (Figure 2). Relatively low U/Th and Ni/Co ratios suggest an upward enhancement in the oxidation state of the bottom water environment on the western side of the intraplatform basin [68,73,75,76] (Figure 2). This interpretation is consistent with redox conditions inferred from Mo content, EF(Mo)/EF(U), and Mo/TOC ratios (Figure 3 and Figure 4a).
It should be noted that in restricted environments, Mo concentrations in the water column may decrease due to the lack of open ocean inputs [80,81,82]. In Unit 1, the Mo content (average of 40.67) and EF(Mo)/EF(U) ratios (5.42 on average) support the anoxic environment indicated by the U/Th and Ni/Co ratios. The extreme point observed in sample W-02 (3242.1 m) (Mo content of 137 ppm, EF(Mo)/EF(U) ratios of 11.06) reflects a possible weakly euxinic environment in the water column (Figure 3). Combining the sea level change curve [36] and various trace elements such as EF(Mo) and EF(U) (Figure 1c and Figure 3), Unit 2 can be divided into two cycles that first increase and then decrease. The relative high points of these cycles (W-20 at 3131 m and W-34 at 3175.7 m) correspond to the occurrence of two transgressive events. The large-scale transgressions connected the intraplatform basin to the open sea, replenishing trace elements (Mo, U, etc.) and increasing the degree of water reduction. A weakly euxinic condition may have occurred at 3175 m (Figure 3), which is consistent with no significant correlation between the TOC and redox-sensitive trace elements such as U/Th, Ni/Al and Cu/Al in this point [83] (Figure 5a–f). Considering the anoxic environment indicated by the U/Th and Ni/Co ratios in Unit 2, the EF(Mo)/EF(U) ratios (average of 5.45) likely indicate dominant anoxic with occasional and weakly euxinic water conditions. This viewpoint is supported by Mo isotope evidence (see details in Section 5.2). Similar short-term euxinic events in anoxic environments have also been observed in the ZY1 well and Jinsha section on the eastern side of the intraplatform basin during the same time [20,31]. In Units 3 and 4, the occurrence of the regression event during Cambrian Age 3 was accompanied by the development of oxidizing bottom water conditions. This interpretation is consistent with the low EF(Mo)/EF(U) ratios (average of 3.09, 3.14) representing an oxic–suboxic environment. From bottom to top, the Qiongzhusi Formation experienced an anoxic–euxinic–oxic–suboxic bottom water environment.
The redox environment of bottom water is closely related to the hydrographic conditions of the basin. The Mo/TOC ratio has been widely used as an indicator of basin water connectivity limitations [81] (Figure 4b). However, due to the possibility of lower Mo concentrations in early Cambrian oceans compared to modern seawater [84,85,86], Cheng et al. [87] suggested that when the Mo/TOC ratio in sediments exceeds 11 ppm/wt.%, good connectivity between the basin and the open sea can be considered. In Unit 1, the relatively high Mo/TOC values (average of 14.52 ppm/wt.%) indicate low basin water restrictiveness, favorable for the existence of anoxic bottom water environments. The Mo/TOC values in Unit 2 correspond well to the two transgression events, indicating increasing basin openness with the occurrence of transgressions. The oxic–suboxic environment in Unit 3 corresponds to the regression during Cambrian Age 3 (Figure 1c and Figure 3), and the weakening connection between the basin and the open sea prevents the replenishment of Mo consumed in the ocean, resulting in a decrease in Mo and U values (average of 8.78 and 7.86, respectively) [81]. In Unit 4, the level of basin restrictiveness further increased, accompanied by a decrease in Mo/TOC values (Figure 4b).

5.2. δ98/95Mo Isotopes and Related Redox Conditions

Mo isotopes in organic-rich sedimentary rocks have been widely used in reconstructing local and global marine redox states because it exhibits high sensitivity to redox conditions [65,67,75,86,88,89,90,91]. There are two main modes of Mo output in the ocean. In oxic environments, iron or manganese oxides preferentially adsorb the light Mo isotope, resulting in significant isotope fractionation between sediments and seawater, up to ~3% fractionation [92,93]. In euxinic environments, when the concentration of H2S in the water solution is high ([H2S]aq > 11 μM), oxyanion molybdate (MoO42−), as the main form of modern seawater Mo, is almost completely converted to oxythiomolybdate species (MoO4-xSx2−, x = 1–4), with little isotope fractionation [66,94]. When the concentration of H2S in the water solution is low ([H2S]aq < 11 μM), the incomplete conversion of molybdate (MoO42−) leads to significant isotope fractionation in the sedimentary Mo isotopes [92,95]. This also explains why Mo isotopes can serve as redox proxies. Before analyzing the Mo isotopes, we eliminated possible detrital effects using the method proposed by Wen et al. [67]. The corrected results show that the measured Mo isotope values (δ98/95Mo) are nearly equivalent to the corrected values (δ98/95Moauth) (Table 1), indicating a minimal impact of detrital Mo on bulk Mo isotopes. Therefore, we directly discuss the measured values in the following discussion.
The presence of weak euxinia in the anoxic bottom water conditions has been confirmed by δ98/95Mo values from the western core of the intraplatform basin. Negative δ98/95Mo values (−0.08‰ to −0.54‰) appear at the bottom of Units 1 and 2 (Figure 3). The occurrence of negative Mo isotopes can usually be attributed to three possible mechanisms: the first possibility is Fe-Mnox adsorption in the oxic environment [96], but the higher U/Th and Ni/Co ratios in Unit 1 and 2 preclude this possibility. The second possibility is Fe-Mnox shuttling [68,87,97,98], similar to the modern Baltic Sea and Cariaco Basin [97,99]. Under the shuttle effect of iron and manganese oxide, lighter Mo isotopes enter the sediment, leading to negative δ98/95Mo values. In this case, euxinic conditions in the sediment result in preferential enrichment of Mo relative to U, producing higher EF(Mo)/EF(U) ratios (~3 × SW) and relatively higher Mo concentrations. At the same time, the lack of support for reductive dissolution inhibits Mn accumulation in the sediment, resulting in a relative deficiency of Mn. The EF(Mo)/EF(U) ratios (7.86) and Mo (55.8 ppm) and Mn (424 ppm) values in sample W-10 at the bottom of Unit 2 support the possibility of Fe-Mnox shuttling. It is worth mentioning that samples W-02 and W-34, mentioned in Section 5.1, also support the possibility of Fe-Mnox shuttling with their negative δ98/95Mo values (0.7‰ and 0.42‰, respectively). The third possibility is based on the incomplete conversion of molybdate (MoO42−) under weakly euxinic water conditions ([H2S]aq < 11μM), causing Mo isotopic fractionation [94,99]. The EF(Mo)/EF(U) ratios of samples with negative δ98/95Mo values in Unit 2 are only about twice that of modern seawater (~1 × SW), which excludes the possibility of Fe-Mnox shuttling. Therefore, the negative δ98/95Mo values in Unit 2 may be attributed to significant fractionation caused by the incomplete conversion of molybdate (MoO42−) under weakly euxinic water conditions. In addition, although Unit 1 is predominantly anoxic, the occurrence of Fe-Mnox shuttling in samples W-02 and W-10, mentioned above, demonstrates the presence of localized euxinic environments in Unit 1. Thus, the appearance of negative δ98/95Mo values in sample W-02 in Unit 1 may also be due to the incomplete conversion of molybdate (MoO42−) under weakly euxinic water conditions or the combined effects of both (such as in Reference [68]).
Except for the negative δ98/95Mo values, samples W-11 (2.23‰) and W-25 (2.37‰) in Units 1 and 2 exhibit relatively high δ98/95Mo values. Similar high values have been reported in profiles from the Yangtze Block and Tarim Basin during the same interval [67,86,87]. The δ98/95Mo values reported by Chen et al. [100] (∼+2.3‰) may reflect the highest δ98/95Mo values of Early Cambrian oceanic seawater, and the two high values are nearly equivalent to contemporaneous seawater δ98/95Mo values (Figure 3), which may indicate smaller isotope fractionation under high-H2S conditions. The Mo isotopes exhibit significant fluctuations (−0.54‰ to 2.37‰) (Figure 3) in Unit 1 and in the lower part of Unit 2 (before the transgression). Except for samples W-02, W-10, and W-34, mentioned earlier, the other samples did not exhibit Fe-Mnox shuttling. Such a large range of fluctuations may reflect isotope fractionation under weakly euxinic water conditions [68,95,99]. Overall, whether it is the extreme points that partially appear or the overall fluctuations in Units 1 and 2, these suggest the development of weakly euxinic conditions in the intraplatform basin.
From 3131 m in Unit 2 to 3000 m in Unit 4, Mo isotopes exhibit a decreasing trend, showing a negative bias since the large-scale transgression of Cambrian Stage 2 (Figure 1c and Figure 3). The average δ98/95Mo value of Ni-Mo sulfide ore layers formed during the transgression in the Maluhe and Dazhuliushui profiles is reported as 1.13 ± 0.14‰ (n = 11), which is consistent with the δ98/95Mo values near the location of the Ni-Mo sulfide ore layers in the W207 well (average of 1.18‰, n = 6) [68,101] (Figure 3). Subsequently, with ongoing regression, the confinement of the intraplatform basin gradually increased, possibly leading to a gradual decrease in Mo isotopic values in the intraplatform seawater to values near those of continental input (+0.7‰) [68,101] (Figure 3).

5.3. Relatively Low Primary Productivity May Be Associated with the Absence of Upwelling

Previous studies suggested that primary productivity can be reconstructed through various geochemical indicators, including TOC values, organic biomarkers, C and N isotopes, and other trace elements such as P, Ba, Cu, and Ni [38,102,103,104,105]. Cu and Ni elements, which are essential nutrients for the growth of phytoplankton in the photic zone, are often associated with OM in sediments [103]. Considering that the assessment of productivity may not be accurate when relying solely on a single indicator [102,106], we studied variations in primary productivity based on multiple indicators (TOC, Cu, Ni) and eliminated the dilutive effects of detrital fractions on Cu and Ni. Cu/Al and Ni/Al showed a strong correlation with TOC (Figure 5e,f) (r = 0.58, p(α) < 0.05 for Cu/Al vs. TOC and r = 0.70, p(α) < 0.05 for Ni/Al vs. TOC), suggesting that most of the Cu and Ni are related to OM and can be used to reconstruct primary productivity in surface waters [20,103]. The shale Cu/Al ratios in the Qiongzhusi Formation minorly decreased from Unit 1 (4.13 × 10−4–10.0 × 10−4, average of 7.20 × 10−4) to Unit 3 (3.0 × 10−4–6.72 × 10−4, average of 4.41 × 10−4), and then increased in Unit 4 (3.71 × 10−4–7.67 × 10−4, average of 5.53 × 10−4), indicating a slight decline in surface water bioproductivity followed by a significant increase upward (Figure 2). A similar trend was also observed in the Ni/Al ratios and TOC values. The Cu/Al (5.47 × 10−4) and Ni/Al (10.2 × 10−4) ratios from well W207 are lower than the corresponding ratios observed in the deep-water slope and basinal facies of well TX-1 (Cu/Al = 31.36 × 10−4, Ni/Al = 29.46 × 10−4) [34]. This finding is consistent with the conclusions in previous studies [20,107] that the productivity of the intraplatform basin facies is lower than the well-developed upwelling events found in the deep-water slope and basinal facies.
Upwelling events are one of the important factors affecting the accumulation of OM and are widely reported in other facies of the Yangtze Platform [23,31,34] (Figure 6a,b and Figure 7). Nevertheless, within the intraplatform basin, the impact of upwelling on OM accumulation remains unclear. Upwelling from the deep sea is often enriched in cadmium (Cd) elements and relatively depleted in cobalt (Co) and manganese (Mn) elements, so when EF(Co)*EF(Mn) < 0.5 or Co*Mn < 0.4, it is considered to indicate the presence of upwelling. Conversely, when EF(Co)*EF(Mn) > 2 or Co*Mn > 0.4, it indicates a restrictive environment [108] (Figure 6a,c). The EF(Co)*EF(Mn) values (0.36–13.84, average of 1.32) and Co*Mn values (0.64–3.40, average of 1.11) of the Qiongzhusi Formation indicate a relatively restricted environment (Figure 6c,d; Table S2), which is further supported by the relatively low Mo/TOC ratios (Figure 3 and Figure 4b).
In the Yangtze Platform, multiple profiles of EF(Co)*EF(Mn) and Co*Mn values within various sedimentary facies reveal a progressive increase in upwelling events from the intraplatform basin towards the deep-water slope and basinal facies. This trend is accompanied by a gradual rise in TOC values (Figure 6a,b and Figure 7). Early findings suggested that upwelling, prevalent within deep-water slope and basinal facies, carried nutrients that effectively enhanced the primary productivity of surface waters, serving as a primary factor for OM accumulation in these regions [23,31,32,34,111]. The EF(Co)*EF(Mn) and Co*Mn values from multiple profiles also corroborate this observation (Figure 6a,b). Therefore, the relatively lower primary productivity in the intraplatform basin facies could potentially be attributed to the absence of upwelling events (Figure 6c,d, and Figure 7). This is in contrast to the deep-water slope and basinal facies, which are characterized by significant upwelling occurrences.

5.4. Mechanisms of OM Enrichment

The enrichment of OM is often influenced by various events, such as sea level changes, upwelling events, and terrigenous input [32,72,86,111,112,113,114,115]. The impact of these events on OM enrichment varies in different geological settings but is ultimately manifested through the control of primary productivity and bottom water preservation conditions [29,72,116].
During the deposition of Unit 1 in Cambrian Stage 2, the strong correlation between redox indices such as U/Th and TOC (R = 0.82, p(α) < 0.05) suggests that the main bottom water conditions are anoxic, providing favorable conditions for the preservation of OM. The relatively high sea level allowed the intraplatform basin to be connected to the open sea, and the input of nutrients sustained high primary productivity. This is consistent with the correlation exhibited by Cu/Al (R = 0.52, p(α) < 0.05), Ni/Al (R = 0.67, p(α) < 0.05), and TOC (Figure 5e,f). Thus, the development of anoxic and high productivity is favorable for increased OM accumulation and preservation (Figure 8A).
Relative sea level changes in the intraplatform basin controlled the redox conditions during the deposition of Unit 2 [117]. Intermittent euxinic and continuous anoxic environments account for the relatively high TOC values. Relatively high primary productivity continuously contributed to OM enrichment with the absence of upwelling events.
During the deposition of Unit 3, as the sea level gradually falls in Cambrian Stage 3, the increasing restriction within the basin led to a reduction in nutrient supply and a decline in primary productivity. At the same time, the predominant bottom water conditions changed from anoxic to suboxic, leading to poor preservation conditions for OM. In addition, the weak negative correlation between Ti/Al and TOC (R = −0.28, p(α) > 0.05) indicated the dilution effect of increasing terrigenous input on OM [115] (Figure 5g). Thus, the low OM content in Unit 3 should be attributed to low productivity, relatively oxidizing conditions, and the dilution of terrigenous input (Figure 8B).
Although Unit 4 is still in an oxidizing environment, its TOC value (0.2–2.5 wt.%, average of 0.9 wt.%) showed a recovery compared to Unit 3. A rapid decrease in Ti/Al ratios indicated a weakening of terrigenous input, which may reflect a transition from arid to humid climate in the intraplatform basin [18,23,118,119] (Figure 2). This change is also consistent with the increase in Rb/Sr ratios and decrease in Sr/Cu ratios (Table S2; Figure 2). The relatively warm and humid climate in Unit 4 promoted the bloom of surface water organisms, which is consistent with the first appearance of Tsunyidiscus niutitangensis in the lower part of Unit 4 [55], indicating relatively higher primary productivity. Overall, although the widespread oxic–suboxic bottom water conditions in the intraplatform basin are unfavorable for the preservation of OM, the increase in primary productivity and the weakening of terrigenous clastic input have relatively compensated for this deficiency (Figure 8C).
Sweere’s analysis of multiple global regions indicates (Figure 6b,d) that Cd/Mo > 0.1 and Co*Mn < 0.4 represent upwelling environments driven by productivity (e.g., Namibian margin, Peruvian margin), while Cd/Mo ratios < 0.1 and Co*Mn > 0.4 reflect restricted basin settings driven by preservation conditions (e.g., the Black Sea). From the Cd/Mo ratios of the four units in this study (Figure 6d), it can be seen that Unit 1 and Unit 2 sample points are mainly located in the “preservation” area, while Unit 3 and Unit 4 samples are mainly located in the “productivity” area.
This is consistent with the TOC values of the shales in Units 1 and 2 being much higher than those in Units 3 and 4. This indicates that preservation conditions are more crucial than productivity conditions in restricted environments that lack upwelling and that they play a critical role in the development of high-quality hydrocarbon source rocks. The enrichment mechanism in the intraplatform basin significantly differs from other facies in the Yangtze Platform, such as the deep-water slope and basinal facies, primarily driven by productivity, or the transitional facies, influenced by both productivity and preservation conditions [20,34,107].

6. Conclusions

(1) During the deposition of the Cambrian Qiongzhusi Formation, the western side of the intraplatform basin within the Yangtze Platform experienced a transition from continuous anoxic conditions with intermittent euxinic to oxic–suboxic bottom water environments. The highly redox-sensitive δ98/95Mo values in Units 1 and 2 support the presence of weakly euxinic conditions in the early Cambrian period, further elucidating the redox environment of the bottom water during this interval.
(2) EF(Co) * EF(Mn) values suggest an absence of upwelling within the intraplatform basin, which diverges from other regions within the Yangtze Platform that showcase substantial upwelling events. The relatively lower primary productivity in the intraplatform basin facies could potentially be attributed to the lack of upwelling events. Primary productivity exhibited a trend of gradual decline from stages 1 to 3, followed by an increase in stage 4.
(3) Qiongzhusi Formation shale exhibits variations in the mechanisms for OM enrichment across distinct units. The lower portion of the Qiongzhusi Formation on the western side of the intraplatform basin (Units 1 and 2) is predominantly influenced by preservation conditions, while the upper portion (Units 3 and 4) is primarily affected by productivity conditions. This indicates that in constrained environments devoid of upwelling, preservation conditions assume a more pivotal role in OM enrichment. This discovery offers valuable insights for the exploration of high-quality hydrocarbon source rocks within the intraplatform basin.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/jmse11101907/s1, Table S1: Total organic carbon (TOC) and Major elements compositions of the lower Cambrian Qiongzhusi Formation; Table S2: Selected trace elements concentrations and enrichment factors of the lower Cambrian Qiongzhusi Formation.

Author Contributions

Conceptualization, E.L. and Q.Z.; methodology, Q.Z.; software, R.Z.; validation, E.L., S.P. and Z.Z.; formal analysis, E.L.; investigation, Q.Z.; resources, S.P.; data curation, Z.J.; writing—original draft preparation, Q.Z.; writing—review and editing, E.L.; visualization, Q.Z.; supervision, H.W.; project administration, S.P.; funding acquisition, H.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Science and Technology Research Project for the Research Institute of Petroleum Exploration and Development, China National Petroleum Corporation (2021YJCQ03).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

We appreciate Research Institute of Petroleum Exploration and Development for providing the data and permission to publish this paper. We are also very grateful to the reviewers and editors for their contributions to improving this paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Paleogeographic map of the Yangtze Block during the Ediacaran–early Cambrian transition (modified form the References [20,23,24]); (b) Early Cambrian global paleogeography (modified form the References [35,36]); (c) Stratigraphic column of the Qiongzhusi Formation in the W207 well (the relative sea level changes are modified from Reference [36]).
Figure 1. (a) Paleogeographic map of the Yangtze Block during the Ediacaran–early Cambrian transition (modified form the References [20,23,24]); (b) Early Cambrian global paleogeography (modified form the References [35,36]); (c) Stratigraphic column of the Qiongzhusi Formation in the W207 well (the relative sea level changes are modified from Reference [36]).
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Figure 2. Vertical variations of total organic carbon (TOC) contents, primary productivity proxies (Cu/Al, Ni /Al), redox proxies (U/Th, Ni/Co), and terrigenous input proxy (Ti/Al).
Figure 2. Vertical variations of total organic carbon (TOC) contents, primary productivity proxies (Cu/Al, Ni /Al), redox proxies (U/Th, Ni/Co), and terrigenous input proxy (Ti/Al).
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Figure 3. Vertical variations of redox proxies (Mo-EF, U-EF, Mo-EF/U-EF, δ98/95Mo, Mo, and Mn) and hydrographic proxy (Mo/TOC).
Figure 3. Vertical variations of redox proxies (Mo-EF, U-EF, Mo-EF/U-EF, δ98/95Mo, Mo, and Mn) and hydrographic proxy (Mo/TOC).
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Figure 4. (a) EF(Mo) versus EF(U) for Qiongzhusi Formation samples [77,78]. The solid line represents the Mo: U ratio in modern seawater. (b) Sediment Mo versus TOC for the Qiongzhusi Formation samples. Four representatives of the modern anoxic basin system (Saanich Inlet, Cariaco Basin, Framvaren Fjord, and Black Sea) are represented by dashed lines in the figure, which reflect different degrees of hydrographic restriction [79].
Figure 4. (a) EF(Mo) versus EF(U) for Qiongzhusi Formation samples [77,78]. The solid line represents the Mo: U ratio in modern seawater. (b) Sediment Mo versus TOC for the Qiongzhusi Formation samples. Four representatives of the modern anoxic basin system (Saanich Inlet, Cariaco Basin, Framvaren Fjord, and Black Sea) are represented by dashed lines in the figure, which reflect different degrees of hydrographic restriction [79].
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Figure 5. Relationships between TOC contents and (a) Mo-EF; (b) U-EF; (c) U/Th ratios; (d) Ni/Co ratios; (e) Ni/Al; (f) Cu/Al ratios; (g) Ti/Al ratios of Qiongzhusi Formation samples.
Figure 5. Relationships between TOC contents and (a) Mo-EF; (b) U-EF; (c) U/Th ratios; (d) Ni/Co ratios; (e) Ni/Al; (f) Cu/Al ratios; (g) Ti/Al ratios of Qiongzhusi Formation samples.
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Figure 6. Cross-plots of (a,c) TOC versus Co-EF × Mn-EF, (b,d) Co (ppm) × Mn (%) versus Cd/Mo ratios of the Qiongzhusi Formation and its equivalents samples [108]. The sample data in (a,b) come from published data of different sections and wells in the Yangtze Block. Data sources: ZY1 [20]; GK1 [46]; Jinsha [31]; Diben [20]; W207 (this study, only part of the sample is shown). c and d show the samples of four different units of the Qiongzhusi Formation in Well W207 (this study).
Figure 6. Cross-plots of (a,c) TOC versus Co-EF × Mn-EF, (b,d) Co (ppm) × Mn (%) versus Cd/Mo ratios of the Qiongzhusi Formation and its equivalents samples [108]. The sample data in (a,b) come from published data of different sections and wells in the Yangtze Block. Data sources: ZY1 [20]; GK1 [46]; Jinsha [31]; Diben [20]; W207 (this study, only part of the sample is shown). c and d show the samples of four different units of the Qiongzhusi Formation in Well W207 (this study).
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Figure 7. Simplified model of organic matter accumulation control during shale deposition of Qiongzhusi Formation in different facies belts of Yangtze block. TOC data of sections and wells are from Wang et al. [40], Wang et al. [42], Cremonese et al. [109], Gao et al. [20,46], Jin et al. [31,39], Chen et al. [24], Wang et al. [9], Yuan et al. [110].
Figure 7. Simplified model of organic matter accumulation control during shale deposition of Qiongzhusi Formation in different facies belts of Yangtze block. TOC data of sections and wells are from Wang et al. [40], Wang et al. [42], Cremonese et al. [109], Gao et al. [20,46], Jin et al. [31,39], Chen et al. [24], Wang et al. [9], Yuan et al. [110].
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Figure 8. Organic matter accumulation models of Qiongzhusi Formation. (A) Units 1 and 2, anoxic bottom water conditions and high primary productivity. (B) Unit 3, suboxic bottom water conditions, low primary productivity, and high terrigenous input. (C) Unit 4, oxic–suboxic bottom water conditions, relatively high primary productivity, warm and humid climate, and low terrigenous input. Data sources: W201 [40]; W207 (this study); Z4 [42]; Xiaotan [39]; ZY1 [20]; GK1 [46].
Figure 8. Organic matter accumulation models of Qiongzhusi Formation. (A) Units 1 and 2, anoxic bottom water conditions and high primary productivity. (B) Unit 3, suboxic bottom water conditions, low primary productivity, and high terrigenous input. (C) Unit 4, oxic–suboxic bottom water conditions, relatively high primary productivity, warm and humid climate, and low terrigenous input. Data sources: W201 [40]; W207 (this study); Z4 [42]; Xiaotan [39]; ZY1 [20]; GK1 [46].
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Table 1. Mo isotope (δ98/95Mo) values of the lower Cambrian Qiongzhusi Formation.
Table 1. Mo isotope (δ98/95Mo) values of the lower Cambrian Qiongzhusi Formation.
SamplesMemberDepthδ98/95Moδ98/95Mo(auth) 12SE 2
m‰‰
W207-90Unit 43003.41−0.10−0.170.04
W207-87Unit 43009.68−0.08−0.140.03
W207-83Unit 43015.33−0.07−0.130.03
W207-76Unit 43031.330.350.330.02
W207-70Unit 43045.350.690.700.03
W207-62Unit 33061.960.921.020.03
W207-56Unit 33074.731.261.370.03
W207-52Unit 33082.831.211.290.03
W207-49Unit 33089.481.171.250.03
W207-45Unit 33098.121.061.080.04
W207-41Unit 33109.601.151.180.02
W207-37Unit 23117.421.241.280.03
W207-34Unit 23131.031.501.520.03
W207-32Unit 23134.001.101.140.04
W207-29Unit 23144.470.720.730.03
W207-26Unit 23152.300.991.030.02
W207-25Unit 23158.592.372.380.03
W207-24Unit 23160.661.251.300.02
W207-22Unit 23168.000.430.470.03
W207-21Unit 23175.100.360.350.02
W207-20Unit 23175.740.420.420.02
W207-19Unit 23179.900.510.510.03
W207-18Unit 23183.420.010.000.02
W207-17Unit 23189.80−0.54−0.580.04
W207-15Unit 13224.681.501.580.03
W207-14Unit 13226.131.691.710.03
W207-11Unit 13230.372.232.410.03
W207-10Unit 13232.92−0.25−0.250.03
W207-05Unit 13239.400.620.620.03
W207-02Unit 13242.100.700.700.03
1 The corrected δ98/95Mo values were calculated following the formula: δ98/95Moauth = [(δ98/95Motot × Motot − δ98/95Modet × Modet)/Moauth] [66,67,68], in which the detrital Mo concentration (Modet) was given by Modet = [(Mo/Al) PAAS × Altot] and the authigenic Mo concentration was calculated by Moauth = Motot − Modet. Mo and Al concentration of PAAS (1.0 ppm, 10%, respectively) [60] and δ98/95Modet value of 0.4‰ [69] were used for detritus correction. 2 2SE means double of Standard Error.
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Zhang, Q.; Liu, E.; Pan, S.; Wang, H.; Jing, Z.; Zhao, Z.; Zhu, R. Multiple Controls on Organic Matter Accumulation in the Intraplatform Basin of the Early Cambrian Yangtze Platform, South China. J. Mar. Sci. Eng. 2023, 11, 1907. https://doi.org/10.3390/jmse11101907

AMA Style

Zhang Q, Liu E, Pan S, Wang H, Jing Z, Zhao Z, Zhu R. Multiple Controls on Organic Matter Accumulation in the Intraplatform Basin of the Early Cambrian Yangtze Platform, South China. Journal of Marine Science and Engineering. 2023; 11(10):1907. https://doi.org/10.3390/jmse11101907

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Zhang, Qiyang, Entao Liu, Songqi Pan, Hua Wang, Zhenhua Jing, Zhengfu Zhao, and Ruiyue Zhu. 2023. "Multiple Controls on Organic Matter Accumulation in the Intraplatform Basin of the Early Cambrian Yangtze Platform, South China" Journal of Marine Science and Engineering 11, no. 10: 1907. https://doi.org/10.3390/jmse11101907

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