Sedimentary Environment and Organic Matter Enrichment Mechanism of the Lower Cambrian Shale in the Northern Margin of the Yangtze Platform

Abstract

Current models of sedimentary environments and organic matter (OM) enrichment for the Lower Cambrian black shales in the Yangtze Platform have not yet incorporated its northern carbonate platform margin where the related research is lacked. This study focuses on the SNZ1 well in the northern carbonate platform margin, utilizing total organic carbon (TOC) content and major and trace element data to reveal the main controlling factors of OM enrichment during the Early Cambrian. The results show that the shale stratum is tentatively ascribed to the Lower Cambrian Stage 3 and that, during its deposition, the redox transitioned from anoxic to suboxic–oxic conditions, the hydrodynamic conditions weakened initially and then strengthened, the primary productivity first increased and then decreased, the paleoclimate shifted from arid–cold to warm–humid conditions, and the terrigenous clastic input gradually diminished. Overall, the OM enrichment is primarily controlled by preservation conditions. By systematically analyzing the data from the intraplatform basin to the deep-sea basin across the Yangtze Block, a model of the sedimentary environments and OM enrichment during the Early Cambrian was suggested. Additionally, this study highlights the intrinsic link between the expansion of oxygenated surface water and the Cambrian explosion. These results provide critical insights for shale gas exploration in this region.

1. Introduction

The Ediacaran–Cambrian transition (ca. 550–530 Ma) was a critical interval in the Earth’s history, profoundly reconstructing the interactions among the various layers of the Earth [1]. This interval is characterized by the break-up of Rodinia and the formation of Gondwana [2], the global ocean oxidation [3], the greenhouse conditions and glacial melting [4], the extinction of Ediacaran fauna, and the “Cambrian explosion” [5]. Under this background, a set of organic matter (OM)-rich black shales was deposited in South China alongside similar deposits in Australia, Canada, France, and Iran, indicating a widespread black shale event in the Early Cambrian [6]. The mechanism of OM enrichment in shales has been extensively debated. Some scholars believe that the OM enrichment primarily depends on anoxic conditions based on the fact that these conditions provide an environment to minimize the consumption of OM, similar to modern environments like the Black Sea [7,8]. Others argue that even under conditions unfavorable for OM preservation, high primary productivity in surface ocean can lead to the deposition and burial of significant amounts of OM, similar to modern environments like the Peruvian margin [9,10]. Actually, in most cases, the preservation and primary productivity could not be considered separately, as they jointly control the entire OM enrichment process [11]. Additionally, terrigenous input, paleoclimate, and hydrodynamic conditions can influence primary productivity and redox conditions, thereby affecting OM enrichment [12].

The Lower Cambrian black shale distributes widely in the Yangtze Block in southern China, and the research on OM enrichment concentrates on the middle to upper Yangtze Block, especially in the Sichuan Basin and its eastern adjacent regions of Hubei, Hunan, and Guizhou provinces [13,14,15], but neglects the northern margin of the Yangtze Platform, leading to a significant gap in the understanding of the OM enrichment mechanism for the Lower Cambrian black shales across the Yangtze Block. Additionally, despite extensive literature discussing the OM enrichment mechanism in these regions, controversy persists regarding the relative importance of “preservation” versus “productivity” [16,17]. Notably, the shale deposition process is subjected to a complex interplay of physical, chemical, and biological processes that exhibit significant variations across different depositional environments, complicating the interpretation of the OM enrichment mechanism [15].

Furthermore, the margin zone of the Yangtze Block carbonate platform during the Early Cambrian exhibits significant potential for depositing high-quality source rocks. Current research about OM enrichment predominantly concentrates on the southeastern margin of the Yangtze Platform [14,18]. However, intense hydrothermal activities in these areas have led to the abnormal enrichment of trace metal and nutrient elements, which in turn affected the redox and primary productivity, creating complex depositional environments, limiting the applicability of the model in the platform-margin settings [13,19].The Hannan paleo-uplift, located along the northern margin of Yangtze Block, spans a transitional zone between the intraplatform basin and the carbonate platform system. The OM enrichment mechanisms under this setting are likely governed by the combined effects of preservation conditions and primary productivity. Previous studies in this region remain limited and fragmented, with paleoenvironmental reconstruction predominantly focusing on isolated parameters while overlooking critical influences, such as hydrodynamic conditions in evaluation metrics. Moreover, the insufficiency of planar comparisons with other regions has rendered such research inapplicable to the holistic analysis of Early Cambrian black shales across the Yangtze Block [20,21].

Therefore, this study conducts the analysis of total organic carbon (TOC) contents and major and trace element data for the shale core samples of the Lower Cambrian shale from the SNZ1 well in the southeastern margin of the Hannan paleo-uplift. On the basis of this, the sedimentary environments and OM enrichment during the Early Cambrian are further discussed by analyzing and incorporating the data from the intraplatform basin to the deep-sea basin across the Yangtze Block.

2. Geological Background

During the Ediacaran to Early Cambrian, the northwestern margin of the middle and upper Yangtze Block was dominated by terranes such as Kangdian and Motianling. The evolution of the middle and upper Yangtze Block was closely linked to the development of the North China Block. Tectonic movements in the Early Cambrian led to the gradual transformation of the northern part of the carbonate platform, near the Qinling ocean, from a rift basin to a passive continental margin [22]. With the breakup of the Rodinia, the Yangtze Block underwent intense stretching, resulting in the formation of the De-An rift in the central area of the middle and upper Yangtze Block [23]. Paleoenvironmental reconstructions indicate that, during the Early Cambrian, the topography of the middle and upper Yangtze Block was characterized by a decreasing trend from the west to the east, with the geomorphological pattern of a combination of uplifts and depressions [24]. Accordingly, the sedimentary paleoenvironment from the southeast to the northwest varied from deep-sea basin, deep-sea slope, carbonate platform, to intraplatform basin [23]. During the middle–late Cambrian, widespread marine transgression occurred. Rapid sea-level rise facilitated an extensive deposition of black shales across the Yangtze Block, particularly within the middle–upper Yangtze Block region [25].

In the Early Cambrian, the southern Shaanxi region, located at the northern margin of the Yangtze Block, experienced stable platform deposition in an extensional geological setting and developed a few depressions from the platform margin to the interior, resulting in the formation of a set of thick black shales [26]. The well SNZ1 lies within the structural transition zone between the western segment of the Dabashan arc belt and the eastern Micangshan structural belt, forming a southwestward convex arc structure [27] (Figure 1a,b). Based on previous research and drilling data, the stratigraphic sequence from bottom to top is composed of the following: the Dengying Formation consisting of dolostone, the Kuanchuanpu Formation consisting of siliceous dolostone, the Guojiaba Formation consisting of carbonaceous shale and argillaceous shale, and the Xiannudong Formation consisting of siliceous limestone and calcareous sandstone [28] (Figure 1c). Notably, the Guojiaba Formation black shale was primarily deposited in the deep-water shelf environment [29], and it serves as the target of this study.

3. Methods and Principles

3.1. Methods

A total of 45 black shale samples in this study were collected from the Lower Cambrian Guojiaba Formation of the SNZ1 well, with a sampling depth range of 1666–1754 m. Prior to the following testing, approximately 20 g of clean samples were crushed to 200 mesh, dried, and stored for the preparation of uses.

The TOC content analysis of the shale samples was conducted at Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, following a widely used protocol. Firstly, HCl solution (10 vol %) was applied to treat the powdered sample (about 100 mg for each) at 60 °C for 12 h to remove inorganic carbon. The sample was further rinsed with deionized water until a complete removal of HCl residues, and then at 60 °C for 24 h. The dried sample was combusted in a Leco CS-230 analyzer under the condition of an oxygen stream at 1350 °C, and the produced CO₂ was quantified by IR detection. The OAS AEB 2151 certified standard (7.45% C, 0.52% N, 0.62% S, Alpha Resources) was used. The analytical precision is better than 10%. The more detailed analysis method information refers to [33,34,35,36].

The major and trace element analysis of the shale samples adhered to a commonly employed procedure and was performed at ALS Laboratory (Guangzhou). The major element contents were determined using an ME-XRF26 X-ray fluorescence (XRF) spectrometer. Firstly, the sample was mixed with the Li₃BO₃-LiNO₃ flux, and then fused at the temperature of 1025 °C for 15 min. After pouring the melt into a platinum mold to form a homogeneous glass disk, the fused sample was analyzed by the instrument to obtain major element concentrations. The trace element measurements were conducted using a coupled plasma optical emission spectroscopy (ME-MS61r). Prior to the analysis, the sample was digested with HClO₄ (70%), HNO₃ (65%), HF (40%), and HCl (36%) under stage-wise heating conditions (80 °C for 1 h, 120 °C for 2 h, 150 for 45 h). Both the major and trace element analyses achieved a precision level of better than 10%. The more detailed analysis method information refers to [37,38,39].

3.2. Principles

3.2.1. Paleoclimate Indicator

Paleoclimate plays a major role in controlling the weathering intensity and type of source rock areas, as well as the mineral composition and sedimentation rate of sediments. Nesbitt et al. proposed using the Chemical Index of Alteration (CIA) to represent the intensity of chemical weathering and the climate conditions of source areas [40]. The CIA is calculated using the following formula:

CIA = [Al₂O₃/(Al₂O₃ + CaO* + Na₂O + K₂O)] × 100

(1)

In this formula, all elemental oxides are converted to molar values. CaO* represents the corrected CaO content in silicates using the P₂O₅ content from apatite. The correction formula is presented as follows [41]:

CaO* = Cao − 10 × P₂O₅/3

(2)

If CaO* < Na₂O, the molar amount of CaO is that of CaO*. If CaO* > Na₂O, the molar amount of Na₂O is used to replace CaO* [42].

Additionally, K-metasomatism in shales and clastic rocks can cause an elevated K₂O content and thus also requires correction. Panahi et al. proposed a method based on the mineral composition of source rocks to correct K₂O content [43]:

K₂O_corr = [m × A + m × (C* + N)/(1 − m)]

(3)

where m is calculated as m=K/(A + C* + N + K), and A, (C* + N), and K represent the molar values of Al₂O₃, (CaO* + Na₂O), and K₂O, respectively. The corrected CIA (CIA_corr) is then calculated as follows:

CIA = [Al₂O₃/(Al₂O₃ + CaO* + Na₂O + K₂O_corr)] × 100

(4)

The CIA and CIA_corr values indicate the cold and dry climate as their range of 50–70, the warm and humid climate as their range of 70–80, and the hot and humid climate as their range of 80–100 [44].

3.2.2. Terrigenous Aeolian Input Indicator

Terrigenous aeolian input influences OM enrichment directly by serving as diluent or providing adsorption sites for OM [45] and indirectly by affecting OM burial rates [46]. Al and Ti are abundant in terrigenous aeolian materials; Al is mainly associated with aluminosilicate clay minerals, while Ti is mainly related to clay and heavy minerals [47]. These elements are minimally affected by diagenesis, making them reliable indicators of terrigenous input conditions [48]. The Ti/Al ratio is the climate-sensitive indicator of terrigenous aeolian input flux, i.e., a higher value indicates a high aeolian flux under the arid and cool climate, while a lower value suggests a high fluvial flux under the warm and humid climate [49].

3.2.3. Redox Conditions Indicator

The burial, cycling, and distribution of redox-sensitive trace elements (RSTEs) are influenced by their chemical properties as well as the redox conditions of water. Under oxic conditions, Mo and U exist as soluble high-valent forms (U⁶⁺, Mo⁶⁺). Conversely, they form insoluble compounds and accumulate in sediments under reducing conditions. Due to their limited mobility during diagenesis, RSTEs are commonly used to reconstruct the modern and ancient redox conditions [50]. In this study, Mo and U are selected as redox proxies.

To quantify the enrichment of redox-sensitive elements, the enrichment factor (EF) X_EF is calculated using the following formula [51]:

X_EF = (X/Al)_SAMPLE/(X/Al)_PAAS

(5)

where X represents the concentration of a given element in the sample, and X_PAAS represents the avg. concentration of that element in the Post-Archean Australian Shale (PAAS) standard. Al_SAMPLE is the concentration of Al in the sample, and Al_PAAS is the avg. concentration of Al in the PAAS standard [52].

Although both Mo and U can be enriched under reducing conditions, their geochemical behaviors differ significantly during sedimentary and diagenetic processes. Mo tends to accumulate in euxinic depositional environments, whereas U preferentially enriches in environments with Fe(II)-Fe(III). Notably, the Particulate Shuttle (PS) mechanism enhances Mo deposition rate, whereas U deposition remains unaffected by this pathway. Therefore, a systematic analysis of their covariation patterns enables robust reconstruction of redox conditions [53].

U/Th ratio is also commonly used as redox proxy. U dissolves in water as U⁶⁺ and is adsorbed by OM-rich clay sediments under reducing conditions, while Th remains largely unaffected by redox conditions. The U/Th ratio of <0.75, 0.75–1.25, and >1.25 indicates oxic, suboxic, and anoxic conditions, respectively [45].

3.2.4. Hydrodynamic Indicator

The evaluation of hydrodynamic conditions relies on the consumption of Co and Mn and the enrichment of Cd and Mo. Sediments deposited in systems with upwelling currents (e.g., Peruvian, Californian, and Namibian margins) are characterized by a Cd/Mo ratio of >0.1 and a Co × Mn value of <0.4 (or a Cd-EF × Mn-EF value of <0.5). In contrast, sediments in restricted marine systems (e.g., Black Sea) are characterized by a Cd/Mo ratio of <0.1 and a Co × Mn value of >0.4 (or a Cd-EF × Mn-EF value of >0.5) [54]. Therefore, this study uses the Co × Mn, Cd/Mo, and Cd-EF × Mn-EF as indicators of hydrodynamic conditions.

3.2.5. Primary Productivity Conditions Indicators

Primary productivity refers to the ability of water, carbon dioxide, and inorganic salts to produce OM through photosynthesis and chemosynthesis, and it is crucial for the accumulation of OM in marine sediments [55]. P is a structural element in DNA, RNA, various enzymes, phospholipids, and other biological molecules, while Zn is vital for eukaryotic organisms to form various metalloproteins and polymerases [50]. After excluding the influence of terrigenous input, the P/Al and Zn/Al ratios can be used to assess the level of primary productivity, their higher values represent a higher primary productivity level.

4. Results and Discussion

4.1. Regional Stratigraphic Framwork

As shown in Figure 1, the core profile of the SNZ1 well displays a stratigraphic succession from the Upper Ediacaran Dengying Formation to the Lower Cambrian Guojiaba Formation. The Guojiaba Formation shows a greater TOC content compared to the Kuanchuanpu and Dengying formations but with no fossil assemblages (Figure 1). Based on the lithological columns, TOC variations, and major and trace element profiles, the interval from 1666 m to 1754 m is subdivided into the following two members (Figure 1 and Figure 2).

The Lower Member (LM: 1688–1754 m) records a gradual rise of sea-level and is dominated by carbonaceous shale with a high TOC content (1.60–15.11%, avg. 4.26%). The major element analysis result shows that the SiO₂ content (53.23–65.32%, avg. 61.42%) maintains a stable upward increase, while the contents of Al₂O₃ (9.75–15.86%, avg. 13.02%) and TiO₂ (0.52–0.85%, avg. 0.72%) display minor fluctuations. The trace elements reveal an upward increase in Ni content (43.90–155.00 ppm, avg. 79.6) and a stable concentration of Cu (27.60–66.90 ppm, avg. 45.30) and Co (14.30–21.70 ppm, avg. 17.00), while the Cr value (70.00–180.00 ppm, avg. 95.00) presents an initial decrease, followed by an oscillatory pattern (Figure 2).

The Upper Member (UM: 1666–1687 m) reflects sustained regression shallow-water shelf conditions, with increasing clay mineral and detrital input. The lithofacies type is mainly argillaceous shale. The TOC content is relatively lower (0.90–3.29%, avg. 2.10%) and decreases upward. The major elements show an upward decrease in SiO₂ content (55.42–58.10%, avg. 56.28%) and an upward increase in Al₂O₃ (14.77–15.87%, avg. 15.26%) and TiO₂ (0.62–0.68%, avg. 0.69%) contents. The trace elements upward demonstrate a decline in Ni (62.3–121 ppm, avg. 93.3) and Cu (51.2–71.9 ppm, avg. 59.5) concentrations, a rise in Cr concentration (110–130 ppm, avg. 118.2) to its peak level, and a variable Co value (17.8–23.2 ppm, avg. 19.4) (Figure 2).

Previous studies on stratigraphic profiles primarily relied on fossil assemblages, δ¹³C_carb, lithostratigraphy, and chronostratigraphy [56,57]. However, the absence of these data in some regions hinders the construction of stratigraphic framework. In this case, the TOC content, as a widely applied indicator in shale, can effectively reflects stratigraphic variations. For instances, Zhang et al. and Tan et al. established the Early Cambrian stratigraphic profiles in the Three Gorges area and across the Yangtze Block, respectively, which validates the reliability of TOC content as a stratigraphic correlation standard [13,58]. In this study, the TOC content of shale is adopted as the primary benchmark, integrated with the available data of fossil assemblages, lithostratigraphy, chronostratigraphy, and marker beds from prior research, to incorporate the SNZ1 well profile into the well-established stratigraphic framework of the Yangtze Block (Figure 3). The Guojiaba Formation is tentatively ascribed to the Lower Cambrian Stage 3 or deposited during the Early Cambrian Age 3.

During LM deposition, the black shale was characterized by widespread Si-P mineralization and Ni-Mo layers in the Yangtze Block. A Re-Os isochron age of 521 ± 5 Ma is obtained from the Ni-Mo layer in the Zhongnan section [59] in the carbonate platform, while U-Pb dating of phosphorite layers in the Bahuang profile records 522.3 ± 3.7 Ma [19]. Thus, these Ni-Mo layers and their Si-P equivalents are widely utilized as markers for the boundary between Stage 2 and Stage 3 [59]. The first appearance of trilobites, notably T. niutitangensis and T. armatus (Figure 3) from the Wutingaspis-Eoredlichia trilobite assemblage in the Jinsha profile, is recognized as the beginning of Stage 3 [60]. Importantly, the Guojiaba Formation in the northern platform margin continued to experience erosional effects from the Zhenba Uplift, with an eastward increase in erosion intensity across southern Shaanxi. In the Ziyang area of eastern southern Shaanxi, the Guojiaba Formation is entirely absent [61]. This tectonic activity suggests a potential erosion of the Ni-Mo layer at the SNZ1 well, implying the deposition of its basal black shale sequence should also postdate 521 Ma.

During UM deposition, a gradual fall in sea levels shallowed the carbonate platform in the Yangtze Block, with a lithological transition from fine-grained sediments (shales) to clastic and carbonate rocks. This shift was well documented in previous studies [56,57]. The regressive phase coincides with a decrease in the TOC content of shale, as evidenced by the data from the upper member of the SNZ1 well (Figure 3). The base of the Mingxinsi Formation in the Jinsha profile contains abundant trilobites (Figure 3), including Kuweichowia, Mayiella, and Drepanuroides, belonging to the Yunnanaspis-Yiliangella assemblage dated to ~515 Ma [60]. This fossil assemblage serves as a key marker for the boundary between the Qiongzhusi Formation and the Mingxinsi Formation [62]. At the beginning of Stage 4, the Songtao profile in the deep-sea slope documents the occurrence of trilobites (e.g., H. orientalis, S. changyangensis, Metaredlichia sp.), while the Longbizui profile yields abundant large sponge spicule fossils (Figure 3). These fossils indicate the gradual expansion of Cambrian Explosion biota into deep-sea environments.

Figure 2. The profiles of major and trace elements of the Guojiaba Formation of the SNZ1 well (lithology symbols refer to Figure 1).

Figure 2. The profiles of major and trace elements of the Guojiaba Formation of the SNZ1 well (lithology symbols refer to Figure 1).

Figure 3. Stratigraphic framework diagram of the Yangtze Block during the Early Cambrian. The data include Jinsha [63,64], Shatan [65,66], GMD1 [67], SNZ1 (this study), ZY1 [15], Songtao [68], and Longbizui [69,70].

Figure 3. Stratigraphic framework diagram of the Yangtze Block during the Early Cambrian. The data include Jinsha [63,64], Shatan [65,66], GMD1 [67], SNZ1 (this study), ZY1 [15], Songtao [68], and Longbizui [69,70].

4.2. Paleoclimate and Terrigenous Clastic Input

CIA is widely used to assess chemical weathering intensity and paleoclimatic conditions, though its reliability may be influenced by source rock composition and sedimentary recycling, thus necessitating feasibility evaluation prior to application. For black shale, the provenance constraints can be typically established through the analysis of major and trace element associations.

In terms of the source input, the TiO₂/Zr ratio can effectively distinguishes three source rock types: mafic igneous rocks (TiO₂/Zr > 200), intermediate igneous rocks (55 <TiO₂/Zr < 195), and felsic igneous rocks (TiO₂/Zr < 55) [71]. As shown in Figure 4a, the studied samples exhibit a low TiO₂/Zr ratio, indicating a dominant felsic igneous provenance.

La, Th, and Sc abundances serve as robust provenance indicators due to their stability during transport, diagenesis, weathering, and metamorphism [72]. Figure 4b demonstrates the granodioritic affinities for the source rocks.

During the Early Cambrian, Motianling and Kangdian terranes were the major source areas surrounding the study area. The Motianling terrane comprises basalt, diorite, and gabbro [72,73], while the Kangdian terrane is characterized by granite, rhyolite, and bentonite [74,75], both of which are consistent with the provenance signatures identified (Figure 4a,b). The U-Pb dating of the Guojiaba Formation sandstone samples from the southwestern Micangshan area reveals a primary age peak of 849–783 Ma, with a secondary age peak of 622–524 Ma, contrasting with the samples from the northwestern with a dominant age of 870–741 Ma [76]. The SHRIMP and LA-ICP-MS dating data of the Motianling terrane from Xiao et al. and Wang et al. reveal a predominant age range between 870 Ma and 741 Ma [72,73], while the SHRIMP dating results of the Kangdian terrane from Zhou et al. and Zhao et al. show the major age clusters focus in the range of 849–783 Ma [74,75]. These results collectively confirm that the two terranes should be the primary sediment sources of the studied area.

On the other hand, the effect of sedimentary recycling can be evaluated using the relationship of Th/Sc vs. Zr/Sc, where the first-cycle sediments follow source rock compositional trends, while the recycled sediments exhibit zircon enrichment trends. Although Figure 4c indicates there is an influence from a portion of the sedimentary recycling, the generally low Zr/Sc ratios suggest a dominant source rock control for the present study samples [77]. In addition, the fine-grained black shale has a relative homogeneous nature, which minimizes the compositional heterogeneity impacts on CIA values [78].

To address diagenetic effects such as K-metasomatism, in this study, the CIA correction was implemented using A-CN-K (Al₂O₃–CaO*+Na₂O–K₂O) ternary diagrams in conjunction with conventional methodologies [78]. In Figure 5, the theoretical weathering trend parallels to the A-CN line, reflecting the concurrent depletion of Na, Ca, and K, which enables the K-metasomatism correction to be made through projecting samples to their original positions [41,44]. As illustrated in Figure 5, the intersection of the weathering trend line with the plagioclase (Pl) vs. K-feldspar (Kfs) line is proximal to granodiorite [79]. Combined with the minimal divergence between ideal and observed weathering trends, it can be believed the K-metasomatism influence is quite limited. The CIA correction results also plot within the low chemical weathering region for the samples investigated in this study (Figure 5). Therefore, the samples in this study can be used to apply CIA and CIA_corr for assessing chemical weathering intensity and paleoclimatic interpretation.

The CIA and CIA_corr values of the LM (CIA: 54.05–63.21, avg. 56.55; CIA_corr: 54.74–64.52, avg. 57.50) collectively indicated the cool–arid climatic conditions (Figure 6). Such cold-arid climate could enhance thermohaline circulation and elevate bio-limiting nutrient fluxes, favoring upwelling development and subsequent proliferation of planktonic organisms in surface water x. However, the upward increases in CIA and CIA_corr at the middle LM, followed by a rapid decline, would be likely resulted from episodic CO₂ releases through hydrothermal vents in the Nanhua Basin of the Yangtze Block during the Early Cambrian tectonic activity. Although the CO₂ pulses could intensify chemical weathering temporarily, an unsustainable CO₂ supply, coupling with rapid consumption during source rock weathering, ultimately would drive the CIA and CIA_corr to fall back again [67,80].

The CIA and CIA_corr values of the UM progressively increase upward (CIA: 62.31–66.50, avg. 64.52; CIA_corr: 63.52–68.15, avg. 65.89), reflecting a gradual climatic transition toward warmer and more humid conditions (Figure 6). This shift corresponds to the northward drift of the Yangtze Block toward equatorial latitudes during 575–515 Ma [81]. Regional studies have demonstrated the coeval strata across the Yangtze Block, such as the Shuijingtuo Formation of the EYY3 well (Hubei), the Qiongzhusi Formation of the ZY1 well (Sichuan Basin), and the Niutitang Formation of the wells RY1 and RY2 (Guizhou), all recorded the transitions from arid–cold to warm–humid regimes during the Early Cambrian Age 3 [14,82,83]. This confirms the regional synchronicity of paleoclimatic trends in the Yangtze Block.

Regarding the terrigenous clastic input, the Ti/Al ratio exhibits a progressive downward trend from the LM (0.049–0.064, avg. 0.059) to the UM (0.046–0.051, avg. 0.049) (Figure 6). This decrease signals a reduction in eolian particle influx and a gradual shift from aeolian-dominated to fluvial-dominated sedimentary regimes, further supporting the paleoclimatic transition toward warm-humid conditions during the Early Cambrian Age 3. Additionally, Figure 2 demonstrates a strong covariance between Al₂O₃ and TiO₂ concentrations from the LM to the UM, and their trend is inversely correlated with sea-level fluctuations, which is attributed to depositional center migration during marine transgression. For instance, the rise in sea levels will redistribute sedimentary loads, with diminished terrigenous clastic deposition [15].

Collectively, for the SNZ1 well, the CIA, CIA_corr, and Ti/Al trends consistently document a climatic transition from cold-arid to warm-humid conditions from the LM to the UM.

Figure 4. Provenance properties and sedimentary recycling discrimination diagram of terrigenous clastics of the Guojiaba Formation of the SNZ1 well. (a) TiO₂ vs. Zr diagram; (b) La-Th-Sc diagram; (c) Th/Sc vs. Zr/Sc diagram (the ranges are from the References [71,84,85]).

Figure 4. Provenance properties and sedimentary recycling discrimination diagram of terrigenous clastics of the Guojiaba Formation of the SNZ1 well. (a) TiO₂ vs. Zr diagram; (b) La-Th-Sc diagram; (c) Th/Sc vs. Zr/Sc diagram (the ranges are from the References [71,84,85]).

Figure 5. A-CN-K (Al₂O₃-CaO* + Na₂O-K₂O) ternary diagram and associated CIA variations of the Guojiaba Formation of the SNZ1 well (Tonalite, granodiorite, and granite data are from Reference [79]); Ka = kaolinite; Chl = chlorite; Gi = gibbsite; Sm = smectite; Mu = muscovite; Pl = plagioclase; Kfs = K-feldspar.

Figure 5. A-CN-K (Al₂O₃-CaO* + Na₂O-K₂O) ternary diagram and associated CIA variations of the Guojiaba Formation of the SNZ1 well (Tonalite, granodiorite, and granite data are from Reference [79]); Ka = kaolinite; Chl = chlorite; Gi = gibbsite; Sm = smectite; Mu = muscovite; Pl = plagioclase; Kfs = K-feldspar.

Figure 6. The profiles of redox, primary productivity, hydrodynamic, paleoclimate, and terrestrial flux proxies of the Guojiaba Formation of the SNZ1 well (lithology symbols refer to Figure 1).

Figure 6. The profiles of redox, primary productivity, hydrodynamic, paleoclimate, and terrestrial flux proxies of the Guojiaba Formation of the SNZ1 well (lithology symbols refer to Figure 1).

4.3. Redox and Hydrodynamic Conditions

The absolute concentrations of RSTEs are commonly employed to characterize redox conditions [86]. However, the high thermal maturity of Lower Cambrian black shale in southern Shaanxi [87] and hydrodynamic influences [88] may compromise the reliability of RSTEs. To mitigate potential biases from single-proxy interpretations, this study incorporates RSTEs EFs and the U/Th ratio.

For the LM, the greater U concentration (13.80–97.00 ppm, avg. 29.30 ppm) relative to PAAS [89] reflects suboxic–anoxic depositional conditions (Figure 6). The Mo concentration (20.8–109.5 ppm, avg. 46.01 ppm) suggests intermittently euxinic environments, consistent with the classification from Scott and Lyons that defines the non-euxinic, intermittently euxinic, and persistently euxinic conditions as Mo < 25 ppm, 25–100 ppm, and >100 ppm, respectively [90]. However, the redox studies from nearby regions (e.g., the SND1 well and the Shatan profile) suggest that euxinic conditions were absent during this period, with the domination of anoxic environments [21,65]. Wu et al. proposed that the expansion of oxygenated surface water during the Early Cambrian would enhance the relative enrichment of Mo and U within anoxic zones [67]. Furthermore, based on the euxinic zone developed near the sediment–water interface in the minimum oxygen zone (OMZ) of the non-sulfide environment at the continental margin proposed by Scott, the Mo and U enrichment degree of the LM is rather close to that in intermittently euxinic conditions but has not exceeded the enrichment degree of persistently euxinic conditions [90]. Due to the distinct geochemical behaviors of Mo and U, the covariation of their EF can be used to infer redox conditions. Using modern seawater (SW) as a reference (Mo-EF: U-EF ratio), a ratio of 0.1–0.3×SW indicates suboxic conditions, 1–3×SW signifies anoxic conditions, and 3–10×SW points to euxinic conditions [53]. The covariation of Mo-EF (27.02–180.56, avg. 68.35) and U-EF (6.00–59.66, avg. 14.19) with TOC content displays a stable upward-increasing trend (Figure 6). The Mo-EF/U-EF ratio (Figure 7) is predominantly below 3×SW (seawater reference), with some samples of < 1×SW, indicating dominantly anoxic conditions rather than euxinic conditions. Thus, euxinic conditions did not likely develop during the LM deposition. The U/Th ratio (1.22–11.27, avg. 2.67) further confirms it would be an anoxic deposition (Figure 6). The hydrodynamic constraints evidenced by an upward-decreasing trend of Co-EF×Mn-EF (0.20–2.16, avg. 0.70) and Co×Mn (0.16–1.60, avg. 0.53), alongside an upward increase of Cd/Mo (0.0018–0.11, avg. 0.023) (Figure 4 and Figure 8), collectively indicate the restriction became weakening. Moreover, previous research indicated that during the late Age 2 to early Age 3, the sea level remained low, with the conditions similar to the Black Sea, and as the sea level rose thereafter, the restriction diminished, with the conditions akin to the Cariaco Basin (Figure 7) [15]. The LM samples in Figure 8 plot between the Black Sea and the Cariaco Basin, near the boundary between the restricted basin and the upwelling zone, and the upper LM samples in Figure 6 overlap with the upwelling indicator line. This suggests hybrid hydrodynamic conditions (e.g., the Cariaco Basin, combining restriction and seasonal upwelling) [91], with limited Cd and Mo enrichment and favored Mn-oxide precipitation [54]. Arthur argued that the rapid sea-level rise during this period perhaps triggered seasonal upwelling [92], delivering nutrients to enhance primary productivity, as evidenced by the high TOC content of the upper LM (15.11% at the peak) (Figure 6). During the deposition of the upper LM, there was a phenomenon of hydrodynamic restriction similar to that of the Cariaco Basin, accompanied by seasonal upwelling. Modern continental margin OMZs where high productivity depletes oxygen, align with the observed redox trend and the OM-rich sedimentation below the chemocline, which further explains the intensified anoxia of the upper LM [88]. Thus, the LM data records it deposited under the anoxic conditions with diminishing hydrodynamic restriction, presenting the intermediate feature between the Black Sea and the Cariaco Basin, and the upper LM more closely resembles the Cariaco Basin.

The UM shows a lower U concentration (8.00–25.30 ppm, avg. 14.25 ppm) (slightly above the PAAS value), with significantly weakened anoxic conditions compared with the LM. The Mo concentration (9.38–42.70 ppm, avg. 27.66 ppm) initially approaches the boundary of non-euxinic conditions and later falls within non-euxinic conditions. The values of Mo-EF (11.22–51.87, avg. 34.44) and U-EF (3.20–10.24, avg. 5.72) of the UM are also notably lower than those of the LM (Figure 6), displaying an declining trend. Although these proxies nominally suggest anoxic conditions, the progressively decreasing Mo and U concentrations, particularly the low U value, indicate that these parameters may be unreliable due to analytical limitations [56]. In particular, the UM samples in Figure 7 fall within the PS (Particulate Shuttle) zone, a feature associated with intermittently suboxic conditions under oxic background [67]. Additionally, rising atmospheric oxygen levels during the late Age 3 of the Early Cambrian likely promoted marine oxygenation [93]. Based on this, for the UM samples, it can be inferred that the evaluation of Mo and U, along with their EF, may be skewed by their low absolute concentrations. The U/Th ratio of the UM (0.81–2.14, avg. 1.24) supports suboxic conditions, contradicting the earlier interpretations of anoxia. The hydrodynamic proxies of UM, including Co-EF×Mn-EF (0.70–1.11, avg. 0.92) and Co×Mn (0.92–1.46, avg. 1.24), exhibit an upward-increasing trend, while the Cd/Mo ratio (0.011–0.078, avg. 0.031) decreases (Figure 6 and Figure 8). In Figure 7, the UM samples deviate from the open marine trend, and in Figure 8, their restriction level resembles the Black Sea, representing the stronger hydrographic restriction compared with the LM. The enhanced hydrographic restriction during the UM deposition hindered the exchange of nutrient and trace elements with the open ocean, leading to the depletion of Mo and U and the decrease in OM burial, a phenomenon termed as the “basin reservoir effect” [94]. This limitation also distorted the RSTEs absolute concentrations and enrichment factors, rendering them unreliable for redox evaluation. Therefore, the UM is characterized by suboxic–oxic conditions with progressively intensifying hydrodynamic restriction, analogous to the Black Sea.

In summary, although some discrepancies exist among redox indicators in distinguishing specific euxinic versus anoxic conditions for the samples of this study, all proxies exhibit the broadly consistent evolutionary trends [95]. From the LM to the UM, the redox conditions transitioned from anoxic to suboxic-oxic conditions, and the hydrodynamic restriction was weakened initially and intensified subsequently, with transient upwelling occurring during the late stage of LM deposition and disappearing in the record of the UM.

4.4. Primary Productivity Conditions

As essential elements for life, the concentrations of P, Zn, Ni, Cu, and Cd recorded in sediments serve as effective proxies for primary productivity [15,50]. This study employs the ratios of nutrients to aluminum (P/Al, Cu/Al, Zn/Al) to evaluate the productivity, where the content of Al was normalized to mitigate the dilution effect from detrital inputs [2].

For the LM samples, the Zn/Al (3.67–110.29, avg. 15.69) and P/Al (119.18–243.81, avg. 147.07) ratios covaried with the TOC content, showing their well-correlated peaks (Figure 6). A comparison of these parameters with modern marine systems (Black Sea: average P/Al: 130, average Zn/Al: 14.4; Peruvian margin: average P/Al: 1322.71, average Zn/Al: 30.34; Gulf of California: average P/Al: 223.90, average Zn/Al: 18.80) [92,96] indicates that the LM productivity level falls between the Black Sea and the Gulf of California. The fluctuations in productivity-related trace elements (P, Zn) across the lower LM may reflect the variability in seawater influx which affects trace element dynamics [97,98]. Notably, both ratios from the middle LM samples are lower than those of the Black Sea (Figure 6), likely reflecting distinct geochemical behaviors. Zn, as a trace metal preferentially utilized by marine organisms, tends to accumulate in biomass. Concurrently, the Early Cambrian Age 3 was a critical phase of the Cambrian explosion, with a rapid increase in biological communities and an excessive consumption of Zn in seawater, which would delay its co-deposition with OM, resulting in a lagged enrichment pattern [56,63,99]. Subsequently, the intensified anoxia in depositional environments promoted Zn sequestration as ZnS [100], and the elevated primary productivity enhanced Zn-organic complexation [50], driving the synchronized peaks in the Zn, Zn/Al, and TOC profiles (Figure 6). The decline in P/Al ratio in the middle LM should be attributed to the enhanced anoxia, since sedimentary phosphorus could be remobilized into the water column under this condition. The regenerated phosphorus stimulated primary productivity, increasing OM flux. This, in turn, intensified oxygen consumption and exacerbated anoxia, forming a coupled feedback loop linking phosphorus cycling, anoxic conditions, and productivity [101]. Subsequently, an excessive phosphorus concentration in the water column triggered authigenic apatite precipitation to elevate the sedimentary P concentration and align the peaks of the P, P/Al, and TOC [50,55]. Compared with the data (Zn/Al: 2.49–25.35, avg. 7.11; and P/Al: 61.41–554.49, avg. 169.70) of the Early Cambrian Age 3 (hydrothermal-active period) from well ND1 (Yichang, Hubei, Yangtze Block), the LM deposition should be influenced by an intense productivity source analogous to hydrothermal activities [102]. Furthermore, the parallel trends of the LM productivity and the sea-level change (Figure 6) suggest the upwelling-driven nutrient supply could reach the carbonate platform margin [15]. This process would be the reason for the Zn/Al ratio of the upper LM in its high-TOC intervals exceeding that of Peruvian margin and its overall mean value being higher than that of Gulf of California, although its P content does not surpass that of the Gulf of California primarily due to the lower P concentration in the sediments under anoxic conditions.

For the UM samples, the Zn/Al ratio ranges from 7.85 to 25.18 (avg. 14.46), and the P/Al ratio ranges from 104.09 to 139.56 (avg. 118.74). A sudden increase in P in the top UM (Figure 6) can be attributed to the re-sequestration of P under oxic conditions [103]. Conversely, the progressive upward decline in Zn across the UM likely reflects the degradation of OM, leading to the breakdown of Zn–organic complexes and the Zn remobilization into the water column [50]. Overall, the Zn/Al and P/Al ratios of the UM resemble those of the modern Black Sea, indicating a sharp deterioration in primary productivity conditions.

In summary, the primary productivity at well SNZ1 transitioned from the moderate to high level of the LM to the low level of the UM.

4.5. OM Enrichment Mechanism in the Northern Margin of the Carbonate Platform

In this study, the main controlling factor of OM enrichment was explored by analyzing the relationships between the TOC content and redox proxies (Mo, U, Mo-EF, U-EF, U/Th), primary productivity proxies (Zn/Al, P/Al), terrigenous input proxies (Ti/Al), and paleoclimate proxies (CIA and CIA_corr) (Figure 9). The Mo-EF, U-EF, and U/Th show the strongest correlations with the TOC content (R² = 0.78, 0.91, and 0.91, p(α) < 0.0001), indicating that anoxic conditions played the most critical role in the OM enrichment throughout the Guojiaba Formation (Figure 9a–c). The P/Al and Zn/Al ratios exhibit weaker correlations with the TOC content (R² = 0.69 and 0.31, p(α) < 0.0001), suggesting that primary productivity had a lesser influence on the OM enrichment compared to redox conditions (Figure 9e–f). For terrigenous input, the Ti/Al ratio shows a poor positive correlation with the TOC content (R² = 0.041, p(α) > 0.05), indicating that aeolian terrigenous clastic input had no significant impact on the OM enrichment during the entire process (Figure 9d). In future research, analytical methods such as machine learning and deep learning will be introduced to further strengthen the analysis of various paleoenvironmental sedimentary elements [104,105].

During the LM deposition, sea-level rise prevented oxygen replenishment from surface water to bottom water [65], resulting in anoxic conditions. In terms of paleoclimate, arid–cold conditions enhanced terrigenous clastic input and delivered more P and Fe into the ocean, moderately boosting primary productivity [100]. Additionally, during the late stage of LM deposition, significant sea-level rise increased the likelihood of nutrient-rich upwelling in the slope region, and the nutrients were transported to the carbonate platform margins, greatly enhancing primary productivity [15,106]. Concurrently, the presence of upwelling accompanied by OMZ further intensified anoxic conditions [107]. Additionally, the coupling of phosphorus cycling, redox conditions, and primary productivity strengthened both preservation and productivity conditions. Thus, the OM enrichment of the LM sequence is mainly controlled by preservation, with localized influences from productivity conditions in the late stage of LM deposition.

During the UM deposition, the gradual oxygenation of the atmosphere in the Early Cambrian affected marine environments [93,108]. A rapid decline in sea level reduced the anoxic extent of bottom water to degrade preservation. The strong hydrodynamic restriction hindered the replenishment of nutrients and critical elements, leading to a severe decline in primary productivity and the onset of the “basin reservoir effect”, characterized by the depletion of elements such as Mo and U [94]. Furthermore, upwelling eliminated with diminishing sea level, further reducing primary productivity. This deterioration of redox and productivity conditions disrupted phosphorus cycling, promoting OM decomposition. Meanwhile, the increase in dilution from aeolian terrigenous clastic input and the shift toward warm–humid climates reduced the supply of P and Fe from terrigenous sources. Therefore, the OM enrichment of the UM is primarily controlled by preservation.

In summary, from well SNZ1 located in southern Shaanxi, the OM enrichment of the northern margin of carbonate platform is mainly controlled by preservation for the LM, with productivity influences for the upper LM, and is primarily controlled by preservation for the UM. OM is of great significance for hydrocarbon resource potential and also exerts important influences on rock mechanics. For instance, organic-rich shales generate natural fractures during hydrocarbon generation [109]. However, further exploration is still required for industrial applications such as hydraulic fracturing [110].

4.6. OM Enrichment Mechanism Across the Yangtze Block

Based on the data from this study and related literature, the paleoenvironmental conditions and the OM enrichment mechanism in the major sedimentary facies of the Yangtze Block during the Early Cambrian were systematically investigated, with focus on the shale sequences of the LM and the UM.

4.6.1. LM Stage Deposition

During the deposition of the LM, the sea level initially resumed rising but gradually declined in its later phase. The intraplatform basin exhibited anoxic conditions (Figure 10). The TOC content is generally elevated, with limited variability (Figure 11a). The hydrodynamic conditions are strongly restricted, exceeding restrictions observed in other regions (Figure 11b). The OM enrichment is primarily controlled by preservation.

Black shales were widely deposited under anoxic conditions across the carbonate platform during this stage, while euxinic environments disappeared by the beginning of the LM (Figure 10). Gao et al. suggested that upwelling during this period likely retreated to the carbonate platform margin (Figure 10 and Figure 11b) [15]. The limited upwelling reduced OM burial and restricted H₂S production via bacterial sulfate reduction (BSR), leading to the gradual disappearance of euxinic conditions. Local exceptions (e.g., SNZ1, GMD1, SL) exhibit the enhanced productivity and preservation due to unique upwelling or hydrothermal influences to result in an elevated TOC enrichment (Figure 10 and Figure 11a). Thus, on the carbonate platform, OM enrichment is primarily controlled by preservation, with the local region co-controlled by both preservation and productivity in its some local areas.

The deep-sea slope to basin area is characterized by anoxic and euxinic conditions, but the range of the euxinic conditions was gradually narrowed. The upper slope (Daotuo, ZK4411) and lower slope (Longbizui) developed a ferruginous condition (Figure 10). This difference likely resulted from the excessive sulfate depletion in the deep sea slope-basin zones due to intense upwelling, which hindered BSR-derived H₂S production and destabilized euxinic environments. The TOC content of this region is higher than that in the other region. The OM enrichment in the deep slope-basin is thus jointly controlled by preservation and productivity.

4.6.2. UM Stage Deposition

The sea level underwent a sustained decline during the UM deposition, which promoted the downwelling of oxygen-rich surface water and the lowering of storm wave base and chemocline. Concurrently, rising atmospheric oxygen concentrations gradually oxygenated the bottom water in shallower regions (Figure 10) [93,108,125]. In the intraplatform basin, the redox had a transition from anoxic to oxic conditions. The weak connectivity with the open ocean, the intensified restriction, and the low TOC content all reflect the deteriorated productivity and preservation conditions (Figure 11a,b). The sedimentary lithology shifts from fine-grained mudstones to clastic and carbonate rocks, further inhibiting OM enrichment [115]. Thus, the OM enrichment mechanism in the intraplatform basin is primarily governed by preservation.

In the carbonate platform, the suboxic–oxic conditions dominated. Progressive shallowing allowed oxygen-rich layers to invade and disrupt anoxic bottom water, leading to gradual oxidation (Figure 10). The hydrodynamic restriction was intensified, with most areas classified as strongly restricted, limiting the exchange of nutrients and trace elements with the open ocean (Figure 11b). Declining sea levels terminated upwelling during this interval, which eliminated productivity enhancement and sustained a low TOC content (Figure 11a). Therefore, the OM enrichment mechanism in the carbonate platform is mainly controlled by preservation conditions.

In the deep-sea slope-basin, the ferruginous–anoxic conditions prevailed, with the euxinic wedge contracting dynamically to the mid–lower slope area, likely restricted to the Yuanjia and its adjacent areas. The suboxic conditions developed in the upper slope region such as the ZK4411 well, further reducing TOC content (Figure 10 and Figure 11b). The redox data from the Longbizui and Diben profiles suggest the oxidation of the deep slope-basin was delayed until the early–middle Age 4, lagging behind the carbonate platform and intraplatform basin (Figure 10). A study from Wu et al. and Xiang et al. also supported this delayed oxidation based on the data of δ¹⁵N and iron speciation [116,126]. These changes mark a transition in the OM enrichment mechanism from jointly controlled by preservation and productivity to preservation-controlled models in the deep-sea slope to basin.

To sum up, from the LM to the UM, the OM enrichment is consistently dominated by preservation in the intraplatform basin. In the carbonate platform, OM enrichment is controlled by preservation, although productivity controlled in local regions. In the deep slope-basin, the OM enrichment mechanism shifts from the preservation and productivity co-controlled to the preservation controlled. Figure 12 presents the conceptual model of the OM enrichment of the Yangtze Block during the Early Cambrian.

4.7. Linkage Between Redox Evolution and the Cambrian Explosion

The Cambrian Explosion primarily occurred from the Fortunian age to the end of Age 3 (539–515 Ma). During this period, most modern animal phyla emerged abruptly and diversified rapidly. The Chengjiang and Qingjiang biotas (~518 Ma) mark the peak of this event [127,128]. While the Early Cambrian organisms were low-oxygen-demanding, the later skeletonized animals required a higher oxygen level [129,130]. Thus, it is widely believed that marine oxygenation drove the Cambrian Explosion. However, debates have persisted regarding the mechanistic links between the oceanic oxidation and the biological radiation [131,132]. Wang et al. suggested that the explosion was associated with a rapid oxygenation of the deep sea, whereas Jin et al. attributed it to the expansion of oxygen-rich surface water, which gradually oxidized anoxic bottom environments to promote biological proliferation [63,69]. Obviously, a further analysis of redox frameworks and fossil records in the Yangtze Block may clarify the role of regional marine oxygenation in this event.

During the early Age 3, the anoxic and euxinic conditions remained dominant in the Yangtze Block, which delayed a widespread oxidation and restricted biological activity and evolution. The fossil records from this interval primarily include low-oxygen-demanding small metazoans (e.g., sponges) and shelly fossils [64,133,134]. By the middle–late Age 3, a declining TOC enrichment on the carbonate platform reflects oxygen-rich surface water incursion into bottom environments. Broad oxidation occurred from the intraplatform basin to the upper deep slope-basin, with the intraplatform basin experiencing suboxic interruptions during this period. This oxidation coincides with the Chengjiang and Qingjiang biotas, both of which are iconic representatives for this explosion, and demonstrates that shallow-water oxygenation triggered gigantism, diversification, and morphological complexity in the Early Cambrian fauna [5,127,128,133,134]. In contrast, the anoxic deep slope-basin preserved only simple metazoans like sponge spicules [69]. By the early–middle Age 4, expanding oxygenation enabled the emergence of trilobites and large sponges in the slope regions (e.g., Songtao and Longbizui profiles; Figure 3) [68,69]. These patterns suggest that regional biotic radiations in the Yangtze Block were unlikely driven by the full deep-sea oxidation but rather promoted by a progressive expansion of oxygenated surface water.

5. Conclusions

This study clarified the paleoenvironmental evolution and controlling OM enrichment factors of the Lower Cambrian Guojiaba Formation shale from the northern margin of Yangtze Platform and incorporated the information from the northern margin into the entire Yangtze Block, discussing the spatiotemporal differentiation in OM enrichment mechanisms across the Yangtze Block during the Early Cambrian Age 3. These findings will provide a scientific basis for the evaluation and exploration of the Lower Cambrian shale gas in the northern margin carbonate platform of the Yangtze Block. The following main conclusions have been obtained.

• The Lower Cambrian Guojiaba Formation of the SNZ1 well in the northern margin of the Yangtze Platform primarily consists of black shale, including two numbers with different lithological characteristics. The lower member (LM) is carbonaceous shale, with a TOC content of 1.60–15.11%. The upper member (UM) comprises clay shale, with a TOC content of 0.90–3.29%. This stratum is tentatively ascribed to the Lower Cambrian Stage 3, i.e., deposited during the Early Cambrian Age 3.
• The redox condition of the northern margin of the Yangtze Platform was anoxic during the LM deposition and suboxic–oxic during the UM deposition. Accordingly, the primary productivity transitioned from a moderate–high level to a relatively low level. Paleoclimate evolved from arid–cold to warm–humid conditions, with decreasing input of terrigenous aeolian detritus. The OM enrichment was primarily the preservation control model, and productivity contribution during the late deposition stage of the LM.
• The OM enrichment mechanism constrained by sedimentary environments for the main sedimentary facies across the Yangtze Block was established during the Early Cambrian Age 3. It varied spatiotemporally. From the LM to the UM, the preservation-controlled remained dominant in the intraplatform basin. There was an overall predominance of preservation conditions controlling the carbonate platform, though local regions productivity support existed, while for the deep slope to basin region, the OM enrichment mechanism transitioned from the preservation and productivity jointly controlled to preservation-controlled.
• Although the multi-well analyses provide regional insights, the reconstruction of paleoenvironments based on single-well geochemical profiles cannot fully resolve basin-scale heterogeneities. Future research integrating machine learning and 3D subsurface modeling is essential to overcome this spatial constraint.