Earliest Cambrian Carbonate Platform Evolution, Environmental Change, and Organic Matter Accumulation in the Northwestern Yangtze Block, South China

Abstract

The earliest Cambrian (ca., 538.8–524.8 Ma) was an important period in geological history witnessing significant environmental change, during which organic-rich facies were developed in the Yangtze Platform, South China. However, the contemporaneous paleogeographic and stratigraphic framework within which the environmental change and organic matter accumulation took place remains poorly understood. We investigate this based on facies, sequence stratigraphic, and geochemical analyses of the lowermost Cambrian Maidiping and Zhujiaqing formations in the northwestern Yangtze Block. The results show that the terminal Ediacaran rimmed platform changed into a foredeep carbonate ramp and backbulge basin after the onset of the earliest Cambrian transgression. Across the Ediacaran–Cambrian boundary, the shallow-marine redox condition rapidly transitioned from relative euxinia to an oxygen-rich state. During the late transgression to highstand normal regression, the foredeep carbonate ramp expanded to the cratonic interior, and nutrients brought by intensified continental weathering and upwelling promoted significant phytoplankton proliferation, an increase in oxygen level and primary productivity, and then organic matter enrichment. During the forced regression, the carbonate ramp gradually changed into a rimmed platform. The weakening continental weathering and expanding anoxic area during the forced to lowstand normal regression led to the significant organic carbon burial in the foredeep basin.

1. Introduction

The earliest Cambrian (i.e., Nemakit-Daldynian age in Siberia; ca., 538.8–524.8 Ma) was an important period in geological history [1,2] (Figure 1a). This period witnessed a substantial increase in O₂ levels both in the atmosphere and oceans [3], a rapid metazoan diversification (i.e., initial Cambrian Explosion) [4], the basal Cambrian (BACE) and Zhujiaqing (ZHUCE) carbon isotope excursions [5] (Figure 1a), and the supercontinent Gondwana assemblage [6] (Figure 1c). These global changes are mechanistically linked [7]. A complex interplay among the evolving continents, atmosphere, oceans and biosphere has been proposed to explain the linked events [8,9]. However, they are only understood in a general sense, and further details are required. South China is among the best regions in the world for studying the coevolution of life and environments during this period [10]. Here, the lowermost Cambrian Zhujiaqing and Maidiping formations in the northwestern Yangtze Block provide nearly continuous records for analysis of the aforementioned changes. Many workers have studied these strata from the perspectives of sedimentology [11], geochronology [12,13,14], paleontology [15,16,17], and element and isotope geochemistry [15,18,19,20,21,22,23], which collectively provide key information for the coevolution of the earliest Cambrian life and marine environment. However, the contemporaneous paleogeographic and stratigraphic framework within which the life and environmental coevolution took place remains poorly understood.

Previous workers suggested that there was a significant increase in oxygen levels in the ocean and on the surface across the Ediacaran–Cambrian transition due to enhanced organic matter burial and efficient photosynthesis [29,30]. However, the fundamental trigger of this oxidation event remains uncertain. The traditional view holds that the intensified continental weathering [22,31] or volcanic hydrothermal activity [22,32] during the early Cambrian may lead to a large influx of nutrients into the global ocean, thereby significantly enhancing marine primary productivity in the photic zone and then promoting the accumulation of O₂. In fact, the improvement of the productivity level may also be related to the upwelling of eutrophic water along the Yangtze slope during transgression [33]. The improvement of marine primary productivity and the enhancement of preservation capacity are often regarded as the keys to the deposition of organic-rich shale in the Lower Cambrian [34]. However, the relationship between marine oxidation and continental weathering, hydrothermal activity, and upwelling, as well as their impact on organic matter enrichment, remains unclear.

Previous workers have conducted extensive studies on the organic matter enrichment mechanism of the lowermost Cambrian organic-rich shale in the Yangtze Block, but most of them are concentrated in the Niutitang Formation of the southeastern basin facies [33,35]. Due to the large burial depth of the Deyang–Anyue Trough in the NW Yangtze Block, the systematic study of the Maidiping Formation inside the trough is limited. Therefore, the transitional Ediacaran–Cambrian carbonate rocks on both sides of the Deyang–Anyue Trough provide a new visual window for exploring the relationship between marine environmental change and organic matter accumulation during this period. Based on facies and sequence stratigraphic analyses of outcrop and subsurface data, this study constructs a sequence stratigraphic framework for the Zhujiaqing and Maidiping formations in the NW Yangtze Block. Within this framework, the C, O, Sr, and Li isotopes and element concentrations of the upper Dengying and Maidiping formations from the Tuanbaoshan section are analyzed and integrated with previously published geochemical data from other outcrop sections or drilled wells. Lastly, we propose a significantly informed model for the earliest Cambrian Yangtze Platform evolution, and then discuss the contemporaneous environmental change, organic matter accumulation, and their underlying controls within the paleogeographic and stratigraphic framework.

2. Geological Setting

During the terminal Ediacaran, a passive margin facing the Proto-Tethys Ocean to the northwest existed in the NW Yangtze Block. The terminal Ediacaran passive margin was transformed into a foreland basin by the subduction of the Proto-Tethys Ocean beneath the NW Yangtze Block during the early Cambrian assembly of the Gondwana supercontinent [28] (Figure 1c,d). During the Ediacaran–Cambrian transition, the NW Yangtze Block was dominated by a carbonate platform deposition of the uppermost Ediacaran Dengying Formation and the lowermost Cambrian Maidiping Formation in a broad, ramp-like area of slow and low subsidence tilting down to the Proto-Tethys Ocean [36]. Meanwhile, a contemporaneous NW–SE-oriented intraplatform trough widening to the NW, named the Deyang–Anyue Trough, occurred in the northwestern Yangtze Block [36] (Figure 1b). Controversially, the Deyang–Anyue Trough was interpreted as an erosional trough [37,38] or eroded valley [39], intracratonic sag [40,41,42,43,44], intracratonic or intraplatform rift [45,46,47], extensional erosional trough [48,49], intracratonic rift–sag [50], collapse valley [51], or paleogeographic embayment [36].

During the deposition of the uppermost Ediacaran Dengying Formation, the NW Yangtze passive margin was dominated by a carbonate platform deposition with significant trough expansion to a width of ca. 400 km [36]. During the deposition of the lowermost Cambrian Maidiping Formation, the NW Yangtze forebulge and backbulge were dominated by the coeval deposition of phosphatic and siliceous dolostones in the carbonate platform and interbedded argillaceous dolostones, cherts, and shales in the intraplatform trough, whereas the NW Yangtze foredeep was dominated by the deposition of interbedded cherts and siliceous shales [28,36]. The Maidiping Formation is erosively overlain by the Qiongzhusi Formation, which gradually coarsens upward from black shale to muddy siltstones and siltstones [28,36].

3. Data and Methods

3.1. Facies and Sequence Stratigraphic Analyses

Facies analysis follows a workflow suggested by Dalrymple [52] on the basis of outcrop and core observations, complemented with petrographic examination. The sequence stratigraphic analysis followed a model-independent methodology suggested by Catuneanu [53]. Nine cored (ZK3-3 and Z4) and/or wireline-logged wells (Z4, HS1, ZY1, GS17, GS1, GT2, and WT1) supplemented with outcrop data from the Meishucun, Laolin, Xiaotan, Huangjiaping, Xianfeng, Maidiping, Tuanbaoshan, Qingping, and Longzikou sections (Figure 1b) were examined for the definition of lithofacies, facies associations, and stratal stacking patterns and the interpretation of depositional environments, systems tracts, and sequence stratigraphic surfaces. Gamma-ray well-log curves and carbon isotope stratigraphic data were used to extrapolate the depositional environmental and sequence stratigraphic interpretations from the cored wells and measured outcrop sections into the uncored wells. The geologic age of the study interval is constrained by small shelly fossil (SSF) zones, radiometric dating, and carbon isotope stratigraphy. A total of 158 samples were collected in an interval of about 160 m of the upper Dengying and Maidiping formations in the Tuanbaoshan section (Table S1).

3.2. Carbon and Oxygen Isotope Analyses

Geochemical analyses were conducted at the Institute of Mineral Resources, Chinese Academy of Geological Sciences. All samples were cleaned and ground to a particle size of less than 200 mesh, then dried at 105 °C for 2 h in an oven. After drying, the sample tubes were heated in the GasBench online preparation facility, ThermoFisher Scientific, Germany at 70 °C for 30 min, followed by the introduction of approximately 0.2 mg of the sample. To eliminate air, high-purity helium was introduced into the sample tubes. The samples then reacted with excess 100% H₃PO₄ in the tubes for 8 h, resulting in the generation of CO₂. This CO₂ was subsequently analyzed in the ThermoFisher Scientific MAT 253 mass spectrometer, Germany to measure the ¹³C/¹²C and ¹⁸O/¹⁶O ratios. Each batch of five samples included a set of standards, such as IAEA-603, NBS-18, and LSVEC. The results were reported relative to the Vienna Pee Dee Belemnite (VPDB) and corrected using linear regression. Measurement precision was 0.1‰ for δ¹³C and 0.2‰ for δ¹⁸O, expressed as 1 standard deviation.

3.3. Lithium, Strontium Isotope, and Elemental Concentration Analyses of Carbonate Leachates

A sequential leaching method was used to minimize non-carbonate contamination in the analysis of lithium (Li) and strontium (Sr) isotopes and elemental concentrations. About 300 mg of bulk carbonate powder was placed in a centrifuge tube and rinsed with 1 N ammonium acetate at 75 rpm for 2 h to remove adsorbed cations (Li, Sr, etc.) After discarding the supernatant, the sample was washed three times with MQ H₂O. The remaining sample was ultrasonically digested with 1.5 N acetic acid at 60 °C for 2 h, centrifuged, and then the supernatant was dried. To expel acetic acid, 0.5 mL of 1 N twice-distilled HNO₃ was added to the dried residue, followed by another 0.5 mL for elemental and isotopic analysis.

Li was purified using a two-step column chromatography method, loading the solution onto a 1 mL cation exchange resin column (Biorad AG50W-X12, Hercules, CA, USA) and separating it with 1 N HNO₃ and 0.2 N HNO₃. The eluate was evaporated and treated with aqua regia for 24 h to eliminate potential matrix effects during the MC-ICP-MS analysis. The Li fraction was dried and dissolved in 3 wt.% (0.5 mol/L) HNO₃ for isotope analysis. Li isotope ratios were measured using MC-ICP-MS (ThermoFisher Scientific, Neptune Plus, Germany) with the standard-sample bracketing method to correct for instrumental mass fractionation, and results were reported in per mil (‰) deviations relative to the L-SVEC standard. The background signal was maintained below 1‰ of the sample signal. The accuracy and reproducibility of the measurements were verified through repeated analyses of reference materials (NASS-7 and JB-2).

For the Sr isotope analysis, a portion of the acetic acid leachate was taken, followed by the addition of the concentrated HNO₃ to the dried residue, which was then dried again and dissolved in 1 mL of 3.5 N HNO₃. Sr separation was performed using Sr-specific resin (Triskem, France, ~100–150 μm), rinsed with 7 mL of 3.5 N HNO₃ and 0.8 mL of Milli-Q water. The eluate was dried and re-dissolved in concentrated nitric acid, then dried again. Finally, the residues dissolved in 1 mL 3% HNO₃ were prepared for the Sr isotope analysis on the ThermoFisher Neptune Plus MC-ICP-M. The ⁸⁷Sr/⁸⁶Sr ratios for reference materials NIST 987 and GBW03105a were found to be 0.710260 ± 0.000007 (2sd) and 0.708935 ± 0.000090 (2sd), respectively.

A split of acetic acid leachate was also analyzed for major and trace elemental concentrations using Agilent 5800 ICP-OES and Agilent 7500 ICP-MS, Santa Clara, CA, USA, with most elements having an accuracy of less than 5%.

4. Results

4.1. Facies Analysis

The description and interpretation of facies associations are summarized from the newly collected outcrop and core data and integrated with previous studies [11,28,54]. The Zhujiaqing and Maidiping formations comprise seven broadly defined facies associations (FA), including the foredeep basin (FA1), backbulge basin (FA2), outer ramp (FA3), mid-ramp (FA4), inner ramp (FA5), prograding slope and platform margin (FA6), and platform interior (FA7). These facies associations arranged in a sequence stratigraphic and chemostratigraphic framework are illustrated in Figure 2a,b and summarized in

Table 1. Summary of facies associations for the lowermost Cambrian Maidiping and Zhujiaqing formations in the NW Yangtze Block.

Facies Associations	Constituent Lithofacies	Description	Interpretation
FA1: Foredeep basin	Bedded chert, siliceous shale, and interbedded chert and siliceous shale	Thinly bedded or interbedded (3−10 cm). Chert beds are structureless. Siliceous shales are finely laminated and contain abundant pyrite bands or nodules. No to very sparse fossil content.	Pelagic chert precipitation and (or alternating with) clay suspension below both storm wave base and euphotic zone in a deep-marine basin setting
FA2: Backbulge basin	Interbedded dolomitic chert and siliceous dolomudstone	Thinly interbed (3−10 cm). Dolomitic chert beds are structureless. Siliceous dolomudstone beds show horizontal lamination or wave-ripple cross-lamination. No to very sparse fossil content.	Alternating dolomitization and silicification below both storm wave base and euphotic zone in a deep-marine basin setting. The wave-ripple cross-lamination indicates episodic storms
FA3: Outer ramp	Interbedded siliceous dolomudstone and shale and interbedded dolomudstone and chert	Thinly interbedded (3−10 cm). Locally containing mud pebbles floating in matrix. Mud clasts are principally derived from underlying beds.	Alternating dolomitization and clay suspension or silicification below storm wave base in an outer ramp setting
FA4: Mid-ramp	Interbedded phosclast grainstone and microsphorite or chert	Sharply based, hummocky to wave-ripple cross-laminated, very fine to fine-grained phosclast beds intercalated with cherts or microsphorites.	Alternating deposition of phosphatic tempestites and fair-weather cherts and microsphorites between fair-weather and storm wave bases in a mid-ramp setting
FA5: Inner ramp	Structureless phosclast grainstone	Amalgamated, thinly bedded phosclast grainstone beds. Each bed is erosively based, structureless, and consists of poorly sorted, sand to gravel-sized phosphatic grains upward cemented by dolomite.	Deposition of amalgamated phosphatic tempestites just above fair-weather wave base
	Interbedded phosclast grainstone and dolomudstone	Thinly interbedded. Each dolomudstone bed shows sharp or distinct contact with the underlying phosclast grainstone bed.	Alternating deposition of phosphatic tempestites and fair-weather dolomudstones just above fair-weather wave base
	Hummocky to low-angle cross-laminated dolograinstone	Thinly bedded. Hummocky to low-angle cross-lamination.	Deposition of dolomitic tempestites in a shallow subtidal shoreface setting
	Interbedded phosclast grainstone and stromatolitic dolobindstone	Very thinly to thinly interbedded. Gradually merging upward succession of stromatolitic dolobindstone beds intercalated with phosclast grainstones.	Deposition of phosclast grainstones by tidal currents and waves alternating with slack-water microbial growth in a shallow subtidal to intertidal flat setting
	Wavy-bedded mixed phosclast grainstone–dolomudstone heterolith	Thinly interbedded. Wavy bedding. Phosclast beds consist of pebbly medium to very coarse phosphatic grains cemented by dolomite.	Alternating deposition of tide and wave-winnowed phosclast grainstones and slack-water dolomudstones in an intertidal flat setting
FA6: Prograding slope and platform margin	Algal dolobindstone, doloframestone, dolograinstone, graded phosclast beds, and dolobindstone or phosphatic breccias	Phosphatic coated grains. Flat-clast conglomerates and breccias. Amalgamated, phosclast beds. Each bed is normally graded and consists of poorly sorted, sand-sized phosphatic grains cemented by dolomite. Algal laminae.	Deposition of turbidites and debrites between fair-weather and storm wave bases in a prograding slope setting, and reefs and sand shoals above the fair-weather wave base in a prograding platform margin setting
FA7: Platform interior	Lime mudstone, dolomudstone, and phosclast rudstone	Thinly bedded. Nodular bedding. Phosclast beds consist of poorly sorted, sand to gravel-sized phosphatic grains cemented by dolomite.	Deposition above fair-weather wave base in an expanding platform interior

.

4.1.1. Facies Association 1: Foredeep Basin

FA1 mainly occurs in the Maidiping Formation in the foredeep province of the NW Yangtze Block and is well exposed both in the Longzikou and Qingping sections (Figure 2a). This facies association predominantly consists of thinly bedded cherts and siliceous shales or thinly interbedded cherts and siliceous shales and has been described in detail by Gu et al. [28] and Xia et al. [54]. The reader is referred to the two references for a thorough treatment of this facies association. The predominance of bedded cherts and siliceous shales in the foredeep province suggests that FA1 represents the deposition below the storm wave base [61] in a foredeep basin setting.

4.1.2. Facies Association 2: Backbulge Basin

FA2 mainly occurs in the Daibu Member of the Zhujiaqing Formation in the backbulge province of the NW Yangtze Block and is well exposed in the Xiaotan, Laolin, and Zhujiaqing sections (Figure 2a). This facies association predominantly consists of thinly interbedded dolomitic cherts and siliceous dolomudstones, i.e., F1 of [11] without skeletal fossils, and it has been described and interpreted in detail by Sun et al. [11]. The reader is referred to Sun et al. [11] for a thorough treatment of this facies association. The predominance of interbedded dolomitic cherts and siliceous dolomudstones in the backbulge province and the absence of skeletal fossils suggests that FA2 represents the deposition below the storm wave base [61] in a backbulge basin setting.

4.1.3. Facies Association 3: Outer Ramp

FA1 in the lower Maidiping Formation passes southwestward into FA3 in the foredeep transition to the forebulge province of the NW Yangtze Block (Figure 2a). FA3 consists dominantly of thinly interbedded siliceous dolomudstones and shales in both the Qingping section and well Z4 [54], which pass southwestward into thinly interbedded dolomudstones and cherts in the Tuanbaoshan section (Figure 3b–f). Herein, the dolomudstone beds contain no to abundant, very fine to fine sand-sized dolopeloids or phospeloids or phosphatic small shelly fossils (SSF; Figure 4a–e). The predominance of dolomudstones with intervening shale and chert beds combined with the northeastward transition to the foredeep basin deposits (FA1) suggests that FA3 represents the deposition below the storm wave base in an outer ramp setting [61,62,63].

4.1.4. Facies Association 4: Mid-Ramp

FA3 passes southwestward into FA4 in the forebulge transition to the backbulge province of the NW Yangtze Block (Figure 2a). FA4 is well exposed in the Maidiping, Xianfeng, Huangjiaping, Xiaotan, and Laolin sections and consists of hummocky and wave-ripple cross-laminated, very fine to fine-grained phosphorites intercalated with microsphorite or chert beds (Figure 5d,e). The hummocky and wave-ripple cross-laminated granular phosphorites are interpreted as tempestites deposited during storm events [64]. The microsphorite and chert beds are interpreted to represent the deposition of pristine phosphorites and cherts during fair-weather periods. The predominance of granular phosphatic tempestites intercalated with fair-weather pristine deposits combined with the northeastward transition to the outer ramp deposits (FA3) suggests that FA4 represents the deposition between the storm and fair-weather wave bases in a mid-ramp setting [61,62,63].

4.1.5. Facies Association 5: Inner Ramp

FA4 passes southwestward into FA5 in the backbulge transition to the cratonic interior of the NW Yangtze Block and is well exposed in the Huangjiaping, Xiaotan, Laolin, Meishucun, and Mingyihe sections (Figure 2a). FA5 consists of granular phosphorite beds intercalated within dolostones that have different expressions. This facies association may appear as amalgamated, structureless granular phosphorite beds passing upward into thinly interbedded granular phosphorites and dolomudstones (Figure 5f). Here, each granular phosphorite bed is erosively based and composed of structureless, poorly sorted sand to gravel-sized phosphatic grains either upward cemented by dolomite or overlain by dolomudstone (Figure 5f). Moreover, FA5 may also appear as very thinly to thinly interbedded granular phosphorites and cryptmicrobial dolobindstones (Figure 5g,h) or as a wavy-bedded mixed granular phosphorite–dolomudstone heterolith (Figure 5i). Locally, hummocky to low-angle cross-laminated dolograinstones are observed (Figure 5j). Both the granular phosphorite beds and the hummocky to low-angle cross-laminated dolograinstones are interpreted as storm-wave-winnowed deposits [64,65]. The cryptmicrobial dolobindstone and dolomudstone beds are interpreted to represent microbial growth and fine-grained deposition during fair-weather periods, respectively. The predominance of carbonate production with frequent granular phosphatic tempestite interbeds combined with the northeastward transition to the mid-ramp deposits (FA4) suggests that FA5 represents the deposition above the fair-weather wave base in an inner ramp setting [61,62,63].

4.1.6. Facies Association 6: Prograding Slope and Platform Margin

FA1 in the upper Maidiping Formation passes southwestward into the FA6 that extends across the NW Yangtze Block (Figure 2a). The lithofacies of FA6 stack vertically to form a distinct coarsening-upward and/or shallowing-upward but highly variable facies succession. In the Tuanbaoshan section, FA6 shows an erosional contact with the underlying FA3 and consists of thinly interbedded dolomudstones and phospeloidal dolowackestones–dolopackstones passing upward into cryptmicrobial dolobindstones and stromatolitic framestones (Figure 3b and Figure 4f–j). In the Huangjiaping section and adjacent well ZK3-3, FA6 displays an erosional contact with the underlying FA4 and consists of dolomite-cemented granular phosphorites passing upward into cryptmicrobial dolobindstones and dolograinstones (Figure 5a,b and Figure 6c,d). In well Z4, FA6 shows a conformable contact with the underlying FA3 and transitions upward from a microsphorite-dominated succession to a granular phosphorite-dominated succession (Figure 2a,b). Locally, flat phosphatic conglomerates and breccias or dolomudstone beds and bands are observed to be intercalated in the microsphorite (Figure 7a,b). The microsphorite typically contains small shelly fossils (SSFs) and sand to silt-sized quartz grains. The SSFs typically have organic matter nuclei and quartz and/or apatite cortices (Figure 8a,b). The granular phosphorite beds are sand to silt-sized phosclastic or phospeloidal grainstone to packstone (Figure 8c,e,g,i). They exhibit normal, inverse, or bi-gradational grading, with structureless, planar to cross-laminations (Figure 7c). Typically, clast or matrix-supported granular phosphorite breccia (Figure 7d,e,h and Figure 8c,d), microsphorite (Figure 8e,f), and cryptmicrobial bindstone (Figure 8g,h) beds are observed to be intercalated within the granular phosphorite. In the Qingping section, FA6 exhibits a conformable contact with the underlying FA1 and transitions upward from the mudstone-intercalated phosphatic turbidites to the siliceous phosphorites [54].

The cryptmicrobial dolobindstones and stromatolitic framestones are interpreted to represent in situ microbial growth in an upper slope to platform margin setting [63,66]. The granular phosphorites and intervening phosphorite breccias are interpreted to represent slope deposits derived from the upper slope and platform margin by sediment gravity flows [63,66]. The upward transition from gravity flow deposits to microbially influenced deposits combined with the northeastward transition to the foredeep basin deposits (FA1) suggests that FA6 represents the deposition above the storm wave base in a prograding slope to platform margin setting [66,67].

4.1.7. Facies Association 7: Platform Interior

FA6 passes southwestward into the FA7 that extends from the forebulge to the backbulge province of the NW Yangtze Block (Figure 2a). FA7 consists predominantly of lime mudstones or dolomudstones (Figure 3a,b, Figure 4l, Figure 5a,b and Figure 6e,g). Both the lime mudstones and dolomudstones typically show nodular bedding (Figure 4l and Figure 6e). Locally, dolorudstone or phosclastic rudstone beds are observed to be intercalated within the dolomudstone or lime mudstone (Figure 4k and Figure 6e,f). Both the dolorudstone and phosclastic rudstone are composed of poorly sorted, subangular sand to gravel-sized clasts cemented by dolomite (Figure 4k and Figure 6e). The predominance of lime mudstones and dolomudstones combined with the northeastward transition to the prograding slope and platform margin deposits (FA6) suggests that FA7 represents the deposition above the fair-weather wave base in a platform interior setting [61,62,63]. The intervening dolorudstone and phosclastic rudstone beds are interpreted to represent tempestites derived from the platform margin.

4.2. Sequence Stratigraphic Analysis

Based on the sequence stratigraphic analysis of FA1 through 7, recognized above, we identify four distinct stratal stacking patterns (SSP1–4) that can be correlated and mapped across the NW Yangtze Block and then used to define systems tracts and sequence stratigraphic surfaces.

4.2.1. Stratal Stacking Pattern 1: Transgressive Systems Tract

SSP1 is well represented in the lower Maidiping and Zhujiaqing formations in erosional contact with the underlying Dengying Formation (Figure 3e and Figure 6a) and consists of a deepening-upward foredeep basin succession (FA1) passing updip through a backstepping and deepening-upward outer ramp succession (FA3) in the transition from the foredeep to forebulge province into a restricted to open marine succession that transitions upward from the backbulge basin (FA2) through the inner ramp (FA5) to mid-ramp (FA4) deposits (Figure 2a,b). The deepening-upward and increasingly open marine trend suggest that SSP1 represents the deposition of a transgressive systems tract TST [61]. The erosional contact of SSP1 with the underlying Dengying Formation represents a combined wave-ravinement surface and subaerial unconformity (WRS/SU). The sharp contact of both the foredeep basin deposits (FA1) with the underlying outer ramp deposits (FA3), and the distal outer ramp deposits (FA3) with the underlying proximal outer ramp deposits (FA3) in the foredeep province, as well as the inner ramp deposits (FA5) with the underlying backbulge basin deposits (FA2) in the backbulge province, is interpreted as a maximum starvation surface MSS [68] that separates the late TST from the early TST [69].

4.2.2. Stratal Stacking Pattern 2: Highstand Systems Tract

SSP2 is well represented in the middle Maidiping and Zhujiaqing formations in conformable with the underlying SSP1 (Figure 3d and Figure 5a,b) and consists of a shallowing-upward foredeep basin succession (FA1) passing updip through a shallowing-upward outer ramp succession (FA3) in the transition from the foredeep to forebulge province, into a shallowing-upward mid-ramp succession (FA4) in the transition from the forebulge to backbulge and a shallowing-upward inner ramp succession (FA5) in the cratonic interior (Figure 2a,b). The shallowing-upward trend following the TST suggests that SSP2 represents the deposition of a highstand systems tract (HST) during highstand normal regression [61]. The conformable contact of SSP2 with the underlying SSP1 represents the maximum flooding surface (MFS).

4.2.3. Stratal Stacking Pattern 3: Falling-Stage Systems Tract

SSP3 is well represented in the upper part of the Maidiping and Zhujiaqing formations and consists of thin (typically 9–18 m thick) and extensive prograding slope to platform margin deposits (FA6) across the NW Yangtze Block passing downdip into foredeep basin deposits (FA1; Figure 2a,b). This stratal stacking pattern exhibits an erosional contact with the underlying mid-ramp (FA4) and outer ramp (FA3) deposits of SSP2 in the forebulge to backbulge province (Figure 3b, Figure 5a,b and Figure 6c,d), but a conformable contact with the underlying outer ramp (FA3) and foredeep basin (FA1) deposits in the foredeep province. The long-distance regression of the thin slope to platform margin deposits (FA6) suggests that SSP3 represents the deposition of a falling-stage systems tract (FSST) during forced regression [70,71]. The erosional and conformable contacts of SSP3 with the underlying SSP2 represent the regressive surface of marine erosion (RSME) of Plint [72] and the basal surface of forced regression (BSFR) of Hunt and Tucker [73], respectively.

4.2.4. Stratal Stacking Pattern 4: Lowstand Systems Tract

SSP4 is well represented in the uppermost Maidiping and Zhujiaqing formations and consists of thick and localized prograding slope to platform margin deposits (FA6) in the foredeep province passing downdip into the foredeep basin deposits (FA1) and updip into extensive platform interior deposits (FA7; Figure 2a,b). This stratal stacking pattern exhibits an erosional or non-depositional contact with the underlying prograding slope to platform margin deposits (FA6) in the backbulge and forebulge provinces (Figure 3b and Figure 5a,b), but a conformable contact with the underlying prograding slope to platform margin (FA6) and foredeep basin (FA1) deposits in the foredeep province. The short-distance regression of the thick slope to platform margin deposits (FA6) following the FSST suggests that SSP4 represents the deposition of a lowstand systems tract (LST) during lowstand normal regression. The erosional or non-depositional contact of the SSP4 with the underlying SSP3 represents the subaerial unconformity (SU) of Sloss et al. [74]. The conformable contact of SSP4 with the underlying SSP3 represents the correlative conformity (CC) of Van Wagoner et al. [75] and Hunt and Tucker [73].

4.3. Geochemical Analysis

4.3.1. Carbon and Oxygen Isotopic Profiles

As shown in Figure 9, the carbon isotope values of carbonates (δ¹³C_carb) in the Tuanbaoshan section remain stable and positive in the upper Dengying Formation. The δ¹³C_carb values of the Maidiping Formation exhibit a negative excursion (down to −2‰) during the late TST, a subsequent modest-positive drift during the HST, and a sharp decline to around −2‰ at the onset of the FSST, followed by a gradual increase to around 4‰ during the FSST and LST. The transitional Ediacaran–Cambrian δ¹³C_carb profile in the Tuanbaoshan section can be correlated with that in the Xiaotan section and other outcrop sections and wells (Figure 2a,b and Figure 10). The δ¹⁸O values of the upper Dengying Formation range from −2.88‰ to −8.64‰, with an average of −5.71‰. In contrast, the δ¹⁸O values of the Maidiping Formation vary from −4.04‰ to −9.94‰, with a mean of −7.20‰. Although the δ¹⁸O values show significant overall variability, there is a gradual decreasing trend from the Dengying Formation to the Maidiping Formation.

4.3.2. Lithium and Strontium Isotopic Profiles

As shown in Figure 9, the δ⁷Li values of the Dengying Formation are highly variable and range from 7.7‰ to 18.3‰, with a mean value of 11.96‰, most of which are above 10‰. In contrast, the δ⁷Li values of the Maidiping Formation are much more stable and range from 6.5‰ to 11.2‰, with a mean of 9.24‰, and the majority being below 10‰. The ⁸⁷Sr/⁸⁶Sr ratios show a progressive decrease from ~0.712 to ~0.708 in the upper Dengying Formation and then an increase to ~0.711 from the late TST to the HST of the Maidiping Formation, followed by a gradual decline to ~0.709 in the LST of the Maidiping Formation.

4.3.3. Element Contents

As shown in Figure 9, the element analysis of the upper Dengying and Maidiping formations shows a significant variation in the P content. The P content of the Dengying Formation is relatively stable and low, primarily ranging from 2 to 200 ppm. In contrast, the P content of the Maidiping Formation is significantly elevated, reaching a maximum of 4490 ppm, with an average value of 700 ppm, and exhibits notable fluctuations in accordance with relative sea-level changes. The calculated Mn/Sr ratios of carbonates range from 0.61 to 9.92 ppm/ppm, with values for the Dengying Formation primarily between 0 and 5 ppm/ppm, whereas those for the Maidiping Formation typically lie between 5 and 10 ppm/ppm. The Ce/Ce* values of the Dengying Formation are relatively stable and range from 0.46 to 0.94, generally exceeding 0.6. In contrast, the Ce/Ce* values of the Maidiping Formation exhibit two cyclic fluctuations and range from 0.33 to 0.77, with most values being below 0.6.

4.3.4. Reliability Evaluation of Geochemical Data

As mentioned above, the selective dissolution of the carbonate fraction was achieved using an acetic acid leaching technique. However, it is important to note that this leaching of carbonates can cause some damage to the aluminum silicate detrital components. Thus, assessing the potential for detrital contamination remains essential. The lithium content in pure carbonates is significantly lower (0–5 ppm) compared to that found in silicates (5–100 ppm) [80,81]. As a result, even a small presence of silicates can noticeably influence the dissolved lithium budget, subsequently altering the lithium isotopic composition of the fluid. In our research, the lithium concentrations in the carbonate fractions varied between 0.01 and 0.6 ppm, averaging 0.24 ppm (Table S1). Furthermore, it has been observed that the Al/(Ca + Mg) ratio in carbonates drops in the range of 0.09–0.98 mmol/mol (Table S1), with an average value of 0.32 mmol/mol, all of which are less than 1 mmol/mol, and no correlation is found between δ⁷Li and Al/(Ca + Mg) ratios of the Dengying and Maidiping formations (Figure 11b), indicating that the δ⁷Li values in these carbonate fractions are generally deemed free of silicate contamination [82,83,84]. Additionally, strontium concentrations in all carbonate fractions ranged from 0 to 92.5 ppm, with no significant relationships observed between ⁸⁷Sr/⁸⁶Sr and Sr content (Figure 11a). This shows that the ⁸⁷Sr/⁸⁶Sr values likely also remained unaffected by any potential silicate leaching [85].

The potential impacts of diagenetic processes can also be carefully assessed. Previous research indicates that dolomite can largely maintain its initial geochemical characteristics as long as the Mn/Sr ratio remains below 10 [86,87], which are exactly the Mn/Sr ratios obtained in this study to meet this standard (Table S1). Within carbonate formations, δ¹⁸O values are more readily altered by diagenesis than δ¹³C values, and a δ¹⁸O measurement greater than −10‰ suggests that the rocks have undergone negligible significant diagenetic alteration [88,89]. The δ¹⁸O values derived from carbonate leachates span from −9.94‰ to −2.88‰, with a mean of −6.35‰, all of which are greater than −10‰ (Table S1). Furthermore, we find no obvious correlation between δ⁷Li and ⁸⁷Sr/⁸⁶Sr ratios with both Mn/Sr ratios and δ¹⁸O (Figure 11c–f). This strongly indicates that the carbonates preserve their original depositional geochemical signatures from seawater [90].

4.4. Sequence Stratigraphic Framework

The results of the sequence stratigraphic analysis show that the lowermost Cambrian Maidiping and Zhujiaqing formations transition from the TST through the HST to the FSST, followed by the LST. The recognized TST, HST, FSST, and LST stack vertically to form the transgressive–regressive (T–R) sequence of Johnson and Murphy [91] and Embry and Johannessen [92]. The combined wave-ravinement surface and subaerial unconformity (WRS/SU) and combined wave-ravinement and maximum regressive surface (WRS/MRS) compose the lower and upper boundaries of the T–R sequence, respectively. The results of the integrated sequence stratigraphic and geochemical analysis show a large negative carbon isotope (both δ¹³C_carb and δ¹³C_org) excursion (i.e., BACE) through the TST, and a pronounced positive excursion (i.e., ZHUCE) through the FSST. This indicates that δ¹³C falling and rising limbs reflect sea-level rises and falls, respectively, and can be used as correlative tools and eustatic proxies [93]. The small shelly fossils (SSFs) occur in differing levels of abundance in the Maidiping and Zhujiaqing formations (e.g., in the Tuanbaoshan section in Figure 4b,c and well Z4 in Figure 8a,b,f). The upper boundary of the Zhujiaqing or Maidiping formation was dated by Yang et al. [60] to 526.2 ± 4.1 Ma in the Meishucun section, by He et al. [14] to 526.8 ± 1.4 Ma near the Xiaotan section, and by Compston et al. [12] to 526.2 ± 1.9 Ma in the Maidiping section. The basin-wide correlation of the interpreted systems tracts and sequence stratigraphic surfaces constrained by the carbon isotope records, SSF zones, and previously dated bentonitic tuffs permits the construction of a sequence stratigraphic framework for the Maidiping and Zhujiaqing formations (Figure 2a,b).

5. Discussion

5.1. Carbonate Platform Evolution

The NW Yangtze Block during the terminal Ediacaran was dominated by a rimmed carbonate platform partitioned by the Deyang–Anyue Trough and flanked by an open, deeper ocean to the northwest and southeast [94,95] (Figure 1b). The latest Ediacaran passive margin was transformed into a foreland basin during the earliest Cambrian assembly of the Gondwana supercontinent [28], in which the most important basin-scale controls on carbonate platform evolution include flexural tectonics related to the Motianling orogenic load, dynamic subsidence related to the Proto-Tethys Ocean subduction-induced corner flow, and sea-level changes [96] (Figure 1d). Flexure of the retro-lithosphere under the Motianling orogenic loading resulted in the partitioning of the foreland system into foredeep, forebulge, and backbulge flexural provinces [28].

The results of facies and sequence stratigraphic analyses suggest that the rimmed carbonate platform on the latest Ediacaran passive margin was converted into a carbonate ramp in the foredeep province and a contemporaneous intraplatform basin in the backbulge province after the onset of the earliest Cambrian transgression. During the early transgression, the rate of flexural uplift outpaced the rate of dynamic subsidence, whereby the forebulge province was subaerially exposed and the foredeep carbonate ramp was separated from, but locally continuous with, the contemporaneous backbulge basin by way of the intervening Deyang–Anyue Trough (Figure 1b, Figure 2a and Figure 12a). During the late transgression to highstand normal regression, the rate of dynamic subsidence outpaced the rate of flexural uplift, by which the forebulge province was submerged and the foredeep carbonate ramp expanded cratonward through the forebulge and backbulge provinces to the cratonic interior (Figure 2a and Figure 12b). In the interim, the exposed forebulge province and cratonic interior, along with the intervening backbulge basin, were flooded and replaced by inner and mid-ramps (Figure 2a and Figure 12b). The recognition of the carbonate ramp model is mainly based on the hydrodynamically defined facies belts that are characterized by the significant and extensive development of grainy tempestites in the inner and mid-ramps [97]. The transition from the rimmed platform to the ramp forced by foreland flexuring has been reported in the Cretaceous Friuli-Adriatic Carbonate Platform of Southern Alps, Italy [98].

After the onset of the forced regression, the microbial reefs and sand shoals on the inner ramp started to prograde basinward and a prograding slope to platform margin was gradually established on top of drowned mid- to outer ramps (Figure 2a and Figure 12c). The forced regression eventually converted the carbonate ramp into a rimmed platform in which there was no significant platform interior aggradation that accompanied this forced regression (Figure 12c). Numerical modeling and field observations indicate that the FSST developed during the forced regression should be more common in tropical carbonates than is currently commonly assumed [99,100]. The FSST is favored by high carbonate production, slow erosion, and slow sea-level fall [99,100]. During the lowstand normal regression, the forced regressive rimmed platform changed into an upstepping and prograding rimmed platform with the significant aggradation of dolomudstone or lime mudstone in the platform interior, which was finally terminated and replaced by the deposition of the Qiongzhusi Shale during the subsequent rapid transgression (Figure 2a and Figure 12d).

5.2. Paleoenvironmental Change

The combination of δ⁷Li and ⁸⁷Sr/⁸⁶Sr in ancient marine carbonates has become a novel tool for tracing weathering mechanisms [101,102,103,104,105]. The secular variation of ⁸⁷Sr/⁸⁶Sr in seawater commonly records the relative ratio of radiogenic strontium from the continental crust to non-radiogenic strontium from hydrothermal alteration of oceanic crust [106,107,108,109]. The long-term change in the Li isotopic composition of seawater (δ⁷Li_sw) is governed by the dynamic equilibrium between Li input and removal fluxes [101]. Generally, the decline in δ⁷Li can be attributed to increased fluxes of low-temperature chemical weathering of continental silicate rocks and/or submarine hydrothermal fluids [110,111]. However, in the Tuanbaoshan section, the overall ⁸⁷Sr/⁸⁶Sr ratios in the late TST of the Maidiping Formation are comparatively lower than those in the upper Dengying Formation (Figure 9), suggesting a decrease in the proportion of radiogenic terrestrial inputs relative to non-radiogenic mantle-derived (hydrothermal) fluxes [112].

Existing research indicates that the Ediacaran–Cambrian transition was marked by accelerated plate spreading and intensified rifting events, with numerous hydrothermal (submarine volcanic) deposits across multiple sections in South China [29,113]. Although the study area is less directly affected by volcanic activity, the fluids released from hydrothermal vents have high ⁶Li (average δ⁷Li value of 8.3‰) and ⁸⁶Sr contents, thereby reducing global ocean lithium isotopes and ⁸⁷Sr/⁸⁶Sr ratios [101]. The increase in ⁸⁷Sr/⁸⁶Sr ratios from the late TST to the HST of the Maidiping Formation (Figure 9) suggests that volcanic activity may have concurrently released large volumes of CO₂, intensifying global greenhouse conditions, and promoting progressive continental weathering that increased the flux of radiogenic terrestrial ⁸⁷Sr into the ocean. The terrestrial lighter lithium inputs could have offset the reduction in hydrothermal contributions, maintaining δ⁷Li at relatively low levels (~10‰). Therefore, it can be inferred that the long-term intensity of continental weathering and associated terrestrial input shows a gradual increase through the TST and HST of the Maidiping Formation, reaching a zenith at the end of the HST, then a gradual decrease through the FSST and LST of the Maidiping Formation.

Since soluble Ce³⁺ in seawater is easily transformed into insoluble Ce⁴⁺ under oxidation conditions, it has obvious redox chemical properties [114,115]. Therefore, the distinct negative shift in Ce/Ce* across the boundary between the Dengying and Maidiping formations in the Tuanbaoshan section indicates that the shallow-marine redox condition rapidly transitioned from relative euxinia during the terminal Ediacaran to an oxygen-rich state during the earliest Cambrian (Figure 9). The two cyclic fluctuations of Ce/Ce* in the Maidiping Formation indicate two cyclic oxygen level changes, which show a high degree of consistency with the Xiaotan section (Figure 10). Furthermore, by integrating the vertical variations of δ²³⁸U [77], δ⁵³Cr [78] and Δ³³S_py [79] of the Zhujiaqing Formation in the Xiaotan section (Figure 10), a unified conclusion can be drawn: the extent of ocean euxinia decreased sharply from the terminal Ediacaran to the earliest Cambrian.

5.3. Organic Matter Accumulation

The negative Δ³³S_py values of the early TST of the Zhujiaqing Formation in the Xiaotan section (Figure 10) indicate upward flux of deep sulfur-rich water mixing with oxygenated shallow water [79]. Due to the euxinic deep water being enriched in ¹²C-depleted organic carbon, episodic upwelling could enhance organic matter remineralization, resulting in negative δ¹³C excursion [21]. Besides upwelling, the release of large amounts of CO₂ by volcanic hydrothermal activity into the ocean would also cause a simultaneous negative shift in both δ¹³C_carb and δ¹³C_org, corresponding to the BACE event [29,116] (Figure 10). The BACE event is most pronounced in the backbulge depozone, where the δ¹³C_carb values can drop to as low as −10‰ to −8‰ (Figure 2a). The release of the large volumes of CO₂ by volcanic activity into the atmosphere likely produced a greenhouse effect that triggered marine transgression, consistent with the global sea-level rise observed in the earliest Cambrian [117]. Volcanic activity and upwelling processes enriched surface waters with phosphorus and other micronutrients, promoting extensive proliferation of photosynthetic microorganisms (e.g., cyanobacteria), thus leading to the increase in primary productivity, which is evidenced by the positive shift in Cd isotopes and N isotopes during the Ediacaran–Cambrian transition in the Xiaotan section (Figure 10), as greater uptake of light Cd and N isotopes by primary producers occurs [21,76]. Enhanced photosynthesis also led to intense surface water oxidation, which promoted biodiversity and the widespread development of SSFs. Although marine primary productivity during the early transgression was significantly higher than before, TOC content in shelf sediments remained relatively low, likely due to poor preservation conditions. Consequently, organic matter enrichment during the early transgression was mainly confined to the foredeep depozone (e.g., Qingping area, wells ZY1 and GS17 in Figure 2a,b).

During the late transgression to highstand normal regression, continental weathering progressively intensified, delivering nutrients and oxidants to the oceans. The Δ³³S_py values remained negative [79], whereas δ¹⁵N and δ⁵³Cr isotopic compositions exhibited continued positive shifts [21,76] (Figure 10), indicating intermittent and sustained upwelling of nutrient-rich, deep-water masses. This upwelling facilitated widespread proliferation of phytoplankton in surface water, maintaining a high level of primary productivity. Consequently, the shallow water oxygen level increased, and significant Ce negative anomalies reappeared. Sediments across the MFS record the highest TOC contents (Figure 2a,b), likely reflecting combined effects of increased primary productivity, enhanced preservation conditions and the adsorption of large amounts of TOC onto terrigenous clay minerals [26]. During this period, extensive organic-rich phosphorite beds were deposited in the oxygenated inner and mid-ramps in association with vigorous upwelling conditions [65,118] (Figure 2a).

During the forced to lowstand normal regression, the parallel positive shifts of δ³⁴S_py and Δ³³S_py observed in the Xiaotan section are consistent with the significant positive excursion of δ¹³C_carb (i.e., ZHUCE; Figure 10), indicating that the anoxic expansion caused a transient increase in the burial of carbon and sulfur in sediments, mainly as organic matter and pyrite [79]. Following sea-level fall, the phosphate mineralization center shifted from the inner and mid-ramps to the foreslope. Meanwhile, the deposition of organic-rich shales shrunk to the foredeep basin. Organic matter of the Maidiping Formation is dominantly preserved as amorphous flocculent organic aggregates derived from algae and phytoplankton [119]. The carbon isotope values of kerogen of the Maidiping Formation range from −36.4‰ to −32.0‰ (averaging −34.3‰), indicating sapropelic (type I) kerogen [119].

6. Conclusions

Based on integrated facies and the sequence stratigraphic and geochemical analysis of outcrop and subsurface data, we have constructed a sequence stratigraphic and chemostratigraphic framework for the Maidiping and Zhujiaqing formations in the NW Yangtze Block, within which to interpret the earliest Cambrian carbonate platform evolution, environmental change, and organic matter accumulation. The rimmed carbonate platform on the terminal Ediacaran passive margin in the NW Yangtze Block changed into a foredeep carbonate ramp and backbulge basin during the earliest Cambrian transgression. The foredeep carbonate ramp was separated from but locally continuous with the contemporaneous backbulge basin by way of the intervening Deyang–Anyue Trough. The shallow-marine redox condition rapidly transitioned from relative euxinia to oxygen-rich state across the Ediacaran–Cambrian boundary. Meanwhile, organic matter remineralization by deep anoxic water upwelling and CO₂ release from volcanic hydrothermal activity led to the occurrence of the BACE event, especially in the backbulge basin. During the late transgression to highstand normal regression of the earliest Cambrian, the foredeep carbonate ramp expanded through the forebulge and backbulge provinces to the cratonic interior, and nutrients brought by intensified continental weathering and upwelling promoted significant phytoplankton proliferation, an increase in oxygen level and primary productivity, and then organic matter enrichment. During the earliest Cambrian forced regression, the prograding slope to platform margin was gradually established on top of drowned mid- and outer ramps, accompanied by the transition from a carbonate ramp to rimmed platform. The weakening continental weathering along with the expanding anoxic area during the earliest Cambrian forced to lowstand normal regression led to significant organic carbon burial in the foredeep basin and the ZHUCE event.