Sedimentary Record of the Bio-Geological Events in Tethys: Insight from the Permian Yangtze Block Breakup in the Sichuan Basin

Abstract

At the end of the Middle Permian Guadeloupe series, the Chinese region recorded the only internationally recognized large igneous provincial eruption event, known as the Emeishan LIP. The Yangtze region of South China records a series of short and almost synchronous geological events that accompanied the development of bio-geological events such as large-scale magmatic intrusion, plate rupture, magnetic pole anomalies, and ecological collapse. These events ultimately triggered the extinction of living organisms. However, the current study leaves several questions unanswered. What was the sequence of geological events? Are the global records of these events synchronized? What is the causal relationship between these events? This study discusses the sedimentary responses to various geological events using biofossils, fluid inclusion, carbon isotopic analysis, and astrochronological analysis. The results reflect the following: (i) Mantle plumes: Mantle plumes act as pathways for heated fluids to ascend from the Earth’s interior. The mantle plume reached the Moho surface in the mid-Wordian and affected the magnetic field at the Earth’s surface; (ii) Magnetic pole anomalies: The anomaly of the Earth’s magnetic poles appeared in the mid-Wordian stage, causing the originally stable plates to begin to split. The sea level changes dramatically, and the ancient landform pattern changes dramatically; (iii) Plate rupture: The rifting of plates accelerated the activity of deep hydrothermal fluids; the hydrothermal fluid gradually infiltrated the paleo-ocean after the J. altudaensis zone; (iv) Emeishan LIP: The volcano erupted at 260 Ma, and eventually led to the mass extinction. We aim to identify the initial triggers of various geological events by analyzing the sedimentary record.

1. Introduction

The significant volcanic event—the Emeishan large igneous province (Emei LIP)—was recorded in the Guadalupian Series of the Middle Permian in China [1,2], which witnessed the development of a series of bio-geological events such as large-scale magmatism, plate rupture, magnetic pole reversal, ecological collapse, and the extinction of pre-Lopingian organisms (about 260 Ma ago) [3,4]. Current research has mostly focused on the pre-Lopingian biological crisis, with the research data concentrated on the Capitanian of the Guadalupian. It is believed that the overlay effect of large-scale tectonic activity, volcanic eruptions, and climate events led to the demise of life [3,4,5]. All these geological events were widely recorded in the Middle Permian strata of the Yangtze region in South China. For example, (i) Layered tuff represents periodic volcanic eruptions; (ii) Upwelling of deep hydrothermal fluids causes the metamorphism of the strata, and large amounts of siliceous rocks are found in the strata; (iii) Plate activity caused the original sedimentary paleo-geomorphology to change, and large amounts of gravity flow sediments are found in the profile, etc. However, the chronological and causal relationships among different geological events and the isochroneity on a global scale have been less studied.

The morphology of conodonts changes over time, so different types of conodonts can be used as a measure of past time. However, the following problems still exist in the study of Permian conodont fossils: (i) Conodont fossils are difficult to compare across different regions. The Middle Permian organisms were clearly divided by paleogeography, and their appearance and evolutionary sequences were different, making it challenging for cross-region comparison. (ii) Missing strata: During the Permian sedimentation period, the supercontinent was uplifted as a whole [5,6,7,8], causing the ancient strata to be lifted to the surface and eroded [9,10,11]. Therefore, the fossil record may be incomplete. The above problems make it difficult to effectively determine the initial activation time of geological events, leading to ongoing debates about the exact sequence of events during the Permian [12,13,14,15].

The northern part of the Sichuan Basin is located on the northwestern edge of the upper Yangtze Craton and is one of the areas with the most complete stratigraphic records and the richest fossil record in the world [5,16,17]. The Yangtze plate is part of the Tethys multi-island ocean system [18,19]. As independent islands, their sedimentary features are more susceptible to tectonic and small-scale sea-level fluctuations, and the records of geological events were well preserved. Therefore, the Permian Yangtze Plate may provide valuable information about both the region’s past rifting activities and the eruption history of the Emei LIP [18,19,20,21].

The identification of conodont sequences can provide a relatively accurate age framework for the study of regional geological events [22,23], such as (i) The size of conodont fossils is determined by the oxygen content in the air, so they can be used to reflect the oxygen concentration at that time. (ii) The morphology of conodonts changes significantly over time and can be used as a timer for the age of the strata. In addition, the wells located in the deep-water basin in the Yangtze region of South China may retain complete astronomical cycle signals [24]. In this study, eight sections and one well with complete Permian exposure were selected for systematic sampling and conodont identification to establish absolute age anchors, combining astrochronological analysis, C isotope analysis, and sedimentological profile measurements. This work aims to (1) Estimate the timing and duration of the conodonts; (2) Explain the evolution of the sedimentary environment of the section; (3) Investigate the long-period behavior of the Earth’s orbital parameters; (4) Explore the coupling relationship between various geological events and their sedimentary responses.

2. Geological Setting

2.1. Geographic Location

The South China Plate is composed of the Yangtze Craton and the Huaxia Craton [19,25] (Figure 1a), and it shows a series of shallow-water carbonate platforms in the Permian. During the middle Permian, active volcanism accelerated the breakup of the Yangtze plate (Figure 1b). The study area belongs to the northwest margin of the Yangtze Craton, located in the northern part of the Sichuan Basin, China (Figure 1b,c).

2.2. Geological Time Relationship

The Permian is divided into the Cisuralian, Guadalupian, and Lopingian series, from bottom to top [18]. The Guadalupian is further divided into the Roadian, Wordian, and Capitanian [4,18]. The stratigraphy of the Maokou Formation under study corresponds to the Guadeloupian depositional period (Figure 2).

2.3. Emei Mantle Plume

Campbell and Griffiths [26] proposed that when a mantle plume reaches the continental lithosphere, it results in crustal uplift and triggers large-scale basalt eruptions approximately 10–20 million years later. The rifting activities and hydrothermal events during the deposition of the Maokou Formation suggest a strong correlation with mantle plume activity in the region [27,28,29]. The mantle plumes in the region continued to be active for 25–30 Ma and eventually erupted to form volcanoes, triggering the pre-Leping biological crisis (260 Ma).

2.4. Yangtze Rifting

A series of northeastern and northwest-trending faults developed in the Sichuan Basin before the Caledonian period (275–260 Ma). During the Middle Permian sedimentation period, the expansion of the Mianlue Ocean led to the reopening of basement faults and the development of a series of extensional rifts in what is now China’s Yangtze Plate [30,31].

The research area has been relatively less affected by crustal vertical movement, allowing for the relatively complete Maokou Formation marine strata to have been deposited. Moreover, the geological and geochemical response to plate rupture and mantle plume activities have been impeccably preserved.

Through astrochronological analysis and identification of conodont fossils, we can establish a relatively accurate age grid. The research timeline is confined to the Roadian to Wordian stages of the Guadalupian [4]. Combined with the analysis of characteristic event-based sedimentary phenomena and successive geochemical tests observed in the profile sedimentology, an attempt is made to recognize the sedimentological records left by various geological events during the Guadeloupean sedimentary period (272.95–265.01 Ma) in the Upper Yangzi region.

2.5. Sedimentary Model

During the Guadalupian sedimentation period, the sedimentary pattern of the Yangtze Plate underwent a significant transformation. Previously, it was a carbonate ramp model, which gradually changed to a steeper carbonate platform model [32]. The cause of the shift in deposition patterns remains unknown [33,34].

3. Methodology

3.1. Conodont Identification

We conducted detailed field investigations of the Up-Yangtze Plate. Eight outcrop profiles (including Maoertang, Sanduizheng, Longfengcun, Wangjiagou, Zhengyuan, and Daliangxiang), combined with one well near the rift trough, were selected to analyze the changes in sedimentary characteristics during that Guadalupian period. Samples for conodont identification analysis were taken from Wordian–Roadian period. A total of 72 carbonate rock and siliceous rock samples were collected, and each sample weighed about 500 g. The collected rock samples were crushed indoors, downsized to around 1–5 cm³, and treated with 5% acetic acid for acid hydrolysis. Every 36 h, the acid was replaced in the container, which was kept at room temperature below 10 °C, and this procedure was repeated three times. The residue obtained from the previous step was washed well and then screened with a 200-mesh stainless steel sieve. Then, it was floated using tetrabromoethane with a density of approximately 2.85 g/mL. Afterward, it was oven-dried at 60 °C, observed under a microscope, and finally attached to a glass carrier. The images were captured using a scanning electron microscope (JEOL Ltd., Akishima, Japan, JSM-35CF), and the analyses were undertaken at China University of Petroleum (Beijing). The results of the conodont output are presented in Figure 3.

3.2. Thin Section and Inclusion Analysis

Based on the biostratigraphic determination of the formation age of the Middle Permian, 52 samples were selected for petrological characteristic observation. These samples were attained from the Roadian–Wordian interval. Thin sections were made at China University of Petroleum (Beijing).

Alizarin red was used for semi-staining to observe the corresponding lithofacies. In the developmental segment of the reefs, 8 thin sections were selected for analysis of the corresponding inclusions, which were identified using a light microscope (DMLP, Leica, Wetzlar, Germany) at the State Key Laboratory of Petroleum Resources and Prospecting, Beijing.

3.3. Time Series Analysis

The Gamma curve, after removal of element U (KTH), can reflect changes in mud and organic matter content through the deposition concentration of radioactive minerals, which may be affected by climate-driven astronomical forcing [35,36]. Therefore, with a sampling density of 0.125 m, the KTH ray from the HB1 well at 5755–5902 m was selected, and the Multi-Window Spectrum Analysis (MWSA) method was used to analyze the spectrum of the data sequence. The peaks represent different astronomical orbital periods, and the peak frequency above the 90% confidence line is mainly selected for analysis.

Sliding window spectrum analysis (E-FFT) and Gaussian band-pass filtering were performed on the data sequence. The 405 kyr long eccentricity period obtained through filtering was used to adjust the short eccentricity period and establish the Middle Permian Roadian–Wordian stage “floating” astronomical age scale. The Age Scale module was used to complete the conversion of the depth domain data sequence to the time domain. The above numerical analysis was performed using Acycle 2.3 software [37].

3.4. C Isotope Analysis

Limestone forms through the consolidation of supersaturated ancient seawater. Therefore, the C isotope values in the limestone represent the amount of dissolved carbon in the seawater at that time. It can be used to indicate environmental fluctuations.

Roadian–Wordian limestone samples were chosen from the Maoertang section and the HB1 well, adhering to the sequence framework. The location where block flow develops was avoided to negate interference from off-site sediments. A total of 46 samples from the Wordian–Roadian period were selected for C isotope analysis.

The samples were drilled and crushed in a tungsten carbide bowl until they were below 0.075 mm for isotope analysis. The instrument used for the isotope test was a stable isotope ratio mass spectrometer (model: MAT253plus). The analyses were undertaken at the China University of Petroleum (Beijing).

4. Results

4.1. Profile Conodont Formation

This study selected well-preserved conodont P1 molecular fossils (Figure 3), which are divided into four genera (Jinogondolella, Clarkina, Sweetognathus, and Hindeodus) and nine species (J. aserrata, J. postserrata, J. altudaensis, C. dukouensis, C. liangshanensis, C. guangyuanensis, S. paralanoeolatus, S. iramicus, and H. minudus) [38,39]. The bases of the Roadian, Wordian, and Capitanian stages are defined by the first appearance of the conodonts J. nankingensis, J. aserrata, and J. postserrata, respectively.

The results show (Figure 4) that J. postserrata zone fossils were found in the limestone with siliceous concretion at the 28th bed of the Maoertang section, the 33th bed of the Sanduizhen section, the 28th bed of the Longfengcun section, and the 28th bed of the Wangjiagou section. The first appearance of J. postserrata should be no later than the location where the stratified siliceous concretions developed. The results of biostratigraphy and lithostratigraphy show a certain correspondence, indicating that the two methods are complementary in determining the geological age of the sediment layers. Since the first appearance of J. postserrata represents the beginning of the Capitanian stage, with this time frame established, we can confine our study to the Roadian to Wordian stages.

4.2. Lithofacies and Sedimentary Facies

4.2.1. Sedimentary Environment

Eight field profiles recorded abrupt changes in lithofacies and depositional environments in the middle of the Wordian, shifting from a carbonate gentle slope model to a carbonate platform model. The Wordian is divided into three layers (SQ1–SQ3) (Figure 5). The sequence stratigraphic characteristics show that the sedimentary characteristics changed significantly during the Wordian SQ2–SQ3 period compared with SQ1 and earlier stages. This transformation process is recorded at the base of the SQ2 stratigraphic sequence in the middle Wordian (Figure 5), showing the superposition of two characteristic sedimentary facies: gravity flow and reef (SQ2–SQ3) (Figure 5). The following describes these two typical deposition phenomena.

4.2.2. Gravity Flow

Gravity flow refers to the phenomenon of sedimentation caused by the instability of sediments due to gravity. Sediments were transported by gravity into deep-water basins. The emergence of gravity flow may indicate that the original ancient landform pattern is no longer stable [31]. This phenomenon is widely observed in the middle of the Wordian (SO2–SQ3) in field profiles (Figure 5). Through section observation, some characteristic sediments such as carbonate olistostrone, debris flow, and their transition types are identified (Figure 6), and typical features are selected for introduction.

• (1). Carbonate olistostrone

Rock collapse can form isolated blocks, which are formed after being transported over a short distance (0–15 km) [34,39]. Due to the short transportation distance, the rock sorting and rounding are poor. The overall appearance is a scattered accumulation of irregular carbonate rock blocks.

• (2). Debris flow

Compared with carbonate olistostrone, the rocks, having been transported over longer distances, are smaller in size (1–20 cm) and show better sorting and roundness. The rock is often pebble-shaped. (Figure 6a).

4.2.3. Reef

After the first large-scale layered mass flow sediments were observed in the section, layered reefs began to develop immediately, and their distribution was noted in each section. The reef can be categorized into three distinct parts based on their morphology: reef front slope, reef core, and back reef (Figure 7).

Reef front and reef front slope: These refer to the windward (open sea) side of the reef body, and there are wave-resistant coral bodies near the wave base. Front- reef collusion can be seen on the front-reef slopes, with poorly rounded and sorted. Marl deposits are in finger-like contact, extending toward the deep depression.

Reef core: This area is long-term located above the water table, undergoing vertical dissolution due to atmospheric precipitation leaching, with selective dissolution of fabrics near the water table.

The dissolution holes were filled with calcite, in which the temperature of the inclusions ranged from 60–75 °C, indicating that the sediment was modified by atmospheric freshwater.

Back reef: Located on the side of the reef facing away from the waves, the main environment tends to be quieter. The mud content increases, particle sorting is better, and the remains of benthic organisms are visible, with biological shells relatively well preserved.

4.3. Carbon Isotope Analysis

Both the profile and the wells recorded a continuous negative deviation in the middle Wordian stage (layer 28 of the Maoertang profile and 5800 m of the HB1 well). This phenomenon was interpreted as the effect of meteoric diagenetic alteration on the exposed carbonate platforms caused by sea-level decrease [40] (Figure 8).

The Maoertang geological profile studied in this study recorded a negative shift in C isotopes from 4.1% to 2.1% during the middle Wordian period. The HB1 well decreased from 4.1% to 1.78% during the same period. The remaining geological profiles located on the Yangtze Plate also recorded a negative drift at approximately 2 points [40,41].

4.4. Astrochronological Analysis

Multi-window spectral analysis of the Roadian–Wordian order KTH data series from the HB1 well, which is located in relatively deep water with relatively stable deposition and minimal influence from exogenous inputs, shows that due to rapid changes in depositional rates, several signals exceed the 90% confidence curve. The signals at 3.39 and 2.59 m exceed the 95% confidence curve, and the peaks at 10.93, 6.80, 2.09, and 1.56 m exceed the 90% confidence curve. Based on the cardinality of the characteristic lithofacies distribution (Figure 5) and using the bottom boundary of the section where the event sediment was first observed as the interface, the HB1 well was divided into two phases: Phase 1 (5800~5902 m) and Phase 2 (5755.375~5800 m). These phases recorded signals at 10.93, 3.39, 2.59 m (4.22:1.31:1) and 6.80, 2.09, 1.56 m (4.35:1.33:1), which are close to the Middle Permian long-eccentricity to short-eccentricity cycle ratio (405:127:95 = 4.26:1.33:1), suggesting that a convincing Milankovitch cycle has been recorded for the Roadian–Wordian order deposits in well HB1 (Figure 9).

The long eccentricity period (405 kyr) is the most stable of the astronomical orbital period parameters [37]. Therefore, using it as a standard for astronomical tuning, it is shown that relatively stable long eccentricity cycles are recorded in both Phase 1 and Phase 2. The rapid environmental changes in a short period of time cause the short eccentricity cycles to fluctuate to some extent, thus completing the conversion of the KTH data series from the depth domain to the time domain. A total of 19 long eccentricity cycles are recorded in the Roadian–Wordian order, with a duration of 8.2 Ma (Figure 10).

5. Discussion

5.1. Durations of Conodont Zones

The fossil dentin is the basis for high-resolution biostratigraphy of the Permian. Due to the pan-continental isolation, the appearance and survival time of the dentin in the Yangzi area may be slightly different from that of the Guadeloupe Mountains (GSSP) in North America [4,42]. The results of dentin output from the South China area are prioritized in the present paper. Among them, the traditional bottom boundary of the Rhodes Order in South China corresponds to the bottom boundary of the Lone Peak Order, for which the volcanic ash at the base was dated to 273.01 ± 0.14 Ma using the CA-ID-TIMS method [4,18]. The bottom boundary of the Ward order is marked by the first occurrence of J. aserrata, but the first occurrences of this marker in both North America and the South China Standard region are unknown. The age of the Capitanian order is based on the first appearance of the J. postserrata belt, with GTS2023 advancing the Capitanian order to 264.28 ± 0.16 Ma in the Guadeloupe Mountain profile in North America. However, the age of the Capitanian order recorded by the first appearance of the J. postserrata belt in the Yangzi Subregion of South China remains at the previously recorded age of 265.1 ± 0.4 Ma [18,43]. Based on this, the duration of the Wordian–Roadian order stratigraphy in South China is 8.1 ± 0.55 Ma, which is close to the result obtained from the present astrochronological analysis (8.2 Ma).

5.2. Illawarra Magnetic Pole Reversal

External magnetic polar anomalies have a significant impact on the Earth’s internal structure, as they can trigger the activity of mantle plume fluids and cause plate movement. This, in turn, leads to an enhancement of crust–mantle interaction, further exacerbating the instability of the Earth’s internal structure and intensifying geomagnetic anomalies. Therefore, since the discovery of the Emei mantle plume group (Figure 11), researchers have recorded a total of five paleomagnetic anomalies [44,45,46] (Figure 11). The fifth anomaly, occurring during the middle Wordian stage of the Guadalupian period, was identified as the Illawarra magnetic pole reversal [47]. The base of the Illawarra magnetic pole reversal zone is isochronous and can, therefore, be used as a reference for comparing geological events around the world (Figure 12).

5.3. Yangtze Region Rifting

The reversal of magnetic poles exacerbates the instability of plate tectonic structures. According to Irving and Parry [48], the magnetic pole reversal caused a gradual transition from the polymerization phase to the extensional rifting of Pangaea. This left a clear spatial and temporal imprint in the present study’s section. It shows a clear change in the lithofacies characteristics of the middle Wordian (Figure 5). In the Yangzi region, C isotopes also recorded a negative drift in the middle-late Wordian after this boundary (Figure 12) [4,49]. The petrographic record of the section shows an upward shallowing sedimentary sequence, with the development of beach bodies and carbonate collapse grains a superposed deposition. Astronomical chronological analysis shows the same results as the lithofacies observations, that is, the initial development of the Yangtze rift occurred at approximately 267.8 Ma (the boundary between stage 1 and stage 2), and the sedimentation rate changed significantly (Figure 9). This indicates that the middle-late Wordian marked a gradual transition from pan-continental to a rifting stage, with sea level declining steadily, albeit with fluctuations during the early stage (Figure 13c).

5.4. Active Mantle Plume Groups

The Great Igneous Province (GIP) formed rapidly during a specific period of geological history, characterized by large-scale intraplate magmatism sourced from the mantle. The GIP emerged during almost all significant rifting events. At the same time, the development of large igneous provinces is also well correlated with biological extinction events that occurred throughout geological history [29,30,50,51].

The expansion of the mantle plume caused uplift of the crust and geological modifications in sediment patterns. For instance, Neoproterozoic mantle plume activity uplifted the region around southern Australia and resulted in the formation of several kilometers deep canyons with sharp edges [30,43,44,45]. During the depositional period of the Maokou Formation, Emei mantle plume activity formed a dome-like uplift that was intermittently activated and led to an extensive sea recession. The current measured section shows sustained layer accumulation of carbonate slides in the Wordian order, indicating a gradual divergence of the palaeomorphic pattern. Additionally, the carbonate terrace gradually replaced the gently sloping slope of carbonate, and the C isotopes show a continuous negative drift in the mid-to-late Wordian order of the Upper Yangzi, which confirms extensive sea recession (Figure 13b). The formation and development ex situ carbonate clasts in Palazzo Adriano, Sicily, Italy—located near the equator—happened simultaneously during the same period [33,34]. According to petrographic features and elemental geochemistry analysis, the active mantle plume at Emei produced a comparable sedimentary record before rifting, which is spread throughout the region.

During the Middle and Late Permian, the Paleotethys oceanic plate continued to subduct eastward along the Jinshajiang fracture zone, resulting in the expansion of the back-arc region [52]. In this context, the mantle heat plume at the core–mantle boundary rose dramatically [51]. The first extensive basaltic mafic overflow was recorded along the Yumnan–Nayong–Xifeng–Wengan area at 263 Ma [34]. This event occurred 2 Ma after the end of the Wordian order deposition. In the study area, the profiles in the study area recorded laminated and densely distributed flint nodules and flint strips, with obvious phase transitions in the early Capitanian (Figure 5 and Figure 13c). The logging curves also recorded significantly increased GR and mud content (Figure 6). These findings indicate that in the early Capitanian, with the paleomagnetic inversion and continued activity of the Emei mantle plume, the degree of rifting in the Yangzi Zone further intensified, and paleomorphic differentiation became more obvious.

In the late Capitanian, above the lower boundary of the J. altudaensis zone, a thin layer of siliciclastic deposition can be observed in the section (Figure 4 and Figure 13c). This indicates that rifting activity caused the sedimentary phase to be reduced to an undercompensated environment. As a result, a large amount of siliceous minerals, partly brought by volcanic eruptions, entered the paleo-ocean, where the temperature decreased, SiO₂ solubility decreased, and the siliceous matter reached the standard for siliciclastic deposition and was deposited to form the silica rock.

The scale-forming sedimentary tuff bands observed in this field section occur at the base of the Lopingian (Figure 4 and Figure 5), corresponding to the C. dukouensis bands and above, suggesting that the large igneous province of Mt. Emei entered an active phase at the beginning of the deposition of the Lopingian. At this time, the energy accumulated in the mantle plume was released, with the descending sedimentary tuffs gradually impacting the northern margin of the Sichuan Basin. During this period, the Emei Great Igneous Province became active, and an event of volcanic eruption severely affected the carbon cycle, causing the mass extinction of organisms (Figure 14).

6. Conclusions

This study obtained the following findings:

• (1). The location of flint-stripped chert development and the fossilized bottom boundary of the J. postserrata zone, based on the results of dentate biostratigraphy and lithostratigraphy, show that the regional section and drilling preserve a complete record of Roadian–Wordian sedimentation;

• (2). The study of petrography shows that the superposition of event gravity flow and reef was preserved in the late stage of the Wordian order, and it is believed that the s pattern was adjusted during this period;

• (3). Time-sequence analyses show similar results, i.e., the Roadian–Wordian order lasted a total of 8.2 Ma, and about 19 of 405 Ka cycles can be identified, which can be divided into stage 1 and stage 2, with obvious changes in depositional rates. The beginning of stage 2 (267.8 Ma) corresponds well with the location of the first development of gravity flow observed in petrography;

• (4). C-isotope analyses show that at about 267.8 Ma, the Yangtze plate recorded a negative drift phenomenon caused by the Illawarra magnetic pole reversal, indicating that the Emei mantle plume, together with rifting activities, led to the range sea recession.

Author Contributions

X.L.: Methodology, Writing—original draft; F.J. contributed to review and editing; Y.G. and X.Z.: contributed to petrological analyses. S.Z.: contributed to geological surveys on 8 sections and made detailed sedimentological interpretations. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures

Figure 1. Tectonic and paleogeographic background of the study area. (a) Paleoceanographic recovery map of the Middle Permian in South China [25]; the Yangtze and Huaxia Craton are located near the equator. (b) Paleogeographic map of the Yangtze region. (c) Field geological plan of the study area.

Figure 2. Stratigraphic characteristics and tectonic background of the study area. (The stratigraphic characteristics and age of the profile in South China are modified from Shen [4]).

Figure 3. Scanning electron microscope images of typical Permian conodonts from the northwest margin of the Sichuan Basin. a: Upper view; b: lateral view; (1) J. aserrata, Maoertang; (2) J. postserrata, Sanduizheng; (3) J. altudaensis, Sanduizheng; (4) C. dukouensis, Daliangxiang; (5) S. paralanoeolatus, Maoertang; (6) S. iramicus, Maoertang; (7) S. movshovitschi, Maoertang; (8) H. minudus, Sanduizheng.

Figure 4. Characteristics of the Permian profile and comparison of conodont zones in the northwest margin of the Sichuan Basin. (See Figure 1b for the location pf the profiles. The profiles show that along the dotted line, the densely developed area of siliciclastic concretions corresponds to the location of the first occurrence of J. postserrata).

Figure 5. Stratigraphic characteristics of the northern section of the Sichuan Basin. (The location of the section is shown in Figure 1b).

Figure 6. Gravity flow fabric characteristics of the Permian section on the northwest edge of the Sichuan Basin. (a–c) Collapse accumulation of carbonate rocks with particles and biogenic fragments in a clear directional arrangement; hydrodynamic forces are progressively weaker from a to c. The higher shale content indicates that the rock formed in a quieter environment. (d–f) Slide accumulation grouping features corresponding to layer 26 of the Wangjiagou profile, layer 27 of the Zhengyuan profile, and layer 26 of the Dawangxiang profile, respectively. The alternating distribution of carbonate olistostrone and debris flows indicates that the ancient landforms began to undergo significant changes over time. (g) Layer 19 of the Maoertang profile, with a soft-sedimentary deformation structure.

Figure 7. Primary sedimentary characteristics of a stratified dissolved body in Sections 27–29 of Longfengcun.

Figure 8. Comparison of C isotopes in the Wordian stage of the Yangtze Craton. The C isotope value show a significant downward trend in the middle Wordian, indicating that the environment underwent drastic fluctuations. The data of Xishui, Nanchuan and Qijiang profiles are from reference [40].

Figure 9. (a) Analysis of the 2 π energy MTM spectrum of the Roadian–Wordian KTH sequence in Well HB1; (b) sliding window analysis with a window width of 20 m. E, e1, and e2 next to the white dashed line represent long eccentricity (405 ka) and short eccentricity (125 ka and 95 ka), respectively; (c,d) 2 π energy MTM spectrum analysis for 5800–5902 m and 5755.375–5800 m, respectively.

Figure 10. Time scale spectrum analysis of the 405 kyr-tuned Radian–Wordian KTH sequence in well HB1. (a) HB1 well profile and astronomical age scale; the Roadian–Wordian order lasted a total of 8.2 Ma, and about 19 cycles of 405 Ka can be identified, which can be divided into stage 1 and stage 2, with distinct changes in depositional rates. (b,c) Time-domain spectral analysis of the 5800–5902 m and 5755.375–5800 m records, respectively. They recorded the same astronomical cycle age (407 ka, 117 ka, and 102 ka).

Figure 11. Global profile reporting of magnetic pole anomalies (modified from Hounslow [47]).

Figure 12. Comparison of C isotopes during the Wordian stage and the Illawarra magnetic pole reversal time [20,40,44,46].

Figure 13. Tectonic-sedimentary pattern of the Guadalupian in the Yangzi plate. (a) Roadian order: relatively flat ancient landform. (b) Mid-late Wordian order: the plates began to break apart. (c) J. postserrata belt: deep hydrothermal fluids reached the moho surface, intensifying plate rifting. (d) J. shannoni belt: mantle plumes arched upward, and the plate rose (e) J. altudaensis belt: the plate split.

Figure 14. Mantle plume group activity model diagram (modified from Bryan [2]). (1) Kungurian: mantle plume activity begins. (2) Roadian order: the mantle plume reaches the Moho surface. (3) Mid-Worian: a large amount of deep hydrothermal fluid reaches the Moho surface. Magnetic poles begin to become disordered, and the plates begin to split. (4) Mid-Worian: process continuation (5) Capitanian: the volcano began to erupt, and creatures began to die.

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