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Article

The Origin of the Upper Cambrian Basin-Scale Massive Dolostones of the Xixiangchi Formation, Sichuan Basin, China

by
Huan Hu
1,2,
Shenglin Xu
1,2,*,
Anqing Chen
1,2,
Long Wen
3,
Benjian Zhang
3,
Xihua Zhang
3,
Fuxiang Li
1,2,
Mengqi Liu
1,2 and
Wei Yong
2,4
1
Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China
2
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China
3
Research Institute of Exploration and Development, PetroChina Southwest Oil and Gas Field Company, Chengdu 610041, China
4
College of Earth Sciences, Chengdu University of Technology, Chengdu 610059, China
*
Author to whom correspondence should be addressed.
Minerals 2023, 13(7), 932; https://0-doi-org.brum.beds.ac.uk/10.3390/min13070932
Submission received: 4 June 2023 / Revised: 10 July 2023 / Accepted: 11 July 2023 / Published: 13 July 2023
(This article belongs to the Special Issue Carbonate Petrology and Geochemistry)
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Abstract

:
The thick Upper Cambrian Xixiangchi dolostones, developed in the Sichuan Basin, are an important deep exploration target, but their genesis is still controversial, which hinders predicting the porous dolomite distribution and related potential hydrocarbon play. Herein, based on the observation and sampling of field outcrops, combined with a microscopic thin section analysis, cathodoluminescence analysis, and geochemical study, their characteristics and genesis were investigated. The results showed that there are mainly three types of dolomite that can be distinguished: (1) fine crystalline dolomite with a low crystallinity (Type 1); (2) granular dolomite with coarse grains, maintaining the original particle structure (Type 2); and (3) grain-texture relict dolomite with a higher degree of crystal form and obvious recrystallization (Type 3). The Type 1 dolomite with a common lamina structure and the highest 87Sr/86Sr ratio implies the most continental-influenced seawater in a supratidal environment around paleouplift, where there is an evaporation pump effect in its formation. The Type 2 dolomite recorded a slightly higher diagenesis temperature and slightly lower brine salinity, which would be formed in a seepage-reflux model in the beach environment of the platform. The lowest REY content and higher dolomite temperature with structural residuals indicate that the Type 3 dolomite is the result of further burial dolomitization during the diagenetic process.

1. Introduction

The so-called “dolomite problem” has always been a complex and controversial scientific issue in carbonate petrography [1,2]. The earliest research dates back to the 18th century [3,4]; due to the complexity of dolomite genesis, up to now, there have been more than ten dolomitization models developed to explain the genesis of different dolomites, such as the seepage-reflux model [2,5], meteoric-marine mixing-zone model [6,7], burial dolomitization model [2], and hydrothermal dolomitization model [8,9], etc. In addition to the above dolomitization models, Vasconcelos et al. [10] first proposed that microbial (thiophilic bacteria) activity can induce dolomite to precipitate directly from fluid. In recent years, dolomitization related to microbial action has attracted more and more attention. Among them, the reaction kinetic factors and fluid source of dolomitization are the most critical issues. In particular, the genesis of large-scale dolomite is the current research focus [11].
The Upper Cambrian Xixiangchi Formation in the Sichuan Basin, South China, contains massive dolostone bodies, referring to continuous dolostone deposition with hundreds of meters in its thickness and hundreds of kilometers in its lateral extent. This kind of dolomite is distributed in the Sichuan Basin and its periphery. There are currently disputes about its proposed genesis models, including the seepage-reflux dolomite model [12], burial dolomitization model [13], hydrothermal dolomitization model [14], penecontemporaneous dolomitization model [15], and mixing water dolomitization model [16]. However, none of the prevailing dolomitization models have been able to adequately account for the genesis of ancient massive dolostones that exhibit hundreds of meters in thickness and are ubiquitously distributed across entire platforms.
Dolostones are the important hydrocarbon reservoirs of the world [17,18,19]. The Sichuan Basin is one of the major foreland basins in China, with great hydrocarbon potential [20,21]. The research study of deep oil and gas exploration in the Sichuan Basin has primarily concentrated on the exploration of the Longwangmiao Formation and Dengying Formation below the Xixiangchi Formation [22,23]. In recent times, there has been a growing interest in the potential of the Xixiangchi Formation for hydrocarbon exploration [24,25,26]. In addition, several boreholes in the Sichuan Basin that have tapped into the Cambrian Xixiangchi Formation have reported significant industrial natural gas flows. For instance, wells such as Wei 12 and Wei 25 have shown promising exploratory prospects [27,28]. The present research paper represents an innovative attempt to undertake a detailed investigation of the genesis of the dolomite in the Cambrian Xixiangchi Formation. The study stands to deepen our understanding of the formation of basin-wide dolostones that have significant implications for geological and petroleum exploration in the Upper Cambrian Xixiangchi Formation. Importantly, this research provides robust scientific evidence to support continued efforts to exploit the hydrocarbon reserves present within the Xixiangchi Formation, thereby advancing the field of petroleum geology.

2. Geologic Setting

The Sichuan Basin is located in the northwestern part of the Yangtze Block in South China. It is bounded by 28°–32° N and 102°–110° E. To the north part of the research area lies the Micangshan dome and Dabashan thrust belt, and to the west is the Songpan-Ganzi fold belt and Longmenshan thrust belt. The Sichuan Basin, as a whole, presents a diamond-shaped appearance along the northeast long axis (Figure 1a). The Sichuan Basin has undergone multi-stage tectonic movements and is a typical multi-cycle cratonic basin, further divided into six secondary tectonic units [29].
There is a set of massive dolomite developed in the Xixiangchi Formation of the Sichuan Basin. Due to multiple tectonic uplifts during the Late Ediacaran to Early Cambrian periods, the Xixiangchi Formation strata in the northwest margin of the Leshan-Longnvsi paleo-uplift were largely eroded. The Xixiangchi Formation thickness varies, with a general thickness of 62–920 m and a lateral extension of 640 km, covering almost the entire Upper Yangtze Block (Figure 1b). The Late Cambrian Upper Yangtze plate is a carbonate platform sedimentary environment (Figure 2), including nearshore tidal flat facies, restricted platform facies, marginal beach facies, and shallow sea shelf facies [32,33,34].
The Cambrian Xixiangchi Formation in the Sichuan Basin is characterized by intricate variations in its lithofacies and the application of diverse criteria for its stratigraphic classification. Therefore, the stratigraphic system division of this horizon is not uniform, and different regions adopt different division schemes [36,37,38]. In this paper, the division scheme of Wen et al. [31] is adopted. The Xixiangchi Formation belongs to the middle–upper Cambrian, and the bottom and top boundaries are integrated with the Gaotai Formation and Ordovician Tongzi Formation, respectively.

3. Samples and Methods

One hundred and two samples were collected from eleven profiles (Figure 2, Tables S1–S3). The samples were observed under a thin-section microscope, cathodoluminescence experiment, fluid inclusion test, and geochemical analyses such as those of the Sr isotope characteristics, C-O isotope characteristics, and REE characteristics.
The cathodoluminescence (CL) was conducted at the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, China. The test conditions were: a voltage of 13 KV, a current of 220 μA, and a working environment with a temperature of 23 °C and a relative humidity of 63%.
Fluid inclusion microthermometry was conducted at the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, China. The homogenization temperatures (Th), final melting temperatures (Tm-ice), and salinity tests were measured using a LINKAM THMGS600 heating–freezing stage for two-phase aqueous inclusions, with an accuracy of ±1 °C for Th and ±0.1 °C for Tm-ice. The experimental environment temperature was 20 °C and the humidity was 25%.
For the Sr isotope analyses, the samples were dissolved in HCl and the measurement was performed using a Triton™ mass spectrometer with an average error of ±1.0 × 10−5. The results of the C-O isotopes were obtained using a MAT-252 isotope mass spectrometer. The test method was the conventional phosphoric acid method and the test accuracy of δ13C and δ18O was ≤0.2‰. All the samples were processed and analyzed at the Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, Peking University, China. The sample test results were accurate and reliable and the accuracy met the requirements of this study.
The REE of the carbonate samples in the target layer of the study area were tested, and the test data were standardized by using North American shale (NASC), which is closely related to seawater and has a similar age. The standardized results and parameters are marked by subscript SN. At present, the difference method is usually used in the calculation of rare earth element anomalies, such as δEu = Eu/Eu* = EuSN/(0.5SmSN + 0.5GdSN). However, when calculating the Ce anomaly, the real anomaly of Ce cannot be calculated by using abnormal La because of the general anomaly of La in marine carbonate rocks [25,39,40]. Therefore, it is more accurate to use δCe = Ce/Ce* = CeSN/(2PrSN – NdSN) to estimate Ce anomalies.

4. Results

4.1. Petrographic Characteristics

4.1.1. Fine Crystalline Dolomite (Type 1)

Type 1 dolomite displays varying degrees of development across different sections. In the field, it is mainly observed as a thin dolomite layer, with some sections containing limited amounts of terrestrial silt. Under the microscope, the crystals of Type 1 dolomite are small, usually less than 50 μm, and their crystal structure and crystal morphology are difficult to identify (Figure 3a–c). Type 1 dolomite is notable for its high organic matter content (Figure 3a). Type 1 dolomite exhibits a compact texture punctuated by several fractures, which are often filled with calcite (Figure 3b). It contains an abundance of well-preserved original structures. Type 1 dolomite displays a moderate cathodoluminescence intensity. Furthermore, in some grained dolomite, the blue luminescence of terrestrial quartz debris is visible (Figure 3d).

4.1.2. Granular Dolomite (Type 2)

Type 2 dolomites are found to be present in three distinct forms: oolitic dolomites (Figure 3e), algal dolomites (Figure 3f), and intraclastic (sandy, gravel) dolomites, the latter of which are predominantly observed as sandy dolomites. These dolomites are predominantly composed of self-shaped and semi-self-shaped crystals, with the crystal size ranging from 50 to 100 μm (Figure 3h). Moreover, type 2 dolomites exhibit slightly more pores and a bright crystal cementation (Figure 3g). The cathodoluminescence analysis revealed type 2 dolomite to be characterized by a medium-intensity luminescence (Figure 3i), which does not significantly differ from that of type 1 dolomite. However, there is no blue luminescence due to the absence of terrestrial debris.

4.1.3. Grain-Texture Relict Dolomite (Type 3)

Type 3 dolomite particles are notably larger than those found in type 2 dolomite, and primarily take on a semi-self-shaped crystalline form with crystal sizes between 100 and 300 μm (Figure 3j–k). Characterized by many pores and contact inlays, Type 3 dolomite features a fog-center-brightening structure (Figure 3k). Additionally, certain occurrences of Type 3 dolomite retain the residual particle structures characteristic of Type 2 dolomite. The cathodoluminescence data reveal that Type 3 dolomite possesses an intensity that is similar to that of both Type 1 and Type 2 dolomite.

4.2. Geochemical Characteristics

4.2.1. Stable Isotope Characteristics

The stable isotope composition statistics are summarized in Table 1. The 87Sr/86Sr ratio of the samples in the study area (0.709084–0.715343, avg. 0.709618) was slightly higher than that of the contemporaneous seawater and normal marine sediments (0.70892–0.70921) [41,42] and lower than that of the crust source 87Sr/86Sr ratio (0.7119) [43]. The average δ13C value of the carbonate rocks in the Xixiangchi Formation of the Sichuan Basin was −0.67‰. The average δ18O value of the carbonate rocks in the Xixiangchi Formation of the Sichuan Basin was −7.73‰ (Table 1).

4.2.2. Rare Earth Element Characteristics

The rare earth element composition statistics are summarized in Table 1. The REE abundance of marine carbonates was much lower than that of clastic rocks, so even 2% terrigenous clastic pollution would significantly change the elemental anomalies of the samples and produce a relatively flat REE pattern. Therefore, in the analysis of the REE data, it was necessary to evaluate whether the sample was contaminated by impurities to obtain an accurate interpretation of the carbonate sedimentary environment and diagenesis. The REY content of the terrestrial sediments (generally > 100 ppm) was much higher than that of the marine sediments [44]. The carbonate rock samples collected in this study had relatively low REY contents (avg. 29.52 ppm), indicating that they were less contaminated by terrigenous debris [45].
The REY concentrations in the limestone and dolomite in the study area were quite different. Limestone had the highest REY concentrations (avg. 80.97 ppm). Type 1 dolomite had lower REY concentrations (avg. 20.81 ppm) than limestone. Type 2 dolomite had lower REY concentrations (avg. 11.79 ppm) than Type 1 dolomite. Type 3 dolomite had the lowest REY concentrations (avg. 9.75 ppm). The average Y concentrations of the limestone (avg. 11.80 ppm), Type 1 dolomite (avg. 2.72 ppm), Type 2 dolomite (avg. 1.40 ppm), and Type 3 dolomite (avg. 1.28 ppm) gradually decreased.
The REE test results of the limestone and dolomite in the study area were standardized, and the overall Eu anomaly (δEu) was 0.97. The average value of the δEu of the Type 1 dolomite was 0.80; Type 2 dolomite was 0.80; and Type 3 dolomite was 0.88. The Ce anomaly (δCe) value of the carbonate in the study area was 0.84. The average value of the δCe of the limestone was 0.85, Type 1 dolomite was 0.83, Type 2 dolomite was 0.86, and Type 3 dolomite was 0.85. The δCe values of various carbonate rocks had little difference. There was a weak negative Ce anomaly in the carbonate rocks in the study area (Table 1).

4.2.3. Fluid-Inclusion Characteristics

Inclusions were extremely difficult to find in Type 1 dolomite (Figure 4a). Inclusions in Type 2 dolomite usually showed a regular distribution (Figure 4b) and irregular shape. The inclusions in Type 3 dolomite usually showed a zonal distribution (Figure 4d) or irregular distribution (Figure 4c), and the shape of the inclusions was irregular.
In Type 2 dolomite, the homogenization temperature (Th) of the fluid inclusions ranged from 68 to 130.2 °C (avg. 102.61 °C), the freezing point temperature ranged from −20 to −6 °C, and the salinity ranged from 8.97 wt% to 14.88 wt% (avg. 12.59 wt%). In Type 3 dolomite, the homogenization temperature (Th) of the fluid inclusions ranged from 50 to 90.8 °C (avg. 71.45 °C), the freezing point temperature ranged from −12.2 to −6 °C, and the salinity ranged from 8.97 wt% to 14.13 wt% (avg. 11.62 wt%) (Table 2).

5. Discussion

5.1. Diagenetic Fluids

Carbon and oxygen isotopes can show and trace the source of dolomitized fluids [46]. Veizer et al. [47] demonstrated that the δ13CVPDB values of Upper–Middle Cambrian calcite fall within the range from −2‰ to +1‰ and that its δ18OVPDB values range from −10‰ to −6‰. Most of the δ13CVPDB and δ18OVPDB isotopes of the limestone and three kinds of dolomite in the study area were within the range of the δ13CVPDB and δ18OVPDB values of Upper–Middle Cambrian (Figure 5). This shows that the carbon isotope of the carbonate rocks in the Upper Cambrian Xixiangchi dolostones developed in the Sichuan Basin preserves the properties of contemporaneous seawater [48,49]. Thus, its diagenetic fluid should be the seawater of the penecontemporaneous period or the original seawater sealed in the pores. The similarity of the δ13CVPDB between the three kinds of dolomite and limestone indicates that dolomite fluid has a certain similarity and inheritance [50].
Unlike carbon and oxygen isotopes, the strontium isotopes in seawater are not fractionated by temperature, pressure, and microbial action [51], and the global isotopic composition of marine strontium is uniform at any age [52]. Therefore, the strontium isotope of marine minerals can better represent the strontium isotope composition and variation trend of diagenetic fluids. The strontium isotopic composition of seawater is mainly controlled by crustal or mantle-derived strontium [42]. Most of the samples in the study area had values higher than the 87Sr/86Sr ratios of the seawater in Middle–Late Cambrian and lower than the crust source 87Sr/86Sr ratios (Figure 6). At the same time, the thin section also exhibited a small amount of terrigenous quartz grains in some samples [53]. Type 1 dolomite had the highest 87Sr/86Sr ratios, suggesting that it was more seriously affected by crustal materials. Type 1 dolomite is a common laminated structure, mainly developed in tidal flat facies. The tidal flat is more susceptible to the influence of terrestrial materials, and this kind of dolomite fluid may be the seawater affected by terrestrial sources, which is evaporated and concentrated on the tide [50].
The distribution pattern and characteristic value of the REE are also important indexes for a diagenetic fluid analysis [44,54,55,56]. The three types of dolomite exhibited almost the same REE distribution pattern as the limestone (Figure 7), indicating that dolomite fluids have similar REE distribution characteristics to contemporaneous limestone. Limestone is weakly affected by diagenesis and often represents the properties of seawater during deposition [14]. Therefore, the fluid source of the three dolomites is contemporaneous seawater. Type 2 dolomite and Type 3 dolomite had a higher salinity than present-day seawater (3.5 wt%) (Figure 8) [8], demonstrating that the salt content of diagenetic fluids is higher than that of normal seawater, that is, their dolomitized fluids should be related to evaporated seawater.
The Eu anomaly is commonly used as a redox indicator of the parent fluid during mineral precipitation or the post-depositional diagenetic fluid influence [57,58]. The positive Eu anomaly occurs when the hydrothermal temperature is higher than 250 °C [59,60,61] and is an important indicator of hydrothermal dolomite, which favors the reduction of Eu3+ to Eu2+ ions by affecting its redox potency of Eu3+/Eu2+. Thus, Eu2+ ions are able to replace Ca2+ ions, which causes positive Eu anomalies in carbonate minerals [59,62]. There was a negative Eu anomaly in several types of carbonate rocks in the study area (Figure 9), which can exclude the diagenesis of an extreme high temperature environment. Negative δ18OVPDB values in dolomite are often used to determine the influence of a high-temperature diagenetic environment [46]. Most of the δ18OVPDB values of the carbonate rocks in the study area were in the range of normal marine sediments of Middle–Late Cambrian (Figure 4). This shows that the diagenesis environment was not obviously affected by a high temperature.
The magnitude of the Ce anomaly can reflect the oxidation degree of a diagenetic environment [63]. This is because the soluble Ce3+ ions in water can be easily oxidized into insoluble Ce4+ ions under oxidizing oxidations, thus facilitating their adsorption as CeO2 onto organic particles, but their depletion in the water [58,63]. Consequently, a negative Ce anomaly reflects oxic shallow seawater conditions or calcites precipitated from seawater-like conditions [55,64]. Generally, the average anomaly of rare earth element Ce in ancient normal seawater is 0.752 [65]. The δCe (0.84) of the samples in the study area was higher than that of normal marine sediments, suggesting that the dolomite diagenesis environment in the study area belongs to an open oxidation environment. This is consistent with the formation of the Xixiangchi Formation in a shallow water platform tidal flat environment in the Sichuan Basin. The alkaline environment of brine with a higher Mg/Ca ratio is conducive to the activation and migration of REE in the process of dolomite [66], so dolomite is a process of REE migration and depletion. The average Y content of limestone, Type 1 dolomite, Type 2 dolomite and Type 3 dolomite in the study area gradually decreased, and it is safe to conclude that the dolomite degree of Type 3 dolomite was more thorough and the formation period was later.

5.2. Dolomitization

Type 1 dolomite preserved the petrological details of limestone, and it was extremely difficult to find inclusions. The characteristics of the dolomitized fluid show the nature of the seawater and, at the same time, its petrological characteristics show that the dolomitization occurred in the syndiagenetic period of the early diagenesis stage [14]. It had the highest 87Sr/86Sr ratio of the three types of dolomite, and its overall high REY content suggests that it is more seriously affected by crustal materials and that the degree of dolomite is low [42,66]. The above characteristics indicate that Type 1 dolomite is penecontemporaneous dolomite produced by the replacement of evaporated seawater [46,67]. It is formed in a near-surface, high-salinity brine environment. It belongs to the evaporation pump dolomitization model in a low-energy tidal flat environment.
The rock details in the Type 2 dolomite were well preserved and show that it was formed in the shallow buried diagenetic period of the early diagenesis stage [14]. As mentioned above, the dolomite fluid exhibited by the 87Sr/86Sr and C/O isotopes was also contemporaneous seawater. However, compared to Type 1 dolomite, the diagenesis temperature of Type 2 dolomite was slightly higher and the salinity of its brine was slightly lower, demonstrating that the depth of the dolomite entered a shallow burial environment [68]. That is, the supratidal high-salinity seawater flows downward to the shallowly buried unconsolidated granular carbonate, which should belong to the seepage-reflux dolomite model. The particles are dolomited during the reflux of diagenetic fluid.
Type 3 dolomite had a lower 87Sr/86Sr ratio, the lowest REY content, and a higher dolomite temperature, reflecting that it had the highest degree of dolomite and the deepest buried environment. Some of the dolomites with a low degree of dolomitization in the type 3 dolomite still had the fog-core and bright edge structure of residual particles, indicating that they were formed by recrystallization on the basis of granular dolomite and showing that it was formed in the late diagenesis [14]. Its diagenetic fluid properties had a certain similarity and inheritance to Type 2 dolomite due to its similar geochemical model [69]. Therefore, it is safe to conclude that Type 3 dolomite is the result of the continued burial of Type 2 dolomite and its dolomitization, or the burial dolomite caused by the original seawater trapped in the pores of the granular limestone.

5.3. Dolomitization Model

We propose a model for the formation of massive dolostones (Figure 10). Under the strong evaporation in a tidal flat, the seawater existing between filled particles is continuously evaporated and concentrated to form a fluid with a high Mg/Ca ratio. This high-magnesium intergranular water, in contact with particles such as aragonite, will inevitably occur by the role of aragonite being explained, namely dolomite [2], and the precipitation of some salt minerals such as gypsum. At the same time, due to capillary action, seawater is continuously replenished to the particles of these sediments. A large area of dolomite will occur in the tidal flat environment, that is, evaporation pump dolomitization [70] (Figure 10a). This type of dolomitization produces fine crystals and is widely distributed in a layered distribution [71].
Inter-granular brine with a high Mg/Ca ratio will be produced due to the evaporation pump. In addition to the appealing surface sediment dolomitization, this high Mg/Ca-ratio fluid is due to its high density and will inevitably flow downward or obliquely downward under the action of gravity [72]. Especially in the carbonate particle accumulation area, the rocks in the early diagenesis stage are weakly compacted and loose, and their porosity and permeability are high, which is conducive to the reflux seepage of high-magnesium salt water along the ramp and down through the bottom of the lagoon and platform edge. They flow through the underlying shallow buried carbonate granular sediment dolomite, thus forming granular dolomite (Figure 10a). In the field, this type of dolomite usually presents as thick massive dolomite [14].
In the burial process of early penecontemporaneous dolomite and reflux-seepage dolomite, with an increase in temperature, dolomitization continues to occur. Because this increase in temperature helps to overcome molecular dynamic barriers, the original high Mg/Ca seawater trapped between the particles interacts with the particles [6]. The granular dolomite formed by reflux seepage in the shallow burial period is further dolomitized, or recrystallized to form grain-texture relict dolomite (Figure 10b).

6. Conclusions

The dolomites of the Cambrian Xixiangchi Formation in the Sichuan Basin can be divided into three types according to their structures, fine crystalline dolomite (Type 1), granular dolomite (Type 2), and grain-texture relict dolomite (Type 3). The dolomitizing fluids are derived from seawater.
The geneses of fine crystalline dolomite, granular dolomite, and grain-texture relict dolomite are evaporation-pump dolomite, reflux-seepage dolomite, and burial dolomite, respectively.
Fine crystalline dolomite is formed in the supratidal environment around the paleo-uplift and is widely distributed in thin layers. Granular dolomite is developed in the platform granular beach environment. Some of them are further buried during the burial process to form grain-texture relict dolomite.

Supplementary Materials

The following supporting information can be downloaded at: https://0-www-mdpi-com.brum.beds.ac.uk/article/10.3390/min13070932/s1, Table S1: The stable isotopic values of the Upper Cambrian Xixiangchi formation different carbonates in the Sichuan Basin, China; Table S2: The rare earth element values of the Upper Cambrian Xixiangchi formation different carbonates in the Sichuan Basin, China; Table S3: The homogenization temperature, ice melting temperature and salinity values of the Upper Cambrian Xixiangchi formation different carbonates in the Sichuan Basin, China.

Author Contributions

Conceptualization, L.W. and B.Z.; methodology, X.Z.; software, M.L.; validation, F.L., M.L. and W.Y.; formal analysis, F.L.; investigation, H.H., M.L. and W.Y.; resources, S.X.; data curation, B.Z.; writing—original draft preparation, H.H.; writing—review and editing, H.H. and A.C; visualization, A.C.; supervision, S.X.; project administration, L.W.; funding acquisition, S.X. and L.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Everest Scientific Research Program of Chengdu University of Technology (Grant No.2022ZF11402).

Data Availability Statement

Data will be made available on request.

Acknowledgments

Thanks to the Ke Feng and Kaidi Hu for their help in the field work of this study. We thank to the Academic Editor for their valuable suggestions. We would like to thank the two referees for their constructive advice.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Geotectonic location of Sichuan Basin (modified after [30]), red star is the geographical location of Beijing, the capital of China. (b) Sequence stratigraphy column of Middle-Upper Xixiangchi Formation in Sichuan Basin (modified after [31]), red box is the research horizon of this paper.
Figure 1. (a) Geotectonic location of Sichuan Basin (modified after [30]), red star is the geographical location of Beijing, the capital of China. (b) Sequence stratigraphy column of Middle-Upper Xixiangchi Formation in Sichuan Basin (modified after [31]), red box is the research horizon of this paper.
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Figure 2. Paleogeographic scheme of the high systems tract of Middle–Later Cambrian tectonic sequence, Upper Yangtze (modified after [35]). Samples were taken from CK—the Loushanguan Formation in Xiongzhu Village, Chengkou City, Chongqing City; HS—the Gengjiadian Formation in Heishui Town, Youyang County, Chongqing City; JZ—the Jingzu in Guizhou Province; LG—the observation profile of Laga Township, Ganluo County, Sichuan Province; LX—the Liugushui Village, Xuekoushan Township, Mabian County, Sichuan Province; NC—the Loushanguan Formation in Nanchuan, Chongqing City; SZ—the Loushanguan Formation in Lengshuixi, Shizhu County, Sichuan Province; TS—the Xiwangmiao-Dahecao Formation in Yoangshan Tuanjie County, Yunnan Province; XS—the Loushanguan Formation in Laodongkou, Xingshan, Hubei Province; YK—the measured profile of the Yankong in Jinsha County, Guizhou Province; and YY—the Loushanguan Formation in Heishui Town, Youyang City, Chongqing City.
Figure 2. Paleogeographic scheme of the high systems tract of Middle–Later Cambrian tectonic sequence, Upper Yangtze (modified after [35]). Samples were taken from CK—the Loushanguan Formation in Xiongzhu Village, Chengkou City, Chongqing City; HS—the Gengjiadian Formation in Heishui Town, Youyang County, Chongqing City; JZ—the Jingzu in Guizhou Province; LG—the observation profile of Laga Township, Ganluo County, Sichuan Province; LX—the Liugushui Village, Xuekoushan Township, Mabian County, Sichuan Province; NC—the Loushanguan Formation in Nanchuan, Chongqing City; SZ—the Loushanguan Formation in Lengshuixi, Shizhu County, Sichuan Province; TS—the Xiwangmiao-Dahecao Formation in Yoangshan Tuanjie County, Yunnan Province; XS—the Loushanguan Formation in Laodongkou, Xingshan, Hubei Province; YK—the measured profile of the Yankong in Jinsha County, Guizhou Province; and YY—the Loushanguan Formation in Heishui Town, Youyang City, Chongqing City.
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Figure 3. Microscopic and cathodoluminescence characteristics of dolomite from the Cambrian Xixiangchi Formation in Sichuan Basin. (a) Type 1 Dolomite from Nanchuan Sanhui Section. (b) Type 1 dolomite from the Yongshan Tuanjie Section, visible fracture development. (c) Type 1 dolomite obtained from Heishui Town, Youyang. (d) Cathodoluminescence picture of (c), showing moderate-intensity luminescence and blue terrigenous debris of Type 1 dolomite and blue luminescence of terrestrial quartz debris (yellow dotted circle). (e) Type 2 dolomite obtained from the section of Jinshayan hole, bright crystal oolitic dolomite. (f) Type 2 dolomite obtained from the section of Jinshayan hole, brilliant crystalline sandy dolomite. (g) Brilliant Oolitic Dolomite from Jingzhu Section, Wuchuan, developed intergranular pores and intergranular dissolved pores. (h) Type 2 dolomite obtained from Jinsha rock hole section. (i) Cathodoluminescence picture of (h), showing moderate-intensity luminescence of Type 2 dolomite. (j) Type 3 Dolomite from the Nanchuan Sanhui Section, developed dissolved pores. (k) Type 3 dolomite from Jinsha rock borehole profile with foggy core brightening structure. (l) Cathodoluminescence piuture of Type 3 dolomite from Heishui Town, Youyang, shows moderate-intensity luminescence.
Figure 3. Microscopic and cathodoluminescence characteristics of dolomite from the Cambrian Xixiangchi Formation in Sichuan Basin. (a) Type 1 Dolomite from Nanchuan Sanhui Section. (b) Type 1 dolomite from the Yongshan Tuanjie Section, visible fracture development. (c) Type 1 dolomite obtained from Heishui Town, Youyang. (d) Cathodoluminescence picture of (c), showing moderate-intensity luminescence and blue terrigenous debris of Type 1 dolomite and blue luminescence of terrestrial quartz debris (yellow dotted circle). (e) Type 2 dolomite obtained from the section of Jinshayan hole, bright crystal oolitic dolomite. (f) Type 2 dolomite obtained from the section of Jinshayan hole, brilliant crystalline sandy dolomite. (g) Brilliant Oolitic Dolomite from Jingzhu Section, Wuchuan, developed intergranular pores and intergranular dissolved pores. (h) Type 2 dolomite obtained from Jinsha rock hole section. (i) Cathodoluminescence picture of (h), showing moderate-intensity luminescence of Type 2 dolomite. (j) Type 3 Dolomite from the Nanchuan Sanhui Section, developed dissolved pores. (k) Type 3 dolomite from Jinsha rock borehole profile with foggy core brightening structure. (l) Cathodoluminescence piuture of Type 3 dolomite from Heishui Town, Youyang, shows moderate-intensity luminescence.
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Figure 4. Photomicrographs showing fluid inclusion petrography. (a) Single distributed elliptical inclusions in Type 1 dolomite (HS12-2). (b) Regularly distributed irregular inclusions in Type 2 dolomite (YK-L-3). (c) Irregularly distributed irregular inclusions in Type 3 dolomite (YK-L-10). (d) Banded irregular inclusions in Type 3 dolomite (HS-16-2).
Figure 4. Photomicrographs showing fluid inclusion petrography. (a) Single distributed elliptical inclusions in Type 1 dolomite (HS12-2). (b) Regularly distributed irregular inclusions in Type 2 dolomite (YK-L-3). (c) Irregularly distributed irregular inclusions in Type 3 dolomite (YK-L-10). (d) Banded irregular inclusions in Type 3 dolomite (HS-16-2).
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Figure 5. δ13C and δ18O values for the Cambrian limestone and dolomite rocks of Sichuan Basin compared to Cambrian seawater.
Figure 5. δ13C and δ18O values for the Cambrian limestone and dolomite rocks of Sichuan Basin compared to Cambrian seawater.
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Figure 6. 87Sr/86Sr of carbonate rocks in our research compared to the Cambrian seawater and to average crustal rocks. The crustal 87Sr/86Sr is from Palmer and Edmond [43]; and the 87Sr/86Sr trend in Cambrian marine carbonates is from Huang et al. [41].
Figure 6. 87Sr/86Sr of carbonate rocks in our research compared to the Cambrian seawater and to average crustal rocks. The crustal 87Sr/86Sr is from Palmer and Edmond [43]; and the 87Sr/86Sr trend in Cambrian marine carbonates is from Huang et al. [41].
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Figure 7. Mean NASC-normalized REE patterns of the XXC Formation dolomites.
Figure 7. Mean NASC-normalized REE patterns of the XXC Formation dolomites.
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Figure 8. Characteristics of fluid inclusions in Xixiangchi Formation of Sichuan Basin.
Figure 8. Characteristics of fluid inclusions in Xixiangchi Formation of Sichuan Basin.
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Figure 9. Scatter diagrams of δEu with δCe.
Figure 9. Scatter diagrams of δEu with δCe.
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Figure 10. Dolomitization models of the Cambrian dolomites in the Sichuan Basin, China. (a) Type 1 Dolomite that was evaporation pump dolomitization and Type 2 Dolomite that was reflux-seepage dolomitization. (b) Type 3 Dolomite that was burial compaction dolomitization.
Figure 10. Dolomitization models of the Cambrian dolomites in the Sichuan Basin, China. (a) Type 1 Dolomite that was evaporation pump dolomitization and Type 2 Dolomite that was reflux-seepage dolomitization. (b) Type 3 Dolomite that was burial compaction dolomitization.
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Table
Table 1. Stable isotopic values and rare earth element statistics of the investigated Upper Cambrian Xixiangchi formation different carbonates in the Sichuan Basin, China.
Table 1. Stable isotopic values and rare earth element statistics of the investigated Upper Cambrian Xixiangchi formation different carbonates in the Sichuan Basin, China.
Lithology87Sr/86Srδ13C‰δ18O‰ΣREE + YYδEuδCe
(VPDB)(VPDB)(PPM)(PPM)(PPM)(PPM)
Limestonen5131316161616
Mean0.709633−1.24−8.9980.9711.800.910.85
Min0.709145−4.70−9.788.991.250.770.73
Mix0.7106212.18−8.04278.539.021.380.91
Type 1 dolomiten18313134343434
Mean0.709838−0.53−7.4320.812.720.800.83
Min0.709084−2.01−11.565.470.810.690.75
Mix0.7153432.24−5.7994.1610.551.020.89
Type 2 dolomiten4131313131313
Mean0.709310−0.50−7.7711.791.400.800.86
Min0.709217−1.62−10.184.940.520.680.76
Mix0.7094270.81−6.0823.362.3610.96
Type 3 dolomiten11151515151515
Mean0.709363−0.63−7.279.751.280.880.85
Min0.709171−1.62−9.124.750.70.710.75
Mix0.7100691.53−6.0815.362.111.620.93
Table
Table 2. Average fluid inclusion values of different carbonates in the Sichuan Basin, China. (Th—homogenization temperature; Tm-ice—ice melting temperature).
Table 2. Average fluid inclusion values of different carbonates in the Sichuan Basin, China. (Th—homogenization temperature; Tm-ice—ice melting temperature).
LithologyTh (°C)Tm-ice (°C)Salinity (wt%)
RangeMeanRangeMeanRangeMean
Type 2 dolomite68~130.2102.61−20~−6−11.198.97~14.8812.59
Type 3 dolomite50~90.871.45−12.2~−6−8.958.97~14.1311.62
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Hu, H.; Xu, S.; Chen, A.; Wen, L.; Zhang, B.; Zhang, X.; Li, F.; Liu, M.; Yong, W. The Origin of the Upper Cambrian Basin-Scale Massive Dolostones of the Xixiangchi Formation, Sichuan Basin, China. Minerals 2023, 13, 932. https://0-doi-org.brum.beds.ac.uk/10.3390/min13070932

AMA Style

Hu H, Xu S, Chen A, Wen L, Zhang B, Zhang X, Li F, Liu M, Yong W. The Origin of the Upper Cambrian Basin-Scale Massive Dolostones of the Xixiangchi Formation, Sichuan Basin, China. Minerals. 2023; 13(7):932. https://0-doi-org.brum.beds.ac.uk/10.3390/min13070932

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Hu, Huan, Shenglin Xu, Anqing Chen, Long Wen, Benjian Zhang, Xihua Zhang, Fuxiang Li, Mengqi Liu, and Wei Yong. 2023. "The Origin of the Upper Cambrian Basin-Scale Massive Dolostones of the Xixiangchi Formation, Sichuan Basin, China" Minerals 13, no. 7: 932. https://0-doi-org.brum.beds.ac.uk/10.3390/min13070932

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