The genesis of dolomites in ancient strata is currently a key topic in the sedimentology of carbonate rocks. Due to the superimposed modification of dolomites by fluids of various stages during and after their formation, the petrologic and geochemical indicators of dolomites measured now are all somewhat “composite”. In other words, the complex geochemical information presented by dolomites will lead to multiplicity of solutions and ambiguity^[1,2,3]. Given this, it is considered a feasible research approach to define the timing of the occurrence of dolomitization based on petrology under constraints of contact relationships between different diagenetic fabrics and dolomites, then to identify diagenetic fluids by rock-mineral analysis (e.g. geochemical analysis).

The Sichuan Basin is a large superimposed petroliferous basin in southwestern China, where a special type of reservoir rock, heterogeneous sucrosic dolomite, has been found in the Lower Permian Qixia Formation in its northwestern part during oil and gas exploration. This sucrosic dolomite is characterized by anhedral-subhedral medium-coarse (300-800 μm) grains^[4,5]. This kind of dolomite is a typical case to investigate the genesis of the deep-buried ancient dolomite^[6]. At present, the genesis of the sucrosic dolomite of the Qixia Formation remains controversial, and the origins reported in literatures include mixed-water dolomitization^[7,8], burial dolomitization^[8,9,10], “basalt leaching”^[11], “hydrothermal sub- basin”^[12], and “tectonic-hydrothermal dolomitization”^{[4-6, 13-15]}. Dolomitization associated with hydrothermal activities during the shallow burial is the most accepted viewpoint. If this is true, dolomite should be distributed along deep large faults developed during the late stage of the Dongwu Movement. However, the exploration practice in the basin reveals the dolomite in the basin doesn’t occur along fault system completely.

In recent years, more and more drilling and outcrop data of the northwestern Sichuan Basin have been accumulated. In particular, both cores taken from Well D6 and Hejialiang section confirm the presence of multiple stages of eogenetic karstification driven by high-frequency sea-level fluctuation, which makes it possible to determine the relative timing of the dolomitization to that of the eogenetic karstification^[16]. Therefore, in this research, starting from analysis of macro and micro features of core samples and field sections of the Qixia Formation in northwestern Sichuan, the relative time sequence of the eogenetic karstification and dolomitization was defined through petrographic analysis of dolomite and close examination of evidence and vertical sequence of eogenetic karstification. Then, the timing and diagenetic fluids of the dolomitization were inferred, and a dolomitization model of the sucrosic dolomite was put forward. We hope that the findings of this research can provide geological basis for predicting the distribution of the Qixia Formation dolomite reservoirs, and the research can serve as a typical case for identifying the genesis of ancient complex dolomites.

The study area is geographically located in the northwestern part of the Sichuan Basin, and tectonically in the transition zone between the West Sichuan piedmont depression belt, Longmenshan fold belt, and Micangshan tectonic belt. The current complex tectonic framework of the study area is the result of superimposed effects of multiple stages of tectonic movement (Fig. 1a). From bottom to top, the Lower Permian is composed of Liangshan and Qixia formations^[17]. Shore-swamp facies sandstone and mudstone deposited first in the Liangshan Formation, and then the system evolved to a carbonate platform due to the transgression during the early Qixia Period. The mid-lower part of the Qixia Formation is primarily composed of limestone and marlstone rhythmic layers, while the mid-upper part mostly consists of packstone or wackestone and laminated or patchy dolomite^[17,18,19]. Specifically, during the depositional period of Qixia Formation, the western part of the study area was NE-SW striking platform-margin mound-shoal and semi-restricted sea; the southwestern and northeastern parts were open-sea platform and intra- platform shoal; and the south part of central-eastern Guangyuan- Wangcang area was intra-platform depression (Fig. 1b). During the early depositional period of the first member of the Qixia Formation (Qi-1 Member), the study area was generally an open environment formed during transgression^[18]. During the mid-late depositional period of the Qi-1 Member, the sea-level dropped, so the top of the Qi-1 Member has widespread karrens, karst caves, and brecciation related to high-frequency exposed surface (Fig. 1c)^[19]. During the deposition of the Qi-2 Member, the sea level maintained relatively low for a long time following a short-lived transgression. By the end of the Qixia Period, the study area was locally exposed^[19]. Therefore, the Liangshan and Qixia Formations in northwestern Sichuan Basin can be generally divided into two complete third-order transgression-regression cycles (Fig. 1c), which correspond to the sections from the Liangshan Formation to the Qi-1 Member, and the Qi-2 Member, respectively. Moreover, the Qi-2 Member can be sub-divided into two sub-members (namely A and B sub-members; Fig. 1c). Under the control of the high stand system tracts of the two transgression-regression cycles, relatively large mound-shoal bodies and semi-restricted sea turned up (Fig. 1b). Furthermore, the local water body might be restricted owing to the stacking and migration of the mound-shoal bodies. Hence, the geological setting was ready for penecontemporaneous dolomitization^[19].

From the perspective of tectonic setting, the northwestern Sichuan area close to the tectonic junction belt was susceptible to tectonic extension^[20], due to the opening and expansion of the Mianlue Ocean during the Early-Middle Permian. With the continuous tensile faulting activities, two passive continental margin normal fault systems (one in nearly NE strike, while the other in nearly EW strike) were formed at the current Longmenshan fold belt and Micangshan tectonic belt (Fig. 1a)^[20,21]. Such tensile fault systems at the plate margin not only, from a macro point of view, controlled the formation filling and carbonate rock sedimentation patterns in the area, but also led to frequent thermal activities in northwestern Sichuan during the Early-Middle Permian, owing to the activities of the fault systems^{[19, 22]}, and the thermal activities provided fluids for hydrothermal dolomitization^{[4-6, 13-15]}.

This study is mainly based on the data of the cores taken from Well D6 and surveyed field sections (e.g. Hejialiang, Changjianggou, and Chejiaba sections, Fig. 1b). The cores and sections were all observed, described, and sampled systematically. A total of 479 thin sections were prepared. Specifically, the conventional thin sections were dyed with the Alizarin Red S solution, while blue epoxy resin was injected into the pores of cast thin sections. The microscopic observation under the cathode luminescence (CL) was completed in the School of Geoscience and Technology, Southwest Petroleum University, China, by using the CL8200 MK5 CL microscope under the working mode of 7-10 kV and 400-500 mA. The fluid inclusion test and carbon and oxygen isotope composition analysis were done in the Southwest Petroleum University division of Key Laboratory of Carbonate Reservoirs, CNPC. The fluid inclusion test was conducted on the Linkam THMSG600 Geology (heating/freezing) Stage (manufactured in Germany), with a temperature range of -196-600 ºC, heating/cooling rates of 0.1-150 ºC/min, and temperature accuracy of 0.1 ºC. For analyzing the carbon and oxygen isotope composition, 1 g of the bulk rock sample for each specimen was cored with a micro drill bit and tested with the MAT 253 Plus isotope ratio mass spectrometer in a super-clean lab. The test procedure is detailed as follows: Oxygen in the gas-tight reaction bottle containing carbonate rock powders was first displaced by high-purity helium, and then 6-8 drops of anhydrous phosphoric acid were injected; the mixture was left reacting at 70 ºC for 1 h, during which the released gaseous CO₂ was entrained by the helium stream into the isotope mass spectrometer for testing. The data of trace elements (including rate earth elements) of this paper are from our previous research^[22]. For inter-well correlation of dolomite horizons, high-precision gamma-ray measurement at an interval of 20 cm was carried out across the Qixia Formation on the Hejialiang and Changjianggou sections.

According to the crystal size and occurrence state, the dolomites of the Lower Permian Qixia Formation in northwestern Sichuan can be mainly classified into three types, namely the very finely to finely crystalline dolomite, medium-coarse crystalline sucrosic dolomite, and the dolomite filling in karst system (Figs. 2 and 3). The former two are the bedrocks of the dolomite filling karst system, and the dolomite filling karst system is generally product of dissolution of the sucrosic dolomite.

3.1.1. Very finely to finely crystalline dolomite (Md1)

At the macro scale, Md1 appears as interbedded grey and dark red dolomite bands (Fig. 2c, 2d). When observed under microscope, this kind of dolomite is often anhedral-subhedral, with a grain size range of 50-150 μm (Fig. 2b), often has crystals with dissolution edges, and silty clasts reducing in sizes due to dissolution between crystal grains (Fig. 2d, 2e). Rhythmic bedding indicating tidal flat (Fig. 2d) and erosion surface structure indicating tidal furrow (Fig. 2e) are observed in this kind of dolomite. The Md1 sends out null-luminance or extremely dark red light signals under cathode luminescence (Fig. 2c).

Md1 occurs less frequently in the study area and is mainly found in the upper part of the Qi-1 Member and the base of the Qi-2 Member. For example, it is commonly seen at the top of each shallowing-upward cycle from the top of the Qi-1 Member to the base of the Qi-2 Member in Well D6 as well as at the top of the single cycle of the Qi-1 Member in the Hejialiang section (Fig. 4).

3.1.2. Medium-coarse crystalline sucrosic dolomite (Md2)

This kind of dolomite is light-grey and blocky, with coarse fracture surface and pinholes in local parts. Due to its sucrosic (sugar particle-like) texture, it is named as sucrosic dolomite (Fig. 2g). Microscope observation shows that its crystals are coarse (300-800 μm) and generally anhedral or subhedral, with occasionally turbid core and bright rim observed (Fig. 2h). The dolomite has intergranular (dissolution) pores in high heterogeneity (Fig. 2g). Under cathode luminescence, the crystal core (the turbid core) gives off non-luminance or extremely dark red light signals, while the crystal edge (the bright rim) sends red light (Fig. 2i). This kind of dolomite may also appear as pinhole- dolomite or residual granular dolomite.

The sucrosic dolomite is discovered in the mid-lower part of the Qi-2 Member and the top of the Qi-1 Member in both wells and outcrop sections at the platform margin in the west of the study area. The sucrosic dolomite layers in the Hejialiang section and Well D6 are the thickest (Fig. 4).

3.1.3. Dolomite filled in karst system (Kfd)

This type of dolomite is generally associated with the sucrosic dolomite. At the macro scale, it is found as dark-grey or black grey karst filling sections or small-scale karren fillings in the sucrosic dolomite (Fig. 3a, 3c, 3d, 3f). It has brecciation features of various shapes, including some nearly in-situ brecciation induced by cutting of the preferential karstification channel (Fig. 3a, 3b). Some dolomite breccias present characteristics of long-distance transportation and rounding (Fig. 3c, 3e); some other dolomite breccias are to some extent plastic and angular (Fig. 3d). This kind of dolomite has crystals commonly characterized by micritization and shrinkage induced by dissolution, and is mostly composed of subhedral-euhedral silty-coarse (30-800 μm) dolomite (Fig. 3b, 3e, 3g). Some crystals of this dolomite feature turbid core and bright rim, and the edge of the bright rim has generally some karst fabrics and cutting features of filling materials (Fig. 3g). Under cathode luminescence, this kind of dolomite often consists of dark red core, red to bright red rim, and intergranular cement (Fig. 3h). This type of dolomite is the product of dissolution of the sucrosic dolomite, and is often seen in the B sub-member of the Qi-2 Member and on the top of the Qi-1 Member in the platform margin facies belt of the western part of the study area. It is especially thicker in Wells D2 and D6 and the Hejialiang section (Fig. 4).

According to the field survey, core description, and well- logging analysis, the Qixia Formation dolomite has the following distribution features of temporal-spatial: (1) The stratigraphic position of the sucrosic dolomite is relatively stable in small regions. For instance, in the Well D6-D3 region (Fig. 4), the sucrosic dolomite is mainly found in the B sub-member of the Qi-2 Member, while it often occurs at the top of the B sub-member and the base of the A sub-member of the Qi-2 Member in the wells and outcrop sections in the north of the Jiange Area (e.g. Well D2 and the Changjianggou section). (2) The stratigraphic position of the sucrosic dolomite tends to be shallower from southwest to northeast. (3) All types of dolomite are 10-30 m thick in total generally, but in some wells, the dolomites developed earlier and are tremen-dously thick. For example, in Well D6, dolomite occurs from the top of the Qi-1 Member to the base of the B sub-member of the Qi-2 Member, with a cumulative thickness of up to 39 m; in the Hejialiang section, the whole Qixia Formation is dolomitized, with a cumulative dolomite thickness of 112 m (Fig. 2).

It can be also seen from Fig. 4 that the Qixia Formation generally tends to overlap in the northeastward direction during early sedimentation period. Due to the stratigraphic overlap, multi-stage sea-level fluctuations, and the overall small geomorphological variation during the Qixia Period, mound- shoal bodies of different stratigraphic horizons had frequent stacking and migration^[21]. Furthermore, during the high-frequency transgression, migration of mound-shoal bodies might lead to some locally restricted environments and varied positions of highlands and restricted areas. Thus, exposure at the end of the high-frequency regression might only result in the dissolution of highlands in some local areas. This may eventually lead to the inconsistent stratigraphic positions of the exposure-induced karst features, and the unclear layer-related dolomitization during the early diagenetic stage.

The dolomitization timing of the sucrosic dolomite in the platform margin of the Qixia Formation has long been controversial. Some researchers suggested that there was no restricted environment for evaporative concentration-reflux infiltration dolomitization during the Qixia Period^[11], and the dolomitization mainly occurred during the burial diagenesis stage^[9]. After analyzing the formation timing of asphalt and siliceous fillings in the dolomite pores and vugs, Chen et al.^[14] and Liu et al.^[23] concluded that the dolomite in the Middle-Lower Permian must be formed earlier than the Jurassic, and most likely prior to the end of the Permian deposition. By using U-Pb dating and other methods, He et al.^[24] speculated that the thermal effect of the Emeishan basalt must have lasted from (263±5) Ma ago to (257±3) Ma ago, and Huang et al.^[4] further pointed out that the dolomitization of the Qixia Formation mainly occurred during this period. With the assistance of the R_o value and inclusion temperature measurement, Zhu et al.^[25] and Jiang et al.^[9] reconstructed the paleo-temperature gradient and inferred that the Middle Permian dolomitization mainly occurred during the period from the end of the Middle Permian to the beginning of the Late Permian (approximately 259 Ma ago). All these views hold that the dolomitization of the Qixia Formation is related to the Dongwu Movement and the thermal activities of the same period, thus the dolomitization is assumed to occur during the peak period of the Dongwu Movement and a period of time thereafter^{[4-6, 13-15]}. However, the aforementioned researches on the dolomitization timing are based on primarily the measurement and judgment of the formation time of the cement fillings in the pores and vugs of the dolomite, which is vulnerable to the multiplicity of solution and ambiguities. In this context, the macro and micro features in direct cross-cutting of diagenetic fabrics provides the most direct evidence for identifying the relative timing of the geological events^[26]. In the following parts, the evidence, vertical sequences, and fabric characteristics of eogenetic karsts formed by high-frequency sea-level fluctuations during the Qixia Period are presented in detail; and the relative timing of the dolomitization is made out by analyzing the cross-cutting relationship between dolomite grains and high-frequency exposure-related karst fabrics.

Early exposed karstification driven by high-frequency sea- level changes are also referred to as the penecontemporaneous karstification or eogenetic karstification^{[16, 27]}. During the eogenetic karst, the top of upward-shallowing cycles primarily consisting of shallow-water carbonate sediments such as those in the tidal flats, grain shoals, and biological reef mounds exposed above the sea level and received freshwater leaching and weathering erosion during the periodic fall of the relative sea level^[28,29]. The eogenetic karst generally lasts from the exposure of the top of one cycle to the end of the initial flooding of the next cycle. The eogenetic karst has obviously different features from the telogenetic karst, because rocks in the early diagenetic stage have loose structures, strong heterogeneity of porosity and permeability, and karst water flow in the rocks controlled by dominant channels during the eogenetic karst^[16]. Based on literature review, eogenetic karst is generally characterized by the following identification signs: (1) Dissolution bodies are honeycomb-shaped, sponge-like, or granophyric; (2) There are abundant dissociated bedrock particles, biological debris, and vadose carbonate sand and mud in the dissolution fillings, the dissociated, broken, and chalky clastic components appear as micrite and micro-sparry crystal under microscope, and there are blurred boundaries between dissolution pores and surrounding rocks; (3) Dissolution of early-formed soft rocks are generally controlled by facies and layers; (4) There is high-frequency superposition of multiple cycles^[29,30].

Exposed surfaces corresponding to multiple cycles are frequently seen in the upper part of the Qi-1 Member and the whole Qi-2 Member (Fig. 5a). Green-gray Md1 filled during initial transgression often covers the exposed surfaces and slight overlaps toward the relative highlands on both sides (Fig. 5b). Earthy yellow and brownish-red centimeter-level (2-10 cm) weathering crusts are frequently seen below the exposed surfaces (Fig. 5b, 5c). Three layers can be recognized in the weathering profile, namely, the main weathered layer composed of clay, dolomite silt, and iron residues, the semi-weathered layer dominated by in-situ breccia formed by cross-cutting of the bedrock by karrens, and the bedrock layer at the bottom (Fig. 5c-5e). Millimeter-level dissolution pores partially or completely filled by vadose silts of Md1 occur sporadically in the bedrock (Fig. 5e). Freshwater leaching, micritization (especially the micritization of dolomite breccias), and granular shrinkage of the sucrosic dolomite are clearly seen in the periphery of filled karst channels (Figs. 3e, 5d, 5h). Saclike caves and honeycomb-shaped, spongy-like, or granophyric dissolution signs (Fig. 5f, 5g), even some dominant karst channels cutting into breccias, are seen in sucrosic dolomite of both field outcrops and drilled cores (Fig. 5f). In addition, near-horizontal caves and cave breccias are also very common in all well blocks of the study area, for example, Wells D6 and D2 and Hejialiang section (Fig. 3c-3d). Generally speaking, eogenetic karst is obviously controlled by high-frequency sea-level fluctuation, and various karst phenomena are closely related to the exposed surface at the end of each cycle and are mostly present in the regression deposits, showing obvious facies-control feature.

Previous studies have demonstrated the high-frequency sea-level changes have strong effect on the eogenetic karst in carbonate platform^[30]. A sea-level eustatic cycle is often comprised of a rapid transgression process followed by a slow regression process^[30,31]. Grain shoals and tidal flats are two commonly seen upward-shallowing sequences with exposed surfaces in the Qixia Formation, and their vertical superimposition suggests frequent sea-level fluctuations and high-frequency exposure. Two upward-shallowing sequences, the 7th coring interval (well depth 7753.50-7759.86 m) of grain shoal and the 5th coring interval (well depth 7744.87-7746.20 m) of tidal flat in Well D6, are taken as examples to illustrated (Figs. 6 and 7).

The grain shoal upward-shallowing sequence of 7754.35-7758.40 m in the 7th coring interval of Well D6 is mainly comprised of wackestone, sucrosic dolomite, and carbonaceous grainstone from the bottom to the top, which respectively correspond to semi-restricted sea, grain shoal, and swamp in the upward-shallowing sequence (Fig. 6b-6i). The section above 7754.35 m is the semi-restricted sea-grain shoal sequence of the next cycle (Fig. 6a). Limestone formed in low-energy environment and dolomitized grainstone interbed in the cores. In one cycle, the energy of the sedimentary water body increased significantly from the initial transgression to the exposure of the top of the grain shoal. A large number of karst fabrics, such as karrens, caves, and breccias (Fig. 6b-6h) can be clearly identified in the coring interval. From the perspective of the karst sequence, the bottom limestone section has only small-scale karrens and dissolution fractures (Fig. 6i), while the relatively tight limestone interval, as the water barrier, controlled the karst features of the high-permeability layers above it. Obvious bedding karst features are observed at the base of the sucrosic dolomite (Fig. 6g, 6h), while vadose karst features such as karst patches and karrens dominate the middle part of the section (Fig. 6e, 6f). Nearly in-situ brecciation caused by dominant channels cutting can be seen below the exposed surface (Fig. 6b-6d, 6j, 6k). In general, the eogenetic karst in the upward-shallowing cycle of Well D6 is obviously facies-controlled. Similarly facies-controlled karsts in the Neogene of Florida, the Ordovician Majiagou Formation in the Ordos Basin of China, and the Permian Maokou Formation in the southern Sichuan Basin were reported^[29,30,31].

The three high-frequency sea-level rise and fall cycles in the interval of 7744.87-7746.20 m in Well D6 are mainly comprised of two parts: (1) the lower part is sucrosic dolomite with residual grain structure, which represents the deposits of dolomitic flat in the lower intertidal zone; (2) the upper part is mainly dark red, light gray dolomitic flat deposits or Md1 formed in algal dolomitic flat, with macroscopically visible algae laminae and rhythmic bedding (Fig. 7). This sequence shows signs of water energy reduction upward and sedimentary characteristics of dolomitic tidal flat. Affected by the frequent and periodic relative fall of the sea level, the top of the tidal flat sedimentary sequence frequently exposed above the sea surface and thus subjected to sedimentary intervals and meteoric freshwater leaching, resulting in many typical eogenetic karst products, including exposed erosion surface, vadose fillings, and breccia (Fig. 7). The karst signs are the most significant at the top of the tidal flat sequence, and quickly reduce at the middle-lower of the sequence. The karst fabrics in the tidal flat sequence are dominated by products of the vadose zone, with no phreatic zone.

The results of isotopic composition analysis of the karst deposits have high consistency with the aforementioned petrological inferences. Dolomite samples were taken from the grain shoal sequence in Well D6 to do carbon and oxygen isotopic analyses. The results show that the carbon and oxygen isotopes present good synergistic variation characteristics. Meanwhile, within a certain range below the exposed erosion surface, δ¹³C and δ¹⁸O values of dolomite samples from the mounds and shoals affected strongly by karstification have some negative excursion. Moreover, such negative excursion weakens downwards as the distance to the exposed surface increases, and shows obvious cyclic features (Fig. 6). This reflects the presence of multiple episodes of meteoric freshwater leaching in the Qixia Formation, which is consistent with the high-frequency exposure characteristics during the early diagenetic stage.

To conclude, multiple stages of karstification and dolomitization occurred in the Qixia Formation of the northwestern Sichuan Basin during the eogenetic stage, and the timing of the dolomitization was significantly earlier than that of the karstification related to high frequency exposure.

According to the field survey, core observation, and thin section analysis, the relationships between dolomites and high- frequency exposed karst fabrics in the Qixia Formation of the northwestern Sichuan Basin have the following features: (1) Macroscopically, the green karren fillings on and below the high-frequency exposed surface cut the dark red dolomite formed early, indicating this dolomite was formed before the karstification resulted from high-frequency exposure; the karren fillings and the overlying matrix dolomite are in gradual transition, which may be related to the filling of the next transgression cycle after the exposure (Fig. 8a, 8b). (2) Microscopically, the gray-green fillings in the karrens and karst caves are mainly composed of dolomite grains shrinking due to dissolution, vadose fillings, or clay (Fig. 5g, 5h), indicating that the dolomitization had completed before the formation of karren and karst cave. (3) Clay fillings that are products of the initial transgression of the late stage can be seen between the sucrosic medium-coarse dolomite crystals with turbid core and bright rim below the exposed surface; and part of the dolomite presents characteristic of refilling after dissolution (Fig. 8c, 8f, 8h). (4) When the grained dolomite is affected by overflow, dolomite in the strong karstification area can be micritized or turned into vadose sand (Fig. 5h); the cutting of dominant channels resulted in brecciation of the dolomite, thus giving rise to sand-gravel clastic dolomite coarser in grain size with dissolution rim and occasionally dedolomitization in some locations (Fig. 8d). (5) The caves formed early are filled with a mixture of dolomite debris and clay, and the sand-gravel clastics comprising the medium-coarse dolomite are obviously rounded (Fig. 8e).

In summary, a lot of diagenetic fabric evidence shows that the large-scale sucrosic dolomite of the Qixia Formation in northwestern Sichuan is cut by the karst fabrics of high-frequency exposure origin; moreover, the karst system fillings mainly consist of sandy and gravel dolomite clasts with dissolution edges and dolomite silts, all these indicate that the medium-coarse dolomite with occasionally turbid core and bright rim had been formed before the eogenetic karst. This is the most direct evidence for determining the timing of the large-scale dolomitization in the Qixia Formation, and it clearly supports the argument that the medium-coarse grain dolomite in Qixia Formation was formed in the penecontemporaneous period, rather than the traditionally believed burial stage.

It is concluded from the above analysis that the Qixia Formation sucrosic dolomite was formed in the penecontemporaneous period. However, it is difficult to explain the genesis of the medium-coarse dolomite with occasional turbid core and bright rim using the penecontemporaneous dolomitization model alone.

The Md1 layers in the platform margin of the study area generally occur at the top of tidal flat sequence, and often have the characteristic rhythmic beddings and bright oxidation colors that are commonly seen in the deposits of intertidal-supratidal environment (Figs. 2a, 2d, and 8a). Moreover, they present dull light under cathodoluminescence (Fig. 2c), which together with the above features suggest that they are formed in high-salinity surface fluids with low Mn²⁺/Fe²⁺, such as salted seawater^[32]. However, the Qixia Formation dolomites in the study area are not found to be associated with evaporite and evaporative minerals. Previous studies have shown that eogenetic dolomites are not necessarily associated with gypsiferous rocks, which may be related to the fact that although the seawater had high Mg/Ca values, the salinity of the seawater wasn’t high enough to cause gypsum precipitation. The dolomitization triggered by this kind of seawater- related fluids should be categorized as brackish water dolomitization or medium-salinity brine backflow dolomitization^[33,34]. This genetic model has been confirmed by previous studies on the Upper Triassic dolomites in the Dachstein platform of Hungary^[34], the Lower Jurassic dolomites in the Peritidal area of the western Mediterranean^[35], and the Lower Cambrian Longwangmiao Formation dolomites in the central Sichuan Basin^[36].

The Md1 layers in the platform margin of the study area generally occur at the top of tidal flat sequence, and often have the characteristic rhythmic beddings and bright oxidation colors that are commonly seen in the deposits of intertidal-supratidal environment (Figs. 2a, 2d, and 8a). Moreover, they present dull light under cathodoluminescence (Fig. 2c), which together with the above features suggest that they are formed in high-salinity surface fluids with low Mn²⁺/Fe²⁺, such as salted seawater^[32]. However, the Qixia Formation dolomites in the study area are not found to be associated with evaporite and evaporative minerals. Previous studies have shown that eogenetic dolomites are not necessarily associated with gypsiferous rocks, which may be related to the fact that although the seawater had high Mg/Ca values, the salinity of the seawater wasn’t high enough to cause gypsum precipitation. The dolomitization triggered by this kind of seawater- related fluids should be categorized as brackish water dolomitization or medium-salinity brine backflow dolomitization^[33,34]. This genetic model has been confirmed by previous studies on the Upper Triassic dolomites in the Dachstein platform of Hungary^[34], the Lower Jurassic dolomites in the Peritidal area of the western Mediterranean^[35], and the Lower Cambrian Longwangmiao Formation dolomites in the central Sichuan Basin^[36].

Meanwhile, the synsedimentary limestone samples have a δ¹³C value range of 1.12‰ to 4.80‰ (on average 2.70‰) and a δ¹⁸O value range of -7.25‰ to -3.02‰ (on average -5.14‰). In comparison, the Md1 samples have a δ¹³C value range of 3.51‰ to 4.11‰ (on average 3.79‰) and a δ¹⁸O value range of -2.99‰ to -0.70‰ (on average -1.59‰) (Fig. 9). It is generally believed that the significantly positive excursion of carbon and oxygen isotopes of the dolomite than those of the synsedimentary seawater indicates that the dolomite is formed in restricted water body with evaporation background^[37]. Compared with synsedimentary seawater^[37] and limestone, the Md1 samples have obviously positive excursions of carbon and oxygen isotopes. Moreover, the oxygen isotopes of the dolomite samples fall in the range of dolomite from seawater precipitation of the same period calculated based on the fractionation equation^[38], which indicates the study area had local evaporation and restriction environment in the platform margin zone during the depositional period of Qixia Formation.

From the perspective of the sedimentary background, the deposits of the Qixia Formation are characterized by high- frequency upward-shallowing shoal-mound sequences and the strata and deposits have overlapping feature towards the highlands (Fig. 4). The growth of upward-shallowing sedimentary sequences could cause superimposition and migration of mounds and shoals, which could further isolate and restrict local sea areas. In this context, the paleoenvironmental basis for the penecontemporaneous evaporative concentration dolomitization and reflux infiltration dolomitization might turn up. Meanwhile, this dolomitization model is inevitably associated with unstable stratigraphic locations of dolomitization, which explains the unstable dolomite horizons revealed by drilling in this area (Fig. 4).

As mentioned above, the sucrosic dolomite was formed before the karstification caused by high-frequency exposure. But dolomite crystals of this kind have turbid core and bright rim occasionally, and bright bands at the edge under the cathode ray (Fig. 2h, 2i), which implies the final formation of the dolomite is related to the multi-stage fluids replacement and cementation in the penecontemporaneous period^[32], but this can’t be explained by the traditional dolomitization model. In this study, a total of 17 primary gas-fluid inclusions were found in the sucrosic medium-coarse dolomite cut by the eogenetic karst fabrics (Figs. 2f and 10a), and another 24 primary gas-fluid inclusions were found in the cement (e.g. saddle dolomite) filling in dissolution pores and fractures. These inclusions are in different shapes (elongated or irregular), generally 2-10 μm in diameter, and have about 5% to 15% of gas in volume. The inclusions in the medium-coarse dolomite cut by the karst fabrics have a homogenization temperature range of 96-124 °C, and an average temperature of 107 °C (Fig. 10b, 10c), while the inclusions from the dolomite cement have a homogenization temperature range of 137-194 °C, and an average temperature of 162 °C (Fig. 10b, 10c). Apparently, both the medium-coarse dolomite and the dolomite cement are products of high-temperature environment. The inclusions from the medium-coarse dolomite have significantly higher homogenization temperatures than the normal seawater during the deposition period, but lower than inclusions from the dolomite cement formed in high-temperature environment^[4]. This means the two kinds of dolomites are both formed in high temperature diagenetic environment. The diagenetic fluid of the medium-coarse dolomite has a δ¹⁸O range of -4.0‰ to 8.3‰ (SMOW)^[4], which is higher than the background δ¹⁸O value of the Early Permian seawater^[38] and similar to that of the dolomite cement (-4.0‰ to 8.3‰; Fig. 10d). This implies that the sucrosic dolomite with occasional turbid core and bright rim was influenced by high-temperature fluids during its formation process. Meanwhile, the high homogenization temperatures of the inclusions from dolomite cement filling in the pores and vugs later also suggest the cement was influenced by multi-stage hydrothermal activities (Fig. 6). The presence of hydrothermal fluids during the penecontemporaneous and subsequent burial stages indicates the rifting activities had been active during the depositional period of Qixia Formation, and multi-stage hydrothermal activities happened thereafter.

Analysis of carbon and oxygen isotope composition (Fig. 9) shows that the Qixia Formation sucrosic dolomite samples have a δ¹³C value range of 0.92‰ to 3.81‰ (on average 2.42‰) and a δ¹⁸O value range of -8.68‰ to -3.43‰ (on average -6.50‰), while the dolomite cement samples have a δ¹³C value of 0.03‰ to 1.68‰ (on average 0.89‰) and a δ¹⁸O value range of -16.35‰ to -9.03‰ (on average -12.20‰). The sucrosic medium-coarse dolomite samples have oxygen isotopic composition similar with limestone and slightly negative oxygen isotope excursion. In general, the sucrosic dolomite has carbon and oxygen isotopic compositions much smaller than those of the dolomite cement deposited by mantle-derived hydrothermal fluids^[4] (Fig. 9). The diagenetic fluid of the sucrosic dolomite isn’t mantle-derived hydrothermal fluid, rather, it has affinity with seawater-derived fluid. Moreover, the slightly negative oxygen isotope excursion might be attributed to the weak influence of the freshwater fluid on the matrix dolomite, which coincides with the karst characteristics in petrology.

The analysis results of trace and rare earth elements^[22] show the sucrosic medium-coarse dolomite generally inherits the partition pattern of the rare earth elements of limestone, and has weak negative excursion of Ce element similar to limestone (Fig. 11), indicating that the sucrosic dolomite was formed in near-surface diagenetic environment with relatively high oxygen content. The dolomite cement has partition pattern of rare earth elements obviously different from that of the sucrosic medium-coarse dolomite, with Ce positive anomaly and extremely significant Eu positive anomaly (Fig. 11), indicating its diagenetic fluid is the mantle-related hydrothermal fluid under restricted reduction conditions. Moreover, the sucrosic dolomite samples have an average δEu of 0.964, which is larger than that of limestone samples (0.820) but much lower than that of the dolomite cement samples (6.217) (Table 1), which suggests that the dolomitization fluid of the sucrosic dolomite has some fluid participating in deep circulation or some hydrothermal fluid mixed in, and hence the δEu of the sucrosic dolomite slightly increases, indicating the sucrosic dolomite was formed during the thermal activities. But the sucrosic dolomite samples have an average Mn/Sr value of only 0.71, significantly lower than 2.0, indicating that the dolomitization is caused by original seawater. Moreover, the sucrosic dolomite samples have an average Ba content of 13.31×10^-6, lower than 20×10^-6, also suggesting that the hydrothermal fluid isn’t the most important diagenetic fluid of this dolomite (Table 1). Together with the REE distribution pattern of limestone, it is inferred that seawater in the penecontemporaneous period is the most important diagenetic fluid of the sucrosic dolomite, which provided the Mg²⁺ required for the dolomitization process. Given the fact that the temperature of seawater in the penecontemporaneous period was generally lower than the temperature required for the formation of dolomite, the heating effect caused by thermal activities is speculated critical for promoting the dolomitization process driven by the seawater-related fluid^{[4, 22]}.

It has been realized based on the regional geological background that the NE-SW-striking highlands in the Zhuyuanba- Kuangshanliang and Tongkou-Shuangyushi belts are mainly ascribed to the activities of multi-level tensile fault blocks formed by the initial tensional rifting of the Dongwu Movement (Fig. 1b). Affected by the undulating geomorphology, the platform margin was isolated and restricted as a result of relative regression during the deposition of the Qi-2 Member, forming an anoxic environment with relatively high salinity^[19]. The limited accommodation space during the high-frequency regression led to the migration and superimposition of shoal bodies on the original landform. This process resulted in further restriction and salinization of seawater during the regression. Meanwhile, the evaporative concentration and reflux infiltration of the heavy brine led to the dolomitization of the mound-shoal sediments, forming the platform margin Md1 unstable in horizon (Fig. 12a). The dolomite formed at this point still had a large number of intergranular pores, framework pores, and intergranular micropores, and was thermodynamically unstable under high temperature or burial conditions^[40,41,42], and was highly vulnerable to diagenetic alteration. Then the surface heat flow of the upper crust in the study area increased abnormally due to the influence of the initial tension rifting during the Dongwu Movement^[25]. The crust was opened by the tensional rifting during the depositional period of Qixia Formation, and the seawater in the western Sichuan platform margin seeping down was heated by the ground temperature along the faults and turned into high-temperature pore water. The high temperature pore water flew upward under the control of the density gradient. This further accelerated the compensational infiltration of the low-temperature deep seawater outside the topographical slope break of the platform margin into the lower part of the permeable layers in the platform margin, making the seawater circulate continuously^{[3, 43]}. When rising into the near-surface permeable layers, the high-temperature fluid of heated seawater reformed the penecontemporaneous dolomite formed early, leading to formation of bright growth rim or recrystallization of the Md1, and eventually, forming the sucrosic dolomite crystal with turbid core and clear rim occasionally (Fig. 12b). In a word, the geothermal anomalies caused by the tectonic activities in the Dongwu Movement provided the high temperature needed for dolomitization, while the continuously circulating seawater provided the source of Mg²⁺. Therefore, this model can be named the dolomitization model of penecontemporaneous seawater circulation combining with hydrothermal activity.

The high-frequency regression driven by the continuous episodic rifting activities led to exposure and dissolution of the platform margin highlands. The penecontemporaneous dolomite was cut by the eogenetic karst fabrics, resulting in karrens, dissolved fractures, dissolved caves, granophyric dissolution, and micritization and crystal shrinkage of dolomite. Especially, the dolomite with turbid core and bright rim could be cut by karst fabrics in local parts (Figs. 3g and 5h). During the next high-frequency transgression, the initial transgression sediments could pour into the early-formed karst system, forming transgressive clay filling between medium-coarse dolomite grains (Fig. 8f-8h), which can’t be explained by the burial hydrothermal dolomitization process. Moreover, it should be noted that after deposition of Qixia Formation, the main episode of the Dongwu Movement might have broken the crust, causing continuous impacts of the multi-episode mantle-source hydrothermal activities on the Qixia Formation. These impacts are embodied by the cement (e.g. saddle dolomite) filling the caves and vugs, and thus the cement has fluid inclusions with higher homogenization temperatures and more significant geochemical characteristics of mantle-derived fluid.

This dolomitization model presented in this paper can explain the genesis of the penecontemporaneous sucrosic dolomite in Qixia Formation and is an extension of the existing theory on the formation of sucrosic dolomite. It demonstrates that the overgrowth of dolomite^[3] caused by multi-phase, long-term reflux infiltration model proposed previously isn’t appropriate to explain the origin of the sucrosic dolomite of the Qixia Formation.

As a kind of high-quality carbonate oil and gas reservoir^[6], the pore-type sucrosic dolomite of the Qixia Formation has been confirmed to have superior reservoir capacity by exploration^{[6, 9, 13-14, 19]}. The exploration breakthrough and production capacity construction of the Shuangyushi Gas Field reveals that this kind of reservoir is distributed in band form along the unstable platform margin. Based on this understanding, two wells, Well PT1 and LS1, were drilled in the western Sichuan Basin in 2020, and both encountered about 20 m thick dolomite reservoirs with pores and vugs. Of them, Well PT1 tested a gas flow of 66.86×10⁴ m³/d, which again proved that this kind of reservoir is distributed along the west margin of the basin in band at large scale. If the dolomite was formed by hydrothermal dolomitization previously proposed, the dolomite reservoir would be generally distributed along hydrothermal channels such as basement faults and their surrounding areas, but the drilling results aren’t like this completely. This is because besides the heat source provided by the tensional rifting activities, the penecontemporneous seawater circulation combining hydrothermal activity dolomitization model presented in this study also depends on the relatively restricted environment formed by the superimposition and migration of shoals, the slope break zone suitable for the circulation of cold and hot seawater, and the early-formed permeable mound and shoal bodies favorable for the flowing of penecontemporaneous diagenetic fluids. Based on this understanding, the platform margin in western Sichuan Basin, including the Longmenshan piedmont belt and its southern margin, which meet all the conditions mentioned above, may have this kind of dolomite reservoir. Moreover, in the upper Yangtze Platform, relative slope break belts that could cause the superimposition and migration of mounds and shoals, such as the Gaoshiti-Moxi Area, may also have this kind of thin-layer dolomite reservoirs in band shape.

On the other hand, the above discovery also confirms the existence of tensional faults reaching the crust and hydrothermal fluid circulation during the depositional period of Qixia Formation, which indirectly reveals that the first episode of the Dongwu Movement started at the end of the deposition of the Qi-1 Member, and the opening of the northern Mianlue Ocean probably also began in this period. The sediments in the intra-platform depressions along the Guangyuan-Wangcang areas could be the sedimentary response to the initial opening of the Mianlue Ocean (Fig. 1b). Therefore, it can be inferred that the inherited continuous sedimentary-tectonic differentiation could make the slope break zones at the margin of the platform depressions potentially favorable exploration zones for the Middle-Lower Permian.

The Lower Permian Qixia Formation in the northwestern Sichuan Basin, China, has mainly sucrosic dolomite with occasional turbid core and bright rim crystals, and secondarily Md1 and dolomite filling in the karst system. These dolomites are distributed in bands, with unstable stratigraphic positions and thicknesses, and tend to gradually elevate toward northeast. In these dolomites, upward shallowing sequences that record multiple periods of high-frequency exposure karstification can be recognized, accompanied by a large number of eogenetic karst features. The early-formed sucrosic medium- coarse dolomite is cut by the early karst fabrics, which indicates the sucrosic dolomite was formed during the penecontemporaneous period. Specifically, in the restricted environment resulted from the superimposition and migration of the platform marginal mounds and shoals, the penecontemporaneous Md1 formed due to evaporative concentration and reflux infiltration was reformed by high-temperature seawater circulating along the faults during the initial Dongwu Movement and turned into sucrosic dolomite finally. On this basis, a dolomitization model that integrates penecontemporaneous seawater circulation and hydrothermal activity has been advanced, which provides a reasonable explanation for band- shaped dolomite with unstable stratigraphic positions in the western Sichuan Basin and thus broadens the exploration field.