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Article

The Development of Dolomite Within a Sequence Stratigraphic Framework: Cambrian Series 2 Changping Formation, Xiaweidian, China

by
Shan Zhong
1,2,
Zhaoqian Liu
1,*,
Zhenkui Jin
3,
Hongyu Tian
4,
Madaki Agwom Istifanus
5 and
Simon C. George
2
1
Key Laboratory of Tectonics and Petroleum Resources (China University of Geosciences), Ministry of Education, Wuhan 430074, China
2
School of Nature Sciences, Macquarie University, Sydney, NSW 2109, Australia
3
State Key Laboratory of Petroleum Resource and Prospecting, College of Geosciences, China University of Petroleum (Beijing), Beijing 102249, China
4
Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China
5
Department of Geology, University of Jos, Jos P.M.B 2084, Plateau State, Nigeria
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(12), 1189; https://doi.org/10.3390/min14121189
Submission received: 16 October 2024 / Revised: 1 November 2024 / Accepted: 12 November 2024 / Published: 22 November 2024
(This article belongs to the Special Issue Life and Carbonate: Biotic and Abiotic Fingerprints in Past and Recent Carbonate Sediments)
Browse Figures
Versions Notes

Abstract

:
The Lower Cambrian Changping Formation in the Western Hills of Beijing hosts tidal flat and lagoonal carbonates comprising dolomites, limestones, and dolomitic limestones, reflecting the processes of dolomite cementation and dolomitization within a sedimentary framework. Based on petrographic textures, two types of dolomites were identified: microcrystalline dolomite and fine-mesocrystalline dolomite. Integrating petrological and geochemical data unveils two diagenetic stages. The initial dolomite formation, attributed to hypersaline fluids, occurred in a supratidal-sabkha setting during the early Cambrian. The dolomitization at the top of the Changping Formation, driven by evaporatively concentrated brines from the overlying Mantou Formation, altered peritidal carbonates. This study evaluates the original sedimentary environment and dolomitization within a sequence stratigraphic context, revealing a correlation between dolomitization episodes and the stratigraphic framework in the study area. Factors influencing this framework profoundly impact depositional environments and material composition, leading to micromorphological differences in dolomites. Sabkha dolomite formation, associated with evaporative pumping, predominates near the base of transgressive systems tracts. Seepage reflux dolomite, often linked with evaporative pumping dolomite, constitutes a vertical cycle in the sequence framework. The sequence from bottom to top is sabkha microcrystalline dolomite, limestone and dolomitic limestone, seepage reflux saccharoidal dolostone, and sabkha dolomite.

Graphical Abstract

1. Introduction

Carbonate rocks are important oil–gas reservoirs, and up to 50% of the world’s carbonate reservoirs are dolomites [1,2], but the impacts of dolomitization on reservoir quality are different, so it is important to study dolomite formation and distribution for exploration prospects [3,4,5,6]. A lot of research on dolomite formation and dolomitization models has been carried out. Generally speaking, dolomites can be divided into two groups: primary dolomite precipitated directly from aqueous solution or secondary dolomite formed through the dolomitization process [7,8]. Due to the absence of low-temperature primary dolomites in modern environments and the experimental difficulties in synthesizing primary dolomite at low temperatures [9,10], the majority of scholars consider that dolostones formed during diagenesis. Many diagenetic models for dolomite have been proposed, including seepage reflux dolomitization [11,12,13,14], sabkha dolomitization [15,16,17], mixed-water dolomitization [2,18,19,20], deep burial dolomitization [21,22,23,24], hydrothermal dolomitization [24,25,26,27,28], and microbial dolomitization [29,30,31]. Although much has been learnt about dolomitization, the conditions and mechanisms by which dolomite forms are still uncertain due to the complexity of the possible origins [14,32,33,34]. Some studies on dolomite genesis have demonstrated that the distribution and characteristics of dolomite can be influenced by the sequence stratigraphic framework [35,36,37,38,39,40]. For example, the formation of widespread dolomite is predominantly subsurface and may be tied to the long-term circulation of seawater through platform sediments, which is controlled by sea-level fluctuations [41,42]. Dolomite is developed in various positions within the sequence stratigraphic framework, but it is often most prominently developed near the maximum flooding surface (MFS) [36,38,39,40,43]. However, it is not clear whether dolomite formation and distribution are universally controlled by the sequence stratigraphic framework.
The Cambrian section at the Xiaweidian outcrop in the Western Hills of Beijing is one of the stratigraphic sections with relatively abundant Cambrian sedimentary phenomena in the North China Platform. The outcrop has been thoroughly studied regarding its lithological characteristics, depositional environment, and sequence stratigraphic, leading to a series of significant insights [35,44,45,46,47,48]. The Lower Cambrian Changping Formation at Xiaweidian in the Western Hills of Beijing contains thick dolostone strata [35,45,48]. However, the distribution of the different types of dolostone and the mechanisms of dolostone formation remain unclear. The innovation of this study lies in its systematic investigation of the genesis and spatial distribution of different types of dolomites in the Changping Formation through detailed petrographic and geochemical data, with a particular focus on the relationship between dolomitization and the sequence stratigraphic framework. We aim to uncover new features and mechanisms of the dolomitization process and propose alternative interpretations to existing theories, providing a novel perspective on the origins and distribution of dolomites.

2. Geological Setting

The Western Hills of Beijing are located on the northern margin of the North China Platform, where the Cambrian system is well developed [49,50]. Due to the deep meanders of the Yongding River, the outcrops are exceptionally well exposed (Figure 1).
In this region, the Lower Cambrian is composed of the Changping and Mantou formations, which unconformably overlies the Neoproterozoic Jingeryu Formation (Figure 2). The Changping Formation is primarily composed of breccia-bearing limestone, oolitic limestone, and mottled (leopard-spot) limestone, which formed in tidal flat and lagoon environments [45,47,48,50,51,52]. Above it, the Mantou Formation is composed from bottom to top of yellow conglomerate, purple-red mudstone, and gray-white micritic dolomite, deposited in a sabkha and tidal flat environment [45,48,50].
The Changping Formation stratum can be subdivided into a third-order sequence, with a TST and an HST [45,47,51,53]. The TST mainly consists of breccia-bearing dolomite and thick limestone and is at the base of the Changping Formation. Oncolitic limestone and dolomitic limestone are situated in the middle part of the Changping Formation, whereas saccharoidal dolomite forms part of the HST at the top of the Changping Formation [45,51]. The Mantou Formation consists entirely of shallow water tidal shelf deposits, forming a single HST that constitutes a third-order sequence [45,48,51].
Most studies in this area have focused on sedimentary environments and sequence stratigraphy [45,46,47,48], but few have studied the relationship between dolomitization models and sequence stratigraphy [35]. In this study, we assess the mineralogy and genesis of the Changping Formation dolomite at the Xiaweidian outcrop and the relationship between the dolomitization model and the sequence by the observation of macroscopic outcrops and rock thin sections and geochemical analyses.

3. Methods and Materials

The Cambrian Xiaweidian outcrop in the Western Hills area of Beijing has a diverse array of sedimentary phenomena and is one of the best stratigraphic profiles within the North China Platform.
The lithology, sedimentary environment, and sequence stratigraphy of the Changping Formation were interpreted from detailed field observations of the Xiaweidian outcrop. A total of fifty-two dolomite and limestone samples that represent the various depositional facies, system tracts, and key sequence stratigraphic surfaces were collected from this outcrop (see Table S1). Based on thin section observations, X-ray diffraction analyses, cathodoluminescence studies, and carbon–oxygen isotope measurements, the characteristics of the different kinds of dolomite and their lithofacies and sedimentary environments were documented.
Thin sections from 43 samples were observed using a Nikon 100-Pol polarizing microscope (Beijing, China), with their classification based on Bissell and Chilingar [54]. A total of 20 thin sections were stained with Alizarin Red to differentiate carbonate minerals (see Table S1). The cathodoluminescence microscopy of 42 polished thin sections was performed on a CITL CL8200 MK5-2 cathodoluminescence system (Beijing, China) with a beam voltage of 17 kV and a current of 500 μA (see Table S1).
Twenty-four dolomite samples were analyzed by a Bruker AXS D2 PHASER X-ray diffractometer (Beijing, China) at the State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum (Beijing), to provide detailed information about the mineralogy and the relative proportions of calcite and dolomite (see Table S1). The degree of cation order (δ) was calculated using the following:
δ = I(d015)/I(d110)
where (I) is the intensity of the superstructure dolomite peaks (015) and (110) in counts per second [55]. In addition, the full width half maximum of the dolomite (104) diffraction peaks (FWHM104) was measured.
Based on the d-spacing d(104) values, the molar concentrations of CaCO3 and MgCO3 in dolomite could also be determined. The formula for the molar calculation is as follows:
NCaCO3 = 333.33d(104) − 911.9
where NCaCO3 is the mole % of CaCO3, and d(104) is the d-value in Å of the measured 104 reflections [56].
Stable carbon and oxygen isotope analyses were carried out on 19 dolomite samples using a Thermo Scientific MAT-253 stable isotope ratio mass spectrometer (Nanjing, China) at the State Key Laboratory, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. About 30–50 mg of carbonate powder was reacted with 100% phosphoric acid at 22 °C ± 1 °C under vacuum to release CO2 from the carbonate minerals. The isotope data are reported in the δ notation, in ‰ relative to the V-PDB (Vienna PeeDee Belemnite) standard. The precision for the stable isotope measurements of both oxygen and carbon was less than 0.08‰. The δ13C and δ18O data were combined to calculate paleo-salinity (Z value) using the following equation [57]:
Z = 2.048∙(δ13C + 50) + 0.498∙(δ18O + 50)
When the Z value is greater than 120, it is an indication of a marine diagenetic environment, and if lower than 120, it implies a continental diagenetic environment [57].

4. Results

4.1. Petrography

Based on observations from the field outcrop profile, the Changping Formation from bottom to top consists of breccia-bearing dolomite, micritic dolomite, breccia-bearing limestone, dolomitic limestone, mottled (leopard-spot) limestone, oncolitic limestone, and saccharoidal dolomite, which are interpreted to have formed in lagoonal environments (Figure 2). Dolomite is concentrated at the bottom and top of the Cambrian Changping Formation strata (Figure 2).
Based on the crystal size and textural characteristics [54,58,59], two types of dolomite textures were identified in the Changping Formation at the Xiaweidian outcrop by microscopic analysis. D1 dolomite is microcrystalline to finely crystalline. D2 dolomite is fine-mesocrystalline, subhedral to euhedral, and anhedral. The two types of dolomite are present in different positions in the Changping Formation. D1 dolomite is mainly concentrated in the lower part of the Changping Formation, including in microcrystalline dolomite and breccia-bearing dolomite. Light gray, thick–massive D2 dolomite is mostly distributed at the top of the Changping Formation.

4.1.1. Microcrystalline and Finely Crystalline Dolomite (D1)

In the outcrop, D1 dolomite rocks are commonly represented by gray or yellowish-brown breccia-bearing dolomites, the breccia of which are commonly angular or subangular microcrystalline dolomite (Figure 2 and Figure 3a,b). D1 dolomites are commonly thin- to medium-layered, with horizontal lamina, and are usually composed of microcrystalline dolomite and finely crystalline dolomites. In the thin section, microcrystalline dolomite has tightly inlaid crystal contacts and relatively homogeneous crystals (Figure 3b,d). Finely crystalline dolomite is composed of subhedral to euhedral dolomite crystals with a crystal size ranging from 10 to 50 μm (mean 20 μm) (Figure 3c,e). D1 dolomites contain abundant bioclastic debris (Figure 3c). Under cathodoluminescence, D1 dolomite crystals commonly exhibit non-luminescence to dull red luminescence (Figure 3f).

4.1.2. Fine-Mesocrystalline Dolomite (D2)

Based on field observation and hand specimens, the top of the Changping Formation contains approximately 20 m of gray to dark gray D2 dolomite (Figure 4a). Microscopically, D2 dolomite contains 100–250 μm diameter crystals with a residual bioclastic structure, such as from trilobites (Figure 4c). When dolomitization is incomplete, dolomite is mainly euhedral to subhedral (Figure 4b). When dolomitization is complete, some of the crystals are anhedral. Some D2 dolomite crystals show a faint cloudy center and clear rim texture under plane-polarized light (Figure 4d). They have a remnant replacement texture, mainly including residual sand-clasts, residual bioclasts, and residual oolites, with clear outlines of the remnant texture (Figure 4c). Under cathodoluminescence, the centers of these dolomite crystals are characterized by dull red luminescence, or have no luminescence, while the rims have bright red luminescence (Figure 4e,f).

4.2. Geochemical Characteristics

4.2.1. Mineralogy and Crystallography

The ordering degrees of dolomites were analyzed by X-ray diffraction to characterize the crystallinity of dolomite crystals and the diagenetic environment. Table 1 shows the X-ray diffraction results for 24 dolomite samples from the Changping Formation at the Xiaweidian outcrop.
The degree of cation order for D1 dolomite varies between 0.50 and 0.76, with an average of 0.65, whereas the degree of cation order for D2 dolomite varies between 0.83 and 0.96, with an average of 0.88 (Table 1).
The CaCO3 (mol %) of D1 dolomite varies between 49.9 and 51.8, with an average of 50.5 (Table 1), whereas the CaCO3 (mol %) of D2 dolomite varies between 48.1 and 50.2, with an average of 49.0 (Table 1). The degree of cation order and CaCO3 (mol %) vary with position in the outcrop (Figure 5).
An analysis of the diffractograms also provided values for the FWHM104 parameter (Table 1, Figure 5). D1 dolomites have FWHM104 values > 0.16 (average 0.170), whereas D2 dolomites have lower FWHM104 values (average 0.156).

4.2.2. Carbon and Oxygen Isotopic Analyses

Table 2 shows the oxygen and carbon isotope values for the dolomite samples. D1 dolomites have an average δ13C of −0.15 ‰ (n = 10, −3.28 to 2.46 ‰) and an average δ18O of −6.5 ‰ (−7.96 to −3.62 ‰). D2 dolomites have an average δ13C of −0.28 ‰ (n = 9, −2.6 to 0.35) and an average δ18O of −7.4 ‰ (−9.45 to −4.70‰). The Z values of D1 dolomites in the Xiaweidian outcrop range from 117.0 to 129.3 (with an average of 123.8), while those of D2 dolomites fall between 117.3 and 124.4 (with an average of 123.0) (Table 2). In general, δ13C and δ18O tend to decrease from the base to the upper part of the Changping Formation (Figure 5).

5. Discussion

5.1. The Sequence Stratigraphy of the Changping Formation

The Changping Formation is a third-order sequence, consisting of a TST and an HST [45]. This sequence stratigraphic framework reflects the relationship between the depositional environments and the dolomitization processes, with sea-level fluctuations playing a major controlling role [35,36,38,39,40,43].
Prior to the deposition of the Changping Formation strata, there was a parallel unconformity on top of the Neoproterozoic Jingeryu Formation, due to a relative sea-level fall (Figure 2 and Figure 6a). This unconformity has been reported on a regional scale [45,51] and is attributed to subaerial exposure, thus defining the lower sequence boundary of the TST in the Changping Formation (Figure 6a). At the base of the Changping Formation, breccia-bearing dolomite (D1) is exposed, with a thickness of about 10 m (Figure 2 and Figure 3a). The breccia exhibits poor sorting and rounding, with angular to subangular clasts (Figure 3a). It has undergone intense dolomitization, with well-formed dolomite crystals. The interstitial material consists of a micritic matrix, partially filled by calcite veins or replaced by dolomite (Figure 3b). The clasts within the breccia are mainly derived from the dolomitic limestone of the Jingeryu Formation, which was originally deposited in a low-energy supratidal flat and later affected by karstification (Figure 6b) [48]. Upwards within the formation, there is a gradual interbedding of breccia-bearing dolomite and dolomitic limestone (about 16m thick; Figure 2), revealing a low-energy supratidal environment. The breccia-bearing dolomite to dolomitic limestone sequence forms the TST of the third-order sequence (Figure 2).
The development of mottled limestone and oncolitic limestone with fossils is pronounced in the middle of the Changping Formation (Figure 6c,d). Burrows, other bioturbation structures, and locally developed wavy-undulated stromatolites are found in this set of strata, which further indicates that the middle of the Changping Formation was deposited in a low-energy intertidal–subtidal environment. This set of oncolitic limestones constitute the early HST of the third-order sequence.
Later, the sea level fell, and overall carbonate production (and carbonate export) increased. Thick lagoonal limestone formed during the late Changping Formation as a consequence of the marine transgression. Later, as the relative sea level fell, the carbonate platform was progressively exposed and altered into saccharoidal dolomite (D2) (Figure 6e) [45,48]. In the upper part of the Changping Formation, thick and massive dolomite with stratiform stromatolites was developed as part of the late HST [45]. Compared to the early HST, the thickness of the individual rock layers decreased, and the intensity of dolomitization increased, indicating an overall trend of shallowing in the depositional environment.
The parallel unconformity between the Changping Formation and the Mantou Formation is interpreted as a sequence boundary (Figure 6e). The TST of the Mantou Formation is a more than 50 m thick reddish-purple mudstone which contains a gypsum breccia, indicating dry weather conditions (Figure 2 and Figure 6f). The breccia was created by the dissolution of gypsum layers under gravitational forces, making it a product of dry climatic conditions. The original rock of the Mantou Formation is interpreted to have formed in a typical sabkha environment. The breccia content gradually decreases from the base to the top of the Mantou Formation, which is interpreted as indicating that evaporation gradually weakened and that dolomite at the top was formed in a supratidal zone. Overall, the Mantou Formation represents a third-order cycle, formed during a hot and dry climate, typical of a sabkha environment.

5.2. The Origin of Dolomite in the Changping Formation

There is convincing petrographic evidence that shows that the dolomitization of the carbonate sediments occurred during various diagenetic stages. There are several lines of evidence. Firstly, D1 dolomite at the base of the Changping Formation is microcrystalline and finely crystalline, usually yellow or light yellow in color, and often occurs in thin layers or laminae. It commonly contains lamination structures and stratified stromatolites. These observations are consistent with it being penecontemporaneous dolomite that formed in a low-energy supratidal flat environment with little hydrodynamic activity (Figure 2 and Figure 3a). In contrast, D2 dolomite at the top of the Changping Formation has a massive structure, was mainly formed by the dolomitization of lagoonal limestones and thus is postcontemporaneous dolomite (Figure 2 and Figure 4a). Secondly, D1 dolomite is characterized by relatively low granularity (5–30 μm) (Figure 3b,c,e). The argillaceous matrix and micritic calcite are the main interstitial materials (Figure 3d). The thin section characteristics show that D1 dolomite formed in a low-energy supratidal environment. The subhedral–xenomorphic crystals in D2 dolomite are between 100 and 250 μm in diameter and have cloudy centers and clear rims, as well as a remnant replacement texture with clear outlines (Figure 4c,d), indicating that D2 dolomite was formed by later replacement. The difference in crystal size between D1 and D2 dolomites indicates that they were likely formed at different diagenetic stages. Thirdly, D1 dolomite has dull red luminescence under cathodoluminescence (Figure 4f), indicating the effect of an oxidizing environment near the surface [60,61]. Under cathodoluminescence, some D2 dolomite samples at the top of the Changping Formation (such as sample Ꞓ1c-39-3) have regions with brighter luminescence (Figure 4d,f). Brighter luminescence is an indication that Mn2+ and Fe2+ contents are higher in D2 dolomite [62], meaning that dolomitization took place in a reducing environment.
The degree of ordering of dolomite is usually related to the mineral composition, the crystallinity of dolomite crystals, and the depositional environment [63,64], although there can be other factors that determine the actual degree of cation order [65]. In the Changping Formation samples, the dolomites are nearly stoichiometric (i.e., with CaCO3% close to 50%), but the degree of cation order is variable (Figure 7). The degree of cation order of D1 dolomite is relatively low (average 0.65) (Figure 7). Dolomite that is formed during early diagenesis usually has a low degree of cation order and very small crystals [62,64,66]. Thus, it is suggested that D1 dolomite formed from hypersaline fluids with a fast crystallization rate and an irregular arrangement of the Ca2+ and Mg2+ ions [62,64,66]. Therefore, D1 dolomite is interpreted to have formed in a penecontemporaneous high-salinity seawater environment. The degree of cation order of D2 dolomite is higher than that of D1 dolomite (average 0.88; Figure 7), and the mole % CaCO3 is near 50%. Compared with D1 dolomite, D2 dolomite has a greater degree of ordering and lower FWHM104 values. This is interpreted as indicating that the diagenetic fluid moved slower during the formation of D2 dolomite, and dolomite had sufficient crystallization time for the molecular layers of CaCO3 and MgCO3 to be arranged regularly [64,66]. This is consistent with previous work that has shown that increased time leads to more cation ordering during dolomitization [67,68].
The δ18O and δ13C values of calcites precipitated from early Cambrian seawater range from −7.0 ‰ to −10.0 ‰ and −2.2 ‰ to 0.4 ‰, respectively, according to analyses of pristine brachiopod shells and other calcitic fossils [69]. It is well known that the oxygen isotope fractionation between calcite and dolomite varies from 1.5‰ to 3.5 ‰ [70]. Dolomites precipitated from early Cambrian seawater should therefore have δ18O values from −3.5‰ to −8.5‰. The measured D1 and D2 dolomite δ18O values (Figure 8) are mainly within the range of those of contemporaneous seawater dolomite. The δ13C and δ18O values of D2 dolomite are similar to those of D1 dolomite, but they are slightly lower. The δ18O composition of dolomites can reflect the deposition temperature or the temperature of dolomitization fluids [66,71]. The δ18O composition of D1 dolomites tends to be more positive than that of D2 dolomite (Figure 8), which is consistent with the effect of low-temperature fluids during early diagenesis [71]. Therefore, D1 dolomite in the lower part of the Changping Formation is interpreted to be the product of evaporation. The δ13C values of D2 dolomites are on average slightly more negative than those of D1 dolomites, consistent with the formation of D2 dolomite during slightly higher temperature diagenesis. The Z values of both D1 and D2 dolomites are mainly >120 (Table 1), suggesting they were formed in a marine diagenetic environment [57]. Furthermore, the average Z value of D1 dolomite is significantly higher than that of D2 dolomite, consistent with the formation of D1 dolomite from a more saline fluid.
The petrographic and geochemical characteristics of D1 dolomites are typical of para-syngenetic sabkha dolomites formed in a dry climate supratidal zone (Figure 9a) [35,45,48]. These are typically formed by the dolomitization of soft supratidal lime dolostone, often in an environment allowing for the evaporation and concentration of seawater to form hypersaline fluids [35]. D2 dolomites are associated with supratidal para-syngenetic evaporative pumping dolostones, so dolomitization most likely occurred via seepage reflux infiltration (Figure 9b). Under the action of gravity, high-density saline seawater is interpreted to have evaporated and concentrated in the supratidal zone or lagoon, resulting in the dolomitization of the underlying limestone [72]. In summary, the formation of dolomite in the Changping Formation occurred in two different diagenetic stages: evaporative pumping dolomitization (D1) and seepage reflux dolomitization (D2).

5.3. A Conceptual Model of Dolomite Formation During the Transgressive System Tract, and the Early Highstand System Tract

The relative sea-level rise during the deposition of the early stage of the Changping Formation (during the TST) is suggested to have caused an increase in sedimentary carbonate production, due to decreased clastic influx. In the early stage of the deposition of the Changping Formation, the climate is inferred to have been warm and humid [48]. The rate of evaporation increased as the climate became hotter and drier, which led to the production of Mg-rich brines that transformed calcium carbonate sediments into D1 (sabkha) dolomites through diagenesis (Figure 9a). The Mg-rich brines refluxed and infiltrated into the Jingeryu Formation, forming seepage reflux dolomite at the top of the Jingeryu Formation (Figure 2). During the sea-level rise during the early stage of the deposition of the Changping Formation, 25 m thick limestone was deposited in the middle of the Changping Formation (Figure 2). The HST of the Changping Formation is characterized by thick–massive lagoonal limestones, which are inferred to have supplemented the required calcium and carbonate ions and nucleation sites. The overlying strata are reddish-purple mudstones of the Mantou Formation, which are interpreted to have formed in a sabkha environment (Figure 9c) [45,48]. Based on outcrop relationships, 35 m thick saccharoidal D2 dolomite is in close contact with the overlying Mantou Formation. The relative sea-level rise (during the early HST) in the early stage of the deposition of the Mantou Formation caused an increase in carbonate sediment production in the late Changping Formation and produced sabkha dolomite at the base of the Mantou Formation [45,48].
The Mg-rich brines that were formed during sabkha dolomitization in the overlying strata refluxed downwards due to gravity, thus forming a vertical sequence of sabkha dolomite and seepage reflux dolomite. These situations enhanced the effective circulation of dolomitizing fluid within the deposited limestones and therefore facilitated dolomitization at the sedimentary surface or near the surface. The dolomitization of limestone in the upper part of the Changping Formation resulted from the reflux infiltration of Mg-rich brines formed in the Mantou Formation sabkha environment (Figure 9b). The rock units from the base to the top are limestone, mottled micrite dolomitic limestones, and seepage reflux saccharoidal dolostone (Figure 9b). D2 dolomites and the underlying mottled limestone and dolomitic limestones at the top of the Changping Formation are inferred to have formed by seepage reflux dolomitization. The unconformity between the Changping Formation and Mantou Formation was a good migration pathway for the Mg-rich brines formed by the sabkha dolomite to reflux into the deeper buried Changping Formation. The Mg-rich brines aided the dolomitization of the Changping Formation limestone, with the greatest degree of dolomitization mostly occurring near the surface. This is also the reason why the top of the Changping Formation is seepage reflux dolomite, with a sufficient downward supply of Mg-rich brines, thereby forming mottled limestone and dolomitic limestone (Figure 9b). The light δ13C and δ18O isotopic values (Table 2; Figure 8) are consistent with the role of meteoric water during the formation of the Changping Formation D2 dolomite [36]. Seepage reflux dolomitization is normally accompanied by evaporative pumping dolomite [35].
In summary, rising sea levels increased the supply of carbonate and reduced that of siliciclastic sediments during the TST and early HST and facilitated the formation of dolomite through three main mechanisms [36,39]. Firstly, the rise in sea level provided sedimentary accommodation space, resulting in large amounts of carbonate deposits being formed, which supplied calcium and carbonate ions and nucleation sites [48]. Other examples include the Carboniferous Um Bogma Formation in the southwestern Sinai region of Egypt, where the TST and early HST were times of high carbonate productivity which generated an active circulating dolomitizing fluid [36]. Secondly, as the salinity of the seawater increased in the sabkha environment, Mg-rich brines were formed, which could supply plenty of magnesium for dolomitization. Near sabkha environments, brines that are converted directly from a restricted environment lead to excessive dolomitization and lower porosity, as observed in the outcrop of the Changping Formation at Dingjiatan in the Western Hills of Beijing [35] and in the Dehluran Field in southwest Iran [38]. Thirdly, sea-level fluctuations affected the sedimentary environment, because meteoric freshwater can dissolve evaporitic rocks and increase porosity [35,36]. Penecontemporaneous exposure controlled by high-frequency sea-level changes is typically characterized by fabric selective dissolution, with the development of intergranular dissolution pores and enhancement of reservoir properties [40].
Accordingly, from a sequence stratigraphic point of view, the Changping Formation dolomite was mainly developed near the unconformity sequence boundaries, which are located at the top of the HST and at the base of the TST (Figure 2). Based on this comprehensive history of the Changping Formation in the Xiaweidian outcrop area, sabkha dolomite is mainly distributed at the top of the retrogressive sequence, or the base of the transgressive sequence, and is associated with underlying fine-mesocrystalline dolomite and dolomitic limestone produced by seepage reflux brines (Figure 9c).
The different dolomitization mechanisms have different impacts on reservoir quality and exploration prospects. The dolomite of the Cambrian Changping Formation is comparable throughout the Beijing area. D1 dolomite exhibits low-salinity sabkha dolomitization characterized by low porosity and permeability [35] and is mainly distributed at the base of the TST. Reflux dolomitization can cause a more complex reservoir formation mechanism. The porosity and permeability of dolomite change with the magnesium content of the brine. Near the sabkha environment, the Mg-rich fluid produced from the restricted environment led to excessive dolomite formation, accompanied by a significant decrease in porosity. The dolomitization fluids far from the restricted sabkha environment have lower magnesium contents, so the dolomite volume decreased, and there were more residual intergranular pores and higher permeability [35]. Therefore, there is a relationship between the porosity of dolomite and the sequence stratigraphy.

6. Conclusions

The Cambrian Series 2 dolomites of the Changping Formation in the Western Hills of Beijing, China, were assessed by field observations, petrography (polarizing microscopy and cathodoluminescence), and geochemical analyses (X-ray diffraction and stable isotopes). Based on textural classification, there are two types of dolomites: microcrystalline dolomite (D1) and fine-mesocrystalline dolomite (D2).
Petrographic and geochemical analyses suggest that the formation of dolomite occurred in two different diagenetic models: D1 dolomite is a typical para-syngenetic sabkha dolomite, which is inferred to have formed by the dolomitization of soft supratidal limestone. Sabkha dolomite (D1) was developed at the base of the TST. D2 dolomite is associated with supratidal para-syngenetic sabkha dolostones and is inferred to have formed by seepage reflux dolomitization.
The Changping Formation dolomite was mainly developed near the basal unconformity boundary of the sequence. Sabkha dolomite is mainly distributed at the top of the HST and at the base of TST and is associated with underlying seepage reflux dolomites and dolomitic limestone.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/min14121189/s1, Table S1: Sample list.

Author Contributions

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

Funding

This work was supported by the Key Laboratory of Tectonics and Petroleum Resources (China University of Geosciences) (Grant No. TPR-2021-22) and the CSC scholarship. The APC was funded by the National Natural Science Foundation of China (No. 41302112) and a National Science and Technology Major Project of China (No. 2016ZX05002-006).

Data Availability Statement

Data is contained within the article or supplementary material.

Acknowledgments

SZ wishes to acknowledge the China Scholarship Council and Macquarie University for awarding them the CSC scholarship and providing financial support for international communication. This work was supported by the Key Laboratory of Tectonics and Petroleum Resources (China University of Geosciences) (Grant No. TPR-2021-22). The authors thank the editor and reviewers for their helpful comments and suggestions.

Conflicts of Interest

Hongyu Tian is an employee of PetroChina. The paper reflects the views of the scientist and not the company.

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Figure 1. A generalized geological map of the Xiaweidian outcrop in the Western Hills area of Beijing, showing the distribution of Cambrian strata [48].
Figure 1. A generalized geological map of the Xiaweidian outcrop in the Western Hills area of Beijing, showing the distribution of Cambrian strata [48].
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Figure 2. A schematic stratigraphic column, showing the sequence stratigraphy of the Cambrian Formation at the Xiaweidian outcrop, China, with the geochemical sampling points (red lines). Fm. = formation. Pt3j = Jingeryu Formation. The height is above the base of the unconformity with the Neoproterozoic Jingeryu Formation.
Figure 2. A schematic stratigraphic column, showing the sequence stratigraphy of the Cambrian Formation at the Xiaweidian outcrop, China, with the geochemical sampling points (red lines). Fm. = formation. Pt3j = Jingeryu Formation. The height is above the base of the unconformity with the Neoproterozoic Jingeryu Formation.
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Figure 3. The characteristics of D1 dolomites from the Xiaweidian outcrop of the Changping Formation. (a) An outcrop photograph of breccia-bearing microcrystalline dolomite, showing angular or subangular brecciation (red ellipses). (b) A photomicrograph in plane-polarized light of the relatively homogeneous crystals of D1 dolomite (sample Є1c-2). (c) A photomicrograph in plane-polarized light of a dolomitic bioclast (sample Є1c-6), consisting of dense microcrystalline dolomite and finely crystalline dolomite. (d) A photomicrograph in plane-polarized light of finely crystalline dolomite, with a calcite vein (sample Є1c-9). (e) A photomicrograph in plane-polarized light of the relatively homogeneous crystals of D1 dolomite (sample Є1c-2). (f) A cathodoluminescence image of the same field of view as (e), showing the non-luminescence of D1 dolomite.
Figure 3. The characteristics of D1 dolomites from the Xiaweidian outcrop of the Changping Formation. (a) An outcrop photograph of breccia-bearing microcrystalline dolomite, showing angular or subangular brecciation (red ellipses). (b) A photomicrograph in plane-polarized light of the relatively homogeneous crystals of D1 dolomite (sample Є1c-2). (c) A photomicrograph in plane-polarized light of a dolomitic bioclast (sample Є1c-6), consisting of dense microcrystalline dolomite and finely crystalline dolomite. (d) A photomicrograph in plane-polarized light of finely crystalline dolomite, with a calcite vein (sample Є1c-9). (e) A photomicrograph in plane-polarized light of the relatively homogeneous crystals of D1 dolomite (sample Є1c-2). (f) A cathodoluminescence image of the same field of view as (e), showing the non-luminescence of D1 dolomite.
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Figure 4. The characteristics of D2 dolomites from the Xiaweidian outcrop of the Changping Formation. (a) An outcrop photograph of fine-mesocrystalline dolomite. Dol = dolomite. (b) A photomicrograph in plane-polarized light (sample Є1c-35) showing D2 dolomites with an intracrystalline pore filled by calcite cement (red). (c) A photomicrograph in plane-polarized light (sample Є1c-29) showing the typical residual bioclastic structure. (d) A photomicrograph in plane-polarized light (sample Є1c-33) showing crystals with cloudy centers and clear rims. (e) A photomicrograph in plane-polarized light of the homogeneous crystals of D2 dolomite (sample Є1c-39-3). (f) A cathodoluminescence image of the same field of view as (e), showing that the cores exhibit dull red or non-luminescence, while the rims have bright red luminescence.
Figure 4. The characteristics of D2 dolomites from the Xiaweidian outcrop of the Changping Formation. (a) An outcrop photograph of fine-mesocrystalline dolomite. Dol = dolomite. (b) A photomicrograph in plane-polarized light (sample Є1c-35) showing D2 dolomites with an intracrystalline pore filled by calcite cement (red). (c) A photomicrograph in plane-polarized light (sample Є1c-29) showing the typical residual bioclastic structure. (d) A photomicrograph in plane-polarized light (sample Є1c-33) showing crystals with cloudy centers and clear rims. (e) A photomicrograph in plane-polarized light of the homogeneous crystals of D2 dolomite (sample Є1c-39-3). (f) A cathodoluminescence image of the same field of view as (e), showing that the cores exhibit dull red or non-luminescence, while the rims have bright red luminescence.
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Figure 5. The degree of cation order, FWHM104, NCaCO3 (%), and stable carbon and oxygen isotope curves of the Changping Formation from the Xiaweidian outcrop, China. See Section 3 for the definitions of the parameters. The height is above the base of the unconformity with the Neoproterozoic Jingeryu Formation. Fm. = formation.
Figure 5. The degree of cation order, FWHM104, NCaCO3 (%), and stable carbon and oxygen isotope curves of the Changping Formation from the Xiaweidian outcrop, China. See Section 3 for the definitions of the parameters. The height is above the base of the unconformity with the Neoproterozoic Jingeryu Formation. Fm. = formation.
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Figure 6. Outcrop photographs of the Changping Formation at the Xiaweidian outcrop (from the base to approximately 90 m). (a) The parallel unconformity (in the direction of the arrow) of the Changping Formation and the Jingeryu Formation. (b) The Neoproterozoic Jingeryu Formation dolomitic limestone. (c) Mottled limestone in the middle part of the Changping Formation. (d) Oncolitic limestone in the middle part of the Changping Formation. (e) Saccharoidal dolomite at the top of the Changping Formation and boundary (the dotted line) between the Changping Formation and the Mantou Formation. (f) Reddish-purple mudstone of the Mantou Formation.
Figure 6. Outcrop photographs of the Changping Formation at the Xiaweidian outcrop (from the base to approximately 90 m). (a) The parallel unconformity (in the direction of the arrow) of the Changping Formation and the Jingeryu Formation. (b) The Neoproterozoic Jingeryu Formation dolomitic limestone. (c) Mottled limestone in the middle part of the Changping Formation. (d) Oncolitic limestone in the middle part of the Changping Formation. (e) Saccharoidal dolomite at the top of the Changping Formation and boundary (the dotted line) between the Changping Formation and the Mantou Formation. (f) Reddish-purple mudstone of the Mantou Formation.
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Figure 7. A cross-plot of the degree of cation order (δ) and mole % CaCO3 from NCaCO3 (%) for microcrystalline D1 dolomite and fine-mesocrystalline dolomite (D2) in the Xiaweidian outcrop of the Changping Formation.
Figure 7. A cross-plot of the degree of cation order (δ) and mole % CaCO3 from NCaCO3 (%) for microcrystalline D1 dolomite and fine-mesocrystalline dolomite (D2) in the Xiaweidian outcrop of the Changping Formation.
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Figure 8. A cross-plot of the δ13OVPDB and δ18CVPDB values (‰) for microcrystalline D1 dolomite and fine-mesocrystalline dolomite (D2) in the Xiaweidian outcrop of the Changping Formation.
Figure 8. A cross-plot of the δ13OVPDB and δ18CVPDB values (‰) for microcrystalline D1 dolomite and fine-mesocrystalline dolomite (D2) in the Xiaweidian outcrop of the Changping Formation.
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Figure 9. Alternate models for the formation of dolomite in the Changping Formation. (a) A sabkha dolomitization model, involving evaporation in a hot and dry climate, leading to D1 dolomite. (b) A seepage reflux dolomitization model, leading to D2 dolomite. (c) Sabkha dolomitization at the bottom of the Mantou Formation and seepage reflux dolomitization at the top of the Changping Formation in a sequence stratigraphic sedimentary rhythm. The positions of the sequence boundaries (SBs), the transgressive system tract (TST), and the highstand system tract (HST) are shown.
Figure 9. Alternate models for the formation of dolomite in the Changping Formation. (a) A sabkha dolomitization model, involving evaporation in a hot and dry climate, leading to D1 dolomite. (b) A seepage reflux dolomitization model, leading to D2 dolomite. (c) Sabkha dolomitization at the bottom of the Mantou Formation and seepage reflux dolomitization at the top of the Changping Formation in a sequence stratigraphic sedimentary rhythm. The positions of the sequence boundaries (SBs), the transgressive system tract (TST), and the highstand system tract (HST) are shown.
Minerals 14 01189 g009
Table 1. The degree of cation order, FWHM104, and mole % CaCO3 based on X-ray diffraction data of the Cambrian Series 2 dolomite samples in the Changping Formation from the Xiaweidian outcrop, China.
Table 1. The degree of cation order, FWHM104, and mole % CaCO3 based on X-ray diffraction data of the Cambrian Series 2 dolomite samples in the Changping Formation from the Xiaweidian outcrop, China.
Rock TypeSampleHeight (m)Degree of Cation Order (δ)FWHM104Mole % CaCO3 from NCaCO3 (%)
Microcrystalline and finely crystalline dolomite (D1)Є1c-14.410.50nd49.6
Є1c-211.510.57nd50.4
Є1c-523.110.710.16251.3
Є1c-628.210.670.16551.4
Є1c-729.810.66nd50.0
Є1c-1137.340.760.17051.4
Є1c-28-156.180.630.16750.2
Є1c-2956.280.61nd49.9
Є1c-29-156.280.74nd49.7
Є1c-29-256.280.650.18951.2
Є1c-3258.280.61nd49.8
Є1c-33-159.550.63nd51.8
Є1c-33-260.550.720.16449.0
Є1c-33-361.550.60nd49.9
Є1c-3566.080.700.17151.2
Average for D1 0.650.17050.5
Fine-mesocrystalline dolomite (D2)Є1c-3768.280.870.15548.8
Є1c-38-273.180.890.15350.2
Є1c-38-373.180.910.15648.7
Є1c-39-174.680.84nd48.9
Є1c-39-277.680.860.15848.1
Є1c-39-379.680.840.15450.0
Є1c-39-488.680.830.16548.8
Є1c-39-591.680.960.15049.0
Є1c-39-693.580.890.15648.9
Average for D2 0.880.15649.0
For the definitions of δ and NCaCO3, please see the text. The height is above the base of the unconformity with the Neoproterozoic Jingeryu Formation. nd = not determined.
Table 2. The stable isotope (δ13C and δ18O) values of the Cambrian Series 2 dolomite samples in the Changping Formation from the Xiaweidian outcrop, China.
Table 2. The stable isotope (δ13C and δ18O) values of the Cambrian Series 2 dolomite samples in the Changping Formation from the Xiaweidian outcrop, China.
Dolomite TypeSampleδ13CVPDB (‰)δ18OVPDB (‰)Paleo-Salinity (Z Value)
Microcrystalline dolomite and finely crystalline (D1)Є1c-3−1.28−7121.19
Є1c-9−3.28−7.2117.00
Є1c-11−0.08−3.92125.18
Є1c-28-1−0.88−7.68121.67
Є1c-29−0.73−7.96121.84
Є1c-29-1−0.46−7.70122.52
Є1c-29-22.02−7.06127.92
Є1c-33-1−0.42−6.68123.11
Є1c-33-22.46−6.17129.27
Є1c-33-31.19−3.62127.94
Average for D1−0.15−6.5123.76
Fine-mesocrystalline dolomite (D2)Є1c-370.34−7.89124.07
Є1c-38-20.04−7.21123.79
Є1c-38-3−0.07−8.40122.97
Є1c-39-1−2.60−9.45117.27
Є1c-39-2−0.35−7.75122.72
Є1c-39-3−0.71−4.70123.51
Є1c-39-40.17−6.69124.32
Є1c-39-50.28−7.62124.08
Є1c-39-60.35−7.23124.42
Average for D2−0.28−7.44123.02
See the text for the definition of the Z value. The height is above the base of the unconformity with the Neoproterozoic Jingeryu Formation.
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Zhong, S.; Liu, Z.; Jin, Z.; Tian, H.; Istifanus, M.A.; George, S.C. The Development of Dolomite Within a Sequence Stratigraphic Framework: Cambrian Series 2 Changping Formation, Xiaweidian, China. Minerals 2024, 14, 1189. https://doi.org/10.3390/min14121189

AMA Style

Zhong S, Liu Z, Jin Z, Tian H, Istifanus MA, George SC. The Development of Dolomite Within a Sequence Stratigraphic Framework: Cambrian Series 2 Changping Formation, Xiaweidian, China. Minerals. 2024; 14(12):1189. https://doi.org/10.3390/min14121189

Chicago/Turabian Style

Zhong, Shan, Zhaoqian Liu, Zhenkui Jin, Hongyu Tian, Madaki Agwom Istifanus, and Simon C. George. 2024. "The Development of Dolomite Within a Sequence Stratigraphic Framework: Cambrian Series 2 Changping Formation, Xiaweidian, China" Minerals 14, no. 12: 1189. https://doi.org/10.3390/min14121189

APA Style

Zhong, S., Liu, Z., Jin, Z., Tian, H., Istifanus, M. A., & George, S. C. (2024). The Development of Dolomite Within a Sequence Stratigraphic Framework: Cambrian Series 2 Changping Formation, Xiaweidian, China. Minerals, 14(12), 1189. https://doi.org/10.3390/min14121189

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