Early diagenetic growth of carbonate concretions in the...

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Sci China Ser D-Earth Sci | Sep. 2008 | vol. 51 | no. 9 | 1330-1339

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Early diagenetic growth of carbonate concretions in the upper Doushantuo Formation in South China and their significance for the assessment of hydrocarbon source rock

DONG Jin1, ZHANG ShiHong1†, JIANG GanQing2, ZHAO QingLe1, LI HaiYan1, SHI XiaoYing1 & LIU JunLai1

1 State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Beijing 100083, China; 2 Department of Geoscience, University of Nevada, Las Vegas, NV 89154-4010, USA

Mineralogical and textural characteristics and organic carbon composition of the carbonate concre-tions from the upper Doushantuo Formation (ca. 551 Ma) in the eastern Yangtze Gorge area reveal their early diagenetic (shallow) growth in organic-rich shale. High organic carbon content (up to 10%) and abundance of framboidal pyrites in the hosting shale suggest an anoxic or euxinic depositional envi-ronment. Well-preserved cardhouse clay fabrics in the concretions suggest their formation at 0－3 m burial depth, likely associated with microbial decomposition of organic matter and anaerobic oxidation of methane. Gases through decomposition of organic matter and/or from methanogenesis created bubbles and cavities, and anaerobic methane oxidation at the sulfate reduction zone resulted in car-bonate precipitation, filling in bubbles and cavities to form spherical structures of the concretions. Rock pyrolysis analyses show that the carbonate concretions have lower total organic carbon (TOC) content but higher effective carbon than those in the host rocks. This may be caused by enclosed or-ganic matter in pores of the concretions so that organic matter was protected from further modification during deep burial and maintained high hydrocarbon generating potential even in over-matured source rock. As a microbialite sensu latu, concretions have special growth conditions and may provide im-portant information on the microbial activities in depositional and early burial environments.

Doushantuo Formation, carbonate concretion, sedimentary environment, hydrocarbon source rock

Abundant carbonate concretions are distributed in the black shale of the Ediacaran Doushantuo Formation and Cambrian Shuijingtuo Formation in South China. These concretions are important in revealing early burial envi-ronments and associated microbial processes. By defini-tion[1], carbonate concretions are made up of detrital material cemented by authigenic carbonate and thus could potentially preserve the original depositional fab-rics[2－4]. Concretionary growth is commonly related to microbial processes. Microbial decomposition of organic matter not only generates abundant authigenic carbon-ates for concretionary growth, but also controls the shapes and compositions of concretions. In the zone of sulphate reduction, anaerobic methane oxidation (AOM,

24SO −

+ CH4 → 3HCO− +HS−+H2O) or organic minerali-

zation ( 24SO − +2CH2O → H2S + 2 3HCO− ) is generally

considered to be a major stage of concretionary growth[5－10]. In this paper, we report mineralogical and textural observations for the carbonate concretions from the upper Doushantuo Formation and discuss the depo-

Received February 18, 2008; accepted June 7, 2008 doi: 10.1007/s11430-008-0107-3 †Corresponding author (email: [email protected]) Supported by the National Natural Science Foundation of China (Grant Nos. 40572019 and 40621002), Prospective Study of China Petroleum and Chemical Corporation (Grant No. G0800-06-ZS-319), Ministry of Education of China (Grant No. NCET-04-0727, ‘the 111 Project’ B07011) and National Science Foundation of USA (Grant No. EAR0745825)

DONG Jin et al. Sci China Ser D-Earth Sci | Sep. 2008 | vol. 51 | no. 9 | 1330-1339 1331

sitional and early burial environments in which the car-bonate concretions were formed.

1 Geological background

In the late Neoproterozoic, the southeastern Yangtze block was interpreted as a southeast-facing, rift to pas-sive continental margin with three major stratigraphic successions: (1) pre-glacial siliciclastic Banxi Group and its equivalents, (2) Cryogenian glacial and interglacial units, and (3) the post-glacial Ediacaran marine carbon-ate and shale successions of Doushantuo and Dengying formations[11－14].

Paleogeographically the Yangtze Gorge area was lo-cated in the interior of the Yangtze Platform and pre-serves the typical sections of the Doushantuo Formation. The Doushantuo Formation directly overlies the Nantuo glacial diamictite and consists of < 200-m-thick carbon-ates and shales (Figure 1(a) and (b)). It is lithologically divided into four members. The lowest member is com-posed of 3－5-m-thick gray dolomite and limestone, which overlies glacial diamictites and is called the Dou-shantuo cap carbonate[12,17,18]. The bottom of the cap carbonate is also the bottom of Ediacaran (or Sinian in China)[19－21]. The second member of Doushantuo For-mation consists of ~ 100-m-thick interbedded black shales and shaly limestones with abundant pea-sized phosphorite or chert nodules. The third member is com-posed of 60－80-m-thick shaly or dolomitic limestones, and thick dolomite. The topmost member consists of ~10-m-thick black shale containing abundant carbonate concretions that are the focus of this study. The Dou-shantuo Formation is overlain by ~ 800-m-thick carbon-ate rocks of the Dengying Formation, which is in turn overlain by Lower Cambrian shales.

Intensive studies have been conducted regarding the litho-, bio-, and chrono-stratigraphy of the Doushangtuo Formation. Large acanthomorph acritarchs, multicellular algae, and cyanobacteria have been found in the phos-phorite-chert nodules from the lower part of this Forma-tion[22－24] and abundant macroscopic algae and putative animal fossils have been reported from the fourth mem-ber, commonly referred to as the Miaohe biota[22,25－27]. Three strongly negative δ 13C anomalies occur in the lowermost, middle and the uppermost of the Doushantuo Formation[13,16,28]. Ash layers near the topmost Dou-shantuo Formation yields a U-Pb zircon age of ~551 Ma,

and the cap carbonate Member has the age of ~635 Ma[15,29].

2 Sedimentary structures and mineral compositions of the Doushantuo concre-tions

Carbonate concretions are randomly distributed in the black shale of the fourth member of the Doushanduo Formation (Figure 1(c)－(e)). Most of them appear as isolated forms but in some cases they align along the sedimentary bedding. The hosting shale shows parallel laminations, whereas the concretions are usually mas-sive, occasionally with faint parallel-lamination. At the contact between the host rocks and concretions, laminae in the host rock bends around the concretion. Concre-tions are usually spherical, ellipsoidal (Figure 1(c)), ob-late (Figure 1(d)) or irregular in shape, with diameters mostly ranging from 0.1 to 5 m and sometimes from 0.05 to 0.1 m. Near the top of the Doushantuo Formation, a few horizons contain large layer-parallel concretions, some of which show clear sedimentary laminations (Figure 1(e)).

X-ray diffraction (Table 1) indicates that concretion-ary cements are mostly dolomite and minor calcite, whereas the detrital components in concretions are mostly quartz, clay and minor K-feldspar. Two types of dolomite can be identified: one is the sparitic dolomite with good crystal shape and the other is microcrystalline, both likely the authigenic precipitates associated with concretion growth. In both concretions and host rocks, the quartz and the clay are the major detrital components and have the same contents, with trace amount of K-feldspars. Similar detrital mineral components in the host rock and concretions suggest that the concretions and host rocks had a detrital source from the same depo-sitional environment.

Many criteria were proposed for the classification of concretionary bodies, including the shape, texture, min-eral component, etc.[7,30－33]. However, with genetic in-terpretations involved, the nomenclature of the concre-tionary bodies become confusing and even misleading. Selles-Martinez[1] discussed these problems and sug-gested a descriptive classification to differentiate con-cretions from other concretionary bodies, especially nodules. Concretions refer to the portion of a sedimen-tary rock which has been cemented differentially by

1332 DONG Jin et al. Sci China Ser D-Earth Sci | Sep. 2008 | vol. 51 | no. 9 | 1330-1339

Figure 1 (a) Simplified geological map of the Yangtze Gorge area (after ref. [15]), Asterisks indicate sampling location; (b) stratigraphic column of the Jiulongwan section (after ref. [16]); (c)－(e) Field photographs of concretions.


Table 1 Analysis of X-ray diffraction for mineral kinds and contents (%) of concretions and host rocks Sample Type Quartz K-feldspar Calcite Dolomite Pyrite Clay 05C1G Concretion 3.1 0.3 5.6 88.2 − 2.8 ZG19G Concretion 2.2 0.2 0.6 94.1 − 2.9 ZG14B Host rock 36.1 4.2 3.6 6.5 3.6 40.7

authigenic minerals and thus differed from their host rock in morphology and lithology, whereas nodules do not incorporate detrital materials during growth, though their shapes are often similar to that of concretions. Similarity of the mineral composition and inheritance of sedimentary structures between the concretions and host rocks prompts the use of the term “concretion” in this paper. This means that the concretions in upper Dou-shantuo Formation are typically made of detrital materi-als cemented by the authigenic carbonates.

3 Microscopic fabrics of the concretions

The most characteristic microfabric of the concretions is the spherical texture, about 100－200 μm in size. The spheres are uniformly distributed throughout the concre-tion and cover the field of view for 80% (Figure 2(a), (b)). There is no difference in thin sections perpendicu-lar or parallel to laminations and spheres are identical from the center to the outer edge of concretions.

Most spheres are independently distributed and some form aggregates. The spheres are filled by sparitic dolomite with clear rhombic shapes. The crystals form an isopachous layer along the edge of the spheres, with crystal growth towards the center. Some voids are still left in the center of spheres and in inter-crystal spaces and amount to 5% (Figure 2(b)). Around the margins of the spherical textures, platy clays display well-devel- oped preferential orientation and form concentric zones. Though few spherical structures exist in the host rocks, concentric rings displayed by platy clays are common (Figure 2(g)).

Inter-sphere spaces in concretions are filled by fine-grained dolomite, quartz, K-feldspar, and clay. After etched with diluted hydrochloric acid, the open network structure of the clastic components could be clearly ob-served under SEM. The grains show cardhouse fabrics displaying edge to face (EF), edge to edge (EE), and face to face (FF) modes of grain contact (Figure 2(d)－ (f))[2,34,35]. In addition, framboid pyrites, 2－3 μm in size, could be observed in the open framework of the card-house structure (Figure 2(f)).

By comparison, the fabric of the host rocks has been intensively influenced by compaction and shows in-creased degree of face-to-face, bedding-parallel align-ment. In addition, the host rocks lack sphere structures, cardhouse fabrics and cementation, and have a lower porosity than the concretions. Occasionally, spherical cavities can be observed in the host rocks; they remain as voids not filled by carbonate cements but in some cases pyrites are found in those voids. Spherical cavities often display oblate shapes indicative of deformation by compaction.

The dolomite in the spherical structures is common in the organodolomitization. It has been suggested that they are formed through anaerobic mineralization of the or-ganic matter, via microbial decomposition[36] or anaero-bic methane oxidation[4]. The spheres can also be formed by isovolumetric mineral phase changes from aragonite to calcite and to dolomite. Considering the nearly spherical shape and their similar size with the gas bub-bles reported from other concretions[3,37], we tend to think that the spherical structures are originally formed as gas bubbles resulted from organic matter decay or methanogenesis and later filled by carbonate minerals. The negative δ13C values (−7‰ to −11‰) of the concre-tions[15,16] are consistent with the addition of 13C-depleted carbon from organic carbon or methane. The cardhouse fabrics are also common in modern ma-rine clay deposits and indicate the physicochemical en-vironment which promotes the flocculation of clay par-ticles[35,38]. The open cardhouse arrangement typical of newly deposited, flocculated clays has been preserved by diagenetic carbonate precipitation during concretion-ary growth[2,7]. The framboid pyrites in the framework of the cardhouse structures possibly suggest an early an-oxic sedimentary environment, likely related to micro-bial processes[39－41]. Pyrites filling in the spherical cavi-ties of the host rocks also indicate anoxic environments.

4 Rock pyrolysis analyses of concre-tions and host rocks

Rock pyrolysis analysis can reveal the maturity and


Figure 2 Optical microscopy and SEM images of concretions and host rocks. (a), (b) Optical microscopy image of concretions; (c)－(f) SEM image of concretions, (d)－(f) lightly etched with dilute hydrochloric acid; (g)－(h) SEM image of host rocks. (b) is the magnification of the dash frame in (a). Ms, sparitic dolomite; M, microcrystalline dolomite; C, clay; Qz, quartz; Py, pyrite; FF, face-to-face mode (of clay contact); EF, edge-to-face mode; EE, edge-to-edge mode; SS: spherical texture.

evolution of organic matter. The measurement was com-pleted in the Institute of Petroleum Exploration and De-velopment, PetroChina, using Oil and Gas Evaluation (OGE) II workstation.

Total organic carbons (TOC) of the host rocks and the concretions are about 9.43% and 3.8%, respectively.

Effective carbons of the host rocks and the concretions are about 0.092% and 0.068%, which are about 0.97% and 1.9% of their TOC (Table 2). In addition, the host rocks and the concretions have similar Tmax (the maxi-mum pyrolysis temperature), which are about 583℃ and 580℃, respectively, indicating high maturity of or-


Table 2 Rock pyrolysis analysis of the Doushantuo concretions and their host shalea) Sample Type TOC (%) Tmax (℃) S1 (mg/g) S2 (mg/g) S3 (mg/g) EC (%) HI (mg/g TOC) HCI (mg/g TOC)

05C1C C 2.88 575 0.54 0.73 27.79 0.11 25 18.75 05C1P C 5.70 582 0.38 0.7 56.11 0.09 12 6.67 05C3G C 3.98 583 0.21 0.25 39.41 0.04 6 5.28 05C3F C 1.80 580 0.1 0.28 17.71 0.03 16 5.56 ZG15D H 4.46 583 0.09 0.26 44.34 0.029 6 2.02 ZG21D H 8.10 579 0.21 0.67 80.23 0.073 8 2.59 ZG23C H 8.06 585 0.54 0.7 79.59 0.103 9 6.70 ZG24G H 8.60 588 0.16 0.7 85.29 0.071 8 1.86 ZG25C H 9.41 587 0.26 0.84 93.23 0.091 9 2.76 ZG19A H 17.97 578 0.15 2.08 177.89 0.185 12 0.83 a) C, concretion; H, host rock; TOC, total organic carbon; Tmax, the maximum temperature of S2; S1, free hydrocarbon; S2, pyrolysis hydrocarbon; S3, re-

sidual organic carbon; EC, effective carbon, (S1+S2)×0.083; HI, hydrogen index, S2/TOC; HCI, hydrocarbon index, S1/TOC.

ganic matter. By reference to previous work on the or-ganic matter abundance standard in carbonates and shales[42], the organic matters in concretions and the host rocks are characterized with high organic matter abun-dance, high residual organic carbon content and low ef-fective carbon content, which suggests that the concre-tions and host shale as source rocks have low hydrocar-bon generating potential. The lower hydrocarbon gen-eration potential is most likely due to the high maturity rather than the type of the organic matter (humic organic matter)[43]. After exceeding the oil generation threshold, oil and gas escaped but the residual organic carbon re-mained[44].

TOC of the host rocks is about 148% times higher than that of the concretions, whereas the effective car-bon of the host rock is only about 36% times higher than that of the concretion. This may be related to the differ-ent types of organic matter in the host rocks and in the concretions. The S1 Rock-Eval parameters (free hydro-carbon) of the concretions and the host rocks are about 0.308 mg/g and 0.235 mg/g, respectively, and the value of the concretions is about 30.9% times higher than that of the host rocks. The S2 Rock-Eval parameters (pyroly-sis hydrocarbon) of the concretions are about 0.49 mg/g, which are about 78.6% lower than those of the host shales (0.875 mg/g). The P-FID (hydrogen flame ioniza-tion detector) spectrums of the concretions and the host rocks are also different (Figure 3). The host rocks have only one temperature peak at about 583℃ in their S2 curve of the P-FID spectrums, while the concretions have two or three temperature peaks in their S2 curve of the P-FID spectrums. The highest temperature peak is close to the Tmax of the host rocks, but the middle tem-perature peak is at about 480℃. Tmax may relate to burial differences and the composition of organic matters. Be-

cause the burial depth of the concretions and the host rocks is identical, the different Tmax may reflect the compositional differences of the organic matter between the concretions and the host rocks.

Hydrogen index (HI) and hydrocarbon index (HCI), which reflects hydrocarbon generating potential per unit organic carbon of the concretion, are different from those of the host rocks. HI and HCI of the concretions are about 70% and 225% times higher than those of the host rocks, respectively. In addition, free hydrocarbon is more than pyrolysis hydrocarbon in the concretions (Ta-ble 2). All these indicate that the concretions have higher hydrocarbon generating potential than the host rocks, though the concretions have lower TOC.

5 Discussion

5.1 Depth of concretionary growth

The porosity variation, the deformation of the host sediment lamination within and around concretions, the septarian fissures and the preservation of uncrushed soft body material such as fossil skeletons, are commonly used as evidence for the time and depth of concretionary growth[8,31,37,45－48]. Applying these criteria, the concre-tions in the Doushantuo Formation are most likely formed by authigenic carbonate cementation of detrital materials during the early diagenesis.

The layering inside the concretions and in the host rocks has been used as an important criterion to judge the relationship between the concretionary growth and the host rock deposition and compaction. If the lamina-tions pass through the host rocks and concretions, it may indicate that cementation occurred after burial compac-tion. If the laminations inside concretions do not change, but laminae in host rocks bend near concretions and


Figure 3 The P-FID spectrum of concretion and host rock. (a) Host rocks; (b) concretions. S1, free hydrocarbon; S2, pyrolysis hydrocarbon; S3, residual organic carbon.

show thickness variations, it may reflects early concretionary growth before compaction. In contrast, al-mond-shaped layering inside the concretions associated with warping of layering in the host rocks suggests syn-chronous concretionary growth during compaction[1]. The Doushantuo concretions do not show laminae across the concretion-host rock boundary. Instead, the hosting shale layers near concretions bend around the concre-tions (Figure 1(c), (d), consistent with early diagenetic growth before burial compaction.

Internal structures and textures of concretions can also help determine the time of concretionary growth[3,8,37]. The spherical texture in the Doushantuo concretions may record gas bubbles in sediments. Duck[3] reported similar textures from the Pleistocene concretions. Those voids are spherical and are uniformly distributed throughout the concretions. They range in diameter from 30 to 350 μm, with 100 to 130 μm being the most common, similar to the ones found in the Dou-shantuo concretions. Berner[49] demonstrated experi-mentally that dissolved carbon dioxide and ammonia liberated by the decomposition of animal protein affect local pH/Eh conditions, creating an alkaline-reducing microenvironment. The rise in pH is accompanied by the release of Ca2+ from seawater to form fatty acid soap which Berner[49] believed was the precursor of CaCO3 concretions. This has been supported by the work of Al-lison[50] who experimentally demonstrated that the for-mation of Ca soap could occur within weeks or months after burial.

If the bubbles were the precursor of spherical textures, then the well-preserved spherical textures and cardhouse clay fabrics of the Doushantuo concretions must have been formed close to the sediment-seawater interface

because these textures can be destroyed easily even with very modest burial loading[51,52]. In modern environ-ments, more than 80% of the cardhouse clay fabrics and gas bubbles are found only in the uppermost 0－3 m of sediments. To preserve such delicate textures in the Doushantuo concretions, they must have formed rapidly at very shallow burial depth[53－55]. Assuming an average sedimentation rate of 5 mm/year, the growth time of concretions may be as short as 400－600 years[37], which could be a first order estimation for the Doushan-tuo concretionary growth.

5.2 Environment and model of the concretionary growth

Studies of modern marine organic-rich sediments show that, under the sediment-water interface, the decomposi-tion of organic matter undergoes a series of reactions in the diagenetic zones of the sediment column[56 － 58]. Firstly, the organic matter is oxidized by dissolved oxy-gen in the sediments. Once oxygen is consumed, oxida-tion processes continue using the next most efficient oxidants, including nitrate, manganese and iron oxide in turn. Then, the sulfate reduction follows. At this stage, sulfate was reduced by CH4 or organic matter and generate sulphides and HCO3

－. Below the sulfate reduction zone, methanogenesis becomes the dominant form of anaerobic respiration and generate CH4. The CH4 dif-fuses upward to the sulfate reduction zone and is used by sulfate-reduction bacteria to produce 3HCO− , leading to carbonate precipitation in pores (Figure 4).

The black shale in the uppermost Doushantuo Forma-tion was likely deposited in anoxic or euxinic environ-ments. In the anoxic and euxinic depositional environ-ments, gas bubbles (ammonia or CO2) formed during


Figure 4 Geochemical classification scheme of modern marine sedimentary environment below sediment-water interface (after ref. [58]) and possible growth model of the Doushantuo concretions. At the initial stage, gas bubbles (ammonia or CO2) associated with the organic matter decomposition were presumably generated.

organic matter decomposition were presumably gener-ated near the sediment-water interface. They were pos-sibly the precursor of the spherical structures in the con-cretions.

In the zone of nitrate, manganese or iron reduction, little 3HCO− was generated and thus the carbonate con-cretion could not grow. In addition, being lack of sul-phide, pyrite could not form through iron reduction, leading to the increase of dissolved iron in the pore wa-ters[49,58]. Instead, in the zone of sulfate reduction, an-aerobic methane oxidation could generate significant

3HCO− and raise pH of pore water. This may promote the precipitation of carbonate and is likely the major stage of concretionary growth[4,10,59,60]. At this stage, the concretions of the Doushantuo Formation may begin to grow by authigenic carbonate cementing detrital materi-als, and the gas bubbles were filled by carbonates to form spherical structures. Dissolved iron reacts with sulphide to form pyrite, as seen in spherical structures in concretions and cavities in hosting shales. The dolomites in the spherical structures could have been primary be-cause methane oxidation and/or organic mineralization may have led to low sulfate concentration, which may promote dolomite precipitation[36,61,62].

Concretions have traditionally been viewed as grow-ing concentrically. However, a number of recent studies indicated that most of the features normally used to support the conventional concentric model could be readily explained by more complex growth[63－65]. Mas-sive textures and pervasive spherical structures of the Doushantuo concretions suggest the pervasive model of the concretionary growth. First, pervasive growth starts with the simultaneous nucleation and precipitation of isolated carbonate crystals; second, adjacent cement crystals form a framework, which is resistant to com-paction. Finally, remaining pore space may have been filled by carbonate cements in relatively deeper burial (Figure 4).

5.3 Potential significance as hydrocarbon source rocks

The concretionary growth is closely related to the mi-crobial processes. Microbial processes control not only the source of carbonates required for concretionary growth, but also the shape and textures of carbonate concretions[5－10]. Concretions in a broad sense can be viewed as a type of microbialite, and their characteristics such as microfabrics and mineral compositions are use-ful to reveal the micro-environments during deposition


and early sediment burial. In addition, concretions are possible hydrocarbon source rocks, too.

The Doushantuo concretions were formed by authi-genic carbonates cementing of the detrital materials. The lower TOC contents but higher effective carbon in con-cretions may suggest that pores in the concretions may have better hydrocarbon preservation potential. Early studies show that carbonates have low absorption capac-ity for organic matter and thus do not favor the preserva-tion of hydrocarbon[66]. However, pores, inclusions, etc., can improve the hydrocarbon generating potential of carbonates[67,68]. Rock pyrolysis analysis shows that the concretions have more free-hydrocarbon than the host rocks and its S2 curve have two or three temperature

peaks. The highest temperature peak is similar to the Tmax of the host rocks, about 580℃; but the middle tem-perature peak is about 480℃, which is similar to the decomposition temperature of dolomite and may repre-sent the releasing temperature of enclosed organic mat-ter[69]. The enclosed organic matter entered in pores and inclusions during carbonate crystallization may become an important source of secondary hydrocarbon genera-tion[67,69]. Therefore, though under the same burial con-dition, pores in concretions could preserve organic mat-ter longer and remain more types of organic matter than those from the host shale. Carbonate concretions may still maintain hydrocarbon generating potential when the hosting source rocks is already over-matured.

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