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Growth characteristics and sedimentary mode of Permian reefs, Lengwu, Tonglu, Zhejiang Province, southern China
Journal of Palaeogeography, 2016, 5(4): 409e422 (00108)
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Lithofacies palaeogeography and sedimentology
Growth characteristics and sedimentary mode of Permian reefs, Lengwu, Tonglu, Zhejiang Province, southern China Ling Liu, Ya-sheng Wu * , Hong-xia Jiang, Hong Liu Institute of Geology and Geophysics, CAS, Key Laboratory of Petroleum Resource Research, Chinese Academy of Sciences, Beijing 100029, China
Abstract Organic reefs are favourable accumulation spaces for hydrocarbons and various mineral resources. A complete Permian organic reef profile about 44 m thick with distribution area no more than 1 km2 is exposed near Lengwu, Tonglu, Zhejiang Province. Examination of outcrops and thin sections revealed that the main reef-building organisms are calcisponges, with inozoans as dominant type. Five types of rocks have been recognized in the reef, and they are calcisponge framestone, calcisponge bafflestone, bindstone, rudstone and bioclastic wackestone. The profile was constructed in three reef-building stages. The thickness of the second stage is largest, followed by the first stage and the third stage is smallest. Each stage started with a framestone or bafflestone, ended as the reef grew near the sea-level, and died because of influx of terrigenous sediments. The development of the Lengwu reef is controlled by biological factors and sea-level changes. Based on the study a reef sedimentary model is established. Keywords Reef, Rock types, Calcisponges, Sedimentary model, Permian, Lengwu, Southern China © 2016 China University of Petroleum (Beijing). Production and hosting by Elsevier B.V. on behalf of China University of Petroleum (Beijing). This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). Received 20 October 2015; accepted 13 June 2016; available online 15 August 2016
1.
Introduction
Reefs are favourable enrichment sites for varieties of mineral resources, and are regarded as a unique “paradise” for hydrocarbon reservoirs (Cai, 2009; Fan, 1996; Michel et al., 1970; Liu, 2007; Lu and Gong, 2007; Xu, 1992; Yang et al., 2002). At the beginning
of the last century (1916), the crude oil production of Well Azul 4 in Mexico prime belt was up to 35 620 t, stimulating people's enthusiasm for reefs research (Song, 2012). The last century was the fastest developing period for reefs research (Zhong et al., 2005). Changes in environmental factors, coupled with the evolution stage of the main reef building organisms, as well as a series of extinction events have
* Corresponding author. E-mail address: [email protected] (Y.S. Wu). Peer review under responsibility of China University of Petroleum (Beijing). http://dx.doi.org/10.1016/j.jop.2016.08.002 2095-3836/© 2016 China University of Petroleum (Beijing). Production and hosting by Elsevier B.V. on behalf of China University of Petroleum (Beijing). This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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controlled the types of ancient reefs (Kiessling et al., 1999; Webb, 1996; Wood, 1995). The diversity and complexity makes the reefs very appealing, but also makes it very difficult to establish widely accepted concepts and classification of reefs (Heckel, 1974). Wood, Dickson, and Kirklandgeoge (1994) made a new appraisal for the ecology of the Permian Capitan Reef, Guadalupe Mountains, Texas and New Mexico, suggesting a detailed ecological reappraisal of the ecology and role of cavities within all parts of the reef, within a well defined stratigraphic and palaeodepth framework. During the Devonian, Permian and Neogene periods, reefs were well developed in China (Liu et al., 2007). In China most studies are regarding the Paleozoic reefs, especially the Permian reefs, and secondly is the Cenozoic reef, since reefs were better developed in the Paleozoic, and less developed in the Cenozoic. The study of Permian reefs in China began in the late 1970s (Wang et al., 1998), with a few formal publications. In the 1980s, Fan (1982, 1996), Fan and Zhang (1987) and Fan and Qi (1990) conducted some detailed studies on the reefs in Lichuan, Hubei Province and other areas in Yunnan, Guizhou, and Guangxi. Their work described the types, petrography, palaeogeography and evolution of the reefs. In addition, Qiang et al. (1985), and Chen et al. (1985) carried out researches on the Permian reefs in Sichuan Basin and eastern Sichuan, focussing mainly on the diagenesis and hydrocarbon reservoir properties of the reefs. In the 1990s, more and more new reefs were discovered, and researches on them involved more aspects of the reefs. Yang and Li (1995) and Xu et al. (1996) made a detailed study on the reef in Cili, Hunan Province, which was constructed not only by calcisponges but also by corals. Subsequently, more Permian reefs or bioherms were found along the mid Yangtze River section, such as Chongyang in Hubei Province, Xiushui in Jiangxi Province, Wuxi in Jiangsu Province and Tonglu in Zhejiang Province. The reef studied in this paper is located at Lengwu, a small village in Yao Lin town, Tonglu County, Zhejiang Province. Since the 1960s, many researchers have studied this reef, such as the Regional Geological Survey of Zhejiang Province, Institute of Geology and Mineral Resources of Zhejiang Province, Petroleum Geology Bureau of East China, Hangzhou Institute of Petroleum Geology, Institute of Geology and Palaeontology CAS, Nanjing University, Tongji University, Zhongshan University, Chinese Academy of Sciences, Petroleum Exploration and Development Research Institute, China National Petroleum Corporation and some other organizations, but there is still much controversy on the nature of the reef. Some researchers
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regarded the reef to have developed on a shelf (Wang, 1987); some believed that it was not a reef, but a biostrome or biological bank (Institute of Geology and Mineral Resources of Zhejiang Province, 1989). Wu (1998) found rudstone mainly consisting of lying skeletons of reef-builders such as calcisponges in the reef, and proposed a reef-developing mode characterized by cycles ended with rudstone formed as the reef grew to the sea-level. Based on recent observations on outcrops and studies of thin sections, this paper deals with the rock types and sedimentary background, and proposes a sedimentary model for the reef development.
2.
Geological background
The studied organic reef is located at Lengwu, Tonglu County, Zhejiang Province (Fig. 1). It is a reef built by calcisponges, and occurred in the Middle Permian Maokouan Lengwu Formation (Table 1). The outcrops of the Lengwu Formation extend in a northeast-southwest direction, for about 1.5 km (Yang, 1995). The base of the formation underlying the reef is gray-black argillite with thin layers of siltstone belonging to the Maokouan Dingjiashan Formation, which contains brachiopods and ammonoid fossils representing continental shelf environment. The strata overlying the reef is the Upper Permian Longtan Formation sandstones and siltstones containing coal seams. The reef has a total thickness of 44 m with distribution area no more than 1 km2. Based on lithology and palaeontological features, the reef profile has been divided into 23 layers from the bottom to the top (Fig. 2).
3.
Reef composition and rock types
The reef is a calcisponge reef. The main reefbuilding organisms are calcisponges, with a small amount of bryozoans and Hydrozoans. The main encrusting organisms are blue-green cyanophytes, Archaeolithophyllum and Tubiphytes. The organisms dwelling in the reef include echinoderms (Echinoidea, crinoids), brachiopods, foraminifers, gastropods, bivalves, and red algae.
3.1.
Reef composition
3.1.1. Reef-building organisms The reef-building organisms of the Lengwu reef are mainly the following groups:
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Fig. 1 Geological location of the Lengwu reef in southern China and sedimentary environments.
1) Calcisponges: They are the main reef-builders of Lengwu reef, which include inozoans, sphinctozoans, and sclerosponges. The inozoans, accounting more than 80%, are dominant in amount (Fig. 3A, B). 2) Bryozoans: The content of bryozoans is low, accounting less than 10% in amount, but constitutes one of the important reef-building groups (Fig. 3C). 3) Hydrozoans: The content of hydrozoans is much less than calcisponges, with about 15% in amount, but constitutes a very important reef-building group. They include two types: one is plate-like, and the other is massive (Fig. 3D). 4) Algae: Algae are distributed generally encrusted on skeletons of calcisponges, hydrozoans and bryozoans and other reef-building organisms, which fix and bind them. The dominant genera are Archaeolithoporella, Archaeolithophyllum, and Tubiphytes (Fig. 3E, F).
3.1.2. Reef-dwelling organisms The reef-dwelling organisms are numerous (Fig. 4), including brachiopods, bivalves, echinoderms (crinoids, echinoids), gastropods, ostracods, and foraminifers.
3.1.3. Filling deposits The deposits filling the interspaces among the reef-building organisms are mainly skeletal debris and a small amount of intraclasts, with a small amount of sparry calcite. The content of the fillings mainly depends on the content of the reef-building organisms: the more of the former then the less of the latter.
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Table 1 Stratigraphic subdivision of the Mid-Upper Permian of the study area.
3.1.4. The framework pores The framework pores, that is, the pores between the reef-builders, have mostly been cemented by calcite in the late burial diagenetic stages.
3.2.
Reef rock types
Five reef rock types were developed in the Lengwu reef. They are framestone, bafflestone, rudstone, bindstone and bioclastic wackestone. Calcisponge framestone and bafflestone are the primary rock types, rudstone is the secondary one and bioclastic wackestone and packstone are the less important types. 1) Framestone The framestone is mainly distributed at Beds 11, 14 and 21 (Fig. 2), with a maximum thickness of 2.5 m. It was formed by wave-resistant skeletons of the in situ reef-building organisms, with micrites and
biological debris filling the interspaces, or being cemented by sparry calcite. The in situ biological skeleton grew densely, often touched each other (The average width of the skeleton is less than 0.1 m) (Wu, 1992). The organisms constructing the framework are mainly inozoans, sphinctozoans, and sclerosponges, hydrozoans, and bryozoans (Fig. 5). The shapes of the inozoans are columnar, branched or conical. The sclerosponges are often columnar and in other shapes. The content of the reef-building organisms is often 40%e60%, occasionally reaching about 80%, with that of reef-dwelling organisms usually 10%e20%, and that of filling materials 20%e40%. A small amount of micrite filled some interspaces between the skeletons, with several generations of sparry calcite cementation filling most interspaces. Since the micrite content is lower and the framestone is resistant to compression, the skeleton pores and a variety of dissolved pores are often preserved, and some of the pores were filled with calcite and bitumen.
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Fig. 2 Lithology and sedimentary facies of Lengwu reef in Tonglu, Zhejiang Province, southern China.
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Fig. 3 Thin-section photographs showing reef-building organisms. AeDendritic calcisponge (longitudinal section); BeColumnar calcisponge (cross section); CeBryozoan; DeHydrozoan; EeFeArchaeolithoporella.
2) Rudstone The term rudstone here mainly referred to the rock composed of unbroken skeletons that have been knocked down by wave or storms. Rudstone is mainly distributed in Beds 3, 15, 17, and 20 in the reef (Figs. 2 and 6AeD). Different from the common grainstones that formed in many geographic areas of a reef, such as reef front slope and reef flat, rudstones are mainly formed by the reef-builders knocked down by wave action. The skeletons in the rudstone in the Lengwu reef are mainly inozoans, sphinctozoans, and hydrozoans. The matrix among them comprise bioclasts and micrite.
sessile calcified organisms such as calcisponges, coralline algae, crinoids. Stromatoporoids, and bryozoans that have the role of baffling sediments. The bafflestone in the studied profile is mainly distributed in Beds 2, 4, 7, 11 and 16 (Figs. 2 and 6EeF). The main baffling organisms in the Lengwu reef include inozoans, sphinctozoans, a small amount of a hydrozoans, and bryozoans. Some baffling organisms have been preserved in situ, but others in random position. The content of the reef-building organisms in bafflestone is relatively low, generally accounting for about 20%e30%, with micrites accounting for about 50% 4) Bindstone
3) Bafflestone The bafflestone is mainly composed of moderately dense skeletons of reef-building organisms and the bioclasts and micrite trapped by them. It is formed by
The bindstone was mainly formed by crustose Rhodophyta (Archaeolithoporella, Archaeolithophyllum) that encrusted or trapped the reef-building organisms such as calcisponges, bryozoans, and calcareous algae.
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Fig. 4 Thin-section photographs showing reef-dwelling organisms. AeEchinoids and trilobite; BeEchinoids and crinoids; CeBivalves; DeTrilobite; EeBioclastic fragments including echinoids, ostracod and foraminifers; FeBioclastic fragments including echinoids, crinoids and brachiopods.
The interspaces between the skeletons were filled by bioclasts and sparry calcite. The content of reefbuilding organisms accounts for 30%e40%, with reefdwelling organisms 10%e20% and infill deposits 40%e 60%. In the Lengwu reef, the bindstone is mainly distributed in Beds 13, 19 and 22 (Fig. 2, Fig. 7EeF). Because the encrusting layers of sheet-like algae are very thick, up to several centimetres, the bindstone resembles stromatolites. 5) Bioclastic wackestone (Packstone) In the Lengwu reef, bioclastic wackestone and packstone are mainly distributed in Beds 1, 5, 6, 7, 8, 12 and 23 (Fig. 2). The bioclasts are mainly echinoderms (crinoids), ostracods, brachiopods, and foraminifers, as well as a small amount of bryozoans, hydrozoans, calcisponges and rhodophytes (Fig. 8). The matrix of the bioclastic wackestone and packstone contains some terrigenous debris.
4.
Subfacies and microfacies
Reefs can be simple or complex. A simple reef develops only one facies: reef core, whereas a complex reef can develop at least two kinds of facies: reef core and reef flank. A more developed complex reef can develop reef core, reef front, back-reef and reef flat facies. The reef flank facies is generally identified by bedded limestone containing reef rock debris or skeletal debris transported from the reef core (James et al., 1984). Our examination of the outcrops did not find any rock that can be attributed to reef flank rudstone. We measured two profiles and both of them show no feature of reef flank. Thus, we suspect that the Lengwu reef did not develop flank facies. Tentatively, we propose that the Lengwu reef developed only reef core facies. The reef core is mainly composed of one or a few types of the following rocks: framestone, bafflestone, and bindstone.
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Fig. 5 Outcrop and thin-section photographs showing framestone. AeOutcrop photograph of framestone; BeThin-section photograph of calcisponges framestone (cross-section); CeBioclasts between calcisponge skeletons; DeArchaeolithoporella (Algae) encrusting the calcisponges (cross-section); EeFeMicrite between calcisponge skeletons E (longitudinal-section), F (cross-section).
In the Lengwu reef, the reef core is mainly composed of framework microfacies, rudite microfacies, baffled microfacies, and bind microfacies with variable prospectivity of forming reservoirs.
and hydrozoans. The matrix include bioclasts, micrite and other debris.
1) Framework microfacies
It is composed of bafflestones. It is formed by the reef-building columnar or branched calcisponges that baffled micrite and bioclasts. Some grew in situ, and some have been knocked down by wave action, reflecting a lower energy depositional environment with relatively deep water and weak wave action. Because of the higher content of micrite, there is little prospect of this microfacies forming good reservoirs.
It is composed of framestones. The framework formed by calcareous calcisponges grew in situ, mostly in massive and columnar forms, reflecting good water circulation. Framework microfacies can form good reservoir space favourable for hydrocarbon accumulation.
3) Baffled microfacies
2) Rudite microfacies 4) Bind microfacies It is composed of biolystone formed by reefbuilding organisms knocked down by wave action or storm as the reef grew to or near the sea-level, reflecting a very shallow water environment. The overturned skeletons include inozoans, sphinctozoans,
It is a massive reef facies mainly composed of bryozoans, Archaeolithoporella and other binding organisms that encrust on overturned or in situ reefbuilding calcisponge and other bioclasts. It reflects a
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Fig. 6 Outcrop and thin-section photographs showing rudstones and bafflestones. AeOutcrop photograph of rudstone; BeDeThin-section photographs of rudstone. Most calcisponges are distributed along the bedding; EeFeThin-section photographs of bafflestones. The matrix between calcisponge skeletons are bioclasts and micrite.
mid-high energy depositional environment with relatively shallow water and moderately strong wave action. With relatively small content of micrite, more skeletal shelter pores, and various dissolution pores are often preserved, whereas some pores were filled by sparry calcite. Even so, this microfacies can form favourable reservoirs.
5.
Reef-developing sequences
Analyzing and summarizing these rock types and microfacies, the Lengwu reef can be divided into three reef-building periods. Correspondingly, the reef includes 3 sequences in ascending orientation, as reflected in organism composition, lithology and microfacies (Fig. 2). All sequences started with a framestone or bafflestone base, representing reef growth
in a suitable environment, and ended with a wackestone top, representing an unsuitable environment near the sea-level. S1 (B2eB9): Before reef development, a layer of wackestone (B1) formed on the local highland of the carbonate platform. When the sea water was moderately deep, the environment became suitable for reef growth, and thus a layer of calcisponge framestone or bafflestone (B2) developed. When hydrodynamic conditions became stronger, the calcisponges were knocked down by wave or storms, and formed a layer of rudstone (B3). When the water became quieter, the conditions became suitable for reef growth, and the calcisponge framestone (B4) formed. The reef stopped growing when an influx of terrigenous sediments occurred, as indicated by the lenticular siltstone of B5eB6. When the environment became suitable again, the reef began to grow again, forming a layer of calcisponge bafflestone (B7). As an influx of terrigenous
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Fig. 7 Outcrop and thin-section photographs showing bindstone and stromatolites. AeOutcrop photograph of bindstone; BeDeThin-section photographs of bindstones. The encrusting organisms are archaeolithoporella, which encrusted on calcisponges, bryozoans, bioclasts, and terrigenous sediments; EeFeThin-section photographs of stromatolites containing filamentous cyanobacteria.
material happened again, the reef stopped growing again, and a layer of bioclastic mudstone (B8) formed. After that, a layer of mudstone with siltstone (B9) formed. The first sequence ended. The thickness of framestone and bafflestone is relatively small, but that of bioclastic wackestone is greater. The thickness of the reef core of early reef-building period is only 5 m. The total thickness of the first sequence (S1) is 16.8 m. S2 (B10eB13): In the second sequence (S2), as the sea-level rose again, the calcisponge framestone and bafflestone formed (B10eB11). The water depth was relatively shallower than the first reef-building stage, the environment became more open and stable, and reef-building organisms, binding organisms grew widely. The binding organisms Archaeolithophyllum and other crustose algae encrusted on columnar calcisponge, hydrozoans and bryozoans to form bindstones, with bioclastic debris in the matrix. As the reef reached the sea-level again, it died due to an influx of terrigenous materials, which formed a bioclastic
wackestone with silty mudstone (B12). As the sea-level declined, a suite of laminated boundstone formed (B13). The thickness of the reef core of the second reef-building stage including B10 and B11 is 6 m. The total thickness of the second sequence is 15.1 m. S3 (B14eB22): In the third sequence (S3), as the sea-level rose again, the water conditions became suitable, and the calcisponge framestone formed (B14). When the hydrodynamic conditions became stronger, the reef-builders were knocked down by wave and storms, and calcisponge rudstone (B15) formed. Similarly, the calcisponge framestone and bafflestone (B16 and B18) formed and the calcisponge rudstone (B17) formed as well. When the water conditions became suitable, calcisponge laminated bindstone and framestone (B19, B20, B21 and B22) formed. Subsequently, due to influx of terrigenous materials, the reef stopped growing, and a layer of wackestone (B23) formed. This stage was characterized by a large number of laminated bindstone formed by Archaeolithophyllum.
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Fig. 8 Outcrop and thin-section photographs showing bioclastic wackestone. AeOutcrop photograph of bioclastic wackestone; BeFeThinsection photographs of bioclastic wackestone including calcisponge debris and echinoderms fragments.
It contains a small amount of calcisponge framestone, with intermittently occurrence of bioclastic packstones. It mainly contained echinoderms, brachiopods, calcareous algae, gastropods and bryozoan debris. After that, the water became much shallower, and the deposits were mainly intertidal laminated bindstone and bioclastic packstone. Finally, the reef-building history of this area ceased, and was followed by the deposition of the Late Permian Longtan Formation sandstone and shale. The water was the shallowest. The thickness of reef core formed in this stage is 3.4 m. The total thickness of the third sequence is 5.1 m.
6.
Reef types and reef sedimentary model
6.1.
Reef types
According to the palaeogeographical locations, reefs can be divided into fringing reefs, platform marginal reefs (rim reefs) and inner reefs (Sun and
Esteban, 1994; Wang et al., 1998). Fringing reef is also called the edge reef, and is characterized by its location close to the edge of a land, without a lagoon separating it from the land. Platform marginal reef refers to the reef that is located at the marginal zone of a carbonate platform. Inner reef refers to the reef that is located in the inner area of a carbonate platform. The distribution area of the Lengwu reef is less than 1 km2. The strata around it are clastic rocks, implying that the sedimentary environments were not far from the land. Thus, the Lengwu reef was developed on highland of a coastal subtidal environment with terrigenous sediments (Wu, 1998). Some beds of the reef contain clastic sands and muds, indicating occasional influx of terrigenous sediments. The deposits overlying the reef are fluvial sandstone, which implies that the location of the reef was very close to the land. Thus, the reef is a fringing reef that developed in the subtidal shallow water shore zone.
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Reef sedimentary model
Reef development is generally controlled by geographic setting and regional structural action (especially contemporaneous faults). This study, in combination with published researches (Feng et al., 1996) on the regional geology of the study area in the Middle Permian, reveals that the development of the Lengwu reef was constrained by geographic and structural features of the study area during the Middle Permian (Fig. 9). In early Middle Permian Chihsian Stage, a transgression occurred from south to north, which formed the broad shallow carbonate shelf environment in southern China (Yangtze carbonate platform). During the Late Middle Permian Maokouan Stage, the enhanced tectonic activity resulted in a significant change in ancient geography. In the Early-Mid Maokouan Stage, the study area was covered by littoral clastic sediments (Dingjiashan formation). In the Late Maokouan Stage, the sea water depth increased, and differential uplift and subsidence movements occurred, which finally led to the formation of a small isolated carbonate platform in Tonglu Lengwu area on the shallow water clastic continental shelf. As long as the terrigenous detrital material supply was reduced, the sea water became clear, and the salinity was normal, and benthonic organisms such as crinoids,
bryozoans, and brachiopods began to colonize, forming a bioclastic debris base suitable for reef-building organisms to grow. As the condition became favourable, the main reef-building organisms such as calcisponges and hydrozoans began to grow and develop into the reef. Since the reef growth environment was near the shore, the calcisponges and hydrozoans were often knocked down by wave or/and storms, and formed calcisponge rudstone. The crustose algae Archaeolithophyllum encrusted the calcisponges to form a laminated bindstone. As sea-level rose again and marine conditions became suitable, the reef growth resumed. Since sea water invaded and retreated repeatedly on the continental shelf, the growth and demise story of the reef repeated, and in total 3 sequences of reef development were formed (Fig. 10). Then a sheet-like cyanobacteria grew on the skeletal debris to form stromalites.
7.
Conclusions
The reef-building organisms of the Middle Permian Maokouan Stage reef in Tonglu County, Zhejiang Province, South China are mainly calcisponge, with hydrozoans, bryozoans and Archaeolithophyllum, cyanobacteria and other algae playing a secondary
Fig. 9 Palaeogeography of southern China in Mid-Permian Maokouan Stage (The red triangle is the study area) (Modified from Feng et al., 1996).
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Fig. 10 Schematic sedimentary model of Lengwu reef, Tonglu County, Zhejiang Province, southern China.
role. The reef-dwelling organisms are numerous, including brachiopods, bivalves, echinoderms (crinoids, echinoids), gastropods, ostracods, and foraminifers. The rock types of Lengwu reef include calcisponge framestone, biolystone, calcisponge bafflestone, bindstone and bioclastic wackestonepackstone. The alternate appearing of these rock types depends on the oscillating sea-level rise and fall that controlled reef development. The development of the Lengwu reef includes three stages. The first and third ones are short in time, while the second is the longest, and can be regarded as the main reef-building stage. The reef sequences can be divided into three sequences, each ended when the reef grew near the sea-level and stopped growing due to the influx of terrigenous materials. The main controlling factors of the reef development are sea-level changes and influx of clastic sediments. When the rate of sea-level rise was identical with the growth rate of the reef-building organisms, the reef kept growing up. When the two rates did not coincide with each other or with influx of terrigenous materials, the reef died.
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L. Liu et al. Wang, W.Y., 1987. Principal features of Permian reef in Lengwu Area, Zhejiang Province. Experimental Petroleum Geology, 9(4), 299e304. Wang, Y.B., Xu, G.R., Zhang, K.X., 1998. Status and new advances in the studies of Permian reefs in China. Geological Science and Technology Information, 17(1), 35e39. Webb, G.E., 1996. Was Phanerozoic reef history controlled by the distribution of non-enzymatically secreted reef carbonates (microbial carbonates and biologically induced cement)? Sedimentology, 43, 947e971. Wood, R., 1995. The changing biology of reef-building. Palaios, 10, 517e529. Wood, R., Dickson, J., Kirklandgeoge, B., 1994. Turning the Capitan reef upside-down- a new appraisal of the ecology of the Permian Capitan reef, Guadalupe Mountains, Texas and New-Mexico. Palaios, 9(4), 422e427. Wu, Y.S., 1992. Structural rock type and structure phase of the reef. Science in China (Series B), 3, 304e310. Wu, Y.S., 1998. Development mode of Permian reef, Lengwu, Tonglu. Marine and Petroleum Geology, 3(2), 11e15. Xu, H., 1992. Comparative study of organic reef in oil-gasbearing basin of China seas and adjacent areas. Marine Geology & Quaternary Geology, 12(4), 41e45. Xu, G.R., Luo, X.M., Huang, S.J., 1996. Late Permian carbonate buildups generating mechanism, Yangtze River area. Earth Sinica, 17(Suppl. l), 120e129. Yang, W.R., 1995. The sedimentary and diagenesis environments of Permian calcisponge mound in Tonglu, Zhejiang. Journal of Stratigraphy, 19(3), 191e198. Yang, W.R., Li, X., 1995. Permian reef type sand controlling factors of reef formation South China. Acta Palaeontologica Sinica, 34(1), 67e75. Yang, X.L., Shen, A.J., Chen, Z.D., 2002. Genetic types for petroliferous system of organic reefs in the Permian system of South China. Acta Petrolei Sinica, 23(3), 6e10. Zhong, J.H., Wen, Z.F., Li, Y., 2005. Organic reefs study: concept, classification, characteristics, history and development. Geological Review, 51(3), 288e300.
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Lithofacies palaeogeography and sedimentology
Growth characteristics and sedimentary mode of Permian reefs, Lengwu, Tonglu, Zhejiang Province, southern China Ling Liu, Ya-sheng Wu * , Hong-xia Jiang, Hong Liu Institute of Geology and Geophysics, CAS, Key Laboratory of Petroleum Resource Research, Chinese Academy of Sciences, Beijing 100029, China
Abstract Organic reefs are favourable accumulation spaces for hydrocarbons and various mineral resources. A complete Permian organic reef profile about 44 m thick with distribution area no more than 1 km2 is exposed near Lengwu, Tonglu, Zhejiang Province. Examination of outcrops and thin sections revealed that the main reef-building organisms are calcisponges, with inozoans as dominant type. Five types of rocks have been recognized in the reef, and they are calcisponge framestone, calcisponge bafflestone, bindstone, rudstone and bioclastic wackestone. The profile was constructed in three reef-building stages. The thickness of the second stage is largest, followed by the first stage and the third stage is smallest. Each stage started with a framestone or bafflestone, ended as the reef grew near the sea-level, and died because of influx of terrigenous sediments. The development of the Lengwu reef is controlled by biological factors and sea-level changes. Based on the study a reef sedimentary model is established. Keywords Reef, Rock types, Calcisponges, Sedimentary model, Permian, Lengwu, Southern China © 2016 China University of Petroleum (Beijing). Production and hosting by Elsevier B.V. on behalf of China University of Petroleum (Beijing). This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). Received 20 October 2015; accepted 13 June 2016; available online 15 August 2016
1.
Introduction
Reefs are favourable enrichment sites for varieties of mineral resources, and are regarded as a unique “paradise” for hydrocarbon reservoirs (Cai, 2009; Fan, 1996; Michel et al., 1970; Liu, 2007; Lu and Gong, 2007; Xu, 1992; Yang et al., 2002). At the beginning
of the last century (1916), the crude oil production of Well Azul 4 in Mexico prime belt was up to 35 620 t, stimulating people's enthusiasm for reefs research (Song, 2012). The last century was the fastest developing period for reefs research (Zhong et al., 2005). Changes in environmental factors, coupled with the evolution stage of the main reef building organisms, as well as a series of extinction events have
* Corresponding author. E-mail address: [email protected] (Y.S. Wu). Peer review under responsibility of China University of Petroleum (Beijing). http://dx.doi.org/10.1016/j.jop.2016.08.002 2095-3836/© 2016 China University of Petroleum (Beijing). Production and hosting by Elsevier B.V. on behalf of China University of Petroleum (Beijing). This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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controlled the types of ancient reefs (Kiessling et al., 1999; Webb, 1996; Wood, 1995). The diversity and complexity makes the reefs very appealing, but also makes it very difficult to establish widely accepted concepts and classification of reefs (Heckel, 1974). Wood, Dickson, and Kirklandgeoge (1994) made a new appraisal for the ecology of the Permian Capitan Reef, Guadalupe Mountains, Texas and New Mexico, suggesting a detailed ecological reappraisal of the ecology and role of cavities within all parts of the reef, within a well defined stratigraphic and palaeodepth framework. During the Devonian, Permian and Neogene periods, reefs were well developed in China (Liu et al., 2007). In China most studies are regarding the Paleozoic reefs, especially the Permian reefs, and secondly is the Cenozoic reef, since reefs were better developed in the Paleozoic, and less developed in the Cenozoic. The study of Permian reefs in China began in the late 1970s (Wang et al., 1998), with a few formal publications. In the 1980s, Fan (1982, 1996), Fan and Zhang (1987) and Fan and Qi (1990) conducted some detailed studies on the reefs in Lichuan, Hubei Province and other areas in Yunnan, Guizhou, and Guangxi. Their work described the types, petrography, palaeogeography and evolution of the reefs. In addition, Qiang et al. (1985), and Chen et al. (1985) carried out researches on the Permian reefs in Sichuan Basin and eastern Sichuan, focussing mainly on the diagenesis and hydrocarbon reservoir properties of the reefs. In the 1990s, more and more new reefs were discovered, and researches on them involved more aspects of the reefs. Yang and Li (1995) and Xu et al. (1996) made a detailed study on the reef in Cili, Hunan Province, which was constructed not only by calcisponges but also by corals. Subsequently, more Permian reefs or bioherms were found along the mid Yangtze River section, such as Chongyang in Hubei Province, Xiushui in Jiangxi Province, Wuxi in Jiangsu Province and Tonglu in Zhejiang Province. The reef studied in this paper is located at Lengwu, a small village in Yao Lin town, Tonglu County, Zhejiang Province. Since the 1960s, many researchers have studied this reef, such as the Regional Geological Survey of Zhejiang Province, Institute of Geology and Mineral Resources of Zhejiang Province, Petroleum Geology Bureau of East China, Hangzhou Institute of Petroleum Geology, Institute of Geology and Palaeontology CAS, Nanjing University, Tongji University, Zhongshan University, Chinese Academy of Sciences, Petroleum Exploration and Development Research Institute, China National Petroleum Corporation and some other organizations, but there is still much controversy on the nature of the reef. Some researchers
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regarded the reef to have developed on a shelf (Wang, 1987); some believed that it was not a reef, but a biostrome or biological bank (Institute of Geology and Mineral Resources of Zhejiang Province, 1989). Wu (1998) found rudstone mainly consisting of lying skeletons of reef-builders such as calcisponges in the reef, and proposed a reef-developing mode characterized by cycles ended with rudstone formed as the reef grew to the sea-level. Based on recent observations on outcrops and studies of thin sections, this paper deals with the rock types and sedimentary background, and proposes a sedimentary model for the reef development.
2.
Geological background
The studied organic reef is located at Lengwu, Tonglu County, Zhejiang Province (Fig. 1). It is a reef built by calcisponges, and occurred in the Middle Permian Maokouan Lengwu Formation (Table 1). The outcrops of the Lengwu Formation extend in a northeast-southwest direction, for about 1.5 km (Yang, 1995). The base of the formation underlying the reef is gray-black argillite with thin layers of siltstone belonging to the Maokouan Dingjiashan Formation, which contains brachiopods and ammonoid fossils representing continental shelf environment. The strata overlying the reef is the Upper Permian Longtan Formation sandstones and siltstones containing coal seams. The reef has a total thickness of 44 m with distribution area no more than 1 km2. Based on lithology and palaeontological features, the reef profile has been divided into 23 layers from the bottom to the top (Fig. 2).
3.
Reef composition and rock types
The reef is a calcisponge reef. The main reefbuilding organisms are calcisponges, with a small amount of bryozoans and Hydrozoans. The main encrusting organisms are blue-green cyanophytes, Archaeolithophyllum and Tubiphytes. The organisms dwelling in the reef include echinoderms (Echinoidea, crinoids), brachiopods, foraminifers, gastropods, bivalves, and red algae.
3.1.
Reef composition
3.1.1. Reef-building organisms The reef-building organisms of the Lengwu reef are mainly the following groups:
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Fig. 1 Geological location of the Lengwu reef in southern China and sedimentary environments.
1) Calcisponges: They are the main reef-builders of Lengwu reef, which include inozoans, sphinctozoans, and sclerosponges. The inozoans, accounting more than 80%, are dominant in amount (Fig. 3A, B). 2) Bryozoans: The content of bryozoans is low, accounting less than 10% in amount, but constitutes one of the important reef-building groups (Fig. 3C). 3) Hydrozoans: The content of hydrozoans is much less than calcisponges, with about 15% in amount, but constitutes a very important reef-building group. They include two types: one is plate-like, and the other is massive (Fig. 3D). 4) Algae: Algae are distributed generally encrusted on skeletons of calcisponges, hydrozoans and bryozoans and other reef-building organisms, which fix and bind them. The dominant genera are Archaeolithoporella, Archaeolithophyllum, and Tubiphytes (Fig. 3E, F).
3.1.2. Reef-dwelling organisms The reef-dwelling organisms are numerous (Fig. 4), including brachiopods, bivalves, echinoderms (crinoids, echinoids), gastropods, ostracods, and foraminifers.
3.1.3. Filling deposits The deposits filling the interspaces among the reef-building organisms are mainly skeletal debris and a small amount of intraclasts, with a small amount of sparry calcite. The content of the fillings mainly depends on the content of the reef-building organisms: the more of the former then the less of the latter.
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Table 1 Stratigraphic subdivision of the Mid-Upper Permian of the study area.
3.1.4. The framework pores The framework pores, that is, the pores between the reef-builders, have mostly been cemented by calcite in the late burial diagenetic stages.
3.2.
Reef rock types
Five reef rock types were developed in the Lengwu reef. They are framestone, bafflestone, rudstone, bindstone and bioclastic wackestone. Calcisponge framestone and bafflestone are the primary rock types, rudstone is the secondary one and bioclastic wackestone and packstone are the less important types. 1) Framestone The framestone is mainly distributed at Beds 11, 14 and 21 (Fig. 2), with a maximum thickness of 2.5 m. It was formed by wave-resistant skeletons of the in situ reef-building organisms, with micrites and
biological debris filling the interspaces, or being cemented by sparry calcite. The in situ biological skeleton grew densely, often touched each other (The average width of the skeleton is less than 0.1 m) (Wu, 1992). The organisms constructing the framework are mainly inozoans, sphinctozoans, and sclerosponges, hydrozoans, and bryozoans (Fig. 5). The shapes of the inozoans are columnar, branched or conical. The sclerosponges are often columnar and in other shapes. The content of the reef-building organisms is often 40%e60%, occasionally reaching about 80%, with that of reef-dwelling organisms usually 10%e20%, and that of filling materials 20%e40%. A small amount of micrite filled some interspaces between the skeletons, with several generations of sparry calcite cementation filling most interspaces. Since the micrite content is lower and the framestone is resistant to compression, the skeleton pores and a variety of dissolved pores are often preserved, and some of the pores were filled with calcite and bitumen.
Growth characteristics and sedimentary mode of Permian reefs
Fig. 2 Lithology and sedimentary facies of Lengwu reef in Tonglu, Zhejiang Province, southern China.
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Fig. 3 Thin-section photographs showing reef-building organisms. AeDendritic calcisponge (longitudinal section); BeColumnar calcisponge (cross section); CeBryozoan; DeHydrozoan; EeFeArchaeolithoporella.
2) Rudstone The term rudstone here mainly referred to the rock composed of unbroken skeletons that have been knocked down by wave or storms. Rudstone is mainly distributed in Beds 3, 15, 17, and 20 in the reef (Figs. 2 and 6AeD). Different from the common grainstones that formed in many geographic areas of a reef, such as reef front slope and reef flat, rudstones are mainly formed by the reef-builders knocked down by wave action. The skeletons in the rudstone in the Lengwu reef are mainly inozoans, sphinctozoans, and hydrozoans. The matrix among them comprise bioclasts and micrite.
sessile calcified organisms such as calcisponges, coralline algae, crinoids. Stromatoporoids, and bryozoans that have the role of baffling sediments. The bafflestone in the studied profile is mainly distributed in Beds 2, 4, 7, 11 and 16 (Figs. 2 and 6EeF). The main baffling organisms in the Lengwu reef include inozoans, sphinctozoans, a small amount of a hydrozoans, and bryozoans. Some baffling organisms have been preserved in situ, but others in random position. The content of the reef-building organisms in bafflestone is relatively low, generally accounting for about 20%e30%, with micrites accounting for about 50% 4) Bindstone
3) Bafflestone The bafflestone is mainly composed of moderately dense skeletons of reef-building organisms and the bioclasts and micrite trapped by them. It is formed by
The bindstone was mainly formed by crustose Rhodophyta (Archaeolithoporella, Archaeolithophyllum) that encrusted or trapped the reef-building organisms such as calcisponges, bryozoans, and calcareous algae.
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Fig. 4 Thin-section photographs showing reef-dwelling organisms. AeEchinoids and trilobite; BeEchinoids and crinoids; CeBivalves; DeTrilobite; EeBioclastic fragments including echinoids, ostracod and foraminifers; FeBioclastic fragments including echinoids, crinoids and brachiopods.
The interspaces between the skeletons were filled by bioclasts and sparry calcite. The content of reefbuilding organisms accounts for 30%e40%, with reefdwelling organisms 10%e20% and infill deposits 40%e 60%. In the Lengwu reef, the bindstone is mainly distributed in Beds 13, 19 and 22 (Fig. 2, Fig. 7EeF). Because the encrusting layers of sheet-like algae are very thick, up to several centimetres, the bindstone resembles stromatolites. 5) Bioclastic wackestone (Packstone) In the Lengwu reef, bioclastic wackestone and packstone are mainly distributed in Beds 1, 5, 6, 7, 8, 12 and 23 (Fig. 2). The bioclasts are mainly echinoderms (crinoids), ostracods, brachiopods, and foraminifers, as well as a small amount of bryozoans, hydrozoans, calcisponges and rhodophytes (Fig. 8). The matrix of the bioclastic wackestone and packstone contains some terrigenous debris.
4.
Subfacies and microfacies
Reefs can be simple or complex. A simple reef develops only one facies: reef core, whereas a complex reef can develop at least two kinds of facies: reef core and reef flank. A more developed complex reef can develop reef core, reef front, back-reef and reef flat facies. The reef flank facies is generally identified by bedded limestone containing reef rock debris or skeletal debris transported from the reef core (James et al., 1984). Our examination of the outcrops did not find any rock that can be attributed to reef flank rudstone. We measured two profiles and both of them show no feature of reef flank. Thus, we suspect that the Lengwu reef did not develop flank facies. Tentatively, we propose that the Lengwu reef developed only reef core facies. The reef core is mainly composed of one or a few types of the following rocks: framestone, bafflestone, and bindstone.
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Fig. 5 Outcrop and thin-section photographs showing framestone. AeOutcrop photograph of framestone; BeThin-section photograph of calcisponges framestone (cross-section); CeBioclasts between calcisponge skeletons; DeArchaeolithoporella (Algae) encrusting the calcisponges (cross-section); EeFeMicrite between calcisponge skeletons E (longitudinal-section), F (cross-section).
In the Lengwu reef, the reef core is mainly composed of framework microfacies, rudite microfacies, baffled microfacies, and bind microfacies with variable prospectivity of forming reservoirs.
and hydrozoans. The matrix include bioclasts, micrite and other debris.
1) Framework microfacies
It is composed of bafflestones. It is formed by the reef-building columnar or branched calcisponges that baffled micrite and bioclasts. Some grew in situ, and some have been knocked down by wave action, reflecting a lower energy depositional environment with relatively deep water and weak wave action. Because of the higher content of micrite, there is little prospect of this microfacies forming good reservoirs.
It is composed of framestones. The framework formed by calcareous calcisponges grew in situ, mostly in massive and columnar forms, reflecting good water circulation. Framework microfacies can form good reservoir space favourable for hydrocarbon accumulation.
3) Baffled microfacies
2) Rudite microfacies 4) Bind microfacies It is composed of biolystone formed by reefbuilding organisms knocked down by wave action or storm as the reef grew to or near the sea-level, reflecting a very shallow water environment. The overturned skeletons include inozoans, sphinctozoans,
It is a massive reef facies mainly composed of bryozoans, Archaeolithoporella and other binding organisms that encrust on overturned or in situ reefbuilding calcisponge and other bioclasts. It reflects a
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Fig. 6 Outcrop and thin-section photographs showing rudstones and bafflestones. AeOutcrop photograph of rudstone; BeDeThin-section photographs of rudstone. Most calcisponges are distributed along the bedding; EeFeThin-section photographs of bafflestones. The matrix between calcisponge skeletons are bioclasts and micrite.
mid-high energy depositional environment with relatively shallow water and moderately strong wave action. With relatively small content of micrite, more skeletal shelter pores, and various dissolution pores are often preserved, whereas some pores were filled by sparry calcite. Even so, this microfacies can form favourable reservoirs.
5.
Reef-developing sequences
Analyzing and summarizing these rock types and microfacies, the Lengwu reef can be divided into three reef-building periods. Correspondingly, the reef includes 3 sequences in ascending orientation, as reflected in organism composition, lithology and microfacies (Fig. 2). All sequences started with a framestone or bafflestone base, representing reef growth
in a suitable environment, and ended with a wackestone top, representing an unsuitable environment near the sea-level. S1 (B2eB9): Before reef development, a layer of wackestone (B1) formed on the local highland of the carbonate platform. When the sea water was moderately deep, the environment became suitable for reef growth, and thus a layer of calcisponge framestone or bafflestone (B2) developed. When hydrodynamic conditions became stronger, the calcisponges were knocked down by wave or storms, and formed a layer of rudstone (B3). When the water became quieter, the conditions became suitable for reef growth, and the calcisponge framestone (B4) formed. The reef stopped growing when an influx of terrigenous sediments occurred, as indicated by the lenticular siltstone of B5eB6. When the environment became suitable again, the reef began to grow again, forming a layer of calcisponge bafflestone (B7). As an influx of terrigenous
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Fig. 7 Outcrop and thin-section photographs showing bindstone and stromatolites. AeOutcrop photograph of bindstone; BeDeThin-section photographs of bindstones. The encrusting organisms are archaeolithoporella, which encrusted on calcisponges, bryozoans, bioclasts, and terrigenous sediments; EeFeThin-section photographs of stromatolites containing filamentous cyanobacteria.
material happened again, the reef stopped growing again, and a layer of bioclastic mudstone (B8) formed. After that, a layer of mudstone with siltstone (B9) formed. The first sequence ended. The thickness of framestone and bafflestone is relatively small, but that of bioclastic wackestone is greater. The thickness of the reef core of early reef-building period is only 5 m. The total thickness of the first sequence (S1) is 16.8 m. S2 (B10eB13): In the second sequence (S2), as the sea-level rose again, the calcisponge framestone and bafflestone formed (B10eB11). The water depth was relatively shallower than the first reef-building stage, the environment became more open and stable, and reef-building organisms, binding organisms grew widely. The binding organisms Archaeolithophyllum and other crustose algae encrusted on columnar calcisponge, hydrozoans and bryozoans to form bindstones, with bioclastic debris in the matrix. As the reef reached the sea-level again, it died due to an influx of terrigenous materials, which formed a bioclastic
wackestone with silty mudstone (B12). As the sea-level declined, a suite of laminated boundstone formed (B13). The thickness of the reef core of the second reef-building stage including B10 and B11 is 6 m. The total thickness of the second sequence is 15.1 m. S3 (B14eB22): In the third sequence (S3), as the sea-level rose again, the water conditions became suitable, and the calcisponge framestone formed (B14). When the hydrodynamic conditions became stronger, the reef-builders were knocked down by wave and storms, and calcisponge rudstone (B15) formed. Similarly, the calcisponge framestone and bafflestone (B16 and B18) formed and the calcisponge rudstone (B17) formed as well. When the water conditions became suitable, calcisponge laminated bindstone and framestone (B19, B20, B21 and B22) formed. Subsequently, due to influx of terrigenous materials, the reef stopped growing, and a layer of wackestone (B23) formed. This stage was characterized by a large number of laminated bindstone formed by Archaeolithophyllum.
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Fig. 8 Outcrop and thin-section photographs showing bioclastic wackestone. AeOutcrop photograph of bioclastic wackestone; BeFeThinsection photographs of bioclastic wackestone including calcisponge debris and echinoderms fragments.
It contains a small amount of calcisponge framestone, with intermittently occurrence of bioclastic packstones. It mainly contained echinoderms, brachiopods, calcareous algae, gastropods and bryozoan debris. After that, the water became much shallower, and the deposits were mainly intertidal laminated bindstone and bioclastic packstone. Finally, the reef-building history of this area ceased, and was followed by the deposition of the Late Permian Longtan Formation sandstone and shale. The water was the shallowest. The thickness of reef core formed in this stage is 3.4 m. The total thickness of the third sequence is 5.1 m.
6.
Reef types and reef sedimentary model
6.1.
Reef types
According to the palaeogeographical locations, reefs can be divided into fringing reefs, platform marginal reefs (rim reefs) and inner reefs (Sun and
Esteban, 1994; Wang et al., 1998). Fringing reef is also called the edge reef, and is characterized by its location close to the edge of a land, without a lagoon separating it from the land. Platform marginal reef refers to the reef that is located at the marginal zone of a carbonate platform. Inner reef refers to the reef that is located in the inner area of a carbonate platform. The distribution area of the Lengwu reef is less than 1 km2. The strata around it are clastic rocks, implying that the sedimentary environments were not far from the land. Thus, the Lengwu reef was developed on highland of a coastal subtidal environment with terrigenous sediments (Wu, 1998). Some beds of the reef contain clastic sands and muds, indicating occasional influx of terrigenous sediments. The deposits overlying the reef are fluvial sandstone, which implies that the location of the reef was very close to the land. Thus, the reef is a fringing reef that developed in the subtidal shallow water shore zone.
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Reef sedimentary model
Reef development is generally controlled by geographic setting and regional structural action (especially contemporaneous faults). This study, in combination with published researches (Feng et al., 1996) on the regional geology of the study area in the Middle Permian, reveals that the development of the Lengwu reef was constrained by geographic and structural features of the study area during the Middle Permian (Fig. 9). In early Middle Permian Chihsian Stage, a transgression occurred from south to north, which formed the broad shallow carbonate shelf environment in southern China (Yangtze carbonate platform). During the Late Middle Permian Maokouan Stage, the enhanced tectonic activity resulted in a significant change in ancient geography. In the Early-Mid Maokouan Stage, the study area was covered by littoral clastic sediments (Dingjiashan formation). In the Late Maokouan Stage, the sea water depth increased, and differential uplift and subsidence movements occurred, which finally led to the formation of a small isolated carbonate platform in Tonglu Lengwu area on the shallow water clastic continental shelf. As long as the terrigenous detrital material supply was reduced, the sea water became clear, and the salinity was normal, and benthonic organisms such as crinoids,
bryozoans, and brachiopods began to colonize, forming a bioclastic debris base suitable for reef-building organisms to grow. As the condition became favourable, the main reef-building organisms such as calcisponges and hydrozoans began to grow and develop into the reef. Since the reef growth environment was near the shore, the calcisponges and hydrozoans were often knocked down by wave or/and storms, and formed calcisponge rudstone. The crustose algae Archaeolithophyllum encrusted the calcisponges to form a laminated bindstone. As sea-level rose again and marine conditions became suitable, the reef growth resumed. Since sea water invaded and retreated repeatedly on the continental shelf, the growth and demise story of the reef repeated, and in total 3 sequences of reef development were formed (Fig. 10). Then a sheet-like cyanobacteria grew on the skeletal debris to form stromalites.
7.
Conclusions
The reef-building organisms of the Middle Permian Maokouan Stage reef in Tonglu County, Zhejiang Province, South China are mainly calcisponge, with hydrozoans, bryozoans and Archaeolithophyllum, cyanobacteria and other algae playing a secondary
Fig. 9 Palaeogeography of southern China in Mid-Permian Maokouan Stage (The red triangle is the study area) (Modified from Feng et al., 1996).
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Fig. 10 Schematic sedimentary model of Lengwu reef, Tonglu County, Zhejiang Province, southern China.
role. The reef-dwelling organisms are numerous, including brachiopods, bivalves, echinoderms (crinoids, echinoids), gastropods, ostracods, and foraminifers. The rock types of Lengwu reef include calcisponge framestone, biolystone, calcisponge bafflestone, bindstone and bioclastic wackestonepackstone. The alternate appearing of these rock types depends on the oscillating sea-level rise and fall that controlled reef development. The development of the Lengwu reef includes three stages. The first and third ones are short in time, while the second is the longest, and can be regarded as the main reef-building stage. The reef sequences can be divided into three sequences, each ended when the reef grew near the sea-level and stopped growing due to the influx of terrigenous materials. The main controlling factors of the reef development are sea-level changes and influx of clastic sediments. When the rate of sea-level rise was identical with the growth rate of the reef-building organisms, the reef kept growing up. When the two rates did not coincide with each other or with influx of terrigenous materials, the reef died.
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