Biostratigraphy and geography of the Ordovician-Silurian Lungmachi ...

SCIENCE CHINA Earth Sciences • RESEARCH PAPER •

December 2011 Vol.54 No.12: 1854–1863 doi: 10.1007/s11430-011-4301-3

Biostratigraphy and geography of the Ordovician-Silurian Lungmachi black shales in South China FAN JunXuan1*, Michael J MELCHIN 2, CHEN Xu1, WANG Yi1, ZHANG YuanDong1, CHEN Qing1, CHI ZhaoLi1,3 & CHEN Feng1 1

State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China; 2 Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada; 3 Algeria Branch of International Oil and Development Company, SINOPEK, Dongying 257000, China Received November 25, 2010; accepted April 20, 2011

Based on the new material of seven Ordovician-Silurian boundary sections investigated recently, together with previously published data, we analyze the temporal and spatial distributions of the Lungmachi black shales, a key petroleum source bed widely distributed in South China. The Lungmachi black shales range in age from the Normalograptus persculptus Biozone of the uppermost Ordovician to the Spirograptus guerichi Biozone of the lower Telychian, and ten graptolite biozones can be recognized within this unit. The basal and upper contacts of the Lungmachi black shales are diachronous. The basal contact ranges from the N. persculptus to the C. cyphus biozones, a span of five graptolite biozones over two stages. The upper contact ranges from the D. pectinatus-M. argenteus Biozone to the Spirograptus guerichi Biozone, which spans four graptolite biozones over two stages. The Yichang Uplift resulted in the formation of the Hunan-Hubei Submarine High in the border area of Hubei, Hunan, and Chongqing. This is supported by a break in sedimentation in this area spanning all or part of the Hirnantian, and in many areas extending into the underlying Katian and overlying Rhuddanian. Comparison of the distribution of the Katian to Rhuddanian strata in this area indicates a growth and subsequent reduction in area of the Hunan-Hubei Submarine High particularly in the Hirnantian to early Rhuddanian. This may partly represent the influence of the process of formation and melting of ice sheet in Ordovician South Pole and consequent sea level change. South China, Ordovician-Silurian, Lungmachi Formation, graptolitic black shales, petroleum source bed Citation:

Fan J X, Melchin M J, Chen X, et al. Biostratigraphy and geography of the Ordovician-Silurian Lungmachi black shales in South China. Sci China Earth Sci, 2011, 54: 1854–1863, doi: 10.1007/s11430-011-4301-3

Llandovery graptolite-bearing strata are widely distributed on the Yangtze Platform (Figure 1), and are mainly assigned to the Lungmachi Formation on the Upper and Middle Yangtze Platform and Kaochiapien Formation on the Lower Yangtze Platform. The lower part of the Lungmachi Formation, the Lungmachi black shales, contains abundant organic matter and is a key petroleum source bed in China. Its thickness and geographic range are important for our under*Corresponding author (email: [email protected])

© Science China Press and Springer-Verlag Berlin Heidelberg 2011

standing of oil reserves. Although it has long been known that the base and top of the Lungmachi Formation are both diachronous, the distribution of the organic-rich black shales have not previously been precisely defined. Accordingly, in the spring and autumn of 2007, we conducted some field work in central and southwest China, mainly in the Hubei, Hunan, and Sichuan provinces, to investigate the base and top boundaries of the Lungmachi black shales. The present paper is the first report of the results of those investigations. Lee and Chao [2] first named the lower 32 m of graptoearth.scichina.com

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December (2011) Vol.54 No.12

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Figure 1 Distribution of the uppermost Ordovician-Lower Silurian Lungmachi black shales on the Yangtze Platform (Revised from Chen et al. [1]). Dots represent the localities with outcrops of Llandovery graptolite-bearing sediments. Circles represent the localities without outcrops of Llandovery graptolite-bearing sediments. Dark-grey area indicates the area with sediments of the Lungmachi black shales. Light-grey area indicates the area with sediments of the Kaochiapien black shales-the contemporary sediments of the Lungmachi black shales on the Lower Yangtze Platform. Broad solid lines indicate the margin of the Yangtze Platform. Dashed rectangle represents the study area, which is also shown in the following geographic figures.

lite-bearing strata of the Sintan Series as the Lungma Shale in the Yangtze Gorge area. According to their fossil list, their Lungma Shale contains at least some early Silurian strata. Subsequently, while studying the Silurian strata at Luoreping, Yichang, Hubei province, Hsieh and Chao [3] indicated that the lower 7 m of black shales of the section, which yielded abundant graptolites, could be correlated with the Lungma Shale. As the name of the Lungma Shale has been used by Hayden in the Jurassic strata in Tibet, Yin [4] suggested using the name of the Lungmachi Shale as a replacement.

1 Localities and material In 2007, in order to estimate the temporal and spatial distribution of the Lungmachi black shales, we investigated several key sections in central and southwest China, including Laomatou, Zhushan County, Qingquan, Shennongjia (Songbai County), Siyangqiao, Badong County, Gaoluo, Xuan’en County, Sanbaoling, Laifeng County, Taiyanghe, Enshi City and Daozimiao, Jingshan County. The biozonations of these sections are described separately as follows. 1.1

Laomatou, Zhushan County, Hubei Province

This section (31°52′19.5′′ N, 110°03′33.5′′ E) lies along the road near the northwest entrance to the Laomatou village,

40 km southwest of downtown Zhushan. A series of Ordovician and Silurian strata are exposed along the countryside road from southeast to northwest, in ascending order, including Fenhsiang, Hunghuayuan, Dawan, Kuniutan, Miaopo, Pagoda, Linhsiang, Wufeng, and Lungmachi formations. The outcrops were considerably influenced by geological structure, especially the black shales near the boundary of the Wufeng and Lungmachi formations, which were folded and brecciated, making it difficult to collect and identify the fossils. We collected one sample from near the base of the Lungmachi black shales (AFU83), which yields Cystograptus vesiculosus (Nicholson), Paraclimacograptus innotatus (Nicholson), and Normalograptus rectangularis (M’Coy). This sample can be assigned to the Cystograptus vesiculosus Biozone. The sample from the top of the Lungmachi black shales (AFU84) contains abundant graptolites, including Spirograptus guerichi Loydell, Štorch and Melchin, Torquigraptus valens (Přibyl and Münch), and Petalolithus fusiformis Chen and Lin, and can be assigned to the Spirograptus guerichi Biozone. 1.2

Qingquan, Shennongjia (Songbai County), Hubei

This section (31°44′37′′ N, 110°45′4.9′′ E) is situated along the road leading to Qingquan Village, 10 km east of Shennongjia (Songbai County). A continuous transition from the lower black shales to the overlying yellowish or grey-green

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shales and mudstones of the Lungmachi Formation can be well observed at this outcrop. The interbedded black shales in the zone of transition to the overlying yellowish or greygreen shales and mudstones (AFU532) yielded Spirograptus guerichi, Streptograptus barrandei (Suess), Monograptus marri Perner, Torquigraptus decipiens (Törnquist), Torquigraptus proteus (Barrande), and Glyptograptus ex gr. tamariscus (Nicholson), which can be assigned to the Spirograptus guerichi Biozone. In the basal Lungmachi black shales, only the second graptolite biozone above the base of the Silurian, the Parakidograptus acuminatus Biozone, can be recognized at present.


(AFU90), except that the apertural spines of thecae cannot be observed on the Gaoluo specimen. The co-occurring graptolites are typical of the Lituigraptus convolutus Biozone. Accordingly, we assign this level to the upper part of the Lituigraptus convolutus Biozone. We collected an abundant, typical Lituigraptus convolutus Biozone fauna (AFU179) 17.5 m below AFU180, including Lituigraptus convolutus, Cephalograptus tubulariformis (Nicholson), Cephalograptus cometa cometa (Geinitz), Pseudorthograptus inopinatus (Bouček), Petalolithus intermedius, and Rastrites pheloides. 1.5

1.3 Siyangqiao, Badong County, Hubei This section (30°50′8′′N, 110°19′29.7′′E) is situated in Siyangqiao Village, 23.5 km south of downtown Badong. The Pagoda and Linhsiang limestones, Wufeng and Lungmachi graptolitic shales are well exposed along the countryside road. The black shales of the basal Lungmachi Formation are significantly weathered to a grey color. According to Ge et al. [5], Akidograptus sp. occurs at this level, which indicates the questionable presence of the Akidograptus ascensus Biozone because akidgraptids may extend to the overlying Parakidograptus acuminatus biozone at many localities in South China. The top of the Lungmachi black shales (AFU90) yields abundant graptolites including Stimulograptus sedgwickii (Portlock), Torquigraptus decipiens, Campograptus obtusus (Rickards), Normalograptus scalaris (Hisinger), and Pseudoretiolites perlatus (Nicholson), which is a typical Stimulograptus sedgwickii Biozone fauna. Above this level are the gray and yellowish shales and silty shales of the upper Lungmachi Formation. 1.4

Gaoluo, Xuan’en County, Hubei

This section (29°43′26.9′′N, 109°29′41.8′′E) lies along the road northwest of Gaoluo, 33 km southeast of Xuan’en. A sequence of Ordovician and Silurian strata is well exposed along the road, including the Nantsinkuan, Fenhsiang, Hunghuayuan, Zitai, Pagoda, Wufeng, and Lungmachi formations, in ascending order. The boundary strata of the Wufeng and Lungmachi formations are washed by mountain springs and it is hard to collect and identify the graptolites from this interval. Ge et al. [6] reported the occurrence of Cystograptus vesiculosus, which indicates the presence of the C. vesiculosus Biozone. The upper part of the Lungmachi Formation is weathered and well preserved. The top of it (AFU180) yields the graptolites Lituigraptus pheloides, Stimulograptus cf. sedgwickii (Portlock), Monoclimacis crenularis (Lapworth), Campograptus ex gr. communis (Lapworth), and Petalolithus palmeus clavatus (Bouček and Přibyl). The fragment of Stimulograptus cf. sedgwickii matches well with the abundant specimens of Stimulograptus sedgwickii from the Siyangqiao section

Sanbaoling, Laifeng County, Hubei

There is a well preserved Ordovician-Silurian boundary section (29°35′55.3′′N, 109°18′45.6′′E) at Sanbaoling Village, Sanhu, Laifeng County, 22 km southwest of Gaoluo. The top of the Wufeng black shales belongs to the D. complexus Biozone, which is disconformably overlain by the Lungmachi black shales. A sample 20 cm above the base of the Lungmachi Formation yields Parakidograptus acuminatus, which indicates the presence of the P. acuminatus Biozone. Therefore, four graptolite biozones from the Katian Paraorthograptus pacificus Biozone to the early Rhuddanian Akidograptus ascensus Biozone are absent at the locality. 1.6

Taiyanghe, Enshi City, Hubei

This section (30°34′7.3′′N, 109°32′53.2′′E) is situated along the road 4 km southeast of Taiyanghe. The Upper Ordovician to Llandovery rocks, including the Pagoda, Linhsiang, Wufeng, and Lungmachi formations, are well exposed along the road. The top of the Wufeng black shales yields Dicellograptus anceps (Nicholson) and Appendispinograptus supernus (Elles and Wood). Ge et al. [5] found D. complexus (=D. szechuanensis Mu) from the same level, which indicates the existence of the D. complexus Biozone. The base of the disconformably overlying Lungmachi black shales belongs to the Coronograptus cyphus Biozone. The upper part of the Lungmachi black shales is not preserved in this new road-cut section. The highest level of the present outcrop (AFU138) yields Agetograptus cf. spiniferus Obut and Sobolevskaya, Pseudorthograptus iropinatus, Pseudoretiolites perlatus (Nicholson), Agetograptus primus, Rastrites approximatus Perner, and Coronograptus gregarius (Lapworth), which are probably indicative of the Demirastrites pectinatus-Monograptus argenteus Biozone. Ge et al. [6] found Stimulograptus sedgwickii from the top of the Lungmachi black shales at this locality and thus proved the existence of the highest biozone of the Aeronian Stage at this section. The disconformity between the Wufeng and Lungmachi black shales results in the absence of six graptolitic biozones from the Katian Paraorthograptus pacificus Biozone to

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the mid Rhuddanian Cystograptus vesiculosus Biozone. 1.7

Daozimiao, Jingshan County, Hubei

This section (30°59′54.5′′N, 113°06′20.9′′E) lies along the road at Daozimiao, Xinshi Town, 3 km southwest of downtown Jingshan. The lower part of the section is continuous outcrop from the Pagoda and Linhsiang limestones to the Wufeng black shales, which are disconformably overlain by the lower part of the Lungmachi black shales. The upper part of the Lungmachi black shales is exposed discontinuously along the road. The uppermost sample of the section (AFU66) belongs to the Lituigraptus convolutus Biozone and yields Pseudoglyptograptus n. sp. sensu Melchin (1989) [7], Lagarograptus inexpeditus and Lituigraptus richteri (Perner). The top of the Wufeng black shales (AFT102) is assigned to the Diceratograptus mirus Subzone of the Paraorthograptus pacificus Biozone, and includes abundant DDO (Dicranograptidae-Diplograptidae-Orthograptidae [8]) fauna, such as Diceratograptus mirus Mu, Appendispinograptus supernus, Dicellograptus ornatus Elles and Wood, Anticostia uniformis (Mu and Lin), Amplexograptus latus (Elles and Wood), and Climacograptus hastatus T. S. Hall. The basal shales of the Lungmachi Formation (AFT103-104) are significantly weathered, obscuring the graptolite specimens. The lowest two samples representing OrdovicianSilurian transition include only N. mirnyensis and N. avitus. N. avitus first occurs in the N. persculptus Biozone and can extend to lower Rhuddanian, and N. mirnyensis spans the Hirnantian and lower Rhuddanian. In the overlying strata, AFT105 yields Normalograptus bicaudatus (Chen and Lin) and N. rhizinus, and AFT105A and AFT106 both yield A. ascensus. Normalograptus bicaudatus (Chen and Lin) first occurs in the A. ascensus Biozone according to records in South China, so the base of the AFT105 collecting layer may indicate the base of the A. ascensus Biozone and AFT102-104 may represent the N. persculptus Biozone. Therefore, only the top of the Wufeng Formation, which coincides with the N. extraordinarius Biozone, is absent at this locality. The basal strata of the Lungmachi black shales are continuous from the uppermost part of the Hirnantian.

2 Biozonation Based on the present material and previously published data, we can establish a continuous graptolite sequence for the Lungmachi black shales from uppermost Ordovician to Early Silurian in this area. This sequence contains 10 graptolite biozones, including one biozone of the Hirnantian Stage, four biozones of the Rhuddanian Stage, four biozones of the Aeronian Stage, and the basal biozone of the Telychian Stage (Table 1). The present graptolite biozonation can be correlated well with previous work [9–12] (Figure 1).


2.1

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Hirnantian Stage (Late Ordovician)

At those sections that are continuous across the OrdovicianSilurian boundary on the Yangtze Platform, the Lungmachi black shales commonly conformably overlie the Kuanyinchiao Formation, which yields an abundant and diverse Hirnantia shelly fauna [13, 14]. The base of the Lungmachi black shales at these localities, such as Wangjiawan (Loc. 110, Figure 1) and Fenxiang (Loc. 109, Figure 1), Yichang, Hubei, and Shichang, Renhuai, Guizhou (Loc. 61, Figure 1), contains a typical N. persculptus Biozone graptolite fauna, including N. persculptus (Elles and Wood), N. avitus (Davies), N. minor (Huang), and Neodiplograptus shanchongensis (Li) [15]. A few components of the Ordovician DDO fauna, which survived from the first, major event of the Late Ordovician graptolite extinction, such as Paraorthograptus pacificus and Amplexograptus latus, may occur in this interval in very low abundance [16, 17]. 2.2 Rhuddanian Stage (Llandovery, Silurian) The Rhuddanian Stage is composed of four graptolite biozones, the Akidograptus ascensus, Parakidograptus acuminatus, Cystograptus vesiculosus, and Coronograptus cyphus biozones in ascending order. Each of these biozones can be recognized by the appearance of the eponymous species. The lower two biozones are widely distributed on the Yangtze Platform [14]. Our zonation for the Rhuddanian differs from that of Mu et al. [18] only in that the N. persculptus Biozone is recognized as the uppermost Ordovician rather than the lowest Silurian, and the overlying Parakidograptus acuminatus Biozone is subdivided into a lower A. ascensus and an upper P. acuminatus biozones. Our biozonation for the Rhuddanian matches the global biozonation employed by Melchin et al. [19]. Since the base of the Silurian System was formally defined [20], the Parakidograptus acuminatus Biozone has been taken as the first graptolite biozone of Silurian, marked by the first appearance of Parakidograptus acuminatus s.l.. However, some graptolite experts [21, 22] suggested that, at the Linn Branch section at Dob’s Linn, the GSSP section for the base of the Silurian System, the Parakidograptus acuminatus s.s. first occurs 1.5 m above the level of the GSSP. The specimens identified as Parakidograptus acuminatus s.l. by Williams [23] that were used to define the base of the P. acuminatus Biozone are best assigned to Parakidograptus praematurus (Davies), which first occurs at the level of the GSSP, together with Akidograptus ascensus. Therefore, the Parakidograptus acuminatus Biozone can be divided into two biozones, the lower Akidograptus ascensus Biozone and the upper Parakidograptus acuminatus Biozone [19]. This revised biostratigaphic definition of the GSSP for the Base of the Silurian System has recently been accepted by the International Commission on Stratigraphy and the International Union of

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Table 1 Biozonation of the Lungmachi black shales on the Yangtze Platform a)

a) The correlation is mainly based on the first appearances of zonal fossils. Because Rastrites and Petalolithus only occur in the Aeronian and higher strata, the C. gregarius Biozone at Tongzi may be correlative with the D. triangulatus Biozone, and the C. cyphus-M. lunata Biozone at Ningqiang at least contains some samples from the overlying strata of the lower Aeronian (Chen Xu, personal communication, 2009). Oktavites communis ranges from the D. triangulatus Biozone to the L. convolutus Biozone, thus the occurrence of O. communis can only indicate early-middle Aeronian in age.

Geological Sciences [24]. The base of the Silurian System is still at the same level of 1.6 m above the base of the Birkhill Shale Formation; however, it is marked by the first appearance of Akidograptus ascensus rather than Parakidograptus acuminatus. This succession of lower Rhuddanian biozones can be recognized in South China, Kazakstan, Bohemian, Poland, Argentina, Tibet, and other regions [14, 25]. Based on study by Chen et al. [14] and our recent study, the FAD of Akidograptus ascensus in South China is synchronous with that at most localities in the world, and thus we can use that to mark the base of the Silurian System in South China. The typical components of the Akidograptus ascensus Biozone fauna include Akidograptus ascensus, Parakidograptus praematurus, N. anjiensis (= N. lubricus), and N. bicaudatus. The common elements of the Parakidograptus acuminatus Biozone fauna include P. acuminatus, Cystograptus ancestralis, and Hirsutograptus sinitzini.

The C. vesiculosus and Coronograptus cyphus biozones can be found in many parts of the Yangtze region and previous systematic and biostratigraphic studies of this interval include those of Chen and Lin [11], Fang et al. [26] and Li [27, 28]. 2.3 Aeronian Stage (Llandovery, Silurian) The Aeronian Stage consists of four graptolitic biozones, including the Demirastrites triangulatus, Demirastrites pectinatus-Monograptus argenteus, Lituigraptus convolutus, and Stimulograptus sedgwickii biozones in ascending order. Most previous graptolite biozonations in South China have recognized only a single biozone in the lower Aeronian, referred to as either the Coronograptus gregarius or Demirastrites triangulatus Biozone [18, 27]. Conversely, in parts of Europe and some other parts of the world, the lower Aeronian can be subdivided into at least three biozones.

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Koren’ et al. [29] and Melchin et al. [19] globally subdivided the lower Aeronian into a combined, lower Demirastrites triangulatus-Demirastrites pectinatus and upper Monograptus argenteus Biozone. We have been able to distinguish the Demirastrites triangulatus Biozone as a distinct biostratigraphic unit here, but more detailed sampling will be required to resolve separate Demirastrites pectinatus and Monograptus argenteus biozones. The upper Aeronian Lituigraptus convolutus and Stimulograptus sedgwickii biozones are equivalent to those defined globally by Koren’ et al. [29] and Melchin et al. [19] and previously employed in South China [18]. We have been able to recognize each of the Aeronian graptolite biozones by the appearance of the eponymous species, except the Demirastrites pectinatus-Monograptus argenteus Biozone, which is indicated by the presence of species such as Rastrites approximatus (Daozimiao and Taiyanghe sections), Monograptus paradenticulatus (Daozimiao section), and Pseudoglyptograptus n. sp. sensu Melchin (1989) [7] (Daozimiao section). The graptolite biostratigraphy of Aeronian strata in South China has already been studied in detail [10, 27]. Some common graptolite genera of Aeronian include Pseudorthograptus, Rastrites, Demirastrites, Metaclimacograptus, Cephalograptus, and Petalolithus.

Figure 2


2.4

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Telychian Stage (Llandovery, Silurian)

Based on present material, we can only recognize the basal graptolite biozone of the Telychian Stage, the Spirograptus guerichi Biozone in the area of Zhushan and Shennongjia of NW Hubei, on the Yangtze Platform. Spirograptus guerichi is the senior synonym of Spirograptus minor (Bouček), which has been reported in Chengkou [12] and Guangyuan [10] of the Sichuan Province, and Ningqiang and Nanzheng of the Shaanxi Province [10].

3 Discussion 3.1

Yichang Uplift

Early Rhuddanian (the A. ascensus to P. acuminatus biozones) black shales of the lower Lungmachi Formation are widely distributed on the Yangtze Platform (Figure 2). Chen et al. [1] indicated that the Yichang Uplift [30] resulted in the formation of the Hunan-Hubei Submarine High in the border area between the Hubei, Hunan provinces and the Chongqing Municipality (Figure 2). In this area, at least the two graptolite biozones of lower Rhuddanian are absent. Our new material, together with some previously published studies [1, 6, 9, 14, 30–32]1), 2), supports this interpretation,

Distribution model of the early Rhuddanian Lungmachi black shales on the Yangtze Platform.

1) Bureau of Geology of Sichuan Province. Geological Report of the Qianjiang Area (with Geological Map of 1:200000) (in Chinese). Chengdu: Bureau of Geology of Sichuan Province, 1975. 1–107 2) Bureau of Geology of Hubei Province. Geological Report of the Yichang Area (with Geological Map of 1:200000) (in Chinese). Wuhan: Bureau of Geology of Hubei Province, 1970. 1–124

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but shows that the pattern and model of the development of this feature are more complicated than previously thought. It is notable that, although the break in sediments in this area is supported by the material present here, the cause of the break is still unknown, which could be the result of no sedimentation at all or a short-term sedimentation following by erosion. Therefore, we still use the term and definition of “Hunan-Hubei Submarine High” by Chen et al. [1] in the present paper. Hirnantian (N. extraordinarius and N. persculptus biozones) sediments are absent at the following localities: Taiyanghe (Enshi), Gaoluo (Xuan’en), Sanbaoling (Laifeng), Siyangqiao (Badong), Qianhe (Wufeng), Huaqiao (Changyang), Xiejiaping and Xizhai (Songzi), Maohutang (Yidu), Longchihe (Shimen), Erfangping (Cili), and Wentang (Zhangjiajie). However, at some localities outside this area, such as Xintan and Longmaxi (Zigui), Huanghuachang and Wangjiawan (Yichang), Ganhegou (Qianjiang) and Jiuxi


(Taoyuan), the Hirnantian strata are well developed. So, the geographic range of the Hunan-Hubei Submarine High during the Hirnantian can be well constrained by the distribution of sites of non-deposition (Figure 3). Early Rhuddanian (A. ascensus and P. acuminatus biozones) sediments are absent at the following localities: Taiyanghe (Enshi), Gaoluo (Xuan’en), Huaqiao (Changyang), Maohutang (Yidu), Longchihe (Shimen), Erfangping (Cili), and Wentang (Zhangjiajie). However, at some localities outside this area, such as Siyangqiao (Badong), Xintan and Longmaxi (Zigui), Huanghuachang and Wangjiawan (Yichang), Xiejiaping (Songzi), Sanbaoling (Laifeng), Ganhegou (Qianjiang) and Jiuxi (Taoyuan), the lower Rhuddanian black shales are well developed. So, the geographic range of the Hunan-Hubei Submarine High can be constrained for the early Rhuddanian (Figure 3). These observations suggest that, from the Hirnantian to the early Rhuddanian, the geographic range of the Hunan-

Figure 3 Temporal and spatial distribution of the Hubei-Hunan Submarine High during the Hirnantian and early Rhuddanian. 1, Siyangqiao, Badong (ref. [6] and the present paper); 2, Taiyanghe, Enshi (ref. [6] and the present paper); 3, Gaoluo, Xuan’en (ref. [6] and the present paper); 4, Sanbaoling, Laifeng (the present paper); 5, Ganhegou, Qianjiang1); 6, Xintan and Longmaxi, Zigui [31]; 7, Wangjiawan, Yichang [9, 14]; 8, Huanghuachang, Yichang [9]; 9, Huaqiao, Changyang [32]; 10, Qianhe, Wufeng [31]; 11, Maohutang, Yidu2); 12, Xiejiaping, Songzi [31]; 13, Xizhai, Songzi [31]; 14, Longchihe, Nishi, Shimen [6]; 15, Erfangping, Cili [1]; 16, Jiuxi, Taoyuan [1]; 17, Wentang, Zhangjiajie [30]; 18, Laomatou, Zhushan (the present paper); 19, Qingquan, Shennongjia (ref. [6] and the present paper); 20, Daozimiao, Jingshan (the present paper).

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Hubei Submarine High was gradually reduced; its border moved inward for several tens of kilometres. It is now widely accepted that there was a global glacial event during the late Katian-early Hirnantian. The gradual growth of the ice sheet at the Ordovician South Pole region during the early-middle Hirnantian [33] resulted in a sustained sea level drop globally, which reached a peak at the middle Hirnantian. Rong [13] suggested that the sea level may have dropped 50–100 m in South China. A significant mass extinction event happened in the marine biota during this time interval. As a result, about 75% of genera and 66% of families became extinct during the Hirnantian in South China [34]. During the mid Hirnantian, when the first and major episode of the graptolite extinction ended, a widelydistributed cool-water Hirnantia shelly fauna flourished on the Yangtze Platform. During the late Hirnantian, the rapid melting of the Gondwana ice sheet resulted in sea level rise, global warming, and the extinction of the cool-water Hirnantia fauna. The period of post-glacial sea level rise continued into the Rhuddanian. The change in the geographic range of the Hunan-Hubei Submarine High in Figure 3 may partly represent the process of ice sheet formation and subsequent melting that influenced sea level and sedimentation patterns in South China. Chen et al. [30] indicated that a regional tectonic movement, the Guangxi movement could also result in absence of the strata in this area. However, the evidence of the effect of the Guangxi movement in this area is still insufficient at present. Our evidence suggests that the Hunan-Hubei Submarine High had already existed before the Hirnantian. The present material from Taiyanghe (Enshi) and Sanbaoling (Laifeng) indicates non-deposition in the Katian P. pacificus Biozone in these localities. Moreover, previously published data show a pattern of absence of sediments for all or part of the Katian in a number of regions such as Siyangqiao (Badong), Gaoluo (Xuan’en), Qianhe (Wufeng), Xiejiaping and Xizhai

Figure 4


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(Songzi), Maohutang (Yidu), Longchihe (Shimen). Outside this area, there are complete Katian sequences at Ganhegou (Qianjiang), Wentang (Zhangjiajie), Jiuxi (Taoyuan), Erfangping (Cili), Xintan and Longmaxi (Zigui), and Huanghuachang and Wangjiawan (Yichang). Although these data are insufficient to describe the detailed evolution of the Hunan-Hubei Submarine High in the Katian, the existence of the submarine highland can be confirmed. Further field work in this area will reveal the geographic ranges of this submarine highland in different intervals in pre-Hirnantian and help establish a more comprehensive model of its evolution. 3.2

Basal boundary of the Lungmachi black shales

The currently available evidence suggests that the basal contact of the Lungmachi black shales takes one of two forms (Figure 4). (1) In some localities, such as Wangjiawan and Fenxiang (Yichang), Shichang (Renhuai) and Hanjiadian (Tongzi), Lungmachi black shales conformably overly the middle Hirnantian Kuanyinchiao limestones, which yields abundant cool-water Hirnantia fauna. The basal strata of the Lungmachi black shales represent the upper Hirnantian N. persculptus Biozone and extend continuously into the Lower Silurian. (2) At other localities the Lungmachi black shales disconformably overly the Katian Wufeng black shales or Linhsiang limestones. The base of the Lungmachi Formation is diachronous from the N. persculptus Biozone to the C. cyphus Biozone. For example, at Daozimiao (Jingshan), the Lungmachi black shale strata of the N. persculptus Biozone disconformably overly Wufeng black shales of the P. pacificus Biozone. At Sanbaoling (Laifeng) and Siyangqiao (Badong), the basal strata of the Lungmachi black shales belong to the A. ascensus Biozone; at Gaoluo

Temporal and spatial distribution of the Lungmachi black shales on the Yangtze Platform.

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(Xuan’en) and Laomatou (Zhushan), the basal strata of the Lungmachi black shales belong to the C. vesiculosus Biozone; at Taiyanghe (Enshi) and Qingquan (Shennongjia), the basal strata of the Lungmachi black shales belong to the Coronograptus cyphus Biozone. 3.3

The upper contact of the Lungmachi black shales

The upper contact of the Lungmachi black shales throughout the study area is marked by a gradational transition to the grey and yellowish shales and siltstones of the upper Lungmachi Formation. However, our biostratigraphic data suggest that this gradational contact is diachronous. The biozonation of the top of the Lungmachi black shales can be classified as follows (Figure 4). (1) At some localities, such as Wangjiawan (Yichang) and Daozimiao (Jingshan), the top of the Lungmachi black shales belongs to the D. pectinatus-M. argenteus Biozone. (2) At localities like Gaoluo (Xuan’en), the top of the Lungmachi black shales belongs to the L. convolutus Biozone. (3) At Taiyanghe (Enshi) and Siyangqiao (Badong), the top of the Lungmachi black shales belongs to the S. sedgwickii Biozone. (4) At localities such as Qingquan (Shennongjia) and Laomatou (Zhushan), the top of the Lungmachi black shales belongs to the Spirograptus guerichi Biozone. According to the presently available data, the upper contact of the Lungmachi black shale reaches a stratigraphically higher level in the northwest part of the Yangtze Platform, which is close to Chengkou (Chongqing City) and the Qinling mobile belt (e.g., Bajiaokou, Ziyang County, Shaanxi Province). To the east toward Daozimiao (Jingshan), and to the south toward Gaoluo (Xuan’en), the top of the Lungmachi black shales occurs at a lower level. Stratigraphic and chemostratigraphic data from a wider range of localities will be required to understand the cause of this diachroneity.


Hunan-Hubei Submarine High in the border area of Hubei, Hunan, and Chongqing. This is supported by a break in sedimentation in this area spanning all or part of the Hirnantian, and in many areas extending into the underlying Katian and overlying Rhuddanian. Comparison of the distribution of the Katian to Rhuddanian strata in this area indicates a growth and subsequent reduction in area of the Hunan-Hubei Submarine High particularly in the Hirnantian to early Rhuddanian, which may partly represent the influence of the process of formation and melting of ice sheet in Ordovician South Pole and consequent sea level change. We thank Yuan Wenwei, Xu Honghe, Chen Junfeng and Chen Jincun for their helps in the fieldwork. This paper benefited considerably from the constructive reviews of Profs. Rong Jiayu, Zhan Renbin, and Feng Hongzhen. The reviewers and Dr. Miao Desui helped us a lot in improving the manuscript. This study was supported by Chinese Academy of Sciences (Grant No. KZCX2-EW-111), National Natural Science Foundation of China (Grant Nos. 40839910 and 40772002), and Natural Sciences and Engineering Research Council of Canada (MJM). We also thank St. Francis Xavier University for providing a James Chair Professorship to FJX. This is a contribution to the Geobiodiversity Database project (www.geobiodiversity.com) and IGCP Project 591.

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4 Conclusions (1) Throughout our study area, the Lungmachi black shales range in age from the Normalograptus persculptus Biozone of the uppermost Ordovician to the Spirograptus guerichi Biozone of the lower Telychian, and ten graptolite biozones can be recognized within this unit. (2) The basal and upper contacts of the Lungmachi black shales are diachronous. The basal contact ranges from the N. persculptus to the C. cyphus biozones, a span of five graptolite biozones over two stages. The upper contact ranges from the D. pectinatus-M. argenteus Biozone to the Spirograptus guerichi Biozone, which spans four graptolite biozones over two stages. (3) The Yichang Uplift resulted in the formation of the

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