China Geology 3 (2018) 354−366

China Geology

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Ordovician sequence stratigraphy and correlation in the Middle-Upper Yangtze region, South China Chuan-shang Wang*, Xiao-feng Wang, Xiao-hong Chen, Zhi-hong Li, Xu-bing Li

Wuhan Center of China Geological Survey, Wuhan 430223, China

A R T I C L E I N F O A B S T R A C T

Article history: The precise correlation of the Ordovician successions in different facies is difficult due to the complex Received 16 July 2018 changes of the tecto-paleogeography in South China. Based on previous studies, the authors recognized 10 Received in revised form 16 July 2018 rd Accepted 16 August 2018 3 grade sequences, i. e., Osq1 to Osq10 in ascending order, by the integrated study of chronostratigraphy, Available online 16 August 2018 biostratigraphy as well as the analysis on the sedimentary facies. The authors are confident to correlate the

Ordovician successions precisely in different facies in an isochronous stratigraphic framework with the Keywords: application of the method of sequence stratigraphy and correlation.

Sequence stratigraphy Correlation ©2018 China Geology Editorial Office. Different facies

Ordovician

1. Introduction The age of the Ordovician rocks is constrained by the graptolite biozonation and the newest chronostratigraphic The Ordovician successions are well developed and framework is given in Table 1 according to the previous extensively distributed in South China, especially in Middle- studies by the Ordovician experts (Wang XF et al., 1987, Upper Yangtze region (Fig. 1). Chronostratigraphic and 1995, 1996, 2001, 2003, 2009; Chen X et al., 1995, 2000, biostratigraphic studies are carried out on the typical 2003, 2006). Ordovician sections with complete successions in various The Three Gorges area is the most typical area for the facies and with diverse fossils in this region (Mu EZ et al., study on the Ordovician stratigraphy in China. The experts 1979; An TX, 1987; Wang XF et al., 1987; Chen X et al., studied the evolution of the Ordovician paleo-environment 1995, 2003; Chen XH et al., 1996, 2002). The Hirnantian and and the facies changes, sea level changes and classification of Dapingian GSSPs have been approved in 2006 in the the sequence stratigraphy, various fossils and biostratigraphy Wangjiawan section and 2007 in the Huanghuachang section of the Ordovician successions, etc. (Chen X et al., 1986; respectively, in the center of the Yangtze craton near Yichang Wang XF et al., 1987; Zeng QL et al., 1983). The systematic since the first GSSP of the Ordovician System, i.e. the studies on the Ordovician sea level changes and sequence Darriwilian GSSP, ratified in 1997 in the Huangnitang section stratigraphy have been carried out since middle 1990s (Wang of Zhejiang Province (Chen X and Bergström SM, 1995; XF et al., 1995, 1996; Li ZM et al., 1997; Chen XH and Wang Chen X et al., 2006; Wang XF et al., 2005, 2009). Although XF, 1999; Wang CS et al., 2002, 2003). rd great progress on the Ordovician chronostratigraphy and Based on previous studies, the authors recognized 10 3 biostratigraphy was achieved, the precise correlation of the grade sequences, i. e., Osq1 to Osq10 (Table 1) in ascending Ordovician successions in different facies is still difficult due order, by the integrated study of chronostratigraphy, to the complex changes of the tecto-paleogeography in South biostratigraphy as well as the analysis on the sedimentary China. The application of sequence stratigraphy and facies (Fig. 2, Fig. 3). correlation makes it possible to precisely correlate Ordovician successions in different facies in an isochronous stratigraphic 2. The classification and correlation on the sequence stratigraphy of the Lower Ordovician framework.

* Corresponding author: E-mail address: [email protected] (Chuan-shang Wang). 2.1. The first 3rd grade sequence of the Ordovician (Osq1) doi:10.31035/cg2018041 2.1.1. The platform facies zone 2096-5192/© 2018 China Geology Editorial Office. The sequence boundary surface of Osq1 coincides with Wang et al. / China Geology 3 (2018) 354−366 355

Table 1. The correlation of the Ordovician successions in the middle-upper Yangtze region.

Chrono- Biostratigraphy Lithostratigraphy stratigraphy

Graptolite zone* Changning Tongzi Zunyi Xiushan Yichang Taoyuan Taojiang Sandu Qidong Sys. Ser. Stage acuminatus Silurian ascensus Longmaxi Longmaxi Zhoujiaxi Fm. Longmaxi Longmaxi Longmaxi Longmaxi persculptus Fm. Fm. Fm. Fm. Fm. Group Guanyinqiao Bed

extraordinarius Guanyinqiao Bed Hirnantian

mirus Wufeng Wufeng Wufeng pacificus typicus Wufeng Fm. Fm. Fm. Wufeng Wufeng Wufeng Fm. Fm. Fm.

Tianmashan complexus Fm. Fm. complanatus Linxiang Jiancaogou Jiancaogou Linxiang Linxiang Nanshichong Fm. Fm. Fm. Fm. Fm. Fm.

quadrimucronatus Upper Ordovician Upper

Katian johnsturpi

Chengbu Fm. pygmaeus Baota Baota Baota Baota Baota spiniferus Fm. Fm. Fm. Fm. Fm.

Nanshichong Fm.

Modaoxi clingani Fm. wilsoni

Shuangjiakou bicornis Fm. Miaopo Yanxi Sandbian gracilis Lankeshan Fm. Fm. murchisoni Fm. Dashaba Shizipu Shizipu Guniutan Guniutan faciculatus Fm. Fm. Fm. Fm. Fm. Lanmutan

Ordovician ellesae Fm. Darriwilian austrodentatus

Baimachong Fm.

clavus caduceus

Middle Ordovician Middle imitatus

Dapingian Daguanshan Meitan Meitan Dawan Dawan Jiuxi Qiaotingzi Tonggao suecicus Fm. Fm. Fm. Fm. Fm. Fm. Fm. Fm.

bifidus

Fm. Qiaotingzi

fruticosus Floian approximatus Honghuayuan Fm.

clonograptus Putaojing Fenxiang Lower Ordovician Lower kiaerograptus adelograptus Fm. Fm. Tongzi Tongzi Tongzi psigraptus Madaoyu Fm. Guotang Fm. Fm. Fm.

angulica Nanjing- Fm.

Tremadocian Fm. Baishuixi Fm. matanensis guan Fm. Baishuixi parabola Wanjuanshu Panjiazui Xilingxia Fm. Fm. taojiangensis Fm. Wuduhe Shenjiawan Cambrian Loushanguan Fm. Tanxi Fm. Sandu Fm. Tanxi Fm. Fm. Fm. Note: * after Wang XF et al.(1987, 1996, 2001, 2003); Chen X et al.(1995, 2000, 2003). the Cambrian–Ordovician boundary. In the Three Gorges area Hisutodontus simplex –Monocostotus sevierensis zone (An of the platform facies zone, the boundary is at the top of TX, 1987; Wang XF et al., 1987), is developed in the Lower Wuduhe Formation consisting of grey thick to massive Mb. of the Xilingxia Formation. The grey medium beded bedded dolomite with nodular chert and chert beds. The sparite dolomitic limestone or dolomite with chert nodules or transgressive systems tract (TST) is a relatively fossiliferous chert bands, containing conodont of the Monocostotus succession, ranging from the Lower Member of the Xilingxia sevierensis zone and cephalopods of the Pseudoectenolites- Formation to the lower part of Upper Member of the Xiadongoceras zone (Wang XF et al., 1987) are developed in Xilingxia Formation (Fig. 4a). The light grey sparrudite and the lower part of the Upper Mb. of the Xilingxia Formation. coarse sparrenite, dolomite yielding the conodonts of the The highstand systems tract (HST) is corresponding to the 356 Wang et al. / China Geology 3 (2018) 354−366

105° 110°

Shaanxi Guangyuan North China Plate

Baokang

Tongjiang Chengkou

River

Sichuan River

Hubei Daxian Wushan

Jialingjiang 2 Wanxian Changjiang River 1

Nanchong Yichang Hanjiang South China Plate Chengdu Enshi 3 Shashi Wufeng 4

30° River

30° Platform facies River Chongqing

Fuling Lishui Cili Tuojiang 5 Sangzhi 10 Dayong Changde 6 Yongshun 8 Nanchuan Dongting Lake 7 Youyang Taoyuan Wuchuan Xiushan Yiyang Yuanling Anhua 11 Jishou Hunan 12 Changsha

Junlian Guizhou Sinan Xiangtan Lianyuan Xuefeng Mt. Zunyi Xinhua

River River

Shaoyang Hengyang

River Yuanjiang Basin facies Xiangjiang Slope facies Zishui Zhijin Guiyang Chengbu Yongxing Dunyun Linglin Sandu 0 100 km

105° 110°

Plate boundary Facies zone boundary Province boundary 9 Section

Fig. 1. Facies zones in the middle-upper Yangtze area and the location of sections. section: 1–Huanghuachang section, Yichang; 2–Wangji- awan section, Yichang; 3–Yazikou section, Changyang; 4–Yuyangguan section, Wufeng; 5–Lianghekou section, Sangzhi; 6–Heishui section, Youyang; 7–Dingshi section, Youyang; 8–Waergang section, Taoyuan; 9–Gaoqiao section, Cili;10–Longtanhe section, Cili; 11–Dafu section, Anhua; 12–Xianxi section, Anhua.

successions of grey medium-thick bedded sparrudite 2.1.2. The slope facies zone dolarenite or dolomitic limestones with nodular chert or chert The Osq1 corresponds to the lower part of the Panjiazui bands of the Upper Xilingxia Formation. The sedimentary Formation. in the slope facies. Thick bedded sparrudite characteristics, the upwards increasing diameter of the micrites and medium-thin bedded limestone are developed at sparrudites and the chert bands or nodules increasing upwards the base of the Panjiazui Formation in Jiuxi area according to as well, demonstrate that the water depth is shallowing the study of Li ZM et al. (1997). Relatively complete Bourma upwards with the sedimentary aggradation and progradation sequence and intensively scouring of the underlying strata can in the Upper Xilingxia Formation. be observed in this succession. Consequently, the lowstand Wang CS et al. (2002) differentiated the Osq1 sequence sparrudite wedge is regarded as the lowstand systems tract into two 3rd grade sequences in previous studies. The authors (LST) of the Osq1. The LST can ’t be recognized in the combine the two sequences into one sequence, Osq1 herein Waergang section of Taoyuan of the same facies zone. Its due to that the time range of the Xilingxia Formation. Is transgressive surface coincides with the sequence boundary, approximately equivalent to a single conodont zone. similar to the platform facies zone.

Platform facies Slope facies Basin facies Lianghekou section

38 Longtanhe .

37 section

Fm

36 Huanghuachang section Sys. 3rd grade Jiuxi Fm. tract sequence 35 Taohuashi Mb. e e e e Guniutan 21 Guniutan 34 Fm. U.Mb.33

. 32

Mb U.

. 31 Fm

M.Mb. Osq5 30 M.Mb Osq4

Middle Ordovician Middle 20 Dawan 26-29 e 19

L. Mb. L. L.Mb.

- 25 e e e . e e e

Fm ee 24 . Waergang ee e section HST 23 ee 18 Fm e

e e e yuan Honghua 22 e Honghua- Fm. yuan e e 17 e 66 16 e e e 65 ee Osq4 e e 15 Yazikou section Wang etal./ChinaGeology 21 e e

e e e 14 Osq4 TST 64 Fm. Madaoyu

Honghuayuan e 20 e e e e 16 e e 63 Madaoyu Fm. Madaoyu

Fenxiang Fm.Fenxiang Fm. Dawan 13 19 e Fenxiang Fm.15 e e e e e 12

Fenxiang Fm. Fenxiang 62 4th 14 11 10 e Mb. HST 18 e e 61 17 U.Mb 9 Xianxi section ee e Osq3 15-16 Dafu section 16-17

3rd 60 Ordovician TST Mb. 14 13 15 Osq3 8 11 12 e 59 7 6 14

58

13 3 (2018) 354−366 Lower 57 10 m 11 5 10 13 56 e 4 12 2nd e e 3 0 9 Mb. HST 2 Fm. Qiaotingzi

2nd 55 Fm. Qiaotingzi Panjiazui Fm. 11 12 Mb. 8 10 Osq2 10-11 10 54 7

9 9 53 9 1st TST 8 6 Osq2 8 Mb. 8 7 5-6 7

Nanjingguan Fm. Nanjingguan 52 e 5 HST 51 P e e 30 m 6 7 e 50 e e 49 4 P Osq1 1st e Fm. Panjiazui 5 e 0

Nanjingguan Fm. Nanjingguan 30 m TST 6 Mb. e e e 20 m e

e Fm. Baishuixi Xilingxia Fm.Xilingxia Fm. Nanjingguan e e 4 e e 3 0 Baishuixi Fm. Baishuixi 4-5 e 0 e 20 m 48 e 4 47 2 3 ee e C Osq2 Tanxi Fm. 0 C 2 3 e e 46 2 e e 3 e Tanxi Fm. 45 20 m Loushanguan Fm. e e e Wuduhe Fm. Osq1 e e e 44

Osq1 0

43 fine sandstone siltstone silty mudstone mudstone calcareous carboniferous chert micrite marl nodular mudstone mudstone limestone e e e 42 e 41 limestone argillaceous bioclastic oolite sparrenite sparrudite dolomitic dolomitedolomite with calcareous limestone lens containing limestone limestone limestone nodular chert dolomite Shenjiawan Fm. mud band 40

Fig. 2. Lower Ordovician sequence stratigraphic classification and correlation across different facies zone in South China. 357 358

Platform facies Slope facies Basin facies

Gaoqiao section Sys. 3rd grade Yuyangguan section tract sequence Ser. Dafu section Xianxi section Longmaxi Fm. Sys. 3rd grade Longmaxi Fm. Si Liangjiang- Wufeng Fm. 20 Si Si tract sequence Dingshi section Liangjiang- cs Osq10 Heishui section 20 Si Si he Fm. he Fm. TST Si Si Linxiang Fm. Longmaxi Fm. Si Si Si Linxiang Fm. e Si Si HST 18-19 + Si Si Si Osq10 19 Wufeng Fm. 19 Si 18 e Si Si Si 34 Osq8-9 CS e Si Si Osq10 Wufeng Fm. Wufeng Fm. Wufeng Fm. e e Si Si Si TST Si Si e e Osq10 HST Si Si Si 33 Linxiang Fm. 28 e Si Si Baota Fm. Osq9 e e Baota Fm. 17 e Si Si Nanshi- Nanshi- e TST e e Si Si Si Osq8 27 18 Si Si Osq8-9 chong Fm. chong Fm. cs e e Osq7 Ordovician Upper e 17 e 17 32 HST Si 16 e Baota Fm. e Si Si

Osq8 26 e Modaoxi Fm. Yanxi Fm. 31 Si Si Si

e e e Osq8-9 Nanshi- 16

Miaopo Fm. 15 e TST 7

. 30 chong Fm. q

14 15 Yanxi Fm. Si Si 29

e Fm s 19

HST 25 e e Osq7 O 28 13 Osq7 . 16 . e TST Osq6-7 24

e Fm

Fm

12 e e

Shizipu 14 e e 11 23 e 27 HST 18 26 22

Guniutan 15 e HST Shizipu 17 13 Osq6 25 Wang etal./ChinaGeology 10 21 12 16

14 Osq6 Fe 9 13 U.Mb 24 Osq6 12 20 19 15 23 U.Mb 8 11 e U.Mb TST Osq6 e TST 22 10 Osq5 14 21 Ordovician Middle

7 e . 18 11

Fm 9 20 M.Mb 10

8 17 . . 16 13 19

M.Mb M.Mb7

HST Fm

Fm

Dawan 20 m 15 6 10 m 9 12 . 14 Fm. Qiaotingzi

5 0 18 3 (2018) 354−366 Dawan

Fm

CS Osq5 Dawan L.Mb 4 16-17 LST 0 13 20 m 3 6 L.Mb L.Mb 2 e 15 5 e Jiuxi TST 4 11 1 e Osq5 2-3 e 11-12 0 8 e e 14 Honghuayuan Fm. e 13 10 10 HST e 12 9 e 9 e 7 TST e 11 Osq5 Honghuayuan Fm.8 e 6 8 Osq5 10 LST 7

20 m 9 6 8 0 Ordovician Lower 7 5 P 30 m 6 Osq1-4 5 4 P 4 5 3 0 30 m 2 ee e

fine sandstone siltstone silty mudstone mudstone calcareous carboniferous chert micrite marl nodular Madaoyu Fm. Fm. Baishuixi mudstone mudstone limestone 4 3 0 Baishuixi Fm. Baishuixi

eee e 2 3 Tanxi Fm. C limestone argillaceous bioclastic oolite sparrenite sparrudite dolomitic dolomite calcareous limestone lens C 2 containing limestone limestone limestone dolomite Tanxi Fm. mud band

Fig. 3. Middle and Upper Ordovician sequence stratigraphic classification and correlation across different facies zone in South China. Wang et al. / China Geology 3 (2018) 354−366 359

(a) (b)

Xinglingxia Fm.

transgressive

surface Wuduhe Fm., Sanyoudong Gr.

1

(c)

Fenxiang Fm.

transgresive surface

(e) Nanjingguan Fm.

(d)

Fig. 4. Sedimentary structures of Lower Ordovician in South China. a–showing the transgressive surface at the boundary between the Xilingx- ia Formation of Ordovician and underlying Wuduhe Formation of Cambrian in Huanghuachang section, Yichang, Hubei; b–the crinoid fauna at the shale of the lowermost Ordovician at Xiufengqiao section, Changyang, Hubei; c–the boundary between the Fenxiang Formation and the Nanjingguan Formation at Huanghuachang section, Yichang, Hubei; d–the slumping structure in the Madaoyu Formation at Gaoqiao section, Cili, Hunan; e–the current ripples showing the current direction in the Madaoyu Formation at Gaoqiao section, Cili, Hunan, China.

2.1.3. The basin facies zone sandstone unit is a flysh-like deposit corresponding to the The Osq1 sequence boundary corresponds to the lowstand turbidite fan. It is worthy to point out that the lithologic change between the Cambrian banded limestones terrestrial debris unlikely originates from the carbonate and the Ordovician grey and light grey fine grain laminated platform and slope in the northwest, but probably was shed sandstone rhythmites of Baishuixi Formation. The laminated from the Cathaysian block in the southeast. 360 Wang et al. / China Geology 3 (2018) 354−366

The 3rd grade sequences cannot be recognized in the basin dolomites of the 2nd member of the Nanjingguan Formation facies. The Osq1 and the subsequent Osq2, Osq3 and Osq4 shows an exposure surface at the top of HST of Osq2. The are equivalent to the succession of sandstones and siltstones sequence boundary between Osq2 and Osq3 coincides with of the Baishuixi Formation. in the basin facies zone in central the transgressive surface of Osq3, which is marked by the Hunnan，China (Fig. 2). appearance of the sparrenite, sparrudite and oolite in the Huanghuachang section. The 30 m thick limestones of the 3rd 2.2. The second 3rd grade sequence of the Ordovician (Osq2) member of the Nanjingguan Formation (TST) and the subsequent 14.9 m bioclastic limestones of the 4th member of 2.2.1. The platform facies the Nanjingguan Formation (HST) make up the Osq3 in the The sequence boundary at the Huanghuachang section, Yangtze Gorges area. Yichang, is in between the top of the Xilingxia Formation, a The exposure surface can be observed in the Yangxi restricted platform deposit of dolomite with chert bands or section of Changyang, Hubei Province, where it is marked by nodular chert, the HST of Osq1; and the base of Nanjingguan the 1.4 m thick dolomitic breccia. The TST is marked by the Formation, a bioclastic limestone containing sparrudite. The appearance of grey medium-thick bedded laminated dolomites sparrudite of the tidal channel deposit indicates the intercalated with a few bioclastic calcareous dolomites transgressive scouring at the base of Nanjingguan Formation indicating intertidal deposits. The subsequent thin bedded Four lithological members, i.e., in ascending order: the micrite is the deposit during the maximum flooding period. limestones of the 1st member, the dolomites of the 2nd The HST of Osq3 consists of dark grey thick bedded micrites member, the limestones of the 3rd member and the bioclastic limestones of the 4th member, are included in the revised containing bioclastics intercalated with the thin-medium Nanjingguan Formation by Wang XF et al. (1996). The bedded bioclastic limestones, and oolites containing limestones of the 1st member of TST and the dolomites of the bioclastics and sparrudites. The fragments of crinoids’ stems 2nd member of the HST make up the 3rd grade sequence and brachiopods are abundant in the HST in the Yangxi (Osq2). The TST ranges from the base of the section. C.angulatus –C.herfurthi conodont zone to the lower part of The Osq3 corresponds to the middle and upper part of the st Glyptoconus quadraplicatus conodont zone (Wang XF et al., 1 member of the Tongzi Formation. The TST is marked by 1987). The HST is marked by the occurrence of the dolomites grey thick bedded micrites. The HST is represented by the of the tidal flat deposits, which are differentiated by the tidal flat facies of laminated argillaceous dolomites, thick dolomitation increasing upwards and the grain size fining bedded dolomites containing sparrenite, sparrudite and upwards from the middle of the Glyptoconus quadraplicatus stromatolite, etc., intercalated with 2 layers of thin bedded conodont zone. fine-grained quart sandstone. The sandstone is probably the In the uplifted area of the platform, the Osq1 is usually LST of the next 3rd grade sequence by the rejuvenation of the missing. The TST of Osq2 starts by the intertidal deposits of rivers. grey thick bedded sparentitic calcareous dolomites in the Yangxi section, Changyang. The thick bedded dolomites at 2.3.2. The slope facies the top of Loushanguan Formation of the tidal flat facies, is in The Osq3 in the Gaoqiao section of the slope facies zone sharp contact with the base of Osq2. The hiatus between these corresponds approximately with the lower to middle part of is the equivalent to the succession of Osq1. the Madaoyu Formation. The TST of the Osq3 mainly The transgressive event is marked by the shallow shelf deposited thin-medium bedded bioclastic micrites intercalated deposits of the ca.10m thick yellow-greenish mudstones with yellow-greenish mudstones. Abundant trace fossils, yielding some trilobites and numerous ossicles of stems and especially Planolites, are found on the bedding surfaces. cups of crinoids. The Osq1 is missing as well between the Current ripples and slumping also can be observed in the TST Osq2 and the thick bedded –massive dolomites of (Fig. 4d, e). The HST is a progradation succession upward at Loushanguan Formation in the Xiufengqiao section, the middle part of the Madaoyu Formation. The Changyang, China(Fig. 4b). characteristics of the packing pattern of the parasequence are thin bedded calcareous mudstones or calcareous mudstones 2.2.2. The slope facies interbedded with limestone lenses in the lower part and thin The TST of Osq2 is made up by the limestones bedded bioclastic limestones, oolites, limestones containing intercalated with argillaceous bands at the top of the Panjiazui sparrudite in the upper part. Formation. In the slope facies, the HST is recognized in the thick bedded limestones with abundant bioclastics in the 2.4. The fourth 3rd grade sequence of the Ordovician (Osq4) lower part of the Madaoyu Formation in the Gaoqiao section, Cili, Hunan, China. 2.4.1. The platform facies Limestones with glauconite content and the argillaceous 2.3. The third 3rd grade sequence of the Ordovician (Osq3) components increasing upward from the base of Fenxiang Formation are regarded as the characters of the TST in 2.3.1. The platform facies Huanghuachang section. The yellow-greenish muddy shales The ca. 1 m thick paleokarst breccias at the top of the yielding abundant benthic graptolites, i.e., Acanthograptus Wang et al. / China Geology 3 (2018) 354−366 361 and Dendrograptus, are interpreted as the condensed section sea level changes and sequence stratigraphy. of Osq4 (Fig. 4c). The upper part of the Fengxiang Formation The Dawan Formation is divided into three members. The in the Huanghuachang section as well as the Yangxi section, lower member is 22 m thick greenish, thin- medium bedded Changyang, mainly contain bioclastic limestones interbedded nodular limestone intercalated with shale, which represents a with mudstones, of which the limestones beds decrease in upward fining and deepening retrogradation succession numbers upward. Shell shoals, crinoids stem shoals, bryozoan characterized by increase of argillaceous debris upwards. The patch reefs or/and algae mounds developed in the upper complete and continued conodont succession, which can be Fenxiang Formation, indicate an obvious progradation divided into four conodont zones, i.e., the B. communis zone, succession. The overlying Honghuayuan Formation is O. evae zone, B. trinngularis zone and B. navis zone in characterised by the Calathium reef deposits. Consequently, ascending order, is easy to correlate with the coeval conodont the upper part of Fenxiang Formation and the overlying faunas in the North Atlantic realm. Honghuayuan Formation constitute the HST of the Osq4 in The graptolites Didymograptellus bifidus, Corymbograp- the Yangtze Gorges area. tus deflexus and Azygograptus suecicus, etc., can be found in shale interbeds in the interval. The 1.6 m thick yellow 2.4.2. The slope facies and the basin facies greenish shale intercalated with limestone lenses at the top of The TST of the Osq4 corresponds to the middle-upper the lower member of the Dawan Formation contain abundant part of the Madaoyu Formation, which mainly includes graptolites and conodonts, indicates the maximum flooding yellow-greenish mud shales interbedded with some bioclastic period. limestones. The HST is recognized in an intercalation of grey- The Asaphid-Pseudocalymene assemblage is present in greenish and light purple bioclastic sparrudites and sparrenites the lower part of the lower member of Dawan Formation and at the top of Madaoyu Formation, which is correlated with the the Asaphid-Raphiophorid assemblage in the upper part of the former Taohuashi Formation in the Gaoqiao section, Cili lower member according to the study by Turvey ST and Zhou County, Hunan Province, of the slope facies. ZY (2002), are apparently the response of the trilobite faunas The Osq4 corresponds to the calcareous muddy siltstones to sea level rise. Similar to the study of the trilobite of the upper part of the Baishuixi Formation with flysh-like assemblage, the brachiopod assemblages show that BA1-BA2 mud-silt rhythm bands in the Anhua area of basin facies zone. in the lower part and BA1-BA3 in the upper part of the Dawan Formation, indicating the sea level rise upward (Zeng 3. The classification and correlation on the sequence QL, 1991). Consequently, the lower member of the Dawan stratigraphy of the Middle Ordovician Formation except the top part is considered as the TST of Osq5 in this paper. 3.1. The first 3rd grade sequence of the Middle Ordovician The 13 m thick purple medium bedded micrites yielding (Osq5) conodonts of the Paroistodus originalis zone and cephalopods 3.1.1. The platform facies of the Bathmoceras-Protocyloceras zone of the middle The base of the Osq5 sequence boundary is consistent member of Dawan Formation, are correlated with the with the boundary between the Honghuayuan Formation and homonymous zone in the North Atlantic realm. the Dawan Formation (Fig. 5a). The top of the Honghuayuan The micrites of the middle member of the Dawan Formation is a diachronous surface, i.e., the graptolite fauna Formation was formerly treated as the HST because of the of the Acrograptus filiformis zone is found in the overlying purple color and gradually increase of calcareous material strata in the lower Meitan Formation at Tongzi, North (Wang CS et al., 2002). The top of the lower member and the Guizhou (Lai CG et al., 1982); the Didymograptellus bifidus middle member are regarded as the condensed section of zone in the coeval strata, the lower of Dawan Formation at Osq5 herein according to the study by Turvey ST and Zhou Yichang, Hubei Province (Wang CS, 2013). ZY (2002), which demonstrated that the Trinucleid The transgression is marked by the deposit of bioclastic assemblage present in the middle member and lower part of limestones containing glauconite. The obvious facies changes the upper member can be interpreted as the deep outer shelf can be observed in the Dawan Formation and coeval strata assemblage. because the west margin of Yangtze Block had been uplifted The upper member of the Dawan Formation, yield the and provided debris for the Middle and Upper Yangtze region graptolite fauna of L. sinodentatus zone in the lower part and during this period. Littoral sandstones are developed in the L. austrodentatus zone in the upper, coexisting the conodont Wuding and Lüquan area, Yunnan Province. The shallow fauna of the L. norrlandicus zone, Lenodus antivariabilis shelf of yellow-greenish muddy shales intercalated with zone, L. variablilis zone, yellow greenish thin-medium muddy siltstones and bioclastic limestones is developed in bedded nodular limestone intercalated with shale, 28 m in north Guizhou Province. The grey, thin bedded micrites of the thickness. The brachiopod assemblage represents the BA4 lower Dawan Formation, former namely the Zitai Formation (Zeng QL, 1991). The trilobite fauna belongs to the Asaphid- in northwest Hunan Province, is obviously different to the Raphiophorid assemblage above the base of the Levisograptus typical Dawan Formation in the Yichang area. sinodentatus graptolite zone in the upper member of Dawan The succession of Dawan Formation is exposed well in Formation (Turvey ST and Zhou ZY, 2002). The biofacies the Yichang area, where it is the typical area for the study of shows the fall of the sea level. 362 Wang et al. / China Geology 3 (2018) 354−366

(a)

Middle Mb.of Dawan Fm.

of Dapingian Lower Mb.of Dawan Fm. The GSSP

(b) (c)

Jiuxi Fm. transgresive

surface

Taohuashi Mb.at the top of Madaoyu Fm.

Fig. 5. The succession of the Middle Ordovician in different facies. a–The GSSP of Dapingian at Huanghuachang section, Yichang; b–show- ing the transgressive surface at the boundary between the Jiuxi Formation and the Madaoyu Formation of Gaoqiao section, Cili; c–the grap- tolite fauna in Qiaotingzi Formation at Dafu section, Anhua, China.

The grey purple thin bedded nodular limestones and light conglomerates, mainly quartz sandstones, most of them less grey thick bedded limestones at the lower Guniutan than 0.2 cm in diameter, distributed on the bedding surface. Formation, yielding abundant cephalopods and the BA3 The sandstones are laminated, interbedded with thin layers of brachiopods, etc., demonstrate a relative sea level drop from light grey mudstones. The light grey medium-thin bedded the upper member of the Dawan Formation to the lower part mudstones, argillaceous slates intercalated with thin siltstone of Guniutan Formation, which consequently treated as the bands, are developed above fine sandstones. HST of Osq5. Another two episodes of turbidite events, which are characterized by the medium-fine quartz sandstones in the 3.1.2. The slope facies and basin facies lower and fine quartz stones intercalated with muddy siltstone The Osq5 corresponds to the lower to middle part of the in the upper part, can be observed above the turbidite event at Jiuxi Formation in the slope facies in the Gaoqiao section, the base of the Qiaotingzi Formation. which includes grey, yellow-greenish silty mudstones (Fig. The TST is a nearly 100 m thick fining upward 5b). The transgressive surface is consistent with the boundary retrogradation succession, corresponding to the yellow between the Jiuxi Formation and the Madaoyu Formation. greenish medium-thick bedded muddy siltstones, silty The TST and HST cannot be distinguished in the slope facies mudstones or silty mud slates bearing graptolite fossils in the area. lower part of the Qiaotingzi Formation above the turbidites. The Osq5 corresponds to the lower to middle part of the The highstand systems tract is an obvious progradation Qiaotingzi Formation in the basin facies in the Xianxi section succession, made up by two parts, i.e., yellow greenish thick and Dafu section, Anhua County, Hunan Province (Fig. 5c). bedded-massive laminated argillaceous siltystones in the The turbidite fan is developed at the base of Qiaotingzi lower part; yellow greenish, grey greenish medium-thin Formation in the Xianxi section, which is regarded as the bedded siltystones, mudstones in the upper. The lamination is LST. The succession of the turbidite fan is developed as more developed, and the thin siltstones or/and fine sandstone follows: grey thick bedded fine grain sandstones containing interbeds increase upwards (Fig. 3). Wang et al. / China Geology 3 (2018) 354−366 363

3.2. The second 3rd grade sequence of the Middle Ordovician the Middle Ordovician and the Upper Ordovician. The (Osq6) transgression surface is a diachronous surface according to the graptolite zones in the different facies. 3.2.1. The platform facies The TST of Osq6 is the succession of the thin bedded 4.1.1. The platform facies nodular limestones, thin bedded nodular limestones The appearance of the graptolite fauna of the H. interbedded with thin bedded marls, which are developed teretiusculus zone and abundant shelly fossils, occurred in the above the sparrudites at the base of the upper Gunitan yellow greenish shale at the base of the Miaopo Formation, Formation in the Three Gorges area. The 4 m thick light grey indicates the initial transgression of the Osq7 in Three Gorges medium-thick bedded limestones at the top of the Guniutan area. The maximum flooding surface is marked by the Formation is the HST of Osq6. The lack or incompleteness of appearance of Nemagraptus gracilis. The yellow-greenish the Hustedograptus teretiusculus graptolite zone or the shale of the N. gracilis zone represents the condensed interval Pygodus serra conodont zone in the Miaopo Formation, which is distributed in the depressions of the platform, shows of the Osq7. The yellow greenish argillaceous siltstones that the uneven surface at the top of the Guniutan Formation containing the abundant benthic fauna of trilobites and is probably the I type sequence boundary. brachiopods without the presence of graptolite faunas above the N. gracilis zone, implying a relatively sea level drop, is 3.2.2. The slope facies and basin facies the HST of the Osq7. The sequence corresponds to the Middle part of the Jiuxi Formation, which is a silty mudstone succession without any 4.1.2. The slope facies zone and the basin facies zone obvious changes in lithology upwards in the slope facies area. The Osq7 in the slope facies zone corresponds to the The Osq6 corresponds to the upper part of the Qiaotingzi Sherenwan Formation, which is now identified as the upper Formation in the basinal facies zone. The LST is part of the Jiuxi Formation. The lithology of the Sherenwan characterized by 3 turbidite successions, of which the typical Formation can be divided into three parts, i.e., the grey turbidite succession is developed in ascending order as greenish silty shales yielding abundant graptolites and follows in Xianxi section, Anhua County: trinucleid trilobite in the lower part; The grey, grey greenish (i) The fine sandstone containing conglomerates at the banded calcareous shales with a few bean-like limestone base. lenses in the middle, yielding the abundant graptolites (ii) The rhythmic succession of fine sandstones Nicholograptus fasciculatus, Climacograptus sp., etc.; and the interbedded with siltstones. The siltstones are yellow grey yellowish, yellow greenish thin bedded limestone greenish, thick bedded, 130 cm approximately of each single intercalated with shale in the upper part, yielding the bed; while the sandstones are light grey-white, thin bedded. (iii) The yellow greenish medium bedded siltstones, graptolite Nicholograptus fasciculatus, Climacograptus sp., argillaceous siltstones, horizontal stratification developed. Aulograptus leeszukangi, etc. (Lai CG et al., 1982). The (iv) The yellow greenish thick bedded silty mudstones. graptolite fauna belongs to Nicholograptus fasciculatus zone The TST of Osq6 is a grey thin bedded argillaceous slate, of mid Darriwilian in age. Lai CG et al. (1982) also noted that 2 –10 cm thick of each single bed, horizontal stratification. the Nemagraptus gracilis had been found at the top of this The laminar sandstones are interbedded in the slate, the formation. Consequently, the Sherenwan Formation spans a frequence of the sandstone interbeds decrease upwards. A long period of time from the late Middle Ordovician to the distal turbidte, is characterized by the rhythmic deposit of the earliest Upper Ordovician. In other words, the transgression grey siltstones, argillaceous siltstones alternated with light of Osq7 is started much early in the slope facies zone. The grey fine sandstone laminae in the lower part, and transgression surface is obviously a diachronous surface from argillaceous siltstones in the upper with increase of the the slope facies zone to the platform facies zone northward. argillaceous component, also can be observed in the TST. In the basin facies zone, Osq7 is corresponding to the thin The HST of Osq6 is a progradation succession of bedded chert intercalated with carboniferous shale of Yanxi coarsening upwards and upward increase of the thickness of Formation, yielding abundant graptolite. It is impossible to the individual beds. The grey thin-medium bedded differentiate the systems tracts. argillaceous siltstones with abundant light grey fine sandstone laminae, horizontal stratification, are developed in the lower 4.2. The second 3rd grade sequence of the Upper Ordovician part; the grey, grey greenish thick bedded to massive siltstone, (Osq8) argillaceous siltstone with plenty of fine sandstone laminas in the upper part (Fig. 3). 4.2.1. The platform facies zone 4. The classification and correlation of the sequence The platform facies zone of the Osq8 in the Three Gorges stratigraphy of the Upper Ordovician area, corresponds to the Baota Formation, which is a succession of grey purple medium-thick bedded bioclastic 4.1. The first 3rd grade sequence of the Upper Ordovician micrites intercalated with a few grey nodular hydromica (Osq7) micrites containing plenty of cephalopods (Fig. 6f). The typical “polygonal marking” structures are well developed in Osq7 is a sequence which crosses the boundary between the micrite, which actually represent shrinkage cracks in deep 364 Wang et al. / China Geology 3 (2018) 354−366 water (Fig. 6d). The brachiopod assemblage BA4-5 (Zeng nodular micrites, yielding the brachiopod, trilobite and a few QL, 1991), the SSI value of the large cephalopod shell and the cephalopod, are developed in the lower part of the Linxiang biofacies of trilobites (Zhou ZY et al., 2000) indicate that the Formation above the Baota Formation. Either the abundance Baota Formation is a deep-water deposit. The water depth is or the diversity of the faunas of the lower part of the Linxiang estimated at up to 100m in Three Gorges area. The Baota Formation obviously is lower than in the Baota Formation. Formation is the new transgression deposit after the The lower part of the Linxiang Formation is regarded as the regression at the top of Miaopo Formation. The light grey HST herein according to sedimentary characteristics and bio- greenish or light purple grey medium-thick bedded bioclastic facies.

(a) (b)

Longmaxi Fm. The O/S boundary

Guanyingqiao Bed The GSSP of Hirnantian

Wufeng Fm.

(d)

(c)

(f) (e)

Wufeng Fm. Katian

Linxiang Fm. Sandbian

Fig. 6. The successions of the Upper Ordovician in different areas. a –The succession across the O/S boundary at Wangjiawan section, Yichang; b–the siltstone of Nanshichong Formation showing the horizontal stratification at the Cili section; c–the Wufeng Formation at Xianxi section, Anhua; d–the shrinkage cracks in the Baota Formation, Puxihe section, Yichang, Hubei; e–the boundary between the Lingxiang Forma- tion and the Wufeng Formation coinciding with the transgressive erosion surface at the Yongshun section, Yongshun; f–the Upper Ordovician succession at Puxihe section, Yichang, China. Wang et al. / China Geology 3 (2018) 354−366 365

4.2.2. The slope facies zone and the basin facies zone that a rise of the sea level from the Dicellograptus Lai CG et al. (1982) claimed that 6m thick medium-thick complanatus zone to the maximum flooding surface in the bedded argillaceous limestones are developed in the Jiuxi Tangyagraptus typicus subzone of the Paraorthograptus section representing the slope facies zone. The authors try to pacificus zone. but did not find the limestones of the Baota Formation in the The HST of Osq10 starts at the base of the Jiuxi area. The coeval strata, the siltstone of the Nanshichong Diceratograptus mirus subzone of the P. pacificus zone, Formation is found in the Gaoqiao section, Cili County, coinciding with a major global extinction event. The FAD of Hunan Province. The Nanshichong Formation is the clastic Normalograptus extraordinarius (Sobolevskaya) marks the wedge generated by the thrust northward of the Cathaysian Hirnantian GSSP level (Fig. 6a). The shelly fossils, i. e. the old land. Hirnantia-Dalmanitina shelly fauna, which is a key element The grey medium-thick bedded mudstones and silty for recognition of the Hirnantian Stage, occurs in the mudstones of the Nanshichong Formation are also found in argillaceous limestones and marks the maximum regression the basinal facies zone, i.e. the Dafu section in Anhua County, due to the Gondwana glaciation. The uppermost Ordovician Hunan Province. Laminations are well developed within graptolite zone, the zone of Normalograptus persculptus, every single bed (Fig. 6b). Each single bed is ca. 70 cm thick. marks the onset of a new transgression event in south China. Trace fossils are extremely abundant in the silty mudstone. The Osq10 corresponds to the chert beds of the Wufeng The Osq8 is corresponds only to the lower part of the Formation in the Gaoqiao section and the Anhua section, Nanshichong Formation according to the graptolite faunas which represent the typical slope facies and basin facies below and above the Nanshichong Formation. sections, respectively (Fig. 6c). The systems tract is impossible to distinguish. The Tianmashan Formation in the 4.3. The third 3rd grade sequence of the Upper Ordovician Qidong and Shuangfeng areas can be regarded as coeval (Osq9) strata. Although the thickness of the Tianmashan Formation is over 1000 m, it corresponds only to the deposition of the The TST of Osq9 is represented by the grey thin-medium Dicellograptus complexus zone. The uppermost Ordovician is micrite intercalated in thin bedded nodular micrite, yielding not represented and the Tianmashan Formation is cephalopods, conodonts, trilobites, etc. of the middle part of unconformably overlain by the Tiaomajian Formation of the the Linxiang Formation in the Three Gorges area. The Middle Devonian. This is the sedimentary response for the trilobites belong to the early Insculptus facies according to Guangxi orogeny movement beyond the topic of this paper, Zhou ZY et al. (2000). which is thoroughly discussed by Chen X et al.(2012, 2013). The argillaceous/calcareous component ratio increases due to increased terrestrial debris input, along with the appearance 5. Conclusions of the yellow greenish, thin bedded nodular limestones of the Nankinolithus trilobite zone. There is a 1m thick mudstone The Ordovician successions are different in various facies layer at the top of the Linxiang Formation at Hongshilin zones in South China. An unconformity surface can be section, Guzhang County and Jiaoziya section, Sangzhi observed at the base of the Ordovician in the platform facies County, Hunan Province. The characteristics of the faunal zone and can be traced in the slope facies zone. There are 4 turnover include the decline of the cephalopod assemblage third grade sequence can be recognized and correlated in and the flourish of benthic trilobite Nankinolithus assemblage different facies zone during the Early Ordovician, although and the Foliomena brachiopod community. All these the response for the sea-level fluctuations is much more phenomena show that the succession of the upper part of obvious in the platform facies zone than in the slope and basin Linxiang Formation possess the characteristics of the HST of facies zone. Osq9. It is witnessed a sustainable large-scale sea-level rising The Osq9 corresponds to the upper part of the during the Middle and Upper Ordovician. The transgression is Nanshichong Formation. But the Osq9 is not well started at late of Early Ordovician, which onlap surface is distinguished with the Osq8 in the slope and basin facies diachronously changed from the level of Acrograptus zones. filiformis zone in northern Guizhou to the level of Dydimograptellus bifidus in western Hubei. The Osq5 to 4.4. The fourth 3rd grade sequence of the Upper Ordovician Osq10 can be recognized and correlated in different facies. (Osq10) The study on the sequence stratigraphy and correlation of this time interval will provide powerful evidence for the analysis The yellow greenish muddy shales, whose thickness of the onset of the evolution of the foreland basin triggered by varies in different areas between 0.2–1 m, due to the initiation the Guangxi orogeny movement. of the transgression at the base of Wufeng Formation (Fig. 6e). The graptolite fauna in the shale corresponds to the D. Acknowledgment complanatus zone. The TST of the Osq10 in the Three Gorges area is made up of black carbonaceous shales intercalated This research is financially supported by the Natural with thin bedded cherts. The graptolite zonation demonstrates Science Foundation of China (41472031) and the Gelogical 366 Wang et al. / China Geology 3 (2018) 354−366

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