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

ABSTRACT

The precise correlation of the Ordovician successions in different facies is difficult due to the complex changes of the tecto-paleogeography in South China. Based on previous studies, the authors recognized 10 3rd grade sequences, i. e., Osq1 to Osq10 in ascending order, by the integrated study of chronostratigraphy,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 application of the method of sequence stratigraphy and correlation.

Keywords:

Sequence stratigraphy

Correlation

Different facies

Ordovician

1. Introduction

The Ordovician successions are well developed and extensively distributed in South China, especially in Middle-Upper Yangtze region (Fig. 1). Chronostratigraphic and biostratigraphic studies are carried out on the typical Ordovician sections with complete successions in various facies and with diverse fossils in this region (Mu EZ et al.,1979; An TX, 1987; Wang XF et al., 1987; Chen X et al.,1995, 2003; Chen XH et al., 1996, 2002). The Hirnantian and Dapingian GSSPs have been approved in 2006 in the Wangjiawan section and 2007 in the Huanghuachang section respectively, in the center of the Yangtze craton near Yichang since the first GSSP of the Ordovician System, i.e. the Darriwilian GSSP, ratified in 1997 in the Huangnitang section of Zhejiang Province (Chen X and Bergström SM, 1995;Chen X et al., 2006; Wang XF et al., 2005, 2009). Although great progress on the Ordovician chronostratigraphy and biostratigraphy was achieved, the precise correlation of the Ordovician successions in different facies is still difficult due to the complex changes of the tecto-paleogeography in South China. The application of sequence stratigraphy and correlation makes it possible to precisely correlate Ordovician successions in different facies in an isochronous stratigraphic framework.

The age of the Ordovician rocks is constrained by the graptolite biozonation and the newest chronostratigraphic framework is given in Table 1 according to the previous studies by the Ordovician experts (Wang XF et al., 1987,1995, 1996, 2001, 2003, 2009; Chen X et al., 1995, 2000,2003, 2006).

The Three Gorges area is the most typical area for the study on the Ordovician stratigraphy in China. The experts studied the evolution of the Ordovician paleo-environment and the facies changes, sea level changes and classification of the sequence stratigraphy, various fossils and biostratigraphy of the Ordovician successions, etc. (Chen X et al., 1986;Wang XF et al., 1987; Zeng QL et al., 1983). The systematic studies on the Ordovician sea level changes and sequence stratigraphy have been carried out since middle 1990s (Wang XF et al., 1995, 1996; Li ZM et al., 1997; Chen XH and Wang XF, 1999; Wang CS et al., 2002, 2003).

Based on previous studies, the authors recognized 10 3rdgrade sequences, i. e., Osq1 to Osq10 (Table 1) in ascending order, by the integrated study of chronostratigraphy,biostratigraphy as well as the analysis on the sedimentary facies (Fig. 2, Fig. 3).

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

2. The classification and correlation on the sequence stratigraphy of the Lower Ordovician

2.1. The first 3rd grade sequence of the Ordovician (Osq1)

2.1.1. The platform facies zone

The sequence boundary surface of Osq1 coincides withthe Cambrian–Ordovician boundary. In the Three Gorges area of the platform facies zone, the boundary is at the top of Wuduhe Formation consisting of grey thick to massive bedded dolomite with nodular chert and chert beds. The transgressive systems tract (TST) is a relatively fossiliferous succession, ranging from the Lower Member of the Xilingxia Formation to the lower part of Upper Member of the Xilingxia Formation (Fig. 4a). The light grey sparrudite and coarse sparrenite, dolomite yielding the conodonts of theHisutodontus simplex–Monocostotus sevierensiszone (An TX, 1987; Wang XF et al., 1987), is developed in the Lower Mb. of the Xilingxia Formation. The grey medium beded sparite dolomitic limestone or dolomite with chert nodules or chert bands, containing conodont of theMonocostotus sevierensiszone and cephalopods of thePseudoectenolites-Xiadongoceraszone (Wang XF et al., 1987) are developed in the lower part of the Upper Mb. of the Xilingxia Formation.

Fig. 1.Facies zones in the middle-upper Yangtze area and the location of sections. section: 1–Huanghuachang section, Yichang; 2–Wangjiawan 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.

Fig. 2.Lower Ordovician sequence stratigraphic classification andcorrelation across different facies zone in South China.

Fig. 3.Middle andUpper Ordovician sequence stratigraphic classification and correlation across different facies zone in South China.

Fig. 4.Sedimentary structures of Lower Ordovician in South China. a–showing the transgressive surface at the boundary between the Xilingxia 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.

The highstand systems tract (HST) is corresponding to the successions of grey medium-thick bedded sparrudite dolarenite or dolomitic limestones with nodular chert or chert bands of the Upper Xilingxia Formation. The sedimentary characteristics, the upwards increasing diameter of the sparrudites and the chert bands or nodules increasing upwards as well, demonstrate that the water depth is shallowing upwards with the sedimentary aggradation and progradation in the Upper Xilingxia Formation.

Wang CS et al. (2002) differentiated the Osq1 sequence into two 3rdgrade sequences in previous studies. The authors combine the two sequences into one sequence, Osq1 herein due to that the time range of the Xilingxia Formation. Is approximately equivalent to a single conodont zone.

2.1.2. The slope facies zone

The Osq1 corresponds to the lower part of the Panjiazui Formation. in the slope facies. Thick bedded sparrudite micrites and medium-thin bedded limestone are developed at the base of the Panjiazui Formation in Jiuxi area according to the study of Li ZM et al. (1997). Relatively complete Bourma sequence and intensively scouring of the underlying strata can be observed in this succession. Consequently, the lowstand sparrudite wedge is regarded as the lowstand systems tract(LST) of the Osq1. The LST can’t be recognized in the Waergang section of Taoyuan of the same facies zone. Its transgressive surface coincides with the sequence boundary,similar to the platform facies zone.

2.1.3. The basin facies zone

The Osq1 sequence boundary corresponds to the lithologic change between the Cambrian banded limestones and the Ordovician grey and light grey fine grain laminated sandstone rhythmites of Baishuixi Formation. The laminated sandstone unit is a flysh-like deposit corresponding to the lowstand turbidite fan. It is worthy to point out that the terrestrial debris unlikely originates from the carbonate platform and slope in the northwest, but probably was shed from the Cathaysian block in the southeast.

The 3rdgrade sequences cannot be recognized in the basin facies. The Osq1 and the subsequent Osq2, Osq3 and Osq4 are equivalent to the succession of sandstones and siltstones of the Baishuixi Formation. in the basin facies zone in central Hunnan，China (Fig. 2).

2.2. The second 3rd grade sequence of the Ordovician (Osq2)

2.2.1. The platform facies

The sequence boundary at the Huanghuachang section,Yichang, is in between the top of the Xilingxia Formation, a restricted platform deposit of dolomite with chert bands or nodular chert, the HST of Osq1; and the base of Nanjingguan Formation, a bioclastic limestone containing sparrudite. The sparrudite of the tidal channel deposit indicates the transgressive scouring at the base of Nanjingguan Formation

Four lithological members, i.e., in ascending order: the limestones of the 1st member, the dolomites of the 2ndmember, the limestones of the 3rdmember and the bioclastic limestones of the 4thmember, are included in the revised Nanjingguan Formation by Wang XF et al. (1996). The limestones of the 1stmember of TST and the dolomites of the 2ndmember of the HST make up the 3rdgrade sequence(Osq2). The TST ranges from the base of the C.angulatus–C.herfurthi conodont zone to the lower part of Glyptoconus quadraplicatus conodont zone (Wang XF et al.,1987). The HST is marked by the occurrence of the dolomites of the tidal flat deposits, which are differentiated by the dolomitation increasing upwards and the grain size fining upwards from the middle of the Glyptoconus quadraplicatus conodont zone.

In the uplifted area of the platform, the Osq1 is usually missing. The TST of Osq2 starts by the intertidal deposits of grey thick bedded sparentitic calcareous dolomites in the Yangxi section, Changyang. The thick bedded dolomites at the top of Loushanguan Formation of the tidal flat facies, is in sharp contact with the base of Osq2. The hiatus between these is the equivalent to the succession of Osq1.

The transgressive event is marked by the shallow shelf deposits of the ca.10m thick yellow-greenish mudstones yielding some trilobites and numerous ossicles of stems and cups of crinoids. The Osq1 is missing as well between the Osq2 and the thick bedded–massive dolomites of Loushanguan Formation in the Xiufengqiao section,Changyang, China(Fig. 4b).

2.2.2. The slope facies

The TST of Osq2 is made up by the limestones intercalated with argillaceous bands at the top of the Panjiazui Formation. In the slope facies, the HST is recognized in the thick bedded limestones with abundant bioclastics in the lower part of the Madaoyu Formation in the Gaoqiao section,Cili, Hunan, China.

2.3. The third 3rd grade sequence of the Ordovician (Osq3)

2.3.1. The platform facies

The ca. 1 m thick paleokarst breccias at the top of the dolomites of the 2ndmember of the Nanjingguan Formation shows an exposure surface at the top of HST of Osq2. The sequence boundary between Osq2 and Osq3 coincides with the transgressive surface of Osq3, which is marked by the appearance of the sparrenite, sparrudite and oolite in the Huanghuachang section. The 30 m thick limestones of the 3rdmember of the Nanjingguan Formation (TST) and the subsequent 14.9 m bioclastic limestones of the 4thmember of the Nanjingguan Formation (HST) make up the Osq3 in the Yangtze Gorges area.

The exposure surface can be observed in the Yangxi section of Changyang, Hubei Province, where it is marked by the 1.4 m thick dolomitic breccia. The TST is marked by the appearance of grey medium-thick bedded laminated dolomites intercalated with a few bioclastic calcareous dolomites indicating intertidal deposits. The subsequent thin bedded micrite is the deposit during the maximum flooding period.The HST of Osq3 consists of dark grey thick bedded micrites containing bioclastics intercalated with the thin-medium bedded bioclastic limestones, and oolites containing bioclastics and sparrudites. The fragments of crinoids’ stems and brachiopods are abundant in the HST in the Yangxi section.

The Osq3 corresponds to the middle and upper part of the 1stmember of the Tongzi Formation. The TST is marked by grey thick bedded micrites. The HST is represented by the tidal flat facies of laminated argillaceous dolomites, thick bedded dolomites containing sparrenite, sparrudite and stromatolite, etc., intercalated with 2 layers of thin bedded fine-grained quart sandstone. The sandstone is probably the LST of the next 3rdgrade sequence by the rejuvenation of the rivers.

2.3.2. The slope facies

The Osq3 in the Gaoqiao section of the slope facies zone corresponds approximately with the lower to middle part of the Madaoyu Formation. The TST of the Osq3 mainly deposited thin-medium bedded bioclastic micrites intercalated with yellow-greenish mudstones. Abundant trace fossils,especially Planolites, are found on the bedding surfaces.Current ripples and slumping also can be observed in the TST(Fig. 4d, e). The HST is a progradation succession upward at the middle part of the Madaoyu Formation. The characteristics of the packing pattern of the parasequence are thin bedded calcareous mudstones or calcareous mudstones interbedded with limestone lenses in the lower part and thin bedded bioclastic limestones, oolites, limestones containing sparrudite in the upper part.

2.4. The fourth 3rd grade sequence of the Ordovician (Osq4)

2.4.1. The platform facies

Limestones with glauconite content and the argillaceous components increasing upward from the base of Fenxiang Formation are regarded as the characters of the TST in Huanghuachang section. The yellow-greenish muddy shales yielding abundant benthic graptolites, i.e., Acanthograptus and Dendrograptus, are interpreted as the condensed section of Osq4 (Fig. 4c). The upper part of the Fengxiang Formation in the Huanghuachang section as well as the Yangxi section,Changyang, mainly contain bioclastic limestones interbedded with mudstones, of which the limestones beds decrease in numbers upward. Shell shoals, crinoids stem shoals, bryozoan patch reefs or/and algae mounds developed in the upper Fenxiang Formation, indicate an obvious progradation succession. The overlying Honghuayuan Formation is characterised by the Calathium reef deposits. Consequently,the upper part of Fenxiang Formation and the overlying Honghuayuan Formation constitute the HST of the Osq4 in the Yangtze Gorges area.

2.4.2. The slope facies and the basin facies

The TST of the Osq4 corresponds to the middle-upper part of the Madaoyu Formation, which mainly includes yellow-greenish mud shales interbedded with some bioclastic limestones. The HST is recognized in an intercalation of greygreenish and light purple bioclastic sparrudites and sparrenites at the top of Madaoyu Formation, which is correlated with the former Taohuashi Formation in the Gaoqiao section, Cili County, Hunan Province, of the slope facies.

The Osq4 corresponds to the calcareous muddy siltstones of the upper part of the Baishuixi Formation with flysh-like mud-silt rhythm bands in the Anhua area of basin facies zone.

3. The classification and correlation on the sequence stratigraphy of the Middle Ordovician

3.1. The first 3rd grade sequence of the Middle Ordovician(Osq5)

3.1.1. The platform facies

The base of the Osq5 sequence boundary is consistent with the boundary between the Honghuayuan Formation and the Dawan Formation (Fig. 5a). The top of the Honghuayuan Formation is a diachronous surface, i.e., the graptolite fauna of the Acrograptus filiformis zone is found in the overlying strata in the lower Meitan Formation at Tongzi, North Guizhou (Lai CG et al., 1982); the Didymograptellus bifidus zone in the coeval strata, the lower of Dawan Formation at Yichang, Hubei Province (Wang CS, 2013).

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

The transgression is marked by the deposit of bioclastic limestones containing glauconite. The obvious facies changes can be observed in the Dawan Formation and coeval strata because the west margin of Yangtze Block had been uplifted and provided debris for the Middle and Upper Yangtze region during this period. Littoral sandstones are developed in the Wuding and Lüquan area, Yunnan Province. The shallow shelf of yellow-greenish muddy shales intercalated with muddy siltstones and bioclastic limestones is developed in north Guizhou Province. The grey, thin bedded micrites of the lower Dawan Formation, former namely the Zitai Formation in northwest Hunan Province, is obviously different to the typical Dawan Formation in the Yichang area.

The succession of Dawan Formation is exposed well in the Yichang area, where it is the typical area for the study of sea level changes and sequence stratigraphy.

The Dawan Formation is divided into three members. The lower member is 22 m thick greenish, thin- medium bedded nodular limestone intercalated with shale, which represents a upward fining and deepening retrogradation succession characterized by increase of argillaceous debris upwards. The complete and continued conodont succession, which can be divided into four conodont zones, i.e., the B. communis zone,O. evae zone, B. trinngularis zone and B. navis zone in ascending order, is easy to correlate with the coeval conodont faunas in the North Atlantic realm.

The graptolites Didymograptellus bifidus, Corymbograptus deflexus and Azygograptus suecicus, etc., can be found in shale interbeds in the interval. The 1.6 m thick yellow greenish shale intercalated with limestone lenses at the top of the lower member of the Dawan Formation contain abundant graptolites and conodonts, indicates the maximum flooding period.

The Asaphid-Pseudocalymene assemblage is present in the lower part of the lower member of Dawan Formation and the Asaphid-Raphiophorid assemblage in the upper part of the lower member according to the study by Turvey ST and Zhou ZY (2002), are apparently the response of the trilobite faunas to sea level rise. Similar to the study of the trilobite assemblage, the brachiopod assemblages show that BA1-BA2 in the lower part and BA1-BA3 in the upper part of the Dawan Formation, indicating the sea level rise upward (Zeng QL, 1991). Consequently, the lower member of the Dawan Formation except the top part is considered as the TST of Osq5 in this paper.

The 13 m thick purple medium bedded micrites yielding conodonts of the Paroistodus originalis zone and cephalopods of the Bathmoceras-Protocyloceras zone of the middle member of Dawan Formation, are correlated with the homonymous zone in the North Atlantic realm.

The micrites of the middle member of the Dawan Formation was formerly treated as the HST because of the purple color and gradually increase of calcareous material(Wang CS et al., 2002). The top of the lower member and the middle member are regarded as the condensed section of Osq5 herein according to the study by Turvey ST and Zhou ZY (2002), which demonstrated that the Trinucleid assemblage present in the middle member and lower part of the upper member can be interpreted as the deep outer shelf assemblage.

The upper member of the Dawan Formation, yield the graptolite fauna of L. sinodentatus zone in the lower part and L. austrodentatus zone in the upper, coexisting the conodont fauna of the L. norrlandicus zone, Lenodus antivariabilis zone, L. variablilis zone, yellow greenish thin-medium bedded nodular limestone intercalated with shale, 28 m in thickness. The brachiopod assemblage represents the BA4(Zeng QL, 1991). The trilobite fauna belongs to the Asaphid-Raphiophorid assemblage above the base of the Levisograptus sinodentatus graptolite zone in the upper member of Dawan Formation (Turvey ST and Zhou ZY, 2002). The biofacies shows the fall of the sea level.

The grey purple thin bedded nodular limestones and light grey thick bedded limestones at the lower Guniutan Formation, yielding abundant cephalopods and the BA3 brachiopods, etc., demonstrate a relative sea level drop from the upper member of the Dawan Formation to the lower part of Guniutan Formation, which consequently treated as the HST of Osq5.

3.1.2. The slope facies and basin facies

The Osq5 corresponds to the lower to middle part of the Jiuxi Formation in the slope facies in the Gaoqiao section,which includes grey, yellow-greenish silty mudstones (Fig.5b). The transgressive surface is consistent with the boundary between the Jiuxi Formation and the Madaoyu Formation.The TST and HST cannot be distinguished in the slope facies area.

The Osq5 corresponds to the lower to middle part of the Qiaotingzi Formation in the basin facies in the Xianxi section and Dafu section, Anhua County, Hunan Province (Fig. 5c).The turbidite fan is developed at the base of Qiaotingzi Formation in the Xianxi section, which is regarded as the LST. The succession of the turbidite fan is developed as follows: grey thick bedded fine grain sandstones containing conglomerates, mainly quartz sandstones, most of them less than 0.2 cm in diameter, distributed on the bedding surface.The sandstones are laminated, interbedded with thin layers of light grey mudstones. The light grey medium-thin bedded mudstones, argillaceous slates intercalated with thin siltstone bands, are developed above fine sandstones.

Another two episodes of turbidite events, which are characterized by the medium-fine quartz sandstones in the lower and fine quartz stones intercalated with muddy siltstone in the upper part, can be observed above the turbidite event at the base of the Qiaotingzi Formation.

The TST is a nearly 100 m thick fining upward retrogradation succession, corresponding to the yellow greenish medium-thick bedded muddy siltstones, silty mudstones or silty mud slates bearing graptolite fossils in the lower part of the Qiaotingzi Formation above the turbidites.

The highstand systems tract is an obvious progradation succession, made up by two parts, i.e., yellow greenish thick bedded-massive laminated argillaceous siltystones in the lower part; yellow greenish, grey greenish medium-thin bedded siltystones, mudstones in the upper. The lamination is more developed, and the thin siltstones or/and fine sandstone interbeds increase upwards (Fig. 3).

3.2. The second 3rd grade sequence of the Middle Ordovician(Osq6)

3.2.1. The platform facies

The TST of Osq6 is the succession of the thin bedded nodular limestones, thin bedded nodular limestones interbedded with thin bedded marls, which are developed above the sparrudites at the base of the upper Gunitan Formation in the Three Gorges area. The 4 m thick light grey medium-thick bedded limestones at the top of the Guniutan Formation is the HST of Osq6. The lack or incompleteness of the Hustedograptus teretiusculus graptolite zone or the Pygodus serra conodont zone in the Miaopo Formation,which is distributed in the depressions of the platform, shows that the uneven surface at the top of the Guniutan Formation is probably the I type sequence boundary.

3.2.2. The slope facies and basin facies

The sequence corresponds to the Middle part of the Jiuxi Formation, which is a silty mudstone succession without any obvious changes in lithology upwards in the slope facies area.

The Osq6 corresponds to the upper part of the Qiaotingzi Formation in the basinal facies zone. The LST is characterized by 3 turbidite successions, of which the typical turbidite succession is developed in ascending order as follows in Xianxi section, Anhua County:

(i) The fine sandstone containing conglomerates at the base.

(ii) The rhythmic succession of fine sandstones interbedded with siltstones. The siltstones are yellow greenish, thick bedded, 130 cm approximately of each single bed; while the sandstones are light grey-white, thin bedded.

(iii) The yellow greenish medium bedded siltstones,argillaceous siltstones, horizontal stratification developed.

(iv) The yellow greenish thick bedded silty mudstones.

The TST of Osq6 is a grey thin bedded argillaceous slate,2–10 cm thick of each single bed, horizontal stratification.The laminar sandstones are interbedded in the slate, the frequence of the sandstone interbeds decrease upwards. A distal turbidte, is characterized by the rhythmic deposit of the grey siltstones, argillaceous siltstones alternated with light grey fine sandstone laminae in the lower part, and argillaceous siltstones in the upper with increase of the argillaceous component, also can be observed in the TST.

The HST of Osq6 is a progradation succession of coarsening upwards and upward increase of the thickness of the individual beds. The grey thin-medium bedded argillaceous siltstones with abundant light grey fine sandstone laminae, horizontal stratification, are developed in the lower part; the grey, grey greenish thick bedded to massive siltstone,argillaceous siltstone with plenty of fine sandstone laminas in the upper part (Fig. 3).

4. The classification and correlation of the sequence stratigraphy of the Upper Ordovician

4.1. The first 3rd grade sequence of the Upper Ordovician(Osq7)

Osq7 is a sequence which crosses the boundary between the Middle Ordovician and the Upper Ordovician. The transgression surface is a diachronous surface according to the graptolite zones in the different facies.

4.1.1. The platform facies

The appearance of the graptolite fauna of the H.teretiusculus zone and abundant shelly fossils, occurred in the yellow greenish shale at the base of the Miaopo Formation,indicates the initial transgression of the Osq7 in Three Gorges area. The maximum flooding surface is marked by the appearance of Nemagraptus gracilis. The yellow-greenish shale of the N. gracilis zone represents the condensed interval of the Osq7. The yellow greenish argillaceous siltstones containing the abundant benthic fauna of trilobites and brachiopods without the presence of graptolite faunas above the N. gracilis zone, implying a relatively sea level drop, is the HST of the Osq7.

4.1.2. The slope facies zone and the basin facies zone

The Osq7 in the slope facies zone corresponds to the Sherenwan Formation, which is now identified as the upper part of the Jiuxi Formation. The lithology of the Sherenwan Formation can be divided into three parts, i.e., the grey greenish silty shales yielding abundant graptolites and trinucleid trilobite in the lower part; The grey, grey greenish banded calcareous shales with a few bean-like limestone lenses in the middle, yielding the abundant graptolites Nicholograptus fasciculatus, Climacograptus sp., etc.; and the grey yellowish, yellow greenish thin bedded limestone intercalated with shale in the upper part, yielding the graptolite Nicholograptus fasciculatus, Climacograptus sp.,Aulograptus leeszukangi, etc. (Lai CG et al., 1982). The graptolite fauna belongs to Nicholograptus fasciculatus zone of mid Darriwilian in age. Lai CG et al. (1982) also noted that the Nemagraptus gracilis had been found at the top of this formation. Consequently, the Sherenwan Formation spans a long period of time from the late Middle Ordovician to the earliest Upper Ordovician. In other words, the transgression of Osq7 is started much early in the slope facies zone. The transgression surface is obviously a diachronous surface from the slope facies zone to the platform facies zone northward.

In the basin facies zone, Osq7 is corresponding to the thin bedded chert intercalated with carboniferous shale of Yanxi Formation, yielding abundant graptolite. It is impossible to differentiate the systems tracts.

4.2. The second 3rd grade sequence of the Upper Ordovician(Osq8)

4.2.1. The platform facies zone

The platform facies zone of the Osq8 in the Three Gorges area, corresponds to the Baota Formation, which is a succession of grey purple medium-thick bedded bioclastic micrites intercalated with a few grey nodular hydromica micrites containing plenty of cephalopods (Fig. 6f). The typical “polygonal marking” structures are well developed in the micrite, which actually represent shrinkage cracks in deep water (Fig. 6d). The brachiopod assemblage BA4-5 (Zeng QL, 1991), the SSI value of the large cephalopod shell and the biofacies of trilobites (Zhou ZY et al., 2000) indicate that the Baota Formation is a deep-water deposit. The water depth is estimated at up to 100m in Three Gorges area. The Baota Formation is the new transgression deposit after the regression at the top of Miaopo Formation. The light grey greenish or light purple grey medium-thick bedded bioclastic nodular micrites, yielding the brachiopod, trilobite and a few cephalopod, are developed in the lower part of the Linxiang Formation above the Baota Formation. Either the abundance or the diversity of the faunas of the lower part of the Linxiang Formation obviously is lower than in the Baota Formation.The lower part of the Linxiang Formation is regarded as the HST herein according to sedimentary characteristics and biofacies.

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 Formation 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.

4.2.2. The slope facies zone and the basin facies zone

Lai CG et al. (1982) claimed that 6m thick medium-thick bedded argillaceous limestones are developed in the Jiuxi section representing the slope facies zone. The authors try to but did not find the limestones of the Baota Formation in the Jiuxi area. The coeval strata, the siltstone of the Nanshichong Formation is found in the Gaoqiao section, Cili County,Hunan Province. The Nanshichong Formation is the clastic wedge generated by the thrust northward of the Cathaysian old land.

The grey medium-thick bedded mudstones and silty mudstones of the Nanshichong Formation are also found in the basinal facies zone, i.e. the Dafu section in Anhua County,Hunan Province. Laminations are well developed within every single bed (Fig. 6b). Each single bed is ca. 70 cm thick.Trace fossils are extremely abundant in the silty mudstone.

The Osq8 is corresponds only to the lower part of the Nanshichong Formation according to the graptolite faunas below and above the Nanshichong Formation.

4.3. The third 3rd grade sequence of the Upper Ordovician(Osq9)

The TST of Osq9 is represented by the grey thin-medium micrite intercalated in thin bedded nodular micrite, yielding cephalopods, conodonts, trilobites, etc. of the middle part of the Linxiang Formation in the Three Gorges area. The trilobites belong to the early Insculptus facies according to Zhou ZY et al. (2000).

The argillaceous/calcareous component ratio increases due to increased terrestrial debris input, along with the appearance of the yellow greenish, thin bedded nodular limestones of the Nankinolithus trilobite zone. There is a 1m thick mudstone layer at the top of the Linxiang Formation at Hongshilin section, Guzhang County and Jiaoziya section, Sangzhi County, Hunan Province. The characteristics of the faunal turnover include the decline of the cephalopod assemblage and the flourish of benthic trilobite Nankinolithus assemblage and the Foliomena brachiopod community. All these phenomena show that the succession of the upper part of Linxiang Formation possess the characteristics of the HST of Osq9.

The Osq9 corresponds to the upper part of the Nanshichong Formation. But the Osq9 is not well distinguished with the Osq8 in the slope and basin facies zones.

4.4. The fourth 3rd grade sequence of the Upper Ordovician(Osq10)

The yellow greenish muddy shales, whose thickness varies in different areas between 0.2–1 m, due to the initiation of the transgression at the base of Wufeng Formation (Fig.6e). The graptolite fauna in the shale corresponds to the D.complanatus zone. The TST of the Osq10 in the Three Gorges area is made up of black carbonaceous shales intercalated with thin bedded cherts. The graptolite zonation demonstrates that a rise of the sea level from the Dicellograptus complanatus zone to the maximum flooding surface in the Tangyagraptus typicus subzone of the Paraorthograptus pacificus zone.

The HST of Osq10 starts at the base of the Diceratograptus mirus subzone of the P. pacificus zone,coinciding with a major global extinction event. The FAD of Normalograptus extraordinarius (Sobolevskaya) marks the Hirnantian GSSP level (Fig. 6a). The shelly fossils, i. e. the Hirnantia-Dalmanitina shelly fauna, which is a key element for recognition of the Hirnantian Stage, occurs in the argillaceous limestones and marks the maximum regression due to the Gondwana glaciation. The uppermost Ordovician graptolite zone, the zone of Normalograptus persculptus,marks the onset of a new transgression event in south China.

The Osq10 corresponds to the chert beds of the Wufeng Formation in the Gaoqiao section and the Anhua section,which represent the typical slope facies and basin facies sections, respectively (Fig. 6c). The systems tract is impossible to distinguish. The Tianmashan Formation in the Qidong and Shuangfeng areas can be regarded as coeval strata. Although the thickness of the Tianmashan Formation is over 1000 m, it corresponds only to the deposition of the Dicellograptus complexus zone. The uppermost Ordovician is not represented and the Tianmashan Formation is unconformably overlain by the Tiaomajian Formation of the Middle Devonian. This is the sedimentary response for the Guangxi orogeny movement beyond the topic of this paper,which is thoroughly discussed by Chen X et al.(2012, 2013).

5. Conclusions

The Ordovician successions are different in various facies zones in South China. An unconformity surface can be observed at the base of the Ordovician in the platform facies zone and can be traced in the slope facies zone. There are 4 third grade sequence can be recognized and correlated in different facies zone during the Early Ordovician, although the response for the sea-level fluctuations is much more obvious in the platform facies zone than in the slope and basin facies zone.

It is witnessed a sustainable large-scale sea-level rising during the Middle and Upper Ordovician. The transgression is started at late of Early Ordovician, which onlap surface is diachronously changed from the level of Acrograptus filiformis zone in northern Guizhou to the level of Dydimograptellus bifidus in western Hubei. The Osq5 to Osq10 can be recognized and correlated in different facies.The study on the sequence stratigraphy and correlation of this time interval will provide powerful evidence for the analysis of the onset of the evolution of the foreland basin triggered by the Guangxi orogeny movement.

Acknowledgment

This research is financially supported by the Natural Science Foundation of China (41472031) and the Gelogical Survey of China (121201009000172111, 121201009000 150017). This paper is also a contribution to the IGCP Project 591, The Early to Middle Paleozoic Revolution. The authors would like to express our sincerely thanks to J. Maletz,anonymous reviewers and Dr. Yan Yang for their valuable suggestions and comments after critical reading of the manuscript.