K-bentonite, black-shale and flysch successions at the ... · Yangtze Block and its implications for the evolution of Gondwana ... collision had occurred long ago, any fossil suture

Gondwana Research 15 (2009) 111–130

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K-bentonite, black-shale and flysch successions at the Ordovician–Silurian transition,South China: Possible sedimentary responses to the accretion of Cathaysia to theYangtze Block and its implications for the evolution of Gondwana

Wenbo Su a,⁎, Warren D. Huff b, Frank R. Ettensohn c, Xiaoming Liu a, Ji'en Zhang d, Zhiming Li e

a School of Earth Sciences and Resources, China University of Geosciences (Beijing), Beijing 100083, Chinab Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013, USAc Department of Earth and Environmental Sciences, University of Kentucky, Lexington, KY 40506-0053, USAd Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, Chinae Faculty of Earth Science, China University of Geosciences (Wuhan), Wuhan 430074, China

⁎ Corresponding author.E-mail address: [email protected] (W. Su).

1342-937X/$ – see front matter © 2008 International Adoi:10.1016/j.gr.2008.06.004

A B S T R A C T
A R T I C L E I N F O
Article history:
The K-bentonite, black shale Received 22 November 2007Received in revised form 4 June 2008Accepted 18 June 2008Available online 2 July 2008
Keywords:Cathaysia BlockYangtze BlockK-bentonite-bearing black shaleFlyschGeochemistryGondwanaOrdovician–Silurian transition

and flysch successions at the Ordovician–Silurian transition in South China havebeen the subject of comprehensive investigations relative to the probable accretion of the Yangtze Block andthe questionable Cathaysia Block. First, the geochemical analyses of K-bentonites show that the parentmagma originated in syn-collisional, volcanic-arc and within-plate tectonic settings, which produced mainlyintermediate-to-felsic series magmas, associated with continuous collision and subduction of paleo-continental blocks/arcs. Further, the regional distribution of K-bentonite thickness indicates that voluminousexplosive volcanism was located in the present southeastern shoreline provinces of China. Secondly,northwestwardly migrating, Ordovician–Silurian, transitional flysch successions, and the accompanyingdiachronous K-bentonite-bearing black-shale interval, as well as the related, overlying, shallowing-upwardsuccession at the interior of the Yangtze Block, developed as an unconformity-bound sequence that mirrorsforeland-basin tectophase cycles in the Appalachian basin. The above features suggest that the sequenceaccumulated in a similar foreland basin, which formed in response to adjacent deformational loading in anorthwesterly migrating orogen located to the southeast. Geochemical and paleocurrent data from theturbiditic flyschoid sandstones also support these depositional settings. Accordingly, it seems that all criteriastrongly support the presence of an Ordovician–Silurian, subduction-related orogen resulting from collisionwith a block to the southeast that must have been the original “Cathaysia Block” of Grabau and later workers.The K-bentonite, black-shale and flysch successions can be regarded as distal, foreland responses to thecontinuous northwestward collision and accretion of the Cathaysia Block to the Yangtze Block. Hence, weprefer to suggest that the suture zone with the sensu stricto Cathaysia Block probably developed alongpreviously identified late Early Paleozoic suture relicts in the shoreline provinces of southeast China. On theother hand, although accretion of fragments with Cathaysian affinities to the Yangtze Block may have begunas early as Middle to Late Proterozoic time, the Ordovician–Silurian orogeny described above probablyreflects the final phase of accretion between the two blocks. Moreover, when combined with similar peri-Iapetan orogenic events in other areas during the same period, this accretion event may have been part of amajor stage of global tectonic reconstruction in the evolution of Gondwana.

© 2008 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

Traditionally, efforts to determine the boundary between twopaleocontinents, or that between a continent and a volcanic island-arcthat were accreted during related orogenic periods, have been basedmainly on the identification of fossil sutures. In such studies, attentionis particularly paid to the occurrence of ophiolite suites, mélanges, and

ssociation for Gondwana Research.

syn-collision magmatism, as well as to the relevant metamorphism.Further, paleobiogeographic data and the polar wandering paths ofthe different blocks also are commonly considered. However, if thecollision had occurred long ago, any fossil suture or related evidencewould have been deformed and severely superimposed duringsubsequent orogenic periods (e.g., South China, see Wang et al.,2005; Li and Li, 2007), and it would be extremely difficult to identifyand determine the exact accretion scenario.

Then the question arises, is it possible to determine the possibleaccretion of continents through the sedimentary record in the relatedstable cratonic areas and in the remains of an adjacent foreland basin?

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112 W. Su et al. / Gondwana Research 15 (2009) 111–130

The answer is, of course, yes. In fact, many people have tried successfullythrough studies of certain sedimentary sequences in various tectonicsettings, such as thewell-knownK-bentonite clusters from theAmericasand Europe (e.g., Huff et al., 1992; Astini et al., 2007), the black/darkshales inNorth America (e.g., Ettensohn,1994), and theflyschdeposits ofthe European Alps (e.g., Hsü, 1972; Sinclair, 1997a). Furthermore, someauthors have used computer models of various kinds to support thedepositional records mentioned above (e.g., Ettensohn, 1994; Sinclair,1997b; Allen et al., 2001; Ettensohn, 2005). In this study,wewould like tointroduce an integrated case study of K-bentonite-bearing black shalesand flysch successions around the Yangtze Block during the Ordovician–Silurian transition in South China, with the goal of illuminating the long-lived debate about the Cathaysia Block and its evolution.

The “Cathaysian Oldland” or “Cathaysia Block” of Grabau (1924)has been considered to be a significant tectonic unit in southeastChina for many years. Although both the definition and extent of this“block” are still debated, this block or small plate is currently situatedon the southeast margin of present-day China and is bordered to thenorthwest by the former Yangtze Block or plate in southeast-centralChina (Fig. 1). Obviously, determining its relationship to the YangtzeBlock and its geologic history has important implications for under-standing the tectonic evolution of both South China and East Asia.

Some workers have suggested that the area of the Cathaysia Blockis a folded orogenic belt rather than a cratonic plate (e.g., Hsü et al.,1988, 1990). However, more recent studies have suggested that therewas a collision zone at the southeast margin of the Yangtze Block thatresulted from the collision of Yangtze and Cathaysia, which began inMiddle to Late Proterozoic time (e.g., Chen et al., 1991; Xu et al., 1992;Li et al., 1995; Chen and John, 1998; Zhao and Cawood, 1999; Li et al.,2002; Ye et al., 2007;Wang et al., 2008; Li et al., 2008a,b) and probablylasted through Early Paleozoic time, although the inferred position ofthe suture is somewhat different in the interpretation of each workeror group (e.g., Wang, 1985; Shui, 1988; Xu et al., 1996; Wang et al.,2005; Fitches and Zhu, 2006). Others, however, do not think that thesuture developed until Early Paleozoic time, and they have suggestedthat the collision occurred at another position to the southeast of thepresent mainland of China and resulted from collision between a“united Yangtze–Cathaysia plate” and an undetermined cratonic plate(e.g., Rong and Chen,1987; Chen et al.,1995a; Chen andMitchell,1996;Rong et al., 2003; Chen et al., 2004). Moreover, still other authors thinkthat there were several terranes or microcontinents southeast of theYangtze Block and that evenmore than three “sutures” or convergencezones may be present in the area (e.g., Guo et al., 1984; Charvet et al.,1996;Wu,1999; Charvet et al.,1999; Hoe and Rangin,1999) (see Fig.1).Metcalfe (e.g., 1996, 2006) has mentioned for years that the “SouthChina Block (including mainly the Yangtze and ‘Cathaysia’) is acomposite terrane” with a complicated history. Obviously, severalmajor problems involving relationships between the Yangtze andCathaysia blocks must still be considered. These problems include:whether or not the Cathaysia Block existed in southeast China; wherethe western boundary of Cathaysia was or what the exact definition ofthe term “Cathaysia Block” is; and what the nature of the accretionprocess was.

In the following discussion, we would like to examine several newlines of evidence that have a definite bearing on the above questions.Oneof these centers on themineralogical and chemical composition anddistribution patterns of altered volcanic ash falls, or bentonites.Ordovician–Silurian K-bentonites in Europe and the Americas haveproven to be very useful tools for probing the tectonic setting of theparent magmas of the relevant volcanoes in addition to serving asunique chronostratigraphic marker beds for the timing of collisionalevents (e.g., Huff et al.,1992; Haynes,1994; Kolata et al.,1996; Huff et al.,1998a,b; Huff et al., 2003; Batchelor et al., 2003; Ramos, 2004; Astini etal., 2007; Zhang et al., 2007).

On the other hand, the huge rhythmic sandy-muddy turbiditesuccessions in marine basins near former plate margins, especially

foreland basins, commonly reflect flysch and related deposits that aretypically correlated with different sedimentary environments inparticular tectonic settings (cf., Bouma, 2004). In addition, not onlycan the position of these deposits in a sequence provide importantinterpretive tectonic information (e.g., Ettensohn, 1994; Sinclair,1997a,b; Allen et al., 2001), but analyses of clastic components,litho-chemical elements from the resulting rocks, and paleocurrentdata from the successions can also provide additional information ondepositional environments, tectonic settings and source-area char-acteristics (e.g., Hsü, 1972; Bhatia, 1983; Bhatia and Crook, 1986; Roserand Korsch, 1988; Bouma, 2004).

Realizing that black- or dark-shale deposition, both in foreland andneighboring intracratonic basins from North America, reflects distincttectophases of deformational activity in any one Paleozoic orogenydue to rapid, loading-related, lithospheric subsidence and sedimentstarvation in the resulting basins, Ettensohn (e.g., 1991, 1994, 1998a,b,2004) proposed a flexural model based on previous studies (e.g.,Karner and Watts, 1983; Quinlan and Beaumont, 1984; Beaumontet al., 1988) that accurately predicts associated stratigraphic succes-sions in the resulting foreland basins. According to this model, black-shale deposition and an underlying unconformity, as well as anoverlying, shallowing- and coarsening-upward, clastic wedge form agenetic sequence that is interpreted to have resulted from forebulgeand foreland-basin migration as a consequence of progressive over-thrust loading in an adjacent orogen during active subduction(Ettensohn, 2005). It is noteworthy that other authors have revealeda similar, stratigraphic, flexural-response process in the Swiss Alps(e.g., Sinclair, 1997a,b; Allen et al., 2001), and the recurring success inmodel application suggests that the flexural model may be reasonablyapplied in other convergent-margin settings.

Theoretically, an active subduction zone should occur with atrench, a volcanic arc, and a foreland or retro-arc basin in time andspace. Although most of these large-scale, tectono-geomorphicfeatures will not be preserved, their presence can be inferred fromstudies of the preserved sedimentary record in both the foreland andneighboring intracratonic basins. Hence, an integrated study of the K-bentonite, black-shale, and associated flysch deposition within thesame stratigraphic framework can be helpful in understanding thelikely position and age of a former, deformational orogen and therelevant plate-movement processes that generated it. Here, we wouldlike to introduce such a study based on new stratigraphic andsedimentologic data from South China near the Ordovician–Siluriantransition and use it to suggest an interpretation relative to theunresolved problems about relationships between Yangtze andCathaysia blocks noted above.

2. Geological setting

During early-mid Early Paleozoic time, depositional facies on theYangtze Block, which includes the present northwestern and centralparts of South China, encompassed three different facies belts, theYangtze region, the Jiangnan (south of Yangtze River) region, and theHua'nan (South China in the narrow sense) region, which representplatform, slope and basinal environments, respectively (Fig. 1). Someworkers prefer to call the first region, in a narrow sense, the “YangtzePlatform,” whereas the latter two regions are commonly joinedtogether with the designation, “slope-basin” or “Zhe-Gan-Xiang-GuiMarginal Sea.” The distribution of these environments, however,apparently changed during Late Ordovician to Early Silurian time (seeWang, 1985; Yang et al., 1986; Wang and Chen, 1995; Mao and Wang,1999; Rong et al., 2003; Chen et al., 2004). Although severelydeformed, what remains of the so-called Cathaysia Block in a broadsense occurs just southeast of the Yangtze Block, but differentviewpoints still prevail about the boundary between the two blocks(see Fig. 1; e.g., Li et al., 1995, 2002; Chen et al., 2004; Fletcher et al.,2004; Wang et al., 2005; Metcalfe, 2006, and references therein).

Fig. 1. Sketch map showing the subdivision of tectonic units and depositional facies in South China, the location of studied outcrops, as well as positions of the main proposed sutures between the Yangtze and Cathaysia blocks. Filled triangles:representative K-bentonite-bearing O–S transitional sections in this paper: 1—Wangjiawan-Huanghuachang, Yichang, Hubei; 2— Jiuxi, Taoyuan, Hu'nan; 3— Beigong, Jingxian, Anhui; 4— Gaojiabian, Jurong, Jiangsu; 5— Luojiang, Chengkou,Chongqing; 6— Nanbazi, Tongzi, Guizhou. Empty triangles : other studied sections in previous years. Filled circles: O–S transitional flysch succession sections in this paper: 7— Luguan, Xinhua, Hunan (Zhoujiaxi Group, Early Llandovery); 8—

Tianmashan, Shuangfeng, Hunan (Tianmashan Formation, mid-late Ashgillian). Relevant province abbreviations: AH: Anhui, CQ: Chongqing, FJ: Fujian, GD: Guangdong, GS: Gansu, GX: Guangxi, GZ: Guizhou, HB: Hubei, HN: Hu'nan, HEN:He'nan, JS: Jiangsu, JX: Jiangxi, QH: Qinghai; SC: Sichuan, SD: Shandong, SAX: Shanxi, SX: Shaanxi, TW: Taiwan, YN: Yun'nan, XZ: Xi'zang (Tibet), ZJ: Zhejiang. Kang-Dian-Qian-Gui Oldland: abbreviation of the “West Sichuan (Kang)-Yun'nan(Dian)-Guizhou (Qian)-Guangxi (Gui) Oldland,” traditional name of the uplands southwest of the Yangtze Platform; Zhe-Gan-Xiang-Gui Sea: abbreviation of the “Zhejiang (Zhe) -Jiangxi (Gan) -Hu'nan (Xiang) -Guangxi (Gui) Sea,” traditionalname of the southeastern Marginal Sea of Yangtze Platform, approximately equivalent to north part of “Hua'nan (South China) Region.”. Proposed suture: ①. Paleozoic–Mesozoic suture position of Hsü et al. (1988, 1990) and Meso-Neoproterozoic suture of Xu et al. (1992), Wang and Mo (1995), Li et al. (1995, 2002), Xu et al. (1996), Zhao and Cawood (1999), Wang et al. (2008), etc.; ②. Paleozoic–Mesozoic suture position of Hsü et al. (1990) and Paleozoic suture of Shui(1988),Wang andMo (1995), Xu et al. (1996),Wang et al. (2005), etc;③. Indefinite Paleozoic suture position of Rong and Chen (1987), Li and Quan (1992), Chen et al. (1995a, 2004), and theMeso-Cenozoic suture of many others, represented bythat of Wang and Mo (1995) and Wang et al. (2005). In addition, Guo et al. (1984) and Wu (1999) proposed more sutures or convergence zones besides those shown at ① and ②.

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During the Ordovician–Silurian transition (late Katian or midAshgill–early Llandovery), a series of grayish-black, siliceous, carbo-naceous shales, interbedded with light-gray calcareous mudstones inmiddle to upper parts of the unit, were deposited widely across theYangtze Block in South China. Since the 1930s, this dark-shalesequence has been one of the most studied stratigraphic intervals inChina because of its abundant fossils and the presence of a prominentmass-extinction event within it (e.g., Sun, 1931; Mu, 1954, 1988; Rongand Chen, 1986, 1987; Wang et al., 1987; Chen et al., 2000, 2005; Ronget al., 2002). Based on original definitions (Chen et al., 1995b; Wanget al., 1996), these dark shales are parts of two different lithostrati-graphic units, named the Wufeng and Longmaxi formations, respec-tively, in ascending order (Fig. 2). Moreover, the Wufeng Formationcan be divided into the lower Graptolite-bearing Shale Member (GSM,late Katian or mid–late Ashgill) and the upper Guanyinqiao Member(GM, ~Hirnantian). So far, more than ten graptolite zones (orsubzones) have been recognized and correlated precisely and widelyin this dark-shale sequence (Chen et al., 2000, 2005; Zhan and Jin,2007).

Based on sequence-stratigraphic interpretations (Su, 1999, 2001;Su et al., 2003a), the GSM, containing mainly the Dicellograptuscomplanatus–Paraorthograptus pacificus graptolite zones (Chen et al.,2000, 2005), has been regarded as the transgressive-through-high-stand systems tracts of a typical sea-level sequence. The overlying GMcontains the interbedded calcareous mudstones mentioned abovealong with the famous cold-water Hirnantia shelly fauna (e.g., Ronget al., 2002), and represents a shelf-margin systems tract (SMST) at the

Fig. 2. Regional stratigraphic framework from mid-Late Ordovician to Devonian time in diffafter Gradstein et al. (2004), Chen et al. (2006), Zhan and Jin (2007); lithostratigraphic subdet al. (1992), Chen et al. (1995b), Wang et al. (1996), Bureau of Geology and Mineral ResearchGSM: Graptolite-bearing Shale Member of Wufeng Fm.; UM: upper member, LM: lower mbearing shales or cherty slates, bearing K-bentonites and abundant graptolites.

margin of the Yangtze Platform, which coincided with the short-lived,global, end-Ordovician, Gondwanan, glacial low-stand. The upper partof the black-shale sequence is included within the lower member ofthe Longmaxi Formation (Fig. 2) and represents the global, post-glacial transgression in early Llandoverian (Rhuddanian) time.

3. K-bentonite, black-shale, and flysch successions during theOrdovician–Silurian transition, South China

Recently, a number of K-bentonite beds have been recognized inthe Ordovician–Silurian transition (mid Ashgill–Rhuddanian; thepresent late-Katian–Rhuddanian, see Chen et al., 2006; Zhan and Jin,2007) interval on the Yangtze Block in south China (Su et al., 2003a,b).Fig. 3 shows some occurrences of the K-bentonite beds at differentplaces in South China.

Furthermore, preliminary geochemical analysis of the K-bentoniteshas suggested a parent magma that produced an explosive eruption ina collisional or accretionary-zone tectonic setting (cf., Su et al., 2003b).These preliminary K-bentonite analyses also suggest a parent magmawith a composition in the trachyandesite to rhyodacite range withsome rhyolite, all of which indicates an origin in volcanic-arc (VA) andsyn-collison (syn-COL) towithin-plate (WA) tectonic settingswhen thedata (Table 1) areplotted in thewidely useddiscriminationdiagrams ofWinchester and Floyd (1977) and Pearce et al. (1984) (Fig. 4A, B).

Along the southeast margin of the Yangtze Block, i.e., in thesoutheastern part of the slope-basin area in Fig. 1, typical flyschoidgraywacke successions have also been identified from both the Early

erent areas around the Yangtze platform, South China. Chronostratigraphic subdivisionivision and correlation compiled mainly after Fu and Song (1986), Liu and Fu (1989), Jinof Hu'nan Province (1997), and Su (2001). GYQM: Guanyinqiao Member of Wufeng Fm.;ember. Darkened zones: interval of black or dark, carbonaceous–siliceous, graptolite-

Fig. 3. Field photos of the K-bentonites at some outcrop sections in South China. A: Lower of the Wufeng Formation, Huanghuachang, Yichang, Hubei (Fig. 1, location1). The K-bentonites are the light-gray layers, and the arrows shows its position. Steel tape as scale, about 100-cm long; B: Guanyinqiao Member of Wufeng Formation, Wangjiawan, Yichang,Hubei (Fig. 1, location1). The K-bentonite is the yellowish layer in themiddle part of the photo, and the arrow shows its position. Note-book as scale, about 20-cm high; C: Lowest partof theWufeng Formation, Gaojiabian, Jurong, Jiangsu (Fig.1, location 4). The K-bentonites are the gray layers, and the arrows shows its position. Note-book as scale, about 20-cm high;D: Lower part of the Longmaxi Formation, Luojiang, Chengkou, Chongqing (Fig. 1, location 5). The K-bentonite is the yellow layer, and the arrow shows its position. Ball-point pen asscale, about 15-cm long; E: Lower part of the Wufeng Formation, Jiuxi, Hu'nan (Fig. 1, location 2). The K-bentonite is the greenish layer, and the arrow shows its position. Ball-pointpen as the scale, about 15-cm long. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

115W. Su et al. / Gondwana Research 15 (2009) 111–130

Llandovery (Rhuddanian) Zhoujiaxi Group and the mid to late Ashgill(Katian to Hirnantian stages) Tianmashan Formation in the central tosouthern part of Hunan Province, South China. Both successionsclearly show a northwestward progradation in space and time,suggesting a northwestwardly advancing orogenic front that “pushed”them cratonward from the southeast.

More interestingly, all the known K-bentonites occur in thegraptolite-bearing black shales of the Ordovician–Silurian transitioninterval (Su et al., 2003a,b), and the black-shale interval also shows anorthwestward, diachronous, onlapping migration from southernHu'nan Province to the western Hubei Province, similar to thegreywacke sequence mentioned above (Fig. 2).

Logically, it is easy to recognize that the in-step migration of K-bentonites and flysch-black-shale couplets during the Ordovician–Silurian transition in south China suggests a kind of parageneticrelationship or coupling of depositional events. Hence, the presence ofK-bentonites with sources in a collision zone to the southeast,different Ordovician–Silurian flysch successions that progradednorthwestwardly from nearly the same source areas at the sametime, and an underlying northwestwardly migrating black-shaleinterval all seem to be related, synchronous responses to the samephase of the orogenic activity farther to the southeast in what musthave been a collision zone at the southeast margin of the YangtzeBlock. We will further explore each of the lines of evidence below.

Table 1Selected trace-element and REE data for Ordovician–Silurian K-bentonites in South China

No. Sample K2O Y Nb Zr Ti Ga Rb Ta Yb

1 2-22-02 4.69 55.2 32.7 273 4383 25.4 173 1.7 4.342 2-22-03 6.07 55.8 92.9 591 5040 29 194 4.1 5.043 2-22-05 5.23 31.0 42.5 333 4714 26.2 178 2.7 3.44 2-22-09 4.68 80.9 73.6 629 4235 30.1 153 3.9 7.525 2-22-10 5.79 78.5 27 508 7024 22.8 151 3.7 10.676 4-13-02 6.57 106.2 450.2 2386 19,190 30.3 225 29.8 10.357 4-13-07 4.87 29.4 26.6 225 5267 24 188 0.7 3.788 4-13-15 7.27 40.5 27.5 372 9781 27.6 213 2.3 7.319 4-14-02 3.10 37.6 95.9 298 14,013 13.4 90 6.3 5.2610 4-14-45 7.18 34.5 20.3 336 1992 23.6 199 3.0 4.8811 7-17-02 7.43 110.5 342.6 1797 6011 40.9 262 25.6 9.2312 7-17-03 5.33 42.5 112 702 4685 2839 197 6.2 4.0413 7-17-04 6.09 100.7 242.5 1079 6832 27.5 211 13.7 7.4714 7-19-01 3.58 62.0 195.4 746 3937 6.1 96 11.4 3.9615 7-19-02 5.07 45.3 26.9 370 9553 25.7 203 2.6 4.7716 7-19-03 3.54 35.6 18.3 243 7778 21.5 139 1.6 3.5217 7-19-04 5.91 39.0 13.6 304 2803 29.3 230 2.6 4.2318 7-19-05 3.96 34.5 13.2 272 3563 20.4 151 2.2 3.8919 7-20-03 4.68 31.3 20.8 164 4240 23.9 185 1.7 3.3220 7-21-25 2.98 68.2 163.6 334 4793 8.2 81 12.0 5.4121 7-21-26 3.90 64.9 190 572 10,005 26.1 167 26.6 7.0922 7-21-27 4.54 45.5 19.5 266 5163 27.8 174 3.0 4.9123 7-26-01 3.24 74.1 169.1 543 11,312 25.4 97 8.3 6.1

Analyses performed at the Institute of Geophysical and Geochemical Exploration, China Geological Survey (Langfang, Hebei). Unit: μg/g (K2O: %). Sample 1–5: Yichang, Hubei; Sample6–9: Tongzi, Guizhou; Sample 10–23: Taoyuan and Taojiang, Hunan Province. Contents of K2O show the potassium-rich characteristics of the K-bentonites. All elements have beenplotted in related discrimination diagrams and suggested similar parent magmas with similar tectonomagmatic settings. For convenience, only the diagrams based on Y, Nb, Zr, Ti(TiO2) are shown as Fig. 4A and B.


3.1. Northwestward distribution of the K-bentonites

In recent years, more than twenty transitional Ordovician–Silurianexposures bearing K-bentonites have been observed in the area of theYangtze block (see Fig. 1), and this coverage provides the opportunity tocreate a preliminary isopach map and probe the potential of the K-bentonite distribution on the block within this time interval. Althoughpreliminary research has shown that most of the K-bentonites can becorrelated across significant distances (Su et al., 2003a), it is still difficultto precisely correlate the K-bentonites bed by bed from one section toanother. Importantly, however, there are some distinct physical surfacesthat can be correlated very precisely with litho-, sequence-, and bio-stratigraphic data in the Wufeng Formation. These surfaces include theboundary between the Wufeng and Linxiang formations, the boundarybetween the two members of the Wufeng Formation, as well as theboundary between the Wufeng and Longmaxi formations. All of theserepresent key surfaces of the related 3rd-order depositional sequencesin this transitional interval, such as the FFS (first flooding surface), themfs (maximum flooding surface) and the SB (sequence boundary) (e.g.,Su et al., 2003a; see Fig. 5). As a result, at the present, all the ash-fall bedsinWufeng Formation at each section can be divided into two parts. Oneis the lower GSM part (GSMP, complanatus–pacificus zones, mid-lateKatian), and the other is the upper GM part (GMP, extraordinarius–persculptus zones, ~Hirnantian). Thus, these two parts of the WufengFormation in different sections can effectively be regarded as respectiveisochronous units and correlated fairly well on the basis of biostrati-graphy and sequence stratigraphy. The combined thickness of K-bentonites in each part of each section can then be calculated andplotted on base maps. Fig. 5 shows fairly well the correlation betweentypical Ordovician–Silurian transition sections on Yangtze Platform.Fig. 6A and B shows the thickness changes of K-bentonites in two suchintervals based on the correlation in Fig. 5.

On the other hand, due to later Siluro–Devonian erosionaltruncation (Fig. 2), the Lower Silurian interval bearing the K-bentonites is commonly diachronous, incomplete, or lacking anisochronous horizon from which to correlate. Thus, both the numberand thickness of K-bentonites in different Lower Silurian sectionsaround the Yangtze Platform are difficult to calculate, and the relevantisopach map for this interval is unavailable at present.

Nonetheless, both Fig. 6A and B show the same clear increase intotal K-bentonite thickness toward the southeast, indicating that theoriginal late Katian (mid-late Ashgill) and the Hirnantian (latestAshgill) volcanic sources must have come from the southeastern partof present South China. Moreover, because all the volcanogenicphenocrysts in the Yangtze K-bentonites are very fined grained(cf., Su et al., 2003b), it seems that the volcanic sources must havebeen somewhat distal from the Yangtze Block, probably somewhere tothe east-southeast of present K-bentonite distribution. Takenwith theother lines of evidence already mentioned, K-bentonite distributionand thickness clearly support some sort of collisional, volcanic-arc-type setting southeast of the present position of the Yangtze Block (seeFigs. 1, 5, and 6A, B).

3.2. Northwestwardly migrating flysch successions

Based on previously published data (Bureau of Geology andMineral Research of Hu'nan Province, 1997), the Zhoujiaxi Group is athick flysch succession of sandstones and siltstones, intercalated withthin-bedded, gray-black, siliceous–carbonaceous shale or mudstoneand contains a few Early Llandovery (Rhuddanian) graptolites. Theunit may be up to 4000 m in thickness. This flysch sequence occursmainly in the central part of Hu'nan Province on the southeasterlysloping basin area of the Yangtze Block (Fig. 1) where it conformablyoverlies the Wufeng Formation, and is, in turn, truncated by LateSilurian/Early Devonian erosion (Fig. 2) (Bureau of Geology andMineral Research of Hu'nan Province, 1997). The present measuredsection is in the west of Xinhua County, Hu'nan Province (see Fig. 1,location 7).

Similarly, the Tianmashan Formation in south-southeastern partsof Hu'nan consists of a thick clastic sequence like the Zhoujiaxi Groupmentioned above, but it lacks mudstone and is locally intercalatedwith moderately thick-bedded, siliceous-carbonaceous shale, bearingmany Ashgillian graptolites. The top of the succession was alsotruncated by Siluro–Devonian erosion (Bureau of Geology andMineralResearch of Hu'nan Province, 1997) (Fig. 2). The section in this study islocated at Shuangfeng County about 100-km east-southeast of Xinhua(see Fig. 1, location 8). Fig. 7 shows outcrop characteristics of the twoflysch successions at related sections, whereas Fig. 8 shows petrologic

Fig. 4. A The Nb/Y–Zr/TiO2 diagram (after Winchester and Floyd, 1977) of the K-bentonites around the O–S boundary in South China (after Su et al., 2003b). B The Y–Nbdiagram (after Pearce et al., 1984) of the K-bentonites around the O–S boundary in SouthChina (after Su et al., 2003b). WPG: within-plate granite; ORG: oceanic-range granite;VAG+synCOLG: volcanic-arc granite and syn-collision granite.


textures in thin-sections of samples from different positions in thesame thick bed of the Zhoujiaxi Group in central Hu'nan.

Recently, both of the above two turbidite successions have beenrestudied and found to show the following characteristics typical offlyschoid graywackes:

a. Complete or incomplete Bouma sequences and other typicalturbiditic depositional textures (see Fig. 7 D);b. Rhythmic or cyclic, fining-upward successions beginning withsandstones, and locally siltstones, and capped with mudstones orshales (see Fig. 7 A, B);

c. Poorly sorted, complex, detrital components typical of gravity-flow processes (see Fig. 8);d. Both successions have wedge-shaped geometries severalhundreds to thousands of meters thick emanating from orogenicsources. The maximum thickness of the Tianmashan Formation ismore than 3000 m, and the Zhoujiaxi Group is more than 4000-mthick (see Bureau of Geology and Mineral Research of Hu'nanProvince, 1997).

To further understand the distribution and provenance of theflysch, the depositional environments involved, and the tectonicsetting of the flysch successions, several preliminary analyses werecompleted following the fieldwork, and they are summarized below.

First, power–law distribution (Carlson and Grotzinger, 2001) ofsingle-bed thicknesses of the sandstones from the flysch were plotted(Fig. 9). The total thickness of Zhoujiaxi Group in Xinhua is about2000 m. However, because parts of the group are covered, just part ofZhoujiaxi Group could be measured bed by bed along the bank of astream near Xinhua County in central Hu'nan (Fig. 1, location 7;Fig. 7A). Only 832 single beds were measured across about 160 m inthickness from this section. Fig. 9 shows the results of the thicknessdistributions from each single bed in the section.

The general outline of the plot is a smooth arc-like curve, while avery short segment of the curve appears to be a straight line. Thisspecific feature is useful for environmental interpretation.

According to Carlson and Grotzinger (2001), in general the arc-likeroller curve with a short straight-line segment represents minorerosion and amalgamation processes in a middle-fan environment(Carlson and Grotzinger, 2001, see their text-figs. 3 to 7, and Tables 1and 2). The present measured section of the Zhoujiaxi Group inXinhua, Hu'nan (Fig.1, location 7; Fig. 7A), consistsmainly of rhythmic,thick-to-thin, sandstone–mudstone couplets, commonly with currentripples on the bedding surfaces and truncations at the bed bottoms,which similarly suggest minor erosion and amalgamation processes.Hence, the present roller curve of the power–law distribution for theZhoujiaxi Group shown in Fig. 5 supports a middle submarine-fandepositional environment. Furthermore, because of the more distalnature of fan sediments, indicated both in the outcrop and in the thin-sections (see Figs. 7 and 8), it is likely that the source area of this flyschmust have been located quite far from its current position.

In addition, analyses of bulk-rock components can be used tointerpret the general tectonic setting of flysch source areas (Fig. 10). Inparticular, Fig. 10 shows tectonic-setting discrimination diagrams forsandstones (after Roser and Korsch, 1988) from the Zhoujiaxi Groupand the Tianmashan Formation. It is notable that both successionsreflect largely passive continental-margin settings, but before suchsettings could have been source areas, they must have been deformedand uplifted during the collision of the Yangtze and Cathaysian blocks.Interestingly, those points from the Zhoujiaxi Group are very close tothe active-continental-margin field in Fig. 10, suggesting that thepassive-margin setting was changing.

Finally, the directional sources of the flysch were determined withthe aid of paleocurrent orientation data. From the same section of theZhoujiaxi Group in Xinhua mentioned above, more than 100 sets ofcross-bed dip orientations, as well as directional aspects of currentripples and flute marks, from the turbiditic-sandstone successionwere measured. Fig. 11A shows clearly a northwestward optimumorientation, based solely on cross-bed dip orientations, with sourceareas to the southeast.

The same types of measurements were also attempted in theTianmashan Formation at the previously mentioned locality. Unfortu-nately, most of this section was covered and only a few cross-bed diporientations could be measured (cf., Fig. 7B). In Fig. 11B, however, thefew recovered orientations also generally show paleocurrent direc-tions similar to those of the Zhoujiaxi Group.

Fig. 5. Correlation between typical Ordovician–Silurian transition K-bentonite-bearing sections in south China. Sections: 1 —Wangjiawan-Huanghuachang, Yichang, Hubei; 2— Jiuxi, Taoyuan, Hu'nan; 3— Beigong, Jingxian, Anhui; 4 — Gaojiabian,Jurong, Jiangsu; 5 — Luojiang, Chengkou, Chongqing; 6 — Nanbazi, Tongzi, Guizhou. For the locations, see Fig. 1.

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Fig. 6. A Isopachmap (in centimeters) ofK-bentonites in theGraptolite-bearingMemberofWufeng Fm., South China. (Approximatelyequivalent to lateKatian ormid-late Ashgill stages, seeFigs. 2 and 5). B Isopach map (in centimeters) of K-bentonites in the Guanyinqiao Member ofWufeng Fm., South China. (Approximately equavalent to Hirnantian Stage, see Figs. 2 and 5).


To summarize, based on the above discussion, both the flyschsuccessions of the Zhoujiaxi Group (Rhuddanian) and the TianmashanFormation (late Katian) reflect deposition on submarine fans (Fig. 9) that

were apparently sourced by deformed passive-margin rocks (Fig. 10).Paleocurrent orientations suggest that both successions had similarsource areas quite far away to the southeast (Fig. 11A, B), and the

Fig. 7. Field photos of the flysch successions in central Hu'nan, South China. A: Outcrop of the Zhoujiaxi Group, Xihua, Hu'nan (Fig.1, location 7). Sitting observer at the left as the scale,about 90-cm high; B: Outcrop of the Tianmashan Formation, Shuangfeng, Hu'nan (Fig.1, location 8). Hammer inwhite cycle as the scale, about 40-cm long; C: Flute casts at the base ofone bed in the Zhoujiaxi Group (Fig. 1, location 8). Ball-point pen in white cycle as the scale, about 15-cm long; D: Deformed cross-bedding (portion C of a Bouma sequence) in theZhoujiaxi Group (Fig. 1, location 8). Bottle-top as the scale, about 3-cm in diameter.


distribution of the successions in time and space (Figs.1 and 2) indicatesthat they prograded successively cratonward toward the northwest.

3.3. Graptolite-bearing black shales and associated depositional successions

In the southeastern part of Hu'nan (see Figs. 1 and 2), the ChengbuFormation is a graptolite-bearing black-shale interval, which is earlyKatian (former late Caradoc to early Ashgill) (cf., Chen et al., 2006;Zhan and Jin, 2007) in age based on graptolites, such as Dicellograptusjohnstrupi, D. complanatus and others (Liu and Fu, 1989; Bureau ofGeology and Mineral Research of Hu'nan Province, 1997). In centralHu'nan (see Figs. 1 and 2), the black-shale interval is the WufengFormation, which contains typical late Katian–early Hirnantian(former mid-late Ashgill) (cf., Chen et al., 2006; Zhan and Jin, 2007)graptolites, including D. complanatus, D. complexus, Normalograptusextraordinarius and others (Bureau of Geology andMineral Research ofHu'nan Province, 1997). However, in northern Hu'nan and westernHubei, as well as other regions to the northwest (see Figs. 1 and 2), theblack-shale interval includes the Wufeng and lower parts of theLongmaxi formations, which reflect an age ranging from late Katian toRhuddanian based on a continuous series of graptolite zones from theD. complanatus through the Coronograptus cyphus zones, or evenmuch higher zones (see Wang et al., 1987; Chen et al., 1995b; Wanget al., 1996; Chen et al., 2000; Chen et al., 2005; Bureau of Geology andMineral Research of Hu'nan Province, 1997; Su, 2001).

Furthermore, in nearby parts of northern Hu'nan and westernHubei provinces on the cratonic platform area, an unconformity,interpreted recently to be a Type I sequence boundary (Su, 1999,2001), occurs at the base of the Wufeng Formation. Yet another

unconformity, moreover, progressively oversteps and truncateslithostratigraphic units, such as the Tianmashan Formation in south-ern Hu'nan, the Zhoujiaxi Group in central Hu'nan, and theHuixingshao and Shamao formations in northern Hu'nan or westernHubei from the southeast to the northwest. This is a pronouncedregional unconformity with a lacuna that encompasses much ofSilurian and Early to Middle Devonian time (Fig. 2), and theunconformity is widely believed to be evidence of the Caledonian-aged Guangxi orogeny in south China (e.g., Wang, 1985; Yang et al.,1986; Chen et al., 1995a; Wang et al., 2005). Another unconformity isalso present between the Wufeng and Longmaxi formations in theborder area between Hu'nan and Hubei (Wang, 1985; Bureau ofGeology and Mineral Research of Hu'nan Province, 1997), as well as inother provinces (Fig. 2). This unconformity occurs locally on a series of“submarine highs” noted recently by Chen et al. (2004) that roughlyparallel the proposed “Yangtze–Cathaysia suture” in the southeast(Fig. 12), and it was initially interpreted to have resulted from tectonicuplift (e.g., Mu, 1954). More recently, however, it was thought to havebeen a response to terminal Ordovician, Gondwanan, glacial draw-down (e.g., Rong and Chen, 1987; Su, 1999, 2001; Rong et al., 2002,2003; Chen et al., 2004; Su et al., 2007), but we will suggest anotherpossible origin in a later section.

Hence, like the overlying, flyschoid, clastic wedge, the graptolitic,black-shale interval near the Ordovician–Silurian transition on theYangtze Block is diachronous and also apparently prograded north-westwardly cratonward across the block in time and space. This kindof cratonward progradation of a black-shale-clastic-wedge couplet(Fig. 2) is a typical response to the cratonward migration of orogenyfrom the present southeast of China that is in good agreement with

Fig. 8. Microscopic features in thin-sections of a turbiditic sandstone from the Zhoujiaxi Group, Xinhua, Hu'nan (see Fig. 1, location 7). A: Outcrop and the sample positions at therelated bed; B: Base part; C: Midlle part; D: Upper part. The three thin-section photos show the petrographic textures in typical graded bedding from a trubiditic sandstone/graywacke unit; note the relatively fine grain size and poorly sorted particles from a distal despositional setting.


the flexural models used by other authors (e.g., Ettensohn, 1994;Ettensohn and Brett, 1998; Allen et al., 2001; Ettensohn and Brett,2002; Ettensohn, 2005).

In fact, in the early 1990s, Li, one of the present authors, and hiscolleague had already suggested tectonic control of Ordovician–Silurianfaciesmigration in South China (Li and Quan,1992). Subsequently, Chenand his colleagues (Chen et al., 1995a; Chen and Mitchell, 1996)suggested a similar Ordovician–Silurian tectonic model for south China,involving a similarly migrating, clastic wedge. Mao and Wang (1999)suggested another similar process but with a different boundaryposition, located near the middle of Guangxi Province since Cambriantime. However, neither of these models was supported by the work ofHsü et al. (1988,1990),which suggested that the “BanxiOcean,” inwhich

Fig. 9. Power–law distribution of the bed thickness of the flysch succession (afterCarlson and Grotzinger, 2001), Zhoujiaxi Group, Xinhua, Hunan (Fig. 1, location 7).

these events could not have occurred, extended across southern China,as early as Early Paleozoic time.

Other possibly applicable models have emerged from the work ofEttensohn (1991, 1994, 1998a,b, 2004, 2005) in the Paleozoicsediments of the Appalachian and Black Warrior foreland basins ofeastern United States. Ettensohn's work, based on the viscoelasticmodels of Beaumont (1981), Beaumont et al. (1987, 1988) andJamieson and Beaumont (1988), suggests that orogenies do notoccur as single, continuous, prolonged events, but as a series of moreor less isolated tectophases that endure for a few tens of million yearsand may migrate in time and space. In addition, Ettensohn showed

Fig. 10. SiO2–w(K2O)/w(Na2O) diagram (after Roser and Korsch, 1988) of flyschoidsandstones from Hunan, South China. ACR: arc of ocean range; ACM: active continentalmargin: PM: passive continental margin. Filled circles: Zhoujiaxi Gr. (S1), Xinhua;Empty circles: Tianmashan Fm. (O3), Shuangfeng (see Fig. 1).

Fig. 11. Rose diagrams of cross-bed orientations from the flysch successions of Zhoujiaxi Group (A) and the Tianmashan Formation (B).


that each tectophase produced a distinctive, unconformity-boundpackage of stratigraphic units in the foreland basin that reflect variousstages of deformational loading and relaxation in the orogen andforelandbasin (Fig.14), and that the series of tectophases during any oneorogeny may produce several stacked tectophase cycles in the adjacentforeland basin. However, Ettensohn also noted that not all cyclesmay becomplete. In some situations, a new tectophase may begin before anearlier one is fully completed, and in other cases, the early erosiveactivity of a succeeding tectophase might destroy later parts of thesedimentary response of the previous tectophase (Ettensohn, 2005).Interestingly, the present Ordovician–Silurian transition sequence insouth China (Fig. 2) is consistent with early parts of one of Ettensohn'stectophase cycles, and when combined with other regional information(Figs.1 and12),with the diachronous timing andmigration of facies (Fig.2), as well as with the tectonomagmatic geochemistry and regionaldistribution of coeval K-bentonites (Figs. 4 and 6), easily supports theidea of Ordovician–Silurian foreland-basin sedimentation on theYangtze Block, related to a subduction-type collisional event to thesoutheast. The likely flexural interpretation of individual units in thismodel (Fig. 13) is briefly discussed below.

A tectophase sequence typically begins with an unconformity thatreflects bulge uplift and moveout, and such an unconformity has beennoted below the Wufeng Formation in northern Hunan and westernHubei Provinces (Fig. 2). The absence of an unconformity at the base ofthe sequence in southeastern and central Hu'nan Province, however, isnot unexpected, because in more proximal parts of the foreland closeto the deformational loading, loading-related subsidence easilyoutstrips bulge-related uplift generating a conformable sequence(Ettensohn, 2005). The overlying black-shale interval (Figs. 2 and 13),including the Chengbu Formation, the Wufeng Formation and thelower member of Longmaxi Formation, as well as the coevaldeposition at the northwestern Yangtze platform (see Fu and Song,1986; Chen et al., 2004; Su et al., 2007), reflects a period of rapidsubsidence and clastic-sediment starvation that accompanies activedeformational loading. Such loading reflects the surface and subsur-face accumulation of folds, thrusts, nappes, and obducted blocks in theorogen and is most prevalent during times of active convergence andsubduction. Because of ongoing subduction, this is also a time of activearc volcanism in the orogen, and hence, K-bentonites, like thosealready noted (Figs. 4 and 6), should be most common in black-shaleparts of the section (Ettensohn, 2005). Also like the black-shale unitsnoted by Ettensohn (1991, 1994, 1998b, 2005) in the Appalachianforeland basin, these south Chinese, Ordovician–Silurian, black-shaleunits migrated cratonward (northwest) in time and space, most likelyin response to the continued cratonward movement of orogeny andaccompanying deformational loading.

Earlier, the presence of an unconformity in the midst of the black-shale sequence, on top of the Guanyinquiao Member of the WufengFormation, wasmentioned (Fig. 2). The occurrence of an unconformityat this position in the black shales is not typical of flexural sequencesand needs further explanation. In fact, the Guanyinquiao or upperMember of the Wufeng Formation occurs during the Late OrdovicianHirnantian glacial drawdown, when due to sea-level drop, shallow-water carbonates and lighter colored shales spread across the YangtzeBlock and much of the foreland basin. The unconformity was littlemore than the natural culmination of this sea-level drop. What isinteresting about the unconformity, however, is the fact that it waslargely confined to a series of “submarine highs” (Chen et al., 2004)that approximately parallel the margin of the foreland basin and theproposed suture between the Yangtze Block and the other continentalblock at present southeast (Fig. 12).

We suggest that these “highs” rather than being just fortuitous, are infact structural highs on the migrating forebulge in front of the forelandbasin. As a forebulge migrates cratonward, basement structuralinhomogeneities may be preferentially uplifted or downdropped(Ettensohn et al., 2002), and during a sea-level drawdown, unconfor-mity development will be enhanced on the uplifted blocks. So theoutline and position of submarine highs (Fig. 12) noted by Chen et al.(2004) not only explain the unusual development of the unconformityat this time, but also add further support to the flexural, foreland-basin-and-bulge model that is responsible for the sedimentary sequencebeing discussed.

Those clastic units overlying the black shales, including theTianmashan Formation, Zhoujiaxi Group, and other coeval units(Figs. 2 and 13) are typical foreland-basin flysch successions thatreflect the cessation of loading in the orogen and the onset of loading-type relaxation in a deepening basinwith two paleoslopes (Fig.1). Likethe black shales, these flysch units also typically prograded distally(northwestward) in time and space (Figs. 2, 11, and 13) as the basinfilled first in more proximal (southeastward) areas (cf., Ettensohn,2004, 2005).

Although upper parts of the tectophase cycle have been eroded awayin southeastern and central Hu'nan Province (Figs. 2 and 13), they arepreserved innorthernHu'nanandwesternHubei provinces in theupperpart of the Longmaxi Formation, in the overlying Luoreping, Shamao,and Xiaoheba-through-Huixingshao formations, and in coeval succes-sions in Guizhou, Sichuan and other provinces, all of which reflectshallowing-upward, normal-shelf to marginal-marine deposition withredbeds across the entire Yangtze Platform (Jin et al., 1992; Chen et al.,1995b; Wang et al., 1996; Bureau of Geology and Mineral Research ofHu'nan Province, 1997; Zhan and Jin, 2007). The presence of redbeds inthis part of the sequence (cf., Rongxi~Huixingshao Formations, see Figs.

Fig. 12. Sketch map showing the inferred relationship between the “submarine highs” of Chen et al. (2004) and the presently inferred bulge-axis/foreland-basin/suture system in South China.

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Fig.13. Simplified Late Ordovician–Early Silurian succession in South China (left), and generalized stratigraphic sequence of a flexural, foreland-basin, tectophase cycle (modified afterEttensohn, 1998a,b, 2004, 2005). For details of the succession, see Fig. 2.


2 and13) is especially diagnostic of the shallowing, normal-to-marginal-marine conditions that accompany final rebound and unloading-typerelaxation in the orogen (Ettensohn, 2004, 2005).

This sequence is then truncated below the prominent, regional,Siluro–Devonian (even Permian locally in northwest Sichuan–south-west Shaanxi; see Figs. 2 and 13) unconformity and may reflectsubsequent bulge uplift and moveout accompanying initiation of themajor Caledonian-aged Guangxi Orogeny, which affected the entiretyof south China (seeWang,1985; Yang et al., 1986;Wang andMo,1995;Wang et al., 2005).

Hence, stratigraphic analysis of the prominent South China blackshales and their associated, overlying depositional succession showsgood agreement with Ettensohn's stratigraphic, flexural-responsemodel and may suggest that a similar evolutionary, tectonic processprobably controlled foreland-basin depositional successions in con-vergent settings on other continents. More importantly, it indicatesthat deposition of the graptolite-bearing, black-shale interval at theOrdovician–Silurian transition in South China was most likely

controlled by flexural, lithosphere subsidence and bulge moveout,which could have only been caused by former plate subduction in thepresent southeast of South China.

3.4. Coupled deposition of K-bentonites and the flysch-black-shaleassociation

When the distribution of flysch successions is plotted on the K-bentonite isopach-distribution map, a very interesting pattern emerges(Fig. 14). The depositional patterns show similar distributional orienta-tions and sources to the southeast. The K-bentonites, however, are morewidely distributed across the Yangtze Platform interior and nearby areasbecause of their windborne nature, and more commonly occur in frontof, or cratonward of, the northwestwardly prograding flysch succession,which is concentrated inmore proximal parts of the foreland-basin area.

It was previously concluded that all the K-bentonites in the sectionswere derived from explosive volcanic eruptions in volcanic-arc (VA)and syn-collision (syn-COL) to within-plate (WP) tectonomagmatic


settings along an active continental margin (Su et al., 2003b) (see Fig.4A, B), and that the flysch successions probably had passivecontinental-margin sources (Fig. 10). Although it is possible forpassive-margin settings to have been subsequently deformed duringchange to an active-margin or collisional regime, it is very unlikely thatthe volcanic arc, collision zone or sutural areas necessary for volcanismwould have occurred on a passive continental margin, despiteevidence indicating that the South China, Ordovician–Silurian K-bentoniteswere derived from ash falls blown by prevailingwinds fromthe present east-southeastwhere only continental and passive-marginsettings exist today. This situation can only be reconciled if during theOrdovician–Silurian transition therewas a suture and subduction zoneto the east-southeast of the present Yangtze Platform. In other words,the presence and distribution of K-bentonites and cratonward-migrating black shales and flysch in a tectophase cycle mean that, atleast during latest Ordovician to Early Silurian time, another plate ormicrocontinent to the east-southeast of present South China wasinvolved in northwestward collision and accretion with the cratonicYangtze Block.

Hence, based our present interpretation, we may now try toanswer some of the primary questions mentioned at the beginning ofthis paper. For example, was this plate or microcontinent theCathaysia Block, and where was the western boundary of theCathaysia Block? Moreover, here we would like to note that if thisformer plate or microcontinent to the east-southeast of present SouthChina was the Cathaysia Block, it should have been bounded on thewest with an Early Paleozoic relict suture along the border of Zhejiangand Fujian, and perhaps into Guangdong or Guangxi, provinces, as was

Fig. 14. Distribution of K-bentonites (~Hirnantian Stage as the representative) and flysch suOrdovician (mid-late Ashgill) flysch fan (e.g., Tianmashan Fm., see Fig. 1, location 8); S1 fan: EThe eastern part of the boundary between the Sino-Korea and the Yangtze blocks has beendeformation and Cenozoic movement on the Tan-Lu Transform Fault.

formerly inferred by several workers (e.g., Wang et al., 1988; Yanget al., 1995; Wang and Mo, 1995; Mao and Wang, 1999; Wang et al.,2005), but which would have been substantially different than theboundary suggested by several other workers (e.g., Hsü et al., 1988,1990; Li et al., 1995; Wu, 1999; Zhao and Cawood, 1999; Li et al., 2002;Rong et al., 2003; Chen et al., 2004; Zhan and Jin, 2007).

4. Discussion

4.1. The term “Cathaysia Block” and its inferred present position

As we now understand, when Grabau (1924) coined the term“Cathaysia,” very few supportive regional geological data wereavailable. Therefore, Cathaysia became a conjectured landmass withthe large area including present-day southeast China, the surroundingshelf seas, the peninsula of Korea, and the islands of Japan. Of course,this understanding is inaccurate based on today's data. Nonetheless,Grabau's idea is still important because it inspires and promotesresearch on the tectonic evolution of South China and East Asia.

Presently, most workers believe that there was such a terrane inpresent southeastern China and nearby areas with a Precambrianbasement about 900–2500 Ma in age (cf., Shui, 1988; Wang and Mo,1995; Chen and John, 1998; Wang et al., 2005). Most recently, Wanet al. (2007) reported early Precambrian inherited and detritalSHRIMP U–Pb zircon ages from northwestern Fujian (CathaysiaBlock), of which a 3.6 Ga detrital grain is the oldest so far found inthis area. This age means that “there may not only be Neoarchaean,but possibly also Meso- and even Eoarchaean rock units, hidden in the

ccssions (O3–S1) near the Ordovician–Silurian transition in South China. O3 fan: Latearly Silurian (early Llandovery) flysch fan (e.g., Zhoujiaxi Gr., see Fig. 1, location 7). Note:reconstructed as a straight line in this map, which was the situation before Mesozoic

Fig.15.Hypothetical map showing the location of major tectonic and paleogeographic features on the Yangtze block and adjacent parts of the suture zone. O3: northwestern boundary of the Late Ordovician flysch-dominated fan complexes;S1: northwestern boundary of the Early Silurian flysch-dominated fan complexes. For abbreviations of relevant provinces and others see Fig. 1.

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Cathaysia Block or adjacent areas” (Wan et al., 2007). Unfortunately,the tectonic base of present southeastern China has been deformedconsiderably by post-Guangxian Orogeny deformation, plutonism,metamorphism, and especially by Mesozoic and Cenozoic volcanismand thrust faulting (e.g., Wang et al., 2005; Li and Li, 2007). Thus, theexact location of the suture zone and the relevant plate construction atthe time were almost completely obliterated, and the primaryproblems mentioned at the beginning continue to be debated.

If our present suggestions above are correct, as Chen et al. (1995a)discussed, it means that during most of Paleozoic time there was nolonger a collision zone or the so-called “Banxi Ocean” (Hsü et al., 1990)along the southeast margin of the Yangtze block, although such acollision zone and adjacent ocean had probably existed during mid-late Proterozoic time (cf., Chen et al.,1991; Xu et al.,1992; Li et al.,1995;Xu et al., 1996; Zhao and Cawood, 1999; Li et al., 2002; Wang et al.,2005; Li et al., 2008a,b).

Nonetheless, the “Banxi-Ocean-model” of Hsü et al. (1990) hastriggered much work in various attempts to understand the tectonicevolution of South China since their first interpretations. During EarlyPaleozoic time, however, it now seems that a new suture wasgenerated farther to the east or southeast of the former Proterozoiccollision zone. In particular, at least during the Ordovician–Siluriantransition, this new orogenic event apparently generated a pro-nounced, diachronous, northwestwardly prograding lithostratigraphicwedge, bounded by K-bentonite-bearing black shales and overlyingflyschoid graywackes in comparisonwith the coeval upward-shallow-ing mixed-facies deposits from the southeast slope-basin to thenorthwest platform-interior of the Yangtze Platform (Fig. 2).

Just as suggested by other authors (Rong and Chen, 1987; Chenet al., 1995a; Chen and Mitchell, 1996; Rong et al., 2003; Chen et al.,2004; Zhan and Jin, 2007), the present study supports the conceptthat the area from the upper Yangtze Block to its southeast borderhad become a consolidated cratonic plate by Early Paleozoic time (cf.,Fig. 1). Both the distribution of K-bentonites and flysch successions (Fig.14) show that there was a continuous and transitional depositionalbasement across this vast area, and that the major suture duringthis period was located farther southeast of flysch and bentonitedeposition.

However, according to other authors (Rong and Chen, 1987; Chenet al., 1995a; Chen and Mitchell, 1996; Rong et al., 2003; Chen et al.,2004; Zhan and Jin, 2007), the entire south and southeast part of China,in addition to the Yangtze Block, belonged to “Cathaysia Land” instead ofto the cratonic plate near the present-day eastern shoreline provinces,and in their reconstructions, “Cathaysia Land” seems to have graduallybecome larger and larger during this time. Ultimately, this interpretationof “Cathaysia Land” expanded to include the entirety of present-dayYun'nan andGuizhou provinces (see Rong et al., 2003; Chen et al., 2004;Zhan and Jin, 2007). Traditionally, however, most of the area included intheir interpretations of “Cathaysia Land” has been included in the upper(western) part of the Yangtze Block (cf.,Wang,1985; Yang et al.,1986). Inthis context, it seems that the name “Cathaysia Land” has merelybecome the name of the paleogeographic area rather than of a tectonicunit (Rong et al., 2003; Chen et al., 2004; Zhan and Jin, 2007); hence, theusage is quite different from that of otherswhouse the name tomean anindependent cratonic plate or block to the southeast that eventuallyaccreted to the Yangtze Block (e.g., Wang, 1985; Shui, 1988; Chen et al.,1991; Li et al., 1995; Xu et al., 1996; Zhao and Cawood, 1999; Li et al.,2002; Wang et al., 2005, and references therein).

On the other hand, some authors prefer to think that there wereseveral terranes or microcontinents to the southeast of the YangtzeBlock (e.g., Guo et al., 1984; Charvet et al., 1996, 1999; Wu, 1999) (seeFig. 1), and they have used the name “Cathaysia Block” to mean a verysmall tectonic unit between the Yangtze Block and other micro-continent-like units in the east. This interpretation, however, isconsiderably different from ours and that of others mentioned above,and certainly inconsistent with that of Grabau (1924).

Noteworthy, however, is the interpretation of Charvet et al. (1996),who suggested that there was more likely an Early Paleozoicremobilization zone along the southern border of the “Jiangnanbelt” (i.e., the ‘Jiangnan Region' in this paper; see Fig. 1), whichresulted from a compressional event involving Proterozoic basement,rather than an Early Paleozoic suture. Moreover, Charvet et al. (1996)preferred that the Early Paleozoic suture occurred farther to the southand east, between the so-called “Jiangnan” and “Wugong-Wuyi”blocks, near the present shoreline of southeast China, and that thedevelopment of this suture caused crustal shortening and shearing onits western side.

In the past two decades, however, several workers have found manynew forms of evidence about the inferred Early Paleozoic suturementioned above in the shoreline provinces of southeast China (e.g.,Wang et al., 1988; Yang et al., 1995). For example, besides the 3.6 GaArchaean detrital grains, Wan et al. (2007) have also identified asignificant tectono-thermal eventwith ametamorphic age of about 458–425 Ma in northwest Fujian (see Fig. 1) that represents high-grademetamorphism in early Paleozoic time. Simultaneously, Wang et al.(2007) presented new zircon U–Pb data (~423 Ma) from rocks formerlythought to be Precambriangneisses on theYunkaimassif at the border ofGuangxi and Guangdong provinces in South China (see Fig. 1), whichwould be roughly equivalent to tectonic processes suggested by severalother workers (e.g., Mao and Wang, 1999). Obviously, the Caledonian-aged Guangxi tectonothermal event on the Yunkai massif is also in goodagreement with the age of metamorphism in northwest Fujian. Thisperiod of synchronous tectonothermal metamorphism correlates fairlywellwithour inferredperiodof LateOrdovician to Early Silurian collision,and is in very good agreementwith our discovery of Ordovician–Siluriantransitional, felsic-intermediate, explosive eruptions to the southeast ofthe Yangtze Block, which gave rise to the K-bentonite deposits on thisblock, as well as to the subduction-generated, northwestwardlyprograding black-shale-and-flyschoid lithostratigraphic wedge. All ofthese studies confirm that there should have been an Early Paleozoiccollision zone trending roughly northeast to southwest along thewestern border areas of both Zhejiang and Fujianprovinces, andpossibleinto the border of Guangdong and Guangxi provinces in southeast China(see Figs. 1 and 12).

Other lines of evidence involve isotopic age dates. With abundantisotopic ages and the other data, Shui (1988) showed earlier thatthere was probably an Early to Middle Proterozoic basement insoutheastern China, even under the vast East and South Seas near theshoreline of China. He preferred to call this large old block “Cathaysia,”using the term of Grabau. Furthermore, the newly reported age of3.6 Ga for Archaean detrital grains (Wan et al., 2007) also supportsthe likely presence of a tectonic block in the area as suggested byseveral other authors. Considering the transitional area between theYangtze Block in a narrow sense and the Cathaysia Block, Wan et al.(2007) suggested that a smaller Proterozoic “Jiangnan Block” orterrane must have existed along with the Yangtze Block to thenorthwest in a southwesterly-opening relict oceanic basin to thesoutheast. From late Sinian (~Ediacaran) through Silurian time,this small block was apparently uplifted during the clockwisedrift (collision) of Cathaysia, resulting in the Hua'nan (South China)Orogeny.

In another words, based on the present work, the formerly inferredEarly Paleozoic relict suture along the western border areas of bothZhejiang and Fujian provinces (and maybe into Guangdong andGuangxi provinces) in southeast China (see Wang et al., 1988; Yanget al., 1995; Wang et al., 2005; Wang et al., 2007) could be regarded asthe western boundary of the sensu stricto Cathaysia Block, and its eastboundary would have been located in the present-day Taiwan trench,which was oriented northeastward into the East Sea and south-westward into the South Sea (cf., Wang et al., 2005; see Fig. 15).Obviously, the present study provides additional evidence supportingthe accretion of the Cathaysia Block to the Yangtze Block during Early


Paleozoic time and helps to clarify themeaning of the name, CathaysiaBlock.

4.2. Relationship between Cathaysia–Yangtze accretion and evolution ofGondwana

According to Huff et al. (cf., 1998b, 2003) and others (e.g., Kolataet al., 1996; Ramos, 2004; Astini et al., 2007), Ordovician explosivevolcanism was closely related to evolution of the Gondwana margin,both in the west and east, and our initial work here may help inunderstanding the situation in South China. It is very likely that thevolcanism and associated deposition discussed above, especially theprograding black-shale and flysch successions in the Yangtze platformand its marginal area (Figs. 2 and 10), were related directly to thecontinuation of collisional process during Early Paleozoic time insoutheastern China. If some paleomagnetic data and continentalreconstructions from this period of South China (e.g., Lin et al., 1985;Scotese and McKerrow, 1990; Wu et al., 1999; Zhang, 2004) areconsidered, we would like to suggest that all phenomena discussedhere may be associated with the beginning of drifting or initial break-up of the several microcontinents surrounding Gondwana. In fact,Veevers (2005, see his text-fig. 2) has suggested that northeasternparts of Gondwana, which included the South China area, may havebegun to separate as early as Middle Devonian time. Although it mayseem contradictory to involve the imminent break-up of Gondwanaand the collision of Cathaysia to the Yangtze Block, the interconnect-edness of tectonic processes due to convection in the asthenospherenecessarily means that the accretion of terranes on one part of acontinent will probably be balanced elsewhere by the near-synchro-nous export of terranes at a divergent boundary (see Veevers, 2005;Vaughan and Pankhurst, 2008).

In comparison with a similar Ordovician–Silurian, bentonite-rich,black-shale and flysch succession in the Appalachian foreland basin,which reflects convergence between Laurentia and Gondwana-derived Avalonian microcontinents (Huff et al., 1992; Ettensohn andBrett, 2002), it is clear that the Ordovician–Silurian, South China,stratigraphic succession similarly reflects convergence and providesimportant ancillary evidence about suture location and the tectonic/paleogeographic setting on the northeast margin of Gondwana. Mostinterestingly, in comparisonwith the stratigraphic succession of SouthChina, the stratigraphic succession of the Famatina Group (EarlyOrdovician) in the Argentine Precordillera (Astini et al., 2007, see theirtext-fig. 3) is also very much in agreement with the flexural-model ofEttensohn (e.g., 1994, 2004, 2005), where it has been shown to be partof a fairly continuous upper-plate, convergent volcanic chain thatfringed western Gondwana (e.g., Huff et al., 1998a,b, 2003).

Obviously, more age and paleomagnetic data need to be accumu-lated and reconsidered before conjectures like this can be made withany certainty. Nonetheless, what is certain is that the Ordovician–Silurian transition was a time of major global tectonic reorganizationthat might reflect the end of Gondwana assembly. Hence, moredetailed work in South China and other places will be necessary inorder to fully understanding the dynamic processes involved duringthis important period in Gondwana evolution.

5. Conclusions

(1) Based on isopach maps, K-bentonites in the Ordovician–Silurian transition sequence on the Yangtze Block were derivedfrom explosive eruptions along a suture farther to the east.Northwestwardly prograding black-shale and flysch succes-sions had similar source areas to the southeast and weredeposited on the former passive margin of the Yangtze Block,which subsided as a foreland basin during the northwestwardaccretion of a continental block or terrane to the Yangtze Block.Both the K-bentonites and flysch reflect sources along a

northeast–southwest-trending line in the present-day shore-line provinces of southeast China. The sources and resultingsediment distribution accurately reflect the northwestwardlyprevailing winds and what must have been a northwestwardlymigrating orogenic belt to the southeast.

(2) The Ordovician–Silurian stratigraphic sequence in South Chinais typical of unconformity-bound, tectophase cycles found inthe Appalachian foreland basin, some of exactly the same age.These cycles have been demonstrated to reflect forelandresponses to tectophases in an adjacent orogen, most likelyreflecting ongoing convergence to the southeast.

(3) Combined with the previously published works, we would liketo suggest that this ongoing suturing or convergence processprobably began initially in Middle to Late Proterozoic time as asuture along the southeast margin of the Yangtze Block withthe so-called Jiangnan belt or block and other fragments withCathaysian affinities. The initial convergence in Middle to LateProterozoic time between the sensu stricto Yangtze Block andthe “Jiangnan Block” or other related fragments, should indicatethe consumption of the so-called “Banxi Ocean.”

(4) Regarding the Ordovician–Silurian tectonism, it probablyreflects the final tectophase of this ongoing convergenceprocess between the Yangtze Block and other continentalfragments. This final tectophase involved an Early Paleozoicsuture near the present-day border area of west Zhejiang andFujian provinces of southeast China. It seems probable that thisis the suture between the Yangtze Block (in broad sense,accreted by the sensu stricto Yangtze, the Jiangnan Block, andother fragments) in northwest and a large cratonic plate to thesoutheast, which probably included the shoreline-provincesarea of present southeast China, the East Sea, as well as otherrelevant areas. We prefer to call this cratonic plate the“Cathaysia Block”, using the original terminology of Grabau.Based on this framework, we suggest the tectonic-paleogeo-graphic setting outlined in Fig. 15.

(5) Finally, just like the peri-Iapetan Orogeny and other areasduring the same period, it is possible that accretion of theCathaysia and Yangtze blocks at the Ordovician–Siluriantransition may have reflected a major tectonic-reconstructivestage of Gondwana evolution.

Acknowledgments

We are very grateful to Prof. Hongzhen Wang for the fruitfuldiscussions. Profs. Gerald R. Baum, Xiaoying Shi, Xunlian Wang arethanked for their assistance in sequence-stratigraphic analyses.We alsogive our sincere thanks to other colleagues at the China University ofGeosciences, China Academy of Sciences, and China Geological Survey.We are also grateful to several IGCP 503 friends for their concerns to thiswork. Prof. M. Santosh read an earlier manuscript version and kindlyprovided several constructive suggestions. We also thank Prof. ZhaoGuochun for his editorial comments and thoughtful handling of themanuscript. Two anonymous reviewers are thanked for their criticalcomments on the first version. This research has been jointly supportedby the Natural Science Foundation of China (49802002, 40372057,40772076), grants from the State Key Laboratory of Paleobiology andStratigraphy (Nanjing), CAS (013101, 033101), and the Sinopec Project(G0800-06-ZS-319). This is also a contribution to IGCP 503.

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K-bentonite, black-shale and flysch successions at the ... · Yangtze Block and its implications for the evolution of Gondwana ... collision had occurred long ago, any fossil suture