Journal of Asian Earth Sciences 50 (2012) 141–149

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Journal of Asian Earth Sciences

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Changhsingian (Late Permian) conodonts from Son La, northwest Vietnamand their stratigraphic and tectonic implications

I. Metcalfe ⇑Earth Sciences, Earth Studies Building C02, School of Environmental and Rural Science, University of New England, Armidale, NSW 2351, AustraliaNational Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC), Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 May 2011Received in revised form 28 December 2011Accepted 6 January 2012Available online 21 February 2012

Keywords:ConodontsChanghsingianPermian–TriassicIndosinian OrogenyVietnam

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⇑ Address: Earth Sciences, Earth Studies Building Cand Rural Science, University of New England, Armida+61 2 67733499; fax: +61 2 67727136.

E-mail address: imetcal2@une.edu.au

Late Permian conodonts are for the first time reported from Vietnam. Pa, Sa, Sb, Sc and M elements of theChanghsingian conodont species Hindeodus julfensis (Sweet) are reported from a 40 cm thick limestone inthe middle part of the Yenduyet Formation near Son La, NW Vietnam. The occurrence of H. julfensis indi-cates a Changhsingian age that is consistent with an interpreted early Changhsingian age for a brachiopodfauna slightly higher in the sampled section. The Son La section is located in the Song Da Rift Zone andoverlies basaltic volcanics considered equivalent to the Emeishan large igneous province basalts that areplume related. The Permian–Triassic boundary in Vietnam is yet to be precisely located biostratigraphi-cally but proxy chemostratigraphic data indicate its likely position in sections at Nhi Tao and Lung Cam,N. Vietnam and correlation with the Global Stratotype Section and Point at Meishan, South China. Therecovered conodonts have a Conodont Colour Alteration Index of 5 and have been heated to c. 600 �Cbut they do not show any evidence of textural alteration due to regional metamorphism such asmicro-folding or stretching that would indicate any direct effects of the compressional IndosinianOrogeny.

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1. Introduction

Much progress has been made in recent years in understandingthe Late Permian–Early Triassic interval globally. This interval in-cludes the ‘‘end-Guadalupian’’ (late-Capitanian) and ‘‘end-Perm-ian’’ (late Changhsingian) mass extinctions that dramaticallyshaped the evolution of life. Conodonts provide one of the bestglobally applicable biostratigraphic schemes for this interval andare used to define the stage boundaries of the Late Permian andEarly Triassic and the base of the Triassic (Fig. 1). Significant recentadvances in U–Pb isotopic dating of volcanic ash beds in Permian–Triassic (P–T) sequences (Mundil et al., 2004, 2010; Ovtcharovaet al., 2006; Lehrmann et al., 2006; Galfetti et al., 2007; Shenet al., 2010, 2011) have now established a relatively robust LatePermian–Early Triassic numerical timescale (see Fig. 1). High-reso-lution conodont zones now provide precise correlation of P–T tran-sitional sequences in the marine environment. Different zonalschemes have been identified for deeper marine environments,which are neogondolellid-dominated, and shallow-marine envi-ronments, which are hindeodid-dominated (Wang, 1996; Mei

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02, School of Environmentalle, NSW 2351, Australia. Tel.:

et al., 1998; Jiang et al., 2007; Chen et al., 2009; Metcalfe andIsozaki, 2009; see Fig. 1).

Late Permian conodonts have not previously been reportedfrom Vietnam. Early Permian conodonts were briefly reported byPham Kim Ngan (1990) and Triassic conodonts were first reportedfrom Vietnam by Bui Duc Thang (1989) who reported an earlyOlenekian (Smithian) shallow-water neospathodid-dominated fau-na from the Bac Thuy Formation at Bac Thuy, northeast Vietnam.More recently, the conodont species Hindeodus parvus (Kozur andPjatakova), the first appearance of which defines the base of theTriassic, has been reported from a carbonate platform sequenceclose to the China border in northeast Vietnam at Nhi Tao (Doanet al., 2004; Nguyen et al., 2004; Algeo et al., 2007). This sequenceappears to form an extension into Vietnam of the Jinxi Platform,one of several large carbonate platforms within the NanpanjiangBasin, located on the southern margin of the South China Block.The occurrence of H. parvus at Nhi Tao is 8 m above the Perm-ian–Triassic boundary as interpreted by the occurrence of otherTriassic faunal elements, magnetic susceptibility curves andchemostratigraphical data (Doan et al., 2004; Algeo et al., 2007).

This paper reports a conodont fauna of Changhsingian age fromnear Son La that is consistent with an interpreted Changhsingianage of biogeographically important brachiopods reported from aslightly higher stratigraphical position in the same section (Shiand Shen, 1998).

Fig. 1. Conodont zonation and U–Pb numerical timescale for the Permian–Triassic transition globally. Partly after Metcalfe and Isozaki (2009). U–Pb isotopic age time scalecompiled from Mundil et al. (2004), Ovtcharova et al. (2006), Lehrmann et al. (2006), Galfetti et al. (2007), Mundil et al. (2010), Shen et al. (2010) and Shen et al. (2011). (Seeabove-mentioned references for further information.)

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2. Geological and tectonic setting

2.1. Tectonic and palaeogeographic setting

Southeast Asia is a complex assembly of Gondwana-derivedcontinental blocks, bounded by suture zones that represent thesites of former ocean basins (Metcalfe, 1996, 1998, 2006). The con-tinental core of mainland Southeast Asia comprises the Sibumasu,Indochina and South China blocks and an island arc, the SukhothaiArc terrane, sandwiched between Sibumasu and South China/Indo-china (Sone and Metcalfe, 2008; Metcalfe, 2011a,b; see Fig. 2).

The Permian–Triassic transitional sequence near Son La is lo-cated within the Song Da Rift Zone NE of the Song Ma Suture Zonethat forms the boundary between the South China Block and theIndochina Block in Vietnam (Sone and Metcalfe, 2008; Metcalfe,2011b; see Figs. 2 and 3A). The sampled section is therefore locatedon the SW margin of the South China Block, within the Song DaPermian–Triassic rift basin. Permian basalts within the Song Da Riftare here interpreted as equivalents of the end-Guadalupian Emei-shan rift basalts and geochemistry of these basalts and associatedPermian–Triassic komatiites suggest the rift basalts are plume re-lated (Hanski et al., 2004; Zhou et al., 2008). The Son La sectionwas located on the palaeo-equator in the Changhsingian (Fig. 4)and a land connection between South China and Indochina is indi-cated by the occurrence of Dicynodon in the Late Permian of Laos(Battail, 2009). In addition, a shallow-marine biogeographic con-nection between South China and Indochina is indicated by theoccurrence of the Tethyan brachiopod Peltichia Jin and Liao, re-ported from the section under discussion here (Shen et al., 1999).Some authors have suggested that Indochina and South China wereseparated by oceanic crust in the Late Permian to Early/MiddleTriassic and that they collided during the Indosinian Orogeny in

the Triassic (Zhang et al., 2006; Zhang and Cai, 2009; Cai andZhang, 2009). I here follow my previous interpretations (seeMetcalfe, 2011b for discussion) that Indochina and South Chinawere already amalgamated in the Permian and that extensionalrifting was occurring in the Song Da region during the LatePermian.

2.2. Permian–Early Triassic stratigraphy and sample location

Permian rocks are distributed sporadically in Vietnam (Fig. 2).The Carboniferous–Permian–Triassic sequence in the Son La region,Vietnam comprises a lower Chiengpac Formation of Upper Carbon-iferous to Lower/Middle Permian age limestones, overlain discon-formably by Middle Permian basalts of the Camthuy Formationwhich are in turn overlain by marine clastic rocks with minor lime-stones of the Upper Permian Yenduyet and Lower Triassic Conoi For-mations (Fig. 3B). The Upper Carboniferous–Lower PermianChiengpac Formation limestones contain fusulinid foraminifera,including the genera Cancellina and Verbeekina indicative of theLower to Middle Permian (BouDagher-Fadel, 2008). The dominantlybasalt Camthuy Formation has been correlated with the Emeishanrift basalts of SW China (Shi and Shen, 1998; Fontaine, 2002) thathave been linked with the end-Guadalipian mass extinction(Wignall et al., 2009). The Upper Permian Yenduyet Formation inthe Son La Pass area comprises a sequence of tuffs, tuffaceous sand-stones, limestones, and argillaceous shales approximately 175 mthick (Thanh and Khuc, 2006, p. 223). A discontinuous red–brownbauxitic layer is found at the base of this formation (Thanh and Khuc,2006; Shi and Shen, 1998) and limestone in its middle part containsthe Changhsingian foraminfers Colaniella parva and Palaeofusulinaprisca (Thanh and Khuc, 2006). A 40 cm thick limestone horizonoccurring in the middle of this formation is exposed in a road cutting

Fig. 2. (A) Tectonic subdivision of Vietnam and adjacent regions of SE Asia (after Sone and Metcalfe, 2008). (B) Distribution of Carboniferous–Permian rocks in Vietnam andlocation of the study area in the Song Da Rift Zone.

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on National Highway 6, between Son La City and Na Co village to thenorthwest (Fig. 3). The conodont fauna reported here is from a singlesample (V2) from this limestone unit. The limestone has yielded avaried invertebrate fauna including forams, rugose and tabulate cor-als, bivalves, gastropods, rare brachiopods and calcareous algae(Dzanh and Tinh, 1995). The presence of the fusulines Codonofusiellasp., Nankinella inflata (Colani), C. parva (Colani), and P. prisca (De-prat), and corals Waagenophyllum pulchrum Hamada, W. simplexWu) suggest a Changhsingian age (Shi and Shen, 1998) based oncomparisons with South China. Shi and Shen (1998) described eightbrachiopod species from shales and siltstones approximately 60 mbelow the top of the Yenduyet Formation above the limestone unitand in the same stratigraphic section on National Highway 6 dis-cussed here. The brachiopod species reported by Shi and Shen werePeltichia kwangtungensis (Zhan), Acosarina minuta (Abich), Rhipidom-ella hessensis King, Schuchertella cf. cooperi Grant, Derbyia sp., Waage-nites soochowensis (Chao), Spinomarginifera chenyaoyanensis Huang,Marginiferinae gen. and sp. indet. The presence of Peltichia kwang-tungensis Zhan and Spinomarginifera chenyaoyanensis Huang wasinterpreted by Shi and Shen (1998) as indicating an early Changhsin-gian age. In addition, the occurrence of the genus Peltichia in the fau-na demonstrates a close biogeographic connection between South

China and Indochina in the Changhsingian and clear Tethyan natureof the Son La fauna (Shen et al., 1999).

3. Microfauna and its implications

The 6 kg limestone sample V2, following acid digestion in buf-fered 5% formic acid, yielded conodonts, ostracods, forams, and ich-thyoliths (shark scales and teeth). Conodont yields are low, withonly 5 elements/kg.

3.1. Conodont fauna and age

The recovered conodont fauna is restricted to elements of a sin-gle species, Hindeodus julfensis (Sweet). The following conodontelements were recovered from the six kilogram V2 sample:

H. julfensis (Sweet).Pa elements: 21.Sa elements: 1.Sb elements: 3.Sc elements: 5.M elements: 1.

Fig. 3. (A) Location of the Son La section. (B) Stratigraphic column showing the conodont and brachiopod horizons of the Yenduyet Formation. (C) The 40 cm thick limestoneexposure that yielded the conodont fauna. Partly after Shi and Shen (1998).

Fig. 4. Palaeogeographic reconstruction for the Late Permian (Changhsingian) showing location of the Son Las section and distribution of the Late Permian terrestrial tetrapodDicynodon. After Metcalfe (2011a,b).

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Fig. 5. Scanning electron micrographs of Hindeodus julfensis (Sweet) Pa elements from sample V2. Scale bar represents 100 lm. (1 and 2) Inner lateral and oral views of V2/1.(3 and 4) Inner lateral and oral views of V2/10. (5 and 6) Outer lateral and oral views of V2/16. (7 and 8) Inner lateral and oral views of V2/3. (9 and 10) Inner lateral and oralviews of V2/12. (11–13) Oblique oral (11, 12) and inner lateral views of V2/19. (14 and 15) Inner lateral and oral views of V2/6. (16 and 17) Outer lateral and oral views of V2/26. (18 and 19) Inner lateral and oral views of V2/2. (20, 21) Inner lateral and oral views of V2/7. (22 and 23) Inner lateral and oral views of V2/9. (24 and 25) Outer lateral andoral views of V2/13.

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The precise stratigraphic range of H. julfensis is unclear but itappears to range from the upper Wuchiapingian (rare) to theChanghsingian (common) but this range is dependent on taxo-nomic interpretations of forms previously referred to this spe-cies but now considered assignable to Hindeodus wordensisWardlaw. Pa elements that exhibit fused denticles forming amicro-serrated ridge-like hump in the posterior part of the ele-ment appear to be restricted to the Changhsingian (Kozur,1995a). Elements of this type are recorded here indicating aChanghsingian age. The limestones yielding the conodonts alsocontain the foraminifers C. parva (Colani), and P. prisca (Deprat)supporting a Changhsingian age. A shallow-water hindeodidbiofacies is indicated and is consistent with the associatedshallow-marine fauna including brachiopods, fusilinids andcorals.

3.2. Taxonomic notes

Genus Hindeodus Rexroad and Furnish (1964).Type species – Hindeodus cristulus (Youngquist and Miller,

1949).H. julfensis (Sweet in Teichert et al., 1973)Fig. 5: 1–25 (Pa elements); Fig. 6: 1–13 (Pa elements), 14–15

(Sa element), 16, 23–24 (Sb elements), 19–22, 25–26(Sc elements) and 17–18 (M element).

Synonymy1973. Anchignathodus julfensis n. sp. Sweet. In Teichert et al.

(1973), Pl. 11, Figs. 10–14.1973. Ellisonia teicherti Sweet. (part) In Teichert et al. (1973),

Pl. 12, Figs. 1–5.

(continued on next page)

Fig. 6. Scanning electron micrographs of Hindeodus julfensis (Sweet). All elements from sample V2. Scale bar represents 100 lm (except 9 where separate scale bar of 20 lm isshown). (1 and 2) Inner lateral and oral views of Pa element V2/4. (3) Oral view of Pa element V2/18. (4) Lateral view of Pa element V2/14. (5–7) Inner lateral, oral and outerlateral views of Pa element V2/8. (8) Lateral views of Pa element V2/15. (9) Detail of Pa element V2/15 fused posterior carina denticles forming a serrated ridge. (10) Lateralview of Pa element V2/27. (11) Lateral view of Pa element V2/31. (12) Lateral view of juvenile Pa element V2/17. (13) Lateral view of Pa element V2/11. (14) Posterior view ofSa element V2/25. (15) Anterior view of Sa element V2/25. (16) Lateral view of Sb element V2/20. (17 and 18) Lateral and oral views of M element V2/22. (19) Lateral view ofSc element V2/28. (20) Lateral view of Sc element V2/24. (21) Lateral view of Sc element V2/23. (22) Lateral view of Sc element V2/21. (23) Lateral view of Sb element V2/29.(24) Lateral view of Sb element V2/30. (25 and 26) Outer lateral and inner lateral views of Sc element V2/5.

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1975. A. julfensis Sweet in Teichert et al. (1973). Kozur et al.(1975), Pl. 1, Figs. 1 and 4; Pl. 2, Figs. 2 and 10.

1977. H. julfensis (Sweet) Sweet (1977), p. 219, Pl. 2, Figs. 7–12.

1978. A. julfensis Sweet, Kozur (1978), Pl. 8, Fig. 14.1987. H. julfensis (Sweet in Teichert et al., 1973), Nestell and

Wardlaw (1987), p. 761–767, Figs. 4.1–4.12, 4.14–4.23,4.25–4.30, 7.15.

1992. H. julfensis (Sweet in Teichert et al., 1973) Kozur (1992),p. 102, Figs. 4–20.

1995. H. julfensis (Sweet in Teichert et al., 1973) Kozur(1995a), p. 67, Pl. 1, Fig. 2.

1995. H. julfensis (Sweet in Teichert et al., 1973) Kozur(1995b), Pl. 6, Fig. 1.

1996. H. julfensis (Sweet in Teichert et al., 1973) Kozur, Plate 1,Fig. 9.

1996. H. julfensis wardlawensis n. subsp. Kozur (1996), p. 90.2000. Non-H. julfensis (Sweet in Teichert et al., 1973) Wardlaw

et al. (2000), Pl. 17–1, Figs. 1–6 (=H. wordensis Wardlaw)

3.2.1. Diagnosis (from Nestell and Wardlaw (1987))The skeletal apparatus of H. julfensis is distinguished by a Pa

(anchignathodan) element with enlarged posterior denticles thatproduce a distinct hump in lateral profile on the posterior thirdof the element, and a Sa (hindeodan) element with fine denticleson a thick element and a bottom profile that is nearly straightand slightly up-turned distally forming a broad and flattened ‘‘W’’.

3.2.2. RemarksFor full description see Nestell and Wardlaw (1987, p. 761–

767). The Pa elements here reported are identical to those de-scribed by Sweet in Teichert et al. (1973) and Nestell and Wardlaw(1987). The enlarged three or four deticles in the posterior onethird of the element form a distinct hump and in some cases be-come fused to form a ridge which is micro-serrated on its uppersurface. This is a feature also seen in its descendent species Hinde-odus changxingensis Wang (Wardlaw and Mei, 1998; Metcalfe et al.,2007). Histological studies (Zhuravlev, 2001) show that the Pa ele-ment is characterized by cores of denticles composed of white

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matter which have short roots slightly increasing toward the ante-rior. The white matter core of the anteriormost denticle is shiftedsignificantly toward the posterior. The element blade is coveredby regular microreticulation texture and the anteriormost denticlebears linear microreticulation texture grading into striations. Theevolutionary precursor of H. julfensis is likely to be H. wordensisWardlaw (Wardlaw, 2000; Wardlaw et al., 2000). The Pa elementof H. wordensis has enlarged carinal denticles in the posterior partof the element but does not develop the characteristic hump seenin lateral profile in H. julfensis.

3.3. Conodont CAI, textural alteration and tectonic implications

The conodonts are black in colour representing a conodont Col-our Alteration Index (CAI) of 5 (Epstein et al., 1977; Rejebian et al.,1987). This indicates that the conodont-bearing limestone hasbeen heated to c. 600 �C. The conodonts exhibit some pitting andre-crystallization due to heating, but do not show textural alter-ation (micro-folding, stretching) due to structural and tectonicdeformation as documented elsewhere in the region (Metcalfe,2003). This is consistent with an Early Triassic thermochronologyevent associated with the collision of Subumasu with Indochina/South China (Carter et al., 2001; Carter and Clift, 2008) rather thana Triassic compressional/collisional event that formed part of theIndosinian Orogeny along the Song Ma zone (Zhang et al., 2006;Zhang and Cai, 2009; Cai and Zhang, 2009).

3.4. Ichthyoliths

The conodont sample residue also contained a number of well-preserved ichthyoliths, including hybodont shark scales and aprobable eugeneodontid tooth. The taxonomy and biostratigraphicutility of these are poorly known and await detailed study. Well-preserved examples of these are shown in Fig. 7. Again, there ap-pears to be no micro-structural deformation or distortion of theseichthyoliths that would suggest orogenic deformation.

Fig. 7. Scanning electron micrographs of ichthyoliths from sample V2. (1–3)Hybodont shark scale, specimen V2/44. (4–6) Hybodont shark scale, specimen V2/47. (7–9) Hybodont shark scale, specimen V2/48. (10) Hybodont shark scale,specimen V2/45. (11) Probable eugeneodontid tooth, specimen V2/46.

4. Permian–Triassic transition in Vietnam and regionalcorrelation

In southern Vietnam, the Permian–Triassic transition is re-corded in South Trungbo and Nambo (Tien and Dickins, 1995)where limestones and marls of the Tathiet Formation with aChanghsingian fauna, including the foraminifers C. parva, and P.prisca are overlain by marls, siltstones and sandstones of the c.900 m thick Song Sai Gon Formation that contains an Induan faunain its lower part including Otoceras phumyi, Ophiceras cf. communeand and Claraia stachei (Tien and Dickins, 1995). It is not yet knownif the transition at the Permian–Triassic boundary sequence in thisregion is complete or if there is a stratigraphic break. The Permian–Triassic transition in northern Vietnam has been documented inWest Bacbo (including the Son La section that is the focus of thispaper) and in two sections in Vietbac, the Nhi Tao and Lung Camsections (Algeo et al., 2007; Son et al., 2007, see Fig. 2). The SonLa section lies within the Song Da Rift Zone and the Nhi Tao andLung Cam sections lie close to the Vietnam–China border on theNorth Vietnam terrane. All these sections have been interpretedas being constructed on South China terrane basement and theNhi Tao and Lung Cam sections are interpreted as forming anextension of the Jinxi carbonate platform of the Nanpanjiang Basin(Algeo et al., 2007). However, recent proposals of a Dian-Qiong Su-ture (Zhang et al., 2006; Zhang and Cai, 2009; Cai and Zhang, 2009)on the SE margin of the Nanpanjiang Basin brings into question theNhi Tao and Lung Cam sections as an extension of the Jinxi plat-form (see Metcalfe, 2011b for discussion). The precise position of

the Permian–Triassic boundary in all three sections is yet to bedemonstrated. The occurrence of H. parvus at Nhi Tao is well abovethe interpreted P–T boundary level using forams and carbon iso-tope proxy data (Fig. 8). The inferred position of the P–T boundaryat Lung Cam is based on d13Ccarb and magnetic susceptibility curvesand limited foraminiferal biostratigraphy (Son et al., 2007; seeFig. 8). Similarly, the inferred boundary level at Nhi Tao is basedon a combination of foraminiferal biostratigraphy, chemostratigra-phy and lithostratigraphy. Further conodont studies of these sec-tions should yield tighter controls on the position of theboundary and extinction levels.

5. Conclusions

Upper Permian (Changhsingian) conodonts are for the first timerecorded from Vietnam. This confirms the Changhsingian age ofbiogeographically important brachiopods in the Yenduyet Forma-tion of northern Vietnam.

The Permian–Triassic biostratigraphic boundary is yet to beprecisely located in Vietnam and is currently indicated by proxychemostratigraphy and magnetic susceptibility signals that arecorrelated with South China. Future intensive conodont studies

Fig. 8. Correlation of the Nhi Tao and Lung Cam Permian–Triassic boundary sections of North Vietnam with the Global Stratotype Section and Point (GSSP) at Meishan (afterAlgeo et al., 2007; Son et al., 2007). Shallo-water biofacies conodont zones at Meishan after Metcalfe and Isozaki (2009).

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on Vietnamese P–T transition sequences should reveal the bound-ary level and confirm continuity or discontinuity of sedimentation.

Colour and textural alteration of the conodonts indicate theyhave been subject to substantial heating to c. 600 �C but that theydo not show any signs of tectonic textural alteration that would beexpected in an orogenic deformational zone. This is consistent withthe Song Da sequence being in a rift environment rather than in anorogenic collision zone during the Late Permian.

Acknowledgements

Facilities provided by the Earth Sciences Discipline, School ofEnvironmental and Rural Science, University of New England isgratefully acknowledged. Scanning electron microscopy of con-odonts and ichthyoliths was undertaken at the Microscopy Unit,Department of Biological Sciences, Macquarie University. I wouldlike to thank Susan Turner and Alain Blieck for comments onichthyoliths.

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