Fish suggests continental connections between the Indochina and South China blocks in Middle Devonian time Tong-Dzuy Thanh D e p a rtm e n t o f G eology, Vietnam National U niversity, 90, Nguyen Philippe Janvier URA 12 du CNRS, L a bo ratoire de P aléontologie, M useum National Trai, Dong Da, Hanoi, Vietnam d ’H istoire N aturelle, 8, rue B u ffon ,7 50 05 Paris, France Ta Hoa Phuong D e p a rtm e n t o f G eology, V ietnam National University, 90, Nguyen Trai, Dong Da, Hanoi, Vietnam ABSTRACT A yunnanolepiform antiarch (placoderm fish) is recorded from the Givetian Dong Tho Formation of Quang Binh Province, central Vietnam This and other fishes from the same locality occur in marginal marine, detrital facies, with plant remains and lingulid bra-chiopods that indicate a nearshore to deltaic environment Yunnanolepiform antiarchs were hitherto known exclusively from the Lower Devonian of the South China block The new occurrence of this group well south of the Song Ma suture suggests close links between the Indochina and South China blocks in Middle Devonian time The massive sandstone ex posures of the Dong Tho Formation may be a southern extension of the Do Son Sandstone Formation of the Hai Phong area, which is located on the South China block INTRODUCTION The marine Devonian faunal assemblages from shallow-water and marginal platform facies of the South China block have long been known to be largely endemic This en-demism is particularly well marked in the Silurian and Early Devonian; some taxa survived until the L ate Devonian Fishes, in particular, are rep rese n ted by several higher taxa (yunnanolepiform and procondylolepi-form antiarchs am ong placoderm s, and youngolepiforms among sarcopterygians) that w ere regarded as unique to the Silurian and Devonian of the South China block A group of jawless fishes, the galeaspids, is especially abundant and diversified in the Si-lurian and Early Devonian of South China, but is now known to occur in the North China block and in northern Tarim (Liu, 1995) These major fish taxa w ere not previously known to occur outside these two blocks, despite an exten sive Devonian vertebrate record from A us tralia, N orth Amer-ica, Europe, and Siberia Y oung (1981, 1990, 1993) therefore coined the name “galeaspid-yunnanolepid province” for this faunal province of A sia in SilurianDevo-nian time The invertebrate faunas, in particular brachiopods, display much the same endemism, for example, the Pragian Eu-ryspirifer tonkinensis and the Howittia wangi faunal assemblages These character istic faunas are easily traced into Vietnam, as far to the south as the Bac Thai and even the Song D a (Black River) areas The Low er Devonian sequence in southern Yunnan, Guangxi, and Bac Bo (northern Vietnam) generally consists o f L ochkovian te rrig enous sediments of Old R ed Sandstone fa cies (the Lianhuashan Form ation in China and the Si Ka Form ation in the Bac Bo), Geology; June 1996; v 24; no 6; p 571-574; figures overlain by m ore marine and pelagic facies throughout the Devonian, with a short detrital (possibly nonm arine) episode in the Givetian or early Frasnian in the east (east ern Bac Bo) In the Song D a area of Vietnam, the fa cies of the Devonian are somewhat differ ent, having deeper facies, and farther south, in central V ietnam (Trung Bo), the classical Devonian lithological sequence of the north reappears with much the same divisions (terrigenous at the base and progressively m ore marine and pelagic toward the top, and a Givetian-early Frasnian detritic episode locally), but quite different invertebrate fau nas The typical Euryspirifer tonkinensis and Howittia wangi faunal assemblages of the north, for example, have never been found in central Vietnam However, some wide spread taxa, in particular among tabulates and corals, have been recorded in both areas (Tong-Dzuy, 1993) Southeast Asia is considered to have form ed by accretion of several terranes, some originating from Gondwana in the early Paleozoic W ithin V ietnam the Song M a suture forms the boundary between the South China block in the north and the In dochina terrace in central and southern V ietnam (Fig 1A) Consistent with this is the marked difference in the Devonian in vertebrate faunas of northern and central V ietnam, which has been interpreted to in- - I _ I _ I _ L A, C H INA 'o ; Dai Giang Formation (Upper Silurian - Lower Devonian) II I II Rao Chan Formation (Lower Devonian) Ban Giang Formation (Eifelian) EZ3 Dong Tho Formation Pre - Lower Carboniferous granite (Givetian - ?Lower Frasnian) Carboniferous □ Quaternary Figure A: General map of Vietnam showing position of main sutures and location of two major exposures of Middle Devonian sandstone discussed here, Do Son Sandstone Formation at Do Son and Dong Tho Formation at Ly Hoa B: Geologic map of Ly Hoa area (from Tong-Dzuy et al., 1994a) Vertebrate localities: 1, outcrop situated near Da Nhay (Tong-Dzuy et al., 1994a); 2, quarry along highway at Ly Hoa Pass 571 dicate some kind of paleogeographical bar rier (e.g., a wide ocean) between the South China and Indochina blocks in Devonian time A fish fauna was recently discovered in the Middle Devonian Dong Tho Formation at Ly H oa, Q uang Binh Province, central Vietnam, in association with lingulid brachiopods, bivalves, and terrestrial plant re mains It comprises at least one of the typical South Chinese fish taxa, the yunnanolepiform antiarchs This discovery strongly sug gests close biogeographical relationships b e tween the South China and Indochina blocks in Devonian time DONG THO AND MUC BAI FORMATIONS Relatively large Devonian exposures are present in the Q uang Binh Province of cen tral V ietnam, east of Cape Ly H oa and south of the G ianh River (Song Gianh) (Fig 1B), in the southern p art of the Truong Son anticlinorium Four Devonian form a tions are recognized in this area: the Early Devonian R ao Chan Form ation, the Eifelian Ban Giang Form ation, and the Givetian-?early Frasnian Muc Bai and Dong Tho formations, which represent two differ ent but probably synchronous facies (TongDzuy, 1993; Tong-Dzuy et al., 1994a) The D ong Tho Form ation consists of massive sandstones, with rare argillaceous lenses, and the Muc Bai Form ation mainly com prises terrigenous sediments in its lower part, and marls, grayish limestones, and in terbedded sandstones and shales in its upper part The Muc Bai Form ation yields a coral and brachiopod fauna of Givetian to possi bly early Frasnian age The Muc Bai F or mation is widespread to the west of this area, whereas the D ong Tho Formation crops out near the m outh of the Gianh River and along the coast The relationship be tween the two formations is still unclear, but it seems that the D ong Tho Form ation is merely a more marginal to deltaic facies of the Muc Bai Formation, as the latter be comes more detrital eastward (Tong-Dzuy, 1993; Nguyen H uu et al., 1995) The Dong Tho Form ation crops out at Cape Ly Hoa, w here it forms a slightly hilly area traversed by the H anoi-H o Chi Minh City highway (Ly H oa Pass) H ere it is almost barren, and only fish remains have been found in one particular level, in association with lingulids and bivalves Some fine-grained sandstone lenses also yield indeterm inate plant re mains, mainly lycophyte leaves and stem fragments F arther to the west, west of Minh Le, black marls interbedded in the Dong Tho Form ation contain relatively large (up to cm in diam eter) stems of lycophytes (of Protolepidodendron and Lepidodendropsis 572 type), as well as a rich spore assemblage that suggests a Givetian age (Tong-Dzuy and Cai, 1995) This spore assemblage is quite similar to that of the Givetian H aikou F or mation of Yunnan The fishes found to date in the Dong Tho Form ation clearly belong to Devonian taxa but are somewhat anach ronistic: they all belong to groups (e.g., yunnanolepiforms, phlyctaeniids, and youngolepiforms) that are either mainly or exclusively known from the Early Devonian on the South China block This discrepancy is provisionally attributed here to ende mism, because there is no evidence that the Dong Tho Form ation extends into the Lower Devonian M oreover, some large plant remains, as yet indeterminable, found in association with the fish remains rule out an Early Devonian age because of their size The cross-bedding of the D ong Tho F or mation is progressively more conspicuous to the east, and this suggests an increasingly deltaic environment Considering the thick ness of this formation, it seems probable that it corresponds to the margin of an im portant landmass that extended east of this area in Middle Devonian time LY HOA FISH FAUNA Fish remains have been found at two places in the outcrop of the Dong Tho F or mation at Cape Ly H oa, at the southern end of Ly H oa beach O ne is a small lens of bio turbated clayey sandstone, opposite the D a Nhay (“dancing rock”; 1, Fig 1B), and the other is a layer of clayey sandstone in a small quarry on the side of the H anoi-H o Chi Minh City highway, at the top of the Ly H oa Pass (2, Fig 1B) The two localities are only km apart, and considering the similar lithology (clayey sandstone) and local struc ture (a 40°SE dip), they probably corre spond to the same level of the formation Fish remains were first discovered from locality (Tong-Dzuy et al., 1994a) They included various poorly preserved placoderm remains and almost com plete thoracic arm or of a phlyctaeniid arthrodire, Lyhoalepis duckhoai, which did not provide biogeographical information, because it is unique in many respects, and does not clearly show any particular affinities with other phlyctaeniid arthrodire genera from China or else where In 1994, the second locality yielded more significant fish remains that could be identified as belonging to antiarchs and sarcopterygians A ntiarchs are a group of placoderms whose pectoral fins are modified into jointed appendages, and which are gen erally restricted to marginal, deltaic, and possibly fresh-water facies They usually show a m arked endemism and are among the biogeographically m ost significant De- Figure A: Anterior ventrolateral plate of tho racic arm or of yunnanolepiform antiarch , Dong Tho Form ation, Ly Hoa, Quang Binh Province, Vietnam (br is brachial articulation) B: Yunnanolepis parvus Zhang, Lower Devo nian or Yunnan, China (after Zhang, 1982), thoracic arm or in ventral view, showing ante rior ventrolateral plate (stippled) and brachial articulation area (br.) Scale bar: mm vonian vertebrates For example, 19 of the 40 antiarch genera are endemic to South China The antiarch m aterial from Ly H oa includes derm al plates of a yunnanolepiform, one of the antiarch groups hitherto known exclusively from the South China block The m ost characteristic of these plates is a right anterior ventrolateral plate of a small yunnanolepiform (Fig 2A), orna m ented with coarse, scattered tubercles The area for the articulation of the pectoral fin is a small recess devoid of any articular device or brachial process (br., Fig 2A), and there is no axillary foramen It is thus similar to that of the classical yunnanolepiforms, such as Yunnanolepis from the Early Devo nian of Y unnan and northern Vietnam (br., Fig 2B) T he proportions of this plate, which is remarkably short and broad, are suggestive of a stout-bodied form, and this accords with the overall shape of several other plates found in the same locality, in particular a short and broad anterior m e dian dorsal plate O ther m aterial consists of isolated plates and plate fragments, among G E O LO G Y , June 1996 which is an anterior m edian dorsal plate, and a posterior median dorsal plate in which the posterior transverse ridge meets the pos terior internal m edian process, as in typical yunnanolepiforms This m aterial certainly belongs to a new species, which will be de scribed subsequently in a monograph, along with the rest of the fauna The sarcopterygian remains consist of derm al bones and scales covered with a layer of cosmine, which is pierced by large pores They are suggestive of the youngolepiforms from the Early-Middle Devonian of the South China block DISCUSSION Y unnanolepiform antiarchs have long been regarded as the most primitive antiarchs because of the simple structure of their brachial articulation, and many other features of their dermal armor, which recall the generalized placoderm condition A l though Janvier and Pan (1982), Young (1988), and Young and Zhang (1992) con sidered that they may be a paraphyletic en semble of generalized antiarchs (not defin able by any unique character), their endemic distribution would be more consistent with monophyly Z hu and Janvier (1996) propose several unique yunnanolepiform features supporting monophyly, which would better accord with an early geographical isolation from other areas with Devonian fishes of this age, namely Gondwana, Euramerica, and Kazakhstan W hether the form de scribed here should be assigned to the Yunnanolepiformes sensu stricto or regarded as a mere close relative of this group is still unde cided, but it is clear that its closest ties are with the South Chinese yunnanolepiforms Considering the fact that antiarchs are among the m ost common Devonian fishes, the occurrence of the yunnanolepiforms ex clusively on the South China and Indochina blocks provides strong evidence for close geographical relationships between these two areas in Devonian time We must, how ever, emphasize the age discrepancy be tween these two occurrences On the South China block, yunnanolepiform antiarchs are restricted to the Lower Devonian and have never been found in stata younger than the late Pragian or Emsian Their new occur rence in the Givetian-?early Frasnian Dong Tho Formation, the age of which is well con strained by underlying and overlying m arine fossiliferous formations, is therefore sur prising A possible explanation is that this area served as a refuge for taxa th at survived from the Early Devonian, as suggested by Tong-Dzuy et al (1994a) for the relatively primitive, usually Early Devonian phlyctaeniid arthrodire Lyhoalepis This may accord G E O LO G Y , June 1996 with the suggestion made by Metcalfe (1996) that the Indochina and South China blocks were connected in the Silurian or Early Devonian, and were then discon nected in the Middle Devonian DEVONIAN FISHES AND TECTONICS It is a well-established view that many terranes were involved in the formation of Southeast Asia, but their num ber and the timing of their accretion are subject to much discussion (Gatinsky and Hutchison, 1987; Hutchinson, 1989; Metcalfe, 1988, 1990, 1993, 1995, 1996; M ouret, 1994; §engor, 1984; §engor and Hsu, 1984; W u et al., 1995) As for peninsular Indochina, there is general agreem ent that south of the South China (Yangtzi, Cathaysia) block, there are two m ajor terranes or blocks, the Sibumasu (Shan-Thai) terrane, and the Indochina (Annamia, Indosinia, Kontum) block A num ber of minor additional terranes have been proposed, such as the Simao, Phuket, Phu Hoat, Song Da, and A nnam ite terranes and/or microplates, but some of these are doubtful The accretion of some of these terranes and blocks to the South China block is now relatively well dated (Hutchinson, 1989; W u et al., 1995), in particular for the late Paleozoic and Triassic accretions However, questions remain as to the timing of the ear lier accretions In Vietnam, only the Song Ma suture is accepted as a suture by all geologists, and marks the northern limit of the Indochina block (Fig 1A) The so-called Song D a su ture, between the “Song D a terran e” and the South China block, is a m atter for de bate (Fig 1A) F arther south, another pos sible suture, the D anang line (Fig 1A), may be Devonian or earlier in age The alleged Triassic accretion of the Song D a terrane to the South China block along the Song D a suture (Fig 1A) as proposed by §engor and H su (1984), is contradicted by the discovery of a typical South Chinese Early Devonian fish and brachiopod assem blage in V an Yen, near H oa Binh, on the shore of the Song D a (Pham Kim and Nguyen Cong, 1995) This fauna comprises the classical com ponents of the Howittia wangi community of the South China block, including yunnanolepiform antiarchs and some galeaspid remains T he similarity of the Devonian invertebrate assemblages of the Song D a area and northern V ietnam (Bac Bo) has long been recognized by p a leontologists (Tong-Dzuy, 1993), and p re cludes the existence of any barrier or large oceanic space between these two areas in Devonian time The occurrence of a typical South Chinese vertebrate assemblage in the Early Devonian terrigenous facies of the Song D a area confirms that, like the more northerly situated areas of the Bac Bo, it belongs to the Devonian South China block A t any rate, if there is a Song D a suture, its Late Silurian closure as suggested by M ouret (1994) would be consistent with the above evidence of Devonian faunas, and as such it would be irrelevant to Devonian paleogeography The age of closure of the Song Ma suture between the Indochina block and the South China block is also a m atter of debate §engor and H su (1984) proposed a Late Triassic age in the tectonic collage also in volving the Song D a suture Hutchison (1989), however, proposed a Visean closure, M ouret (1994) considered it to be middle Carboniferous, and Laveine et al (1994); on the basis of paleobotanical data, provided evidence for Indochina-N orth and South China links existing in the Early Carbonif erous The new discovery of yunnanolepiform antiarchs south of the Song M a suture provides evidence for a Middle Devonian continental connection between the In dochina and South China blocks T hat these Indochinese antiarchs could be relicts of an Early Devonian South Chinese group sug gests that this contact occurred much ear lier, possibly in the Late Silurian or Early Devonian, and that a later (?M iddle Devo nian) isolation occurred (Metcalfe, 1996) Again, it is very unlikely that such continen tal m argin-bound fishes as yunnanolepiform antiarchs could have dispersed across a wide ocean Otherwise, yunnanolepiforms would also occur in Gondwanan regions (Australia) or Kazakhstan This preliminary survey of the Ly H oa fish fauna raises the question of the nature of another Devonian fish-bearing formation, the Do Son Sandstone Form ation (Janvier et al., 1994) This is a small “Old R ed Sand stone” facies exposure forming the Do Son peninsula, south of H Phong in northern V ietnam (Fig 1A) It is bounded by faults and surrounded by N eogene sediments of the R ed River delta, so its tectonic relation ship to the Devonian of the rest of the Bac Bo is obscure Nevertheless, it is now dated as late Givetian, thanks to the discovery of fishes, bivalves, and plants (Janvier et al., 1989; Long et al., 1990; Tong-Dzuy et al., 1994b; Tong-Dzuy and Cai, 1995) The lithology and the plant assemblage of the Do Son sandstone are strikingly similar to those of the Dong Tho Formation The plant as semblage consists of large stems of Colpodexylon and Lepidodendropsis and abundant remains of psilophytes Although no typi cally South Chinese fish taxa have been found in the Do Son sandstone, some of the antiarchs show similarity to forms from the 573 South China block, whereas others are unique to this locality We suggest that the D o Son Sandstone and Dong Tho form a tions may be rem nants of the same shore line, on the western margin of a landmass th at extended the Indochina and South China blocks eastward and may now be rep resented by p art of H ainan and possibly the W est Borneo basem ent (Hutchison, 1989) An additional occurrence of these late Givetian sandstones may be represented farther north in the eastern Bac Bo by the Givetian detrital upper m em ber of the Duong Dong Form ation or the Tan Lap Form ation (Tong-Dzuy, 1993), which yield lycophyte stems and the same kind of planorbid-like gastropods (still indeterm inate) as those found in the Do Son Sandstone Formation It thus seems that the geological histories of the Indochina and South China blocks in Devonian time are strongly linked, as sug gested by continent-bound vertebrate fau nas The difference in Devonian inverte brate faunas between the South China block (Yunnan, Guangxi, Bac Bo, and Song D a area) and the Indochina block (Trung Bo) remains a riddle but may provisionally be assigned to a difference in latitude or envi ronm ent A t any rate, the vertebrates and plants found in the marginal and deltaic fa cies of the two blocks provide evidence for close biogeographical links as early as the Givetian We can thus predict that the Lower Devonian red sandstones of the Tan Lam Form ation in the Quang Tri area (cen tral V ietnam ), which have hitherto yielded only lingulid brachiopods, may contain much the same fish assemblage as the red sandstone and shales of the LochkovianPragian Si Ka Form ation of the Bac Bo CONCLUSIONS The occurrence of a yunnanolepiform an tiarch in the Dong Tho F orm ation of central Vietnam suggests that the Indochina block was linked by continental connections to the South China block during Givetian time, and probably somewhat earlier How and w here this contact occurred is unknown, but if the Dong Tho Form ation belongs to the same detritic ensemble as the contem pora neous Do Son Sandstone Form ation of the H Phong area, as suggested here, it may possibly have been through a large eastern portion of the Indochina block, now dism an tled by the opening of the South C hina-E ast Sea Bringing the D o Son and Ly H oa sand stones into the closest possible position may provide an estim ation of the am plitude of the eastward displacement (or extrusion) of the Indochina block since Devonian time 574 ACKNOWLEDGMENTS Supported by the Research Program in Funda m ental N atural Sciences of Vietnam , grant KT 04, and the National G eographic Society, grant 5089/93 Photographs by D Serrette, Paris W e thank I M et calfe for providing unpublished information REFERENCES CITED Gatinsky, Y G., and Hutchison, C S., 1987, Cathaysia, Gondwanaland, and the Paleotethys in the evolution of continental Southeast Asia: Geolog ical Society of Malaysia Bulletin, v 20, p 179-199 Hutchinson, C S., 1989, Geological evolution of Southeast Asia: Oxford M onographs on G eol ogy and Geophysics, v 13, p 1-349 Janvier, P., and P an Jiang, 1982, Hyrcanaspis bliecki n g., n 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Journal of V er tebrate Paleontology (in press) M anuscript received Novem ber 2, 1995 Revised m anuscript received M arch 12, 1996 M anuscript accepted M arch 25, 1996 G E O L O G Y , June 1996 ... suture by all geologists, and marks the northern limit of the Indochina block (Fig 1A) The so-called Song D a su ture, between the “Song D a terran e” and the South China block, is a m atter... with the above evidence of Devonian faunas, and as such it would be irrelevant to Devonian paleogeography The age of closure of the Song Ma suture between the Indochina block and the South China. .. with the South Chinese yunnanolepiforms Considering the fact that antiarchs are among the m ost common Devonian fishes, the occurrence of the yunnanolepiforms ex clusively on the South China and