Fish suggests continental connections between the Indochina and South China blocks in Middle Devonian time Tong-Dzuy Thanh Department of Geology, Vietnam National University, 90, Nguyen Trai, Dong Da, Hanoi, Vietnam Philippe Janvier URA 12 du CNRS, Laboratoire de Paléontologie, Museum National d’Histoire Naturelle, 8, rue Buffon,75005 Paris, France Ta Hoa Phuong Department of Geology, Vietnam 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 Late Devonian. Fishes, in particular, are represented by several higher taxa (yunnanolepiform and procondylolepi-form antiarchs among placoderms, and youngolepiforms among sarcopterygians) that were 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 were not previously known to occur outside these two blocks, despite an exten- sive Devonian vertebrate record from Aus- tralia, North Amer-ica, Europe, and Siberia. Young (1981, 1990, 1993) therefore coined the name “galeaspid-yunnanolepid province” for this faunal province of Asia in Silurian- Devo-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 Da (Black River) areas. The Low- er Devonian sequence in southern Yunnan, Guangxi, and Bac Bo (northern Vietnam) generally consists of Lochkovian terrig- enous sediments of Old Red Sandstone fa- cies (the Lianhuashan Formation in China and the Si Ka Formation in the Bac Bo), overlain by more marine and pelagic facies throughout the Devonian, with a short de- trital (possibly nonmarine) episode in the Givetian or early Frasnian in the east (east- ern Bac Bo). In the Song Da area of Vietnam, the fa- cies of the Devonian are somewhat differ- ent, having deeper facies, and farther south, in central Vietnam (Trung Bo), the classical Devonian lithological sequence of the north reappears with much the same divisions (terrigenous at the base and progressively more 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 formed by accretion of several terranes, some originating from Gondwana in the early Paleozoic. Within Vietnam the Song Ma suture forms the boundary between the South China block in the north and the In- dochina terrace in central and southern Vietnam (Fig. 1A). Consistent with this is the marked difference in the Devonian in- vertebrate faunas of northern and central Vietnam, which has been interpreted to in- -I _________ I _________ I _________ L. A, CHINA 'o. ; Dai Giang Formation (Upper Silurian - Lower Devonian) II I II Rao Chan Formation (Lower Devonian) Ban Giang Formation (Eifelian) EZ3 Dong Tho Formation (Givetian - ?Lower Frasnian) Carboniferous Pre - Lower Carboniferous granite □ Quaternary Figure 1. 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. Geology; June 1996; v. 24; no. 6; p. 571-574; 2 figures. 571