The Đồng Văn section was first described by J.Deprat [2] as "Bắc Bun – Summit 1500 section". The author of this paper published the data on Devonian and Carboniferous stratigraphy of this section in a recent issue of this journal [15]. In this paper conodont zones found in the Si Phai and Tốc Tát formations (corresponding to the Mã Pì Lèn "Série" of Deprat ) will be presented.

The conodont fossils of the Đồng Văn section were first discovered and determined by Phạm Kim Ngân [12, 13]. They belong to the Emsian of Lower Devonian (perbonus Zone – 1 sample), the Givetian of Middle Devonian (varcus Zone – 1 sample) and the Famennian of Upper Devonian (marginifera Zone – 1 sample).

In his publication on the fossils of tentaculites and conodonts found in the Đồng Văn area Tạ Hòa Phương [14] has been distinguishing 7 tentaculite zones in the "Nà Quản and Bằng Ca formations" namely: Nowakia acuaria, N. zlichovensis, N. praecursor, N. barrandei, N. elegans, N. cancellata, N. richteri, N. otomari zones (the total amount of these above formations is just the Si Phai Formation according to recent opinions of the author of this paper) [15, see Table 1]. At that time conodont fossils were preliminarily studied, but participated in the orientation of later studies.

After 5 times of study from 1990 to 1996 in the Đồng Văn area, hundreds samples were collected in a thicker and thicker network by the author. In the interval from Emsian (D₁) to Tournaisian (C₁) 17 conodont zones and 1 conodont association were discovered. The brief description of these units in ascending order with the correlation to lithostratigraphic units (Table 1) is given as follows.

The excavatus Zone is the former gronbergi Zone established by K.Weddige (24), that was renamed according to the taxonomic revision. In the Đồng Văn section it was found in a bed of grey, medium-bedded limestone of the lower part of the Si Phai Formation, higher than the Nowakia acuaria and N. zlichovensis zones of Dacryoconarids. Its lower boundary is marked by the first appearance of Pol. excavatus excavatus (Carl & Gandl), but in this section, corresponding to this level there is only Pandorinella steinhornensis (Ziegler), a widespread species in the zone. In the higher stratigraphic level have been met Pol. excavatus grongergi Klapper & Johnson and Pol. nothoperbonus Mawson (early form).

The nothoperbonus Zone occurs in limestone beds of the lower part of the Si Phai Formation, corresponding to the N. elegans Zone of Dacryoconarids. It begins by the appearance of Pol. nothoperbonus Mawson (typical form). Besides, there still are Pandorinellina steinhornensis (Ziegler) and Icriodus n. sp..

The inversus Zone is present in limestone beds of the lower part of the Si Phai Formation, in the same level of the cancellata Zone of Dacryoconarids. The lower boundary of the zone is marked by the first appearance of Pol. inversus Klapper & Johnson. In the Đồng Văn section the fossil assemblage includes Pol. inversus Klapper & Johnson, Pol. nothoperbonus Mawson (late form, in abundance), Pandorinella steinhornensis (Ziegler).

The serotinus Zone occurs in the beds of the middle part of the Si Phai Formation, corresponding to the level of the N. richteri Zone of Dacryoconarids. Its beginning is marked by the appearance of Pol. serotinus Telford. The zone includes Pol. serotinus Telford, Pol. inversus Klapper & Johnson, Pol. bultyncki Weddige and Pol. inversus Klapper & Johnson (late form).

The patulus Zone exists in the beds of the middle part of the Si Phai Formation. It begins by the appearance of Pol. costatus patulus Klapper. The fauna assemblage includes Pol. costatus patulus Klapper, Pol. pseudofoliatus Wittekindt and Pol. serotinus Telford.

The varcus Zone occurs in the beds of grey, thin-bedded limestone of the upper part of the Si Phai Formation. Its beginning is marked by the appearance of Pol. varcus Stauffer. The conodont assemblage of this zone in the Đồng Văn section is rather abundant with Pol. varcus Stauffer, Pol. kluepfeli Wittekindt, Pol. ovatinodosus Ziegler & Klapper, Pol. ansatus Ziegler & Klapper, Pol. hemiansatus Bultynck, Pol. linguiformis Hinde, Pol. weddigei Clausen, Leuteritz & Ziegler, Pol. timorensis Klapper, Philip & Jackson and Pol. klapperi Clausen, Leuteritz & Ziegler.

The cristatus Zone is present in the beds of grey, thin-bedded limestone of the upper part of the Si Phai Formation, just above the varcus Zone. Its beginning is marked by the appearance of Pol. cristatus Hinde. The conodont assemblage of this zone in the Đồng Văn section is still poor. Apart from the index species of the zone, there is only Schmidtognathus sp., but abundant individuals of the tentaculite Homoctenus (H.10/11), are not detailedly determined yet.

The disparilis Zone occurs in the beds of grey, thin-bedded limestone of the upper part of the Si Phai Formation. It begins by the appearance of Klapperina disparilis Ziegler & Klapper. The conodont assemblage of this zone includes, apart from the index species of the zone, Pol. cristatus Hinde, Schmidtognathus sp. and Homoctenus sp. (H.10/12).

The transitans Zone exists in the beds of grey, thin-bedded limestone of the uppermost part of the Si Phai Formation. Its beginning is marked by the appearance of Palmatolepis transitans Muller. The conodont assemblage of this zone includes, apart from the index species of the zone, Mesotaxis falsiovalis Sandberg, Ziegler & Bultynck, Icriodus difficilis Ziegler & Klapper and many specimens of the tentaculite Homoctenus sp.. It is possible to distinguish this zone from the underlying falsiovalis Zone in further more detailed study.

Table 2. Devonian-Carboniferous conodont zones found in the Đồng Văn section

The hassi Zone occurs in the beds of pinkish-grey, thin-bedded and banded limestone of the lowermost part of the Tốc Tát Formation. Its lower boundary is marked by the appearance of Palmatolepis hassi Muller & Muller. The abundant conodont assemblage of this zone is characterized by, apart from the index species of the zone, Pa. proversa Ziegler, Pa. punctata (Hinde), Pa. amplificata Klapper, Kuzmin & Ovtanova, Pa. proversa đ Pa. simpla, Pa. simpla Ziegler & Sandberg, Pa. ljaschenkoae Ovtanova, Pa. domanicensis Ovtanova, Ancyrodella gigas Youngquist, An. nodosa Ulrich & Bassler and Ag. aff. primus Ji.

To date, the index species of the zone has not been found yet in the Đồng Văn section. However, the conodont assemblage corresponding to the rhenana Zone occurs in the banded limestone of the lower part of the Tốc Tát Formation, including: Pa. subrecta Miller & Youngquist, Pa. ljaschenkoae Ovtanova, Pa. cf. jamieae Ziegler & Sandberg, Pol. uchtensis Ovtanova & Kuzmin, Pol. lodinensis Polsler, Ancyrodella nodosa Ulrich & Bassler and Pol. brevilamiformis Ovtanova.

The triangularis Zone occurs in the beds of banded limestone of the lower part of the Tốc Tát Formation. Its lower boundary is marked by the appearance of Pa. triangularis Sanneman. The conodont assemblage of this zone is characterized by, apart from the index species of the zone, Pa. superlobata Branson & Mehl, Pa. clarki Ziegler, Pa. delicatula delicatula Branson & Mehl, Pa. protorhomboidea Sandberg et Ziegler, Pa. cf. regularis Cooper, Pa. clarki đ Pa. minuta and Pa. minuta minuta Branson & Mehl.

The rhomboidea Zone occurs in the beds of banded limestone of the lower part of the Tốc Tát Formation, near the Si Phai slope. Its lower boundary is marked by the appearance of Pa. rhomboidea Sanneman, as the upper – by the first appearance of Pa. marginifera Helms. The conodont assemblage of this zone is poor and includes, apart from the index species of the zone, Pa. glabra elongata Holmes and Pa. minuta minuta Branson & Mehl.

In the stratigraphy of Upper Devonian of Việt Nam the marginifera Zone is a widespread zone and occurs in the thickest beds in comparing with other zones. It exists in banded limestone of the middle part of the Tốc Tát Formation. Its lower boundary is marked by the first appearance of Pa. marginifera Helms. The conodont assemblage of this zone is characterized by, apart from the index species of the zone, Pa. glabra glabra, Pa. glabra pectinata Ziegler, Pa. minuta minuta Branson & Mehl, Pa. rhomboidea Sanneman and Polygnathus purus purus Voges.

The index species of the uppermost Famennian conodont zones (Pa. postera, Pa. expansa, Pa. praesulcata) have not been found yet in the Đồng Văn section. The Pa. sigmoidalis – Pa. gonioclymeniae Association corresponds to the above zones. It occurs in the beds of banded limestone of the uppermost part of the member 1 of the Tốc Tát Formation, including: Pa. perlobata schindewolfi Muller, Pa. gracilis gracilis Branson & Mehl, Pa. gracilis sigmoidalis Ziegler, Pa. gonioclymeniae Muller, Pseudopolygnathus marburgensis trigonicus Ziegler, Ps. micropunctatus Ziegler, Polygnathus nodocostata Branson & Mehl, Pol. glaber Ulrich & Bassler, Pol. glaber Ulrich & Bassler, Pol. vogesi Ziegler, Bispathodus stabilis (Branson & Mehl).

The duplicata Zone occurs in the beds of grey limestone belonging to the basal part of the member 2 (Tu Sán Member) of the Tốc Tát Formation. Its lower boundary is marked by the appearance of Siphonodella duplicata (Branson & Mehl). The conodont assemblage of this zone is characterized by, apart from the index species of the zone, Si. sulcata (Huddle), Pol. communis communis Branson & Mehl, Pol. purus purus Voges, Bispathodus aculeatus aculeatus (Branson & Mehl), Pseudopolygnathus primus, Ps. inaequalis and Ps. cf. brevipennatus Ziegler. The presence of Si. sulcata in the assemblage shows that the base of this member is very near to the D/C boundary (see Table 2).

The crenulata Zone was found in the beds of grey limestone belonging to the lower part of the Tu Sán Member, Tốc Tát Formation. Its lower boundary is marked by the first appearance of Siphonodella crenulata (Cooper). The conodont assemblage of this zone includes, apart from the index species of the zone, Si. duplicata (Branson & Mehl) and Si. lobata (Branson & Mehl).

The isosticha Zone occurs in the beds of grey limestone belonging to the middle part of the Tu Sán Member, Tốc Tát Formation. Its lower boundary is marked by the first appearance of Siphonodella isosticha (Cooper). The conodont assemblage of this zone includes, apart from the index species of the zone, Si. cooperi Hass and Si. obsoleta Hass, Si. crenulata (Cooper).

1) The Đồng Văn section is the fullest and most typical Devonian section in this extreme north area of Việt Nam. It contains all Devonian stratigraphic units of the area, namely: Si Ka, Bắc Bun, Mia Lé, Si Phai and Tốc Tát formations, with different groups of fossils (vertebrates, tabulates, rugoses, brachiopods, bivalves, trilobites, tentaculites, conodonts, etc...). The two uppermost formations of the section, namely Si Phai and Tốc Tát, belong to the deep-water facies, containing in abundance fossils of the Dacryoconarid (tentaculites) and conodont groups, which are of good stratigraphic significance..

2) The Pragian / Emsian chronostratigraphic boundary has not been determined on the conodont basis yet due to the lack of materials. In the basal bed of the Si Phai Formation many specimens of Nowakia acuaria, the tentaculite characterizing the Pragian sediments of many places in the world, have been found [14, 15]. The brachiopods of the Euryspirifer tonkinensis Assemblage characterizing the underlying Mia Lé Formation correspond clearly to the Pragian [22].

3) The fossils of Dacryoconarids and conodonts characterizing the Eifelian have not been found and need further investigations.

4) The Givetian / Frasnian boundary occurs in the lower part of the falsiovalis Zone. This zone has not been found in the Đồng Văn section, but its two adjacent zones, namely the upper transitans Zone and lower disparilis Zone have been met in the beds of grey, thin-bedded limestone of the upper part of the Si Phai Formation. In fact, the index species of the falsiovalis Zone occurs in a same sample together with Palmatolepis transitans of the transitans Zone; therefore it is possible to find the falsiovalis Zone bearing the above said boundary. Similarly, the Frasnian / Famennian boundary can not be accurately delineated because only the triangularis Zone, the lowermost Famennian zone, has been found, whereas the linguiformis Zone (uppermost zone of Frasnian) does not occur.

5) The Devonian / Carboniferous boundary has great prospect to be discovered in further detailed studies. The Pa. sigmoidalis – Pa. gonioclymeniae Association is presently considered as the uppermost Devonian conodont assemblage in Việt Nam (in Đồng Văn, Hạ Lang, Đŕ River lower basin and Quảng Bình areas). As for the duplicata Zone, the second conodont zone from the base of Carboniferous, it has been found at the base of the Tu Sán Member of the Tốc Tát Formation (see Table 1 and 2). But, just in the duplicata Zone many specimens of the Siphonodella sulcata occur. Therefore, in the Đồng Văn section the problem of Devonian / Carboniferous boundary has more and more materials for clarifying it; in further studies it is hopefull to find this boundary in the studied section. However, it is to note that the conodont zones are not fully present in every place in the world, for example, in the stratotype of the Frasnian / Famennian boundary in the southeast of Montagne Noire (France), the linguiformis Zone which is the uppermost zone of Frasnian, does not occur.

This paper is completed in the framework of the Project 72.08.01 supported by the Programme of Basic Scientific Research. Its author would like to express his sincere thanks to the Prof. Dr. Tống Dzuy Thanh, Hà Nội National University, for his trusty opinions in the improvement of the manuscript; the Prof. Dr. W. Ziegler, Senckenberg Research Institute for Sciences, Federal Republic of Germany for his help in the determination of Vietnamese conodont collection; the Engineer Lê Văn Giang, Division of Geological Mapping of North Việt Nam for his help during the collection of fossils in the northern slope of Tu Sán Mt; the geologists Đoŕn Nhật Trưởng and Nguyễn Đức Toàn for their help during field investigations.

1. Bourret R., 1922. Etudes géologiques sur le Nord-Est du Tonkin. Bull. SGI, XI/I : 326 p. Hà Nội.

2. Deprat J., 1915. Etudes géologiques sur la région septentrionale du Haut Tonkin. Mém. SGI, IV/4 : 176 p. Hà Nội.

3. Dovzhikov A.E. (Editor), 1965. Geology of North Việt Nam. Gen. Dept of Geology, Hà Nội, 584 p. (in Russian).

4. Dương Xuân Hảo, Nguyễn Thơm, Nguyễn Đức Khoa, 1975. New data on the biostratigraphy of Middle Paleozoic. Tuyển tập công trình nghiên cứu về địa tầng : 66-105. Nxb Khoa học kỹ thuật, Hà Nội (in Vietnamese).

5. Đặng Trần Huyên, 1976. Paleontological materials of the Tốc Tát Formation. Địa chất, 128 : 12-17. Hà Nội Nội (in Vietnamese).

6. Đặng Trần Huyên, 1979. New data on the stratigraphy of Devonian in the Đồng Văn area. Địa chất, 142 : 22-24. Hà Nội Nội (in Vietnamese).

7. Đặng Trần Huyên, 1985. The beds containing Nowakiidae (Tentaculites) in Devonian sediments of Việt Nam and their correlation in the confines of SE Asia. Tuyển tập báo cáo HN KHKT Địa chất Việt Nam lần 2 : 32-40. Hà Nội Nội (in Vietnamese).

8. Jacob C., Bourret R., 1920. Itinéraire géologique dans le Nord du Tonkin. Bull. SGI, IX/1 : 49 p. Hà Nội.

9. Lê Văn Đệ, Nguyễn Thơm, 1974. On the Upper Devonian sediments in the Đồng Văn area. Địa chất, 115 : 13-15. Hà Nội (in Vietnamese).

10. Paris F., Ta Hoa Phuong, Baudu V., 1993. Découverte de chitinozoaires et de scolécodontes dans l’Emsien du Vietnam (coupe de Dong Van - Nho Que, Province de Ha Giang). N. Jb. Geol. Palaeont. Mh.. H.10 : 596-606. Stuttgart.

11. Phạm Đình Long, 1973. On the stratigraphy of Devonian in the Hạ Lang Zone, Cao Bằng Province. Địa chất, 106 : 1-7. Hà Nội (in Vietnamese).

12. Phạm Kim Ngân, 1981. Possible use of conodont fossils in the study on Paleozoic and Triassic stratigraphy in Việt Nam. TC Các khoa học Trái đất, 3/2 : 49-52. Hà Nội Nội (in Vietnamese).

13. Phạm Kim Ngân, 1985. On the formation of red-veined limestone in the Đồng Văn area. Thông tin KHKT ĐC, 6 : 16-20. Viện BTLT ĐC, Hà Nội (in Vietnamese).

14. Tạ Hoà Phương, 1994. New discovery of Devonian and Lower Carboniferous pelagic fossils in Dong Van area (Ha Giang Province, Việt Nam). Proc. Intern. Symp. on Strat. Corr. SE Asia, Bangkok : 62-68.

15. Tạ Hoà Phương, 2000. Stratigraphy of Devonian and Carboniferous in the Đồng Văn section. TC Địa chất, A /Phụ trương 2000 : 2-9. Hà Nội (in Vietnamese).

16. Tạ Hoà Phương, Đoŕn Nhật Trưởng, 1995. Preliminary studies on the boundaries of Famennian stage in Việt Nam. Proc. IGCP Symp. on Geology of SE Asia, Hà Nội : 94-104.

17. Tạ Hoà Phương, Đoŕn Nhật Trưởng, 1998. General outlines of Famennian sediments in Việt Nam. TC Địa chất, 245 : 1-9. Hà Nội (in Vietnamese).

18. Tạ Hoà Phương, Lê Văn Giang, 1996. New data on the Lower Devonian stratigraphy in the Yên Minh – Mậu Duệ area, Hà Giang. TC Địa chất, 237 : 6-13. Hà Nội (in Vietnamese).

19. Tống Duy Thanh, 1979. Stratigraphy of the Lower Devonian in the Bắc Bộ region. TC Các khoa học về Trái đất, I/1 : 2-8. Hà Nội (in Vietnamese).

20. Tống Duy Thanh, Đặng Trần Huyên, Lương Hồng Hược, Nguyễn Đình Hồng, 1985. Biostratigraphy of Silurian-Devonian in Việt Nam. Tuyển tập BCHN KHKT Địa chất Việt Nam lần 2, 2 : 5-21. Hà Nội (in Vietnamese).

21. Tống Duy Thanh (Editor), 1986. The Devonian in Việt Nam. Nxb Khoa học kỹ thuật, Hà Nội, 141 tr.. Hà Nội (in Vietnamese)

22. Tống Duy Thanh, 1993. Main features of Devonian in Viet Nam with remarks on paleobiogeography. J. Geology, B/1-2 : 3-18. Hà Nội.

23. Tong-Dzuy Thanh, Ta Hoa Phuong, 1994. New data for correlation of Early Devonian bearing the Euryspirifer tonkinensis fauna in Vietnam and South China. Newsletter 11. Subcomm. on Devonian Stratigraphy.

24. Weddige K., 1977. Die Conodonten der Eifel-Stufe im Typusgebier und in benachbarten Faziesgebieten. Senckenbergiana lethaea, 58 : 271-419. Frankfurt am Main.

25. Yolkin E., Weddige K., Izokh N., Erina M., 1994. New conodont zonation (Lower Devonian). CFS 168, Frankfurt am Main, 115 pp.

26. Ziegler W., Sandberg C., 1990. The late Devonian Standard conodont zonation. CFS 121, Frankfurt am Main, 115 pp.