T¹ hßa ph­¬ng

The §ång V¨n section was first described by J.Deprat [2] as "B¾c 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 Tèc T¸t formations (corresponding to the M· P× LÌn "SÐrie" of Deprat ) will be presented.

The conodont fossils of the §ång V¨n section were first discovered and determined by Ph¹m Kim Ng©n [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 §ång V¨n area T¹ Hßa Ph­¬ng [14] has been distinguishing 7 tentaculite zones in the "Nµ Qu¶n and B»ng 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 §ång V¨n 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 §ång V¨n 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 §ång V¨n 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 §ång V¨n 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 §ång V¨n 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 §ång V¨n section

To date, the index species of the zone has not been found yet in the §ång V¨n section. However, the conodont assemblage corresponding to the rhenana Zone occurs in the banded limestone of the lower part of the Tèc T¸t 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 Tèc T¸t 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 ® Pa. minuta and Pa. minuta minuta Branson & Mehl.

The rhomboidea Zone occurs in the beds of banded limestone of the lower part of the Tèc T¸t 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 ViÖt 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 Tèc T¸t 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 §ång V¨n 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 Tèc T¸t 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 S¸n Member) of the Tèc T¸t 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 S¸n Member, Tèc T¸t 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 S¸n Member, Tèc T¸t 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 §ång V¨n section is the fullest and most typical Devonian section in this extreme north area of ViÖt Nam. It contains all Devonian stratigraphic units of the area, namely: Si Ka, B¾c Bun, Mia LÐ, Si Phai and Tèc T¸t 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 Tèc T¸t, 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 LÐ 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 §ång V¨n 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 ViÖt Nam (in §ång V¨n, H¹ Lang, §µ River lower basin and Qu¶ng B×nh 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 S¸n Member of the Tèc T¸t Formation (see Table 1 and 2). But, just in the duplicata Zone many specimens of the Siphonodella sulcata occur. Therefore, in the §ång V¨n 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. Tèng Dzuy Thanh, Hà Néi 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 Lª V¨n Giang, Division of Geological Mapping of North ViÖt Nam for his help during the collection of fossils in the northern slope of Tu S¸n Mt; the geologists §oàn NhËt Tr­ëng and NguyÔn §øc Toàn for their help during field investigations.

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