NGUYÔN H÷U HïNG

The Upper Devonian sediments in North ViÖt Nam (Fig.1) are widely distributed and well exposed with diversified lithofacies (littoral, continental shelf, continental slope and deep-water) and abundant benthonic fossils (brachiopods, corals, stromatoporoids, bivalves, crinoids, bryozoans) and pelagic fossils (conodonts, tentaculites, foraminiferas).

In order to introduce in general the presence of Upper Devonian sediments in North ViÖt Nam, a summary of research history, stratigraphic units, extinction of macrofossils and the F/F boundary are presented in this paper.

The Soviet geologists A. E. Dovjikov et al. [4] have denied the presence of Frasnian and Famennian sediments in the areas where the French geologists have showed. Because of that brachiopod fossils identified by H. Mansuy [8, 9], E. Patte [13, 14], J. H. Hoffet [6] as Frasnian species Spirifer zigzag Roemer, Rhynchonella cuboides Kayser, Uncinulus cuboides Sow. have been reviewed by Z­¬ng Xu©n H¶o, O. I. Nikiforova [2]; Z­¬ng Xu©n H¶o, M. A. Rzonsniskaia [3] as Silurian species Retziella weberi Nik., Eospirifer lynxoides (Nal.) and Early Devonian Hysterolites wangi Hou, Pletorhyncha chieni Zuong et Rzons. At the same time, the Soviet geologists [4] have established two new formations for Frasnian sediments in ViÖt Nam: the B¶n Coáng Formation in the H¹ Lang Structuro-facial Zone (Northeast ViÖt Nam), composed of light-grey, thich-bedded limestone containing Clavidictyon (?) cf. milcreckense Gall. et Ehlers, Gypidula sp., Devonoproductus sp., Atrypa ex gr. desquamata Sow., Spinatrypa sp., Meristella ?, Athyris sp., Striatopora cf. zeaporoides Dub., Alveolites sp., Parathurammina sp., Bisphaera sp., Vicinusphaera sp.; the §éng Thê Formation in the Tr­êng S¬n Structuro-facial Zone (north of Central ViÖt Nam), composed of sandstone containing Mytilarca sp., Nucula sp., Schizophoria sp., Atrypa reticularis L., Spinulicosta spinulicosta Hall, Stropheodonta sp., Fenestella sp., Semicoscinium sp., Sycidium sp.. Beside these two formations of Frasnian sediments, some Frasnian species of stromatoporoids Amphipora patokensis Riab., A. patokensis var. minor Riab. and some representatives of genus Quasiendothyra showing the Late Famennian-Early Tournaisian age have been found in the coastal zone of Northeast ViÖt Nam (the Duyªn H¶i Structuro-facial Zone). Based on the fact that, the stromatoporoid fossils in Southeast Asia are not studied in detail and the Famennian and Tournaisian sediments are completely absent in North ViÖt Nam, the limestone containing the above stomatoporoids were assigned to Eifelian-Givetian, in addition the limestone containing Quasiendothyra Zone were assigned to Visean by Soviet geologists [4]. After 1965's, many data of biostratigraphy have been largely collected during the regional geological and mineral survey. Many biostratigraphical sections have been studied in detail. Many groups of fauna have been identified and studied. Many special investigations were done, among them the investigation on the Frasnian, Famennian and Tournaisian sediments are most intensive with 6 stratigraphical units established for Frasnian-Famennian beds.

Established by NguyÔn H÷u Hïng [10]. It is composed of two parts. The lower part consists of 100-250 m thick black limestone containing many fossils of tabulates, rugoses, stromatoporoids, foraminiferas belonging to Amphipora laxeperforata, Stachyodes costulata, Tikhinella Assemblages (Frasnian). The upper part consists of 100-120 m thick light, thin-bedded, oolitic limestone and dolomite. Distributed in L¹ng S¬n area (the S«ng HiÕn Structuro-facial Zone [4]), conformably overlying the T©n LËp Formation (D₂g-D₃fr tl), but having unclear contact with the overlying B¾c S¬n Formation (C-P bs).

Established by Ph¹m §×nh Long [15]. It is composed of 160-290 m thick variegated, red-violet, green-grey, striped limestone interbedded with manganese ore-bearing beds yielding conodonts ranging from Palmatolepis haasi Zone to Palmatolepis sigmoidalis Zone, foraminiferas of Quasiendothyra communis Zone and brachiopds Lingula subparallela Sand., Schizophora bistriata Tchern., Barroisella sp., Echinoconchus bistriatus (Hall), Praewaagenoconcha sp., Spinulicosta spinulicosta (Hall), Donella minima Rotai, Athyris sulcifera Nal., Cyrtospirifer sp.; bivalves Posidonia venusta (Munst.), P. nalivkini Sad.. Conformably overlying the B»ng Ca Formation (D₃fr bc) and conformably? underlying the Lòng NËm Formation (C₁ ln). Distributed in H¹ Lang, Tèc T¸t, Trïng Kh¸nh areas, Cao B»ng Province; §ång V¨n area, Hµ Giang Province of Northeast ViÖt Nam (the S«ng HiÕn Structuro-facial Zone); B¶n C¶i are, S¬n La Province of Northwest ViÖt Nam (the S¬n La Structuro-facial Zone [4]).

Established by NguyÔn H÷u Hïng [11]. It is composed of three parts. The lower part comprises 50-170 m thick black limestone containing many fossils of tabulates, rugoses, stromatoporoids belonging to Stachyodes radiata Assemblage (Uppermost Givetian). The middle part is composed of 120 m thick marl interbedded with light-grey limestone yielding brachiopods of Calvinaria cracoviensis Assemblage, stromatoporoids of Stachyodes costulata Assemblage (Frasnian). The upper part comprises 50-80 m thick, striped limestone yielding conodonts of Palmatolepis glabra glabra Zone. Conformably overlying the Huæi L«i Formation (D_1-2 hl), unconformably underlying the La Khª Formation (C₁ lk). Distributed in M­êng SÐn area, NghÖ An Province of north of Central ViÖt Nam (the Tr­êng S¬n Structuro-facial Zone [4]).

Established by NguyÔn H÷u Hïng [9, 23]. It is composed of three beds. The beds 1 comprise 20-30 m thick, light-grey to grey limestone containing many fossils of tabulates, rugoses, stromatoporoids, tentaculites, algae belonging to Stachyodes costulata Assemblage (Frasnian); conodonts of Palmatolepis gigas Zone. The beds 2 comprise 7,2 m thick dark-grey medium-bedded limestone interbedded with argillaceous, cherty shales yielding many conodonts ranging from Upper rhenana to marginifera Zones. The beds 3 consist of 40 m thick nodular and striped limestone yielding Upper Famennian conodonts Palmatolepis marginifera marginifera Helms, Pa. glabra glabra Ulrich et Bassler, Pa. glabra distorta Branson et Mehl.. Conformably overlying the §éng Thê Formation (D₂g-D₃fr ®t). Contact with overlying strata is not clear. Distributed in Quy §¹t area, Qu¶ng B×nh Province of North of Central ViÖt Nam (the Tr­êng S¬n Structuro-Facial Zone [4]).

Established by TrÇn ThÞ ChÝ ThuÇn et H. Fontaine [22]. It is composed of three parts. The lower part comprises 100-120 m thick black, thick-bedded limestone containing many stromatoporoids, tabulates, rugoses, brachiopods, foraminiferas belonging to Amphipora laxeperfora, Stachyodes costulata, Tikhinella, Cyrospirifer and Uchtospirifer Assemblages (Frasnian). The middle part consists of 50 m thick light-grey dolomite, not yielding fossils. The upper part comprises 120m thick grey, thin-bedded limestone with the brachiopod fauna Yunnanellina hanburyi David.), Y. aff. triplicata Grabau, Y. sp.. Unconformably overlying the §¹i Giang Formation (S₂ ®g). Contact with overlying beds is not clear. Distributed in T©n L©m, Cï Bai areas, Qu¶ng TrÞ Province; Mü §øc area, Qu¶ng B×nh Province of North of Central ViÖt Nam (the Tr­êng S¬n Structuro-Facial Zone [4]).

In order to reflect the mass extinction of macrofossils during the Givetian and Frasnian times in of North ViÖt Nam, the following statistics based on the documents of D­¬ng Xu©n H¶o et al. [3], Dovjikov et al. [4], Tèng Dzuy Thanh et al. [21], NguyÔn H÷u Hïng [9, 10, 11], Vò Khóc et al. [23] are presented.

About 32 species, 11 genera have been collected from Givetian sediments of the Trµng Kªnh, Nµ Qu¶n, Môc Bµi Formations: Atrypa desquamata Sow., A. vulgariformis Alek., Spinatrypa aspera (Schloth.), S. quidatensis Zuong et Rzons., S. aff. quidatensis Zuong et Rzons., S. aspera Dalm., S. asperaeformis Alek., S. vassinensis Rzons., Desquamatia magna (Grabau), D. ventricosa (Kell.), D. kansuensis (Grabau), D. zonataeformis Alek., D. parazonata (Kell.), D. ex gr. desquamata Sow., Undispirifer undiferus (Roemer), Gypidula calceola Frech., G. vietnamica Zuong et Rzons., G. biplicata (Schn.), Chonetipustula orientalis Zuong et Rzons., Emanuella cf. cicer (Ech.), E. takwanensis (Kayser), E. ronensis (Mans.), E. cf. ronensis (Mans.), E. volhynica Kelus, E. tumida Ljasch., E. haugi Patte, Ambocoelia umbonata Conrad., Schizophoria striatula Schloth. S. bistiata Tschern., Stringocephalus burtini Defr., S. cf. burtini Defr., Ivdelina (Procerulina ) nalivkini And..

22 species, 11 genera from Frasnian sediments of the B¶n Coáng, T©n L©m, §éng Thê, Kú S¬n Formations: Emanuella ronensis (Mans.), Schizophoria ivanovi Tchern., S. cf. striatula Schloth, S. sp., Mucrospirifer novosibiricus vietnamicus Zuong et Rzons., Athyris concentricus V.Buch., Gypidula cf. planisinosa Grabau, G. sp., Devonoproductus sp., Atrypa cf. desquamata Sow., A. reticularis L., A. ex gr. desquamata Sow., Uchtospirifer nalivkini Ljasch., U. cf. concentricus Ljasch., Meristella sp., Desquamatia sp., Hypothyridina cuboides Sow., Camarotoechia polita Ljasch., Calvinaria crakoviensis Bal., Leiorhynchus laevis Gurich., Ladogia volgensis Ljasch..

7 species, 5 genera from Lower Famennian sediments of the Phong Son and T©n L©m Formations: Yunnanella hsikuangshaensis Tien, Y. sp. Yunnanellina hanburyi (David), Y. aff. triplicata Grabau, Tenticospirifer tenticulum (Ver.), T. cf. vilis kwangsiensis Tien, Cyrtospirifer postarchiaci Nalivkin, Athyris concentrica (Buch).

8 species, 8 genera from Upper Famennian sediments of the Tèc T¸t Formation: Lingula subparallela Sand., Barroisella sp., Schizophoria bistriata Tchern., Echinoconchus bistriatus (Hall), Praewaagenoconcha sp., Donella minima Rotai, Cyrtospirifer sp., Athyris sulcifera Nal..

34 species, 21 genera have been collected from Givetian sediments of the Nµ Qu¶n, Trµng Kªnh, B¶n P¸p, Kú S¬n and Môc Bµi Formations: Temnophyllum mansuyphylloides Khoa, T. raocaiensis Khoa, T. varicum Khoa, T. waltheri Yoh, T. aff. waltheri Yoh, Neostrigonophyllum heterophylloides (Frech), Thamnophyllum aff. virgatum Soshk., Tabulophyllum sp., Sinospongophyllum conicum (Wang), S. cf. planotabulatum Yoh, Dendrostella trigemme (Quest), Hexagonaria bongbutensis Khoa, H. arctica (Meek), H. darwini (Frech), Zoelasma mirabilis (Khoa); Cuctienophyllum cuctienense (Khoa), Iowaphyllum chuaense Khoa, I. medium Khoa et Tong-dzuy, Grypophyllum wedekindi Middleton, G. isactis (Frech), G. cf. carinatum (Soshk.), Nalivkinella tomiensis (Bul.), Sociophyllum halangense Khoa, Cystiphylloides cf. kwangsiense (Yoh), Marisastrum aff. thomasi (Stairb.), Pseudomicroplasma cf. fongi (Yoh), P. cf. uralica (Soshk.), Aulacophyllum cf. cylindricum Buch., Phillipsastrea sp., Macgeea cf. multizona Reed, M. aff. puchra Spass., Argutastrea bongbutensis Khoa, Solipetra cf. vietmamica Font., Linstroemia sp..

4 species, 4 genera from the Lower and Middle Frasnian sediments of the Xãm Nha and T©n L©m Formations: Temnophyllum isetense (Soshk.), Alaiophyllum incomptum (Hill et Jell), Hexagonaria aff. lavali (Mans.), Disphyllum sp..

7 species, 5 genera from the Upermost Famennian sediments of the Phong Nha and Phong S¬n Formations: Halongophyllum halongense Khoa, Cystophrentis kolaohoensis Yu, C. grandis Kwang, C. roniewiczae Khoa, Cobainophyllum curvatum Khoa, Amygdalophyllum amygdalum Xu, Zaphrentoides sp..

52 species, 19 genera have been found in Givetian sediments of the Trµng Kªnh, Nµ Qu¶n, B¶n P¸p, Kú S¬n, Môc Bµi Formations: Thamnopora polygonalis (Mans.), T. polyforata (Schloth.), T. boloniensis (Gros.), T. nicholsoni (Frech), T. densata Tchud., T. angusta Lec., T. polytrematiformis Tong-dzuy, T. aff. compacta Tchud., Cladopora gracilis (Salee), C. elegans Dubat., C. aff. vermicularis Dubat., C. cylindrocellularis Dubat., Trachypora dubatolovi Tong-dzuy, T. circulipora Kayser, Striatopora aff. magnifica Tchud., Natanophyllum rarum Tchud., N. bandonicum Ta-Fuong et The-Dzan, Caliapora batterbyi (M.E.H.), C. reducta Yanet, C. robusta Thom, Scoliopora denticulata (M.E.H.), S. dubrovensis Dubat., S. formosa Tchud., S. muricata Tchud., S. conferta Erm., Alveolites admirabilis Tong-dzuy, A. lemiscus Smith., A. taenioformis Schlut., A. aff. maillieuxi Lec., A. aff. smithi Lec., A. tischnoffi Dubat., A. mauritanus Le Maitre, Alveolitella polenovi (Peetz), A. aff. karmakensis (Tchern.), A. polonica Now., A. cella Tchud., Crassialveolites crassus Lec., C. crassiformis (Sok.), C. incrassatus Dubat., Aulocystis polonicus Now., A.? kheloensis Tong-dzuy, Coenites subramosus Lec., C. multus Tchud., C. quydatensis Tong-dzuy, C. ramosus (Miron.), C. simakovi Dubat., C. tenella Gur, C. multicellus Ta-Fuong et The-Dzan, C. dubatolovi Yanet, Grabauites sp., Remesia kettneri Tong-dzuy, Chaetetes yunnanensis (Mans.), Chaetetes (Lithophyllum) inflatus Lec., Pachytheca aff. abdita Yanet., Heliolites altaiformis Dubat., H. ex gr. porosus (Goldf.).

7 species, 5 genera from the Lower and Middle Frasnian sediments of the B¶n Coáng, Má Nhµi, Xãm Nha Formations: Thamnopora strelinaensis Dubat., T. cf. polyforata Schloth., Scoliopora aff. denticulata (M.E.H.), Cladopora aff. crassa Yanet., Striatopora cf. zeaporoides Dubat., Alveolites suborbicularis Lamarck, A. sp., Gracilopra polonica Stasinska et Nowinski.

6 species, 3 genera from Uppermost Famennian sediments of the Phong Nha, Phong S¬n and Nói Voi Formations: Syringopora distans Fisch., S. geniculata haiphongensis Mans., S. reticulata Goldf., S. serenei Font., Roemeripora aff. terraenovae Smirnova, Chaetetes sp..

56 species, 22 genera have been reported from the Givetian sediments of the Trµng Kªnh, Nµ Qu¶n, B¶n P¸p, Kú S¬n and Môc Bµi Formations: Amphipora ramosa (Phill.), A. angusta Lec., A. angustaeformis Hung, A. monstrosa Hung, A. rudis Lec., A. aff. blokini Yav., A. ramosa minor Riab., A. difficilis (Yav.), Vacuustroma thanhlangense Hung et Mistiaen, V. concentrica Hung et Mistiaen, V. minuta Hung et Mistiaen, Euryamphipora sp., Dendrostroma oculatum (Nich.), D. minutum Hung, Idiostroma crassum Lec., I. fililaminatum Lec., I. roemeri Nich., I. quidatensis Hung et Mistiaen, Stachyodes radiata Lec., A. singularis Yav., S. caespitosa Lec., S. laichauensis Khrom., S. verticillata (M'Coy), S. gracilis Lec., Actinostroma clathratum Nich., A. bifarium Nich., A. undulata Mans., A. densatum Lec., A. salairicum Yav., A. expansum (Hall et Whitfield), Gerronostroma mareitchevi Flerova, Clathrocoilona actinostromoides (Lec.), C. obliterata (Lec.), C. irregularis (Lec.), C. solida Yav., Flexiostroma flexiosum V.Khalf., F. crispum V. Khalfina, F. tabulatum Hung, Syringostroma ? grossum Hung, Ferestromatopora krupennikovi var. talovensis Yav., Syringostromella zintchenkovi tenuis (V. Khalfina), Stromatopora cooperi Lec., S. concentrica Goldf., S. hupschii (Barg.), S. orientalis Khrom., Salairella khalfinae Hung, Hermatoporella maillieuxi (Lec.), H. porosum (Lec.), H. cf. cimacense Lec., H. crassa (Lec.), H. tenuilamellatum (Lec.), Stictostroma ivdeliense (Yav.), Trupetostroma belgica Khromych et Hung, Parallelopora dartingtonensis (Cater), P. sp., Hermatostroma parksii Lec., Tienodictyon cf. rarum Bogoyav..

31 species, 12 genera have been collected from Lower and Middle Frasnian sediments of the Trµng Kªnh, Má Nhµi, Kú S¬n, Xãm Nha, T©n L©m Formations: Amphipora laxeperforata Lec., A. pervesiculata Lec., A. ramosa minor Riab., A. patokensis Riab., A. minima Ermak., A. patokensis minor Riab., A. tschussovense (Yav.), A. pinguis Yav., A. moravica Zuk., A. franca Ermak., A. recta Yav., Stachyodes costulata Lec., S. paralleloporoides Lec., S. lagowiensis Gogol., S. crebrum Stearn, Idiostroma crassum Lec., Dendrostroma cumulus (Yav.), Clavidictyon cubaiense Hung, C.? cf. milcreckense Galloway et Ehlers, Hermatoporella chucaense Hung, Salairella ? hybridina Yang et Dong, Stromatopora ? spissa (Lec.), Clathrocoilona saginata (Lec.), C. pertabulata (Lec.), Actinostroma vovojensis Riab., A. petrovi Riab., A. dehorneae Lec., A. clathratum Nich., A. crassepilatum Lec., Atelodictyon trauscholdi (Riab.), A. tuberculatum (Riab.), Tienodictyon tschussovense (Yav.).

13 species, 6 genera from the Uppermost Famennian sediments of the Phong Nha, Phong S¬n, Nói Voi Formations: Labechia densata Gorsky, L. kurganensis Yav., Stylostroma ramosum Gorsky, S. sinense (Dong), S. cf. convexa (Yav.), S. huanjangensis (Dong), Platiferostroma hybridium (Dong), P. phongnhaense Hung et Mistiaen, Vietnamostroma vietnamense Hung et Mistiaen, Rosenella miniarensis Riab., R. aff. miniarensis Riab., Clavidictyon regulare Dong, C. luochengense Dong.

2 species, 2 genera have been found from the Givetian sediments of the Môc Bµi Formation: Nuculoidea aff. lodanensis (Beush.), N. sp. A.

2 species, 2 generahave been found from the Frasnian sediments of the §éng Thê Formation: Mytilarca sp., Nucula sp..

2 species, 1 genus from Upper Famennian sediments of the Tèc T¸t Formation: Posidonia venusta (Munst.), P. nalivkini Sad..

5 species, 4 genera have been reported from the Givetian sediments of the Môc Bµi Formation: Vietnamicrinus stukalinae Huyen, Floricrinus floreus (Yelt.), F. cf. floreus (Yelt.), Schyschcatocrinus cf. creber (J. Dubat.), Hexacrinites ? aff. argutus J. Dubat.

2 species, 2 genera have been found from the Givetian sediments of the Môc Bµi Formation: Rhombopora mucbajensis Modz., Fistulipora banjangensis Modz..

This statistics shows that, 190 species and 91 genera became extinct in Upper Givetian sediments, 49 species and 26 genera became extinct below the F/F boundary. Among 34 species belonging to 22 genera reported from Famennian sediments, besides the two genera Cyrtospirifer and Tenticospirifer of brachiopods passing the F/F boundary, almost all the rest belong to new branches: the new family Yunnanellidae of brachiopods appearing in Early Famennian, the new rugose family Cystophrentidae appearing in Late Famennian and reappearence of stromatoporoid genera Labechia, Stylostroma, Vietanmostroma which appeared early in the Upper Ordovician. The extinction of these fossils is a very good sign for recognizing the F/F boundary beds in sections of Frasnian-Famennian sediments.

Generally, the conodont fossils in ViÖt Nam have not been studied in detail. At present 16 conodont zones have been reported from Devonian sediments of continental slope and deep-water facies. According to T¹ Hßa Ph­¬ng, NguyÔn H÷u Hïng [19] and T¹ Hßa Ph­¬ng [20], they are Polygnathus excavatus, Po. nothoperbonus, Po. inversus, Po. serotinus, Po. varcus, Po. patulus, Palmatolepis disparilis, Pa. cristatus, Pa. transitans, Pa. hassi, Pa. rhenana, Pa. linguiformis, Pa. triangularis, Pa. crepida, Pa. rhomboidea, Pa. marginifera, Pa. sigmoidalis. In the F/F boundary beds, the conodont fossils are best studied with 13 index species of linguiformis Zone: Pa. linguiformis Muller, Pa. rhenana Bischoff, Pa. rheana brevis Ziegler, Pa. rhenana Muller, Pa. gigas Miller & Youngquist, Pa. subrecta Miller & Youngquist, Pa. hassi Muller & Muller, Pa. juntinaensis Han, Pa. ederi Ziegler & Sandberg, Pa. eureka Ziegler & Sandberg, Pa. foliacea Youngquist, Ancyrodella nodosa Ulrich & Bassler, A. ioides Ziegler and 7 index species of triangularis Zone: Pa. triangularis Sannemann, Pa. superlobata Branson & Mehl, Pa. clarki Ziegler, Pa. delicatula delicatula Branson&Mehl, Pa. delicatula protorhomboidea Sandberg & Ziegler, Pa. regularis Cooper, Pa. minuta minuta Branson & Mehl have been reported from the Xãm Nha and §ång V¨n sections.

A high abudance of index species of two conodont zones linguiformis and triangularis in some sections of slope and deep water facies are very good markers for drawing the F/F boundary in North ViÖt Nam.

The Frasnian / Famennian boundary in North ViÖt Nam has been recognized in many sections of the Trµng Kªnh, Má Nhµi, Tèc T¸t, Kú S¬n, Xãm Nha and T©n L©m Formations.

For sediments of littoral and continental shelf facies, the recognition of this boundary can be based on the mass extinction of macrofossils, such as brachiopods, tabulate corals, rugose corals and stromatoporoids. In the Má Nhµi, B¶n Má, Phè Hoµng sections, where there are sediments of the Má Nhµi Formation, the F/F boundary of these sections was first reported by NguyÔn H÷u Hïng [10]. The lower part of these sections is composed of dark-grey limestone containing many stromatoporoids, tabulates belonging to the Stachyodes costulata, Amphipora laxeperforata, A. moravica Assemblages. The sudden disappearance of these assemblages from the middle part has been recognized below the thin bedded limestone without fossils. The F/F boundary is supposedly drawn in these limestone beds.

The recognition of the F/F boundary in the continental slope and deep-water facies is based on the presence of liguiformis and triangularis conodont zones. In North ViÖt Nam, this boundary is first reported by Ph¹m Kim Ng©n [16]. According to this author, the F/F boundary is drawn between grey thin-bedded argillaceous limestone yielding the gigas, triangularis conodont zones and light striped limestone yielding the crepida, marginifera conodont zones in the Xãm Nha section of the Xãm Nha Formation. T¹ Hßa Ph­¬ng and NguyÔn H÷u Hïng [19] proposed to draw the F/F boundary between conodont zones linguiformis and triangularis, that means this boundary is drawn in the beds 2 corresponding to thin-bedded argillaceous limestone beds in Ph¹m Kim Ng©n’s description. The beds 2 is composed of dark- grey, medium-bedded limestone interbedded with thin-bedded argillaceous limestone; 5.25 m in thickness, containing many important conodont taxa that have been found, from the Lower rhenana Zone to triangularis Zone of the F/F boundary beds.

For the study on mass extinction of macrofossils below the F/F boundary in the littoral facies and drawing the F/F boundary in the continental slope and deep-water facies, three following sections are proposed:

The road No.10 Section in KiÕn An area (the Duyªn H¶i Structuro-facial Zone [4]) is a typical section of littoral facies for the study on F/F boundary. It is situated about 10 km southwest from H¶i Phßng City, on the right side of the Highway No.10. The section is composed of 5 members:

Member 1 consists of 75 m thick black, medium-bedded limestone yielding a few remains of algae, crinoids, ostracods and spongies.

Member 2 consists of 45 m thick dolomitic limestone and dolomite yielding a few remains of Amphipora sp., Stromatopora sp..

Member 3 consists of 10 m thick grey limestone yielding in abundance stromatoporoid remains of the species Clathrocoilona saginata (Lec.) and a few algae belonging to solonoporids and Devonoscale-Jansaella.

Member 4 consists of 40 m thick black shale interbedded with marl.

Member 5 consists of 80 thick light-grey, thick-bedded limestone yielding the rugose Cystophrentis sp., foraminiferas Quasiendothyra kobeitusana (Raus.), Q. communis regularis Lip., Q. communis communis (Raus.), Septatournayella rauserae Lip., S. lebedeva Pojark., Septaglomospiranella opulenta Durk., S. kazakhstanica Reitl., Septabrunsiina sp..

The recognition of F/F boundary is based on the extinction of macrofossils Clathrocoilona saginata (Lec.), an index species of Middle Frasnian in member 3 and many rugoses and foraminiferas of the genera Cystophrentis and Quasiendothyra, the index fossils of late Famennian in member 6. The F/F boundary is supposedly drawn in member 4.

The Xãm Nha Section located in Quy §¹t area, Qu¶ng B×nh Province (the Tr­êng S¬n Structuro-facial Zone [4]) belonging to the Xãm Nha Formation (D₃ xn) is typical for the continental slope and continental shelf facies; it is situated in about 2,7 km west of Quy §¹t Townlet. It is composed of three members:

Member 1 comprises 20-30m thick grey-light, thick-bedded limestone yielding many stromatoporoids Stachyodes costulata Lec., S. paralleloporoides Lec., S. lagowiensis Gogol., Anostylostroma? crassa Hung (in col.), Amphipora sp., rugoses Temnophyllum cf. lantenoisi (Mans.), T. isetense (Sosh.) and Heterocoral fossils, tabulates Alveolites sp., Thamnopora sp., Scoliopora sp., conodonts belonging to Palmatolepis rhenana Zone.

Member 2 comprises 7,2 m thick grey, medium-bedded limestone interbedded with cherty shales yielding conodonts ranging from the rhenana to marginifera Zones.

Member 3 consists of 40 m thick light-grey, medium-bedded limestone and striped limestone yielding many conodonts of marginifera Zone.

The F/F boundary has been proposed to be drawn in member 2, between the linguiformis and triangularis Zones.

The §ång V¨n Section is situated about 1.8 km northeast of the §ång V¨n Townlet, §ång V¨n District, Hµ Giang Province of East B¾c Bé (the S«ng HiÕn Structuro-facial Zone [4]) belonging to the Tèc T¸t Formation (D₃ tt). Differing from of Xãm Nha Formation, the Tèc T¸t Formation is characterized by greyish-red limestone of deep-water sedimentary environment. It lies conformably upon the Nµ Qu¶n Formation (D_1-2 nq) and under the Lòng NËm Formation (C₁ ln). Four members of the Tèc T¸t Formation in this section can be distinguished:

Member 1. 21 m in thickness, including black, thick- to medium-bedded limestone interbedded with cherty shale yielding stromatoporoids Stachyodes costulata Lec., algae Umbella sp., tentaculites Homoctenus sp., foraminiferas Eogeinitzina sp., Multiseptida sp., Nodosaria sp., Eonodosaria sp., Tikhinella sp. and conodonts of the varcus Zone.

Member 2. 24 m in thickness, including grey, thin-bedded limestone yielding conodonts of the transitans Zone.

Member 3. 40 m in thickness, green, grey-red, grey-yellow striped and nodular limestone with conodonts ranging from rhenana to triangularis Zone.

Member 4. 140 m in thickness, with nodular and light-grey striped limestone containing conodonts ranging from marginifera to sigmoidalis Zone.

The F/F boundary is proposed to be drawn in member 3, between linguiformis and triangularis Zones.

The Upper Devonian sediments in North ViÖt Nam are widely distributed and developed in littoral, continental shelf, continental slope and deep-water facies. The mass extinction of macrofossils below the F/F boundary is well expressed in many sections of littoral and continental shelf facies. The abundance of conodont zones in the F/F boundary beds have been recognized in many sections of continental slope and deep-water facies. They are good sign for the study on the F/F boundary in North ViÖt Nam.

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