Silurian and Devonian in Vietnam—Stratigraphy and facies

August 1, 2017 | Author: Nguyen Cuc | Category: Geology, Geophysics, Stratigraphy, Geodynamics, Vietnam, Devonian, Geomatic Engineering, South China, Continental shelf, Deep water, Facies, Marine Environment, Long Day Care, Shallow Water, Devonian, Geomatic Engineering, South China, Continental shelf, Deep water, Facies, Marine Environment, Long Day Care, Shallow Water
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Silurian and Devonian in Vietnam—Stratigraphy and facies Tong-Dzuy Thanh a,∗ , Ta Hoa Phuong a , Janvier Philippe b , Nguyen Huu Hung c , Nguyen Thi Thu Cuc a , Nguyen Thuy Duong a a b c

Laboratory of Historical Geology, Vietnam National University, 334 Nguyen Trai Str., Thanh Xuan District, Hanoi, Viet Nam UMR7207 du CNRS, Muséum National d’Histoire Naturelle, 8 rue Buffon, 75005 Paris, France Institute of Geosciences and Mineral Resources, Thanh Xuan District, Hanoi, Viet Nam

a r t i c l e

i n f o

Article history: Received 6 August 2010 Accepted 10 October 2011 Available online xxx Keywords: Silurian Devonian Stratigraphy Facies Viet Nam

a b s t r a c t Silurian and Devonian deposits in Viet Nam are present in several zones and regions, including Quang Ninh, East Bac Bo, and West Bac Bo Zones of the Bac Bo Region, the Dien Bien-Nghe An and Binh Tri Thien Zones of the Viet-Lao Region, and the South Trung Bo, and Western Nam Bo Zones of the South Viet Nam Region (Fig. 1). The main lithological features and faunal composition of the Silurian and Devonian Units in all these zones are briefly described. The Silurian consists of deep-water deposits of the upper parts of the Co To and Tan Mai Formations in the Quang Ninh Zone, the upper parts of the Phu Ngu Formation in the East Bac Bo Zone and the upper parts of the Long Dai and Song Ca Formations in the Viet-Lao Region. Shallow water facies Silurian units containing benthic faunas are more widely distributed, including the upper part of the Sinh Vinh and Bo Hieng Formations in the West Bac Bo Zone, the Kien An Formation in the Quang Ninh Zone, and, in the Viet-Lao Region, the Dai Giang Formation and the upper part of the Tay Trang Formation. No Lower and Middle Devonian deposits indicate deep water facies, but they are characterized by different shallow water facies. Continental to near shore, deltaic facies characterize the Lower Devonian Song Cau Group in the East Bac Bo Zone, the Van Canh Formation in the Quang Ninh Zone, and the A Choc Formation in the Binh Tri Thien Zone. Similar facies also occur in the Givetian Do Son Formation of the Quang Ninh Zone, and the Tan Lap Formation in the East Bac Bo Zone, and consist of coarse terrigenous deposits—cross-bedded conglomerates, sandstone, etc. Most Devonian units are characterized by shallow marine shelf facies. Carbonate and terrigenous-carbonate facies dominate, and terrigenous facies occur in the Lower and Middle Devonian sections in some areas only. The deep-water-like facies is characteriztic for some Upper Devonian formations in the Bac Bo (Bang Ca and Toc Tat Formations) and Viet-Lao Regions (Thien Nhan and Xom Nha Formations). These formations contain cherty shale or siliceous limestone, and fossils consist of conodonts, but there are also brachiopods and other benthos. They were possibly deposited in a deep water environment on the slope of the continental shelf. Most Devonian units distributed in the North and the Central Viet Nam consist of self shallow water sediments, and apparently they were deposited in a passive marginal marine environment. The coarse clastic continental or subcontinental deposits are distributed only in some areas of the East Bac Bo and of the Quang Ninh zones of the Bac Bo Region, and in the south of the Binh Tri Thien Zone. This situation suggests the influence of the Caledonian movement at the end of the Silurian period that called the Guangxi movement in South China. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction The Silurian and Devonian deposits of Viet Nam were first discovered and described by French geologists of the Geological Survey of Indochina (Saurin, 1956). Later, they have been systematically studied by Dovjikov et al. (1965), Tong-Dzuy Thanh (1993),

∗ Corresponding author. Tel.: +84 1696456546; fax: +84 438583061. E-mail address: [email protected] (Tong-Dzuy T.).

Tong-Dzuy Thanh et al. (1988), Vu Khuc and Bui Phu My (1990), and Tong-Dzuy Thanh and Vu Khuc (2006). Major contributions in the description of Silurian and Devonian units have been made by geologists from the Geological Mapping Division, during the last decades of the 20th century (1:200,000 geological maps). Stratigraphic units have been well established and their correlations are based on abundant faunas. Their study shows that Silurian and Devonian deposits are widespread in the North and the Central parts of the country, whereas in the South there are only terrigenous deposits of suspected Devonian age. New data on the Silurian

0264-3707/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jog.2011.10.001

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Fig. 1. Zonal distribution of the Silurian and Devonian deposits of Viet Nam. 1. Bac Bo Region: 1a. Quang Ninh Zone; 1b. East Bac Bo Zone; 1c. West Bac Bo Zone. 2. Viet-Lao Region: 2a. Dien Bien-Nghe An Zone; 2b. Binh Tri Thien Zone. 3. South Viet Nam Region: 3a. Trung Bo Zone; 3b. West Nam Bo Zone. Regional and zonal boundaries: I. Song Chay Fault; II. Dien Bien-Lai Chau Fault; III. Song Ma Fault; IV. Rao Nay Fault; V. Tam Ky-Phuoc Son Fault.

and Devonian stratigraphy have also been obtained recently, and contribute to processing the sequence of some sections and completing their distribution areas based on new discoveries. The Silurian is characterized by marine deposits only, with deep water facies in some regions and areas and shallow-water facies in others. In contrast, the Devonian mainly comprises continental and shallow-water marine deposits, and deep-water like facies of the Upper Devonian in some areas. The difference between the various sedimentary components and facies of the deposits depends on the distribution areas that reflect the paleoenvironment conditions of their sedimentary deposition processes. On the basis of the lithological components and facies, the distribution of the Silurian and Devonian deposits can be differentiated into regions and zones as follows (Fig. 1): 1. The Bac Bo Region (Northern Region) extends over almost all the Northern part of the country (the Tonkin of older publications). It is limited in the West by the Song Ma and Dien Bien-Lai Chau faults and includes three zones: 1a. The Quang Ninh Zone includes the northeast coastal area of Bac Bo, including the islands of the Ha Long Bay; 1b. The Eastern Bac Bo Zone extends eastward from the Song Chay fault and is limited by the Quang Ninh Zone in the East; 1c. The Western Bac Bo Zone is located between the Song Chay Fault in the East, the Dien Bien-Lai Chau Fault in the West, and Song Ma Fault in the Southwest.

2. The Viet-Lao Region, which is located between the Song Ma and Dien Bien – Lai Chau Fault in the Northwest and the Tam Ky – Phuoc Son Fault in the South, is divided into two zones – the Dien Bien – Nghe An and Binh Tri Thien zones, respectively. 2a. The Dien Bien-Nghe An Zone extends from the Dien Bien-Lai Chau and Song Ma Faults in the Northern and Eastern sides and the Rao Nay Fault in the South. 2b. The Binh Tri Thien Zone is located between the Rao Nay Fault in the North and the Tam Ky-Phuoc Son one in the South. 3. The South Viet Nam Region, which extends southward from the Tam Ky-Phuoc. Son Fault, includes two zones that are separated by a presumed boundary. 3a. The South Trung Bo Zone (Southern part of the Central Viet Nam) extends from the Tam Ky-Phuoc Son Fault to the East Nam Bo area. 3b. The Western Nam Bo Zone includes the territory of Kien Giang Province and the Vietnamese part of the Gulf of Thailand. The stratigraphic units will be presented below for each region and its respective zone. 2. Stratigraphic units 2.1. Silurian See (Fig. 2).

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Fig. 2. The distribution in regions and zones of the Silurian and Devonian Units of Viet Nam.

2.1.1. Bac Bo region 2.1.1.1. Quang Ninh Zone. In the Quang Ninh Zone, the most important Silurian deposits are distinguished by deep-water facies containing graptolite faunas and form the upper part of the Ordovician-Silurian formations in the northeastern coastal area and in the Co To archipelago. The shallow-water deposits containing benthic fauna are exposed in a small area of the Kien An District, in the vicinity of Hai Phong City. The Tan Mai Formation (O-S tm) is characterized by rhythmic and banded structures, including terrigenous sediments. It is about 7 km broad an extends discontinuously for hundreds of kilometers along the southeast side of the NE-SW trending Yen Tu–Tien Yen–Tan Mai Fault, and crops out in the Cai Chien and Vinh Thuc islands. The stratotype of the formation extends along the road from Mong Cai Town through Tan Mai to Than Pun villages. It has been subdivided into two parts: the lower one is 1000–1200 m thick and comprises grey, greenish-grey, fine- to medium-grained quartzitic micaceous sandstone with flattened pebbles interbedded with micaschist, micaceous siltstone, and phyllite. The latter gradually increases upwards in the section. In the upper part (about 1000 m thick) occur green-grey mica schist, micaceous phyllite with some interbeds of micaceous sandstone. Graptolites (Bohemograptus tenuis, Lobograptus cf. crinitus, Neoculograptus inexpectatus, Pristiograptus pseudodubius) occur in the siltstone of the upper part along with sporomorphs (Acanthotriletes sp., Lophozonotriletes sp., Simozonotriletes sp., Stenozonotriletes sp., and Punctatisporites sp.), notably in the Nam Ha–Truong Thu section. Silurian fossils have also been found in the upper part of the formation. Yet the Ordovician age of this part is only inferred from the thickness of beds that underlie the Silurian fossiliferous layers (Nguyen Cong Luong, 2001a,b). The Co To Formation (O-S ct) is mainly characterized by polymictic, cross-bedded sandstone bearing fragments of felsic effusives,

claystone with banded and rhythmic structure, and tuffaceous interbeds and gritstone. It is therefore readily distinguished from the Tan Mai and Phu Ngu formations of the same age. The total thickness of the Co To Formation reaches over 1000 m (Fig. 3). The Co To Formation occurs on over 40 islands and islets of Co To Archipelago, situated in the extreme northeast of Bac Bo (Tonkin) Gulf. It extends along a NE–SW trend, from Lo Chuc San, through Thanh Lan to Co To Islands. The 710 m thick Thanh Lan Section is composed of two parts: the lower part is composed of mixed deep marine sediments intercalated with shallow marine facies, and the upper part, made up by fine-grained, banded sediments that indicate deep marine facies. Early Silurian graptolites (Streptograptus exiguus, Monograptus priodon) have been collected from the silty claystone of the uppermost levels of the lower part. Apart from these species, other graptolites were also reported from Co To Island (Spirograptus cf. minor, S. cf. turriculatus, S. cf. regularis, Oktavites aff. O. planus, Campograptus communis, Demirastites sp., Climacograptus sp., Monograptus ex gr. pandus, Streptograptus sp. (Tran Van Tri et al., 1972, 1975, 1977). Con Ngua Island also yielded Pristiograptus cyphus, Pristiograptus sp., Pseudoclimacograptus sp. (in Nui Nhon Islet), and Demirastrites triangularis (Nguyen Huy Mac and Pham The Hien, 1972), and Lo Chuc San Island yielded Streptograptus exiguous and Monoclimacis linarsoni. All these graptolites indicate an Early Silurian age and are located in the lower beds of the upper part of the formation. The unfossiliferous lower part is rather thick, consisting of deep marine sediments and possibly corresponds to a relatively long period; therefore, the Co To Formation is approximately considered as Ordovician–Early Silurian in age (Tong-Dzuy Thanh and Vu Khuc (2006). The Kien An Formation (S4 kn) crops out in several hills in the Kien An District, near Hai Phong City (Fig. 4). According to Tong-Dzuy Thanh et al. (2001) the tripartite sequence of the Kien An Formation consist of a 120 m thick lower part of blue-grey marl, mudstone

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Fig. 3. Correlation of the Silurian deposits in the Bac Bo Region. (A) West Bac Bo Zone: 1–3. Sinh Vinh Formation (O3 -S sv); 4. Bo Hieng Formation (S4 bh); (B) East Bac Bo Zone: 1–5. Phu Ngu Formation (O-S pn); (C) Quang Ninh Zone: 1–4. Co To Formation (O-S ct) (The legends in this figure are for Fig. 6 and Fig. 9 also).

and yellowish-grey sandstone containing Nikiforovaena cf. ferganensis and indeterminate rugosans, and a 320 m. thick middle part composed of grey, quartzitic sandstone with dark purplish-red mudstone interbeds which yields Retziella weberi, Nikiforovaena ferganensis and Howellella sp. The upper part of the section, on the north slope of Xuan Son Mountain, consists of about 100 m of thick-bedded dark grey limestone with shale and marl interbeds in the uppermost layers, which yield an abundant Retziella weberi assemblage. At the same level, but on the north slope of Tien Hoi Mountain, the brachiopods Retziella weberi, Nikiforovaena ferganensis, Howellella bragensis, Howellella sp. (Vu Khuc and Bui Phu My, 1990) were recorded, along with abundant corals, notably Favosites admirabilis, Xiphelasma sp., Nipponophyllum sp., and Mesofavosites sp. No lower boundary of the Kien An Formation has ever been observed, and its relations with the Co To and Tan Mai Formation are unknown. It possibly unconformably underlies the Lower Devonian Van Canh Formation. The Kien An Formation is referred to the Late Silurian, (Late Ludlow or Pridolian) (Tong-Dzuy Thanh et al., 2001). 2.1.1.2. Eastern Bac Bo Zone. In Eastern Bac Bo Zone, the Silurian deposits are represented by deep-water facies containing graptolite faunas which occur in the central area of the zone. They make up the upper part of the Ordovician-Silurian Phu Ngu Formation. The Phu Ngu Formation (O-S pn) is characterized by banded, mainly fine-grained rhythmic terrigenous sediments interbedded

with effusives of mafic to felsic and alkaline composition (Fig. 3). The Formation occurs in two bands, the first of which extends from Dai Tu (Thai Nguyen Province) to Cho Ra (Bac Can Province). The second band extends from Luong Mt through Ngoi Nac, Vinh Tuy to Huong Son areas (Ha Giang Province). With a thickness of 2300–2400 m, the Phu Ngu stratotype is composed of two distinct parts. The 1100 m thick lower part includes argillaceous shale, cherty-argillaceous shale rhythmically interbedded with silty sandstone, locally with lenses of limestone and mafic effusives. The 1200 m thick upper part comprises silty sandstone interbedded with argillaceous shale and chertyargillaceous shale, locally with lenses of limestone and felsic effusives. Graptolites (e.g., Climacograptus sp., Glyptograptus sp., Monoclimacis sp.) have been collected from the uppermost levels of the lower part, along with poorly preserved trilobites and brachiopods (Fig. 3). In Na Ri area (Bac Can Province), the formation occurs in small exposures with a thickness of about 250 m, including grey silty sandstone, greenish-grey marl, black, thin-banded argillaceous shale containing the graptolites Diplograptus sp., Climacograptus latus, Climacograptus cf. C. scolaris, Ptilograptus sp., Glyptograptus sp., the trilobites Agnostus perrugatus, Remopleurides aff. R. salteri, and the bryozoan Fenestella sp. All these fossils are of Ordovician to Early Silurian age, but occur in a fairly low position in the sections, but the part the section that overlies the fossil-bearing beds is rather thick and characterized by deep marine facies, therefore the formation can reasonably be

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Fig. 4. Distributed area (A) and stratigraphic sequence (B) of the Upper Silurian Kien An Formation (S4 kn) in Kien An District (Hai Phong City).

dated as Ordovician-Silurian (Pham Dinh Long, 2001; Nguyen Kinh Quoc, 2001; Tong-Dzuy Thanh and Vu Khuc, 2006). 2.1.1.3. Western Bac Bo Zone. In the Western Bac Bo Zone the Silurian deposits include the upper parts of the Ordovician-Silurian Sinh Vinh Formation, and the Upper Silurian Bo Hieng Formations. They mainly consist of shallow-water facies deposits, and deepwater deposits are limited to the west of Son La Province (Ket Hay Formation). The Sinh Vinh Formation (O3 —S sv) (Fig. 3) is mainly characterized by light grey, thickly bedded dolomitic limestone, exposed in the lower section of the Song Da River in discontinuous narrow bands. In the Sinh Vinh Section, along the Song Da River, the Formation is clearly distinguished by the three following parts: (1) The 40 m thick lower part, made up by conglomerate, whose quartzite pebbles are derived from the underlying Ben Khe Formation; (2) The 400 m thick middle part, made up by calcareous sandstone and mudstone, dolomite, sandy limestone and marl bearing the corals Reuschia sp., Plasmoporella kiaeri, Favositella alveolata; (3) The 350 m thick upper part, which consists of light grey, thickly bedded dolomitic limestone and calcareous shale interbeds bearing the corals Favosites sp., F. aff. F. forsbesi, F. ex gr. F. gothlandica, F. cf. F. hisingeri, F. cf. F. coreaniformis, Mesofavosites sp., Squameofavosites sp., and Parastriatopora sp. The total thickness of this section is 800 m, but in other sections its thickness reaches 1000–1200 m.

Apart from the above mentioned fauna, some other fossils have been collected, such as the corals Reuschia sp., Favositella alveolata, and Favosites sp., F. aff. F. forbesi var. similis, F. ex gr. F. gothlandicus, F. cf. F. hisingeri, F. cf. F. coreaniformis, Squameofavosites sp., Mesofavosites sp., Parastriatopora sp., Multisolenia ex gr. M. tortuosa, and the crinoid Bistrowicrinus quinquelobatus. The Bo Hieng Formation (S4 bh) is distributed in the limited area of the lower section of the Song Da River basin, and it conformably rests upon the Sinh Vinh Formation and under the Lower Devonian Song Mua Formation (Fig. 3). The 400 m thick Bo Hieng Formation, mainly consists of dark green marly shale interbedded with black-grey limestone and, in its lower part, some banded argillaceous shale containing brachiopods (Retziella weberi, Orbiculoidea tonkinensis), and bivalves (Modiomorpha brevis, M. sp., Cymatonota cf. C. yunnanensis, Dysodonta sp., Actinopteria sp.). The upper part is composed of black, thin- to medium-bedded limestone and marl in the upper part, containing Retziella weberi, Tadschikia xuanbaoi, Howellella sp., Scoliopora sp., and Favosites kunjakensis (Ta Hoa Phuong and Le Van Manh, 1996). In the Nui Tau Mt Section (Yen Lap, Phu Tho Province), the black-grey limestone interbedded with argillaceous shale yielded corals (Palaeofavosites aff. balticus, Favosites kernihoensis, Aphyllum aff. sociale) and brachiopods (Retziella weberi, Ferganella aff. Turkestanica) (Nguyen Huu Hung et al., 1988), which indicate a Late Silurian age of the formation (Tong-Dzuy Thanh et al., 2001). The

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Fig. 5. Distributed area and stratigraphic sequence of the Silurian Dai Giang Formation in Quang Tri Province.

lithology and fauna of the Bo Hieng Formation is similar to that of the Kien An Formation described above. The Ket Hay Formation (S1-2 kh) occurs in a narrow band in the Southwestern area of Son La Province and discontinuous exposures northwestward from Son La to Tuan Giao, all of them being probably in close relation to the Song Ca Formation (see below—Viet-Lao Region). The 230 m thick Ket Hay Formation consists of thinbedded siliceous shale, interbedded with siliceous siltstone, shale, sericite-chlorite schist, and calcareous-siliceous schist. Most of the graptolites it has yielded (Demirastrites triangulates, Hedrograptus cf. H. rectangulatus, Diplograptus cf. D. modestus) are Early Silurian ˜ in age (Le Thanh Huu and Vu Xuan Luc, 2005; Tran Van Tri and Vu Khuc, 2009).

2.1.2. Viet-Lao Region The Silurian deposits in the Viet-Lao region include the upper part of the Ordovician-Silurian Song Ca Formation, and the Tay Trang Formation of Late Silurian-Early Devonian age in the Dien Bien-Nghe An Zone. In the Binh Tri Thien Zone, they are composed

of the upper part of the Ordovician-Silurian Long Dai Formation, and the Upper Silurian Dai Giang Formation. 2.1.2.1. Dien Bien-Nghe An Zone. The Song Ca Formation (O3 -S2 sc) crops out along two main strips, the first of which extends from Muong Xen and Tuong Duong to Quy Chau, Nghe An Province, south of the Song Ca River. The second strip extends from the Vietnam-Laos border through Nghe An to Ky Anh, Ha Tinh Province. The formation is about 2100–2300 m thick and mainly consists of argillaceous shale, sandstone, and some beds of black, and bedded limestone. The graptolites Monoclimacis vomerinus and Pristiograptus kweichihensis occur in the argillaceous shale of its upper part, and indicate an Early Silurian age. Besides, some Silurian spores also occur in this formation, such as Zonosphaeridium sp. and Leiotrichoides sp. Since the Early Silurian graptolites occur in the upper part of the formation, its lower part may therefore tentitavely be referred to the Upper Ordovician (Tong-Dzuy Thanh and Vu Khuc, 2006). The Tay Trang Formation (S3 -D2 e tt) is distributed in Lai Chau and Dien Bien areas, Northwest Bac Bo, extending to the West of Nghe

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An Province (North Trung Bo), so it includes all the components of the Tay Trang Formation introduced by Dovjikov et al. (1965) and the Huoi Nhi Formation erected by Nguyen Van Hoanh (Le Duy Bach and Nguyen Van Hoanh, 1995); therefore, the Tay Trang name has priority. The 800 m thick Tay Trang Formation consists of: (1) 540 m thick grey, fine- to middle-grained sandstone, interbedded with ash grey to dark grey, thin-bedded siltstone, and dark grey clay shale; (2) 260 m thick, dark grey, thin bedded clay shale, dark grey siltstone with some dark grey, and fine-grained, thin bedded sandstone. The formation yields fossils that indicate an age ranging from the Ludlow (Silurian) to the Eifelian (Middle Devonian). They include, in the West of Nghe An Province, graptolites (Bohemograptus bohemicus, Monograptus thomasi, M. yukonensis, M. uniformis) (Nguyen Van Phuc, 1998), and dacryoconarids (Nowakia cancellata, N. holynensis, Homoctenus sp.), and, in the Tay Trang area (Dien Bien Province, Western Bac Bo Zone), cephalopods (Erbenoceras sp., Anetoceras sp.), the dacryoconarid Nowakia sp. (cf. N. acuaria), and the brachiopod Chonetes ex gr. tenuicostata. The Tay Trang Formation (= Huoi Nhi Fm.) rests unconformably upon the Song Ca Formation (O-S sc) and conformably under the Huoi Loi Formation (D1-2 hl) (Tong-Dzuy Thanh and Vu Khuc, 2006). 2.1.2.2. Binh Tri Thien Zone. The Long Dai Formation (O2 -S2 ld) comprises terrigenous sediments of rhythmic structure, interbedded with acidic to intermediate effusive, exposed in the south of the Rao Nay Fault. In the Ban Ho–Vit Thu Lu stratotype, the formation is up to 1250 m thick, and its lower part (about 1000 m thick) consists of rhythmic intercalation of sandstone, siltstone and argillaceous shale, clay-cherty shale with some interbeds of andesite, which yield the Late Ordovician trilobites Cyclopyge sp., Microparia (?) sagaviaformis, Ogygiocaris sp., Nileus sp., and the graptolite Diplograptus sp. Its upper part is composed of black argillaceous shale, clay-chlorite shale with some interbeds of sandstone and gritstone containing the Late Silurian graptolites Neodiversograptus nilssoni, Monograptus sp., and Pristiograptus sp. The Le Ky section, which reaches 1930 m in thickness, displays the same lithological components, but begins with 90 m thick conglomerate beds, and is distinguished by the tuffaceous sandstone in its middle part, and the lack of andesite interbeds. Llandovery graptolites (Demirastrites convolutus, Monograptus halli, Oktavites spiralis) have been collected from the middle part of the section, whereas its upper part yields Bohemograptus bohemicus, Monoclimacis sp. (Nguyen Xuan Duong, 1996; Tong-Dzuy Thanh and Vu Khuc, 2006). The Dai Giang Formation (S3-4 dg) is exposed in the Quang Binh and Quang Tri Provinces, with thickness of about 1100 m (Fig. 5), and its sequence begins with gritstone and coarse-grained sandstone, unconformably overlying the Long Dai Formation. The formation consists essentially of sandstone, siltstone and calcareous sandstone, with limestone and marl beds in its upper part. A benthic fauna has been collected mainly from the middle and the upper parts of the formation, and includes trilobites (Cromus beaumonti, Metacalymene sp., Praedechenella aff. vietnamica, Encrinurus cf. sinicus), brachiopods (Retziella weberi, R. altaica, Nikiforovaena vietnamica, “Howellella” cf. lynxoides), corals (Multisolenia cf. formosa, Nipponophyllum anmaense), bivalves (Schizodus? myducensis, Sphenotus antecedens, Modiomorpha paracrypta, Goniophora dianensi), and fish remains referred to the placoderm Myducosteus anmaensis and an indeterminate acanthodian (Nguyen Xuan Duong, 1996; Tong-Dzuy Thanh and Vu Khuc, 2006). 2.2. Devonian See (Fig. 2; Fig. 6). Devonian deposits are widespread in Viet Nam, but their lithology and sequences, especially in the Lower Devonian, are different

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in the Bac Bo, Viet-Lao, and South Viet Nam regions. The most complete Devonian sections are observed in the Bac Bo region, whereas separate, discontinuous sections are met with only in some areas of the South Viet Nam region (Tong-Dzuy Thanh and Vu Khuc, 2006). In the Bac Bo region, the resemblances between the Devonian sections is conspicuous and well characterized, both lithologically and faunally, thereby indicating that they were formed in a single, large basin. In the Quang Ninh, Eastern Bac Bo and Western Bac Bo zones, the Devonian sections are composed of two conspicuous parts, the lower one mainly consisting of terrigenous sediments, and the upper one of carbonate. However, detailed differences between these sections can be noticed. Notably, in the Eastern the Bac Bo Zone, the Lower Devonian is characterized by Red Beds that unconformably overly the Lower Paleozoic, while in the Western Bac Bo there is a conformity between the Upper Silurian Bo Hieng Formation and the Lower Devonian Song Mua Formation (Fig. 6). 2.2.1. Bac Bo Region 2.2.1.1. Quang Ninh Zone. In the Quang Ninh Zone, the Devonian deposits consist of the Van Canh, Duong Dong, Do Son, and Trang Kenh Formations, successively. The uppermost of the Devonian sequences in this zone displays the lowermost part of the Upper Devonian-Lower Carboniferous Con Voi and Pho Han formations. The Van Canh Formation (D1 vc) is the lowermost formation of the Devonian sequence in the Quang Ninh Zone, and crops out in the Van Canh islet, Tra Ban, Ngoc Vung, and other islands in the Bai Tu Long (Fai Si Lung) Bay, and on the Do Son Peninsula (Nguyen Huu Hung et al., 2007). In the Van Canh type section, the formation consists of a 200 m thick series of alternating sandstone and mudstone beds, which contains plant remains (Tainiocrada? sp. cf. T. decheniana, and Psilophytites? sp.). In this section, the Van Canh Formation conformably underlies the Duong Dong Formation, which yields the brachiopod Tulynetes nonpoensis. On the Do Son Peninsula, the Van Canh Formation unconformably underlies the Do Son Formation. In previous works, the Ngoc Xuyen mudstone and sandstone that has yielded the eurypterids Rhinocarcinosoma dosonensis and Hughmilleria sp., isolated placoderm fish plates (referred to a primitive antiarch and a “Wangolepis”-like placoderm) (Brady et al., 2002), and Lower Devonian sporomorphs, were considered as the lower part of the Do Son Formation (Tong-Dzuy Thanh and Vu Khuc, 2006). They are now referred to the Van Canh Formation (Nguyen Huu Hung et al., 2007), but their eurypterid and vertebrate fauna seems more suggestive of the Uppermost Silurian (Brady et al., 2002; Janvier et al., 2003). The Duong Dong Formation (D1 –D2 e dd) is exposed in Kinh Mon (Hai Duong Province), Duong Dong (Trang Kenh, Hai Phong City) and Dong Trieu (Quang Ninh Province), and in Chau Dop, Tra Ban, and Ngoc Vung islands (Ha Long Bay). The new data provided by Nguyen Huu Hung et al. (2007) show that the crossbedded quartzitic sandstone, siltstone and shale of the upper part of the Duong Dong Formation described in the previous works (Tong-Dzuy Thanh and Vu Khuc, 2006) should be referred to the overlying Do Son Formation, and that the brachiopod- and coral-bearing limestone lenses of its uppermost beds belong in fact to the Trang Kenh Formation. This accords with the original description of the formation by Nguyen Quang Hap (1967). According to Nguyen Huu Hung et al. (2007), the Duong Dong Formation in the Lo Son area consists of: (1). 50 m thick clay shale and mudstone containing brachiopods; (2) 70 m thick fine-grained sandstone and interbeds of shale containing brachiopods, and (3) 80 m thick fine-grained sandstone, mudstone interbeds containing poorly preserved brachiopods. The fossils from the Duong Dong Formation are mainly brachiopods, and have been collected from many localities, such as the Lo Son, Duong Dong and Hiep Son (Kinh Mon) areas, and on Ngoc Vung and Van Canh islands

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Fig. 6. Correlation of the Devonian deposits in the Bac Bo Region. (A) West Bac Bo Region: 1. Upper Silurian Bo Hieng Formation (S4 bh); 2. Song Mua Formation (D1 sm); 3. Ban Nguon Formation (D1 bn); 4. Ban Pap Formation (D1 p-D3 fr bp); 5. Bang Ca Formation (D2 gv-D3 fr bc); 6. Toc Tat Formation (D3 fm-C1 t tt). (B) East Bac Bo Zone: 1. Ordovician Lutxia Formation unconformably is underlying the Si Ka Formation; 2. Si Ka Formation (D1 sk)—Red beds containing vertebrate remains; 3. Bac Bun Formation (D1 bb); 4. Mia Le Formation (D1 ml) containing abundant Pragian benthos of the Euryspirifer tonkinensis assemblage; 5. Ban Pap Formation (D1 p-D3 fr bp); 6. Bang Ca Formation (D2 gv-D3 fr bc); 7. Toc Tat Formation (D3 fm-C1 t tt). (C) Quang Ninh Zone: 1. Upper Silurian Kien An Formation (S4 kn); 2. Van Canh Formation (D1 vc); 3. Duong Dong Formation (D1 -D2 e dd); 4. Do Son Formation (D2 gv-?D3 fr ds); 5. Trang Kenh Formation (D2 gv-D3 tk.

(Bai Tu Long Bay). The brachiopods are referred to Euryspirifer cf. tonkinensis, Indospirifer kwangsiensis, Schellwienella cf. S. lantenoisi, Stropheodonta cf. S. interstrialis, Athyris concentric, Atrypa ex gr. reticularis, Bacbochonetes sp., Retichonetes sp., Bacbochonetes janvieri, Perichonetes mutabilis, and occur along with the bivalves Pterinopecten sp. Carinatina cf. C. arimaspa, Desquamatia sp. The Duong Dong Formation conformably overlies the Van Canh Formation, and unconformably underlies the Do Son Formation. It is referred to the Lower Devonian—Eifelian. The Do Son Formation (D2 gv-?D3 fr ds) crops out in the Do Son Peninsula and some islands of Bai Tu Long Bay. On the mainland of the Quang Ninh Zone, it is usually exposed along with the Duong Dong Formation and has sometimes been incorrectly referred to part of the latter. It was first described by Lantenois (1907) as the “Do Son Sandstone” (Grès de Do-Son), which occurs along the beaches of Do Son Peninsula (Hai Phong City). Later, it has been described as a tripartite formation (Tong-Dzuy Thanh and Vu Khuc, 2006). Our new investigations show that the Do Son Formation includes two parts which correspond to Lantenois’s “Do Son Sandstone”, and a lower part of the “tripartite formation”, which is now referred to the Van Canh Formation. The sequence of the formation is thus as follows: - 150–200 m thick, thick-bedded, locally cross-bedded quartz gravelstone at the base, overlain by quartzitic sandstone, coarsegrained sandstone, sometimes with interbeds of chocolatecoloured siltstone. This member is most clearly exposed at the back of the Do Son Market and along the road towards Beach 2, particularly around the Van Huong Quay. Some bivalves, such as: Schizodus (?) sp., Ptychopteria (Actinopteria) hunanensis and Goniophora sp. were collected on the mountain slope situated in the back of the hamlet lying between the Do Son Market and the

main office of the Do Son Townlet. Vertebrate remains (Vietnamaspis trii, Briagalepis sp., Bothriolepis cf. B. gigantea) have been found along with the lycophyte Bergeria or Knorria (cf. Lepidodendropsis sp.) on the shore of the Van Huong Quay area (Janvier and Tong-Dzuy Thanh, 1998; Long et al., 1990; Tong-Dzuy Thanh and Cai Chong-yang, 1995). - 200 m thick, brown, medium-bedded, locally cross-bedded sandstone, dark chocolate-coloured siltstone interbedded with grey, greenish-grey, yellow-brown weathering clay shale,. This barren member is clearly visible on the road towards the Casino building, from near the “Bao Dai Villa” to the southernmost end of the peninsula. The sedimentology and fossils of the Do Son Formation indicate a deltaic to coastal facies. However, the outcrops of this formation are scattered over a large area and display a great diversity of facies that is characteriztic for deltaic environments. Therefore it deserves investigation. The unconformity between the Do Son Formation and the underlying Duong Dong Formation is only suggested by the presence of gravelstone beds at the base of the former formation, but is conformably underlying the carbonate Trang Kenh Formation. The Trang Kenh Formation (D2 gv-D3 tk) is largely distributed in the Quang Ninh Zone, including islands of Ha Long Bay, and consists of limestone containing abundant corals, stromatoporoids and some brachiopods. In previous works, the Trang Kenh Formation was described as a tripartite unit, which is composed of two limestone parts, and a middle cherty part. Its total thickness varies from 250 m to 300 m. The new study by Nguyen Huu Hung et al. (2007) shows that this formation only consists of limestone, the cherty part belonging in fact to the Pho Han Formation of Upper Devonian-Lower Carboniferous age. The Trang Kenh Formation

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yields Amphipora ramosa, A. rudis, Crassialveolites crassus, Scoliopora denticulata, Caliapora battersbyi, Alveolites admirabilis, and the brachiopod Stringocephalus burtini, which all suggest a Givetian age. However, the upper part of the formation yields stromatoporoids and corals (Amphipora laxeperforata, A. pervesiculata, Stachyodes costulata, S. fasciculate, which indicate a Givetian-Frasnian age. In the Phuc Son–Nui Han area, Famennian conodonts (Palmatolepis subperlobata, P. triangularis, P. glabra glabra, P. subrecta, P. perlobata) have been recorded from the uppermost beds of the Trang Kenh Formation (Tong-Dzuy Thanh and Vu Khuc, 2006; Nguyen Huu Hung et al., 2004). The Trang Kenh Formation conformably rests between the underlying Do Son Formation, and the overlying Con Voi and Pho Han Formations, which are described below. The Con Voi Formation (D3 fm-C1 cv) mainly consists of limestone and some thin beds of siliceous limestone that occur in the Mt Nui Con Voi Kien An and An Lao Districts, Hai Phong City. The Pho Han Formation (D3 fm-C1 ph) includes limestone, siliceous limestone and chert in Cat Ba Island (Ha Long Bay), and is the coeval with the Con Voi Formation. These two formations are thus distinguished by siliceous components: some thin beds in the Kien An Formation, and more diffuse ones in the Pho Han Formation. The fauna of the Con Voi Formation mainly consists of brachiopods, corals, and some foraminifera, whereas that of the Pho Han Formation includes many conodonts and a rich benthic fauna, such as corals (Fuchungopora sp., Syrigopora distans, Tetraporinus sp.) and many brachiopods in the lower beds of the formation. Both formations were dated as Upper Famennian-Lower Carboniferous, but most of them are Lower Carboniferous in age, their Famennian part being negligible in the sections and may be referred to the Trang Kenh Formation. It is possible that the Con Voi and Pho Han Formations represent different sections of a single formation that displays a variable number of siliceous components (Tong-Dzuy Thanh and Vu Khuc, 2006). 2.2.1.2. Eastern Bac Bo Zone. The Song Cau Group (D1 sc) includes the Si Ka Formation (red, subcontinental facies) and the Bac Bun Formation (deltaic facies grading upwards into shallow marine facies). The Si Ka Formation (D1 sk) is exposed in the Viet Nam–China frontier areas of Ha Giang Province and in Lang Son Province, whereas in the Cao Bang and Thai Nguyen Provinces, it is composed of part of the sequences of the Song Cau Group. In its stratotype (Fig. 3), the Si Ka Formation consists of greenish-grey siltstone interbedded with clay shale, greenish marly shale, thin-bedded, fine-grained limestone, and grey siltstone with interbeds of violet chocolate marly shale. The total thickness of the formation is about 650 m. According to Hoang Xuan Tinh, the basal conglomerate beds are locally up to 5–10 m and met with in the frame of the Geological Map (1:200,000) of the Bao Lac Map sheet (Hoang Xuan Tinh, 2001). Its sections are also observed in Dong Mo (Lang Son Province) and Trang Xa (Thai Nguyen Province), where the red sandstone is an important component in the sequence. Early publications on this formation briefly mentioned (e.g., the fishes “Asterolepis” sp., “Homosteus” sp., and the plant “Bythotrephis aff. antiquate” (Saurin, 1956). During the last two decades the vertebrates from the Si Ka Formation have been shown to belong to the two major fish groups that also populate the Lower Davonian of southern China, namely the Galeaspida and the antiarchan Placodermi, to which can be added various Sarcopterygii. Neither Asterolepis, nor Homosteus have been confirmed in the fauna. To date, the upper beds of the Si Ka Formation consisting of sandstone in Thai Nguyen Province has yielded the galeaspid Bannhuanaspis vukhuci and fragmentary remains of various Polybranchiaspis-like species, along with various yunnanolepiform and procondylolepiform antiarchs (Procondylolepis sp., Yunnanolepis spinulosa), and youngolepidid sarcopterygian remains (Janvier et al., 1993; TongDzuy Thanh et al., 1994; Janvier and Ta Hoa Phuong, 1999). The Si Ka

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Fig. 7. Lower Devonian Si Ka Formation unconformably rests upon the Ordovician Lutxia Formation (Left cliff on the road Dong Van-Lung Cu (Photo: Ta Hoa Phuong).

Formation unconformably rests upon different levels of the Lower Paleozoic deposits (Fig. 7) and conformably underlies the Bac Bun Formation. The Bac Bun Formation (D1 bb) (Fig. 4) consists of grey, chocolate, variegated weathered clay shale and marly shale, siltstone containing vertebrates in the stratotype (Dong Van District, Ha Giang Province). West of Dong Mo (Lang Son Province), it comprises a member of grey carbonate phosphate containing well-preserved vertebrates, and the overlying clay shale and siltstone member contains abundant brachiopods. In Trang Xa (Thai Nguyen Province), it consists of grey limestone and marly shale containing corals, brachiopods and vertebrates. The thickness of the formation varies from some tens to 500 metres. The distribution areas of this formation are much the same as for the Si Ka Formation, i.e. in the Ha Giang, Cao Bang, Lang Son, and Thai Nguyen Provinces. Vertebrate-bearing faunas of the lower member of the Bac Bun Formation include, in the stratotype, the galeaspids Polybranchiaspis sp. cf. P. liaojaoshanensis, numerous antiarchs (Yunnanolepis deprati, Y. bacboensis, Chuchinolepis dongmoensis, Vanchienolepis langsonensis), the petalichthyid Tongdzuylepis vietnamensis and the sarcopterygian Youngolepis cf. Y. praecursor. The Dong Mo section yields much the same vertebrate fauna with, in addition, a still indetermined lungfish (Tong-Dzuy Thanh and Janvier, 1990, 1994; Tong-Dzuy Thanh and Vu Khuc, 2006). The upper member of the formation yields brachiopods (Howittia wangi, Cyrtochonetes indosinensis, Parachonetes zeili) (Racheboeuf and Tong-Dzuy Thanh, 2000). The lithology and faunal composition of the Bac Bun Formation indicate a littoral to deltaic facies grading upwards into a shallow marine facies. The Mia Le Formation (D1 ml) is widespread in the Eastern zone of Bac Bo region and consists of greenish-grey siltstone interbedded with clay shale and marl, with some thin interbeds of argillaceous limestone, and grey limestone in its uppermost part (Figs. 6 and 8). The thickness of the formation in the Lung Co–Mia Le stratotype (Dong Van District, Ha Giang Province) reaches 500 m, but in other sections, it can decrease to a few hundred metres, as in the Ha Lang area (Cao Bang Province), where it is only about 200 m. This formation is the most fossiliferous unit in the Paleozoic of Viet Nam, containing hundreds of species of corals, brachiopods, trilobites, crinoids, and other groups, all indicating a Pragian age. They are components of the well-known Euryspirifer tonkinensis Assemblage (Tong-Dzuy Thanh and Ta Hoa Phuong, 1994), which is dominated by brachiopods (e.g., Euryspirifer tonkinensis, Undispirifer aff. pseudoaculiatus, Elymospirifer kwangsiensis, Dicoelostrophia annamitica, Atrypa aff. reticularis, Schellwienella douvillei, S. lantenoisi, Athyris? tiaomachiensis, Parachonetes zeili) and corals (e.g., Favosites

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Fig. 8. Stratigraphic sequence of the Lower Devonian Lung Cu-Ma Le section (Dong Van District, Ha Giang Province).

goldfussi, F. styriacus, F. pencolei, F. preplacenta, Squameofavosites cechicus, Sqf. giganteus, Sqf. sokolovi, Sqf. delicatus, Emmonsia yenlacensis, Squameopora vukhuci, Echyropora grandiporosa, Coenites bulvankerae, Heliolites praeporosus), but also includes the trilobite Proetus sp. and bivalves (e.g., Pterinea mieleensis). The Mia Le Formation lies conformably between the Bac Bun (D1 bb) and Ban Pap (D1-2 bp) formations (Tong-Dzuy Thanh and Vu Khuc, 2006). The Khao Loc Formation (D1-2 kl) is characterized by its carbonate composition and is limited to the Khao Loc Structure Westward to Ha Giang Province. It consists of 15 m thick dark grey, yellow-grey, red-brown weathering marl and clay shale containing abundant vertebrates, and 800 m thick, black-grey limestone interbedded with argillaceous limestone that yield corals and Amphipora sp. The vertebrate fauna notably includes galeaspids (Polybranchiaspis liaojaoshanensis, Laxaspis yulongsus), antiarchs (Yunnanolepis sp., Minicrania lissa, Heteroyunnanolepis qujingensis), the acanthothoracid placoderm Hagiangella goujeti, and the sarcopterygian Youngolepis praecursor (Janvier and Ta Hoa Phuong, 1999; Racheboeuf et al., 2005). The lowermost limestone beds of the formation yield corals (e.g., Favosites stellaris, F. vilvaensis, Squameofavosites enormis, S. vanchieni, S. baolacensis, S. spongiosus, Thamnopora beliakovi, Cladopora cf. C. yavorskyi, Parastriatopora champungensis, Caliapora cf. C. primitive) and the brachiopods Howittia wangi and Howellella ex gr. crispa. The middle part of the

section yields Striatopora sp., Thamnopora kolodaensis, T. cf. T. siavis, Alveolitella cf. A. polenowi, Coenites sp., and Corolites haoi (TongDzuy Thanh et al., 1988; Tong-Dzuy Thanh and Vu Khuc, 2006). The corals, brachiopods, and vertebrates from the base of the formation are to be correlated with the Bac Bun and Mia Le formations (Lower Devonian), whereas the corals from its middle part indicate a Middle Devonian age. The Ban Pap Formation (D1 p-D3 fr bp) is characterized mainly by grey, thin to medium bedded limestone, widely extending in both Eastern and Western Bac Bo Zones, and will be presented hereinafter for all Bac Bo Region. The total thickness of the formation reaches 1200 m, but in certain sections, it may only be 300–400 m thick. The lower part of the Ban Pap Formation mainly consists of dark grey, thin- to medium-bedded, bituminous limestone containing Emsian stromatoporoids and corals, such as Amphipora acerba alaiskiensis, A. agreste vojachia, Favosites styriacus, F. stellaris, F. regularissimus, Emmonsia yenlacensis, Cladopora yavorskyi. The middle part of the Ban Pap Formation includes dark grey, medium-bedded, fine-grained limestone, and its upper part grey to light grey, thick-bedded to massive fine-grained limestone. Although the boundary between the lower and the upper parts is difficult to identify on the basis of the lithology, their respective faunas are readily distinguished. Its middle part yields characteriztic

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Eifelian fossils, such as the dacryoconarids Nowakia sp., Viriatellina irregularis, V. dalejensis, and the corals Favosites robustus, Squameofavosites cf. S.kulkovi, and Thamnopora cf. T. micropora. The upper part of the Ban Pap Formation consists of grey to light grey, thick-bedded to massive fine-grained limestone, containing a Givetian fauna, such as the stromatoporoids Amphipora ramose and Actinostroma clatratum, the corals Thamnopora polygonalis, T. nicholsoni, Trachypora dubatolovi, Caliapora battersbyi, Crassialveolites crassiformis, Coenites quydatensis, Scoliopora denticulata, Dendrostella trigemme, Grypophyllum cf. G.carinatum, Macgea ex gr. multizonata, and the brachiopods Stringocephalus burtini and Atrypa (Desquamatia) sp. Moreover, Early Frasnian species of the Stachyodes costulata Assemblage occur in some sections. It is worth noting that in small areas of Dong Van (Ha Giang Province) there are limestone, clayish limestone, marl, black shale and chert interbeds, containing numerous pelagic fossils. In the lower beds are Lower Devonian fossils (Nowakia acuaria, N. barrandei, conodonts from the Polygnathus excavatus-Pandorinellina steinhornensis, P. nothoperbonus zones, and there are still the corals Favosites regularissimus and Heliolites sp. In upper beds occur Givetian species, such as Palmatolepis varcus and others. These carbonate deposits containing pelagic fauna can correlate with the lower and middle parts of the Ban Pap Formation, and were described as the Si Phai Formation by Dang Tran Huyen (1979), but they can reasonably be considered as components of a member – the Si Phai Member – within the Ban Pap Formation. The Ban Pap Formation conformably rests between the Mia Le and Toc Tat Formations. Most of the fauna indicate an EmsianGivetian age, but in the top part of Dong Van (Ha Giang Province) and Chi Lang (Lang Son Province) sections, there are Early Frasnian fossils; so, the upper boundary of the formation is diachronic, and the formation should be referred to the Emsian-Frasnian (TongDzuy Thanh and Vu Khuc, 2006). Tan Lap Formation (D2 gv-D3 fr tl) extends over some areas of the Chi Lang District, Lang Son Province, and is composed of 35–80 m thick conglomerate, arkosic gritstone, and arkosic sandstone interbedded with black siltstone. In the sandstone were collected plant remains that resemble Bergeria or Knorria (i.e. “Lepidodendropsis” sp.), and are similar to those also recorded from the Do Son Formation. The Tan Lap Formation is an insert in the limestone of the Ban Pap Formation; its basal conglomerate rests unconformably upon Givetian limestone, and conformably on its limestone containing an Early Frasnian Stachyodes costulata Assemblage (Fig. 2) (Nguyen Huu Hung et al., 1991; Tong-Dzuy Thanh and Vu Khuc, 2006). The Bang Ca Formation (D2 gv-D3 fr bc) is characterized mainly by cherty shale, calcareous cherty shale and clay shale, and reaches a thickness of about 200–250 m. This formation occurs in the Ha Lang area, West of the Cao Bang Province, and in the Dong Van and Yen Minh areas of the Ha Giang Province, it is also exposed in the Van Yen area, Son La Province, in the West of Bac Bo. The Givetian age of the formation is based on the brachiopod Stringocephalus burtini, and by conodonts (Polygnathus xylus xylus, P. varcus and P. linguiformis) collected in the Ban Cai area (West Bac Bo). The Frasnian age is based on foraminifera (Eonodosaria evlanensis, Eogeinitziana aff. E. rara, Nanicella aff. N. uralica, and Tikhinella multiformis) and conodonts of the Palmatolepis hassi and P. rhenana Zones (Pham Dinh Long, 2001; Ta Hoa Phuong, 1998). The Toc Tat Formation (D3 fm-C1 t tt) mainly consists of variegated (red-violet, brown-red, greenish-grey) limestone, cherty-clay shale and marl and argillaceous limestone. Calcareous cherty shale interbeds are also frequent in the formation, especially on the top of the sections. It is usually exposed in narrow bands, like the uppermost part of the section in all the distribution areas of the Devonian. In the East of Bac Bo, the Toc Tat Formation occurs in the Trung Khanh District (Cao Bang Province), where its shale

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contains manganese ore, and in some areas there are important economic seams. In the Dong Van Plateau (Ha Giang Province), and the West of Bac Bo, in the Van Yen area (Son La Province). The total thickness of the formation varies from 260–280 m to 350 m. The fossils collected in the Toc Tat Formation comprise bivalves, brachiopods, foraminifera, and conodonts, most of them, especially the conodonts (Palmatolepis minuta, P. glabra, P. marginifera, P. gracilis, P. sigmoidalis), indicating a Famennian age. However, in the uppermost part of the formation occurs a Tournaisian fauna, with such conodonts as Siphonodella sinensis, S. duplicate, S. isosticha, and such foraminifera as Chernyshinella glomifomis, C. triangularis, C. uralica (Pham Dinh Long, 2001; Ta Hoa Phuong, 1998). The Toc Tat Formation conformably rests upon the Bang Ca Formation, and is covered unconformably by the Lower Carboniferous Lung Nam Formation. 2.2.1.3. Western Bac Bo Zone. Except for the lower part of the Lower Devonian, the Western Bac Bo Zone shows Devonian units that are similar to those of Eastern Bac Bo Zone. In other words, from the upper part of the Lower Devonian, the lithology of the synchronous deposits in two zones of the Bac Bo Region resembles each other. Actually, as described hereinbefore, the Ban Pap, Bang Ca, and Toc Tat Formations extend in both Eastern and Western zones of the Bac Bo Region. Therefore, in this section about the Western Bac Bo Zone we only describe the Devonian Song Mua, Ban Nguon, and Nam Pia Formations. The Song Mua Formation (D1 sm) is distributed in the lower section of the Song Da River and is characteriztically very thick (from 1500 m to 2300 m, depending on the areas), it mainly includes black clay shale, and is thus clearly distinguished from other Devonian units of Viet Nam (Nguyen Vinh, 1978; Tong-Dzuy Thanh and Vu Khuc, 2006). Based on its lithology and fauna, the Song Mua Formation can be subdivided into three parts. The 600–1500 m thick lower part consists of black, black-grey, thin-bedded calcareous clay shale containing poorly preserved fossils. The 500 m thick middle part is composed of grey to black marly shale containing brachiopods (Mesodouvillina aff. M. subinterstrialis, Iridistrophia aff. I. praeumbracula, Howellella sp., Schellwienella sp., Camarotoechia sp., Platyorthis sp.) and bivalves (Pteria sp., Sphenotus? sp., Posidonia sp.). The 600 m thick upper part of the formation includes black, black-grey clay shale, and quartzitic sandstone containing numerous Howittia cf. wangi, Pugnacina sp., Lingula sp., Sphenotus? cf. S. spatulata, Mytilarca? sp., Pteria (Actinopteria) sp.). The Song Mua Formation rests conformably upon the Bo Hieng Formation, and conformably underlies the Ban Nguon Formation. The Ban Nguon Formation (D1 bn) occurs in the lower section of Song Da River basin, where it is exposed side by side with the underlying Song Mua Formation. It is mainly composed of argillaceous shale, quartzitic sandstone, and limestone. In its stratotype, the formation consists of argillaceous shale and quartzitic sandstone, and marly shale containing abundant brachiopods and bivalves. In the Hoa Binh-Tu Ly section, its thickness reaches 750 m, and its carbonate components include corals and stromatoporoids. The fauna of the formation belongs to the Pragian Euryspirifer tonkinensis Assemblage. Its characteriztic species are corals (Favosites goldfussi, Squameofavosites attenuatus, Caliapora macroporosa, Heliolites vulgaris, H. barrandei, Pseudozonophyllum aff. P. zmeinogorskiensis), stromatoporoids (Stromatoporella rara, Tienodictyon sp., Trupetostroma sp., Salairella sp., Clathrocoilona sp.) and brachiopods (Dicoelostrophia annamitica, Euryspirifer tonkinensis, Nervostrophia rzonsnickajae, etc.). The Ban Nguon Formation rests conformably upon the Song Mua and beneath the Ban Pap formation. By its stratigraphic relations and faunal composition, the Ban Nguon Fm. can be regarded as synchronous with the Mia Le Fm. of the Eastern Bac Bo Zone, but differs from the latter by

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its lithological components (Nguyen Xuan Bao, 1978; Tong-Dzuy Thanh and Vu Khuc, 2006). The Nam Pia Formation (D1 np) is widespread in West Bac Bo, from the upper course of the Da River (Lai Chau Province), following the eastern margin of the Song Ma Structure to the West and East of Thanh Hoa Province. With a thickness of about 350 m, the Nam Pia Formation consists of two parts. The lower part includes puddingstone, conglomerate and gritstone, and coarse-grained quartzite, with some interbeds of clay shale in the top. The upper part is composed of black-grey clay shale interbedded with marl and quartzite, marl and limestone lenses in the top. Their faunas are distinguished by two assemblages corresponding to the two levels, respectively. The former one includes brachiopods belonging to the Iridistrophia praeumbracula Assemblage of the lowermost level of the Devonian. The latter one contains corals and brachiopods belonging to the Pragian Euryspirifer tonkinensis Assemblage. The most characteriztic of them are the corals Favosites brusnitzini, F. stellaris, F. alpina, F. sublatus, F. ottiliae, Squameofavosites cechicus, S. baolacensis, Emmonsia yenlacensis, Cladopora rectilineata, Lecomptia ramose, and the brachiopods Euryspirifer tonkinensis, Schellwienella lantenoisi, Dicoelostrophia annamitica, Nervostrophia rzonsniskajae, etc. The Nam Pia Formation rests in “pseudo-conformity” upon sandstone of the Dong Son Formation (O1 ds) and conformably underlies the Ban Pap Formation (Bui Phu My, 1978; Tong-Dzuy Thanh and Vu Khuc, 2006). The Ta Khoa Formation (D1 tk) is composed of two parts. The lower part consists of grey, thin-bedded sandstone and quartzitic sandstone intercalated with grey sericite schist and sericite-biotite schist, and the upper part consists of sericitized black schist. Brachiopods (Stropheodonta annamitica, Spirifer sp., Atrypa reticularis, Orthis sp., Leptaena sp. indet., Chonetes sp. indet.) (Le Thac Xinh and

Hoang Tri Nghi, 1964) and phacopid trilobites have been collected at Ban Mong Village in dark schist belonging to the upper part of the formation (Phan Cu Tien et al., 1977). In addition, Nguyen Huu Hung (in Tong-Dzuy Thanh and Vu Khuc, 2006) has reported Howellella cf. H. crispa, Levenea cf. L. depressa, Fasculiptera sp., Howellella cf. H. crispa, Howellella sp., Dicoelostrophia sp., Atrypa sp., the bivalves Modiomorpha oblonga and Modiolopsis cf. M. wuxuanensis, the trilobites Ancanthopyge cf. A. yiminensis, the crinoids Hexacrinites (?) trangxaensis, Hexacrinites sp., and the bryozoan Fenestella sp. The thickness of the Ta Khoa Formation is variously assessed: more than 3000 m, according to some geologists, or less than 2000 m according to others. The contact of the Ta Khoa with the underlying formation has never been observed, and its relation to the overlying formation is either conformable according to some geologists, or uncomformable to others. The fauna that has been collected from the upper part of the Ta Khoa Formation corresponds to the Euryspirifer tonkinensis Assemblage, and can thus be correlated with the level of the Mia Le and Ban Nguon formations, of Pragian age. 2.2.2. Viet-Lao Region (Fig. 9) See Fig. 9. 2.2.2.1. Dien Bien-Nghe An Zone. The Huoi Loi Formation (D1-2 hl) is distributed in some areas of the West of Nghe An Province, notably in the Muong Xen area and as a slender wedge form in Cua Rao, and it can be divided into two parts. The lower part is about 300 m thick and composed of light grey, medium bedded sandstone, and upwards in the sequence are reddish brown thin interbeds of sandstone and argillaceous shale. The latter yields Howellella sp., Stropheodonta sp., Hexacrinites? Humilicarinatus, Lissocrinus sp. The upper part consists of 200 m

Fig. 9. Correlation of the Devonian deposits in the Viet-Lao Region. (A) South of Binh Tri Thien Zone: 1. Upper Silurian Dai Giang Formation (S3-4 dg); 2. A Choc Formation (D1 ac)—Red beds containing inarticulate brachiopods; 3. Tan Lam Formation (D2 gv-C1 tl); 4. Phong Son Formation (D3 -C1 ps). (B) North of the Binh Tri Thien Zone: 1. Rao Chan Formation (D1 rc); 2. Ban Giang Formation (D1 -D2 e bg); 3. Muc Bai Formation (D2 gv mb); 4. Dong Tho Formation (D2 gv-D3 fr dt); 5. Thien Nhan Formation (D3 tn); 6. Xom Nha Formation (D3 xn). (C) Dien Bien-Nghe An Zone: 1. Ordovician-Silurian Song Ca Formation (O3 -S2 sc); 2. Tay Trang Formation (S3 -D2 e tt); 3. Huoi Loi Formation (D1-2 hl); 4. Nam Can Formation (D2 gv-D3 nc).

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thick sandstone interbedded with limestone lenses, and contains bivalves (Glossites sp., Sphenotus sp., Archaeocardium sp.), stromatoporoids (Stromatopora concentrica, Amphipora ramose) and corals (Thamnopora cf. T. polygonalis, Crassialveolites crassus, Temnophyllum sp.). The relation of the Huoi Loi Formation to the underlying formation is not observed, and it conformably underlies the Nam Can Formation. The fauna from its lower part indicates a Late Emsian age, and that from its upper part suggests an Eifelian to Givetian age. In sum, the Huoi Loi Formation can be referred to the Emsian–Givetian (Nguyen Van Hoanh, 1995). The Thien Nhan Formation (D3 tn) occurs in a narrow NW-SE trending band, extending from the northeast of Huong Son, the southwest of Thanh Chuong, Nam Dan (Nghe An Province), Duc Tho to Ky Anh districts (Ha Tinh Province). With the thickness of about 800 m, it consists of dark grey siliceous shale and interbeds of argillaceous-siliceous shale, fine-grained sandstone, and siltstone. Fossils are rare and poorly preserved, consisting of the conodonts Ozarkodina (Spathognathodus) sp. and Palmatolepis sp., of Late Devonian age, and entactiniid and albaillellid radiolarians that indicate a Middle-Late Paleozoic age. Similar deposits occur in the Tuyen Hoa and Minh Hoa Districts, (Quang Binh Province), where black argillaceous shale, chert and siltstone, siliceous siltstone, and thin manganese interbeds, are used to describe the Ngoc Lam Formation. Here were found the conodont Palmatolepis subrecta and the brachiopod Calvinaria cf. megistanu. The lower and upper boundaries of the Thien Nhan Formation have not been observed, but the “Ngoc Lam Formation” conformably rests between the underlying sandstone of the Dong Tho Formation (D2 gv-D3 fr dt), and the overlying banded limestone and chert of the Xom Nha Formation (D3 xn) (Tong-Dzuy Thanh and Vu Khuc, 2006). The Nam Can Formation (D2 gv-D3 nc) is restricted to the western Ky Son District (Nghe An Province) in a narrow, 20 km long band, and possibly extends into the Xieng Khoang Province of Laos. Its thickness reaches 600 m. The Nam Can Formation comprises two parts. The lower part consists of grey, bedded limestone, siliceous limestone, thin interbeds or lenses of chert, containing Amphipora sp., Trachypora sp., Stachyodes radiata, Amphipora ramosa minor, and the corals Thamnopora polyforata and Thamnophyllum sp. The upper part includes mainly dark grey limestone with many calcite veins, and interbeds of argillaceous shale, containing Stachyodes radiata, Amphipora ramosa minor, the corals Thamnopora polyforata and Thamnophyllum sp., late Frasnian conodonts (Palmatolepis gigas, P. foliacea, P. cf. P.triangularis, Ancyrognathus triangularis, Polygnathus xylus, Spathognathodus sp.), and Famennian conodonts (Nothognathella sp., Palmatolepis glabra glabra, P. perlobata schindewolfi, P. trachytera) (Le Duy Bach and Nguyen Van Hoanh, 1995). The Nam Can Formation has conformable relations to the underlying Huoi Loi Formation. Its upper boundary usually shows a tectonic contact, but seems to be conformable with the overlying Lower Carboniferous chert. 2.2.2.2. Binh Tri Thien Zone. The A Choc Formation (D1 ac) shows a restricted distribution in Quang Tri Province, crops out from A Choc to Ta Puong villages (Huong Hoa District), and in Tan Lam area it is exposed along the Road № 9. It was termed Tan Lam Formation in former Vietnamese publications (Dang Tran Huyen et al., 1980), but the name Tan Lam was used earlier by Tran Thi Chi Thuan and H. Fontaine (1968) for the Devonian Tan Lam Limestone (Calcaire de Tan Lam) distributed in the same Tan Lam area (Quang Tri Province). The formation is composed of sandstone and siltstone interbedded with some argillaceous shale of chocolate colour, and its thickness does not exceed 100 m. The lowest beds consist of sandstone,

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conglomerate and pebble-bearing sandstone, with 3–5 cm large, more or less rounded pebbles made of quartz, quartzite, sandstone, siltstone, chert and sericite schist. The Early Devonian age of the formation is suggested by inarticulate brachiopods Lingula aff. L. loulaensis, L. cf. muongthensis, L. cf. L. cornea, L. aff. L. hawkei, Lingula sp. The A Choc Formation rests unconformably upon the Silurian Dai Giang Formation, but its upper boundary is still unknown (Tong-Dzuy Thanh and Vu Khuc, 2006). The Rao Chan Formation (D1 rc) is characterized by black shale, calcareous shale and sandstone, and limestone lenses, which are widespread in the West of Quang Binh and Ha Tinh Provinces. The thickness of the formation reaches more than 2000 m. The corals that have been collected in the limestone of the lower part of the formation include Pachyfavosites cf. P. delicatus, Alveolites sp., Coenites cf. simakovi, Heliolites cf. H. insolens, Chaetetes ninae. Desmidopora? sp., Dohmophyllum sp. The upper part of the formation yields brachiopods (Desquamatia vijaica, Levenea aff. L. transversa) and the crinoid Lissocrinus curtus. The Rao Chan Formation is referred to the Lower Devonian; it pseudoconformably overlies the Dai Giang Formation (S3-4 dg), and conformably underlies the Ban Giang Formation (D1 -D2 e bg) (Tran Tinh, 1996). The Ban Giang Formation (D1 -D2 e bg) is about 700 m thick and crops out in the West of Quang Binh and Ha Tinh Provinces. It consists of sandstone, quartzitic sandstone, siltstone interbeds, and some calcareous argillaceous shale, limestone lenses in its upper part. The fauna it yields comprises species ranging from the Lower Devonian (Emsian) to the Middle Devonian. Emsian species from the lower part of the formation include corals (Calceola sandalina, Stringophyllum sp.), crinoids Hexacrinites biconcavus, H. humilicarinatus), brachiopods (Atrypa auriculata, Chonetes sp.), and trilobites Dechenella (Basidechenella?) sp. The limestone of the upper part of the formation yields Middle Devonian species, such as corals (Gephuropora vietnamica, Thamnopora nicholsoni, Cladopora gracilis, Crassialveolites cf. C. multiperforatus, Thamnophyllum khelopense, Pseudogrypophyllum stenotabulatum, Heliolites pororus, Paraheliolites cf. P. hanusi) and strmatoporoids (e.g., Stromatopora hupschii). The Ban Giang Formation conformably rests between the underlying Rao Chan Formation (D1 rc) and the overlying Muc Bai Formation (D2 gv mb) (Tong-Dzuy Thanh and Vu Khuc, 2006). The Muc Bai Formation (D2 gv mb) occurs mainly in NW–SE bands in the west of Ha Tinh and Quang Binh Provinces and consists of terrigenous-carbonate deposits containing an abundant benthic fauna. With the thickness of about 800 m, the sequence of this formation is characterized by the intercalation of marly limestone, calcareous shale, sandstone, siltstone, argillaceous shale, in which the fauna is usually met with in calcareous host rocks. The conformable relation of the Muc Bai Formation with the underlying Ban Giang Formation and the overlying Dong Tho Formation is well observed. Among the abundant fossils from the Muc Bai Formation, the most characteriztic ones indicate a Givetian age, such as the brachiopods (e.g., Stringocephalus burtini, Bornhardtina sp., Schizophoria striatula, Gypidula biplicata, Emanuella takwanensis) and the corals (Thamnopora nicholsoni, Caliapora battersbyi, Scoliopora denticulata, Iowaphyllum chucaense, Dendrostella trigemme). In addition, the formation yields numerous stromatoporoids (e.g., Amphipora ramosa minor, Amphipora minima, Hermatoporella porosum, Flexiostroma flexiosum, Stachyodes concent, Clathrostroma actinostromoides) (Tran Tinh, 1996; Tong-Dzuy Thanh and Vu Khuc, 2006). The Dong Tho Formation (D2 gv-D3 fr dt) is characterized by coarse sediments with a thickness of about 400 m and clearly divided into two parts (Dovjikov et al., 1965). The lower part (150 m thick) consists of dark grey argillaceous shale, sometimes with interbeds of marly shale and sandstone, and the upper part (250 m

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Fig. 10. Stratigraphic sequence of the Upper Devonian Xom Nha Formation.

thick) is composed of light grey, yellowish grey sandstone, sometimes with shale interbeds. The Dong Tho Formation conformably overlies the Muc Bai Formation (D2 gv mb), and conformably underlies the “Ngoc Lam Formation”, the latter being an equivalent of the Thien Nhan Formation (D3 tn) (Tong-Dzuy Thanh and Vu Khuc, 2006). The Dong Tho Formation yields brachiopods (Atrypa reticularis, Spinulicosta spinulicosta, Stropheodonta sp., Megachonetes sp., Schizophoria cf. S. ivanovi, Adolfia sp.) and bryozoans Fenestella sp.) In the Hoi Da Stream, near Minh Le Village, an abundant flora has been found in the siltstone and shale. The macrofossils have been referred to Lepidodendropsis sp. and Protolepidodendron sp., and the sporomorphs to e.g., Apiculatisporites sp., Gymbosporites magnifica, Costazonotriletes latidentatus, Archaezonotriletes variabilis, Tholisporites densus, Aneurospora goensis, Ancyrospora involucra, and A. tenicaulis. From the sandstone of the Ly Hoa Pass there are vertebrates Lyhoalepis duckhoai, and Vukhuclepis lyhoaensis (TongDzuy Thanh et al., 1994, 1997).

The Xom Nha Formation (D3 xn) is about 200 m thick, and characterized by dark grey, light grey stylolitic limestone containing abundant corals, stromatoporoids and conodonts, and it crops out in the West of Quang Binh Province, as well as in the Chuc A area, Ha Tinh Province (Fig. 10). The lower part of the formation contains Frasnian fossils, such as stromatoporoids (Stachyodes costulata, S. angulata, S. paralleloporoides; corals Scoliopora denticulata, Thamnopora polyforata, Temnophyllum isetense) and the conodont Palmatolepis gigas. Fossils from the upper part include Famennian conodonts of the Palmatolepis triangularis, P. marginifera and P. crepida Zones. The Xom Nha Formation conformably rests upon the cherty shale and carbonaceous shale of the “Ngoc Lam Formation” (= Thien Nhan Formation-D3 tn), and shows an unconformity with the overlying Lower Carboniferous La Khe Formation (Nguyen Huu Hung et al., 1995). The Tan Lam Formation (D2 gv-C1 tl) was termed Cu Bai Formation in former Vietnamese publications (Nguyen Xuan Duong,

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1996), but the name Tan Lam Limestone was used earlier by Tran Thi Chi Thuan and H. Fontaine (1968) for this limestone. The formation consists mainly of black-grey limestone, light-grey dolomitic limestone occurring sporadically in the West of Quang Tri Province, and in the Le Thuy, Phong Nha areas of Quang Binh Province. The fauna collected from the Tan Lam, Cu Bai, and A Choc sections (Quang Tri Province) suggests for this formation has an age that ranges from the Givetian to the Famennian. In the lower part of the formation, it includes Givetian stromatoporoids (e.g., Amphipora ramosa, A. pinguis) and corals (Dendrostella trigemme, Argutastrea lavali, Disphyllum sp.). The middle part of the formation yields Frasnian foraminifera (Tournayella jubra, Eonodosaria sp.), stromatoporoids (Stachyodes costulata, Amphipora laxeperforata), and brachiopods (Uchtospirifer nalivkini). Famennian brachiopods (Tenticospirifrer minor, Cyrtospirifer postarchiaci, Cyrtiopsis graciosa, Uchtospirifer nalivkini, Yunnanellina sp.) occur in the upper part of the formation (Tong-Dzuy Thanh and Vu Khuc, 2006). The Le Thuy and Phong Nha areas (Quang Binh Province) limestone, possibly belonging to the uppermost Tan Lam Formation, contains Famennian-Lower Carboniferous fossils, such as foraminifera (e.g., Eoendothyra communis, Quasiendothyra kobeitusana, Tournayella jubra, Parathurammina suleimanovi, Septatournayella pseudocamerata), the coral Pseudouralinia aff. P. tangpakonensis and the brachiopod Crurithyris urii. The thickness of the formation in Tan Lam area is about 150 m, and the limestone corresponding to its uppermost part in Le Thuy and Phong Nha areas reaches about 170–300 m, so the total thickness of the Tan Lam Formation reaches 500 m and more. The Tan Lam Formation rests unconformably upon the Dai Giang Formation (S3-4 dg), the upper boundary of the formation is not clear. The Phong Son Formation (D3 -C1 ps) occurs only in Thua Thien (Hue Province), with few natural exposures, but drilling information shows that it extends in NW-SE trending bands from the south of Hue City. The formation consists of ash and dark grey limestone regularly intercalated with black clay shale containing fossils of Famennian age in its lower part and Lower Carboniferous ones in its upper part. Famennian fossils from the lower part comprise brachiopods (e.g., Yunnanella hsikwangshaensis, Yunnanellina cf. Y. hanburyi, Uchtospirifer sp., Athyris concentrica, Tenticospirifer tenticulum). The upper part yields a Late Famennian–Early Carboniferous fauna, with such foraminifera as Septabrunsiina cf. minuta, S. cf. S. rauserae, Septabrunsiina cf. S. kazakhtanica, Septatournayella cf. S. segmentata, or Chernyshnella sp., and the corals Syringopora sp. and Pseudouralinia sp. (Tong-Dzuy Thanh and Vu Khuc, 2006).

2.2.3. South Viet Nam Region 2.2.3.1. South Trung Bo Zone (southern part of Viet Nam Central). The Cu Brei Formation (D1 cb) consists of 400 m thick terrigenouscarbonate sediments, distributed only in a narrow (6 km long, 3 km wide) strip in the Cu Brei Mountain area (Yaly Commune, Sa Thay District of Kon Tum Province), i.e. in the western margin of the Kon Tum Block. The formation is divided into two parts: the lower part consists of conglomerate, gritstone and sandstone, argillaceous shale, some argillaceous schist interbeds, the upper—green grey talc schist, limestone, dolomitised limestone, marly shale containing the coral Squameofavosites aff. S. spongiosus, stromatoporoids (Syringostroma cf. S. densum, Amphipora cf. A. raris, A. cf. A. raritalis, Simplexodictyon cf. S. artyschtence, Stromatopora cf. S. boriarchinovi), and other undeterminable algae and crinoids. The Cu Brei Formation rests unconformably on the biotite-hornblende granitoid Dien Binh (384 ± 17; 418 ± 12 Ma by K/Ar method), and the mentioned coral, stromatoporoid assemblage indicates the Early Devonian age of the formation (Tong-Dzuy Thanh et al., 2007).

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2.2.3.2. Western Nam Bo Zone. The Hon Heo Formation (D? hh) mainly consists of sandstone, clay shale, occurring scatteredly in the Kien Giang Province, in coastal areas and in some islands of the Gulf of Thailand, such as Hon Trem, Hon Chong, Hon Heo, Bai Ot, Hon Mot, Ba Lua Islands, etc. Fossils are rare and represented by pooly preserved plant remains referred to, e.g., Taeniocrada? sp., Psilophyton sp. (Tong-Dzuy Thanh and Vu Khuc, 2006). Since the formation is scattered over the islands, its stratigraphic relations to the adjacent formations are unknown. The Hon Heo Formation is supposedly referred to the Devonian on the basis of the mentioned plant remains, but the study of Paleozoic spores in Viet Nam is inadequate, so the age of this formation is still a matter of controversy. 3. Facies distribution 3.1. Facies of the Silurian deposits Silurian deposits in Viet Nam are differentiated into deep-water facies containing graptolites and shallow water shelf facies. The deep-water facies are characterized for the upper parts of the Ordovician-Silurian formations, for instance, Co To and Tan Mai formations in the Quang Ninh Zone, Phu Ngu Formation in the East Bac Bo Zone, and Long Dai Formation in the Binh Tri Thien Zone, Song Ca Formation, and the lower part of the Tay Trang Formation in the Viet-Lao Region. The shallow water shelf facies is widespread for most Silurian formations, such as the Kien An Formation in the Quang Ninh Zone, the Sinh Vinh and Bo Hieng Formations in the West Bac Bo Zone, and the Dai Giang Formation in the Binh Tri Thien Zone. 3.1.1. Deep water facies of Lower Silurian deposits in the Bac Bo Region (Fig. 11) In the Quang Ninh Zone, both the Co To and Tan Mai Formations have been dated as Ordovician-Silurian, but fossils collected in their upper parts indicate the Early Silurian only. The deep-water facies of these formations have been proved by their fine-grained components, rhythmic structure in its sequence, and by graptolites assemblages, such as Streptograptus exiguus, Monograptus priodon, Spirograptus cf. minor, Oktavites aff. O. planus, Campograptus communis, Demirastrites sp., Climacograptus sp., Monograptus ex gr. pandus, Pristiograptus cyphus, Pseudoclimacograptus sp., Demirastrites triangularis, and Monoclimacis linarsoni (Nguyen Cong Luong, 2001a,b). In the Co To Formation, the sections are turbidite-like and belong to a dissected island arc environment (Pham Thanh Binh and Nguyen Cong Luong, 1999). The studies by Nguyen Xuan Khien (2000) have shown that the clastic components of the Co To Formation were deposited in a forearc sedimentary basin, and originated from a regenerated orogenic process. The Tan Mai Formation, also shows tuffaceous materials and rhythmic structure in its sequence, but these are different from those observed in the Co To Formation, the fragments of felsic effusive being absent in the components of sediments, and fine-grained rocks being less abundant than in the Co To Formation. Thus, perhaps the Tan Mai formation was attributed to the back-arc marine basin situated near an old continental margin (Nguyen Xuan Khien, 2000; Tong-Dzuy Thanh and Vu Khuc, 2006). In the East Bac Bo Zone, the Phu Ngu Formation reaches a thickness of more than 2000 m, consisting of argillaceous shale, cherty-argillaceous shale rhythmically interbedded with silty sandstone, and felsic effusive. The formation was dated as Ordovician-Silurian, but fossils from the sections indicate an Ordovician-Early Silurian age. Most of these fossils are graptolites such as Climacograptus sp., Glyptograptus sp., Monoclimacis sp., and Diplograptus sp., Climacograptus latus, Climacograptus cf. C. scolaris,

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Fig. 11. Sketch of the facial profile of the Silurian and Devonian deposits of the Bac Bo Region in the present situation.

Ptilograptus sp., Glyptograptus sp., and the trilobite Agnostus perrugatus. All these lithological and faunal characteriztics indicate a deep-water facies of the deposits in an island arc environment in the sedimentary process of the Phu Ngu Formation (Pham Dinh Long, 2002; Tong-Dzuy Thanh and Vu Khuc, 2006). 3.1.2. Deep water facies of Silurian deposits the Viet-Lao Region (Fig. 12) In the Dien Bien-Nghe An Zone, the deep-water facies of the Silurian deposits are expressed in the the upper part of the Ordovician-Silurian Song Ca Formation and perhaps in the lower part of the Upper Silurian-Eifelian Tay Trang Formation. The deep-water facies of the Song Ca Formation are evidenced by its fine-grained deposits reaching more than 2000 m in thickness, and containing Early Silurian graptolites in the upper part, such as Monoclimacis vomerinus and Pristiograptus kweichihensis. The deep-water facies are also characterized for the lower part of the Silurian-Eifelian Tay Trang Formation, which consists of thinbedded siltstone, dark grey clay shale, and fine -grained sandstone containing the graptolites Bohemograptus bohemicus, M. thomasi, and M. yukonensis. Thus, in the Dien Bien-Nghe An Zone the deepwater facies characterizing all Silurian deposits includes the Lower Silurian of the upper part of the Song Ca Formation and the Upper Silurian in the lower part of the Tay Trang Formation. Incidentally, it is worth noting that the Lower Silurian Ket Hay Formation (Le Thanh Huu and Vu Xuan Luc, 2005), distributed along the northeast side of the Song Ma structure of the present limits of the West Bac Bo Zone, is also characterized by

deep-water facies. Originally, this formation was a Silurian component of the Song Ca Formation in the Dien Bien-Nghe An Zone, but was displaced to the northeast edge of the Song Ma structure and wedged into the bodies of the Neoproterozoic Nam Co Formation by an imbricate fault (personal communication from Prof. Tran Van Tri). In the Binh Tri Thien Zone, the deep-water facies is composed of the upper part of the Ordovician-Lower Silurian Long Dai Formation, which consists of black argillaceous shale, clay-chlorite shale with some interbeds of tuffaceous sandstone, and gritstone containing Silurian graptolites Neodiversograptus nilssoni, Pristiograptus sp., Demirastrites convolutus, Monograptus halli, Oktavites spiralis, Bohemograptus bohemicus, and Monoclimacis sp. (Nguyen Xuan Duong, 1996). In summary, the deep-water facies is observed in Lower Silurian in the Quang Ninh and East Bac Bo Zones, where some tuffaceous sediments or effusive beds are interbedded in the sequences and possibly indicate the arc basin of the sedimentary environment. In the Viet-Lao Region, the deep-water facies is characteriztic for all Silurian deposits of the Dien Bien-Nghe An Zone, and the Lower Silurian only in the Binh Tri Thien Zone. In the West Bac Bo Zone there is no Silurian deep-water facies, all Silurian deposits in this zone are characterized by shallow water facies. 3.1.3. Shallow water facies of the Silurian deposits in the Bac Bo Region (Fig. 11) In the Quang Ninh Zone, the shallow water facies of the Upper Silurian includes the Kien An Formation, which occurs in a small

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area near Hai Phong City. Silty sandstone, marly shale, and limestone containing abundant benthos fauna of the formation such as corals and brachiopods indicate the shallow water facies of the deposits. Among the recorded fossils the brachiopods of the Retziella weberi Assemblage play an important role; its components, especially the index species, are widespread in Upper Silurian of Asia and Australia (Tong-Dzuy Thanh et al., 2001). In the West Bac Bo Zone, all Silurian deposits (upper part of the Sinh Vinh Formation, and the Bo Hieng Formation) are composed of carbonate deposits containing an abundant benthic fauna. The upper part of the Sinh Vinh Formation consists of dolomitic limestone and limestone containing Late Silurian corals, e.g. Favosites sp., F. aff. F. forsbesi, F. ex gr. gothlandica, F. cf. F. hisinger, F. cf. F. coreaniformis, Mesofavosites sp., Squameofavosites sp., and Parastriatopora sp. The Upper Silurian Bo Hieng Formation consists of marly shale, limestone and some banded argillaceous shale containing corals, brachiopods, and bivalves. Among the corals there are also tabulates, such as Palaeofavosites aff. P. balticus, Favosites kernihoensis, and the rugosan Aphyllum aff. A. sociale. Remarkable fossils in this formation are representatives of the Retziella weberi Assemblage, and bivalves Modiomorpha brevis, M. sp., Cymatonota sp. (aff. C. yunnanensis), Dysodonta sp., and Actinopteria sp. (Ta Hoa Phuong and Le Van Manh, 1996; Tong-Dzuy Thanh and Vu Khuc, 2006). Thus, carbonate sediments and abundant benthic fauna clearly indicate the shallow water shelf facies of the Silurian deposits in the West Bac Bo Zone. In the East Bac Bo Zone, the Upper Silurian is either lacking or failed to be identified in the sections, and the Lower Devonian Red Beds unconformably overly the older Silurian deposits.

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Deep-water facies in the Devonian deposits of Viet Nam are rarely observed, and most Devonian units belong to different kinds of shallow water facies.

conformably overlies the Upper Silurian Bo Hieng Formation. Both the Bo Hieng and Song Mua formations belong to the shallow water facies, although their sedimentary components differ. The former consists of carbonate-terrigenous deposits that yield a benthic fauna and belongs to the shallow water shelf facies. The latter, the thick Song Mua Formation, includes shale and sandstone bearing brachiopods in the upper part, and showing possible shallow water facies of offshore environment. Other Silurian-Devonian continuous sections have been recorded in the Tay Trang Formation in the Dien Bien-Nghe An Zone of the Viet-Lao Region. The lower part of this formation consists of thin-bedded siltstone, clay shale, and fine- to middle-grained sandstone containing deep-water Upper Silurian fossils, such as graptolites (Bohemograptus bohemicus, M. thomasi, M. yukonensis), and the dacryoconarids Nowakia cancellata and N. holynensis. The upper part of the formation mainly consists of siltstone, and sandstone containing Lower Devonian brachiopods and cephalopods that indicate a shallow water facies. Lower Devonian “Red Beds”: The lowermost part of the Lower Devonian in the East Bac Bo Zone of the Bac Bo Region and in the Binh Tri Thien Zone of the Viet-Lao Region begins by “Red Beds”, which unconformably overly the older sediments. In the Western areas of the East Bac Bo Zone (Fig. 11), the Si Ka Formation consists of conglomerate, greenish siltstone, clay shale, and marly shale, and chocolate marly shale containing vertebrate remains (Hoang Xuan Tinh, 2001). Upwards, the lower part of the Bac Bun Formation consists of grey, chocolate, variegated weathering clay shale and marly shale, siltstone containing abundant remains of vertebrates (see above, in the description of the Si Ka and Bac Bun Formations). The upper part of the Bac Bun Formation consists of shale, marly shale and limestone containing abundant shallow water benthos of the Howittia wangi Assemblage, such as Howittia wangi, Cyrtochonetes indosinensis, Parachonetes zeili (Racheboeuf and Tong-Dzuy Thanh, 2000). The lithological composition and fauna of the Si Ka and Bac Bun Formations (Song Cau Group) show that they probably belong to a transitional environment, ranging from lagoonal, coastal and deltaic facies to onshore shallow marine facies. It is noteworthy that in a small area, northwestward of the East Bac Bo Zone, where the Khao Loc Formation (D1-2 kl) extended, the lower Devonian is distinguished from sections of the Song Cau Formation described above. There, the “Red Beds” are absent, and the lower parts of the Khao Loc Formation include marl and marly shale containing vertebrates that can be correlated with those of the lowermost beds of the Devonian of Yunnan (China), such as Youngolepis praecursor, Polybranchiaspis liaojaoshanensis, Minicrania lissa, Laxaspis yulongsus, and Heteroyunnanolepis qujingensis (Tong-Dzuy Thanh et al., 1995; Janvier and Ta Hoa Phuong, 1999). Lower Devonian “Red Beds” are also present in the south of the Binh Tri Thien Zone (Viet-Lao Region), which unconformably overly the Upper Silurian Dai Giang Formation. Here, the A Choc Formation consists of gritstone, conglomerate and sandstone, siltstone of chocolate colour containing brachiopods (Lingula aff. L. loulaensis, L. cf. L. muongthensis, L. cf. L. cornea, L. aff. L. hawkei, Lingula sp). The lithology and faunal composition suggests that the A Choc Formation was deposited in a continental or brackish water environment (Tong-Dzuy Thanh and Vu Khuc, 2006).

3.2.1. Lower Devonian The lowermost Devonian sections are distributed into two kinds. The first one is represented by the sections that are continuous since the Upper Silurian, and the second one by the Devonian sections that are separated from older deposits by an unconformity. In the areas where the sections are continuous since the Upper Silurian, there is no sudden vertical change in the facies of deposition. In the west Bac Bo Zone, the Lower Devonian Song Mua Formation

3.2.2. Lower Devonian-Eifelian shallow water marine facies From the upper Lower Devonian to the Eifelian most of Viet Nam was covered by marine shallow water deposits, except in the uplift zones, such as the Red River uplift and the Kontum Block. Terrigenous deposits extend on the territory of the Quang Ninh and Binh Tri Thien Zones, whereas terrigenous-carbonates and carbonates occur in the other zones (Fig. 11).

3.1.4. Shallow water facies of Silurian deposits in the the Viet-Lao Region (Fig. 12) In the Viet-Lao Region, the shallow water facies is characteriztic for Upper Silurian deposits in the Binh Tri Thien Zone (Dai Giang Formation), but this facies is lacking in the Silurian of the Dien BienNghe An Zone (Fig. 12). The Dai Giang Formation (S3-4 dg) consists of gritstone and coarse-grained sandstone at its base, which unconformably overlies the Long Dai Formation. The main composition of the formation consists of terrigenous deposits, such as sandstone, and limestone, marl beds in its upper part. Its benthic fauna include trilobites (Cromus beaumonti, Metacalymene sp., Praedechenella aff. P. vietnamica, Encrinurus cf. E. sinicus), brachiopods of the Retziella weberi Assemblage, and bivalves (Schizodus? myducensis, Sphenotus antecedens, Modiomorpha paracrypta, Goniophora dianensi) (Nguyen Xuan Duong, 1996). In addition the Dai Giang Formation yielded placoderm (Myducosteus anmaensis) and acanthodian fish remains (Tong-Dzuy Thanh et al., 1997; Janvier and Tong-Dzuy Thanh, 1998). The composition of the sediments and the benthic fauna indicates that the Dai Giang Formation corresponds to a shallow water facies deposited in a littoral environment. 3.2. Facies of the Devonian deposits

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Fig. 12. Sketch of the facial profile of the Silurian and Devonian deposits of the Viet-Lao Region in the present situation.

3.2.2.1. Terrigenous deposits of shallow water marine facies.. In the Quang Ninh Zone (Fig. 11), the Duong Dong Formation (D1 -D2 e dd) includes mudstone, fine-grained quarzitic sandstone containing, e.g., brachiopods, bivalves of the Euryspirifer tonkinensis Assemblage. In the Binh Tri Thien Zone, both the Rao Chan (D1 rc) and Ban Giang (D1 -D2 e bg) formations mainly consist of shale and sandstone, carbonate components present in thin calcareous argillaceous shale interbeds or limestone lenses only. All the fossils recorded are benthic. For instance, those from the Rao Chan Formation are corals (Pachyfavosites cf. P. delicatus, Coenites cf. C. simakovi, and Heliolites cf. H. insolens, Chaetetes ninae. Dohmophyllum sp.), brachiopods (Desquamatia vijaica, Levenea aff. L. transversa) and the crinoid Lissocrinus curtus. The fossils from the Ban Giang Formation are more diverse, with brachiopods, trilobites, crinoids, and some corals in the thin limestone lense.

3.2.2.2. Terrigenous-carbonate and carbonate deposits of shallow water marine facies. Starting from the upper Lower Devonian, carbonate components predominate in the Devonian sections in the Bac Bo Region, and are increasing upwardly, becoming the main component of the Eifelian sections in most zones. Devonian carbonate components appear earliest in the northeastern area of the East Bac Bo Zone, from the lowermost beds of the Lower Devonian in the Khao Loc Formation, which in marly argillaceous shale yield vertebrate remains (see above). Upwards, from the Givetian level with corals of the Euryspirifer tonkinensis Assemblage, the Khao Loc Formation only consists of carbonate sediments.

The terrigenous-carbonate facies are widespread in upper Lower Devonian of the Bac Bo Region, in the Mia Le Formation of East Bac Bo, and the Ban Nguon and Nam Pia formations of West Bac Bo (Fig. 11). The faunas are abundant, and there are more than 300 species of corals, brachiopods, bivalves, trilobites, and crinoids belonging to the Euryspirifer tonkinensis Assemblage. This faunal community is the most abundant and diversified among the Devonian faunas in the Bac Bo Region and in Viet Nam as a whole. Judging from the lithological composition and faunal assemblage, the upper Lower Devonian terrigenous-carbonate deposits in the East Bac Bo, and West Bac Bo Zones were probably deposited in a shallow marine shelf environment. 3.2.2.3. Middle Devonian carbonate facies. The Middle Devonian carbonate deposits occur in almost all zones of the Bac Bo and Viet-Lao Regions. In the East Bac Bo and West Bac Bo Zones, they compose the Eifelian-Givetian parts of the Ban Pap Formation and the Khao Loc Formation, containing an abundant fauna of, e.g., stromatoporoids, corals, and some brachiopods. The same also occurs in the North of the Viet-Lao Region, i.e. in the Dien Bien-Nghe An Zone, where limestone is the main component of the Huoi Loi and Nam Can Formations, but here some terrigenous composition also occurs as thin interbeds. Limestone appears later as the unique component, in the Givetian of the Quang Ninh Zone (Trang Kenh Formation) and in the Binh Tri Thien Zone, in the Tan Lam Formation. Further northward in this zone, limestone appears even later, in the upper Givetian, as in the Muc Bai Formation (Fig. 12). The faunal community in the Eifelian formations mainly includes corals, such as, e.g., Favosites styriacus, F. stellaris, F.

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regularissimus, Emmonsia yenlacensis, Squameofavosites bohemicus, S. enormis, and S. vanchieni. The dominant taxa in the Givetian collections are the representatives of the Caliapora batersbyi Assemblage, such as, e.g., Amphipora ramosa, Actinostroma clatratum, Thamnopora polygonalis, T. nicholsoni, Caliapora battersbyi, Scoliopora denticulata, and Dendrostella trigemme, and the brachiopod Stringocephalus burtini. The carbonate composition and coral fauna demonstrate that the Devonian carbonate deposits in the Bac Bo and Viet-Lao Region represent a shallow water shelf facies. Exceptionally, in small areas of Dong Van (Ha Giang Province) are limestone, clayey limestone, marl, black shale and chert interbeds, containing an abundant pelagic fauna. In the lower beds are Lower Devonian fossils of the Nowakia acuaria and N. barrandei zones, conodonts of the Polygnathus excavatus, Pandorinellina steinhornensis, and Polygnathus. nothoperbonus zones, and still the corals Favosites regularissimus and Heliolites sp. In the upper beds, Givetian species have been found, such as, e.g., Palmatolepis varcus and others. The carbonate and siliceous composition and the pelagic fauna suggests that the “Si Phai Member” of the Ban Pap Formation was deposited in an environment corresponding to the slope of the continental shelf. Specific case of the Do Son and Tan Lap Formations (Fig. 11): The Do Son Formation mainly consists of conglomerate at its base, crossbedded sandstone, and shale interbeds that occur in the Quang Ninh Zone. The Tan Lap Formation is composed of conglomerate, arkosic gritstone, and arkosic sandstone interbedded with black siltstone that extend in small areas of the East Bac Bo Region (Bac Son area). Both the Do Son and Tan Lap formations contain continental plant remains (Lepidodendropsis), and in the Do Son Formation there still are bivalves, and vertebrates Vietnamaspis trii, Briagalepis sp., Asterolepis sp. (Janvier et al., 1989; Long et al., 1990; Nguyen Huu Hung et al., 1991, 2007; Janvier and Tong-Dzuy Thanh, 1998). The lithological composition and fauna indicate the continental and deltaic facies of the Do Son and Tan Lap formations. Both the Do Son and Tan Lap formations unconformably overly the older deposits, the Do Son Formation upon the Eifelian deposits of the Duong Dong Formation, and the Tan Lap Formation upon the Givetian limestone of the Ban Pap Formation. These coarse sediments of the Tan Lap and Do Son formations can be regarded as transgression molasses deposited after a local elevation; a similar situation occurred in South China (personal communication from Hou Hong-fei to TongDzuy Thanh). The case of the adjoining Muc Bai and Dong Tho Formations (Fig. 12): The Muc Bai and Dong Tho Formations contiguously extend in the the Binh Tri Thien Zone, the former mainly consisting of carbonate deposits, and the latter of coarse sandstone and argillaceous shale. Both formations contain a benthic fauna, but in the Muc Bai Formation, apart from brachiopods, there is a large number of coral colonies, while in the Dong Tho Formation there is only a shelly fauna (brachiopods, bryozoans, etc.) and plant remains (Lepidodendropsis sp., Protolepidodendron sp.) in some siltstone and shale interbeds. One can therefore suggest that these formations were deposited in a shallow water environment of the same basin, the Dong Tho Formation representing a coastal facies, and the Muc Bai Formation the onshore, shelf shallow water facies.

3.2.3. Facies of the Upper Devonian deposits In the Upper Devonian, the shelf shallow water facies are characterized by a large number of carbonate formations in the Bac Bo and Viet-Lao regions. In the Quang Ninh Zone (Figs. 11 and 12), one can notice the upper part of the Trang Kenh Formation, in the North of the Dien Bien-Nghe An Zone, the upper part of the Nam Can Formation, and in the Binh Tri Thien Zone, the upper part of the Tan Lam and Phong Son formations. At the same time, there appear carbonate-siliceous deposits containing conodonts which indicate

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a deep-water facies in some areas of both the Bac Bo and Viet-Lao Regions. The deep-water facies in the Bac Bo Region begin with the Givetian-Frasnian Bang Ca Formation, which consists of cherty shale, calcareous cherty shale clay shale, occurring in the Ha Lang area of the East Bac Bo Region and in the lower section of the Da River basin. The fauna consists of conodonts of the Palmatolepis hassi and P. rhenana Zones, and Polygnathus xylus xylus, P. varcus, P. linguiformis, and also the brachiopod Stringocephalus burtini. The Toc Tat Formation (D3 fm-C1 t tt) that overlies the Bang Ca Formation is also characterized by deep-water facies, which is evidenced by its composition, consisting of variegated limestone, cherty-clay shale, marl, argillaceous limestone, and frequent calcareous cherty shale interbeds. Fossils of this formation mainly include conodonts, such as Palmatolepis minuta, P. glabra, P. marginifera, P. gracilis, and P. sigmoidalis, but there are also bivalves, brachiopods, and foraminifera. A similar condition is found in the Binh Tri Thien Zone and the southern area of the Dien Bien-Nghe An Zone, where the Thien Nhan and the Xom Nha formations extend as small bands. The Thien Nhan Formation consists of argillaceous-siliceous shale, chert, fine-grained sandstone, and siltstone, in which were found conodonts (Ozarkodina (Spathognathodus) sp., Palmatolepis sp.) and brachiopods. Similar deposits occur in the Tuyen Hoa and Minh Hoa Districts, (Quang Binh Province), where occur black argillaceous shale, chert and siltstone, siliceous siltstone, thin manganese interbeds, which are used to characterize the Ngoc Lam Formation. Here have been found the conodont Palmatolepis subrecta and the brachiopod Calvinaria cf. C. megistanu. The Xom Nha Formation consists of dark grey, light grey stylolitic limestone containing abundant corals (Scoliopora denticulata, Thamnopora polyforata, Temnophyllum isetense), stromatoporoids (Stachyodes costulata, S. angulata) and conodonts (Palmatolepis gigas, P. triangularis, P. marginifera). Thus, in the above-mentioned Upper Devonian formations, the benthic fauna and conodonts coexist in the carbonate sediments. These carbonate formations were possibly deposited in a deep water environment on the slope of the continental shelf.

3.2.4. Facies of the Devonian deposits in the South Viet Nam Region Devonian deposits rarely occur in the South Viet Nam Region, except in isolated areas. Moreover, their stratigraphic relations with the underlying and overlying deposits are unknown; therefore, they are scarcely studied. Fossiliferous Devonian sediments only occur in a small area in the West of the Kon Tum Block, in the southern part of Central Viet Nam. There, the Cu Brei Formation (D1 cb) consists of terrigenous sediments in its lower part, with conglomerate, gritstone in the bottom, and carbonate deposits containing corals, stromatoporoids in its upper part (Tong-Dzuy Thanh et al., 2007). Perhaps the terrigenous-carbonate of the Cu Brei Formation belongs to an onshore, shallow water facies that is similar to the ones in Viet-Lao Region. The Hon Heo Formation in western Nam Bo Zone (Kien Giang Province) mainly consists of sandstone. It is supposedly referred to the Devonian and belongs to the subcontinental facies, based on the presence of plant remains referred to Psilophyton sp. and Taeniocrada? sp.

4. Remarks 4.1 Due to the extensive investigations carried out in the last decades, new achievements of the Silurian and Devonian stratigraphy of Viet Nam are well ascertained. All Silurian and Devonian units have been well described on the basis of the

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4.2

4.3

4.4

4.5

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study of the lithology, paleontology, facies of the sediments, and of their stratigraphic sequences. On the stratigraphic relations of the Silurian and Devonian units of Viet Nam - In all studied sections of Viet Nam there are no gaps or unconformities between the Ordovician and Silurian have been observed. The conformable relation is obviously expressed in the sequences of the Ordovician–Silurian formations, such as the Co To (O-S ct) and Tan Mai (O-S tm) formations in the Quang Ninh Zone, the Phu Ngu Formation (O-S pn) in the East Bac Bo Zone, the Sinh Vinh Formation (O-S sv) in the West Bac Bo Zone, and the Song Ca (O3 -S2 sc) and Long Dai (O2 -S2 ld) formations in the Viet-Lao Region. - In the East Bac Bo and the Quang Ninh zones of the Bac Bo Region, and in the Binh Tri Thien Zone of the Viet-Lao Region the lowermost units of the Devonian with the basal conglomerate are unconformable lying upon the Silurian units. While in the West Bac Bo Zone and in the Dien Bien-Nghe An Zone of the Viet-Lao Region the relation between Silurian and Devonian units seem to be conformable. - There are no gaps or unconformable relations are expressed in the sequence between the Devonian and Carboniferous units. In the most cases the Silurian deposits consist of the sandstone, shale and limestone containing benthos fauna, which confirm the marine shallow water environment of the sedimentation process of these units. The deep-water facies of the Silurian deposits is characterized for the Co To and Tan Mai formations in the Quang Ninh Zone, and the Phu Ngu Formation in the East Bac Bo Zone only. Perhaps the Co To and Tan Mai formations in the Quang Ninh Zone were taken shape, in a forearc sedimentary basin, and originated from a regenerated orogenic process (Nguyen Xuan Khien, 2000). This situation is similar to the Silurian deposits in the adjacent area of the Southeast China (Wang Hongzhen, 1985). By the lithological and paleontological compositions the Devonian deposits of the Bac Bo Region, especially of the East Bac Bo Zone, are similar to the ones of the Guangxi (South China). This similarity is obviously expressed in the Si Ka, Bac Bun and Mia Le formations of the Bac Bo Region and the Lianhuashan, Nakaoling and Yukiang formations, respectively, of the South China (Wu Yi et al., 1987; Yang Si-pu et al., 1981). Most Devonian units distributed in the North and the Central Viet Nam consist of self shallow water sediments, and apparently they were deposited in a passive marginal marine environment. The coarse clastic continental or subcontinental deposits are distributed only in some areas of the East Bac Bo and of the Quang Ninh zones of the Bac Bo Region, and in the south of the Binh Tri Thien Zone. This situation suggests the influence of the Caledonian movement at the end of the Silurian period that called the Guangxi movement in South China.

Acknowledgments The authors are grateful to the National Foundation for Sciences and Technology Development of Viet Nam (NAFOSTED) for the effective support to the Projects 105.01.79.09 and 105.06.60.09. We are thankful to Prof. A.J. Boucot (Oregon State University, USA) and Prof. Yu Chang-min (Institute of Palaeontology and Geology, Nanking. China) for their valuable criticisms and recommendations. Nguyen Thi Thu Cuc and Nguyen Thuy Duong would like to thank the TRIG A project of the Hanoi University of Science (Vietnam National University, Hanoi, Viet Nam) for the support to study mission at the University of Greifswald, Germany. We are very grateful to an anonymous reviewer for their valuable comments and suggestions, which helped us to improve the final version of

this manuscript. Interesting discussions of Prof. Tran Van Tri (Geological Association of Viet Nam) in the preparation of the paper are much appreciated.

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