1. Introduction
Graptolites have been widely documented from lower and middle Palaeozoic strata of Vietnam (Nguyen Van Phuc, Reference Van Phuc2002), including from the Lower, Middle and Upper Ordovician. However, until recently there has been limited illustration of this material in the literature. Museum collections in Vietnam are equally scant, with scarce assemblage data to support published identifications. Here we record graptolites with illustrations and museum repositories from NE Vietnam, from a region lying to the northeast of the Red River Fault Zone in an area that formed part of the South China plate during the early Palaeozoic (Tran Ngoc Nam, Reference Nam1995, and references therein). The graptolites characterize, for the first time, strata within the Than Sa Formation that are assignable to the Lower Ordovician Tremadocian and Floian stages. The graptolite assemblages are important in NE Vietnam: firstly to establish the regional stratigraphy of the middle Cambrian – Lower Ordovician Than Sa Formation (Tran Ngoc Nam, Reference Nam1995); secondly to provide biostratigraphical control for mapping in a region of complicated tectonics and tropical vegetation cover; and thirdly to offer correlation for the stratigraphy of NE Vietnam with South Chinese graptolite assemblages (e.g. Zhang & Chen, Reference Zhang and Chen2003) and those of central Vietnam (Nguyen Van Phuc, Reference Van Phuc2002). Targeted recovery of graptolites in this region of NE Vietnam, as part of ongoing studies by the Vietnam Institute of Geosciences and Mineral Resources, may provide an important resource for enhanced geological mapping, akin to the use of graptolites in exploring the structurally complex lower Palaeozoic terranes of Britain (Rushton, Stone & Hughes, Reference Rushton, Stone and Hughes1996).
2. Materials and repositories
Figured fossils are curated in the collection of the Geological Museum, no. 6, Pham Ngu Lao, Hanoi, Vietnam (numbers BT6/515 to BT20/515). They are from two localities in NE Vietnam, Dinh Ca and Ban Chau, both in Vo Nhai District, Thai Nguyen Province (Figs 1, 2). Graptolites occur in weathered laminated mudstones (Fig. 2) collected from exposure, and are preserved either as iron oxide, or in some instances with periderm remaining. In addition to specimens in Hanoi, a representative suite of graptolites from Dinh Ca is curated at the Oxford University Museum of Natural History, Oxford, UK.
Figure 1. Geographical location and geological context for graptolite localities at Dinh Ca and Ban Chau, Vo Nhai District, NE Vietnam. (a) Locus of collecting area in NE Vietnam; (b) geographical position of graptolite localities in Thai Nguyen Province; (c) geographical markers at Dinh Ca; (d) geographical markers at Ban Chau; (e) geological context of Dinh Ca graptolite sub-localities (from Geological and Mineral Resources Map of Vietnam 1:200,000 Lang Son Sheet F-48-XX111); (f) geological context of the Ban Chau trilobite and graptoloid locality (from Geological and Mineral Resources map of Vietnam 1:200,000 Tuyen Quang Sheet F-48-XXII).
Figure 2. (a, b) Graptolite-bearing horizons in the small roadside quarry to the south of the Suoi Lu bridge, Dinh Ca (top left, sub-locality ‘a’; and top right, sub-locality ‘b’, with ‘a’ in the background); (c) track section on the un-metalled road at Ban Chau; (d) laminated mudstones in the track; and (e) graptoloid-bearing horizon at Ban Chau.
3. Graptolite biostratigraphy in the Lower Ordovician Than Sa Formation
The lower Palaeozoic succession of NE Vietnam formed part of the marine sedimentary deposits of the South China plate (Tran Ngoc Nam, Reference Nam1995, and references therein), which lay in the palaeo-tropics during the early Palaeozoic (Cocks & Torsvik, Reference Cocks and Torsvik2013). The Than Sa Formation forms a major part of this succession, being c. 1200 m thick (Tran Ngoc Nam, Reference Nam1995; 1:200K Lang Son Sheet F-48-XX111) and succeeding the Mo Dong Formation, c. 400 m thick, from which the agnostoid Ptychagnostus atavus has been reported (Chernysheva in Tran Van Tri et al. Reference Van Tri, Tinh, Son and An1964): if this record is confirmed it signifies the mid-Cambrian Drumian Stage (see Rushton et al. Reference Rushton, Brück, Molyneux, Williams and Woodcock2011). The Than Sa Formation is succeeded by Middle Ordovician rocks of the Na Mo Formation, c. 400 m thick, that contain the calymenid trilobite Vietnamia douvillei (Mansuy, Reference Mansuy1908). Despite its great thickness and extensive regional crop, there are limited biostratigraphical data to constrain the age of the Than Sa Formation itself. Graptolites discovered from two new localities at Dinh Ca and Ban Chau (Figs 1, 2) provide important new data to assign strata within the upper part of the Than Sa Formation to the Lower Ordovician Tremadocian and Floian stages.
3.a. Dinh Ca
To the south of the small town of Dinh Ca, graptolites are sourced from laminated mudrocks in two sub-localities lying approximately along strike (by c. 15 m) in a small roadside quarry (at 21°44′11″N, 106°04′43″E) immediately south of the Suoi Lu Bridge (Figs 1, 2). This locality is on the western edge of Geological and Mineral Resources Map of Vietnam 1:200,000 Lang Son Sheet F-48-XX111. The graptolite assemblage from within the quarry (at sub-locality ‘a’ of Fig. 2) includes Tetragraptus approximatus Nicholson (Figs 3b, 4d), which is a distinctive component of pan-tropical Early Ordovician graptolite assemblages worldwide; its first appearance is a marker for the base of the Floian Stage of the Lower Ordovician (Cocks, Fortey & Rushton, Reference Cocks, Fortey and Rushton2010, p. 174; Fig. 5). Other Tetragraptus species present at sub-locality ‘a’ include T. acclinans Keble (Figs 3d, 4c), and a possible T. quadribrachiatus (J. Hall), together with Etagraptus cf. harti (T. S. Hall) (Fig. 3e), Expansograptus constrictus (J. Hall) (Figs 3a, 4b), E. similis (J. Hall) (Figs 3c, 4a) and possible ‘Didymograptus’ sinensis Lee & Chen (Fig. 4e–g). The biostratigraphical age suggested by the overlap of T. approximatus and E. constrictus is equivalent to the Australasian Bendigonian 1 (Be1) Stage (VandenBerg & Cooper, Reference VandenBerg and Cooper1992) or, in international terms, lower Floian (Fig. 5), but the level of overlap is above the earliest international occurrences of T. approximatus, and thus indicates a horizon immediately above the approximatus Biozone. This stratigraphical level is also suggested by the tentative identification of ‘Didymograptus’ sinensis, which occurs at the level of the filiformis Biozone in South China, the biozone immediately succeeding the approximatus Biozone in the Ningkuo Shale (Mu et al. Reference Mu, Ge, Chen, Ni and Lin1979, p. 140).
Figure 3. Photographic images of graptolites from sub-locality ‘a’ in the small quarry at Dinh Ca: (a) Expansograptus constrictus (J. Hall), BT8/515a; (b) Tetragraptus approximatus Nicholson, BT9/515a; (c) Expansograptus similis (J. Hall), BT10/515a; (d) Tetragraptus acclinans Keble, BT11/515a; (e) Etagraptus cf. harti (T. S. Hall), BT12/515a. Scale bars are 5 mm.
Figure 4. Camera lucida figures of graptolites from sub-locality ‘a’ in the small quarry at Dinh Ca: (a) Expansograptus similis (J. Hall), BT10/515a; (b) Expansograptus constrictus (J. Hall), BT8/515a; (c) Tetragraptus acclinans Keble, BT11/515a; (d) Tetragraptus approximatus Nicholson, BT9/515a; (e–g), ‘Didymograptus’ sinensis Lee & Chen?, BT13/515a – BT15/515a respectively. Scale bar is 5 mm.
Figure 5. Graptolite-based biozones and subdivisions for the Lower Ordovician of Britain, Australia and South China (Yangtze Platform), with the likely biostratigraphical level of collections at Ban Chau (Rhabdinopora?) and Dinh Ca (T. approximatus). Australasian stages from VandenBerg & Cooper (Reference VandenBerg and Cooper1992). Chinese biozonation is based on Zhang & Chen (Reference Zhang and Chen2003); for pre-H. copiosus graptolite biozonation see Mu et al. (Reference Mu, Ge, Chen, Ni and Lin1979).
The graptolite material from the roadside margin of the quarry (sub-locality ‘b’ of Fig. 2) has a more poorly preserved assemblage of Tetragraptus spp. and Expansograptus spp. that may represent the same biostratigraphical level.
3.b. Ban Chau
The small village of Ban Chau, Sang Moc Commune, is c. 30 km to the northwest of Dinh Ca (Fig. 1), and there a single graptoloid was collected from within the Than Sa Formation in the un-metalled road at 21°53′41″N, 105°58′44″E (Figs 1, 2). The locality is on the eastern edge of Geological and Mineral Resources map of Vietnam 1:200,000 Tuyen Quang Sheet F-48-XXII. The single graptoloid resembles Rhabdinopora (Fig. 6a) and was recovered from laminated mudstones that also contain poorly preserved possible agnostoids. Cleavage distortion and the paucity of the collection preclude specific identification of the graptoloid, and re-collecting has not recovered additional specimens. Rhabdinopora flabelliformis subspecies are characteristic of earlier Ordovician strata, and form a succession of assemblages ranging up through the earlier Tremadoc (Cooper et al. Reference Cooper, Maletz, Wang and Erdtmann1998), or the Cressagian Substage (Cocks, Fortey & Rushton, Reference Cocks, Fortey and Rushton2010). Fine stratigraphic discrimination is not possible at Ban Chau, but the presence of a possible Rhabdinopora is nonetheless important in that it suggests that these strata of the Than Sa Formation may be lower Tremadocian Stage in a succession where late Cambrian trilobites are already reported (Pham Kim Ngan & Tran Huu Dan, Reference Ngan and Dan2004). Some 60 m down succession from the graptoloid-bearing locality (at 21°53′41″N, 105°58′44″E) are strata within the Than Sa Formation that yield a trilobite cephalon suggestive of the late Cambrian genus Charchaqia Troedsson, Reference Troedsson1937 (Fig. 6b); Pham Kim Ngan & Tran Huu Dan (Reference Ngan and Dan2004) also reported Charchaqia from this area. This suggests that the succession in the road track at Ban Chau encompasses the local Cambrian–Ordovician boundary within the Than Sa Formation, and that the transition to laminated mudstones occurs possibly at a low level in the Tremadocian.
Figure 6. Fossils from the track section at Ban Chau: (a) a graptoloid rhabdosome thought to be a Rhabdinopora? preserved in discoid aspect, but deformed tectonically; it shows many stipes and a few dissepiments, BT7/515a; (b) silicone cast of external mould of flattened cephalon with two thoracic segments referred to Charchaqia? sp., BT6/515b. Scale bars are (a) 2 cm; (b) 0.5 cm.
4. Taxonomic notes on the graptolites
The following notes relate to graptolites from Dinh Ca, sub-locality ‘a’ (Figs 1, 2). The nomenclature follows that of Maletz (Reference Maletz2014).
4.a. Tetragraptus Salter
Tetragraptus approximatus Nicholson is represented by three definite examples (BT9/515, BT17/515, BT18/515) that show the typical H-shaped rhabdosome. The first two thecae form a ‘funicle’ c. 2.3 mm long, and each of them gives rise to an extended stipe. In the figured example (Figs 3b, 4d) the stipes are curved and diverge distally at a low angle, as in examples shown by Williams & Stevens (Reference Williams and Stevens1988, fig. 20V, GG), in contrast to the straight, parallel stipes shown by Cooper (Reference Cooper1979, pl. 6a). The stipes widen to c. 1.6 mm at the eighth theca. Thecae are of simple dichograptid form, numbering about nine or ten in 10 mm. This species is very widely distributed in graptolite-bearing deposits of earliest Floian age.
There is one good specimen of Tetragraptus acclinans Keble (Figs 3d, 4c), which is of similar form to T. approximatus but is distinguished by its outward-splayed stipes (Cooper, Reference Cooper1979, pl. 6a). The present specimen, though rather small, is distinguished from the similar form T. akzharensis Tzaj (Williams & Stevens, Reference Williams and Stevens1988, p. 36; Fortey, Reference Fortey2011, p. 224), by the greater width of its main stipes (c. 1.7 mm at 10 mm) and slightly more widely spaced thecae. T. acclinans is quite widely distributed (Williams & Stevens, Reference Williams and Stevens1988, p. 35) and occurs in the higher parts of the range of T. approximatus.
A small specimen of Tetragraptus (BT16/515), though poorly preserved, is provisionally referred to the stratigraphically long-ranging and widely distributed species T. quadribrachiatus (J. Hall).
4.b. Etagraptus Ruedemann
A graptolite of tetragraptid form is doubtfully referable to Etagraptus harti (T. S. Hall) on account of its narrow stipes, widening from 0.3 to 0.5 mm in 10 mm. The funicle is a little shorter than that of E. harti figured by Cooper & Fortey (Reference Cooper and Fortey1982, fig. 64); the preservation is poor and does not show any thecae clearly (Fig. 3e).
4.c. Expansograptus Bouček & Přibyl
Expansograptus constrictus (J. Hall) is represented by at least three specimens (BT8/515, BT19/515, BT20/515). The figured specimen has a sicula c. 2.3 mm long; the stipes are very slightly reclined near the sicula and are c. 1.7 mm wide at the first theca. Distally the stipes become horizontal and at theca 5 have expanded to a width of 2.3–2.5 mm, which is maintained (Figs 3a, 4b). There are nine thecae in 10 mm, thecae being inclined at c. 40°. This form agrees fairly well with the description by Williams & Stevens (Reference Williams and Stevens1988, p. 48).
Expansograptus similis (J. Hall) is similar to E. constrictus, but the sicula is a wide cone c. 1.7 mm long (Figs 3c, 4a). The stipes are narrower than E. constrictus, widening from 1.2 to 1.8 mm. There are ten thecae in 10 mm, inclined at c. 35°, thus inclined more steeply than in the description by Cooper & Fortey (Reference Cooper and Fortey1982, p. 238).
4.d. Didymograptus s.l.
Several small proximal ends of a slender declined didymograptid are poorly preserved, but appear to be comparable to ‘Didymograptus’ sinensis Lee & Chen (Fig. 4e–g). The sicula is 1.6–1.7 mm long. The two stipes are declined, making an angle of 135–150°; they are slender, c. 0.7–0.8 mm wide, and the thecae are rather closely spaced, having a two-theca repeat distance (2TRD) of 1.5 mm, equivalent to 13 thecae in 10 mm. Lee & Chen's (Reference Lee and Chen1962, pp. 24, 29, pl. 3, figs 12–16) species is based on good material, but the present material is too poorly preserved to be assigned to the same species with any certainty.
5. Taxonomic note on the trilobite from Ban Chau
One diagenetically flattened cephalon with two thoracic segments attached is comparable to Charchaqia Troedsson, because the glabella is nearly parallel-sided and rounded anteriorly, and shows very little sign of the glabellar and occipital furrows (Fig. 6b). The librigenae show that the palpebral lobe is small and the postocular suture is long, oblique and nearly straight; one librigena shows a fragmentary genal spine. The specimen appears to differ from the type species, C. norini Troedsson (Reference Troedsson1937, p. 48, pl. 6, figs 1–12), because the preocular sutures are more strongly divergent forwards, and in this respect it is more like Charchaqia lata Troedsson (Reference Troedsson1937, pl. 6, figs 13–16) or the cranidium attributed to C. norini, as figured by Peng (Reference Peng1984, pl. 3, fig. 8). Charchaqia is recorded from China in late Cambrian strata in Sinkiang, W Zhejiang and NW Hunan, associated with Hedinaspis and Lotagnostus punctatus. These records suggest a late Furongian level, near the top of the Jiangshanian Stage or the base of the highest, as yet unnamed, stage of the Cambrian, referred to as Stage 10.
6. Regional correlation and significance
Previous field analysis demonstrates that the Than Sa Formation is divisible into a lower ‘sub-formation’ of mudstones, siltstones and sandstones, and an upper ‘sub-formation’ of mudstones, siltstones and bedded limestone (1:200K Lang Son Sheet F-48-XX111, and Tuyen Quang Sheet F-48-XXII). These subdivisions likely do not illustrate the overall complexity of the Than Sa sedimentary succession, and further work is needed to resolve its lithological and sedimentological variety and the overall history of its genesis. The sedimentological complexity is evident from the recognition of laminated mudstones with graptolites that represent a previously unreported lithofacies from the Than Sa Formation. At Ban Chau the change from underlying siltstones to laminated mudstones is recognizable in the un-metalled road a few tens of metres down succession from the graptoloid-bearing locality at 21°53′41″N, 105°58′44″E (Fig. 2c–e), and a formational boundary might be defined at this level if it can be mapped regionally. At Dinh Ca the contact of graptolite-bearing mudstones with the underlying succession is obscured, so that the lower transition cannot yet be discriminated: at this locality underlying rocks of supposed late Cambrian age are siltstones, but these outcrop north of the Suoi Lu Bridge (Fig. 1) and there is no indication of rocks of intervening Tremadocian age, or of the lowermost Floian (equivalent to the T. approximatus Biozone).
Tran Ngoc Nam (Reference Nam1995) interpreted the lower Palaeozoic succession of NE Vietnam as indicative of deep basin sedimentation. The trilobites and graptolites identified within the Than Sa Formation are consistent with that interpretation, though the sedimentary succession as a whole suggests greater complexity. Tran Ngoc Nam (Reference Nam1995) also depicted deep-water facies in central Vietnam, accumulating at this time on the Indochina plate (also known as the Annamia plate) that may have been in close geographical proximity to South China during the Early Ordovician (Cocks & Torsvik, Reference Cocks and Torsvik2013). The Lower Ordovician (Tremadocian) graptolites ‘Dictyonema’ flabelliforme graptolithinum, Acanthograptus sinensis and Ptilograptus plumosus have previously been reported from the lower part of Que Tan section, Que Son District of central Vietnam (Nguyen Van Phuc, Reference Van Phuc2002), but the record from Vo Nhai District is the first for graptolites of approximately comparable age from NE Vietnam. The new graptolite ages therefore afford enhanced correlation of the lower Palaeozoic sedimentary successions between the Indochina and South China plates, and aid correlation with approximately comparably aged successions in South China (Zhang & Chen, Reference Zhang and Chen2003).
Acknowledgements
This research was supported by the Vietnamese cooperative project ‘Applied research [for] sequence stratigraphy methodology for the Middle Cambrian – Lower Ordovician sedimentary rocks in the Northeast part of Vietnam’ (project number TNMT.03.48 to Phong), a Grant-in-Aid from the Japan Society for the Promotion of Science (16K05593 to Komatsu), and a Leverhulme Trust International Network Grant (IN-2014-025 ‘Assembling the Early Palaeozoic terranes of Japan’ to Williams). We are grateful to the two reviewers for constructive comments on the manuscript, to Richard Fortey for comments on the trilobite from Ban Chau, and to Chris Stocker for making the silicone cast of the trilobite specimen.
