Early Ordovician (Tremadocian and Floian) Graptolites from the Than Sa Formation, Northeast Vietnam

Geol. Mag. 155 (7), 2018, pp. 1442–1448. c Cambridge University Press 2017 1442 doi:10.1017/S0016756817000310 Early Ordovician (Tremadocian and Floian) graptolites from the Than Sa Formation, northeast Vietnam

∗ ADRIAN RUSHTON , MARK WILLIAMS‡, NGUYEN DUC PHONG§†, TOSHIFUMI KOMATSU¶, DAVID SIVETER‡, JAN ZALASIEWICZ‡, DINH CONG TIEN§, NGUYEN VIET HIEN§, NGUYEN HUU MANH§ & GENGO TANAKA|| ∗ Department of Earth Sciences, The Natural History Museum, Cromwell Road, London SW7 5BD, UK ‡Department of Geology, University of Leicester, Leicester LE1 7RH, UK §Vietnam Institute of Geosciences and Mineral Resources (VIGMR), 67 Chien Thang, Van Quan, Ha Dong, Ha Noi, Vietnam ¶Faculty of Advanced Science and Technology, Kumamoto University, 2-39-1, Kurokami, Chuo-ku, Kumamoto 860-8555, Japan ||Center for Marine Environment Studies, Kumamoto University, Kamiamakusa City, Kumamoto 861-6102, Japan

(Received 11 October 2016; accepted 7 March 2017; ﬁrst published online 24 April 2017)

Abstract – The lower Palaeozoic marine succession of NE Vietnam accumulated on the South China plate. Despite historical works dating to French colonial times, the stratigraphy and palaeontology of the succession is poorly constrained. Chief amongst the lower Palaeozoic lithostratigraphical divi- sions is the Than Sa Formation, a c. 1200 m thick succession of clastic rocks of Cambrian and early Ordovician age. Newly collected graptolites (including Rhabdinopora?sp.andTetragraptus approximatus) from the upper part of the formation identify strata assignable to the Tremadocian and Floian stages of the Lower Ordovician. The same succession also probably records the Cambrian–Ordovician boundary. Our analysis identiﬁes one or more intervals of graptolite-bearing laminated mudstones in the upper part of the Than Sa Formation that may be widespread in NE Vietnam during the early Tremadocian. Keywords: Graptolites, Ordovician, Floian, Tremadocian, South China plate, Vietnam

1. Introduction tectonics and tropical vegetation cover; and thirdly to offer correlation for the stratigraphy of NE Viet- Graptolites have been widely documented from lower nam with South Chinese graptolite assemblages (e.g. and middle Palaeozoic strata of Vietnam (Nguyen Van Zhang & Chen, 2003) and those of central Vietnam Phuc, 2002), including from the Lower, Middle and (Nguyen Van Phuc, 2002). Targeted recovery of grap- Upper Ordovician. However, until recently there has tolites in this region of NE Vietnam, as part of on- been limited illustration of this material in the literat- going studies by the Vietnam Institute of Geosciences ure. Museum collections in Vietnam are equally scant, and Mineral Resources, may provide an important re- with scarce assemblage data to support published iden- source for enhanced geological mapping, akin to the tifications. Here we record graptolites with illustrations use of graptolites in exploring the structurally complex and museum repositories from NE Vietnam, from a lower Palaeozoic terranes of Britain (Rushton, Stone & region lying to the northeast of the Red River Fault Hughes, 1996). Zone in an area that formed part of the South China plate during the early Palaeozoic (Tran Ngoc Nam, 1995, and references therein). The graptolites char- 2. Materials and repositories acterize, for the first time, strata within the Than Sa Formation that are assignable to the Lower Ordovi- Figured fossils are curated in the collection of the cian Tremadocian and Floian stages. The graptolite as- Geological Museum, no. 6, Pham Ngu Lao, Hanoi, semblages are important in NE Vietnam: firstly to es- Vietnam (numbers BT6/515 to BT20/515). They are tablish the regional stratigraphy of the middle Cam- from two localities in NE Vietnam, Dinh Ca and Ban brian – Lower Ordovician Than Sa Formation (Tran Chau, both in Vo Nhai District, Thai Nguyen Province Ngoc Nam, 1995); secondly to provide biostratigraph- (Figs 1, 2). Graptolites occur in weathered laminated ical control for mapping in a region of complicated 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 †Author for correspondence: [email protected] Hanoi, a representative suite of graptolites from Dinh

Downloaded from https://www.cambridge.org/core. University of Athens, on 25 Sep 2021 at 02:14:05, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756817000310 Early Ordovician graptolites from Vietnam 1443

a b 105° 45’ E 106° 00’ E Bac Kan 22° 00’ N

China Dinh Hoa Ban Lang Son Chau

Vo Nhai Hanoi 21° 45’ N Phu Luong Dinh Ca ThaiThai NguyenNguyen Prov.Prov. Thai Nguyen Dai Tu Province Dong Hy Thai Nguyen 21° 30’ N Song Phu Binh Cong Vietnam 105° 30’ E Pho Yen Bac Giang Vinh Phuc 0 1000 km 10 km Ha Noi

c 1B d Khuoi Tinh Khuoi Xom Stream Dong Chan Dinh Ca Stream Trilobite Ban Chau Sub-locality b

Sub Thanh Tien -locality a Graptoloid Ban Truong

0 5 km 0 2 km Khuoi Tat Stream

e f C-P D C-O D Fig. 1d

C-O1 1B Fossil site C-O D

C-O1 C-O Fossil site D O2 Fig. 1c D D 0 10 km D 0 2 km

Legend

Fossil localities 1B Route number C Cambrian O1 Lower Ordovician

O2 Middle Ordovician D Devonian C Carboniferous P Permian

Figure 1. (Colour online) 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. (Colour online) (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.

Ca is curated at the Oxford University Museum of to constrain the age of the Than Sa Formation itself. Natural History, Oxford, UK. 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 3. Graptolite biostratigraphy in the Lower Sa Formation to the Lower Ordovician Tremadocian Ordovician Than Sa Formation and Floian stages. The lower Palaeozoic succession of NE Vietnam formed part of the marine sedimentary deposits of the 3.a. Dinh Ca South China plate (Tran Ngoc Nam, 1995, and references therein), which lay in the palaeo-tropics dur- To the south of the small town of Dinh Ca, grap- ing the early Palaeozoic (Cocks & Torsvik, 2013). The tolites are sourced from laminated mudrocks in two Than Sa Formation forms a major part of this succes- sub-localities lying approximately along strike (by c. sion, being c. 1200 m thick (Tran Ngoc Nam, 1995; 15 m) in a small roadside quarry (at 21° 44 11 N, 1:200K Lang Son Sheet F-48-XX111) and succeeding 106° 04 43 E) immediately south of the Suoi Lu the Mo Dong Formation, c. 400 m thick, from which Bridge (Figs 1, 2). This locality is on the western the agnostoid Ptychagnostus atavus has been reported edge of Geological and Mineral Resources Map of Vi- (Chernysheva in Tran Van Tri et al. 1964): if this re- etnam 1:200,000 Lang Son Sheet F-48-XX111. The cord is confirmed it signifies the mid-Cambrian Dru- graptolite assemblage from within the quarry (at sub- mian Stage (see Rushton et al. 2011). The Than Sa locality ‘a’ of Fig. 2) includes Tetragraptus approx- Formation is succeeded by Middle Ordovician rocks imatus Nicholson (Figs 3b, 4d), which is a distinct- of the Na Mo Formation, c. 400 m thick, that contain ive component of pan-tropical Early Ordovician grap- the calymenid trilobite Vietnamia douvillei (Mansuy, tolite assemblages worldwide; its first appearance is 1908). Despite its great thickness and extensive re- a marker for the base of the Floian Stage of the gional crop, there are limited biostratigraphical data Lower Ordovician (Cocks, Fortey & Rushton, 2010,

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) Tetra- graptus acclinans Keble, BT11/515a; (e) Etagraptus cf. harti (T. S. Hall), BT12/515a. Scale bars are 5 mm.

p. 174; Fig. 5). Other Tetragraptus species present at 21° 53 41 N, 105° 58 44 E(Figs 1, 2). The locality sub-locality ‘a’ include T. acclinans Keble (Figs 3d, is on the eastern edge of Geological and Mineral 4c), and a possible T. quadribrachiatus (J. Hall), to- Resources map of Vietnam 1:200,000 Tuyen Quang gether with Etagraptus cf. harti (T. S. Hall) (Fig. 3e), Sheet F-48-XXII. The single graptoloid resembles Expansograptus constrictus (J. Hall) (Figs 3a, 4b), Rhabdinopora (Fig. 6a) and was recovered from lamin- E. similis (J. Hall) (Figs 3c, 4a) and possible ‘Didy- ated mudstones that also contain poorly preserved pos- mograptus’ sinensis Lee & Chen (Fig. 4e–g). The sible agnostoids. Cleavage distortion and the paucity biostratigraphical age suggested by the overlap of of the collection preclude specific identification of the T. approximatus and E. constrictus is equivalent to graptoloid, and re-collecting has not recovered addi- the Australasian Bendigonian 1 (Be1) Stage (Vanden- tional specimens. Rhabdinopora flabelliformis subspe- Berg & Cooper, 1992) or, in international terms, lower cies are characteristic of earlier Ordovician strata, and Floian (Fig. 5), but the level of overlap is above the form a succession of assemblages ranging up through earliest international occurrences of T. approximatus, the earlier Tremadoc (Cooper et al. 1998), or the Cres- and thus indicates a horizon immediately above the ap- sagian Substage (Cocks, Fortey & Rushton, 2010). proximatus Biozone. This stratigraphical level is also Fine stratigraphic discrimination is not possible at suggested by the tentative identification of ‘Didymo- Ban Chau, but the presence of a possible Rhabdino- graptus’ sinensis, which occurs at the level of the pora is nonetheless important in that it suggests that filiformis Biozone in South China, the biozone imme- these strata of the Than Sa Formation may be lower diately succeeding the approximatus Biozone in the Tremadocian Stage in a succession where late Cam- Ningkuo Shale (Mu et al. 1979, p. 140). brian trilobites are already reported (Pham Kim Ngan The graptolite material from the roadside margin & Tran Huu Dan, 2004). Some 60 m down succession of the quarry (sub-locality ‘b’ of Fig. 2) has a more from the graptoloid-bearing locality (at 21° 53 41 N, poorly preserved assemblage of Tetragraptus spp. and 105° 58 44 E) are strata within the Than Sa Form- Expansograptus spp. that may represent the same ation that yield a trilobite cephalon suggestive of biostratigraphical level. the late Cambrian genus Charchaqia Troedsson, 1937 (Fig. 6b); Pham Kim Ngan & Tran Huu Dan (2004) also reported Charchaqia from this area. This suggests 3.b. Ban Chau that the succession in the road track at Ban Chau en- The small village of Ban Chau, Sang Moc Commune, compasses the local Cambrian–Ordovician boundary is c. 30 km to the northwest of Dinh Ca (Fig. 1), and within the Than Sa Formation, and that the transition there a single graptoloid was collected from within to laminated mudstones occurs possibly at a low level the Than Sa Formation in the un-metalled road at in the Tremadocian.

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 (1988, fig. 20V, GG), in contrast to the straight, parallel stipes shown by Cooper (1979, 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, 1979, pl. 6a). The present specimen, though rather small, is distinguished from the similar form T. akzharensis Tzaj (Williams & Stevens, 1988, p. 36; Fortey, 2011, 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, 1988, 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). Figure 4. Camera lucida figures of graptolites from sub-locality ‘a’inthesmallquarryatDinhCa:(a)Expansograptus similis (J. Hall), BT10/515a; (b) Expansograptus constrictus (J. Hall), 4.b. Etagraptus Ruedemann BT8/515a; (c) Tetragraptus acclinans Keble, BT11/515a; A graptolite of tetragraptid form is doubtfully referable (d) Tetragraptus approximatus Nicholson, BT9/515a; (e–g), to Etagraptus harti (T. S. Hall) on account of its nar- ‘Didymograptus’ sinensis Lee & Chen?, BT13/515a – BT15/515a respectively. Scale bar is 5 mm. row 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 (1982, fig. 64); the pre- 4. Taxonomic notes on the graptolites servation is poor and does not show any thecae clearly The following notes relate to graptolites from Dinh Ca, (Fig. 3e). sub-locality ‘a’ (Figs 1, 2). The nomenclature follows that of Maletz (2014). 4.c. Expansograptus Boucekˇ & Pribylˇ Expansograptus constrictus (J. Hall) is represented 4.a. Tetragraptus Salter by at least three specimens (BT8/515, BT19/515, Tetragraptus approximatus Nicholson is represen- BT20/515). The figured specimen has a sicula ted by three definite examples (BT9/515, BT17/515, c. 2.3 mm long; the stipes are very slightly reclined

Figure 5. Graptolite-based biozones and subdivisions for the Lower Ordovician of Britain, Australia and South China (Yangtze Plat- form), with the likely biostratigraphical level of collections at Ban Chau (Rhabdinopora?) and Dinh Ca (T. approximatus). Australasian stages from VandenBerg & Cooper (1992). Chinese biozonation is based on Zhang & Chen (2003); for pre-H. copiosus graptolite biozonation see Mu et al.(1979).

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 (1937, 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 (1937, pl. 6, figs 13–16) or the cranidium attributed to C. norini, as figured by Peng (1984, 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 Figure 6. Fossils from the track section at Ban Chau: (a) a grap- records suggest a late Furongian level, near the top toloid rhabdosome thought to be a Rhabdinopora?preservedin discoid aspect, but deformed tectonically; it shows many stipes of the Jiangshanian Stage or the base of the highest, and a few dissepiments, BT7/515a; (b) silicone cast of external as yet unnamed, stage of the Cambrian, referred to as mould of flattened cephalon with two thoracic segments re- Stage 10. ferred to Charchaqia? sp., BT6/515b. Scale bars are (a) 2 cm; (b) 0.5 cm. 6. Regional correlation and significance Previous field analysis demonstrates that the Than Sa Formation is divisible into a lower ‘sub-formation’ of near the sicula and are c. 1.7 mm wide at the first theca. mudstones, siltstones and sandstones, and an upper Distally the stipes become horizontal and at theca 5 ‘sub-formation’ of mudstones, siltstones and bedded have expanded to a width of 2.3–2.5 mm, which is limestone (1:200K Lang Son Sheet F-48-XX111, and maintained (Figs 3a, 4b). There are nine thecae in Tuyen Quang Sheet F-48-XXII). These subdivisions 10 mm, thecae being inclined at c. 40°. This form likely do not illustrate the overall complexity of the agrees fairly well with the description by Williams & Than Sa sedimentary succession, and further work is Stevens (1988, p. 48). needed to resolve its lithological and sedimentological Expansograptus similis (J. Hall) is similar to E. con- variety and the overall history of its genesis. The sed- strictus, but the sicula is a wide cone c. 1.7 mm long imentological complexity is evident from the recog- (Figs 3c, 4a). The stipes are narrower than E. constric- nition of laminated mudstones with graptolites that tus, widening from 1.2 to 1.8 mm. There are ten thecae represent a previously unreported lithofacies from the in 10 mm, inclined at c. 35°, thus inclined more steeply Than Sa Formation. At Ban Chau the change from un- than in the description by Cooper & Fortey (1982, derlying siltstones to laminated mudstones is recog- p. 238). nizable in the un-metalled road a few tens of metres down succession from the graptoloid-bearing locality at 21° 53 41N, 105° 58 44E(Fig. 2c–e), and a formational boundary might be defined at this level if it can be mapped regionally. At Dinh Ca the con- 4.d. Didymograptus s.l. tact of graptolite-bearing mudstones with the un- Several small proximal ends of a slender declined derlying succession is obscured, so that the lower didymograptid are poorly preserved, but appear to be transition cannot yet be discriminated: at this loc- comparable to ‘Didymograptus’ sinensis Lee & Chen ality underlying rocks of supposed late Cambrian (Fig. 4e–g). The sicula is 1.6–1.7 mm long. The two age are siltstones, but these outcrop north of the stipes are declined, making an angle of 135–150°; they Suoi Lu Bridge (Fig. 1) and there is no indication are slender, c. 0.7–0.8 mm wide, and the thecae are of rocks of intervening Tremadocian age, or of the rather closely spaced, having a two-theca repeat dis- lowermost Floian (equivalent to the T. approximatus tance (2TRD) of 1.5 mm, equivalent to 13 thecae in Biozone). 10 mm. Lee & Chen’s (1962, pp. 24, 29, pl. 3, figs 12– Tran Ngoc Nam (1995) interpreted the lower Pa- 16) species is based on good material, but the present laeozoic succession of NE Vietnam as indicative of material is too poorly preserved to be assigned to the deep basin sedimentation. The trilobites and grap- same species with any certainty. tolites identified within the Than Sa Formation are

consistent with that interpretation, though the sed- Lee,C.K.&Chen, X. 1962. Cambrian and Ordovician imentary succession as a whole suggests greater graptolites from Sandu, S. Gueizhou (Kueichou). Acta complexity. Tran Ngoc Nam (1995) also depicted Palaeontologica Sinica 10, 12–35, pls 1–3 (in Chinese deep-water facies in central Vietnam, accumulating at with English summary). Maletz, J. 2014. The classification of the Pterobran- this time on the Indochina plate (also known as the chia (Cephalodiscida and Graptolithina). Bulletin of Annamia plate) that may have been in close geograph- Geosciences 89, 477–540. ical proximity to South China during the Early Ordovi- Mansuy, H. 1908. Contribution à la Carte Géologique cian (Cocks & Torsvik, 2013). The Lower Ordovician de l’Indo-Chine: Paléontologie. 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Viet- relation of the lower Palaeozoic sedimentary succes- nam National University Journal of Science, Natural sions between the Indochina and South China plates, Sciences and Technology 18, 38–50. Peng, S.-C. 1984. Cambrian–Ordovician boundary in and aid correlation with approximately comparably the Cili–Taoyuan border area, north-western Hunan aged successions in South China (Zhang & Chen, with descriptions of relative trilobites. In Stratigraphy 2003). and Palaeontology of Systemic Boundaries in China. Cambrian–Ordovician Boundary (1), 285–405, 18 pls. Acknowledgements. This research was supported by the Nanjing Institute of Geology and Palaeontology, Aca- Vietnamese cooperative project ‘Applied research [for] se- demia Sinica. Anhui Science and Technology Publish- quence stratigraphy methodology for the Middle Cambrian – ing House. Lower Ordovician sedimentary rocks in the Northeast part of Pham Kim Ngan &Tran Huu Dan. 2004. 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