New discovery of late early ordovician graptolites and discussion on stratigraphic sequence of paleozoic sediments in the Dinh Ca - Trang Xa Section, Vo Nhai Disctrict, Thai Nguyen Province, Northeast Vietnam

Vietnam Journal of Earth Sciences, 39(1), 76-86, DOI: 10.15625/0866-7187/39/1/9229 76 (VAST) Vietnam Academy of Science and Technology Vietnam Journal of Earth Sciences New discovery of late early ordovician graptolites and dis- cussion on stratigraphic sequence of paleozoic sediments in the Dinh Ca - Trang Xa Section, Vo Nhai Disctrict, Thai Nguyen Province, Northeast Vietnam Nguyen Huu Hung*, Nguyen Trung Minh, Doan Dinh Hung, Nguyen Ba Hung Vietnam National Museum of Nature, Vietnam Academy of Science and Technology Received 19 July 2016. Accepted 9 February 2017 ABSTRACT Sandstone, quartzitic sandstone rhythmically interbedded with siltstone, calcareous siltstone and cherty shale are largely exposed in the Dinh Ca - Trang Xa area, Vo Nhai District, Thai Nguyen Province, NE Vietnam. In previous studies, they were described as Mo Dong or Than Sa Formations (Middle-Upper Cambrian). The graptolite faunas newly discovered from the Dinh Ca Market - Na Mo - Dong Bo cross section are composed of Expansograptus urba- nus (Monsen), Expansograptus abnormis (Hsu), Tetragraptus quadribrachiatus (Hall), T. approximatus Nicholson, T. pacificus Ruedemann, T. decipiens T.S. Hall, Schizograptus sp., belonging to the quadribrachiatus Biozone, indi- cating Floian Stage or late Early Ordovician. This serves as the basis for age determination and for establishing new lithostratigraphic units termed as the Dinh Ca Formation (O1 dc) and the Ba Nhat Member. The stratigraphic se- quence of Paleozoic sediments in the Dinh Ca - Trang Xa area is also discussed in this paper. Keywords: Graptolite faunas, late Early Ordovician, Dinh Ca Formation, Ba Nhat Member, Thai Nguyen, Vietnam. ©2017 Vietnam Academy of Science and Technology 1. Introduction1 The location of graptolite Ordovic fauna collection and study of stratigraphic sequence including Ordovician, Devonian, and Carbon- iferous-Permian is shown in Figure 1. The ar- ea consists of low hilly and mountainous are- as, ranging in elevation from 50 - 500m, and comprising mainly terrigenous, carbonate and cherty shale rocks. They are largely exposed *Corresponding author, Email: hungcsdt@yahoo.com around the Dinh Ca Town, and in the Tien Phong, Phu Thuong, Ba Nhat, Tan Thanh, Na Mo, Dong Bo, Mo Dinh, Dong En, Lang Den, Trang Xa villages of Vo Nhai District, Thai Nguyen Province, about 30 km northeast of Thai Nguyen City, and 100 km from Ha Noi. Geographically, the study area is in the north- eastern region of Viet Nam, belongs to the Song Hien Structure-facial Zone (Dovjikov et al., 1965), the Bac Son Miogeosynclinal Fold Zone (Le Duy Bach, 1985); or the Early Paleozoic Intracontinental Orogenic Belt of Nguyen Huu Hung, et al./Vietnam Journal of Earth Sciences 39 (2017) 77 Northeast Viet Nam (Tran Van Tri, Vu Khuc (Eds.) 2011). In this study, the new findings of Ordovi- cian graptolite faunas are used to propose a new stratigraphic nomenclature for the Ordo- vician succession. The stratigraphic sequence of Paleozoic sediments in the Dinh Ca-Trang Xa area are also discussed here. Figure 1. Sketch map showing outcrops of Paleozoic sedimens in the Dinh Ca - Trang Xa area 2. Geological setting The Dinh Ca - Trang Xa area is chiefly Paleozoic sediments, which were mapped by Doan Ky Thuy et al. (1976, 2000) at 1:200.000 scale of the Lang Son Geological Sheet (Figure 2), and divided into the follow- ing lithostratigraphic units (Doan Ky Thuy (Ed.) 1976, 2000): The Mo Dong Formation (ε2 md) composed of quartzite, quartzitic sandstone, siltstone, clay shale, 300 m thick, is considered as the lowermost formation of Cambrian sediments in the Dinh Ca - Trang Xa area. The Than Sa Formation (ε3 ts) is composed of two sub-formations, with the Lower Sub- formation (ε3 ts1) comprising clay shale, thin beds of polymictic sandstone, 500 m thick, and the Upper Subformation (ε3 ts2) contain- ing polymictic sandstone, grey clayish shale, 350 m thick. The formation lies conformably on the Mo Dong Formation (ε2 md), and has a tectonic contact with the Song Cau Group (D1 sc) and the Mia Le Formation (D1 ml). The Na Mo Formation (O nm) composed of sandstone, quartzitic sandstone, clay shale, and siltstone yielding trilobite Ectenoglossa Vietnam Journal of Earth Sciences, 39(1), 76-86 78 sp., Lochodomas aff. yohi, Ceraurinus sp., 200-225 m thick overlies conformably the Than Sa Formation (ε3 ts), and shows tectonic contact with the Song Cau Group (D1 sc). Figure 2. The Dinh Ca - Trang Xa area shown in Lang Son geological and mineral resources Sheet map at 1:200,000 scale (Doan Ky Thuy et al., 2000) Nguyen Huu Hung, et al./Vietnam Journal of Earth Sciences 39 (2017) 79 The Song Cau Group (D1 sc) comprising basal conglomerate, gritstone, sandstone, silt- stone, marlaceous shale bearing fish Youngo- lepis cf. praecusor, Yungolepis sp., 370 m thick has a tectonic contact with the Than Sa Formation (ε3 ts), Na Mo Formation (O nm), Bac Son Formation (C-P bs), and underlies conformity the Mia Le Formation (D1ml). The Mia Le Formation (D1 ml) composed of marl, siltstone, and limestone containing brachiopod Euryspirifer tonkinensis, tabulate Favosites goldfussi, bivalve Goniopora cf. spatulata, 150-300 m thick overlies conform- ably the Song Cau Group (D1 sc), and shows tectonic contact with the Than Sa Formation (ε3 ts). The Bac Son Formation (C-P bs) is com- posed of massive limestone, oolitic limestone, clayish limestone, yielding foramiferan Ne- oschwagerina margaritae, Fusulinella bocki, 550 m thick. It has a tectonic contact with the Song Cau Group (D1 sc). 3. Materials and methods Five hundred paleontological samples were collected from this area. Among these two hundred specimens are graptolite fossils, some were determined by Xu Chen (Laboratory of Palaeontology and Stratigraphy, Nanjing Ins- titute of Geology and Palaeontology, Chinese Academy of Sciences; 39 East Beijing Road, Nanjing 210008, China). The brachiopod, po- riferan, cnidarian, mollusc, echinoderm, bryo- zoan and chordata (fish fossils) data cited in this study were determined by Kobayashi T. (1960), Sokolov B. S. (1965); Tong Duy Thanh (1967), Tong Duy Thanh, Janvier Ph. (1994), Duong Xuan Hao, Pham Kim Ngan, Luong Hong Huoc, Dang Tran Huyen (1980), Le Hung (1977, 1980), Nguyen Van Liem (1978), Nguyen Huu Hung (1982) and this study. The section bearing graptolite faunas of late Early Ordovician is exposed along the road, in south-southwestern direction from Dinh Ca Market to Dong Bo-Na Mo cross section (Figure 1 and Figure 4). In general, this section reveals an anticlinorium structure, which consists mainly of sandstone, siltstone, calcareous shale and siliceous shale rhythmically intercalated with thick-bedded sandstone. In the previous studies, this strati- graphic sequence was confusingly described by Dovjikov et al. (1965) as of the Mo Dong Formation (ε3 md) and showed on the North Vietnam geological map at 1:500,000 scale; or listed to the Than Sa Formation (ε3 ts) and shown on the Lang Son geological map at 1:200,000 scale (F-48-XXIII) by Doan Ky Thuy et al. (1976, 2000) . Member 1: Containing medium-grained sandstone, light-colored, 4-5 cm bedded; yielding some crinoid stems, not well preservation, interbeds are light-colored siltstone, and black-grayish shale, thin-bedded yielded graptolite faunas. The total thickness is about 150m Member 2: 50 m thick. The medium- grained sandstone, greenish-grey, thick- bedded; no fossils found. Member 3: Including greenish-grey micaceous siltstone, with interbeds of brownish shale; no fossils found. The total thickness is about 150 m Member 4: Being named as Ba Nhat Member (new name) for the sediments yield- ing graptolite faunas. The base is characterized by fine-grained sandstone, 1-2 cm bedded. Directly overlying is grey siliceous shale intercalated with light-colored silstone, and black-grey calcareous shale, 1-2 mm bedded with abundant graptolite fau- nas (Figure 3). This member is isolated with Vietnam Journal of Earth Sciences, 39(1), 76-86 80 Member 3 by minor fault. The total thickness is about 20 m. Member 5: Comprising brownish quartzitic sandstone, thick-bedded of the total thickness about 50 m, no fossils found. Member 6: Including grey-greenish silt- stone, thin-bedded with the total thickness about 8 m, no fossils found. Member 7: Containing brownish sand- stone, medium-bedded, having a thickness of about 10 m; no fossils found. Member 8: Containing greenish-grey silt- stone, thin-bedded with the total thickness about 8 m no fossils found. Member 9: Comprising brownish sand- stone, thick-bedded with the total thickness about 80 m, no fossils found. Member 10: Including white-grey, greenish- grey siltstone, thin-bedded, intercalated with fine-grained sandstone with the tsotal thickness about 60 m, no fossils found. Figure 3. A. Outcrop containing late Early Ordovician graptolite faunas at the Ba Nhat Village; B. Light-colored silt- stone including graptolite faunas Tetragraptus quadribrachiatus Biozone In summary, 200 grapptolite specimens were collected in the Ba Nhat Member. Some of the samples have recently been identified by Xu Chen as Expansograptus urbanus (Monsen) (Figure 5A), E. abnormis (Hsu) (Figure 5B), Tetragraptus quadribrachiatus (Hall) (Figure 5D-C), Tetragraptus. approxi- matus Nicholson (Figure 5E-F), Tetragraptus. pacificus Ruedemann (Figure 6A-C), T. de- cipiens T.S. Hall (Figure 6D), Schizograptus sp. (Figure 6E), belonging to the quad- ribrachiatus Biozone. This graptolite biozone is completely disappeared at the top of the Ba Nhat Member. Figure 4. The Dinh Ca Market - Na Mo - Dong Bo cross section Nguyen Huu Hung, et al./Vietnam Journal of Earth Sciences 39 (2017) 81 Figure 5. A. Expansograptus urbanus (Monsen), specimen no.CS155.3.1; B. Expansograptusabnormis (Hsu), spe- cimen no.CS155.3.9; C-D. Tetragraptus quadribrachiatus (Hall) C-specimen no.CS155.3.2, D-specimen no.CS155.3.10.; E-F. Tetragraptus approximatus (Nicholson); E-specimen no.CS155.3.3; F-specimen no.CS155.2.2.The all specimens are housed in the Vietnam National Museum of Nature, Vietnam Academy of Sci- ence and Technology, 18 Hoang Quoc Viet Street, Cau Giay District, Ha Noi Capital Vietnam Journal of Earth Sciences, 39(1), 76-86 82 Figure 6. A-C. Tetragraptus pacificus Ruedemann; A - specimen no.CS155.2.3; B-specimen no.CS155.2.4; C - specimen no.CS155.2.5; D. Teragraptus decipiens T.S. Hall, specimen no.CS155.1.1; E. Schizograptus sp., spec- imen no. CS155.2.6. The specimens are housed at the Vietnam National Museum of Nature, VAST, 18 Hoang Quoc Viet Street, Cau Giay District, Ha Noi 4. Stratigraphic sequence in the Dinh Ca- Trang Xa area The stratigraphic sequence of Paleozoic sediments in the Dinh Ca - Trang Xa area is shown in Figure 8. 4.1. Late Early Ordovician sediments in the Dinh Ca Formation (newly defined) The late Early Ordovician graptolite faunas newly found from the Dinh Ca - Trang Xa stratigraphic section have provided im- portant information relative to the Ordovician graptolite faunas and regional stratigraphy of the Lower Paleozoic sediments in Viet Nam. The stratigrapical and biogeoraphical signifi- cances of graptolite biozone are described as follows: - Tetragraptus quadribrachiatus (Hall) is geographically widespread in the world. It Nguyen Huu Hung, et al./Vietnam Journal of Earth Sciences 39 (2017) 83 was reported in the works of Hall (1858), Elles and Wood (1901), Ruedemann (1904, 1947), Berry (1960), Cooper (1979), Cooper and Fortey (1982), Tsai (1974), and Carter (1988). It was found in Alaska, Western Can- ada, New Zealand, Spitsbergen and Soviet Union. Recently in the Ordovician bi- ochemostratigraphic chart (International Sub- comission on Ordovician Stratigraphy, 2009) (Bergstöm 2008, Xu Chen et al. 2009), the graptolite Tetragraptus approximatus Zone is considered as the base of stage slice Fl1. Above it is the conodont Oepikodus evae Zone, belonging to Fl2. This interval corre- sponds to the lower part of Floian Stage or late Early Ordovician. - Tetragraptus approximatus Nicholson is considered as the global graptolite Tetragrap- tus approximatus Biozone, that is the base for the global Florian Stage (Bergstöm, 2008). In Southwest Sweden, Tetragraptus approxi- matus Nicholson appeared at the lower half of the Second Stage of Lower Ordovician, in- cluding Tetragraptus phyllograptoides, Did- ymograptus balticus (Bergstöm, 2004). - Expansograptus abnormis (Hsu) appears at the base of the Hentang Member corre- sponding to the late Early Ordovician, and completely disappears at the Upper Member of the Ningkuo Formation, corresponding to the Middle Ordovician in Zhejiang Province, South China. The other forms are Tetragrap- tus pacificus Ruedemann, T. decipiens T.S. Hall, Expansograptus urbanus (Monsen), Schizograptus sp., generally known in the Lower-Middle Ordovician stratigraphy of North America, Europe and China. In sum- mary, most of the graptolite faunas from Ba Nhất Member showed closest relationships to the Floian (late Early Ordovician) graptolite species in the world. At present, there are a number of fossil-free gaps (or not yet found) at the section of Dinh Ca Formation; but hopefully the fossils will be found in both the lower and upper parts so that age of the Dinh Ca Formation will be completely defined. The sediments yielded late Early Ordovi- can graptolites cited above may not be viewed as Upper Cambrian or of the Mo Dong For- mation (ε3 md) as in the geologic map of northern Vietnam at 1:500,000 scale by Dovjikov et al. or the Than Sa Formation (ε3ts) as in the Lang Son geologic sheet at 1:200,000 by Doan Ky Thuy et al. (1976, 2000). Therefore, we propose new lithostrati- graphic units for terrigenous sediments of the Lower Paleozoic stratigraphy. The Dinh Ca Formation (newly named) named after Dinh Ca Town, and the Ba Nhat Member (newly named) named after Ba Nhat Village, where graptolite faunas found. They are character- ized by rhythmic sediments, including sand- stone with intercalations of siltstone, cherty shale, and calcareous shale yielded graptolite faunas of late Early Ordovician age. The strat- igraphic section from the Dinh Ca Market (21o45’08’’N, 106o04’42’’E) to the Dong Bo-Na Mo cross section (21o43’47’’N, 106o04’18’’E) is chosen as the stratotype for the Dinh Ca Formation (Figure 4). The Ba Nhat Member is characterized by thin beds of sandstone, siltstone intercalated with siliceous shale and calcareous shale yielding abundant graptolite faunas of late Early Ordovician age. In the Song Hien Structure-facial Zone of Dovjikov et al. (1965) in Northeast Viet Nam, the Dinh Ca Formation (O1 dc) is distin- guished from the Than Sa Formation (ε3 ts), which was established by Tran Van Tri et al. (1964) in Than Sa area, Vo Nhai District, Thai Nguyen Province by the presence of graptolite biozones of late Early Ordovician age. It is also distinguished from the Phu Ngu For- mation of Late Ordovician-Early Silurian age, which was established by Pham Dinh Long (1968) in the Phu Ngu area, Dinh Hoa Dis- trict, Thai Nguyen Province by its sediments, which are free of acidic effusives but contain- ing older graptolite biozones. The detailed sedimentary distributions in the Dinh Ca For- mation in the Song Hien Structure-facial Vietnam Journal of Earth Sciences, 39(1), 76-86 84 Zone, also elsewhere in Northeast Viet Nam should be studied in future. 4.2. Middle-Late Ordovician sediments of the Na Mo Formation (O2-3 nm) Sediments of the Na Mo Formation widely exposed in the southern side of the Dinh Ca- Na Mo section (see Figure 4) are composed of thick-bedded quartz sandstone with interbeds of sandstone, siltstone containng trilobite, brachiopod faunas of Middle Ordovician age. In the upper part of the formation are inter- beds of light-colored limestone (Figure 7A, B) which in the E. D. Vaxilevxkaja’s original de- scription (Dovjikov et al., 1965), these lime- stones yielded heliolitoid coral Plasmoporella of Late Ordovician age. Presently, limestone beds are well exposed along the local road, to the left hillside, about 300m southwest of the Dong Bo-Na Mo cross section. The contact with the underlying Dinh Ca Formation is cut by minor faults running in east-west direction. Figure 7. A. Light-colored limestone, thick-bedded at left hillside; B. Light-colored limestone of the Na Mo For- mation widely exposed to the left of the road, km 3.7 toward Trang Xa 4.3. Devonian sediments The Devonian sediments are largely spread. Five lithostratigraphic units are being proposed here. At the southern side of the sec- tion in the Dong En Village exposed cropped out are red conglomeratic sandstone beds yielding some fish remains, asigned to the Si Ka Formation (D1 sk). It lies unconformably on the Na Mo Formation, but locally, some of the beds are cut by small-scaled faults. The Early Lochkovian age of the Si Ka Formation is based on the correlation with stratotype of the formation bearing fish, crutacean, plant fossils in the Dong Van area, Ha Giang Prov- ince. Directly above are calcareous siltstone yielded abundant brachiopod Howellella mer- curi, Howittia wangi; fish Yunnanolepis cf. praecusor, Y. meemannae, Chuchinolepis cf.dongmoensis, Zeaspis sp., Nostolepis sp., Langdenia campylognathus; tabulate Fa- vosites aff. intricatus, Squameofavosites ko- lymaensis, Thamnopora incerta showing Late Lochkovian-Early Pragian age, being assigned to the Bac Bun Formation. Overlying con- formably the Bac Bun Formation are marble and dark-grey calcareous shale of the Mia Le Formation (D1 ml), including abundant bra-chiopod, coral, bivalve, stromtoporoid, crinoid faunas belonging to the Euryspirifer tonkinen- sis assemblage of the Late Pragian-Early Emsian age. Overlying conformably the Mia Le Formation are mainly limestones bearing abundant coral and stromatoporoid faunas suggesting the Late Emsian-Early Eifelian age, and assigned to the Na Quan Formation (D1-2nq). Found in the northeastern side of the section, at the Dinh Ca bridge is a tectonic fault of NE-SW direction, which spreads along a stream of north flank of the terri- genous mountain range of the Dinh Ca For- mation (O1 dc). On the valley from the Dinh Ca Town to the Ve Bridge exposed are light- colored limestones yielding poriferan Amphi- Nguyen Huu Hung, et al./Vietnam Journal of Earth Sciences 39 (2017) 85 pora, Stachyodes, Trupetostroma, Chaetetes and coral, brachiopod fossils belonging to the Na Quan Formation (D1-2 nq). Directly above, on the hills around the Dinh Ca Town out- cropped are siliceous shale bearing tentacu- litid Stiatostyliolina sp., Viriatellina sp., Styli- olina sp. showing the Late Emsian-Frasnian age, and assigned to the Na Tuong Formation (D1-3 nt). 4.4. Carboniferous-Permian sediments Along the northwestern direction near the Committee Building of Vo Nhai District, above the Na Tuong Formation cropped out are mainly thick-bedded limestones of the Bac Son Formation (C-P2 bs) bearing abundant foraminifers of zones Dainella, Endothy anopsis, Millerella, Profusulinella, Fusulinel- la, Obsoletes, Triticites, Schwagerina, Ro- bustoschwagerina, Misellina and Neoschwa- gerina. The Bac Son Formation ranges from the Lower Carboniferous (Visean stage) to the upper Middle Permian (Capitanian stage), and has the tectonic contact with the Na Tuong Formation. In general, the structure of the stratigraph- ic section from the Committee Building of Vo Nhai District, northsouthern direction passing by the Dinh Ca Market to Trang Xa Village (Fig. 8) is an anticlinorium, whose core is filled by sediments of the Dinh Ca Formation, bearing global Tetragraptus quadribrachiatus graptolite Biozone of late Early Ordovician age. Figure 8. Geologic section from the Committee Building of Vo Nhai District passing by the Dinh Ca Market to the Trang Xa Village 5. Conclusion The discovery of Early Ordovician grapto- lithina plays an important role for stratigraph- ic correlation of Lower Paleozoic sediments in Northern Vietnam. It serves as the basic data for establishing new lithostratigraphic unit termed as the Dinh Ca Formation in the north- eastern region of Vietnam. Acknowledements The study was conducted as part of the project “Paleontological collection in Vietnam territory from 2015 to 2020” funded by Vi- etnam Academy of Science and Technology coded BSTMV.28/15-18 to whom we sincere- ly acknowledge. The authors are grateful to Prof. Acad. Xu Chen, Institute of Geology and Palaeontology, Chinese Academy of Sciences for identification of the graptolite faunas. 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