OUTLINES ON STRATIGRAPHY OF THE JURASSIC
NON-MARINE RED BEDS IN THE NORTH VIỆT NAM
AND ADJACENT AREAS

NGUYỄN XUÂN KHIỂN¹, JINGENG SHA²

¹Việt Nam Institute of Geosciences and Mineral Resources, Thanh Xuân, Hà Nội.
²Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences,
39 East Beijing Road, Nanjing 210008, China.

Abstract: The Jurassic non-marine red beds in Việt Nam and adjacent areas are of special importance in the forming and evolving history of the earth crust in the studied regions. The studying degree of these sedimentary formations, of which there is the problem of stratigraphy, however is still much different among territorial regions leading to the limited understanding on the nature, as well as the relationship between them. In the North Việt Nam, the Jurassic non-marine red beds are distributed in almost all structural zones. Due to the material differentiation and various sedimentary environments among the zones, they are described in 7 stratigraphic units. In the South China, especially in the territory of Guangxi Province, where the characteristics of structural geology are closely related to that in the North Việt Nam, the Jurassic non-marine red beds are described in the Tientang, Daling formations, the Shiti Group and the Upper Jurassic series. In the Laotian territory particularly, the degree of study on these formations is still in a preliminary level through geological mapping projects at medium scale. This paper focuses on an introduction to current research status and new results on stratigraphy of Jurassic non-marine red beds in the North Việt Nam and adjacent areas.

I. INTRODUCTION

In various scales, the Jurassic non-marine red beds in Việt Nam are the important component making up almost all structural units over the north of the territory. They were formed in the beginning stage of an arid paleoclimatic regime which widely outspread over the North Việt Nam and adjacent areas, almost accompanied by non-marine red beds subsequently being formed in Cretaceous, as well as the Triassic underlain coal-bearing sediments; they play an especially significant role in the forming and development history of the earth crust of the territory. These sediments have, therefore, attracted the attention of many home and oversea geologists long ago (A. Jamoida, 1962; A. E. Dovjikov, 1965; Tạ Hoàng Tinh & Phạm Đình Long, 1966; Nguyễn Trí Vát, 1969; Trần Văn Trị, 1977; Vũ Khúc, 1989, 2002; Nguyễn Xuân Khiển, 2003...). As far as nowadays, however, many stratigraphic and lithological problems are just clarified.

II. STRATIGRAPHY OF JURASSIC NON-MARINE RED BEDS IN THE NORTH VIỆT NAM

Due to the differentiation of material composition and depositional environments between regions, the Jurassic non-marine red beds have been described in 7 stratigraphic units, namely: the Hà Cối Formation (J_1-2 hc) in East Bắc Bộ, Nậm Pô Formation (J₁ np) in Westernmost Bắc Bộ, Nậm Thếp (J₁ nt) Formation in West Bắc Bộ and the Bãi Dinh (J_1-2 bd), Núi Xước (J₁ nx), Động Trúc (J_1-2 đt), Nước Rụng (J₃ nr) Formations in North Trung Bộ.

1. Hà Cối Formation (J_1-2 hc)

The Hà Cối Formation (A. Jamoida, 1962) is largely distributed in the form of superimposition in the Quảng Ninh, An Châu, and Sông Hiến structural zones. Its stratotype was established in the Hà Cối area (Hải Hà, Quảng Ninh), where one can obviously observe its unconformity upon sericitic shale of the Ordovician-Silurian Tấn Mài Formation. There, the section of the formation includes 3 parts:

- Lower part: composed mainly of coarse-grained sediments starting with basal conglomerate, grading upward into whitish-gray coarse quartzose sandstone, quartzitic sandstone interbedded with some brown-red and dark gray siltstone containing coal materials, about 400 m thick. Siltstone contains flora fossils Anomozamites sp., Equisetum sp.

- Middle part: composed of reddish and purple-brown siltstone interbedded with some fine- to medium-grained sandstone with cross-bedding in some places; 480 m in thickness. At the Cái Lầm Village, flora fossils Coniopteris sp., Equisetites sp. and Podozamites sp. were found together with fresh-water fauna fossils Amussiam sp., Baidestheria sp. and a few sporomorphs of Ginkgo sp., Benettites sp., Protoconiferus sp., Chomotriletes sp., Laevigatosporites sp.. Among the above fossils, Coniopteris is a genus that occurred only in Lower Jurassic.

- Upper part: consisting of coarse-grained sediments interbedded with fine-grained ones with the presence of sandstone, quartzitic sandstone grading upwards into purple, brownish-red or reddish siltstone, 306 m in thickness. In some places, sandstone has a cross-bedding structure.

The total thickness of the formation in this section is 1176 m.

Although the Hà Cối Formation section has the lithological differentiation in different areas, but the formation, in general, was created in the non-marine environment. In the outcrops, the variation is often shown in the increase or decrease of coarse-grained beds. In the Thái Nguyên area, for instance, the section includes mainly coarse sediments with horizontal bedding on the top of the low mountain ranges, therefore, Fromaget (1942) had named them as “Upper Sandstone”.

Such a section is exposed along the road from Phúc Thuận to Liên Xá, Trại Cau, including purely quartzose sandstone, gravelstone bearing interbeds of quartz conglomerate with gray shale and reddish-brown siltstone in the upper part. From gray shale, leaf imprints of Czekanowskia cf. rigida, Cladophlebis haiburnensis, Sagenopteris sp., Phoenicopteris sp. of Jurassic age have been collected. The total thickness of the formation in this section is 1850 m.

The Hà Cối Formation has its lower boundary clearly observed at Hà Cối, particularly in the section extending from Tấn Mài to Hà Cối. There, the basal conglomerate bed of oligomictic composition covers unconformably shale of the Ordovician-Silurian Tấn Mài Formation. In the Thái Nguyên area, basal conglomerate of the formation unconformably overlies the Norian-Rhaetian coal-bearing sediments of the Văn Lãng Formation. During the survey on the section in the Tràng Vinh dam foot (Móng Cái, Quảng Ninh) and Bản Chắt - Pò Háng (Đình Lập, Lạng Sơn), we firstly discovered the member of brownish-purple gravelstone and gritstone at the lower part of the formation (gravel and grit are up to 60-75%) much rich in bivalve fossils Cuneopsis sp., Cuneopsis (Eocuneopsis) dehuaensis Guo ... that were met in the Lower Jurassic Zhangkezhai Formation in Yunnan (China), and Pseudocardinia sp. of Early Jurassic age. Based on the above fossils and stratigraphic correlation, the Hà Cối Formation has been attributed to Early-Middle Jurassic.

2. Nậm Thếp Formation (J₁ nt)

Nguyễn Vĩnh (1972) established this unit to describe the red beds that unconfomably overlie the Norian-Rhaetian coal-bearing sediments distributed in the Nậm Thếp, Huổi Xa, Nậm Peng streams and scattered in Quỳnh Nhai, Mường Vọ, Núi Tọ, and Đầm Đùn areas with a close relation in space with the Suối Bàng Formation (T₃n-r sb).

The stratotype of the formation is established in the Nậm Thếp valley and along the Khe Mảng stream with the sedimentary composition including 2 members:

Member 1: pinkish, medium-grained sandstone interbedded with some siltstone and reddish-brown, thin-bedded shale, 200 m in thickness.

Member 2: claystone and reddish to purple-brown siltstone with some interbeds of fine sandstone, 300 m in thickness.

The total thickness of the formation in this section is 500 m.

The section situated along the Nậm Peng stream includes 3 members:

Member 1: thick-bedded sandstone interbedded with some brownish claystone containing leaf prints of Coniopteris cf. clavipes (Heer), Otozamites cf. indosinesis Zeiller of Early Jurassic age and some shells of Estheria. Thickness: 230 m.

Member 2: grayish-brown claystone interbedded with a little reddish-brown siltstone and pinkish sandstone; thickness: 90 m.

Member 3: medium-grained sandstone interbedded with quartzose gritstone, banded siltstone and some reddish-brown claystone, containing Estheria; thickness: 110 m.

The total thickness of the formation in this section is 430 m.

The Nậm Thếp Formation rests conformably upon the Norian-Rhaetian coal-bearing Suối Bàng Formation, observed in Huổi Xa (Điện Biên) and Mường Vọ (Hòa Bình) areas. Based on the collected fossils in the Nậm Peng section, among them the most significant is Coniopteris cf. clavipes, and the conformable relation upon the Suối Bàng Formation, the Nậm Thếp Formation is dated as Early Jurassic.

3. Nậm Pô Formation (J₁ np)

The Nậm Pô Formation (Dovjikov A. et al, 1965) is widely distributed in the Nậm Vì, Bản Yên, Nậm Chà, Nậm Ngà, Nậm Long, Nậm Khao and Tăng Ngá areas, occupying almost all the area of southwestern half of the Mường Tè District, Lai Châu Province. Its stratotype has been mapped along the Nậm Pô Stream of the Mường Nhé District (situated in the Mường Tè structural zone). According to Lê Hùng [2], the formation is divided into 4 members:

Member 1: thin-bedded, fine-grained, oligomineralic sandstone interbedded with gritstone and reddish-brown or brownish siltstone; thickness: 400 m. Siltstone in the Rông Hái Stream yields sporomorphs Selaginella tabagaensis Bolkh, Pagiophyllum sp. and Lophotriletes sp..

Member 2: reddish-brown siltstone interbedded with some oligomineralic brownish-gray or reddish-purple sandstone; thickness: 350 m.

Member 3: greenish-gray, medium-grained sandstone interbedded with reddish brown, medium-bedded siltstone and claystone; thickness: 400 m, containing sporomorphs Palaeoconiferus asaccatus Bolkh, Tricolpites sp. and Balteisporites sp.

Member 4: reddish-brown siltstone interbedded with lighter coloured sandstone, gritstone and claystone of same colour; thickness: 300 m.

The total thickness of the formation is 1450 m.

The Nậm Pô Formation conformably overlies the Norian-Rhaetian coal-bearing Suối Bàng Formation, observed in the Huổi Sáy area and along the road from Nà Pheo Village to Nậm Chim Stream. Its Early Jurassic age is determined by this stratigraphic relation and our first discovery of bivalve fossils Cuneopsis (Eocuneiopsis) aff. yunnanensis Guo and Pseudocardinia sp. in the purplish gray sandstone of the Nà Khoa - Nà Hỳ section; Cuspidaria sp. (aff. C. minleana Guo), Corbula sp., Modiolus sp. in the dark gray siltstone of the Nậm Ngà - Nà Hỳ section (Mường Lay, Điện Biên) and combined with the previously found sporomorphs.

4. Núi Xước Formation (J₁ nx)

The Núi Xước Formation (Nguyễn Chí Hưởng, 1998) is widely distributed in south Thanh Hóa - north Nghệ An and Hà Tĩnh areas. The section of formation in the Núi Xước area, extending along the south Thanh Hóa - north Nghệ An coastline includes 3 parts:

Lower part: consisting mainly of coarse sediments including gray, medium- to coarse-grained quartzose sandstone, thick-bedded gritty sandstone, quartzose conglomerate, in some places, with interbeds or lenses of gritstone, reddish-brown siltstone; some beds contain silicified tree-trunks; thickness: 630-910 m.

Middle part: composed of yellowish gray, thick-bedded, fine- to medium-grained sandstone, sometimes containing quartzose gritstone interbedded with reddish-brown and unclearly bedded claystone, somewhere interbedded with coaly shale containing many flora fossils, such as Coniopteris sp., Radicites sp., Equisetites sp. and Podozamites sp. of Early Jurassic age; thickness: 220-250 m.

Upper part: composed mainly of pinkish, thick-bedded quartzose gritstone - conglomerate, pinkish, thick-bedded, sometimes, cross-bedded, medium- to coarse-grained sandstone; thickness: 120 m.

The total thickness of the formation in this section is 870-1050 m.

The Núi Xước Formation conformably overlies the Norian-Rhaetian Đồng Đỏ Formation observed just in Hải Lễ and many other localities. It is aged as Early Jurassic based on the above fossils and its relations with Upper Triassic coal-bearing sediments.

5. Động Trúc Formation (J_1-2 dt)

The Động Trúc Formation (Phạm Đình Trưởng and Lê Thanh Hựu, 1994) is distributed in small areas in the north of the deep-seated fault separating the two Hoành Sơn and Trường Sơn zones on the territory of Tuyên Hóa District, Quảng Bình Province.

Its characteristic section in the Động Trúc mountain side includes 2 members:

Member 1: reddish-brown conglomerate grading upwards into quartzose gritstone, with the pebble composition mainly from quartzite, quartz, chert and rhyolite, 10-30 m thick.

Member 2: medium-grained sandstone, reddish-brown siltstone containing worm traces. Thickness: 130-150 m. These sediments contain the sporomorphs Picea aff. latens Bolch., Pinus sp., Podozamites sp., Myrtus sp., Classopollis sp., Hammamelis sp., Myrica sp., Fagaceae gen. et sp., of Jurassic age.

6. Bãi Dinh Formation (J_1-2 bd)

The Bãi Dinh Formation (Phạm Huy Thông et al, 2000) is one of the formations filling up the Nậm Theun Depression on the Lao territory. Within the Vietnamese territory it is distributed mainly in narrow strips in the Mụ Giạ mountain foot (Dân Hóa, Tuyên Hóa, Quảng Bình). Its section has been divided into two parts: the lower part consisting of marine deposits, that, in a similar section within the Laotian territory, contain marine bivalve fossils of Pliensbachian, Early Jurassic. The upper part is composed of non-marine red beds containing fresh-water bivalves and sporomorphs of, possibly, Middle Jurassic age. The total thickness reaches 260-400 m.

In the Bãi Dinh - Y Leng section extending along the road No.12 and the Cao Ai stream valley, the red beds are composed mainly of reddish-brown siltstone interbedded with pinkish, thick-bedded, medium-grained sandstone; 100-150 m thick. In this section, fresh-water bivalves have been found, including: Nakamuranaia sp., Peregrinoconcha sp., together with sporomorphs Coniopteris aff. divaricata, Lycopodium subrotundum, Davallia aspecta, Protoquercus sp., Laevigatosporites sp., among them, Coniopteris is often considered to be a representative of Early-Middle Jurassic.

The Bãi Dinh Formation unconformably overlies different older sediments (Fig. 1).

Figure 1. Geological cross-sections showing the relationship between the Bãi Dinh Formation and older ones at Bãi Dinh area.

7. Nước Rụng Formation (J₃ nr)

The Nước Rụng Formation (Phạm Huy Thông et al, 2000) is distributed in a narrow area in the upstream of the Nước Rụng and Cao Ai stream in the Mụ Giạ Pass area. Its typical section extends along the Nước Rụng Stream at an approximate altitude of 700-1600 m, including 3 members:

Member 1: polymictic purplish conglomerate, 2 m thick, grading gradually upwards into thick-bedded, polymineralic sandstone, purplish-brown, thick-bedded or, sometimes, banded siltstone; thickness: 100-250 m. Siltstone contains sporomorphs Classopollis sp., Cycadopites nitidus, Bacutricolpites centricus, Quadraeculina sp. of Jurassic age.

Member 2: polymineralic sandy siltstone interbedded with some brownish or grayish-purple siltstone, about 80 m thick.

Member 3: thick-bedded, polymineralic sandstone, thick-bedded, purplish-gray siltstone, in some places, interbedded with lenses of polymineralic gritstone, 150-200 m in thickness. It is to note that, the cement of sandstone contains the Cu (174 ppm), Ni (196 ppm) and Cr (439 ppm) mineralizations, as seen in the Upper Jurassic Nam Phouan Formation in the Nam Theun basin (Central Lao) [8].

The total thickness of the formation is 330-350 m.

The Nước Rụng Formation is supposedly dated as Late Jurassic, based partly on the palynological data and partly on the correlation to the Upper Jurassic Nam Phouan Formation [8] in adjacent Central Lao area, that is also Cu-mineralized.

The stratigraphic correlation of Jurrassic non-marine red beds in the North Việt Nam is shown as follows (Fig.2):

Figure 2. Correlation of Jurassic non-marine red beds in North Việt Nam.

III. STRATIGRAPHIC CORRELATION OF JURASSIC NON-MARINE RED BEDS IN NORTH VIỆT NAM WITH THOSE IN ADJACENT AREAS

In adjacent areas of the North Việt Nam, especially in South China, Upper and Central Lao, there are also Jurassic non-marine red beds. In spite of various degrees of research, the correlation between them could contribute to clarify the paleogeographic conditions of these areas, as well as the forming and existing history of these formations.

1. South China region: particularly on the territory of Guangxi Province, where the geological structure is closely related to that in North Việt Nam [7]. There, the Jurassic non-marine red beds are described in the following stratigraphic units:

- Tiantang Formation: of Lower Jurassic, including conglomerate, breccia-conglomerate interbedded with reddish-brown siltstone and claystone, containing poorly preserved flora remains. Thickness: 22-572 m.

- Daling Formation: of Lower Jurassic, composed of gray, reddish-brown shale, sandstone and gray argillaceous limestone interbedded with some beds of coaly shale. These sediments contain freshwater bivalves, like Hunanella guanyintanensis, Margaritifera guanyintanensis, the crustacean Darwinula incurvata and some flora remains of Podozamites sp.. Thickness: 85-915 m.

Table 1. Stratigraphic correlation of Jurassic
non-marine red beds in North Việt Nam

Age		East Bắc Bộ	West Bắc Bộ		North Trung Bộ
Jurassic	J3					Nước Rụng Fm (330-530m)
	J2	Hà Cối Fm (1176-1850 m)				Bãi Dinh Fm (upper part) (100-150 m)	Động Trúc Fm (180 m)
	J1
			Nậm Thếp Fm (430 m)	Nậm Pô Fm (1450 m)	Núi Xước Fm (870-1050 m)

- Shiti Group: composed mainly of greenish gray sandstone, pebble-bearing coarse sandstone interbedded with some reddish-brown shale and silty shale containing freshwater bivalves Psilounio ovalis, Cuneopteris johannisboehmi. Thickness: 112-522 m.

- Upper Jurassic series: including brownish-gray quartzose sandstone, reddish-brown siltstone containing flora fossil Cupressinocladus sp., 869 m thick.

2. Upper Lao region: The degree of research on the red beds in this region is still limited. However, it is preliminarily shown that there is a similarity with the Jurassic non-marine red beds in West Bắc Bộ, Việt Nam. With existing data, they have been subdivided into:

- Lower-Middle Jurassic red beds: including reddish-brown siltstone and claystone interbedded with sandstone; thickness: about 500-600 m. They conformably overlie the Upper Triassic coal-bearing formation.

- Upper Jurassic - Lower Cretaceous red beds: including conglomerate, gritstone, polymineralic sandstone containing manganese nodules, interbedded with reddish-brown siltstone and claystone, 350 m thick. They unconformably overlie the Lower-Middle Jurassic sediments.

3. Central Lao region: These sediments are the important constitutions filling the Nam Theun basin that are described in the Nam Phouan formation of Late Jurassic age.

- Nam Phouan Formation: distributed in narrow bands in Na Kay, Thakhek, Boualapha areas (Khammouan Province) and Pakkadinh area (Bolikhamxay Province). The typical section of the formation extends along the Nam Phouan Stream, including 3 members:

Member 1: greenish-gray, medium-bedded fine sandstone, interbedded with some thin beds of blackish-gray sandstone; thickness: over 100 m.

Member 2: purplish-brown, medium-bedded siltstone; 15 m thick.

Member 3: brownish and grayish purple sandstone containing poorly preserved remains of ostracods. In the middle of the member, there is some thin-bedded purplish- brown siltstone. Thickness: 280 m.

The total thickness of the formation is about 400 m.

In the Na Hin - Nong Kok section, conglomerate of the Nam Phouan Formation unconformably overlies limestone of the Carboniferous - Permian Khammouan Formation and terrigenous sediments of the Early Carboniferous Boualapha Formation. Near the Khoun Kham Village, a 25 m - thick member of this conglomerate unconformably overlies also limestone of the Khammouan Formation; limestone pebbles contains Carboniferous foraminifera fossils. Along the left bank of the Mekong River (Nam Khong River) is rather common the phenomenon that, sediments of the Nam Phouan Formation started with a basal conglomerate bed of various thickness, unconformably overlying older sediments, including the marine facies ones of the Lower-Middle Jurassic Ban Lao Formation. The Late Jurassic age of the Nam Phuoan Formation has been supposed on the basis of this stratigraphic relationship.

The scheme of stratigraphic correlation between Jurassic non-marine red beds in North Việt Nam with those of adjacent areas is presented in Table 2.

Table 2. Scheme of stratigraphic correlation between Jurassic non-marine
red beds in the North Việt Nam and those in adjacent areas.

Age		North Việt Nam			South China	North Lao	Central Lao
Jurassic Jurassic	J3			Nước Rụng Fm (330-530m)	J3 (869 m)	J3-K1 (350 m)	Nam Phouan Fm (400 m)
	J2	Hà Cối Fm (1176-1850m)		Bãi Dinh Fm (upper part) (100-150 m) Động Trúc Fm (180 m)	Shiti (112-522 m)	J1-2 (500-600 m)
	J1		Nậm Thếp Fm (430-500 m) Nậm Pô Fm (1450 m) Núi Xước Fm (870-1050 m)
					Daling Fm (85-915 m) Tiantang Fm (22-572 m)

IV. DISCUSSION AND CONCLUSIONS

1) The Jurassic non-marine red beds in North Việt Nam and adjacent areas have two principal types of section:

- Continuously developed from the Norian-Rhaetian coal-bearing formations and begun from Early Jurassic Epoch (the Nậm Thếp, Nậm Pô, Núi Xước formations, and that in Upper Lao). It means that, in these cases, the depositional environment had a transition from sub-continental to pure continental.

- In unconformity upon various older geological bodies (the Hà Cối, Động Trúc, Nam Phouan Formations). These formations were formed when the regions had changed to non-marine regime.

2) Due to various degrees of research, the age of some formations is still supposed. The results on lithofacial, palaeogeographic researches, as well as of new stratigraphic methods (stratigraphic paleomagnetism, seismostratigraphy, lithostratigraphy, geochemical stratigraphy) are still much limited or not efficiently applied yet.

3) Almost all Jurassic non-marine red beds in Việt Nam and adjacent areas have the distributive areas and a close relation with Cretaceous red beds. The Jurassic red beds bear many features of the regime of non-marine deposition, arid climate changing gradually to the regime of purely non-marine deposition with the typical arid climate in Cretaceous.

Acknowledgements

The authors would like to express their sincere thanks to Prof. Vũ Khúc for his full support during the research work and for his valuable informations.

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