Tèng Duy Thanh¹, Vò Khóc²

Abstract. On basis of the "Stratigraphic Code" the authors attempt to assess the Paleozoic and Mesozoic stratigraphic units described in B¾c Bé (North ViÖt Nam). All the valid units are presented in two tables of this paper (tab. 1 and tab. 2), the change of unit names, the explanation of assessing these units, and the description of new units are introduced in the text.

Mesozoic. The "T©n L¹c Formation" is actually considered the upper part of the Cß Nßi Formation, lower boundary of which is diachronous. Consisting mainly of felsic effusive compositions, Lòng Tr©u, B×nh Liªu, Tiªn Yªn, Pß HÌn and Lµng Cæng Formations are synonyms of the Kh«n Lµng Formation, while the Yªn B×nh Formation is a valid unit. The §iÒm He Formation was established recently on the basis of lentiform limestone at the base of the Nµ KhuÊt Formation, while synonyms of the last are S¬n D­¬ng, D­¬ng H­u and Tr¹i S­u formation names, and NËm ThÕp and Huæi Xa are invalid names corresponding to the NËm P« Formation. The discovery of Gigantopteris remains result in the excluding most parts of terrigenous and limestone components from the "V¨n ChÊn Complex" and the remaining felsic effusive rocks with some thin tuffaceous sandstone belong to real V¨n ChÊn Formation. With the presence of mafic components, the Suèi BÐ Formation distinguishes from the V¨n ChÊn one, as for NËm Qua, Tó LÖ, B¶n H¸t, and M­êng Kim formation names, they are only irrationally used for different parts of the V¨n ChÊn Formation.

During century period, since the time when the geology of ViÖt Nam’s territory has been regularly studied, nearly 8.000 research works on the geology of ViÖt Nam were published. Important successes in stratigraphy have been achieved, which play the key role in understanding structure and geological history of country’s entrails.

In the twentieth century over 500 stratigraphic unit names have been published or proposed, and a lot of them became popular in the Vietnamese geological language. Before the 50’s decade of last century stratigraphic researches were realized by French and other foreign geologists, which worked with different styles of their countries. At that time, a great number of stratigraphic units were described in different stratigraphic conceptions. The stratigraphic terms have been rapidly increased at last time, when the geological mapping works (at 1/200 000 and 1/50 000) were developed.

Though a stratigraphic code has been approved and published [25] many junior geologists appear to prefer creating new stratigraphic terms, instead of the available ones. Some junior geologists understood the lithological properties of the lithostratigraphic unit as a rigid character and usual describe new unit and give a new term, although there is only a facies change of an acquainted unit. In result, the number of stratigraphic terms increases rapidly causing a complicated picture in geological information. In this situation, an assessment on the validity of stratigraphic units of ViÖt Nam becomes a pressing work, and the present authors try to give an initial revision of Paleozoic and Mesozoic units in North ViÖt Nam (B¾c Bé).

It is difficult to realize the revision on stratigraphic units in lacking an official commission, so all results of assessing stratigraphic unit terms presented in this paper are only a first contribution to this field of North ViÖt Nam. Working in this aim, the present authors have based on the statutes of the "Stratigraphic Code" approved by Geological Department of ViÖt Nam [25].

By tradition, stratigraphy is studied in the frame of present structural schemas, and in each geological region, stratigraphic units from oldest to youngest will be described. Stratigraphic units were formed in a determined sedimentary basin, so they must be studied in the relationship with the basin environment where they were formed, but the last could not be a stable element through geological times. There are not any sedimentary basin fixed through all geological history, and the description of all stratigraphic units from oldest to youngest in the frame of a present structural unit seems to be irrational mode. As a result in this paper geological regions, in which stratigraphic units will be presented, have not been identical in the schemes of Paleozoic and Mesozoic.

In each scheme (Tab.1, Tab.2) of this paper, valid stratigraphic units are introduced, as for the change of the unit name and the explanation of assessing these names, and the description of new units – they will be presented in detail in the text. Due to the lack of data, some questionable problems in the stratigraphic division and unit dating are not solved yet, but they are also showed in the paper for studying in the time to come.

Lower Paleozoic

Cambrian deposits containing Planolites in Th¸i Nguyªn and B¾c K¹n Provinces were first described by E. Patte [16], latter Vasilevskaia E. D. has described them as the Má §ång Formation [3]. The Má §ång Formation is characterized by sandstone, interbeds of red-brown clay shale and chocolate, variegated, stripped silty sandstone, calcareous sandstone. Apart from Planolites remains; Middle Cambrian trilobite Ptychagnostus atavus was collected from the ThÇn Sa village. Just after the mapping works of Vasilevskaia E. D., TrÇn V¨n TrÞ did the exploration in the same area and described the ThÇn Sa Formation, which is characterized by the same rock sequences, so ThÇn Sa and Má §ång Formation should be synonyms [35]. The paper in which the ThÇn Sa Formation is described, had been rapidly published in the §Þa ChÊt Journal [31] a year before the publication of Má §ång Formation [3]. In this real unexpected situation, the Má §ång Formation became a junior synonym of the ThÇn Sa one.

Lithostratigraphically, the ThÇn Sa Formation is much similar to the Bång S¬n one, described by Bourret R. [1] as "SÐrie de Bong-Son." However, the Bång S¬n Formation seems include some layers belonging to the S«ng CÇu Group (Lower Devonian), so the Bång S¬n term has not the priority, and almost all its rock components belong to the ThÇn Sa Formation.

Zhamoida A. I. and Ph¹m V¨n Quang [3] have described the C« T« and the TÊn Mµi Formations in the Coastal Zone of B¾c Bé, and dated the first as Neogene, and the second as Cambrian – Ordovician. Later the first formation was redescribed by TrÇn V¨n TrÞ et al. [33], and the second – by NguyÔn C«ng L­îng [35], with discovered graptolites indicating Ordovician – Silurian age of both formations. The C« T« Formation was considered as a synonym of the TÊn Mµi one [36], because of its age was dated as the same Ordovician – Silurian. However, C« T« and TÊn Mµi formations distinctly distinguish each from other by their lithological compositions; in the first tuffaceous sandstone and rhyolitic tuffs play important role, while these components are absent in the second formation. Therefore, both formations are worthy considered as valid units in stratigraphic terms of ViÖt Nam.

Devonian

The §¹i ThÞ Formation, described by Ph¹m §×nh Long, NguyÔn Kinh Quèc, Hoµng Xu©n T×nh [35], consists of sericite - quartz schist, sericite schist, interbeds of recrystallized limestone, silty sandstone, interbeds of sericite - quartz schist, lustrous phyllite and thin beds of quartzitic sandstone.

So, the lithological components of Pia Ph­¬ng and §¹i ThÞ Formations are similar to those of the Mia LÐ Formation, their slight difference expresses only the higher metamorphism level of the two first formations. Furthermore, in Pia Ph­¬ng and §¹i ThÞ Formations the collected fossils belong to the same Euryspirifer tonkinensis Assemblage that characterized the Mia LÐ Formation.

Mentioned data show that Pia Ph­¬ng and §¹i ThÞ Formations are junior synonyms of the Mia LÐ Formation. Here could be included and the most parts of the Cèc X« Formation that are characterized by the same lithological and fauna compositions (see also Tèng Duy Thanh, TrÇn V¨n TrÞ, T¹ Hoµ Ph­¬ng, NguyÔn H÷u Hïng [23]).

The B¶n P¸p Formation was established by NguyÔn Xu©n Bao [14] in the S«ng §µ basin, characterized by Devonian grey bedded limestone. Later, Ph¹m §×nh Long [17] described the Nµ Qu¶n Formation in Cao B»ng Province, which is characterized by the same rock properties as of the B¶n P¸p one, and which D­¬ng Xu©n H¶o called as "Nµ Qu¶n" without section description. The B¶n P¸p Formation was described five years sooner than the Nµ Qu¶n Formation, so the priority belongs to the B¶n P¸p name, and the Nµ Qu¶n name should be a synonym of the B¶n P¸p.

The Phia Khao Formation was established and dated as Silurian – Early Devonian by NguyÔn Kinh Quèc [9]. Later it was irrationally considered as a component of the Pia Ph­¬ng Formation [36]. It consists mainly of recrystallized limestone, which were described as "Eifelian – Givetian Stages" [3] and Khe Lau Formation [23], and by new observation and fossil study it should be correlated with the B¶n P¸p Formation in the current conception [23].

§Æng TrÇn Huyªn [4] described the Si Phai (Chi Phai) Formation, which consists of limestone, marl, and chert interbeds, overlying the Mia LÐ Formation, and occurring only in the §ång V¨n area. It should be considered as a facies change of the B¶n P¸p Formation, that in it apart from pelagic fossils, one of us collected recently Tabulates such as Heliolites sp. and other indeterminable Corals. Other synonym of the B¶n P¸p Formation is the "Má Nhµi Formation" [8], which consists of Frasnian limestone containing abundant Stromatoporoids, siliceous limestone, and occurring only in the B¾c S¬n area. It is worthy to note that Devonian limestone containing shallow water fossils is a distinguished character of the B¶n P¸p Formation in B¾c Bé, so the Má Nhµi Formation must be considered as the upper part of the B¶n P¸p one.

The Trµng Kªnh Formation characterized by Devonian dark grey limestone was described by NguyÔn Quang H¹p [11] in the coastal zone "Duyªn H¶i"; its junior synonym is the Lç S¬n Formation described by Ph¹m V¨n Quang (1969), which is characterized by the same limestone and occurred in the same area. In previous works, Tong-Dzuy Thanh [26] has considered this formation as a component of the B¶n P¸p Formation, but new data from investigations of NguyÔn H÷u Hïng, §oµn NhËt Tr­ëng and T¹ Hoµ Ph­¬ng and of us prove that it distinguishes clearly from the B¶n P¸p Formation. First, the Trµng Kªnh Formation distinguishes from the B¶n P¸p one by the presence of a siliceous member in the middle part and of the light variegated limestone Famennian age in the top of its sections. Second, if benthos and pelagic fossils indicate Early Devonian – Frasnian age of the B¶n P¸p Formation, the Trµng Kªnh Formation has diachronous boundaries – from Givetian or Frasnian of the lower boundary to Famennian of the upper one.

Vasilevskaia E. D. [3] has described the B¶n Cáng (B¶n Coáng) Formation, which distinguishes from the Nµ Qu¶n one by containing Frasnian, but not Middle Devonian fossils, and by light grey but not dark grey limestone. Later, Givetian brachiopods and corals were discovered from limestone of the B¶n Coáng Formation, therefore the B¶n Cáng Formation was considered as a synonym of the Nµ Qu¶n [2, 21, 22] or B»ng Ca ones [17].

New data of NguyÔn H÷u Hïng, §oµn NhËt Tr­ëng and of us prove that the B¶n Coáng Formation clearly distinguishes by its rock properties and age of its fossils. First, the B¶n Coáng Formation is dated as Upper Givetian – Famennian, so it could be correlated with the upper part of "Nµ Qu¶n Formation" and B»ng Ca, Tèc T¸t Formations including. Second, the B¶n Coáng Formation is characterized by shelf facies, containing benthos fossils such as foraminifers, corals, and brachiopods that clearly distinguishes from the deep-water facies with conodont, dacryoconarid fossils of B»ng Ca and Tèc T¸t Formations. Third, the B¶n Coáng is characterized by diachronous lower boundary, which overlies the upper level of the "Nµ Qu¶n Formation," or the B»ng Ca Formation, as for its upper boundary, it may correspond to the upper boundary of the Tèc T¸t Formation.

Therefore, the B¶n Coáng Formation could not be considered as a synonym of "Nµ Qu¶n Formation" or of B»ng Ca Formation and it is worthy to be a valid formation.

In West B¾c Bé (S«ng §µ Basin) NguyÔn Xu©n Bao [14,15] established the §a Niªng Formation and dated it as Givetian – Frasnian, which consists also of ash grey thin-bedded limestone and siliceous interbeds. On the basis of abundant foraminifer and conodont assemblages from its hypostratotype in the S«ng Mua upstream, §oµn NhËt Tr­ëng [24] has dated this formation as Tournaisian – Visean and correlated it with the Lòng NËm Formation in East B¾c Bé. So the Lòng NËm Formation closely resembles the §a Niªng one, and it rules out a possibility that the first is synonymous with the second.

Ng« Quang Toµn [7] described the Phè Hµn Formation in the C¸t Bµ Island (H¹ Long Bay), which consists also of ash grey thin-bedded limestone and siliceous interbeds, and in which Tournaisian - Visean Conodont and Foraminifer fossils were collected.

Describing Lower Carboniferous limestone at the "Nói Con Voi" mount, near H¶i Phßng City, NguyÔn Quang H¹p [11] has given the name the Con Voi Formation. French geologists described this Lower Carboniferous limestone mountain as "Calcaires noires de la Montagne de l’ElÐphant" and had dated it as Visean [20]. But there are two different limestone beds in the same Con Voi Limestone Mountain. The first consists off ash grey limestone, containing Lower Carboniferous foraminifers and corals, which correspond to the Con Voi Formation. The second is composed of underlying dark grey dolomite, and more or less recrystallized limestone containing Frasnian Amphipora and foraminifers that belong to the upper part of the Trµng Kªnh Formation [24].

Thus from four synchronous formations, the Con Voi one distinguished itself by limestone of shallow water, all the rests (Lòng NËm, §a Niªng and Phè Hµn Formations) are characterized by chert and limestone containing conodonts and foraminifers of deep-water facies. Lòng NËm and §a Niªng Formations are characterized by the same rock properties - dark grey limestone, thin cherty shale interbeds, and both of them contain Tournaisian - Visean conodonts and foraminifers, for that reason they are possible synonyms.

As for the Phè Hµn Formation, a thick member of mudstone and shale (90-115 m) is distinguished among its limestone and cherty shale components. Moreover, foraminifers and brachiopods from its lower and middle beds show that this formation seems to be a little older than the Lòng NËm and §a Niªng Formations, its lower boundary may fall into upper Devonian. Therefore, the Phè Hµn Formation may be distinguished formation of Upper Devonian – Lower Carboniferous only in the Coastal Zone of B¾c Bé (Duyªn H¶i).

In East B¾c Bé (Cao B»ng Province) successive stratigraphic units – B»ng Ca, Tèc T¸t and Lòng NËm Formations, which occur mainly in the Trµ LÜnh, Tèc T¸t, H¹ Lang areas of the Trïng Kh¸nh District, have common characteristics in their deposits and fossils. First, all of them are composed of grey limestone, chert shale, and manganiferous calcaro-cherty shale, and second – collected foraminifers and conodonts show that they belong to the deep-water facies. These common characteristics reflect a sedimentary cycle in deep water environment of the region, so three successive units – B»ng Ca, Tèc T¸t, and Lòng NËm Formations occurring mainly in the Trïng Kh¸nh District – should shape a tripartite unit namely the Trïng Kh¸nh Group. Respectively, three formations B»ng Ca, Tèc T¸t and §a Niªng in West B¾c Bé should be considered as components of the Trïng Kh¸nh Group, though there manganiferous elements are not observed yet.

The B¾c S¬n Formation was first described by NguyÔn V¨n Liªm [12] for grouping all Lower Carboniferous (Visean) – Lower Permian limestone that is widespread in North ViÖt Nam, and this term became largely in use in geological language of country. However, since 1925 [6] the B¾c S¬n name (Bacsonien) has been in use and is well known not only in ViÖt Nam, but and in the world as a Quaternary unit (Holocene) by its eminent prehistoric instruments. Avoiding the confusion in geological naming, the name of Carboniferous B¾c S¬n Formation appeared later than the one of Quaternary B¾c S¬n (Bacsonien), so it must be replaced by other suitable name.

In West B¾c Bé NguyÔn Xu©n Bao [15] has described two upper Paleozoic formations – the §¸ Mµi Formation Carboniferous – Early Permian in age, and the overlying B¶n DiÖt one Middle Permian in age. The first was named after the §¸ Mµi limestones (Calcaire de Da May) [20] and is characterized by light grey limestone containing Visean – Early Permian Foraminifers. According to the description of NguyÔn Xu©n Bao [15], the overlying B¶n DiÖt Formation is composed of two parts – shale and sandstone, effusive rocks in lower part, and limestones in the upper. Lithostratigraphically, the thick lower part (shale and sandstone, and effusive rocks – 210 - 500m thick) worthy composes a formation – the B¶n DiÖt one. As for the limestone part in the top, it should belong to the §¸ Mµi Formation, and in this case the terrigenous-effusive B¶n DiÖt Formation is as only an intercalating unit in the §¸ Mµi limestone Formation.

From above mentioned data, the §¸ Mµi Formation is characterized by the same light grey limestone and of the same Early Carboniferous (Visean) – Early Permian age, which are characteristics of the B¾c S¬n Formation. Considering that the §¸ Mµi Formation was sooner described than the B¾c S¬n one, the priority belongs to the §¸ Mµi name, and it is the suitable name for replacing the B¾c S¬n name.

Triassic

In West B¾c Bé, the Cß Nßi Formation was described in the monograph "Geology of North ViÖt Nam" [3] as the lowest unit in the Mesozoic section in the Ninh B×nh and S¬n La Zones of this region. But later, it has been determined as located in the S¬n La and Thanh Hãa Zones and dated as Early Triassic, while in the Ninh B×nh and S«ng §µ Zones the Triassic section begins by mafic effusives described as Viªn Nam Formation of Induan age, which grades upward into terrigeno-carbonate beds of the T©n L¹c Formation of Olenekian age. Recent studies have been showing that the T©n L¹c Formation lithologically and paleontologically is similar to the upper part of the Cß Nßi Formation, and it is not necessary to establish it. This means, in the Ninh B×nh and S«ng §µ Zones the Lower Triassic begins by the Viªn Nam Formation which grades upward into the upper part of the Cß Nßi Formation. So, the T©n L¹c Formation becomes an invalid unit.

Kh«n Lµng Formation and its synonyms - Lòng Tr©u, B×nh Liªu, Tiªn Yªn, Pß HÌn and Lµng Cæng Formations.

Felsic effusives in the An Ch©u Depression of East B¾c Bé were all referred to Jurassic on the Geological Map of North ViÖt Nam at 1:500,000 scale (1963). NguyÔn Kinh Quèc et al. [10] first distinguished that they are composed of two formations: one conformably underlies the Middle Triassic Nµ KhuÊt Formation; another unconformably overlies it. The lower corresponds to the Lòng Tr©u Formation previously, but insufficiently described by TrÞnh Thä [38], therefore it was redescribed by these geologists as Kh«n Làng Formation of Anisian age; the upper – described as Tam Lung Formation of Late Jurassic age.

In the south of this depression, felsic effusives, their tuffs and terrigenous interbeds distributed in the B×nh Liªu - Tiªn Yªn area were assigned to the S«ng HiÕn Formation in the Geological Map of ViÖt Nam at 1:500,000 scale (1988). During the geological mapping at 1:50,000 scale of this area TrÇn Thanh TuyÒn et al. [29] proposed to limit the distribution of the above formation in the S«ng HiÕn Depression, and basing on newly found Anisian fauna described these volcanogenic beds as B×nh Liªu Formation. Later, TrÇn Thanh TuyÒn considered that effusives in the B×nh Liªu area grade upward into subvolcanic rocks, therefore they must be assigned to a subvolcanic complex – the B×nh Liªu Complex. At the mean time, the section composed of an intercalation of effusives and tuffaceous beds was described by him as Tiªn Yªn Formation, and the terrigenous beds lying beneath the B×nh Liªu Complex – as Pß HÌn Formation [30]. At that time, a section similar to the Tiªn Yªn Formation distributed in the Ba ChÏ area was described by Lª Hïng et al. [Geological Archives] as Lµng Cæng Formation.

In our opinions, the assignment of subvolcanic rocks distributed in the B×nh Liªu area to the B×nh Liªu Subvolcanic Complex is a reasonable thing. But, after that the remaining part can be compared with the Kh«n Làng Formation which is distributed in the same geological structure, of the same lithological and petrographical composition, of the same Anisian age, although in the one or another locality it differs by the quantity of interbeds of effusives that can be regarded as intraformational change. Therefore, we propose to consider the Kh«n Làng Formation as valid unit, and the others – Lòng Tr©u, Tiªn Yªn, Pß HÌn and Làng Cæng formations as its synonyms.

In the uplifted L«-G©m Zone, there is a small area of terrigenous sediments bearing Middle Anisian ammonoids that was formerly assigned to different stratigraphic unit: either to the Nµ KhuÊt Formation [3], or to the S«ng HiÕn Formation [32]. But, detailed studies have been showing that these assignments are not reasonable, because the studied sediments do not contain calcareous interbeds and belong to lower stratigraphic level in comparing with the Nà KhuÊt Formation, and do not contain volcanogenic interbeds and belong to higher stratigraphic level in comparing with the S«ng HiÕn Formation. Recently, they were described as an independent stratigraphic unit – the Yªn B×nh Formation [34].

At the base of the Nµ KhuÊt Formation there is a lentiform body of black-grey, medium-bedded limestone. In the §iÒm He - Lïng Pa area its thickness reaches the maximum of about 200 m. Apart from it, in the Nµ KhuÊt Formation there is not any other interbeds of limestone. Therefore, recently §Æng TrÇn Huyªn has been proposing to establish an independent formation for this limestone – the §iÒm He Formation. It is reasonable, and we include it in the stratigraphic scheme.

The Nµ KhuÊt Formation distributed in the An Ch©u Depression has a stable lithological composition – terrigenous sediments with calcareous siltstone, calcareous sandstone in the lower part of the formation. However, in the Geological Map of the East B¾c Bé Coal Basin at 1:200,000 scale Ph¹m V¨n Quang et al. [18] established some formations – the S¬n D­¬ng, D­¬ng H­u and Tr¹i S­u formations, having the same lithological characteristics as that of the Nµ KhuÊt Formation. Later, geological mapping work at 1:200,000 scales in this area did not use these units, and they have been considered as synonyms of the last formation.

Jurassic-Cretaceous

The felsic effusives, their tuffs, interbeds of black coaly shale, coaly siltstone and lenses of motley-coloured limestone distributed in the Tó LÖ Volcanic Depression of West B¾c Bé were firstly described by NguyÔn VÜnh et al. [13] as "V¨n ChÊn Complex" including the NËm Qua, Tó LÖ and B¶n H¸t formations, and dated as Late Jurassic - Early Cretaceous. According to these geologists it is unconformably covered and, locally, penetrated by volcanic and subvolcanic rocks of the Ngßi Thia Complex. In the monograph "Geology of ViÖt Nam. Vol. 1. Stratigraphy" [36] the "V¨n ChÊn Complex" was described as V¨n ChÊn Formation and its componential formations – as its parts, because volcanites of these "formations" consist of the same orthophyre, quartz orthophyre and sedimentary beds – the same black-grey carbonaceous shale, carbonaceous siltstone, tuffaceous rocks and lenses of limestone. It is to note that the Suèi BÐ Formation described by NguyÔn Xu©n Bao in the V¹n Yªn Map sheet [14] was also included in the V¨n ChÊn Formation as its uppermost part, although in the Suèi BÐ there are interbeds of mafic effusives.

In recent studies in the framework of geological mapping at 1:50,000 scale NguyÔn §¾c §ång found leaf imprints of the Gigantopteris Flora of Late Permian age in the beds of black-grey carbonaceous shale, carbonaceous siltstone of the studied formation (unpublished yet data). Based on this finding the main part of its sedimentary beds, including limestone lenses, has been considered as of the basement of the Tó LÖ Depression, among them the carbonaceous beds containing Permian flora belong to the Yªn DuyÖt Formation, limestone lenses exposed in the east margin of the depression – to the §ång Giao Formation, and those exposed in the west margin – to the M­êng Trai Formation. The remaining part including the members of effusives and tuffaceous sandstone, tuffaceous siltstone can be called as "V¨n ChÊn Formation", because: 1) under this name geologists always think about volcanogenic beds, and the exclusion of flora-bearing sediments does not influence the concept on the formation; 2) V¨n ChÊn is a large district where there are many sections from which we can select a good lectostratotype for the formation. Other proposals, such as "Tó LÖ" and "M­êng Kim", do not conform to the Stratigraphic Codex: 1) Tó LÖ was used in different cases. It was used by NguyÔn VÜnh for a componential formation of the V¨n ChÊn Complex, but by authors of the monograph [37] for a complex including the two V¨n ChÊn and Ngßi Thia formations. So, this name may cause confusion in the understanding of the content of the formation; 2) M­êng Kim is a young synonym of the V¨n ChÊn Formation.

The Suèi BÐ Formation has another, more mafic petrographical composition. Therefore, it must be kept as an independent formation. According to recent studies in the framework of geological mapping at 1:50,000 scale and of thematic research its distribution is not limited in the Tó LÖ Depression and, locally, erroneously described as interbeds of the Ladinian M­êng Trai Formation. According to new data it has been referred to Upper Jurassic, and is covered unconformably by the V¨n ChÊn Formation. The former Ngßi Thia Formation was recently assigned to a subvolcanic complex.

The continental red beds distributed in the §iÖn Biªn Zone was recently described as NËm Bay Formation. It differs from the Yªn Ch©u Formation by the absence of interbeds of evaporite minerals, that, possibly, shows a younger age – Early Cretaceous. At present it is supposedly referred to Undifferentiated Cretaceous.

Acknowledgment. This paper was completed in the frame of the project "Assessing the validity of stratigraphic units of ViÖt Nam" (721101), funded by the Ministry of Sciences and Technology, ViÖt Nam. The present authors express their deep thanks to the Commission of Natural Sciences and the Commission of Earth Sciences for their supports.