TrÇn v¨n b¹n¹, bountheung phengthavongsa²,
vò khóc³, vò ch©u¹, inpong homsombath²

On the territory of Laos PDR there are long ago well known Mesozoic depressions, such as the Dong Hen Basin in Lower Laos becoming famous by the discovery of dinosaur remains by French geologists in Mouang Phalane [3, 4], and of great deposits of mineral salt and anhydrite in Dong Hen; and the Vientiane Basin, famous by its deposits of potassic salt. Recently, through the geological mapping and mineral resources investigation at 1:200,000 scale of Middle Laos (1995-2000, editor - TrÇn V¨n B¹n) a Middle-Upper Mesozoic sedimentary basin has been appearing clearly in the Nam Theun River basin with the length of over 200 km and a width of about 15-20 km, lying in the territory of the Bolikhamxay and Khammouan provinces with the southeastern end extending a bit to the Mô Gi¹ area of ViÖt Nam. On the present structural plan it does not connect with the Vientiane and Dong Hen basins, but they have many common characteristics, possibly controlled by general tectonic activities of the region.

In this paper the authors would like to present basic features of the stratigraphy of this depression and preliminarily to interpret its development history on the basis sedimentary characteristics. The factual materials described in the paper were collected just by them in the field trips realized in the framework of the Project Geological mapping and mineral resources investigation at 1:200,000 scale of Middle Laos (1995-2000).

The Nam Theun Basin (Fig. 1) is filled up with Mesozoic sediments of Early Jurassic to Late Cretaceous age. These beds unconformably rest upon the Paleozoic basement composed of Upper Paleozoic carbonate-terrigenous beds with the direct contact observed in many places, where Lower Jurassic basal conglomerate unconformably rests clearly upon Carboniferous-Permian limestone.

 The sediments filling up the Nam Theun Basin are subdivided into 2 parts: 1) the lower one is described as Ban Lao Formation of Early-Middle Jurassic age; 2) the upper - as Nam Theun Group including 4 formations, such as the Nam Phouan Formation of Late Jurassic age, Nam Xot Formation of Early Cretaceous age, Nam Noy Formation of undiscriminated Cretaceous age, and Nong Boua Formation of Late Cretaceous age. Thick seams of mineral salt and anhydrite occur in the Nong Boua Formation. The description of the formations is given as follows.

LOWER-MIDDLE JURASSIC

The stratotype of the formation was studied in details by Vò Khóc and Vò Ch©u in the Ban Lao Village area, situated in north Gnommalat District, Khammouan Province. There, the beds have the monoclinal attitude covering limestone mounts of the karstic topography in the south of the Na Kay Plateau. The Lower-Middle Jurassic sediments plunge gradually northwards under the younger Jurassic-Cretaceous formations.

The section is subdivided into 2 subformations.

Lower subformation: including grey, more or less carbonate sediments containing marine bivalves, subdivided into 4 members.

- Member 1: dark grey, thin-bedded arkosic siltstone, 1 m thick.

- Member 2: grey, medium-bedded clayey limestone and silty limestone containing marine bivalves Cardinia sp., Goniomya sp., Hiatella cf. rotunda (Terquem) of Early Jurassic age; 5 m thick.

- Member 3: grey, medium-bedded calcareous sandstone, 2.5 m thick.

- Member 4: grey, thin-bedded marl interbedded with dark grey, locally banded, medium-bedded clayey limestone, 83 m thick; marl yields Hiatella sp..

The total thickness of the lower subformation in this section is 91.5 m.

Upper subformation: including an intercalation of chocolate-coloured, thin- to medium-bedded siltstone and fine- to medium-grained sandstone grading upwards into the upper part, where sandstone plays the main role; thickness: about 200 m.

The total thickness of the Ban Laos Formation in this section reaches 291.5 m.

The characteristics of the above section is fairly stable in a long band stretching from Ban Lao, Ban Bo eastwards through Xiang Dao to the Mô Gi¹ Pass area on the ViÖt Nam - Laos border. In the Ban Bo Village area, along the Nam Mac Stream, the beds of greenish-grey clayey limestone of the lower part of the section yields the following marine bivalves: Cuspidaria aff. C. annamitica (Saurin), Cardinia sp., Burmesia sp., close to fossils found in the member 2 of the Ban Lao Section. Near the Xiang Dao Village, grey, yellowish-grey weathering, thin-bedded marl exposed in a dry small stream yields following marine bivalves Pleuromya aff. P. concentrica Saurin.

TrÇn V¨n B¹n, Vò Khóc and Vò Ch©u have observed the basal beds of the formation in the Na Hin - Nong Kok Section situated on the side of the Road No.8. There, the formation begins by thick-bedded conglomerate with the pebbles mainly from limestone, some quartz and quartzitic sandstone, 5 m thick, lying directly upon grey, thick-bedded to massive Carboniferous-Permian limestone. The upper boundary of the formation has been clearly observed at this section, where the Nam Phouan Formation containing sedimentary copper mineralization of Late Jurassic age unconformably rests upon the described formation through a thin basal conglomerate bed.

The most part of the above cited fossils occurs in the lower beds of the §¨k Kr«ng Formation distributed in the Hoµ Huúnh area, Kh¸nh Hoµ Province. This bivalve assemblage has been dated as Pliensbachian of Early Jurassic. Based on these materials the Ban Lao Formation has been dated as Early-Middle Jurassic with the correlation with the Thä L©m Group of the N«ng S¬n Depression, Middle Trung Bé, ViÖt Nam. This group has also the lower part of marine sediments of Early Jurassic age grading upwards into the upper part of continental red beds of Middle Jurassic age.

UPPER JURASSIC - CRETACEOUS

As said above, the Nam Theun Group is subdivided into 4 formations, namely (in ascending order): Nam Phouan, Nam Xot, Nam Noy and Nong Boua formations.

UPPER JURASSIC

The stratotype of the Nam Phouan Formation was studied along the Nam Phouan Stream; besides there still are supplementary sections at Na Hin - Nong Kok, Nam Xot, Ban Bo, Phon Dou - Sop Sa and Nam Noy (Khammouan Province).

The Nam Phouan section is situated in 2 km west of the Ton San Village (Kham Keut District, Bolikhamxay Province), studied in details by TrÇn V¨n B¹n and Vò Ch©u. In this area the base of the section is poorly seen, but the upper part is continuously exposed. The beds in the section are monoclinal and sloping. They are subdivided into 3 members.

- Member 1: greenish-grey, medium-bedded, fine-grained arkosic sandstone with some interbeds of black-grey sandstone containing much organic matter, over 100 m thick.

- Member 2: violet-grey, medium-bedded siltstone, 15 m thick.

- Member 3: violet-grey to violet-brown, medium- to coarse-grained, locally fine-grained sandstone with some interbeds of violet-brown, thin-bedded siltstone in the middle part, 280 m thick. Sandstone beds in the middle part of the member yield some poorly preserved fossils of ostracod.

The total thickness of the formation in this section reaches nearly 400 m.

Along the Road No.8, near the Na Hin - Nong Kok Section, Vò Ch©u, Lª V¨n §iÒu and Savoi have observed the basal conglomerate bed of the formation unconformably resting upon black-grey Lower Carboniferous limestone of the Boualapha Formation. Near the Khoun Kham Village, this basal bed has the thickness up to 25 m, lying unconformably upon grey Carboniferous-Permian limestone of the Khammouan Formation, and Carboniferous fusulinids occur in limestone pebbles. This conglomerate bed grades upwards into violet-brown siltstone and ostracod-bearing sandstone similar to the member 3 of the Nam Phouan Section.

As along the left side of the Mekong River the Nam Phouan Formation generally begins by the basal conglomerate bed unconformably resting upon many older formations, inclusive of the Lower-Middle Jurassic Ban Lao Formation. The Late Jurassic age of the Nam Phouan Formation has been proposed on the basis of this stratigraphic relation.

LOWER CRETACEOUS

The stratotype of the formation has been designated in the basin of the Nam Xot Stream, situated in the Na Kay District, Khammouan Province; the supplementary sections were studied along the Nam Noy, Nam On and Nam Kading streams. The Nam Xot Section was studied in details by Vò Khóc and Vò Ch©u from the Song Khon Village to the Nam Xot Mouth where the stream empties into the Nam Theun River. It has been subdivided into 4 members.

- Member 1: light-coloured, thick-bedded quartz gritstone with well rounded grains, 30 m thick.

- Member 2: brownish, medium- to thick-bedded, locally cross-bedded, fine-grained, but in the middle part, medium- to coarse-grained quartz sandstone, 330 m thick. Medium-grained sandstone of the middle part yields a small list of sporomorphs with: Cyathidites sp., Classopollis sp., Ovalipollis sp., Pinus sp., Abies sp., Cedrus sp. of Early Cretaceous age.

- Member 3: violet-brownish, thin- to medium-bedded silty claystone, 50 m thick.

- Member 4: violet-brownish, medium- to coarse-grained quartz sandstone, 50 m thick.

The total thickness of the formation in this section reaches 460 m.

In other sections the Nam Xot Formation has the same lithological characteristics and section structure as in the Nam Xot Section, it means the formation begins by a thin basal gritstone bed grading upwards into mainly coarse-grained sediments, but the granulity becomes finer towards the upper part.

The Nam Xot Formation conformably lies upon the Nam Phouan Formation and beneath the Nam Noy Formation. Based on palynological materials, it has been referred to Lower Cretaceous.

UNDIFFERENTIATED CRETACEOUS

The stratotype of the formation was designated in the Nam Noy Stream, from the Sop On Village to the Kaoi Village. In this area the beds have a very sloping attitude; the section is subdivided into 4 members.

- Member 1: gravelstone and gritstone with the grains mainly from quartz, poorly rounded and sorted, about 20 m thick.

- Member 2: light-brown, massive, medium-grained quartz sandstone interbedded with light chocolate-coloured, thick-bedded arkosic sandstone, 50 m thick.

- Member 3: light-violet siltstone, chocolate-coloured, thin- to medium-bedded siltstone, 440 m thick; on the bedding surface of siltstone often occur primary dessication cracks.

- Member 4: brownish, thick-bedded fine-grained sandstone with some interbeds of chocolate-coloured, thin- to medium-bedded silty claystone, 50 m thick.

The total thickness of the formation in this section reaches 540 m.

The described characteristics of the section together with the thickness of the formation are stable in the studied area. However, no fossils have been found in it.

The Nam Noy Formation conformably rests upon the Nam Xot Formation and beneath the Nong Bua Formation. It has been considered as including the upper part of Lower Cretaceous and lower part of Upper Cretaceous.

UPPER CRETACEOUS

The stratotype of the formation is a natural section exposed along the Nam Xot Stream, near the Nong Boua area of exploration for mineral salt, from the Nam Theun to the Nam Xot villages. In this section the basal bed has been met showing the whole amount of the formation, but the seams of salt and anhydrite are not preserved, therefore the section in the borehole No.DD-1 drilled in Nong Boua is considered as parastratotype.

According to NguyÔn V¨n Canh and Bountheung Sayaseng, the section along the Nam Xot Stream includes 4 members.

- Member 1: violet-pinkish to grey, fine-grained quartz sandstone non-bearing or with very rare mica flakes, locally cross-bedded with small amplitude, 370 m thick.

- Member 2: light chocolate-coloured, fine-grained sandstone interbedded with darker coloured, medium-bedded siltstone, 285 m thick.

- Member 3: violet-pinkish, fine-grained sandstone interbedded with banded sandstone and chocolate-coloured claystone, about 100 m thick.

- Member 4: brownish-grey, thin-bedded fine-grained sandstone, 20 m thick.

The section drawn after drilling materials from the borehole DD-1 in the Nong Boua exploration area is as follows:

- Member 1 (157.9 - 155 m): chocolate-coloured fine-grained sandstone, 2.9 m thick.

- Member 2 (155 - 153.33 m): greyish to dull-white anhydrite, 1.67 m.

- Member 3 (153.33 - 68.06 m): dull-white crystallized halite, 85.27 m thick.

- Member 4 (68.06 - 61.76 m): white, friable gypsum, 6.3 m thick.

- Member 5 (61.76 - 12.53 m): greenish-grey claystone grading upwards into chocolate-coloured claystone interbedded with greenish-grey sandstone, 49.23 m thick.

The total thickness of the formation in the above section reaches 675 m, but in borehole - 154.4 m, possibly because the hole does not reach the lower part of the formation.

Along the Nam Theun River, especially in the section from Nikhom to Taleo villages, NguyÔn ThÕ ViÖt and Khamxeng Vilaykham found a section of the described formation exposed on the river side, whose upper part contains a gypsum member in a length of 50 m with some seams, among them the thickest is of 5 m in thickness. As in the Thangon borehole drilled in the south of Thakhek, a seam of white-grey halite of 70.4 m in thickness occurs at the depth of 200 m, that grades upwards into a white-grey anhydrite seam of 6 m in thickness.

Thus, the lower boundary of the formation has been observed only in the Nam Xot section in Nong Boua area, where the studied beds conformably rest upon the Nam Noy Formation. The upper boundary has been met in many places, showing that the formation is covered by Cenozoic formations aged from Pliocene-Pleistocene to Quaternary.

Based on the evaporite-bearing characteristics the Nong Boua Formation can be correlated with the Thangon Formation in the Vientiane Basin, the Dong Hen Formation in the Dong Hen Basin, as well as the upper part of the Ban Nayo Formation in the Khorat Basin. These formations have been dated as Late Cretaceous on the basis of paleontological materials. Therefore the Nong Boua Formation has been referred to Upper Cretaceous.

The Nam Theun sedimentary basin was formed on a basement composed mainly of Upper Paleozoic subdivided at present time into the Boualapha Formation of Early Carboniferous age and Khammouan Formation of Carboniferous-Permian age. Participating in this basement there still is a felsic volcanogeno-sedimentary formation of Middle Triassic age with a restricted distribution. Upper Triassic sediments are completely absent, including the Norian-Rhaetian coal-bearing formation which is widespread in North Laos.

The formation of this basin began at the beginning interval of Early Jurassic. The fossils found near the base of the Ban Lao Formation are of Pliensbachian age, but they occur far from the basal conglomerate bed, showing that the forming time of the basin may begin since Sinemurian. This time corresponds to the forming time of the N«ng S¬n and §µ L¹t basins in Central and South ViÖt Nam.

The sea stage existing in the Nam Theun Basin extended to the end of Early Jurassic. Around this time the sea regressed, and at the beginning of Middle Jurassic the continental regime appeared in the basin. This event coincides also with that happening in the N«ng S¬n Basin of Central ViÖt Nam, but differs from that happening in the most part of the §à L¹t Basin of South ViÖt Nam, where the marine regime continued to Middle Jurassic. It is to note again that, Lower Jurassic sediments of marine facies were first discovered by us in thus high latitude in the Indochina Peninsula (near the 18 N latitude), while in former works they were recognized only in the N«ng S¬n Basin (about the 16 N latitude).

During the Late Jurassic - Cretaceous stage, when the continental regime dominated in the Nam Theun Basin, two periods can be distinguished based on the sedimentary characteristics. The first of them extended from Late Jurassic to about middle Early Cretaceous. In this time interval the relief of the basin, as well as of surrounding areas, still bore the features of a young topography, the streams were still strong forming in the basin thick members of coarse-grained sediments, among them many beds are cross-bedded with large amplitude characterizing the streamline deposition. During this period the climatic conditions were still favourable for the development of organisms, that is expressed by the existence of dark grey beds containing much organic matter in the Nam Phouan Formation. These climatic conditions existed also in South Laos forming favourable conditions for the development of dinosaur packs in the Mouang Phalane area as well as of abundant assemblage of large-sized brackish-water bivalves in this region.

The second period extended from the last half of Early Cretaceous to the end of Cretaceous, during that the relief of the basin and surrounding areas becomes older and older forming large planation surfaces and large lakes of brackish water (dead seas), causing the formation of thick members of fine-grained sediments; the cross-bedding of these beds is of small amplitude caused by waves. During this period the climatic conditions transformed into arid ones with the evaporisation as main process, creating favourable conditions for forming thick members of mineral salt and anhydrite in the Nong Boua Formation, at the same time annihilating the organic world that was abundant before. The sediments formed during this period do almost not contain fossils.

The Mesozoic Nam Theun sedimentary basin became a plateau as we see at present under the action of neotectonic activities.

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