Bïi phó mü¹, trÇn hång lÜnh²,
khiÕu v¨n gi¸p², hoµng ®×nh kh¶m¹

he Thæ Chu Archipelago is the southwesternmost territorial of the ViÖt Nam country, situated in 160 km northwest of the Cµ Mau Cape and 100 km southwest of the Phó Quèc Island, including 8 islands of different sizes. Formerly, on the administrative side it belongs to the An Xuyªn Province (Cµ Mau), but at present, to the Kiªn Giang Province. The largest island is just the Thæ Chu one, called as Thæ Ch©u by local inhabitants. On former maps it was named as Poulo Panjang. It has an area of nearly 10 km². The others are quite smaller with the emerged area of some square metres (Hßn Kh«) to one square kilometre (Hßn Tõ). Their names are as follows: Hßn Kh«, Hßn Hµng (named also as Hßn Chim, Hßn Nh¹n), Hßn KÌo Ngùa (also as Hßn Xanh), Hßn Tõ, Hßn Cao, and the two last islands lying at 50 km northeast of the Thæ Chu one, Hßn Cao C¸t and Hßn M« (also as Hßn C¸i Bµn) (Fig. 1).

Up to present day there are inhabitants only on the Thæ Chu Island, which is organized as the Thæ Chu Commune of the Phó Quèc District. On this island, there are quay, lighthouse, school, medical station and post office. Some boats keep the regular contact with the district and the mainland. There are convenient roads on the island.

In the island water for living requirements is taken from wells digged in the Quaternary terrace of 2 m to some tens metres in height in comparing with the sea level. Locally, water flows out from red beds, but its output is very small.

Geologically, the investigation on the archipelago is still very limited. In 1951, basing on some samples collected by a French naval unit E. Saurin assigned the sediments distributed on the Thæ Chu Island to the "Upper Sandstone" of Jurassic-Cretaceous age.

In 1967, during his investigations on the archipelago H. Fontaine first found the fossils of silicified tree trunk (studied by C. Serra) and considered the sandstone beds on the Thæ Chu Island as equivalent to the upper part of the Khorat Group in Thailand, i.e. Jurassic in age.

On the Geological Map of ViÖt Nam at 1:500,000 scale [6] these sandstone beds were assigned to the Phó Quèc Formation of Undifferentiated Jurassic age (J pq). In the National Geological maps at 1:200,000 scale [3] they were assigned also to this formation, but dated as Early Cretaceous (K₁ pq). Recently, after the assignment of the Phó Quèc Formation to Late Miocene by TrÞnh Dz¸nh et al (1999; Geological Archives) the dating problem of sediments on the Thæ Chu Archipelago becomes acute: they are also Miocene in age or keep their Cretaceous datation ?

The islands of the Thæ Chu Archipelago are all formed by sandstone interbedded with some siltstone and gravelstone. In general, they are weakly folded.

The beds on the Hßn M« (12 m high) and Hßn Cao C¸t (12 m high) islets plunge eastwards with the dip of 10-15o. As the attitude of the beds on the Thæ Chu Island it is nearly horizontal. The average height of this island is about 150 m. In general, the dip of the beds does not surpass 20o.

In February 2000, in the framework of the study on living water for the Phó Quèc and Kiªn H¶i island districts, Kiªn Giang Province realized by TrÇn Hång LÜnh of the Hydrogeological Party No.803 five boreholes were drilled on the Thæ Chu Island (TK.1-TK.5), one on the Hßn Tõ Islet (TK.6). Investigations were carried on in the Thæ Chu, Hßn Tõ, Hßn Cao C¸t, Hßn Xanh and Hßn Nh¹n islands. The sediment section on the Thæ Chu Island can be considered as most characteristic for all the archipelago.

According to drilling materials, the section can be subdivided into 8 members as follows (in ascending order).

Member 1: greyish, firmly cemented, medium- to coarse-grained sandstone, 10.5 m thick; found in the hole TK.2.

Member 2: greyish, locally brownish, cross-bedded, fine-grained sandstone interbedded with siltstone containing carbonized plant debris, 29.8 m thick; found in the holes TK.1, TK.2, TK.3 and TK.6.

Member 3: greenish-grey siltstone with interbeds of thin-bedded, firmly cemented fine-grained sandstone (whetstone), 12.2 m thick; found in the holes TK.1, TK.2, TK.3 and TK.6. A palynological assemblage occurs in the hole TK.1 with: Lycopodiumsporites sp., Cyathidites sp., Sphagnumsporites sp., Polypodiites sp., Laevigatosporites sp., Osmundasporites sp., Nevisisporites sp., Cheiropleuria sp., Gleicheniacidites sp., Brachyphyllum sp., Hymenophyllum sp., Myoporumpollenites sp., Monocolpopollenites sp. (sample TK.1/3). The sample TK.1/5 furnish some others: Cicatricosisporites sp., Dictyophyllidites sp., Coupersisporites sp., Leiotriletes sp., Tricolpopollenites sp..

Sporomorphs and microcospic plant debris occur also at the depth of 17 m of the hole TK.6, Hßn Tõ Islet (sample TK.6/17). They are flattened and carbonized. Beside the above species there still are Classopollis sp., Araucariacites sp., Quercidites sp., Staplinisporites sp., Schizaeacites sp., Eucommiidites sp., Cupressacites sp., Taurocusporites sp., Klukisporites sp..

Member 4: brownish-grey, medium- to thick-bedded, often cross-bedded, medium- to coarse-grained sandstone containing quartz gravels and pebbles in the lower part, 31.2 m thick; found in the holes TK.1, TK.2, TK.3, TK.4 and TK.6. Silicified tree trunks occur in sandstone of the uppermost part of this member exposed in the ChiÕn Th¾ng Beach, Thæ Chu Island, about 10-15 m above the sea level. Besides, these fossils appear also in Ng­ Beach, NhÊt Beach and Vßng Beach. Especially, in the Hßn Xanh Islet where greenish-grey fine-grained sandstone is widespread on the surface silicified tree trunk of big size and length is present in many places.

Although the above fossil does not exist in drilling core but, the observation in the field allows to determine that it occurs in the upper part of the member 4 or the lower part of the member 5. The collection is abundant, but there is not specialist on this fossil kind in the country. The collection of H. Fontaine just in the Thæ Chu Island was studied by Serra C. (1969) and determined as Prototaxoxylon asiaticum, Protopodocarpoxylon orientale, Pr. paraorientale.

Member 5: thin- to medium-bedded fine-grained sandstone interbedded with brown siltstone containing much plant debris, 25.5 m thick; found in the hole TK.4 and TK.5. Samples from the hole TK.4 yield sporomorphs, such as: Taurocusporites sp., Cibotium sp., Cicatricosisporites sp., Brachyphyllum sp., Tricolpopollenites sp., Eucommiidites sp., Ginkgocycadophytus sp., Araucariacites sp., Lycopodiumsporites sp., Lygodiumsporites sp., Gleicheniacidites sp.. Besides, there still are fresh- to brackish-water algae, such as: Leiofusa sp., Metaleiofusa sp., Veryhachium sp. (TK.5/65).

Member 6: brown to motley-coloured, thin-bedded siltstone; towards the hole TK.5 there is an interbed of greenish-grey, locally pinkish, fine-grained sandstone, 32.3 m thick; found in the holes TK.4 and TK.5. Siltstone from the hole TK.5 yields many sporomorph fossils and microcospic plant debris. They are flattened, and include Taurocusporites sp., Cicatricosisporites sp., Tricolpopollenites sp., Monocolpopollenites sp., Laevigatosporites sp., Eucommiidites sp., Gleichiniacidites sp., Cupressacites sp., Lycopodiumsporites sp., Lygodiumsporites sp..

Beside, there still are green algae and some dinoflagellates (TK.5/4-69).

Member 7: brownish, fine- to medium-grained sandstone interbedded with grey siltstone and some thin-bedded claystone in the uppermost part, 29.7 m; found in the hole TK.4 and TK.5.

Member 8: brownish pebble-bearing gritstone grading upwards into brown-grey, coarse-grained sandstone containing quartz grit and gravel, then yellowish, medium-grained sandstone, 18.7 m thick; found only in the hole TK.5.

The thickness of the formation in boreholes is 197 m.

The following beds occur on the surface including brownish-grey, thick-bedded, medium-grained sandstone with some interbeds of white-grey siltstone and clayey siltstone, 40 m thick. So, the total thickness of the formation reaches 237 m.

According to the lithological characteristics this section can be divided into 3 rhythms. The rhythm 1 includes the members 1-3, the rhythm 2: members 4-7 and, the rhythm 3: member 8 upwards. They consist of sediments of lagoonal, fluvio-deltaic facies. The section is covered by Quaternary terraces. Archeological vestiges of the Sa Huúnh Culture occur in the terrace of 20 m in height.

Studying the silicified tree trunks of the Fontaine’s collection Serra C. [5] referred them to Mesozoic. In their "Review of the geology and mineral resources of Kampuchea, Lao and Viet Nam" H. Fontaine and D.R. Workman [2] considered the fossils of tree trunk collected from "Red Beds" and "Upper Sandstone" in Kampuchea and ViÖt Nam as Middle Jurassic to Middle Cretaceous (with the tripartite subdivision of Cretaceous). The recent palynological collection from the boreholes was studied by TrÇn H÷u DÇn, according to whom "this assemblage is often met from Cretaceous to Neogene in many places of the world. In ViÖt Nam it has been found in Oligocene-Miocene sediments". The spores and pollens remains occur in two levels: under and above the silicified tree trunks. Therefore, the Cretaceous age of fossil-bearing sediments is most appropriate.

Many geoscientists often correlate the Thæ Chu red beds with the beds exposed in the Phó Quèc Island and in the Elephant Mt (South Cambodge). In this mountain French geologists collected spores and pollens determined by them as of Early Cretaceous age.

In recent years sediments in the Thæ Chu Archipelago have been usually assigned to the Phó Quèc Formation, which was recently dated as Late Miocene based on the collection of leaf imprints and spores and pollens gathered just in the Phó Quèc Island. This new situation leads to the need to separate the beds in Thæ Chu for establishing a new formation - the Thæ Chu Formation of Cretaceous age (K tc)*. It is composed of 3 parts: lower, middle and upper, corresponding to 3 sedimentary rhythms as described above. The authors of this paper consider that it is the Cretaceous section of the Phó Quèc - Thæ Chu Structuro-facial Zone. It is not completely similar to the Cretaceous sections occurring in ViÖt Nam. As for the relation between the sediments exposed in the Phó Quèc and Thæ Chu Islands, it needs further stratigraphic study in the Phó Quèc Island, at the same time to carry on new paleontological study on silicified tree trunks found in both Phó Quèc and Thæ Chu islands.

On the occasion of publishing this paper, the authors would like to express their sincere thanks to the Directorate of the Division of Hydrogeology and Ingineering Geology of South ViÖt Nam and the Chief of the Hydrogeological Party No.803 for their help during the investigation in the field.

1. Fontaine H., 1967. Note sur l’Archipel de Tho Chau. Archives gÐologiques du Viet Nam, 10 : 17-22. Sai Gon.

2. Fontaine H., Workman D.R., 1978. Review on the geology and mineral resources of Kampuchea, Lao and Vietnam. Proc. 3^rd Reg. Conf. GeoSEA : 359-608. Bangkok.

3. NguyÔn Ngäc Hoa (editor), 1996. Geological and mineral resources map of ViÖt Nam at 1:200,000 scale. Nam Bé Plain Sheet Series: Phó Quèc - Hµ Tiªn Sheet. Côc §Þa chÊt ViÖt Nam, Hµ Néi (in Vietnamese).

4. Ramingwong Tavisadki, 1978. A review of the Khorat Group of Thailand. Proc. 3^rd Reg. Conf. GeoSEA : 763-774. Bangkok.

5. Serra C., 1969. Sur des bois fossiles de l’Archipel de Tho Chau (golfe de Thailande). Archives gÐologiques du Viet Nam, 12 : 1-15. Sai Gon.

6. Vò Khóc, Bïi Phó Mü (coeditors), 1989. Geology of ViÖt Nam. Vol. 1. Stratigraphy. Tæng côc Má vµ ®Þa chÊt, Hµ Néi (in Vietnamese).