地学雑誌 Journal of Geography 103 (6) 623-636 1994

Preliminary Study of Sedimentation in

Lake Tonle Sap,

Shinji TSUKAWAKI *, Masafumi OKAWARA **, Kim-Leang LAO * * * and Motohiko TADA **

Abstrac t Tonle Sap, the largest lake in the Indochina Peninsula, lies in central Cambodia. Unique sedimentation is expected in the lake due to drastic changes in its water area between the rainy and dry seasons. As the first step to examine the sedimentary processes of the lake, bottom sedi- ments of the lake and the Tonle Sap , and surface soils of alluvial deposits were collected and examined in order to reveal the origin of the lake bottom sediments. As the results it becomes clear that clay minerals in the bottom sediments of the northern part of Lake Tonle Sap are derived both from surface soils on the alluvial deposits around the lake, and metamorphic and granitic rock bodies lying in the River basin. The latter is trans- ported, as suspended sediments by backward current in the rainy season, into the lake from the Tonle Sap River. The presence of marine creatures in bottom sediments of the lake suggests that the lake was closely connected with the sea during the last sea-level high stand. Furthermore, there is a strong possibili ty that annual changes due to alternating rainy and dry seasons will be recorded in the lake bottom sediments over the long geological period.

control of seasonal fluctuations in water level I. Introduction due to alternating rainy and dry seasons, char-

Lake Tonle Sap, the largest lake in the acteristics of the tropical monsoon region. In Indochina Peninsula, lies in central Cambodia. addition, the lake has been closely associated The lake is known as "the elastic water with the lives and culture of the Cambodian world", because its water area expands drasti- people since the Khmer Dynastic periods. cally in rainy seasons. It is also known as "the Consequently, it is expected that climatic and mud ocean" which refers to deep brown colour environmental changes, and their relationships of the water all year round due to a large with the rise and fall of various civilization amount of muddy suspended sediments con- in the Indochina Peninsula can be reconstructed

tained in water. Such settings hold out a over the period based on analysis of the lake

promising prospect for investigations of unique bottom sediments. sedimentary processes in the lake under the Bathymetry, water quality and aquatic re-

* Department of , College of Liberal Arts, Kanazawa University ** Department of Civil and Environmental Engineering , Faculty of Engineering, Iwate University *** Environmental Center , Japan Quality Assurance Organization

62 Fig. 1 Lake Tonle Sap and topographic features of Cambodia Dotted line around the lake indicates flooded areas in the rainy season. sources of Lake Tonle Sap have been reported of the largest on the Asian continent, (Mitusio et al., 1970 ; Kottelat, 1985 ; Lao, and the surface soils of alluvial deposits in the 1992, 1993 ; Matsui et al., 1993). However, northern part of the Tonle Sap Basin which no geological and sedimentological investiga- seem to be one of the important sources of tions have been carried out in the lake, except bottom sediments of the lake. for brief descriptions of bottom sediments of II. Lake Tonle Sap and Natural the lake and adjacent rivers (Tsukawaki and Environments of Cambodia Moriai, 1993 b ; Tsukawaki and Lao, 1994) . As the Cambodian political unrest has persisted Figure 1 shows topographic features of Cam- since the 1970's, only a limited survey can be bodia. Lake Tonle Sap lies in the central carried out. Therefore, as the first step in a part of the Tonle Sap Basin, a gentle topo- series on pursuing the above-mentioned sub- graphic depression with a NW-SE trend, in jects, the present paper illustrates preliminarily central Cambodia. The Cardomon Chain and the sedimentary processes of the lake based the Elephant Mountains with the highest peak mainly on the sedimentological and biological of 1,744 m above sea level, and the Dangrek properties of bottom sediments in the northern Escarpment, 765 m above sea level, all com- part of the lake and the Tonle Sap River which posed mainly of sedimentary rocks of the Upper connects the lake to the Mekong River, one Indosinias Formation ranging in age from Cre-

624 taceous to Palaeocene (Gubler, 1933 ; Saurin, inundated during high stands of water level as

1935 ; United State Geological Survey, 1971 ; well as paddy fields (Pl.-3, 4).

Workman, 1979) form the western, southern There is a network of numerous rivers and and northern boundaries of the basin, streams flowing directly into the lake from its respectively. The overwhelming part of the surroundings, but their flow is adequate only basin is covered mainly with alluvial deposits in the rainy season, except for a certain num- composed of unconsolidated silt, sand, gravel, ber of rivers, such as the River laterite (United State Geological Survey, 1971) originating in the Kulen Mountains which are and a variety of soil types such as acidic litho- a salient topographic high in the northern

soil and lateritic clay which are typical soil part of the Tonle Sap Basin. Small rivers and

types in a humid tropical region (Bridges, streams around the lake partly in the

1978 ; Garami and Kertai, 1993). The thick- rainy season. Many muddy deltas form in the

ness of the alluvial deposit reaches more than vicinity of river mouths in the coastal area of

a few Eundred metres at the in the lake in the rainy season (Pl.-5).

Viet Nam, whereas up to 15 m at Again in the rainy season, the flow of water

City (Workman, 1979). Several topographic which contains a large amount of muddy sus-

highs such as the Kulen Mountains, Phnom pended sediment derived from surface soils

Bakheng, , Phnom Dei and Phnom distributed in the Tonle Sap Basin through

Krom are scattered in the northern part of the this group of rivers joins water of the Mekong

basin (P1.-1, 2). River, originating in and bringing much melted

The of Cambodia is controlled main- snow from the northern slopes of the Hima-

ly by the tropical monsoon system influenced layas in the western part of China together with

by such local topographic highs as the Cardo- the heavy monsoon rain. Therefore, water of

mon Chain and the Dangrek Escarpment. the Tonle Sap River flows backwards to the

Therefore the dry season (November to May) lake in the rainy season, and water level of

is clearly distinguished from the rainy season the river increases more than 8 m in compari-

(May to November) with a brief semidry season son with it in the dry season in Phnom Penh

in July, year in, year out (Fig. 2). City (Pl.-7, 8). On the other hand, in the

Lake Tonle Sap is long, narrow and gourd- dry season, the inflow from the rivers decreases

shaped, having a NW-SE longitudinal axis markedly and water flows out from the south-

of 120 km and a maximum width of 40 km in eastern part of the lake through the Tonle

the dry season. The water area of about 3,000 Sap River to the Mekong River.

km2 in the dry season swells more than three- The lake water is reddish brown to yellow-

fold (about 10,000 km2) in the rainy season ish brown, and extremely low in transparency

(National Astronomical Observatory, 1993), thus (50-100 cm) even in the dry season (Mitusio the lake lacks fixed coastal lines. At low water et al., 1970). The temperature of surface water

level the depth is small wholly (3 to 6 m) is extremely high (26-30 •Ž) all year round (Mitusio et al., 1970) and at high water level and the pH is almost neutral (around 7.0) (Mi- the deepest parts are no more than 14 m tusio et al., 1970 ; Lao, 1992, 1993). Two (National Astronomical Observatory, 1993). hundred and fifteen species of fishes which come Forests in the coastal areas of the lake are under 127 genera and 47 families including

625 626 balloonfishes and rays which are generally made into a thin section and examined under

marine forms are recognized in the lake, the the microscope. Since the contents of micro-

Mekong River and adjacent rivers in Cambodia organic remains in the samples are insufficient

(Kottelat, 1985). for statistical analysis, the results is short of

a preliminary grasp. III. Sampling Method and Analytical For clay mineralogy, samples were first Procedures disaggregated in distilled water, followed by

The ;samples used for the present study were ultrasonic procedures for four hours, then clay

collected in August 1992 at the northern part fractions were concentrated by a centrifuge.

of Lake Tonle Sap and north of which Almost pure clay fractions almost free from

is the central sanctuary of Thom, and impurities were dried at room temperature,

March 1993 at the Tonle Sap River in Phnom and prepared for X-ray analysis. X-ray anal-

Penh City. Sampling devices were a cylindri- ysis was carried out on oriented powder smear-

cal aluminium can, 6 cm in diameter and 12 ed on a glass slide. X-ray diffraction patterns

cm long, with an 1 kg steel weight at the lake, for each of the samples were recorded with a

and a Seki-type grab sampler, 500 ml in vol- Mac Science type MXP-3 AHF X-ray powder

ume, at the Tonle Sap River. Surface soil diffractometer equipped with a graphite mono-

samples at Bayon were collected by a 6.5 m chromator, using Cu-Kƒ¿ radiation, tube voltage

long hand auger. Hydrogen ion exponent of 40 kV, tube current 20 mA, silt system of 0.5 •‹-

the samples was measured made with a Horiba 0.3 mm and scanning speed of 2 •‹/min. X-ray

type D-13 pH meter on about one gramme analysis was performed for untreated speci-

of moist samples dispersed in 5 ml of pure mens and treated specimens such as ethylene

water immediately after collected. glycol-solvated, glycerol-solvated, HC1-treat-

A smear slide was prepared and examined ed, K-saturated, Mg-saturated, and one hour

under the microscope for muddy sediments. heating treated at 300 •Ž and 600 •Ž specimens,

For sandy sediments, the entire sample was respectively.

heated over 24 hours at about 60 •Ž, and its IV. General Features of Samples and dry weight was measured. Then it was washed Sampling Sites repeatedly to remove mud, and dried and weighed again to obtain proportional mud General features of the bottom sediments

content. The remaining coarse material was from Lake Tonle Sap, the Tonle Sap River and

Plate 1. A view of the northern part of the alluvium plain in the Tonle Sap Basin looking west from the summit of in August 1992 (WB : W est Baray). 2. A view of the northern part of the alluvium plain in the Tonle Sap Basin looking north east from the summit of Phnom Bakheng in August 1992 (KL : Kulen Mountains, PD : Phnom Dei). 3. An aerial view of flooded forests in the southeastern coastal area of Lake Tonle Sap in August 1992. Dark spots in the lake are flooded trees. 4. An aerial view of flooded paddy fields south of the Tonle Sap River (TSR) in the rainy season, about 30 km northwest of Phnom Penh City in August 1992 (MR : Mekong River). 5. An aerial view of developing muddy deltas in the rainy season on the western coastal area of Lake Tonle Sap in August 1992. 6. The auger drilling site north of Bayon, in in August 1992. 7. Tonle Sap River in Penh City in the rainy season (August 1992, arrow indicates Chroy Chanvar Bridge). 8. Tonle Sap River in Phnom Penh City in the dry season (March 1993, arrow indicates Chroy Chanvar Bridge).

627 surface soils in the Tonle Sap Basin based on the south of the village of Chongkneas which Tsukawaki and Moriai (1993 b), Tsukawaki and is located about 13 km south of Siem Reap City Lao (1994) and Tsukawaki and Moriai (1993 a), alongside the Siem Reap River leading to the respectively, are as follows. lake (Fig. 3). Depths at these sites are about 1) Lake Tonle Sap 3 m. Only the farthest site (LTS-3) yielded Bottom sediments were collected in Lake bottom sediments the other two sites produced Tonle Sap at three sites (LTS-1, 2 and 3) off to only fresh leaves of land plants which are prob- ably flooded forests existing under the water. The lake water at each of the sites was very muddy and yellowish brown, being about 30 cm in transparency, with pH values of 7.5-7.7. The bottom sediments at site LTS-3 are com- posed of the top 1 cm moderate brown mud and lower 2 cm thick dark greenish grey sandy mud. The lower sandy mud is oxidized by exposure to the air, becoming light brown in colour in a few days. The mud contents of the upper and the lower layers are 98 and 84 Fig. 2 Average monthly mean temperatures and %, respectively. There is no great difference rain falls during the past 10 years in in sediment composition between the upper and Siem Reap City in the west of Cambodia (drawn from Garami and Kertai, 1993) lower layers. Under the microscope, yellowish

Fig. 3 Sediment sampling sites in the northern part of Lake Tonle Sap off the village of Chongkneas and the surface soil sam- pling site at Bayon of Angkor Thom

628 (a) (b)

(c)

Fig. 4 (a) General features of the junction of the Mekong, Tonle Sap and Bassac Rivers on the east of Phnom Penh City, (b) sediment sampling sites in the Tonle Sap River, and (c) a bottom profile of the Tonle Sap River along the sampling sites brown clays predominate muddy sediments of Phnom Penh City (Fig. 4(a) and 4(b)). The both layers, in addition to small amounts of river flows together with the Mekong River quartz and biogenic materials including dia- about 4 km below the sampling sites. The toms, pollen and sponge spicules. Sandy sedi- width of the Tonle Sap River at the sampling ments consist mainly of poorly sorted fine- sites is about 300 m and water depths are up grained sands, predominantly irregular-shaped to 5.8 m (Fig. 4(c)). Around the sampling quartz and lateritic rock fragments. Biogenic sites, the east bank of the river shows natural material:, include comparatively abundant dark topography, but the west bank has artificially brown plant debris and pale brown insect frag- been embanked. The water at each of the ments a:, well as an incomplete specimen of sites was pale greenish grey, being about 2 m ostracoda and a fossil planktonic foraminifer. in transparency, with pH values of 6.7-7.0. 2) Tonle Sap River No distinct internal structures were observed Bottom samples were taken in the Tonle Sap in these samples. River at five sites (TSR-1, 2, 3, 4 and 5), The sediments at all sites, about 4 cm thick, about 700 m above the Chroy Chanvar Bridge are moderately brown mud or sandy mud. (Japan Bridge) between the village of Daem The mud contents of these samples are high Kortimuoi and Russey Keo Oil Preserver in in the range of 71-99 %. Moderatel- brown

629 clays predominate muddy sediments, in addition to small amounts of quartz and biogenic materi- als such as diatoms, pollen and sponge spicules. Nearly all sandy sediments are very well sorted fine-grained sand, predominantly irregular- shaped quartz, feldspar, muscovite and biotite, with a small amount of magnetite and lithic fragments. Biogenic materials include compar- atively abundant black and brown plant debris and insect fragments. A complete carapace of the ostracode Candonocypris? sp. was detected from sample TSR-5. 3) Surface sois of Bayon, Angkor Thom Surface soil samples of the alluvial deposits in the Tonle Sap Basin were taken at an auger hole about 30 m north of Bayon of Angkor Thom in Siem Reap Province (Fig. 3; P1.-6). Angkor Thom, constructed in the beginning of the 13 th century (Khmer Dynastic times), is one of the largest Angkor monuments. The Siem Reap River which flows southward into Lake Tonle Sap along the embankment of the eastern moat of Angkor Thom, and a network of numerous watercourses connects the moat with the river and ancient water reservoirs such as W est Baray and moats of Angkor Vat (e. g. Lao, 1992). Tsukawaki and Moriai (1993 b) mentioned that surface soils, particularly clay fractions, are washed out by surface and ground Fig. 5 A soil columnar section at the auger waters due to heavy rainfall in the rainy season hole north of Bayon of Angkor Thom and driven into the Siem R eap River. on the Siem Reap River in the Tonle Sap Basin Auger drilling proceeded to a depth of 620 The location of Bayon is indicated in cm, above the ground water level at that time. Fig. 3. Arrows indicate sampling hori- The soil at the drilling site is composed of zons of BY 420 and BY 560 (simplified from Tsukawaki and Moriai, 1993 b). yellowish grey clay, silty clay, sandy clay and clayey sand with thin intercalations of reddish 420) and the reddish brown clay layer at a brown clay (Fig. 5) which indicates a probable depth of 560 cm (BY 560). surface of temporarily static ground water (Tsu- The mud contents and pH values of the sam- kawaki and Moriai, 1993 b). Samples for the ples BY 420 and BY 560 are 91 % and 98 %, present study were taken from the yellowish and 5.6 and 5.5, respectively. Pale brown grey sandy clay layer at a depth of 420 cm (BY clays predominate both samples, in addition to

630 small amounts of quartz. Sandy sediments con- solvated specimens suggests the presence of a sist mainly of fine-grained quartz and lateritic small amount of chlorite. The sharp peak at

rock fragments. No biogenic materials are 10.0 A and its high order reflexion do not

recognized in the samples except for fine vege- change by glycolations and 300 •Ž heating,

tational roots. indicating that the peak at 10.0 A is attributed

to illite. The basal peak at 7.1 A which is V. Clay Mineralogy not affected by HCl-treatment is attributed to

1) Lake Tonle Sap kaolin.

Figure 6 shows X-ray diffraction patterns 2) Tonle Sap River

for untreated and treated specimens of the Figure 7 shows X-ray diffraction patterns

lower greenish grey layer of sample LTS-3 for untreated and treated specimens of 2 cm

from the northern part of Lake Tonle Sap. thick surface sediments of sample TSR-2 from

The untreated specimen has a broad reflexion the Tonle Sap River in Phnom Penh City.

peak around 14.3 A and sharp peaks at 10.0, The untreated specimen has reflexion peaks at

7.1, 4.25 and 3.35 A. The sharp peaks at 14.4, 10.1, 7.2 and 3.34 A. The peak at 3.34 A

4.25 and 3.35 A are attributed to impure quartz. is attributed to impure quartz. After ethylene

Broad peak shifts to about 17 A by ethylene glycol-solvation, the peak at 14.4 A separates glycol-solvation and also by glycerol-solvation into weak peaks at 17 and 14 A, thus the dif-

of Mg-saturated specimen indicates that the fuse peaks are attributed to montmorillonite.

broad p eak is attributed to montmorillonite. The weak peak at about 14 A increased its However, a small peak at about 14 A observed in untreated, ethylene-solvated and glycerol-

Fig. 6 X-ray diffraction patterns for oriented Fig. 7 X-ray diffraction patterns for oriented aggregates of sample LTS-3 from the aggregates of sample TSR-2 from the northern part of Lake Tonle Sap Tonle Sap River in Phnom Penh City

631 intense, and its spacing is slightly contracted broad reflexion peak from 14 to 10 A, strong to 14 A after heating at 600 •Ž, while higher peaks at 7.2 and 3.58 A, and weak peaks at order reflexions disappeared. In addition, the 4.97, 4.19, 3.35, 2.39 and 2.00 A. After eth- peak at 14.4 A does not change by K-satura- ylene glycol-solvation, the broad peak sepa- tion, indicating that the peak is attributed to rates into 17 and 10 A. The diffuse peak at chlorite. The basal peak at 7.2 A is not af- 17 A is attributed to montmorillonite. The fected by HCl-treatment although the intensity reflexion peak at 10 A does not change by weakened slightly. Therefore, the basal reflex- glycolations, and heating at 300 •Ž and 600 •Ž, ion peak at 7.2 A is attributed to chlorite thus the 10 A reflexion and its higher order overlapping with kaolin. The 10.1 A reflex- reflexions are attributed to illite. The strong ion peak does not change by HCl-treatment peak at 7.2 A does not change by HCl-treat- and 300 •Ž heating, indicating that the reflex- ment, but collapses after heating at 600 •Ž, ion peak is attributed to illite. indicating that the 7.2 A reflexion peak and its

3) Surface soils of the Tonle Sap Basin higher order reflexion are attributed to kao-

lin. Figures 8 and 9 show X-ray diffraction pat- terns for untreated and treated specimens of X-ray diffraction patterns of BY 560 are very

similar to those of BY 420. The untreated spec- samples BY 420 and BY 560, from surface soils

imen shows a broad peak from 10 to 14 A, at Bayon, Angkor Thom locating on the allu- strong peaks at 7.2 and 3.58 A, and weak vial deposits in the Tonle Sap Basin. peaks at 4.97, 4.18, 3.35, 2.38 and 2.00 A. The untreated specimen of BY 420 shows a

Fig. 8 X-ray diffraction patterns for oriented, Fig. 9 X-ray diffraction patterns for oriented aggregates of sample BY 420 from sur- aggregated sample BY 560 from surface face soil at Bayon soil at Bayou

632 Fig. 10 X-ray diffraction patterns for oriented aggregates of untreated samples from Lake Tonle Sap (LTS-3), the Tonle Sap River (TSR-2) and surface soils at Bayon (BY 420 and BY 560)

These peaks show behavior coincident with the samples from the Tonle Sap River and those of BY 420, following a variety of chem- Lake Tonle Sap. They generally do not ascribe ical treatment. Therefore, clay minerals of chlorite to the products by natural weather- BY 560 consist mainly of montmorillonite, ing even under acid conditions in humid tropi- illite and kaolin. cal region (Shirozu, 1969 ; Sudo, 1974 ; Nishi- yama, 1985 ; Kodama, 1986). Therefore, con- VI. Discussion sidering sediment transport in water systems 1) Provenance of lake bottom sediments in Cambodia, it is highly probable that the The X-ray diffraction patterns of all the chlorite detected in the lake sediments is the analyzed samples are summarized in Fig. 10. derivative of river sediments. Tsukawaki and Illite, kaolin, montmorillonite and chlorite were Lao (1994) demonstrated that the sediments determined as clay minerals in the samples. of the Tonle Sap River in Phnom Penh City Kaolin and illite were commonly observed in are those derived mainly from Precambrian to every sample, and were particularly abundant Palaeozoic metamorphic and granitic rock in surface soil samples from the northern part bodies distributed in the Mekong River basin in of the Tonle Sap Basin. Montmorillonite was Viet Nam and transported through the frequent ty detected in surface soil samples from Mekong River into the Tonle Sap River by a the Tonle Sap Basin. It was also determined strong back current in the rainy season. Ma- as minor expansive clay mineral in the sam- tsui et al . (1993) noted that there is no great ples from the Tonle Sap River and Lake Tonle difference in water quality among Lake Tonle Sap, and was rather abundant in the sample Sap, the Tonle Sap River and the Mekong from the lake than the river. River, and concluded that the Mekong River Chlorite characteristically occurred only in water with muddy suspended material flows

633 into Lake Tonle Sap through the Tonle Sap comparatively abundant plant fragments, and River in the rainy season. Consequently, the the colour indicative of deposits under reduced chlorite detected in the bottom sediments of the conditions for a certain period of time, suggests lake should be derived from metamorphic rock a possible deposit during the dry season when bodies distributed in the Mekong River basin the sedimentation rate was markedly low. through the river by strong back currents in From these observations, there is a possibility the rainy season. that seasonal changes due to the alternating Taking all these factors into consideration, rainy and dry seasons are recorded in the mod- it becomes clear that clay minerals of the bot- ern and ancient bottom sediments of Lake tom sediments in the northern part of the lake Tonle Sap. are those derived both from the surface soils 3) Marine creatures in Lake Tonle Sap lying in the northern part of the Tonle Sap Three and eight diatom species were recog- Basin and rock bodies distributed in the Me- nized in the sediments from Lake Tonle Sap basin through rivers such as the and the Tonle Sap River, respectively (Tsu- Siem Reap River and back currents of the kawaki and Moriai, 1993 a; Tsukawaki and Tonle Sap River during the rainy season. Lao, 1994) . Among these diatom species are On the other hand, the composition and typical fresh water species, having floating or grain size of sandy sediments of Lake Tonle bottom dwelling habits. However, Actino- Sap are widely different from those of the cyclus cf. normanii or Thalassiosira sp. recog- Tonle Sap River. Muscovite and biotite deriv- nized in the sediments from the northern part ed from rock bodies in the Mekong River of Lake Tonle Sap are typical floating marine basin were not detected in the sample of the forms. In addition, sponge spicules were usu- lake, accordingly it is expected that the sandy ally present in the samples from the lake and sediments transported by back currents of the the river. This discovery shows clearly that Tonle Sap River in the rainy season do not sponges, being rare in fresh water, do exist reach to the northern part, but to the southern in the lake and the river. Apart from sponges, part, of the lake. it is known that there are a certain number of 2) Seasonal changes recorded in lake marine fishes such as balloonfishes and rays bottom sediments living in the lake (Kottelat, 1985). The pres- In the approximately 3 cm thick sediments ence of such marine creatures suggests that collected in the northern part of Lake Tonle there was a high stand of the global sea level Sap, it was confirmed that the lower dark green- which closely connected Lake Tonle Sap to the ish grey sandy mud layer is covered by an sea, and these marine creatures have been kept 1 cm thick moderately brown mud layer. The in the inland water since then. lake water at the time of sampling (the rainy VII. Conclusions and Open Subjects season) was thick with muddy suspended sedi- ments showing the same colour with the upper 1. Clay minerals in the bottom sediments layer of the bottom sediments. Thus, it is of Lake Tonle Sap are derived both from the clear that the upper layer consists of mud parti- surface soils of the alluvial deposits in the cles settled from the lake water. On the other Tonle Sap Basin and rock bodies in the Me- hand, the lower sandy mud layer containing kong River basin. They are those transported

634 into the lake through the Tonle Sap River of mica and chlorite minerals. NendoKagaku(J. with flooding waters in the rainy season. Clay Sci. Soc., Japan), 26, 67-77 (in Japanese with Englishabstract). Since, only one sample from the lake was Kottelat, M. (1985) : Fresh-water fishes of Kampu- examined in the present study, systematic sedi- chea. A provisory annotated check-list. Hydro. ment sampling and examinations should be biologia, 121, 249-279. Lao, K.-L. (1992) : The inland water environment carried out over the whole lake. of the Angkor region •\Preliminary survey of

2.There is a large possibility that seasonal water quality•\. Renaissance Culturelle du Cam- changes due to the alternating rainy and dry bodge, Sophia Univ., Inst. Asian Cult., 6, 109-128 (in Japanese), 200-213 (in English) and 347-362 (in seasons are recorded in the bottom sediments Cambodian). of Lake Tonle Sap. The presence of marine Lao, K.-L.(1993): Current status of water environ- creatures such as marine diatoms, sponges and ment in the Angkor Region. Renaissance Culturelle du Cambodge, Sophia Univ., Inst. Asian Cult., 8, fishsuggests that Lake Tonle Sap was closely 128-136 (in Japanese) and 363-369 (in English). connected with the sea in the past. A long Matsui, S., Kawaguchi, H., Doi, A. and Matsuda, core sampling should be carried out in the T.(1993): A report on the environment and water resources of LakeTonle Sap. Soc. Civil Eng., centralpart of the lake in order to develop Japan, Ann. Meet. Abstract, 139-145 (in Japanese thehistcry of the lake. with English abstract).

Mitusio, H., Ohno, M. and Meas, S. A.(1970): Lim-

Acknowledgements nological study of the Mekong water system,

Cambodia. Res. Rep. Kochi Univ., Nat. Sci., 16, Theau:hors express their sincere appreciation to 59-68. Professor Y. Ishizawa, Sophia University and mem- National Astronomical Observatory ed. (1993): bers of the 8 th and 9 th Angkor Survey Mission of Chronological scientific tables (Rika-nenpyo). Sophia University, Mr. Sun Kun, Ministry of Infor- Maruzen, Tokyo,1037p. (inJapanese). mation and Culture, the Royal Government of Cam- Nishiyama, T.(1985): Clay minerals produced by bodia,Mr, Chuch Phoeurn and Mr. Koum Sorith, weathering and their properties. Nendo Kagaku(J.

University of Fine Arts, Phnom Penh and Mr. S. ClaySci. Soc., Japan), 25, 101-106 (in Japanese Tanikawa, P. I. T. Travel (Cambodia) for various with English abstract). helps in carrying out this study. They wish to thank Saurin, E.(1935): Etudes geologiques sur 1'Indo- Professor T. Tamura, Tohoku University, Mr. T. chine du sud-est (Sud Annam, Cambodge oriental). Yoneda, Hokkaido University, Professor T. Miyagi, Bull. Surv.Geol. Indochine, 22, 1-419 (in Tohoku Gakuin University and Dr. T. Irizuki, Aichi French). University of Education for their various suggestions Shirozu, H.(1969): Genesis and chemicalcompo- forimprovement of this paper. They are grateful sition of chlorite. Sci. Rep., Fac. Sci., Kyushu to Professor K. Ishizaki, Tohoku University and Dr. Univ., Geol., 9, 105-111 (in Japanese with English T.Maruyama, Yamagata University for helpful abstract). suggestions during the course of this work. Sudo, T.(1974): Clay mineralogy. Iwanami Sho-

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トン レサ ッ プ湖 の 堆 積 作 用 に 関 す る予 察 的 研 究

塚脇 真二* 大河原 正文** ラオ キム リァン*** 多田 元彦**

カ ンボジア中部 に位置す る トン レサ ップ湖 は, 湖 の周辺 に分布 す る沖積層の堆積物組成 か ら同湖 雨季 と乾季 とで その冠水面積が3倍 以 上に も変化 にお ける堆積作 用の予 察的検討 を行 なった。 し,季 節的水位 変動 に支配 された特 異な堆積作用 そ の結果,粘 土鉱物組成 に もとづき,ト ン レサ の存在が予想 され る。 また,同 湖 はイ ン ドシナ半 ップ湖北部の湖 底堆積物 は,沖 積層 な らび にメコ 島最大 の湖で あ り,湖 底堆 積物 には同半島にお け ン河水 系 に分布 する地 質岩体 の双方 を起源 と し, る地域的気候 ・環境 変動が記録 されてい る可能性 堆積物 はお もに雨季 に湖 に流入す ることが判明 し が ある。そこで,ト ン レサ ップ湖研究 の第一 段階 た。 また,同 湖 での存在 が確認 され た海洋性 レリ と して,湖 の北部,同 湖 とメコン河 とを連絡 し雨 ックか ら,過 去 の高海水準期 における同湖 の南 シ 季 に同湖へ と逆流す る トン レサ ップ川,な らびに ナ海 との連絡 が推 定 された。 さらに,湖 底堆 積物 *金 沢 大 学教 養 部 地 学 教 室 に雨季 と乾季 との季節 変化記録 の可能性が示 され **岩 手 大 学 工学 部建 設 環 境 工学 科 た。 ***(財)日 本 品質 保 証機 構 環 境 計 画 セ ンタ ー

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