laterites of sri lanka

Mineral. Deposita 17, 245-256 (1982)

MINERALIUM DEPOSITA © Springer-Verlag 1982

Laterites of Sri Lanka- A Reconnaissance Study

K. Dahanayake

Department of Geology, University of Peradeniya, Peradeniya, Sri Lanka

Laterites occur extensively in the lowlands of Sri Lanka whereas these are observed

as localized outcrops in the uplands and the highlands. In lateritic weathering profiles, Al-rich zones are found between and beneath harder Fe-rich layers. The processes of lateritisation and bauxitisation have produced residual products enriched in both Fe20 3 and AI20 3. In the lowlands and uplands the lateritisation processes have prevailed and the bauxitisation trend is more pronounced in the highlands.

Les gisements lat~ritiques s' observent largement dans les "lowlands" du Sri Lanka. Mats dans les "uplands" et "highlands" les affleurements lat~ritiques soni rares. Aux profils d' alteration les zones riches en A1 se trouvent entre ou sous les zones plus dures mats riches en Fe. Les deux processus de lat~ritisation et bauxitisation ont donn~ lieu aux produits r~siduels riches en Fe~O, ainsi qu' en AI20~. Ii a ~t~ observ8 que le proeessus de lat~ritisation se prodZii~plus souvent dans ~es "lowlands" et "uplands" tandis que la bauxitisation est plus repandue aux "highlands".

INTRODUCTION

The term laterite as defined by Buchanan (1907) represents a rock variety among many which look different although formed in similar ways. The main process of laterite formation is the relative or abso- lute enrichment of AI, Fe, Mn, Ni and Cr. Such a weathering process occurring under tropical or sub-tropical climatic conditions may lead to the formation of a range of products prominent among them in Sri Lanka being Fe-rich, AI- rich and Ni-rich laterites. Thus a given rock may be upgraded in a metal con-

tent by leaching of silica during the

weathering processes. In this work, the term lalerite is used to denote a rock whose Fe and A1 contents are enriched. The term lateritisation is employed to designate such processes, which result in the residual enrichment of Fe20 3 and the term bauxitisation is reserved

for those giving residues rich in AI20 3. Since the laterites studied have shown to be rich in both these oxides, these par- ameters have been utilized for the pur- pose of this work. In Sri Lanka up to now little work has been carried out on later- ires rich in Fe and AI. Herath (1963 and 1973) studied the mineralogy of some laterites and associated industrial clays.

0026-4598/82 /0017/0245/$02 .40

246 K. Dahanayake

b

• Lowlands

• Uplonds

x HighIends

•40 • 39

Colombo '

e41

e38

o,~3

• Anurodhopuro

• 42

e46

LOWLANDS

%35 ~.

eKandy?2 I e7 A j~A k

p.. . ,s6 ~ 7 ~ L \

• A 3 o \ 'JJ A - I to " l

29 J25~.," t~uw~ro Eliya

• L 'yx . X X {" • x x x 16 15

UPL ANDS ~52

LOWLANDS

A53

5?

Hatera

0 40kin E

Fig. i. Map of Sri Lanka showing the three physiographic regions. The num- bered circles, tri-

angles and crosses refer to sampling points in the lowlands, uplands and highlands respective- ly

Dissanayake and Vitanage (1977) discuss- laterites collected from a large number ed the preliminary investigations for of localities. bauxites in Sri Lanka. Dissanayake (1979) studied the mineralogy and chemical corn- The present study aims at determining position of some laterites. Dahanayake the degree to which the processes of la- and Dissanayake (1979) discussed the dis- teritisation and bauxitisation have been tribution and chemical composition of active in different physiographic regions

Laterites of Sri Lanka 247

PHYSIOGRAPHY, CLIMATE AND GEO- The three distinct physiographic regi- LOGY OF SRI LANKA ons were considered to be three pene-

plains by Wadia (1943) who attributed On the basis of height and slope charac- their existence to successive block up- teristics and the observable regional lifts (Fig. 2). The same author recogniz- topographic discontinuities the island of ed a general accordance of summit levels

Sri Lanka can be divided into three physio- of erosion remnants within the peneplains

graphic regions (Vitanage, 1970); lowlands,and also the existence of vigorous water- uplands and highlands (Fig. i). The lowlands have elevations ranging from 0 to 270 m above MSL; isolated ridges with gentle slopes (0-i0 °) occur in a vast coastal area dissected by sluggish streams and rivers. The uplands lie at elevations of 270 to 1060 m and are characterized by parallel ridge and valley topography and also by highly dissected plateaux with narrow arenas, amphithe- aters and domes. The slopes range from ]0 to 35 ° along the ridges and are con- trolled generally by lithology and structure. In the upland regions, well developed steep scarps are common and waterfalls are prominent along them. The arena floors are flat and undulating with gentle slopes ranging from 0 to i0 °. The

highlands are typified by a series of well defined plateaux which are rimmed by mountain peaks and ridges with elevations ranging from 910 to 2420 m above I~ISL. High level topographic discontinuities are common at the boundaries of plateaux.

falls along the plain boundaries. Vitanage (1972) referred to the different morpholo- gical regions as erosion surfaces.

The present climatic conditions of Sri Lanka can be described as tropical humid. In the study areas where mean annual rainfall lies between 2000 to 3000 mm the SW and NE monsoons as well as intermittent thunderstorms arrive more or less regularly. In most parts of the lowlands the higher rainfall figures are attained and in the uplands and highlands the pre- cipitation is less and approaches the lower figures. The temperature varies from 15 to 32°C throughout a given year in the different physiographic regions.

The island of Sri Lanka is underlain mainly by Precambrian metamorphic rocks except for the narrow coastal strips which contain Jurassic and Mio- cene sediments (Fig. 3). The Preeam- brian rocks have been divided into

Adam's Peak pidurutalagala ((2190 rn ) ( 2460 rn )

~.."~"--4,-,j "v~ ' ~ " ~ t F ~ F ~ ~ ~ .~,..,,~.L.-- ~ ,/" '-""- ~ ---v--

F F = Faults

(Af£er Wad~a, 1943)

Fig. 2. Three peneplains of Sri Lanka postulated by Wadia (1943) - an idealized cross section across Sri Lanka

248 K. Dahanayake

LEGEND

~ : : ~ Limestones & Sandstones. MIOCENE

~ - ~ Sandstones. Silfstones & - JURASSIC

~ ~ l " ~"\ ~ Highland ~ r s ~" ~.~ , . ,

( - ~ " ~ l . ~ . . ( f ~ ~ So.,h.e, ~--;., Ss , ~ ~,," ',...-. ~ ( , ";- k..~" X [+ "1-[ Granites

• 0 " , , , ' ; . 2 7 . " I ~ I'li,~i,I Doler,te

( - - V ' "> "i," / I ,-~.,()

,i / / .' / \ ' ; i l i i t

' , :;<" ' { 3 , ' : t 1 i ~ t.,<ond,: " ' ." l ! : ' i" ' ) t>¢" ~ \ \ " \ ' \ S i j ~ " ' _ ~

Colcmbo ~\ "Kadawaia. \ \ ,Nuwara E/G~ J" ' / -o<'\\ ~.,,,,\ \ - , , ~_ :, /

\kX",. x, . \ \.....-"'; s, , J /

o io ~,o km

~S "'~~ ~ v, -o . ,uds,ones ./" .~ I J~-} I 7ayan ~. rnplex

Highlan ' Group ~ PRECAMBRIAN

Southwestern Group J

Fig. 3. Map of Sri Lanka showing the major geological d'ivisions and the loca- tions of weathering profiles studied.

three major units by Cooray (1978): Vijayan Complex, Highland Group and Southwestern Group. Vijayan Complex consists of granites, gneisses, migma- tites and pegmatites. Highland Group comprises quartzites, charnockites, gneisses, khondalites, granulites and marbles. Cordierite gneisses, charnockites, quartzites and calc-silicate rocks form the Southwestern Group.

TECTONIC HISTORY OF SRI LANKA

Wadia (1943) noted that the hilly terrain in Central Sri Lanka represented fault mountains formed by block uplifts occur-

ring in stages giving a "low" and "high" topography (Fig. 2). The same author believed that the lowest peneplain was the oldest of the three post Jurassic peneplains (ist, 2nd and 3rd peneplains), where Gondwana deposits akin to those of Deccan landmass were deposited. The second and third peneplains are bounded by scarp faces of considerable relief (500-1000m) and these abrupt structural features are characterized by rings of waterfalls. The scarp faces are attributed to faulting by the same author (1943) and those formed around the second peneplain were thought to have formed during the time of tectonic activity which faulted


the Jurassic basins at Tabbowa and Andi- gama in NW Sri Lanka (Fig. 3). The 3rd peneplain, now forming the highest mountains of the Central Highlands, was considered to be the youngest formed probab- ly during Tertiary.

Thus it is believed that the Sri Lankan Preeambrian with mostly NNE and NNW trending ridges had been subjected to at least two successive uplifts caused by faulting. However direct evidences of faulting are not very much observed. Vitanage

(1959) observed shearing characteristics in the rocks of the area bordering the 2nd and 3rd peneplains. The eastern border area between the 2nd and 3rd peneplains is also characterized by a series of hot springs, dolerite dikes and serpen- tinite intrusive bodies (Munsinghe and Dissanayake, 1979). The western border too shows intrusive characteristics and zircon and hornblende granites (Cooray, 1978), carbonatite bodies have been re- ported in this area,

Vitanage (1970, 1972) suggested a series of vertical movements since the Precambrian upto the present with cul- minating phases in the Jurassic and the Miocene. The same author noted small scale displacements (of the horst-graben type) with NW and NE orientations while mapping a 8 km long tunnel of the Central Highlands which lies in the western bound- ary of the 2nd peneplain of Wadia (1943). Remarkable occurrences of horst-graben structural feature can be frequently observed in this region.

Thus it is seen that the Precambrian basement area of Sri Lanka as well as the area between Sri Lanka and India (underlain by Cauvery basin) have been characterized by continuous tectonic activity since the Upper Gondwana period. Observations by Cantwell et al. (1978) of stratigraphic sections in the Cauvery basin area towards N and NW of Srf Lan- ka suggest a reduction in the degree of thickness variations of stratigraphic in- tervals towards the end of Oligocene. Thus offshore stratigraphy tends to indicate that tectonism in Cauvery basin was waning towards the end of Oligocene.

However onshore observations in Sri

Lanka such as the hot water springs as-

sociated with a host of other structural

features attest to the continuation of tec-

tonism even upto this date.

METHOD OF STUDY

As a pilot investigation, samples were collected from different horizons to study variation in texture in three representative lateritic weathering profiles selected from the lowlands, uplands and highlands of Sri Lanka. Chemical analyses of the samples show the existence of an Al-rich layer between and beneath harder Fe- rich horizons. The texture of theAl-rich horizon is not significant in that it occurr- ed frequently as a clayey horizon. Based on this investigation, samples were collected from 72 lateritic weathering profiles located in the different physiographic regions of Sri Lanka (Fig. i). The sampling was carried out in the exposed (topmost harder vesicular)porous horizons of the weathering profiles located on road cuttings. The samples were analysed for their chemical composition and the results indicated a dominant pre-

sence of the three oxides - Fe203, AI20 3 and SiO 2.As such, their relative percentages were computed and the results were plotted in a ternary diagram with the oxides as end members (Fig. 7).

MODES OF OCCURRENCE OF LAT- ERITES

In the lowlands of Sri Lanka, laterite capped ridges are extensively developed. The weathering profiles can be observed

to depths reaching a maximum of 39m. In the recently sunk wells and road cuts, the laterite horizons with vesicular texture are found to depths of 10m and the remainder is formed of lithomarge and bedrock. The bedrock is commonly biotite gneiss, charnockite or garnetiferous gneiss.

In the uplands and highlands, laterites are sparsely distributed along slopes of ridges bordering the valleys. The observed weathering profiles show lateritic layers whose thicknesses rarely exceed

250 K. Dahanayake

o-

0 . . " " '

2 m - " ' . " . .

• . . . •

~ ~ • ~

" 7 , , . ,

3 m - - - . . . . .

~e~% Al 2 % Si% 0 to 50"/, 0 to 50"/. 0 to 50"/.

! I

\

I vesicular hard laterit ic cap

biscuit - like softer laterlte

concretionary clayey l a t e r i t e

nodular quartz grains

clayey l a t e r i t e

lifhornarge

weathered bed rock

(biotite gneiss ?)

Fig. 4. A lateritic weathering profile (Profile i) from the lowlands (Kadawata, Sri Lanka). Graphs indicate the relative vertical variations

of Fe203, AI203 and SiO 2

5m. Vesicular texture is not common. The lithomarge and decomposed bedrock are poorly developed.

The vertical distribution of lithologic horizons within three representative

Table I. Relative percentages of FegO3, A1203 and SiO 2 in the lat%r~ic samples of Profile l-lowlands, Kadawata, Sri Lanka (from top to bottom) (see also Fig. 4)

Depth below Fe203 AI203 SiO 2 surface

Om 25 34.31 Om 75 42.08 Im 40 26.42 Im 80 38.83 2m 20 34.67 2m 60 47.98

32 42 32 10 38 31 32 67 35 07 28 O1

33.25 25.80 35.26 28.49 30.25 24.00

weathering profiles from the lowlands, uplands and highlands are given below:

Profile I (Lowlands) (Fig. 4) - Table 1

From top to bottom: vesieular hard lateritic cap with pisolitic and vermicular textures (ira); biscuitlike (platy hard pan- like structures in a clayey matrix) softer laterite (. 8m); concretionary clayey laterite (. 8m); clayey laterite with overlying layer of nodular quartz (. 3m); lithomarge (. 3m); weathered biotite gneiss.

Profile 2 (Uplands) (Fig. 5) - Table 2

From top to bottom: vesicular laterite with pisolitic texture (I, 25m); vermicular laterite with associated quartz grains (. 7m); concretionary clayey laterite (. 7m); biscuit-like hard laterite (. 6m); vermicular soft laterite (. 5m);


o- ~

t m -

/J

2m- ~,e:% p , .~..o. ~.

" "O ' " " " ~

5rn. ~ t

%% %% si% 0 to 50°/, 0 foSO'L 0 to 50%

I [ I

vesicular herd /eterific cap

vermicular soft /oterlte with associqted free quartz grains

concretionory clayey leterite

biscuit- like massive herd lafer(te

vermicular softer laterite

free quartz grains

lithornerge

weathered bed rock

( garne t iferous gneiss)

Fig. 5. A lateritie weathering profile (Profile 2) from the uplands, (Kandy, Sri Lanka). Graphs indicate the relative vertical variations of Fe203, AI203 and SiO 2

Table 2. Relative percentages of Fe203, AI203 and SiO 2 in the lateritic samples of Profile 2-uplands, Kandy, Sri Lanka (from top to bottom) (see also Fig. 5)

Depth below Fe203 AI203 SiO 2 surface

Om 40 30.41 19.68 49.89 Im 35.07 23.19 41.72 Im 75 30.37 22.48 47.13 2m 40 31.96 27.03 41.OO 3m 25 32.02 24.44 43.53 3m 60 38.90 11.19 49.90

lithomarge with an overlying layer of nodular quartz (.5m); weathered garnetiferous gneiss.

Profile 3 (Highlands) (Fig. 6) - Table 3

From top to bottom: black humic soil (. 5m); lateritic soil with iron concretions (. 3m); mottled zone (locally biscuit-like texture) (. 5m); mottled zone, locally biscuit-like), associated with isolated quartz grains (. 9m); lithomarge (. 4m); weathered garnetiferous gneiss.

252 K. Dahanayake

• . . . • • ,

!.:i:.. :.' :/

z

C'o ~ °

.2 " ~.,)

0 to 50% t 0 to 50°/° I 0 to 50"/, z

i/ CHEMICAL COMPOSITION OF LAT- ERITES

The results of chemical analysis of samples collected from the three re-

%%

hum/c soil

la ler i t ic soil with iron concretions

motl[ed zone locally biscuit 7ike)

moft led zone

(locally biscuit-Tike)

~th i~olated uartz grains

ifhomarge

weathered bed rock ( garnefiferous gneiss)

Fig. 6. A lateritic weathering profile (Profile 3) from the highlands (Nuwara Eliya, Sri Lanka) Graphs indicate the relative vertical vari-

ations of Fe203, AI203 and SiO 2

presentative profiles for varying textures show a distinct Al-rich horizon in each of them (Figs. 4-6, Tables 1-3). In the profile of the lowlands, the AI- rich zone shows increasing AI203 and

• Lowlands

• Uplands

x Highlands

sio2

I

~ xo

• ~ • • A

A x

• 0

x

• A A

t Fo 50

Lateritisatlon increasing )

23

Fig. 7. Diagram showing the relative percentages of Fe203, AI203 and SiO2, in the lateritic samples collected from the three physiographic regions of Sri Lanka


Table 3. Relative percentages Fe203, AI203 and SiO 2 in the lateritic samples of Profile lands, Nuwara Eliya, top to bottom) (see

of SiO 2 contents for decreasing Fe203. In the profile of the uplands high AI203

3-high- and Fe20 3 values are shown for decreas- Sri Lanka (from ing SiO 2. In the highlands profile, high

also Fig. 6) AI203 content with appreciable decrease

AI203 SiO2 of Fe203 was noted though with high quantities of SiO 2.

The data of chemical analyses of iat- 19.92 28.75 15.83 48.26 critic samples collected from weather- 22.12 71 . 52 ing profiles of different physiographic 24.08 60.67 regions were computed to determine the

relative values for Fe203, AI203 and

Deep below Fe203 surface

Om 75 51.53 Im 20 35.89 Im 50 06.35 2m 15.25

Table 4. Relative percentages of Fe203, AI203, and SiO 2 in the lateritic samples of Sri Lanka (see also Fig. 7)

Sample Fe203 AI203 SiO 2 Sample Fe203 AI203 SiO 2 No. No.

O1 02 03 04 O5 06 O7 O8 09 I0 11 12 13 14 15 16 17 18 19 2O 21 22 23 24 25 26 27 28 29 3O 31 32 33 34 35 36

33.61 27.60 35.70 16.60 20.90 26.25 21 86 21 09 46 39 20 47 36 81 16 06 11 02 11 52 10 72 13 18 12 01 16.50 12.05 21.20 16.07 21.39 11.96 14.33 20.83 09.80 25.93 09.46 20.69 28.43 24.56 10.79 O6.71 13.92 13.45 04.87

40.81 46.69 27.30 28.50 28.30 32.50 28.95 30.77 1 8 . 0 1 2 6 . 2 3 2 3 . 6 5 3 0 . 4 7 25.07 24.03 29.34 32.30 32.84 20.84 2 8 . 1 5 2 9 . 9 3 34.02 30.90 37.97 29.09 31.94 31.99 26.40 34.83 29.06 5 0 . 1 6 2 2 . 8 0 2 7 . 8 1 3 7 . 2 7 2 6 , 4 2 2 5 . 9 3 0 9 . 5 1

25.95 25.70 36.90 54.7O 50.60 53.75 49.18 48.12 35 59 53 28 39 52 43 46 63 90 64 43 59 93 54.51 55.13 62.65 59.79 48.84 49.89 47.69 50.06 56.57 47.38 5 8 . 1 9 4 7 . 6 6 5 5 . 6 9 5 0 , 2 3 2 1 . 3 9 5 2 . 6 3 6 1 . 3 8 56.02 59.64 60.61 85.60

37 35.04 38 40.13 39 36.75 40 32.01 41 03.96 42 03.43 43 23.87 44 15.29 45 46.92 46 24.50 48 46.25 49 33.74 5O 45.49 51 28.94 52 24.41 53 15 77 54 11 29 55 17 14 56 10 56 57 23 96 58 68 15 59 13 19 60 37 73 61 32 96 62 27.94 63 28.50 64 28.54 65 33.32 66 18.13 67 36.34 68 34.61 69 27.68 70 40.81 71 12.72 72 21.81 73 09.05

13.90 11 92 16 43 18 16 11 88 19 94 18 O9 31 58 24.20 27.10 32.44 48.65 32.27 27.26 31.11 45.62 28.92 32.18 30.04 27.54 14.60 29.83 31 .08 39.68 30.77 31 .97 20.10 25.31 20.29 28.19 27.44 31 .79 26.20 38.06 59.2O 64,06

51 .05 47.93 46.80 49.81 84.14 76.61 58.02 53.]2 28.86 48.38 21 .30 17.59 22.22 43.79 44.46 38.60 59.78 50.66 59.39 48.49 17.23 56.96 31 .17 27.35 41 .28 3 9 , 5 1 41 .34 41 . 3 5 61 . 5 6 3 5 , 4 5 3 7 . 9 4 40.52 32.97 49.20 1 8 . 9 8 26.88

254 K. Dahanayake

Table 5. Chemical and mineralogical composition of some laterite samples from the physiographic regions (modified ater Dissanayake, and Vitanage, 1977)

Chemical Beruwala Kandy Nuwara compo- (low- (up- Eliya sition lands) lands) (high-

lands)

AI203 31.1 41.8 46.7

SiO 2 40.2 13.4 19.6

Fe203 10.4 15.4 06.6

TiO 2 01.9 O1.4 00.3

MnO 2 - _ _

Loss on 16.2 26.0 25.2

ignition

Mineralogical composition

Gibbsite 26.5 59.0 63.5

Goethite 06.8 19.2 08.2

Hematite 05.0 - -

Kaolinite 32.6 - -

Metahal- - 09.5 12.2 loysite

Quartz 25.2 09.0 14.O

Ilmenite 01.0 01.4 -

Anatase O1.4 O1.4 00.3

Pyrolusite - - -

Adsorp- tion O1.5 O1.6 01.4 water

SiO 2. The values so obtained were plotted on a ternary diagram with the three oxides as its end-members (Fig. 7, Table 4). The following observations can be made from the ternary diagram:

(i) The process of lateritisation is more pronounced in the lowlands and the

uplands.

(it) Significant number of samples shows a trend towards bauxitisation.

(iii) Bauxitisation trend is most notice- able in the highlands.

MINERAL COMPOSITION OF LATERITES

The observations regarding the bauxitisation trend can only be justified by a sys- tematic mineralogical analysis of the lateritic samples. The available combined chemical and mineralogical data of some random lateritic samples located in weathering profiles from lowlands (Beruwala), uplands (Kandy) and highlands (Nuwara Eliya) are given in Table 5. In spite of the random nature of these samples, the data tend to confirm the earlier observations based on the chemical analysis of laterites:

DISCUSSION

The available field and laboratory data suggest that both lateritisation and bauxitisation processes have been active in Sri Lanka. Lateritisation is restricted to the lowlands and the uplands whereas bauxitisation shows a general pattern in all three physiographic regions with a marked trend in the highlands.

In the lowlands, the deep weathering profiles show thick lithomarge and decomposed bedrock horizons which indicate the more or less static nature of the regional water table. The Fe-rieh layer is well developed and the lateritisation processes have been active. However the development of the Al-rich horizon, though with appreciable amounts of SiO 2 could be due to intermittent fluctuations of the groundwater levels. Such oscilla- tions would no doubt have favoured both lateritisation and bauxitisation processes, the latter being perhaps due to better localized leaching at relatively higher elevations. Further detailed studies on later- ires at such elevations could reveal stron- ger levels of bauxitisation.

In the uplands, with less developed lithomarge and decomposed bedrock horizons, the leaching processes have been more active to generate moderate to strong levels of both lateritisation and bauxitisation. However in the highlands the process of bauxitisation has prevailed. It is interesting to note that a significant majority of the highland


samples showed an exclusive trend towards bauxitisation. But this tendency has been conspicuous by its weak to moderate nature despite the existence of better drainage conditions and potential parent materials in the highlands. It is

believed that this could be due to the bi- assed nature of sampling which was done at weathering profiles along road cuttings. The roads being located for the most part along contours adjacent to the valleys, the drainage conditions of the weathering profiles could well have been poor. Therefore the results would obvi- ously manifest insufficient leaching with higher silica values.

Acknowledgements. The writer wishes to thank his colleague, Dr. C. B. Dissana- yake for the useful suggestions throughout this study. Dr. J. Berjaud, Scientific Counsellor of the French Embassy in Sri Lanka kindly provided necessary funds to get samples analysed at CRPG, France under the Sri Lanka-French Technical Co-operation Programme. Mr.L.R.K. Perera is thanked for his assistance in the field and laboratory. A grant from the National Science Council of Sri Lan- ka is also gratefully acknowledged. For drafting the figures and typing the manu- script the writer is grateful to Miss Ja- yanthi Wijesekera and Mr, K. Dunuhappa- wa.

CONCLUSION

The present exercise has been carried out on a reconnaissance basis with much emphasis on the harder and more lateritic horizons of the weathering profiles. A detailed chemical and mineralogical study of the various horizons of the profiles would reveal the exact degree of bauxitisation prevalent in Sri Lankan laterites.

As for the age of laterites and the associated processes, a study of the three distinct erosion/peneplain surfaces, cor- relation and determination of the ages of the surfaces are necessary. This could be realized perhaps by the application of Rb/Sr and 87Sr/86Sr isochron tech- niques (Dash and Hills, 1972) for dating weathering profiles. However the available structural evidence suggests that these surfaces could date back to a period extending from the late Mesozoic to early Tertiary. Towards the end of the Tertiary, tectonism in Sri Lanka was on the decline and relatively stable conditions had set in. It is probable that Sri Lankan laterites commenced to form during this epoch. Perhaps it is interesting to note here the suggestions (Va- leton, 1972; and Subramanian, 1978) that in the neighbouring Indian Peninsula, Tertiary had been characterized by tectonic and climatic conditions conducive to lateritisation.

REFERENCES

Buchanan, F. : A journey from Madras through the Countries of Mysore, Canara and Malabar. East India Co., London 2, 440-441 (1807)

Cantwell. T., Brown, T.W., Mathews, D. G. : Petroleum Geology of the North- west offshore area of Sri Lanka. Pro- ceedings Seapex Conf. Singapore (1978)

Cooray, P. G. : Geology of Sri Lanka Pre- cambrian. Proceedings, GEOSEA III Bangkok, Thailand, 701-710 (1978)

Dahanayake, K., Dissanayake, C.B. : The distribution and the chemical composition of some laterites of Sri Lan- ka. Abstr. International Seminar on lateritisation processes, Trivandram, India i0 (1979)

Dash, J. E. , Hills, A. F. : Distinguishing modern from relict weathering profiles, Nature 236, 73 (1972)

Dissanayake, C.B., Vitanage, P.W.. Exploration for bauxites in Sri Lanka, Preliminary investigations, J. Geol. Soc. India 18, 338-343 (1977)

Dissanayake, C. B. : Mineralogy and chemical composition of some laterites of SriLanka. Geoderma 23, 147-155 (1979)

Herath J.W. : The mineralogical composition of some laterites and lateritic soils of Ceylon. Ind. Mineral 4, 36-42 (1963)

256 K, Dahanayake :Laterites of Sri Lanka

Herath, J. W. : Industrial clays of Sri Lan- ka. Econ. Bull. No.l, Geol. Surv. Sri Lanka (1973)

Munasinghe, T., Dissanayake, C.B.: Is the Highland-Eastern Vijayan Boun- dary in Sri Lanka a possible mineral- ized belt? Econ. Geol. 74, 1495-1496 (1979)

Subramanian, K.S. : How old are later- ires in the Indian Peninsula? A sugges- tion, J. Geol. Soc. India 19, 269-272 (1978)

Valeton I. : Bauxite, Development in soil sciences I, North Holland: Elsevier 1972

Vitanage, P. W. : Geology of the country around Polonnaruwa, Mere. Geol. Surv. Ceylon i, (1959)

Vitanage, P. W. : A study of the geomor- phology and morphotectonics of Cey- lon, Proceedings 2nd Seminar Geo-

chem. Prospect. Methods. UNESCO, N.Y. 391-405 (1970)

Vitanage, P. W. : Post-Preeambrian uplifts and regional neotectonic movements in Ceylon. Proceedings 24th Int. Geol. Congress (Montreal) 3, 642-754 (1972)

Wadia, D. N. : The three superposed peneplains of Ceylon: their physiography and geological structure. Ceylon Dept. of Mineralogy, Prof. Paper i, 25-38 (1943)

Received November I0, 1981

Dr. Kapila Dahanayake Head-Department of Geology University of Peradeniya Peradeniya Sri Lanka

laterites of sri lanka

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