The Postgraduate Institute of Archaeology University of Kelaniya
Diploma in Palaeobiology
Geology of Sri Lanka S.M.Aravinda Ravibhanu Sumanarathna
Introduction to History of Geology in Sri Lanka
The frist account of Ceylon precrambrian was done by Wadia (1929). He was instrumental in completing the draft geological map covers entire Sri lanka, although some work had been previously by Ananda coomaraswamy (1903,1904,). Coates (1935) made more systematic clarification of rock. Most of the geological work in the early times was based on morphological variation with less weight on mineralogical & petrological identification. The geological knowledge of sri lanka was able to mark a substantial improvement after intensive geological mapping and the structural analyses of rock unit late sixties. Improvement of the understanding of Sri Lanka geology was facilitated further by the addition of scientific explanation to the field knowledge gained earlier 1. Adam ‘ s Three Peneplains Theory The idea of the geology of Sri Lanaka was initially come from the morphology of Sri Lankan terrain and peneplains concept. A peneplains is defined as a “Plain” produced by a long period of weathering and erosion. The geo morphology of sri lanka can best be described as three major peneplain as described by Frank Dawson Admas in 1929. The lowest peneplains surrounds the cental hill country in all side, and is a flat, sometimes gently undulating plain stretching to the cosat. It has as average height of less than 30m. But the rise around 90 m to 120m above sea level at the inland boundary .The middle peneplain rises from this inner edge of the lower peneplain in a steep step of about 300m, and reaches a maximum elevation of 760m above sea level: it is best seen in the south and east of the island.Within it and rising from it in another steep step of 910m to 1200m is the highest peneplain at a general level of 1500 m to 1800m, but rising at some places up to 2100m to 2450m. The all three peneplains towards the south can best be viewed from Beragala junction in Haputhalaee. Adams thouth that the highest peneplain was the oldest of the three and that the island has been rising thought out its geological history in a slow vertical movement, exposing more and more land to atomospheric erosion and denudation.
climatic zones
The highest peneplain is least like a peneplain composed of complex platues, mountain chains, massifs and basins. The southern margin of the heist peneplain is demarcated bu Horton plains, Elk plains, Moon plain (Nuwara- Eliya), Kandapola, Sita eliya plains and comprised of southern mountain in Sri Lanka. ( Piduruthalagala, Kirigal poththa, Thotupola). The southern margin of the highest peneplain is demarcated by the Adam’s Peak in the west, Namunukula mountain in the east world’s end in the south Ramboda- Pussellawa in the north east. The boundary between highest peneplain and the middle peneplain can best be observed at the world’s end. Number of waterfalls like Diyaluma and Bambarakanda, drops over its edges to the middle peneplain. Two cliffs, namely Haputhale Gap are also resulted from the steep rice from middle to highest peneplain.
Escarpments can be observed in many places defining the boundary between the middle and lower peneplains. Hairpin bends at KandyMahiyangana road mark the northern boundary whereas, the steep east facing escarps of the knuckles massif demarcate the north west boundary. As one looks southwards from the Beragala Juntion, Uggalkaltota escarpment can be seen. Uggalkalthota escarpment separates second and third peneplain at the south of Sri Lanka. Alagalla, near Rambukkana- Kadugannawa separates the Kandy plateau from the lowest penelpain at the south west.
B
A
Fig:02: Geo morphology of Sri
The lowest peneplain stretches from coast to coast in the north part of Sri Lanka and from Trincomalee to Hambanthota at the east and the south. Same features of lowest peneplain can be observed in the western Sri Lanka as well. Some isolated hills, popularly known as erosional remnants can be observed in the third peneplain (eg. Sigiriya, Yapahuwa ect.)
2. Wadia’s Block Uplift Theory During the times of D.N.Wadiya, who acted as the Gov.Mineralogist of Ceylon, had written a Memo in 1941 suggesting the highland were formed comparatively later by the vertical uplift of the large block of crust along very large faults, in terms of “Block Uplift”. In contrast, he proposed that the highest peneplain is the youngest,not the oldest as Adams suggested.
Lower Vijaya Series
Upper Khondalite series
Lower Vijaya Series
Fig:03: Formation of three Peneplains through Block Uplift Wadiya recognized two-fold rock sequences; lower vijaya series (igneous rock) and upper khondalite series (metamorphosed sedimentary rock).He believed the coates(1935) idea,much younger khondalite series is associated with sri lankan highlands where as older vijayan series located at the lowland. The idea of coates was nicely matched with wadia’s story of block uplift, which can be used to interpret the formation of khondalite series as younger group.Wadia recognized that charnockites granites and zircon granites as parts of the vijaya gneiss. In the late forties geologists were in a point that recognition of rock types are important than recoginition of Sri lankan basement as a whole.Fernando (1948) came with a suggestion to the wadia’s theory which suggested that the vijaya gneisses are basement rock of the khondalight found in the central hills.He further suggested that the gneisses fround in Sri Lankan basement were not uniform and preferred to classify them as heterogeneous basement were not components of various origins.He did not gave more details on the origin of the “Sri Lankan basement”.
Overview of Sri Lankan Geology Within Basement Geology of Sri Lanka Sri Lankan geology has received increasing attention in the last two decades.The early and late 1980’s geological works done by Japanese and German research group led by Prof. M Yashida & Prof . A. Kroner respectively shew new units,on the Sri Lankan geology . As a result of this , nomenclature of the rock units, as described in the special issue of the Journal of Precambrian Research on emphasis on Sri Lanka, is now used as the latest account on the Geology of Sri Lanka The island of Sri Lanka consists of crystalline and foliated metamorphic rocks of pre Cambrian age, 1000my to 2500my old. These rocks account to more than 90% of the surface and are divided into two distinct groups named Highland series and the Vijayan series. The highland series consists of a succession of meta-sedimentary rocks, quartzite, marble, white granulite and granulitic gneiss, charnockite and gneiss, garnet-sillimanitegraphite-gneiss, garnet-biotite gneiss and biotite gneiss. The argillaceous rocks of the Highland series found in the south western area of Sri Lanka consists of more feldspathic and garntiferous rocks and wollastonite rock. The younger Vijayan series which further sub divided into Eastern Vijayan and western Vijayan consists mainly of migmatitic microclinehornblende-biotite gneiss and calcareous gneiss. Marble and quartzite are rare in eastern Vijayan rocks and they also occupy the eastern and south eastern low lands and uplands. The western Vijayan rocks are characterized by the occurrence of pink-feldspar–hornblende
granites, migmatites and gneiss ss and also occur in northwestern and northernn lowlands l and uplands(Cooray 1984).
Lanka The Basement Geology of Srii L The Proterozoic bastment of Sri Sr Lanka exposes substantial parts of the loweer continental crust. Four different unit weree distinguished on the basis of isotopical, geoc ochronological, geochemical and petrological con onstraints.There are five major complex in Sri Lanka La (Cooray 1994). 1.Highland complex, 2.The 2 Vijayan Complex, 3.The Wanni Com omplex, 4.The Kadugannawa Complex., 5.Limes estone complex The Highland Complex(HC), an association of interlayered, predominantly gran ranulite- facies, granitoid gneisses ( metamorpho hosed igneous rock – orthogenisses ) and clastic to calcareous shallow – water metasedimen ent (metamorphosed sedimentary rock – paragn agneisses) .The gneisses were ubiquitously intruded i by mafic dykes that are now w structurally concordant(layered parallel to gneissic) g with their host rocks.The HC consi sist mainly of interbeded meta – pelites, quar artzite, marble, metabasites and charnockites.. C Calc- siticate gieisses, sapphirine –bearing granulites, gr cordierite –bearing gneisses and corun undum bearing gneisses are exposed in minor qu quantities. Some granulites are exposed in the sou southern part of VC near buttala and katharaga gama.They comprise rocks similar to those of HC and are interpreted as tectonic nappes nam amely:Buttala klippe, Katharagama klippe, Kuda da oya klippe
Fig:04: Simplifed Geology mapp of Sri Lanka showing
The Wanni Complex (WC), an upper amphibolite to granulite- facies assemblage of 770- 1100 Ma granitoid, grabbroic, charnockitic and enderbitic gueisses, migmatites, minor clastic meatsediments ,including garnet- cordierite gueisses , as well as late to post – tectonic granites. The Characteristic feature of rock in Wanni Complex is the absence of thick marble and quartzite bands, which is the dominant feature in HC. The Vijayan Complex(VC) , an upper amphibolites- facies suite of 1000 -1030 Ma calc – alkaline granitoid gneisses, including augen- gneisses, with minor amphibolites layers (derived from mafic dykes) and sedimentary xenoliths such as metaquartzite and calc – silicate rock.The Kadugannawa Complex (KC), rock of the kadugannawa complex are seen in the cores of the six doubly plunging synforms,which were name as Arenas by Vithange (1972). The dominant rocks of the KC are hornblende –biotite gneisses, granitic, grandioritic and tonalitic association, which are identical, rock types of the WC as well.In some recent publications, kadugannawa complex is included in the WC because of this similarity. The rock from VC,WC, and KC, which are predominantly of orthogneisses, yield relatively younger deposition ages at 1.1 Ma ago.This implies that igneous activity had occurred after the deposition of rock of HC .
Formation of Sri Lankan Curst and Rocks Gondwanaland, was the southernmost of two supercontinents that formed part of the Pangaea supercontinent from approximately 510 – 200 million year ago.The theory of plate tectonics suggests that most of the landmasses and related rocks which formed most of Antartica, Australia, Madagascar, Mozambique and Tanzania, Indaia and Sri Lanka were part of a single southern landmass called East Godwanaland.About 20 million years ago during the Miocene an arm of the Tethys Sea stretched down the west coast of india, a long narrow gulf and gradually encroached on the land surface between India and Sri Lanka for the frist time, into an island,froming a narrow strip of sea, that we referred as the Palk Strait and Gulf of mannar today.Cooray (1967) belived that, the island has retained its original outline and remained more or less above the sea for ten million years or so since the end of Miocene.
Fig:05: Gondwana Fragments
After the Miocene rock deposited and uplifted above the sea, there was another period of geologically inactivity, except fro weathering and erosion.Then in the last million years or so , there were slight changes of sea level much wind activity and climetic variations during which the quaternary sediments were laid down, mainly on the miocenc limestone, and also party on the metamorphic basement.
Fig:06: Formation of Cauvery basin between Sri Lanka and India during the break up of Gondwana.
Igneous Rock of Sri Lanka. Rocks with igneous origin are very rarely explosed in the Sri Lanka crust. The series of serpentinites (ultramafic igneous rock) are reported in many places especially along the HC/VC boundary in Ussangoda & Ginigalpelessa.Ussangoda has one of largest serpentine outcrops of the five known occurrences in the HC/VC geological boundary(three of those serpentinites area of Sri lanka are Ginigalpelessa & Indikolapelessa both close to Udawalawa and Ussangoda near Nonagama junction). This tectonic boundary strectches from the SE coast curving in NE direction to the coast in Trincomalee. It is suggested that the HC plate had been overd – thrusted over VC during the Pan- Africa event(The event called the collision of East and West Gondwana). This model explains that during or after the period of overthrusting , the serpentinte rich ultramafic magma was intruded to the crust and belives to be formed green coloured serpentinites rock including Ussangoda Serpentinties.It seems that from the Cambrian to the early Permian , Sri Lankan Precrambian crust underwater magmatic activites , after the peak event occurring in the late Jurassic to early cretaceous, forming the series of ultramafic rock along the HC/VC boundary.The carbonate – rich rock from carbonatite magma occurs as several low hills at Eppawala.This deposit is now popularly know as “Eppawala Apatite Deposit” . Several Dolerite dikes of igneous origin are present on the eastern side of the island, intrusive into the best know being Galllodai dike near welikanda. The pure intrusive granites are reported to be found in many places including Tonigala,Ambagaspitiya and Arangala, but later confirmed that they have been metamorphosed too some extent in the recent times.
Sedimentary deposite in Sri Lanka Jurassic Deposits: After the premian age, there is no evidence for any kind of deposition in Sri Lankan crust, perhaps the agitated environment during that period may prevent any thick deposition.Collision of plates or continuous subduction or obduction of lithospheric plates may prevent such deposition.In Sri Lanka such deltaic deposites are found as three small basin.This is the next immediate deposition reported after resting the Precambrian crust in the calm environment.Sediments were deposited in the early Jurassic period forming the siltstone, mudstone and arkosic sandstone now exposed on the Thabbowa,Andigama and pallama, The beds are faulted into the wanni complex basement rocks as horst and graben structure. Miocence Diposits: After the Jurassic rocks formed. Little has happened geologically to the Sri Lankan crust unit about 20 Ma. The northern and north –western part were submerged under the sea during the detachment of the Indian Peninsula with Sri Lankan crust in the Miocence epoch. A thick series of sediment, mostly the a fossiliferrous limestone was then deposited, which we now know see underlying the whole of the Jaffna peninsula and surrounding island and the north western coastal belt extending southwards to beyond puttalm.This is popularly known as Wanathawilluwa limestone, and it is almost flat bedded in highly fossiliferrous with gratropods, and foraminifera.A small deposit of miocenc rock is present at Minihagalkanda on the coast.The miocene rocks rest unconformably on the eroded bastment of the crystalline complex. These rocks exhibit a range of sedimentary structures, produced by soft sediment deformation.They from minor features, such as the hills at Arukakkalu and kudriramalai.Outcrops of limestone are also well exposed in the Parappukkadantan and Adampan area on the mainland near Mannar island.Sediment similar to Tabbowa,Adigama,Pallama beds were recently found to be present in the drill cores in the mannar area,lying below 10 metres of Miocene beds.It is still possible that these types of deposits of Jurassic age may exist in the faulted basins within the crystalline besement, hidden by latter deposits of Miocene and Quaternary age. Quaternary Deposits: Formation of laterite belongs to Pleistocene epoch .The laterite is a mottled deep red, yellow, or reddish brown ferruginous earth showing vesicular structure, the vesicles (cavities within the rock) are often lined by paler material.It is extensively developed in Colombo district and long the southwest coast, extending down to Mathara & Thangalla.The laterite is clearly the alteration product of underlying crystalline rocks due to fluctuation of water table which dissolve silicate minerals while keeping iron- bearing minerals in the lateritic profile. Fluctuation of water table is high in the wet zone of Sri Lanka and hence laterites are mostly obsevered in the wet zone. The typical laterite profile shows the transition from partly decomposed granites or gneisses throught an intermediate zone of kaolin and angular quartz to the typical cellular laterites of high porosity and permeability, usually capped by loose layer of small ferruginous nodules.
Summary There are five major complex in Sri Lanka (Cooray 1994). Highland complex, The Vijayan Complex, The Wanni Complex, The Kadugannawa Complex, Limestone complex. The rock units found in Sri Lanka crust are described here in the chronological order with their probable origin during the break up of the Gondwana. More than 90 percent of rock found in the Sri Lanka rocks are belongs to crystalline metamorphic rock with Precambrian age.Since then some diposition were recorded according to the geological timescale with some unconformities.
References Adams,F.D. (1929). The Geology of Sri Ceylon,Canad. J. Research, vol1:pp.411-525 Coates, J.S., (1935). The Geology of Ceylon. Spolia Ceylanica. 19 (2): pp101-187. Cooray, P.G., (1963). The Erunwala Gravel and the probable significance of its ferricrete cap. The Ceylon Geographer, 17: pp39-48. Cooray, P.G., (1967). An Introduction to the Geology of Sri Lanka. Ceylon National Museum Publication, Colombo, pp 184-176. Cooray, P.G., (1968). A note in the occurrence of beachrock along the west coast of Ceylon. Jour. of Sedimentary Petrology, 38: pp650- 654. Cooray, P.G., (1968): The geomorphology of the part of the northwestern coastal plain of Ceylon. Zeitschrift fur Geomorphologie, NF Supplement 7,pp95-113. Cooray P.G., 1984. An Introduction to the Geology of Sri Lanka. 2nd Revised Edition. Ceylon National Museum Publication, Colombo, pp 140- 142.
Cooray, P.G. and Katupotha, J., 1991. Geological evolution of the coastal zone of Sri Lanka. Proc. Symposium on "Causes of Coastal Erosion in Sri Lanka", CCD/GT7., Colombo, Sri Lanka, 9-11, Feb. 1991 Cooray P.G., (1984).The geology of Sri Lanka,Natn.Museums.Sri Lanka, pp.340 Dahanayake,K and Jayaawardena, S.K.(1979).Study of red and brown earth Deposites of North west Sri Lanka,J.Geolo. Soc. India, 20:433 440 Wadia, D.N. (1945).The three superposed peneplains of Ceylon,Cey.Dept.Mineralogy records, Prof. Pap.,1, pp 25- 32 Yashida, M., Funaki, M., Vithange, P.W. (1992). Proterozoic to Mesozoic East Gondwana : the juxtaposition of India, Sri Lanka and Antarctica, Tectonics 11: 381- 391
