by Lee Chai Peng
Welcome to Langkawi, the Island of Legends. The Langkawi group of 104 islands located some 30 km off the coast of Perlis and 112 km north of Penang is a paradise for geologists. One finds within it some of the best and most interesting exposures of Palaeozoic rocks in Malaysia ranging in age from Cambrian to Permian. These sedimentary formations consisting of both clastics and carbonates had been deposited within differing palaeoenvironments and under different palaeoclimatic conditions to provide a highly variable set of rocks to tease the minds of geologists who attempt to understand and interpret them. In addition, these sedimentary rocks had been intruded by younger granites that produced interesting metamorphic imprints at their contacts. These rocks had also undergone at least two phases of tectonic deformation that created interesting geological structures for study.
Since the publication of Jones’ 1966 Geological Map of Langkawi and his 1981 Geological Survey Malaysia District Memoir 17 on the “Geology and mineral resources of Perlis, North Kedah and the Langkawi Islands”, many more studies had been carried out on various aspects of its geology. Many new outcrops have been created by the recent development projects on the island and these are incorporated into this field guide to enable you to sample the various sedimentary formations as well as the igneous, metamorphic and structural geology of this geologists’ paradise.
The Stratigraphy of Langkawi
The Palaeozoic sedimentary rocks in Langkawi had been placed under four formations. They are, from the oldest to the youngest, the Machinchang, Setul, Singa and Chuping formations. Their geographical distribution and stratigraphic relationships are given in Fig. 1 and Fig. 2 respectively. They are mostly shallow marine shelf type deposits forming part of a belt that extends from south China down through Burma, Thailand, northwest Peninsular Malaysia and north Sumatra.
Its name is derived from the conspicuous long serrated quartzite ridge of five peaks aptly called Gunung Machinchang or Mat Cincang meaning “chopped-up mountain”. This is a thick succession of predominantly arenaceous rocks with minor interbeds of conglomerates, phyllitic slates and tuffs. The base of the formation is not exposed and the oldest part of the formation is the pelitic rocks exposed in the core of the Machinchang Anticline at Teluk Datai. The top of the formation has several thin calcareous horizons before passing unconformably up into the succeeding Setul Formation limestone near Teluk Sabong.
The Machinchang Formation is poorly fossiliferous and only poorly preserved fragments of trilobites and brachiopods had been found. Better preserved fossilized saukid trilobites and orthid brachiopods are found on Tarutao Island just 5 km north of Langkawi in Thailand which give a late Cambrian to early Ordovician age to the uppermost part of the Machinchang Formation. Fission track studies on zircon gives an age of 555±37 Ma. Interesting trace fossils and sedimentary structures such as various types of cross-beddings, convolute bedding and load casts can be seen in the upper parts of the formation at Pasir Tengkorak and Pulau Jemurok.
Palaeocurrent studies on the abundant cross-beddings found in the formation gave a predominantly westward downcurrent direction towards the palaeoshore because of the dominance of beach-barrier shoreline processes acting at that time. The overall depositional environment from the facies sequence is that of a high-destructive, wave-dominated delta which had built over an offshore shelf deposit to produce a series of barrier-beach sands cut by small channels.
The Setul Formation is made up of predominantly dark coloured shelfal limestone with minor black detrital bands. It is largely metamorphosed on the main island but good fossiliferous outcrops of the limestone are present in the vicinity of Pulau Langgun. The impure limestone and hardened black mudstones contain a rich fauna of both larger fossils such as gastropods, bivalves, nautiloids, brachiopods, trilobites and graptolites as well as microfossils such as conodonts, ostracods and tentaculitids.
The Setul Formation ranges in age from early Ordovician to early Devonian. It is divisible into a Lower Setul Formation of Ordovician age occurring below the Lower Detrital Band on Pulau Langgun and a Silurian to Devonian Upper Setul Formation above the band. The top of the formation is represented by the Upper Detrital Band, a largely arenaceous unit with minor argillaceous beds at Teluk Memplam on Pulau Langgun. The formation has been estimated to be about 1500 m thick.
The Singa Formation, named after Pulau Singa Besar, is a clastic unit typified by crudely laminated, dark grey, poorly sorted mudstones with scattered dropstone horizons. The dropstones that range in size from granules to boulders are mainly of sandstone with subordinate limestone, vein quartz, granitic, volcanic and metamorphic rocks. They are probably glacial dropstones derived from some palaeo-Gondwana margin in the Carboniferous. Fossils are only common in its uppermost parts but burrows and soft sediment deformation structures can be found in many parts. Long paired vertical burrows exceeding a meter are found on Pulau Tepor. The brachiopods found in the uppermost part of the formation associated with dropstone horizons at Kilim, Batu Asah and Pulau Singa Besar are “cold-water” forms of Permian age. Other fossils found associated with them are bryozoa, bivalves, gastropods, corals and crinoid ossicles. Sandstone beds are few and often calcareous. The Singa Formation is best exposed in the southwestern part of Langkawi. The basal contact with the Rebanggun Beds is unexposed and the top is gradational into the overlying Chuping Formation limestones that are exposed on Pulau Singa Besar. The age of the formation is probably Carboniferous to early Permian because it is bracketted between the uppermost Devonian Rebanggun Beds and the basal Permian Chuping Formation.
The Chuping Formation is a unit of thickly bedded to massive, light coloured limestones that sits conformably above the Singa Formation in the southeastern part of the island. It can be easily distinguished from the dirtier and darker coloured Setul limestone. The Chuping limestone is fossiliferrous in its basal part with fusulines, brachiopods, bivalves, gastropods, corals, bryozoans and algae giving a late early Permian or early Middle Permian age. Chert nodules are common at its base in Pulau Singa. The famous Langkawi marble is a white saccharoidal ornamental stone quarried from metamorphosed equivalents of this limestone in southeastern Langkawi and Pulau Dayang Bunting.
Two granitic bodies are found in the islands. The Raya and Tuba granites cover some 113 km2 of the islands. The huge Gunung Raya massif which rises to 878 m forms the highest part of the island. It is late Cretaceous in age while the smaller pluton in Pulau Tuba is late Triassic in age. The granites are petrographically similar and hard to distinguish in the field. Both are porphyritic biotite granites with tourmaline clots and veins. Large K-feldspar phenocrysts and spherical quartz phenocrysts are common. Alignment of the large K-feldspar crystals is common at the margins of the plutons. Some acid volcanics or tuffs occur as thin, fine grained, pale green layers in the Machinchang Formation indicative of acid volcanism during Cambrian times.
The Langkawi area had undergone deformation and low grade regional metamorphism during the late Permian producing at least one set of low angled cleavages in the older Machinchang, Setul and Singa formations. These rocks had been metamorphosed to slates, phyllites, quartzites and fine grained marbles in the process. Contact metamorphism adjacent to the granites had recrystallized some of them to hornfels, mica schists, coarse marbles and tough metaquartzites.
The main structures that can be observed in the field are faults, folds and joints. Larger scale structures could be worked out from the dips and strikes of beds and cleavages found in the rocks aided by remote sensing. The major fold axes seen in the Machinchang area and on the western part of Pulau Dayang Bunting are oriented north-south while those in the northeastern part of the island are oriented northwest-southeast.
Three episodes of deformation had affected the rocks. The earliest occurred in early Devonian time about 370 Ma producing overturned to recumbent folds in the Lower Palaeozoic Machinchang and Setul formations by its east-west compression. A second weaker deformation in the Late Permian with north-south compression produced more complex structures in the Lower Palaeozoic rocks and simple open to slightly asymmetrical folds in the Upper Palaeozoic rocks. The third deformation resulted from east-west compression in post–Permian times producing the present day north-south folds accompanied by major faults the most notable of which is the Kisap Thrust in the eastern part of the island which caused the Lower Palaeozoic Setul Formation to be thrusted over the Upper Palaeozoic Chuping Formation. Normal faulting developed following late Triassic granite intrusion and the region has been tectonically stable thereafter.
Localities to visit on Langkawi Island
Locality 1 : Datai Road and Machinchang North Coast.
This scenic road along the eastern and northern edges of the east Machinchang area has several roadcuts and access to coastal exposures that allows one to visit the different parts of the very thick Machinchang Formation. Ripple marked sandstones, slump bedding and load casts as well as beautiful honeycomb weathering of sandstones can be seen at the coastal exposure at the picnic area at Pasir Tengkorak while thick trough cross-bedded quartzites can be seen at the roadcut near the northernmost point of the Machinchang area. One should also look northwards across the sea to the Thai island of Tarutao to see the beautiful cuestas formed by the extension of the Machinchang rocks into Thailand.
The eastern limb of the Machinchang Anticline is somewhat more disturbed structurally than the western limb. This can be clearly observed if one were to take a boat ride along the northern coast from Pulau Jemurok in the east to Tanjung Chinchin in the west. It is worthwhile to land at Teluk Sabong near Pulau Jemurok to examine the transitional beds from clastic Machinchang to carbonate Setul and Pulau Jemurok itself to view the storm deposited sandstone beds as well as beautiful Dictyodora trace fossils and fragmentary trilobite lag deposits in the northwestern part of the small island.
Locality 2 : Pasir Hitam (Black Sands Beach).
This popular tourist spot is named after the black placer sands on the beach. The black sand is composed of mostly tourmaline with minor amounts of ilmenite and zircon derived from the eroding margins of the Raya Granite and concentrated by the repeated winnowing away of the lighter minerals such as quartz and micas or clays by the beach processes. Souvenirs, hawker food and drinks are available at this stop. One could also get a spectacular sunset view of the Machinchang hills from this spot.
Locality 3 : Tanjong Rhu.
This stop takes us into Setul limestone country. One can see the dark coloured, well bedded limestone outcropping irregularly as headlands and islands around Tanjong Rhu. Outcrops of the limestone enroute are unfortunately not very exciting and generally lacking in fossils compared to the unmetamorphosed and tectonically less disturbed exposures around Pulau Langgun.
Locality 4 : Telaga Air Hangat (Hot Spring)
A small hot spring issuing salty water at around 50° to 60° C is located near the contact between the Setul Formation and Devonian red beds in the northern part of Langkawi Island. The hot water is probably produced by sea water seeping through faults connected to the hot basement Raya Granite nearby. This locality has been developed into a tourist complex with craft shops, dining facilities and daily cultural shows.
Locality 5 : Kisap Thrust near Kilim
This locality is a roadcut across a zone of intensely brecciated greyish Chuping limestone. The actual contact between the Chuping and adjacent Setul limestone is however not seen at this outcrop.
Locality 6 : Kedah Marble Industries, Bukit Batu Putih A brief stop at this showroom will allow one to view and purchase souvenirs made from the famous Langkawi marble varieties quarried from various parts of the island group.
Locality 7 : Batu Asah
Dark coloured, poorly bedded and sometimes bioturbated mudstone and siltstone of the Singa Formation are found in the cut hillsides around the housing estate here. Rare pebbly and fossiliferous horizons are exposed near the top of the hill behind the housing estate. Fossils found in the more calcareous intervals include early Permian brachiopods indicative of a cool temperate climate at the time of deposition. Other fossils found are bryozoans, bivalves, gastropods, ammonites, crinoids and corals.
Locality 8 : Gunung Raya
The roadcuts along the scenic road up Gunung Raya expose the Raya Granite and roof-pendants of metamorphosed Singa Formation rocks near the top of the mountain. There are several vantage points for viewing the various parts of the island enroute. Granitic sills and tourmaline quartz veins can be seen near the top of the mountain.
Locality 9 : Behind Underwater World, Tanjung Mali, Pantai Chenang
Several isolated dropstones of quartzite and granite embedded in typical dark coloured hardened Singa Formation silty mudstone can be seen at this outcrop. Tight slump folds are visible especially at the southern end of this coastal outcrop.
Locality 10 : Telaga Tujuh (Seven Wells Waterfall)
This popular waterfall with seven wells or potholes eroded into the granite is located at the upper part of the waterfall located at the southern boundary of the Machinchang area. The granite is porphrytic with aligned feldspar phenocrysts. The contact between the Machinchang Formation quartzites and the Raya Granite is exposed at the lower part of the base of the waterfall. Dark coloured xenoliths and spotted hornfels are present at the contact.
Localities to visit on other islands
Locality 11: Pulau Langgun
One can hire a fishing or tourist boat from Tanjong Rhu to visit Pulau Langgun and Pulau Tanjong Dendang. A brief stop can be made at Gua Cerita to visit the cave before proceeding to Pulau Langgun. For those who are interested in fossils, the northwestern coast of Pulau Langgun has the most interesting section from the top of the Ordovician Lower Setul Limestone exposed near Teluk Memplam, going into the Lower and Upper Detrital Bands and ending in the red mudstones of the Rebanggun Beds. This section can be walked through during low tide and the weathering of the clastic rocks has produced some of the most interesting geoart to the gastropod limestones exposed at the narrow neck of Pulau Tanjong Dendang is strongly recommended. A return journey to Tanjong Rhu via the mangrove creeks surrounded by spectacular limestone karst of the Sungai Kisap which takes one through a low sea arch and a newly cut channel through a limestone ridge is not to be missed.
Locality 12: Pulau Dayang Bunting (Pregnant Maiden Island)
This island is so named because its profile from a certain view resembles that of a pregnant lady lying on her back. Tasik Dayang Bunting or the Lake of the Pregnant Maiden located in the southwestern coast of this large island to the south of Langkawi is a popular tourist stop for swimming and canoeing. It is a big freshwater lake surrounded by steepsided karstic limestone had formed in a collapsed sinkhole that had developed in the light coloured Chuping Limestone which forms the western half of the island. A white marble quarry can be observed in the northwestern part of the island where the limestone-granite contact is located. The eastern part of the island is occupied by Setul Limestone that has been thrusted against the Chuping Limestone.
Locality 13 : Pulau Singa Besar
The northern coast of this island is worth exploring as it shows the different facies of the Singa Formation inclusive of some thin sandstone beds and dropstone horizons interbedded with the dark mudstones that typify the formation. The transition of the Singa to the Chuping Formation can be seen in the northeastern corner of the island. The basal Chuping limestone is riddled with odd shaped chert nodules that have been selectively exposed by the differential weathering of the limestone. An animal sanctuary has been created on this island with the introduction of local wild animals and island hoppers regularly stop at the park HQ to view the exhibition on local wildlife and trek around the island.
Locality 14 : Pulau Ular
This longish island is located a short distance from the Awana Portabello Resort at Tanjung Malai. It is accessible by hiring a boat from the nearby fishing village. The yellowish thinly bedded mudstones and siltstones exhibit slump folding in the northwestern part of the island. A beautiful wave-cut platform separates it from the darker coloured pebbly mudstones and sandstones in the eastern part of the island. Angular limestone blocks together with rounded quartzite pebbles are found in the dropstone horizons near the eastern end of the island. Also seen are abundant trace fossils in the layer beneath the horizon with angular limestone dropstones.
Locality 15 : Pulau Tuba
Take a ferry across from the Kuah jetty to the fishing village located at the northeastern part of Pulau Tuba. A very interesting occurrence of a Tuba granite sill that has intruded into a black detrital band at the base of a hill of Setul Limestone can be seen to the west of the long jetty leading to the village. The granitic intrusion has contorted parts of the dark detrital band into small irregular folds and reacted with the limestone to produce greenish brown skarn deposits of interesting minerals like vesuvianite and grossularite. The folded detrital band can only be accessed at low tide.
1) Jones, C.R. 1981. Geology and mineral resources of Perlis, North Kedah and the Langkawi Islands. Geological Survey Malaysia District Memoir 17, 275 pages. (includes a detailed geological map of Langkawi).
2) Marilah Sarman, Ibrahim Komoo and Kadderi Md . Status Permuliharaan Sumber Geologi Di Malaysia. In: Ibrahim Komoo,Mohd Shafeea Leman, Kadderi Md Desa & Ibrahim Abdullah (eds.) Warisan Geologi Malaysia (Geological Heritage of Malaysia). LESTARI UKM, p.13-46. (has a comprehensive bibliography of geological research papers on Langkawi ).
Di petik dari : https://gsmpubl.wordpress.com/2014/09/02/geological-field-guide-to-langkawi/