Malaysia comprises a broad range of rock types, from the sands and silts of the coastal plains to the granite of the Main Range and limestone outcrops of the Langkawi Islands. Geologists group them into units according to their type, age and environment of deposition. Formation is the most common unit used, each with its own geographical name.
Peninsular Malaysia, which forms part of the Sunda Shield, is the spine of the Peninsula. Its Triassic fold-mountain belt continues from eastern Myanmar through Thailand, Peninsular Malaysia, the Banka and Billiton Islands, and eastwards into Indonesian Borneo. The Triassic and older strata are essentially marine as opposed to the post Triassic rocks which are characteristically non-marine or continental in nature.
In Peninsular Malaysia, all the systems ranging from the Cambrian to the Quarternary, that is from 570 million years to about 10,000 years ago, are represented.
Throughout the Palaeozoic and Mesozoic eras, sedimentation was continuous; and due to the basin’s instability, major breaks are apparent within and between the Palaeozoic, Mesozoic and Cenozoic group of rocks. Almost half of Peninsular Malaysia, notably in the Main Range, is occupied by granitoids. This granitic emplacement coincides with the culmination of the late Triassic orogenic event during which all the older strata were folded and deformed.
Regional metamorphism is widespread and most of the Palaeozoic and Mesozoic rocks show slight to moderate deformation but the grade has never been higher than the green schist facies which differentiate one rock from another in appearance and composition. The contact metamorphosed rocks generally form narrow aureoles around the igneous bodies. Major mineralisation occurred during the granitic emplacement and commonly associated with faulting. Faulting is common in all rocks. At least three sets of faults have been recognised on a regional scale, the youngest of which occurred during the post-Early Creataceous period.
The Macincang Formation in the northwest part of the Peninsula provides the oldest evidence of sedimentation. It consists of shallow-water, current-bedded deposits in the Langkawi Island. By the Ordovician period, the extent of the basin could have trangressed as far as Melaka. By the Silurian period, thick successions of limestone and graptolitic shales were laid down. Volcanic activities also occurred and mainly acidic tuffs were deposited in Kedah and Northern Perak. During the Devonian, sediments continued to be deposited. They occur as thick succession of limestone in Central Perak and as clastics in the northwest. East of the Main Range, in the foothills regions of western Pahang and Southwestern Kelantan, they consist of graptolitic shales, cherts, quartzites and intraformational conglomerates with minor intrusives of ophiolitic rocks. Sediments of the Upper Palaeozoic e.g. Kenny Hill, Singa and Kati formations are uncomformable over the Lower Palaeozoic sequences.
Thick formations of Lower Carboniferous limestone in central Pahang and carboniferous shales with limestone lenses in east Pahang provide the earliest indications of the formation of the basin to the east of the Main Range. Sedimentation here was typically shallow marine and, in Kelantan, was probably continuous till Early Permian. The sediments deposited consist of four main facies: argillaceous, volcanic, calcareous and arenaceous. Here, sedimentation with intermittent volcanism appears to have continued from Carboniferous through the Permian and Triassic periods. The general relationship of the Triassic with the Permian is one of conformity. However, in Kelantan, Lower Triassic beds most probably overlie the uppermost Permian conformably. Lower Triassic limestones are common, but following that, the strata became more arenaceous and argillaceous in character. The Middle and Upper Triassic periods are characterised by a flysch-type sedimentation. Widespread volcanic activity with the eruption of andesite and other intermediate to acid tuffs and lavas occurred in the axial basin. The Upper Triassic orogeny which was also accompanied by granitic intrusions saw the cessation of marine sedimentation in the Peninsula.
Post Triassic sediments are mainly continental in character and are described as molasses. These Upper Jurassic-Lower Cretaceous sediments in the Malay Peninsula overlie the older rocks with marked unconformity. The sedimentary basins occupy a zone on the eastern portion of the Peninsula from Gunung Gagau in the north to Gunung Panti in the south. The sediments consisting essentially of sandstone, conglomerate and shales with minor coal seams and volcanics, show fluvial, lalcustrine and deltaic conditions of deposition.
The Tertiary rocks are distributed onshore as isolated lalcustrine basins underlying the Quarternary deposits and offshore areas mainly as thick continental areno-argillaceous sequences. The Quarternary deposits which consist mainly of unconsolidated to semi-consolidated gravel, sand, clay and silt occupy the coastal terrains and floors of some of the inland valleys. In the Kinta and Klang Valleys, the alluvium contains valuable concentrations of tin ore.
Sabah, situated at the northern tip of Borneo, is geologically complex. The oldest rocks are the Early Triassic metamorphic rocks of the Crystalline Basement, found mainly in eastern Sabah. Large bodies of granite, granodiorite, tonalite, ultramafic and mafic rocks intrude into the metamorphic rocks. The ultramafic bodies are distinctly elongated and commonly aligned east-west along the general metamorphic foliation trend.
During the Early Cretaceous period, limestone was deposited in several localities on an emerging basement in eastern Sabah. By Late Cretaceous, thick clastic and calcareous sediments, chert, limestone and volcanic rocks were deposited over a large part of northern Sabah. Deposition continued until the Eocene epoch.
By early tertiary, an elongated northeast trending marine trough already existed, extending from the Kalimantan border into western and northern Sabah. Deposition of thick sequences of sandstone and mudstone occurred uninterrupted into the Upper Miocene epoch until it was terminated by folding and uplift, accompanied by the intrusion of the Kinabalu Batholith.
During this major Late Miocene tectonic event, slump deposits and pyroclastics accumulated in several deep basins in eastern Sabah, followed by the deposition of sandstone and mudstone with minor amounts of limestone and coal in a chain of circular to sub-circular shallow basins. Rapid uplift in the Late Miocene epoch resulted in the formation of conglomerate at Lahad Datu and cessation of deposition in the area, except in the easternmost part – the Dent Peninsula – where Pliocene sediments were deposited in coastal swamps and shallow-marine waters.
From the Late Miocene to Quarternary epochs, extensive volcanism and associated shallow intrusions along the Semporna Peninsula and a batholith-size granitic intrusion at Mount Kinabalu occurred. The post-tectonic volcanic rocks that erupted in the Semporna Peninsula are typical of the calc-alkaline Pacific island arc type, being rich in soda-lime feldspar and generally low in potash. The early eruptions are mainly andesite, dacite and basalt. Several volcanic cones are still recognisable, and hot springs – remnants of volcanism – occur at several places in the Semporna Peninsula.
Quarternary deposits, consisting of coarse gravel, sand, silt, clay, peat and coral accumulated along the coast and are now found in raised terraces and in inland plains in Tenom, Klias, Padas valley, and the Sook-Keningau plains.
Most metallic mineral deposits and occurrences in Sabah occur along a central belt stretching from the northern islands of Banggi and Malawali, through Taritipan, Mount Kinabalu and the Labuk valley to the upper Segama valley – Darvel Bay area and Semporna Peninsula.
In Sarawak, the oldest formations date back to 300 million years. These ancient rocks form part of the West Borneo Basement which is the exposed part of Sundaland in Southwest Borneo, and is thus related to continental South-East Asia. The Basement is built up of Palaeozoic and Mesozoic rocks. Most of Sarawak is underlain by younger Tertiary sedimentary rocks especially the region northeast of the Lupar river.
Among the more prominent geomorphorlogical landforms are the many caves developed in limestone formations including the famed Niah Caves and Mulu Caves; and the high waterfalls developed over Tertiary sedimentary rocks of the Usun Apau Plateau and Hose Mountains in the interior of the state.
The Mulu Caves, developed in the Melinau Limestone during the Tertiary period, stand out as one of the most spectacular cave systems of the world.
* The Sunda Shelf and its continental rim is generally referred to a geographical region covering an area 1.8 million square kilometres wide, known as Sundaland, that extends from Southeast Asia southwards. The Sunda Shelf itself comprises the wide and relatively shallow continental shelf in the southern part of the South China Sea between the Malay Peninsula and Borneo, and in the Java Sea between Java and Borneo.
About one and a half million to 10,000 years ago, during the Pleistocene period, the Earth went through several cooling phases followed by warming. The cooling down phase is referred to as the glacial period or the Ice Age, which saw the expansion of the polar icecaps. As the ice extended across land and seas, a considerable amount of water was locked in the icecaps causing a lowering of the sea level on a global scale. As the sea level dropped, the Sunda Shelf dried and the extensive landmass created, called Sundaland, effectively merging Sumatra, the Malay Peninsula, Borneo and Java. Subsequently, during the warming periods, the polar icecaps melted and the Sunda Shelf was drowned again. Present-day Sundaland is so extensively flooded that only the higher terrain is visible above sea level. As we know it today, the Malay Peninsula is separate from Borneo and Sumatra.