LA CLASIFICACIÓN POR MATERIAS Y CC.AA DE LOS IMPUESTOS PROPIOS

It is recognized that Pre-Tertiary basement outcrops widely in the Barisan Mountains in the western part of Sumatra and in the Tin islands (i.e., Bangka, Singkep, Bintan and Belitung) in the eastern part of Sumatra. These Pre-Tertiary rocks are variably metamorphosed and were termed as the “Barisan-Schiefer” and the “Old-Slates

Formation” in Central Sumatra, and the “Crystalline Schists” in the Lampung area at the southern Sumatra.

Occurrences of granites in the eastern part of Sumatra and the Tin islands suggest that a large part of Sumatra is underlain by a highly differentiated Pre-Carboniferous crystalline continental crust, the age of which extends back into the Precambrian (Barber and Crow, 2005). However, Eubank and Makki (1981) obtained Rb-Sr ages of 426+41.5 Ma (Silurian) and 335+43 ma (Early Carboniferous) from granites in the basement beneath the Central Sumatra Basin. These granites suggest that an older basement underlies eastern Sumatra, although the Proterozoic and Lower Paleozoic rocks occur in the Malaysian Langkawi Islands only some 30 km to the NE of Sumatra (Barber and Crow, 2005). Oil wells drilled in eastern Sumatra intersected sedimentary rocks containing palynomorphs from near the Devonian-Carboniferous boundary (Eubank and Makki, 1981).

A stratigraphic scheme which divided the Pre-Tertiary rocks into: the

Carboniferous-Permian Tapanuli Group, the Permo-Triassic Peusangan Group and the Jurassic-Cretaceous Woyla Group was proposed by Cameron et al.(1980) and Pulunggono and Cameron (1984). However, this stratigraphic terminology should be applied only to northern Sumatra where the units were first defined (Barber and Crow, 2005).

The oldest sedimentary rocks that have been reliably dated are Carboniferous- Permian, although Devonian rocks have been reported from a borehole in the Malacca Strait, and undated gneissic rocks in the Barisan Mountains may represent a Pre-

Carboniferous continental basement. All the older rocks, which lie mainly to the northeast of the Sumatran Fault System (SFS), show some degree of metamorphism, mainly to low-

______________________________________________________________________________________________

grade slates and phyllites, but younger Permo-Triassic sedimentary and volcanic rocks are less metamorphosed (Barber et al., 2005). The area to the southwest of the Sumatran Fault System (SFS) is composed largely of variably metamorphosed Jurassic-Cretaceous rocks. The Permo-Triassic is further divided by Barber and Crow (2005) into two units, Mid-Late Permian and Mid-Late Triassic. The Pre-Tertiary basement is cut by granite plutons that range in age from Permian to Late Cretaceous.

Locally within the Barisan Mountains the basement is intruded by Tertiary igneous rocks and is overlain to the east and west by volcaniclastic and siliciclastic sedimentary rocks in hydrocarbon-(oil and gas) and coal-bearing Tertiary sedimentary basins. These basins have back-arc, fore-arc and inter-arc relationships to the Quaternary to Recent volcanic arc. Lavas and tuffs from these young volcanoes overlie the older rocks throughout the Barisan Mountains and, in particular cover an extensive area in North Sumatra around Lake Toba. Recent alluvial sediments occupy small grabens within the Barisan Mountains, developed along the line of Sumatra fault and cover lower ground throughout Sumatra (Barber et al., 2005).

Figure 2.3. The main geological evolution of Sumatra and magmatism events from its dispersal from Gondwana to the collision of the Bentaro-Saling Oceanic Volcanic Arcs during Paleozoic to Mesozoic period (modified from Crow, 2005).

(a) The opening of Meso-Tethys in the Early Permian. In this period the West Sumatra Block is still in position between Cathaysia and the Greater Sula Spur, while East Sumatra is in the Sibumasu block.

(b) Sumatra and the Malay Peninsula in the Mid-Late Triassic showing the Pahang Volcanic Belt (V, volcanic localities) in the Semantan Basin.

(c) Subduction of the Meso-Tethys in the Jurassic-Early Cretaceous showing the Plutonic Arc- Woyla Foreland Assemblage in the east and Bentaro-Saling Arc with the Woyla Accretionary Complex in the west.

(d) Arrival in the Late Cretaceous of the Bentaro-Saling Oceanic Arc that has collided with and has been overthrust onto, Sumatra as the Woyla Nappe. This collision event terminated subduction in the Late Cretaceous.

______________________________________________________________________________________________

2.5 Magmatism of Sumatra

2.5.1 Pre Tertiary magmatism

Magmatism in Sumatra occurred during several periods. The oldest magmatism is represented by a Silurian granite (426 ± 41.5 Ma), found in a borehole in the basement beneath the Central Sumatra Basin (Eubank and Makki, 1981). However, the Sibolga granite (264±6 Ma) and Ombilin granite (256±6 Ma), which are exposed along northwestern Sumatra, suggest Permian magmatism (McCourt et al., 1996).

Mesozoic and Cenozoic magmatism can be divided into several major events; Triassic-Early Jurassic (247-180 Ma), Middle Jurassic-Early Cretaceous (169-129 Ma), Mid- Late Cretaceous (120-75 Ma) and Cenozoic (65-0). They represent long-lived magma generation throughout the last 200 Ma (McCourt et al., 1996; Crow, 2005). Following the collision between the Sibumasu and East Malaya blocks during the Triassic, horst and graben structures were formed parallel to the orogen axis where the granites of the Main Range and Eastern Province of the SE Asia granite belts were subsequently emplaced (Crow, 2005).

Within the Malaysia peninsular, the Triassic-Early Jurassic magmatism is identified as I-type for the Eastern Province and S-type for the Central Range Province granitoids (Cobbing et al., 1992; Gasparon and Varne, 1995; Cobbing, 2005). The Pahang volcanics in east Malaya represent the volcanic carapaces of Eastern Province granites emplaced within the Semantan Basin (Crow, 2005) (Figure 2.3.b). In Sumatra, the Middle Triassic-Early Jurassic granites (250-143 Ma) are scattered along and east of Barisan Mountains and within the Tin Islands. These granites were mainly emplaced at 220 Ma, suggesting that most of the plutonism was post-orogenic and occurred at the southern-end extension of the SE Asia granites belt (Cobbing et al., 1992; Gasparon and Varne, 1995; McCourt et al., 1996; Cobbing, 2005). On mainland Sumatra, the Middle Triassic-Early Jurassic granites are identified as I-type, while in Tin Islands these granites are both I- and S-types. The

occurrence of Meso-Tethys subduction along the margin of the West Sumatra Block is believed to contribute to the presence of I-type granites in Sumatra during this period (Figure 2.3b).

The Middle Jurassic-Early Cretaceous (169-129 Ma) magmatism formed an extensive I-type plutonic arc that can be correlated with the Western Province of SE Asia granites (McCourt et al., 1996). These granites are associated with subduction of Meso- Tethys to the northeast and the accretion documented within the Woyla accretionary

complex (Figure 2.3c). According to Crow (2005) another arc known as the Bentaro-Saling oceanic island arc was also formed along the western margin of the Meso-Tethys. In the latest Early Cretaceous both arcs were accreted towards the Sundaland (Figure 2.3c).

Resuming of Meso-Tethys subduction in the Mid-Late Cretaceous (Figure 2.3d) formed the Cretaceous and Cenozoic Sumatra arcs, including a new plutonic arc formed on the Woyla Nappe and the margin of the West Sumatra Block (Crow, 2005). The Mid-Late Cretaceous (120-75 Ma) was characterized by I-type granitoid magmas (McCourt et al., 1996) that were likely the continuation of the western Burma Arc or the Central Valley Province of SE Asia granites (McCourt et al., 1996; Cobbing, 2005). This magmatic belt continues to the Meratus Complex in South East Kalimantan, forming the Middle-Late Cretaceous Sumatra-Meratus Arc (Carlile and Mitchell, 1994).

2.5.2 Tertiary magmatism

Bellon et al. (2004) suggested that the continuation of Cenozoic magmatism in Sumatra began at 65 Ma after a general absence of magmatism during 75-65 Ma. Cenozoic volcanism occurs along the Barisan Mountains and the western coast of Sumatra (Figure 2.1 and 2.4). Main volcanic episodes recognized in Tertiary Sumatra are during the

Paleocene (65-c.50 Ma; Bellon et al., 2004; Crow, 2005), Late Middle Eocene (c.46-40 Ma), Late Eocene-Late Oligocene (c.38-24 Ma), Late Early Miocene-Middle Miocene (c.22-14 Ma), Late Miocene-Pliocene (6-1.8 Ma) and Quaternary (1.8-0 Ma) (Crow, 2005). However, the youngest plutonic rock is 5±0.2 Ma (McCourt et al., 1996). Geochemical signatures of arc magmas suggest that they are of I-type continental margin type rather than the island arc type of Java (Whitford, 1975; McCourt et al., 1996; Crow, 2005). Therefore, this arc magma provided an abundance of young silicic volcanic rocks associated with caldera- forming events involving the melting of upper crustal material (Whitford, 1975; Hamilton, 1979; Gasparon and Varne, 1995; Gasparon, 2005).

2.5.3 Quaternary volcanism

The morphology and type of Quaternary volcanic activity was compiled by Neumann van Padang (1951) and Kusumadinata (1979). The Quaternary volcanoes along the Sunda-Banda arcs of Sumatra are a well-known example of subduction-related

volcanism (Figure 2.1 and 2.2). Volcanic rocks associated with the active volcanic arc in Sumatra range in composition from calc-alkaline basalts to andesites and dacites, typical of

______________________________________________________________________________________________

a volcanic arc built on continental crust (Gasparon, 2005). Gasparon (2005) identifies four major Pliocene to Quaternary pyroclastic deposits in Sumatra, namely the Lampung and Ranau tuffs in south Sumatra, the Padang tuffs in central Sumatra, and the Toba tuffs in north Sumatra. The Toba tuffs are associated with a very large eruption that formed the caldera now occupied by Lake Toba. Toba caldera is the earth’s largest Quaternary caldera some 35 km x 100 km in dimension, formed during four major Pleistocene eruptions that

produced over 25,000 km3 _{of ejecta (Kusumadinata, 1979).}