The Geology of Indonesia/Sumatra

Sumatra Island is the northwest oriented physiographic expression, lied on the western edge of Sundaland, a southern extension of the Eruasian Continental Plate (Fig. 2.1). The Sumatra Island has an area of about 435,000 km2, measuring 1650 km from Banda Aceh in the north to Tanjungkarang in the south. Its width is about 100-200 km in the northern part and about 350 km in the southern part. The main geographical trendlines of the island are rather simple. Its backbone is formed by the Barisan Range which runs along the western side. This region divides the west and the east coast. The slope towards the Indian Ocean is generally steep, consequently the west belt is mostly mountainous, with the exception of two lowland embayments in north Sumatra which are about 20 km wide. The eastern belt of the island is covered by broad, hilly tracts of Tertiary formations and alluvium lowlands. At Diamond Point, in Aceh, this low eastern belt has a width of about 30 km; its width increases to 150-200 km in central and south Sumatra.

The Sumatra island is interpreted to be constructed by collision and suturing of discrete micrcontinents in late Pre-Tertiary times (Pulunggono and Cameron 1984, Barber 1985). At the present-day, the Indian Ocean Plate is being subducted beneath the Eurasian Continental Plate in a N20oE direction at a rate of between 6 and 7 cm/yr (Fig. 2.2). This zone of oblique convergence is marked by the active Sunda Arc-Trench system which extends for more than 5000 km, from Burma in the north to where the Australian Plate is in collision with Eastern Indonesia in the south (Hamilton 1979). The basinal configuration of Sumatra is directly related to the presence of the subduction-induced non-volcanic forearc and the volcano-plutonic backarc, the morpho-structural backbone of the Island.

In general the region can be divided into 6 regions (Fig. 2.1):

1. Sunda outer-arc ridge, located along the active margin of the Sunda forearc basin and separate it from the trench slope.

2. Sunda forearc basin, lying between the accreting non-volcanic outer-arc ridge with submerged segments, and the volcanic back arc of Sumatra.

3. Sumatra back arc basins including North, Central and South Sumatra basin. The system developed as distinct depressions at the foot of the Barisan range.

4. Barisan mountain range, occupies the axial part of the island and is composed mainly of Permo-Carboniferous to Mesozoic rocks.

5. Sumatra intra-arc or intermontane basin, separated by subsequent uplift and erosion from this former depositional area, thus with similar lithologies to the fore-and backarc basins.

2.1. SUNDA OUTER-ARC RIDGE

The Sunda non-volcanic outer-arc ridge marks the western margin of the Sunda Forearc Basin of West Sumatra. This chain of islands and sea-floor rises, between 100 and 150 km off the coast of West Sumatra, forms a structurally controlled topographic ridge nearly 200 km wide (Karig et al., 1979), that extends from the Andaman Sea to the southeast of Java. Nias, Simeulue, and Banyak Island lithologies represent the stratigraphy of the Sunda ourter-arc ridge in general. The geology of the Sunda outer-arc ridge is represented by Nias and Simeulue Island in this chapter.

2.1.1. NIAS (Adopted from Situmorang, B & Yulihanto, B., 1992)

Nias Island is located approximately 125 km off the west coast of Sumatra (Fig. 2.1) and it has been frequently cited as a classic model of an accretionary complex (Fig. 2.3). Nias lithologies were divided into two principal units, the Oyo complex and the Nias Beds (Fig. 2.4). The contact between the two units has not been observed in the field.

2.1.1.1. OYO COMPLEX MELANGE

The Oyo Complex is described by Moore and Karig (1980) as a tectonic melange. On Nias, outcrops of Oyo Complex are seen as isolated blocks and boulders in river sections, along road sections and coastal exposures. The Complex is composed of sedimentary blocks, including conglomerates, sandstones and siltstones, with subordinate mafic plutonic rocks, pillow basalts and cherts (Harbury et. al., 1990). Sandstone blocks form the dominant clast type in the SW part of the island, while pillow basalts and gabbros form some largest blocks (up to 200 m diameter) cropping out mostly along the west coast of the Nias Island (Fig. 5).

Texturally, the sediment boulders are sub to mature clastic with mainly subangular to rounded and well sorted sediments, and are either grain supported or matrix supported. In the area where the mélange is present, landslips are common to occur and the fresh matrix of the Oyo Complex can be observed. Good outcrop of melange is exposed in central Nias (Moi River) and SW Nias. The matrix forms a typical scaly clay, with a high density of curved, polished shear planes. The age of the Oyo Complex remains unresolved by paIeontological analysis.

2.1.1.2. NIAS BEDS

Overlying the Oyo Complex, with probable unconformable contact, are a series of clastic sediments of shallow to deep marine deposits of Nias Beds which are well exposed along the eastern part of the island (Fig 2.4 & 2.5). It consists of coarse to fine sandstone, conglomerate, mudstone, shale and limestone. The age of the Nias Beds has been interpreted by previous authors as Early Miocene-Pliocene. On the contrary, Situmorang & Yulihanto (1992) fieldwork indicates that the lower part of the Nias Beds is Upper Oligocene in age.

2.1.2. SIMEULUE

Simeulue lies slightly off-strike and to the northwest of Nias (Fig. 2.1). This island shares a broadly comparable geology with Nias, of melange overlain by interbedded sandstone and siltstone sequences, with parts of the succession dominated by bioclastic limestones. Although lithological variations do exist, the most notable differences between the two islands is one of structural style.

2.1.2.1. SIBAU GABBRO GROUP

The oldest rocks exposed on the island are represented by the Sibau Gabbro Group (Situmorang et al. 1987; Fig. 2.4)). The Sibau Gabbro Group is composed mainly of metaigneous lithologies with predominantly transitional contacts. The ophiolite correlates closely with a partially defined gravity high in this area indicating that the basic igneous rocks form a major body, extending to a depth of several kilometers (J. Milsom, pers. commun. 1990). Lithologies identified within the group include gabbros, meta-dolerite and meta-volcanics, all with abundant chlorite and pumpellyite suggesting that these rocks are all low-grade metamorphics. Rock dating suggest that the Sibau Gabbro Group and Baru Melange Formation were metamorphosed between Late Eocene and Early Oligocene (Harbury & Kallagher, 1991).

2.1.2.2 BARU MELANGE FORMATION

Situmorang et al. (1987) describe the Baru Melange formation as being in structural (thrust) contact with basalts at the top of the Sibau Gabbro Group (Fig. 2.4). Blocks within the melange include fine-grained, micaceous sandstone some of which are fractured; very well-consolidated, weakly sheared, micaceous mudstone, poorly-sorted meta-greywacke; iron-rich meta-dolerite; brecciated meta-basalt; meta- volcanics and calcite-rich, lithic and crystal tuft’s. Blocks within the melange may be in excess of 10 m in diameter. Smaller blocks of 5 – 10 cm in diameter are commonly enclosed within a sticky blue/grey clay matrix containing organic material, or within a cleaved mudstone matrix. No bedding or other sedimentological characteristics, within the blocks of the melange or the clay matrix, can be used to determine the stratigraphical base or top of the Baru Melange Formation. The apparent random distribution of blocks of different lithology within the outcrop area suggests that the melange is unsorted. The thickness of the formation is estimated to be approximately 200 m.

2.1.2.3 AI MANIS LIMESTONE FORMATION

The Ai Manis Limestone Formation forms a NW – SE orientated ridge in the east central part of Simeulue. The formation is approximately 260 – 350 m thick and consists of both biostromal, biohermal (composed of in situ corals) and bioclastic limestones. The major part of the formation consists of bioclastic