Page 14 - Sumatra Geology, Resources and Tectonic Evolution

Chapter 1

Introduction and previous research

A. J. BARBER, M. J. CROW & J. S. MILSOM

Sumatra, with an area of 473 606 km 2 is the largest island in the Geologically, Sumatra forms the southwestern margin of the
Indonesian archipelago and the fifth largest island in the world. Sunda Craton, which extends eastwards into Peninsular Malaysia
The island stretches across the equator for 1760 km from NW to and into the western part of Borneo (Fig. 1.2). A Pre-Tertiary
SE, and is up to 400 km across (Fig. 1.1). Administratively, and basement is exposed extensively in the Barisan Mountains
for the purposes of this Memoir, Sumatra includes the Mentawai (Fig. 1.4) and in the Tin Islands of Bangka and Billiton. The
islands from Simeulue to Pagai, which with Enggano form a oldest rocks which have been reliably dated are sediments of
forearc chain to the SW, and the 'Tin Islands' of Bangka and Carboniferous-Permian age, although Devonian rocks have
Billiton and the Riau islands to the east. The backbone of the been reported from a borehole in the Malacca Strait, and
main island is formed of the Barisan Mountains, which extend undated gneissic rocks in the Barisan Mountains may represent
the whole length of Sumatra in a narrow belt, parallel to, and a Pre-Carboniferous continental crystalline basement. All the
generally only a few tens of kilometres, from the SW coast. The older rocks, which lie mainly to the NE of the Sumatran
main peaks (which are mainly Quaternary or Recent volcanoes) Fault System, show some degree of metamorphism, mainly to
commonly rise 2000 m above sea level, culminating in Mt low-grade slates and phyllites, but younger Permo-Triassic sedi-
Kerinci at 3805 m. Short, steep river courses drain the Barisans ments and volcanics are less metamorphosed. The area to the
towards the SW, often cuttting deep gorges, while towards the SW of the fault is composed largely of variably metamorphosed
east the rivers follow long meandering courses across broad Jurassic-Cretaceous rocks. The Pre-Tertiary basement is cut by
coastal plains and swamps to the Malacca Straits, which separate granite plutons that range in age from Permian to Late Cretaceous.
Sumatra from the Malay Peninsula, or to the Java Sea. Eastwards, Locally within the Barisans the basement is intruded by Tertiary
across the Java Sea, lies the almost equally large island of Borneo igneous rocks and is overlain to the NE and SW by volcani-
(Indonesian Kalimantan), and Java lies immediately to the SE clastic and siliciclastic sediments in hydrocarbon- (oil and gas)
across the narrow Sunda Strait. and coal-bearing Tertiary sedimentary basins. These basins have
The Malacca Strait and the Java Sea form the southern parts backarc, forearc and interarc relationships to the Quaternary to
of the Sunda Shelf (Fig. 1.1). Across the shelf the seafloor is Recent volcanic arc. Lavas and tufts from these young volcanoes
shallow with a depth of less than 200 m and remarkably flat. overlie the older rocks throughout the Barisans and, in particular
Virtually the whole of the shelf was exposed at the peak of cover an extensive area in North Sumatra around Lake Toba
the last glaciation. To the SW, Sumatra is separated from a (Fig. 1.4). Recent alluvial sediments occupy small grabens
linear ridge with emergent islands extending from Simeulue within the Barisan Mountains, developed along the line of the
in the north to Enggano in the south, by marine basins more Sumatran Fault and cover lower ground throughout Sumatra.
than 1000 m deep, which increase to a depth of more than These alluvial sediments are of fluvial origin immediately
2000m in the south. To the SW of the ridge the seafloor adjacent to the Barisans, but pass into swamp, lacustrine and
slopes steeply into the Sunda Trench, 5000 m deep in the coastal deposits towards the northeastern and southwestern
NW, deepening to >6000 m towards Java in the SE. The floor margins of the island.
of the Indian Ocean, with a depth of about 5000 m, lies to the
SW beyond the trench, extending all the way to to India and
History of geological research in
the east coast of Africa. Immediately to the west of Sumatra
the floor of the Indian Ocean is covered by the thick sediments Sumatra before-WWII
of the Nicobar Fan, the currently inactive eastern lobe of the
Bengal Fan, composed of debris eroded from the Himalayas. During the late nineteenth and early twentieth centuries Sumatra
The fan is separated from the main part of the Bengal Fan to was explored by geologists and engineers working for mining
the west by seamounts of the north-south trending Ninety- and petroleum companies under the auspices of the Bureau of
East Ridge (Fig. 1.2). Mines in the Dutch East Indies Colonial Administration. In 1925
In terms of present-day tectonics Sumatra forms the active south- a 'Palaeobotanic Expedition to Djambi (Jambi)' was undertaken
western margin of the Sunda Craton (Sundaland), the southeastern to collect samples of the 'Djambi Flora'. This early work is
promontory of the Eurasian Plate (Fig. 1.2). The relative 7.7 cm a- summarized by Rutten (1927) in his 'Lectures on the Geology
NNE-directed motion of the Indian Ocean results in oblique (c. 45 ~ of the Netherlands East Indies'. Between 1927 and 1931 the
subduction at the Sunda Trench. Seismic profiles across the land- Netherlands Indies Geological Survey conducted a mapping
ward side of the Sunda Trench imaged the removal of packages programme in South Sumatra with the production of a series of
of sediment from the downgoing plate to build a forearc ridge accre- sixteen 1:200 000 Geological Map Sheets (e.g. Musper 1937),
tionary complex (Hamilton 1979; Karig et al. 1980) (Fig. 1.3). and carried out other geological studies in Central and Northern
Oblique subduction results in the northwestward movement of a Sumatra (Musper 1929; Zwierzijcki 1922a, b, 1930a). Unfortu-
'sliver' plate (Curray 1989), decoupled both from the downgoing nately, as a result of the global economic depression, this
Indian Ocean Plate and the Sundaland Plate, along the Wadati- mapping programme was discontinued in 1933, before the
Benioff seismic zone, which dips northeastwards at c. 30 ~ and mapping of the whole island was complete. However, the cessa-
along the vertical Sumatran Fault System. The Wadati-Benioff tion of fieldwork provided an opportunity to publish the results
zone intersects the fault at a depth of some 200 km. The active of the 1925 Palaeobotanic expedition to Djambi (Zwierzijcki
Sumatran Fault System runs the whole length of the Sumatra, 1930a; Jongmans & Gothan 1935). Exploration by mining and
through the Barisan Mountains, from Banda Aceh to the Sunda petroleum companies continued throughout Sumatra, but for
Strait, and is paralleled by a line of Quaternary volcanoes, mainly commercial reasons most of the reports remained confidential
quiescent, but some currently active (Fig. 1.4). and unpublished. However, some of the results, notably for