Ecology of Sulawesi. Tony Whitten
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Chapter One
Physical, Biological and
Human Background
GEOLOGY
Geological History
The geology of an area and its geological history are the major determinants of the soils, plants and animals that occur there. For this reason a brief account is given below concerning the physical conditions and history of Sulawesi.
About 250 Ma1 ago the earth comprised of two great continents: Laurasi-comprising present-day North America, Europe and much of Asia and Gondwanaland-comprising present-day South America, Africa, India, Australia, Antarctica and the remainder of Asia. Until the last few years the once widely accepted view of the geological history of Indonesia and surrounding regions was that the western half (Malay Peninsula, Sumatra, Java, Borneo and western Sulawesi) had been part of Laurasia, separated until recently from the eastern half (eastern Sulawesi, Timor, Seram, Buru, etc.), which had been part of Gondwanaland, by the broad Tethys Ocean.
This picture has had to change in the light of recent geological and palaeontological evidence. The current view, not without its critics however, is that southern Tibet, Burma, Thailand, Peninsular Malaysia and Sumatra were once part of Gondwanaland and rifted from the northern Australia-New Guinea continental margin some 200 Ma ago. This continental fragment then formed a dissected land connection between Australia and Asia and may have carried with it an evolving higher-plant flora (Audley-Charles 1987). Western Sulawesi together with Sumatra, Borneo, and land that would later form the islands of the Banda Arc2 are considered to have separated from Gondwanaland in the middle Jurassic (Audley-Charles 1983). Australia broke away from Antarctica much later, perhaps in the early Cretaceous (90 Ma), and Australia, New Guinea, and east Sulawesi proceeded to travel northwards at about 10 cm per year. At least part of eastern Sulawesi probably separated from New Guinea before its mid-Miocene collision with western Sulawesi after which the eastern half began to emerge as an island (Audley-Charles 1987) (fig. 1.1; table 1.1).
Figure 1.1. Changing locations of components of Southeast Asia since the first rifting from east Gondwanaland, showing schematic ocean spreading ridges, subduction trenches (triangles) and continental margins (dotted lines). Present coastlines shown for reference only. Horizontal lines: 0°, 30° and 60° South.
After Audley-Charles 1987
The most dramatic event in Indonesian geological history occurred in the Miocene when the northward-drifting Australian Plate caused the bending to the west of the eastern part of the Banda Arc. This westward movement, coupled with the westward thrust along the east-west Sorong fault system from western Irian Jaya, modified the two major landmasses that would form the peculiar shape of Sulawesi we recognize today. It has been proposed that this collision occurred 19-13 Ma ago (Sasajima et al. 1980; Audley-Charles 1987). The Banggai-Sula Islands formed the continental platform section of the east Sulawesi fragment. The Talaud Islands and the small islands of Mayu and Tifore between North Sulawesi and Halmahera are probably also part of the collision suture that formed between Sundaland and Gondwanaland (Audley-Charles 1987).
Thus eastern Sulawesi was like a spearhead that hit western Sulawesi and caused the southwest peninsula to rotate anticlockwise by about 35°, thereby opening the Gulf of Bone (Haile 1978) and causing the northern peninsula to pivot around its northern end, rotating clockwise through nearly 90°. This would have caused subduction3 along the North Sulawesi Trench in Gorontalo Bay (Otofuji et al. 1981) and obduction4 of the ultrabasic rocks of east and Southeast Sulawesi over the erosion debris or molasse deposits of younger rocks.
The physical history of eastern Indonesia has made it one of the most geologically complex regions in the world (Audley-Charles 1981). It is this complexity and the strange shape of Sulawesi, described variously as an orchid, a demented spider and a wobbly 'K', which have long attracted the interest of geologists and others (Davis 1976; Otofuji et al. 1981).
Sulawesi comprises three distinct geological 'provinces' brought together by movements of the earth's crust as described above. These are West and East Sulawesi (divided by the north-northwest fault between Palu and the Gulf of Bone-the Palu-Koro fault), and the Banggai-Sula province comprising the Tokala region behind Luwuk on the northeast peninsula, the Banggai Islands, Butung Island and the Sula Islands (actually part of the political province of Maluku) (fig. 1.2).
West Sulawesi is underlain in the south by a basement of schists (metamorphic rocks of continental origin that split easily along their mineral plates) and ultrabasic rocks (derived from the mantle), and in the north by schists and gneiss (banded, coarse, metamorphic rocks that do not split easily). These are overlain by marine sediments including limestone (primarily calcium carbonate from animal shells), sandstones (consolidated sand), cherts (a compact flint-like variety of silica) and shales (thin layers of consolidated mud, clay and silt). Volcanism began in the Eocene but became widespread in the Miocene, and the volcanic arc that ran from the south to the north deformed the existing sedimentary rocks. The ash and dust from the volcanoes mixed with eroded sedimentary rock derived from the uplifting of the eastern part of the Sulawesi collision zone, to form the Celebes molasse, a generally poorly-consolidated conglomerate rock of gravels, sands, silts and muds formed in terrestrial or shallow-water environments. Molten igneous rocks such as granite and diorite have forced their way by intrusion into Miocene and older rocks at a number of locations (Sukamto 1975a, b; Otofuji et al. 1981). East Sulawesi consists mainly of basic and ultrabasic igneous rocks associated with schists in the west and with Mesozoic limestones in the east and the south.
Figure 1.2. Geological map.
After Katili 1978
The Banggai-Sula province comprises a basement of Palaeozoic metamorphic rocks intruded by granites. Triassic and Permian effusive rocks were deposited locally on the basement. These rocks are overlain by Mesozoic shale, sandstone, conglomerate and marl (consolidated mud and calcium carbonate) deposited in both continental and marine shelf environments. Butung Island is included in this province by virtue of distinctive Jurassic shales which it shares with Banggai-Sula but which do not occur in East