The Unique Tectonic Setting ofSulawesi, IndonesiaPetrologyThe Western Arc: The petrology of Sulawesi is quite varied from one part of the island to another. The western arc of the island, which includes the southern and northern arms, as well as the western portion of central Sulawesi, is composed of Cretaceous subduction complexes which have been overlain by sediments that may be from an outer arc basin. The arc basin sediments are overlain as well by Upper Paleogene continental shelf deposits which include quartzose sandstone, shale, marly shale, coal, shallow water platform limestone, and redbeds. The ages of these clastics range from Lower Paleogene to Upper Eocene. These basin and shelf deposits have been overlain by volcanic and sedimentary rocks of Neogene age, and intruded by those same Neogene volcanics. The Neogene volcanism apears to be due to a westward subduction which affected both the East and Southeast Arms. There are a variety of volcanic rock types, such as balsalt, andesite, dacite, rhyolite, and many alkali rocks in which leucite may be found. The earliest volcanism is thought to have begun 18 Ma and produced basaltic lava. Approximately 9 Ma, andesitic eruptions began, followed by dacite eruptios 4-5 Ma. Various types of granitic magmas produced batholiths and stocks on the North Arm. All grades of metamorphic rocks are also found in this region. The ages of these igneous and metamorphic rocks are thought to range from 5-14 million years, with the majority of the magmatism occuring in the middle and late Miocene. In the southwestern section of the South Arm there are two erosional windows which expose ultramafic and metamorphic rocks of the basement complex. The rocks, which likely formed in a region of subduction, include greenschist, hornblende-mica schist, chloritic phyllite, and lesser amounts of ecogite, glaucophane schist, and quartzite. Muscovite from a garnet schist sample was dated to 111 m.y. by the K-Ar techneque. In the northeast portion of the southern arm there are two exposed areas where melange is found. These are chaotic deposits of fragments of feldspathic sandstone and siltstone, red chert, red limestone, sheared shale, and the various metamorphic and ultramafic rocks described earlier. In one area, Bantimala, there is radiolarian red chert which is interbedded with a schist-pebble conglomerate, overlying schist. The chert is then overlain, or grades into a siliceous shale. Radiolaria from the chert and shale have been dated to the Early Cretaceous. The chert and shale are overlain by a mixture of clastic rocks, primarily shale, siltstone, and graywacke, as well as some minor limestone deposits. The Eastern Arc The eastern arc, which is composed of the eastern central Sulawesi and the East and Southeast Arms has a much different assemblage of rocks. This part of Sulawesi is made up of subduction complexes and ophiolite fragments, considered to be products of a west-dipping subduction during the Late Cretaceous, Paleogene, and Neogene. Glaucophane schist, is common in the western portion of central Sulawesi. These high pressure, high temperature blueschist-facies rocks are juxtaposed upon both unmetamorposed rocks and metamorphic rocks of completely different temperature/pressure gradients, indicating large offsets between the bounding faults. The only rocks in this assemblege that could be dated were the radiolarian cherts which dated to the Middle or Upper Miocene. The eastern belt of sedimentary and low grade metamorphic rocks contains both pelagic and terrigenous clastic rocks. The clastics include limestone, shale, red and gray chert, and sandstone. The rocks can be dated by their Triassic, Jurassic, Cretaceous and possibly Tertiary fossils. The metamorphics include slate, schist, quartzite, and phyllite, intercalated with ophiolite fragments. Ophilites cover much of the East Arm and the northeast part of the Southeast arm. There are also Mesozoic and Tertiary sedimentary rocks amoungst the ophiolites. The sedimentary rocks are much as previously described in the western belt, and their percentages increase in a eastward direction towards the Gulf of Tolo. The eastern secton of the East Arm contains abundant ophiolites with lesser amounts of sedimentary rocks of Mesozoic age to the northwest. To the southeast however, Mesozoic, Paleogene, and Miocene strata, mixed with ophiolite fragments, form a northwest-dipping imbricated complex. Coral reefs in this area have been uplifed as high as 700 m above sea level.
Geologic map of Sulawesi (Katili 1978)
Geologic map of the Southeast Arm of Sulawesi and neighboring islands (Hamilton 1979)
Geologic map showing area of imbricated ophioliote fragments and Mesozoic/Paleogen sedimentary rocks (Hamilton 1979)
Geologic map of Sulawesi (Katili 1978) |
