General Lecture: Geology of The Eastern Part of Sualwesi Indonesia, In Relation To The Geological Resources Prof. Dr. Surono
The eastern part of Sulawesi and its surrounding area comprise: continental fragments, the Eastern Sulawesi Ophiolite Belt (as oceanic fragments) and molasse sediments (named Sulawesi Molasse). These continental and oceanic terranes had collided each other during latest Oligocene – Middle Miocene times and than they are covered by the Sulawesi Molasse.
The largest continental fragment in the eastern part of Sulawesi occurs in the East Arm, which is named as the Banggai-Sula continental fragment by Sukamto (1975). The second biggest continental fragment is located in the Southeast Arm of Sulawesi called as the Southeast Sulawesi continental fragments (Surono, 1996). Stratigraphic features of the both continental fragments are similar each other. These continental fragments compose of metamorphic basement, with minor granitic/aplitic intrusions, Mesozoic clastic- and carbonate-sequences, and Paleogene carbonates. A primary palaeolatitude of 20o S has been determined on Late Triassic sandstone of the Meluhu Formation in the Southeast Sulawesi continental fragment (Surono and Bachri, 2002), that was consistent with the location of the northern margin of the Australian continent during the Late Triassic. The Meluhu Formation was deposited within a palaeoclimate of a warm palaeotemperature and a high rainfall condition. A similar palaeoclimate has been determined for the northern Australian continent as indicated by the presence of widespread Late Triassic coal measures (Frakes and Rich, 1982; Quilty, 1984). These evidences strongly indicate that the Southeast Sulawesi fragment was originated from the northern margin of the Australian continent (Papua New Guinea region). Stratigraphic similarity between the Southeast and Banggai-Sula continental fragments indicates that they were derived from a same source, probably the Kubor anticline in the northern margin of Australia.
The Eastern Sulawesi Ophiolite Belt consists of ophiolite and associated pelagic sedimentary rocks. The ophiolite, which consists of lherzolite, harzburgite, dunite, wherlite, gabbro and serpentinite (Surono et al., 1993; Rusmana et al., 1993; Simandjuntak et al., 1994; Surono and Sukarna, 1995), is associated with the pelagic sedimentary rocks. This ophiolite was probably formed in a mid oceanic ridge from Late Cretaceous to Eocene in an area with latitude of 17o-24o S (Simandjuntak, 1986; Surono and Sukarna, 1995). In the field, the ophiolite is thrusted over the continental fragments. A palaeomagnetic study of thirty one ophiolite samples from the East Arm of Sulawesi indicates that the ophiolite formed at 17o-24o S (present position at 0.6o-1.7o S) in the Late Cretaceous and suffered a post-Cretaceous clockwise rotation of about 60o (Mubroto, 1988). On the other hand, a palaeomagnetic study of a single exposure of Late Jurassic-Early Cretaceous chert from the northern part of the Southeast Arm having a primary magnetisation indicates a palaeolatitude of 42o S (Haile, 1978). Haile interpreted that the chert occurred as a pelagic cover on the ophiolite suite. Both sets of palaeomagnetic results indicate that the ophiolite of the East and Southeast Arms formed at latitude of 17o-42o S, most probably at 17o-24o S.
The Sulawesi Molasse deposited after the collision between the continental fragments and the ophiolite belt, is widely distributed throughout the eastern part of Sulawesi and consists of coarse‑ to fine-grained clastic sequences with shallow marine carbonate sequences. Boulders of pink granite, found in the Early Miocene molasse sequences on Selabangka and Manui Islands, may have been derived from the Banggai‑Sula continental fragment. The molasse in the Southeast Arm is slightly older (Early Miocene-Pliocene) than in the East Arm (Late Miocene-Pliocene).
Based on stratigraphic similarity, probably before collision with the ophiolite belt the Banggai-Sula and Southeast Sulawesi continental fragments had joined together as a Great Banggai-Sula continental fragment. Because of the oblique collision with the ophiolite belt, the Great Banggai-Sula had been broken to be the Banggai-Sula, Southeast and some smaller continental fragments in the eastern part of Sulawesi. The collision had occurred during latest Oligocene time in the Southeast Arm and it was in Middle Miocene in the East Arm.
The oceanic fragments, which were from Pacific, contain mineral resources, especially nickel, chrome, and platinum. The continental fragments, derived from Australian continent, have hydrocarbon potential.
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