Sedimentology and tectonics of the collision complex in the east arm of Sulawesi Indonesia

Abstract

An imbricated Mesozoic to Palaeogene continental margin sequence is juxtaposed with ophiolitic rocks in the East Arm of Sulawesi, Indonesia. The two tectonic terranes are bounded by the Batui Thrust and Balantak Fault System, which are considered to be the surface expression of the collision zone between the Banggai-Sula Platform and the Eastern Sulawesi Ophiolite Belt. The collision complex contains three distinctive sedimentary sequences : 1) Triassic-Palaeogene continental margin sediments, ii) Cretaceous pelagic sediments and iii) Neogene coarse clastic sediments and volcanogenic turbidites. (i) Late Triassic Lemo Beds consisting largely of carbonate-slope deposits and subsidiary clastics including quartz-rich lithic sandstones and lensoidal pebbly mudstone and conglomeratic breccia. The hemipelagic limestones are rich in micro-fossils. Some beds of the limestone contain bivalves and ammonites, including Misolia, which typifies the Triassic-Jurassic sequence of eastern Indonesia. The Jurassic Kapali Beds are dominated by quartzose arenites containing significant amounts of plant remains and lumps of coal. The Late Jurassic sediments consist of neritic carbonate deposits (Nambo Beds and Sinsidik Beds) containing ammonites and belemnites, including Belemnopsis uhligi Stevens, of Late Jurassic age. The Jurassic sediments are overlain unconformably by Late Cretaceous Luok Beds which are predominantly calcilutite with chert nodules rich in microfossils. The Luok Beds are unconformably overlain by the Palaeogene Salodik Limestones which consist of carbonate platform sediments rich in both benthic and planktonic foraminifera of Eocene to Early Miocene age. These sediments were deposited on the continental margin of the Banggai-Sula Platform. (ii) Deep-sea sediments (Boba Beds) consist largely of chert and subsidiary calcilutite rich in radiolaria of Cretaceous age. These rocks are part of an ophiolite suite. (iii) Coarse clastic sediments (Kolo Beds and Biak Conglomerates) are typical post-orogenic clastic rocks deposited on top of the collision complex. They are composed of material derived from both the continental margin sequence and ophiolite suite. Volcanogenic Lonsuit Turbidites occur in the northern part of the East Arm in Poh Head and unconformably overlie the ophiolite suite. Late Miocene to Pliocene planktonic foraminifera occur in the intercalated marlstone and marly sandstone beds within these rocks. The collision zone is marked by the occurrence of Kolokolo Melange, which contain exotic fragments detached from both the ophiolite suite and the continental margin sequence and a matrix of calcareous mudstone and marlstone rich in planktonic foraminifera of late Middle Miocene to Pliocene age. The melange is believed to have been formed during and after the collision of the Banggai-Sula Platform with the Eastern Sulawesi Ophiolite Belt. Hence, the collision event took place in Middle Miocene time. The occurrence of at least three terraces of Quaternary coraline reefs on the south coast of the East Arm of Sulawesi testifies to the rapid uplift of the region. Seismic data suggest that the collision might still be in progress at the present time.