Agustiyanto, Dwi Atmo; (1996) The geology and tectonic evolution of the Obi region, eastern Indonesia. Masters thesis (M.Phil), UCL (University College London).

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Abstract

Obi is located in eastern Indonesia in the zone of convergence between the Australian, Philippine Sea and Eurasian plates, and is situated within strands of the Sorong Fault system at the Australian-Philippine Sea plate boundary. A new stratigraphy and geological map of Obi have been constructed using new data collected during fieldwork, supplemented by information acquired from aerial photographic studies, biostratigraphic dating, K-Ar dating, petrography, mineral and whole rock geochemistry. The oldest rocks in Obi belong to the Tapas Metamorphic Complex which is probably of Palaeozoic or greater age. The Tapas metamorphic rocks include phyllites, schists and gneisses with mineral assemblages typical of regional metamorphism at greenschist to amphibolite facies. Triassic and Jurassic sedimentary rocks of the Soligi Formation and the Gomumu Formation, which comprise micaceous sandstones and black shales, are interpreted to rest unconformably on the Tapas metamorphic rocks. These rocks are considered to be derived from the Australian continental margin and formed part of its Mesozoic passive margin. Ophiolitic rocks, of supposed Jurassic age, form the basement of most of the Obi region. These rocks are unconformably overlain by Cretaceous volcaniclastic rocks, limestones and mudstones of the Leleobasso Formation. Similarities between the stratigraphies of Obi, east Halmahera, Waigeo and other islands of the North Moluccas, are consistent with a common origin as part of a single plate, the Philippine Sea plate. Subduction of the Indo-Australian plate beneath the Philippine Sea plate in the early Tertiary led to arc volcanic activity which resulted in diorite intrusions and the deposition of the volcaniclastic Oligocene Anggai Formation above the ophiolitic rocks. Early Neogene collision between the Australian continent and the Philippine Sea plate arc caused uplift which was followed by deposition of limestones of the Fluk Formation. In the Late Miocene the region underwent uplift and erosion, associated with renewed arc volcanic activity which produced the Woi and the Guyuti Formations. When arc activity ceased, limestones of the Anggai Formation were deposited. Juxtaposition of the ophiolitic and continental rocks in south Obi probably occurred in the Late Neogene. Pliocene thrusting on Obi, as on Halmahera, has dramatically contracted the late Neogene arc, and on Obi carried the Woi Formation arc over the Guyuti Formation forearc. This juxtaposition was accompanied and followed by uplift, tilting, erosion forming conglomerates and sandstones of the Plio-Pleistocene South Obi Formation. A brief summary of the geological history is synthesised with regional tectonic events and the large scale tectonic development of eastern Indonesia to provide a model of the tectonic evolution of the Obi region.

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