THE GEOLOGY AND TECTONIC EVOLUTION OF THE OBI REGION, EASTERN INDONESIA BY DWIATMO AGUSTIYANTO A thesis submitted for the degree of Master of Philosophy at the University of London Department of Geological Sciences University College London 1995 ProQuest Number: 10055427 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest. ProQuest 10055427 Published by ProQuest LLC(2016). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code. Microform Edition © ProQuest LLC. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 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. ACKNOWLEDGEMENTS I would like to take this opportunity to express my deep sense of gratitude and sincere thanks to my supervisor Prof. Robert Hall, for all valuable guidance, encouragement, useful discussions, practical suggestions and his patience with me throughout the course of my research. I am deeply indebted. My sincere gratitude to Drs. Gary Nichols, Tim Charlton, Jason Ali, Jeffrey Malaihollo, Emily Forde, Giz Mariner, Hillary Downes, Diane Cameron, Simon Baker and Andy Beard for all assistance and encouragement. I thank Drs. Irwan Bahar and Rab. Sukamto who have give me the support an opportunity to improve my knowledge and to finish my study. I also thank my Indonesian colleagues Sardjono, Agus Guntoro and Alit Ngakan who give me support, encouragement, discussion and comment which useful in arranging this thesis. I wish to thank my mother Mrs. Sugiarti Moerhardjo and my mother in law Mrs. Tum Silowati Hani Sarono and all my family for their supporting and sincere praying. My very special sincere thanks to my wife Dwi Harini for encouraging, understanding, assistance and all the hard time she has been through. Special thanks to my children Ivan Yudha Nugraha Putra and Ayu Puspaningrum for their understanding, prayers and encouragement. CONTENTS 1. IN TR O D U C TIO N ................................................................................................................................................................ 1 1.1 Pream ble ...............................................................................................................................................................................1 1.2 A im of this stu d y ............................................................................................................................................................... 1 1.3 Geographic setting ..........................................................................................................................................................4 1.3.1 Location......................................................................................................................................................................... 4 1.3.2 Morphology and Relief............................................................................................................................................. 4 1.3.3 Settlement and Culture..............................................................................................................................................6 1.3.4 Climate, Flora and Fauna........................................................................................................................................6 1.3.5 Accessibility and Logistics........................................................................................................................................7 1.4 Review of Previous work ..............................................................................................................................................8 1.4.1 Investigations Prior to the Second World War...................................................................................................8 1.4.2 Post-War Studies of Obi..........................................................................................................................................10 1.4.3 Regional Tectonic Investigations..........................................................................................................................11 1.4.4 London University-GRDC Investigations.......................................................................................................... 18 1.5 Present Tectonic Framework of the A rea Surrounding Obi..................................................................22 1.5.1 Halmahera Arc..........................................................................................................................................................22 1.5.2 Molucca Sea Plate....................................................................................................................................................23 1.5.3 Philippine Sea Plate................................................................................................................................................ 23 1.5.4 Sorong Fault Zone....................................................................................................................................................23 1.5.5 O b................................................................................................................................................................................. i 24 1.6 Me t h o d s ............................................................................................................................................................................. 24 /. 6.1 Sampling Methods....................................................................................................................................................24 1.6.2 Pétrographie Methods.............................................................................................................................................24 1.6.3 Photogeological Interpretation............................................................................................................................24 1.6.4 Microprobe Methods............................................................................................................................................... 26 1.6.5 X-ray Fluorescence (XRF) Methods................................................................................................................... 26 1.6.6 K-Ar Methods...........................................................................................................................................................