Wednesday, October 3, 1:10pm

Dr. Bona Situmorang

Head of the Petroleum Division, Dept. of Geology, University of Trisakti, Jakarta, Indonesia

Strike-slip duplexes and basin formation: observation from the North Sumatra Backarc, Ombilin Intermontane and West Sumatra Forearc Basins


Interpretation of seismic data from the Aru Area (NSB), detailed field mapping in the Ombilin Basin and results of marine geophysical cruises in the forearc basin off West Sumatra suggest that the formation of those basins are closely associated with the strike-slip duplex system (SDS).

During Late Oligocene-Early Miocene times, SDS was active in the NSB to form N-S orientated transtensional half grabens, with the main depocenters situated in the western part of Aru Area. Subsequently, deposition of lacustrine and fluvial sediments of Bampo and Parapat Formations took place, followed by inversion tectonics in the Middle Miocene. Inversion resulted in shifting of the depocenters toward east.

Similar mechanism appears to occur in the Ombilin Basin. The graben formation due to transtentional movements occurred in the Paleocene-Eocene, when the deposition of the lacaustrine, alluvial fans and fluvial sediments of Sangkarewang and Brani Formations took place. Inversion tectonics took place in the Early Oligocene associated with the deposition of fluvial sediments of the Sawahlunto and Sawahtambang Formations. Transtensional episode was again active in the Late Oligocene-Early Miocene, and deposition of shallow-deep marine sediments of the Ombilin Formation was prevailed in the Neogene grabens situated further southeast of the Paleogene depocenters.

The results of marine geophysical cruises in the West Sumatra forearc basin and field mapping on the Simeulue and Nias Islands confirmed the occurrence of strike-slip duplexes. N-S trending grabens were formed due to Late Oligocene transtentional movements in which sediments were deposited unconformably upon the Pre-Oligocene accretionary complex.

The present complex structures in those basins were formed due to the Plio-Pleistocene orogeny.

Biography of the Speaker

Bona Situmorang is a graduate of the University of Gadjah Mada, Yogyakarta, Indonesia (Ir. Degree in geology, 1968). He received his PhD in geology from Chelsea College, University of London in 1982. He was the Head of the R & D Division in Exploration and Exploitation Technology of Lemigas (the Indonesian R & D Center for Oil and Gas Technology) in Jakarta, before his appointment as Technical Director of the Australia-Indonesia Joint Authority for the Timor Gap Zone of Cooperation (A-I JATG) based in Darwin, Australia, in 1995. In 1998 he was relocated to Jakarta Head Office as the Executive Director or the AI-JATG, until the office was closed down due to the transferred of Indonesian authority over East Timor to the UN. He is presently Vice Dean (Cooperation and HRD) at the Faculty of Mineral Technology, Trisakti University, and since 1997 he has also been a member of the International Editorial Board of the Marine and Petroleum Geology. He is a member of the AAPG, IPA and Ikatan Ahli Geologi Indonesia (IAGI).