Offshore Tertiary Sedimentary Basins in Indonesia
Abstract
Ringkasan. Cekungan-cekungan sedimen berumur Tersier yang kaya akan minyak dan gas bumi terutama terdapat di bagian barat Indonesia. Cekungan-cekungan ini dahulunya disebut sebagai "ideogeosynclines", dan terdapat sekeliling tepi suatu masa daratan pre-Tersier yang diperkirakan dan dinamakan "Sunda Shelf". Dewasa ini daerah tersebut ditutup oleh laut-laut Epikontinental yang dangkal (Laut Jawa, Laut China Selatan). Bahagian luarnya cekungan-cekungan ini dibatasi oleh busur kepulauan volkanik dalam. Penyelidikan-penyelidikan seismic lautan yang sangat luas, yang dilakukan baru-baru ini dan disusul oleh pengeboran telah membuktikan adanya penerusan dari beberapa cekungan ini kelepas pantai (antara lain Sumatra Utara, Jawa Barat, Jawa Timur dan Kalimantan Timur). Lebih penting lagi penyelidikan tersebut telah membuka tabir bahwa paparan Sunda yang semula diperkirakan bertindak sebagai sumber-sumber sedimen selama "Tersier" dalam kenyataannya terdiri dari banyak cekungan-cekungan sedimen dan peninggian-peninggian yang memisahkannya, dan hanya daerah antara Bangka, Belitung, serta kepulauan Natuna-Anambas adalah daratan Sunda yang sebenarnya. Peninggian-peninggian ini bertindak sebagai sumber dari berbagai lapisan sedimen dan beberapa diantaranya diwakili oleh pulau-pulai pre-Tersier antara lain Bangka, Belitung dan Karimun Jawa. Pematahan bongkah dari batuan dasar ternyata bertanggung jawab atas perkembangan cekungan-cekungan ini, dan juga mengendalikan pengendapan maupun tektonik dari lapisan Tersier yang menutupinya. Selanjutnya explorasi lepas-pantai juga telah membuktikan adanya lapisan-lapisan sedimen tersier di antara Busur Kepulauan Volkanik dalam (misalkan Sumatra) dan busur Non-volkanik luar (pulau Nias dan lain-lain). Sampai saat ini belum terdapat cukup data-data untuk membahasa cekungan sedimen lepas-pantai di Indonesia Timur. Penyelidikan-penyelidikan geofisika laut dan pengeboran masih dalam kemajuan dewasa ini. Lokasi-lokasi pengeboran lepas-pantai masih sangat banyak yang direncakan untuk tahun-tahun yang akan datang. Sehingga masih terlalu pagi untuk membahas implikasi-implikasi geologi yang luas.
Abstract. Prolific Tertiary sedimentary basins are mainly located in Western Indonesia. They have previously been described as "ideo-geo-synclines", and are situated around the periphery of a supposed pre-Tertiary land-mass the "Sunda Shelf", now largely covered by shallow epicontinental seas, (Java Sea, South China Sea). Outward they are bordered by the Inner Volcanic island arc. Recent extensive marine seismic investigations and subsequent drilling have confirmed the extension of some of these basins offshore (e.g. North Sumatra, West Java, East Java and East Kalimantan). More important, however, these surveys have revealed that the "Sun Shelf" which supposedly served as source of sediments during the Tertiary, actually consist of numerous sedimentary basins and intervening uplifts, leaving the area between Bangka-Billiton and Natuna-Anambas Islands as the Sunda-land proper. These uplifts served as source of sediments and some are represented by pre-Tertiary islands, e.g. Bangka, Billiton and Karimun Jawa. Basement block-faulting is apparently responsible for the development of these basins, and controlled depositions as well as tectonics of the Tertiary cover. Further offshore exploration has also confirmed the presence of Tertiary sediments between the Inner Volcanic islands (c.q. Sumatra) and the outer non-volcanie arc (c.q. Nias). There is no sufficient data to date to describe offshore sedimentary basins is eastern Indonesia. Marine geophysical surveys and drilling are still in progress at present. Numerous offshore drilling locations are scheduled for several years, so that it is still too preliminary to discuss the broad geological implications.
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