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Geology of Southeast Asia with particular reference to the South China Sea
A Geology and Hydrocarbon
Potential of the South China Sea
1075
Entr(ly Printed
Vol. 6. No. II. pp. IO77-1091. in Gnat Britain.
1981
036&5442/8l/
I 11077~ISSOZ.OO/O Pergamon Press Ltd.
GEOLOGY OF SOUTHEAST ASIA WITH PARTICULAR REFERENCE TO THE SOUTH CHINA SEA J. A. KATILI Ministryof Mines and Energy, Jakarta, Indonesia
Abstract-The concentric arrangement of the Phanerozoic arc-trench systems of the western part of Southeast Asia has been previously interpreted in the light of plate tectonics as a migration of subduction zones related to the oceanic spreading centers located in the Indian Ocean and the South China Sea respectively. The Sumatra, Java, and Meratus arcs, with ages ranging from Paleozoic to the present time, were generated by spreading centers situated in the Indian Ocean, whereas the Natuna and Palawan arcs had their origin in the spreading center located in the South China Sea. The Tertiary Banda arc was generated by a spreading center in the Indian Ocean whereas the more or less Tertiary Sulawesi arc had its origin in the spreading center located in the Pacific Ocean. In the South China Sea region, the results of recent exploration drilling on the Reed Bank supports the previous concept that the intermediate shelfal region at a depth of about 2ooOm represents the fragmented remains of continental crust. Oceanic crust emplaced between 32 and 17 million years (my) occupies the abyssal depths. Magnetic patterns of the basin show an east-west alignment of the spreading axis. Reconstruction by closure of this basin would place the northeastern Palawan-Mindano microcontinent against the China Shelf adjacent to the eastern Macclesfield Banks. Prior to this event, southerly directed Cretaceous and Paleogene subduction was Ltive beneath the northern Sundaland, which already existed as an accreted crystalline core dating into the Paleozoic era. The geotectonic evolution of Southeast Asia is reconstructed with particular reference to the South China Sea Region. Several dominant basin types have been recognized on the margins of the South China Sea. These basins as a whole contain an estimated 56% or 20 billion barrels of the remaining undiscovered oil reserves in Southeast Asia.
INTRODUCTION
Little is known of the geology, geophysics and hydrocarbon potential of the South China Sea. A model has been suggested and elaborated’-3 which envisages opposing subduction zones in the western Indonesian region and two spreading centers, a major one situated in the Indian Ocean and a minor one presumably located in the South China Sea. Suggestions that the crust beneath the deep part of the China basin is oceanic& have been confirmed by refraction studies.’ Lineated magnetic anomalies have been recognized in the area west of Luzon and reexamined.’ However, the formation of the South China Sea Basin by separation of Borneo from the Asian mainland’ could not be supported by geological evidence.’ The purpose of this paper is to present more recent data on the geology and geophysics of the South China Sea obtained from earlier research by scientific institutions and exploration results of oil companies. An attempt will be made to reconstruct the geotectonic evolution of Southeast Asia with particular reference to the South China Sea and to discuss the hydrocarbon potential of this area based on the latest findings of the oil industry. Suggestions will be made for future joint research. A REGIONAL VIEW OF THE SOUTH CHINA SEA
The South China Sea is that part of the Pacific Ocean which lies generally west of the Philippines and Borneo (Kalimantan) covering an area of approx. 200 km by 1000km. It is comprised essentially of the central deep water South China Basin surrounded by broad, shallow continental crust to the north, west and southwest, and limited to the southeast by the Northwest BorneolPalawan Trenches. The eastern boundary is well defined by the active north-south Manila Trench. Manila trench (Fig. 1) This trench, some 5OOOmdeep, is separated from Luzon in the Philippines by an outer arc ridge and sediment filled outer arc basin. Mantle earthquakes indicate the presence of an active 1077
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J. A. KATILI
MB.
Mocclesfleld
P.G. Poracrl Grcup
from Gape .SWI~Q'80
Fig. 1. Tectonic basin styles in the South China Sea.
Benioff zone dipping eastwards. Volcanic rocks erupted above this zone range in composition from tholeiitic in the west, through talc-alkaline in the middle to high potassium in the east. Palawan/Northwest Borneo trenches (inactive) (Fig. 1) These trenches lie to the southeast of the South China Basin northwest of Borneo and Palawan in the region of the limit of the continental slope. A pronounced bathymetric feature is associated with the Northwest Borneo trench although it is virtually aseismic and devoid of active volcanoes. The Tertiary Palawan arc faces southeast and separates the South China Sea from the Sulu Sea. The islands on the ridge are composed of ophiolite and imbricated Tertiary (mainly Palaeogene) melange, overlain by Miocene and younger strata. In the northern part of the ridge PalaeozoiclMasozoic continental crustal rocks are present. Shoal areas to the west of Palawan in the South China Sea may be similar continental fragments. This Palawan continental slope area is covered by a few hundred meters of virtually undeformed sediments which continue out onto the South China Sea abyssal plain. The seafloor north of the shelf and trench has highly irregular topography with plateaus, ridges and seamounts covered by water depths locally less than 2000 m, but with intervening depressions up to 2000m deep. In the deeper areas the irregular topography comprises basement with a thin sedimentary veneer. Recent turbiditic sediments have locally filled in irregularities. The Palawan Trench is filled with deformed sediments. Pliocene and younger strata of the continental slope prograded across the trench system and are essen!ially undeformed.
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The Sarawak Basin (Fig. 2) This large continental shelf basin covers a cresent shaped area off northwestern Borneo and extends northward to link up with the basins off the coast of Vietnam. It is separated from the West Natuna/Malay Basin region by the north-south trending Natuna Arc. The stratigraphy and structural history of this basinal area is variable along and across the basin axial zone. Interbedded sand, carbonate, shale and clay, both marine and non-marine, interfinger complexly. Off the 3orneo coast at least 50OOm of Eocene to Lower Miocene shaly section is present, dominantly marine in the northwest and transitional to the southeast. An overlying younger section thickens gradually offshore and is dominantly deltaic. Shale diapirs and Neogene growth faulting form anticlinal features. Shelf carbonates and reefs are present. Continental shelf basins continue north along the Vietnamese and Chinese coastal areas. The South China Basin (Fig. 1 and 2) This basin is characterized by a central northeast-southwest oriented bathymetric deep at about 4000 m. Particularly in a southern direction this basin is bordered by a broad intermediate
INDO
0
CHINA
Sedimentary
basins
with
200m
water
depth
Fig. 2. Relationship of the West Natuna basin to the oceanic spreader.
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shelfal area lying at an average depth of 2000m. A continental shelf at 200m or less is most extensive to the southwest where it comprises part of the Sundaland cratonic platform. These morphologic features represent the components of a complex tectonic history. Contributions to its understanding have become available in recent years from research and oil exploration. The intermediate shelfal region, at 2000 m is believed to represent the fragmented remains of continental crust. Such areas include the bathymetric highs of the Spratley Islands, the Paracel Islands and Macclesfield Bank. Support for this concept has come from exploration drilling on the Reed Bank, west of northern Palawan. Here unmetamorphosed early Tertiary and Lower Cretaceous continental associated sediments have been proved. Taylor and Hayes (in press) have suggested that the Reed Bank and the Permian and Mesozoic rocks of the northern Palawan Islands belong to the same microcontinental fragment. lo Eocene sandstones present on Mindoro may be part of a formation of similar age and rock type recorded in the Reed Bank section. The Ulugan fault divides the northern Palawan microcontinent from the Tertiary subduction system of southwestern Palawan.” Here subduction ceased in late Miocene.” The north-south trend of the Ulugan fault is possibly an indication of a transform extending into the midTertiary ocean crust of the central South China Sea. Oceanic crust emplaced between 32 and 17 my (Fig. 3) is identified as occupying the abyssal depths.” Magnetic patterns in the northern, most extensive part of the basin give an east-west alignment of the spreading axis. Reconstruction by closure of this basin places the Palawan-Mindoro microcontinent against the China Shelf adjacent to the eastern Macclesfield Banks and also indicates a sedimentary provenance from the China mainland. Published well information from offshore southern Taiwan,” supports incipient disruption of the China Sea Shelf during Cretaceous and early Tertiary in Oligocene time. Such data are our incentive to examine other bathymetric highs within the intermediate shelf zone for their hydrocarbon potential in Mesozoic and Paleogene sedimentary sections. The extent of the pre-Oligocene South China Shelf and its relation to Borneo which abuts its southeastern margin is not clear. The subsequent southwards displacement of microcontinents suggests a probable affinity with the mainland China Shelf. The southern perimeter of the basin has been the site of southerly directed subduction ranging from Cretaceous to Eocene age or even earlier. Recognition of this med-Tertiary spreading event has been a major contribution to interpretation of the South China Sea region. Prior to this event, southerly directed Cretaceous and Paleogene subduction was active beneath northern Sundaland, which already existed as an accreted crystalline core dating into the Paleozoic. Paleomagnetic studies” concluded that Borneo has rotated anticlockwise through 50” since Cretaceous time. This rotation was probably associated with subduction beneath northwestern Borneo. Subduction along the Lupar-Natuna are ceased by the end of Eocene timer3 and appears to have become progressively inactive along the North Borneo -_--1_ margm. Modification to the early subduction-dominated regime resulted from the mid-Tertiary spreading. Though in its broadest part, the spreading ridge is established as east-west, the overall trend of the bathymetric deep is northeast/southwest and there were changes probably both in spreading direction and in propagation of oceanic spreading to the southwest. As yet uncorrelated magnetic signatures have been recorded in this part of the basin.r4 The northeast-southwest axial alignment of the oceanic floor is projected into the West Natuna Basin.” This alignment is interpreted as a rift continuation of the South China Sea spreading tectonics into the continental crust of the Sunda Platform, quite independent of the pre-existing Lupar-Natuna subduction zone (Fig. 1). A regionally high geothermal gradient of 2.8”F/100ft’5 is recorded in the West Natuna basin and is cited as possibly representative of thinned continental crust and precursor of seafloor spreading. Earliest sediments in the West Natuna Basin are of Oligocene age coinciding with the initiation of spreading. Following early northeast-southwest trending faulting and subsidence in the basin which was highlighted by the similarly aligned sedimentary wedge, spreading became inverted during late Miocene time.r3 Considering the probable southwesterly propagation of oceanic spreading this episode could represent the final relaxation of the extensional regime, However the apparent offset between the Khorat and Natuna portions of the overall late Mesozoic batholithic arc may well have contributed a wrench-type compressional element.
Geology of Southeast Asia
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Southeast Asia has been interpreted in the light of plate tectonics as a migration of subduction zones related to the oceanic spreading centers located in the Indian Ocean and the South China Sea respectively (Fig. 4). Malayan
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Geology of southeast Asia
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The Tertiary subduction zone is near the surface in the Mentawai Islands and the submarine ridge south of Java, and the corresponding volcano-plutonic arc situated along the west coast of Sumatra can be traced to the south of Java. Another Tertiary subduction zone is located in the southwestern part of Palawan. The Late Cretaceous-Early Tertiary volcano-plutonic arc present in Sumatra does not continue into Java, but passes north of it, running parallel to the subduction zone northeast of Java. These two zones converge in the Meratus mountains in southeast Kalimantan. The occurrence of Late Cretaceous-Early Tertiary granites on the islands of the Sunda Shelf and in West Kalimantan suggests the existence of a double island arc system. The distribution pattern of the ophiolites in the island off western Sumatra, West Java, and southeast Kalimantan also strongly indicates the existence of two opposing subduction zones in this region. Late Triassic-Jurassic volcanics are found in Serian, Matan and Ketapang and on the tin islands whereas the corresponding subduction zones are represented by the Gumai, Garba, Aceh and Natuna Ophiolites. These features also point to the existence of two opposing arc-trench systems in this region. The presence of Permian and early Triassic volcanic and granitic rocks in the Malay Peninsula and presumably in western Kalimantan and the abundant occurrence of Permian volcanic rocks in Sumatra point to a double volcanic arc with opposing subduction zones similar to Cretaceous systems. A double arc-trench system might also have existed during Late Carboniferous to Early Permian time as evidenced by the volcanics and granites occurring in the Padang Highlands, Batang Sangir, Jambi, western Kalimantan, and the eastern Malay Peninsula. The Sumatra, Java and Meratus arcs were generated by spreading centers situated in the Indian Ocean whereas the Natuna and Palawan arcs had their origin on the spreading center located in the South China Sea. The non-volcanic outer arc of eastern Indonesia can be subdivided into two main parts, the Banda outer arc subduction zone consisting of Timor, Tanimbar, Ceram, Buru and Buton, and the Sulawesi subduction zone comprising the Southeast and east Sulawesi arms, the submarine Maju ridge and the Talaud islands. The corresponding volcano-plutonic arc consists of the inner volcanic Banda arc and the western arm of Sulawesi. The Banda arc was generated by a spreading center in the Indian Ocean whereas the Sulawesi arc had its origin in the spreading center located in the Pacific Ocean. In Late Carboniferous-Early Permian time, a subduction zone, dipping in the direction of the Asian continent, existed in or west of Sumatra. Andesitic volcanism and granitic emplacement in Sumatra accompanied this subduction process. Andesitic, basaltic and granitic rocks encountered in the eastern part of the Malay Peninsula and western Borneo indicate that at the same time a minor subduction zone dipping towards the southwest may have existed at the northeastern margin of the continent. Subduction continued or shifted slightly oceanwards in Permian-Early Triassic time. In Late Triassic-Jurassic time the subduction zone at the southwestern continental margin 4 .“. . . . * .* SmItea sllgntly again tOWardS the indian Ocean. The Weii-deveioped, broad voicano-piutonic arc of the Malay Peninsula and the Indonesian tin islands suggest that the Benioff zone was then probably shallower than the older subduction zones. Another minor subduction zone with opposing dip developed at the same time, presumably along the Lupar line in Sarawak,lh.17 indicating a slight migration of the northeastern subduction zone towards the South China Sea. The corresponding volcanic arc is represented by the Serian volcanic rocks and the Triassic volcanics encountered in drill holes in Sunda Shelf. In Late Cretaceous-Early Tertiary time both the southwestern and the northeastern subduction zones became larger as they moved towards the Indian Ocean and the South China Sea, respectively. Emergence of the Sundaland occurred while subduction of oceanic crusts continued under Borneo. Volcanic activity and emplacement of granites took place along the edge of the crater, including Natuna and Anambas islands. Towards the end of the Cretaceous period a rift system of the Gondwana continent developed into a major spreading axis, separating India and Australia from the Gondwana block. During Tertiary time the arc-trench system in Indonesia became fully developed. The new spreading center in the Indian Ocean generated an arc-trench system that stretched from the northwestern tip of Sumatra to Buru and Buton via Java, the Lesser Sunda Islands, Timor, Tanimbar, Kai, and Ceram. The arc at that time comprised an approx. 6000 km-long Tertiary
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subduction zone dipping at a relatively steep angle towards the continent. Intensive volcanism was associated with this renewed subduction and its products are now well exposed along the west coast of Sumatra, the south coast of Java and the Lesser Sunda Islands. Granitic rocks in Sumatra, Java, Flores, Alor, and Ambon also belong to this Tertiary volcano-plutonic arc. Subduction along the Lupar-Natuna line on the southwestern margin of the South China Sea, ceased at the end of Eocene time. The Gulf of Thailand including the west Natuna subbasin was formed by the collapse of the Sundaland mass. In Oligocene time andesitic volcanism started again in Sumatra and lasted until early Miocene time. It has been suggested that the mid-oceanic ridge that migrated northward along the east-side of the Ninety East ridge collided with the western end of the “Old Sunda trench” in Middle to Late Miocene time (10-20 my BP).‘6*‘7The ridge then jumped to the back arc region, and opened the Andaman Sea. Right-lateral strikeslip movement along the Sumatran fault system was initiated during Middle Miocene time.” Emplacement of the serpentized ultramafic rock in northern Sumatra occurred at about the same time. In the South China Sea basin new oceanic crust was emplaced resulting in the separation of the Reed Bank from the Macclesfield Bank. The seafloor spreading which lasted until Early Miocene time caused subduction of the southward advancing crustal block beneath the Borneo-Palawan Ridge. The northern Palawan micro continent drifted further south along the Ulugan transform fault resulting in attachment of this micro continent to the south Palawan subduction zone. The collision of the Reed Bank with south Palawan could have resulted in the emplacement of ophiolites on Palawan in Miocene time.’ In Miocene time or perhaps even earlier a north-south trending, east-facing island arc began to form 600 km east of Borneo, originating from a spreading center situated in the Pacific Ocean; this island arc marked a new pattern of subduction. The emergence of the SulawesiPhilippine island-arc system coincided with the change in directional movement of the Pacific Plate to west-northwest in Eocene Oligocene time.’ In Middle to Late Miocene time this north-south trending Sulawesi-Mindanao subduction zone migrated farther eastward and created the Eastward-facing Halmahera island arc. This arc could not be developed further south as its growth was hampered by the northwardadvancing Australian continent with New Guinea attached to its northern border. Subduction ceased, presumably at the end of the Miocene Epoch and the Indonesion non-volcanic outer arc, Mentawai-Nias, Timor, Tanimbar, Kei, Buru, Ceram, and Buton was uplifted. In southwestern Palawan, subduction aIso ceased in late Miocene time. In Pliocene time the subduction zone west of Sumatra and south of Java shifted oceanward to the present Sumatra-Java trench. However Late Cenozoic to recent volcanism migrated in opposite directions as the dip of the Benioff was much shallower than that of the previous zone. It has been suggested that Irian Jaya is being subducted beneath the Banda Sea, that subduction is not continuous around the arc, and that the north-eastward directed subduction in rl_ A-.. .__.._I_ :_ __-___r_, lrum I?-_- rL_ __...I_...-_~ . ..l_..._*J__ L-I_... LII~:nru LIUU~I’ IS separa~eu int: suutnwaru SUDUUCLIU~ D~IUWo_____ Leram L.. my a^ r_,__~___trdnslurm t_..,* adult in the neighborhood of the Banda Islands.‘9 The most dramatic event in the geologic history of Indonesia took place in Pliocene” when the northward-advancing Australian continent coupled with the counterclockwise rotation of New Guinea and caused the westward bending of the east-west trending Banda arc. The subsequent westward thrust along the Sorong fault system severely modified the east-facing Sulawesi arc into a K-shaped pattern.“,2’.22 The origin of the loop-shaped Banda arc, the age of the Banda Sea and the peculiar K-shape of Sulawesi and Halmahera remain topics of controversy. Two main hypotheses for Banda arc evolution differ regarding the age assumed for the oceanic crust of the Banda Sea and in the inferred past spatial and tectonic relationship between the Banda arc and Sulawesi. The hypothesis presented here including the rolling up of the Banda arc and the westward thrust of the Sulawesi arc holds that the Banda Sea represents an old trapped oceanic crust. Other hypotheses suggest a Late Tertiary age for part of the Banda Sea crust and that the Banda volcanic arc was originally continuous with the Sulawesi volcanic arc to the north, and associated with a west dipping subduction zone. “92’,22Late Tertiary back-arc spreading west of the Banda arc caused the eastward migration of this arc away from the Sulawesi arc creating the Banda Sea. The curvature of the arc reflects the progressive collision of the arc with the curved continental margin of northwestern Australia and Irian Jaya.
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Buton does not belong to the east arm of Sulawesi, since the Triassic autochthonous series present in the island of Timor, Ceram, Buru, and Buton do not occur in the eastern arm of Sulawesi. The Triassic autochthonous series are widely recognized as representing the source of hydrocarbon indications occurring in Timor (mudvolcanoes), Ceram(oi1) and Buton (asphalt). Paleomagnetic studies on the north arm of Sulawesi shows a clockwise rotation of nearly 40” between Early and Late Miocene time about an axis near the neck of Sulawesi, between the south and north arms,21,23slightly earlier in time (Silver, personal comm. 1979). Figures 5-7 show the tectonic elements of western Indonesia and the South China Sea such as volcano-plutonic arcs, subduction zones and the various Phanerozoic basins. Although these figures differ in detail from the schematic sections presented in Fig. 4 they nevertheless reinforce earlier opinion regarding the concentric arrangement of the Phanerozoic volcanoplutonic arcs and the existence of opposing dips of paleo-Benioff zones in the western Indonesian region.‘”
HYDROCARBON
POTENTIAL
OF THE
SOUTH
CHINA
SEA
The South China Sea area is already ringed by hydrocarbon finds. Recent oil discoveries have been made off Palawan in the Miocene Nido reef complex. In a clockwise direction, oil and gas occur offshore Sabah, Brunei, and Sarawak in a variety of Tertiary elastic and carbonate reservoirs; oil and a giant gas field are known in the acreage east of the Natuna arch. To the north, discoveries have been made off the Vietnamese coast and in Chinese waters of the Gulf of Tonkin. The hydrocarbon potential as related to type and structural style of sedimentary basins closely follows the associated tectonic events. Certain basin types classified on a genetic format appear better suited to contain appreciable hydrocarbon accumulations than others. Potential oil reserves were assessed for eleven genetic basin types identified in Southeast Asia. The total anticipated reserves of oil in basins of Southeast Asia amount to 70 billion barrels. The basin types of oceanic margin, wrench, rift and continental margin, which include the dominant types bordering the South China Sea, as a whole were estimated to contain some 56%, or 20 billion barrels of the remaining undiscovered reserves in Southeast Asia. The rift and drift margin of the China continental shelf and micro-continental fragments have individual characteristics reflecting their former grains and sedimentation styles before and after breakup as well as in the ensuing drift phase. The Mesozoic and early Tertiary sedimentary section of the Reed Bank is an addition to the ubiquitous Oligocene and younger sedimentary sequence throughout most of Southeast Asia. The exploration successes of the Reed Bank/Palawan micro continent contrast with the poor record in the remainder of the archipelago where basins are termed as oceanic-arc related. These latter basins generally lack quartoze reservoirs and have patchy carbonate developments. TineWest Natuna and offshore Mekong basins are rift-reiated, and aiigned with the extensionai axis of the South China Sea. By world-wide analogy, rift basins are inherently rich in hydrocarbons, benefiting from an often semirestricted depositional environment and reservoirs in both horstblock structures and overlying draped sediments. The northwest Borneo basin complex is considered to be at least partly underlain by a remnant of oceanic crust that was subducted beneath the Sundaland. Such crust is more readily loaded than buoyant continental type particularly by point source sediment infill, with a consequent thick section and syn-sedimentary tectonics. The Baram Delta and the Mahakam Delta of eastern Kalimantan are examples of this extremely rich basin type. Sediments immediately east of Natuna are positioned over the former subduction complex” and structural styles are generally basement related. They lack the potential of the adjacent oceanic margin of Borneo. The Gulf of Tonkin is believed to have a wrench related basin style. The syn-sedimentary tectonics are postulated to be associated with Tertiary movement of the Red River fault which is accommodating lateral stress from the Himalayan collision belt.” CONCLUDING
REMARKS
Future research for better understanding of the South China Sea Basin might include (1) further magnetic profiling to update and enable correlation of previously recorded linear anomalies in the central portion of the basin ; (2) seismic profiling to delineate possible areas of subsided
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continental fragments in the flanks of the basin; (3) investigations of other bathymetric highs within the intermediate shelf zone for their hydrocarbon potential in Mesozoic and Paleogene sedimentary sections; (4) detailed investigation of the age of the Danau formation in west Kalimantan and absolute age dating of granites occurring in this region; and (5) investigations of the age, distribution pattern, and geochemistry of the Plio-Pleistocene volcanics in western Sarawak and Sabah in order to determine whether these features can be related to continuing subduction of the South China Sea crust. Additional geological and geophysical data from the Indochina and China plate margins should be made available to the international scientific community in order to enable a complete understanding of the tectonic evolution of Southeast Asia and in particular the South China Sea Basin. A new transect north and parallel to transect I (c) might be added to the existing SEATAR programme.
Acknowledgements-The author expresses his appreciation to Mr. Pulunggono of PERTAMINA for assistance and valuable advice during the preparation of this paper. The author’s deepest gratitude is extended to Dr. Gage of CONOCO, Indonesia, for providing new data of the South China Sea region and for giving permission to publish some of the magnificant cross-sections of the western Indonesian-South China Sea region prepared by the oil industry. Finally the author thanks Messrs. Hartono and Sudradjat from the Geological Survey of Indonesia for the suggestions they offered in improving this paper.
REFERENCES I. J. A. Katili, A review of geotectonic theories and tectonic maps of Indonesia, Earth Science Reoiew, 7, 143 1971. 2. J. A. Katili, On fitting certaingeological and geophysical features of the Indonesian island arc to the new global tectonics. In The Western Pacific tsland Arcs, Marginal Seas, Geochemistry (Edited by P. J. Coleman), pp. 287-305.University of Western Australia Press 1973. _ 3. J. A. Katili, Geochronology of West Indonesia and its implication on plate tectonics, Tectonphysics 19, 195 1973. 4. M. L. Parke Jr., K. 0. Emery, R. Szymankiewicz, and L. M. Renolds, Structural framework of the continental margin of the South China Sea, American Association of Petroleum Geologists Bulletin 55,723 1971. 5. Z. Ben-Avraham and S. Uyeda, The evolution of the China Basin and the Mesozoic palaeogeography of Borneo, Earth and Planetary Science Letters, 18, 365. 6. K. 0. Emery and Z. Ben-Avraham, Structure and stratigraphy of the China Basin, American Association of Petroleum Geologists Bulletin, 56, 839 1972.
7. W. J. Ludwig, R. E. Houtz, and N. Kumar, Profiler-Sonobuoy measurements in the South China Sea Basin, J. Geophysical Res. 84,3505 1979.
8. C. 0. Bowin, R. S. Lu, C. S. Lee and H. Schouten, Plate convergence and accretion in the Taiwan-Luzon region, American Association ofPetroleum Geologists Bulletin, 62, 1645 1978; D. E. Hayes and B. Taylor, South China Sea: Evolution of a marginal basin (abstract) EOS, Transactions of the American Geophysical Union, 60(18),389 1978. 9. C. S. Hutchison, Ophiolite in Southeast Asia, Geological Sot. Am. Bull. 86,797 1975. 10. B. Taylor and D. E. Hayes, Evolution of the South China Basin through Tertiary seafloor spreading, Tectonic/geologic Evolution of Southeast Asia (Edited by D. E. Hayes). America1Geophysical Union Monograph, in press. 11. W. Hamilton, Subduction in the Indonesia region. In Island arcs, deep sea trenches and back-arc basins, Maurice Ewing Series, America1 Geophysical Union 1, I5 1977. 12. N. S. Haile, M. W. McElhinny and J. McDougall,Palaeomagnetic data and radiometric ages from the Cretaceous of West Kalimantan (Borneo), and their significance in interpreting regional structures, Journal of the Geological Society 133,133 1977.
13. J. F. White, Jr., and R. S. Wing, Structural development of South China Sea with particular reference to Indonesia, Proceedings of the 7th Annual Convention of the Indonesian Petroleum Association, Jakarta, 1978, pp. 159-64 1978. 14. B. Taylor;personal communication to M. Gage 1980. 15. M. Gage and R. S. Wing, Southeast Asian basin types versus oil opportunity, Proceedings of the 9th Annual Convention of the Indonesian Petroleum Association, Jakarta, 1980 in press. 16. T. Eguchi, S. Uyeda and T. Maki, Seismotectonics and tectonic history of the Andaman Sea, Tectonophysics 57, 35 1979.
17. C. S. Hutchison, Tectonic evolution of Sundaland: A phanerozoic synthesis, Proceedings oftheRegiona1 Conferenceon the Geology of SE Asia, Geological Society of Malaysia Bulletin, 6, 61 1973.
18. B. G. N. Page, Serpentinites of northern Sumatra (abstract). CCGP/SEATAR
Workshop on the Sumatra Transect,
Parapat.
19. K. R. Cardwell and B. L. Isacks, Geometry of subducted lithosphere beneath the Banda Sea in Eastern Indonesia from seismicity and fault plane solutions, J. Geophysical Res. 83, 2825 1978. 20 J. A. Katili, Volcanism and plate tectonics in the Indonesian islands arcs, Tectonophysics, 26, 165 1975. 31 &,. J. A. Katili, Past and present geotectonic position of Sulawesi, Indonesia, Tectonophysics, 45,289 1978. 22. D. J. Carter, M. G. Audley-Charles and A. J. Barber, Stratigraphical analysis of island arc-continental margin collision in eastern Indonesia, Journal of the Geological Society of London, 132, 179 1976. 23. S. Sasajima, S. Nishimura, K. Hirooka, Y. Otofuji, T. V. Leeuwen and F. Hehuwat, Paleomagnetic studies combined with fission track datings on the western arc of Sulawesi, East Indonesia, Tectonophysics, 64,163 1980. This suggestion is similar to that of Katili, but slightly earlier in time (Eli Silver, personal communication).
Geology of Southeast Asia
1091
24. P. Molnar and P. Tapponier, Cenozoic tectonics of Asia, effects of a continental collision, Science, 189,419 1975. 25. M. Gage, personal communication 1980. 26. Sudradjat, personal communication 1980. 27. J. A. Katili, Geological environment of the Indonesian mineral deposits: A plate tectonic approach, Publication Teknikal Series Geological Ekonomi (Geological Survey of Indonesia), 7, 1974.Also United Nations ESCAP, CCOP Technical Bulletin, 9, 39 1975.
Potential of the South China Sea
1075
Entr(ly Printed
Vol. 6. No. II. pp. IO77-1091. in Gnat Britain.
1981
036&5442/8l/
I 11077~ISSOZ.OO/O Pergamon Press Ltd.
GEOLOGY OF SOUTHEAST ASIA WITH PARTICULAR REFERENCE TO THE SOUTH CHINA SEA J. A. KATILI Ministryof Mines and Energy, Jakarta, Indonesia
Abstract-The concentric arrangement of the Phanerozoic arc-trench systems of the western part of Southeast Asia has been previously interpreted in the light of plate tectonics as a migration of subduction zones related to the oceanic spreading centers located in the Indian Ocean and the South China Sea respectively. The Sumatra, Java, and Meratus arcs, with ages ranging from Paleozoic to the present time, were generated by spreading centers situated in the Indian Ocean, whereas the Natuna and Palawan arcs had their origin in the spreading center located in the South China Sea. The Tertiary Banda arc was generated by a spreading center in the Indian Ocean whereas the more or less Tertiary Sulawesi arc had its origin in the spreading center located in the Pacific Ocean. In the South China Sea region, the results of recent exploration drilling on the Reed Bank supports the previous concept that the intermediate shelfal region at a depth of about 2ooOm represents the fragmented remains of continental crust. Oceanic crust emplaced between 32 and 17 million years (my) occupies the abyssal depths. Magnetic patterns of the basin show an east-west alignment of the spreading axis. Reconstruction by closure of this basin would place the northeastern Palawan-Mindano microcontinent against the China Shelf adjacent to the eastern Macclesfield Banks. Prior to this event, southerly directed Cretaceous and Paleogene subduction was Ltive beneath the northern Sundaland, which already existed as an accreted crystalline core dating into the Paleozoic era. The geotectonic evolution of Southeast Asia is reconstructed with particular reference to the South China Sea Region. Several dominant basin types have been recognized on the margins of the South China Sea. These basins as a whole contain an estimated 56% or 20 billion barrels of the remaining undiscovered oil reserves in Southeast Asia.
INTRODUCTION
Little is known of the geology, geophysics and hydrocarbon potential of the South China Sea. A model has been suggested and elaborated’-3 which envisages opposing subduction zones in the western Indonesian region and two spreading centers, a major one situated in the Indian Ocean and a minor one presumably located in the South China Sea. Suggestions that the crust beneath the deep part of the China basin is oceanic& have been confirmed by refraction studies.’ Lineated magnetic anomalies have been recognized in the area west of Luzon and reexamined.’ However, the formation of the South China Sea Basin by separation of Borneo from the Asian mainland’ could not be supported by geological evidence.’ The purpose of this paper is to present more recent data on the geology and geophysics of the South China Sea obtained from earlier research by scientific institutions and exploration results of oil companies. An attempt will be made to reconstruct the geotectonic evolution of Southeast Asia with particular reference to the South China Sea and to discuss the hydrocarbon potential of this area based on the latest findings of the oil industry. Suggestions will be made for future joint research. A REGIONAL VIEW OF THE SOUTH CHINA SEA
The South China Sea is that part of the Pacific Ocean which lies generally west of the Philippines and Borneo (Kalimantan) covering an area of approx. 200 km by 1000km. It is comprised essentially of the central deep water South China Basin surrounded by broad, shallow continental crust to the north, west and southwest, and limited to the southeast by the Northwest BorneolPalawan Trenches. The eastern boundary is well defined by the active north-south Manila Trench. Manila trench (Fig. 1) This trench, some 5OOOmdeep, is separated from Luzon in the Philippines by an outer arc ridge and sediment filled outer arc basin. Mantle earthquakes indicate the presence of an active 1077
1078
J. A. KATILI
MB.
Mocclesfleld
P.G. Poracrl Grcup
from Gape .SWI~Q'80
Fig. 1. Tectonic basin styles in the South China Sea.
Benioff zone dipping eastwards. Volcanic rocks erupted above this zone range in composition from tholeiitic in the west, through talc-alkaline in the middle to high potassium in the east. Palawan/Northwest Borneo trenches (inactive) (Fig. 1) These trenches lie to the southeast of the South China Basin northwest of Borneo and Palawan in the region of the limit of the continental slope. A pronounced bathymetric feature is associated with the Northwest Borneo trench although it is virtually aseismic and devoid of active volcanoes. The Tertiary Palawan arc faces southeast and separates the South China Sea from the Sulu Sea. The islands on the ridge are composed of ophiolite and imbricated Tertiary (mainly Palaeogene) melange, overlain by Miocene and younger strata. In the northern part of the ridge PalaeozoiclMasozoic continental crustal rocks are present. Shoal areas to the west of Palawan in the South China Sea may be similar continental fragments. This Palawan continental slope area is covered by a few hundred meters of virtually undeformed sediments which continue out onto the South China Sea abyssal plain. The seafloor north of the shelf and trench has highly irregular topography with plateaus, ridges and seamounts covered by water depths locally less than 2000 m, but with intervening depressions up to 2000m deep. In the deeper areas the irregular topography comprises basement with a thin sedimentary veneer. Recent turbiditic sediments have locally filled in irregularities. The Palawan Trench is filled with deformed sediments. Pliocene and younger strata of the continental slope prograded across the trench system and are essen!ially undeformed.
Geology of Southeast Asia
1079
The Sarawak Basin (Fig. 2) This large continental shelf basin covers a cresent shaped area off northwestern Borneo and extends northward to link up with the basins off the coast of Vietnam. It is separated from the West Natuna/Malay Basin region by the north-south trending Natuna Arc. The stratigraphy and structural history of this basinal area is variable along and across the basin axial zone. Interbedded sand, carbonate, shale and clay, both marine and non-marine, interfinger complexly. Off the 3orneo coast at least 50OOm of Eocene to Lower Miocene shaly section is present, dominantly marine in the northwest and transitional to the southeast. An overlying younger section thickens gradually offshore and is dominantly deltaic. Shale diapirs and Neogene growth faulting form anticlinal features. Shelf carbonates and reefs are present. Continental shelf basins continue north along the Vietnamese and Chinese coastal areas. The South China Basin (Fig. 1 and 2) This basin is characterized by a central northeast-southwest oriented bathymetric deep at about 4000 m. Particularly in a southern direction this basin is bordered by a broad intermediate
INDO
0
CHINA
Sedimentary
basins
with
200m
water
depth
Fig. 2. Relationship of the West Natuna basin to the oceanic spreader.
1080
J. A. KATILI
shelfal area lying at an average depth of 2000m. A continental shelf at 200m or less is most extensive to the southwest where it comprises part of the Sundaland cratonic platform. These morphologic features represent the components of a complex tectonic history. Contributions to its understanding have become available in recent years from research and oil exploration. The intermediate shelfal region, at 2000 m is believed to represent the fragmented remains of continental crust. Such areas include the bathymetric highs of the Spratley Islands, the Paracel Islands and Macclesfield Bank. Support for this concept has come from exploration drilling on the Reed Bank, west of northern Palawan. Here unmetamorphosed early Tertiary and Lower Cretaceous continental associated sediments have been proved. Taylor and Hayes (in press) have suggested that the Reed Bank and the Permian and Mesozoic rocks of the northern Palawan Islands belong to the same microcontinental fragment. lo Eocene sandstones present on Mindoro may be part of a formation of similar age and rock type recorded in the Reed Bank section. The Ulugan fault divides the northern Palawan microcontinent from the Tertiary subduction system of southwestern Palawan.” Here subduction ceased in late Miocene.” The north-south trend of the Ulugan fault is possibly an indication of a transform extending into the midTertiary ocean crust of the central South China Sea. Oceanic crust emplaced between 32 and 17 my (Fig. 3) is identified as occupying the abyssal depths.” Magnetic patterns in the northern, most extensive part of the basin give an east-west alignment of the spreading axis. Reconstruction by closure of this basin places the Palawan-Mindoro microcontinent against the China Shelf adjacent to the eastern Macclesfield Banks and also indicates a sedimentary provenance from the China mainland. Published well information from offshore southern Taiwan,” supports incipient disruption of the China Sea Shelf during Cretaceous and early Tertiary in Oligocene time. Such data are our incentive to examine other bathymetric highs within the intermediate shelf zone for their hydrocarbon potential in Mesozoic and Paleogene sedimentary sections. The extent of the pre-Oligocene South China Shelf and its relation to Borneo which abuts its southeastern margin is not clear. The subsequent southwards displacement of microcontinents suggests a probable affinity with the mainland China Shelf. The southern perimeter of the basin has been the site of southerly directed subduction ranging from Cretaceous to Eocene age or even earlier. Recognition of this med-Tertiary spreading event has been a major contribution to interpretation of the South China Sea region. Prior to this event, southerly directed Cretaceous and Paleogene subduction was active beneath northern Sundaland, which already existed as an accreted crystalline core dating into the Paleozoic. Paleomagnetic studies” concluded that Borneo has rotated anticlockwise through 50” since Cretaceous time. This rotation was probably associated with subduction beneath northwestern Borneo. Subduction along the Lupar-Natuna are ceased by the end of Eocene timer3 and appears to have become progressively inactive along the North Borneo -_--1_ margm. Modification to the early subduction-dominated regime resulted from the mid-Tertiary spreading. Though in its broadest part, the spreading ridge is established as east-west, the overall trend of the bathymetric deep is northeast/southwest and there were changes probably both in spreading direction and in propagation of oceanic spreading to the southwest. As yet uncorrelated magnetic signatures have been recorded in this part of the basin.r4 The northeast-southwest axial alignment of the oceanic floor is projected into the West Natuna Basin.” This alignment is interpreted as a rift continuation of the South China Sea spreading tectonics into the continental crust of the Sunda Platform, quite independent of the pre-existing Lupar-Natuna subduction zone (Fig. 1). A regionally high geothermal gradient of 2.8”F/100ft’5 is recorded in the West Natuna basin and is cited as possibly representative of thinned continental crust and precursor of seafloor spreading. Earliest sediments in the West Natuna Basin are of Oligocene age coinciding with the initiation of spreading. Following early northeast-southwest trending faulting and subsidence in the basin which was highlighted by the similarly aligned sedimentary wedge, spreading became inverted during late Miocene time.r3 Considering the probable southwesterly propagation of oceanic spreading this episode could represent the final relaxation of the extensional regime, However the apparent offset between the Khorat and Natuna portions of the overall late Mesozoic batholithic arc may well have contributed a wrench-type compressional element.
Geology of Southeast Asia
B a
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1082
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Southeast Asia has been interpreted in the light of plate tectonics as a migration of subduction zones related to the oceanic spreading centers located in the Indian Ocean and the South China Sea respectively (Fig. 4). Malayan
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Geology of southeast Asia
1083
The Tertiary subduction zone is near the surface in the Mentawai Islands and the submarine ridge south of Java, and the corresponding volcano-plutonic arc situated along the west coast of Sumatra can be traced to the south of Java. Another Tertiary subduction zone is located in the southwestern part of Palawan. The Late Cretaceous-Early Tertiary volcano-plutonic arc present in Sumatra does not continue into Java, but passes north of it, running parallel to the subduction zone northeast of Java. These two zones converge in the Meratus mountains in southeast Kalimantan. The occurrence of Late Cretaceous-Early Tertiary granites on the islands of the Sunda Shelf and in West Kalimantan suggests the existence of a double island arc system. The distribution pattern of the ophiolites in the island off western Sumatra, West Java, and southeast Kalimantan also strongly indicates the existence of two opposing subduction zones in this region. Late Triassic-Jurassic volcanics are found in Serian, Matan and Ketapang and on the tin islands whereas the corresponding subduction zones are represented by the Gumai, Garba, Aceh and Natuna Ophiolites. These features also point to the existence of two opposing arc-trench systems in this region. The presence of Permian and early Triassic volcanic and granitic rocks in the Malay Peninsula and presumably in western Kalimantan and the abundant occurrence of Permian volcanic rocks in Sumatra point to a double volcanic arc with opposing subduction zones similar to Cretaceous systems. A double arc-trench system might also have existed during Late Carboniferous to Early Permian time as evidenced by the volcanics and granites occurring in the Padang Highlands, Batang Sangir, Jambi, western Kalimantan, and the eastern Malay Peninsula. The Sumatra, Java and Meratus arcs were generated by spreading centers situated in the Indian Ocean whereas the Natuna and Palawan arcs had their origin on the spreading center located in the South China Sea. The non-volcanic outer arc of eastern Indonesia can be subdivided into two main parts, the Banda outer arc subduction zone consisting of Timor, Tanimbar, Ceram, Buru and Buton, and the Sulawesi subduction zone comprising the Southeast and east Sulawesi arms, the submarine Maju ridge and the Talaud islands. The corresponding volcano-plutonic arc consists of the inner volcanic Banda arc and the western arm of Sulawesi. The Banda arc was generated by a spreading center in the Indian Ocean whereas the Sulawesi arc had its origin in the spreading center located in the Pacific Ocean. In Late Carboniferous-Early Permian time, a subduction zone, dipping in the direction of the Asian continent, existed in or west of Sumatra. Andesitic volcanism and granitic emplacement in Sumatra accompanied this subduction process. Andesitic, basaltic and granitic rocks encountered in the eastern part of the Malay Peninsula and western Borneo indicate that at the same time a minor subduction zone dipping towards the southwest may have existed at the northeastern margin of the continent. Subduction continued or shifted slightly oceanwards in Permian-Early Triassic time. In Late Triassic-Jurassic time the subduction zone at the southwestern continental margin 4 .“. . . . * .* SmItea sllgntly again tOWardS the indian Ocean. The Weii-deveioped, broad voicano-piutonic arc of the Malay Peninsula and the Indonesian tin islands suggest that the Benioff zone was then probably shallower than the older subduction zones. Another minor subduction zone with opposing dip developed at the same time, presumably along the Lupar line in Sarawak,lh.17 indicating a slight migration of the northeastern subduction zone towards the South China Sea. The corresponding volcanic arc is represented by the Serian volcanic rocks and the Triassic volcanics encountered in drill holes in Sunda Shelf. In Late Cretaceous-Early Tertiary time both the southwestern and the northeastern subduction zones became larger as they moved towards the Indian Ocean and the South China Sea, respectively. Emergence of the Sundaland occurred while subduction of oceanic crusts continued under Borneo. Volcanic activity and emplacement of granites took place along the edge of the crater, including Natuna and Anambas islands. Towards the end of the Cretaceous period a rift system of the Gondwana continent developed into a major spreading axis, separating India and Australia from the Gondwana block. During Tertiary time the arc-trench system in Indonesia became fully developed. The new spreading center in the Indian Ocean generated an arc-trench system that stretched from the northwestern tip of Sumatra to Buru and Buton via Java, the Lesser Sunda Islands, Timor, Tanimbar, Kai, and Ceram. The arc at that time comprised an approx. 6000 km-long Tertiary
1084
J. A. KATILI
subduction zone dipping at a relatively steep angle towards the continent. Intensive volcanism was associated with this renewed subduction and its products are now well exposed along the west coast of Sumatra, the south coast of Java and the Lesser Sunda Islands. Granitic rocks in Sumatra, Java, Flores, Alor, and Ambon also belong to this Tertiary volcano-plutonic arc. Subduction along the Lupar-Natuna line on the southwestern margin of the South China Sea, ceased at the end of Eocene time. The Gulf of Thailand including the west Natuna subbasin was formed by the collapse of the Sundaland mass. In Oligocene time andesitic volcanism started again in Sumatra and lasted until early Miocene time. It has been suggested that the mid-oceanic ridge that migrated northward along the east-side of the Ninety East ridge collided with the western end of the “Old Sunda trench” in Middle to Late Miocene time (10-20 my BP).‘6*‘7The ridge then jumped to the back arc region, and opened the Andaman Sea. Right-lateral strikeslip movement along the Sumatran fault system was initiated during Middle Miocene time.” Emplacement of the serpentized ultramafic rock in northern Sumatra occurred at about the same time. In the South China Sea basin new oceanic crust was emplaced resulting in the separation of the Reed Bank from the Macclesfield Bank. The seafloor spreading which lasted until Early Miocene time caused subduction of the southward advancing crustal block beneath the Borneo-Palawan Ridge. The northern Palawan micro continent drifted further south along the Ulugan transform fault resulting in attachment of this micro continent to the south Palawan subduction zone. The collision of the Reed Bank with south Palawan could have resulted in the emplacement of ophiolites on Palawan in Miocene time.’ In Miocene time or perhaps even earlier a north-south trending, east-facing island arc began to form 600 km east of Borneo, originating from a spreading center situated in the Pacific Ocean; this island arc marked a new pattern of subduction. The emergence of the SulawesiPhilippine island-arc system coincided with the change in directional movement of the Pacific Plate to west-northwest in Eocene Oligocene time.’ In Middle to Late Miocene time this north-south trending Sulawesi-Mindanao subduction zone migrated farther eastward and created the Eastward-facing Halmahera island arc. This arc could not be developed further south as its growth was hampered by the northwardadvancing Australian continent with New Guinea attached to its northern border. Subduction ceased, presumably at the end of the Miocene Epoch and the Indonesion non-volcanic outer arc, Mentawai-Nias, Timor, Tanimbar, Kei, Buru, Ceram, and Buton was uplifted. In southwestern Palawan, subduction aIso ceased in late Miocene time. In Pliocene time the subduction zone west of Sumatra and south of Java shifted oceanward to the present Sumatra-Java trench. However Late Cenozoic to recent volcanism migrated in opposite directions as the dip of the Benioff was much shallower than that of the previous zone. It has been suggested that Irian Jaya is being subducted beneath the Banda Sea, that subduction is not continuous around the arc, and that the north-eastward directed subduction in rl_ A-.. .__.._I_ :_ __-___r_, lrum I?-_- rL_ __...I_...-_~ . ..l_..._*J__ L-I_... LII~:nru LIUU~I’ IS separa~eu int: suutnwaru SUDUUCLIU~ D~IUWo_____ Leram L.. my a^ r_,__~___trdnslurm t_..,* adult in the neighborhood of the Banda Islands.‘9 The most dramatic event in the geologic history of Indonesia took place in Pliocene” when the northward-advancing Australian continent coupled with the counterclockwise rotation of New Guinea and caused the westward bending of the east-west trending Banda arc. The subsequent westward thrust along the Sorong fault system severely modified the east-facing Sulawesi arc into a K-shaped pattern.“,2’.22 The origin of the loop-shaped Banda arc, the age of the Banda Sea and the peculiar K-shape of Sulawesi and Halmahera remain topics of controversy. Two main hypotheses for Banda arc evolution differ regarding the age assumed for the oceanic crust of the Banda Sea and in the inferred past spatial and tectonic relationship between the Banda arc and Sulawesi. The hypothesis presented here including the rolling up of the Banda arc and the westward thrust of the Sulawesi arc holds that the Banda Sea represents an old trapped oceanic crust. Other hypotheses suggest a Late Tertiary age for part of the Banda Sea crust and that the Banda volcanic arc was originally continuous with the Sulawesi volcanic arc to the north, and associated with a west dipping subduction zone. “92’,22Late Tertiary back-arc spreading west of the Banda arc caused the eastward migration of this arc away from the Sulawesi arc creating the Banda Sea. The curvature of the arc reflects the progressive collision of the arc with the curved continental margin of northwestern Australia and Irian Jaya.
Geology of Southeast Asia
1085
Buton does not belong to the east arm of Sulawesi, since the Triassic autochthonous series present in the island of Timor, Ceram, Buru, and Buton do not occur in the eastern arm of Sulawesi. The Triassic autochthonous series are widely recognized as representing the source of hydrocarbon indications occurring in Timor (mudvolcanoes), Ceram(oi1) and Buton (asphalt). Paleomagnetic studies on the north arm of Sulawesi shows a clockwise rotation of nearly 40” between Early and Late Miocene time about an axis near the neck of Sulawesi, between the south and north arms,21,23slightly earlier in time (Silver, personal comm. 1979). Figures 5-7 show the tectonic elements of western Indonesia and the South China Sea such as volcano-plutonic arcs, subduction zones and the various Phanerozoic basins. Although these figures differ in detail from the schematic sections presented in Fig. 4 they nevertheless reinforce earlier opinion regarding the concentric arrangement of the Phanerozoic volcanoplutonic arcs and the existence of opposing dips of paleo-Benioff zones in the western Indonesian region.‘”
HYDROCARBON
POTENTIAL
OF THE
SOUTH
CHINA
SEA
The South China Sea area is already ringed by hydrocarbon finds. Recent oil discoveries have been made off Palawan in the Miocene Nido reef complex. In a clockwise direction, oil and gas occur offshore Sabah, Brunei, and Sarawak in a variety of Tertiary elastic and carbonate reservoirs; oil and a giant gas field are known in the acreage east of the Natuna arch. To the north, discoveries have been made off the Vietnamese coast and in Chinese waters of the Gulf of Tonkin. The hydrocarbon potential as related to type and structural style of sedimentary basins closely follows the associated tectonic events. Certain basin types classified on a genetic format appear better suited to contain appreciable hydrocarbon accumulations than others. Potential oil reserves were assessed for eleven genetic basin types identified in Southeast Asia. The total anticipated reserves of oil in basins of Southeast Asia amount to 70 billion barrels. The basin types of oceanic margin, wrench, rift and continental margin, which include the dominant types bordering the South China Sea, as a whole were estimated to contain some 56%, or 20 billion barrels of the remaining undiscovered reserves in Southeast Asia. The rift and drift margin of the China continental shelf and micro-continental fragments have individual characteristics reflecting their former grains and sedimentation styles before and after breakup as well as in the ensuing drift phase. The Mesozoic and early Tertiary sedimentary section of the Reed Bank is an addition to the ubiquitous Oligocene and younger sedimentary sequence throughout most of Southeast Asia. The exploration successes of the Reed Bank/Palawan micro continent contrast with the poor record in the remainder of the archipelago where basins are termed as oceanic-arc related. These latter basins generally lack quartoze reservoirs and have patchy carbonate developments. TineWest Natuna and offshore Mekong basins are rift-reiated, and aiigned with the extensionai axis of the South China Sea. By world-wide analogy, rift basins are inherently rich in hydrocarbons, benefiting from an often semirestricted depositional environment and reservoirs in both horstblock structures and overlying draped sediments. The northwest Borneo basin complex is considered to be at least partly underlain by a remnant of oceanic crust that was subducted beneath the Sundaland. Such crust is more readily loaded than buoyant continental type particularly by point source sediment infill, with a consequent thick section and syn-sedimentary tectonics. The Baram Delta and the Mahakam Delta of eastern Kalimantan are examples of this extremely rich basin type. Sediments immediately east of Natuna are positioned over the former subduction complex” and structural styles are generally basement related. They lack the potential of the adjacent oceanic margin of Borneo. The Gulf of Tonkin is believed to have a wrench related basin style. The syn-sedimentary tectonics are postulated to be associated with Tertiary movement of the Red River fault which is accommodating lateral stress from the Himalayan collision belt.” CONCLUDING
REMARKS
Future research for better understanding of the South China Sea Basin might include (1) further magnetic profiling to update and enable correlation of previously recorded linear anomalies in the central portion of the basin ; (2) seismic profiling to delineate possible areas of subsided
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7
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continental fragments in the flanks of the basin; (3) investigations of other bathymetric highs within the intermediate shelf zone for their hydrocarbon potential in Mesozoic and Paleogene sedimentary sections; (4) detailed investigation of the age of the Danau formation in west Kalimantan and absolute age dating of granites occurring in this region; and (5) investigations of the age, distribution pattern, and geochemistry of the Plio-Pleistocene volcanics in western Sarawak and Sabah in order to determine whether these features can be related to continuing subduction of the South China Sea crust. Additional geological and geophysical data from the Indochina and China plate margins should be made available to the international scientific community in order to enable a complete understanding of the tectonic evolution of Southeast Asia and in particular the South China Sea Basin. A new transect north and parallel to transect I (c) might be added to the existing SEATAR programme.
Acknowledgements-The author expresses his appreciation to Mr. Pulunggono of PERTAMINA for assistance and valuable advice during the preparation of this paper. The author’s deepest gratitude is extended to Dr. Gage of CONOCO, Indonesia, for providing new data of the South China Sea region and for giving permission to publish some of the magnificant cross-sections of the western Indonesian-South China Sea region prepared by the oil industry. Finally the author thanks Messrs. Hartono and Sudradjat from the Geological Survey of Indonesia for the suggestions they offered in improving this paper.
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17. C. S. Hutchison, Tectonic evolution of Sundaland: A phanerozoic synthesis, Proceedings oftheRegiona1 Conferenceon the Geology of SE Asia, Geological Society of Malaysia Bulletin, 6, 61 1973.
18. B. G. N. Page, Serpentinites of northern Sumatra (abstract). CCGP/SEATAR
Workshop on the Sumatra Transect,
Parapat.
19. K. R. Cardwell and B. L. Isacks, Geometry of subducted lithosphere beneath the Banda Sea in Eastern Indonesia from seismicity and fault plane solutions, J. Geophysical Res. 83, 2825 1978. 20 J. A. Katili, Volcanism and plate tectonics in the Indonesian islands arcs, Tectonophysics, 26, 165 1975. 31 &,. J. A. Katili, Past and present geotectonic position of Sulawesi, Indonesia, Tectonophysics, 45,289 1978. 22. D. J. Carter, M. G. Audley-Charles and A. J. Barber, Stratigraphical analysis of island arc-continental margin collision in eastern Indonesia, Journal of the Geological Society of London, 132, 179 1976. 23. S. Sasajima, S. Nishimura, K. Hirooka, Y. Otofuji, T. V. Leeuwen and F. Hehuwat, Paleomagnetic studies combined with fission track datings on the western arc of Sulawesi, East Indonesia, Tectonophysics, 64,163 1980. This suggestion is similar to that of Katili, but slightly earlier in time (Eli Silver, personal communication).
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24. P. Molnar and P. Tapponier, Cenozoic tectonics of Asia, effects of a continental collision, Science, 189,419 1975. 25. M. Gage, personal communication 1980. 26. Sudradjat, personal communication 1980. 27. J. A. Katili, Geological environment of the Indonesian mineral deposits: A plate tectonic approach, Publication Teknikal Series Geological Ekonomi (Geological Survey of Indonesia), 7, 1974.Also United Nations ESCAP, CCOP Technical Bulletin, 9, 39 1975.