PRE-TERTIARY AND TERTIARY FAULT SYSTEMS AS A FRAMEWORK OF THE SOUTH SUMATRA BASIN; A STUDY OF SAR-MAPS PDF

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zy © IPA, 2006 - 21st Annual Convention Proceedings, 1992 IPA 92-11.37 zyxwv PROCEEDINGS INDONESIAN PETROLEUM ASSOCIATION Twenty First Annual Convention , October 1992 PRE-TERTIARY AND TERTIARY FAULT SYSTEMS AS A FRAMEWORK OF THE SOUTH SUMATRA BASIN; A STUDY OF SAR-MAPS A. Pulunggono * Agus Haryo S....

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SOUT H SUMAT RA BASIN PROVINCE, INDONESIA: T HE LAHAT /TALANG AKAR-CENOZOIC T OTA… Aji Lesmana ANCIENT TALANG AKAR DEEPWAT ER SEDIMENT S IN SOUT H SUMAT RA BASIN: A NEW EXPLORAT ION P… Iman Argakoesoemah dokumen.t ips_ makalah-cekungan-sumat era-selat an.docx Sayyidil Mursalin

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© IPA, 2006 - 21st Annual Convention Proceedings, 1992

IPA 92-11.37

PROCEEDINGS INDONESIAN PETROLEUM ASSOCIATION Twenty First Annual Convention , October 1992 PRE-TERTIARY AND TERTIARY FAULT SYSTEMS AS A FRAMEWORK OF THE SOUTH SUMATRA BASIN; A STUDY OF SAR-MAPS A. Pulunggono * Agus Haryo S. ** Christine G. Kosuma ***

ABSTRACT

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Recent analyses of SAR-maps covering South Sumatra show WNW-ESE, N-S, NW-SE and k N 30"E dominant trends (or fault strikes).

Similar trends were recognized earlier (1990-1991) from Landsat studies and aerial photo/SAR interpretations backed up by geological field data in the Garba Area and Negara Batin (Lampung) sub-basin. Previous seismic work in the Palembang Basin (also) revealed the WNW-ESE, N-S and N 30"E trends as major deepseated faults. The complex lineament pattern on the maps are seen as a combined expression of structural phenomena induced by main diastrophic events (or tectonic phases) within this part of Sundaland. Therefore, to distinguish structural patterns of same genetic origin or related to a particular tectonic phase in this region with a convergent plate tectonic setting since the Jurassic, is necessary. The principle of rockfailure with inherent main stress direction and intensity due to the colliding and subducting Indian oceanic plate upon Sundaland is integrated in the study. The distribution of granitoid intrusives of Jurassic and Cretaceous ages previously mapped by the Directorate of Geology and also known from oil companies drillings, has been reviewed in this study and proved to be of utmost importance to explain the geological evolution of Sundaland since Jurassic times.

* Freelance Geologist ** Trisakti University * * * P.T. Indo Georeka Nusantara

Ellipsoid models for Mid-Mesozoic (Upper Jurassic Lower Cretaceous), Upper Cretaceous - Lower Tertiary and Mid-Tertiary (Mid-Miocene) main tectonic phases, clearly discern wrench, normal and upthrust components for each phase among the recognized dominant trends.

Hence, the conclusion that initiation of the South Sumatra back-arc basin took place in the Paleogene by way of subsiding "block-areas" along an established framework of primarily WNW-ESE (Lematang) and N-S trending strike-slip faults of Pre-Tertiary origin which became rejuvenated as normal faults with the start of (Tertiary) basinal history. Compressive tectonics in the Neogene marked the South Sumatran back-arc basin development a.o. inducing inversion of faulted block areas along the WNW-ESE trending faults. The NW-SE (Barisan or Semangko) trend clearly offsets the WNW-ESE trend and presently is encountered as an active strike-slip faultzone at crestal parts of the Barisan Mountain Range. INTRODUCTION

The geodynamics of the South Sumatra backarc basinal area, especially with regard to the evolution of the Palembang Basin, has been of special interest within these last two decades or so because of its particular position within a long- term convergent setting of the Indian Ocean plate which subducts below the southwestern edge of continental Sundaland. Since first exploration activities here started in the last decade of the previous century, a substantial amount of geological, geophysical, drilling and other data have been obtained, but only since the early seventies in this

340 century and with accessibility to a wealth of new data, studies pertaining to the area of a more regional character and in line with the new view of tectonics could be carried out.

by the Oil companies and systematic mapping by the Directorate added a of new material not only with regard to the stratigraphy and structure of Tertiary sedimentary formations but also on the pre-Tertiary of South Sumatra whereby, in the course of time, it became clear that Tertiary (sedimentary) basinal development is inseparable from, and in a way controlled by the pre-Tertiary basement's dynamics (Figure 1). Nevertheless, with the South Sumatra backarc basinal area vastly stretching from the Malacca Strait in the North and Northeast to the Barisan Mountain Range in the South and Southwest, the Tigapuluh Mts as its western boundary and the Lampung High as its eastern edge, and with similar and related histories of the Palembang Basin and the Jambi Basin to its Northwest, the Garba sub-basin at its southern margin and the Negara Batin-Lampung sub-basin in its southeastern extension, it is thought necessary to conduct a study on a much more regional scope for a deeper insight with regard to the mechanism of (South Sumatra)backarc basin forming within the context of plate tectonics in the region. Since the WNW-ESE trending and seismically detected 155 km long Lematang Fault in the Palembang Basin has been described for the first time in 1969, this particular k N300"E trending steeply dipping basement-rooted (strike-slip) fault which subsequently became a normal (or "growth") fault and later on subject to compressional tectonics during the Tertiary basinal evolution, has been a matter of speculation regarding its origin and connection with the popularly known "Sumatra Trend" marked by the NWSE (or k N320"E) trending Barisan Mountain Range and its Barisan Fault Zone (Pulunggono, 1969, 1983) (Figure 2). At the most westnorthwestern site of detection, the Lematang Fault shows a vertical throw of ca. 1500 m which means that this fault still extends into WNW direction over a considerable distance. In 1974, de Coster already noted the existence of belts of Late Paleozoic and Mesozoic age trending WNW-ESE (or k N300"E) with major faults probably as boundaries between the belts in the South Sumatra backarc region (de Coster, 1974). Its obliquity to the longitudinal (NW-SE directed) axis of the isle of Sumatra is obvious (Figure 3). The 1984-1987 field geological mapping by Trisakti University in the Garba Area revealed the existence of a WNW-ESE (k N300"E) trending strike-slip fault, called the Saka Fault while Landsat studies of the area

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show that this 80 km long fault is truncated by the NW-SE trending Barisan Fault Zone at its northwestern end (Haryo, A . S . , 1986). . Recent (1990) SAR photo and seismic surveys in the Negara Batin-Lampung sub-basin also show an obvious WNW-ESE (+ "300"E) trending major lineament (or fault) which may be the eastern continuation of the Saka Fault (Petrocorp-Trisakti University, 1991) (Figure 4)

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Another conspicuous trend of major magnitude in the South Sumatra backarc basinal area are North-South trending fault (faultzone) a.0. encountered in the Palembang Basin (Pendopo area) where this 50 km long and 5 km wide half-graben is known as the LaganLenggaran Fault or popularly as the "Benakat gulley".

The N-S trending Lagan-Lenggaran Fault is seen offsetting right-laterally the WNW-ESE trending Lematang Fault over a distance of ca. 12 km. Its southern prolongation is known as the Kikim Fault. At top basement, the Kikim Fault is traced over a distance of ca. 20 km and seismic cross-sections show a considerable vertical displacement (throw) of ca. 800 m. Further east, the N-S trending and ca. 300 km long east coast of Lampung Province as a likely faultzone is obvious while lately, field investigations in the Negara Batin sub-basin showed the existence of a N-S trending depression. This paper is basically an analysis of recent SAR (Synthetic Aperture Radar) maps which practically covers the whole of South Sumatra comprising not only the South Sumatra backarc basinal area, but also Bangka Island in the Northeast, the Barisan Mountain region in South Sumatra and part of the province of West Sumatra. The objective of this paper is not only to recognize obvious lineament patterns from the maps which are seen as a combined surface expression of structural phenomena induced by several diastrophic events within this part of Sundaland and to discern them into structural patterns of the same genetic origin (or the same tectonic phase), but also to collate the results of these SAR analyses with important subsurface (mainly structural) phenomena recognized within the Tertiary and the pre-Tertiary of the South Sumatra backarc basinal area i.e. the aforesaid WNW-ESE or f N300" fault trend, the North South trending fault zones, the NW-SE trending Barisan Fault Zone and other geologic data such as pre-Tertiary lithology, distribution of intrusive rocks etc., obtained from previous exploration activities and studies and which are seen as key parameters of the region's geodynamics.

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At this scope of study, the obtained patterns and likely systems are instrumental not only for a better insight of backarc basin development, but also towards a better understanding of the evolution of Sundaland itself since Jurassic times with the inception of a subducting oceanic plate below Proto-Sundaland. RESULT OF ANALYSES

The conducted analyses of the SAR-maps show the existence of a structural pattern which consist of lineaments (or faults) and fold structures. Lineaments

A pattern of five long lineaments stretching linearly and continuously in a WNW-ESE (or ESE-WNW) direction is recognized. These lineaments are each traced over a distance of hundreds of kilometers, ending more or lessabruptly at the N-S trending east coastal area of Lampung Province (Figure 5). From North to South these lineaments of major magnitude are respectively :

A ESE-WNW trending lineament stretching from the Lampung coastal area Southeast of the town Palembang along the greater part of the Musi River and further Westnorthwest until a site north of Lake Kerinci. The estimated total length of this lineament is 630 km. A ESE-WNW trending lineament stretching from the Lampung coastal area East of the town Prabumulih and along a line which coincides with the Lematang Fault and further Westnorthwest until it is truncated by the Barisan Mountain with the offsetted prolongation west of the Barisan Mountain range reaching the coast line of West Sumatra. A ESE-WNW trending lineament stretching from the coastal region East of the town Baturaja (Kepayang area) to the town Lahat in westnorthwestern direction and then further Westnorthwest until it is truncated by the Barisan Mountains with its offsetted prolongation west of the Barisan Mountain range reaching the coastline of Bengkulu. A ESE-WNW trending lineament stretching from the Lampung coastal area east of the Negara Batin sub-basin and along the entire Saka Fault and then further to the Westnorthwest until it is truncated by the Barisan Mountain range with its offsetted prolongation west of the mountain range reaching the coastline of Bengkulu.

5) A ESE-WNW trending lineament at the Southeastern end region of the Lampung Province starting from the coastal region with its Westnorthwestern prolongation to be traced in the Indian Ocean. Measured strikes on these ESE-WNW trending major lineaments are respectively in the range of: 1) N285"E - N300"E, 2) N288"E - N295"E, 3) N280"E - N295"E, 4) N280" - N295"E and 5) N275"E - N295"E.

Thus, five parallelly aligned lineaments of major magnitudes and trending ESE-WNW or WNW-ESE are recognized. These five major lineaments may be called respectively : The Musi lineament, the Lematang lineament, the Kepayang lineament, the Saka lineament and the Lampung South Coast lineament (Figure 6 ) . Another obvious pattern of lineaments of major size has also been recognized from the SAR-maps analyses. These consist of three North-South trending lineaments and from west to east are respectively: 1) A zone of at least two parallel aligned N-S lineaments stretching from South Sumatra's northern coastal area southward to the Pendopo-Limau area where it is already known as the pronounced feature of the "Benakat gulley". From hereon it stretches further south as a clear continuous lineament, along the already mentioned Kikim Fault bypassing the town Muara Enim and following the straight N-S trending upper course of the Lematang river to end at a site northwest of the Garba Mts. 2) A N-S trending zone of lineaments starting from the north of the town Palembang to the western boundary region of the Negara Batin sub-basin in Lampung. In this paper from hereon called the North-South Palembang lineament. 3) A pronounced N-S trending zone of lineaments stretching along the entire east coast of Lampung Province. A third lineament trend worth mentioning is a SSW-NNE directed (or N30"E) trend. On the lineament map, this trend is obviously seen only in the southern and southeastern part of the Palembang backarc basin (Garba Mts and Negara Batin sub-basin), but geophysical and drilling data from the Limau area clearly depict this N30"E trend as deepseated normal faults with substantial vertical displacements.

Furthermore, the produced lineament map shows that all ESE-WNW trending major lineaments are at their WNW-ern end truncated by the Northwest-Southeast oriented Barisan Fault Zone.

342

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To simplify discussions pertaining to the content of this paper, the abovementioned ESE-WNW trending major lineaments will be mentioned as the N300"E trendor the Lernatang Trend. The North South Trend is a clear trend, but the orientation of the Barisan Fault Zone along the entire length of the west coast of Sumatra may be better described as the N320"E Trend or may be simply called the Barisan Trend. Interestingly the N300"E trend (the Lematang Trend) is seen to dominate the entire island of Bangka. Less distinct but still seen are North-South trending lineaments. The N300"E trend's prolongation to westnorthwestern direction or to South Sumatra's northern coastal region is not clear. The North-South lineaments are seen to dissect the Lematang Trend through right-lateral offsets. This is clearly seen in the Pendopo area and possibly also in the Kepayang area and in the area southwest of the Negara Batin sub-basin. At the latter place the NorthSouth Trend as well as the N300"E trend are also dissected by the N30"E directed lineaments and also by the Barisan Trend. The Barisan Trend is a pronounced feature over the entire length of the Barisan Mountain Range notably in its crestal part. However, no distinct lineaments trending N320"E or in that order are seen in the backarc basin region except at some places in the Garba Mts where it is seen to offset N300"E trending faults.

PATTERNS IN THE PRE-TERTIARY (BASEMENT) ROCK FORMATIONS The Tertiary sedimentary rock formations in the South Sumatra backarc basinal area are unconformable underlain by rock formations of pre-Tertiary age and which primarily consist of Mesozoic igneous rocks and of Paleozoic and Mesozoic metamorphics and carbonates. These complex of "basement" rocks were intensively folded and faulted and also intruded by igneous rocks during the Mesozoic. These rocks are observed as outcrops in the Barisan Mountains, locally outcropping areas of pre-Tertiary rock formations within the backarc basinal region namely the Tigapuluh Mts, the Duabelas Mts, the Garba Mts, the Bukit Pendopo area and Bukit Batu east of Palembang and are also known from drilling activities which penetrate basement. Data from drillings show that basement lithology is dominantly quartzite, phyllite, metagreywacke, occasionally limestones and granitoids.

In reviewing data of the pre-Tertiary complex compiled by the Directorate of Geology and data obtained from

oil companies drilling activities in the region, an obvious pattern with regard to the distribution of the granitoids is observed. Radiometric age determinations on several rock samples of these intrusive rocks carried out by the Directorate of Geology as well as by the oil companies show that in the area under study and going from north to south, a distribution of successively older to younger age granitoids in a parallel linear pattern and trending WNW-ESE each, can be distinguished (Figure 7).

As its most WNW-ern site, granitoids of Jurassic age are outcropping at the Gumanti-Liki Mts, east of the town Padang. In ESE-ern direction, Jurassic granitoid rocks are encountered as outcrop in the Tigapuluh Mts and in the Duabelas Mts. Further to the ESE, drillings at Kluang Utara-49 and Tanjung Laban-1 proved the existence of Jurassic granitoids as basement. The Bukit Batu granitoid outcrop east of Palembang is determined to be of Jurassic age. In the offshore region of the Java Sea further southeast, Jurassic granitoids constitute basement at the Sari-1 well. The Tigapuluh Mts complex is situated more northwards with regard to the Duabelas Mts but in itself is a WNW-ESE trending structural element. WAr age determinations upon the granitoids here show a Lower Jurassic age (180 k 7 m.y.). The Duabelas Mts granites, situated further south and in a perfect linear position with the Kluang Utara, Tanjung Laban and Bukit Batu granitoids in the east and with the GumantiLiki granitoids in the west, are dated to be of Late Jurassic age (159 k 6 m.y.).

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The granitoids at well Kluang Utara-49 and at well Tanjung Laban-1 respectively show radiometric ages of 153 (k 5 m.y.) and 149 (+ 4 m.y.) which thus show Upper Jurassic ages.

Parallel and to the south of the abovementioned WNW-ESE trending distribution of Jurassic granitoids, geological fieldwork and drilling data revealed the existence of WNW-ESE directed and (also) linearly aligned outcrops and subcrops of granitoids of Cretaceous age. Starting from the north of Gunung Bolang (southeast of Padang) to ESE-ern direction, granitoids of Cretaceous age are encountered at Batang (or river) Siliti and at Batang Gumanti. Further ESE are the granitoid outcrops of the Tembesi-Rawas Mts. From hereon to ESE-ern direction, (oil companies) drillings show the existence of Cretaceous granitoids as basement rocks in the Pendopo-Limau area, (granites and granodiorites). Granite and granodiorite intrusions have been found as basement rocks at drillings in the Java Sea Sunda Basin where radiometric age datings show ages which vary from Lower Cretaceous to Upper

343 Cretaceous (115,s m.y. to 84,7 m.y.). In 1976, the Directorate of Geology interpreted the granitoids southeast of Padang and those in the Limau-Pendopo area to be Cretaceous in age. Granites southeast of Lake Singkarak situated NNE of Padang, have been age dated by way of isotopic methods and found to be 112 k 24 m.y. old which shows to a Lower Cretaceous. age. (Katili, 1962). It is thus reasonable to interpret the granitoid intrusions in the Limau-Pendopo area which were never radiometrically age dated, to be of Lower Cretaceous age instead, due to its obvious position linearly and in a WNW-ESE direction between the granitoids southeast of Lake Singkarak and the Sunda Basin granitoids. In addition to this, radiometric age determinations on andesite samples from well Lembak A-1, situated ca 15 km north of the Limau area, show a Lower Cretaceous age of 121 i 2 m.y. (Pertamina, 1980). Distinct Upper Cretaceous and Late Cretaceous ages (79,9 ?L 1,3 m.y. and 89,3 i 1,7 m.y.) have been determined for the granites of the Garba Mts which is situated 80 km. south of the Pendopo-Limau area and which is bound to its south by the WNW-ESE trending Saka Trend (Trisakti University, 1985).

It is obvious that in the context mentioned above, s...