Pulonggono Structure

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

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 inter- pretations 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 rock- failure 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 en- countered 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 south- western 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

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

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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.

show that this 80 km long fault is truncated by the NW-SE trending Barisan Fault Zone at its north- western end (Haryo, A. S . , 1986). .

Recent (1990) SAR photo and seismic surveys in the by the Oil companies and systematic Negara Batin-Lampung sub-basin also show an obvious

WNW-ESE (+ "300"E) trending major lineament (or mapping by the Directorate added a of new material not only with regard to the stratigraphy fault) which may be the eastern continuation of the 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

Saka Fault (Petrocorp-Trisakti University, 1991) (Figure 4)

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 Lagan- Lenggaran Fault or popularly as the "Benakat gulley".

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 sub- sequently 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 NW- SE (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

The N-S trending Lagan-Lenggaran Fault is seen off- setting 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 para- meters 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 Prabu- mulih 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 westnorth- western 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 Bengku- lu.

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 South- eastern 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

N280" - N295"E and 5) N275"E - N295"E. - N300"E, 2) N288"E - N295"E, 3) N280"E - N295"E, 4)

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 line- aments 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.

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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 line- aments. The N300"E trend's prolongation to westnorth- western 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 North- South 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, meta- greywacke, 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 Gumanti- Liki granitoids in the west, are dated to be of Late Jurassic age (159 k 6 m.y.).

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

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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).

On Bangka island, granitoids had been determined to be of Upper Triassic age by radiometric methods namely -t 195 - i 210 m.y. (Priem et al, 1955; Jones et al, 1977). However these tinbearing granites which intrude Upper Paleozoic to Trassic rocks do not show any particular pattern of distribution.

INTERPRETATION AND DISCUSSIONS

Based on data mentioned in the previous chapter, the following interpretations can be forwarded:

The Musi, Lematang, Kepayang, Saka and Lampung Southern Coast lineaments are faults or faultzones of major magnitudes especially in length. The Lematang lineament and the Saka lineament show large vertical displacement which may also be the case with regard to the other lineaments mentioned above.

In conjunction with the striking distribution of the granitoid intrusions of various ages which are spread out parallel and practically along the length of the Musi, Lematang and Saka lineaments, the five above- mentioned lineament or faults can be also arranged on the basis of age of origin namely that the most northern situated Musi lineament is the oldest while the most southern situated Saka lineament is the youngest, with the Lampung South Coast lineament even younger.

It is obvious that in the context mentioned above, subduction related granitoid intrusions and related faults or faultzones were responsible for the regional expansion or progressive continentalization of (old) Sundaland to southwestern directi'on.

The abovementioned also show that a WNW-ESE trending magmatic arc existed in Jurassic times which formed the southern margin of Sundaland then. Subse- quently, the Cretaceous (Lower and Upper) magmatic arc(s) formed the southern margin of Sundaland.

The Musi, Lematang, Kepayang, Saka and Lampung South Coast (?) lineaments of Jurassic to Upper Cretaceous age are interpreted to be strike-slip faults (or faultzones) on the basis of : a) their remarkable length, b) their continuity, c) the result of an oblique convergence of the subducting Indian Ocean plate against the WNW-ESE trending margin of continental Sundaland in Jurassic to Upper Cretaceous times.

Within the aforementioned context, the Musi lineament and the Lematang lineament are clearly related to the Jurassic and (Lower) Cretaceous granitoid intrusive rocks and thus are of Jurrasic to (at least) Lower Cretaceous age. Then, in Jurassic times commencing northwestward moving Indian Ocean plate as a driving mechanism for compression and concomitant control of tectonic features, converged obliquely against the WNW-ESE trending margin of Sundaland inducing magmatism and accompanying strike-slip faults (or faultzones) which is now seen as the Musi lineament and the Lematang lineament (With a N30"W directed Indian Ocean plate converging onto a WNW-ESE trending margin of Sundaland, the angle of oblique convergence is ca 30"). The subduction zone during (Upper) Jurassic and (Lower) Cretaceous times is best fitted by a WNW-

ESE trending zone situated south of the Lematang lineament and which southwards stretches until the Saka lineament. (Figure 8) This zone is analogous to the Lingsing Series and Saling Series of Musper (1937) and van Bemmelen (1949) and to the Woyla Terrains of Cameron (1980) and Pulunggono and Cameron (1984).

With the Saka lineament as the direct south margin of the Upper Cretaceous Garba granites and in the same WNW-ESE orientation as the older Lematang lineament to its north, it is shown that at least, until the Late Cretaceous (Campanian, with a Garba granite sample radiometrically dated 79.9 k 1,3 m.y.) (Trisakti University, 1985), the Indian Ocean plate is still converging onto a WNW-ESE trending Cretaceous margin of Sundaland obliquely in a N30"W direction.

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The North-South directed Lenggaran-Lagan fault(zone) or Benakat gulley on the lineament map is shown to offset the Lematang Trend right laterally, thus inter- preted to be younger than the N300"E trend of Late Jurassic to Lower Cretaceous age. It continues south- wards as the Kikim fault. The Lemat Formation (also called "young-Lemat" which mainly consist of tuffs and sandstones) of Late Paleocene-Early Oligocene age is regarded as the earliest "infill" sediments in the Benakat gulley and its further continuation to the south (de Coster, 1974). On this basis, a Late Paleocene to Eocene age has been assigned to the origin of the Benakat gulley.

However, after the recognition of the three N-S trending lineaments of major size, other aspects come into light which may support the opinion of an older than Late Paleocene age for these N-S trending lineaments including the origin of the Benakat gulley and the Kikim fault:

a. The large vertical Fsplacement of ca. 1700 m measured from more recent seismic sections across the Kikim fault is indicative for a fault which is deep-seated (or deeply rooted) in the pre-Tertiary (basement) rock formations. (Harding, 1983, Suhendan A.R., 1984) (Figure 9).

b. The straightforward north-south trending lineament which incorporates the Lenggaran-Lagan fault/ Benakat gulley and the Kikim fault is now recog- nized as a fault (zone) with such a notab1,e length namely in the order of ca 300 km or more while the N-S Palembang lineament to its east and the one which stretches along the entire east coast of Lampung, are also in the order of hundreds of kilometres and presumably, may also have large vertical displacements. These straight north-south aligned structural elements, stretching over a distance of ca 300 km or more, and at places (Benakat gulley - Kikim, west part Negara Batin sub-basin) known to contain Tertiary sedimentary infill, may be classed together with the 265 km long Bengkalis Depression of the Central Sumatra Basin, described by Moulds in 1989. Like the Bengkalis Depression, the distal southern ends of those north-south running megafaults in South Sumatra described in this paper are obscured by so-called tectonic overprinting of younger age of the Barisan Tectonic Event (Moulds,1989).

The distal northern end of these lineaments are seen on the map as progressively becoming blurred which is probably due to the swampy coastal environment. Thus the possibility that these major North-South (or South-North) trending lineaments

still continue over considerable distances to the north or to the south, is not out of the question. It is furthermore noted that the highest present day heat flows measured in the South Sumatra backarc basin are obviously found and mapped as a north- south stretching broad zone lying in between and possibly bounded on both sides by the westernmost situated major lineament described in this paper and the North-South Palembang lineament (Figure 5 & Figure 6A).

c. Based upon the characteristics mentioned above, the North-South trending lineaments (faults or faultzones) are structural elements of the same magnitude as the N300"E trending major faults in the South Sumatra region. These two sets of linear and parallel aligned major faults (or faultzones) with the one dissecting (and offsetting) the other in a more or less regular pattern, strongly suggest an inter-related character of the Lematang (N300"E) trend - North-South lineament configuration.

d. The oldest rocks found in the South Sumatra basin that post-date the Mesozoic and Paleozoic strata (basement) are tuffaceous sandstones, conglomerates, breccias and clays, grouped together as the "Kikim Tuffs" or "old Lemat" and are assigned a Late Cretaceous to early Paleocene age. These are found in the Tamiang-2, Lemat-1 and Lemat-2 wells, the Laru wells and in exposures in the Gumai mountains (de Coster, 1974). It is now shown on the lineament map that the Tamiang-2 well is situated obviously along a north-south lineament (or lineaments) which clearly is the northern prolongation of the Lenggaran-Lagan fault or Benakat gulley. The Lemat wells situated to the southeast of well Tamiang-2, are now also known to be situated alongside the North-South Palembang lineament. On the basis of these findings, it may be concluded that the North-South trending major lineaments in South Sumatra in places contain deposits of a Late Cretaceous to early Paleocene age. So the forming of the grabens themselves took place in pre-Tertiary times.

e. Nevertheless, inter-relation of the Lematang Trend (i.e. the Lematang Fault) with the North-South trending lineament, i.e. the Benakat gulley and Kikim fault, can only be more solidly explained by a model which reasonably relates origin and age and is relevant to the geological history of the region. By applying the ellipsoid model to the area, the following successive stages are recorded:

1. Due to the obliquely N30"W oriented converg- ing Indian Ocean plate as a driving mechanism

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gulley, practically all presently observed anticlinal structures in the South Sumatra backarc basin have their longitudinal axes aligned to the "old grain" of the N300"E Trend. The Barisan Mountains had already become a full mountgin range in Plio-Pleistocene times (Figure 13).

for compression against the margin of Sunda- land (primary stress) with resulting simple shear, the N300"E Musi and Lematang linea- ments were induced as active right-lateral strike-slip faults during Upper Jurassic - Lower Cretaceous times (Figure 10). Its first order antithetic direction is the North-South trending lineament which includes the Lenggaran-Lagan fault (or Benakat gulley) and Kikim fault, which is of left-lateral character in the same period but is not actively moving. Thus during this time, a fracture (or parallel fractures) in north-south direction have been induced already (Figure 11).

2. A different stress regime is recorded during Upper Cretaceous - Lower Tertiary times whereby the north-south oriented lineaments (fractures) were subject to extension, thus forming grabens or depressions a.o. the Benakat gulley. The N300"E Lematang lineament (i.e. the Lematang Fault) ceased to be a strike-slip fault and the Lematang Depression situated east of the Kikim fault, became progressively a pro- nounced feature also because with the start of the Tertiary sedimentary history, the Lematang fault that had already ceased to exist as a strike- slip fault became a normal fault. SW-NE (N30"E) running normal faults also were generated and dissect the Limau area along its longitudinal axis so that horst and grabens characterize its (pa1eo)topography.

3. The N320"E trending Barisan lineament may have been initiated already during Lower Tertiary times, but only in Mid-Miocene time is this structural element seen as a row of mountains with an active strike-slip fault.

4. A compressive regime marked the Mid-Miocene (i.e. Upper Mid-Miocene) until recent times whereby at places (Lematang fault, Saka fault) so-called inversions took place. (Figure 12). The Lenggaran-Lagan (Benakat gulley) - Kikim fault became subject to wrench movements in right-lateral sense. Folding and upthrusting caused by compressive movements became pro- gressively pronounced in the backarc basinal areas and reached a peak during Plio-Pleis- tocene times. So, during Mid Miocene time and onwards, north-south directed wrench move- ments as well as compressive movements are recognized in the backarc basinal area. It is interesting to note that except at places where north-south directed right lateral wrench move- ments had been active such as in the Benakat

Abovementioned interpretations and conclusions also support previous opinions or models which have suggested that the North-South aligned Sumatra grabens originally crossed the Barisan Mountains or originally extended southwards right to the edge of the continental plate (Howles, 1986, Moulds, 1989), except that in this paper an older age for the origin of the North-South trending lineament in South Sumatra is inferred. Field geological mapping by Trisakti University also show that south of the Saka Fault (Garba area), the Tertiary sedimentary formations are still found to extend southward, only to be covered by Quaternary and Recent sediments of the Barisan Mountains.

CONCLUSIONS

The framework of the South Sumatra backarc basinal area can be traced within the pre-Tertiary geological history of the region.

Depocentres of the Tertiary sediments in the South Sumatra basinal area are situated in depressions or " 1 0 ~ ~ " which basically are bounded by N300"E and North-South directed faults of major size of pre- Tertiary origin and which since the start of Tertiary sedimentary history were rejuvenated until about Mid-Miocene time.

Notwithstanding faults which originated during the Tertiary, structuring (i.e. folding) of the Tertiary sedimentary layers have taken place along the pre- Tertiary "grain" or a system of faults created by the Lematang Trend, the North-South lineaments and also Southwest-Northeast running faults.

In conjunction with geological, geophysical and drilling data, SAR map analyses of this study have proven to be very instrumental in obtaining a more thorough understanding of South Sumatra's regional structural framework.

The three recognized North-South running linea- ments in South Sumatra are now seen as elements of major size. The westernmost situated North- South lineament comprising the Lenggaran-Lagan faultmenakat gulley and Kikim fault and the North-South Palembang lineament to its east,

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originated within the pre-Tertiary. These two major north-south trending lineaments seemingly are structural boundaries of a north-south trending mega-depression which is the locus of high heatflow and where most of the Tertiary oil in South Sumatra have been generated and produced.

An inter-relation between the N300"E Lematang lineament, and the North-South trending lineament which incorporates the Benakat gulley and Kikim fault, is explainable by way of applying the ellipsoid model.

The evolution of Sundaland since Upper Jurassic times was a regional expansion and a process of accretion and continentalization to Southwestern direction by way of longterm oblique subduction of the Indian Ocean plate against the Sunda continent with WNW-ESE directed retreating (Southwest- ward or Indian Ocean-wards moving) trenches until, at least, Upper Cretaceous times.

The Barisan Tectonic Event overprints the earlier induced framework of WNW-ESE and North- South directed lineaments and except at crestal regions and regions in close surroundings of the Barisan Mountain Range, played a minimal role with regard to the origin and subsequent develop- ment of the South Sumatra backarc basin.

ACKNOWLEDGMENTS

We thank Dr. Paul Suharto, Deputy Head of BAKOSURTANAL, for his permission to carry out this study with the use of Synthetic Aperture Radar maps and P.T. Indo Georeka Nusantara for their assistance in obtaining the maps.

REFERENCES

Bemmelen, R.W. van., 1949. The Geology of Indonesia, Martinus Nijhoff, The Haque, vol, 1A, pp 732.

Cameron, N.R., and 4 others, 1980. The Geological Evolution of Northern Sumatra, Proceedings Zndo- nesian Petroleum Association, 9, 149-181.

Coster, G.L. de, 1974. The Geology of the Central and South Sumatra Basins. Proceedings Indonesian Petroleum Association, 3.

Harding, T.P., 1983. Structural Inversion at Rambutan Oil Field South Sumatra Basin, in Seismic Expression of Structural Styles, A. W. Bully, ed.: AAPG Studies in Geology #15, Vo1.3.

Haryo A. S. , 1986. Peta Kelurusan (berdasarkan citra Landsat), daerah Garba - Danau Ranau dan sekitar- nya, Sumatra Selatan. Publikasi Intern, Univ. Trisakti.

Katili, J.A., 1962. Age determination by isotopic methods of granites southeast of Singkarak lake in Geologic Map of the Solok Quadrangle, Sumatra, Silitonga P. H. and Kastowo, 1975: Direktorat Geologi, Bandung.

Moulds, P.J., 1989. Development of the Bengkalis Depression, Central Sumatra and its subsequent deformation - A model for other Sumatra graben? Proceedings Indonesian Petroleum Association, 18, 217-245.

Pulunggono, A. 1969. Basement configuration in the South Palembang basinal area; its significance to depositional conditions and oil-trapping. N t h Petroleum Symposium, Canberra.

Pulunggono, A , , 1983. Sistem Sesar Utama dan Pembentukan Cekungan Palembang. Tesis S-3, Znstitut Teknologi Bandung; tidak diterbitkan.

Pulunggono, A. and Cameron, N.R., 1984, Sumatran Microplates, their characteristics and their role in the evolution of the Central and South Sumatran Basins. Proceedings Indonesian Petroleum Association, 13, 121- 144.

Suhendan, A.R. , 1984. Middle Neogene Depositional Environments in Rambutan Area, South Sumatra. Proceedings Indonesian Petroleum Association, 13, 63-73.

Universitas Trisakti, 1990. Geologi Daerah Garba dan Sekitarnya, Sumatra Selatan. Publikasi Univ. Trisakti. 57 hal.

Yarmanto and Karsani Aulia, 1988. The seismic expression of wrench tectonics in the Central Sumatra Basin. Ikatan Ahli Geologi Indonesia, Seventeenth Annual convention, Jakarta.

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