PROCEEDINGS INDONESIAN PETROLEUM ASSOCIATION Thirteenth Annual Convention, May 1984
SUMATRAN MICROPLATES, THEIR CHARACTERISTICS AND THEIR ROLE IN THE EVOLUTION OF THE CENTRAL AND SOUTH SUMATRA BASINS
A. Pulunggono* N.R. Cameron**
ABSTRACT The pre-Tertiary framework of Sumatra consists of a
mosaic of continental and oceanic micmplates accreted in the late Triassic when the Mergui, Malacca and East Malaya Microplates were joined together to form Sundaland. Fur- ther accretion involving the west coast Woyla Terrains fol- lowed in the late Mesozoic. Concurrent magmatism and faulting affected much of Sundaland.
The continental Mergui Microplate occupies the central core of Sumatra from Aceh to southern Jambi and is characterised by a complex history which included older and younger Palawoic granite plutonism, late Permian arc volcanism and the widespread deposition of Permo-Carboni- ferous "pehly mudstones". A NW-SE t o N-S trending Triassic suture complex, traceable at subcrop from Riau to the Palembang district and named the Mutus Assemblage, separates the continental Malacca Microplate to the east. This microplate is poorly known, but appears to be domi- nated by low grade metasediments cut in the east by gra- nites that represent the continuation of the Triassic Main Range Granites of the Malay Paninsula. The East Malaya Microplate is characterized by Permo-Triassic arc magmatism and lies east of a line joining Kundur and NE Bangka. The boundary is a complex one associated with mafic and ultra- mafic rocks related to the Raub-Bentong Line of peninsula Malaya. The Woyla Terrains consist of tectonised, Jurassic and Cretaceous arc volcanics and ophiolites.
The major zone of basement weakness during the de- velopment of the back-arc Central and South Sumatra Basins was the Mutus Assemblage. This region experienced Miocene alkaline magmatism and, due to a combination of high heat flow and the early growth of structures, is the site o f t 95% of the two basin's oil production. Young Tertiary structures in this zone are related to wrenching in the north; and in the south, to compressional reactivation of cross cutting WNW-ESE faults formed during the accretion of the adjacent Woyla Terrains.
INTRODUCTION In the last ten years systematic mapping by the Direc-
torate of Geology in Bandung and the publication by the oil companies of information on the subcrop basement has added a wealth of new material on the pre-Tertiary of
* Pertamina E & P/Trisakti University ++ Conow Indonesia
Sumatra. It is the objective of this paper to collate and summarise these new results and to show, with an example from the oil-rich Central and South Sumatra Basins, how reviews of this type have practical applications. A secondary objective is to briefly examine how ideas on the pre-Ter- tiary evolution of Sumatra have advanced and to demonstra- te, as geological fashions change, how the microplates and their boundaries have been variously interpreted. Emphasis will be placed on the Lematang Line in South Sumatra, the former Jambi Thrust of Dutch authors.
THE CHANGING PATTERN OF IDEAS ON SUMATRA Ideas on the origin of Sumatra have been closely linked
to Sundaland, the partly submerged, southeastern out- growth of Asia occupied by Sumatra, West Malaysia and much of Kalimantan. It was regarded as the largest and most coherent shelf in theworld (Bemmelen, 1949) and was considered to consist of a stable pre-Tertiary core surroun- ded by zones of progressively younger rocks. Initially the outer zones were explained on the basis of Vening Meinesz's downbuckling hypothesis (Bemmelen, 1949). Subsequently Bemmelen used his Undation Theory to explain the concen- tric rings, arguing on the basis of gravity anomalies, that they were "crustal waves'' radiating from a core area centred in the Anambas Islands in the South China Sea. His ideas were cogently presented and as they united so many seeming- ly unrelated facets of Indonesian geology they were accepted without serious reservation until the development of plate tectonics in the late 1960's.
The first comprehensive plate tectonic models w e d built by Katili (1973 and 1975). He was also impressed with the apparent concentricity of the belts ringing Sundaland and showed how they could be explained by two opposing sets of oceanward migrating Benioff zones. With the flood of new information that became available in the late 1970's from Sumatra, West Malaysia and Thailand, it became clear that a "fixist" interpretation for the evolution of Sundaland was no longer tenable and that highly "mobilistic" models similar to those successfully pursued in eastern Indonesia were required. Rapid changes in traditional ideas followed and it is now agreed that Sundaland is not a single entity, but consists of a complex mosaic of constantly moving fragments or microplate,s.
SUMATRAN AND SOUTH EAST ASIAN MICROPLATES For the purposes of this paper microplates are defined
as discrete fragments of the majox plates; that is, regionally
homogeneous terrains separated by megafaults (Nelson, 1983) which extend to the base of the, lithosphere. They had: (1) separate histories until brought together by sutur- ing, (2) similar histories until separated by rifting or trans- forms, (3) briefly dissimilar histories whilst separated ,by short-lived rifts.
The boundary faults in,the first and third type of setting are frequently occupied by ophiolites. However, their pre- sence is not mandatory as crustal extension during short- lived rifting episodes may have been insufficient for ophio- lite emplacement. Also, ophiolites may have been destroy- ed either during suturing or in subsequent strike-slip epi- sodes.
Sumatra is built, because of its location in a region of long term plate convergence (Hamilton, 1979), of combi- nations of the first and third types of microplates. There is increasing evidence that these microplates originated from GondwBnaland following episodes of rifting and possibly also transform faulting. Current models for these events are described in papers by Parker and Gealey (1983) and Stauffer (1983).
An accretionary origin is aIso accepted for South East Asia as a whole (Mitchell, 1977 and 1981). Primary assem- bly was essentially complete by the end of the Triassic. Two of the main sutures cross Sumatra (Figure 1) and se- parate the Mergui, Malacca and East Malaya Microplates. Much of the evidence for the suture separating the Malacca and East Malaya Microplates exists in West Malaysia. Here the tin-bearing Main Range Granites are considered to have formed as crustal melts as the host Mergui Mircoplate was overridden along the Raub-Bentong Line by the East Mala- ya Microplate (Mitchell, 1977 and 1981).
The present p t t e rn of microplate boundaries in Sumatra differs considerably from that existing at the end of the Mesozoic. It is defined by the Andaman Sea rifts to the north and the 1650 krn long, right lateral Sumatra Fault System which runs the length of the island. Together, they separate the elongate Burma Microplate to the west from the Sundaland or S.E. Asian Plate to the east (Curray et al. 1979; Aldiss and Ghazali, 1984).
The growth of the Sumatran Fault System has consider- ably modified the end-Mesozoic boundaries, especially in Aceh (Curray et al., 1979,Haile, 1979). This should be born in mind when examining Figure 1 which depicts the present configuration of the pre-Tertiary microplate boundaries.
THE MERGUI MICROPLATE Following the reconnaissance mapping of northern Su-
matra, Cameron et al. (1980) were able to show that Stauf- fer’s (1974) West Malaya Block consisted of a number of components, the largest of which is the continental Mergui Microplate. Its extension and limits south of the Equator were subsequently defined by Pulunggono (1983). The stratigraphy is remarkably uniform and the individual units tabulated in Figure 2 can be traced throughout Sumatra and into NW Malaysia, western Thailand and beyond.
OLDER PALAEOZOIC GRANITE BASEMENT RbSr dates of 426 2 4 1 5 Ma and 335 2 43 Ma from
granites beneath the Central Sumatra Basin (Eubank and Makki, 1981) represent the oldest basement identified in Sumatra. Proof of the validity of these dates exists a t Cucut # 1 well where a Rb-sr age of 348 2 €0 Ma has been obtain- ed from granite cEasts within the ”pebbly mudstones” of the PermoCarboniferous Bohorok Formation (Koning and Darmono, 1984).
Hints of an even older basement exist. A whole rock Rb- Sr age of 500 100 Ma was calculated for the Bohorok Formation in the Alas valley to the NW of Medan (Beckin- sale, pers. comm., 4/10/80; Cameron et al.,1982a). Possible Precambrian leucotonalite clasts with a K-Ar age of 1029 5 15 Ma are present in the related ”pebbly mudstone” of the Singa Formation of the LangkawiIslands in NW Malaysia (Stauffer and Snelling, 1977). Early Palaeozoic plus probable Precambrian sediments are exposed irf the Langkawi Islands and the Thai islands to the north (Jones, 1973; Hutchison, 1982).
THE PERMO-CARBONIFEROUS TAPANULI GROUP The Tapanuli Group, which was defined by Cameron
et al. (1980), corresponds to Bemmelen’s (1949)~tructural zonations ”I and II”. Though it occupies a large area north of the Equator, it is divisible into only three main units, the most conspicuous of which is the ”pebbly mudstones” of the Bohorok Formation. The KluetKuantan Formations consist of marzne slates and quartz-rich metasandstones. The Alas Formation is a shelfal limesto8e sequence. South of the Equator the Group is separated into a number of local formations, ail of which can be closely matched to the three main units of northern Sumatra (Rosidi et al., 1976 and Figure 2). Cameronet al.(1980), using preliminary fossil determinations, placed the Alas Formation in the early Permian. Its true age is now known to be early Carbo- niferous (late Visean), both in the Alas wlley (Fossil Loca- tion l on Figure l : Metcalfe, 1983) and east of Bukittinggi {Fossil Location 2 : Fontaine, 1981; Metcalfe, 1983). Pos- sibly also related is the Asahan # A1 well faune from north Sumatra (Fossil Location 3) which is stated by Adinegoro and Hartoyo (1974) to be Devonian to Mississippian in age.
The Bohorok Formation at Cucut # 1 well (Fossil Loca- tion 4) contains an early to middle Carboniferous flora (KO- ning and Darmono, 1984). Transitional Kluet-Bohorok, calcareous metasediments at P a n g u ~ r a n on the west shore of lake T6ba (Fossil Location 5) contain a shallow marine fauna (Aldiss et d., 1983) rich in bryozoa which are closely related to the early Permian (Sakmarian) Bryozoa Bed at Phuket (Garson et d.,1975; Waterhouse, 1982).
Cameron et A’s (1980) deduction that the Bohorok Formation is of glacial origin is supported by new sedimen- tological studies of the Carboniferous Singa and Kubang Pa- su Formations of NW Malaysia and the Pfiuket Group of SW Thailand (Stauffer and Mantajit, 1981; Waterhouse, 1982; Stauffer, 1983).
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terminate against the western Mutus Assemblage faults in the Saka 4 well . Bukit Pendopo area. In West Sumatra the volcanics were described in detail by Katili (1969) who named them the Silungkang Formation. Identical rocks in Jambi are mapped as the Palepat formation (Rosidi, et al., 1976).
The interbedded fusulinid limestones are of early (Ar- tiykian) to late Permian (Kazanian) age (Hahn and Weber, 1981; Fontaine, 1981; Rock etal., 1983). The much dis- cussed Jambi flora is most probably Artinskian (Asama et al., 1975). A K-Ar radiometric age of 248 + 10 Ma exists for the Silungkang Formation (Nishimura et d., 1978).
The Triassic sediments in nothern Sumatra mainly be- long to thei widely developed Kualu Formation, which be- neath the Central Sumatra Basin is believed to pass 1ateraIly into the Mutus Assemblage of Eubank and Makki (1981). Environments are mostly deep water with brown radiolari- an cherts, Halobia shales and rhythmic turbidites predomi- nating. Shallow (?)water limestones, some of which m y be reefal, are also recorded. Very similar sediments known as the Tuhur Formation are exposed in West Sumatra, just south of the Equator (Silitonga and Kastowo, 1975).
The faunas indicate the Kualu, Tuhur and related forma- tions are of middle (mainly Carnian) to late (Norian) Trias- sic age (Kobayashi and Masatini, 1968;Metcalfeet al., 1979; Fontaine, 1981). Shghtly older, Anisian to Ladinian faunas are recorded from the Gle Situtup limestones of Central Aceh (Fossil Location 7: Cameron el al., 1983). No eviden- ce for the early Triassic or the uppermost Permian has been found and as Turner (pers. comm., 16/12/83) has identified Permian coral clasts in Tuhur equivalent sediments in the Lubuksikaping -Quadrangle (Rock et al., 1983) there was probably a period of uplift and erosion at this time.
EARLY PERMIAN DEFORMATION OF THE TAPANULI GROUP
Consistent structural evidence throughout northern Su- matra indicates that the Tapanuli Group is more highly de- formed than the overlying Peusangan Group (Cameron
. et al., 1980). Though the unconformity between the two groups remains to be demonstrated, support for tectonism at this time exists in the 2 6 4 5 6 Ma RbSr age of the meso- zonal Sibolga Granite (Aspden et al., 1982). Probably re- hted granites are :
Ombilin ranite (RbSr) 256 2 6 Ma (Silitonga and Kastowo, 1975;
1981) (K-Ar) 287 t 3 . 5 Ma Hahn and Weber,
(Rb-Sr) 276 $20 Ma (Katili, 1973)
c: Setiti Granite
298 39 Ma
There is a widespread hiatus at this time elsewhere on the Mergui Microplate (Hutchison, 1982), but a break of si- milar magnitude to that in Sumatra has not been described. Indeed Gobbett (1973) states that the contact in the Lang- kawi Islands between the Singa Formation and the overlying Chuping Formation limestones is conformable. This con- tact does look conformable, but it may be illusionary be- cause the Singa Formation, is cleaved and thermally meta- morphosed whilst the Chupping Formation appears to be non metamorphosed. An argon-loss event within the Singa Formation, dated at 264 5 4 Ma by Stauffer and Snelling (1977), is taken to represent the date of the metamorphism. Evidence for a sizeable break above the S iga exists on the adjacent mainland in Kedah where Metcalfe (pers. comm., 3/1/81) has shown a spectacular unconformity is present at Gunung Keriang (Fossil Location 6) within the prevously interpreted, unbroken sequence of Triassic Kodiang limes- tones (Burton, 1973). Conodonts reveal the lower limes- tones are Sakmarian in age, the upper limestones are early Triassic (middle Scythian).
On Phuket Island, the Phuket Group exhibits a similar style of low grade metamorphism to that seen in the Singa Formation. Waterhouse (1982) records an early Permian hiatus betweeen the Phuket Group and the overlying Rat Buri Group which has a basal, middle Permian (Kunguri- an) fauna. It may also prove significant that Veevers (1969) describes an unconformity between glaciomarine Sakmarian sediments and late Permian marine beds in the Bonaparte Basin (NW Australia).
THE PEUSANGAN GROUP The Peusangan Group‘was divided by Cameron et al.
(1980) into two units: a Permian arc and fringiiig reef asso- ciation related to an eastward inclined Benioff zone, and a marine, middle to late Triassic association. The Permian association is present in central Aceh and in an elongate, fault-determined strip occupying the western edge of the Microplate from West Sumatra to SW Jambi. This belt. which forms Tobler’s (1 922) ”Vor Barisan’’ and Bemmelen’s (1949) ”Zone III”, was shown by Pulunggono (1983) to
THE TRIASSIC MAGMATIC ARC The West Sumatra,Triassic succession on both sides of
the Equator includes horizons of green volcanic wackes and waterlain tuffs (Rock et al., 1983; Turner, pers. comm., 1983). The associated volcanic arc has not been identified and it is unclear whether it lay to the east or west of the Triassic volcanogenic sediments. Evidence for an eastern position may exist in the Sumpur area where there is a sizeable body of hornblende biotite granodiorite (?) with concordant, middle Triassic RbSr and K-Ar ages of 215 - t 3 Ma and 206.5 5 2.5 Ma (Silitonga and Kastowo, 1975; Hahn and Weber, 1981),A K-Ar date of 197.6 2 2.4 Ma from a biotite hornblende granite (?) i~ the postPeusangan Muara Sipongi Intrusions (Rock et d., 1983) could define the northern continuation of the arc. A reset K-Ar date of 21 1 5 5 Ma from the. Sibolga Granite (Aspden et al., 1982) indicates contemporaneous tectonism.
A western arc would have been 6estroyed during the accretion of the Woyla Terrains. A possible remnant may exist as the Schist Member of the Muarasoma Formation (Rock, el al., 1983). This unit appears to have been derived from felsic volcanics and exhibits refolding and a higher degree of deformation than the surrounding Woyla Group. Triassic magmatism is also recorded from the Langkawi
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Islands, but this is related to the Raub-Bentong Line suture. Rb-Sr ages of 209 2 6 and 217 5 8 Ma are quoted by Bignell and Snelling (1 977).
POST-TRIASSIC SUTURING EVENTS The younger Mesozoic history of the Mergui Microplate
after its suturing with the Malacca Microplate was initially erosional. K-Ar ages of 186 - 189 5 2 Ma were obtained
. from the Rokan Granite (Eubank and Makki, 1981; Rock et al., 1983); the petrography of the granites suggesting these are reset dates related to cataclasis. Similar events may be responsible for the K-Ar dates of 180 5 7 and IS9 - + 4 Ma from the Tiaapuluh and Duabelas Mountain granites (Simandjuntak et al., 1981) of South Sumatra.
A more dynamic environment, defined by an eastwards marine transgression and the renewal of subduction, existed from about the middle Jurassic. These events, however, remain poorly documented and the following paragraphs will be subject to revision. The change is attributed t o the onset of the processes that led in the late Cretaceous to the accretion of the Woyla Terrains.
The Telukkido Formation in West Sumatra (Fossil Loca- tion 8 : Rock et al., 1983) consists of shallow to darginal marine beds with a probable middle Jurassic flora. Reworked Cretaceous floras in the Tertiary of the west Central Sumatra Basin indicate yGunger Mesozoic sediments were also once present (Turner, pers. comm., 16/12/83). South of the Equator the Tabir Formation consists of reddened m-arine beds with some tuffs. I t is possibly of late Jurassic (Kim- meridgian) age (Fontaine et d., 1982).
Contemporaneous plutonism is recorded in West Sumatra by a Rb-Sr date of 145 % 4 Ma from the Tanjung Gadang Intrusive Complex (Silitonga and Kastowo, 1975; Hahn and Weber, 1982) which was emplaced at shallow depth into a major volcanic edifice. A related intrusion is the horn- blende biotite granodiorite of the Atar pluton to the M y : this has a K-Ar age of 147 2 2.4 Ma. Continued plutonism is marked by the mesozonal Lassi complex (Rb-Sr age 112 - t 2.4 Ma: Katili, 1973), the granite at Balam South Field (Rb-Srage94,522.8 Ma: Eubank and M a w , 1981),and the UIai Intrusions just.north of the Equator (K-Ar age 89.6 Ma: Hehuwat and Sopaheluwakan, 1978,Rocket al., 1983). Reset K-Ar ages are also recorded:
Panyabungan Batholith 121 2 1 Ma (Rock et al., 1983) Kiri Granite 134 2 1 Ma (Eubank and-Makki, 1981)
Widespread early Cretaceous, tin granite emplacement js described from western Thailand (Beckinsale et al., 1979; Hutchison and Taylor, 1979) and is attributed to con- temporaneous reactivation events within Sundaland. In the Phuket area the granites have a Rb-Sr age of 124 4 Ma.
THE lzuTUS ASSEMBLAGE The Mutus Assemblage was first described from Central
Sumatra Basin well records by Eubank and Makki (1981). It separates, along the "W-SSE trending Kerumutan Line, the Mergui and Malacca Microplates, and comprises Kualu Formation lithologies, plus argillites, red-mauve shales, basalt, and in the west tuffs. Near the Equator sericite
chlorite schists are also present. A Triassic age was assumed because of a'K-Ar date of 222 5 3 Ma from the western tuffs at Duri Field and the similarity of the sediments to the Kualu Formation.
THE SOUTH SUMATRA SUCCESSION Pulunggono (1983) demonstrated the subcrop existence
in the South Sumatra Basin of a body of rocks, the "Man- dala Antara", analogous in its setting to the Mutus Assem- blage. It occupies an approximately N-S belt separating the Mergui and Malacca Microplates, and consists principal- ly of severely propylitised volcanics and tuffs of inter- mediate to ?mafic composition. Argillites, thin limestones and sericite chlorite schists are also present. Well records of gabbros and serpentinites exist, but these rock types re- main to be verified. As this assemblage of rocks pre-dates the younger Mesozoic Kluang limestones (Coster, 1974), a Mutus equivalent age is favoured.
THE NW MALAYSIA CONTINUATION Eubank and Makki (1 98 1) depict the Mutus Assemblage
as striking across the MaIacca Strait towards the Triassic Semanggol Formation outcrop of NW Malaysia (Figure 3 and Burton, 1973). These sediments, which were correlated on the basis of lithology and age by Cameron et al., (1980) with the Kualu Formation, are complexly disturbed by strike-slip tectonism. They occupy a 30 km wide, N-S trough which separates a typical Mergui Microplate successi- on to the west from a fundamentally different terrain do- minated by shelfal Silurian to early Permian limestones (Gobbett, 1973; Jones, 1973; Hutchison, 1982; Metcalfe, 1983). This break meets our definition of a microplate boundary and we regard the trough as representing the northern continuation of the Mergui-Malacca Microplate sut ure .
A MODEL FOR THE MUTUS ASSEMBLAGE The origin of the Mutus Assemblage is problematic,
mainly because there is insufficient information on the petrography and chemistry of the volcanics. The magnitude of the break across the Kerumutan Line at first' suggests the Mergui and Malacca Microplates had separate histories prior to suturing. However, this would require the abun- dance of tuffs to be explained by the development of a westward inclined Benioff zone during suturing. Since an eastward inclined Benioff zone existed in western Sumatra at this time, this model seems improbable. A preferable alternative is that the Mutus Assemblage is the product of back-arc rifting and volcanism. The disparity in geology across the Kerumutan Line In this case would require strike- slip faulting, either during closure or in the younger Meso- zoic. A modern analogue is the Andaman Sea which, if it closes, will most probably do so along existing strike-dip faults. A less extreme example is the present Central SU- matra Basin.
The Tertiary evolution of the Central Sumatra Basin indicates there is a transitional boundary with the Mergui Microplate occupied by a strip of attenuated continental crust. Its presence is revealed by deep N-S to "W-SE
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Pematang Formation grabens (Mertosono and Nayoan, 1974; Eubank and Makki, 1981) and the NNW-SEE orientation of the Sihapas, Duri-Bekasap Delta System de- pocentre (wongsosantiko, 1976). Though this attenuation could be solely the result of early Tertiary extension, both models for the Mutus Assemblage would predict Triassic disruption of the western boundary of the Mergui Micro- plate, either by comprmsion during suturing or by attenua- tion during the development of the back arc basin.
The Mutus Assemblage boundaries in the northerm portion of the South Sumatra Basin are schematic due to the kick of well control in the deep Jambi Sub-Basin. The limited evidence points to a series of rifts and grabens in this region. The width of the Mutus Assemblage appears to be severaly reduced to the NE of the Tigapuluh Moun- tain boundary faults. It is not known whether this is a pri- mary feature or the result of later strike-slip disruption.
POST-CLOSURE EVENTS The younger Mesozoic history of the Mutus Assemblage
is identical to that of the Mergui Microplate. Post-Mutus sediments are not known from the Central Sumatra Basin, but in South Sumatra the Kluang limestones were laid down following a marine transgression from the west (Figure 4). The limestones are believed, in the absence of contact meta- morphism, to overlie a cataclased granite at Kluang Field (NK 49 well: Pulunggono, 1983) and are shown as ? Creta- ceous on Coster’s (1974) map. A Jurassic age is also possible as the 5 150 Ma year K-Ar date for the granite may relate to the cataclasis.
Widespread pre-Tertiary plutonism is recorded from nu- merour, wells in the Pendop+Prabumulih region of South Sumatra. The granites and granodiorites have not been dated, but as they are elongated parallel to the end-Creta- ceous, Lematang Line suture they are most probably Creta- ceous in age. Their setting and petrographic descriptions suggest they are subduction related (Pulunggono, 1983). Regional, WNW-ESE faults which cut through the Mutus Assemblage further north (Cosier, 1974; Pulunggono, 1983) are considered to be of the same general age, and are regarded as rupture lines formed during the accretion of the Woyla Terrains to Sundaland. Similar faults would have de- veloped in the Mergui and Malacca Microplates, but their age is less easily verified because their trend sub-parallels the preexisting tectonic grain. Far more conspicuous in these areas are NE-SW and N-S cross faults which were shown by Pulunggono (1983) to be also of Crefaceous age. The latter faults, since they parallel the Mutus Assemblage, may have first formed in the Triassic. The Cretaceous tec- tonism is considered to be the cause of the reset K-Ar ages obtained from Mutus Assemblage volcanics (Pulunggono, 1983) :
Tuff, Batang # 1 well Fractured propylitised andesite
71 Ma.
breccia, Lembak # A1 well 121 & 2 Ma.
It is not clear whether the K-Ar date of+ 140 f iom the gra- nite basement in Tanjung Laban f I well is an emplacement or reset age.
THE MALACCA MICROPLATE The Malacca Microplate succession is generally poorly
known in Sumatra as, except offshore, it is almost entirely present at subcrop.
PRE-TR~SSIC SUTURING EVENTS The pre-suture succession consists predominantly of
quartzites (meta-sandstones), slates and phyllites (Figure 5) . They were labelled by Eubank and Makki (1981) the ”Quartzite Terrain” and by Pulunggono (1983) the ”Quart- zite PhyUite Terrain”. As the post-suture succession is non- metamorphosed these rocks cantlot be younger than Trias- sic. More precise age control is lacking, except at Pusaka # 1 well (Fossil Locatim 10 : Koning and Darmono, 1984) where shales contain a flora which straddles the Devonian - Carboniferous boundary. The adjacent granite at Idris # 1 well has a Rb-Sr age of 295 Ma. A probably reset K-Ar age of 247 2 10 Ma was obtained from phyllitic basement at Berembang # 2 well in south Sumatra (Katili, 1973). Strip- ed tourmaline muscovite schists and gneisses surround the small catazmal Danai Granite which is exposed in low coastal hills opposite Kundur Isfand (Cameron et a1.,1982c).
Offshore o n Kundur Island pre-granite shales, sandstones and reddened conglomerates are mapped as the Papan Formation (Germeraad, 1941; Cameron el d., 1982~) . Very similar Lithologies on Bangka were grouped as the Bangka Formation by Zwierzycki (1930) and Katili (1967). Meta- morphics, mainly consisting of muscovite schists, are also present offshore. The majority of these are spatially associa- ted with the postulated southern continuation of the Raub- Bentong Line and present mapping is insufficiently precise to determine which microplate they belong to. A similar situation exists with the late Triassic (Norian) and Permian faunas of NE Bangka. The lithologies of these rocks, how- ever. best fit those of the East Malaya Mircoplate.
A pre-granite clastic succession is also developed in the Malacca region of West Malaysia. Jones (1973) refers to two units: older, possibly early Palaeozoic argillites, cherts and pelitic schists, and younger metasandstones which may be of Triassic age. The older sequence was given an arbitrary early Palaeozoic age as it lithologically resembles the Silurian and older pelitic schists which underlie the shelfal limestones of the Kuala Lumpur region (Stauffer, pers. comm., 30/12/82).
Volcanics are not recorded, except within the Raub-Ben- tong Line (Jones, 1973) and it would appear that both margins of the Malacca Microplate were essentially passive during the Palaeozoic. The palaeogeographical relationship of the shelfal limestone succession in the Kuala Lumpur Ipoh region with the clastic succession to the west is unknown. Tentatively it is assumed the limestones were laid down on the seaward margin of the clastics.
THE TRIASSIC COLLISIONAL GRANITES A beh of tin-bearing granites, representing the southern
extension of the Malayan Main Range Granites, is present between Kundur and Billiton Islands. Rb-Sr ages from both
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West Malaysia and Sumatra show the ganites were emplaced in the middle Triassic at about 217 Ma (F'riem et al.,l975; Bignell and Snelling, 1973;Katili, 1980). The associated tin mineralisation in Billiton is dated at 2 198 Ma by Priem et al., 1975).
The large massif of granite plotted by\ Coster (1974) from wells penetrating basement beneath the Tertiary Lam- pung High may also be of lcriassic age as the small Bukit Ba- tu outcrop SE of Palembang is tin-bearing and has a similar mineralogy to the Bangka granites (Katili, 1974). -
THE RAUB-BENTONG LINE The Raub-Bentong Line of medial West Malaysia sepa-
rates two very different terrains. It is accompanied by a severely dismembered ophiolite (mainly serpentinites and horn-blende schists and is accepted as a Mcroplate boundary (Mitchell 1977 and 1981; Hutchison and Taylor, 1979; Stauffer, 1983).
Its trace south of West Malaysia remains to be fixed. We do not concur with Stauffer's (1983) alignment through Singapore. Instead it is tentatively plotted on Figure 1 as passing between Kundur and Karimun Islands, through the centre of Singkep Island and thence across the NE corner of Bangka before it is lost in the Java Sea.Trueophio- lites have not been recorded, but the suggested trace is accompained by the same change of geology as is seen in West Malaysia. The Kundur-Karimun boundary is marked by the Merak Complex which consists of metagabbros, diallagites and microfolded hornblende schists (Raadshooven and Swart, 1942; Cameron et al., 1982 b and c). A belt of identical hornb:ende schists crosses central Singkep lsland (Raadshoven and Swart, 1942); Bakri, 1982). In NE Bang ka discontinuous serpentinites are present at Pemali Mine (Suyono and Clarke, 1982) and Belinyu No. 17 pit within a narrow belt of dynamothermal, ?talc bearing schists (Katili, 1967). At the latter locality trace element analysis showed the serpentinities contain 1500 ppm Ni and 1280 ppm Cr, values typical of mantle ultramafics (Cameron et al., 1982~).
Cobbing (pers. comm. 1/84) argues against our boundary as Main Range type granites in some cases lie north of the proposed suture trace, for example the Klabat Granite of NE Bangka. He has also mapped East Malaya Microplate- type granites to the south of the suture line in S . Bangka. Given Mitchell's model for the origin of the Main Range Granites, we see no reason why these granites shouldn't have broken through the suture line to crystallise in the upper plate. The southern anomalous granites are harder to explain. Possibly the suture zone is more complex than shown and is Qccupied by lensoid fragments of both micro- plates.
POST-COLLISIONAL EVENTS Pose-collisional events follow the previously established
pattern for this period. Sediments comprise the Kluang lime- stones at subcrop in South Sumatra, and in the offshore islands, the littoral to fluviatile sandstones of the flat lying Bintan Formation (Katili, 1967; Suyitno, 1977). Cretace- ous and Jurassic radiometric dates are recorded from Su- matra. They are K-Ar ages (KatiIi, 1973; Eubank and Makki,
198 1) and may be reset : Granite near Pakning 1 1 2 5 2 M a Granite near Pakning ~ 1 2 2 2 2 Ma Granite at Bungsu Field 1502 2 M?'
+ 167Ma P. Berhala microgabb_ro (?) -
Clear evidence of reset Jurassic and C r e t a e s Y-Ar ages was presented by Bignell and Snelling (1977) for 'the Main Range Granites.
THE EAST MALAYA MICROPLATE Rocks occupying this microplate lie east of the Raub-
Bentong Line. On Karimun the pre-coilisional Malarco Formation consists of limestone? and rhyodacitic volcanics which are lithologically closely related to the Permo Carbo- niferous Raub Group and the Pahang Volcanic Series of West Malaysia (Figure 6; ,Raadshooven ang-fhvart, 1942; Haile et at., 1977; Hutchison, 1982; Cameron el al., 1982b). The tuffaceous Permian portion of the Bangka Formation most probably represents a continuation of the Malarco Formation. The fossiliferous Triassic portion. is ,considered to be related to the Jelai and Jurong Formation? of southern Malaysia and Singapore (Burton, 1973).
Widespread synvolcanic rn&matism with Rb-Sr peaks of 222 5 5 and 250 2 4 Ma (Bignell and Snelling, 1977) occur- red in the east of peninsular Malaysia and Singapore. The Singapore Granite is dated at 224 5 9 Ma. ThKpofi-Malarco Formation, Karimun Granite closely resembles thesd Eastern Belt plutons (Hutchison and Taylor, 1939; Cameron et al., 1 98 2 b) .
In the east of the Malay peninsula in the Kuantan regi- on the Mersing Beds consist of a thick, slightly regionally metamorphosed sequence of shales, sandstones and minor limestone (Gobbett, 1973; Hutchison, 1982; Stauffer, pers. comm., 30/12/82). These rocks are poorly known, but include a Visean fauna plus late Carboniferous tilloids with a probable Glossopteris flora (Stauffer and Mantajit, 1981). The implications of th6 flora in relation to the Raub- Bentong Line are discussed by Stauffer (1983).
Post-collisionid sedimentation is dominated by the con- tinental Bintan Formation of offshore Riau, and the Tem- beling Formation and Gagau Group in West Malaysia (Burton, 1973). Plants from Bintan Island (Fossil Location I1 : Iwai et aI., 197.5) have a Neocomian age. Parts of this unit may be older as the Tembeling Formation extends from the' Rhaetic to ?Late-Jurassic. The Gagau Group spans the interval from late Jurassic to early Cretaceous. The latter unit is associated with extensional tectonism and i s accompanied by trachytes. Granite magmatism fol- lowed in the late Cretaceous (Hutchison, 1973):
Granite in Johore Granite in Malacca
83 2 30 R b S r 87 2 30 Rb-Sr
THE WOYLA TERRAINS The Woyla Terrains occupy Tobler's (1922) "Schiefer
Barisan" and Bemmelen's (1949) "Zone IV". They are di- visible in northern Sumatra into: (1) a volcanic arc associ- ation with fringing reefs and turbidites, (2) a variably dis-
membered ophiolite association (Figure 7; Cameron et al, 1980). These assemblages are also present south of the Equ- ator (Pulunggono, 1983) where they were first recognised in the Gumai Mountains. Musper (1937) named the arc association the Saling Series and the ophiolite iassociation the Lingsing Series. A third assemblage is present in SW Jambi against'the Lematang Line suture. It is characterised by a lack of volcanics and consists predominantly of slates with some metasandstones and coralline limestones. It was first described by Tobler (1922) andis currently separated
• into two units (Fontaine et al., 1982), the Rawas and Asai Formations. The former corresponds to T obler's "Variegat- ed Slate Zone", the latter to his "Lustrous and Dull Slate Zones". The eastern setting of these sediments indicates they are most probably a transitional or foreland associati- on and that they represent the deep water equivalent of the Kluang limestones (Figure 4).
The absolute age span of the Woyla group is unknown. In NW Aceh the oldest recorded sediments consist of reworked, ?Oxfordian limestones within the ophiollte association (Fossil Location 12: Bennett et al., 1981b). The Rawas and Asai Formations contain? Toarcian (Fossil Loca- tion 13), Bathonian (Fossil Location 14) and Valanginian (Fossil Location 15) faunas (Hashimoto et al~, 1975; Fon- taine et al., 1982). Elsewhere the arc and opholite associati- on limestones have yielded non-specific, late Jurassic - early Cretaceous faunas.
THE EASTERN BOUNDARY OF THE WOYLA TER- RAINS
In Aceh the eastern contact with the Woyla Terrains is the Kla Line, an eastward underthrust suture. It was ex- tensively modified in the Tertiary and in central Aceh it was overridden by the late Oligocene-earliest Miocene Takengon Line thrusts (Cameron et al., 1980 and 1983). A related suture, the Lematang line, is present in Jambi. It continues at subcrop into the South Sumatra Basin where Pulunggono (1983) showed from seismic it developed as the 1000 m, re- versed Lematang Fault.
The trace of the Lematang Line immediately NW of Jambi Province is poorly controlled due to an extensive cover of Tertiary and Quaternary volcanics. Examination of the eastern limit of Woyla Group serpentinites on the Painan Quadrangle (Rosidi et al., 1976) reveals it is probably loca- ted just east of the upper Batang Hari valley. Further north it passes to the west of the Lassi complex (Silitonga and Kastowo, 1975) and thence through Lake Singkarak and Bukittinggi (Kastowo and Leo, 1975). At the Equator it becomes the Gadis-Pasaman Fault Zone of Rock et al., (1983).
Its trace east of the termination of the Lematang Fault is conjectural. Most probably it continues as the Kepayan.g Fault. Thereafter, Cretaceous K-Ar dates (84.7-115.8 Ma) from Woyla-like metamorphics forming the basement be- neath the Sunda and NW Java Basins (Katili, 1973; Pulung- gono. 1983) point to it passing into the Java Sea on an
127
easterly track, approximately in the position theorised by Coster (I 974):
The presence of low grade, continental metasediments with a K-At age of 213 ± 11 Ma beneath the Pamanukan High in West Java (Patmosukismo and Yahya, 1974)sug- gests there may be in this area a fragment of the Malacca Microplate embedded within the Woyla Terrains. It is tempting to speculate that the other basement highs in West Java, notably the N-S a!ign.ed Seribu Platform (Todd and Pulunggono, 1971), are also continental fragments.
CHANGING IDEAS ON, TIIE ORIGIN OF THE LEMA- TANG LINE
The Lematang Line was interpreted by Dutch geologists as a regional thrust rooted in the offshore Riau islands (Fi- gure 8 ; Zwierzycki, 1930; Bemmelen, 1949). This view was founded on two observations. Firstly the contact was observed to be flat lying. Secondly it separated the meta- morphic "Schiefer Barisan" to the SW from the fion-meta- morphosed "Vor-Barisan" to the NE. The latter was corre- lated on the basis of their lithologies and faunas with t h e Raub Group/Pahang Volcanic Series of West Malaysia and the Riau tin islands. Since there was an intervening belt of metamorphics (the Tapanuli Group), the most rational ex- planation, for a generation of geologists tiained in the Alps, was to. postulate that the "Vor Barisan" was emplaced along a major westward directed gravity slide, the trace of which in eastern Sumatra had been removed by erosion. Their in- terpretation was supported by the abundance of granites in the purported root zone (Zwierzy#ki, 1930).
Katili (1970) was the first to seriously challenge the or- thodox view on the Jambi Thrust. After showing that the horizontal contact was a local phenomenon related to superficial movements (Figure 8), he demonstrated that
t h e fault was of transcurrent origin and that it typically
had a near vertical hade. This interpretation, which was sub- sequently developed by Posavec et a1.,(1973), remains valid. thougk it is now believed that this particular fault also de-:, fines a microplate boundary. The Dutch authors were, there- fore, correct in treating the Lematang Line as a fundamental break, but because of their belief in the worldwide app~- cability of alpine tectonics, they were unable to discern the true nature of the contact.
THE WESTERN BOUNDARY OF THE WOYLA TER- RAINS
The western• boundary of the Woyla Terrains lies between the Sumatran coast and the modern trench slope break (Ka- rig et al., 1978). On Figure 1 it is abitrarfly extended out to the western limit, but it is recognised, in view of'the ?late Cretaceous- Palaeocene subduction complex at Ciletuh Bay in West Java (Thayyib et al, 19J7), that this position may be incorrect. However, in the Andaman and Nicobar Islands probable Woyla equivalent rocks extend to the west
128
of the trench slope break where they are tectonically in- corporated into the late Cretaceous Andaman Ophiolite (Karunakaran, et d., 1964; Bandyopadhyaya, e t al., 1973; Aldiss, pers. comm., 11 /SO).
THE SMULEH AND NATAL CONTINENTAL FRAG MENTS
In NW Aceh the arc a&ociation lies both to the east and west of the ophiolite association. Cameron et a1.,(1980) postulated that the western portion of the arc overlies an older continental block which they named the Sikuleh Continental Fragment. The evidence (Bennett el al.,1981a) for continental crust is :
1. a continental clastic sequence of quartzites, grey phyllites and metasiltstones, not unlike the Kluet Formation, underlies the Woyla Group.
2. the post-Woyla Sikuleh Batholith is unusual for a calc- alkaline body in consisting predominantly of granites.
3. early Tertiary rhyolites and Mo-bearing breccia pipes.
Evidence for the Natal Continental Fragment is less conclusive. The case rests principally on the presence of a relatively undeformed arc sequence, consisting of proximal volcanic wackesinterbedded with and intruded by andesites and lying, as in Aceh, to the west of the ophiolite associati- on. The pre-Woyla basement is not exposed, 6ut the post- Woyla Air Bangis Intrusions include granites which may be tin-bearing (Rock et al., 1983).
MODELS FOR THE WOYLA TERRAINS Cameron e t al., (1980) are of the opinion that the Aceh
ophiolite association prior to tectonism consisted of an un- broken sequence. As it is bounded on both sides by con- temporaneous arc volcanics, they interpreted the Woyla Terrains as forming in a marginal basin (intra-arc) setting. The Sikuleh Continental Fragment would, therefore, rep- resent a rifted fragment of Sundaland. Their analogue was the southern Andaman Sea (Curray et al., 1978 and 1979) where it appears post-middle Miocene rifts (Figure 1) have split and separated the Woyla Teirains. Since regional ex- tensional tectonics and high heat flow commonly precede rifting, they went on to suggest that this model could alsc explain the rift setting of the Gagau Group in West Malaysia and the subsequent emplacement, in West Malaysia and SW 'hailand, of non-subduction related granites. This modei also offers an explanation for the widespread reset K-Ar ages within Sundaland as it would predict that at least some of the endCretaceous, compressional faults originated as tensional faults in the Jurassic.
An alternative hypothesis was developed by Pulunggono (1983) who argued that the broken nature of the Lingsing Series and Saling Series (his Accretion Terrain), plus the po- sition of the subduction related granites to the NE of the Lematang Line best fitted a subduction complex model. A
modern analogue would be the subduction compIexes lying between the Java Trench and the present arc to the north. This is also Barber's @ers. comm., 16/12/83) preferred mo- del for the Woyla Terrains in West Sumatra, though here it could be argued, as did Rock et al., (1983), that the tecto- nic disruption in this area is the product of subsequent strike- slip tectonism.
In view of the length of Sumatra it is conceivable that both hypotheses are correct, the marginal basin model applying to the north, the subduction complex model to the south. Strike changes of this type exist along the Cali- fornia coast and between Java and the Andaman Sea. As with the Mutus Assemblage, the answer will only become apparent with further research.
END-CRETACEOUS METAMORPHISM AND PLU- TONISM
A major end-Cretaceous episode of metamorphism, strike- slip faulting and plutonism affects the entire Woyla Terrains belt. Deformation of this type would be a natural consequen- ce of Pulunggono's 1983 model. In the case of Cameron et ai's (1980) marginal basin model, it. was attributed to a closure event related to the change in spreading direction in the Indian Ocean between 1 I0 and 85 Ma (Hamilton, 1979). The degree of deformation and metamorphism is very vari- able, but tends to be highest near the plutons and major strike-slip faults where gneisses and schists were formed. Radiometric dates are closely bunched :
Manunggai Batholith 87 Ma (K-Ar) (Hehuwat (West Sumatra) and Sopaheluwakan, 1978;
Lampung granites - + 88 Ma (Rb-Sr) (Katili,
Sikuleh Batholith (NW Aceh) 97.7 5 0.7 Ma (K-Ar)
Rock et al., 1983)
(South Sumatra) 1973)
Bennett el al., 198 1 a)
Jaleuem Formation slates within the ophiolite associati- 30 Ma (Beckinsale, on in NW Aceh have a Rb-Sr age of 99
pers. comm, 4/10/80; Cameronet al., 1983).
THE ROLE OF THE MUTUS ASSEMBLAGE IN THE
MATRA BASINS
The Tertiary back-arc Central Sumatra (CSB) and South Sumatra (SSB) Basins, as can be seen from Figure 9, exhibit a more complex top basement configuration than that of the North Sumatra Basin. Comparison with Figure 1 reveals the major cause of the greater complexity in the CSB and SSB is the nature of the pre-Tertiary basement. Thus, because the North Sumatra Basin is underlain by the homogenous Mergui Microplate it consists essentially of a single depocentre lying parallel t o the Barisan Mountains
EVOLUTION OF THE CENTRAL AND SOUTH SU-
129
and the Mergui Ridge magmatic arc (Curray et al., 1979). In contrast the CSB is floored by two microplates qeparated by the Mutus Assemblage: three microplates, plus the Mu- tus Assemblage, underlie the SSB.
Munggono (1983) demonstrated how the Lematang Line and the Woyla Terrains influenced both the deposition of the Talang Akar Formation and the growth of the Lematang Fault. Here, we examine ihe role of a second basement unit, the Mutus Assemblage. It is of considerable interest, as re- gardless of its origin, it represented in the Tertiary a zone of weakness between the rigid Mergui and Malacca Microplates. Its effects are most obvious in the CSB where it is aligned at a high angle to the regional stress field. During the early extensional growth phase of this basin it was the locus of rifting and high heat flow. During the postmiddle Miocene compressional phase it absorbed the main deformation and 'strike-slip movements. In the SSB it was also the locus of ex- tensional tectonism and high heat flow. However, in the sub- sequent compressional phase, the cross cutting, end-Creta- ceous faults were better orientated to absorb stresses and the resulting surface pattern of young folds completely obscures the critical role of the Mutus Assemblage. It is only when the N-S aligned facies variations of the older Tertiary (Coster, 1974), and the positions of the oiI fields are examined in relation to heat flow (Pulunggono, 1983) that its paramount importance becomes apparent.
'IHE CENTRAL SUMATRA BASIN In the CSB, the review paper by Eubank and Makki
(1981) shows, particularly from the regional seismic secti- ons, that the continental" Mergui and Malacca Microplates are associated with a stable block-type Tertiary history. The Pematang Formation in these areas is either missing or poor- ly developed, whilst the overlying Sihapas Formation is thin. Even where the ensuing younger Tertiary is thick, com- pressional folds are rare and youthful. Tectonism is essential- ly confined to vertical and regional tilt movements. Heat flow, except in the region of recent uplift, is for the CSB low (Carvalho et d., 1978).
The Mutus Assemblage and the attenuated western strip of the Mergui Microplate have very different histories and are associated with early Tertiary horsts and grabens, com- pressional folds, and above all for the petroleum industry, high heat flow and an early onset of structuring. Figure 10, taken from Hasan et al's 1977-78 paper on the giant Minas Field, shows an example of early structuring: others are given by Eubank and Makki (1981).
Maximum arc volcanicity, and in the back-arc basins maximum transgression, occurred at the early-middle Mio- cene boundary between plankton zones N7 and ?N9. This period would have been the tinie of maxinium crustal ex- tension and highest mantle heat input (Eubank and Makki, 1981, Figure 3). It is marked in the CSB by at least seven extrusivelintrusive igneous centres which are located
above the Mutus Assemblage and which are the result of deep seated alkaline (shoshonitic) magmatism. K-Ar dates lie in the range 12 to 17.5 Ma (with the likely degree of deute- ric alteration Ke suspect the older dates represent the age of emplacement). At Merak # I the lavas are interbedded with N8 sediments.
THE SOUTH SUMATRA BASIN An identical history characterises the portion of the SSB
underlain by the Mutus Assemblage (Pulunggono, 1983). It began with the rifting events that lead to the deposition of the Lahat and Talang'Akar Formations. This first phase is illustrated in Figure 11 which is taken from Hutapea's (1974) paper on Abab Field: examples of early structural growth are given by hlunggono (1983). It continued with early Miocene sill emplacement into the Gumai Formation at Tempino and Sungai Gelam Fields, K-Ar dates'from Tempino indicating they were intruded between 1 1.1 and 16.2 Ma (the older dates are the most likely): Probably related sills cut the Talang Akar at Plajawan # 1 well. Thin sections prepared from cuttings reveal the magmas were alkaline (Rock, pers. comm., 4/1/84 and 7/3/84). Re- examination of Thamrin et a l . ~ 1979 heatflow data show- ed that the highest present day heat flows are not associated as previously claimed with the Basin's margins, but are re- lated to the subcropping Mutus Assemblage.
OIL AND THE MUTUS ASSEMBLAGE Early structuring and long term high heat flow provide
optimum conditions for the generation and trapping of Tertiary oiis. Examination of production records (mainly Enright et al., 1983) for the CSB and SSB reveals_+95%of their cumulative output (Table l), that is? 65%of Indone- sia's total production, is from fields either located directly over the Mutus Assemblage or in areas considered to have been affected by related Triassic tectonism.
Figure 12, which depicts this relationship, is particularly revealing in the SSB as the Mutus Assemblage trend enclos- es two types of fields, those related to the transgressive Ta- iang Akar Formation and those related to the regressive Air Benakat Formation. The degree of control exercised on the local scale can be seen in the Pendopo-Prabumulih district where only these portions of the elongate surface anticlines (Zwierzycki, 1930) overlying or immediately adjacent to the Mutus Assemblage are productive.
FINAL STATEMENT A deliberate attempt has been ma& to avoid the tempta-
tion of discussing the detailed derivation and movement history of the microplates. Instead we have concentrated on building a compendium of information on pre-Tertiary Sumatra. Much remains to be done, but wehope the frame- work we have erected wilI prove sufficient to show where the problems lie. Their answer is not only of academic in- terest, but has considerable implications for the petroleum
Table 1 : Comparison between the cumulative producti- on of the Mutus Assemblage related fields and the gross Central and South Sumatra Basins production. Figures principally derived from Enright et al., 1983.
industry and the understanding of the metallogenesis of SE Asia, particularly the unusually rich tin mineralisation.
NO mention has been made of the megafaults which are not sutures. They include the faults defining the increasing- ly economically important, N-S aligned BengkaIis Trough with its high heat flows (Lee, 1982), the Takung Fault of West Sumatra (Posavec et al., 1973) and the deep Martona Trough faults west of the Central Sumatra Basin (Cameron, 1983). Most appear to be related to the suturing events and can be regarded as “Continental Thrusts’’ of the type des- cribed by Allegre et al. (1984) from the Himalayas: other are more enigmatic and require further study. The abundan- ce of megafaults suggestsfuture research will show the micro- plates are not truly homogeneous, but consist of clusters of like fragments or chips. In the meantime caution is advised in postulating additional sub-units before the detailed strati- graphy of Sumatra is more fully understood. There is a tendency these days to ignore classical geological principles and to attribute any abrupt changes across major faults to suturing and suspect terrains. It would be regretable if pre- conceptions were once again allowed to retard geological progress.
ACKNOWLEDGEMENTS This paper is published with the permission of Pertamina
and the Continental Oil Gmpany of Indonesia. Pulunggono wishes to acknowledge Professor Katili and R.P. Koesoema- dinata of ITB @andung! for their guidance and inspiration during the preparation of his doctoral thesis on South Su- matra. He also wishes to thank Z.A. Kamili and Z. Achmad of Pertamina, 1. Wasti of Asamera (South Sumatra) Ltd.,
and E. Harm and Sudarmono of P.T. Stanvac Indonesia for fruitful discussions held between 1980 and 1983. N.R. Ca- meron wishes to acknowledge his former IGS-DMR colleagu- es who worked with him in Bandung between 1975 and 1980, especially D.A. Aldiss, J.A. Aspden, M.C.G. Clarke and N.M.S. Rock. Their ideas helped to mature many of the concepts advanced in this paper. He also wishes to acknowledge exchanges of information with: (1) R. D. Beckinsale, E.J. Cobbing and C.R. Jones of the British Geological Survey, (2) A. J. Barber of Chelsea College (Lon- don), (3) P.I. Turner of Cambridge University, (4) 1. Met- calfe and P. H. Stauffer of the Department of Geology, UN- versity of Malaya, (5) N. Mantajit of the Gedogical Survey Division of the DMR in Bangkok, (6) R. T. Eubank and T. Koning of P.T. Caltex Pacific Indonesia. This paper includes previously unpublshed observations made during visits by N. R. Cameron to West Malaysia sponsored by the Uni- versity of Malaya and Phuket sponsored by the DMR in Bangkok. This paper was typed by NelIeke Tuwahatu. The figures
were prepared by A. Dochri, Sabeni and R.I. Subrata (Co- now), and Sutadi and Tukiran (Pertamina).
CAPTIONS
Figure 1 :
Figure2 -:
Figure 3 :
Microplate boundaries, radiometric dates and principal text locations. Me@ Microplate geology. Mutw Assemblage geology.
131
Figure4 :
Figure 5 :
Figure 6 :
Figure7 :
Figure8 :
Figure 9 :
Figure 10 :
Figure 11 :
Figure 12 :
Cartoon section showing preand post-suture relationships in South Sumatra. MaIacca Microplate geology. East Malaya Microplate geology. Woyla Terrains geology. Past and present interpretations of the te- matang Line. Simplified top basement configuration of the Sumatran back-arc basins. An example from Minas Field (CSB) of early structuring above the Mutus Assemblage (Source Hasan et al., 1977-78). An example from Abab Field (SSB) of early structuring above the Mutus Assemblage (Source Hutapea, 1974). Heat flow and position of oil fields in relati- on to the Mutus Assemblage.
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p. 125-140.
Figure 1 Microplate boundaries, radiometric dates and principal tex t loco tions ( Principal Sources: Comeron et al,1980; Curray et 01,1979 ; Eubank and Mokki, 1981 ; Hamilton, 1979 ; Korig , e t 0 1 , 1980 ; Pulunggono, 1933.)
134
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440 -
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SUMATRA WEST MALAYSIA /THAILAND Principol Radiometric
