doi:10.1144/GSL.SP.2005.004.01.21 1974; v. 4; p. 365-378 Geological Society, London, Special Publications

Michael Geoffrey Audley-Charles

Sulawesi

Geological Society, London, Special Publications

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© 1974 Geological Society of London

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SULAWESI

MICHAEL GEOFFREY AUDLEY-CHARLES Department of Geology, Imperial College, Prince Consort Road, London S.W.7

CONTENTS

A. General data on the segment 365

B. Subdivision of the segment 374

C. Structural relationships of the Sulawesi Arcs with neighbouring regions 376

D. References 377

A. G E N E R A L D A T A ON T H E S E G M E N T

1. T H E SEGMENT STUDIED

The island of Sulawesi--formerly called Celebes--occupies a critical but very complex position at the junction of the Alpine-Himalayan and Circum-Pacific chains. It links along strike with both the Philippine and the Banda orogens, but lies between the Borneo and the West Irian orogens. The island is one of the most mountainous in the Indonesian Archipelago and is poorly known geologically, because of which the present article is largely in essay form.

Segment: the Sulawesi orogen has a strike length, excluding small Islands, of about 1300 km. The west margin of the belt consists of an arc open in the south to the south-east, which reverses its direction of curvature in the north to form an arc open to the north-west. In the south it is separated from the Sunda Shelf by the 2000 m deep Makassar Strait and in the north it is adjacent to the Celebes Sea. The east margin of the belt abuts against the Banggai and Sula Islands (part of a stable positive area called the Sula Spur) and, to the south, against the northern part of the Banda Sea. The width of the orogenic belt in Sulawesi, measured between these margins, varies from 500 km to 270 km, and averages 350 km.

History: the oldest rocks known in Sulawesi are sedimentary rocks which have been metamorphosed in pre-U. Triassic times. Then, there seems to have been a period of general uplift and erosion in Sulawesi. Mesozoic-Cenozoic orogenic movements commenced during U. Jurassic and L. Cretaceous times with vertical movements, perhaps associated with some folding, in the Butung Archipelago and the South-east and South Arms of Sulawesi. It is possible there were fold movements at the end of the Triassic in the west of Central Sulawesi and in parts of the East Arm, but palaeontological control is un- satisfactory and the rocks have been mapped on a reconnaissance basis only. The first phase of strong folding began at the end of the Cretaceous and was completed during the L. Eocene in most parts of Sulawesi and in the Butung Archipelago (Bemmelen 1949). A major phase of folding and thrusting of M. Miocene age is reported from the East Arm, and may also have occurred in Central Sulawesi and the North Arm. An important phase of folding (locally with overthrusting) during the late Pliocene is reported throughout Sulawesi and the Butung Archipelago.

The stratigraphy and structure of Sulawesi are poorly known, for the density of reconnaissance geological mapping in the island is low. All published geological maps appear to be based on surveys carried out before 1942 (see Roy 1961 ; Tectonic Map of Eurasia 1966; U.S.G.S. 1965). Few topographical maps were available; aerial photography was not then employed so that many of the published maps show only the geology of the immediate vicinity of the traverses. Only three papers dealing with regional geology of parts of Sulawesi

366 S U L A W E S I

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Figure 1. T h e s t r u c t u r a l zones o f the Su lawes i o r o g e n i c bel t , the i r c o r r e l a t i o n w i t h those o f the B a n d a Arcs o r o g e n i c bel t , a n d the i r r e l a t i o n s h i p w i t h t he S u l a S p u r ( s h o w n b y coar se d o t t e d o r n a m e n t ) .

S U L A W E S I 367

appear to have been published since Van Bemmelen's The Geology of Indonesia of 1949. A further impediment to understanding the structural history is the often uncertain age of sedimentary, igneous and meta- morphic rocks. Many of the published ages of the Cenozoic strata are based on foraminifera determined before 1948 and names are probably in need of revision to bring them into line with recent work.

Each of the four arms of Sulawesi appears to have a different geological history, but it has long been recognized that the North Arm, the western part of the Central region and the South Arm have fundamental features in common which distinguish them from the South-east and East Arms.

The present article has been prepared from a literature review: the writer has not visited Sulawesi.

2. SHAPE OF THE O R O G E N IN PLAN

33 The Sulawesi orogenic belt (including the North Celebes Arc and

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i NN,-.'b % - ~ t Figure 2. The distribution of the arcs of the Sulawesi orogenic belt and Banda Arcs orogenic belt. l = O u t e r Sunda Arc; 2 = I n n e r Sunda Arc; 3 = O u t e r Banda Arc; 4 = Inner Banda Arc; 5 = Tukang Besi-Buru Arc; 6 = Salajar-Lucipara Arc; 7 = East Gelebes Arc; 8 = Western Celebes Arc; 9 = North Moluccan Arc; 10 = Nor th Celebes Arc; 11 = Ha lmahera Arcs. S = Sorong Faul t Zone; TA = Tarera-Aidoena Fault Zone; T = Tawael ia Faul t Zone; FS = Fossa Sarasina.

The area of coarse dot ted o rnament represents the Sula Spur.

the western part of the Salajar-Lucipara Arc) curves through 270 ° and has a strike length of 1900 km, or of 1300 km if those arcs of small islands are omitted. The lengths of radii of curvature of the principal arcs are 500 km for the 'outer' arcs (East Celebes and Tukangbesi Arcs). In the north the curvature of the arcs is reversed: the radius of the 'inner' North Celebes Arc is 400 km and that of the 'outer' North Moluccan Arc is 550 km.

3. SURFACE SHAPE OF THE SULAWESI BELT IN ELEVATION

30 Highest 5% of the ground: 2000 to 2500 m above sea level. Sulawesi is one of the most mountainous islands of the Indonesian Archipelago. Most of the highest land is in Central Sulawesi, but all the arms are mountainous and nearly 40% of the land area is over 500 m above sea level.

33-4 Plio-Pleistocene raised reef limestones have been reported from the East Arm at various heights. Bemmelen (1949, p. 151) quoted heights of 400 m, 500 m, 1000 m, and possibly even 2000 m in the western part of the East Arm (cf. Loczy 1934). An old alluvial surface 20 to 30 m above the present one occurs on the north coast of the East Arm and indicates minor subrecent uplift (Koolhoven 1930, p. 207).

4. GEOPHYSICAL DATA

38 Gravity data are available as a map on a scale of 1:10,000,000 with a 50 mgl contour interval for the marine areas surrounding Sulawesi (Kuenen, Umbgrove & Vening Meinesz 1934). A revised isogam map for the whole of Indonesia has since been published (Bruyn 1951). It incorporates the gravity data of Vening Meinesz and data from land based surveys, is on a scale of 1:10,000,000 and has a contour interval of 25 mgl.

41 There is isostatic inequilibrium over most of Sulawesi and this is greatest in the East Celebes and North lV[oluccan Arcs. 42-3 As far as can be judged from available data the general gravity field shows pronounced concordance with the main tectonic and topographic trends of Sulawesi.

There are several outstanding features of the gravity field: very strong negative values are coincident with the marine section of the North Moluccan Arc; strong positive values coincide with some deeper parts of the surrounding seas, e.g. Celebes Sea and Banda Sea; strong positive anomalies are coincident with the marine gulfs which separate the inner and outer arcs of Sulawesi and the outer arc from Borneo. Bruyn (1951, p. 605) remarked that ' I t is certainly possible to recognize on the profiles across Sulawesi the characteristics of a double arc. Here (i.e. all Sulawesi except the Minahasa section of the North Arm), however, the curvature is reversed so that the Vening Meinesz

368 SULAWESI

minimum is found on the concave side of the arc. No active volcanoes are present (in the section of reversed curvature). The high positive values coinciding with the deep water of the Makassar Strait are comparable with those of the Banda Sea. As soon as the curvature returns to normal (i.e. the minimum on the convex side of the arc) active volcanoes reappear, namely Una-Una and those on the North Arm of Sulawesi and the Sangihe Islands. Here the profiles show the standard sequence of maximum and minimum axes, troughs and ridges.'

5. PRESENT-DAY A C T I V I T Y

69 The Sulawesi region is seismically active, and the northern half is very active. Gutenberg & Richter (1954, pp. 60-61) commented that seismic activity in the Celebes 'is well above average for the Pacific belt; intermediate shocks are very numerous and some of them (notably in northern Celebes) are among the largest known. Deep shocks are fairly frequent.' Fitch & Molnar (1970) remarked that 'The occurrence of earthquakes between 300 and 500 km depths in the Banda Sea region is an indication that the lithosphere is continuous there. This activity may be a consequence of the marked contortion in the seismic zone.' The difficulty of interpreting important features of the seismic behaviour of Sulawesi and the Banda Arcs in terms of regional tectonics was pointed out by Fitch & Molnar: 'A zone of intense intermediate-depth activity paralleling the northern peninsula of Celebes forms the southern terminus of the deep-focus activity beneath the Philippine arc. There are similarities between the con- figuration of this zone and the seismic zone beneath the Banda Sea. Both regions have intense activity extending from the surface nearly vertically to approximately 200 km. Below 200 km activity is less, and it is along a plane which dips approximately 45 ° beneath the respective arcs.' 'East of the Celebes Sea is a complicated seismic region associated with an island arc paralleling the northern peninsula of Halmahera. A cross section of the activity in this region shows that the zone beneath the Celebes Sea dips towards the west while another zone extending only to intermediate depth dips towards the south-east. This eastward-dipping zone is very unusual, and its relation to the tectonics of the region is not apparent to us.' More recently, further maps of the seismic epicentres have been published (Fitch 1970). Hatherton & Dickinson (1969) figured seismic isobaths to the north, south and east of Sulawesi.

6. T I M E RELATIONS

85 The oldest undeformed rocks in Sulawesi are of Quaternary age. They are mainly raised reef limestones, alluvium and coastal plain deposits, many of which are highly fossiliferous.

83 The youngest deformed rocks are late Pliocene in age. Throughout

Sulawesi and the archipelagos to the north and south Pliocene rocks have been folded. Palaeontological evidence is not sufficiently precise to be certain that these movements did not continue into the Pleistocene (see Bemmelen 1949, pp. 149-52, 389-436; Vlerk & Dozy 1934). Before the age of the tectonic events in the Cenozoic rocks of Sulawesi can be correlated reliably with those in other Cenozoic fold belts the faunas need to be reviewed in the light of the biostratigraphy of Blow (1969). 9t-5 Phases of mobility

10. Plio-Pleistocene uplift: Geomorphological evidence for this event has been reported from all parts of Sulawesi. The process is probably still continuing.

9. Late Pliocene phase: This was important and widespread. An unconformity with gentle folds occurs in the East Arm (Kundig 1956; Loczy 1933-4; Umbgrove 1938; Bemmelen 1949), in the South Arm (Umbgrove 1938), in Central Sulawesi (Witkamp 1940; Vlerk & Dozy 1934), and in the South-east Arm (Umbgrove 1938; Bothe 1927). In the Butung Archipelago Bothe (1927) and Hetzel (1936) reported strong folding and thrusting of Pliocene age. The evidence for the age of these tectonic events rests mainly on the foraminifera determined at that time. It seems likely that the age of the folding would still be regarded as Pliocene, but in view of the anomalous directions of movement, the faunas might profitably be reviewed in the light of Blow's (1969) work. The movements might range into the Quaternary.

8. Middle Miocene phase: In the East Arm this is the major orogenic phase (Kundig 1956) and involved strong folding and thrusting. The presence of an unconformity between U. and L. Miocene, which may represent M. Miocene uplift, erosion and possible folding, was reported from the Gorontalo and Minahasa sections of the North Arm (Koperberg 1929-30; Bemmelen 1949). An unconformity between the Oligo-Miocene and the 'Celebes Molasse', which Umbgrove (1938) regarded as a M. Miocene Tf2 unconformity, was reported from Central Sulawesi by Witkamp (1940) and may possibly represent an orogenic phase there. The report of a transgression in the 'post-Tertiary-f' (i .e.M. Miocene) in the South-east Arm by Bothe (1927) may be related to M. Miocene or possibly U. Miocene movements. Local erosion in M. Miocene time in the Butung Archipelago was reported by Bothe (1927).

7. Lower Miocene: An unconformity at the base of the Miocene Tondo Beds was reported from Butung (Hetzel 1936; Bothe 1927). Umbgrove (1938) reported an unconformity of L. Miocene (Te4) age from Central Sulawesi and the East Arm, but Kundig (1956) made no mention of this unconformity in the East Arm.

6. Oligocene: An unconformity at the base of the Oligocene is reported from Central Sulawesi (Bemmelen 1949). In the northern part of the South Arm the main phase of folding and thrusting is said to occur in the Oligocene (Rutten 1927; Bemmelen 1949).

SULAWESI 369

An unconformity at the base of the L. Miocene Tondo Beds of Butung (Bothe 1927) may represent Oligocene movements. The uncertainty results from the lack of a sufficiently diagnostic microfauna.

5. End-Cretaceous to Eocene phase: In the South-east Arm a major unconformity is present between the Cretaceous and the Neogene (Eocene and Oligocene rocks are missing) and represents a period of strong folding, uplift and erosion (Bothe 1927; Brouwer et al. 1934; Hetzel 1936; Bemmelen 1949). Here and in the nearby

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island of Kabaena evidence suggests there was an important phase of metamorphism (glaucophane facies) penecontemporane- ous with these end-Cretaceous orogenic movements in the South- east Arm (Roever 1950, 1953, 1956). A major unconformity between the Cretaceous Tobelo Beds and the L. Miocene Tondo Beds is widespread in Butung Archipelago where it is considered to have resulted from a period of folding, uplift and erosion (Bothe 1927; Hetzel 1936). The local occurrence in Butung of the Wani Beds of either Eocene or Oligocene age, unconformable on but folded with the underlying Cretaceous Tobelo Beds, indicates an important orogenic phase at the end of the Cretaceous, perhaps extending into Eocene or even Oligocene time. In the South Arm an important unconformity between Cretaceous and Eocene strata represents a period of folding and erosion; in the northern part it marks the onset of andesite volcanism (Rutten 1927; Brouwer et al. 1934; Bemmelen 1949). Owing to the lack of precisely diagnostic faunas there is some doubt about the time limits of these movements. In the East Arm the actual contact between the Cretaceous and Eocene has not been observed. Kundig (1956) thought it probable that there was a depositional gap between the Cretaceous and the Palaeocene, and that this interval coincided with the main phase of ophiolitic extrusions. The rapid change from abyssal facies in the Cretaceous to nerific facies in the Eocene seems to indicate strong vertical uplift at the end of the Cretaceous in the East Arm. In the western part of Central Sulawesi an important unconformity resulting from fold movements and erosion separates Cretaceous and basal Tertiary strata (which have locally been determined as Eocene) according to Witkamp (1940) and Bemmelen (1949, p. 409).

4. Intra-Cretaceous phase: In part of the South Arm a siliceous facies of probable Cretaceous age is overlain unconformably by a greywacke-sandstone facies that is also thought to be Cretaceous on the basis of its stratigraphical position and analogous litho- logy (Rutten 1927; Brouwer et al. 1934; Bemmelen 1949). If these facies have been correctly identified as Cretaceous there is evidence for a period of folding and erosion in the southern part of the South Arm during the Cretaceous.

3. Upper Jurassic--Lower Cretaceous phase ?: The Cretaceous Tobelo Beds in Butung are in contact with the U. Jurassic Rumu Beds and with other older Mesozoic formations. It is uncertain whether this has resulted from tectonic complications or from a depositional unconformity at the base of the Cretaceous Tobelo Beds (Hetzel 1936; Bothe 1927; Bemmelen 1949). In the East Arm there is no unconformity between the Jurassic and Cretaceous but Kundig (1956) considered that there must have been a phase of great downwarping during U. Jurassic to L. Cretaceous time, because of the sudden change in facies from neritic Jurassic to bathyal Cretaceous.

2. Lower Jurassic phase ?: In the west of the East Arm Loczy

370 S U L A W E S I

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t Lk

A TANIMBAR IPs--

]k Active v o l c a n o 4 0 0 " ~ - - ~ ~ ~ __ . . . . . ._ Faults and f ~ ~ " I o O "iolites postulated submarine

~ ~ M'"~r~ I ~ Granitic intrusions ~ Sula Spur

~ 0 ~ ~ - ~ - I~11 Direction of increasing ~ - n- - grade of metamorphism ~ Main Islands

~ + ~ ~ ~ ) + ~ . . . - - T --- '~ ~ Direction of thrusting 250kin I II I

! ., I

Figure 4. The regional distribution of seismic and gravity features of the Sulawesi and Banda Arcs orogenic belts and their relationship to overthrmting, ophiolites, acid plutonics, and recent volcanism.

SULAWESI 371

(1933-4) considered that the Triassic rocks were folded prioI to the deposition of Jurassic strata. Umbgrove (1935) suggested that in the west of Central Sulawesi there was a phase of folding after the Triassic, followed by erosion after which marine sedi- mentation recommenced during the Cretaceous. No rocks of definitely Triassic age have been reported from western Sulawesi, however. So although it is possible that there were fold movements and erosion during the Jurassic in these parts of Sulawesi, it cannot be regarded as proved.

1. Pre-Upper Triassic uplift and erosion appears to have been widespread in Sulawesi and Butung. There may have been associated folding but this has not been established. The evidence for these events has been discussed by Bemmelen (1949).

87-9 The initiation of mobility associated with the Mesozoic-Cenozoic orogeny could be regarded as the marine transgression near the beginning of the U. Triassic that appears to be a widespread pheno- menon in Sulawesi.

81 The oldest rocks deformed for the first time during the Mesozoic-Cenozoic orogeny are fossiliferous Upper Triassic strata reported from the East Arm by Loczy (1933-4) and Kundig (1956). Loczy (1933-4) and Kutassy (1934) regarded some rocks in the western part as Permo-Carboniferous but Hetzel (1935) referred to them as 'very doubtful Permo-Carboniferous'. In the east of Central Sulawesi, fossiliferous U. Triassic was reported to be the lowest exposed member of a continuous succession up into the Cretaceous (Umbgrove 1938). Fossiliferous U. Triassic Baito Beds in the South-east Arm and the Einto Beds in Butung (Bothe 1927) are the oldest rocks deformed for the first time by this orogeny there.

78 Basement rocks: Crystalline schists of pre-U. Triassic age have been reported from the south-west of the Gorontalo section of the North Arm (Koperberg 1929, 1930), from the Neck of the North Arm (Egeler 1947), from the western and eastern parts of Central Sulawesi (Kundig 1932, Williams 1937; Brouwer 1941; De Roever 1947; Bemmelen 1949, fig. 171), from the South-east Arm (Dieckmann & Julius 1925; Koolhoven 1932; Brouwer et al. 1934; Hetzel 1936; Bemmelen 1949), from Butung (Bothe 1927; Hetzel 1936) and from Kabaena (De Roever 1953). From the South Arm they were described as pre-Cretaceous crystalline schists (Rutten 1927; Brouwer 1934; Bemmelen 1949). The age of these basement rocks has been determined from the unconformable relationship of the unmetamorphosed Meso- zoic (U. Trias or locally Cretaceous) on the metamorphic basement.

To the east of the Sulawesi orogenic belt very similar basement rocks to those just described have been reported from the Sula Islands (pre-Liassic crystalline schists, Brouwer 1921); they are generally regarded as lateral extensions of the pre-U. Triassic crystalline schists of the Banggai Islands (Bemmelen 1949; Klompe 1954, 1957; Kundig 1956). There seems to be strong evidence for regarding, as Klompe (1954) did, the Banggai Islands as belonging to the Sula Spur. They

thus lie outside the Sulawesi orogenic belt. To the west of the Sulawesi orogenic belt basement rocks are not known to crop out within 200 km of Sulawesi.

8. S T R U C T U R A L R E L A T I O N S

m-17 Two major faults have been recognized. The Tawaelia Graben zone (Brouwer 1947) strikes N-S in Central Sulawesi and probably extends north along strike to cut the North Arm and southwards along the west side of the Bone Gulf, passing just east of Salajar. The age of commencement of movements along this structure is not known, but it separates zones 2 and 3 and so seems to be an important feature related to the evolution of the belt. I t appears to have in- fluenced the distribution of sedimentary facies on either side since the Cretaceous and it limits the distribution of metamorphic facies. The extent of its movements are unknown, but it appears to have acted as a graben during Neogene and probably earlier times. Mylonite occurs along part of the fault zone where igneous rocks are present.

The other major faul t -- the Fossa Sarasina--also appears to behave as a graben in Sulawesi, where it has formed a fault valley. I t appears to strike NNW into the Makassar Strait. I t may be much younger than the Tawaelia structure, but its age of initiation is uncertain. The Fossa Sarasina and other faults in Sulawesi have been discussed recently by Katili (1970).

9. R E V I E W OF O R O G E N I C D E V E L O P M E N T

132 GEOPHYSICAL EVIDENCE OF OROGENIC STRUCTURE

Fitch (1970) stated that data gathered from shallow focus earth- quakes from 1961 to 1968 show that thrusting and normal faulting are the dominant modes of deformation in the Philippines, Sulawesi and Bands orogenic belts. The conclusion reached by Ritsema & Veldkamp (1968) that strike-slip faulting is dominant was based on data that Fitch (1970) claimed was inadequate by modern standards. Another recent paper is that of Hatherton & Dickinson (1969.)

Mechanism solutions from submarine seismic zones on the convex sides of the North Celebes and North Moluccan Arcs show evidence for underthrusting of lithosphere beneath the island arcs. Fitch stated that there 'is no evidence from focal mechanisms supporting the existence of underthrusting along the eastern end of the Sunda Arc (i.e. the Banda Arcs) even though a well-developed inclined seismic zone exists beneath the arc in this region . . . the close approach of the Australian continent may have disrupted the simple under- thrusting in the region'. Also 'inclined seismic zones beneath the (Bands, North Celebes and North Moluccan) arcs show that activity at deep and intermediate depths is in general confined to narrow slab-like regions of the upper mantle' . He claimed that seismic evidence from this region and other inclined seismic zones 'supports

372 SULAWESI

the hypothesis that slabs of lithosphere constituting lateral inhomo- geneities in the upper mantle beneath island arc structures act as stress guides'.

On the basis of shallow focus seismic activity, Fitch (1970) defined boundaries of plates of lithosphere that differ in detail from the plate boundaries proposed by Le Pichon (1968) largely by the introduction of the Philippine Sea plate (Fig. 5). Fitch has drawn the tentative boundary of the Pacific and Asian plates cutting through the East Arm and North Arm of Sulawesi. This interpretation seems un- satisfactory to the present writer on the basis of the published accounts of the geology of these regions.

Figure 5. Plates of lithosphere within the Sulawesi and Banda Arcs orogenic belts (from Fitch 1970). Plate boundaries are defined here by shallow focus seismic activity, shown by the solid curve and by a peeked line where the boundary is poorly defined.

134 THE OVERALL EVOLUTION OF THE SULAWESl OROGENIC BELT

Pre-Upper Triassic history: The oldest rocks in Sulawesi are pre-U. Triassic crystalline schists with intrusives. The schists have been produced by regional metamorphism of mainly sedimentary rocks, the age of deposition and provenance of which are unknown. The true age of the pre-U. Triassic metamorphism is not known. Kundig (1956) and Roever (1956) referred to the crystalline basement rocks as Palaeozoic or pre-Palaeozoic, but apparently without evidence other than regional considerations. In the South Arm of Sulawesi the crystalline schists can only be shown to be pre-Cretaceous in age, but their metamorphism is likely to have been of the same age as elsewhere. Possible Permian history: The apparent absence of Permian rocks from Sulawesi is surprising in view of the extensive Permian marine deposits

in parts of the Banda Arc (Timor and Left). Brouwer (1919) thought that the crystalline schists in the Lesser Sunda Islands (e.g. Timor) might, at least in part, be Permian in age on the basis of a reported gradation between them and fossiliferous rocks in Left. Bemmelen (1949) doubted the reality of this gradation and postulated tectonic wedges to explain their association. The field relations in Left need re-examination. Syn-orogenic (Triassic-Jurassic) history: L. and M. Triassic strata are not known in Sulawesi, although their presence was postulated by Loczy (1933-4). U. Triassic neritic limestones and shales and a paralic facies occur in zone 4. There may have been a period of erosion in zones 2 and 3 during the U. Triassic. In zone 4 sedimentation from the U. Triassic to the L. Cretaceous seems to have been fairly continuous in shallow marine environments. Relatively gentle fold movements may have occurred locally during this period. Syn-orogenic (Cretaceous) history: During the Cretaceous there must have been considerable downwarping of zone 4 for the facies change rapidly from neritic to bathyal, suggesting a correlation with the development of the Outer Banda Arc. In zone 2 Cretaceous deposits appear to belong to two very different facies; one is neritic-paralic, the other bathyal and similar to that of zone 4. I t is possible that tectonic dislocations, presently unrecognized, may account for this anomalous association. Alternatively, the east and north of zone 2 may have been downwarped to provide a bathyal environment similar to zone 4, whilst the west of zone 2 remained close to sea level producing neritic sedimentation. Another possibility is that the bathyal environment has been mis-identified. Perhaps the calcilutites and siliceous deposits were laid down in a quiet sea of moderate depth in which little terrigenous sediment entered, so that deposits similar to bathyal facies were produced. Syn-orogenic (late Cretaceous-L. Eocene) history: At the end of the Cretaceous and possibly extending into Eocene times, there was a major orogenic phase which resulted in folding and local over- thrusting. It has been identified in zones 2, 3 and 4, although its effects appear to vary greatly along the strike of the zone, e.g. it appears not to have had much influence, beyond possible uplift, in the East Arm (zone 4). Accompanying this orogenic phase was a major ophiolite eruption in zone 4. Greenschist metamorphism probably occurred during this time in zones 2, 3 and 4. In zones 3 and 4 there was also a glaucophane-schist metamorphism contemporaneous with this phase. Granodiorite intrusions into zone 2 occurred at this time as well as volcanic activity during the Eocene. The direction of folding and thrusting, as well as the development of ophiolites in a belt around the Sula Spur, seem to indicate the important role that feature was playing in the evolution of this region. In addition the downwarping of zone 4 and perhaps parts of zones 2 and 3 in a belt around the Sula Spur suggests a direct comparison with the U. Cretaceous downwarping of the Outer Banda Arc in front of the Australian shelf. Orogenic movements were widespread in the Banda Arcs at this time.

S U L A W E S I 373

I I

' ' ; : " . ~,~ "' " "1- ': .. . . . . ~"~ , ", ,, .. ~ : ' . . -~-. : . ! ,

P .~, ,, • _~.. ~. ~ ' + . : ~ - ~ "-- .. :.. .... . + + + ". - , ; ,-..: t~.~.,-,.- .... • ".-F ± .: "" . "'

. . . . . .~ ~ "l'_Lt---~ ' " . .;'1":i --J:~.'.:'~--~'_ " - ,- ÷ ' r . I . . . . . ":

UNA UNA I S . ~

.. :~- f... GAI

~+ ÷ . ~ . . _ /~--,-- ." •

+ ar','-_~-- " ~

.~ " . : -

• t - , __--

_ SULA IS.

IOOkm i !

Ouat . . . . . y a,,uv,om ~" ~.-.~,.'," :at" Reefs etc

(~ i : ! "" _ ~ P Palaeogene

• . x ~ [ ) f ~'~11 und i f fe ren t ia ted

i ~ B U T U N G IS. ~ In te rmed ia te to basic suite, .!: • KABAENA IS. . . ~

inc lud ing some in t rus ives I I Q TUKANG PLUTONIC ROCKS

t 0 ~ BESI IS. ~ i" SALAJAR IS. ~ ~ ' ~

Gran i te & granodiorite

Basic rocks, inc lud ing oph io l i tes

V 'i" " Figure 6. Geological sketch map of Sulawesi (redrawn from a geological map published by the U.S.G.S. 1965). Blank areas in

Sulawesi and the Sula Islands are areas for which no geological maps are known.

374 SULAWESI

Syn-orogenic (M. Eocene-L. Miocene) history: Eocene to Oligocene neritic deposits, mainly shelf limestone in zones 3 and 4, are thin and often absent. This may be due to a period of uplift and erosion following the folding and introduction of ophiolites. In zone 2 a paralic facies de- veloped during Eocene to Oligocene times as well as some lime- stones. The Eocene volcanic activity continued through Oligocene and into Miocene times.

During the Oligocene the main orogenic phase of folding and thrusting occurred in the north of the South Arm (zone 2). There may also have been some fold movements in zone 3 and part of zone 4. Granodiorites and diorites were intruded into zone 2 during Eocene to Oligocene times. During the L. Miocene shelf limestones were widely deposited in Sulawesi, in zones 2, 3 and 4. Syn-orogenic (M. Miocene) history: The main orogenic phase in part of zone 4 (East Arm), involving folding and thrusting, occurred at this time. There is a suggestion of some fold movements at this time in zone 3 and the north of zone 2. The direction of thrusting was towards the Sula Spur. The M. Miocene was the time of a major orogenic phase in the Banda Arcs. @n-orogenic (U.Miocene-Pliocene) history: A molasse facies was de- veloped in zones 2, 3 and 4 during this interval, perhaps as a re- sponse to uplift and erosion following the M. Miocene folding phase. Very similar behaviour at this time is found in the Banda Arcs. Locally neritic and littoral depgsits developed in zones 2, 3 and 4 and in zone 2 volcanoes were again active and important intrusions of granodiorites occurred. Syn-orogenic (late Pliocene) history: Towards the end of the Pliocene there was a widespread period of folding in all parts of Sulawesi (zones 2, 3 and 4). Thrusts developed in the Butung Archipelago and the movements appear to be towards the west. This seems to be an anomalous feature of the regional tectonics and may indicate the waning importance of the southern part of the Sula Spur, perhaps associated with its subsidence in that region, where the sea is now more than 4000 m deep. The U. Miocene to Pliocene granodiorite intrusions of zone 2 may be associated with this orogenic phase. In zone 2 there was volcanic activity at this time. The late Pliocene interval also saw a period of folding throughout most of the Banda Arc. Post-orogenic (Quaternary) history: The Quaternary history of zones 2 and 4 appears to have been one of uplift and erosion, following the late Pliocene fold movements. The behaviour of zone 3 appears anomalous. The part of zone 3 in Central Sulawesi seems to have been uplifted and eroded, as were the Tongian Islands, whilst most of the zone is sub- marine and may have subsided. Zone 1 subsided during the Quater- nary (Krause (1966). Zone 5 by analogy with zone 1 also possibly subsided. It seems likely that in general the Quaternary evolution of the region has emphasized the relief between the marine and non- marine zones by adjacent processes of uplift and subsidence. Volcanic activity has occurred during the Quaternary in the nolthern part of zone 2 and in zone 3 (Una Una).

135 METAMORPHISM IN THE SULAWESI OROGENIG BELT

Metamorphic rocks occur in all the four arms of Sulawesi, in some small nearby islands such as Kabaena and Butung and in several parts of Central Sulawesi. The considerable literature on these rocks has been summarized by Rutten (1927), Brouwer (1947) and Bemmelen (1949) and the most recent publications are by Egeler (1947, 1948) and Roever (1947, 1950, 1953 and 1956). The general conclusions reached by Roever (1953, 1956) are as follows: (1) The oldest phase of regional metamorphism in Sulawesi occurred before the U. Triassic and probably produced the epidote-amphibolite facies of Central Sulawesi (zone 3) and the amphibolite facies of the South-east Arm, Butung and Kabaena (zone 4). This regional metamorphic phase, which may even be pre-Palaeozoic according to Kundig (1956), belongs to an orogeny older than the Mesozoic-Cenozoic orogeny. (2) A younger regional metamorphic phase which produced green- schists and glaucophane schist facies in zones 3 and 4 is probably of late Cretaceous or early Eocene age. (3) A phase of strong over- thrusting in Kabaena (zone 4), which was post-Eocene in age (or possibly late Eocene or even mid-Eocene, as Kundig (1956) suggested for the East Arm) produced dynamic metamorphism. This dynamic metamorphic phase may have also occurred contemporaneously in the North Arm (zone 2) (Brouwer et al. 1934) and in the South-east Arm (zone 4). According to Kundig (1956) the greenschists and amphi- bolites in the East Arm (zone 4) were associated with the eruption of the ophiolites.

B. S U B D I V I S I O N OF T H E S E G M E N T

The Sulawesi orogenic belt is here divided into five structural zones delimited somewhat arbitrarily at the 1000 m isobaths. The Banggai Islands have been included in the Sula Spur, which is regarded as a separate structural feature. Zone 1 is wholly submarine, but has distinct northern and southern parts, the Celebes Sea Basin (Krause 1966) and the Makassar Strait Fracture. At the south of the Makassar Strait the sea floor shallows and opposite Makassar the zone may be cut off" or pinch out. I f zone 1 of the Sulawesi orogenic belt connects with zone 1 of the Banda Arc orogenic belt, then it must do so at 500 m between the Doang- doang shoals and the coast near Makassar. There is some geophysical evidence for regarding this broad ridge between the South A r m - - Paternoster Islands and east Java as a structural feature connecting the Western Celebes Arc with the Inner Sunda Arc of Java. The geo- logical history and igneous activity of these two regions offer support for this idea. Zone 5 (the Foredeep) will be considered next, for like zone 1 it is wholly marine. Where zone 5 lies on the convex, outer side of the Banda Arc orogenic belt it is a deep water zone always over 1000 m

SULAWESI

Table 1

ZONES AND ELEMENTS IN SULAWESI

375

Age and fades of the Elements

Zones

1 2 3 4 5

Celebes Sea Basin

W. Celebes Arc and N. Celebes Arc

Intra-Celebes E. Celebes Arcs Zone Arc Foredeep

Western Part of Sula Spur

Quaternary Volcanics Quaternary Littoral and neritic Quaternary Evidence of subsidence LATE-PLIOCENE OROGENIC PHASE Pliocene Littoral and neritic Miocene-Pliocene Granodiorite intrusive, U. Miocene-Pliocene Molasse U. Miocene-Pliocene Paralic U. Miocene-Pliocene Littoral and neritic U. Miocene-Pliocene Volcanics MID-MIOCENE OROGENm PHASE Oligo-L. Miocene Volcanics Oligo-L. Miocene Neritic Eocene-Oligocene Granite, granodiorite and

diorite intrusives Eocene-Oligocene Neritic Eocene-Oligocene Paralic Eocene Volcanics U. Mesozoic-Eocene Glaucophane schists U. Mesozoic-Eocene Greenschists U. CRETACEOus-L. EOCENE OROGENIC PHASE U. Cretaceous-L. Eocene Ophiolites U. Cretaceous-L. Eocene Granodiorite intrusives U. Mesozoic Bathyal U. Mesozoic Neritic L. Mesozoic Neritic and paralic

UNCONFORMITY BELOW THE MESOZOIC SUCCESSION Pre-U. Triassic Basement crystalline schists

X X

X X X

M M M + S x x x x × × x

x x W W S+W x x x x x

x x

x not Obi x only Obi

X X

X X

S S + W W x x only in Obi

x x not Obi x ? in Obi

x Juras, but no Trias

x x x x x x

Note: x =present Principal orogenic phases: S=strong; M=moderate; W=weak.

and locally over 4000 m deep. In the Sulawesi region zone 5 lies on the concave side of the inner arc, and its disappearance from most of the Sulawesi orogenic belt may be associated with this reversal of geometry. The in terpre ta t ion of the relative positions of zone 5 and the Sula Spur in the Nor the rn Banda Sea is open to discussion, however. The identif icat ion of the deeper water ( > 1000 m) immedi- ately west of the Ha lmahe ra Islands as the cont inuat ion of zone 5 of the Sulawesi orogenic belt is one that might not find general acceptance, but it does seem to occupy the outside of the convex outer arc (of the Nor th Moluccan Arc). This region between Sulawesi and I r ian Barat is geophysically complicated (el. Fitch 1970; Ha the r ton & Dickinson

1969) and geologically poorly documented, so several a l ternat ive tectonic interpretat ions are possible. Al though few epicentres of in termediate and deep focus seismic ear thquakes are reported from this par t of zone 5 (of. Fitch 1970), we may not expect geophysical analogy with zone 5 of the Banda Arcs, because the nea rby deep Phil ippine t rench and the volcanoes of Ha lmahe ra have no equivalents on the convex side of the zone 5 of the Banda Arcs. Zone 2. The western limits of zone 2 are drawn arbi t rar i ly at the 1000 m isobath, except south of Makassar where (as discussed for zone 1 above) the l imit is taken at the 500 m isobath unti l near the t rench in the Flores Sea. From there the margins of zone 2 cont inue

376 SULAWESI

eastward as part of the Salajar-Lucipara Arc at much greater depths in the Banda Sea. The inner margin of zone 2 is drawn at the 1000 m isobath until the Banda Sea. This eastern margin coincides locally with the submarine fault scarp on the east of the South Arm which extends from the Tawaelia graben in Central Sulawesi to the east of Salajar. In the north the margin separates the Sangihe ridge and islands from the Gorontalo Trough and the Sangihe Trough. The northern part of zone 2 (Northern Celebes Arc) is characterized by a line of active volcanoes and a zone of epicentres of intermediate focus earthquakes. Zone 3 may be called the Intra-Celebes Arcs zone. Mostly it is submarine and is deeper than 1000 m or even than 9000 m. Two land areas are involved in this zone, one is the Poso zone of Central Sulawesi and the other is the Tongian Islands and the volcano of Una Una. On Sulawesi the western margin of the zone is coincident with the Tawaelia graben and its northern and southern submarine exten- sions. In the south this zone is continuous with the Intra-Banda Arcs trough (zone 3 of the Banda Arcs). Zone 3 is coincident with a zone of positive isostatic gravity anomalies over most of its length in the Sulawesi belt, but this feature does not continue into zone 3 of the Banda orogenic belt. Zone 4 is the East Celebes Arc, which to the south and north continues with structures called the Tukang Besi Arc and the North Moluccan Arc respectively. At the southern end the arc is inter- preted to be continuous along strike with the Outer Banda Arc (zone 4 of the Banda Arcs orogenic belt. The zone is characterized in both the Sulawesi and Banda orogenic belts by a strong zone of negative isostatic gravity anomalies. The seismic isobaths drawn by Hatherton & Dickinson (1969) suggest important differences between zone 4 of the Banda Arcs and part of zone 4 of Sulawesi (the South- east Arm). Their contours suggest, however, that the East Arm of Sulawesi may occupy a similar position to an inclined seismic zone as that indicated for the Outer Banda Arc. The Sula Spur: Stille (1945) and Klompe (1954) regarded the Sula Spur as a detached part of the Australian continent. Kundig (1956) referred to it as a 'small stable intra-orogenic craton'. Wegener (1966) interpreted it as a structure due to New Guinea penetrating the region of the Banda Arcs by the north-west drift of Australia relative to Asia. Fitch (1970) claimed that seismic evidence suggests the Sula Spur 'currently includes one or more plates of lithosphere'.

The north margin of the western part of the Sula Spur is drawn at the 1000 m isobath. Klompe (1954), and others later, took this margin through the middle of Obi so as to exclude the ophiolites. The present writer follows Kundig (1956) and Fitch (1970) and considers on the basis of the geological history, the isostatic gravity map and the shallow focus seismic activity that the northern limit of the Sula Spur should be drawn north of Obi. Eastward from Obi the northern limit is here drawn at the north edge of the Obi-Pisau-Kafiau-Batanta ridge, which on the basis of topography, the isostatic gravity anomaly

map and with some support from shallow focus seismic activity (Fitch 1970), appears to be a more consistent interpretation than that offered by Klompe (1954).

One problem is the relationship of the possible western extension of the Sorong Fault zone, to the north edge of the Sula Spur. Visser & Hermes (1962) suggested the fault extended south of Obi and south of the Sula Islands. Alternatively it may extend south of Obi but north of the Sula Islands, so that west of Obi it could coincide with the north limit of the Sula Spur, or it could occur north of Obi separating that island from the Halmahera Islands; this last interpretation might account for a line of shallow focus seismicity along the postulated extension (Fitch 1970). The south margin of the west part of the Spur presents the greatest problem; the positive isostatic gravity anomaly extends south into the north of the Banda Sea where the sea floor drops rapidly to 4000 m south of the Sula Islands. The present writer follows Klompe (1954) in drawing this south margin around the southern limit of the gravity anomaly; this seems to accord most closely with the tectonic behaviour of the East Celebes Arc and the Outer and Inner Banda Arcs, although this hypothesis receives no support from present seismic activity.

C. S T R U C T U R A L R E L A T I O N S H I P S OF T H E S U L A W E S I A R C S W I T H

N E I G H B O U R I N G R E G I O N S

Sulawesi separates four major geological provinces: to the west and south-west are the Larger Sunda Islands and their shelf, partly represented by Borneo and Java; to the south and south-east are the Lesser Sunda Islands which form the Banda Arcs; to the east is the Vogelkop and Bomberai of Irian Barat (formerly called west New Guinea); to the north are the Philippines which form part of the island chains that mark the boundary of the western Pacific. Each different part of Sulawesi appears to have geological affinity with some important aspects of one or more of these four major geological provinces. On the basis of the linear belt of strongly negative gravity anomalies that characterize the East and South-East Arms of Sulawesi, the East Celebes Arc has been regarded (Kuenen et al. 1934) as an extension of the Outer Banda Arc (structural zone 4). I t is believed to continue northwards into the North ~vIoluccan Arc (Visser & Hermes 1962). There is uncertainty about the correlation on gravity data between Butung at the south of the South-East Arm and Buru because this part of the Banda Sea does not display the strong negative gravity values that characterize zone 4 elsewhere. Westerveld (1955) noted that the submarine floor of this part of zone 4 was marked by a series of strong ridges and trenches striking NW-SE, which he suggested probably marked the sites of transverse faults striking in that direction. The absence of the strong negative gravity values may be associated with such faults. The correlation of the Outer Banda Arc with the

SULAWESI 377

East Celebes Arc has also been proposed on the basis of these lands possessing a Mesozoic (and possibly Palaeozoic) history that dis- tinguishes them from all the surrounding regions. Umbgrove (1938) a n d Teichert (1939) referred to this zone 4 as the T i m o r - - E a s t Celebes Geosyncline, and Teichert thought it an extension of the Westra l ian Geosyncline of the Carnarvon basin.

The South A r m of Sulawesi, together with the Palu zone of the central region and the North Arm, expose granite intrusions and acid extrusives which suggests some affinity between this zone 2 and the larger Sunda Islands (Borneo, J a v a & Sumatra) . In contrast, in the East and South-East Arms extensive outcrops of ultrabasic and basic igneous rocks predominate , which as Brouwer (1930) observed are characteristic of the Banda Arcs.

The North A r m of Sulawesi is usually regarded as a southerly extension of the chains of island arcs around the western Pacific. The Phi l ippine Arc passes south of M i n d a n a o into a submar ine ridge with islands and then into the Minahasa portion of the North Arm. This correlation is based on the strong topographical feature with accompanying chain of active volcanoes (zone 2) which separates the belt of strong negative gravity anomalies of the North Moluccan Arc (zone 4) from the strong positive anomalies of the Celebes Sea (zone 1).

The structural relationship of the South A r m and the western part of Central Sulawesi (zone 2) to zones 3 and 4 is uncertain. The south of zone 2 lacks active volcanoes (Neumann van Padang 1951) and strong gravity anomalies in Sulawesi. The acid igneous rocks of this region have suggested some possible affinity with the Larger Sunda Islands to the west and south, but Westerveld (1955) proposed that this region was an extension of the Inner Banda Arc. He suggested that the southern part of the South A r m was part of an arc that connected Salajar with the Banda Islands through a series of submar ine ridges with a few islands such as Komba , Gunungap i and the Lucipara Islands. This arc is characterized by weak positive isostatic anomalies and locally in the Banda and Flores Seas by active volcanoes. At the present t ime there seems to be insufficient avai lable geological or geophysical evidence to support or reject this hypothesis.

Two major strike slip faults, called the Sorong Faul t zone and the Tarera-Aidoena Faul t zone in I r ian Barat by Visser & Hermes (1962), m a y through their possible westward extension have influenced the development of the major arcuate zone of the East Celebes Arc and its extensions into the Phi l ippine and Banda Arcs.

Stille (1945) and Klompe (1954) suggested that the region of the Sula and Banggai Islands where crystalline schists crop out, m a y repre- sent a detached fragment of the Austral ian continent. The western par t of this Sula Spur is characterized by a strong positive gravity anomaly. In Figs. 1, 2 and 4 the eastward extension of this Spur is shown to include the is land of Obi and a south-eastward extension is suggested to include Misool and part of south-western Bomberai . Stille (1945) regarded Misool as part of the Sula Spur and Bemmelen (1949, p. 458) considered that Misool does not belong to the Banda Arcs but is par t

of what he called the ' foreland belt ' that included Sula, Ob i and western Bomberai . Visser & Hermes (1962) and Hermes (1968) considered that Misool and western Bomberai form part of the Banda Arc pr in- cipally because they were submerged dur ing most of Mesozoic time. The present author favours the interpretat ion that regards Misool and western Bomberai as part of the Sula Spur and distinct from the Banda Arcs.

Al though the definition of the limits and correlation between the various structural zones of Sulawesi and the ne ighbour ing island arcs is open to debate, it is general ly acknowledged (Umbgrove 1949; Holmes 1965) that Sulawesi represents the junc t ion of two major Ter t iary orogenic belts, one is the H i m a l a y a n - I n d o b u r m a n - I n d o n e s i a n orogenic belt and the other a b ranch of the Circum-Pacif ic orogenic belt.

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