Geologically, Sulawesi Island and its surrounding area is a complex region. The complexity was caused by convergence between three lithospheric plates: the northward-moving Australian plate, the westward-moving Pacific plate, and the south-southeast-moving Eurasia plate. Regional structures, which affects the island of Sulawesi and the surrounding area, are shown in Figure 8.1. The Makassar Strait, which separates the Sunda Platform (part of the Eurasia Plate) from the South Arm and Central Sulawesi, formed by sea-floor spreading originating in the Miocene (Hamilton, 1979, 1989; Katili, 1978, 1989). North of the island is the North Sulawesi Trench formed by the subduction of oceanic crust from the Sulawesi Sea. To the southeast convergence has occurred between the Southeast Arm and the northern part of the Banda Sea along the Tolo Thrust (Silver et al., 1983a, b). Both major structures (the North Sulawesi Trench and Tolo Thrust) are linked by the Palu-Koro-Matano Fault system. Based on lithologic association and tectonic development, Sulawesi and its surrounding islands are divided into 3 geological provinces (Fig. 8.2): (1) the Western Sulawesi Volcanic Arc; (2) the Eastern Sulawesi Ophiolite Belt and its associated pelagic sedimentary covers; and (3) continental fragments derived from the Australian continent (Hamilton, 1978, 1979; Sukamto and Simandjuntak, 1983; Metcalfe, 1988, 1990; Audley-Charles and Harris, 1990; Audley-Charles, 1991; Davidson, 1991). The contacts between those provinces are faults.
WESTERN SULAWESI VOLCANIC ARC
The Western Sulawesi Volcanic Arc extends from South Arm through the North Arm (Fig. 8.2). In general, the arc consists of Paleogene-Quaternary plutonic-volcanic rocks with Mesozoic - Tertiary sedimentary rocks and metamorphic rocks. In this chapter the stratigraphy is divided into South Sulawesi and North Sulawesi system.
The Geology of eastern and western South Sulawesi is distinctly different, and these two areas are separated by the NNW-SSE trending Walanae Depression. South Sulawesi is structurally separated from the rest of the Western arc of Sulawesi by a NW-SE trending depression which passes through Lake Tempe (Fig 8.3, van Leeuwen, 1981). Figure 8.4 is a compilation of the formation names and ages of lithologies in South Sulawesi, used by various workers. For reference a geological map and the stratigraphy of South Sulawesi are presented in Fig 2.4 & 2.6. The following sections describe the geology of South Sulawesi through time.
Mesozoic basement complex
The basement complex is exposed in two areas: in the western half of South Sulawesi near Bantimala and Barru, and consists of metamorphic, ultramafic and sedimentary rocks (Fig. 8.4). Metamorphic lithologies include amphibolite, eclogite, mica-schists, quartzites, chlorite-feldspar and graphite phyllites (t’Hoen & Zeigler, 1917; Sukamto, 1975; 1982; Berry & Grady, 1987). K/Ar dating on muscovite-garnet and quartz- muscovite schists, both from the Bantimala basement complex yielded 111 Ma (Obradovich, in Hamilton, 1979) and 115 + 7 Ma (Parkinson, in Hasan, 1991) respectively. Wakita et al.,(1994) have dated five schist samples from the Bantimala complex and one from Barru complex using K/Ar analyses and yielded an age of 132- l 14 Ma and 106 Ma respectively. This data suggests a late early Cretaceous age for the emplacement of the basement in south Sulawesi. The sequence unconformably overlying and tectonically intercalated with the metamorphic lithological units consists of red and grey siliceous shales, feldspathic sandstones and siltstones, radiolarian cherts, serpentinized peridotite, basalt and diorites (Sukamto, 1975; 1982; Hamilton, 1979; van Leeuwen, 1981; Wakita et al., 1994). Radiolaria extracted from the cherts have been dated as late Albian (latest early Cretaceous: Pessagano, in Sukamto, 1975) or late Albian to early Cenomanian (Wakita et al., 1994). The presence of similar metamorphic rocks in Java, the Meratus mountains in SE Kalimantan and Central Sulawesi suggest that the basement complex in south Sulawesi may be a dismembered fragment of a larger early Cretaceous accretionary complex (Parkinson, 1991).
Late Cretaceous sedimentation
The late Cretaceous sediments include the Balangbaru (Sukamto, 1975;1982; Hasan, 1991) and Marada Formations (van Leeuwen, 1981) in the western and eastern parts of west South Sulawesi respectively (Fig. 8.4). The Balangbaru Formation unconformably overlies the basement complex and is composed of interbedded sandstones and silty-shales, with less important conglomerates, pebbly sandstones and conglomeratic breccias (Sukamto, 1975; 1982; Hasan, 1991). The Marada Formation consists of an arenaceous succession of alternating impure sandstones, siltstones and shales (van Leeuwen, 1981). The sandstone are mostly feldspathic greywacke which are locally calcareous composed of subangular to angular grains of quartz, plagioclase, and orthoclase with subordinate biotite, muscovite and angular lithic fragments embedded in a matrix of clay minerals, chlorite and sericite (van Leeuwen, 1981). Graded bedding is occasionally present in sandstone and the sandstone and siltstone. Coarser units of the Balangbaru Formation contain sedimentary structures typical of gravity flow deposits, including the chaotic fabric of debris flows, graded bedding and sole marks indicative of turbidites (Hasan, 1991). The lithologies and fauna of the Balangbaru and contemporaneous Marada Formations to the east (van Leeuwen, 1981; Sukamto, 1982) are typical of an open marine, deep neritic to bathyal environment (van Leeuwen, 1981; Sukamto, 1982; Hasan, 1991). The Marada Formation is interpreted to be the distal equivalent of the Balangbaru Formation, based on lithological and grain size considerations (van Leeuwen, 1981). The tectonic setting of the Balangbaru Formation is interpreted to be a small fore-arc basin on the trench slope (Hasan, 1991).
Volcanics of Paleocene age occur in restricted areas of the eastern part of South Sulawesi and unconformably overlie the Balangbaru Formations (Sukamto, 1975). In the Bantimala region these volcanics have been called Bua Volcanics (Sukamto, 1982); Langi Volcanics in Biru area (van Leeuwen, 1981; Yuwono et al., 1988). This formation consists of lavas and pyroclastic deposits of andesitic to trachy-andesitic composition with rare intercalations of limestone and shale towards the top of the sequence (van Leeuwen, 1981; Sukamto, 1982). Fission track dating of a tuff from the lower part of the sequence yielded a Paleocene age of + 63 Ma (van Leeuwen, 1981). The calc-alkaline nature, and enrichment of certain light rare earth elements, suggests that the volcanics were subduction related (van Leeuwen, 1981; Yuwono, 1985), probably from a west dipping subduction zone (van Leeuwen, 1981).
Eocene to Miocene volcanism and sedimentation
The Malawa Formation is composed of arkosic sandstones, siltstones, claystones, marls and conglomerates, intercalated with layers or lenses of coal and limestone. This formation occurs in the western part of South Sulawesi and unconformably overlies the Balangbaru Formation and locally the Langi Volcanics (Fig. 2.5, Sukamto, 1982). A Palaeogene age for this formation is inferred from palynomorphs (Khan & Tschudy, in Sukamto, 1982) whilst ostracods suggest an Eocene age (Hazel, in Sukamto, 1982). The Malawa Formation is inferred to have been deposited in a terrestrial/marginal marine environment passing transgressively upwards into a shallow marine environment (Wilson, 1995). The Tonasa Limestone Formation conformably overlies the Malawa Formation or the Langi Volcanics. This Formation consists of four members ’A’, ’B’, ’C’ and ’D’ from bottom to top. The ’A’ member comprises well bedded calcarenite, the ’B’ member is composed of thickly-bedded to massive limestone, the ’C’ member consists of a thick sequence of detrital limestone with abundant foraminifera and the ’D’ member is characterised by the abundant presence of volcanic material and limestone olistoliths of various ages (van Leeuwen, 1981; Sukamto, 1982). The age of the Tonasa Formation is Eocene to middle Miocene (van Leeuwen, 1981; Sukamto, 1982; Wilson, 1995). A ramp type margin is inferred for the southern margin of the Tonasa Formation, and the Tonasa Carbonate Platform is composed mainly of shallow water facies, whilst redeposited facies predominated the northern margin (Wilson, 1995). The Malawa and Tonasa Formations have a widespread distribution over the western part of South Sulawesi (Wilson, 1995). These formations do not outcrop east of the Walanae Depression (Fig. 2.4.) apart from a small outcrop of the Tonasa Limestone Formation at Maborongnge (Sukamto, 1982; Wilson, 1995). The Salo Kalupang Formation is present in the eastern part of South Sulawesi (Fig. 2.4). This formation consists of sandstones, shales and claystones interbedded with volcanic conglomerates, breccias, tuffs, lavas, limestones and marls (Sukamto, 1982). Based on foraminifera dating techniques, the age of the Salo Kalupang Formation is believed to range from the early Eocene to the Late Oligocene (Kadar, in Sukamto, 1982 and Sukamto & Supriatna, 1982). This formation is contemporaneous with the Malawa Formation and the lower part of the Tonasa Formation (Sukamto, 1982). The Kalamiseng Formation outcrops to the east of the Walanae Depression (Fig. 2.4) and comprises of volcanic breccias and lavas, in the form of pillow lavas or massive flows. These are interbedded with tuffs, sandstones and marls (Sukamto, l982; Sukamto & Supriatna, 1982; Yuwono et al., 1987). The lavas are characterised by spillitic basalts and diabases which have been metamorphosed to a greenschist facies (Yuwono et al., 1988). The Bone mountains have been interpreted as part of an ophiolitic sequence based on high gravity anomalies and the marine MORB nature (Yuwono et al., 1988). K/Ar dating on pillow lavas of the Kalamiseng Formation gave late Eariy Miocene ages (17.5+ -0.88 and 18.7+ -0.94, Yuwono et al., 1988) and this may represent an emplacement age of the suggested ophiolitic suite (Yuwono et al., 1988). Intrusive bodies are exposed in the eastern part of the Biru area and Tonasa-I (Sukamto, l982) where dating by fission track yielded an age of Early Miocene (van Leeuwen, 198l). Yuwono et al., (1987) relate these intrusive bodies to calc-alkaline volcanics in the lower member of Camba Formation and suggests that both were derived from early Miocene subduction. However, this is inconsistent with a mid- Miocene (Sukamto & Supriatna, l982) or middle to late Miocene age (Sukamto, 1982) suggested by foraminifera in marine sediments interbedded with the volcaniclastics. The lower member of the Camba Formation consists of tuffaceous sandstone, interbedded with tuff, sandstone, claystones, volcanic conglomerates and breccia, marls, limestones and coals (Sukamto, 1982; Sukamto & Supriatna, 1982). The Bone Formation has been reported by Grainge & Davies (1985) from the Kampung Baru-I well in the Sengkang area (Fig. 2.5) where it comprises bioclastic wackestone and fine grained planktonic foraminifera packstones interbedded with calcareous mudstone. The limestones have been dated as early Miocene (N6-N8) in age (Grainge & Davies, 1985).
Miocene to Recent volcanism and sedimentation
The upper member of the Camba Formation described here as the Camba Volcanics, is located in the Western Divide Range forming the ’backbone’ (Fig. 2.4). This member consists of volcanic breccias and conglomerates, lavas and tuffs interbedded with marine sediments (Sukamto, 1982; Sukamto & Supriatna, 1982). Foraminiferal dating suggests a middle to late Miocene age (Sukamto, 1982) for the Camba Volcanics. The Lemo Volcanics unconformably overlie the upper Miocene Walanae Volcanics in the Biru area (van Leeuwen, 1981), K/Ar dating for Lemo Volcanics yielded an age of Pliocene (Yuwono, et al., 1988). Although Sukamto (1982) included the Lemo Volcanics as part of the Camba Volcanics, this is unlikely since the age range of Camba Volcanics is only up to the late Miocene. The lower part of Camba Volcanics (Fig. 2.5) is thought to be equivalent to the mid- Miocene Sopo Volcanics in the Biru area (van Leeuwen, 1981). The upper part of the Camba Volcanics is thought to be analogous to the Pammesurang Volcanics from the Biru area, described by van Leeuwen (1981). Yuwono et al., (1988) subdivided the Camba Volcanics into two members: Camba IIa of alkali potassic nature and Camba IIb of alkali ultrapotassic nature. Based on K/Ar dating the age of the Camba II Volcanics is determined as late Miocene (9.91 + 0.5 Ma – 6.27 + -0.31 Ma, Yuwono et al., 1988). The volcanic units of Miocene to Pleistocene age in South Sulawesi have been discussed by Yuwono et al., (1987). These includes the Baturape volcanics, a series of alkali potassic extrusive and intrusive lithologies, where K/Ar analyses yields 12.8 + 0.64 Ma (mid Miocene, Yuwono et al., l988); the Cindako volcanics have the same characteristics as the Baturape Volcanics, but K/Ar dating yielded an age of 8.2+ 0.4l Ma for the Cindako Volcanics (late Miocene, Yuwono et al., 1987). These two volcanic units are grouped together by Sukamto (l982) who suggested an upper Pliocene age on the basis that they both unconformably overlie the Camba Formation. The Soppeng Volcanics are inferred to have a late Miocene age (Yuwono et al., 1987), however, Sukamto (1982) interpreted these volcanics as early Miocene in age since they are conformably overlain by rocks of the Camba Formation. The Parepare Volcanics are remnants of a strato-volcano composed of alternating lava flows and pyroclastic breccias dated by K/Ar analyses as late Miocene (Yuwono et al., 1987). The lavas are intermediate to acidic in composition (Yuwono et al., 1987). The Plio/Pliestocene volcanics of the strato volcano of Lompobatang occupies the southern-most portion of south Sulawesi rising to a height of 2,871 m. These volcanics consist of silica undersaturated in alkali potassic and more acidic silica saturated shoshonitic lava flows and pyroclastic breccias (Yuwono et al,, 1987). The mid-Miocene to Pleistocene volcanic rocks in South Sulawesi, including the upper member of the Camba Formation, have a predominantly alkaline nature, interpreted by Yuwono et al., (1987), as a result of partial melting of the upper mantle (phlogoplite- bearing peridotite) which was previously enriched in incompatible elements by metasomatism’ (Yuwono et al., 1987). This may possibly have been linked to previous subduction in early Miocene times in a ’distensional intraplate context’ (Yuwono et al.,1987). Van Bemmelen (1949) suggested that the alkali nature of these volcanics is caused by ’excessive assimilation of the older limestones into melt’ and incorporation of continental material into a subduction-related volcanic arc (Katili, 1978). Neogene magmatism in western central Sulawesi has been related to lithospheric thickening and melting (Coffield et al., 1993; Bergman et al., 1996). The bimodal nature of Neogene igneous lithologies in this area is thought to be from the melting of ancient mantle peridotite and crust yielding alkaline basaltic (shoshonitic) and granitic composition melts respectively (Coffield et al., 1993; Bergman et al., 1996) The late Miocene sedimentation is marked by the development of the Tacipi Formation (see section 2.3.2). The Middle Miocene-Pliocene (Grainge & Davies, 1983) Tacipi Formation forms the subject of the present study and is therefore not discussed further in this section. The Walanae Formation (see section 2.3.2) is locally unconformable on the Tacipi Formation and in places, the two units interdigitate. The Walanae Formation is dated as mid-Miocene to Pliocene (N9-N20 Sukamto, 1982) based on foraminifera, or alliteratively as predominantly Pliocene (up to N2l), with the basal units probably Late Miocene in age (Grainge A Davies, 1985). In the East Sengkang Basin the Walanae formation can be divided into two interval: a lower interval made up of calcareous mudstone and an upper interval which is more arenaceous. The lower intervals outcrops intensively in the southern part of the basin which in places interfinger with reef talus of the Tacipi Formation. Limestones on the southern tip of South Sulawesi (Figs. 2.2; 2.5) and on the island of Selayar are named the Selayar Limestone which is a member of the Walanae Formation (Sukamto & Supriatna, 1982). The Selayar Member is composed of coral limestone and calcarenite with intercalations of marl and calcareous sandstones. This carbonate unit ranges from upper Miocene to Pliocene in age (N16-N19, Sukamto & Supriatna, 1982). Sukamto & Supriatna (1982) reported that an interfingering relationship between the Walanae Formation & Selayar limestone occurs in the Selayar Island. Terrace, alluvial, lacustrine and coastal deposits occur locally in South Sulawesi. Recent uplift in South Sulawesi is characterized by raised coral reef deposits (van Leeuwen 1981; Sukamto,1982).
In Central Sulawesi, Late Miocene to Recent potassic calc-alkaline magmatism occur notably along the left lateral Palu-Koro Fault Zone (Priadi et al., 1999). This granitoid is supposed to be correlated with the collision of Banggai-Sula micro-continent with Sulawesi Island in Middle Miocene but the detailed studies about its genesis and its ascent mechanism are still limited. Based on their petrological aspects, association with other rocks/formations, degree of alteration, and chemical characters, this Neogene granitoid can be classified into at least three groups, from old to young, and they demonstrate a systematical change in their features: Coarse and KF-megacrystal bearing granitoid (Granitoid-C) is distributed in the northern and southern limits of Palu-Koro areas. They can be easily recognized as they present coarse equigranular or coarse and containing KF-megacrystals. Several K-Ar age dating indicate its ages ranging from 8.39 Ma to 3.71 Ma. Two petrographic characters can be distinguished: granitoid containing biotite and hornblende as mafic minerals (4.15-3.71 Ma and 7.05-6.43 Ma), and granitoid containing biotite as major mafic mineral (8.39-7.11 Ma). Medium mylonitic-gneissic granitoids (Granitoid-B) are exposed relatively in the central areas (around Palu-Kulawi). They are all present medium grained granitoids and sometimes contain xenoliths. This granitoid can also be subdivided into hornblende-biotite and biotite bearing granitoids. The former is distributed in the southern part (Saluwa-Karangana) and dated of 5.46-4.05 Ma. Whereas the latter which is dated 3.78-3.21 Ma exposes around Kulawi. Fine and biotite-poor granitoid (Granitoid-A) represent the youngest granitoid in Palu-Koro area (3.07-1.76 Ma), they occur as small dykes cutting the other granitoids. The rocks are clear, white and containing few biotites as single mafic minerals, most of them are concentrately exposed between Sadaonta-Kulawi in the central parts. Together with these aplitic dykes are also found lamprophyric dykes (minnette type). Pre-Neogene Gneissic Granitoid (Granitoid-D) is found in certain limited areas around Toboli. Based on geological map of Sukamto et al. (1973) its distribution can be extrapolated to extend north-south in Toboli-Kasimbar areas. It mainly consists of granites that are composed of quartz, K-feldspar, plagioclase and muscovite. The occurrence of muscovite and its older age (96.37 Ma), makes this granitoid differ from the others. Laterally these granitoids present a relative circular distribution with Granitoid-A around Kulawi as the focus and rimmed by Granitoid-B and C. The oldest Granitoid-D elongates north-south at the eastern part of the concentric distribution (Figure-1).
The North Sulawesi Arc, defined primarily on the basis of distribution of Lower Miocene arc-related rocks, extends for about 500 km onshore, from 121o E to 125 o 20’ E, and has a relatively constant width of 50 – 70 km, with elevations up to 2065 m. Higher elevations up to 3225 m are present at the neck of Sulawesi. The evolution of the North Sulawesi Arc may be divided into two main stages, with respect to the mid- Miocene collision of the arc with the Sula Platform: (1) west-directed subduction during the Early Miocene, and (2) post-collisional rifting and uplift of the arc, and inception of subduction along the North Sulawesi Trench during the Late Miocene to Quaternary. Geological relationships, paleontology (summarized on published 1: 250,000 maps) and preliminary K – Ar dating (Lowder and Dow 1978, Villeneuve et al. 1990, Perello 1992, Priadi, pers. commun. 1991) suggest two main periods of magmatic activity during the Neogene and Quaternary, namely, 22 – 16Ma (Early Miocene) and younger than 9 Ma (Late Miocene – Quaternary), i.e. pre- and post-collision of the arc with the Sula Platform. Pliocene and active Quaternary volcanicity belonging to the Sangihe Arc (Fig. 1) conceals much of the Early Miocene geology near Manado (Fig. 4). Small exposures of andesite and diorite below Quaternary volcanic cover on the Sangihe islands, north of Manado, suggest that older arc volcanics continue offshore, possibly to Mindanao (Fig. 1), and form the basement to the present-day Sangihe Arc. Neogene arc-related volcanic rocks are absent between Tolitoli and Palu in the neck of Sulawesi (Fig. 4), partly due to high uplift rates and deep erosion. Lower Miocene granitoids are not known, and there seems to be little evidence that the Early Miocene arc extended into the neck. Despite this, it is still inferred that the Early Miocene Benioff zone extended beneath the neck, and south to an intersection with the paleo-Palu – Matano transform fault (Fig. 1). In Western Sulawesi, south of Makale (Fig. 1), potassic alkaline (or shoshonitic) magmatism related to rifting rather than subduction was dominant during the Neogene (Yuwono et al. 1985, Leterrier et al. 1990, Priadi et al. 1991).
EASTERN SULAWESI OPHIOLITE BELT
The ophiolite complex and its pelagic sedimentary cover in the East and Southeast Arms of Sulawesi was named the Eastern Sulawesi Ophiolite Belt by Simandjuntak (1986). The belt comprises mafic and ultramafic rocks together with pelagic sedimentary rocks and melange in places. Ultramafic rocks are dominant in the Southeast Arm of Sulawesi, but mafic rocks are dominant farther north, especially along the northern coast of the East Arm (Smith, 1983; Simandjuntak, 1986). A complete ophiolite sequence was reported by Simandjuntak (1986) in the East Arm, including ultramafic and mafic rocks, pillow lavas and pelagic sedimentary rocks dominated by deep-marine limestone and bedded chert intercalations. Much of the complex is highly faulted and tectonised with blocky exposures. Based on limited geochemistry data (16 basalt samples), the Eastern Sulawesi Ophiolite Belt was probably of mid-oceanic ridge origin (Surono, 1995).
SOUTH EAST SULAWESI
The Southeast Sulawesi continental terrain occupies a large area in the Southeast Arm of Sulawesi, whereas the ophiolite belt is mainly restricted to the northern part of this arm (Fig. 2). The continental terrane, which trends northwest- southeast, is bounded by the Lawanopo Fault in the northeastern edge and by the Kolaka Fault in southwestern edge (Figs 1-2). The terrain is separated from the Buton Terrain by a thrust fault, and at the eastern end there is an older ophiolite suite thrusting over. The continental terrane comprises metamorphic basernent, with minor aplitic intrusions, Mesozoic clastic and carbonate strata, and Paleogene limestone (Fig. 2). The basement mainly consists of low-grade metamorphic rocks. The clastic sedimentary sequences consist of the Late Triassic Meluhu Formation. Paleogene limestone units include the Tamborasi Formation and Tampakura Formaticm (Figs 2, 3).
The low-grade metamorphic basement rocks form the dominant component in the Southeast Arm (Fig. 2). Tbe age of metamorphism is not clear yet. However, there are recognized an older metamorphic epidote-amphibolite kcies and a younger low grade dynamo-metamorphic glaucophane schist facies. The older metamorphism was related to burial, whereas the younger metamorphism was caused by large scale overthrusting when the Southeast Sulawesi continental terrane collided with the ophiolite belt, The metamorphic rocks were intruded by aplite and overlain by quartz-latite lava in places, especially along the western coast of Bone Gulf.
Mesozoic sedimentary rocks
In Kendari area, the basement rocks are unconformably overlain by the Late Triassic Meluhu Formation (Figs 2,3), which consists of sandstone, shale and mudstone. The Meluhu Formation composes of 3 members: from oldest to youngest they are the Toronipa, Watutaluboto and Tuetue Members. The Toronipa Member consists of meandering river deposits and is dominated by sandstone intercalated with conglomeratic sandstone, mudstone and shale. The Watutaluboto Member is a tidal-delta deposit dominated by mudstone intercalated with thin beds of sandstone and conglomerate. The Tuetue Member consists of mudstone and sandstone passing up into shallow marginal marine marl and limestone. Sandstone in the Toronipa Member consists of litharenite, sublitharenite and quartzarenite derived from a recycle orogen source The ubiquitous metamorphic rock fragments in the sandstone indicates that the source area for the Meluhu Formation was dominated by metamorphic basement. The metamorphic rocks were probably covered by a thin sedimentary succession. The small percentage of volcanic fragments in the formation suggests that volcanic rocks also formed a thin layer with limited lateral extent in the source area. The rare felsic igneous fragments were probably derived from dykes and/or si1ls that intruded the rnetamorphic basement. The Meluhu Formation is time equivalent to the Tinala Formation of the Matarombeo Terrain and the Tokala Formation in Siombok Terrain (Figs 2,4). Lithologically, these three formations are similar, with clastic-dominated sequences in their lower parts and become carbonate-dominated in the higher part of the formations. Halobia and Daonella in the Meluhu, Tinala and Tokala Formations indicate a Late Triassic age. The presence of ammonoids and pollen in the Tuetue Member of the Meluhu Formation strongly supports this interpretation. The clastic sedimentary sequence of the Tinala Formation (Fig. 4), in the Matarombeo Terrane, is successively overlain by the fine-grained clastic Masiku Formation and the carbonate-rich Tetambahu Formation. Molluscs, ammonites and belemnites are abundant in the lower part of the Tetambahu Formation and indicate a Jurassic age. The upper part of the formation contains cherty limestone and chert nodules rich in radiolarians. The radiolames suggesting a Jurassic-Early Cretaceous age. In the East Arm, the Tokala Formation of the Siombok and Banggai-Sula Terranes (Fig. 4), consists of limestone and marl with shale and chert intercalations. Steptorhynchus, Productus and Oxytoma are present in the formation that suggest a Permo-Carbonaferous age. However, Misolia and Rhynchonel1a are found within a limestone bed in the formation indicating a Late Triassic age. Due to lithological similarity between this formation and the upper Meluhu Formation, a Late Triassic age is most probable for the Tokala Formation age, while the Pamo-Carboniferous age probably represents a basement age. The Tokala Formation is overlain by the pink granitic conglomerate of the Nanaka Formatian, which may have been derived from the widespread granitic basement in the Banggai-Sula Islands. The overlying Nambo Formation consists of sandstone and shale containing common belemnites and ammonites indicating a Jurassic age.
Paleogene limestone sequences of the Tampakura Formation (400m thick) unconformably overlie the Meluhu Formation in the Southeast Sulawesi Continental Terrane. The formation consists of oolite, lime mudstone, wackestone and locally packstone, grainstone and framestone. In the lowest part of the formation, there is a clastic strata consisting of mudstone, sandstone and conglomerate. The formation contains foraminiferas indicating a Late Eocene-Early Oligocene age. Nanoflora in the formation indicating a broad Middle Eocene to Middle Miocene age. Thus deposition of the formation must have taken place during the Late Eocene-Early Oligocene. Initial deposition was in a delta environment where siliciclastic materials were dominant. A reduction in clastic sediment supply allowed an intertidal-subtidal carbonate facies to develop extensively on a low relief platform. Carbonate buildups, dominated by coralline 6amestone, and elongate carbonate sand bodies or barriers formed a rimmed shelf that protected and enclosed the carbonate tidal flat environment and isolated it from direct marine influence. Reflux dolomitizations took place in the intertidal- supratidal zones as Mg-rich fluids moved back towards the sea The similar Paleogene carbonate sequence of the Tamborasi Formation was deposited in shallow marine environments. Based on their ages and lithologies, the Tampakura and Tamborasi Formation (probably also the Lerea Formation in the Matarombeo) were probably deposited on a single broad shallow marine shelf, The shelf surrounded an island composed of metamorphic and granitic basement and Mesozoic clastic successions (Meluhu, Tinala and Tetambahu Formations). Equivalent units in the East Arm (the Banggai-Sula Terrane) include the Eocene-Oligocene limestone of the Salodik Formation, which interfingers with marl in the Poh Formation (Figs 1, 4).
The oldest rock formation of Triassic age is called Tokala formation. This consists of limestone and marl with intercalations of shales and cherts, regarded as being deposited in a deep sea environment. Another rock facies of the same age deposited in a shallow sea is formed by Bunta formation consisting of altered fine-grained clastic sediments such as slate, metasandstone, silt, phyllite and schist. In the East Arm of Sulawesi is also found the so called Ophiolite complex of late Jurassic to Eocene age which originated from an oceanic crust (Simandjuntak, 1986). This complex is found in a tectonic contact with Mesozoic sediments and consists of mafic and ultramafic rocks such as harzburgite, lherzolite, pyroxenite, serpentinite, dunite, gabbro, diabase, basalt and microdiorite. These rocks under went several times of deformations and displacements from their original place of which the last one was of Middle Miocene age. The Tokala and Bunta formations are unconformably overlain by Nanaka formation consisting of coarse-grained well-bedded clastic sediments such as conglomerate, sandstone with intercalations of silts and coal lenses. Among the fragments within the conglomerate are found red granite, metamorhpic rocks and chert which presumably originated from the socalled banggai-sula microcontinent (Simandjuntak, 1986). The age of this formation is assumed as Lower to Middle Jurassic and it was formed in a paralic environment. Conformably overlying the Nanaka formationis found the Nambo formation of Middle to Upper Jurassic age. This shallow marine unit consists of fine clastic sediments of sandy marl and marl containing belemnite and Inoceramus. The Upper Jurassic to Upper Cretaceous Matano formation consists of limestone with intercalations of chert, marl and silt. Unconformably overlying the Nambo formation are found the Salodik and Poh formations which interfingers each other. These formations are of Eocene to Upper Miocene in age. The Salodik formation consists of limestone with intercalation of marl and sandstone containing quartz fragments. The abundance of corals, algae and larger foraminifera found in this formation suggest that it was formed in a shallow marine environment. The Salodik formation is in a fault contact with the Ophiolite Complex. The Poh formation consists ofmarl and limestone with sandstone intercalations. The foraminifera assemblage of this formation indicating an age of Oligocene to the lower part of Upper Miocene. Nanno planktons within this formation suggest Oligocene to Middle Miocene age. The Molasse of Sulawesi which consists of Tomata, bongka, Bia, Poso,Puna and Lonsio formations (Surono, 1989) is of Middle Miocene to Pliocene. The Molasse contains conglomerate, sandstone, silt, marl and limestone, deposited in paralic to shallow marine facies. It overlies unconformably the Salodik and Poh formations as well as the Ophiolite complex. The Middle Miocene to Late Pliocene Bualemo volcanics interfinger with the Lonsio formation of the Molasse and consist of pillow lava and volcanic rocks. Unconformably overlying the Molasse of Sulawesi is the Pleistocene Luwuk formation, consisting of coral reef limestone with intercalations of marl in its lower part.
The Sulawesi Molasse was deposited after the collision between the continental fragments and the ophiofite belt. Tbe molasse is widely distributed throughout eastern Sulawesi and consists of coarse- to fine-grained clastic sequences with minor shallow marine carbonate sequences in places. The molasse in the Southeast Arm was divided into the conglomerate-dominated Alangga and Pandua Fonnations, a marl and limestone sequence of the Boepinang Formation, limestone of the Eemoiko Formation, and coarse- to fine-grained clastic strata of the Langkowala Formation. Boulders of pink granite found in the Early Miocene molasse sequences on the northern coast of the Southeast Arm and on Selabangka and Manui Islands may have been derived from the Banggai-Sula Islands. The molasse in the Southeast Arm is slightly older (Early Miocene) than in the East Arm where the collision between the Banggai-Sula continental terrane and the East Sulawesi ophiolite belt resulted in the deposition of Late Miocene molasse.
The continental fragments in the Sulawesi region, including Central and Southeast Sulawesi, Banggai-Sula and Buton, are believed to have been derived from part of the northern Australian continent (Pigram et al., 1985; Metcalfe, 1988, 1990; Audley-Charles and Harris, 1990; Audley-Charles, 1991; Davidson, 1991; Surono, 1997). They probably broke off from the Australian continent in the Jurassic and moved northeast to their present position. Audley-Charles and Harris (1990), Metcalfe (1990) and Audley-Charles (1991) termed them allochthonous continental terranes. Metamorphic rocks are distributed widely in the eastern part of Central Sulawesi, the Southeast Arm and the island of Kabaena. The metamorphic rocks can be divided into amphibolite and epidote-amphibolite facies and a low grade dynamometamorphic group of glaucophane or blueschist facies (deRoever, 1947, 1950). The amphibolite and epidote-amphibolite facies are older than the radiolarite, ophiolite and spilitic igneous rocks which are found in the metamorphic belt of the Central Sulawesi Province, while the glaucophane schist, on the other hand, is younger. The glaucophane schist is consistent with a high pressure and low temperature petrogenesis but these rocks have only had a reconnaissance petrological examination. Glaucophane becomes more abundance westward (Sukamto, 1975b). Except in Buton, the metamorphic rocks were intruded by granitic rocks in the Permo-Triassic. In the Southeast Sulawesi, Banggai-Sula and Buton Microcontinents metamorphic rocks form the basements of the Mesozoic basins. These rocks are unconformably overlain by thick units of Mesozoic sedimentary rocks, dominated by limestone in Buton and siliciclastic rocks in the Southeast Sulawesi and Banggai-Sula Microcontinents. Paleogene limestone is found on all of the microcontinents (Smith, 1983; Surono, 1986, 1989a, b; Supandjono et al., 1986; Surono and Sukarna, 1985; Garrad et al., 1989; Soeka, 1991). In the Late Oligocene-Middle Miocene time, westward-moving slices of one or more Indonesian-Australian microcontinents collided with the ophiolite complex of East and Southeast Sulawesi. The collision produced melange and an imbricate island arc zone of Mesozoic and Paleogene sedimentary strata from the microcontinents, with overthrust slices of ophiolite (Silver et al., 1983a, b). During the collision, local sedimentary basins formed in Sulawesi. After the collision, basins became more widely developed throughout Sulawesi. Sedimentation in the Southeast Arm began earlier (Early Miocene) than in the East Arm (Late Miocene, Smith, 1983; Surono, 1989a, b). Both these sequences are commonly referred to as the Sulawesi Molasse (Sarasin and Sarasin, 1901) and consist of a major clastic succession and minor reefal limestone. Most of the molasse was deposited in a shallow marine environment but in some places it was deposited in fluvial to transitional environments (Simandjuntak et al., 1981a, b, 1984; Surono et al., 1983; Rusmana et al., 1988; Surono, 1989a, b, 1996).
Bone Basin is located between south and southeast arm of Sulawesi, interpreted as a composite basin, with its origin as a subduction complex and suture between Sundaland and Gondwana-derived microcontinents, which subsequently evolved as a submerged intramontane basin. Tectonic and stratigraphic evolutions of the Bone Basin are still poorly understood due to limited data. A new model based on surface geology, seismic and single well data is presented for the tectonic and stratigraphic evolution of Bone Basin. During Early Tertiary or older, a westward subduction complex was probably developed to the east of western Sulawesi and Bone Basin was in a fore arc setting. A collisional event occurred between Australian-derived microcontinents and the Early Tertiary accretionary complex during Middle Miocene resulting in eastward obduction of the accretionary complex during Middle Miocene resulting in eastward obduction of the accretionary complex onto the microcontinents. The westerly continental moving microcontinents then collided against and partly was subducted beneath the western Sulawesi during Late Miocene. The compression from the collision propagated a major back-thrust system westward to the subduction zone generating foldbelts as indicated by the west-verging Kalosi and Majne fold belts. The two colliding plates then were locked up during the Pliocene and the continued plate convergence was accommodated by strike-slip movements along the Walanae, Palukoro and other faults. In the southern part of Bone Basin, westerly movement of the microcontinents did not reach the collision stage with western Sulawesi. Instead, Southeast Sulawesi was rotated eastward resulting in a major extensional fault cutting along the middle of the Bone Basin (Sudarmono, 1999). Stratigraphic record is very limited as only one well was drilled in the basin. The well indicates that the northern part of the Bone Basin basically consist of two marine sedimentary packages separated by a major Pliocene unconformity, which are pre-collision and post-collision sediments. The pre-collision sediments is of Late to Lower Miocene age consisting of predominantly calcareous claystone with rare limestone beds in the upper part and a conglomeratic layer in the lowermost part. The post-collision sediment is a syn-orogeneic sequence consisting of interbedded sands and clays with a few thin sporadic lenses of lignites. The lowermost part of the package and overlying the major Pliocene unconformity is a layer of fine to coarse grain sandstones grading to conglomerates (Sudarmono, 1999).
Sulawesi Island and its surroundings is one of the most complicated active margin in term of geology, structure and tectonic as well. The region represents a center of triple junction plate convergene, due to the interaction of three major earth crusts (plates) in Neogene times (Simandjuntak, 1992). This convergence gave rise to the development of all type of structures in all scales, including subduction and coliision zone, fault and thrust and folding. At present most of the Neogene structures and some of the pre-Neogene structures are still being activated or reactivated. The major structures include Minahasa Trench, Palu-Koro Fault System and its spalys of Balantak-Sula Fault, Matano Fault, Lawanopo Fault, Kolaka Fault and Kabaena Fault, Batui Thrust, Poso Thrust and Walanae Fault.
The Minahasa Trench is surfece expression of Benioff zone, inwhich the Sulawesi Sea crust being subducted beneath the North Arm of Sulawesi in late Palogene times (Fitch, 1970; Katili, 1971; Cardwell and Isacks, 1978; Hamilton, 1979; McCaffrey et al, 1983; Simandjuntak, 1993a). The subduction seems to be culminated in Neogene comtemporaneously with the west-southwest dipping collision zone between the Eastern Sulawesi Ophiolite Belt against the Banggai-Sula Platform along the Batui Thrust in the south. Seismicity suggests that at the present the Minahasa Trench seems to be dying out (Mc Caffrey et al, 1983; Kertapati et al, 1992). Simandjuntak (1988) suggested that recently, the eastern portin of the subduction zone seems to have been reactivated and produced the Minahasa Volcanic arc.
PALU-KORO FAULT SYSTEM
The Palu-Koro Fault System for the first time is defined by Sarasin (1901), and Rutten (1927) described the fault zone stretched on nearly N-S direction for at least 300 km long in Central Sulawesi. Sudrajat (1981) described that the Palu-Koro Fault stretchs from west Palu City to the Bone Bay in the southeast for some 250 km long and calculated the transcurrent movement in the ranging of 2-3.5 mm to 14-17 mm/year. Tjia (1981) analysed the rate of up-lifting of coralline reefs within the fault zone of some 4.5 mm/year. Indriastuti (1990) calcaulated the means of horizontal maovement of 1.23 mm/year. Bemmelen (1970) and Katili (1978) suggested that the northern portion of the fault system is dominated by vertical movement whereas the southern part is dominated by sinistral wrench movement. Walpersdorf et al (1997) on the basis of interferometric GPS analysis found out that the sinistral wrench movement of the Palu-Koro Fault System on te rate of 3.4 mm/year. Seismicity shows that at the present the Palu-Koro Fault being at least segmently reactivated (Kertapati et al, 1992; Soehaemi and Firdaus, 1995 ). Simandjuntak (1993a, b) thought that the Palu-Koro Fault System continued to Bone Bay, cut across the Flores Thrust and terminated in the Timor Trough in the south and to the north is termianted in Minahasa Trench. He also pointed out that during the history of fault movement, the Palu-Koro Fault was dominated by a sinistral transpressional movement, giving rise to the up-lifting of the mountain ranges along the fault zone. Althought in recent time the fault system was subjected to a transtensional sinistral wrenching causing the development of graben like basins such as Palu Valley and small lakes in many parts along the fault zone. He also further suggested that the development of Bone Bay was magnified by the sinistral transtensioanl movement of Palu-Koro Fault System in very late Neogene time. The Palu-Koro Fault System in Sulawesi is connected with Sorong Fault System in Irian Jawa via Balantak-Sula Fault, Matano-South Buru Fault. To the south the Palu-Koro Fault merges with the Lawanopo Fault. Kolaka Fault and Kabaena Fault (Simandjuntak, 1993a)..
Simandjuntak (1993a) defined that the Batui Thrust is surface expression of the collision zone between Banggai-Sula Platform against Eastern Sulawesi Opiolite Belt in Neogene time. The thrust bounds the ophiolite belt in the hanging wall from the micro-continents in the foot wall regims. The thrust can be obsrved clearly on the landsat imagery of the region (Hamilton, 1979). The thrust strechts from Balantak in the eastern tip of the East Arm of Sulawesi to the SW in Morowali, Tomori Bay. The thrust is disrupted and cut across by a number strike-slip fault, Toili Fault, Ampana Fault and Wekuli Fault. Its continuation further to the south in central, Southeast Arm, Buton and Kabaena Islands seems to have been greatly disrupted and modified by post-collision faults and hence it can not be traced as a continuous thrust zone. Seismicity shows that at present the thrust might be reactivated (McCaffrey et al, 1983; Kertapati et al, 1992). The occurrence of at least three terraces of Quaternary coraline reefs along the southern coast of the East Arm of Sulawesi also testifies the recent reactivation of the thrust (Simandjuntak, 1986, 1993a).
Poso Thrust is defined as structural contact zone between the Central Sulawesi Metamorphic Belt (CSMB) and the Western Sulawesi Magmatic Belt (Bemelen, 1949; Hamilton, 1979; Simandjuntak et al, 1991; Simandjuntak et al, 1992). The thrust is believed to have instrumented the up-thrusting of high pressure metamorphics (CSMB) from the depth in Benioff zone on to the top of magmatic belt in Neogene times. Seismicity suggests that at the present the thrust is no longer active (Kertapati et al, 1992). However, the recent earth quake in the west coast of Tomini Bay indicates that at least the northern portion of the thrust being reactivated.
The Walanae Fault is defined as a sinistral wrench faulting trending in NW-SE direction, cut across the South Arm of Sulawesi. The fault seems to be continued further to the northwest cut across Makassar Strait and merged with the Phaternoster-Lupar suture in Kalimantan and to the south is terminated in the Flores Thrust. In Quaternary the fault seems to have been reactivated transtensionally causing the development of Walanae Depression. Seismicity suggests that at the present the fault is no longer active or dying out.
NOTES ON THE MAKASSAR STRAIT
Katili (1978) suggested that the Makassar Strait was tectonically developed due to the rifting of the region with axis trending nearly N-S direction parallel to the long axis of the strait. Situmorang (1983), on the basis of seismic reflection profile across Makassar Strait found out that no a new developing oceanic crust beneath of the Tertiary sequences at the sea floor of the strait. He further suggested that the basement of the strait is more likely of continental crust. The occurrence of the Neogene fissured volcanics in and along the Lupar-Phaternoster suture and other parts in the interior of Kalimantan (Bergman et al, 1988; Harahap, 1996; Hutchison, 1996; Simandjuntak, 1999) and a similar sosonitic volcanics in wsetern South Arm of Sulawesi (Pryadi, 199 ) suggest the development of extensional tectonic in the region on Neogene times. The development of Makassar Strait more likely being related with the extensional tectonic occurring in many parts of central Indonesia in Neogene times.
TECTONIC DEVELOPMENT OF SULAWESI
The peculiar ‘K’ shaped of Sulawesi Island may indicates the complexity of geology and tectonics of the region. On the basis of data obtained on geology and geophysics Simandjuntak (1993) summarized the tectonic evolution of Sulawesi and its surroundings, which is related with the (re)occurrence of a number types of tectonism, including a) Cretaceous Cordileran type subduction, b) Mesozoic tectonic divergence, c) Neogene Tethyan type collision and d) Quaternary double opposing collision.
CRETACEOUS CORDILERAN TYPE SUBDUCTION
A Cretaceous Cordileran type subduction is recorded by the development of a west-dipping Benioff zone in and along western Sulawesi, inwhich the proto- Banda Sea crust subducted beneath south-southest margins of Sunda Shield (SE Eurasian Craton). The occurrence of Late Cretaceous high pressure metamorphic rocks in the Central Sulawesi Metamorphic Belt, the Cretaceous-Paleogene melange wedges associated with metamorphics and ophiolitic rocks, the Paleogene volcanics in the Westren Sulawesi Magmatic Belt and the ophiolites in the Eastern Sulawesi Ophiolite Belt are thought to have been developed during and subsequent to this subduction (Simandjuntak, 1980). The presence of Late Cretaceous-Paleogene flysch sediments associated with basaltic lavas may represent an upper trench slope sequences during this palte convergence.
MESOZOIC TECTONIC DIVERGENCE
Meanwhile, further to the south-southeast, subsequently after the Permo-Triassic thermal doming the northern continental margins of Australia were rifted due essentially to the extensional tectonic. The continental fragments, then were detached and displaced north-northwestwards to form the present micro-continents in the Banda Sea region (Pigram & Panggabean, 1984), including the Banggai-Sula Paltform, Tukangbesi-Buton Platform and Mekongga Platform (Simandjuntak, 1986), During the history of the detacheement and northwestwards displacement, the continental blocks were fragmented to form those micro-continents occurring in the Banda Sea region. And by the Neogene times, some of the micro-continents were collided with the subduction complex and ophiolite belt in the western margin of Banda Sea region. The tectonic divergence seems to be essentially dominated by a transcurrent-transformal displacement along the line of Sorong Fault System together with its splays of steep faults in the region (Simandjuntak, 1986, 1993).
NEOGENE TETHYAN TYPE COLLISION
The north-northwestwards moving continental fragments (micro-continents) of Banggai-Sula Platform, Tukangbesi-Buton Platform and Mekongga Platform collided with the subduction complex (CSMB) and the ophiolite belt (ESOB) in Neogene times. This tectonic convergence is typically Tethyan collision inwhich the the platforms underplated the ophiolite belt and subduction complex. At present the collision zone is marked by the occurrence of Neogene melange wedges in places along the Batui Thrust in the East Arm of Sulawesi (Simandjuntak, 1986). The collision characteristically produced no volcanic arc and geometrically without the development of fore-arc and back-arc basinal setting (Simandjuntak, 1988). The end products of this collision is characteristicaaly marked by the obduct-ing (up-thrusting) of the ophiolite suite onto the margins of the micro-continents and the thrusting-up of the subduction complex (CSMB) over the Western Sulawesi magmatic arcs (Simandjuntak, 1991; Bergman et al, 1996). The Papua New Guinea Ophiolite Belt is also emplaced by an obduction tectonics (Davies, 1976). During the end and subsequent to this collision, the deposition of post-orogenic coarse clatics of mostly molasse type sediments took place in the Late Neogene times. The molasses are mostly marine, but partly are terrestrials as indicated by the occurrence of lensoidal lignites, which seems to have been acummulated in an isolated and fault-bounded graben like basins especially in the interior of Central Sulawesi. The marine molasse at least partly seem to have been deposited in a submarine fan environmental setting.
QUATERNARY DOUBLE OPPOSING COLLISION
At present an active volcanics in and along the Minahasa-Sangihe Volcanic Arc appears to have been initiated by the development of a double- opposing subduction in northern Sulawesi in Neogene and reactivated in Quaternary. The plate convergence is marked by the development of south-southeastwards-dipping subducted crust of Sulawesi Sea beneath the North Arm of Sulawesi couples with the westward-dipping subducted crust of Maluku Sea in the north with its southern continuation along the Batui Thrust, inwhich the Banggai-Sula Platform underpalted the Eastern Sulawesi Ophiolite Belt in the East Arm of Sulawesi (Simandjuntak, 1991). On the basis micro-seismicity analysis McCaffrey et al (1983) suggest that the southern collision might be (re) activated at the present time. The occurrence of at least three terraces of Quaternary reefal limestones in and along the southern coast of the East Arm of Sulawesi testifies the reactivati-on of thie plate convergence and the rapid uplifting of the region.