Basement Character and Basin Formation in Gorontalo Bay, Sulawesi, Indonesia: New Observations from the Togian Islands

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Basement character and basin formation in Gorontalo Bay, Sulawesi, Indonesia: new observations from the Togian Islands

M. A. COTTAM1*, R. HALL1, M. A. FORSTER2 & M. K. BOUDAGHER-FADEL3 1SE Asia Research Group, Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey, UK 2Department of Earth Sciences, Australian National University, Canberra, ACT, 0200, Australia 3Department of Earth Sciences, University College London, London, UK *Corresponding author (e-mail: [email protected])

Abstract: We present a new stratigraphy for the Togian Islands, Sulawesi, and interpret the age, character and evolution of Gorontalo Bay. At its western end the bay is underlain by continental crust. The central part is underlain by Eocene to Miocene oceanic and arc rocks, although the area south of the Togian Islands could have continental crust of the Banggai-Sula microcontinent thrust beneath this and the East Arm ophiolite. Gorontalo Bay was not a significant deep bathymetric feature before the Miocene. Field relationships indicate a latest Miocene to Pliocene age for inception of the basin. Medium-K to shoshonitic volcanism in the Togian Islands is not due to subduction but reflects crustal thinning and extension in the Pliocene and Pleistocene, causing the underlying mantle to rise, decompress and melt. Extension is continuing today and is probably the cause of volcanism at Una-Una. Volcanic activity migrated west with time and volcanic products have been offset by dextral strike-slip displacement along the Balantak Fault. Extension and subsidence was driven by rollback of the subduction hinge at the North Sulawesi Trench with a possible contribution due to flow of the lower crust.

Gorontalo Bay is one of the most enigmatic basins underlain by ophiolitic crust equivalent to the East in East Indonesia. It is relatively deep with water Arm ophiolite, situated in front of the North Arm depths up to 2000 m, and Hamilton (1979) showed volcanic arc that has been thrust south onto the up to ﬁve kilometres of sediment in its western Banggai-Sula microcontinent. depocentre. It is surrounded by land on three sides The Togian Islands, situated in the centre of and receives large volumes of sediment from Gorontalo Bay (Fig. 1), offer a unique opportunity nearby mountains up to three kilometres high. to investigate aspects of the basin’s origin and evol- Miocene carbonates are widespread in these areas ution. The archipelago forms a broadly WSW–ENE (van Leeuwen & Muhardjo 2005) and suggest that trending ridge that continues to the west as a sub- the deep basin formed since their deposition but marine feature. Geological maps of the islands the timing and mechanism of basin inception show igneous rocks and contrasting interpretations remain unclear. of them. Ku¨ndig (1956) reported andesitic intrusive The nature and age of the crust beneath rocks in the central islands, and older ophiolitic Gorontalo Bay is also unknown. To the north, the rocks in the eastern islands – suggesting a possible North Arm of Sulawesi is interpreted as a volcanic link to the East Sulawesi Ophiolite. In contrast, arc built on Eocene oceanic crust (Taylor & van Rusmana et al. (1982) reported widespread tuffs Leeuwen 1980; Elburg et al. 2003; van Leeuwen and sedimentary formations of Mio-Pliocene age. & Muhardjo 2005). In contrast, at the western end The volcanic rocks could therefore be part of the of Gorontalo Bay, there are two kilometre high ophiolite, could form part of the North Arm volcanic mountains with young metamorphic ages and evi- arc, or could be subduction-related products that dence of continental crust, Miocene extension and predate the collision (Garrard et al. 1988; Davies core complex formation (Sukamto 1973; Elburg 1990) of the Banggai-Sula microcontinent with the et al. 2003; van Leeuwen et al. 2007). To the south, East Arm. the East Arm of Sulawesi comprises ophiolitic rocks The Togian Islands are also close to the isolated of the East Sulawesi Ophiolite (Simandjuntak 1986; active volcano of Una-Una, just NW of the Togian Monnier et al. 1995; Bergman et al. 1996; Parkinson archipelago, which has a K-rich chemistry and 1998; Kadarusman et al. 2004). Silver et al. (1983b) erupted violently in 1983 (Katili et al. 1963; Katili suggested that Gorontalo Bay was a fore-arc basin, & Sudradjat 1984). It is not a typical subduction

From:Hall, R., Cottam,M.A.&Wilson, M. E. J. (eds) The SE Asian Gateway: History and Tectonics of the Australia–Asia Collision. Geological Society, London, Special Publications, 355, 177–202. DOI: 10.1144/SP355.9 0305-8719/11/$15.00 # The Geological Society of London 2011. 178 M. A. COTTAM ET AL.

Fig. 1. Tectonostratigraphic provinces of Sulawesi. Modified after Hall & Wilson (2000), Calvert (2000) and van Leeuwen & Muhardjo (2005). volcano in position (about 200 km above the previous authors in separating this ‘terrane’ in to Benioff zone) and, if related to this subduction, is two different entities based on the recognition of unusual in being the only volcano. significant differences in age and character of We present a stratigraphy for the Togian Islands rocks (e.g. Taylor & van Leeuwen 1980; Calvert based on new field observations and dating. In many 2000; Elburg et al. 2003). We adopt the terms cases dating was restricted by the intense tropical Western Sulawesi Province and Northern Sulawesi weathering typical of SE Asia, and/or a lack of Province (e.g. van Leeuwen et al. 2007; see datable material. We combine these new data with Fig. 1). The position of the boundary between these earlier studies and observations of the physio- provinces remains uncertain (Elburg et al. 2003). graphy, bathymetry and seismicity of the northern Sulawesi region, to elucidate the Cenozoic history Western Sulawesi Province of Gorontalo Bay. The Western Sulawesi Province (Fig. 1) represents a continental margin segment (van Leeuwen et al. Tectonic setting 2007). It has a metamorphic basement that includes the Malino and Palu Metamorphic Complexes, Sulawesi comprises a complex association of mag- exposed at the NW and SW corners of Gorontalo matic arcs, metamorphic rocks (varying in grade Bay respectively (Elburg et al. 2003; van Leeuwen from low to high), ophiolites and microcontinental et al. 2007). These rocks form part of an arcuate fragments that have been variously assembled and zone of dismembered accretionary complexes deformed during the Late Mesozoic and Cenozoic (Parkinson 1998) and continental fragments, meta- (e.g. Audley-Charles 1974; Hamilton 1979; Hall morphosed in the mid-Cretaceous during emplace- 2002). It has been subdivided into four tectonostra- ment along the SE margin of Sundaland by NW tigraphic terranes separated by major faults (e.g. directed subduction (Parkinson 1998). Hamilton 1979). The composition of the terranes The basement is overlain by weakly metamor- surrounding the study area is described below. phosed Upper Cretaceous sedimentary rocks of the Following recent studies (e.g. Calvert 2000; van Latimojong Formation, which are in turn overlain Leeuwen & Muhardjo 2005; van Leeuwen et al. by a sequence of weakly metamorphosed Palaeo- 2007) we do not use the term Western Sulawesi gene sedimentary rocks and subordinate volcanic Plutono-Volcanic Arc Terrane. Instead, we follow rocks belonging to the ‘Older Series’ of Elburg TOGIAN ISLANDS AND GORONTALO BAY 179 et al. (2003). The exact nature of the contact epiclastic rocks of the calc-alkaline ‘CA Suite’, (depositional or faulted) is not known. Close to the and associated Early–Middle Miocene marine sedi- study area these rocks include the Tinombo For- ments (Elburg et al. 2003). mation (Brouwer et al. 1947), fore-arc basin sediments characterized by a transition from syn-rift East Sulawesi Ophiolite sedimentation to platform carbonates and deeper marine sedimentation between the Late Eocene The East Sulawesi Ophiolite (Fig. 1) comprises a and Middle Miocene (Coffield et al. 1993; Wilson sequence of dunite, lherzolites and harzburgites, & Bosence 1996; Calvert 2000). The contempora- ultramafic cumulates, layered gabbros, isotropic neous Tinombo Formation volcanic rocks (c.51to gabbros, sheeted dykes and basaltic pillows and 17 Ma) range from basalt to rhyolite and include lavas (e.g. Simandjuntak 1986; Parkinson 1991, dykes, volcanic piles and co-magmatic intrusive 1998). Field mapping (Kadarusman et al. 2004) stocks (Elburg et al. 2003). and geophysical studies (Silver et al. 1978) sug- Intrusive and extrusive rocks of the ‘Younger gest an abnormally large reconstructed strati- Series’ (Elburg et al. 2003) include an acidic graphic thickness of at least 15 km. The origin of high-K calc-alkaline (CAK) suite of plutons the East Sulawesi Ophiolite has been variously (Kavalieris et al. 1992) and comagmatic volcanic attributed to a typical mid-oceanic ridge (e.g. rocks (van Leeuwen et al. 1994; Elburg et al. Soeria-Atmadja et al. 1974; Simandjuntak 1986), 2003), and a high-K calc-alkaline, shoshonitic and supra-subduction zone (Monnier et al. 1995; ultra-potassic alkaline (HK) suite of dykes, small Bergman et al. 1996; Parkinson 1998) and oceanic stocks and less common extrusive rocks (Elburg plateau settings (Kadarusman et al. 2004). K–Ar et al. 2003). dating of the ophiolite ranges in age from Cretac- eous to Eocene (Simandjuntak 1986). They are Northern Sulawesi Province interpreted to reflect Cretaceous, specifically Cenomanian, ocean floor with younger seamounts The Northern Sulawesi Province (Fig. 1) comprises (Simandjuntak 1986). K–Ar dating (Parkinson a dominantly tholeiitic Tertiary volcanic arc built 1998) has been interpreted to suggest intra-oceanic on Eocene oceanic crust (Taylor & van Leeuwen thrusting of the ophiolite at c. 30 Ma. 1980; Elburg et al. 2003; van Leeuwen & Muhardjo 2005). Volcanism was driven by the northward sub- Microcontinental fragments duction of Indian Ocean lithosphere beneath the North Arm (e.g. Hall 1996, 2002; Rangin et al. The Banggai-Sula block (Fig. 1) has a basement of 1997). The Papayato Volcanic rocks are the pro- Palaeozoic or older metamorphic rocks intruded ducts of this arc, a bimodal suite of mafic and by Permo-Triassic granites associated with acid vol- felsic volcanic rocks cut by co-magmatic stocks of canic rocks. These rocks are overlain by undated, gabbro and diorite (Trail et al. 1974; Kavalieris probably Lower Jurassic, terrestrial sediments and et al. 1992; van Leeuwen et al. 1994; Elburg et al. by Jurassic and Cretaceous marine shales and lime- 2003) belonging to the ‘Older Series’ of Elburg stones. In the western parts of the islands there are et al. (2003). Limited isotopic and palaeontological Eocene to Neogene limestones (Garrard et al. ages suggest a Middle Eocene to earliest Miocene 1988; Supandjono & Haryono 1993; Surono & age (van Leeuwen et al. 2007) making them the Sukarna 1993). broad age equivalent of the Tinombo Formation in The block is a continental fragment derived from the Western Sulawesi Province. However, contrast- northern Australia (e.g. Audley-Charles et al. 1972; ing volcanic–sedimentary proportions suggest that Hamilton 1979; Pigram et al. 1985) which collided they were formed in different tectonic environments with a subduction margin represented by the ophio- (van Leeuwen et al. 2007). lites and associated rocks of East Sulawesi. The Papayato Volcanic rocks are overlain by a Hamilton (1979) suggested it was sliced from New thick series of Neogene volcanic rocks and volcani- Guinea and carried westward along a strand of the clastics of calc-alkaline composition and cut by Sorong Fault system and this view has become co-magmatic intrusives (‘CA Suite’ of Polve´ et al. widely accepted and incorporated in many tectonic 1997), which are accompanied by marine sedimen- models (e.g. Pigram et al. 1985; Garrard et al. tary rocks (Kavalieris et al. 1992) that include well- 1988; de Smet 1989; Daly et al. 1991; Smith & bedded shallow marine sediments and limestones of Silver 1991; Hall et al. 1995; Hall 1996, 2002). Early to Middle Miocene age (e.g. Sukamto 1973; The collision is generally thought to have occurred Norvick & Pile 1976; Ratman 1976). All these in the Neogene (Simandjuntak & Barber 1996) but rocks are cut by Neogene volcanic rocks belonging a wide range of ages has been suggested includ- to the ‘Younger Series’ (Elburg et al. 2003). They ing Late Oligocene or Early Miocene (Milsom include andesitic and dacitic stocks, dykes and et al. 2001), within the Miocene (Hamilton 1979), 180 M. A. COTTAM ET AL.

Early to Middle Miocene (Bergman et al. 1996), have collided with East Sulawesi or that a single Middle Miocene (Sukamto & Simandjuntak 1983; large microcontinent may have been fragmented Simandjuntak 1986), Middle Miocene to Pliocene during oblique collision. (Garrard et al. 1988) and Late Miocene (Silver Recent work has cast doubt on the existence of a et al. 1983b; Davies 1990; Smith & Silver 1991; subduction-related volcanic arc in West Sulawesi Parkinson 1998). Buton–Tukang Besi has been during most of the Palaeogene and Neogene suggested to be another microcontinental fragment (Polve´ et al. 1997; Elburg et al. 2003). There is (Hamilton 1979) that collided in the Early or also little evidence for a collision that affected Middle Miocene (Fortuin et al. 1990; Smith & West Sulawesi (Hall & Wilson 2000; Calvert & Silver 1991), after strike-slip faulting sliced it from Hall 2007), and it is now known that the North New Guinea. Banda basin formed by oceanic spreading during Although these microcontinents are small, their the Middle Miocene (Hinschberger et al. 2000). collisions are often interpreted to be responsible Spakman & Hall (2010) have proposed a tectonic for widespread deformation in Sulawesi and model for the Banda and Sulawesi region that recon- Borneo. Westward thrusting of the central Sulawesi ciles these and other observations with earlier metamorphic belt, a foreland fold and thrust belt in interpretations, and offers an alternative to the pre- west Sulawesi, deformation in the Makassar Straits, viously accepted idea of slicing of continental deformation in the Meratus Mountains, and inver- slivers from New Guinea. There was an Early sion in the Kutei basin have been attributed to the Miocene collision of the Sula Spur with the North collision (e.g. van de Weerd & Armin 1992; Arm volcanic arc and East Arm ophiolite, and this Coffield et al. 1993; Simandjuntak & Barber 1996; continental area was then fragmented during exten- Pubellier et al. 1999; McClay et al. 2000). Many sion caused by subduction rollback into the Banda authors suggest the collision, or collisions, followed embayment. westward subduction of ocean lithosphere (e.g. Garrard et al. 1988) interpreted to have produced a magmatic arc in West Sulawesi (e.g. Hamilton Celebes Molasse 1979; Parkinson 1991) or alternatively post- Pre-Miocene rocks of the different provinces are collisional magmatism (e.g. Bergman et al. 1996; unconformably overlain by the Celebes Molasse – Polve´ et al. 1997; Elburg et al. 2003). a weakly to moderately consolidated association The age of collision is difficult to determine and of interbedded sedimentary formations that is wide- could vary within Sulawesi. It requires dating of ter- spread across Sulawesi (Sarasin & Sarasin 1901; restrial clastic rocks (‘Celebes Molasse’) that rest van Bemmelen 1949). Sediments include conglom- unconformably on deformed sedimentary, meta- erate, quartz sandstone, greywacke and mudstone morphic and ophiolitic rocks. In the East Arm with subordinate intercalations of breccia, marl Umbgrove (1938) reported a Lower Miocene and coral limestone (e.g. van Bemmelen 1949; van unconformity, Brouwer et al. (1947) recorded iso- Leeuwen et al. 2007). They have been interpreted clinal folding of Early to Middle Miocene age, and to reflect deposition in a coastal alluvial plain Ku¨ndig (1956) interpreted a Middle Miocene oro- environment situated along the flanks of rapidly genic phase followed by molasse sedimentation uplifting and eroding mountains (Calvert 2000). and later Pliocene folding. Hamilton (1979) The Celebes Molasse was originally interpreted to reported that ‘lower Miocene strata are fully relate to a single Miocene collision (Ku¨ndig involved in the imbrication and upper Miocene 1956). More recently it has been suggested to be clastic rocks were derived from the thrust belt’. diachronous across Sulawesi, representing several Other authors have reported Middle Miocene tectonic events (Hall & Wilson 2000). Within folding and thrusting (e.g. Audley-Charles et al. West Sulawesi and the East Arm it is interpreted 1972; Audley-Charles 1974; Katili 1978; Parkinson to represent latest Miocene to Plio-Pleistocene 1991). Surono (1995) suggested that conglomerates uplift and erosion (Hall & Wilson 2000). from the SE Arm are the oldest Lower to Middle Miocene parts of the Langkowala Formation which rests unconformably upon the ophiolite. In Buton, Stratigraphic observations Smith & Silver (1991) interpreted a deformed complex including Upper Eocene or Lower Oligo- We present a new stratigraphy (Fig. 2) for the cene pelagic limestones to be overlain by Lower western, central and eastern Togian Islands Miocene conglomerates, but because of the lack of (Fig. 3). Based on new field observations and lab- ophiolite detritus interpreted the conglomerates to oratory analyses, we define three new units, the be the product of erosion associated with slicing of Walea Formation, Peladan Formation and Benteng the block from New Guinea rather than collision. Intrusives, and integrate them with the previously They suggested that separate microcontinents may recognized Lamusa Formation (Rusmana et al. TOGIAN ISLANDS AND GORONTALO BAY 181

Fig. 2. Schematic Neogene stratigraphy of the western, central and eastern Togian Islands, incorporating the age ranges derived in this study. Age (Ma) from Gradstein et al. (2004); PZ, Planktonic Foraminiferal biozones from BouDagher-Fadel (2008); LS, Far East Letter Stages from BouDagher-Fadel (2008). Note that the timescale is not linear.

1982, 1993), Bongka Formation (Rusmana et al. (Rusmana et al. 1982, 1993) and Luwuk Formation 1993) of the Celebes Molasse (Sarasin & Sarasin (Garrard et al. 1988). Our new stratigraphy ranges 1901; van Bemmelen 1949), Lonsio Formation in age from possible Mesozoic basement rocks 182 .A COTTAM A. M. TAL. ET

Fig. 3. Simplified geological map of the Togian Archipelago, modified from Rusmana et al. (1982, 1993) based on new field observations. Island names in bold italics; population centres in regular. Open circles and/or underlined dip measurements indicate locations examined in this study. Other structural information from Rusmana et al. (1982, 1993). Arrows and bold numbers (all prefixed RTG-) highlight the location of samples explicitly discussed in the text, for which GPS locations (decimal degrees) are listed in the inset table. TOGIAN ISLANDS AND GORONTALO BAY 183 through to Quaternary deposits (Fig. 2). The most Volcanic rocks occur along the west coast of complete section is seen in the eastern islands the southern peninsula of Walea Bahi and in coastal (Walea Kodi and Walea Bahi; Fig. 3) where base- outcrops along the channel between Walea Bahi ment rocks, possibly of Eocene to Oligocene age, and Walea Kodi. They include breccias, pillow are overlain by Middle Miocene, Pliocene and lavas and more massive and layered lavas of basal- Quaternary strata. The central and western islands tic to andesitic composition. The rocks have a fine- expose more restricted sections dominated by Plio- grained groundmass of feldspar, pyroxene and cene volcaniclastics and clastics respectively. altered olivine + phenocrysts of plagioclase feldspar + amygdales (up to 1 cm) of zeolite and/ Lamusa formation or calcite. Large blocks (1 0.6 m) of breccia are exposed in the beach along the west coast of the Indurated sedimentary rocks of different types occur southern peninsula of Walea Bahi. There are large, in several small exposures at the southern end of the sub-rounded, clasts of dark grey (c. 10 cm) and channel between the islands of Batu Daka and green (c. 6 cm) material within a light grey matrix. Togian (Fig. 3). The rocks are weakly bedded and The clasts have within them feldspar phenocrysts dip to the north. Lithologies include calcareous and amygdales of low-grade epidote-rich alteration sandstones, interbedded with non-calcareous sand- products. Further north, pillows are exposed in stones and dark mudstones, and dark, fine-grained several outcrops along the channel between Walea recrystallized limestones. They are heavily brec- Bahi and Walea Kodi, often forming small head- ciated and crushed. All lithologies are cut by small lands. Pillows are grey greenish in colour, weather- extensional faults. No fossils or sedimentary struc- ing to grey brown. In most places they are heavily tures were identified. The formation has a min- weathered and altered with late-stage alteration imum thickness of 3 m, but neither the top, nor the along fractures. Where relatively fresh, pillows base was seen. Following Rusmana et al. (1993) show spectacular teardrop shapes (around 30 cm we assign these rocks to the Mesozoic Lamusa across), picked out by dark, glassy chilled rims of Formation. Their highly indurated and veined char- between 0.5 and 3 cm and fine-grained interpillow acter is consistent with the Mesozoic age suggested material (Fig. 4b), which provide right way-up by Rusmana et al. (1993) and suggests that they may criteria. Pillows contain abundant zeolite and/or form part of the basement of the Togian Islands. calcite amygdales up to 1 cm in size; chilled rims contain small (5 mm) amygdales and alter to rusty Walea formation coloured skins where weathered. More massive, layered lavas are also present; individual flows are Arc-related volcanic and volcaniclastic rocks are marked by craggy tops and brecciated areas. observed in exposures along the western coast of Rusmana et al. (1982) reported similar pillow Walea Bahi and eastern coast of Walea Kodi. lavas, breccias, conglomerates and sandstones They include volcanic breccias, pillow lavas and from Poh Head (Fig. 1), at the east end of the East arc-derived volcanogenic sediments. Well-bedded Arm, and within the eastern Togian Islands, assign- volcanogenic sedimentary rocks are exposed as a ing them both to the Miocene Malik Formation. large, possibly fallen, block on the west coast of Simandjuntak (1986) assigned basaltic rocks from the southern peninsula of Walea Bahi. Medium- Poh Head to the basalt zone of the Balantak Ophio- grained, feldspar-rich, grey-brown beds are inter- lite, and suggested a Late Cenomanian to Eocene bedded with green and blue-grey units with a fine- age based on K–Ar ages. In a later revision, grained green matrix on a scale of c. 5 cm. All Rusmana et al. (1993) assigned these rocks to the show internal stratification and possible grading. Cretaceous Mafic Complex, whilst those in the Further north, just south of a large coastal embay- eastern Togian Islands were reassigned to the Mio- ment, a larger outcrop exposes an in-situ section Pliocene Lonsio Formation (see below). of gently dipping (19–298 to the east) volcanogenic Based on new observations we assign the basal- sediments (Fig. 4a) including interbedded sands tic lavas and volcanogenic sedimentary rocks of and silts, some of which are calcareous. Mostly the Togian Islands to the Walea Formation, a new beds are laterally persistent with normal grading, formation named from the type localities on the parallel and cross-lamination and ripple cross islands of Walea Bahi and Walea Kodi. Neither bedding. Bedding parallel bioturbation and water the top, nor the base, of the Walea Formation is escape structures are evident in the more sandy observed but there is a minimum thickness of 5 m layers. Finer-grained siltstones dominate the upper of pillows and 7 m of volcanogenic sediments. part of the exposed sequence. The volcanogenic The total thickness of the formation is probably sedimentary rocks are interpreted to have been much greater. The exact age of the Walea Formation deposited as turbidites and debrites in a deepwater is unknown, but it is the stratigraphically oldest and arc-related setting. structurally lowest unit seen in the eastern islands. 184 M. A. COTTAM ET AL.

a b

30 cm

20 cm

20 cm ~2 m

Fig. 4. Field photographs of the Walea Formation and Lonsio Formation. (a) Arc-related (?) volcaniclastic sediments of the Walea Formation. (b) Basaltic pillows of the Walea Formation exposed on the west coast of Walea Bahi. (c) Well-bedded tuffs of the Lonsio Formation. Coarser tuff units (centre of image beneath pen) show rough stratiﬁcation, dewatering and cross bedding. Finer tuff units (upper and lower sections of photograph) are more massive, have irregular bases and show an increase in joint density towards the upper boundary (lower section of photograph). (d) Syn-sedimentary folding and faulting within the Lonsio Formation.

Volcanic arc sedimentary rocks have not been Peladan formation reported from the East Arm ophiolite. Their association with basaltic lavas is more similar to the oldest Hard, indurated limestones occur on (at least two) rocks known from the North Arm which formed in small islands situated around 250 m off the central an intra-oceanic arc between the Middle Eocene west coast of Walea Bahi. Lithologies include and earliest Miocene. micritic wackestones and packstones with TOGIAN ISLANDS AND GORONTALO BAY 185 planktonic and benthic foraminifera and fine- c.308 dips to the NE/NNE) are seen in a more grained volcanogenic material (Table 1). These out- extensive cliff outcrop on a small island east of crops define the type section for the new Peladan the village of Katupat. Lithologies at this location Formation. Benthic and planktonic foraminifera include laminated siltstones and sandstones, and indicate shallow inner platform or fore-reef shelf pebble conglomerates containing well-rounded peb- and deeper inner platform environments (Table 1). bles (up to 2 cm) dominated by ophiolitic material The sequence is well-bedded on a decimetre scale, (basalts, dolerites, gabbros and serpentinite) with up to a maximum of around 1 m (mode c. 30 cm) some chert and limestones. The silts and sands and dips gently towards the north. The sequence contain abundant, highly oxidized, plant mate- has a minimum observed stratigraphic thickness of rial. The formation has a minimum thickness, as around 12 m. The top and base of the sequence is observed in outcrop, of 15 m but neither the top not seen and the true thickness may be much nor the base of the unit is seen. greater. No other structure (folding/faulting) was Similar deposits, but coarser still in grain size, observed. In places the beds have a rubbly texture were observed in roadside outcrops on the northern interpreted to reflect re-working of components coast of the East Arm of Sulawesi, west of the town prior to deposition. Some thin (c. 10 cm), finer of Bunta (Rusmana et al. 1982, 1993; this study). grained horizons appear not to have been reworked. Here they comprise coarse, massive, sandstones In places the limestone are partially dolomitized. with pebble-rich horizons that include large clasts Early–Middle Miocene limestones of a similar (up to 3 cm) of red chert and cobbles (up to age and character are reported from the North and 15 cm) of basalts, dolerites, gabbros, metagabbros East Arms of Sulawesi (e.g. Sukamto 1973; Norvick and serpentinite with some limestones. Again, the & Pile 1976; Rusmana et al. 1982; Garrard et al. sequence dips north at moderate angles of c.308. 1988; van Leeuwen & Muhardjo 2005). In the We observed a minimum stratigraphic thickness of Togian Islands Rusmana et al. (1982) previously around 20 m, although neither the top nor the base assigned these rocks to the Salodik Formation and of the unit was seen. Based on strong lithological suggested a Late Paleocene to Early Miocene age. and compositional similarities between these rocks Later, Rusmana et al. (1993) reassigned them to and those within the Togian Islands, we follow the Lonsio Formation tuffaceous units. Rusmana et al. (1993) in assigning all of these Micropalaeontological analyses of larger fora- rocks to the Bongka Formation of the Celebes minifera and planktonic foraminifera were per- Molasse. In northern Sulawesi palaeontological formed on five samples of the Peladan Formation dating of the Celebes Molasse suggests a Late (Table 1). Nannofossil dating was not attempted. Early Pliocene to Mid Pleistocene age (Norvick & We correlate the standard Planktonic Foraminiferal Pile 1976; Ratman 1976; Hadiwijoyo et al. 1993; biozones (PZ) with the ‘Letter Stages’ (LS) of the Chamberlain & Seago 1995). Late Miocene– Far East (as defined by BouDagher-Fadel 2008), Pliocene ages have been reported for the East Arm relative to the geological timescale of Gradstein (Surono & Sukarna 1996). et al. (2004). Analyses indicate a late Middle The Celebes Molasse has been interpreted as Miocene age (PZ: Late N12 – Early N17; LS of alluvial fan and coastal fan delta deposits that the Far East: Late Tf2 – Early Tg). Based on their reflect the deposition of locally sourced sediment lithology and age we assign these rocks to the new in alluvial plain environments with a marginal Peladan Formation, named for one of the two marine influence (Calvert 2000). In contrast, the islands on which they were observed. ophiolitic material observed in the Togian Islands has no local source, and such material can only Bongka formation (Celebes Molasse) have been derived from the East Arm Ophiolite. Based on the relative grain size and shared structural Weakly to moderately consolidated interbedded characteristics (gentle north dip), we suggest that sediments with characteristic lithic-rich horizons outcrops of the Bongka Formation within the East occur in heavily weathered outcrops along the chan- Arm and the Togian Islands represent proximal nel between the islands of Batu Daka and Togian. (coarser) and distal (finer) alluvial fan deposits They are sub-rounded, green-brown, medium- respectively, both having been transported north grained sandstones with bands of coarser, angular from the interior of the East Arm. lithic fragments, medium-grained sandstones with a slabby, bedded character, and brecciated material Lonsio formation with possible ultrabasic content. Petrographical analyses reveal a matrix of serpentinite-rich mate- Volcaniclastic rocks are extensively exposed in rial. The sequence dips moderately (c.308) to the coastal outcrops on the northern peninsula of Tala north. Sediments with a coarser grain size, but of Teoh, the north coast of Togian and the west coast comparable composition and structure (moderate of Walea Kodi. They are grain-supported rocks Table 1. Biostratigraphical age, facies and palaeoenvironmental analyses of the Peladan formation 186

Sample ID Depositional Microfacies Components Age (PZ/LS)* (based environment on ﬁrst appearance)

RTG 18 A Shallow inner Micritic packstone of planktonic and Benthic foraminifera: Cycloclypeus indopaciﬁc, Katacycloclypeus Late N12/Late Tf2 platform/ benthic foraminifera. Micritic martini, Amphistegina spp., Cycloclypeus pillaris, Cycloclypeus fore-reef patches reworked into the matrix. spp., Sphaerogypsina spp., Lepidocyclina spp., Lepidocyclina shelf (Nephrolepidina) spp., L.(Nephrolepidina) angulosa Planktonic foraminifera: Sphaeroidinellopsis spp., Globorotalia praemenardii, Globigerinoides spp., Globorotalia peripheroacuta, Globorotalia praefohsi, Globoquadrina altispira, Planorbulinella solida Globoquadrina spp., Globoquadrina dehiscens, Echinoid spp., fragments of rodophyte algae. RTG 18 B Shallow inner Micritic packstone of larger benthic Benthic foraminifera: Cycloclypeus spp., Cycloclypeus pillaria, Late N12/Late Tf2 platform/ foraminifera Cycloclypeus carpenteri, Amphistegina spp., Discogypsina discus. fore-reef Textularia spp., Carpenteria spp., Katacycloclypeus annulatus, shelf Planorbulinella spp. Planktonic foraminifera: Dentoglobigerina COTTAM A. M. altispira, Globigerinoides primordius, Globigerina spp., Globigerinoides quadrilobatus, Orbulina suturalis, Globorotalia praemenardii, Echinoid spp., fragments of rodophyte algae and corals, Gastropods, fragments of bryozoa. RTG 18 C Shallow inner Micritic packstone of recrystallized Benthic foraminifera: Cycloclypeus spp., Amphistegina spp., N12 and younger/Tf2

platform/ algae and benthic foraminifera. Textularia spp., Miliolid spp., Sphaerogypsina spp. Planktonic and younger AL. ET fore-reef Micritic patches reworked into the foraminifera: Globigerinoides quadrilobatus, Orbulina spp., shelf matrix. Globigerinoides spp., Globorotalia menardii, fragments of rodophyte algae, Lithophyllum spp., Lithothamnium spp., Gastropods, Echinoid spp., rare fragments of bryozoa. RTG 18 D Shallow inner Micritic packstone of foraminifera Benthic foraminifera: Cycloclypeus pillaria, Planorbulinella solida, Late N12 – Early platform/ and algae. Gypsina spp., Sphaerogypsina spp., Elphidium spp., Nodosaria N17/Tf3 – Early fore-reef spp. Planktonic foraminifera: Globoquadrina spp., Tg shelf Globigerinoides trilobus, Globigerinoides spp., Orbulina suturalis, Globorotalia conoidea, Globorotalia menardii, Globorotalia scitula, Gastropod spp., fragments of bryozoa, fragments of coral. RTG 18E Relatively Micritic wackestone of foraminifera. Benthic Foraminifera: Lepidocyclina spp., Carpenteria spp., Late N12 – Early deeper inner Reworked patches of micrite are Cycloclypeus spp., Cycloclypeus pillaria, Operculina spp., N13/Late Tf2 – platform also present. Heterostegina spp., Gypsina spp., Planorbulinella larvata, Lagena Early Tf3 spp., Textularia spp. Planktonic foraminifera: Globoquadrina altispira, Globorotalia spp., Globorotalia scitula, Globoquadrina dehiscens, Globorotalia menardii, Globoquadrina dehiscens, Globorotalia fohsi, Ostracod spp., Gastropod spp.

*We correlate the standard Planktonic Foraminiferal biozones (PZ) with the ‘Letter Stages’ (LS) of the Far East (as defined by BouDagher-Fadel 2008), relative to the biostratigraphical timescale (as defined by Gradstein et al. 2004) TOGIAN ISLANDS AND GORONTALO BAY 187 with a carbonate (dominantly sparry) matrix. Micro- single eruption or input from several eruptions. fossils and algal fragments are also embedded Dewatering structures within coarse units suggest within the matrix; their abundance varies between rapid loading by the subsequent fine units. Jointing units (Table 2). present near the upper boundary of the finer units The volcaniclastics are well-bedded, commonly may be syn- or post-depositional. at the decimetre scale, with a maximum bed thick- Based on their striking similarity to tuffaceous ness of around 3 m. Two main bed types alternate units observed on Poh Head (Simandjuntak 1986; at a range of scales. Individual beds appear laterally A. J. Barber, pers. comm. 2009) we assign these persistent at the outcrop scale. Stratified beds are rocks to the Lonsio Formation of Rusmana et al. typically around 10 to 30 cm in thickness and (1982, 1993). Micropalaeontological analyses of show parallel lamination of fine to coarse sand. larger foraminifera and planktonic foraminifera They contain rare horizons of small (up to fine were performed on five tuff samples from the pebbles) angular lithic fragments (Fig. 4c). In Lonsio Formation (Table 2). Nannofossil dating places the beds show spectacular dewatering struc- was not attempted. Foraminiferal assemblages tures, and may be wholly or partly cross-bedded, range from N4 and younger (PZ) and Te and producing an irregular upper surface. The base of younger (LS), and constrain a Late Miocene to the beds is almost universally planar. Stratified Early Pliocene age (PZ: N19; LS: Early Th). units are overlain by fine-grained cream coloured material, which show little variation in grain size Benteng Intrusives or internal structure (Fig. 4c). Beds range in thickness from cm scale to a maximum of 3 m. Their Intrusive rocks of intermediate composition are bases are commonly irregular, reflecting the topo- exposed in isolated outcrops, along the northern graphy of the stratified layer below, and they and southern coasts of Togian Island. They occur display an increase in joint density towards the as small intrusions, often forming topographic upper boundary, which is characteristically planar. highs and small islands. We infer the presence of In places the finer beds may be very thin, or entirely additional intrusive bodies within the interior of absent from the sequence. Overall, the sequence Togian Island based on the presence of isolated dips gently in various directions. Locally, the steep-sided topographic highs visible from the rocks dip steeply and show intense syn-sedimentary coast as shown on the map of Ku¨ndig (1956). The folding and faulting (Fig. 4d), interpreted to rocks have a fine to medium grained light-grey reflect soft sediment deformation. The sequence groundmass with phenocrysts of phlogopite mica has a minimum stratigraphic thickness of around (up to 7 mm) + feldspar (6–7 mm) + hornblende 20 m, however, the top and base of the sequence (1–3 mm) + mafic xenoliths (up to 2 cm). In places is not seen and the true thickness is probably feldspar phenocrysts are concentrated into ‘trails’ much more. up to 20 cm long. Orthogonal sub-horizontal and Comparable volcaniclastic rocks are observed on sub-vertical joints spaced at around 20 to 50 cm, Poh Head, where they include thick sequences of and resulting in a characteristic blocky appearance, coarse stratified units (this study). Rusmana et al. suggest intrusion at shallow depths. In places the (1982, 1993) described these rocks as tuffaceous rocks are cut by east–west trending brittle faults, sediments and assigned them to the Lonsio For- producing breccia zones around 1 m wide. mation. Simandjuntak (1986) interpreted similar These rocks are classified (Fig. 5; Table 3) as volcanogenic sediments from the East Arm as trachydacites and trachyandesites on the total megacyclic turbidites, and assigned them to the alkalis v. silica (TAS) diagram of Le Maitre (1989) Lonsuit Turbidites of the Batui Group. We interpret (they are syenites on TAS diagrams adapted for plu- these rocks as tuffaceous sediments that reflect rapid tonic rocks (e.g. Wilson 1989)) and belong to the aqueous reworking of primary volcaniclastic mate- alkaline magma series (Kuno 1966; Irvine & rial during deposition in a shallow marine environ- Baragar 1971). They have an extremely K-rich ment soon after eruption. Microfossil observations chemistry and plot within the shoshonitic field of suggest depths less than 200 m. Stratification Rickwood (1989) on a K2O v. SiO2 diagram. Intru- reflects crude sorting of coarse ash during settling sive intermediate rocks were first recognized on through the water column; cross-bedding may reflect Togian Island by Ku¨ndig (1956), who identified turbidity currents formed by ash initially held in rocks of andesitic composition. These were sub- suspension. Finer-grained ash settled more slowly sequently misidentified as basaltic (Rusmana et al. through the water column, draping topography in 1982) or volcaniclastic (Rusmana et al. 1993) in the underlying coarse units. Pumice is largely character. We assign these intrusive rocks to the absent and may have been floated off and not pre- new Benteng Intrusives, named for the village of served (e.g. Freundt 2003). The repeated sequence the same name in south central Togian Island of coarse and fine tuff may reflect pulses within a (Fig. 3). 188

Table 2. Biostratigraphical age, facies and palaeoenvironmental analyses of the Lonsio formation

Sample ID Depositional environment Microfacies Components Age (PZ/LS)* (based on ﬁrst appearance)

RTG 25 Inner neritic, planktonic & shelf Sparitic packstone of volcanic Globoquadrina altispira, Globoquadrina spp., N19/Early Th benthic foraminifera drifted/ sediments rich in embedded Orbulina spp., Globorotalia margaritae, reworked into volcanic deposits. planktonic foraminifera and rare Globorotalia scitula, Sphaeroidinellopsis larger benthic and algae subdehiscens, Globigerinoides trilobus, fragments Globigerinoides quadrilobatus, Globorotalia acostaensis, Fragments of rodophyte algae,

Elphidium spp. COTTAM A. M. RTG 26 Inner neritic Sparitic packstone of volcanic Globigerinoides spp. N4 and younger/Upper sediments with rare embedded Te and younger planktonic foraminifera RTG 27 Sparitic packstone of volcanic Globigerinoides spp. N4 and younger/Upper sediments with rare embedded Te and younger planktonic foraminifera RTG 30 Inner neritic, planktonic & shelf Sparitic packstone of volcanic Catapsydrax spp., Orbulina universa, N19/Early Th AL. ET benthic foraminifera drifted/ sediments rich in embedded Globoquadrina dehiscens, Pulleniatina reworked into volcanic deposits. planktonic foraminifera and rare primalis, Globoquadrina altispira, larger benthic and algae Globorotalia globosa, Globorotalia fragments humerosa, Globorotalia mayeri, Globorotalia scitula, Globigerinoides sacculifer, Globigerinoides quadrilobatus, Elphidium spp., Amphistegina spp., Heterostegina spp., Asterigerina spp. RTG 36 Inner neritic Sparitic packstone of volcanic Orbulina universa, Globigerinoides spp., N4-N19/Upper Te – sediments with rare embedded Globigerinoides quadrilobatus, Early Th planktonic foraminifera Globoquadrina spp.

Five high-purity mica separates from four samples of the Benteng Intrusives were dated using 40Ar/39Ar techniques. Samples were crushed, graded using disposable nylon cloth sieves in a brass collar and separated using conventional elec- tromagnetic techniques. High-purity mineral separates were handpicked from the 63–250 mm fraction, and for RTG-12 from the .250 mm fraction, thus any contamination in the analyses is assumed to be due to intra-grain alteration and/or contaminants. All analyses were undertaken in the Argon Labora- tory of the Research School of Earth Sciences, The Australian National University, using the furnace step-heating technique (Table 4). Samples were irradiated at the McMaster Nuclear Reactor, McMaster University, Canada using Sanidine 92– 176 from Fish Canyon Tuff, Colorado (K/Ar reference age 28.10 + 0.04 Ma) as the Fluence Monitor (Spell & McDougall 2003). Ages were calculated using the 40K abundances and decay constants of Steiger & Ja¨ger (1977). Uncertainties in isotopic ratios and ages are quoted at the 1s level. For all samples plots of 36Ar/40Ar v. 39Ar/40Ar demonstrate the presence of one main gas population, with varying amounts of contaminants (such as excess argon), and a large atmospheric argon component – particularly in the coarser grained samples (Fig. 6). The oldest ages are preserved in the high-temperature heating steps of coarse- grained (.250 mm) biotite from samples RTG12 Fig. 5. Major element classification diagrams for the (2.40 + 0.01 Ma; MSWD (mean square of weighted volcanic rock samples analysed in this study. (a) Total deviation) 1.58) and RTG31 (2.02 + 0.01 Ma; alkalis (K2O þ Na2O) v. silica (SiO2) diagram. Field MSWD 0.01) (Fig. 6). However, significant atmos- boundaries are those of Le Maitre (1989): 1, andesite; 2, pheric argon contents, and evidence of argon loss dacite; 3, trachyandesite; 4, trachydacite. Subdivision and possible younger events render the meaning into alkaline and sub-alkaline series: dashed curved line of these ages ambiguous. Analysis of fine-grained – Irvine & Baragar (1971); solid curved line – Kuno (250–63 mm) biotite from sample RTG12 contains (1966). (b)K2O v. SiO2 diagram. Series boundaries and nomenclature: dashed lines and bold italics, Le Maitre significantly less atmospheric argon than the (1989); solid lines and nomenclature in parentheses, after coarser-grained biotite and produced a reliable, con- Rickwood (1989). sistently flat spectrum of 1.80 + 0.01 Ma (MSWD 3.95) (Fig. 6). This analysis provides the best age for this sample and the most robust age for the Table 3. Major element data (weight %) for samples Benteng Intrusives. Analyses of fine-grained mica of the Benteng Intrusives analysed in this study from two other samples gave robust Pleistocene ages. Despite disturbance during the initial heat- Sample ID RTG08 RTG09 RTG12 RTG31 ing steps (linked to variation in Ca), over 50% of the gas emitted from RTG08 produced a strong SiO2 63.36 63.14 58.97 61.39 plateau with an age of 1.52 + 0.02 Ma (MSWD Al2O3 15.52 15.43 15.85 14.54 0.3) (Fig. 6). Except for several contaminated inter- Fe2O3 3.92 3.77 4.09 5.82 vening steps, analysis of fine-grained biotite from MgO 2.25 2.37 2.97 2.24 RTG09 would have produced a similar plateau, CaO 3.67 3.77 4.38 2.21 Na O 4.41 4.62 3.88 3.44 giving an age of 1.68 + 0.09 Ma (MSWD 3.9) 2 with a younger age of 1.37 + 0.02 Ma evident K2O 5.40 5.34 6.51 6.46 TiO2 0.34 0.33 0.76 0.69 (Fig. 6). P2O5 0.42 0.41 0.45 0.43 Based on our new field observations and labora- MnO 0.09 0.09 0.06 0.12 tory analyses we interpret these rocks as shallow Total 99.38 99.26 97.92 97.34 level stocks and dykes of Late Pliocene to Early Pleistocene age. The observed and inferred intrusive 190

Table 4. 40 Ar/39 Ar step heating analyses

Temp Ar36 err Ar37 err Ar38 err Ar39 err Ar40 err Ar40 *Ar40 */ Cumulative Calculated age Ca/K (8C) (mol) (%) (mol) (%) (mol) (%) (mol) (%) (mol) (%) (%) Ar39 (K) Ar39(%) Ma + 1s.d.

Sample RTG-08 (R1) Biotite 600 4.19E-16 0.87 2.04E-15 3.90 1.79E-16 3.38 6.52E-15 0.24 1.30E-13 0.25 4.60 0.92 0.71 2.87 + 0.78 0.59 650 1.47E-16 1.46 1.89E-15 6.16 1.40E-16 3.73 6.08E-15 0.56 4.77E-14 0.64 8.90 0.70 1.37 2.20 + 0.36 0.59 700 2.48E-16 0.73 1.96E-15 3.02 2.12E-16 1.75 1.28E-14 0.20 8.16E-14 0.21 10.00 0.64 2.76 2.00 + 0.14 0.29 750 4.99E-16 0.48 2.07E-15 8.62 3.92E-16 0.29 2.52E-14 0.11 1.57E-13 0.16 5.50 0.34 5.49 1.08 + 0.16 0.16 800 1.02E-15 0.24 2.88E-15 2.87 7.11E-16 1.25 4.07E-14 0.09 3.24E-13 0.11 6.60 0.53 9.92 1.66 + 0.09 0.14 840 1.28E-15 0.38 4.70E-15 3.48 8.54E-16 0.21 4.97E-14 0.09 4.00E-13 0.11 5.30 0.43 15.32 1.35 + 0.13 0.18 890 2.90E-15 0.26 7.08E-15 2.45 1.66E-15 0.91 9.11E-14 0.11 9.10E-13 0.13 5.80 0.58 25.22 1.83 + 0.12 0.15 930 2.91E-15 0.41 9.44E-15 3.38 2.09E-15 0.54 1.27E-13 0.11 9.24E-13 0.18 6.60 0.48 39.02 1.51 + 0.14 0.14 970 2.49E-15 0.34 7.38E-15 0.88 2.93E-15 0.63 1.99E-13 0.12 8.37E-13 0.17 11.60 0.49 60.61 1.53 + 0.06 0.07 COTTAM A. M. 1020 1.94E-15 0.48 5.66E-15 3.74 2.42E-15 0.47 1.66E-13 0.09 6.55E-13 0.11 12.10 0.48 78.64 1.50 + 0.08 0.06 1070 8.81E-16 0.67 6.81E-15 1.57 1.67E-15 0.64 1.21E-13 0.13 3.11E-13 0.20 15.50 0.40 91.83 1.25 + 0.06 0.11 1140 2.98E-16 0.73 4.17E-14 0.64 9.95E-16 0.72 7.28E-14 0.08 1.19E-13 0.14 28.10 0.46 99.74 1.44 + 0.06 1.09 1200 5.94E-17 2.41 1.04E-13 1.06 4.91E-17 0.80 2.00E-15 0.50 1.25E-14 0.53 45.10 2.94 99.95 9.20 + 0.90 103.00 1350 4.00E-17 3.20 1.85E-14 5.07 1.27E-17 20.02 4.88E-16 0.61 1.04E-14 0.70 4.20 0.92 100.00 2.89 + 4.44 74.20 Total 1.51E-14 2.16E-13 1.43E-14 9.20E-13 4.92E-12 0.49 1.53 + 0.10 AL. ET Lambda K40 ¼ 5.5430E-10 J ¼ 1.7413E-3 +0.413 Sample RTG-09 (R2) Biotite 600 9.22E-16 0.55 2.25E-15 5.24 3.05E-16 0.99 7.96E-15 0.28 2.78E-13 0.31 2.10 0.72 0.32 2.26 + 0.94 0.54 650 5.83E-16 0.63 1.99E-15 2.74 2.54E-16 0.60 1.06E-14 0.21 1.78E-13 0.26 2.90 0.49 0.75 1.53 + 0.35 0.36 700 6.71E-16 0.45 3.90E-15 2.72 4.28E-16 0.66 2.20E-14 0.13 2.08E-13 0.16 4.30 0.41 1.65 1.27 + 0.15 0.34 750 1.09E-15 0.72 4.56E-15 2.40 7.13E-16 1.27 4.12E-14 0.38 3.45E-13 0.43 6.70 0.56 3.32 1.77 + 0.23 0.21 800 4.73E-15 0.36 1.40E-14 4.79 2.73E-15 0.69 1.52E-13 0.09 1.48E-12 0.11 5.40 0.52 9.50 1.64 + 0.13 0.17 840 4.03E-15 0.37 1.01E-14 1.43 2.31E-15 0.26 1.28E-13 0.12 1.26E-12 0.13 5.10 0.50 14.70 1.56 + 0.18 0.15 930 3.69E-15 0.56 1.43E-14 1.65 3.58E-15 0.59 2.38E-13 0.22 1.23E-12 0.34 11.10 0.58 24.37 1.80 + 0.11 0.11 970 1.92E-15 0.39 1.14E-14 2.18 4.60E-15 0.91 3.39E-13 0.10 7.54E-13 0.19 23.60 0.53 38.13 1.65 + 0.03 0.06 1020 1.40E-15 0.60 1.07E-14 2.75 4.10E-15 0.33 3.07E-13 0.11 5.74E-13 0.13 26.70 0.50 50.58 1.57 + 0.03 0.07 1070 2.84E-15 0.40 4.04E-14 1.27 9.22E-15 0.40 7.00E-13 0.06 1.16E-12 0.11 26.40 0.44 78.98 1.37 + 0.02 0.11 1140 2.07E-15 0.80 2.64E-13 0.22 5.79E-15 0.35 4.27E-13 0.09 8.00E-13 0.12 25.60 0.48 96.31 1.50 + 0.04 1.17 1200 7.59E-16 1.23 4.84E-13 0.36 9.02E-16 1.33 4.59E-14 0.21 2.19E-13 0.32 19.80 0.95 98.16 2.99 + 0.20 20.20 1350 4.57E-16 0.84 1.07E-13 0.60 6.62E-16 1.33 4.55E-14 0.09 1.53E-13 0.14 18.40 0.62 100.00 1.95 + 0.08 4.47 Total 2.52E-14 9.67E-13 3.56E-14 2.46E-12 8.64E-12 0.50 1.57 + 0.06 Lambda K40 ¼ 5.5430E-10 J ¼ 1.7378E-3 +0.413 Sample RTG-12 (coarse-grained) (R3) Biotite 600 2.35E-15 0.47 4.36E-16 16.96 4.87E-16 1.48 2.39E-15 0.25 6.94E-13 0.26 -0.20 0.00 0.08 0.00 + 4.90 0.35 650 1.90E-15 0.63 4.40E-16 22.82 4.18E-16 0.35 4.43E-15 0.32 5.60E-13 0.33 -0.10 0.00 0.23 0.00 + 2.69 0.19 700 2.57E-15 0.45 2.14E-15 5.57 6.49E-16 0.88 1.20E-14 0.08 7.64E-13 0.10 0.60 0.40 0.65 1.27 + 1.05 0.34 750 4.70E-15 0.45 5.49E-15 1.01 1.24E-15 1.91 2.49E-14 0.30 1.41E-12 0.33 1.20 0.68 1.50 2.15 + 1.08 0.42 800 1.15E-14 0.28 6.25E-15 2.98 2.93E-15 0.83 5.73E-14 0.11 3.43E-12 0.12 1.20 0.70 3.46 2.20 + 0.58 0.21 840 8.35E-15 0.38 5.59E-15 5.75 2.80E-15 0.28 9.55E-14 0.17 2.50E-12 0.21 1.10 0.30 6.74 0.94 + 0.36 0.11 900 1.19E-14 0.50 2.25E-15 5.51 5.12E-15 0.45 2.24E-13 0.35 3.61E-12 0.45 2.10 0.34 14.40 1.08 + 0.38 0.02 980 1.51E-14 0.43 3.31E-15 10.36 9.69E-15 0.98 5.49E-13 0.17 4.79E-12 0.24 6.70 0.58 33.22 1.83 + 0.13 0.01 1020 6.52E-15 0.29 2.40E-15 11.44 6.72E-15 0.51 4.40E-13 0.08 2.22E-12 0.14 12.80 0.65 48.30 2.04 + 0.05 0.01 1060 3.49E-15 1.03 2.18E-15 11.95 5.04E-15 0.53 3.52E-13 0.19 1.28E-12 0.23 18.30 0.67 60.37 2.09 + 0.12 0.01 1100 2.81E-15 0.98 2.34E-15 4.76 5.28E-15 0.76 3.82E-13 0.08 1.14E-12 0.16 25.90 0.77 73.47 2.42 + 0.07 0.01

1200 2.76E-15 0.43 1.71E-14 0.79 9.57E-15 0.26 7.21E-13 0.06 1.38E-12 0.08 39.70 0.76 98.19 2.40 + 0.02 0.05 BAY GORONTALO AND ISLANDS TOGIAN 1350 1.94E-16 2.36 2.49E-15 5.92 6.83E-16 1.77 5.27E-14 0.53 9.48E-14 0.64 38.50 0.69 100.00 2.17 + 0.09 0.09 Total 7.41E-14 5.25E-14 5.06E-14 2.92E-12 2.39E-11 0.65 2.03 + 0.13 Lambda K40 ¼ 5.5430E-10 J ¼ 1.7442E-3 +0.426 Sample RTG-12 (ﬁne-grained) (R4) Biotite 600 3.70E-16 1.23 2.29E-15 12.90 1.88E-16 2.11 8.05E-15 0.25 1.18E-13 0.29 7.40 1.09 0.32 3.39 + 0.51 0.54 650 3.83E-16 0.85 4.55E-15 1.32 2.74E-16 2.01 1.62E-14 0.18 1.23E-13 0.22 7.90 0.60 0.97 1.87 + 0.19 0.53 700 3.24E-16 1.14 6.82E-15 3.16 4.00E-16 0.98 2.74E-14 0.20 1.14E-13 0.25 15.60 0.65 2.07 2.02 + 0.13 0.47 750 7.46E-16 1.34 2.17E-14 4.21 1.59E-15 2.31 1.15E-13 0.17 2.92E-13 0.19 24.20 0.62 6.65 1.92 + 0.10 0.36 800 2.83E-16 0.87 7.56E-15 1.05 9.40E-16 0.45 7.21E-14 0.39 1.26E-13 0.45 32.60 0.57 9.53 1.78 + 0.04 0.20 850 3.30E-16 1.68 7.82E-15 3.81 1.60E-15 0.54 1.21E-13 0.10 1.72E-13 0.14 41.70 0.59 14.37 1.85 + 0.04 0.12 890 3.08E-16 1.10 5.27E-15 4.20 2.46E-15 0.78 1.85E-13 0.16 2.03E-13 0.23 52.90 0.58 21.75 1.81 + 0.02 0.05 930 2.46E-16 0.90 3.26E-15 4.00 3.56E-15 0.29 2.72E-13 0.12 2.37E-13 0.17 66.30 0.58 32.62 1.80 + 0.01 0.02 970 3.62E-16 1.10 2.77E-15 13.41 4.57E-15 0.44 3.50E-13 0.34 3.17E-13 0.38 63.50 0.58 46.61 1.80 + 0.01 0.02 1020 5.43E-16 0.74 3.14E-15 0.94 4.57E-15 0.34 3.51E-13 0.08 3.71E-13 0.10 54.20 0.57 60.66 1.79 + 0.01 0.02 1070 5.92E-16 0.72 6.44E-15 2.47 4.14E-15 0.25 3.15E-13 0.07 3.58E-13 0.10 48.90 0.56 73.27 1.74 + 0.01 0.04 1140 9.20E-16 0.59 4.58E-14 0.58 6.96E-15 0.18 5.33E-13 0.13 5.91E-13 0.15 52.40 0.58 94.61 1.81 + 0.01 0.16 1200 2.96E-16 0.73 8.36E-14 0.72 1.71E-15 0.35 1.29E-13 0.09 1.61E-13 0.14 48.90 0.61 99.78 1.90 + 0.02 1.23 1350 5.45E-17 2.59 6.65E-15 1.25 8.01E-17 4.55 5.50E-15 0.41 1.88E-14 0.44 17.00 0.58 100.00 1.81 + 0.24 2.30 Total 5.76E-15 2.08E-13 3.30E-14 2.50E-12 3.20E-12 0.58 1.81 + 0.02 Lambda K40 ¼ 5.5430E-10 J ¼ 1.7305E-3 +0.356 (Continued) 191 192

Table 4. Continued

Temp Ar36 err Ar37 err Ar38 err Ar39 err Ar40 err Ar40 *Ar40 */ Cumulative Calculated age Ca/K (8C) (mol) (%) (mol) (%) (mol) (%) (mol) (%) (mol) (%) (%) Ar39 (K) Ar39(%) Ma + 1s.d.

Sample RTG-31 (coarse-grained) (R6) Biotite 470 6.17E-17 4.37 1.04E-16 24.22 2.80E-17 4.16 1.17E-15 0.50 1.82E-14 0.51 -0.30 0.00 0.07 0.00 + 1.83 0.17 510 3.02E-16 1.19 3.13E-16 15.37 1.50E-16 4.28 7.16E-15 0.20 9.00E-14 0.24 0.70 0.08 0.49 0.26 + 0.50 0.08 550 7.04E-16 0.89 3.85E-16 2.84 3.39E-16 0.32 1.65E-14 0.21 2.08E-13 0.23 -0.40 0.00 1.45 0.00 + 0.36 0.04 600 1.68E-15 0.67 1.12E-15 3.27 9.63E-16 0.59 5.15E-14 0.18 5.10E-13 0.21 2.30 0.22 4.46 0.70 + 0.22 0.04 COTTAM A. M. 650 2.96E-15 0.36 2.58E-15 0.45 1.99E-15 0.56 1.12E-13 0.11 8.98E-13 0.14 2.50 0.20 10.98 0.63 + 0.10 0.04 700 4.48E-15 0.35 4.28E-15 2.73 3.23E-15 0.47 1.91E-13 0.05 1.40E-12 0.10 5.30 0.39 22.14 1.20 + 0.09 0.04 750 5.25E-15 0.28 4.91E-15 5.91 3.87E-15 0.58 2.26E-13 0.07 1.66E-12 0.12 6.40 0.47 35.36 1.47 + 0.08 0.04 790 3.87E-15 0.81 3.03E-15 2.15 2.68E-15 0.79 1.60E-13 0.66 1.23E-12 0.79 6.90 0.53 44.72 1.65 + 0.28 0.04 840 4.88E-15 0.73 3.76E-15 5.11 3.48E-15 1.11 1.99E-13 0.15 1.56E-12 0.16 7.40 0.59 56.32 1.83 + 0.17 0.04

890 2.65E-15 0.91 2.99E-15 4.52 2.19E-15 0.73 1.35E-13 0.10 8.55E-13 0.16 8.10 0.52 64.20 1.61 + 0.17 0.04 AL. ET 950 3.94E-15 0.57 3.62E-15 7.25 3.00E-15 1.10 1.73E-13 0.26 1.28E-12 0.29 8.80 0.65 74.31 2.03 + 0.16 0.04 1000 3.55E-15 0.73 3.19E-15 3.05 2.32E-15 0.38 1.32E-13 0.17 1.14E-12 0.21 7.50 0.65 82.01 2.01 + 0.19 0.05 1050 5.05E-15 0.54 4.65E-15 5.84 3.17E-15 1.20 1.69E-13 0.21 1.59E-12 0.31 6.10 0.58 91.90 1.80 + 0.13 0.05 1100 2.52E-15 0.77 6.00E-15 2.63 1.74E-15 2.32 1.01E-13 0.14 8.18E-13 0.22 8.90 0.72 97.79 2.25 + 0.18 0.11 1200 5.26E-16 1.56 6.02E-14 0.42 6.73E-16 1.57 3.66E-14 0.15 1.79E-13 0.19 16.30 0.80 99.93 2.49 + 0.21 3.13 1350 4.62E-17 13.90 1.28E-15 8.90 4.96E-17 1.36 1.26E-15 1.19 1.75E-14 1.21 22.50 3.14 100.00 9.76 + 1.91 1.93 Total 4.25E-14 1.02E-13 2.99E-14 1.71E-12 1.35E-11 0.52 1.62 + 0.16 Lambda K40 ¼ 5.5430E-10 J ¼ 1.7286E-3 +0.426 TOGIAN ISLANDS AND GORONTALO BAY 193

5.0 5.0 RTG-08 RTG-09

4.0 4.0

3.0 3.0 Age (Ma) 2.0 2.0

1.0 1.0

0.0 0.0 0 20 40 60 80 100 0 20 40 60 80 100 Cumulative % 39Ar released Cumulative % 39Ar released 5.0 5.0 5.0 RTG-12 F RTG-12 C RTG-31

4.0 4.0 4.0

3.0 3.0 3.0 Age (Ma) 2.0 2.0 2.0

1.0 1.0 1.0

0.0 0.0 0.0 0 20406080100 0 20406080100 0 20 40 60 80 100 Cumulative % 39Ar released Cumulative % 39Ar released Cumulative % 39Ar released

Fig. 6. 40Ar/39Ar age spectra plots for biotite step-heating analyses performed on four samples from the Benteng Intrusives. For sample RTG12 separate analyses were undertaken on coarse (.250 mm; RTG-12 C) and ﬁne (63–250 mm; RTG-12 F) mica.

bodies follow a broadly north–south trend through Quaternary age and assign them to the Luwuk the centre of Togian Island, supporting the spatial Formation (Garrard et al. 1988). observations of Ku¨ndig (1956), and indicating a possible structural control on their intrusion. Discussion Luwuk formation The Togian Islands offer a unique opportunity to Reefal limestones are found throughout the archipe- investigate Gorontalo Bay. Our new stratigraphy lago, and dominate outcrop in the western islands offers insight into several aspects of the basin (e.g. Batu Daka). They occur as high cliffs and including the nature of its basement rocks, its age raised terraces of poorly bedded, rubbly limestones and its mode of formation. containing broken coral fragments. The limestones have been uplifted to heights of around 200 m Basement rocks beneath Gorontalo Bay within the archipelago and to more than 300 m on the East Arm (Garrard et al. 1988). Following Based on geophysical evidence, Silver et al. (1983b) Rusmana et al. (1982) we allocate these rocks a suggested that much of Gorontalo Bay is underlain 194 M. A. COTTAM ET AL. by basement rocks belonging to the East Sulawesi by continental crust (Elburg et al. 2003; van Ophiolite (East Sulawesi Ophiolite). Beneath these Leeuwen & Muhardjo 2005; van Leeuwen et al. may be continental basement rocks belonging to 2007) as far east as 1218E (Fig. 1). This material the leading edge of the Banggai-Sula microconti- forms the eastern margin of Sundaland and is prob- nental block (Silver et al. 1983a; Hall & Wilson ably of Australian origin (van Leeuwen & Muhardjo 2000). Other hypotheses are that the bay is underlain 2005), but was accreted to Sundaland during the by oceanic crust of the Northern Sulawesi Province mid-Cretaceous (Parkinson 1991; Parkinson et al. (Monnier et al. 1995) or that the basement of Goron- 1998; Hall 2009) and is not part of the Banggai- talo Bay comprises a complex amalgamation of at Sulu block. least two tectonostratigraphic provinces. The posi- Continental crust probably continues north from tion of the Togian archipelago in the middle of the Banggai-Sulu microcontinent beneath the Gorontalo Bay provides an opportunity to test these Molucca Sea (Silver et al. 1983b; Watkinson et al. hypotheses. 2010). Beneath Gorontalo Bay earthquake hypocen- Our new ﬁeld observations suggest that the tres (Engdahl et al. 1998) deﬁne the southern edge central part of Gorontalo Bay, including the of the westward-subducting Molucca Sea plate. This Togian Islands, is underlain by oceanic and arc is a very sharp, almost WNW–ESE, line (Fig. 7a) basement of the Northern Sulawesi Province rather that we interpret as the former continental– than the continental basement of the Banggai- oceanic crust boundary between the Molucca Sea Sula Block. The Walea Formation represents the and the Banggai-Sula block. The position of the basement within the Togian Islands. Its age is not line implies that continental crust continues north known but it is inferred to be older than the from the Banggai-Sula Islands to the centre of the Middle Miocene limestones against which it is eastern part of Gorontalo Bay. How far west faulted. The formation comprises an association of beneath Gorontalo Bay the continental crust con- volcanic rocks and subordinate volcanogenic sedi- tinues is uncertain; the oil that seeps through ments that we suggest represent the products of a the ophiolite north of the thrust complex in the submerged volcanic arc rather than an ophiolite, as East Arm (Ku¨ndig 1956) suggests continental base- previously interpreted (Ku¨ndig 1956). A similar ment may extend at least west to about 1228E association of volcanic rocks and subordinate volca- (Fig. 8). niclastics is reported within the Papayato Volcanic rocks of the North Sulawesi Province (Elburg Miocene carbonate platform et al. 2003; van Leeuwen et al. 2007), and is consistent with the suggestion that the basement of Miocene carbonate rocks are widespread in northern this province continues southwards beneath the Sulawesi. They include the Middle Miocene lime- archipelago. Volcanic rocks (breccias, pillows and stones of the Peladan Formation reported here, car- lavas) similar to those of the lower parts of the bonates of the Buol Beds in NW Sulawesi (Ratman Walea Formation are also reported from the Cre- 1976; van Leeuwen & Muhardjo 2005), the Salodik taceous Balantak Ophiolite of East Sulawesi Formation within the East Arm (Rusmana et al. (Simandjuntak 1986; A. J. Barber, pers. comm. 1982), and limestones observed around Palu and 2009), but they do not show the same association the western Toli–Toli region (Sukamto 1973; with contemporaneous volcaniclastic sediments. Norvick & Pile 1976; van Leeuwen & Muhardjo Geochemical and/or geochronological analyses of 2005). Benthic and planktonic foraminifera indicate the Walea Formation, and comparison with the deposition within inner platform/fore-reef shallow (Middle Eocene to Early Miocene) Papayato Vol- marine environments during the late Early to canic rocks (North Sulawesi Province of Elburg Middle Miocene (van Leeuwen & Muhardjo 2005; et al. 2003) and the (Cretaceous) Balantak Ophiolite this study). Jablonski et al. (2007) report submerged (East Sulawesi Ophiolite) would help to resolve this carbonate reefs in Gorontalo Bay based on seismic issue but the rocks are so deeply weathered that observations which they interpreted as Oligocene obtaining suitable material has not so far been to Middle Miocene in age. The distribution of possible. Miocene carbonate rocks suggests that Gorontalo Field investigations and geochemical analyses Bay was an area of extensive carbonate platform suggest that the western end of the bay is underlain deposition during the Miocene. It was probably

Fig. 7. Earthquake hypocentres in Eastern Indonesia based on the dataset of Engdahl et al. (1998). (a) Black crosses denote all hypocentres, those assigned to the westward subducting Molucca Sea Plate are highlighted with blue dots, those assigned to subduction at the North Sulawesi Trench are highlighted in green. Hypocentres associated with volcanism at the Una-Una volcano are shown in purple. Red box denotes the line of section illustrated in (b). (b) North–south cross section though Gorontalo Bay and the Una-Una volcano. Hypocentres associated with volcanism at the Una-Una volcano (purple dots) are notably shallower than those related to the downgoing slab. TOGIAN ISLANDS AND GORONTALO BAY 195

Fig. 7 (Continued)(c) Earthquake hypocentres assigned to the Molucca Sea Plate coloured based on depth. To aid clarity, hypocentres less than 75 km depth are not shown. Colouration shows that the slab dips gently to the NW but is sharply terminated along its southern edge in a steep upturned lip. Black crosses denote hypocentres at depths greater than 75 km elsewhere in the region. They are almost entirely absent in the Banggai-Sula plate. 196 M. A. COTTAM ET AL.

Fig. 8. Detailed bathymetry of Gorontalo Bay, modified from Jablonski et al. (2007). Topography based on SRTM (Shuttle Radar Topographic Mission) data (courtesy of NASA, NGA & USGS). characterized by contiguous shallow marine plat- Head, with westward subduction implied in front forms, but was certainly not a continuous deep of them. However, it has also been suggested that bathymetric feature at this time. In west Sulawesi collision between microcontinental blocks and carbonate deposition terminated by the end of the East Arm began earlier, between the latest the Middle Miocene (van Leeuwen & Muhardjo Oligocene and Late Miocene (e.g. Audley-Charles 2005). 1974; Sukamto & Simandjuntak 1983; Daly et al. 1991; Parkinson 1991; Smith & Silver 1991; Rapid Pliocene uplift Bergman et al. 1996; Milsom 2000; Hall 2002; van Leeuwen et al. 2007; Spakman & Hall 2010). The clastic sediments of the Bongka Formation If so, collision significantly predated the rapid record localized rapid uplift and erosion of the uplift and erosion of the East Arm during the East Arm in the latest Miocene to Pliocene latest Miocene to Pliocene, which must have a dif- (Surono & Sukarna 1986; Hall & Wilson 2000), ferent cause. instigating the development of the high (in places .3 km) present-day topography. In some cases, Basin subsidence the sudden influx of clastic material may have been directly responsible for the reduction of car- Seismic surveys (Jablonski et al. 2007) and multi- bonate areas from large platforms to isolated pin- beam surveys of Gorontalo Bay show present-day nacle reefs (Jablonski et al. 2007). water depths up to 2000 m in the western part of Uplift has previously been attributed to collision the basin and .2700 m in the eastern part (Fig. 8). between the Banggai-Sula microcontinent and the Sediment thicknesses within these areas may be as East Arm (e.g. Garrard et al. 1988; Davies 1990; great as 10 km (Jablonski et al. 2007). There is Hall 1996; Calvert 2000; Hall & Wilson 2000; van a bathymetric high area that links the East Arm Leeuwen & Muhardjo 2005). This interpretation and the Togian Archipelago, with water depths followed Hamilton’s (1979) proposal of slivers of of between 500 and 1200 m (Fig. 8), which may continental crust moving west from the Bird’s continue across the entire bay to the North Arm. TOGIAN ISLANDS AND GORONTALO BAY 197

This feature appears to have a broadly NW– 1996; Hall & Wilson 2000) for inception of SE trend. the basin. Seismic data have been used to suggest that the basin formed in a predominantly extensional tec- Cause of subsidence and uplift tonic environment dominated by east–west trending extensional faults (Jablonski et al. 2007). It was The broadly contemporaneous nature of basin subsi- interpreted to have formed in the Eocene as a dence and uplift and erosion at the ﬂanks suggests failed rift arm (Jablonski et al. 2007). We infer a that these two processes are inherently linked. much younger, Pliocene age of formation of the Together, the rapid latest Miocene to Pliocene deep basin. uplift (c. 3 km) and subsidence (.2 km) in and We interpret deposits of the Bongka Formation around Gorontalo Bay has produced an exceptional (Celebes Molasse) observed in the Togian Islands total elevation contrast of more than 5 km in less and the East Arm as distal and proximal equivalents than 6 Ma. The thickness of sediment in the of a Pliocene alluvial fan building out from the East central part of the bay (up to 10 km) suggest much Arm. Seismic data reveal thick (up to 2 seconds greater differential movements. TWT (two-way travel time)) north–south trending The North Sulawesi subduction zone probably lobes of sediment that we infer to be submerged developed in the last 5 Ma (Silver et al. 1983a; parts of this fan (Fig. 9). Prograding fan delta Surmont et al. 1994). We suggest that palaeomag- deposits of similar age are also interpreted from netic data (Surmont et al. 1994), seismic data elsewhere in the basin (Jablonski et al. 2007). (Silver et al. 1983a; Jablonski et al. 2007) and These observations imply that basin subsidence plate tectonic modelling (Silver et al. 1983b; (from close to sea level to present-day water Hall 1996, 2002) indicate that the region has been depths of 500 to 1500 m) occurred after deposition in extension since the Early Pliocene, with the of the fan. The age of the Celebes Molasse in the North Arm moving away from the East Arm. East Arm therefore provides a maximum, latest We interpret Global Positioning System (GPS) Miocene to Pliocene age (e.g. Surono & Sukarna measurements of present-day motions (Walpersdorf

Fig. 9. Thickness of the sedimentary fill in Gorontalo Bay, modified from Jablonski et al. (2007). Thickness is based on two-way travel-time in seconds (TWT s) between water bottom and basement isochron (Jablonski et al. 2007). Topography based on SRTM (Shuttle Radar Topographic Mission) data (courtesy of NASA, NGA & USGS). 198 M. A. COTTAM ET AL. et al. 1998; Vigny et al. 2002; Socquet et al. 2006) to Post-Pliocene tectonics indicate that this extension continues today. There- fore one possible cause of subsidence is extension Tuffaceous rocks of the Lonsio Formation are also of the upper plate that was driven by rollback of known from the East Arm (Rusmana et al. 1982, the subduction hinge at the North Sulawesi 1993; this study), around 150 km SE of the Togian Trench. However, the extremely rapid rates and Islands. Following Simandjuntak (1986), we large amounts of uplift and subsidence in the suggest that Poh Head has been offset to the SE region suggest that significant flow of lower crust, along the Balantak Fault. Based on satellite from beneath the basin towards topographically images, field observations and seismic data, we elevated areas, may also have contributed (Hall interpret this structure as a steeply dipping, right- 2010). lateral, strike-slip fault that can be traced offshore to the east, where it terminates in a zone of dextral et al. Young volcanism transpression (Watkinson 2011). To the west of Poh Head the position of the fault is not known, The rocks of the Lonsio Formation and Benteng but it may bend to the north, possibly linking to Intrusives record young volcanism in Gorontalo the fault that we infer between the islands of Bay during the late Neogene. Although the Togian Walea Kodi and Walea Bahi. The distribution and Islands are in the right position for a subduction- ages of the volcanic rocks in the Togian Islands related volcanic arc ahead of a westward-moving and Poh Head could therefore be explained by post- Banggai-Sula microcontinent, volcanism does not depositional dextral faulting, or by westward appear to be subduction related. Such volcanic migration of the volcanic centre with time. activity should have preceded the East Sulawesi– Banggai-Sula microcontinent collision. The Conclusions youngest age suggested for this is end Miocene (c. 5 Ma) but the dates we have for the Togian We interpret Gorontalo Bay to be underlain by a Islands and Poh Head volcanic activity are Pliocene composite basement comprising several different or younger. The composition of the volcanic rocks is tectonostratigraphic provinces. The western end of not typical of most subduction-related volcanism. Gorontalo Bay is underlain by continental crust The Benteng Intrusives are extremely rich in potass- added to the eastern margin of Sundaland in the ium (Fig. 5; Table 3), they are shoshonites using the mid Cretaceous. The central part of the bay, includ- scheme of Rickwood (1989). Earthquake hypocen- ing the Togian Islands, is underlain by oceanic base- tres beneath Una-Una volcano (Fig. 7b) show that ment of the Northern Sulawesi Province. It is volcanism is unrelated to subduction beneath the possible that the area south of the Togian Islands North Arm, being much further west and much has continental crust at depth, with a thrust contact shallower than hypocentres related to the down- beneath the Northern Sulawesi volcanic basement going slab. and East Arm ophiolite, as suggested by oil seeps High-K compositions are characteristic of small through the ophiolite on land. degrees of partial melting of anomalous (metasoma- In the Miocene, Gorontalo Bay was an area of tized or enriched) material in the upper mantle (e.g. extensive carbonate deposition, characterized by Wilson 1989). We infer a similar origin for the contiguous shallow marine carbonate platforms. It Benteng Intrusives and suggest that rapid exten- was not a significant, continuous, deep bathymetric sional thinning of the crust beneath Gorontalo Bay feature in the Miocene. Instead, broadly contem- caused the upper mantle to rise, decompress and poraneous flank uplift and basin subsidence give a melt. The resulting K-rich melts were intruded maximum latest Miocene to Pliocene age for the into the crust as a series of shallow level stocks inception of the deep basin. and dykes. Present-day high-K volcanism at Una- Volcanism in the Togian Islands is unrelated Una suggests that volcanism has evolved to a rela- to subduction that preceded collision of the tively less K-rich chemistry, possibly reflecting Banggai-Sula microcontinent. Instead, it records increased amounts of partial melting, and has rapid extension of the crust in the Pliocene and moved WNW over time. Plio-Pleistocene, causing the underlying mantle to The tuffaceous rocks of the Lonsio Formation rise, decompress and melt. We interpret GPS obser- represent the products of extrusive volcanism, vations (Socquet et al. 2006) to indicate extension is reworked during deposition in a shallow marine continuing today and is probably the cause of vol- environment during the latest Miocene and Early canism at Una-Una. Volcanic activity has migrated Pliocene (N19). They are significantly older (as west towards Una-Una during the Pleistocene and much as 3 million years) than the Benteng Intru- deposits of the Pliocene volcanic episode may sives, and appear to be derived from a different – have been offset by dextral strike-slip displacement or unknown volcanic centre. along the Balantak Fault. TOGIAN ISLANDS AND GORONTALO BAY 199

Rapid subsidence associated with crustal thin- PSC area and associated development of a Tertiary ning was driven by rollback of the subduction petroleum system, South Sulawesi, Indonesia. In: Pro- hinge at the North Sulawesi Trench. The unusual ceedings Indonesian Petroleum Association, 22nd Annual Convention, 679–706. character of volcanism in the Togian Islands is not Daly Cooper Wilson Smith due to subduction but reflects crustal thinning and , M. C., , M. A., , I., ,D.G.& Hooper, B. G. D. 1991. Cenozoic plate tectonics and extension. The extreme rates of uplift and sub- basin evolution in Indonesia. Marine and Petroleum sidence observed in and around Gorontalo Bay Geology, 8, 2–21. (producing an elevation contrast of .5 km) suggest Davies, I. C. 1990. Geology and exploration review of flow of lower crust may also have contributed. the Tomori PSC, Eastern Indonesia. In: Proceedings Indonesian Petroleum Association, 19th Annual The industrial member companies of the SE Asia Research Convention, 41–68. Group Consortium provided financial support for our work. Elburg, M., van Leeuwen, T., Foden,J.&Muhardjo The authors thank Benjamin Sapiie and Alfend Rudyawan 2003. Spatial and temporal isotopic domains of (Institute Teknologi Bandung) for facilitating our work in contrasting igneous suites in Western and Northern Indonesia. M.A. Forster acknowledges the support of an Sulawesi, Indonesia. Chemical Geology, 1999, Australian Research Fellowship provided by the Austra- 243–276. lian Research Council (ARC) associated with the Discov- Engdahl, E. R., van der Hilst,R.&Buland, R. 1998. ery grants DP0877274, and additional support from the Global teleseismic earthquake relocation with Research School of Earth Sciences at The Australian improved travel times and procedures for depth deter- National University. eArgon software written by Gordon mination. 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