IPA12-G-029

PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Thirty-Sixth Annual Convention & Exhibition, May 2012

A NEW INTERPRETATION OF GORONTALO BAY, SULAWESI

Parinya Pholbud*

Robert Hall* Eldert Advokaat*

Pete Burgess* Alfend Rudyawan*

ABSTRACT Recent seismic and multibeam data, combined with information from land, provide the basis for a new interpretation of western Gorontalo Bay. The stratigraphy is divided into three parts. The basement (Unit A) is proposed to be Sundaland continental crust, below a major unconformity interpreted to be either Mid Eocene or Early Miocene in age. Above the unconformity is a sequence up to 6 sec TWT divided into two parts. The lower part (Units B and C) is interpreted as quartz-rich marine sediments, with little volcanic debris, derived from granites and continental basement of western Sulawesi. There is a minor unconformity at the base of the upper part (Units D to F). We suggest Units D and E are carbonates. Unit D has major clinoforms indicating water depths less than 200m. At the top of Unit E are linear bands of pinnacle reefs which step back towards the north. They mark rapid subsidence which began at about 5 Ma and are partly buried by Pliocene–Recent deposits (Unit F) in the basin centre. This may include quartzose siliciclastic material, carbonate sediment and volcanic ash. Given the observed subsidence the absence of significant faulting in the basin is noteworthy. We suggest the cause of extension is lithosphere-scale low angle normal faulting driven by subduction roll-back at the North Sulawesi trench. This is consistent with the existence of young rapidly exhumed core complexes and low angle detachments on land. Hydrocarbons are leaking from the south side of the bay. Important considerations for a petroleum system are: a basement of predominantly highly thinned Australian continental crust accreted to the Sundaland margin in the mid Cretaceous; a quartzose lower sequence with possible sources * SE Asia Research Group

from coals at the base or organic matter in sands; an upper sequence of shallow marine carbonates with little volcanic material, and a bay that has subsided significantly only since about 5 Ma. INTRODUCTION Gorontalo Bay (Figure 1) is one of several enigmatic basins in East Indonesia. It is a semi-enclosed sea, separating the mountainous North Arm from Central Sulawesi Metamorphic Belt and the East Arm Ophiolite (Figure 2). To the west it is bounded by the narrow Sulawesi Neck. Early researchers (Ahlburg, 1913; Rutten, 1927) reported water depths up to 2000m in the western part of the bay and rapid deepening within short distances of the coast but until recently there was little information on the bathymetry or geology of the bay. Ideas about the nature of Gorontalo Bay are widely divergent (Figure 3). Silver et al. (1983a) suggested that the eastern part of Gorontalo Bay (Figure 3A) is a forearc basin, with a relatively thin sedimentary cover above oceanic crust in the North Arm forearc. They considered that there was a north-dipping oceanic slab connected to the Banggai–Sula Block, based on “extreme low-gravity values” near the Batui Thrust and hypocentres dipping northwards to a depth of about 100 km (McCaffrey & Sutardjo, 1982). Kadarusman et al. (2004) suggested that the western part of Gorontalo Bay is underlain by oceanic crust of the East Sulawesi Ophiolite (Figure 3B) with a thin sedimentary cover, thrust over the Banggai–Sula Block. In contrast, Hamilton (1979) estimated a five kilometre thick sediment infill in the western part of the bay, which is supported by recent seismic surveys (Jablonski et al., 2007) that show up to 6 sec TWT of sediments below the sea bed. Jablonski et al. (2007) considered Gorontalo Bay to be a lateral equivalent of the Makassar Strait,

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based on relative constant thickness of interpreted Upper Miocene deposits, following relatively young uplift of the Neck which separated the two basins. They interpreted the basement to be cut by numerous faults related to Eocene extension of continental crust which included fragments rifted from Australia and added to the Sundaland margin in the Cretaceous. In this paper we discuss the geology of Gorontalo Bay based on recently acquired seismic (courtesy of Fugro MCS and Searcher Seismic) and multibeam data (courtesy of TGS). We present a seismic interpretation and consider the implications for the tectono-stratigraphic evolution of Gorontalo Bay in the context of its regional setting. Geological setting Sulawesi is located at the triple junction where the Pacific–Philippine and the Indian–Australian plates are subducted below the Eurasian plate (Figure 1). As result of this convergence, Sulawesi is composed of numerous fragments derived from these major plates. They include metamorphic complexes, magmatic provinces, volcanic arcs, ophiolites and micro-continental fragments and are commonly (e.g. Katili, 1978; Hamilton, 1979; Taylor & van Leeuwen, 1980; Sukamto & Simandjuntak, 1983) grouped into a number of major tectonic units (Figure 2): 1. Western Sulawesi Plutono-volcanic Province,

the eastern continental margin of Sundaland (Hamilton, 1979; Polvé et al., 1997; van Leeuwen et al., 2007) formed largely of Australian fragments accreted in the Cretaceous (Jablonski et al., 2007; Hall et al., 2009).

2. North Sulawesi Volcanic Province, an intra-

oceanic volcanic island arc of dominantly tholeiitic composition underlain by oceanic crust (Rangin et al., 1997; Elburg et al., 2003; van Leeuwen & Muhardjo, 2005).

3. Central Sulawesi Metamorphic Belt, a terrane

made up predominantly of phyllites and schists of both continental and oceanic origin (Rutten, 1927; Brouwer, 1947; Parkinson, 1991, 1998a, b).

4. East Sulawesi Ophiolite, a dismembered

sequence including dunite, lherzolites and harzburgites, ultramafic cumulates, layered and isotropic gabbros, sheeted dykes and basaltic

pillow lavas (Kündig, 1956; Simandjuntak, 1986; Parkinson, 1998b; Kadarusman et al., 2004).

5. Microcontinental fragments of Australian origin

(Pigram et al., 1985), including the Banggai–Sula (Garrard et al., 1988; Supandjono & Haryono, 1993) and Tukang Besi–Buton (Davidson, 1991; Smith & Silver, 1991) Blocks, previously considered to be separate fragments sliced from New Guinea and carried west along the Sorong Fault (Hamilton, 1979). These are now interpreted as part of the Sula Spur (Klompé, 1954) that collided with the North Arm in the Early Miocene (Hall, 1996, 2002) and subsequently fragmented (Spakman & Hall, 2010; Hall, 2011).

Based on the references cited above, the relationships between these different provinces and the tectonic development of Sulawesi can be summarised as follows. The metamorphic complexes in West and Central Sulawesi were emplaced or formed as consequence of mid-Cretaceous accretion of Australian continental fragments to Sundaland. During the Early Eocene western Sulawesi rifted away from Borneo forming the Makassar Strait at the same time as oceanic spreading in the Celebes Sea. From the Middle Eocene to Late Oligocene the North Arm Volcanic Arc formed as consequence of north-directed subduction of the Indian Ocean beneath the North Arm. In the Early Miocene, the East Sulawesi Ophiolite was emplaced by northwest-directed obduction during collision of the Sula Spur with the North Arm. The Central Sulawesi Metamorphic Belt includes metamorphic rocks associated with the ophiolite, the Sundaland margin and the Banggai–Sula Block. Regional extension, from Sulawesi eastwards, commenced in the Middle Miocene, associated with rollback of subduction into the Banda embayment. There was widespread magmatic activity in the Western Sulawesi Plutono-volcanic Province, probably mainly extension-related, from the Middle Miocene producing high-K calc-alkaline acidic igneous rocks. Southwards subduction of the Celebes Sea under the North Arm is interpreted to have started around 5 Ma. Subduction rollback of the North Sulawesi subduction zone initiated a new phase of extension in northern Sulawesi which has been active until the present-day. The geology of Gorontalo Bay and surrounding areas of West Sulawesi, the Neck, North Arm and

East Arm reflects this complex history. The stratigraphy of these areas is summarised in Figure 4 based on observations by Sukamto (1973), Ratman (1976), Bachri et al. (1993), Rusmana et al. (1993), Surono et al. (1993), Apandi & Bachri (1997), Simandjuntak et al. (1997), van Leeuwen & Muhardjo (2005), and Cottam et al. (2011). Basement rocks of different types and ages are exposed in West Sulawesi, the Neck and the western end of the North Arm. These are mainly granites and metamorphic rocks with protolith ages which are Cretaceous or older. Cretaceous low grade metasediments are known from western Sulawesi and undated low grade metasediments are found in the Togian Islands. The basement rocks are overlain unconformably by Paleogene mainly clastic sediments in western Sulawesi (Budung-Budung Formation) to very low grade metasediments (Tinombo Formation) in the Neck. These are considered to pass into the Papayato Volcanics of the North Arm which resemble Paleogene pillow lavas and volcaniclastic sandstones (Walea Formation) found in the Togian Islands. To the southeast Paleogene rocks, which are predominantly shallow marine limestones, are found in the collision complex between the East Arm Ophiolite and the Banggai–Sula Block. There is a widespread but poorly dated unconformity recording collision of the Banggai–Sula Microcontinental Block, East Arm Ophiolite and North Arm Volcanic Arc in the Early Miocene. There are limited exposures of Neogene rocks in all the areas surrounding Gorontalo Bay. In the North Arm there are a variety of acid eruptive and intrusive rocks, and some clastic sedimentary rocks. In the Neck there are granite bodies of the Dondo Suite. Radiogenic isotope analyses (Priadi et al., 1993; Priadi et al., 1994; Bergman et al., 1996; Polvé et al., 1997; Elburg & Foden, 1999; Polvé et al., 2001; Elburg et al., 2003) and U–Pb dating (van Leeuwen et al., 2007) of the igneous rocks in the Palu and Neck regions indicate melting of lower crustal material of Australian affinity. In the Togian Islands there are Middle Miocene shallow marine limestones, and similar rocks are known from the East Arm east of the Togian Islands and in the Banggai–Sula Islands. Pre-Miocene rocks throughout Sulawesi are unconformably overlain by the Celebes Molasse (Sarasin & Sarasin, 1901) a poorly lithified sequence of interbedded conglomerates, sandstones, and mudstones with subordinate intercalations of

breccia, tuffs and limestones. Originally this was considered to be a single formation formed after a Miocene collision (Kündig, 1956) but recent studies have shown that it is diachronous across Sulawesi (Hall & Wilson, 2000), locally very lithologically variable, and it probably records several episodes of uplift and erosion around Gorontalo Bay in the latest Miocene to Plio-Pleistocene. K–Ar, Ar–Ar and Apatite fission track ages indicate rapid uplift in the Pliocene (van Leeuwen & Muhardjo, 2005; Bellier et al., 2006) contemporaneous with sinistral movement on the Palu-Koro Fault, and 20° clockwise rotation of the North Arm around a pole close to Manado interpreted from geological observations (Silver et al., 1983b) and palaeomagnetic data (Surmont et al., 1994). This has been linked to subduction at the North Sulawesi Trench. New tectonic models of the region indicate an important role for Neogene extension, related initially to subduction rollback in the Banda Arc (Spakman & Hall, 2010) and later to subduction rollback of the North Sulawesi subduction zone (Hall, 2011). Extension at different times is manifested in several ways. The Malino Complex northwest of Gorontalo Bay is suggested to be an extensional metamorphic core complex (Kavalieris et al., 1992; van Leeuwen et al., 2007) of Miocene age. West of Gorontalo Bay, mountains more than 1 km high form the narrow Neck which exposes medium to high-grade continental metamorphic rocks with young ages (Sukamto, 1973; Elburg et al., 2003; van Leeuwen et al., 2007). Pinnacle reefs seen on seismic sections from Gorontalo Bay are interpreted by Jablonski et al. (2007) to be Late Miocene or younger and correlations with the Middle–Upper Miocene Peladan Formation on the Togian Islands. These suggest that Gorontalo Bay was a shallow marine area until the latest Miocene with rapid subsidence from the Early Pliocene (Cottam et al., 2011). Medium-K to shoshonitic volcanism on the Togian Islands is suggested to be the result of crustal thinning in the Plio-Pleistocene (Cottam et al., 2011). South of Gorontalo Bay, young core complexes have been identified at the Tokorondo Mountains and the Pompangeo Mountains based on morphological features observed on SRTM images (Spencer, 2010, 2011). GPS measurements (Walpersdorf et al., 1998; Vigny et al., 2002; Socquet et al., 2006) indicate that this motion is continuing at the present day and potentially accommodates major top-to-the-north extensional exhumation in Central Sulawesi (Spencer, 2010, 2011).

DATASET AND METHODOLOGY This project was provided with a 2D seismic data set by Fugro MCS and Searcher Seismic (Figure 5). The Gorontalo 2D Non-Exclusive Seismic Survey was acquired in 2006 with a total length of 8,009 km. The survey was acquired utilizing long offset seismic cabling with recording to between 8 and 12 sec TWT. The key processing parameters for the seismic data are SRME and Kirchhoff PSTM. The spacing between each seismic line was approximately 7 km. Seismic data were interpreted using SMT Kingdom Suite on a Windows workstation. Multibeam data (Figure 6) were provided by TGS. This coverage was acquired using a Kongsberg Simrad EM120 Multibeam Echo Sounder using 191 beams at equidistant spacing. Positioning control used a C-Nav Starfire DGPS. During processing, positioning, tidal and calibration corrections were applied, random noise and artefacts were removed, and a terrain model using a 25 m bin size was gridded and exported to ESRI format. Multibeam data were further processed in ER Mapper to remove voids and generate a digital elevation model (DEM) and used to display seafloor topography in 3D. Seismic Units The seismic and multibeam data show that the Gorontalo Bay changes character at about 12130’E (Figure 6). East of this longitude the basin is separated into two parts by a shallow ridge that includes the modern volcano of Una-Una and the Togian Islands and there is a relatively narrow, broadly E–W orientated northern deep area north of the Togian Islands. West of 12130’E the basin is much wider and deeper. This western part of Gorontalo Bay is divided into two sub-basins by a NE-trending wide ridge capped by drowned carbonate reefs we name the Lalanga Ridge after a small island with reefs at its SW end. The SE sub-basin (Poso Basin) is slightly shallower (Figure 7) than the NW sub-basin (Tomini Basin) with a maximum depth of 1800m compared to 2000m, and contains much less sediment (about 3 sec TWT compared to 6 sec TWT). Although there is a relatively close spacing of seismic lines it is not possible to correlate from the larger Tomini Basin to the smaller Poso Basin across the Lalanga Ridge. In this project we therefore focused on the broader and deeper Tomini Basin which contains the much thicker sequence.

Six key seismic horizons were interpreted in Tomini Basin to define seismic megasequences and geological structures. The seismic units identified were named from base to top as Units A, B, C, D, E and F (Figure 8). Interpreted horizons were mapped to construct two-way-time thickness maps and the seismic stratigraphy of each unit was studied. Digital seafloor topography from multibeam data was correlated with interpreted seismic horizons to define the paleoshelf-edge of carbonates within the basin. UNIT A The top of Unit A is marked by an angular unconformity which is the deepest seismic horizon mappable through the whole of Tomini Basin. The unit is characterized by complex zone of moderate to strong reflectors (Figures 9 and 10). Locally there are reflectors that are parallel or sub-parallel to the unconformity surface whereas in other places there are reflectors that are oblique to it with discordances up to 10. At the NW end of some seismic lines there are NW-dipping normal faults below the unconformity surface (Figure 9), but apart from these there is little evidence for rifting during deposition of units above the unconformity. UNIT B The Tomini Basin essentially begins with deposition of Unit B. This package onlaps the basal unconformity of Horizon A. This unit consists of moderate to strong, continuous, parallel reflectors, which dip at a low angle towards to the west in the western part of the basin and are sub-horizontal in the east. Unit B is mainly thicker than 1 sec TWT and reaches a maximum thickness of approximately 2.3 sec TWT in the basin centre (Figure 11) and thins toward the basin flanks. A ridge crosses the basin (Figures 9 and 10) which is orientated broadly E-W (Figure 11) and divides Unit B into two depocentres. There are no signs of faulting associated with the ridge which appears to have been a pre-existing feature that was gradually onlapped and buried by sediment transported from the west. UNIT C Unit C lies conformably above Unit B, and is separated from it by a strong reflector. It varies in thickness between 1 and 2 sec TWT. This unit is characterized by moderate to strong continuous reflectors with small offset normal faults. These

normal faults are only observed in seismic lines that are oriented NE-SW and are less common in the western part of the basin. They are almost entirely confined to Unit C although a few cut down into Unit B. A few faults have been inverted forming small anticlinal structures within the unit. Units B and C are restricted to a relatively narrow basin (compared to the present western Gorontalo Bay) of about 50 km width which is elongate and trends approximately ENE-WSW. Unit B clearly thins to the NE consistent with a western source. This is less clear for Unit C which seems to thin slightly towards the central part of the basin (Figure 10) but then begins to thicken slightly towards the NE. The unit also appears to thin towards the SE. These observations suggest sediment input predominantly from the north from both the W and E ends of the basin. There is little to indicate the environment of deposition of Units B and C. The well bedded character and lateral continuity of reflectors for considerable distances suggest a marine setting. There are no signs of slumping or disturbance indicating rapid deepening or unstable slopes. Almost all the faults in Tomini Basin are small offset normal faults restricted to Unit C, and are observed only in E-W and NE-SW lines. This suggests they are a response to flexure associated with deformation that resulted in the Horizon C unconformity. Units B and C appear to fill in a pre-existing basin topography which was gradually buried rather than created by rifting. UNIT D Unit D is separated from underlying units by an angular unconformity of Horizon C. Units B and C are tilted gently to the north with a dip of a few degrees subparallel to the northern slope of the Lalanga Ridge. Reflectors are truncated by the unconformity surface (Figure 9) north of the Lalanga Ridge. On the north-eastern end of the ridge is a large rimmed carbonate reef complex (Figure 13). Unit D appears to be the lateral equivalent of the lower part of the reefs as both rest unconformably on the unconformity surface. It reaches a thickness of about 1 sec TWT and contains a series of clinoforms reflectors that prograde towards the basin centre (Figure 14). We interpret this unit as shallow water carbonates, largely prograding to the south but with a thinner equivalent package prograding northwards from the Lalanga Ridge (Figure 9). This is based on the strata geometries observed in the clinoforms and the overlying carbonate strata (e.g. Oberlin & Ginsburg,

1987; Faradic, 2006; Dorset, 2010). The clinoforms appear to be strongly progradational linear features (rather than point sourced locate features as would be expected with a siliciclastic delta system on this scale) and are interpreted to pass upwards into an apparently genetically related unit with large-scale stacking patterns changing from strongly progradational to aggradations to retro gradational, and terminating as a series of drowned pinnacle features. The basin shape changed and widened during deposition of Unit D and the northern basin margin changed from an ENE-WSW to a NNE-SSW orientation. UNIT E Unit E is characterised by moderate amplitude reflectors that are generally sub-horizontal with locally shallow dips to the northwest. It shows gradational and retro gradational stacking patterns with many pinnacle reefs in its upper part (Figure 9 and Figure 10) which we interpret to represent a shallow marine environment for the whole of the unit. The thickness increases toward the northwest with a maximum of 2.4 sec TWT and the unit thins significantly in the basin centre and pinches out as it onlaps the Lalanga Ridge to the south. During deposition of Unit E the NNE-SSW northern margin shelf edge rotated to a NE-SW orientation add the shelf edge retreated to the north marked by multiple lines of sub-parallel pinnacles and implying episodic subsidence of the basin. UNIT F Unit F is the youngest sequence in Tomini Basin. It has relatively low amplitude flat reflectors in the basin centre where it is thinly bedded, up to 0.7 sec TWT thick, and appears to be fed from the west. In the northern part of the basin it is thinner with a thickness up to 0.4 sec TWT and it buries or partly buries many of the pinnacle reefs. In this part of the basins sediment appears to have entered the basin from several sources in the north. The depth of the pinnacle reefs varies from 2000m at the basin centre to approximately 1000m at the northern limit of the multibeam coverage (Figure 14) indicating subsidence of at least 2000m since the formation of the oldest reefs at the top of Unit E close to the basin centre. Although most of the sediment appears to be derived from the west, the presence of Pliocene volcanic tuffs in the Togian Islands (Cot tam et al., 2011), and evidence of explosive acid volcanic activity in the North Arm (Bachri et al. 1993; Apandi & Bachri, 1997), suggest the material that was sourced from the north, and sediment in the

eastern part of Tomini Basin, may include a significant volcanic component.

UNIT AGES AND BASIN HISTORY

Because there are no wells in Gorontalo Bay the ages of horizons must be interpreted from other data. This presents some difficulties because the very thick sequence in the Tomini Basin cannot be traced on to land, and in the surrounding areas of the North Arm, Neck and East Arm, exposures are predominantly basement rocks. The almost 6 sec TWT of section (Figure 10) must terminate abruptly to the west, presumably at a fault, because on land there are high-grade schists and granites in the Neck. There is a NW-SE fault on land, cutting across the peninsula NW of Poso, where there are slivers of Miocene limestones, probable Pliocene volcanic tuffs, and Neogene conglomerates and sandstones. This fault, which we name the Tambarana Fault (Figure 15), is the termination of the Tomini Basin and is probably a sinistral strike-slip fault that links to the NW with the Palu-Koro Fault. We suggest Horizon E is Early Pliocene in age. In the Togian Islands Cottam et al. (2011) interpreted deposits of the Bongka Formation (Figure 4), which is part of the Celebes Molasse, to be the distal parts of a Pliocene alluvial fan building out from the East Arm. The Bongka Formation in the Togian Islands and East Arm contains abundant distinctive ophiolitic debris which can only have come from the East Arm ophiolite. Seismic data (Jablonski et al., 2007) show thick (up to 2 sec TWT) north-south trending lobes of sediment inferred to be submerged parts of this fan (Cottam et al., 2011). These observations imply that subsidence of the Poso Basin (from near sea level to present-day depths of 500 to 1500 m) occurred after deposition of the fan. The latest Miocene to Pliocene age of the Celebes Molasse in the East Arm (e.g. Surono & Sukarna, 1993; Hall & Wilson, 2000) therefore indicates rapid subsidence since 5 Ma. All the sediment in the Poso Basin (up to 3 sec TWT) above the buried pinnacle reefs and limestones on the SE side of the Lalanga Ridge (Figure 7) would therefore be Pliocene to Recent in age. This would also suggest a very asymmetrical subsidence from approximately sea level to about 3 km in the centre of Tomini Basin (2 km water + 0.7 sec TWT sediment) but about 6 km in Poso Basin (1.8 km water + 3 sec TWT sediment). The rapid subsidence of both the Tomini and Poso Basins and probable contemporaneous uplift on land are likely to have a common cause. Spencer

(2010, 2011) has identified extensional detachment faults in central Sulawesi (Figure 15) which he suggested are kinematically linked to the Palu-Koro fault. These have exhumed middle to lower crustal metamorphic rocks in the Tokorondo and Pompangeo Mountains (Figure 15). These elevated areas have provided sediment to the Poso Basin (Figure 6 and Figure 7). The subsidence of western Gorontalo Bay, the asymmetry of the Poso Basin, and the extensional detachments in Central Sulawesi to the south are well explained by a simple shear detachment model (Wernicke, 1985; Lister et al., 1986). We propose the extension is very young, indicated by preservation of grooves on the exhumed footwalls (Spencer, 2010), and GPS vectors (Socquet et al. 2006) parallel to the grooves that suggest it is continuing today, and was driven by rollback of the North Sulawesi subduction zone. The interpreted carbonates of Units D and E were deposited above the angular unconformity of Horizon C (Figures 8, 9, 10). There are two possible ages for this unconformity: either about 23 Ma or 15 Ma; the different ages for the seismic units interpreted from these alternatives are shown on Figure 4. The older age of about 23 Ma is based on the interpretation of the unconformity as due to collision of the Banggai–Sula Microcontinental Block with the North Arm Volcanic Arc causing emplacement of the ophiolite. In this model Middle Miocene shallow marine carbonates found in the Togian Islands (Cottam et al., 2011; Figure 4) are interpreted to have been deposited on the unconformity surface after a period of erosion following the collision that produced a wide shallow marine shelf across the whole of Gorontalo Bay. The younger age of about 15 Ma is based on the interpretation of the unconformity as the result of regional extension. van Leeuwen et al. (2007) suggested that the Malino Metamorphic Complex at the western end of the North Arm, immediately north of Tomini Basin, is a core complex exhumed in the Miocene. K/Ar and 40Ar/39Ar cooling ages of 23–11 Ma, indicate exhumation from 27–30 km before eruption of unconformably overlying volcanic rocks at 7 Ma. Sulawesi and the Banda region experienced widespread extension from about 15 Ma (Spakman & Hall, 2010; Hall, 2011) and it is therefore possible that the core complex formation is related to this phase of extension. In this interpretation, the angular unconformity of Horizon C age of about 15 Ma, is still consistent

with the Middle Miocene age of carbonates in the Togian Islands. Units C and B were deposited between the angular unconformity of Horizon C and the unconformity of Horizon A (Figures 8, 9, 10). If the older age of 23 Ma is chosen for Horizon C, Horizon A is most likely to be Middle Eocene (c. 45 Ma), an unconformity age recognised in all the areas close to Gorontalo Bay (Figure 4), which would be correlative with the age of rifting in the Makassar Straits as suggested by Jablonksi et al. (2007). This is not such an obvious correlation as it appears at first sight. First, if the North Arm is restored to its pre-Pliocene position by rotation of 20 along the Palu-Koro Fault the North Makassar Basin and Gorontalo Bay no longer join up in such a straightforward way. They must have been separated by an emergent area to account for the eastern basin margin to the Makassar Straits observed in western Sulawesi (Calvert, 2000; Calvert & Hall, 2007). Second, in contrast to the North Makassar Basin there is no evidence of rifting above Horizon A as discussed above. If the younger age of 15 Ma is chosen for the upper unconformity the deeper unconformity is more likely to be an Early Miocene unconformity (c. 23 Ma), related to collision of the Banggai–Sula Microcontinental Block further east. This would account for some of the features of the seismic data, such as the absence of evidence for rifting and deposition of Unit B on an irregular topography. However, it would mean that subsidence began soon after collision, with no obvious cause. This would require a very thick sequence of clastic sediments to be deposited in a very short time interval (up to 4 sec TWT in c. 8 Ma), and would imply the source was to the SW in Central Sulawesi if the North Arm is restored to its pre-Pliocene position. If Central Sulawesi was an elevated area in the Early Miocene it is difficult explain why there was no significant pulse of clastic sediment carried to the west into western Sulawesi at this time (Calvert, 2000; Calvert & Hall, 2007). We consider that an equally good case can be made for both age interpretations shown in Figure 4 and at present we can see no reason to prefer either. A number of studies are currently underway to provide new data that can test these interpretations. IMPLICATIONS FOR HYDROCARBONS Hydrocarbons are leaking from the south side of the bay. In the East Arm (Figure 15) oil seeps are

reported near Dolang (Rusmana et al., 1993) and gas seeps shown by Kündig (1956) near Tanjung Api, north of Ampana, remain active today. Clearly, given the uncertainties about the age of the sequence and lack of detailed knowledge of the structure of the basins Gorontalo Bay remains a frontier prospect. However, there are a number of features that offer a more positive view of Tomini Basin from a hydrocarbon perspective than previously. There is much more sediment than predicted by the interpretations (compare Figure 3 with Figure 9) of Silver et al. (1983a) and Kadarsuman et al. (2004) and there are structures below Horizon A inconsistent with suggestions that western Gorontalo Bay is underlain by oceanic crust or the East Arm Ophiolite. The most likely basement for Tomini Basin is continental crust similar to that exposed in the Neck, the Malino Complex, and the Tokorondo Mountains, i.e. metamorphic rocks and granites with protoliths that are probably pre-Mesozoic with an Australian origin (van Leeuwen et al., 2007). These formed part of the eastern Sundaland margin from the Early Cretaceous. The eastern part of Gorontalo Bay could be underlain by oceanic crust as suggested by Silver et al. (1983a), although based on observations from the North Arm and Togian Islands (Cottam et al., 2011) it is more likely to be underlain by Paleogene arc volcanics which are potentially underthrust by continental crust of the Banggai–Sula block (Ferdian et al., 2010; Watkinson et al., 2011).

Whatever the age of the lower sequence (Units B and C) these are sediments mainly derived from the west or southwest and their sources are likely to be mainly metamorphic and granitic rocks similar to those that form the Neck and the Tokorondo Mountains of Central Sulawesi. Unit B thins eastwards, and may terminate completely further east (Kerr, 2010), suggesting the possibility that in its early stages Tomini Basin could have been partially or fully enclosed. Sources at the base of the sequence could include lacustrine shales or coals. Sediments of Units B and C are expected to be quartzose with good reservoir properties and rich in organic matter.

We have no evidence to indicate the depth of water in which Units B and C were deposited. They were deformed before deposition of Unit D, although dips are gentle and there are few obvious structural traps. Our suggestion that Unit D clinoforms are carbonates implies that the whole of western Gorontalo Bay was very shallow from Horizon C to Horizon D and the thick sequences of carbonates

between 1 and 2 sec TWT total thickness which are potential reservoirs. The bay remained shallow until about 5 Ma.

Rapid subsidence began at about 5 Ma and rates of subsidence eventually exceeded rates of carbonate production, killing all the reefs except for rare pinnacles on the Lalanga Ridge and close to the present coastlines, and exceeding rates of clastic supply to the Tomini and Poso Basins, so that they are now between 1800m and 2000m deep in their deepest parts. Extension exhumed the deep crust in onshore metamorphic core complexes at the same time as subsidence in Gorontalo Bay which is probably underlain by highly thinned continental crust. In the Togian Islands Pliocene (Cottam et al., 2011) magmatism and Pleistocene-Recent (Katili & Sudradjat, 1984) volcanic activity of Una-Una may be an expression of thinning which induced small amounts of melting of the underlying mantle. We propose that extension of the region was driven by rollback of the North Sulawesi subduction zone, accommodated by movements on the Palu-Koro and Tambarana Faults. More work is required to understand the detailed structural development of Gorontalo Bay and the heatflow implications of crustal/lithosphere extension models and this is now underway. Only in Unit F do we anticipate that there may be a significant volcanic component, which is likely to increase in importance towards the north and east, resulting from magmatism in the North Arm, Togian Islands or Una-Una.

ACKNOWLEDGEMENTS We thank Fugro MCS and Searcher Seismic for the seismic data and TGS for multibeam data, and Chris Elders and Simon Suggate for advice and help. Parinya Pholbud’s MSc programme was funded by PTTEP. REFERENCES Ahlburg, J. 1913. Versuch einer geologischen Darstellung der Insel Celebes. Neue Folge Band 12, Heft 1 Jena, Gustav Fisher 12, 172 pp. Apandi, T. & Bachri, S. 1997. Geological Map of the Kotamobagu Sheet, Sulawesi 1:250,000. Geological Research and Development Centre, Bandung. Bachri, S., Sukido & Ratman, N. 1993. Geological Map of the Tilamuta Sheet, Sulawesi 1:250,000. Geological Research and Development Centre, Bandung.

Bellier, O., Sebrier, M., Seward, D., Beaudouin, T., Villeneuve, M. & Putranto, E. 2006. Fission track and fault kinematics analyses for new insight into the Late Cenozoic tectonic regime changes in West-Central Sulawesi (Indonesia). Tectonophysics 413, 201-220. Bergman, S. C., Coffield, D. Q., Talbot, J. P. & Garrard, R. J. 1996. Tertiary tectonic and magmatic evolution of Western Sulawesi and the Makassar Strait, Indonesia: Evidence for a Miocene continent-continent collision. In: Hall, R. & Blundell, D. J. (Eds.), Tectonic Evolution of SE Asia. Geological Society of London Special Publication, 106, 391-430. Brouwer, H. A. 1947. Geological explorations in Celebes – summary of results. In: Brouwer, H. A. (Ed.), Geological Explorations in the Island of Celebes. North Holland Publishing Company, Amsterdam, 1-64. Calvert, S. J. 2000. The Cenozoic evolution of the Lariang and Karama basins, Sulawesi. Indonesian Petroleum Association, Proceedings 27th Annual Convention, 505-511. Calvert, S. J. & Hall, R. 2007. Cenozoic Evolution of the Lariang and Karama regions, North Makassar Basin, western Sulawesi, Indonesia. Petroleum Geoscience 13, 353-368. Cottam, M. A., Hall, R., Forster, M. & Boudagher Fadel, M. 2011. Basement character and basin formation in Gorontalo Bay, Sulawesi, Indonesia: New observations from the Togian Islands. In: Hall, R., Cottam, M. A. & Wilson, M. E. J. (Eds.), The SE Asian Gateway: History and Tectonics of the Australia-Asia collision. Geological Society of London Special Publication, 355, 177-202. Davidson, J. W. 1991. The geology and prospectivity of Buton Island, Southeast Sulawesi, Indonesia. Indonesian Petroleum Association, Proceedings 20th Annual Convention, 209-234. Droste, H. 2010. High-resolution seismic stratigraphy of the Shu’aiba and Natih formations in the Sultanate of Oman: implications for Cretaceous epeiric carbonate platform systems. In: van Buchem, F. S. P., Gerdes, K. D. & Esteban, M. (Eds.), Mesozoic and Cenozoic Carbonate Systems of the Mediterranean and the Middle East: Stratigraphic and Diagenetic Reference Models. Geological Society of London Special Publication, 329, 145-162.

Eberli, G. P. & Ginsburg, R. N. 1987. Segmentation and coalescence of Cenozoic carbonate platforms, north-western Great Bahama Bank. Geology 15, 75-79. Elburg, M. & Foden, J. 1999. Sources for magmatism in central Sulawesi: geochemical and Sr-Nd-Pb isotopic constraints. Chemical Geology 156, 67-93. Elburg, M., van Leeuwen, T., Foden, J. & Muhardjo 2003. Spatial and temporal isotopic domains of contrasting igneous suites in Western and Northern Sulawesi, Indonesia. Chemical Geology 199, 243-276. Farzadi, P. 2006. The development of Middle Cretaceous carbonate platforms, Persian Gulf, Iran: constraints from seismic stratigraphy, well and biostratigraphy. Petroleum Geoscience 12, 59-68. Ferdian, F., Watkinson, I. M. & Hall, R. 2010. A structural re-evaluation of the North Banggai-Sula area, eastern Sulawesi. Proceedings Indonesian Petroleum Association, 34th Annual Convention, IPA10-G-009 1-20. Garrard, R. A., Supandjono, J. B. & Surono 1988. The geology of the Banggai-Sula microcontinent, Eastern Indonesia. Indonesian Petroleum Association, Proceedings 17th Annual Convention, 23-52. Hall, R. 1996. Reconstructing Cenozoic SE Asia. In: Hall, R. & Blundell, D. J. (Eds.), Tectonic Evolution of SE Asia. Geological Society of London Special Publication, 106, 153-184. Hall, R. 2002. Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: computer-based reconstructions, model and animations. Journal of Asian Earth Sciences 20, 353-434. Hall, R. 2011. Australia-SE Asia collision: plate tectonics and crustal flow. In: Hall, R., Cottam, M. A. & Wilson, M. E. J. (Eds.), The SE Asian gateway: history and tectonics of Australia-Asia collision. Geological Society of London Special Publication, 355, 75-109. Hall, R. & Wilson, M. E. J. 2000. Neogene sutures in eastern Indonesia. Journal of Asian Earth Sciences 18, 787-814. Hall, R., Clements, B. & Smyth, H. R. 2009. Sundaland: Basement character, structure and plate

tectonic development. Proceedings Indonesian Petroleum Association, 33rd Annual Convention, IPA09-G-134 1-27. Hamilton, W. 1979. Tectonics of the Indonesian region. USGS Professional Paper 1078, 345pp. Jablonski, D., Priyono, R., Westlake, S. & Larsen, O. A. 2007. Geology and exploration potential of the Gorontalo Basin, Central Indonesia - eastern extension of the North Makassar Basin? Indonesian Petroleum Association, Proceedings 31st Annual Convention, 197-224. Kadarusman, A., Miyashita, S., Maruyama, S., Parkinson, C. D. & Ishikawa, A. 2004. Petrology, geochemistry and paleogeographic reconstruction of the East Sulawesi Ophiolite, Indonesia. Tectonophysics 392, 55-83. Katili, J. A. 1978. Past and present geotectonic position of Sulawesi, Indonesia. Tectonophysics 45, 289-322. Katili, J. A. & Sudradjat, A. 1984. The devastating 1983 eruption of Colo Volcano, Una-Una Island, Central Sulawesi, Indonesia. Geologisches Jahrbuch, Reihe B 75, 27-47. Kavalieris, I., van Leeuwen, T. M. & Wilson, M. 1992. Geological setting and styles of mineralization, north arm of Sulawesi, Indonesia. Journal of Southeast Asian Earth Sciences 7, 113-130. Kerr, A. 2010. The tectono-stratigraphic evolution of central Gorontalo Bay, eastern Indonesia. MSc Thesis, Royal Holloway University of London. Klompé, T. H. F. 1954. The structural importance of the Sula Spur (Indonesia). Indonesian Journal of Natural Sciences 110, 21-40. Kündig, E. 1956. Geology and ophiolite problems of East Celebes. Verhandelingen Koninklijk Nederlands Geologisch en Mijnbouwkundig Genootschap, Geologische Serie 16, 210-235. Lister, G. S., Etheridge, M. A. & Symonds, P. A. 1986. Detachment faulting and the evolution of passive continental margins. Geology 14, 246-250. McCaffrey, R. & Sutardjo, R. 1982. Reconnaissance microearthquake survey of Sulawesi, Indonesia. Geophysical Research Letters 9, 793-796.

Parkinson, C. D. 1991. The petrology, structure and geologic history of the metamorphic rocks of Central Sulawesi, Indonesia. PhD Thesis, University of London. Parkinson, C. 1998a. An outline of the petrology, structure and age of the Pompangeo Schist Complex of central Sulawesi, Indonesia. Island Arc 7, 231-245. Parkinson, C. 1998b. Emplacement of the East Sulawesi Ophiolite: evidence from subophiolitic metamorphic rocks. Journal of Asian Earth Sciences 16, 13-28. Parkinson, C. D., Miyazaki, K., Wakita, K., Barber, A. J. & Carswell., D. A. 1998. An overview and tectonic synthesis of the pre-Tertiary very-high-pressure metamorphic and associated rocks of Java, Sulawesi and Kalimantan, Indonesia. Island Arc 7, 184-200. Pigram, C. J., Surono & Supandjono, J. B. 1985. Origin of the Sula Platform, eastern Indonesia. Geology 13, 246-248. Polvé, M., Maury, R. C., Bellon, H., Rangin, C., Priadi, B., Yuwono, S., Joron, J. L. & Atmadja, R. S. 1997. Magmatic evolution of Sulawesi (Indonesia): Constraints on the Cenozoic geodynamic history of the Sundaland active margin. Tectonophysics 272, 69-92. Polvé, M., Maury, R.-C., Vidal, P., Priadi, B., Bellon, H., Soeria-Atmadja, R., Joron, J.-L. & Cotten, J. 2001. Melting of lower continental crust in a young post-collision setting: A geochemical study of Plio-Quaternary acidic magmatism from central Sulawesi (Indonesia). Bulletin de la Société Géologique de France 172, 333-342. Priadi, B., Polvé, M., Maury, R., Soeria-Atmadja, R. & Bellon, H. 1993. Geodynamic implications of Neogene potassic calc alkaline magmatism in central of Sulawesi: geochemical and isotopic constraints. Proceedings, 22nd Annual Convention of the Indonesian Association of Geologists (IAGI) 1, 59-81. Priadi, B., Polvé, M., Maury, R. C., Bellon, H., Soeria-Atmadja, R., Joron, J. L. & Cotten, J. 1994. Tertiary and Quaternary magmatism in Central Sulawesi: chronological and petrological constraints. Journal of Southeast Asian Earth Sciences 9, 81-93.

Rangin, C., Maury, R. C., Polvé, M., Bellon, H., Priadi, B., Soeria-Atmadja, R., Cotten, J. & Joron, J. L. 1997. Eocene to Miocene back-arc basin basalts and associated island arc tholeiites from northern Sulawesi (Indonesia): implications for the geodynamic evolution of the Celebes basin. Bulletin de la Société Géologique de France 168, 627-635. Ratman, N. 1976. Geologic Map of Tolitoli Quadrangle, North Sulawesi 1:250,000. Geological Research and Development Centre, Bandung. Rusmana, E., Koswara, A. & Simandjuntak, T. O. 1993. Geology of the Luwuk Sheet, Sulawesi 1:250,000. Geological Research and Development Centre, Bandung. Rutten, L. M. R. 1927. Voordrachten over de geologie van Nederlandsch Oost Indie. Wolters, Groningen, 839 pp. Sarasin, P. & Sarasin, F. 1901. Materialien zur Naturgeschichte der Insel Celebes. Vierter Band: Entwurf einer Geografisch-Geologischen Beschreibung der Insel Celebes. G.W. Kreidel's Verlag, Wiesbaden, Germany, 344 pp. Silver, E. A., McCaffrey, R., Joyodiwiryo, Y. & Stevens, S. 1983a. Ophiolite emplacement and collision between the Sula platform and the Sulawesi island arc, Indonesia. Journal of Geophysical Research 88, 9419-9435. Silver, E. A., McCaffrey, R. & Smith, R. B. 1983b. Collision, rotation, and the initiation of subduction in the evolution of Sulawesi, Indonesia. Journal of Geophysical Research 88, 9407-9418. Simandjuntak, T. O. 1986. Sedimentology and tectonics of the collision complex in the East Arm of Sulawesi, Indonesia. PhD Thesis, University of London, 374 pp. Simandjuntak, T. O., Surono & Supandjono, J. B. 1997. Geological Map of the Poso Quadrangle, Sulawesi 1: 250,000. Geological Research and Development Centre, Bandung. Smith, R. B. & Silver, E. A. 1991. Geology of a Miocene collision complex, Buton, eastern Indonesia. Geological Society of America Bulletin 103, 660-678. Socquet, A., Simons, W., Vigny, C., McCaffrey, R., Subarya, C., Sarsito, D., Ambrosius, B. & Spakman, W. 2006. Microblock rotations and fault coupling in

SE Asia triple junction (Sulawesi, Indonesia) from GPS and earthquake slip vector data. Journal of Geophysical Research 111, doi 10.1029/2005JB003963. Spakman, W. & Hall, R. 2010. Surface deformation and slab-mantle interaction during Banda arc subduction rollback. Nature Geoscience 3, 562-566. Spencer, J. E. 2010. Structural analysis of three extensional detachment faults with data from the 2000 Space-Shuttle Radar Topography Mission. GSA Today 20 August, 4-10. Spencer, J. E. 2011. Gently dipping normal faults identified with Space Shuttle radar topography data in central Sulawesi, Indonesia, and some implications for fault mechanics. Earth and Planetary Science Letters 308, 267-276. Sukamto, R. 1973. Reconnaissance Geological Map of Palu Quadrangle, Sulawesi 1:250,000. Geological Research and Development Centre, Bandung. Sukamto, R. & Simandjuntak, T. O. 1983. Tectonic relationship between geologic provinces of western Sulawesi, eastern Sulawesi and Banggai-Sula in the light of sedimentological aspects. Bulletin Geological Research and Development Centre, Bandung 7, 1-12. Supandjono, J. B. & Haryono, E. 1993. Geology of the Banggai Sheet, Sulawesi - Maluku 1:250,000. Geological Research and Development Centre, Bandung. Surmont, J., Laj, C., Kissal, C., Rangin, C., Bellon, H. & Priadi, B. 1994. New paleomagnetic constraints on the Cenozoic tectonic evolution of the North Arm of Sulawesi, Indonesia. Earth and Planetary Science Letters 121, 629-638. Surono & Sukarna, D. 1993. Geology of the Sanana Sheet, Maluku, 1:250,000. Geological Research and Development Centre, Bandung. Surono, Simandjuntak, T. O., Situmorang, R. L. & Sukido 1993. Geological Map of the Batui Quadrangle 1:250,000. Geological Research and Development Centre, Bandung.

Taylor, D. & van Leeuwen, T. M. 1980. Porphyry-type deposits in Southeast Asia. In: Ishihara, S. & Takenouchi, S. (Eds.), Granitic Magmatism and Related Mineralization. Mining Geologist Japan Special Issue, 8, 95-116. van Leeuwen, T. M. 1981. The geology of southwest Sulawesi with special reference to the Biru area. In: Barber, A. J. & Wiryosujono, S. (Eds.), The Geology and Tectonics of Eastern Indonesia. Geological Research and Development Centre, Bandung, Special Publication, 2, 277-304. van Leeuwen, T. & Muhardjo 2005. Stratigraphy and tectonic setting of the Cretaceous and Paleogene volcanic-sedimentary successions in northwest Sulawesi, Indonesia: implications for the Cenozoic evolution of Western and Northern Sulawesi. Journal of Asian Earth Sciences 25, 481-511. van Leeuwen, T., Allen, C. M., Kadarusman, A., Elburg, M. & Michael Palin, J. 2007. Petrologic, isotopic, and radiometric age constraints on the origin and tectonic history of the Malino Metamorphic Complex, NW Sulawesi, Indonesia. Journal of Asian Earth Sciences 29, 751-777. Vigny, C., Perfettini, H., Walpersdorf, A., Lemoine, A., Simons, W., Loon, D. v., Ambrosius, B., Stevens, C., McCaffrey, R., Morgan, P., Bock, Y., Subarya, C., Manurung, P., Kahar, J., Abidin, H. Z. & Abu, S. H. 2002. Migration of seismicity and earthquake interactions monitored by GPS in SE Asia triple junction: Sulawesi, Indonesia. Journal of Geophysical Research 107, doi:10.1029/2001JB000377. Walpersdorf, A., Rangin, C. & Vigny, C. 1998. GPS compared to long-term geologic motion of the north arm of Sulawesi. Earth and Planetary Science Letters 159, 47-55. Watkinson, I. M., Hall, R. & Ferdian, F. 2011. Tectonic re-interpretation of the Banggai-Sula–Molucca Sea margin, Indonesia. In: Hall, R., Cottam, M. A. & Wilson, M. E. J. (Eds.), The SE Asian gateway: history and tectonics of Australia-Asia collision. Geological Society of London Special Publication, 355, 203-224. Wernicke, B. 1985. Uniform-sense normal simple shear of the continental lithosphere. Canadian Journal of Earth Sciences 22, 108-125.

Figure 1 – Location of the study area (red box).

Figure 2 – Principal tectonic subdivisions of Sulawesi.

Figure 3 – Previous interpretations of Gorontalo Bay from (A) Silver et al. (1983a); (B) Kadarusman et al.

(2004), and (C) Jablonski et al. (2007). Location of cross sections A, B and C shown in inset map.

Figure 4 – Stratigraphy of areas of Sulawesi surrounding Gorontalo Bay modified from Bachri et al. (1993);

Rusmana et al. (1993); Surono et al. (1993); Apandi & Bachri (1997); van Leeuwen & Muhardjo (2005) and Cottam et al. (2011). The two interpretations (1 and 2) of the age of seismic units of this study are discussed in the text.

Figure 5 – Seismic grid used in this study. The main focus of this study was the area west of 12130’E. LR

marks the island and reefs of Lalanga.

Figure 6 – Multibeam coverage of Gorontalo Bay. LR is at the SW end of the Lalanga Ridge which

separates the southeastern sub-basin (Poso Basin) from the northwestern sub-basin (Tomini Basin). Tomini Basin was the focus of this study.

Figure 7 – Uninterpreted seismic lines crossing the Lalanga Ridge and showing the southeastern and

northwestern sub-basins. G06-208 (above) is west of G06-210 (below).

Figure 8 – Seismic units and horizons identified in this study.

Figure 9 – Uninterpreted and interpreted seismic line crossing the Tomini Basin from NW to SE showing the seismic units identified in this study.

Figure 10 – Uninterpreted and interpreted seismic line crossing the Tomini Basin from SW to NE showing

the seismic units identified in this study.

Figure 11 – Time structure maps for the principal seismic horizons mapped in this study.

Figure 12 – Time thickness maps for the seismic units mapped in this study.

Figure 13 – Oblique 3D image of seafloor of western Gorontalo Bay from multibeam coverage, looking

northeastwards.

Figure 14 – Detail of seismic line in Tomini Basin centre showing prograding clinoforms of Unit D.

Figure 15 – Principal structural features of western Gorontalo Bay and surrounding areas based on this

study and mapping on land using SRTM imagery and from our fieldwork.