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Back to Session IPA12-G-029

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

A NEW INTERPRETATION OF BAY,

Parinya Pholbud* Robert Hall* Eldert Advokaat* Pete Burgess* Alfend Rudyawan*

ABSTRACT from coals at the base or organic matter in sands; an upper sequence of shallow marine carbonates with Recent seismic and multibeam data, combined with little volcanic material, and a bay that has subsided information from land, provide the basis for a new significantly only since about 5 Ma. interpretation of western Gorontalo Bay. The stratigraphy is divided into three parts. The basement (Unit A) is proposed to be INTRODUCTION continental crust, below a major unconformity interpreted to be either Mid or Early Gorontalo Bay (Figure 1) is one of several Miocene in age. Above the unconformity is a enigmatic basins in East . It is a semi- sequence up to 6 sec TWT divided into two parts. enclosed sea, separating the mountainous North The lower part (Units B and C) is interpreted as Arm from Metamorphic Belt and quartz-rich marine sediments, with little volcanic the East Arm Ophiolite (Figure 2). To the west it is debris, derived from granites and continental bounded by the narrow Sulawesi Neck. Early basement of western Sulawesi. There is a minor researchers (Ahlburg, 1913; Rutten, 1927) reported unconformity at the base of the upper part (Units D water depths up to 2000m in the western part of the to F). We suggest Units D and E are carbonates. bay and rapid deepening within short distances of Unit D has major clinoforms indicating water the coast but until recently there was little depths less than 200m. At the top of Unit E are information on the bathymetry or geology of the linear bands of pinnacle reefs which step back bay. towards the north. They mark rapid subsidence which began at about 5 Ma and are partly buried by Ideas about the nature of Gorontalo Bay are widely Pliocene–Recent deposits (Unit F) in the basin divergent (Figure 3). Silver et al. (1983a) suggested centre. This may include quartzose siliciclastic that the eastern part of Gorontalo Bay (Figure 3A) material, carbonate sediment and volcanic ash. is a forearc basin, with a relatively thin sedimentary cover above oceanic crust in the North Arm forearc. Given the observed subsidence the absence of They considered that there was a north-dipping significant faulting in the basin is noteworthy. We oceanic slab connected to the Banggai–Sula Block, suggest the cause of extension is lithosphere-scale based on “extreme low-gravity values” near the low angle normal faulting driven by subduction Batui Thrust and hypocentres dipping northwards to roll-back at the trench. This is a depth of about 100 km (McCaffrey & Sutardjo, consistent with the existence of young rapidly 1982). Kadarusman et al. (2004) suggested that the exhumed core complexes and low angle western part of Gorontalo Bay is underlain by detachments on land. Hydrocarbons are leaking oceanic crust of the East Sulawesi Ophiolite (Figure from the south side of the bay. Important 3B) with a thin sedimentary cover, thrust over the considerations for a petroleum system are: a Banggai–Sula Block. In contrast, Hamilton (1979) basement of predominantly highly thinned estimated a five kilometre thick sediment infill in Australian continental crust accreted to the the western part of the bay, which is supported by Sundaland margin in the mid Cretaceous; a recent seismic surveys (Jablonski et al., 2007) that quartzose lower sequence with possible sources show up to 6 sec TWT of sediments below the sea bed. Jablonski et al. (2007) considered Gorontalo * SE Research Group Bay to be a lateral equivalent of the Strait, based on relative constant thickness of interpreted pillow lavas (Kündig, 1956; Simandjuntak, Upper Miocene deposits, following relatively young 1986; Parkinson, 1998b; Kadarusman et al., uplift of the Neck which separated the two basins. 2004). They interpreted the basement to be cut by numerous faults related to Eocene extension of 5. Microcontinental fragments of Australian origin continental crust which included fragments rifted (Pigram et al., 1985), including the Banggai– from and added to the Sundaland margin Sula (Garrard et al., 1988; Supandjono & in the Cretaceous. Haryono, 1993) and Tukang Besi– (Davidson, 1991; Smith & Silver, 1991) Blocks, In this paper we discuss the geology of Gorontalo previously considered to be separate fragments Bay based on recently acquired seismic (courtesy of sliced from and carried west along Fugro MCS and Searcher Seismic) and multibeam the Sorong (Hamilton, 1979). These are data (courtesy of TGS). We present a seismic now interpreted as part of the Sula Spur interpretation and consider the implications for the (Klompé, 1954) that collided with the North tectono-stratigraphic evolution of Gorontalo Bay in Arm in the Early Miocene (Hall, 1996, 2002) the context of its regional setting. and subsequently fragmented (Spakman & Hall, 2010; Hall, 2011).

Geological setting Based on the references cited above, the Sulawesi is located at the triple junction where the relationships between these different provinces and Pacific–Philippine and the Indian–Australian plates the tectonic development of Sulawesi can be are subducted below the (Figure 1). summarised as follows. The metamorphic As result of this convergence, Sulawesi is composed complexes in West and Central Sulawesi were of numerous fragments derived from these major emplaced or formed as consequence of mid- plates. They include metamorphic complexes, Cretaceous accretion of Australian continental magmatic provinces, volcanic arcs, ophiolites and fragments to Sundaland. During the Early Eocene micro-continental fragments and are commonly western Sulawesi rifted away from forming (e.g. Katili, 1978; Hamilton, 1979; Taylor & van the at the same time as oceanic Leeuwen, 1980; Sukamto & Simandjuntak, 1983) spreading in the . From the Middle grouped into a number of major tectonic units Eocene to Late Oligocene the North Arm Volcanic (Figure 2): Arc formed as consequence of north-directed subduction of the beneath the North 1. Western Sulawesi Plutono-volcanic Province, Arm. In the Early Miocene, the East Sulawesi the eastern continental margin of Sundaland Ophiolite was emplaced by northwest-directed (Hamilton, 1979; Polvé et al., 1997; van obduction during collision of the Sula Spur with the Leeuwen et al., 2007) formed largely of North Arm. The Central Sulawesi Metamorphic Australian fragments accreted in the Cretaceous Belt includes metamorphic rocks associated with (Jablonski et al., 2007; Hall et al., 2009). the ophiolite, the Sundaland margin and the Banggai–Sula Block. Regional extension, from 2. North Sulawesi Volcanic Province, an intra- Sulawesi eastwards, commenced in the Middle oceanic volcanic of dominantly Miocene, associated with rollback of subduction tholeiitic composition underlain by oceanic into the Banda embayment. There was widespread crust (Rangin et al., 1997; Elburg et al., 2003; magmatic activity in the Western Sulawesi Plutono- van Leeuwen & Muhardjo, 2005). volcanic Province, probably mainly extension- related, from the Middle Miocene producing high-K 3. Central Sulawesi Metamorphic Belt, a terrane calc-alkaline acidic igneous rocks. Southwards made up predominantly of phyllites and schists subduction of the Celebes Sea under the North Arm of both continental and oceanic origin (Rutten, is interpreted to have started around 5 Ma. 1927; Brouwer, 1947; Parkinson, 1991, 1998a, Subduction rollback of the North Sulawesi b). subduction zone initiated a new phase of extension in northern Sulawesi which has been active until the 4. East Sulawesi Ophiolite, a dismembered present-day. sequence including dunite, lherzolites and harzburgites, ultramafic cumulates, layered and The geology of Gorontalo Bay and surrounding isotropic gabbros, sheeted dykes and basaltic areas of , the Neck, North Arm and East Arm reflects this complex history. The breccia, tuffs and limestones. Originally this was stratigraphy of these areas is summarised in Figure considered to be a single formation formed after a 4 based on observations by Sukamto (1973), Miocene collision (Kündig, 1956) but recent studies Ratman (1976), Bachri et al. (1993), Rusmana et al. have shown that it is diachronous across Sulawesi (1993), Surono et al. (1993), Apandi & Bachri (Hall & Wilson, 2000), locally very lithologically (1997), Simandjuntak et al. (1997), van Leeuwen & variable, and it probably records several episodes of Muhardjo (2005), and Cottam et al. (2011). uplift and erosion around Gorontalo Bay in the latest Miocene to Plio-. 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 K–Ar, Ar–Ar and Apatite fission track ages indicate granites and metamorphic rocks with protolith ages rapid uplift in the Pliocene (van Leeuwen & which are Cretaceous or older. Cretaceous low Muhardjo, 2005; Bellier et al., 2006) grade metasediments are known from western contemporaneous with sinistral movement on the Sulawesi and undated low grade metasediments are - Fault, and 20° clockwise rotation of the found in the . The basement rocks are North Arm around a pole close to overlain unconformably by Paleogene mainly interpreted from geological observations (Silver et clastic sediments in western Sulawesi (Budung- al., 1983b) and palaeomagnetic data (Surmont et al., Budung Formation) to very low grade 1994). This has been linked to subduction at the metasediments (Tinombo Formation) in the Neck. North Sulawesi Trench. New tectonic models of the These are considered to pass into the Papayato region indicate an important role for Neogene Volcanics of the North Arm which resemble extension, related initially to subduction rollback in Paleogene pillow lavas and volcaniclastic the Banda Arc (Spakman & Hall, 2010) and later to sandstones (Walea Formation) found in the Togian subduction rollback of the North Sulawesi Islands. To the southeast Paleogene rocks, which subduction zone (Hall, 2011). Extension at different are predominantly shallow marine limestones, are times is manifested in several ways. The Malino found in the collision complex between the East Complex northwest of Gorontalo Bay is suggested Arm Ophiolite and the Banggai–Sula Block. to be an extensional metamorphic core complex (Kavalieris et al., 1992; van Leeuwen et al., 2007) of Miocene age. West of Gorontalo Bay, mountains There is a widespread but poorly dated more than 1 km high form the narrow Neck which unconformity recording collision of the Banggai– exposes medium to high-grade continental Sula Microcontinental Block, East Arm Ophiolite metamorphic rocks with young ages (Sukamto, and North Arm Volcanic Arc in the Early Miocene. 1973; Elburg et al., 2003; van Leeuwen et al., There are limited exposures of Neogene rocks in all 2007). Pinnacle reefs seen on seismic sections from the areas surrounding Gorontalo Bay. In the North Gorontalo Bay are interpreted by Jablonski et al. Arm there are a variety of acid eruptive and (2007) to be Late Miocene or younger and intrusive rocks, and some clastic sedimentary rocks. correlations with the Middle–Upper Miocene In the Neck there are granite bodies of the Dondo Peladan Formation on the Togian Islands. These Suite. Radiogenic isotope analyses (Priadi et al., suggest that Gorontalo Bay was a shallow marine 1993; Priadi et al., 1994; Bergman et al., 1996; area until the latest Miocene with rapid subsidence Polvé et al., 1997; Elburg & Foden, 1999; Polvé et from the Early Pliocene (Cottam et al., 2011). al., 2001; Elburg et al., 2003) and U–Pb dating (van Medium-K to shoshonitic volcanism on the Togian Leeuwen et al., 2007) of the igneous rocks in the Islands is suggested to be the result of crustal Palu and Neck regions indicate melting of lower thinning in the Plio-Pleistocene (Cottam et al., crustal material of Australian affinity. In the Togian 2011). South of Gorontalo Bay, young core Islands there are Middle Miocene shallow marine complexes have been identified at the Tokorondo limestones, and similar rocks are known from the Mountains and the Pompangeo Mountains based on East Arm east of the Togian Islands and in the morphological features observed on SRTM images Banggai–Sula Islands. (Spencer, 2010, 2011). GPS measurements (Walpersdorf et al., 1998; Vigny et al., 2002; Pre-Miocene rocks throughout Sulawesi are Socquet et al., 2006) indicate that this motion is unconformably overlain by the Celebes Molasse continuing at the present day and potentially (Sarasin & Sarasin, 1901) a poorly lithified accommodates major top-to-the-north extensional sequence of interbedded conglomerates, sandstones, exhumation in Central Sulawesi (Spencer, 2010, and mudstones with subordinate intercalations of 2011). DATASET AND METHODOLOGY Six key seismic horizons were interpreted in Tomini Basin to define seismic megasequences and This project was provided with a 2D seismic data geological structures. The seismic units identified set by Fugro MCS and Searcher Seismic (Figure 5). were named from base to top as Units A, B, C, D, E The Gorontalo 2D Non-Exclusive Seismic Survey and F (Figure 8). Interpreted horizons were mapped was acquired in 2006 with a total length of 8,009 to construct two-way-time thickness maps and the km. The survey was acquired utilizing long offset seismic stratigraphy of each unit was studied. seismic cabling with recording to between 8 and 12 Digital seafloor topography from multibeam data sec TWT. The key processing parameters for the was correlated with interpreted seismic horizons to seismic data are SRME and Kirchhoff PSTM. The define the paleoshelf-edge of carbonates within the spacing between each seismic line was basin. approximately 7 km. Seismic data were interpreted using SMT Kingdom Suite on a Windows UNIT A workstation. The top of Unit A is marked by an angular Multibeam data (Figure 6) were provided by TGS. unconformity which is the deepest seismic horizon This coverage was acquired using a Kongsberg mappable through the whole of Tomini Basin. The Simrad EM120 Multibeam Echo Sounder using 191 unit is characterized by complex zone of moderate beams at equidistant spacing. Positioning control to strong reflectors (Figures 9 and 10). Locally there used a C-Nav Starfire DGPS. During processing, are reflectors that are parallel or sub-parallel to the positioning, tidal and calibration corrections were unconformity surface whereas in other places there applied, random noise and artefacts were removed, are reflectors that are oblique to it with discordances and a terrain model using a 25 m bin size was up to 10. At the NW end of some seismic lines gridded and exported to ESRI format. Multibeam there are NW-dipping normal faults below the data were further processed in ER Mapper to unconformity surface (Figure 9), but apart from remove voids and generate a digital elevation model these there is little evidence for rifting during (DEM) and used to display seafloor topography in deposition of units above the unconformity. 3D. UNIT B Seismic Units The Tomini Basin essentially begins with The seismic and multibeam data show that the deposition of Unit B. This package onlaps the basal Gorontalo Bay changes character at about 12130’E unconformity of Horizon A. This unit consists of (Figure 6). East of this longitude the basin is moderate to strong, continuous, parallel reflectors, separated into two parts by a shallow ridge that which dip at a low angle towards to the west in the includes the modern volcano of Una-Una and the western part of the basin and are sub-horizontal in Togian Islands and there is a relatively narrow, the east. Unit B is mainly thicker than 1 sec TWT broadly E–W orientated northern deep area north of and reaches a maximum thickness of approximately the Togian Islands. West of 12130’E the basin is 2.3 sec TWT in the basin centre (Figure 11) and much wider and deeper. This western part of thins toward the basin flanks. A ridge crosses the Gorontalo Bay is divided into two sub-basins by a basin (Figures 9 and 10) which is orientated broadly NE-trending wide ridge capped by drowned E-W (Figure 11) and divides Unit B into two carbonate reefs we name the Lalanga Ridge after a depocentres. There are no signs of faulting small island with reefs at its SW end. The SE sub- associated with the ridge which appears to have basin ( Basin) is slightly shallower (Figure 7) been a pre-existing feature that was gradually than the NW sub-basin (Tomini Basin) with a onlapped and buried by sediment transported from maximum depth of 1800m compared to 2000m, and the west. contains much less sediment (about 3 sec TWT compared to 6 sec TWT). Although there is a UNIT C relatively close spacing of seismic lines it is not possible to correlate from the larger Tomini Basin Unit C lies conformably above Unit B, and is to the smaller Poso Basin across the Lalanga Ridge. separated from it by a strong reflector. It varies in In this project we therefore focused on the broader thickness between 1 and 2 sec TWT. This unit is and deeper Tomini Basin which contains the much characterized by moderate to strong continuous thicker sequence. reflectors with small offset normal faults. These normal faults are only observed in seismic lines that 1987; Faradic, 2006; Dorset, 2010). The clinoforms are oriented NE-SW and are less common in the appear to be strongly progradational linear features western part of the basin. They are almost entirely (rather than point sourced locate features as would confined to Unit C although a few cut down into be expected with a siliciclastic delta system on this Unit B. A few faults have been inverted forming scale) and are interpreted to pass upwards into an small anticlinal structures within the unit. Units B apparently genetically related unit with large-scale and C are restricted to a relatively narrow basin stacking patterns changing from strongly (compared to the present western Gorontalo Bay) of progradational to aggradations to retro gradational, about 50 km width which is elongate and trends and terminating as a series of drowned pinnacle approximately ENE-WSW. Unit B clearly thins to features. The basin shape changed and widened the NE consistent with a western source. This is less during deposition of Unit D and the northern basin clear for Unit C which seems to thin slightly margin changed from an ENE-WSW to a NNE- towards the central part of the basin (Figure 10) but SSW orientation. then begins to thicken slightly towards the NE. The unit also appears to thin towards the SE. These UNIT E observations suggest sediment input predominantly from the north from both the W and E ends of the Unit E is characterised by moderate amplitude basin. reflectors that are generally sub-horizontal with locally shallow dips to the northwest. It shows There is little to indicate the environment of gradational and retro gradational stacking patterns deposition of Units B and C. The well bedded with many pinnacle reefs in its upper part (Figure 9 character and lateral continuity of reflectors for and Figure 10) which we interpret to represent a considerable distances suggest a marine setting. shallow marine environment for the whole of the There are no signs of slumping or disturbance unit. The thickness increases toward the northwest indicating rapid deepening or unstable slopes. with a maximum of 2.4 sec TWT and the unit thins Almost all the faults in Tomini Basin are small significantly in the basin centre and pinches out as it offset normal faults restricted to Unit C, and are onlaps the Lalanga Ridge to the south. During observed only in E-W and NE-SW lines. This deposition of Unit E the NNE-SSW northern suggests they are a response to flexure associated margin shelf edge rotated to a NE-SW orientation with deformation that resulted in the Horizon C add the shelf edge retreated to the north marked by unconformity. Units B and C appear to fill in a pre- multiple lines of sub-parallel pinnacles and existing basin topography which was gradually implying episodic subsidence of the basin. buried rather than created by rifting. UNIT F UNIT D Unit F is the youngest sequence in Tomini Basin. It Unit D is separated from underlying units by an has relatively low amplitude flat reflectors in the angular unconformity of Horizon C. Units B and C basin centre where it is thinly bedded, up to 0.7 sec are tilted gently to the north with a dip of a few TWT thick, and appears to be fed from the west. In degrees subparallel to the northern slope of the the northern part of the basin it is thinner with a Lalanga Ridge. Reflectors are truncated by the thickness up to 0.4 sec TWT and it buries or partly unconformity surface (Figure 9) north of the buries many of the pinnacle reefs. In this part of the Lalanga Ridge. On the north-eastern end of the basins sediment appears to have entered the basin ridge is a large rimmed carbonate reef complex from several sources in the north. The depth of the (Figure 13). Unit D appears to be the lateral pinnacle reefs varies from 2000m at the basin centre equivalent of the lower part of the reefs as both rest to approximately 1000m at the northern limit of the unconformably on the unconformity surface. It multibeam coverage (Figure 14) indicating reaches a thickness of about 1 sec TWT and subsidence of at least 2000m since the formation of contains a series of clinoforms reflectors that the oldest reefs at the top of Unit E close to the prograde towards the basin centre (Figure 14). We basin centre. Although most of the sediment appears interpret this unit as shallow water carbonates, to be derived from the west, the presence of largely prograding to the south but with a thinner Pliocene volcanic tuffs in the Togian Islands (Cot equivalent package prograding northwards from the tam et al., 2011), and evidence of explosive acid Lalanga Ridge (Figure 9). This is based on the strata volcanic activity in the North Arm (Bachri et al. geometries observed in the clinoforms and the 1993; Apandi & Bachri, 1997), suggest the material overlying carbonate strata (e.g. Oberlin & Ginsburg, that was sourced from the north, and sediment in the eastern part of Tomini Basin, may include a (2010, 2011) has identified extensional detachment significant volcanic component. faults in central Sulawesi (Figure 15) which he suggested are kinematically linked to the Palu-Koro UNIT AGES AND BASIN HISTORY fault. These have exhumed middle to lower crustal Because there are no wells in Gorontalo Bay the metamorphic rocks in the Tokorondo and ages of horizons must be interpreted from other Pompangeo Mountains (Figure 15). These elevated data. This presents some difficulties because the areas have provided sediment to the Poso Basin very thick sequence in the Tomini Basin cannot be (Figure 6 and Figure 7). The subsidence of western traced on to land, and in the surrounding areas of Gorontalo Bay, the asymmetry of the Poso Basin, the North Arm, Neck and East Arm, exposures are and the extensional detachments in Central predominantly basement rocks. The almost 6 sec Sulawesi to the south are well explained by a simple TWT of section (Figure 10) must terminate abruptly shear detachment model (Wernicke, 1985; Lister et to the west, presumably at a fault, because on land al., 1986). We propose the extension is very young, there are high-grade schists and granites in the indicated by preservation of grooves on the Neck. There is a NW-SE fault on land, cutting exhumed footwalls (Spencer, 2010), and GPS across the peninsula NW of Poso, where there are vectors (Socquet et al. 2006) parallel to the grooves slivers of Miocene limestones, probable Pliocene that suggest it is continuing today, and was driven volcanic tuffs, and Neogene conglomerates and by rollback of the North Sulawesi subduction zone. sandstones. This fault, which we name the Tambarana Fault (Figure 15), is the termination of The interpreted carbonates of Units D and E were the Tomini Basin and is probably a sinistral strike- deposited above the angular unconformity of slip fault that links to the NW with the Palu-Koro Horizon C (Figures 8, 9, 10). There are two possible Fault. ages for this unconformity: either about 23 Ma or 15 Ma; the different ages for the seismic units We suggest Horizon E is Early Pliocene in age. In interpreted from these alternatives are shown on the Togian Islands Cottam et al. (2011) interpreted Figure 4. deposits of the Bongka Formation (Figure 4), which is part of the Celebes Molasse, to be the distal parts The older age of about 23 Ma is based on the of a Pliocene alluvial fan building out from the East interpretation of the unconformity as due to Arm. The Bongka Formation in the Togian Islands collision of the Banggai–Sula Microcontinental and East Arm contains abundant distinctive Block with the North Arm Volcanic Arc causing ophiolitic debris which can only have come from emplacement of the ophiolite. In this model Middle the East Arm ophiolite. Seismic data (Jablonski et Miocene shallow marine carbonates found in the al., 2007) show thick (up to 2 sec TWT) north-south Togian Islands (Cottam et al., 2011; Figure 4) are trending lobes of sediment inferred to be submerged interpreted to have been deposited on the parts of this fan (Cottam et al., 2011). These unconformity surface after a period of erosion observations imply that subsidence of the Poso following the collision that produced a wide Basin (from near sea level to present-day depths of shallow marine shelf across the whole of Gorontalo 500 to 1500 m) occurred after deposition of the fan. Bay. The latest Miocene to Pliocene age of the Celebes Molasse in the East Arm (e.g. Surono & Sukarna, The younger age of about 15 Ma is based on the 1993; Hall & Wilson, 2000) therefore indicates interpretation of the unconformity as the result of rapid subsidence since 5 Ma. All the sediment in the regional extension. van Leeuwen et al. (2007) Poso Basin (up to 3 sec TWT) above the buried suggested that the Malino Metamorphic Complex at pinnacle reefs and limestones on the SE side of the the western end of the North Arm, immediately Lalanga Ridge (Figure 7) would therefore be north of Tomini Basin, is a core complex exhumed Pliocene to Recent in age. This would also suggest a in the Miocene. K/Ar and 40Ar/39Ar cooling ages of very asymmetrical subsidence from approximately 23–11 Ma, indicate exhumation from 27–30 km sea level to about 3 km in the centre of Tomini before eruption of unconformably overlying Basin (2 km water + 0.7 sec TWT sediment) but volcanic rocks at 7 Ma. Sulawesi and the Banda about 6 km in Poso Basin (1.8 km water + 3 sec region experienced widespread extension from TWT sediment). about 15 Ma (Spakman & Hall, 2010; Hall, 2011) and it is therefore possible that the core complex The rapid subsidence of both the Tomini and Poso formation is related to this phase of extension. In Basins and probable contemporaneous uplift on this interpretation, the angular unconformity of land are likely to have a common cause. Spencer Horizon C age of about 15 Ma, is still consistent with the Middle Miocene age of carbonates in the reported near Dolang (Rusmana et al., 1993) and Togian Islands. gas seeps shown by Kündig (1956) near Tanjung Api, north of , remain active today. Clearly, Units C and B were deposited between the angular given the uncertainties about the age of the unconformity of Horizon C and the unconformity of sequence and lack of detailed knowledge of the Horizon A (Figures 8, 9, 10). If the older age of 23 structure of the basins Gorontalo Bay remains a Ma is chosen for Horizon C, Horizon A is most frontier prospect. However, there are a number of likely to be Middle Eocene (c. 45 Ma), an features that offer a more positive view of Tomini unconformity age recognised in all the areas close Basin from a hydrocarbon perspective than to Gorontalo Bay (Figure 4), which would be previously. correlative with the age of rifting in the Makassar Straits as suggested by Jablonksi et al. (2007). This There is much more sediment than predicted by the is not such an obvious correlation as it appears at interpretations (compare Figure 3 with Figure 9) of first sight. First, if the North Arm is restored to its Silver et al. (1983a) and Kadarsuman et al. (2004) pre-Pliocene position by rotation of 20 along the and there are structures below Horizon A Palu-Koro Fault the North Makassar Basin and inconsistent with suggestions that western Gorontalo Bay no longer join up in such a Gorontalo Bay is underlain by oceanic crust or the straightforward way. They must have been East Arm Ophiolite. The most likely basement for separated by an emergent area to account for the Tomini Basin is continental crust similar to that eastern basin margin to the Makassar Straits exposed in the Neck, the Malino Complex, and the observed in western Sulawesi (Calvert, 2000; Tokorondo Mountains, i.e. metamorphic rocks and Calvert & Hall, 2007). Second, in contrast to the granites with protoliths that are probably pre- North Makassar Basin there is no evidence of rifting Mesozoic with an Australian origin (van Leeuwen above Horizon A as discussed above. et al., 2007). These formed part of the eastern Sundaland margin from the Early Cretaceous. The If the younger age of 15 Ma is chosen for the upper eastern part of Gorontalo Bay could be underlain by unconformity the deeper unconformity is more oceanic crust as suggested by Silver et al. (1983a), likely to be an Early Miocene unconformity (c. 23 although based on observations from the North Arm Ma), related to collision of the Banggai–Sula and Togian Islands (Cottam et al., 2011) it is more Microcontinental Block further east. This would likely to be underlain by Paleogene arc volcanics account for some of the features of the seismic data, which are potentially underthrust by continental such as the absence of evidence for rifting and crust of the Banggai–Sula block (Ferdian et al., deposition of Unit B on an irregular topography. 2010; Watkinson et al., 2011).

However, it would mean that subsidence began Whatever the age of the lower sequence (Units B soon after collision, with no obvious cause. This and C) these are sediments mainly derived from the would require a very thick sequence of clastic west or southwest and their sources are likely to be sediments to be deposited in a very short time mainly metamorphic and granitic rocks similar to interval (up to 4 sec TWT in c. 8 Ma), and would those that form the Neck and the Tokorondo imply the source was to the SW in Central Sulawesi Mountains of Central Sulawesi. Unit B thins if the North Arm is restored to its pre-Pliocene eastwards, and may terminate completely further position. If Central Sulawesi was an elevated area in east (Kerr, 2010), suggesting the possibility that in the Early Miocene it is difficult explain why there its early stages Tomini Basin could have been was no significant pulse of clastic sediment carried partially or fully enclosed. Sources at the base of the to the west into western Sulawesi at this time sequence could include lacustrine shales or coals. (Calvert, 2000; Calvert & Hall, 2007). Sediments of Units B and C are expected to be quartzose with good reservoir properties and rich in We consider that an equally good case can be made organic matter. for both age interpretations shown in Figure 4 and at present we can see no reason to prefer either. A We have no evidence to indicate the depth of water number of studies are currently underway to in which Units B and C were deposited. They were provide new data that can test these interpretations. deformed before deposition of Unit D, although dips are gentle and there are few obvious structural IMPLICATIONS FOR HYDROCARBONS traps. Our suggestion that Unit D clinoforms are carbonates implies that the whole of western Hydrocarbons are leaking from the south side of the Gorontalo Bay was very shallow from Horizon C to bay. In the East Arm (Figure 15) oil seeps are Horizon D and the thick sequences of carbonates between 1 and 2 sec TWT total thickness which are Bellier, O., Sebrier, M., Seward, D., Beaudouin, T., potential reservoirs. The bay remained shallow until Villeneuve, M. & Putranto, E. 2006. Fission track about 5 Ma. and fault kinematics analyses for new insight into the Late Cenozoic tectonic regime changes in West- Rapid subsidence began at about 5 Ma and rates of Central Sulawesi (Indonesia). Tectonophysics 413, subsidence eventually exceeded rates of carbonate 201-220. production, killing all the reefs except for rare pinnacles on the Lalanga Ridge and close to the Bergman, S. C., Coffield, D. Q., Talbot, J. P. & present coastlines, and exceeding rates of clastic Garrard, R. J. 1996. Tertiary tectonic and magmatic supply to the Tomini and Poso Basins, so that they evolution of Western Sulawesi and the Makassar are now between 1800m and 2000m deep in their Strait, Indonesia: Evidence for a Miocene continent- deepest parts. Extension exhumed the deep crust in continent collision. In: Hall, R. & Blundell, D. J. onshore metamorphic core complexes at the same (Eds.), Tectonic Evolution of SE Asia. Geological time as subsidence in Gorontalo Bay which is Society of London Special Publication, 106, 391- probably underlain by highly thinned continental 430. crust. In the Togian Islands Pliocene (Cottam et al., 2011) magmatism and Pleistocene-Recent (Katili & Brouwer, H. A. 1947. Geological explorations in Sudradjat, 1984) volcanic activity of Una-Una may Celebes – summary of results. In: Brouwer, H. A. be an expression of thinning which induced small (Ed.), Geological Explorations in the Island of amounts of melting of the underlying mantle. We Celebes. North Holland Publishing Company, propose that extension of the region was driven by Amsterdam, 1-64. rollback of the North Sulawesi subduction zone, accommodated by movements on the Palu-Koro and Calvert, S. J. 2000. The Cenozoic evolution of the Tambarana Faults. More work is required to Lariang and Karama basins, Sulawesi. Indonesian understand the detailed structural development of Petroleum Association, Proceedings 27th Annual Gorontalo Bay and the heatflow implications of Convention, 505-511. crustal/lithosphere extension models and this is now underway. Only in Unit F do we anticipate that Calvert, S. J. & Hall, R. 2007. Cenozoic Evolution there may be a significant volcanic component, of the Lariang and Karama regions, North Makassar which is likely to increase in importance towards Basin, western Sulawesi, Indonesia. Petroleum the north and east, resulting from magmatism in the Geoscience 13, 353-368. North Arm, Togian Islands or Una-Una. Cottam, M. A., Hall, R., Forster, M. & Boudagher ACKNOWLEDGEMENTS Fadel, M. 2011. Basement character and basin formation in Gorontalo Bay, Sulawesi, Indonesia: We thank Fugro MCS and Searcher Seismic for the New observations from the Togian Islands. In: Hall, seismic data and TGS for multibeam data, and Chris R., Cottam, M. A. & Wilson, M. E. J. (Eds.), The Elders and Simon Suggate for advice and help. SE Asian Gateway: History and Tectonics of the Parinya Pholbud’s MSc programme was funded by Australia-Asia collision. Geological Society of PTTEP. London Special Publication, 355, 177-202.

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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 12130’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.