The Poso Basin in Gorontalo Bay, Sulawesi: Extension Related to Core Complex Formation on Land

IPA14-G-297

PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Thirty-Eighth Annual Convention & Exhibition, May 2014

THE POSO BASIN IN GORONTALO BAY, SULAWESI: EXTENSION RELATED TO CORE COMPLEX FORMATION ON LAND

Giovanni Pezzati* Robert Hall* Peter Burgess* Marta Perez-Gussinye*

ABSTRACT part of Poso Basin, suggests that the development of Poso Basin may be related to extension associated Gorontalo Bay is a semi-enclosed sea between the with the rapid development of metamorphic core North and East Arms of Sulawesi. It is surrounded complexes on land. by land on three sides, separating a northern volcanic province from metamorphic rocks to the INTRODUCTION south and west, and ophiolites to the southeast. Seismic analysis and literature research suggest a Poso Basin is one of the subbasins of Gorontalo possible Early Miocene origin for Gorontalo Bay, Bay, a semi-enclosed sea surrounded by land on following Sula Spur collision which resulted in three sides (Figure 1), separating North Sulawesi terrestrial conditions. In the western part of from Central Sulawesi and East Sulawesi to the Gorontalo Bay there are two subbasins: the northern south. To the west, the Neck of Sulawesi separates Tomini Basin and the southern Poso Basin, which Gorontalo Bay from the Makassar Strait. have different histories. This study presents a new geological interpretation of the Poso Basin based on The presence of Miocene carbonates in these areas recent multibeam and 2D seismic data. (van Leeuwen and Muhardjo, 2005; Cottam et al., 2011) suggests that the bay is Miocene in age. The seismic stratigraphy of Tomini Basin shows progressive deepening of the area, with deposition However, no drilling has been carried out in the largely under marine conditions (Unit B and C), basin and the age and the nature of the crust beneath followed by a regional uplift event (Middle-Late Gorontalo Bay is still unknown. Miocene?) that caused local subaerial erosion (east Lalanga Ridge) with development of an Studies on land show metamorphic core complexes unconformity and shallowing of the central part of of Neogene to Recent age to the north and south of the basin. Renewed subsidence formed shallow the bay (van Leeuwen et al., 2007; Spencer, 2010, marine environments (Unit D and E). Subsidence 2011). They indicate rapid exhumation, possibly later accelerated, causing backstepping of the shelf contemporaneous with subsidence in Gorontalo edge, drowning of pinnacle reefs and subsidence to Bay. Recent multibeam and 2D seismic data show present depths of 2 km in the basin centre (Unit F). narrow coastal shelf and water depths up to 2000 m The Poso Basin is much younger than Tomini (Pholbud et al., 2012). The presence of pinnacle Basin. The deeper part of the sequence is probably structures, interpreted as pinnacle reefs, also the time equivalent of Unit D in Tomini Basin. suggests rapid subsidence of the bay.

However, rapid subsidence is recorded by a thick The seismic dataset gives a unique opportunity to sequence of up to 3 sec TWT which is the unravel the geological evolution of Gorontalo Bay equivalent of the thinner Units E and F in Tomini and North Sulawesi. Two sub-basins characterize Basin. Subsidence is interpreted to be related to the western part of Gorontalo Bay: the Tomini uplift nearby. A large north-dipping potential low Basin and the Poso Basin. They are separated by a angle normal fault, identified below the southern submarine ridge, called the Lalanga Ridge, which shows apparent continuity with the Togian Islands (Figure 3 and Figure 4). Tomini Basin is * Royal Holloway University of London characterized by six units (Pholbud et al., 2012), from the lowest Unit A, to the uppermost Unit F Australian origin to the east of Sulawesi and that shows the most recent sedimentary processes. interpreted them to have been sliced from New Guinea (e.g. Hamilton, 1979; Pigram & The seismic data show that Tomini Basin and Poso Panggabean, 1984; Pigram et al., 1985; Silver et al., Basin have very different sedimentary histories. The 1985). Hall (1996, 2002, 2011) and Spakman and Lalanga Ridge could have presumably played a role Hall (2010) suggested these were extended in the development of different sedimentary fragments of the Sula Spur (Klompé, 1954). environments or different sedimentation rates According to this interpretation, the Sula Spur within the two basins. Compared to the seismic collided with the North Arm volcanic arc and lines in Tomini Basin, the Poso Basin lines emplaced the East Arm ophiolite in the Early terminate very close to its southern coastline, Miocene and was subsequently fragmented by providing useful information for possible extensional processes in the late Neogene. correlations between the geology on land and offshore. This, the comparison of Poso Basin with During the Miocene and Pliocene, Sulawesi Tomini Basin and their very different histories underwent rapid uplift (van Leeuwen and provide valuable insights into the tectonic and Muhardjo, 2005; Bellier et al., 2006; Cottam et al., sedimentary processes that characterized the 2011) and subsidence, generating the present strong development of western Gorontalo Bay, in elevation contrast in the Gorontalo Bay area (~3 km particular the relationship between rapid uplift on uplift and <2 km subsidence). A south-dipping land and rapid subsidence offshore. We present new subduction of Celebes Sea under the North Arm observations from interpretations of seismic lines (Silver et al., 1983a; Surmont et al., 1994) acquired in Poso Basin, suggesting that the developed in the last 5 Ma. Hall (2011) suggested exhumation of the Pompangeo metamorphic that Sulawesi has been extending since the Early complex (MMC) on the southern margin of Pliocene because of the rollback of the subducting Gorontalo Bay is related to subsidence and Celebes Sea slab beneath the North Arm. Present extension in the Poso Basin. day data, such as seismic activity measurements, show that Sulawesi is characterized by high level of GEOLOGICAL SETTING seismicity in the region of the North Arm and by extensional regimes in the south area of Tomini Sulawesi Gulf (Beaudouin et al., 2003).

Sulawesi is composed of numerous fragments Gorontalo Bay (Audley-Charles, 1974; Katili, 1978; Hamilton, 1979; Taylor and Van Leeuwen, 1980; Sukamto and The geological events that shaped Gorontalo Bay Simandjuntak, 1983; Hall, 1996, 2002) that are and the relationship between Gorontalo Bay and the commonly subdivided in five main tectonic units or surrounding areas are still unclear. Silver et al. provinces (Figure 1). The Western Sulawesi (1983a) interpreted Gorontalo Bay, its eastern Province formed a part of the Mesozoic eastern sector in particular, as a fore-arc basin underlain by continental margin of Sundaland (Hamilton, 1979; oceanic crust and a relatively thin sedimentary Polvé et al., 1997; van Leeuwen et al., 2007). The cover. In contrast, Hamilton (1979) suggested a North Sulawesi Province is dominantly a Cenozoic thicker sedimentary cover of ~5 km further west intra-oceanic volcanic arc built on Eocene oceanic that has been partially supported by Jablonski et al. crust (Taylor and van Leeuwen, 1980; Rangin, (2007). Jablonski et al. (2007), through the analysis 1997; Elburg et al., 2003; van Leeuwen and of recently acquired seismic data, showed a Muhardjo, 2005). The Central Sulawesi sedimentary thickness up to 6 sec TWT and Metamorphic Belt broadly separates the Sundaland interpreted Gorontalo Bay as a lateral equivalent of margin of the Western Sulawesi Province from the the Makassar Strait. Jablonski et al (2007) consider East Arm ophiolite and the Banggai-Sula block that the basement of Gorontalo Bay is made of (Parkinson, 1991). The East Sulawesi Ophiolite has continental crust and cut by Eocene extensional been interpreted as oceanic crust formed at a typical faults. Kusnida et al. (2009) used magnetic anomaly middle ocean ridge (Soeria-Atmadja et al., 1974; data to propose a possible rift-related genesis of the Simandjuntak, 1986), in a supra-subduction zone basin. Pholbud et al. (2012) show that there is no setting (Monnier et al., 1995; Bergman et al., 1996; evidence for rifting. Instead, they suggested that the Parkinson,1998b) or in an oceanic plateau rapid subsidence of both the Tomini and Poso (Kadarusman et al., 2004). Many authors have Basins and the uplift on land could be explained by recognised various microcontinental fragments of a simple shear detachment model (Wernicke, 1985; Lister et al., 1986). An extensional regime and marked by a clear erosional unconformity crustal thinning is also supported by observations of (Unconformity 1) on the northeastern flank of the volcanic products on Una Una and the Togian basin (on the Lalanga Ridge) generated by the uplift Islands (Cottam et al., 2011). of same ridge.

DATASET AND METHOD Unit 0 is the first unit that rests on Unconformity 1 (Figures 4 and 6) in the northeastern area of Poso Both multibeam and 2D seismic data were analysed Basin. Unit 0 indicates a shallow water marine in this study. The multibeam dataset (Figure 3) was environment after the formation of Unconformity 1. acquired by TGS using a Kongsberg Simrad EM The area (northeast Poso Basin) underwent a rapid 120 Multibeam Echo Sounder with 190 beams at subsidence that provoked the drowning of the equidistant spacing. C-Nav Starfire DGPS was used carbonate platform and the pinnacles. It is divided for positioning control. During the processing, into two parts. Subunit 0a is recognised in the east corrections were applied for positioning and tidal and all along the top of the Lalanga Ridge, where calibrations. Noise and artifacts were removed and a the seismic lines suggest the possible formation of terrain model was gridded using a 25 m bin size. pinnacle reef-carbonate rims and a local small carbonate platform. On the northeastern side of The 2D seismic data set was acquired by Fugro Poso Basin near the Lalanga Ridge, this unit seems MCS and Searcher Seismic, with a total length of to rest on Unconformity 1, showing that its 8009 km of Gorontalo 2D non-exclusive seismic sedimentation started after the event that generated survey collected in 2006 along the whole Gorontalo the unconformity. Subunit 0b is characterized by Bay. In the Poso Basin the survey was acquired low-moderate amplitude reflectors that cover the utilizing a long offset seismic cabling with rough morphology of the pinnacles. It is not clear recording to between 8 and 12 sec TWT; the space when this subunit was deposited, but we suggest between each line was approximately 10 to 15 km that it is probably approximately contemporaneous (Figure 3). with subunit 0a, indicating a general drowning of the carbonates. The key parameters for the processing of the seismic data are surface-related multiple elimination Unit 1 is characterized by moderate-strong (SRME) and Kirchoff prestack time migration reflectors showing relatively good lateral continuity. (PSTM). The seismic interpretation was done with The deposition of this unit occurred in the manual picking of horizons, unconformities and southwestern part of Poso Basin (Figure 4), while faults, using the IHS Kingdom Suite on a Windows its north-eastern part and the top of the Lalanga workstation. Ridge were still characterized by carbonate buildups (possibly in shallower water). Unit 1 started to RESULTS: OBSERVATIONS OF THE POSO deposit together or slightly after Unit 0 in the BASIN western Poso Basin. The seismic data show that after an initial phase of possible contemporaneous In this paper we present observations about deposition (Unit 1 rests on and onlaps Unit X on the stratigraphy and tectonic structures that have been southwestern flank of the Lalanga Ridge), Unit 1 identified in Poso Basin, focusing mainly on its started to onlap and cover Unit 0. southwestern area. The main units that we identified from the seismic data are described below. Unit 2 is present only in the north area of southwestern Poso Basin (Figure 4) and is Stratigraphy characterized by moderate amplitude reflectors with very good lateral continuity. This unit rests Unit X is the lowest unit and is characterized by conformably on Unit 1. Both Unit 1 and Unit 2 are moderate to strong reflections, but internal folded in the southwest part of the basin. An geometries are not clearly visible. Generally the top erosional unconformity is visible on top of the fold. of Unit X is characterized by strong reflectors. Locally (Figures 4 and 5) there are some deep Unit 3 is visible only in the north area of strong reflectors that are possible to identify in southwestern Poso Basin, like Unit 2 (Figure 4). several lines. The correlation and interpolation of Also like Unit 2, it is characterized by strong to these deep reflectors suggest the possible presence moderate reflectors. It onlaps Unit 2 and it shows of a deep feature that shows a general north- geometries typical of contourite deposits. It seems northwest dip direction. The top of this unit is to be almost contemporaneous with Unit 4. Unit 4 is characterized by low-moderate amplitude angle normal faults previously mentioned. The reflectors, it downlaps onto Unit 0 (Figure 4) and is interpolation and correlation of these reflectors in present mainly in eastern Poso Basin. Preliminary different lines suggests the presence of a deep interpretations suggest this unit to be younger than feature dipping towards the north-northwest (Figure Unit 1 and Unit 2. In the northeastern area of the 7). The connection of this feature with possible basin, this unit has been clearly deformed and normal fault planes dipping towards the northeast eroded. An erosional unconformity is evident. suggests it is potentially an extensional feature. We suggest that this deep feature can be interpreted as a Unit 5 rests conformably on Unit 4 and downlaps low angle fault plane dipping north-northwest. onto the angular unconformity on top of Unit 3 (Figure 4). Apart from minor transparency when This low angle fault appears to have generated a compared to Unit 4, this unit has exactly the same large offset of Unit X, mostly in terms of horizontal characteristics. We identified it as an independent extension (over 20 km), but possibly also in terms unit not only because of the minor transparency, but of vertical displacement. The dip of the reflectors because it was not involved in the folding and suggests that this potential fault extends at least erosion that removed large parts of Unit 4. from beneath the Lalanga Ridge to the southern coast of the Poso Basin. Noteworthy is the presence STRUCTURAL INTERPRETATION of the Pompangeo Metamorphic Core Complex in Central Sulawesi, right on the coast of Poso Basin, The seismic line (Figure 5) in the central part of the (Figures 2 and 3) that has been interpreted as a basin, shows that the top of Unit X (characterized metamorphic core complex (Spencer, 2010, 2011). by strong reflections) is offset vertically by up to According to Spencer (2011) the lowest dips of the 500 ms which may be caused by the activity of footwall of the Pompangeo Metamorphic Core normal faults. Unfortunately, the quality of the Complex are from 4° to 7°. This seems consistent seismic data does not allow a clear identification of with the north-northwestward dipping reflectors normal faults within the sedimentary fill of the identified in Poso Basin. Spencer (2011) suggested basin. Strong reflectors within Unit X dip gently that lineations visible on SRTM images show that towards the north-northeast and seem consistent the unroofing of the core complex occurred by with the offset visible in Unit X. Our seismic extensional movement towards the north-northwest. analysis suggests that these strong reflectors are fault planes of the normal faults. It is possible to In view of the proximity of the Pompangeo identify at least four possible important low angle Metamorphic Core Complex to the Poso Basin and normal faults in the centre of the basin, dipping the similar orientation of the north-northwest dip of northeastward (Figure 5). Some of these faults, in the identified deep reflectors to the north-northwest the southwestern area of the basin, seem also to to south-southeast trending lineations visible on the generate local tilting in the overlying Unit 1 Pompangeo Metamorphic Core Complex, we (Figure 5). suggest that the core complex exhumation and the potential low angle normal fault are related. In the southeastern area (Figure 6) Unit X and Unit 1 look deformed, especially between the central part CONCLUSIONS of the basin and the southern coast, while the flank of the Lalanga Ridge appears to be undeformed. Comparison of seismic lines acquired in the Poso Normal faults are clearly visible in the centre of the Basin show they are quite different, especially the basin. They deform Unit 1, Unit 2 and Unit 3 and two southwest-northeast trending lines (one along are visible in many different lines, over a distance the axis of the basin and one along its north margin, of more than 30 km. The area where these faults are on the southern flank of the Lalanga Ridge). The present seems to separate the relatively undeformed lines in the southwestern area (close to Sulawesi flank of the Lalanga Ridge from the more deformed Neck) and the northeastern area (close to the Togian southern flank of the Poso Basin. Islands) are also very different. This shows that the Poso Basin had a complex evolution both in terms Strong reflectors dipping north-northwest have been of tectonics and sedimentation. observed and based on a possible 3000m/s velocity for the overlying sediments, would be dipping at an We suggest that the observed deep feature in Poso angle of approximately 9° (Figure 6). They seem to Basin can be considered as a low angle normal fault correspond to other deep reflectors present beneath associated with the exhumation of the Pompangeo the flank of the Lalanga Ridge and with the low Metamorphic Core Complex and is probably the main detachment fault. We propose that the activity (Eds.), Tectonic Evolution of SE Asia. Geological on the low angle normal faults that led to Society of London Special Publication, 106, 391- exhumation of the Pompangeo Metamorphic Core 430. Complex also played a major role in the extension and subsidence of Poso Basin. The deformation is Cottam, M. A., Hall, R., Forster, M. & Boudagher- mostly present in the southwestern area of the basin, Fadel, M. 2011. Basement character and basin where stratigraphy and tectonic structures are highly formation in Gorontalo Bay, Sulawesi, Indonesia: complex. The lack of deformation on the Lalanga New observations from the Togian Islands. In: Hall, Ridge flank may indicate that it forms part of the R., Cottam, M. A. & Wilson, M. E. J. (Eds.), The hanging wall of the north-northwest dipping low SE Asian Gateway: History and Tectonics of the angle normal fault. Australia-Asia collision. Geological Society of London Special Publication, 355, 177-202. Since the exhumation of a core complex represents a regional scale event, we consider it likely that the Elburg, M., van Leeuwen, T., Foden, J. & Muhardjo formation of the Pompangeo Metamorphic Core 2003. Spatial and temporal isotopic domains of Complex and the Poso Basin also affected the contrasting igneous suites in Western and Northern evolution of Tomini Basin. It is probable that uplift Sulawesi, Indonesia. Chemical Geology 199, 243- of the Pompangeo Metamorphic Core Complex and 276. the related subsidence of the Poso Basin are directly linked to subsidence in western Gorontalo Bay and Hall, R. 1996. Reconstructing Cenozoic SE Asia. the drowning of carbonate platforms in Tomini Bay. In: Hall, R. & Blundell, D. J. (Eds.), Tectonic Evolution of SE Asia. Geological Society of ACKNOWLEDGMENTS London Special Publication, 106, 153-184.

We thank Nicola Scarselli, Jurgen Adam and Ian Hall, R. 2002. Cenozoic geological and plate Watkinson for discussion of seismic interpretation tectonic evolution of SE Asia and the SW Pacific: and tectonics. Thanks to Dominique Tanner for computer-based reconstructions, model and paper review Fugro, Searcher Seismic and TGS are animations. Journal of Asian Earth Sciences 20, thanked for providing the data used in this study. 353-434.

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Figure 1 – Tectonic Provinces of Sulawesi. The study area is highlighted in light blue. Figure modified after Pholbud et al., 2012.

Figure 2 – Tectonic sketch map of south-western Gorontalo Bay (modified after Pholbud et al., 2012).

Figure 3 – a) Gorontalo Bay multibeam and SRTM imagery of central and North Sulawesi. b) Multibeam data of Poso Basin, showing the location of the seismic dataset

Figure 4 – Seismic Line 1 acquired along the southern flank of Lalanga Ridge.

Figure 5 – Seismic Line 2 acquired along the NE-SW axis of Poso Basin. The deformation of Unit A is clearly visible. Deep reflectors highlighted by circles are visible also in the other lines.

Figure 6 – Seismic Line 3 crosses Poso Basin from the north coast of Central Sulawesi to the southern flank of Lalanga Ridge.

Figure 7 – 3D view of low angle normal faults crossing Line 2 and Line 3 in Poso Basin.