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 Gorontalo Bay is a semi-enclosed sea between the North and East Arms of Sulawesi. It is surrounded by land on three sides, separating a northern volcanic province from metamorphic rocks to the south and west, and ophiolites to the southeast. Seismic analysis and literature research suggest a possible Early Miocene origin for Gorontalo Bay, following Sula Spur collision which resulted in terrestrial conditions. In the western part of Gorontalo Bay there are two subbasins: the northern Tomini Basin and the southern Poso Basin, which have different histories. This study presents a new geological interpretation of the Poso Basin based on recent multibeam and 2D seismic data. The seismic stratigraphy of Tomini Basin shows progressive deepening of the area, with deposition largely under marine conditions (Unit B and C), followed by a regional uplift event (Middle-Late Miocene?) that caused local subaerial erosion (east Lalanga Ridge) with development of an unconformity and shallowing of the central part of the basin. Renewed subsidence formed shallow marine environments (Unit D and E). Subsidence later accelerated, causing backstepping of the shelf edge, drowning of pinnacle reefs and subsidence to present depths of 2 km in the basin centre (Unit F). The Poso Basin is much younger than Tomini Basin. The deeper part of the sequence is probably the time equivalent of Unit D in Tomini Basin. However, rapid subsidence is recorded by a thick sequence of up to 3 sec TWT which is the equivalent of the thinner Units E and F in Tomini Basin. Subsidence is interpreted to be related to uplift nearby. A large north-dipping potential low angle normal fault, identified below the southern

* Royal Holloway University of London

part of Poso Basin, suggests that the development of Poso Basin may be related to extension associated with the rapid development of metamorphic core complexes on land.

INTRODUCTION Poso Basin is one of the subbasins of Gorontalo Bay, a semi-enclosed sea surrounded by land on three sides (Figure 1), separating North Sulawesi from Central Sulawesi and East Sulawesi to the south. To the west, the Neck of Sulawesi separates Gorontalo Bay from the Makassar Strait. The presence of Miocene carbonates in these areas (van Leeuwen and Muhardjo, 2005; Cottam et al., 2011) suggests that the bay is Miocene in age. However, no drilling has been carried out in the basin and the age and the nature of the crust beneath Gorontalo Bay is still unknown. Studies on land show metamorphic core complexes of Neogene to Recent age to the north and south of the bay (van Leeuwen et al., 2007; Spencer, 2010, 2011). They indicate rapid exhumation, possibly contemporaneous with subsidence in Gorontalo Bay. Recent multibeam and 2D seismic data show narrow coastal shelf and water depths up to 2000 m (Pholbud et al., 2012). The presence of pinnacle structures, interpreted as pinnacle reefs, also suggests rapid subsidence of the bay. The seismic dataset gives a unique opportunity to unravel the geological evolution of Gorontalo Bay and North Sulawesi. Two sub-basins characterize the western part of Gorontalo Bay: the Tomini Basin and the Poso Basin. They are separated by a submarine ridge, called the Lalanga Ridge, which shows apparent continuity with the Togian Islands (Figure 3 and Figure 4). Tomini Basin is characterized by six units (Pholbud et al., 2012),

from the lowest Unit A, to the uppermost Unit F that shows the most recent sedimentary processes. The seismic data show that Tomini Basin and Poso Basin have very different sedimentary histories. The Lalanga Ridge could have presumably played a role in the development of different sedimentary environments or different sedimentation rates within the two basins. Compared to the seismic lines in Tomini Basin, the Poso Basin lines terminate very close to its southern coastline, providing useful information for possible correlations between the geology on land and offshore. This, the comparison of Poso Basin with Tomini Basin and their very different histories provide valuable insights into the tectonic and sedimentary processes that characterized the development of western Gorontalo Bay, in particular the relationship between rapid uplift on land and rapid subsidence offshore. We present new observations from interpretations of seismic lines acquired in Poso Basin, suggesting that the exhumation of the Pompangeo metamorphic complex (MMC) on the southern margin of Gorontalo Bay is related to subsidence and extension in the Poso Basin. GEOLOGICAL SETTING Sulawesi Sulawesi is composed of numerous fragments (Audley-Charles, 1974; Katili, 1978; Hamilton, 1979; Taylor and Van Leeuwen, 1980; Sukamto and Simandjuntak, 1983; Hall, 1996, 2002) that are commonly subdivided in five main tectonic units or provinces (Figure 1). The Western Sulawesi Province formed a part of the Mesozoic eastern continental margin of Sundaland (Hamilton, 1979; Polvé et al., 1997; van Leeuwen et al., 2007). The North Sulawesi Province is dominantly a Cenozoic intra-oceanic volcanic arc built on Eocene oceanic crust (Taylor and van Leeuwen, 1980; Rangin, 1997; Elburg et al., 2003; van Leeuwen and Muhardjo, 2005). The Central Sulawesi Metamorphic Belt broadly separates the Sundaland margin of the Western Sulawesi Province from the East Arm ophiolite and the Banggai-Sula block (Parkinson, 1991). The East Sulawesi Ophiolite has been interpreted as oceanic crust formed at a typical middle ocean ridge (Soeria-Atmadja et al., 1974; Simandjuntak, 1986), in a supra-subduction zone setting (Monnier et al., 1995; Bergman et al., 1996; Parkinson,1998b) or in an oceanic plateau (Kadarusman et al., 2004). Many authors have recognised various microcontinental fragments of

Australian origin to the east of Sulawesi and interpreted them to have been sliced from New Guinea (e.g. Hamilton, 1979; Pigram & Panggabean, 1984; Pigram et al., 1985; Silver et al., 1985). Hall (1996, 2002, 2011) and Spakman and Hall (2010) suggested these were extended fragments of the Sula Spur (Klompé, 1954). According to this interpretation, the Sula Spur collided with the North Arm volcanic arc and emplaced the East Arm ophiolite in the Early Miocene and was subsequently fragmented by extensional processes in the late Neogene. During the Miocene and Pliocene, Sulawesi underwent rapid uplift (van Leeuwen and Muhardjo, 2005; Bellier et al., 2006; Cottam et al., 2011) and subsidence, generating the present strong elevation contrast in the Gorontalo Bay area (~3 km uplift and <2 km subsidence). A south-dipping subduction of Celebes Sea under the North Arm (Silver et al., 1983a; Surmont et al., 1994) developed in the last 5 Ma. Hall (2011) suggested that Sulawesi has been extending since the Early Pliocene because of the rollback of the subducting Celebes Sea slab beneath the North Arm. Present day data, such as seismic activity measurements, show that Sulawesi is characterized by high level of seismicity in the region of the North Arm and by extensional regimes in the south area of Tomini Gulf (Beaudouin et al., 2003). Gorontalo Bay  The geological events that shaped Gorontalo Bay and the relationship between Gorontalo Bay and the surrounding areas are still unclear. Silver et al. (1983a) interpreted Gorontalo Bay, its eastern sector in particular, as a fore-arc basin underlain by oceanic crust and a relatively thin sedimentary cover. In contrast, Hamilton (1979) suggested a thicker sedimentary cover of ~5 km further west that has been partially supported by Jablonski et al. (2007). Jablonski et al. (2007), through the analysis of recently acquired seismic data, showed a sedimentary thickness up to 6 sec TWT and interpreted Gorontalo Bay as a lateral equivalent of the Makassar Strait. Jablonski et al (2007) consider that the basement of Gorontalo Bay is made of continental crust and cut by Eocene extensional faults. Kusnida et al. (2009) used magnetic anomaly data to propose a possible rift-related genesis of the basin. Pholbud et al. (2012) show that there is no evidence for rifting. Instead, they suggested that the rapid subsidence of both the Tomini and Poso Basins and the uplift on land could be explained by a simple shear detachment model (Wernicke, 1985;

Lister et al., 1986). An extensional regime and crustal thinning is also supported by observations of volcanic products on Una Una and the Togian Islands (Cottam et al., 2011). DATASET AND METHOD Both multibeam and 2D seismic data were analysed in this study. The multibeam dataset (Figure 3) was acquired by TGS using a Kongsberg Simrad EM 120 Multibeam Echo Sounder with 190 beams at equidistant spacing. C-Nav Starfire DGPS was used for positioning control. During the processing, corrections were applied for positioning and tidal calibrations. Noise and artifacts were removed and a terrain model was gridded using a 25 m bin size. The 2D seismic data set was acquired by Fugro MCS and Searcher Seismic, with a total length of 8009 km of Gorontalo 2D non-exclusive seismic survey collected in 2006 along the whole Gorontalo Bay. In the Poso Basin the survey was acquired utilizing a long offset seismic cabling with recording to between 8 and 12 sec TWT; the space between each line was approximately 10 to 15 km (Figure 3). The key parameters for the processing of the seismic data are surface-related multiple elimination (SRME) and Kirchoff prestack time migration (PSTM). The seismic interpretation was done with manual picking of horizons, unconformities and faults, using the IHS Kingdom Suite on a Windows workstation. RESULTS: OBSERVATIONS OF THE POSO BASIN In this paper we present observations about stratigraphy and tectonic structures that have been identified in Poso Basin, focusing mainly on its southwestern area. The main units that we identified from the seismic data are described below. Stratigraphy Unit X is the lowest unit and is characterized by moderate to strong reflections, but internal geometries are not clearly visible. Generally the top of Unit X is characterized by strong reflectors. Locally (Figures 4 and 5) there are some deep strong reflectors that are possible to identify in several lines. The correlation and interpolation of these deep reflectors suggest the possible presence of a deep feature that shows a general north-northwest dip direction. The top of this unit is

marked by a clear erosional unconformity (Unconformity 1) on the northeastern flank of the basin (on the Lalanga Ridge) generated by the uplift of same ridge. Unit 0 is the first unit that rests on Unconformity 1 (Figures 4 and 6) in the northeastern area of Poso Basin. Unit 0 indicates a shallow water marine environment after the formation of Unconformity 1. The area (northeast Poso Basin) underwent a rapid subsidence that provoked the drowning of the carbonate platform and the pinnacles. It is divided into two parts. Subunit 0a is recognised in the east and all along the top of the Lalanga Ridge, where the seismic lines suggest the possible formation of pinnacle reef-carbonate rims and a local small carbonate platform. On the northeastern side of Poso Basin near the Lalanga Ridge, this unit seems to rest on Unconformity 1, showing that its sedimentation started after the event that generated the unconformity. Subunit 0b is characterized by low-moderate amplitude reflectors that cover the rough morphology of the pinnacles. It is not clear when this subunit was deposited, but we suggest that it is probably approximately contemporaneous with subunit 0a, indicating a general drowning of the carbonates. Unit 1 is characterized by moderate-strong reflectors showing relatively good lateral continuity. The deposition of this unit occurred in the southwestern part of Poso Basin (Figure 4), while its north-eastern part and the top of the Lalanga Ridge were still characterized by carbonate buildups (possibly in shallower water). Unit 1 started to deposit together or slightly after Unit 0 in the western Poso Basin. The seismic data show that after an initial phase of possible contemporaneous deposition (Unit 1 rests on and onlaps Unit X on the southwestern flank of the Lalanga Ridge), Unit 1 started to onlap and cover Unit 0. Unit 2 is present only in the north area of southwestern Poso Basin (Figure 4) and is characterized by moderate amplitude reflectors with very good lateral continuity. This unit rests conformably on Unit 1. Both Unit 1 and Unit 2 are folded in the southwest part of the basin. An erosional unconformity is visible on top of the fold. Unit 3 is visible only in the north area of southwestern Poso Basin, like Unit 2 (Figure 4). Also like Unit 2, it is characterized by strong to moderate reflectors. It onlaps Unit 2 and it shows geometries typical of contourite deposits. It seems to be almost contemporaneous with Unit 4.

Unit 4 is characterized by low-moderate amplitude reflectors, it downlaps onto Unit 0 (Figure 4) and is present mainly in eastern Poso Basin. Preliminary interpretations suggest this unit to be younger than Unit 1 and Unit 2. In the northeastern area of the basin, this unit has been clearly deformed and eroded. An erosional unconformity is evident. Unit 5 rests conformably on Unit 4 and downlaps onto the angular unconformity on top of Unit 3 (Figure 4). Apart from minor transparency when compared to Unit 4, this unit has exactly the same characteristics. We identified it as an independent unit not only because of the minor transparency, but because it was not involved in the folding and erosion that removed large parts of Unit 4. STRUCTURAL INTERPRETATION The seismic line (Figure 5) in the central part of the basin, shows that the top of Unit X (characterized by strong reflections) is offset vertically by up to 500 ms which may be caused by the activity of normal faults. Unfortunately, the quality of the seismic data does not allow a clear identification of normal faults within the sedimentary fill of the basin. Strong reflectors within Unit X dip gently towards the north-northeast and seem consistent with the offset visible in Unit X. Our seismic analysis suggests that these strong reflectors are fault planes of the normal faults. It is possible to identify at least four possible important low angle normal faults in the centre of the basin, dipping northeastward (Figure 5). Some of these faults, in the southwestern area of the basin, seem also to generate local tilting in the overlying Unit 1 (Figure 5). In the southeastern area (Figure 6) Unit X and Unit 1 look deformed, especially between the central part of the basin and the southern coast, while the flank of the Lalanga Ridge appears to be undeformed. Normal faults are clearly visible in the centre of the basin. They deform Unit 1, Unit 2 and Unit 3 and are visible in many different lines, over a distance of more than 30 km. The area where these faults are present seems to separate the relatively undeformed flank of the Lalanga Ridge from the more deformed southern flank of the Poso Basin. Strong reflectors dipping north-northwest have been observed and based on a possible 3000m/s velocity for the overlying sediments, would be dipping at an angle of approximately 9° (Figure 6). They seem to correspond to other deep reflectors present beneath the flank of the Lalanga Ridge and with the low

angle normal faults previously mentioned. The interpolation and correlation of these reflectors in different lines suggests the presence of a deep feature dipping towards the north-northwest (Figure 7). The connection of this feature with possible normal fault planes dipping towards the northeast suggests it is potentially an extensional feature. We suggest that this deep feature can be interpreted as a low angle fault plane dipping north-northwest. This low angle fault appears to have generated a large offset of Unit X, mostly in terms of horizontal extension (over 20 km), but possibly also in terms of vertical displacement. The dip of the reflectors suggests that this potential fault extends at least from beneath the Lalanga Ridge to the southern coast of the Poso Basin. Noteworthy is the presence of the Pompangeo Metamorphic Core Complex in Central Sulawesi, right on the coast of Poso Basin, (Figures 2 and 3) that has been interpreted as a metamorphic core complex (Spencer, 2010, 2011). According to Spencer (2011) the lowest dips of the footwall of the Pompangeo Metamorphic Core Complex are from 4° to 7°. This seems consistent with the north-northwestward dipping reflectors identified in Poso Basin. Spencer (2011) suggested that lineations visible on SRTM images show that the unroofing of the core complex occurred by extensional movement towards the north-northwest. In view of the proximity of the Pompangeo Metamorphic Core Complex to the Poso Basin and the similar orientation of the north-northwest dip of the identified deep reflectors to the north-northwest to south-southeast trending lineations visible on the Pompangeo Metamorphic Core Complex, we suggest that the core complex exhumation and the potential low angle normal fault are related. CONCLUSIONS Comparison of seismic lines acquired in the Poso Basin show they are quite different, especially the two southwest-northeast trending lines (one along the axis of the basin and one along its north margin, on the southern flank of the Lalanga Ridge). The lines in the southwestern area (close to Sulawesi Neck) and the northeastern area (close to the Togian Islands) are also very different. This shows that the Poso Basin had a complex evolution both in terms of tectonics and sedimentation. We suggest that the observed deep feature in Poso Basin can be considered as a low angle normal fault associated with the exhumation of the Pompangeo Metamorphic Core Complex and is probably the

main detachment fault. We propose that the activity on the low angle normal faults that led to exhumation of the Pompangeo Metamorphic Core Complex also played a major role in the extension and subsidence of Poso Basin. The deformation is mostly present in the southwestern area of the basin, where stratigraphy and tectonic structures are highly complex. The lack of deformation on the Lalanga Ridge flank may indicate that it forms part of the hanging wall of the north-northwest dipping low angle normal fault. Since the exhumation of a core complex represents a regional scale event, we consider it likely that the formation of the Pompangeo Metamorphic Core Complex and the Poso Basin also affected the evolution of Tomini Basin. It is probable that uplift of the Pompangeo Metamorphic Core Complex and the related subsidence of the Poso Basin are directly linked to subsidence in western Gorontalo Bay and the drowning of carbonate platforms in Tomini Bay. ACKNOWLEDGMENTS  We thank Nicola Scarselli, Jurgen Adam and Ian Watkinson for discussion of seismic interpretation and tectonics. Thanks to Dominique Tanner for paper review Fugro, Searcher Seismic and TGS are thanked for providing the data used in this study. REFERENCES  Audley-Charles, M. G. 1974. Sulawesi. In: Spencer, A. M. (Ed.), Mesozoic-Cenozoic Orogenic Belts. Geological Society of London Special Publication, 4, 365-378. Beaudouin, T., Bellier, O. & Sebrier, M. 2003. Present-day stress and deformation fields within the Sulawesi Island area Indonesia: geodynamic implications. Bulletin de la Société géologique de France, 174, 305-317. 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.

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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.