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IPA05-G-161

PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Thirtieth Annual Convention & Exhibition, August 2005

BASEMENT ARCHITECTURE AND SEDIMENTARY FILL

OF THE NORTH MAKASSAR STRAITS BASIN

Siti Nur’Aini* Robert Hall**

Christopher F. Elders**

ABSTRACT The North Makassar Straits Basin (NMSB) is the deepest Cenozoic basin in Indonesia. Debate continues about whether it is underlain by continental or oceanic crust, and whether it originated as a rift or foreland basin. 2D seismic lines, gravity and well data were used to study the basement architecture and sedimentary fill of the basin. Based on those data, the NMSB is interpreted to have originated as a strongly segmented extensional basin, associated with half graben and graben structures. Mapping has revealed a system of NNW-SSE lineaments and faults intersecting the Top Basement seismic reflector. The lineaments and faults are arranged in en-echelon patterns and bound disconnected NNW-SSE trending depocentres. From comparison with sandbox analogue models, the syn-rift fault system in the NMSB seems most likely to have been produced by oblique rifting. The principal extension direction was east-west, at an angle of approximately 60° to pre-existing basement structures. Rifting of this part of the southeast Eurasian continental margin occurred during the Middle to Late Eocene. The crust beneath the NMSB is interpreted to be continental. The post-rift section is subdivided into three megasequences. Post-rift Megasquence 1 was deposited during the rapid subsidence of the basin from the Late Eocene to Late Oligocene. Uplift of Kalimantan in the Early to Middle Miocene generated Post-rift Megasequence 2, an interval characterised by a prograding delta system. Late Miocene Post-rift Megasequence 3 is interpreted to contain a turbidite interval in the central part of the basin. Subsidence and deep water marine sedimentation continued to the present-day in the central NMSB but from the Early Pliocene, sediment * ENI Indonesia Ltd. ** SE Asia Research Group, Royal Holloway University of London

supply from the east increased significantly as result of the westward propagation of the West Sulawesi fold and thrust belt. The NMSB became a foreland basin only from the Early Pliocene. INTRODUCTION The Makassar Straits contains the deepest Cenozoic basin in Indonesia, the North Makassar Straits Basin (NMSB). There is more than 12 kilometres of sedimentary fill and a record of continuous sedimentation from the Eocene until the present day (Situmorang, 1982; Guntoro, 1999; Moss and Chambers, 1999). Large hydrocarbon deposits have been discovered beneath the Mahakam delta and in the deepwater offshore area (Figure 1). Water depths in the central NMSB, also known as the offshore Kutai Basin, exceed 2000 metres. The origin of the Makassar Straits is still controversial and there are several hypotheses about the nature of the underlying crust and the formation of the deep central basin. There are no wells that penetrate the deeper sequences close to the basin axis and seismic and gravity data have been used as the basis of this and previous interpretations. Some authors suggest the NMSB formed on continental crust (e.g. Burollet and Salle, 1981) whereas others have argued that it is oceanic crust (e.g. Hamilton, 1979; Cloke, 1997; Cloke at al., 1999; Guntoro, 1999; Fraser et al., 2003). The straits have been interpreted to have formed by Cenozoic extension (Hamilton, 1979; Cloke, 1997; Cloke at al., 1999; Moss and Chambers, 1999) which is thought to have been initiated during the Middle Eocene (van de Weerd and Armin, 1992; Hall, 2002). This has been suggested to have been driven by subduction of the Pacific plate (Guntoro, 1999). Others have suggested that the Makassar Straits represent a remnant oceanic

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basin (Malecek et al., 1993) and some authors have concluded the Makassar Straits Basin is a Neogene foreland basin formed after Early Miocene continent-continent collision (Coffield et al., 1993; Bergman et al., 1996). The aim of this study was to investigate the basement architecture and sedimentary fill of the NMSB with the aim of contributing to the debate about its formation. DATASET AND METHODOLOGY Ninety-eight 2D seismic lines (provided by TGS-NOPEC Geophysical Company) combined with well and gravity data were used in this study. The Panca-1 and Pamukan Bay wells were used to calibrate the seismic interpretation in the study area (Table 1). Based on correlation of markers from those wells to the study area, estimates of ages of key horizons were made (Figure 2A) and their stratigraphic relationships interpreted (Figures 2B). BASEMENT ARCHITECTURE The free-air gravity map (Figure 1B), based on Smith and Sandwell (1997) and derived from GeoSat and ERS-1 data, shows two gravity lows: an elongated low northeast of the Paternoster Platform and an irregular low between the Mahakam delta and the Mangkalihat High. The elongated low continues to the southeast and forms a narrow trough between the northeast Paternoster Platform and the South Sulawesi coast. Further southeast, the trough is linked to the South Makassar Straits Basin. The Top Basement pick shows irregular topography. In the south of the study area this can easily be interpreted as due to faulting, supported by the presence of clear syn-rift wedges in the hanging walls of extensional faults (Figures 3 to 7). Further north the structure at basement level is less well imaged due to the presence of thicker, more structurally complex overburden. However, by analogy with better imaged areas further south, basement topography can be similarly interpreted in terms of extensional faults. The basement structure map shows the NMSB has a rhomboidal shape with the longer axis orientated NNE-SSW (Figure 4). Within that overall shape, however, individual structural lineaments trend predominantly NNW-SSE (Figure 8) and have

created a major depression which is approximately 150 km in length. Half graben and graben structures and lineaments are arranged in an en-echelon pattern and are parallel to sub-parallel. The overlapping fault tips indicate relay structures along which the fault polarity is often reversed (Figure 5). The faulting has produced a series of disconnected NNW-SSE trending structurally low areas separated by relay ramps and other accommodation zones. In the area near the edge of the Paternoster Platform, en-echelon faults display zones of overlapping splays. SEDIMENTARY FILL AND BASIN RECONSTRUCTION The sedimentary rocks deposited above the basement of the NMSB are divided here into three tectono-megasequences: syn-rift, post-rift and foreland basin packages (Figure 2B). Syn-rift package (Middle to Late Eocene) The syn-rift sequence is characterized by sub-parallel, discontinuous, moderate to low amplitude reflectors. In some places the syn-rift packages include divergent, prograding and submarine fill. The submarine fill associated with channel and overbank complexes is seen clearly near the edge of Paternoster Platform (Figure 6). Divergent fill can be seen close to the West Sulawesi where the section thickens towards the hanging wall side of a normal fault (Figure 7). Another type of syn-rift feature in this structural low area is a slope fan complex that accumulated in front of a steep fault. Carbonate build-ups are interpreted in the study area on the top of horsts and tilted fault blocks. They exhibit strong to low amplitude character, are discontinuous and chaotic, and have mounded or conical shapes. They grew as sea level rose and migrated towards land (eastward) as a back-stepping carbonate shelf-edge. Fraser et al. (2003) have interpreted these features as volcanic centres. However, if they are volcanoes they have an unusual form for oceanic crust. Basaltic volcanic centres in oceanic rifts are commonly elongate fissures which erupt low viscosity magmas and do not form conical edifices. The features are apparently situated on the tilted fault blocks (Puspita et al. 2005) and have internal parallel reflectors. They also appear to form NNE-SSW and NW-SE linear ridges. Because of the spacing of the seismic lines it is not possible to

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identify for certain the 3D shape of these structures but we prefer the hypothesis of carbonate build-ups on tilted fault blocks to that of volcanic edifices. Post-rift sedimentation (Early Oligocene to Pleistocene) a. Post-rift 1 (Oligocene) The post-rift 1 package was deposited between the Top Late Eocene (red) and Top Late Oligocene (blue) horizons. The seismic character of this package is typified by parallel to variable, continuous to discontinuous patterns, and moderate to high amplitudes. In places it also displays transparent seismic reflectors probably associated with shale-rich sections. The change to reflector patterns that blanket previous topography suggests that the basin underwent subsidence during the Early to Late Oligocene as in the Kutei Basin (e.g. Feriansyah et al., 1999). b. Post-rift 2 (Early to Middle Miocene) There are two principal sedimentary packages between the Top Late Oligocene (blue) and Top Middle Miocene (pink) horizons. The lower sequence is interpreted as progradational and is characterized by dips that gradually decrease up section. The prograding system indicates that the basin received a major influx of sediments from land on the Kalimantan side of the NMSB from the Early Miocene (Figure 3). Seismic stratigraphic features such as mounded forms with bi-directional and uni-directional downlap and discontinuous, transparent internal reflections characterise the upper (Middle Miocene) sedimentary package (Figure 7). It is suggested that gravity flow mechanisms delivered turbidite deposits into the deeper parts of the basin at this time. c. Post-rift 3 (Late Miocene) The post-rift 3 package is characterized by parallel to sub-parallel continuous seismic reflectors, consistent with the continued deposition of deepwater marine sediments. The post-rift 3 megasequence reflection package continues to the sea floor in the central parts of the NMSB, unaffected by the deformation seen on the east side of the basin.

Foreland Basin (Early Pliocene to Pleistocene) On the east side of the basin a fold and thrust belt is developed. The thrust faults are mostly west verging and have created north-south trending anticlines separated by syn-kinematic mini-basins. The mini-basins contain localized divergent fill and onlap fill where the sediments were derived in part from the erosion of anticlinal crests (Figure 3). In other areas along strike, the fold and thrust belt is not so well developed (Figure 7). There, the eastern NMSB has been regionally uplifted and the unconformity between the pre-uplift and post-uplift sections can be seen to coincide with the Top Late Miocene reflector. The youngest sediments involved in the folding appear to be very young, perhaps Pleistocene or even Holocene. Significant volumes of sediment clearly started to enter the basin from the Sulawesi side in the Early Pliocene (Puspita et al., 2005). DISCUSSION Oblique Rifting The Eocene syn-rift fault pattern in the NMSB is characterized by short rift border faults which are highly segmented and form en-echelon fault arrays parallel to the zone of rifting. Several discontinuous NNW-SSE depocentres developed with the fault zone. A change in polarity of faults can also be seen in the study area. Along strike, the faults that developed within this area seem to be discontinuous. These observations are interpreted as indicative of oblique extension of a basement with a pre-existing fabric. Oblique rifting of a sequence in which there are pre-existing basement faults leads to a complex pattern in which new faults are typically not perpendicular to the extension direction. By comparison with the analogue models of McClay and White (1995), the fault system in the NMSB seems most likely to represent oblique rifting with a principal extension direction E-W, at an angle of approximately 60° to pre-existing faults (Figure 8). This direction is different to the NW-SE extension direction proposed by Hamilton (1979). The NMSB is interpreted as an extensional basin during the Middle to Late Eocene. Following the cessation of rifting, the basin underwent subsidence in the Early Oligocene to Late Miocene. There was an increased influx of sediment into the basin after the beginning of uplift on Kalimantan during the Late

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Oligocene and widespread inversion in East Kalimantan in the Early Miocene (e.g. Feriansyah et al., 1999) At the beginning of the Early Pliocene, plate re-organisation caused the formation of a convergent mountain belt in eastern Indonesia, including Sulawesi, the Moluccas and the Banda Arc (Hall, 2002). This compressional event is considered to be a major force for creating the foreland basin in the NMSB. East-west seismic lines clearly demonstrate the westward vergence of the fold and thrust belt system (Figure 6). The new seismic data also show that the folding and uplift was initiated in the Early Pliocene (Figures 6 and 7). It is therefore interpreted that the North Makassar Straits Basin became a foreland basin no earlier than Early Pliocene and it continues as such today. Nature of the Basement We interpret the crust beneath the NMSB to be continental. At the deepest levels on all seismic lines half-graben and graben structures can be seen. The pattern of faults which can be traced into the deepest parts of the basin suggest oblique rifting, implying the existence of a pre-existing crust with a structural fabric which controlled the orientation of faults. Beneath the Top Basement horizon reflections can often be seen suggesting a complex structure more similar to continental basement than oceanic crust. The volcanic edifices interpreted by other authors do not have the form expected of oceanic basalt magmatism. Even if they are volcanoes, which we doubt, they resemble conical volcanoes developed in rifted (but not those in which extension has reached the point of oceanic crust formation) and arc settings. These structures appear to have a linear character and resemble tilted fault blocks capped by carbonate build-ups. CONCLUSIONS The study of basement architecture and sedimentary fill of the North Makassar Straits Basin provides the following conclusions. The NMSB can be divided into 3 or 4 tectono-megasequences, depending on the location in the basin. Basinwide, pre-rift, syn-rift and post-rift megasequences can be interpreted. Only on the eastern side of the NMSB can the fourth

megasequence, the Plio-Pleistocene foreland basin megasequence, be seen. The basement architecture of the NMSB is characterised by half graben and graben structures interpreted to have formed during the early rifting phase of the southeast Eurasian continental margin (Middle Eocene). The basement faults predominantly trend NNW-SSE and are arranged en-echelon. They bound disconnected depocentres and terminate along strike in accommodation structures such as relay ramps, interlocking fault tips and zones where fault polarity reverses. Based on the analogue models of McClay and White (1995), the extensional fault systems in the NMSB seem most likely to have formed as a result of oblique rifting. The principal extension direction is interpreted as east-west, forming an angle of approximately 60° with pre-existing faults. The Middle to Late Eocene syn-rift megasequence displays a variety of stratal geometries, including parallel, divergent and prograding, and is interpreted to have been deposited in a submarine environment. Carbonate build-ups developed on the top of structural highs or tilted fault blocks. The post-rift megasequence is sub-divided into three units: Post-rift 1 was deposited from the Late Eocene until the Late Oligocene and created a thick shale-rich section. It represents a period when the basin underwent thermal subsidence. Post-rift 2 records the effects of the uplift in Kalimantan after the Late Oligocene and has been divided into two units. A prograding delta system forms part of the lower unit. A turbidite interval is interpreted in the Middle Miocene section. Post-rift 3 was deposited during continued subsidence of the basin from the Middle Miocene until the present day in the central NMSB. The unit is predominantly deep water shales. A foreland basin phase was initiated by the uplift of West Sulawesi from the Early Pliocene and is seen only on the east side of the basin. This tectonic event formed a thrust and fold belt system which has moved progressively westward. It also produced a switch in sedimentary provenance, from Kalimantan to Sulawesi, for the eastern NMSB. Very young sediments are involved in both the anticlines and the adjacent syn-kinematic sedimentary basins. This indicates that compression has continued until very recently and quite probably continues today.

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ACKNOWLEDGEMENTS We thank TGS-NOPEC Geophysical Company, and Peter Baillie in particular, and Migas for permission to use the seismic data and friends (Simon, Helen, Paul), and IT staff at RHUL (Mark and Frank). The MSc programme of Siti Nur’Aini, during which this project was undertaken, was supported by a British Council-ENI jointly funded Chevening scholarship. REFERENCES Bergman, S.C., Coffield, D.Q., Talbot, J.P., and 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., and Blundell, D.J (Eds.). Tectonic Evolution of SE Asia. Geological Society of London Special Publication, 106, p. 391–430. Cloke, I.R., 1997. Structural controls on the basin evolution of the Kutai Basin and Makassar Strait, Indonesia. PhD Thesis, University of London 374 pp. Cloke, I.R., Milson, J. and Blundell, D.J.B., 1999. Implications of gravity data from East Kalimantan and the Makassar Strait: a solution to the origin of the Makassar Strait? Journal of Asian Earth Sciences, 17, p. 61–78. Cloke, I.R., Moss, S.J and Craig, J., 1999. Structural controls on the evolution of the Kutai Basin, East Kalimantan. Journal of Asian Earth Sciences, 17, p. 137–156. Coffield, D.Q., Bergman, S.C., Garrard, R.A., Guritno, N., Robinson, N.M., and Talbot, J., 1993. Tectonic and stratigraphic evolution of the Kalosi PSC area and associated development of a Tertiary petroleum system, South Sulawesi, Indonesia. Proceedings Indonesian Petroleum Association, 22nd Annual Convention, p. 679–706. Feriansyah, L.T., Chambers, J.L.C., Dewantohadi, S.H. Syaiful, M., Priantono, and Imanhardjo, D.N. 1999. Structural and stratigraphic framework of the Palaeogene in the northern Kutei Basin East Kalimantan. Indonesian Petroleum Association, 27th Annual Convention, p. 443-455.

Fraser, T.H. and Ichram, L. A., 1999. Significance of Celebes Sea spreading centre to the Paleogene petroleum systems of the SE Sunda Margin, Central Indonesia. Proceedings Indonesian Petroleum Association, 27th Annual Convention, p. 431–441. Fraser, T.H., Jackson, B.A., Barber, P.M., Baillie, P. and Myers, K., 2003. The West Sulawesi fold belt and other new plays within the North Makassar Strait – a prospectivity review. Proceedings Indonesian Petroleum Association, 29th Annual Convention, p. 429-450. Guntoro, A., 1999. The formation of the Makassar Strait and the separation between SE Kalimantan and SW Sulawesi. Journal of Asian Earth Sciences, 17, p. 79–98. 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, p. 353–431. Hamilton, W., 1979. Tectonics of the Indonesian region, US Geological Survey, Professional Paper 1078, 345 pp. Malecek, S.J., Reaves, C.M., Atmaja, W.S., and Widiantara, K.O., 1993. Seismic stratigraphy of Miocene and Pliocene age outer shelf and slope sedimentation in the Makassar PSC, Offshore Kutei Basin. Proceedings Indonesian Petroleum Association, 22nd Annual Convention, p. 345–371. McClay, K.R and White, M. 1995. Analogue modelling of orthogonal and oblique rifting. Marine and Petroleum Geology, 12, p. 137–151. Mitchum, R.M., Vail, P.R., and Sangree, J.B. 1977. Stratigraphic interpretation of seismic reflection pattern in depositional sequences. In: Payton, C.E (Ed.), Seismic stratigraphy – application to hydrocarbon exploration. American Association of Petroleum Geologists, Memoir 26, p. 117– 133. Moss, S.J., and Chambers, J.L.C., 1999. Tertiary facies architecture in the Kutai Basin, Kalimantan, Indonesia. Journal of Asian Earth Sciences, 17, p. 157–181.

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Pertamina BPPKA, 1996. Petroleum geology of Indonesian basins: principles, methods and applications: Kutei Basin. v. XI. Pieters, P.E., Trail, D.S., and Supriatna, S., 1987. Correlation of Early Tertiary rocks across Kalimantan, Indonesian. Proceedings Indonesian Petroleum Association, 16th Annual Convention, p. 291–306. Puspita, S.D. Hall, R., and Elders, C.F. 2005. Structural styles of the offshore West Sulawesi fold belt, North Makassar Straits, Indonesia. Proceedings Indonesian Petroleum Association, 30th Annual Convention (this volume). Rose, R., and Hartono, P., 1978. Geological evolution of the Tertiary Kutei – Melawi Basin Kalimantan Indonesia. Proceedings Indonesian Petroleum Association, 7th Annual Convention, p. 225–251. Satyana, A.W., Nugroho, J., and Surantoko, I. 1999. Tectonic control on the hydrocarbon habitats of the Barito, Kutei and Tarakan Basins, Eastern Kalimantan, Indonesia: major dissimilarities in adjoining basins. Journal of Asian Earth Sciences, 17, p. 99–122. Situmorang, B., 1982. The formation of Makassar Basin as determined from subsidence curves.

Proceedings Indonesian Petroleum Association, 11th Annual Convention, p. 83–108. Sandwell, D. T., W. H. F. Smith 1997. Marine gravity anomaly from Geosat and ERS 1 satellite altimetry, Journal of Geophysical Research, v. 102 , No. B5, p. 10039-10054. van Bemmelen R.W., 1970. The geology of Indonesia. 2nd edition Government Printing Office. Nijhoff, The Hague, 732 pp. van de Weerd, A.A., and Armin, R.A., 1992. Origin and evolution of the Tertiary hydrocarbon bearing basins in Kalimantan (Borneo), Indonesia. Bulletin, American Association of Petroleum Geologists, 76, p. 1778–1803. van de Weerd, A.A., Armin, R.A., Mahadi, S., and Ware, P.L.B., 1987. Geological setting of the Kerendan gas and condensate discovery, Tertiary sedimentation and paleogeography of the Northwestern part of the Kutei Basin, Kalimantan, Indonesia. Proceedings Indonesian Petroleum Association, 16th Annual Convention, p. 317-338. Wain, T., and Berod, B., 1989, The tectonic framework and paleogeographic evolution of the Upper Kutei Basin. Proceedings Indonesian Petroleum Association, 18th Annual Convention, p. 55–78.

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TABLE 1

TIME AND DEPTH CONVERSION FOR THE PANCA-1 AND PAMUKAN BAY-1 WELLS

Panca-1 (1997)

Pamukan Bay-1 (1970) Stratigraphic

Markers Depth (m)

TWT (ms)

Stratigraphic Markers Depth

(m) TWT (ms)

Top Early Pleistocene 549.0 588.0 Top Early Pleistocene 39.0 35.5 Top Late Pliocene 1060.7 1116.0 Top Late Pliocene 48.8 45.0 Top Early Pliocene 1353.3 1368.0 Top Late Miocene 2286.0 2028.0

Top Middle Miocene 4057.0 3108.0 Top Middle Miocene 79.2 76.0 Top Early Miocene 419.4 413.5 Top Late Eocene 1503.6 937.6

Top Basement 1683.4 1057.8

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Figure 1 - A. The location of the study area and the 2D seismic dataset from four surveys (CM01, CM02, MDD99, and NM93) used for this study. Important and oil & gas discoveries, and key wells referred to in the text, are also shown. B. Free-air gravity map of the Makassar Straits Basin based on Sandwell & Smith (1997).

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Figure 2a - Panca-1 well tied to the A-A’ regional line.

Figure 2b - Chronostratigraphic chart for the central and eastern parts of the North Makassar Straits Basin.

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Figure 3 - East-west regional seismic line across the study area showing half-graben and graben developed during early extension of the North Makassar Straits Basin. A is uninterpreted and B is the interpreted line. Different fault colors represent different fault orientations or generation.

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Figure 4 - The structure maps for six horizons showing the NNW-SSE lineaments identified in the study area. The white dashed lines are the lineaments. The orange line shows the present-day thrust front of the deformed sediment wedge on the east side of the Makassar Straits.

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EW

7048m

C C’

D’D

A

TWT msecs

TWT msecs

C’

D D’

EW

Top Basement

Top Late MioceneTop Middle Miocene

Seabed

Top Late OligoceneTop Late Eocene

7048m

Mass-flow depositshorst

Back-stepping carbonate shelf edge

Changing polarity of faults

C

7089mB 7089m

horst

Back-stepping carbonate shelf edge

100km

N

C C’D D’

TWT msecs

TWT msecs

4000

3000

1000

1500

2500

3500

4500

5500

6500

7500

8500

2000

500

6000

8000

7000

5000

0

4000

3000

1000

1500

2500

3500

4500

5500

6500

7500

8500

2000

500

6000

8000

7000

5000

0

9000

Figure 5 - Two parallel seismic lines (C-C’ & D-D’) illustrating the changing polarity of faults along strike.

A: uninterpreted. B: interpreted. Different fault colors represent different fault orientations or generation.

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Figure 6 - Line F-F’ showing the syn-kinematic basin fill (onlap fill and divergent fill) produced during thrusting and folding in West Sulawesi from the Early Pliocene. The NMSB became a foreland basin above the brown Top Late Miocene horizon.

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Figure 7 - Seismic line G–G’ displays the prograding fill and divergent fill that developed within the syn-rift

packages and the turbidite lobe systems developed in the post-rift 2 unit during the Early to Middle Miocene. Sediment from Sulawesi started to fill the basin during the Early Pliocene.

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Figure 8 - Fault distribution at Top Basement level compared to faults produced in analogue models of orthogonal extension (α = 90°) and oblique extension (α = 60°). Analogue models are from McClay & White (1995).