2009 Hall Etal 2009 N Makassar Straits

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The North Makassar Straits: what lies beneath?

Robert Hall1,*, Ian R. Cloke1,2, Siti Nuraini1,3, Sinchia Dewi Puspita1,4,

Stephen J. Calvert1,5

and Christopher F. Elders1

1SE Asia Research Group, Department of Earth Sciences, Royal Holloway University of London, Egham,Surrey TW20 0EX, UK

2Tullow South Africa, 7 Coen Steytler Avenue, Cape Town 8001,South Africa3ConocoPhillips Indonesia, Jl. Jend. Gatot Subroto Kav 9-11, Jakarta 12930, Indonesia

4Chevron, Jl. Asia Africa No. 8, Jakarta 10270, Indonesia5Marathon Oil Company, Jalan TB Simatupang No. 41, Jakarta 12550, Indonesia

*Corresponding author (e-mail: [email protected])

ABSTRACT: It has been accepted for many years that eastern Borneo and westernSulawesi were close together in the Late Cretaceous but the mechanism and age offormation of the Makassar Straits, which now separate them, have been the subjects

of much debate. Geological studies on land show that the straits formed by Eocenerifting. However, the nature of the crust beneath the straits remains controversial.The southern parts are likely to be underlain by extended continental crust but, inthe northern Makassar Straits, it is more difficult to decide. Water depths are up to2500 m, there is a very thick sedimentary cover, the basement is not well imagedon seismic lines and there is no way of directly sampling it. Field studies fromthe Borneo and Sulawesi margins have provided the basis for reconstructing thedevelopment of the straits, and suggesting they are underlain by oceanic crust. Therift and its margins are asymmetrical and wide, with up to 400 km of stretched cruston the Borneo side and about 200 km on the Sulawesi side, separated by about 200km of the deepest crust in the northern Makassar Straits. Gravity data and flexuralmodelling on the Borneo side suggest a junction between continental and oceanic

crust beneath the Mahakam delta. The oceanic crust is inferred to be of MiddleEocene age, similar to the Celebes Sea to the north; apparent conical structures onseismic lines have been interpreted as volcanic edifices. However, the earliestbackstripping studies suggested thinned continental crust in the central straits andthis has been supported by interpretations of new seismic data from the offshorearea west of Sulawesi. Half-graben and graben are interpreted beneath thicksediments, there are low-angle extensional faults, and lineaments crossing basementcan be traced into the deepest parts of the straits. These structures suggest an originby oblique rifting of continental crust in which the apparent conical structures areinterpreted as carbonate build-ups on tilted fault blocks.

KEYWORDS: Eocene, rifting, continental crust, oceanic crust, Borneo, Sulawesi

INTRODUCTION

The Makassar Straits ( Fig. 1) separate Borneo from Sulawesi inIndonesia and are situated within a complex tectonic region atthe edge of the Eurasian plate. The northern straits are thedeepest, with water depths of almost 2.5 km, and they open tothe north into the Celebes Sea. The southern part of the straitsare shallower, with water depths mainly less than 2 km and theycontinue south into the shallow shelf area of the East Java Sea.

The Makassar Straits are today the main passage in the transferof water and heat from the Pacific to the Indian Ocean, via theIndonesian Throughflow, which is thought to play an import-ant role in the modern global climate system and have been an

important influence on biogeographical patterns. Wallace(1869) shows a line up the Makassar Straits, beginning between

the islands of Bali and Lombok, and then crossing the CelebesSea, which separated Asian and Australasian faunas and is nowknown as Wallaces Line.

There are two deep basins in the straits (Fig. 2). TheNorth Makassar Basin is bounded to the south by thenorthern edge of the Paternoster Platform, often shown as aNWSE-trending fault, with a narrow deep channel connect-ing it to the South Makassar Basin. To the east is thePalu-Koro Fault, a major sinistral strike-slip fault runningNNWSSE from onshore West Sulawesi into the CelebesSea, and to the north is the Mangkalihat Peninsula which isalso often shown as bounded by active NWSE-trendingfaults. On the western side of the North Makassar Straits is

the Kutai Basin, the largest and deepest (15 km) basin inIndonesia ( Rose & Hartono 1978) and one of the richest

Petroleum Geoscience, Vol. 15 2009, pp. 147158 1354-0793/09/$15.00 2009 EAGE/Geological Society of LondonDOI 10.1144/1354-079309-829


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hydrocarbon provinces in Indonesia. The first deep-waterproduction of oil and gas in Indonesia came from the easternedge of the Kutai Basin, on the western side of the MakassarStraits, at the foot of the Mahakam delta.

On the eastern side is West Sulawesi, with so far unprovenhydrocarbon resources, but where oil seeps have been known

for many years (see Calvert & Hall 2007). The young fold beltin the deep-water region just offshore of West Sulawesi is nowthe target of active exploration. The character of the basementbeneath the Makassar Straits is important for the petroleumsystem since it will determine the subsidence history, thermalhistory and, consequently, source-rock maturation, as well asthe style of traps. If continental, it is likely there are Eocenelacustrine source rocks, tilted fault blocks and carbonate andclastic reservoirs. If there is oceanic crust beneath the MakassarStraits, Miocene organic material transported into deep water

would probably be required for the petroleum system to work.

MODELS OF FORMATION

East Borneo and West Sulawesi were part of a single area in theLate Mesozoic (e.g. Katili 1978; Hamilton 1979) but wereseparated during the Cenozoic by the opening of the MakassarStraits. There has been debate about the age of formation of thestraits (e.g. early accounts (Katili 1978; Hamilton 1979;

Hutchison 1989) favoured Miocene separation of Sulawesi andBorneo) but an Eocene age is now generally accepted. Themechanism of opening has also been the subject of controversyand its cause remains uncertain. Most authors have favoured anextensional origin for the straits (Katili 1978; Hamilton 1979;Situmorang 1982a, b; Wissmann 1984; Cloke 1997; Guntoro1999), with the Middle Eocene as the age of rifting (Situmorang1982a, b; Hall 1996; Moss et al. 1997; Guntoro 1999; Moss &Chambers 1999; Calvert & Hall 2003, 2007) but no agreementabout basement type. Hamilton (1979) showed an oceanicspreading centre down the entire length of the straits andinterpreted several NWSE transform faults. Hall (1996) pro-posed that oceanic spreading in the Celebes Sea during theMiddle Eocene propagated towards the southwest into the

northern straits. Fraser & Ichram (2000) interpreted there to beoceanic crust beneath the northern straits and southern straitsas far south as latitude 6S. The Makassar Straits have also beeninterpreted as a remnant oceanic basin (Malecek et al. 1993), aback-arc basin (Parkinson 1998), and Guntoro (1999) sug-gested that extension was due to trench rollback and sinkingof a plate subducting east of a magmatic arc beneath

West Sulawesi. Others have argued that rifting never reachedthe stage of oceanic spreading (e.g. Burollet & Salle 1981;

Fig. 1. Geography of Borneo and Sulawesi and surrounding regions.Small black filled triangles are volcanoes from the SmithsonianInstitution, Global Volcanism Program (Siebert & Simkin 2002), andbathymetry is simplified from the GEBCO (2003) digital atlas.Bathymetric contours are at 200 m, 2000 m, 4000 m and 6000 m.

Wallaces Line is the dashed line; this is the original biogeographicalboundary identified by Wallace (1860) but an alternative was latertraced north to the west of Mindanao from the northern MakassarStraits by Huxley (1868). These were named Wallaces Line by

Huxley (1868), as discussed by Mayr (1944).

Fig. 2. (A) Physical setting andbathymetry of the North MakassarBasin shown on the global topographicdigital elevation model (DEM)of Becker & Sandwell (2007).(B) Two-dimensional seismic datacoverage used in this study (NurAiniet al. 2005; Puspita et al. 2005) with

sediment thicknesses in the offshoreKutai Basin from Moss & Chambers(1999), modified in the central part ofthe straits using the seismic data.

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Situmorang 1982a, b). The inversion on the west side of thestraits in East Kalimantan (Moss et al. 1997; Moss & Chambers1999) from the Early Miocene, and eastward propagation offolding and thrusting, have suggested to some that the straitshave a flexural origin. On the east side there was developmentof a fold-and-thrust belt in Western Sulawesi during theMiocene (Coffield et al. 1993; Bergman et al. 1996; Guritno et al.1996) or later (Calvert 2000a, b; Calvert & Hall 2003, 2007)and, thus, the straits have been interpreted as a foreland basinformed after Early Miocene continentcontinent collision inSulawesi (Coffield et al. 1993; Bergman et al. 1996) in responseto thrust loading on one or both sides.

The South Makassar Straits are almost certainly underlain bycontinental crust. They are relatively narrow and water depthsare mainly less than 2 km. There are shelves with thin sedimen-tary cover above basement to the east and west. Granite andpre-Cenozoic metamorphic rocks are known from boreholesthat reach the basement. Although Hamilton (1979) interpretedan oceanic spreading centre, apparently based on the shape andmorphology of the basin, as did Fraser & Ichram (2000),

back-stripping and gravity modelling by Situmorang (1982a, b)indicate continental crust is much more likely, and this is nowgenerally accepted. Recent seismic lines show up to 7 s ofsediment above tilted fault blocks and half-graben in the centralparts of the straits (Johansen et al. 2007), with a thickPalaeogene syn-rift sequence above possible pre-Eocene rocks.

Here, we focus on the North Makassar Straits where thenature of basement is much less clear. Since the crust beneaththe Celebes Sea, just to the north, is oceanic, it is plausible thatthe North Makassar Straits are underlain by oceanic crust (e.g.Cloke et al. 1999b; Guntoro 1999) but others have put forwardarguments in favour of attenuated continental crust (e.g.Burollet & Salle 1981; Situmorang 1982a, b). We summarize theobservations from land on each side of the straits, which

explain the age and character of the margins, and outline theinterpretation of basement in the Makassar Straits based onland observations and gravity modelling. We then outline theobservations of the offshore region based mainly on seismicdata from the deep-water central and eastern straits. We thentry to provide an objective assessment of the main argumentsfor oceanic and continental crust beneath the North MakassarStraits.

THE GEOLOGY ON EAST AND WEST SIDES OFTHE STRAITS

Field observations cannot directly show the nature of thebasement in the central deepest-water part of the straits, but

they do reveal the history and dimensions of the rifted zone,which broadly support suggestions of a relatively narrow centralarea of oceanic crust with passive margins to the west and east(Fig. 3).

West side: Kutai

In Borneo the basement consists of metamorphic, sedimentaryand granitic to gabbroic rocks of Devonian to Cretaceous age(van Bemmelen 1949; Pieters et al. 1987; van de Weerd &

Armin 1992; Hall et al. 2008). Continental rocks are broadly inthe west and, to the east, there are ophiolitic rocks, cherts andassociated deep-marine sediments of Cretaceous age. To the

west of the North Makassar Straits Basin is the Kutai Basin

(Pieters et al. 1987, 1993; Wain & Berod 1989; van de Weerd &Armin 1992; Cloke 1997; Moss et al. 1997; Moss & Chambers1999), which began to form in the Middle Eocene, andCenozoic rocks rest unconformably upon deformed Cretaceous

turbidites and ophiolites. Possible PaleoceneEocene volcanicrocks are reported from wells just south of the Mangkalihatpeninsula (Sunaryo et al. 1988). Rifting formed a series ofnorth- and NNE-orientated graben and half-graben (Cloke et al.1997; Moss & Chambers 1999) which contain terrestrialdeposits in their deepest parts, and pass up through marginalmarine into marine deposits. The western depocentres generallybecame marine later than those further east. By the LateEocene most of the Kutai Basin was dominated by quiet marineshale deposition with foraminiferal shoals and other carbonatebuild-ups on the crests of shallow-marine tilted fault blocks.Extension ceased by the end of the Eocene and there wasregional subsidence with widespread deposition of basinalshales during the Oligocene, while locally carbonate depositioncontinued on basement highs and near to the basin margins.

There is an important unconformity in the Upper Oligocene,interpreted as due to uplift in central Kalimantan at about25 Ma (Moss & Chambers 1999) and possible renewed exten-sion in the basin (Cloke et al. 1997, 1999a, c). In the EarlyMiocene large amounts of clastic sediments were shed into the

Kutai Basin due to erosion of the Borneo highlands to the westand later and by inversion of older parts of the basin margins tothe north and west (van de Weerd & Armin 1992; Chambers &Daley 1997; Ferguson & McClay 1997; Moss et al. 1998).Inversion migrated from west to east during the Early andMiddle Miocene (Ferguson & McClay 1997; Moss et al. 1997;McClay et al. 2000). In the Late Miocene there was anotherphase of inversion, westward progradation of the Mahakamdelta and exhumation of anticlines close to the present coast,removing up to 3000 m of section. Beneath the present mouthof the delta is the centre of the Mahakam depocentre wherethere is an estimated sediment thickness of more than 14 km(Moss & Chambers 1999).

East side: West Sulawesi

Immediately east of the North Makassar Basin are the Lariangand Karama regions of West Sulawesi where there are twomajor unconformities: one between basement rocks andEocene shelf sediments, and a younger one between LowerPliocene shelf sediments and Plio-Pleistocene syn-orogenicsediments (Sukamto 1973; Hadiwijoyo et al. 1993; Ratman &

Atmawinata 1993; Calvert 2000a, b; Calvert & Hall 2003, 2007).The Mesozoic basement consists of metamorphic rocks uncon-formably overlain by less deformed Upper Cretaceous darkshales and volcanic rocks. These are at least 1000 m thick andconsidered laterally equivalent to basement in other parts of

western Sulawesi, and interpreted to be the deposits of a

forearc basin situated to the west of a west-dipping subductionzone (van Leeuwen 1981; Hasan 1991), or a passive margin(Hall 2009). Non-marine sediments at the base of the lowerCenozoic sections could be as old as the Paleocene, but theoldest dated sediments are marine and record a transgression inthe Middle Eocene that must post-date the initiation of riftingin the region. The Eocene sediments were deposited in grabenand half-graben in both marginal marine and marine environ-ments. The post-rift subsidence phase had started by the LateEocene. In the Late Eocene carbonate shoals and shelf mud-stones developed on both margins of the Makassar Straits and,by the end of the Oligocene, most of West Sulawesi was an areaof shelf carbonate and mudstone deposition. The lowermostMiocene has not been found, but there is little evidence in West

Sulawesi for either a significant regional unconformity, or inputof orogenic sediment. Instead, throughout the Early Mioceneand in places until the Middle or Late Miocene, carbonates andmudstones were deposited on a shallow-marine continental

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Fig.

3.T

ruescalecross-sectionsacrosstheMakass

arStraitsatthepresentdayandinterpretedforthelatePalaeogene,

modifiedfrom

Calvert&

Hall(2007).

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margin. Further south, in the south arm there is no evidence fora significant break in marine deposition.

Only during the Pliocene did the character of sedimenta-tion across the whole of western, central and eastern Sulawesichange significantly. Uplift and erosion was followed by thedeposition of coarse clastics derived from an orogenic belt tothe east. To the west of the orogenic belt there wassyn-orogenic sedimentation, inversion and folding abovePalaeogene half-graben, detachment folding and thrusting,and the development of intra-basinal unconformities andmini-basins, which has propagated west into the MakassarStraits. In the Late Pliocene the Lariang and Karama regionsof West Sulawesi changed from a passive margin to a forelandbasin setting and sedimentation rates doubled.

Width of the rift

Interpretations based on the onshore investigations havefavoured oceanic crust in the central part of the north MakassarStraits. The crustal-scale cross-sections (Fig. 3) interpreted

from field observations and seismic data, first by Cloke (1997)for the Kutai Basin in eastern Borneo and, later, by Calvert(2000b) for the Lariang and Karama Basins of West Sulawesi,show that the Eocene Makassar Straits was highly asymmetricaland an extremely wide rift. The zone of extension of the

western margin (Kalimantan) was up to 400 km wide, and thatof the eastern margin ( Sulawesi) was approximately 200 km

wide, and the two reconstructed margins are separated by thedeep-water zone about 200 km wide. Typically the width of thezone of extended continental crust is 50150 km (Louden &Chian 1999), although it can be as large as 400500 km (e.g.Orphan Basin, Keen et al. 1987; northern Gulf of Thailand,Chantrapresert 2000). Moss & Chambers ( 1999) suggested thegreat width of the rifted zone may reflect a weak warm

lithosphere, possibly due to Cretaceous collisions in the region.But the great width of the areas of extension in Borneo andSulawesi, plus the additional width of the deepest-water part ofthe central straits, strengthened the suggestions of Cloke (1997)and Cloke et al. (1999b) that the central North Makassar Straits

were underlain by oceanic crust.

Gravity and back-stripping

Free-air gravity (Fig. 4 ) shows there is a broad gravity lowbeneath the central North Makassar Basin. This includes anelongated low northeast of the Paternoster Platform thatfollows the narrow trough connecting the North and SouthMakassar Basins, and an irregular low between the Mahakam

delta and the Mangkalihat Peninsula. There is large gravity highbeneath the Mahakam delta depocentre. Cloke et al. (1999b)interpreted the gravity data to suggest that oceanic crust laybeneath the central straits. They supported this by an investi-gation using gravity modelling and flexural back-stripping witha crustal model interpreted from seismic sections of the KutaiBasin. They argued that the free-air gravity high beneath theMahakam delta was produced by rapid thinning of the crustfrom 30 km beneath the Kutai Basin on land to 14 km in thecentre of the North Makassar Straits, as also suggested byGuntoro (1999). They then applied the method of Watts(1988) to determine the contribution to the calculated gravityanomaly from rifting (crust and upper mantle) and the sedi-ment load (Fig. 5A) which they suggested indicated an under-

lying crust with an elastic thickness (Te) of 20 km. Such anelastic thickness is similar to that of oceanic crust with an age of48 Ma. Comparison of the calculated anomaly with modelledanomalies for different elastic thickness corresponding to

oceanic crust of different ages and thinned continental crustwith T

etypical of rifts (Fig. 5B), shows that larger or smaller

values ofTe

produced anomalies which matched less well to theobserved anomaly. From this Cloke et al. (1999b) concludedthat the central Makassar Straits was underlain by oceanic crust

with a spreading centre situated to the east.

OFFSHORE NORTH MAKASSAR STRAITS

Until recently there were only a few regional seismic linescrossing parts of the Makassar Straits (Wissmann 1984;Bergman et al. 1996; Samuel et al. 1996) and these were mainlyof poor quality and showed little that helped to interpret thenature of the basement. They did show thick sediments, theundeformed central parts of the straits, and fold-and-thrustbelts converging from the west and east sides. However, in thelast few years more than 10 000 km of new data have beenacquired or reprocessed by TGS-NOPEC GeophysicalCompany during seismic surveys covering large parts of theNorth Makassar Straits (Fig. 2). These data have been inter-preted in different ways and used to suggest possible volcanicstructures (Baillie et al. 2000), a basement of oceanic crust(Fraser et al. 2003) or continental crust (NurAini et al. 2005;Puspita et al. 2005).

Bathymetry and morphology

The bathymetry in the North Makassar Straits reflects someobvious features of the deeper structure. The seafloor in the

Fig. 4. Free-air gravity map of the Makassar Straits Basin based onSmith & Sandwell (1997).



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central North Makassar Straits is flat and undeformed (Fig. 6).In the north the water depth is almost 2500 m and is about200 m less in the south. Depths decrease towards thecarbonate-dominated Paternoster Platform in the south and theMangkalihat Peninsula in the north. To the west, the seafloorrises gradually to the very shallow East Kalimantan Shelf,crossing the front of the Mahakam delta. In the east, theseafloor shallows towards western Sulawesi, rather moreabruptly than on the west side, reflecting folding and thrusting

of a deformed zone that is now described as the Offshore WestSulawesi Foldbelt.From south to north, this deformed zone can be divided

into three provinces (Fig. 6): the Southern Structural Province

(SSP), Central Structural Province (CSP) and Northern Struc-tural Province (NSP) based on seafloor characteristics, subsur-face deformation, in particular the character and position of thedeformation front. The SSP is south of 130#S and is c. 4080 km wide. The CSP is very narrow, about 30 km wide and liesbetween latitude 050#S and 130#S. The NSP is north oflatitude 050#S and is between 50 km and 100 km wide. Itsnorthern limit is not seen. At the very northern edge of theNorth Makassar Straits is the Palu-Koro fault which can be

traced offshore but is seen on only a few seismic lines at thelimit of our dataset.The area immediately east of the deformed zone is beyond

the ends of the seismic lines but there is a rapid decrease in

Fig. 5. Results of flexural modellingand back-stripping, modified fromCloke et al. (1999b). (A) Profiles showthe calculated (a) crust thickness and(b) sediment thickness anomalies whichcombine to provide the (c) totalanomaly, compared to the (d) observedfree-air and Bouguer gravity anomalybased on an elastic thickness (T

e) of

20 km. (B) Comparison of the

observed free-air and Bouguer anomalyand the calculated anomaly expected fordifferent values of T

e: values typical of

rift basins (Te=5 and 10 km) and old

oceanic crust (Te=50 km). The best

match is for Te=20 km, which is close

to that expected for 48 Ma oceaniccrust. Thin dashed lines show thetectonic subsidence/uplift (TSU ) andback-stripped Moho based on aTe=20 km.

Fig. 6. Three-dimensional view ofbathymetry of the North MakassarStraits and topography of WesternSulawesi produced by mergingbathymetric data from seismic data withthe global bathymetry of Smith &Sandwell (1997) and Shuttle Radar

Topography Mission (SRTM )topographic data for onshore Sulawesi.

The cliff-like feature east of theOffshore West Sulawesi Foldbelt,especially clear behind the SouthernStructural Province (SSP), is partly anartefact of the merged bathymetric data.Nevertheless, a very steep rise to theshelf must be present as there is a veryshort distance between the 1000 mbathymetric contour and the coast.CSP, Central Structural Province; NSP,Northern Structural Province.

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water depth. Combining bathymetric data from the seismiclines and topography from the Shuttle Radar TopographyMission (SRTM) shows a steep shelf edge with a narrow shelf.Onshore, there is a narrow coastal plain and, just east of thecoast, pre-Pliocene rocks, including pre-Cenozoic basement,have been brought to the surface by young thrusting (Calvert2000a, b). Approximately 100 km east of the coast, the moun-

tains are up to 3000 m high. Depths of almost 2500 m in theNorth Makassar Straits are separated from mountains of3000 m by less than 200 km horizontal distance.

Structure

The Cenozoic sedimentary sequence in the central part of theNorth Makassar Straits is undeformed and separated from theOffshore West Sulawesi Foldbelt by a change in slope at whichthere are folds, and blind and emergent thrusts and back-thrusts, which we describe as a deformation front since itsexpression varies from place to place (Figs. 6 and 7). The NSPand SSP are sedimentary wedges with different characteristics

separated by the basement high of the CSP, gradually onlappedeastwards by a thin sediment cover which is little deformed.

In the NSP there are no clear continuous reflectors, andthrusts and folds are not well developed. The seismic datasuggest distributed deformation, small individual structuralunits, small graben associated with shale diapirism, and a steepsharp deformation front. All these features suggest relativelyhomogeneous mud-rich sediments. In the SSP, thrust faults andfolds are very obvious, and growth strata between thrust faultscan be distinguished. There is a gentler overall slope to thesediment wedge and no steep deformation front. The strongand weak amplitude reflectors in the deformed successionsuggest interbedded sands and muds. The characteristics ofthe NSP and SSP are similar to those in deformed areas of

the Timor Trough south of Timor (Karig et al. 1987) wherevariations along-strike in internal character and slope angleshave been interpreted as due to differences between mud-dominated and sand-dominated sequences.

Basal unconformity

In the central part of the straits, and beneath the two sedimentwedges, the deeper structure can be mapped out. However,tracing seismic horizons and interpreting structure becomesmore difficult with increasing depth, due to the thickness ofoverlying sediments in the undeformed area (up to 4 s TWT),

and the thickness and deformation of the sediment wedges ofthe NSP and SSP (up to 7 s TWT). A very prominent reflectoron most seismic lines and the deepest distinctive horizon thatcan be mapped throughout most of the area is here termed thebasal unconformity (Fig. 7). Above it, strata are flat lying andmostly parallel to sub-parallel in the central part of the straits.In contrast to the bathymetry, the basal unconformity isdeepest in the south, close to the Paternoster Platform. Itascends gradually towards the Mangkalihat Peninsula in thenorth, and descends to the east, towards West Sulawesi.

Guntoro (1999) identified this reflector as top acousticbasement. Baillie et al. (2000) interpreted it as the top of oceaniccrust or thinned continental crust. However, even thoughlocally this horizon resembles the top of basement, in many

places there are observable structures beneath it which havebeen interpreted as volcanic edifices, or rift-related topographywith carbonate build-ups. Puspita et al. (2005) suggested this isthe top of the syn-rift sequence.

The most striking features to emerge from mapping thebasal unconformity and contouring the surface are a number ofNWSE and NNESSW features (Fig. 7). At the southern endof the area is a rhomb-shaped depression, about 100 km wide,immediately north of the Paternoster Platform. This extendsfrom the undeformed central straits to beneath the deformedbelt, and remains at a similar depth across most of the straits.North of this depression is an irregular higher area of similar

width.The NWSE to NNESSW features are not associated with

faults in the overlying sediments. However, at the north andsouth end of the North Makassar Basin are the MangkalihatPeninsula and the Paternoster Platform. These are often shownto be bounded by NWSE faults. Both remained structural

Fig. 7. (A) Map of the surface of thebasal unconformity by Puspita et al.(2005), showing the two main rhombicdepressions crossing the straits in aNWSE orientation. Note the

apparently continuous ridges trendingNNESSW and NWSE that havebeen interpreted as volcanic structuresand tilted fault blocks. (B) The samesurface contoured, with contours inseconds TWT.



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highs during the Cenozoic with almost continuous shallow-water carbonate deposition, whereas from the Middle Eocenethe Kutai Basin subsided to great depths. This suggests somedeep structural control with normal faulting in the Eocene.

There are linear features on the contoured surface trendingbroadly NNESSW. Because of the spacing of seismic lines itis not possible to say with certainty that these features arecontinuous, but they appear to be asymmetrical, broadlycontinuous structures that have been interpreted as faultblocks (Puspita et al. 2005) and are probably the features that

were interpreted as extrusive centres (Fraser et al. 2003).

Mapping beneath the basal unconformity

Especially at the southern end of the southern rhombicdepression, it is possible to map structures beneath the basalunconformity (NurAini et al. 2005). The fault pattern (Fig. 8)is characterized by short rift border faults which are highlysegmented and form en-echelonfault arrays parallel to the zone ofrifting. In the area near the edge of the Paternoster Platform,

en-echelonfaults display zones of overlapping splays. A change inpolarity of faults can also be seen in some areas. Overlappingfault tips suggest relay structures along which the fault polarityis reversed. The faulting has produced a series of disconnectedNNWSSE-trending, structurally low, areas interpreted to beseparated by relay ramps and other accommodation zones.

Along-strike, the faults are discontinuous and associated withsmall half-graben and graben.

In the south of the area the map of top basement shows anirregular topography interpreted as due to faulting, supportedby the presence of clear syn-rift wedges in the hanging walls ofextensional faults. Further north the structure at basement levelis less well imaged due to the presence of the overlying thick,structurally complex, sedimentary wedges. However, by analogy

with better imaged areas further south, basement topographycan be similarly interpreted in terms of extensional faults.These observations have been interpreted as indicating

oblique extension of a basement with a pre-existing fabric(NurAini et al. 2005). Oblique rifting of a sequence in whichthere are pre-existing basement faults leads to a complexpattern in which new faults are typically not perpendicular tothe extension direction. By comparison with the analoguemodels of McClay & White (1995), the fault system in theNorth Makassar Straits seems most likely to represent obliquerifting with a principal extension direction EW, at an angle ofapproximately 60 to pre-existing faults ( Fig. 8 ). This directionis different to the NWSE extension direction proposed byHamilton (1979).

DISCUSSION: CONTINENTAL OR OCEANICCRUST?

The nature of the basement to the central part of the MakassarStraits can be interpreted only indirectly, because the very thicksediment cover and the great depth to basement means that nodirect sampling is possible. The deep unconformity in thestraits is most likely the top of the syn-rift sequence and marksthe beginning of the sag phase of dominantly thermal subsid-ence that continued during the Oligocene (Situmorang 1982a,b; Bergman et al. 1996; Guritno et al. 1996).

The oceanic crust interpretation is favoured by the greatwidth of the extended zone and, in particular, the 200 km width

of the deepest part of the straits where depths are close to2.5 km water depth and there are several kilometres of almostundisturbed flat-lying sediments above the basement. Back-stripping and flexural modelling indicate that the best fit to

observed gravity profiles is achieved if the basement is oceaniccrust of about 50 Ma age, which is very similar to thatdemonstrated for the Celebes Sea to the north. In contrast,typically stretched continental crust in such deep rifts wouldhave a much smaller elastic thickness. At the level of basement,NWSE-trending structures are interpreted as transform offsetsof the spreading axis, and the triangular structures seen belowthe basal unconformity are interpreted as volcanic edifices. Inprofile, these and other features on seismic lines resemblesubmarine volcanoes, sills and dykes imaged in 3D surveys onthe northeast Atlantic margin (Davies et al. 2002).

The continental crust interpretation is favoured by theobservations that rifting structures can be seen below the basalunconformity. Half-graben and graben are evident in places,and the pattern of faulting mapped below the basal unconform-ity is similar to that expected from oblique extension of abasement with a pre-existing NWSE fabric (NurAini et al.2005). The NWSE lineaments which segment the basin areinterpreted to be Cretaceous or Paleocene structures, which inplaces may have been reactivated. The northern margin of the

Paternoster Platform is clearly a major steep fault with about2 km of normal offset of the Eocene and the large displacementis inconsistent with an oceanic transform fault. Except for thePaternoster Fault, none of the NWSE features crossing thestraits in our dataset show any sign of having been active faultsduring the Cenozoic, although on land, mapping in WestSulawesi (Calvert, 2000b) shows structures with similar orien-tation that continue towards the deformed zone offshore whichprobably segmented the area during the early Cenozoic and

were reactivated as strike-slip faults during Pliocene to Recentdeformation. The triangular structures seen below the basalunconformity are interpreted as carbonate build-ups on tiltedfault blocks.

The Makassar Straits and its margins are asymmetrical with

a wider region of extended crust on the west side (Borneo)compared to the east side (Sulawesi), as clearly seen in therestored crustal sections of Figure 3. This resembles profilesacross the southern Labrador Sea, where asymmetric passivemargins are separated by a narrow zone of ocean crust (Louden& Chian 1999). Such asymmetry, however, may also result fromsimple shear, and extended regions of similar dimensions areknown to be underlain by thinned continental crust (e.g.

Wernicke & Burchfiel 1982; Ebinger et al. 1991). The greatwidth of the extended region may reflect heating and weakeningof the lithosphere during Mesozoic accretionary events (Moss& Chambers 1999) or long-term subduction beneath Sundaland(Hall & Morley 2004; Hyndman et al. 2005).

The flexural modelling makes a good case for oceanic crust,

and a central high strength oceanic lithosphere juxtaposedagainst weaker continental crust is consistent with thedeformed zones on the west and east sides of the straits.However, the elastic thickness of continental crust is muchmore variable and less predictable than oceanic crust (Burov &Diament 1995, 1996; Watts 2001), and the modelled result ofTe=20 km is well within the range of values from continents

and their margins (e.g. Bechtel et al. 1990; Watts & Burov 2003;Prez-Gussiny et al. 2008). Elastic thickness varies with tem-perature, rheology and lithospheric structure, and a higher T

e

could reflect an olivine-rich rheology or strong layer due toCretaceous ophiolite accretion. The volcanic structures inter-preted from 3D seismic data by Davies et al. (2002) are about1 km across, whereas those in the Makassar Straits are typically

between 4 km and 10 km across. Their width and height wouldbe unusual for basaltic volcanoes, which would be expected toform much lower and small gradient volcanoes, whereas theirdimensions are very similar to tilted fault blocks from other

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rifts. These structures are shown deliberately uninterpreted onFigures 9, 10 and 11, so that readers can compare alternativeinterpretations for themselves.

The undisputed graben and half-graben seen in parts of thestraits indicate extended continental crust, and mapping ofthe straits indicates the likelihood of oceanic basement beneaththe eastern and central straits is low. This is the area whereCloke et al. (1999b) located the supposed spreading centre. Inthis part of the North Makassar Straits we have been unable toidentify the many extrusive centres mapped by Fraser et al.(2003) in the area covered by our seismic dataset. Furthermore,

we find it difficult to see on what observations the discontinu-ous spreading centre and transforms shown by Fraser et al.(2003) are based. In the northern part of the North MakassarBasin where they show the NESW-trending spreading centreoffset by NWSE transform faults in the deep central part ofthe straits, we interpret there to be NNWSSE-trending faults(NurAini et al. 2005) forming graben and half-graben (Fig. 8).Further south, the spreading centre and transform faults ofFraser et al. (2003) are at several kilometres depth beneath the

sedimentary wedges of the Offshore West Sulawesi Foldbeltwhere we were unable to map the basement with confidence.The form of the oceanic basin interpreted by Fraser et al. (2003)is also questionable. In their Late Eocene reconstruction theyinterpret a symmetrical oceanic basin at the northern end of theNorth Makassar Basin with the spreading centre in the middle,

whereas 200 km further south the spreading centre is offset tothe east and is beneath the present coast of West Sulawesi. Theeastern part of their symmetrical oceanic basin shows oceaniccrust to lie beneath the 3842.5 Ma shelf edge. Oceaniclithosphere follows a predictable agedepth relationship (Par-sons & Sclater 1977) and, even allowing for differencesobserved in small marginal basins (Wheeler & White 2002), theocean crust would be expected to be at depths of more than

3 km within a few million years of formation. The palaeogeo-graphical reconstruction of Fraser et al. (2003) therefore impliesseveral kilometres of Eocene sediment and huge westwardprogradation of the Eocene Sulawesi shelf edge to move fromtheir inferred Eocene basin floor fan domain above the formerspreading centre to their deltaic influences and coals anddeltaic sediments onshore SW Sulawesi. For these reasons weconsider that the reconstruction of Fraser et al. (2003) isimprobable. Nevertheless, extended continental crust in theeastern and central straits is not incompatible with the presenceof some oceanic crust beneath the Mahakam depocentre on the

west side of the straits where the flexural modelling of Clokeet al. (1999b) was carried out.

CONCLUSIONS

The Makassar Straits formed by rifting. Extension began in theMiddle Eocene and formed graben and half-graben above

which is an important unconformity of probable Late Eoceneage. The unconformity marks the top of the syn-rift sequence.Structures can be seen beneath the unconformity which couldbe carbonate build-ups on tilted fault blocks or volcanicedifices. Thermal subsidence continued during the Oligocene.Flexural subsidence due to loading on the west and east sidesmay have deepened the straits, as inversion in eastern Kaliman-tan migrated east and the Mahakam delta prograded east since

Fig. 8. (a) Fault pattern and interpreted structure mapped at top

basement level by NurAini et al. (2005); (b) faults produced inanalogue models of oblique extension (=60) by McClay & White(1995). (c) Three-dimensional interpretation of these structures andunderlying crust.



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the Early Miocene, while folding and thrusting of westernSulawesi migrated west since the Early Pliocene.

The authors of this paper cannot agree on the nature of the

basement beneath the straits and similar disagreement is likelyto continue amongst those working in the area. From alarge-scale tectonic perspective, the difference between oceanicand continental crust is not very important, although the

strength of a central oceanic area may have been of importancein localizing deformation since the entire BorneoSulawesiregion went into compression at the beginning of the Pliocene.From a hydrocarbon prospectivity point of view, the differenceis of greater significance. If there is a widespread syn-riftsequence across the whole or most of the straits there arepotentially a greater number of exciting exploration targets indeeper water. If the straits are floored mainly by oceanic crust,success will depend on organic material carried into deep waterand distributed through sand-rich sequences (Saller et al. 2006)

which appears a riskier system. Drilling results and observationsbased on new deep-water 3D seismic surveys may strengthenthe arguments on one side or the other.

The authors thank the consortium of oil companies who havesupported projects in SE Asia for many years. Colleagues inIndonesia at the Pusat Survei Geologi, Bandung and Lemigas, Jakartaare thanked for help with fieldwork, as are Peter Baillie, TGS-NOPEC Geophysical Company, and MIGAS, for permission to useseismic data. The MSc programmes of Siti NurAini and SinchiaDewi Puspita were supported by Chevening Scholarships from the

British Council and ENI.

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