1999-the formation of the makassar strait and the separation between se kalimantan and sw...

The formation of the Makassar Strait and the separationbetween SE Kalimantan and SW Sulawesi

Agus Guntoro

Fakultas Teknologi Mineral, Jurusan Teknik Geologi, Universitas Trisakti, Jl. Kyai-Tapa-Grogol, Jakarta, Indonesia

Received 10 December 1997; accepted 20 June 1998

Abstract

The formation of the Makassar Strait, situated between southeast (SE) Kalimantan and western Sulawesi, is still subject of

much debate. Di�erent authors have proposed several hypotheses to explain its evolution. The only agreement between thoseseveral hypotheses is that SE Kalimantan and western Sulawesi once lay close together and that their separation is due to theopening of the Makassar Strait. The age and driving mechanism for this opening are, however, still poorly understood. The

strait separates the stable core of the Eurasian Plate to the west from the very active region of the triple junction of three largeplates to the east. To the north the strait is bounded by the Sulawesi Sea and to the south by the East Java Sea. The strait isroughly 100±200 km wide and 300 km long and is usually divided into the North and South Makassar basins, separated by the

Paternoster Fault. The present study interprets the history of the Makassar Strait using seismic re¯ection pro®les and gravitymodels, in addition to the compilation of geological information. Implications for the origin of rifting is also discussed. Theresult of the present study indicates that Makassar Strait was formed by the vertical sinking of a subducting oceanic plate to the

east of western Sulawesi, leading to trench roll-back. This vertical sinking was accommodated by extension and rifting ofcontinental crust above the subduction zone at a previous site of collision, causing the opening of Makassar Strait. The time ofthis trench roll-back marks the cessation of subduction. # 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction

The Makassar Strait is situated between SEKalimantan and western Sulawesi (Fig. 1), and liesgeographically at the boundary between the WesternIndonesian Province and the Eastern IndonesianProvince. The origin and geological framework of theMakassar Strait have been considered by manyauthors, either in detailed studies of the strait itself orin compilations of the regional geology. Some of sev-eral ideas about the evolution of Makassar Strait areas follows: Katili (1978) proposed that opening tookplace in the Quaternary along the Paternoster Fault,with the formation of oceanic crust. Rose andHartono (1978) attributed the formation of the basinto counterclockwise rotation of Kalimantan during theLate Cretaceous and Early Palaeogene. Hamilton(1979) suggested that the the Makassar Strait wasformed by sea ¯oor spreading in the Mid-Tertiary.Burrolet and Salle (1981) argued from the presentdepths of the Makassar Basin that it is a rhombo-chasm formed on rigid continental or intermediate

crust. Situmorang (1982) explained the origin of theMakassar Basin in terms of stretching, from theLower-Middle Eocene to Lower Miocene, andsuggested that it is now underlain by attenuated conti-nental crust. Daly et al. (1991) attributed the strait toback-arc extension along the Paci®c margin, reactivat-ing earlier Meratus thrust terranes. Bergman et al.(1996) suggested that in the Neogene the MakassarStrait experienced thrust loading, forming thrust beltson both sides of the strait, leading to the formation ofa foreland basin.

2. Bathymetry

The Makassar Strait (Fig. 2) is a symmetrical zoneof depression (median valley), ¯anked by upliftedtopography on each side. It is ¯anked by the mountain-ous region of SE Kalimantan in the west and by westernSulawesi in the east. Along the SE Kalimantan margin,the continental shelf is wide and gentle, with water depthless than 200 m, and is referred to as the Paternoster

Journal of Asian Earth Sciences 17 (1999) 79±98

1367-9120/99 $ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S0743-9547(98 )00037 -3

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Platform (Situmorang, 1982) forming the easternmostpart of the Sunda Shelf. In contrast, o� western Sulawesi,the shelf is narrow, with steep continental slopes descend-ing to a maximum depth of more than 2000 m.

The bathymetry of the Makassar Strait shows sev-eral features interpreted to be structurally controlled.The strait can be divided into the North and SouthMakassar Strait basins, which are separated by thesinistral Paternoster Fault (Katili, 1978; Situmorang,1982). The North Makassar Basin is 340 km fromnorth to south, 100 km wide, from east to west, andhas water depths varying from 200 to 2000 m. The axisof the basin trends N±S or NNE±SSW. The SouthMakassar Basin is 300 km from N±S, 100 km widefrom E±W and has water depths varying between 200to 2000 m. The axis of the South Makassar Basintrends NE±SW.

3. The comparison between the geology of SEKalimantan and Western Sulawesi

The main tectonic control upon the geology of east-ern Kalimantan and western Sulawesi is believed to bethe collision between the Eurasian Plate and

Australian microcontinental blocks in the Cretaceous(Sikumbang, 1990). Several authors have proposedthat southeast (SE) Kalimantan and southwest (SW)Sulawesi were parts of a single plate during theCretaceous and that separation between these tworegions occurred during the Tertiary. This hypothesisis inferred on the basis of similarities of geologicalrecords of the two areas.

4. Geology of SE Kalimantan

Kalimantan is usually regarded as having been astable craton since the Middle-Late Cainozoic(Hamilton, 1979), following formation by amalgama-tion of several unrelated terranes. The area can be sub-divided geologically, into ®ve major units, namelyWest and Central Kalimantan, Southeast Kalimantan,Northeast Kalimantan, North Kalimantan andNorthwest Kalimantan (van Bemmelen, 1949). Thedevelopment of this area was in¯uenced mainly bysubduction and collision, accompanied by basementcomplex emplacement.

The geology of southeast Kalimantan is in¯uencedby subduction and collision during the Cretaceous.

Fig. 1. Location map of the Makassar Strait situated between southeast Kalimantan and western Sulawesi.

A. Guntoro / Journal of Asian Earth Sciences 17 (1999) 79±9880

The geology and stratigraphy (Fig. 3) of SEKalimantan in the Meratus Region have been summar-ised by Sikumbang (1990), from whom most of the in-formation given below is taken.

4.1. Basement complexes

Pre-Tertiary basement complexes in the MeratusMountains have a NE±SW structural lineation (Fig. 4).They consist of the Meratus Ophiolite and meta-

morphic rocks of Early Aptian (116 Ma) and EarlyAlbian (108 Ma) age, respectively. This association ofrocks is believed to have formed in a subduction zone.

4.2. Meratus ophiolite

The ophiolite consists of ultrama®c rocks, gabbroicrocks, plagiogranite and microdiorite. Ultrama®c rocksare disrupted, sheared and serpentinized, and locallyexhibit boudinage structures.

Fig. 2. Bathymetric and seismic location map of the Makassar Strait. Line PAC 201 is shown in Fig. 5a and b, Line PAC 202 is shown in Fig. 6,

Line MCP 05 is shown in Fig. 7, Line MK1 and MK3 are shown in Fig. 8.

A. Guntoro / Journal of Asian Earth Sciences 17 (1999) 79±98 81

4.3. Metamorphic rocks

Metamorphic rocks in the Meratus Mountains havebeen designated as the Harun Schist and the PelaihariPhyllite. The distinction between the two is madesolely on the basis of metamorphic grade, since inboth cases the age of metamorphism seems to havebeen Early Albian (108.4 Ma).

4.4. Sedimentary rocks

The oldest sedimentary rocks in the Meratus areaare the Paniungan and Batununggal formations ofBerriasian±Barremian and Barremian±Aptian age, re-

spectively. Both formations were deposited in a shal-low marine to slope setting on the southeastern marginof the Sunda continent. The Paniungan Formationconsists largely of mudstone with intercalations ofsandstone and minor limestone. The BatununggalFormation is divided into three di�erent units; auto-chthonous (intact limestone), para-authochthonous(thrust sheet) and allochthonous (exotic blocks). Theformation occurs in the northeastern and southeasternparts of the Meratus Mountains. In the northeast it islargely covered by in situ and undeformed amygdaloi-dal lava ¯ows.

The Alino Group, which is considered to be derivedfrom a volcanic island arc of Albian to Early

Fig. 3. Summary of stratigraphic framework and geological evolution of the Meratus Mountains (SE Kalimantan) (Sikumbang, 1990).


Cenomanian age, can be divided into the Pudak

Formation and the Keramaian Formation. The Pudak

Formation consists mainly of coarse volcaniclastic

deposits with limestone blocks. Most of the volcanic

materials were derived from erosional disintegration

and fragmentation of lavas. They are occasionally

intermixed with pre-existing sedimentary material (i.e.

limestone of the Batununggal Formation and sand-

stone of the Paniungan Formation) and with igneous

material (e.g. ma®c and ultrama®c rocks of the

Meratus Ophiolite).

The Keramaian Formation consists of alternating

volcaniclastic sandstone and mudstone and chert with

or without radiolarian skeletons. It overlies the Pudak

Formation conformably.

The Manunggul Group includes all the Upper

Cretaceous sedimentary strata of the region, as well as

andesitic lavas, rhyolitic volcanics and pyroclastics

that occupy a trough-like basin in the central axis of

the Meratus Mountains. The group is subdivided into

the Pamali, Benuariam Volcanic, Tabatan,

Rantaulajung, and Kayujohara Volcanic formations.

4.5. Plutonic rocks

There are two exposures of plutonic rocks in theMeratus Mountains. The ®rst is the Rimuh Pluton, inthe Tambak±Tamban Range, the second the KintapPluton about 10 km north of Kintap. These plutonicrocks can be related to a west-dipping subduction zonein the Early Cretaceous±Early Tertiary. The earlyUpper Cretaceous or pre-Upper Turonian (91 Ma)Rimuh Pluton is associated with volcanics of thePitanak Formation. The Kintap Pluton (95 Ma) isintrusive into both the Meratus Ophiolite and theAlino Group.

5. Geology of southwest Sulawesi

Sulawesi consists of four diverging arms named thesouth, north, east and southeast arms, each of whichrecords a very di�erent and complicated geological his-tory (see Fig. 4). The complicated geology of Sulawesi,consisting of various lithologies and structures withdi�erent histories and origins, leads to the conclusion

Fig. 4. Structural map of the Makassar Strait, SE Kalimantan and western Sulawesi (Fault data are derived from Simandjuntak, 1990; Biantoro

et al., 1992; Bergman et al., 1996).


that the island is composed of several di�erent ter-ranes, however, the history of amalgamation of each

terrane still remains subject of debate. Based on the

terrane concept Sulawesi is generally divided into fourmajor terranes or belts: i.e. the Banggai±Sula

Microcontinent (BSM); the Eastern Sulawesi OphioliteBelt (ESOB); the Central Sulawesi Metamorphic Belt

(CSMB) and the Western Sulawesi Plutono-Volcanic

Belt (WSPVB).

Southwest Sulawesi is part of the Western SulawesiPlutono-Volcanic Belt (WSPVB) which is characterised

by biotite schist, extensive massifs of granodioritic

rocks, and sediments which were in general depositedcloser to shore than those of the Eastern Ophiolite

Belt (van Bemmelen, 1949). Katili (1978) suggestedthat the WSPVB formed the magmatic arc related to

Tertiary subduction in the east. van Leeuwen (1981)

states that the ages of the volcanic rocks in theWSPVB vary from Palaeogene to Quaternary.

The geology and stratigraphy of southwest Sulawesi

have been described by many authors (Sukamto, 1978;

Hamilton, 1979; Parkinson, 1991) and can be summar-ised as follows. The basement of the province (the

Bantimala Complex) crops out in two small windows(Bantimala and Barru). It consists of serpentinised

peridotites, intercalated by thrusts, with highly

deformed metaclastic greenschist and epidote amphibo-lites, and a tectonic melange of unmetamorphosed

pelagic and terrigenous sediments, gabbros, amphibo-lites and blueschist (Parkinson, 1991). K±Ar radio-

metric dating yielded a metamorphic age of 111 Mafor the schist (Hamilton, 1979).

Unconformably overlying the basement complex are¯ysch sediments of the Cretaceous Balangbaru and lat-

erally equivalent Marada Formation. This is overlainunconformably by the Palaeocene±Eocene Langi

Formation, consisting of propylitized volcanic rocks

(Wakita et al., 1996). The Eocene Malawa Formation,consisting of marine siliciclastics, shale and coal, over-

lying the Langi Formation conformably. The MiddleEocene±Middle Miocene Tonasa Formation interdigi-

tates with the upper part of Malawa Formation, and

consists mainly of limestone forming a transgressivesequence. The Middle to Late Miocene Camba

Formation conformably overlies the TonasaFormation and consists of volcanic and volcaniclastic

rocks.

Miocene and younger volcanic and plutonic rocks

are dominant in the South Arm of Sulawesi and havebeen interpreted as a magmatic belt resulting from the

development of a subduction-related volcanic arc

(Sukamto, 1978; Hamilton, 1979). Yuwono et al.(1988) interpreted the magmatic arc as the result of

post-collisional rift-related magmatism. Bergman et al.(1996) suggested the magmatic arc as a result of the

lithospheric melting due to continent±continentcollision.

6. Geological summary

Many of the authors who have worked in the areahave drawn attention to the similarities in the strati-graphy of SE Kalimantan and SW Sulawesi (Katili,1978; Sikumbang, 1990; Wilson and Bosence, 1995).The relationship between the stratigraphy of the twoareas may be summarized as follows. In SEKalimantan metamorphic rocks are overlain by theAlbian±Early Cenomanian Alino Formation (deep seasediments and basic volcanics) and its neritic equival-ent, the Jurassic (?) to Early Cretaceous PaniunganBeds. The Upper Cretaceous Manunggal Formationoverlies unconformably the above rock units. The simi-lar sequences are also found in SW Sulawesi wheremetamorphic rocks are also unconformably overlainby a series of Jurassic (?) to Early Cretaceous siliceousshale, sandstone and radiolarian chert, which is locallymetamorphosed. On the basis of the sedimentology,tectonic style and regional setting, Sikumbang (1990)suggested that the Manunggul Group of SoutheastKalimantan was deposited in a pull-apart basin devel-oped within a strike±slip zone initiated during orshortly after arc±continent collision. The ManunggalGroup can be correlated with the BalangbaruFormation of SW Sulawesi (Hasan, 1987).

The Tertiary stratigraphy of western Sulawesi is alsoconsidered to be comparable with that of many of theTertiary basins in neighbouring east Kalimantan(Katili, 1978; Wilson and Bosence, 1995).

Similarities between the pre-Tertiary basement com-plexes of SE Kalimantan and SW Sulawesi have beenproposed by many authors, not only from geologicalpoint of view, but also from geophysical viewpoints,including seismic and geomagnetic characteristics(Hamilton, 1979; van Leeuwen, 1981; Sikumbang,1990, Parkinson, 1991).

Because of the similarities described above it is fre-quently suggested that SE Kalimantan and SWSulawesi, were positioned closer together in the LateCretaceous, supporting the hypothesis that theMakassar Strait was formed by the later separation ofthe two areas. However; the timing and the mechanismof this separation are still not clear, these problems areinvestigated in this paper.

7. Seismic interpretation

Structural interpretations and seismic stratigraphyfor the North and South Makassar basins have beenderived from seismic re¯ection pro®les PAC 201, PAC


202, MCP 05, MK1 and MK 3 (see Fig. 2 for lo-cation). PAC 201 and PAC 202, representing Northand South Makassar basins, respectively, are multi-

channel seismic pro®les. The sections are displayed asline drawing interpretations. Analysis is based on theprocedures of Vail et al. (1977).

Fig. 5. (a) Line drawing and its interpretation of western segment of line PAC 201 showing normal faults indicating extensional basin. Arrows

mark cycle terminations on onlap, downlap and toplap which provide criteria for recognition of sequence boundaries. Letters H1±H6 designate

the top of seismic sequence. (b) Seismic line drawing and interpretation of eastern segment of line PAC 201 showing extensive thrust faults after

the formation of horizons H5 forming Neogene foreland basin. Arrows mark cyscle terminations onlap, downlap, toplap which provide criteria

for recognition of sequence boundaries. Letters H1±H6 designate the top of seismic sequence.


8. Seismic stratigraphy of lines PAC 201 and PAC 202

Line PAC 201, situated in the North MakassarBasin, can be separated into eastern and western seg-ments on the basis of structural regimes. The westernsegment (Fig. 5a) extends from SP 1200 to SP 4000and displays thick sediments controlled by acousticbasement faults. In contrast, the eastern segment(Fig. 5b), from SP 0 to SP 1400, exhibits extensivewest-directed thrust faulting and basement cannot betraced clearly, due to widespread multiples and di�rac-tions. PAC 202, situated in the South Makassar Basin,is approximately 200 km to the south of PAC 201.Unlike PAC 201, where there is extensive thrust fault-ing in the east, sediments on this line have not beena�ected by thrust faults. Both line drawing and in-terpretation of line PAC 202 are shown on Fig. 6.

Seismic sequence analysis shows that the re¯ectorscan be divided into six seismic sequences which can begrouped into three major units; acoustic basement(Sequence 1), syn-rift sediments (Sequence 2) and post-rift sediments (Sequence 3±6). In the ®gures, theseismic sequence boundaries are shown as horizonsH1±H6. The three units are described below.

8.1. Acoustic basement (Seismic Sequence 1)

The oldest recognised seismic sequence is character-ised by an absence of re¯ections and is interpreted asacoustic basement. The contact with the overlying sedi-ments is di�cult to trace, especially in the eastern seg-ments of Line PAC 201 where it is obscured bydi�ractions and multiples. This contact is marked asH1 but, in general, it can only be identi®ed at a few lo-cations. To estimate the basement depth, interval vel-

ocity data were used where available, the boundarybetween acoustic basement and the overlying sedimentsbeing placed at depths at which there was an extremevelocity contrast. The greatest depths are in the middleof the line, such as on Line PAC 201 from SP 1470 toSP 1600, where horizon H1 was not seen as it lies dee-per than the maximum time recorded (8 s TWT). Thehorizon shallows to the west and is displaced by nor-mal faults, forming half-graben structures.

8.2. Syn-rift unit (Seismic Sequence 2)

Unconformably overlying Seismic Sequence 1 isSeismic Sequence 2. This sequence is characterised byparallel±subparallel re¯ectors, with poor to fair conti-nuity and low to medium amplitude. Re¯ection geome-try suggests a concordant sequence boundaryrelationship at the top, and onlap at the base, againstH1 (Line PAC 201, SP 1800 to SP 1650 and Line 202,SP 3900 to SP 3650). Following the criteria of Vailet al. (1977), these re¯ection characteristics are inter-preted as indicating a shelf depositional environment.The thickness of the sequence varies, suggesting in®ll-ing of a faulted and irregular basement. This is thebasis for inferring that the sediments are rift-related.The faults cut the basement but do not disturb the pre-sent-day sea ¯oor, indicating a limit to the period oftectonic activity. The top of this syn-rift sequence(Seismic Sequence 2) is designated H2.

8.3. Post-rift unit

Overlying Seismic Sequence 2, which is consideredto be a syn-rift unit, are Seismic Sequences 3±6. These

Fig. 6. Seismic line drawing and interpretation of line PAC 202 showing basement faults, suggesting extensional basin. Arrows mark cycle ter-

minations on onlap, downlap and toplap which provide criteria for recognition of sequence boundaries. Letters H1±H6 designate the tops of seis-

mic sequences.


sequences have not been a�ected by normal faults andare therefore considered to be post-rift sediments.

8.4. Seismic Sequence 3

This sequence is bounded by horizons H2 and H3,and exhibits parallel to subparallel bedding, with poorto fair continuity and high to medium re¯ection ampli-tude; in some parts amplitudes are low. The variationin amplitude and frequency may indicate a lithologicalfacies change, which could relate to a decreasing rateof subsidence. The lower boundary shows downlap tothe top of Seismic Sequence 2 (Boundary H2). Thesere¯ector characteristics can be taken as indicating ashelf to shelf margin depositional environment (Vailet al., 1977).


This sequence is bounded by horizons H3 and H4,and is dominated by parallel and locally sub parallelre¯ections, with fair to good continuity and mediumto high re¯ection amplitude. The unit is characterisedby the presence of local mound-like re¯ector patternsfrom SP 3400 to SP 3200, SP 3050 to SP 2850 and SP2250 to SP 2150 (on Line PAC 201) and SP 3450 to3550 (on Line PAC 202) which are interpreted as car-bonate mounds. The upper boundary is marked bytoplap to horizon H5 from SP 2450 to SP 2250 (onLine PAC 201). The re¯ector characteristics are classi-®ed as indicating a shelf to shelf margin depositionalenvironment.


This sub unit is bounded by horizons H4 and H5and displays parallel con®gurations with fair to goodcontinuity and medium to high re¯ection amplitude.Discontinuous re¯ectors are present with low to med-ium amplitude, whilst continuity is observed with med-ium to high amplitude. These re¯ection characteristicsare typical of a shelf depositional environment and in-dicate a shallow marine shelf deposit. The unit can stillbe recognised in the eastern segment, although thisregion is distorted by thrust faulting.


This sub unit is bounded by horizons H5 and H6and shows parallel con®gurations with good continuityand medium to high re¯ection amplitudes. The re¯ec-tion characteristics are classi®ed as indicating a shelfdepositional environment. In the eastern segment ofLine PAC 201 from SP 1300 to SP 0 the sequence canbe subdivided into sub-sequences con®ned to localbasins in which horizontal re¯ectors onlap to the topof Horizon H5, and this sub unit was deposited asonlapping ®ll.

9. Seismic interpretation line MCP 05

This line in the North Makassar Basin (Fig. 7) hasbeen interpreted and published as a line drawing byKatili (1978). It lies at about 28S, trends E±W and isapproximately 275 km long. It is crossed close to itscentre by Line PAC 201. The line drawing producedby Katili (1978) did not show any detail of the re¯ec-tors and it is therefore di�cult to correlate his in-terpretation with those of PAC 201 and 202, in termsof sequence stratigraphy. However, the section doesshow an extensional basin forming a graben structure.The re¯ection con®guration within this graben is par-allel and continuous, suggesting uniform rates of depo-sition on a uniformly subsiding base (Vail et al., 1977).The top of basement is at its deepest in the middle ofthe graben, at approximately 5 s TWT (6 km); sedi-ment occupies about 2 s TWT beneath more than 2km of water. The external form of this sedimentarysequence appears to indicate onlapping in®ll. Thesequence was deposited at a uniform rate on a uni-formly subsiding basin ¯oor. To the west of this gra-ben is a basement high with depth varying between 2 sTWT and 0.5 s TWT, controlled by normal faults. Thesequence shows parallel-divergent con®gurations withcontinuous re¯ectors over this high. To the east of thegraben are folded sediments, suggesting compressionaltectonics in this part of the line, as opposed to thecentral and western part which show extensionaltectonics.

Fig. 7. Seismic line drawing interpretation of line MCP 05 across the Makassar strait showing the rifting of the Makassar Strait causing separ-

ation between SE Kalimantan and SW Sulawesi (after Katili, 1973).


10. Seismic interpretation line MK1 and MK3

These lines (Fig. 8) in the North Makassar Basinhave been interpreted and published as line drawingsby Burrolet and Salle (1981). As with MCP 05, the in-terpretations were not drawn in detail and it is di�cultto correlate the sequence stratigraphy with lines PAC201 and 202.

Line MK 3 is situated in the northernmost part ofthe North Makassar Basin (see Fig. 2). In the east (SP2200 to the end of the line at SP 3300), the basementis high and from SP 2950 to 3100 it forms the sea¯oor at approximately 2.5 s TWT (1850±1900 m).Between SP 2300 and the western end of the line, base-ment is not shown but must drop sharply from 4 sTWT to more than 6 s TWT. The overlying sedimentshave a generally uniform thickness of more than 3 sTWT and display parallel con®gurations, with moder-ate to good continuity.

Line MK 1 is parallel to PAC 201 and 25 km to thenorth (see Fig. 2). The compressional zone at the east-ern margin, which is dominant on PAC 201, is notobserved. Acoustic basement was not detected continu-ously along the pro®le. It is present in the eastern part(SP 2400 to SP 2900) at about 4.5 s TWT, but in thewestern part, towards the axial trough, it is seen onlydiscontinuously, reaching a depth of 7.5 s TWT insome locations. The sediments display parallel con-®gurations, apparently with moderate to good continu-ity.

The interpretations of lines MK1 and MK3 suggesta history of sedimentation similar to that seen onLines PAC 201 and PAC 202, indicating that thewhole Makassar Basin formed by rifting and was sub-

sequently modi®ed by thrust faulting along the easternmargin of the North Makassar Basin.

11. Structural interpretation

From the seismic pro®les presented above, and alsofrom the interpretation of other seismic pro®les acrossthe Makassar Strait obtained from the literature(Situmorang, 1982; Katili, 1978; Pertamina, 1985), thestructural setting of the Makassar Strait can bededuced as follows. The centres of the North andSouth Makassar Basins have similar structures, show-ing that the major tectonic regime is extensional, withnormal faults displacing the older syn-rift sediments,but not disturbing the younger post-rift sediments.However, the western and eastern sides of the Northand South Makassar Basins have di�erent structures.The North Makassar Basin is limited by active reversefaults on both sides (Bergman et al., 1996; Pertamina,1985). On the western side, Pertamina (1985) show aseries of reverse faults dipping both to the east andwest and displacing all the stratigraphic units, up tothe youngest sediments. While on the eastern side, aseries of reverse faults dip to the east (see Fig. 5b);mostly these faults do not displace the youngestsequence. On the other hand the South MakassarBasin is bounded by normal faults.

12. Correlation of well data with seismic sequences

The age of the seismic units identi®ed on lines PAC201 and PAC 202 cannot be determined directly.

Fig. 8. Seismic line drawing of line MK1 and MK3 showing deep basin of the North Makassar Basin (after Burollet and Salle, 1981).


However, well data from two wells on the eastern edgeof the Paternoster Platform (TT 1 and TT 2) and re-gional studies provide some age and stratigraphic con-trol (Fig. 9).

Regionally, the top of the Early Miocene carbonatereef has been used as an acoustic marker in the area tothe south, and also in the East Kalimantan basinalarea (Situmorang, 1982). On the basis of this knowl-edge, the carbonate reefs, recognised by their moundedexternal form in seismic sequence 4 on seismic section

PAC 201, can be used to locate the top of the EarlyMiocene. Using this assumption, the other sequencescan be correlated with the well data.

The top of sequence 1 (H1) is equivalent to horizonC1 of Situmorang (1982), which is the pre-Tertiarybasement, consisting of gabbros and dolerites (WellTT 2). Sequence 2 (between horizons H1 and H2) isequivalent to the Late Eocene syn-rift sediments. Thetop of this unit, designated H2, marks the end of therifting phase, which was followed by basin subsidence

Fig. 9. Lithologies of Well TT1 and TT2 and correlation to the seismic horizons.


and the deposition of post-rift sediments. The openingof the Makassar Strait can be related to the depositionof Sequence 2. Sequence 3 (between horizons H2 andH3) is equivalent to the Lower Oligocene conglomera-tic limestone. The top of Sequence 4 (Horizon H4) isequivalent to horizon C2, the Early Miocene carbonatereef, of Situmorang (1992). Sequence 5 is equivalent tothe Early to Middle Miocene deep marine shales andmarls and Sequence 6 is equivalent to the Plioceneshallow marine limestone.

13. Gravity data

There are two di�erent free-air anomaly maps forthe Makassar Strait. The ®rst, produced by SIPM(Shell), was reproduced by Situmorang (1982) withoutany indication of the formula used to calculate gravityvalues. The two contour maps are based on di�erentdata, with the free-air anomaly values shown di�er-ently on the two maps. To ®nd the magnitude of thedi�erence, the two maps were overlapped and at everycrossing point the free-air anomaly values were com-pared. The di�erence is approximately constant, theEdcon (1991) values being greater than the SIPMvalues by 50 mGal. This di�erence is thought to be re-lated to the fact that the SIPM data was not tied toany international system (J. Milsom, personal com-munication, 1994). In order to integrate the two maps(Fig. 10), 50 mGal were added to the SIPM values.

14. Qualitative gravity interpretation

The free-air anomaly map of the Makassar Strait ischaracterised by negative free-air anomalies along theaxial depression and positive free-air anomaliestowards the continental shelves of Kalimantan andSulawesi. Free-air anomaly values thus re¯ect bathy-metry. Major features seen in the gravity data havebeen named as follows: Laut High, Mahakam High,Paternoster High, Paternoster Lineament, NorthMakassar Low and South Makassar Low.

The Laut High is centred on Laut Island, close tothe Meratus Mountains and trends NE±SW. The free-air anomaly ranges from +40 to +70 mGal. Thishigh is interpreted as indicating the presence of highdensity ultrama®c rocks of the basement complex,close to the surface. Ophiolites are present on LautIsland and in the Meratus Mountains (Sikumbang,1990). To the northeast of the Laut High, theMahakam High has a N±S trend which changes shar-ply to E±W at about 18N. The free-air anomaly valuesrange from +40 to +120 mGal. The Laut High andMahakam High are in the same trend, but they are

not continuous, and they are separated by a steepNW±SE gradient, the Paternoster Lineament.

Parallel and to the east of the Laut High, thePaternoster High forms an elliptical closure, elongatedin a NNE±SSW direction, in a region where the broadcontinental shelf of the southeast Kalimantan marginlies in a water depth of less than 200 m. The free-airanomaly values range from +50 to +70 mGal.Parallel and to the east of the Paternoster High is along and narrow free-air anomaly low which trendsroughly N±S. At about 38S this is o�set by thePaternoster lineament, dividing it into the South andNorth Makassar Lows. The North Makassar Low hasfree-air anomaly values ranging from 0 to ÿ40 mGal,and the South Makassar Low has free-air anomalyvalues ranging from 0 to ÿ50 mGal. These low free-airanomalies indicate the presence of thick low-densitysedimentary rocks. The North and South Makassarlows are de®ned by steep gravity gradients, attributedto faulting, at the contacts with the Mahakam andPaternoster highs.

15. Gravity models

Two gravity models of the Makassar Strait havebeen constructed using the GM-SYS Gravity modelingprogram. The models are constrained by interpretationof the seismic re¯ection pro®les PAC 201 and PAC202, from the North and South Makassar basins, toshed light upon the bathymetry and thickness of sedi-ments. Densities have been assigned for seawater, sedi-ments, upper crust and oceanic crust as follows. Thedensity of seawater is taken as 1.03 Mg mÿ3. Theaverage density of the sediments has been estimatedusing the average interval velocities from seismic pro-®les PAC 201 and 202 and radiosonobuoy data at lo-cations 38 and 39, close to the Makassar Strait(Guntoro, 1995), converted using the density±velocitycurve of Nafe and Drake (1962), to be 2.3 Mg mÿ3.The density of the upper crust below the sediments istaken as 2.67 Mg mÿ3 and density of the upper mantleis taken as 3.3 Mg mÿ3. The oceanic crust was mod-eled using density of 2.85 Mg mÿ3. The reference crus-tal model has a density of 2.67 Mg mÿ3 and athickness of 30 km.

15.1. Gravity modeling PAC 201

This cross section is taken across the MakassarStrait in a NE±SW direction along seismic re¯ectionline PAC 201, but extends up to the coasts ofKalimantan and Sulawesi on either side. The result ofmodeling is shown in Fig. 11. There is a good matchbetween observed and calculated gravity values. Inparticular, the anomaly in the axial trough is simply


explained by the presence of a sedimentary basin, indi-cating that the low is mainly caused by the sedimentsbetween down-faulted blocks. The lowest free-air

anomaly, of ÿ30 mGal, occurs close to the centre ofthe basin. Otherwise the model seems di�cult tomatch with the gravity data. Towards the shelves o�

Fig. 10. The free-air anomaly map of the Makassar Strait and Bouguer for onland SW Sulawesi (Sources; Simamora and Marzuki, 1990;

Situmorang, 1982; Edcon, 1991).


Kalimantan and Sulawesi the values increase sharplyto about +50 mGal. The minimum free-air anomalyin the centre of the basin is due to the presence of ap-proximately 8 km of low density (2.3 Mg mÿ3) sedi-ments. In this model, the centre of the Makassar Straitis underlain by oceanic crust, and in the eastern partthe oceanic crust has been subducted beneath SWSulawesi, whereas on the western side of MakassarStrait, oceanic crust has not been not subducted. Thedepth to the upper mantle is 25 km towardsKalimantan and 18 km towards Sulawesi anddecreases to 12 km in the centre of the basin.

15.2. Gravity modeling PAC 202

This model has been prepared along seismic linePAC 202, but is extended up to the coast of LautIsland on the Kalimantan side and to the coast ofSulawesi to the east. The result of the modeling isshown in Fig. 12.

The lowest free-air anomaly, of about ÿ40 mGal,occurs in the middle of the basin, with steep gradientstowards the continental shelves of Kalimantan andSulawesi. The minimum free-air anomaly coincideswith the centre of the basin, where the water depth isabout 2 km and where there are about 8 km of sedi-ments. The free-air anomaly increases to as much as+70 mGal towards Kalimantan and Sulawesi. In thismodel, the centre of the South Makassar Basin isunderlain by oceanic crust, and in the eastern part theoceanic crust was subducted beneath SW Sulawesi.The depth of the upper mantle beneath the shelf o�

Kalimantan is about 25 km and beneath the shelf ofSulawesi is about 20 km but in the centre of the basinis only about 15 km.

16. Crustal structure

Deep water areas in the Makassar Strait correspondto areas of low free air anomalies, shallow bathymetrycorresponds to areas of high free air anomaly. Gravitymodeling shows, however, that the Moho is shallowbeneath the axial trough and deepens towards theshelves, especially toward the SE Kalimantan shelf(Figs. 11 and 12) and that there is a change in thethickness of the crust, excluding the post-extensionalsediments cover, from the continental shelf regions(25±28 km) to the axial trough (5±12 km). It issuggested that the changes in crustal thickness are dueto deformation by extensional thinning. The crustalthickness in the axial region indicates the presence ofoceanic crust. Seismic refraction data in the SouthMakassar Basin show basement velocities rangingfrom 3.56 to 5.69 km sÿ1 (Prasetyo and Dwiyanto,1986), which are typical velocities for continental crust,but could possibly be derived from the upper part ofoceanic crust. Situmorang (1982) suggested that astretching value of 2.9 is the lower limit for the for-mation of oceanic crust in the south Makassar Basin.He calculated that the Makassar Basin stretching fac-tor was between 2 and 2.9 and that the basin had notyet developed oceanic crust. In contrast, the doleritesand gabbros in Well TT 2 in the Makassar Strait are

Fig. 11. Gravity model PAC 201 in the North Makassar Basin showing the presence of oceanic crust subducted toward SW Sulawesi.


typical of an ophiolite sequence, suggesting the possi-bility of the presence of oceanic crust. The presentgravity models also suggest typical oceanic crustalthicknesses. In view of the well data and gravitymodels, it is suggested that the central part of theMakassar Basin is underlain by oceanic crust.

17. Tectonic implications and the evolution of theMakassar Strait

Seismic refraction and re¯ection surveys and gravitymodeling, as outlined above, support an Eocene exten-sional model for the Makassar Basin. Prior to exten-sion the region is thought to have undergonecompression due to the collision between SEKalimantan and SW Sulawesi, which also producedthe uplift of the Meratus Range in the LateCretaceous. This compressional phase is thought tohave thickened the crust, as normally happens in com-pressional regimes.

From seismic re¯ection interpretation, the top of thebasement reaches a depth of 10 km and is overlain bysediments up to 8 km thick. The water depth in theaxial trough reaches 2.2 km. Seismic stratigraphic ana-lyses suggest that the Makassar Basin has subsidedslowly and has experienced continuous sedimentationsince the Eocene. This argument is supported by thedepositional environmental data from Wells TT 1 andTT 2, which showed continuous sedimentation duringthe Tertiary. The sediments were deposited in a neriticenvironment in the Eocene and a neritic to sub-neriticenvironment in the Late Eocene to Middle Miocene.

Shallow marine carbonates were deposited from theMiddle Miocene to the Recent (Situmorang, 1982).This observation leads to the interpretation that riftingwas followed by thermal subsidence causing the basinto subside slowly and continuously.

The following is a history of the formation ofMakassar Strait, based on seismic interpretation, grav-ity data and models presented in this paper, in ad-dition to available geological information from theregion. Fig. 13 shows a possible plate tectonic recon-struction of the evolution of the Makassar Strait fromLate Cretaceous to Late Miocene. These reconstruc-tions summarise the geological and geophysical datafrom the region and also integrate previous modelsfrom Hamilton (1979), Daly et al. (1991) andParkinson (1991).

The Cretaceous basement complexes in Java, theJava Sea, SE Kalimantan and SW Sulawesi arebelieved to constrain the geometry of subductionbetween the East Java Sea microplate and the Indo-Australian Plate in the Early Cretaceous (Fig. 13a).The trends on magnetic and gravity maps are also con-tinuous from Java to SE Kalimantan through the JavaSea (Guntoro, 1995), and palaeomagnetic results alsoindicate that SW Sulawesi lay close to its present pos-ition from the Jurassic to Early Cretaceous, relative toEast Kalimantan (Haile, 1978). From a sedimentarypoint of view, there is strong evidence for the presenceof west-dipping subduction at the eastern margin ofthe East Java Sea microplate and the close positioningof SW Sulawesi and SE Kalimantan. The AlinoFormation, the Paniungan beds, the ManunggulFormation and the plutonic and volcanic rocks in the

Fig. 12. Gravity model PAC 201 in the South Makassar Basin showing the presence of oceanic crust subducted toward SW Sulawesi.


Fig.13.(a)TheSW

SulawesiMicrocontinentwassubducted

beneath

SE

Kalimantan(East

JavaSea

Microplate),generatinganaccretionary

complex.(b)TheSW

SulawesiMicrocontinentcol-

lided

withSEKalimantan.Thiseventresulted

intheupliftoftheMeratusMountainsandtheem

placementofthebasementcomplexes.Thecontinuingmovem

entofthePaci®cPlate

wasaccom-

modatedbytheform

ationofnew

subductionto

theeast

ofSW

Sulawesi.(c)Thewhole

areaoftheMeratusMountainswasuplifted

andassociatedwithblock

faulting.O�

totheeast

the

Banggai±Sula

Microcontinentwasapproaching.(d)Theverticalsinkingofthesubductingplate

causingback-arc

spreadingandtheopeningoftheMakassarStraitwiththeform

ationofoceanic

crust.(e)ThecollisionoftheEast

Sulawesiophiolite

withSW

Sulawesicausingtheterm

inationoftheriftingin

theMakassarStrait.(f)ThecollisionoftheBanggai±Sula

Microcontinentwith

East

Sulawesicausingthechangeofsubductionpolarity

inwhichtheoceanic

crust

intheeasternpart

oftheMakassarStrait

wassubducted

beneath

SW

Sulawesi.Further

on,thecollision

causedtheanticlockwiserotationofSW

Sulawesicausingthecompressionin

theNorthMakassarBasinandthisledto

theform

ationofaseries

ofeast

dippingthrust

faults.


Meratus Mountains are considered elements of theCretaceous subduction complex (Katili, 1978;Sikumbang, 1990). There is also the similarity betweenthe Manunggul Formation (SE Kalimantan) and theBalangbaru Formation (SW Sulawesi), suggesting thatthese two areas once lay close together (Hasan, 1990).

In the Late Cretaceous, the Paci®c Plate pushed wes-tern Sulawesi against SE Kalimantan causing the clo-sure of the intervening oceanic basin, ®nally leading tocollision (Sikumbang, 1986). This event resulted in theuplift of the Meratus Mountains and the emplacementof basement complexes in the Meratus Range and SWSulawesi (Fig. 13b).

Shortly following this collision, the passive margineast of western Sulawesi changed to an active margin,to accommodate compression from the continuingwestward movement of the Paci®c Plate. West-dippingsubduction was active again, forming the PompangeoSchist Complex in central Sulawesi and is thought tohave been responsible for the volcanic activity in SEKalimantan. The Pompangeo Schist Complex (centraland SE Sulawesi) and the Bantimala Complex (SWSulawesi) are both K/Ar dated as mid-Cretaceous andsome authors (Sikumbang, 1986; Parkinson, 1991)have interpreted them as part of the same accretionaryterrane. However, the Bouguer and free-air anomalycontours associated with the two complexes havedi�erent trends (Guntoro, 1995), suggesting di�erentbasement con®gurations. The Bantimala Complex isdominated by NNE±SSW trends, linking it to Java,the Java Sea and SE Kalimantan, whereas orientationsin the Pompangeo Schist Complex are N±S to NNE±SSW and continue southwards towards Flores(Guntoro, 1995).

In the Paleocene, the whole area of the Meratusmountains was uplifted and a�ected by block faulting(Fig. 13c).

In the Eocene the subducting plate to the east ofwestern Sulawesi is thought to have experienced verti-cal sinking, leading to trench-rollback (Fig. 13d). Thisvertical sinking was accommodated by extension andrifting of the continental crust above the subductionzone at a previous site of collision, causing the openingof Makassar Strait by the formation of oceanic crustwithin a back-arc setting. The time of this trench roll-back marks the cessation of volcanic activity beneathWest Sulawesi. Igneous intrusions are rarely imaged inseismic re¯ection surveys (Figs. 5 and 6) but are seenon line PAC 202 at SP 600±SP 800. Rifting was ac-companied by the deposition of syn-rift sediments(Seismic sequence 2, PAC 201 and PAC 202).

In the Early Oligocene, collision and obduction ofthe East Sulawesi Ophiolite against SW Sulawesi, assuggested by Parkinson (1991), may have terminatedthe rifting of the Makassar Strait (Fig. 13e) and havebeen followed by the deposition of post-rift sediments

(Seismic Sequences 3 to 6, Seismic lines PAC 201 and202). Thrust faults which intercalate oceanic crust withthe Oligocene Peleru Melange Complex in centralSulawesi, were a result of this collision. Later thisthrust complex was covered by Miocene sediments.This model provides an explanation for the westwardthickening of the ophiolite in eastern Sulawesi(Simandjuntak, 1990), the emplacement of the PeleruMelange Complex beneath the ophiolite (Parkinson,1991), the presence of undeformed sedimentary rocksin eastern Sulawesi, and also relates the opening of thebasins in the Makassar Strait to the trench-rollback.

Continuous volcanic activity in SW Sulawesi duringthe Neogene is interpreted as due to active subductioneast of Sulawesi conveying the Banggai±SulaMicrocontinent westwards after detachment from NewGuinea (Fig. 13f) (Hamilton, 1979). Movement alongthe Sorong Fault displaced the Banggai±Sula micro-continent which collided with East Sulawesi, probablyin the Middle Miocene (Hamilton, 1979; Silver et al.,1981; Simandjuntak, 1990; Parkinson, 1991; Guntoro,1996). This collision is marked by the formation of theKolokolo melange in East Sulawesi (see also Fig. 4),which contains fragments detached from both theophiolite suite and the continental margin sequence(Simandjuntak, 1990). The boundary between thesetwo terranes is placed at the Batui±Balantak Fault(Simandjuntak, 1990; Silver et al., 1981). The collisionalso reactivated the Median Line as a thrust fault andemplaced the Pompangeo Schist Complex above theWestern Sulawesi Plutono-Volcanic Belt. As a result ofthis collision a new subduction zone was developed tothe west of the collision zone to accommodate the con-tinuing movement of the Paci®c Plate and to cause thepassive margin on the eastern side of the MakassarStrait to become an active margin, whereas the westernmargin of the Makassar Basin remained passive. Thisinterpretation is also supported by gravity models (seeFigs. 11 and 12). From palaeomagnetic measurementsHaile (1978) and Sasajima et al. (1981) have suggestedthat SW Sulawesi has been rotated anticlockwise. Thisanticlockwise rotation of SW Sulawesi is regarded asanother consequence of the collision. Later on, contin-ued anticlockwise rotation of SW Sulawesi caused theeastern margin of the North Makassar Basin to experi-ence compression more intensively than the easternmargin of the South Makassar Basin, the di�erentialmovement being taken up along the Paternoster Fault(Fig. 14).

The zone of young compression, a�ecting strata upto horizon H5 (Mid-Miocene), converted the easternpart of the North Makassar Basin into a forelandbasin, as shown on line PAC 201, MCP 05 and twoother seismic lines P610 and P614 located close to linePAC 201, interpreted by Situmorang (1982), but noton other seismic lines further to the south. Therefore,


thrust faulting was not due to a regional compressiveregime, but ®ts with a rotation model of SW Sulawesiwhich caused compression in the North MakassarBasin. To the west of the North Makassar Strait, thisphenomenon can also be seen in the Kutai Basin(Biantoro et al., 1992) but here the time of com-pression is in the Pliocene. Therefore, these two thrustfault systems cannot have a common origin. Biantoroet al. (1992) suggested that formation of an anticlinor-ium and thrust faults in the Kutai Basin were due tothe interaction of two major strike±slip faults; theSangkulirang and Paternoster faults which movedduring the Plio-Pleistocene as a result of the collision

of the Indo-Australian Plate with the Banda-SundaArc in the Pliocene. This collision also caused inver-sion in the marginal basins and the development ofback thrusts along the southern margin of theSoutheast Kalimantan.

The Banggai±Sula collision is also thought to haveruptured the subduction zone, which ceased to operatein the north, but persisted further south, where it isnow marked by oceanic depths along the line of theTolo Thrust (see also Fig. 4).

The model outlined above can to some extentexplain the development of a carbonate platform onthe SE Kalimantan Shelf (Paternoster Platform), the

Fig. 14. (a) A cartoon showing the indentation model of Tapponnier et al. (1982) showing the sequence of faults to accommodate the indenta-

tion. (b) The same model is applied to the collision between Sulawesi and Banggai±Sula Microcontinent causing anticlockwise of the North

Makassar Basin forming a series of thrust faults as can be seen in (c).


presence of volcanics in SW Sulawesi and the presenceof east-dipping thrust faults in the eastern part ofNorth Makassar Basin. Katili (1978) also suggestedthe presence of a remnant subduction zone dipping tothe east, toward SW Sulawesi, in the south ofMakassar Strait from a seismic re¯ection pro®le to thewest of Ujung Pandang. The subduction of oceaniccrust in the North Makassar Basin dipping to the eastbeneath SW Sulawesi is believed to be responsible fora series of east-dipping thrust faults, seen in seismicpro®les in the North Makassar Basin. These east-dip-ping thrust faults are also present in the Pre-TertiaryBantimala basement complex and are in contrast towest-dipping structures which formed as a result ofwestward oceanic subduction toward SE Kalimantanduring Cretaceous time (Wakita et al., 1996), up to theMiocene Camba Formation, and is interpreted to bein¯uenced by this subduction.

18. Conclusions

1. The North and South Makassar basins show simi-larities in stratigraphic framework and tectonicstyles which can be explained by similarities in geo-logical environments. Rift structures can beobserved in the centre of both basins, characterisedby basement faults forming horst and graben struc-tures. A major di�erence between these two regionsis that there is a compressional zone marked bythrust faults with associated folds a�ecting allTertiary sediments on the eastern side of the NorthMakassar Basin and Plio-Pleistocene sediments inthe western part of North Makassar Basin. In con-trast in the South Makassar Basin is still dominatedby extensional structures.

2. The extensive thrust faults of western and easternparts of the North Makassar Basin have di�erentmechanism: thrust faults in the western part,onshore East Kalimantan, were formed during thePlio-Pleistocene and are related to a coupling sys-tem between Sangkulirang and Paternoster faults,whilst thrust faults in the eastern part, onshore ando�shore western SW Sulawesi, occurred duringMiocene±Pliocene and are related to the anticlock-wise rotation of SW Sulawesi, causing the NorthMakassar Basin to experience compression on itseastern ¯ank.

3. The centre of Makassar Strait is underlain by ocea-nic crust as inferred from gravity data and gravitymodels, and later the oceanic crust on the easternpart was subducted eastward beneath SW Sulawesi,with a change from a passive margin to an active

margin. In contrast the SE Kalimantan Shelf hasremained a passive margin.

4. Seismic interpretation and gravity models, in ad-dition to the geological information, support anEocene extensional model for the Makassar Strait.The opening of Makassar Strait during the Eocenewas due to the vertical sinking of the subductingplate, situated east of western Sulawesi magmaticarc which led to trench rollback.

Acknowledgements

This paper was part of my Ph.D. research atUniversity College London and was sponsored byBritish Petroleum Exploration, Jakarta, Indonesia. Theauthor would like to thank British Petroleum, for their®nancial support during this research, and also toJohn Milsom, my supervisor at University CollegeLondon.

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