The formation of the makassar strait guntoro (1999)

The formation of the Makassar Strait and the separation
between 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 those
several hypotheses is that SE Kalimantan and western Sulawesi once lay close together and that their separation is due to the
opening 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 large
plates 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 is
roughly 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 gravity
models, in addition to the compilation of geological information. Implications for the origin of rifting is also discussed. The
result 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 of
continental crust above the subduction zone at a previous site of collision, causing the opening of Makassar Strait. The time of
this trench roll-back marks the cessation of subduction. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction
The Makassar Strait is situated between SE
Kalimantan and western Sulawesi (Fig. 1), and lies
geographically at the boundary between the Western
Indonesian Province and the Eastern Indonesian
Province. The origin and geological framework of the
Makassar Strait have been considered by many
authors, either in detailed studies of the strait itself or
in compilations of the regional geology. Some of sev-
eral ideas about the evolution of Makassar Strait are
as follows: Katili (1978) proposed that opening took
place in the Quaternary along the Paternoster Fault,
with the formation of oceanic crust. Rose and
Hartono (1978) attributed the formation of the basin
to counterclockwise rotation of Kalimantan during the
Late Cretaceous and Early Palaeogene. Hamilton
(1979) suggested that the the Makassar Strait was
formed by sea ¯oor spreading in the Mid-Tertiary.
Burrolet and Salle (1981) argued from the present
depths of the Makassar Basin that it is a rhombo-
chasm formed on rigid continental or intermediate
crust. Situmorang (1982) explained the origin of the
Makassar Basin in terms of stretching, from the
Lower-Middle Eocene to Lower Miocene, and
suggested that it is now underlain by attenuated conti-
nental crust. Daly et al. (1991) attributed the strait to
back-arc extension along the Paci®c margin, reactivat-
ing earlier Meratus thrust terranes. Bergman et al.
(1996) suggested that in the Neogene the Makassar
Strait experienced thrust loading, forming thrust belts
on both sides of the strait, leading to the formation of
a foreland basin.
2. Bathymetry
The Makassar Strait (Fig. 2) is a symmetrical zone
of depression (median valley), ¯anked by uplifted
topography on each side. It is ¯anked by the mountain-
ous region of SE Kalimantan in the west and by western
Sulawesi in the east. Along the SE Kalimantan margin,
the continental shelf is wide and gentle, with water depth
less 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
PERGAMON

Platform (Situmorang, 1982) forming the easternmost
part 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 South
Makassar Strait basins, which are separated by the
sinistral Paternoster Fault (Katili, 1978; Situmorang,
1982). The North Makassar Basin is 340 km from
north to south, 100 km wide, from east to west, and
has water depths varying from 200 to 2000 m. The axis
of the basin trends N±S or NNE±SSW. The South
Makassar Basin is 300 km from N±S, 100 km wide
from E±W and has water depths varying between 200
to 2000 m. The axis of the South Makassar Basin
trends NE±SW.
3. The comparison between the geology of SE
Kalimantan and Western Sulawesi
The main tectonic control upon the geology of east-
ern Kalimantan and western Sulawesi is believed to be
the collision between the Eurasian Plate and
Australian microcontinental blocks in the Cretaceous
(Sikumbang, 1990). Several authors have proposed
that southeast (SE) Kalimantan and southwest (SW)
Sulawesi were parts of a single plate during the
Cretaceous and that separation between these two
regions occurred during the Tertiary. This hypothesis
is inferred on the basis of similarities of geological
records of the two areas.
4. Geology of SE Kalimantan
Kalimantan is usually regarded as having been a
stable 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, namely
West and Central Kalimantan, Southeast Kalimantan,
Northeast Kalimantan, North Kalimantan and
Northwest Kalimantan (van Bemmelen, 1949). The
development of this area was in¯uenced mainly by
subduction and collision, accompanied by basement
complex emplacement.
The geology of southeast Kalimantan is in¯uenced
by 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 SE
Kalimantan 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 Meratus
Mountains have a NE±SW structural lineation (Fig. 4).
They consist of the Meratus Ophiolite and meta-
morphic rocks of Early Aptian (116 Ma) and Early
Albian (108 Ma) age, respectively. This association of
rocks is believed to have formed in a subduction zone.
4.2. Meratus ophiolite
The ophiolite consists of ultrama®c rocks, gabbroic
rocks, plagiogranite and microdiorite. Ultrama®c rocks
are disrupted, sheared and serpentinized, and locally
exhibit 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 have
been designated as the Harun Schist and the Pelaihari
Phyllite. The distinction between the two is made
solely on the basis of metamorphic grade, since in
both cases the age of metamorphism seems to have
been Early Albian (108.4 Ma).
4.4. Sedimentary rocks
The oldest sedimentary rocks in the Meratus area
are the Paniungan and Batununggal formations of
Berriasian±Barremian and Barremian±Aptian age, re-
spectively. Both formations were deposited in a shal-
low marine to slope setting on the southeastern margin
of the Sunda continent. The Paniungan Formation
consists largely of mudstone with intercalations of
sandstone and minor limestone. The Batununggal
Formation is divided into three di€erent units; auto-
chthonous (intact limestone), para-authochthonous
(thrust sheet) and allochthonous (exotic blocks). The
formation occurs in the northeastern and southeastern
parts of the Meratus Mountains. In the northeast it is
largely covered by in situ and undeformed amygdaloi-
dal lava ¯ows.
The Alino Group, which is considered to be derived
from 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 the
Meratus Mountains. The ®rst is the Rimuh Pluton, in
the Tambak±Tamban Range, the second the Kintap
Pluton about 10 km north of Kintap. These plutonic
rocks can be related to a west-dipping subduction zone
in the Early Cretaceous±Early Tertiary. The early
Upper Cretaceous or pre-Upper Turonian (91 Ma)
Rimuh Pluton is associated with volcanics of the
Pitanak Formation. The Kintap Pluton (95 Ma) is
intrusive into both the Meratus Ophiolite and the
Alino Group.
5. Geology of southwest Sulawesi
Sulawesi consists of four diverging arms named the
south, north, east and southeast arms, each of which
records a very di€erent and complicated geological his-
tory (see Fig. 4). The complicated geology of Sulawesi,
consisting of various lithologies and structures with
di€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).
Source, File: The Formation of the Makassar Strait - Guntoro (1999).pdf (p5_83)

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 four
major terranes or belts: i.e. the Banggai±Sula
Microcontinent (BSM); the Eastern Sulawesi Ophiolite
Belt (ESOB); the Central Sulawesi Metamorphic Belt
(CSMB) and the Western Sulawesi Plutono-Volcanic
Belt (WSPVB).
Southwest Sulawesi is part of the Western Sulawesi
Plutono-Volcanic Belt (WSPVB) which is characterised
by biotite schist, extensive massifs of granodioritic
rocks, and sediments which were in general deposited
closer to shore than those of the Eastern Ophiolite
Belt (van Bemmelen, 1949). Katili (1978) suggested
that 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 the
WSPVB 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 Ma
for the schist (Hamilton, 1979).
Unconformably overlying the basement complex are
¯ysch sediments of the Cretaceous Balangbaru and lat-
erally equivalent Marada Formation. This is overlain
unconformably 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 Middle
Eocene±Middle Miocene Tonasa Formation interdigi-
tates with the upper part of Malawa Formation, and
consists mainly of limestone forming a transgressive
sequence. The Middle to Late Miocene Camba
Formation conformably overlies the Tonasa
Formation and consists of volcanic and volcaniclastic
rocks.
Miocene and younger volcanic and plutonic rocks
are dominant in the South Arm of Sulawesi and have
been 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±continent
collision.
6. Geological summary
Many of the authors who have worked in the area
have 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 two
areas may be summarized as follows. In SE
Kalimantan metamorphic rocks are overlain by the
Albian±Early Cenomanian Alino Formation (deep sea
sediments and basic volcanics) and its neritic equival-
ent, the Jurassic (?) to Early Cretaceous Paniungan
Beds. The Upper Cretaceous Manunggal Formation
overlies unconformably the above rock units. The simi-
lar sequences are also found in SW Sulawesi where
metamorphic rocks are also unconformably overlain
by a series of Jurassic (?) to Early Cretaceous siliceous
shale, sandstone and radiolarian chert, which is locally
metamorphosed. On the basis of the sedimentology,
tectonic style and regional setting, Sikumbang (1990)
suggested that the Manunggul Group of Southeast
Kalimantan was deposited in a pull-apart basin devel-
oped within a strike±slip zone initiated during or
shortly after arc±continent collision. The Manunggal
Group can be correlated with the Balangbaru
Formation of SW Sulawesi (Hasan, 1987).
The Tertiary stratigraphy of western Sulawesi is also
considered to be comparable with that of many of the
Tertiary 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 been
proposed by many authors, not only from geological
point 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 SW
Sulawesi, were positioned closer together in the Late
Cretaceous, supporting the hypothesis that the
Makassar Strait was formed by the later separation of
the two areas. However; the timing and the mechanism
of this separation are still not clear, these problems are
investigated in this paper.
7. Seismic interpretation
Structural interpretations and seismic stratigraphy
for the North and South Makassar basins have been
derived 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 North
and South Makassar basins, respectively, are multi-
channel seismic pro®les. The sections are displayed as
line drawing interpretations. Analysis is based on the
procedures 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 Makassar
Basin, can be separated into eastern and western seg-
ments on the basis of structural regimes. The western
segment (Fig. 5a) extends from SP 1200 to SP 4000
and displays thick sediments controlled by acoustic
basement faults. In contrast, the eastern segment
(Fig. 5b), from SP 0 to SP 1400, exhibits extensive
west-directed thrust faulting and basement cannot be
traced 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 been
a€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¯ectors
can be divided into six seismic sequences which can be
grouped into three major units; acoustic basement
(Sequence 1), syn-rift sediments (Sequence 2) and post-
rift sediments (Sequence 3±6). In the ®gures, the
seismic sequence boundaries are shown as horizons
H1±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 as
acoustic basement. The contact with the overlying sedi-
ments is dicult to trace, especially in the eastern seg-
ments of Line PAC 201 where it is obscured by
di€ractions and multiples. This contact is marked as
H1 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 boundary
between acoustic basement and the overlying sediments
being placed at depths at which there was an extreme
velocity contrast. The greatest depths are in the middle
of the line, such as on Line PAC 201 from SP 1470 to
SP 1600, where horizon H1 was not seen as it lies dee-
per than the maximum time recorded (8 s TWT). The
horizon 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 is
Seismic Sequence 2. This sequence is characterised by
parallel±subparallel re¯ectors, with poor to fair conti-
nuity and low to medium amplitude. Re¯ection geome-
try suggests a concordant sequence boundary
relationship at the top, and onlap at the base, against
H1 (Line PAC 201, SP 1800 to SP 1650 and Line 202,
SP 3900 to SP 3650). Following the criteria of Vail
et 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 the
basis 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 of
tectonic 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 considered
to 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 and
are 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 poor
to fair continuity and high to medium re¯ection ampli-
tude; in some parts amplitudes are low. The variation
in amplitude and frequency may indicate a lithological
facies change, which could relate to a decreasing rate
of subsidence. The lower boundary shows downlap to
the top of Seismic Sequence 2 (Boundary H2). These
re¯ector characteristics can be taken as indicating a
shelf to shelf margin depositional environment (Vail
et al., 1977).
This sequence is bounded by horizons H3 and H4,
and is dominated by parallel and locally sub parallel
re¯ections, with fair to good continuity and medium
to high re¯ection amplitude. The unit is characterised
by the presence of local mound-like re¯ector patterns
from SP 3400 to SP 3200, SP 3050 to SP 2850 and SP
2250 to SP 2150 (on Line PAC 201) and SP 3450 to
3550 (on Line PAC 202) which are interpreted as car-
bonate mounds. The upper boundary is marked by
toplap to horizon H5 from SP 2450 to SP 2250 (on
Line PAC 201). The re¯ector characteristics are classi-
®ed as indicating a shelf to shelf margin depositional
environment.
This sub unit is bounded by horizons H4 and H5
and displays parallel con®gurations with fair to good
continuity 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 characteristics
are typical of a shelf depositional environment and in-
dicate a shallow marine shelf deposit. The unit can still
be recognised in the eastern segment, although this
region is distorted by thrust faulting.
This sub unit is bounded by horizons H5 and H6
and shows parallel con®gurations with good continuity
and medium to high re¯ection amplitudes. The re¯ec-
tion characteristics are classi®ed as indicating a shelf
depositional environment. In the eastern segment of
Line PAC 201 from SP 1300 to SP 0 the sequence can
be subdivided into sub-sequences con®ned to local
basins in which horizontal re¯ectors onlap to the top
of Horizon H5, and this sub unit was deposited as
onlapping ®ll.
9. Seismic interpretation line MCP 05
This line in the North Makassar Basin (Fig. 7) has
been interpreted and published as a line drawing by
Katili (1978). It lies at about 28S, trends E±W and is
approximately 275 km long. It is crossed close to its
centre by Line PAC 201. The line drawing produced
by Katili (1978) did not show any detail of the re¯ec-
tors and it is therefore dicult to correlate his in-
terpretation with those of PAC 201 and 202, in terms
of sequence stratigraphy. However, the section does
show 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 of
the graben, at approximately 5 s TWT (6 km); sedi-
ment occupies about 2 s TWT beneath more than 2
km of water. The external form of this sedimentary
sequence appears to indicate onlapping in®ll. The
sequence 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 s
TWT and 0.5 s TWT, controlled by normal faults. The
sequence shows parallel-divergent con®gurations with
continuous re¯ectors over this high. To the east of the
graben are folded sediments, suggesting compressional
tectonics in this part of the line, as opposed to the
central and western part which show extensional
tectonics.
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 Basin
have been interpreted and published as line drawings
by Burrolet and Salle (1981). As with MCP 05, the in-
terpretations were not drawn in detail and it is dicult
to correlate the sequence stratigraphy with lines PAC
201 and 202.
Line MK 3 is situated in the northernmost part of
the North Makassar Basin (see Fig. 2). In the east (SP
2200 to the end of the line at SP 3300), the basement
is 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 s
TWT to more than 6 s TWT. The overlying sediments
have a generally uniform thickness of more than 3 s
TWT and display parallel con®gurations, with moder-
ate to good continuity.
Line MK 1 is parallel to PAC 201 and 25 km to the
north (see Fig. 2). The compressional zone at the east-
ern margin, which is dominant on PAC 201, is not
observed. 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 the
western part, towards the axial trough, it is seen only
discontinuously, reaching a depth of 7.5 s TWT in
some locations. The sediments display parallel con-
®gurations, apparently with moderate to good continu-
ity.
The interpretations of lines MK1 and MK3 suggest
a history of sedimentation similar to that seen on
Lines PAC 201 and PAC 202, indicating that the
whole Makassar Basin formed by rifting and was sub-
sequently modi®ed by thrust faulting along the eastern
margin of the North Makassar Basin.
11. Structural interpretation
From the seismic pro®les presented above, and also
from the interpretation of other seismic pro®les across
the Makassar Strait obtained from the literature
(Situmorang, 1982; Katili, 1978; Pertamina, 1985), the
structural setting of the Makassar Strait can be
deduced as follows. The centres of the North and
South Makassar Basins have similar structures, show-
ing that the major tectonic regime is extensional, with
normal faults displacing the older syn-rift sediments,
but not disturbing the younger post-rift sediments.
However, the western and eastern sides of the North
and South Makassar Basins have di€erent structures.
The North Makassar Basin is limited by active reverse
faults on both sides (Bergman et al., 1996; Pertamina,
1985). On the western side, Pertamina (1985) show a
series of reverse faults dipping both to the east and
west and displacing all the stratigraphic units, up to
the youngest sediments. While on the eastern side, a
series of reverse faults dip to the east (see Fig. 5b);
mostly these faults do not displace the youngest
sequence. On the other hand the South Makassar
Basin is bounded by normal faults.
12. Correlation of well data with seismic sequences
The age of the seismic units identi®ed on lines PAC
201 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 edge
of 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 carbonate
reef has been used as an acoustic marker in the area to
the south, and also in the East Kalimantan basinal
area (Situmorang, 1982). On the basis of this knowl-
edge, the carbonate reefs, recognised by their mounded
external form in seismic sequence 4 on seismic section
PAC 201, can be used to locate the top of the Early
Miocene. Using this assumption, the other sequences
can be correlated with the well data.
The top of sequence 1 (H1) is equivalent to horizon
C1 of Situmorang (1982), which is the pre-Tertiary
basement, consisting of gabbros and dolerites (Well
TT 2). Sequence 2 (between horizons H1 and H2) is
equivalent to the Late Eocene syn-rift sediments. The
top of this unit, designated H2, marks the end of the
rifting 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 opening
of the Makassar Strait can be related to the deposition
of Sequence 2. Sequence 3 (between horizons H2 and
H3) is equivalent to the Lower Oligocene conglomera-
tic limestone. The top of Sequence 4 (Horizon H4) is
equivalent to horizon C2, the Early Miocene carbonate
reef, of Situmorang (1992). Sequence 5 is equivalent to
the Early to Middle Miocene deep marine shales and
marls and Sequence 6 is equivalent to the Pliocene
shallow marine limestone.
13. Gravity data
There are two di€erent free-air anomaly maps for
the Makassar Strait. The ®rst, produced by SIPM
(Shell), was reproduced by Situmorang (1982) without
any indication of the formula used to calculate gravity
values. The two contour maps are based on di€erent
data, with the free-air anomaly values shown di€er-
ently on the two maps. To ®nd the magnitude of the
di€erence, the two maps were overlapped and at every
crossing point the free-air anomaly values were com-
pared. The di€erence is approximately constant, the
Edcon (1991) values being greater than the SIPM
values by 50 mGal. This di€erence is thought to be re-
lated to the fact that the SIPM data was not tied to
any 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 is
characterised by negative free-air anomalies along the
axial depression and positive free-air anomalies
towards the continental shelves of Kalimantan and
Sulawesi. Free-air anomaly values thus re¯ect bathy-
metry. Major features seen in the gravity data have
been named as follows: Laut High, Mahakam High,
Paternoster High, Paternoster Lineament, North
Makassar Low and South Makassar Low.
The Laut High is centred on Laut Island, close to
the Meratus Mountains and trends NE±SW. The free-
air anomaly ranges from +40 to +70 mGal. This
high is interpreted as indicating the presence of high
density ultrama®c rocks of the basement complex,
close to the surface. Ophiolites are present on Laut
Island and in the Meratus Mountains (Sikumbang,
1990). To the northeast of the Laut High, the
Mahakam High has a N±S trend which changes shar-
ply to E±W at about 18N. The free-air anomaly values
range from +40 to +120 mGal. The Laut High and
Mahakam High are in the same trend, but they are
not continuous, and they are separated by a steep
NW±SE gradient, the Paternoster Lineament.
Parallel and to the east of the Laut High, the
Paternoster High forms an elliptical closure, elongated
in a NNE±SSW direction, in a region where the broad
continental shelf of the southeast Kalimantan margin
lies in a water depth of less than 200 m. The free-air
anomaly values range from +50 to +70 mGal.
Parallel and to the east of the Paternoster High is a
long and narrow free-air anomaly low which trends
roughly N±S. At about 38S this is o€set by the
Paternoster lineament, dividing it into the South and
North Makassar Lows. The North Makassar Low has
free-air anomaly values ranging from 0 to À40 mGal,
and the South Makassar Low has free-air anomaly
values ranging from 0 to À50 mGal. These low free-air
anomalies indicate the presence of thick low-density
sedimentary rocks. The North and South Makassar
lows are de®ned by steep gravity gradients, attributed
to faulting, at the contacts with the Mahakam and
Paternoster highs.
15. Gravity models
Two gravity models of the Makassar Strait have
been constructed using the GM-SYS Gravity modeling
program. The models are constrained by interpretation
of the seismic re¯ection pro®les PAC 201 and PAC
202, from the North and South Makassar basins, to
shed light upon the bathymetry and thickness of sedi-
ments. Densities have been assigned for seawater, sedi-
ments, upper crust and oceanic crust as follows. The
density of seawater is taken as 1.03 Mg mÀ3
. The
average density of the sediments has been estimated
using 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±velocity
curve of Nafe and Drake (1962), to be 2.3 Mg mÀ3
.
The density of the upper crust below the sediments is
taken as 2.67 Mg mÀ3
and density of the upper mantle
is taken as 3.3 Mg mÀ3
. The oceanic crust was mod-
eled using density of 2.85 Mg mÀ3
. The reference crus-
tal model has a density of 2.67 Mg mÀ3
and a
thickness of 30 km.
15.1. Gravity modeling PAC 201
This cross section is taken across the Makassar
Strait in a NE±SW direction along seismic re¯ection
line PAC 201, but extends up to the coasts of
Kalimantan and Sulawesi on either side. The result of
modeling is shown in Fig. 11. There is a good match
between observed and calculated gravity values. In
particular, 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 sediments
between down-faulted blocks. The lowest free-air
anomaly, of À30 mGal, occurs close to the centre of
the basin. Otherwise the model seems dicult to
match 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 sharply
to about +50 mGal. The minimum free-air anomaly
in the centre of the basin is due to the presence of ap-
proximately 8 km of low density (2.3 Mg mÀ3
) sedi-
ments. In this model, the centre of the Makassar Strait
is underlain by oceanic crust, and in the eastern part
the oceanic crust has been subducted beneath SW
Sulawesi, whereas on the western side of Makassar
Strait, oceanic crust has not been not subducted. The
depth to the upper mantle is 25 km towards
Kalimantan and 18 km towards Sulawesi and
decreases to 12 km in the centre of the basin.
15.2. Gravity modeling PAC 202
This model has been prepared along seismic line
PAC 202, but is extended up to the coast of Laut
Island on the Kalimantan side and to the coast of
Sulawesi to the east. The result of the modeling is
shown in Fig. 12.
The lowest free-air anomaly, of about À40 mGal,
occurs in the middle of the basin, with steep gradients
towards the continental shelves of Kalimantan and
Sulawesi. The minimum free-air anomaly coincides
with the centre of the basin, where the water depth is
about 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 this
model, the centre of the South Makassar Basin is
underlain by oceanic crust, and in the eastern part the
oceanic 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 of
Sulawesi is about 20 km but in the centre of the basin
is only about 15 km.
16. Crustal structure
Deep water areas in the Makassar Strait correspond
to areas of low free air anomalies, shallow bathymetry
corresponds to areas of high free air anomaly. Gravity
modeling shows, however, that the Moho is shallow
beneath the axial trough and deepens towards the
shelves, especially toward the SE Kalimantan shelf
(Figs. 11 and 12) and that there is a change in the
thickness of the crust, excluding the post-extensional
sediments cover, from the continental shelf regions
(25±28 km) to the axial trough (5±12 km). It is
suggested that the changes in crustal thickness are due
to deformation by extensional thinning. The crustal
thickness in the axial region indicates the presence of
oceanic crust. Seismic refraction data in the South
Makassar Basin show basement velocities ranging
from 3.56 to 5.69 km sÀ1
(Prasetyo and Dwiyanto,
1986), which are typical velocities for continental crust,
but could possibly be derived from the upper part of
oceanic crust. Situmorang (1982) suggested that a
stretching 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 not
yet developed oceanic crust. In contrast, the dolerites
and 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 present
gravity models also suggest typical oceanic crustal
thicknesses. In view of the well data and gravity
models, it is suggested that the central part of the
Makassar Basin is underlain by oceanic crust.
17. Tectonic implications and the evolution of the
Makassar Strait
Seismic refraction and re¯ection surveys and gravity
modeling, as outlined above, support an Eocene exten-
sional model for the Makassar Basin. Prior to exten-
sion the region is thought to have undergone
compression due to the collision between SE
Kalimantan and SW Sulawesi, which also produced
the uplift of the Meratus Range in the Late
Cretaceous. This compressional phase is thought to
have thickened the crust, as normally happens in com-
pressional regimes.
From seismic re¯ection interpretation, the top of the
basement reaches a depth of 10 km and is overlain by
sediments up to 8 km thick. The water depth in the
axial trough reaches 2.2 km. Seismic stratigraphic ana-
lyses suggest that the Makassar Basin has subsided
slowly and has experienced continuous sedimentation
since the Eocene. This argument is supported by the
depositional environmental data from Wells TT 1 and
TT 2, which showed continuous sedimentation during
the Tertiary. The sediments were deposited in a neritic
environment in the Eocene and a neritic to sub-neritic
environment in the Late Eocene to Middle Miocene.
Shallow marine carbonates were deposited from the
Middle Miocene to the Recent (Situmorang, 1982).
This observation leads to the interpretation that rifting
was followed by thermal subsidence causing the basin
to subside slowly and continuously.
The following is a history of the formation of
Makassar Strait, based on seismic interpretation, grav-
ity data and models presented in this paper, in ad-
dition to available geological information from the
region. Fig. 13 shows a possible plate tectonic recon-
struction of the evolution of the Makassar Strait from
Late Cretaceous to Late Miocene. These reconstruc-
tions summarise the geological and geophysical data
from the region and also integrate previous models
from Hamilton (1979), Daly et al. (1991) and
Parkinson (1991).
The Cretaceous basement complexes in Java, the
Java Sea, SE Kalimantan and SW Sulawesi are
believed to constrain the geometry of subduction
between 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 Java
Sea (Guntoro, 1995), and palaeomagnetic results also
indicate that SW Sulawesi lay close to its present pos-
ition from the Jurassic to Early Cretaceous, relative to
East Kalimantan (Haile, 1978). From a sedimentary
point of view, there is strong evidence for the presence
of west-dipping subduction at the eastern margin of
the East Java Sea microplate and the close positioning
of SW Sulawesi and SE Kalimantan. The Alino
Formation, the Paniungan beds, the Manunggul
Formation 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) The SW Sulawesi Microcontinent was subducted beneath SE Kalimantan (East Java Sea Microplate), generating an accretionary complex. (b) The SW Sulawesi Microcontinent col-
lided with SE Kalimantan. This event resulted in the uplift of the Meratus Mountains and the emplacement of the basement complexes. The continuing movement of the Paci®c Plate was accom-
modated by the formation of new subduction to the east of SW Sulawesi. (c) The whole area of the Meratus Mountains was uplifted and associated with block faulting. O€ to the east the
Banggai±Sula Microcontinent was approaching. (d) The vertical sinking of the subducting plate causing back-arc spreading and the opening of the Makassar Strait with the formation of oceanic
crust. (e) The collision of the East Sulawesi ophiolite with SW Sulawesi causing the termination of the rifting in the Makassar Strait. (f) The collision of the Banggai±Sula Microcontinent with
East Sulawesi causing the change of subduction polarity in which the oceanic crust in the eastern part of the Makassar Strait was subducted beneath SW Sulawesi. Further on, the collision
caused the anticlockwise rotation of SW Sulawesi causing the compression in the North Makassar Basin and this led to the formation of a series of east dipping thrust faults.
A.Guntoro/JournalofAsianEarthSciences17(1999)79±9894
Source, File: The Formation of the Makassar Strait - Guntoro (1999).pdf (p16_94)

Meratus Mountains are considered elements of the
Cretaceous subduction complex (Katili, 1978;
Sikumbang, 1990). There is also the similarity between
the Manunggul Formation (SE Kalimantan) and the
Balangbaru Formation (SW Sulawesi), suggesting that
these 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 to
collision (Sikumbang, 1986). This event resulted in the
uplift of the Meratus Mountains and the emplacement
of basement complexes in the Meratus Range and SW
Sulawesi (Fig. 13b).
Shortly following this collision, the passive margin
east of western Sulawesi changed to an active margin,
to accommodate compression from the continuing
westward movement of the Paci®c Plate. West-dipping
subduction was active again, forming the Pompangeo
Schist Complex in central Sulawesi and is thought to
have been responsible for the volcanic activity in SE
Kalimantan. The Pompangeo Schist Complex (central
and SE Sulawesi) and the Bantimala Complex (SW
Sulawesi) are both K/Ar dated as mid-Cretaceous and
some authors (Sikumbang, 1986; Parkinson, 1991)
have interpreted them as part of the same accretionary
terrane. However, the Bouguer and free-air anomaly
contours associated with the two complexes have
di€erent trends (Guntoro, 1995), suggesting di€erent
basement con®gurations. The Bantimala Complex is
dominated by NNE±SSW trends, linking it to Java,
the Java Sea and SE Kalimantan, whereas orientations
in 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 Meratus
mountains was uplifted and a€ected by block faulting
(Fig. 13c).
In the Eocene the subducting plate to the east of
western Sulawesi is thought to have experienced verti-
cal sinking, leading to trench-rollback (Fig. 13d). This
vertical sinking was accommodated by extension and
rifting of the continental crust above the subduction
zone at a previous site of collision, causing the opening
of Makassar Strait by the formation of oceanic crust
within a back-arc setting. The time of this trench roll-
back marks the cessation of volcanic activity beneath
West Sulawesi. Igneous intrusions are rarely imaged in
seismic re¯ection surveys (Figs. 5 and 6) but are seen
on 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 of
the East Sulawesi Ophiolite against SW Sulawesi, as
suggested by Parkinson (1991), may have terminated
the rifting of the Makassar Strait (Fig. 13e) and have
been followed by the deposition of post-rift sediments
(Seismic Sequences 3 to 6, Seismic lines PAC 201 and
202). Thrust faults which intercalate oceanic crust with
the Oligocene Peleru Melange Complex in central
Sulawesi, were a result of this collision. Later this
thrust complex was covered by Miocene sediments.
This model provides an explanation for the westward
thickening of the ophiolite in eastern Sulawesi
(Simandjuntak, 1990), the emplacement of the Peleru
Melange Complex beneath the ophiolite (Parkinson,
1991), the presence of undeformed sedimentary rocks
in eastern Sulawesi, and also relates the opening of the
basins in the Makassar Strait to the trench-rollback.
Continuous volcanic activity in SW Sulawesi during
the Neogene is interpreted as due to active subduction
east of Sulawesi conveying the Banggai±Sula
Microcontinent westwards after detachment from New
Guinea (Fig. 13f) (Hamilton, 1979). Movement along
the Sorong Fault displaced the Banggai±Sula micro-
continent which collided with East Sulawesi, probably
in the Middle Miocene (Hamilton, 1979; Silver et al.,
1981; Simandjuntak, 1990; Parkinson, 1991; Guntoro,
1996). This collision is marked by the formation of the
Kolokolo melange in East Sulawesi (see also Fig. 4),
which contains fragments detached from both the
ophiolite suite and the continental margin sequence
(Simandjuntak, 1990). The boundary between these
two terranes is placed at the Batui±Balantak Fault
(Simandjuntak, 1990; Silver et al., 1981). The collision
also reactivated the Median Line as a thrust fault and
emplaced the Pompangeo Schist Complex above the
Western Sulawesi Plutono-Volcanic Belt. As a result of
this collision a new subduction zone was developed to
the west of the collision zone to accommodate the con-
tinuing movement of the Paci®c Plate and to cause the
passive margin on the eastern side of the Makassar
Strait to become an active margin, whereas the western
margin of the Makassar Basin remained passive. This
interpretation is also supported by gravity models (see
Figs. 11 and 12). From palaeomagnetic measurements
Haile (1978) and Sasajima et al. (1981) have suggested
that SW Sulawesi has been rotated anticlockwise. This
anticlockwise rotation of SW Sulawesi is regarded as
another consequence of the collision. Later on, contin-
ued anticlockwise rotation of SW Sulawesi caused the
eastern margin of the North Makassar Basin to experi-
ence compression more intensively than the eastern
margin of the South Makassar Basin, the di€erential
movement being taken up along the Paternoster Fault
(Fig. 14).
The zone of young compression, a€ecting strata up
to horizon H5 (Mid-Miocene), converted the eastern
part of the North Makassar Basin into a foreland
basin, as shown on line PAC 201, MCP 05 and two
other seismic lines P610 and P614 located close to line
PAC 201, interpreted by Situmorang (1982), but not
on other seismic lines further to the south. Therefore,

thrust faulting was not due to a regional compressive
regime, but ®ts with a rotation model of SW Sulawesi
which caused compression in the North Makassar
Basin. To the west of the North Makassar Strait, this
phenomenon 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 thrust
fault systems cannot have a common origin. Biantoro
et al. (1992) suggested that formation of an anticlinor-
ium and thrust faults in the Kutai Basin were due to
the interaction of two major strike±slip faults; the
Sangkulirang and Paternoster faults which moved
during the Plio-Pleistocene as a result of the collision
of the Indo-Australian Plate with the Banda-Sunda
Arc in the Pliocene. This collision also caused inver-
sion in the marginal basins and the development of
back thrusts along the southern margin of the
Southeast Kalimantan.
The Banggai±Sula collision is also thought to have
ruptured the subduction zone, which ceased to operate
in the north, but persisted further south, where it is
now marked by oceanic depths along the line of the
Tolo Thrust (see also Fig. 4).
The model outlined above can to some extent
explain the development of a carbonate platform on
the 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 presence
of east-dipping thrust faults in the eastern part of
North Makassar Basin. Katili (1978) also suggested
the presence of a remnant subduction zone dipping to
the east, toward SW Sulawesi, in the south of
Makassar Strait from a seismic re¯ection pro®le to the
west of Ujung Pandang. The subduction of oceanic
crust in the North Makassar Basin dipping to the east
beneath SW Sulawesi is believed to be responsible for
a series of east-dipping thrust faults, seen in seismic
pro®les in the North Makassar Basin. These east-dip-
ping thrust faults are also present in the Pre-Tertiary
Bantimala basement complex and are in contrast to
west-dipping structures which formed as a result of
westward oceanic subduction toward SE Kalimantan
during Cretaceous time (Wakita et al., 1996), up to the
Miocene Camba Formation, and is interpreted to be
in¯uenced by this subduction.
18. Conclusions
1. The North and South Makassar basins show simi-
larities in stratigraphic framework and tectonic
styles which can be explained by similarities in geo-
logical environments. Rift structures can be
observed in the centre of both basins, characterised
by basement faults forming horst and graben struc-
tures. A major di€erence between these two regions
is that there is a compressional zone marked by
thrust faults with associated folds a€ecting all
Tertiary sediments on the eastern side of the North
Makassar Basin and Plio-Pleistocene sediments in
the western part of North Makassar Basin. In con-
trast in the South Makassar Basin is still dominated
by extensional structures.
2. The extensive thrust faults of western and eastern
parts of the North Makassar Basin have di€erent
mechanism: thrust faults in the western part,
onshore East Kalimantan, were formed during the
Plio-Pleistocene and are related to a coupling sys-
tem between Sangkulirang and Paternoster faults,
whilst thrust faults in the eastern part, onshore and
o€shore western SW Sulawesi, occurred during
Miocene±Pliocene and are related to the anticlock-
wise rotation of SW Sulawesi, causing the North
Makassar Basin to experience compression on its
eastern ¯ank.
3. The centre of Makassar Strait is underlain by ocea-
nic crust as inferred from gravity data and gravity
models, and later the oceanic crust on the eastern
part was subducted eastward beneath SW Sulawesi,
with a change from a passive margin to an active
margin. In contrast the SE Kalimantan Shelf has
remained a passive margin.
4. Seismic interpretation and gravity models, in ad-
dition to the geological information, support an
Eocene extensional model for the Makassar Strait.
The opening of Makassar Strait during the Eocene
was due to the vertical sinking of the subducting
plate, situated east of western Sulawesi magmatic
arc which led to trench rollback.
Acknowledgements
This paper was part of my Ph.D. research at
University College London and was sponsored by
British Petroleum Exploration, Jakarta, Indonesia. The
author would like to thank British Petroleum, for their
®nancial support during this research, and also to
John Milsom, my supervisor at University College
London.
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