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IPA05-G-161
PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION
Thirtieth Annual Convention & Exhibition, August 2005
BASEMENT ARCHITECTURE AND SEDIMENTARY FILL
OF THE NORTH MAKASSAR STRAITS BASIN
Siti Nur’Aini*
Robert Hall**
Christopher F. Elders**
ABSTRACT
The North Makassar Straits Basin (NMSB) is the
deepest Cenozoic basin in Indonesia. Debate
continues about whether it is underlain by continental
or oceanic crust, and whether it originated as a rift or
foreland basin. 2D seismic lines, gravity and well data
were used to study the basement architecture and
sedimentary fill of the basin. Based on those data, the
NMSB is interpreted to have originated as a strongly
segmented extensional basin, associated with half
graben and graben structures. Mapping has revealed a
system of NNW-SSE lineaments and faults
intersecting the Top Basement seismic reflector. The
lineaments and faults are arranged in en-echelon
patterns and bound disconnected NNW-SSE trending
depocentres. From comparison with sandbox
analogue models, the syn-rift fault system in the
NMSB seems most likely to have been produced by
oblique rifting. The principal extension direction was
east-west, at an angle of approximately 60° to preexisting basement structures. Rifting of this part of
the southeast Eurasian continental margin occurred
during the Middle to Late Eocene. The crust beneath
the NMSB is interpreted to be continental. The postrift section is subdivided into three megasequences.
Post-rift Megasquence 1 was deposited during the
rapid subsidence of the basin from the Late Eocene to
Late Oligocene. Uplift of Kalimantan in the Early to
Middle Miocene generated Post-rift Megasequence 2,
an interval characterised by a prograding delta
system. Late Miocene Post-rift Megasequence 3 is
interpreted to contain a turbidite interval in the central
part of the basin. Subsidence and deep water marine
sedimentation continued to the present-day in the
central NMSB but from the Early Pliocene, sediment
*
**
ENI Indonesia Ltd.
SE Asia Research Group, Royal Holloway University of London
supply from the east increased significantly as result
of the westward propagation of the West Sulawesi
fold and thrust belt. The NMSB became a foreland
basin only from the Early Pliocene.
INTRODUCTION
The Makassar Straits contains the deepest Cenozoic
basin in Indonesia, the North Makassar Straits Basin
(NMSB). There is more than 12 kilometres of
sedimentary fill and a record of continuous
sedimentation from the Eocene until the present day
(Situmorang, 1982; Guntoro, 1999; Moss and
Chambers, 1999). Large hydrocarbon deposits have
been discovered beneath the Mahakam delta and in
the deepwater offshore area (Figure 1). Water depths
in the central NMSB, also known as the offshore
Kutai Basin, exceed 2000 metres.
The origin of the Makassar Straits is still
controversial and there are several hypotheses about
the nature of the underlying crust and the formation of
the deep central basin. There are no wells that
penetrate the deeper sequences close to the basin axis
and seismic and gravity data have been used as the
basis of this and previous interpretations. Some
authors suggest the NMSB formed on continental
crust (e.g. Burollet and Salle, 1981) whereas others
have argued that it is oceanic crust (e.g. Hamilton,
1979; Cloke, 1997; Cloke at al., 1999; Guntoro, 1999;
Fraser et al., 2003). The straits have been interpreted
to have formed by Cenozoic extension (Hamilton,
1979; Cloke, 1997; Cloke at al., 1999; Moss and
Chambers, 1999) which is thought to have been
initiated during the Middle Eocene (van de Weerd
and Armin, 1992; Hall, 2002). This has been
suggested to have been driven by subduction of the
Pacific plate (Guntoro, 1999). Others have suggested
that the Makassar Straits represent a remnant oceanic
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basin (Malecek et al., 1993) and some authors have
concluded the Makassar Straits Basin is a Neogene
foreland basin formed after Early Miocene continentcontinent collision (Coffield et al., 1993; Bergman et
al., 1996).
The aim of this study was to investigate the basement
architecture and sedimentary fill of the NMSB with
the aim of contributing to the debate about its
formation.
created a major depression which is approximately
150 km in length. Half graben and graben structures
and lineaments are arranged in an en-echelon pattern
and are parallel to sub-parallel. The overlapping fault
tips indicate relay structures along which the fault
polarity is often reversed (Figure 5). The faulting has
produced a series of disconnected NNW-SSE
trending structurally low areas separated by relay
ramps and other accommodation zones. In the area
near the edge of the Paternoster Platform, en-echelon
faults display zones of overlapping splays.
DATASET AND METHODOLOGY
Ninety-eight 2D seismic lines (provided by TGSNOPEC Geophysical Company) combined with well
and gravity data were used in this study. The Panca-1
and Pamukan Bay wells were used to calibrate the
seismic interpretation in the study area (Table 1).
Based on correlation of markers from those wells to
the study area, estimates of ages of key horizons were
made (Figure 2A) and their stratigraphic relationships
interpreted (Figures 2B).
BASEMENT ARCHITECTURE
The free-air gravity map (Figure 1B), based on Smith
and Sandwell (1997) and derived from GeoSat and
ERS-1 data, shows two gravity lows: an elongated
low northeast of the Paternoster Platform and an
irregular low between the Mahakam delta and the
Mangkalihat High. The elongated low continues to
the southeast and forms a narrow trough between the
northeast Paternoster Platform and the South
Sulawesi coast. Further southeast, the trough is linked
to the South Makassar Straits Basin.
The Top Basement pick shows irregular topography.
In the south of the study area this can easily be
interpreted as due to faulting, supported by the
presence of clear syn-rift wedges in the hanging walls
of extensional faults (Figures 3 to 7). Further north
the structure at basement level is less well imaged due
to the presence of thicker, more structurally complex
overburden. However, by analogy with better imaged
areas further south, basement topography can be
similarly interpreted in terms of extensional faults.
The basement structure map shows the NMSB has a
rhomboidal shape with the longer axis orientated
NNE-SSW (Figure 4). Within that overall shape,
however, individual structural lineaments trend
predominantly NNW-SSE (Figure 8) and have
SEDIMENTARY FILL AND BASIN
RECONSTRUCTION
The sedimentary rocks deposited above the basement
of the NMSB are divided here into three tectonomegasequences: syn-rift, post-rift and foreland basin
packages (Figure 2B).
Syn-rift package (Middle to Late Eocene)
The syn-rift sequence is characterized by sub-parallel,
discontinuous, moderate to low amplitude reflectors.
In some places the syn-rift packages include
divergent, prograding and submarine fill. The
submarine fill associated with channel and overbank
complexes is seen clearly near the edge of Paternoster
Platform (Figure 6). Divergent fill can be seen close
to the West Sulawesi where the section thickens
towards the hanging wall side of a normal fault
(Figure 7). Another type of syn-rift feature in this
structural low area is a slope fan complex that
accumulated in front of a steep fault.
Carbonate build-ups are interpreted in the study area
on the top of horsts and tilted fault blocks. They
exhibit strong to low amplitude character, are
discontinuous and chaotic, and have mounded or
conical shapes. They grew as sea level rose and
migrated towards land (eastward) as a back-stepping
carbonate shelf-edge. Fraser et al. (2003) have
interpreted these features as volcanic centres.
However, if they are volcanoes they have an unusual
form for oceanic crust. Basaltic volcanic centres in
oceanic rifts are commonly elongate fissures which
erupt low viscosity magmas and do not form conical
edifices. The features are apparently situated on the
tilted fault blocks (Puspita et al. 2005) and have
internal parallel reflectors. They also appear to form
NNE-SSW and NW-SE linear ridges. Because of the
spacing of the seismic lines it is not possible to
484
identify for certain the 3D shape of these structures
but we prefer the hypothesis of carbonate build-ups
on tilted fault blocks to that of volcanic edifices.
Post-rift sedimentation (Early Oligocene to
Pleistocene)
a. Post-rift 1 (Oligocene)
The post-rift 1 package was deposited between the
Top Late Eocene (red) and Top Late Oligocene (blue)
horizons. The seismic character of this package is
typified by parallel to variable, continuous to
discontinuous patterns, and moderate to high
amplitudes. In places it also displays transparent
seismic reflectors probably associated with shale-rich
sections. The change to reflector patterns that blanket
previous topography suggests that the basin
underwent subsidence during the Early to Late
Oligocene as in the Kutei Basin (e.g. Feriansyah et
al., 1999).
Foreland Basin (Early Pliocene to Pleistocene)
On the east side of the basin a fold and thrust belt is
developed. The thrust faults are mostly west verging
and have created north-south trending anticlines
separated by syn-kinematic mini-basins. The minibasins contain localized divergent fill and onlap fill
where the sediments were derived in part from the
erosion of anticlinal crests (Figure 3). In other areas
along strike, the fold and thrust belt is not so well
developed (Figure 7). There, the eastern NMSB has
been regionally uplifted and the unconformity
between the pre-uplift and post-uplift sections can be
seen to coincide with the Top Late Miocene reflector.
The youngest sediments involved in the folding
appear to be very young, perhaps Pleistocene or even
Holocene. Significant volumes of sediment clearly
started to enter the basin from the Sulawesi side in the
Early Pliocene (Puspita et al., 2005).
DISCUSSION
Oblique Rifting
b. Post-rift 2 (Early to Middle Miocene)
There are two principal sedimentary packages
between the Top Late Oligocene (blue) and Top
Middle Miocene (pink) horizons. The lower sequence
is interpreted as progradational and is characterized
by dips that gradually decrease up section. The
prograding system indicates that the basin received a
major influx of sediments from land on the
Kalimantan side of the NMSB from the Early
Miocene (Figure 3). Seismic stratigraphic features
such as mounded forms with bi-directional and unidirectional downlap and discontinuous, transparent
internal reflections characterise the upper (Middle
Miocene) sedimentary package (Figure 7). It is
suggested that gravity flow mechanisms delivered
turbidite deposits into the deeper parts of the basin at
this time.
c. Post-rift 3 (Late Miocene)
The post-rift 3 package is characterized by parallel to
sub-parallel continuous seismic reflectors, consistent
with the continued deposition of deepwater marine
sediments. The post-rift 3 megasequence reflection
package continues to the sea floor in the central parts
of the NMSB, unaffected by the deformation seen on
the east side of the basin.
The Eocene syn-rift fault pattern in the NMSB is
characterized by short rift border faults which are
highly segmented and form en-echelon fault arrays
parallel to the zone of rifting. Several discontinuous
NNW-SSE depocentres developed with the fault
zone. A change in polarity of faults can also be seen
in the study area. Along strike, the faults that
developed within this area seem to be discontinuous.
These observations are interpreted as indicative of
oblique extension of a basement with a pre-existing
fabric. Oblique rifting of a sequence in which there
are pre-existing basement faults leads to a complex
pattern in which new faults are typically not
perpendicular to the extension direction. By
comparison with the analogue models of McClay and
White (1995), the fault system in the NMSB seems
most likely to represent oblique rifting with a
principal extension direction E-W, at an angle of
approximately 60° to pre-existing faults (Figure 8).
This direction is different to the NW-SE extension
direction proposed by Hamilton (1979).
The NMSB is interpreted as an extensional basin
during the Middle to Late Eocene. Following the
cessation of rifting, the basin underwent subsidence in
the Early Oligocene to Late Miocene. There was an
increased influx of sediment into the basin after the
beginning of uplift on Kalimantan during the Late
485
Oligocene and widespread inversion in East
Kalimantan in the Early Miocene (e.g. Feriansyah et
al., 1999) At the beginning of the Early Pliocene,
plate re-organisation caused the formation of a
convergent mountain belt in eastern Indonesia,
including Sulawesi, the Moluccas and the Banda Arc
(Hall, 2002). This compressional event is considered
to be a major force for creating the foreland basin in
the NMSB. East-west seismic lines clearly
demonstrate the westward vergence of the fold and
thrust belt system (Figure 6). The new seismic data
also show that the folding and uplift was initiated in
the Early Pliocene (Figures 6 and 7). It is therefore
interpreted that the North Makassar Straits Basin
became a foreland basin no earlier than Early
Pliocene and it continues as such today.
Nature of the Basement
megasequence, the Plio-Pleistocene foreland basin
megasequence, be seen.
The basement architecture of the NMSB is
characterised by half graben and graben structures
interpreted to have formed during the early rifting
phase of the southeast Eurasian continental margin
(Middle Eocene). The basement faults predominantly
trend NNW-SSE and are arranged en-echelon. They
bound disconnected depocentres and terminate along
strike in accommodation structures such as relay
ramps, interlocking fault tips and zones where fault
polarity reverses. Based on the analogue models of
McClay and White (1995), the extensional fault
systems in the NMSB seem most likely to have
formed as a result of oblique rifting. The principal
extension direction is interpreted as east-west,
forming an angle of approximately 60° with preexisting faults.
We interpret the crust beneath the NMSB to be
continental. At the deepest levels on all seismic lines
half-graben and graben structures can be seen. The
pattern of faults which can be traced into the deepest
parts of the basin suggest oblique rifting, implying the
existence of a pre-existing crust with a structural
fabric which controlled the orientation of faults.
Beneath the Top Basement horizon reflections can
often be seen suggesting a complex structure more
similar to continental basement than oceanic crust.
The volcanic edifices interpreted by other authors do
not have the form expected of oceanic basalt
magmatism. Even if they are volcanoes, which we
doubt, they resemble conical volcanoes developed in
rifted (but not those in which extension has reached
the point of oceanic crust formation) and arc settings.
These structures appear to have a linear character and
resemble tilted fault blocks capped by carbonate
build-ups.
The Middle to Late Eocene syn-rift megasequence
displays a variety of stratal geometries, including
parallel, divergent and prograding, and is interpreted
to have been deposited in a submarine environment.
Carbonate build-ups developed on the top of
structural highs or tilted fault blocks.
CONCLUSIONS
A foreland basin phase was initiated by the uplift of
West Sulawesi from the Early Pliocene and is seen
only on the east side of the basin. This tectonic event
formed a thrust and fold belt system which has moved
progressively westward. It also produced a switch in
sedimentary provenance, from Kalimantan to
Sulawesi, for the eastern NMSB. Very young
sediments are involved in both the anticlines and the
adjacent syn-kinematic sedimentary basins. This
indicates that compression has continued until very
recently and quite probably continues today.
The study of basement architecture and sedimentary
fill of the North Makassar Straits Basin provides the
following conclusions.
The NMSB can be divided into 3 or 4 tectonomegasequences, depending on the location in the
basin. Basinwide, pre-rift, syn-rift and post-rift
megasequences can be interpreted. Only on the
eastern side of the NMSB can the fourth
The post-rift megasequence is sub-divided into three
units: Post-rift 1 was deposited from the Late Eocene
until the Late Oligocene and created a thick shale-rich
section. It represents a period when the basin
underwent thermal subsidence. Post-rift 2 records the
effects of the uplift in Kalimantan after the Late
Oligocene and has been divided into two units. A
prograding delta system forms part of the lower unit.
A turbidite interval is interpreted in the Middle
Miocene section. Post-rift 3 was deposited during
continued subsidence of the basin from the Middle
Miocene until the present day in the central NMSB.
The unit is predominantly deep water shales.
486
ACKNOWLEDGEMENTS
We thank TGS-NOPEC Geophysical Company, and
Peter Baillie in particular, and Migas for permission
to use the seismic data and friends (Simon, Helen,
Paul), and IT staff at RHUL (Mark and Frank). The
MSc programme of Siti Nur’Aini, during which this
project was undertaken, was supported by a British
Council-ENI jointly funded Chevening scholarship.
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TABLE 1
TIME AND DEPTH CONVERSION FOR THE PANCA-1 AND PAMUKAN BAY-1 WELLS
Stratigraphic
Markers
Top Early Pleistocene
Top Late Pliocene
Top Early Pliocene
Top Late Miocene
Top Middle Miocene
Panca-1
(1997)
Depth
TWT
(m)
(ms)
549.0
588.0
1060.7
1116.0
1353.3
1368.0
2286.0
2028.0
4057.0
3108.0
Top Early Pleistocene
Top Late Pliocene
Pamukan Bay-1
(1970)
Depth
TWT
(m)
(ms)
39.0
35.5
48.8
45.0
Top Middle Miocene
Top Early Miocene
Top Late Eocene
Top Basement
79.2
419.4
1503.6
1683.4
Stratigraphic
Markers
76.0
413.5
937.6
1057.8
489
490
Figure 1 - A. The location of the study area and the 2D seismic dataset from four surveys (CM01, CM02, MDD99, and NM93) used for this study.
Important and oil & gas discoveries, and key wells referred to in the text, are also shown. B. Free-air gravity map of the Makassar
Straits Basin based on Sandwell & Smith (1997).
Figure 2a - Panca-1 well tied to the A-A’ regional line.
Figure 2b - Chronostratigraphic chart for the central and eastern parts of the North Makassar Straits Basin.
491
492
Figure 3 - East-west regional seismic line across the study area showing half-graben and graben developed during early extension of the North
Makassar Straits Basin. A is uninterpreted and B is the interpreted line. Different fault colors represent different fault orientations or
generation.
493
Figure 4 - The structure maps for six horizons showing the NNW-SSE lineaments identified in the study area. The white dashed lines are the
lineaments. The orange line shows the present-day thrust front of the deformed sediment wedge on the east side of the Makassar Straits.
W
E
C
C’
TWT
msecs
7048m
D
D’
TWT
msecs
A
W
E
C
C’
0
500
1000
1500
2000
2500
3000
TWT
msecs
3500
4000
4500
N
Back-stepping
carbonate shelf edge
5000
5500
C
D
6000
Mass-flow
deposits
6500
horst
7000
C’
D’
7500
8000
8500
Changing polarity of faults
7048m
0
500
1000
100km
D
D’
1500
2000
2500
3000
TWT
msecs
3500
4000
4500
5000
Back-stepping carbonate shelf edge
5500
Seabed
6000
6500
Top Late Miocene
horst
7000
B
7500
8000
8500
9000
7089m
7089m
Top Middle Miocene
Top Late Oligocene
Top Late Eocene
Top Basement
Figure 5 - Two parallel seismic lines (C-C’ & D-D’) illustrating the changing polarity of faults along strike.
A: uninterpreted.
B: interpreted. Different fault colors represent different fault orientations or generation.
494
Figure 6 - Line F-F’ showing the syn-kinematic basin fill (onlap fill and divergent fill) produced during thrusting and folding in West Sulawesi
from the Early Pliocene. The NMSB became a foreland basin above the brown Top Late Miocene horizon.
495
Figure 7 - Seismic line G–G’ displays the prograding fill and divergent fill that developed within the syn-rift
packages and the turbidite lobe systems developed in the post-rift 2 unit during the Early to Middle
Miocene. Sediment from Sulawesi started to fill the basin during the Early Pliocene.
496
497
Figure 8 - Fault distribution at Top Basement level compared to faults produced in analogue models of orthogonal extension (α = 90°) and oblique
extension (α = 60°). Analogue models are from McClay & White (1995).