The Geology of The Geology of Indonesia/ Indonesia/

Makassar StraitMakassar Strait

►The Makassar Strait, located The Makassar Strait, located along the eastern margin of along the eastern margin of Sundaland, between Kalimantan Sundaland, between Kalimantan and Sulawesi, forms a distinct and Sulawesi, forms a distinct physiographic border between physiographic border between the western Indonesia stable the western Indonesia stable cratonic landmass and the cratonic landmass and the complex collage of the eastern complex collage of the eastern Indonesia archipelagoIndonesia archipelago

►. It has been a focus of attention of . It has been a focus of attention of scientific community since at least scientific community since at least the nineteenth century, when the nineteenth century, when Wallace (1864) established the Wallace (1864) established the Wallace Line longitudinally along Wallace Line longitudinally along the strait. The line is a biodiversity the strait. The line is a biodiversity boundary between the Asiatic fauna boundary between the Asiatic fauna in the west and the Australian in the west and the Australian fauna in the east and southeast. fauna in the east and southeast.

. The Makassar strait is bounded towards the . The Makassar strait is bounded towards the north by the long lateral Palu-Koro fault, which north by the long lateral Palu-Koro fault, which

separates this basin from the Sulawesi sea.separates this basin from the Sulawesi sea. ►The Makassar strait is The Makassar strait is

divided into the North divided into the North Makassar and South Makassar and South Makassar basin by another Makassar basin by another lateral fault, lateral fault, so called so called Paternoster faultPaternoster fault

►. The occurrence of these two . The occurrence of these two faults is clearly reflected by faults is clearly reflected by the steep gradients indicated the steep gradients indicated by the bathymetric contours by the bathymetric contours A thick sequence of relatively A thick sequence of relatively undisturbed Neogene and undisturbed Neogene and probably Paleogene probably Paleogene sediments showing good sediments showing good lateral continuity were lateral continuity were deposited in the basin.deposited in the basin.

TectonicsTectonics

►The Makassar Strait occupies The Makassar Strait occupies the continental shelf, slope the continental shelf, slope and rise areas between the and rise areas between the islands of Kalimantan and islands of Kalimantan and Sulawesi Sulawesi

►This region is situated between This region is situated between the cratonised Paleozoic and the cratonised Paleozoic and Mesozoic rocks of the Sunda Mesozoic rocks of the Sunda Shield in the west and the late Shield in the west and the late Tertiary volcanic arc of Sulawesi Tertiary volcanic arc of Sulawesi in the east. The latter can be in the east. The latter can be classified as a continental classified as a continental margin of the Pacific type due margin of the Pacific type due to its tectonic mobility (Beck to its tectonic mobility (Beck and Lehner, 1974). and Lehner, 1974).

►The strongly positive isostatic The strongly positive isostatic gravity anomaly over the gravity anomaly over the Makassar trough which was Makassar trough which was recognised by Vening Meinesz recognised by Vening Meinesz (1954) and confirmed by (1954) and confirmed by Mobil’s marine reconnaissance Mobil’s marine reconnaissance survey (1970-1971) and survey (1970-1971) and Schwartz et al. (1973), has led Schwartz et al. (1973), has led to to the conclusion that oceanic the conclusion that oceanic crust may underlie the trough. crust may underlie the trough.

►. According to these . According to these authors, it is uncertain authors, it is uncertain whether oceanisation of the whether oceanisation of the trough resulted from trough resulted from tensional rifting or was due tensional rifting or was due to compressional stresses.to compressional stresses.

►Much of the evidence supports Much of the evidence supports the first interpretation. The the first interpretation. The deep offshore seismic survey deep offshore seismic survey performed by Total-CFP over performed by Total-CFP over the Makassar Strait in 1974 the Makassar Strait in 1974 showed that no features showed that no features characteristic of a subduction characteristic of a subduction zone occur at the northeaster zone occur at the northeaster edge of the abyssal plain of edge of the abyssal plain of the Strait. the Strait.

A hypothetical A hypothetical triple-triple-junction rift-systemjunction rift-system is is

proposed, to explain the proposed, to explain the oceanisation of the crust in oceanisation of the crust in

the Makassar trough the Makassar trough

►The sequence of formation The sequence of formation of a divergent triple-of a divergent triple-junction was discussed by junction was discussed by Thompson (1976), and was Thompson (1976), and was applied to explain the applied to explain the origin of the Mahakam origin of the Mahakam Delta complex in East Delta complex in East Kalimantan (Weimer, 1975) Kalimantan (Weimer, 1975) in terms of aulacogenin terms of aulacogen

Crustal upwarping is though to have Crustal upwarping is though to have occurred along the East Kalimantan occurred along the East Kalimantan

continental margin followed by continental margin followed by fracturing and the formation of three fracturing and the formation of three

rift armsrift arms

►The less active east-west rift The less active east-west rift arm was further developed as a arm was further developed as a graben (Melawi and Ketungau graben (Melawi and Ketungau basins), while the more active basins), while the more active north-south rift arm caused north-south rift arm caused South Sulawesi to drift South Sulawesi to drift eastward, resulting in early sea-eastward, resulting in early sea-floor spreading. New oceanic floor spreading. New oceanic crust was then formed in the crust was then formed in the area which now underlies the area which now underlies the present Makassar trough present Makassar trough (Weimer, 1975).(Weimer, 1975).

►The The similarities between similarities between Cretaceous basement rocks Cretaceous basement rocks and the overlying Eocene-and the overlying Eocene-Oligocene section between Oligocene section between south-eastern Kalimantan south-eastern Kalimantan and southwestern Sulawesi and southwestern Sulawesi (Hamilton, 1974),(Hamilton, 1974), suggest suggest that the rift-system that the rift-system probably opened probably opened during during mid-Tertiary timemid-Tertiary time

►Murphy (1976), suggested that Murphy (1976), suggested that the South Arm of Sulawesi was the South Arm of Sulawesi was a continental splinter rafted a continental splinter rafted from the pre-Tertiary Sunda from the pre-Tertiary Sunda core; the similarities between core; the similarities between the shapes of the coastlines the shapes of the coastlines from Palu to the south in from Palu to the south in Sulawesi, and from Sulawesi, and from Sangkulirang to the south in Sangkulirang to the south in Kalimantan support this Kalimantan support this hypothesis. hypothesis.

The North Makassar and the The North Makassar and the South Makassar basins can South Makassar basins can be classified as marginal seas be classified as marginal seas (Murphy, 1975) based on the (Murphy, 1975) based on the fact that the Makassar trough fact that the Makassar trough is underlain by oceanic crust, is underlain by oceanic crust, and is flanked to the west by and is flanked to the west by the Asian continental margin the Asian continental margin and to the east by the and to the east by the volcanic arc of Sulawesi.volcanic arc of Sulawesi.

StratigraphStratigraphyy

►Straigraphy of the basin is Straigraphy of the basin is interpreted base on seismic interpreted base on seismic reflection profiles and Taka reflection profiles and Taka Talu 1 & 2, drilled by Union Talu 1 & 2, drilled by Union Carbide in 1970. The Carbide in 1970. The stratigraphic sequences have stratigraphic sequences have been described by Guntoro been described by Guntoro (1999).(1999).

Acoustic Basement Acoustic Basement (Seismic Sequence 1)(Seismic Sequence 1)

►The oldest recognised seismic sequence The oldest recognised seismic sequence is characterised by an absence of is characterised by an absence of reflections and is interpreted as reflections and is interpreted as acoustic basement. The contact with acoustic basement. The contact with the overlying sediments is difficult to the overlying sediments is difficult to trace, especially in the eastern trace, especially in the eastern segments of Line PAC201 where it is segments of Line PAC201 where it is obscured by diffractions and multiples. obscured by diffractions and multiples.

►. This contact is marked by H1 . This contact is marked by H1 but, in general, it can only be but, in general, it can only be identified at a few locations. To identified at a few locations. To estimate the basement depth, estimate the basement depth, interval velocity data were used interval velocity data were used where available, the boundary where available, the boundary between acoustic basement and between acoustic basement and the overlying sediments being the overlying sediments being placed at depths at which there placed at depths at which there

was an extreme velocity contrast.was an extreme velocity contrast.

►The greatest depths are in the The greatest depths are in the middle of the line, where horizon middle of the line, where horizon H1 was not seen as it lies deeper H1 was not seen as it lies deeper than the maximum time than the maximum time recorded (8s TWT). recorded (8s TWT).

►The horizon shallows to the westThe horizon shallows to the west and is displaced by normal faults, and is displaced by normal faults, forming half-graben structures. forming half-graben structures.

►The top of sequence 1, which is The top of sequence 1, which is the pre-Tertiary basement, the pre-Tertiary basement, consisting of Cretaceous gabbros consisting of Cretaceous gabbros and dolerites in TT-1 and TT-2 and dolerites in TT-1 and TT-2 well.well.

Syn-rift unit (Seismic Sequence 2)Syn-rift unit (Seismic Sequence 2)

►Unconformably overlying Unconformably overlying Seismic Sequence 1 in Seismic Seismic Sequence 1 in Seismic Sequence 2. Sequence 2.

►This sequence is charactrised by This sequence is charactrised by parallel-subparallel reflectors, parallel-subparallel reflectors, with poor to fair continuity and with poor to fair continuity and low to medium amplitude. low to medium amplitude.

Reflection geometry suggests a Reflection geometry suggests a concordant sequence boundary concordant sequence boundary

relationship at the top, and onlap at relationship at the top, and onlap at the base, against H1. These the base, against H1. These reflection characteristics are reflection characteristics are

interpreted as indicating a shelf interpreted as indicating a shelf depositional environment and well depositional environment and well data (TT-1 and TT-2) indicate Late data (TT-1 and TT-2) indicate Late

Eocene age.Eocene age. The thickness of the sequence The thickness of the sequence

varies, suggesting infilling of a varies, suggesting infilling of a

faulted and irregular basementfaulted and irregular basement. .

. This is the basis for inferring that the . This is the basis for inferring that the sediments are rift-related. The faults cut sediments are rift-related. The faults cut the basement but do not disturb the pre-the basement but do not disturb the pre-

tectonic activity.tectonic activity.

►The top of this syn-rift sequence The top of this syn-rift sequence (Seismic Sequence 2) is designated (Seismic Sequence 2) is designated H2, marks the end of the rifting H2, marks the end of the rifting phase, which follwoed by basin phase, which follwoed by basin subsidence and the deposition of subsidence and the deposition of post-rift sediments. The opening of post-rift sediments. The opening of the Makassar Strait can be related the Makassar Strait can be related to the deposition of Sequence 2.to the deposition of Sequence 2.

Post-rift unitPost-rift unit

►Overlying Seismic Sequence 2, Overlying Seismic Sequence 2, which is considered to be a syn-which is considered to be a syn-rift unit, are Seismic Sequence rift unit, are Seismic Sequence 3-6. These sequences have not 3-6. These sequences have not been affected by normal faults been affected by normal faults and are therefore considered to and are therefore considered to be post-rift sediments.be post-rift sediments.

Seismic Sequence 3Seismic Sequence 3► This sequence is bounded by horizon H2 This sequence is bounded by horizon H2

and H3, and exhibits parallel to and H3, and exhibits parallel to subparallel bedding, with poor to fair subparallel bedding, with poor to fair continuity and high to medium reflection continuity and high to medium reflection amplitude; in some parts amplitude; in amplitude; in some parts amplitude; in some parts amplitudes are low. The some parts amplitudes are low. The variation in amplitude, and frequency variation in amplitude, and frequency may indicate a lithological facies may indicate a lithological facies change, which could relate to a change, which could relate to a decreasing rate of subsidencedecreasing rate of subsidence. .

►The lower boundary shows The lower boundary shows downlap to the top of Seismic downlap to the top of Seismic Sequence 2 (Boundary H2). Sequence 2 (Boundary H2). These reflector These reflector characteristics can be taken characteristics can be taken as indicating a shelf margin as indicating a shelf margin depositional environment depositional environment which is equivalent to the which is equivalent to the Lower Oligocene Lower Oligocene conglomeratic limestone.conglomeratic limestone.

Seismic Sequence 4Seismic Sequence 4►This sequence is bounded by horizons This sequence is bounded by horizons

H3 and H4, and is dominated by H3 and H4, and is dominated by parallel and locally sub-parallels parallel and locally sub-parallels reflections, with fair to good continuity reflections, with fair to good continuity and medium to high reflection and medium to high reflection amplitude. The unit is characterised by amplitude. The unit is characterised by the presence of local mound-like the presence of local mound-like reflector patters which are interpreted reflector patters which are interpreted

as carbonate mounds.as carbonate mounds.

►A correlation to TT-2 and A correlation to TT-2 and TT-1 well indicate Early TT-1 well indicate Early Miocene age at the top of Miocene age at the top of this sequence. The upper this sequence. The upper boundary is marked by boundary is marked by toplap to horizon H5. The toplap to horizon H5. The reflector characteristics are reflector characteristics are classified as indicating a classified as indicating a shelf to shelf margin shelf to shelf margin depositional environment.depositional environment.

Seismic Sequence 5Seismic Sequence 5

►The Seismic Sequence 5 is The Seismic Sequence 5 is bounded by horizons H4 and bounded by horizons H4 and H5 and displays parallel H5 and displays parallel configurations with fair to configurations with fair to good continuity and medium good continuity and medium to high reflections to high reflections amplitude. amplitude.

This sequence is equivalent to the This sequence is equivalent to the Early to Middle Miocene deep marine Early to Middle Miocene deep marine shales and marls in well TT-1 and TT-shales and marls in well TT-1 and TT-

2.2.

►Discontinuous reflectors are Discontinuous reflectors are present in shallower part with present in shallower part with low to medium amplitude, low to medium amplitude, whilst continuity is observed whilst continuity is observed with medium to high amplitude. with medium to high amplitude.

►These reflection characteristics These reflection characteristics are typical of shelf depositional are typical of shelf depositional environment and indicate a environment and indicate a shallow marine shelf depositshallow marine shelf deposit. .

►The unit can still be recognized The unit can still be recognized in the in the eastern segmenteastern segment, , although this region is distorted although this region is distorted by thrust faulting.by thrust faulting.

►Seismic Sequence 6Seismic Sequence 6►This sub unit is bounded by This sub unit is bounded by

horizons H5 and H6 and horizons H5 and H6 and shows parallel shows parallel configurations with good configurations with good continuity and medium to continuity and medium to high reflection amplitudes. high reflection amplitudes.