The Geology of Indonesia/Makassar Strait

Contributors: A. Guntoro, H. Darman

The Makassar Strait, located along the eastern margin of Sundaland, between Kalimantan and Sulawesi, forms a distinct physiographic border between the western Indonesia stable cratonic landmass and the complex collage of the eastern Indonesia archipelago. It has been a focus of attention of scientific community since at least the nineteenth century, when Wallace (1864) established the Wallace Line longitudinally along the strait. The line is a biodiversity boundary between the Asiatic fauna in the west and the Australian fauna in the east and southeast. The Makassar strait is bounded towards the north by the long lateral Palu-Koro fault, which separates this basin from the Sulawesi sea. The Makassar strait is divided into the North Makassar and South Makassar basin by another lateral fault, so called Paternoster fault. The occurrence of these two faults is clearly reflected by the steep gradients indicated by the bathymetric contours A thick sequence of relatively undisturbed Neogene and probably Paleogene sediments showing good lateral continuity were deposited in the basin.

Tectonics
The Makassar Strait occupies the continental shelf, slope and rise areas between the islands of Kalimantan and Sulawesi (Figure 27-1). This region is situated between the cratonised Paleozoic and Mesozoic rocks of the Sunda Shield in the west and the late Tertiary volcanic arc of Sulawesi in the east. The latter can be classified as a continental margin of the Pacific type due to its tectonic mobility (Beck and Lehner, 1974). The strongly positive isostatic gravity anomaly over the Makassar trough which was recognised by Vening Meinesz (1954) and confirmed by Mobil’s marine reconnaissance survey (1970-1971) and Schwartz et al. (1973), has led to the conclusion that oceanic crust may underlie the trough. According to these authors, it is uncertain whether oceanisation of the trough resulted from tensional rifting or was due to compressional stresses.

Much of the evidence supports the first interpretation. The deep offshore seismic survey performed by Total-CFP over the Makassar Strait in 1974 showed that no features characteristic of a subduction zone occur at the northeaster edge of the abyssal plain of the Strait. A hypothetical triple-junction rift-system is proposed, to explain the oceanisation of the crust in the Makassar trough (Figure 27-4). The sequence of formation of a divergent triple-junction was discussed by Thompson (1976, fig. 15), and was applied to explain the origin of the Mahakam Delta complex in East Kalimantan (Weimer, 1975) in terms of aulacogen. Crustal upwarping is though to have occurred along the East Kalimantan continental margin followed by fracturing and the formation of three rift arms. The less active east-west rift arm was further developed as a graben (Melawi and Ketungau basins), while the more active north-south rift arm caused South Sulawesi to drift eastward, resulting in early sea-floor spreading. New oceanic crust was then formed in the area which now underlies the present Makassar trough (Weimer, 1975).

The similarities between Cretaceous basement rocks and the overlying Eocene-Oligocene section between south-eastern Kalimantan and southwestern Sulawesi (Hamilton, 1974), suggest that the rift-system probably opened during mid-Tertiary time. Murphy (1976), suggested that the South Arm of Sulawesi was a continental splinter rafted from the pre-Tertiary Sunda core; the similarities between the shapes of the coastlines from Palu to the south in Sulawesi, and from Sangkulirang to the south in Kalimantan support this hypothesis. The North Makassar and the South Makassar basins can be classified as marginal seas (Murphy, 1975) based on the fact that the Makassar trough is underlain by oceanic crust, and is flanked to the west by the Asian continental margin and to the east by the volcanic arc of Sulawesi.

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

Acoustic Basement (Seismic Sequence 1)

The oldest recognised seismic sequence is characterised by an absence of reflections and is interpreted as acoustic basement. The contact with the overlying sediments is difficult to trace, especially in the eastern segments of Line PAC201 where it is obscured by diffractions and multiples. This contact is marked by H1 but, in general, it can only be identified at a few locations. To estimate the basement depth, interval velocity 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, where horizon H1 was not seen as it lies deeper than the maximum time recorded (8s TWT). The horizon shallows to the west and is displaced by normal faults, forming half-graben structures. The top of sequence 1, which is the pre-Tertiary basement, consisting of Cretaceous gabbros and dolerites in TT-1 and TT-2 well.

Syn-rift unit (Seismic Sequence 2)

Unconformably overlying Seismic Sequence 1 in Seismic Sequence 2. This sequence is characterised by parallel-subparallel reflectors, with poor to fair continuity and low to medium amplitude. Reflection geometry suggests a concordant sequence boundary relationship at the top, and onlap at the base, against H1. These reflection characteristics are interpreted as indicating a shelf depositional environment and well data (TT-1 and TT-2) indicate Late Eocene age. The thickness of the sequence varies, suggesting infilling 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-tectonic activity. The top of this syn-rift sequence (Seismic Sequence 2) is designated H2, marks the end of the rifting phase, which followed by basin subsidence and the deposition of post-rift sediments. The opening of the Makassar Strait can be related to the deposition of Sequence 2.

Post-rift unit

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

Seismic Sequence 3

This sequence is bounded by horizon H2 and H3, and exhibits parallel to subparallel bedding, with poor to fair continuity and high to medium reflection amplitude; in some parts amplitude; 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 reflector characteristics can be taken as indicating a shelf margin depositional environment which is equivalent to the Lower Oligocene conglomeratic limestone.

Seismic Sequence 4

This sequence is bounded by horizons H3 and H4, and is dominated by parallel and locally sub-parallels reflections, with fair to good continuity and medium to high reflection amplitude. The unit is characterised by the presence of local mound-like reflector patters which are interpreted as carbonate mounds. A correlation to TT-2 and TT-1 well indicate Early Miocene age at the top of this sequence. The upper boundary is marked by toplap to horizon H5. The reflector characteristics are classified as indicating a shelf to shelf margin depositional environment.

Seismic Sequence 5

The Seismic Sequence 5 is bounded by horizons H4 and H5 and displays parallel configurations with fair to good continuity and medium to high reflections amplitude. This sequence is equivalent to the Early to Middle Miocene deep marine shales and marls in well TT-1 and TT-2. Discontinuous reflectors are present in shallower part with low to medium amplitude, whilst continuity is observed with medium to high amplitude. These reflection characteristics are typical of shelf depositional environment and indicate a shallow marine shelf deposit. The unit can still be recognized in the eastern segment, although this region is distorted by thrust faulting.

Seismic Sequence 6

This sub unit is bounded by horizons H5 and H6 and shows parallel configurations with good continuity and medium to high reflection amplitudes. The reflection characteristics are classified as indicating a shelf depositional environment. In the eastern segment, the sequence can be subdivided into two sub-sequences confined to local basins in which horizontal reflectors on lap to the top of Horizon H5, and this sub unit was deposited as onlapping fill. Seismic sequence 6 is equivalent to the Pliocene shallow marine limestone of TT-1 and TT-2 well.