6.7 CORRELATION OF WELL DATA WITH SEISMIC SEQUENCES
6.9.3.3 Gravity modelling FAC
This model has been prepared along seismic line PAG 202, but is extended up to the coast o f Laut Island on the Kalimantan side and to the coast o f Sulawesi. The result of modelling is shown in figure 6.12.
The lowest free-air anomaly, of about -40 mGal, occurs in the middle o f the basin, with steep gradients towards the continental shelves of Kalimantan and Sulawesi. The
minimum free-air anomaly coincides with the centre o f the basin, where the water depth is about 2 km and where there are about 8 km of sediments. The free-air anomaly increases to as much as +70 mGal towards Kalimantan and Sulawesi. The depth of the upper mantle beneath the shelves off Kalimantan and Sulawesi is about 25 km but in the centre of the basin is only about 15 km.
LA
Seism ic profile PAC 201
G e n e r a t e d w i t h GM-SYS 100 G r a v i t y fO C D E -M > (U
^
- 100 0 = O b s e r v e d , - = C a l c u l a t e d North M akassar BasinThe extent of seismic line PAC 201
0 5 10
Crust
D=0
15 Q . 20 QUpper mantle
D=+0.4
25 30 31 19 69 119 169 219 269 319 V. E. = 3 . 1 3 D i s t a n c e (Km)Seism ic profile PAC 202 G e n e r a t e d w i t h GM-SYS 100 G r a v i t y 00 >> +J > fO s_ CD
0 = O b s e r v e d , - = C a l c u l a t e d South M ak assar Basin
- 10 0
The extent of seismic line PAC 202
0 Sea water, 5
Sediments
D=-0.37
10Crust
D=0
15 4 - > a . 20 QUpper m antle
D=-t-0.4
25 30 36 14 64 114 164 214 264 3 1 4 364 V. E. = 3 . 6 3 D i s t a n c e (Km)6,9,3.4 Gravity modelling MCP 5
This model has been prepared along a line which crosses the Makassar Strait in a nearly E-W direction at latitude 20°S, i.e. along line seismic MCP 5, and is 275 km long. The thickness o f sediments in the model was estimated using the average velocity from seismic line PAG 201, which was situated close to MCP 5. The result of modelling is shown in figure 6.13. The lowest free-air anomaly of -30 mGal coincides with the deepest water (2.1 km) and with the thickest sediments in the centre of basin (6.2 km). To the west, towards Kalimantan, the free-air anomaly increases sharply, reaching +50 mGal. This high free-air anomaly coincides with, and may be caused by, the basement uplift interpreted from the seismic profile. To the east of the axial trough, towards the shelf off Sulawesi, the free-air anomaly also increases sharply to +50 mGal. This increase is probably associated with a basement complex uplift.
The depth to the mantle is 28 km off Kalimantan and 25 km off Sulawesi; beneath the axial trough the depth decreases to 17 km.
6.10 CRUSTAL STRUCTURE
Deep water areas in the Makassar Strait correspond to areas of low free air anomalies, shallow bathymetry corresponds to areas o f high free air anomaly. Gravity modelling shows, however, that the Moho is shallow beneath the axial trough and deepens towards the shelves (Fig.6.10-13) 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 he crustal thickness changes are due to deformation by extensional thinning. The crustal thickness in the axial region indicates thinned continental crust. It is, however, still not certain whether the extension has generated oceanic crust. Seismic refraction data in the South Makassar Basin show basement velocities ranging from 3.56 to 5.69 km/sec (Prasetyo 1990), 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 formation of oceanic crust in the south Makassar Basin.
Situmorang (1982) calculated that the Makassar Basin stretching factor was between 2 and 2.9 and that the basin had not yet developed oceanic crust. In contrast, the dolerites
4^ 00 Seismic profile M CP 05
W 0^
Sea water Basement 100 G r a v i t y G e n e r a t e d w i t h GM-SYS to <T3 CD E O- 0 = O b s e r v e d , - = C a l c u l a t e d -50North M akassar Basin
0 Sea water. Sedim ents D=-Q.37 5 10
Crust
D=0
15 CL 20 QUpper mantle
D=+O.A 25 30 28 22 72 122 172 222 272 V. E. = 2 . 8 2 D i s t a n c e (Km)and gabbros in Well TT 2 in the Makassar strait are typical of an ophiolite sequence, suggesting the presence of oceanic crust. The present gravity models also suggest typical oceanic crust thicknesses. In view o f the well data and gravity models, it is suggested that the central part of the Makassar Basin is underlain by oceanic crust.
6.11 TECTONIC IMPLICATIONS
Seismic reflection surveys and gravity modelling, as outlined above, support an Eocene extensional model for the Makassar Basin. Prior to extension the region is thought to have suffered 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 collision is thought to have thickened the crust, as normally happens in
compressional regions.
From seismic reflection interpretation, 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 stratigraphie analyses suggest that the Makassar Basin has subsided slowly and has experienced continuous sedimentation since the Eocene. This argument is supported by the depositional environment data from Wells TT 1 and TT 2, which showed continues sedimentation during the Tertiary. The sediments were deposited in a near shore 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 extension in the Makassar Strait can be explained by a sinking of the subducting plate east o f western Sulawesi, leading to trench roll-back. 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. The time of this trench roll-back marks the cessation of subduction. Seismic reflection surveys (Figs. 6.5 and 6.6) rarely image effects o f igneous intrusion, except on line PAC 202 at SP 600 - SP 800. Rifting was accompanied by the deposition o f syn-rift sediments (Seismic
sequence 2, PAC 201 and PAC 202). In the Early Oligocene, the rifting may have terminated and have been followed by the deposition of post-rift sediments (Seismic sequences 3 to 6, seismic line PAC 201 and 202).
Young compressional zones (thrust faults) affecting strata up to horizon H5
(Mid-Miocene) formed a foreland basin as a crustal flexure in the eastern part of the Makassar Basin (Line PAC 201). This phenomenon can also be seen in the Kutai Basin (Biantoro et al. 1992), to the west of the Makassar Strait. The fact that the Makassar Basin is flanked by these two east-dipping thrust faults is difficult to understand because the basin is not disturbed at all, and on PAC 201 the thrusts die out from west to east, as in the Kutai Basin. Biantoro et al. (1992) suggested that anticlinorium and thrust faults in the Kutai Basin are due to the interaction o f two major strike-slip faults; the
Sangkulirang and Paternoster faults. The same explanation is also suggested for the formation of thrust faults in the eastern part of the Makassar Strait since the thrust faults observed on the seismic profiles also occur between these two major strike-slip faults. The collision of the NW Australian Continental margin with the Sunda Trench and Banda Arc is thought to be responsible for the formation o f the major dextral strike-slip Paternoster and Sangkulirang faults.