The Geology of Indonesia/Sulawesi Sea
The Sulawesi Sea covers an area of 400,000 km2, with mean water depth of somewhat over 5 km and crustal thickness of about 6-7 km (Murauchi et al., 1973). The basin is bordered on the East by Sangihe Arc and Southern Philippines, on the south by the north arm of Sulawesi, on the west by Kalimantan and on the north by the Sulu Archipelago. Subduction occur in the Cotabato Trench (northeastern) and southern part of the Sulawesi basin (North Sulawesi trench).
Tectonics
editIn the view of Karig (1971), the Sulawesi Sea is a backarc basin, whereas Lee & McCabe (1986) interpret the Sulawesi Basin as a fragment of an ancient larger ocean basin of which Banda, Sulawesi, and Sulu Seas are only remnant, and now divided by mobile collision belts and subduction zones. They identified magnetic anomalies 30-33, trending 65oE and representing crustal ages of 65-72 Ma, indicating a Maastrichtian episode of sea-floor spreading. Hutchison (1989) agreed on Lee & McCabe interpretation for ocean floor basalts (Chert-Spilite Association) occur commonly uplifted on south-east Sabah and the Sulu Archipelago, and adjacent parts of the Philippines, and where dated, they have a Late Cretaceous age, equivalent to anomalies 30-31. The lineations progressively decrease in age towards the NW, continuing the parallel pattern found in the Banda Sea. Their parallelism and regularity suggest that the Sulawesi Sea also represents a piece of Indian Ocean lithosphere, trapped behind younger arc-trench systems. Silver & Rangin (1991) suggested three alternative models for the Sulawesi Sea basin: a fragment of an older ocean basin (e.g. the northern Indian Ocean, West Philippine Sea Basin or the Molucca Sea), a back-arc basin, or a basin formed by rifting from a continental margin (such as SE Asia). Hall (1996) has presented a model for the tectonic history of the region and indicate that Sulawesi Sea Basin and West Philippine Sea Basin were formerly connected. Nichols & Hall (1999) tested this model and conclude that the basement age, chemistry and the nature of the overlying stratigraphic succession are consistent with the formation of the Sulawesi Sea and West Philippine Sea basins as part of the same basin during middle Eocene time (50-37 Ma). This basin formed by spreading behind a north-dipping subduction zone accommodating the northward movement of the Indian Ocean Plate. Spreading in the Sulawesi Sea- West Philippine Sea Basin ceased in the late Eocene and by the end of the late Oligocene the two areas were becoming separated, firstly by strike-slip and later by convergent plate boundaries.
Stratigraphy
editThe stratigraphy of the Sulawesi Sea is based on two holes drilled by ODP Leg 124 (Rangin et al., 1990). The first (site 767) was located in one of the deepest parts of the basin in the northeast in a water depth of 4900 m; continuous coring provided nearly 800 m of sedimentary section down to basaltic basement. The second hole (site 770) was on a shallower are of raised basement in a water depth of just over 4500 meters and was 50 km the NNE of hole 767. A thinner sedimentary succession was encountered (420 m) and this was continuously cored only in the lower 80 m, as the primary objectives of this hole were to sample the basement lithologies. The following stratigraphic description is summarized from Nichols and Hall (1999).
Middle Eocene to Middle / Upper Miocene
The basement encountered in both holes is basalt with a strong MORB affinity, indicated by the geochemistry of the rocks (Silver and Rangin, 1991; Spadea et al., 1996) The lower part of the succession drilled at site 767 consisted of 80 m of brown pelagic claystone containing radiolaria, fish teeth and manganese micronodules. Rates of sedimentation of this unit were slow at 2-6 m/Ma. The same rates of sediment accumulation were found at Site 770 over a similar thickness of strata but deposition is considered to have occurred in shallower water as the preservation of calcareous nannovossils indicates deposition at or above the carbonate compensation depth. The Eocene to Oligocene CCD in the Sulawesi Sea would have been at a depth between these two sites (Smith et al., 1990).
Middle / Upper Miocene
A sharp change in the character of the succession drilled at Site 767 occurs at about 700 m below the sea floor. The brown colored pelagic claystone is replaced by a bioturbated, green-grey hemipelagic claystone, although there is no change in the clay mineralogy at this point (Smith et al., 1990). At about 650 m below sea level, the first distinct turbidites occur in the succession. The majority of the redeposited units are made up of silty claystone and are interpreted as distal fine grained turbidite deposits (Betzler et al., 1991), but there are a number of thicker units (up to 3.6 m) which consist of quartz sand and silt in sharp-based, normally-graded units. Carbonaceous material is common within these quartz sandstones, mostly as finely disseminated coalified plant remains occurring along laminae, but also as larger (over 1 cm) coal clasts. Rates of sedimentation at Site 767 are one to two orders of magnitude higher (up to 109 m/Ma) in this section of the stratigraphy as a consequence of the influx of turbidites. The most likely source of the quartz turbidites would have been from uplifted terrains in southern and eastern Borneo as they are the only substantial area of continental crust adjacent to the Sulawesi Sea at that time. A change in the bathymetry in the Sulawesi, due to the formation of the North Sulawesi Trench in late Miocene caused a diversion of turbidites into the southern part of the basin (Nichols and Hall, 1999) In addition to the influx of siliciclastic turbidites, redeposited limestone beds make up about 10% of this stratigraphic interval. These are a few centimetres to tens of centimetres thick and are typically graded carbonate siltstone and calcareous claystone withsmall amounts of terrigenous clastic material. These carbonate turbidites contain foraminifera and nannofossils which provide age constraints on the upper part of this unit. Neither the quartz-rich or carbonate turbidites were recovered at site 770. This absence of redeposited units may be because the gravity currents were restricted to the deeper parts of the basin, although the discontinuous coring at this site may mean that turbidite units were missed.
Upper Miocene – Holocene
The upper half of the cored stratigraphic succession at Site 767 is late Miocene to Holocene in age. Sediments consist mainly of hemipelagic silts and clays: the silts are of volcanic lithic and crystal fragments, and the clay fraction is dominated by smectite. Thin ash layers occur in the Pleistocene part of the succession. The proportion of carbonate in the sediments is very low (3-4%: Smith et al., 1990) as a consequence of continued residence of this part of the basin below the CCD through to the present day. Sediment accumulation rates for this interval ranges from 33 to 60 m/Ma. Coring was incomplete for this interval at Site 770, but the claystone recovered contained a higher proportion of carbonate as nannofossils and marl, than was encountered at Site 767. Sedimentation at this shallower site was therefore above or close to the CCD during this period.