BONE BASIN - SOUTH CELEBES

BONE BASIN, SOUTH CELEBES

Sulawesi (Celebes) Island is one of the five largest islands in the Indonesian archipelago, having a distinctive shape like the letter "K". Sulawesi Island is located in an area with complex tectonics in the meeting zone between the Eurasian, Indo-Australian and Pacific plates (Hamilton, 1979; Silver et al., 1983).

Sulawesi Island is formed from a tectonic zone that traverses north-south (Sukamto, 1975). The zones are started from west to east namely West Sulawesi Tertiary Volcanic Bow, Minahasa-Sangihe Quaternary Volcanic Bow, Central Sulawesi Cretaceous-Paleogene Metamorphic Path, East Sulawesi Cretaceous Ofiolite Path and association of cover pelagic sediments and continental micro fragments of the Paleozoic continent originating from the Plate Continent of Australia). Contact between these tectonic provinces is large faults (Figure 2).

In the northern part of the island of Sulawesi there is a Trench in North Sulawesi formed by subduction of Sulawesi's oceanic oceanic crust, while in the southeast part of Sulawesi there is a Tolo Fault triggered by subduction between the southeast arm of Sulawesi Island and the northern Banda Sea, where the two main structures are connected by Palu fault -Koro and Matano In the western part of Sulawesi there is the Makassar Strait that separates the western arm of Sulawesi with the Sunda Bow which is part of the Eurasian plate which is thought to have formed from the expansion of the ocean floor in the Miocene period, while in the east there are continental fragments which move from sliding faults New Guinea (Hall and Willson 2000, in Armstrong, 2012).

Location of Bone Basin, Sulawesi (modification from Camplin and Hall, 2014).

The Bone Basin is located in the Gulf of Bone (Figure 1), where the western and eastern parts are limited by the Arm of West Sulawesi and the Arm of East Sulawesi, the northern part is bordered by Central Sulawesi and the southern part is bordered by the Java Sea. Bone Bay covers an area of ​​about 30,000 square kilometers. The water depth in Bone Bay ranges from 200 to 3,000 meters. The Bone Basin is cut by several faults such as; The Palu-Koro Fault, and the Walanae Fault, and flanked by two heights namely Bonerate high in the west and Kabaena high in the east, resulting in various types of rocks mixed so that the stratigraphic position becomes very complicated.

In 2011 the Marine Geological Research and Development Center (P3GL) conducted geological and geophysical surveys in the waters of the Gulf of Bone (Figure 1) using the Geomarin III Research Ship (Sarmili, 2011). The main focus of this paper is to interpret the sedimentation process in Bone basins based on reflected seismic data. The difference in the reflector character in this reflective seismic will be interpreted as a reference for any type of sediment and its stratigraphic relationship in the deposition process.

 Sulawesi Geological Map (Hall and Wilson, 2000)

Bone Basin is divided into 6, (six) rock units according to Yulihanto (2004), namely claystone unit A, limestone unit B, volcanic rock unit C, limestone unit D, sandstone unit E and the youngest unit sediment filler F (Channel-filled sediment) .

Sequence A is equated with claystone which is the lowest sequence of the Bone basin and has the oldest age based on the appearance of the Malawa Formation in the Western Arm and the pelagic and ofiolite sediments in the East Arm in the proportional column (Figure 12), ie aged Eocene with this unit thickness reaching 450 meters with a depth of around 3986 meters below sea level.

Sequence B is equated with Limestone which is estimated to represent units from the Tonasa Formation and the Oligocene Tampakura Formation (Figure 11) with a thickness of variation reaching 900 meters with the deepest depth around 3407 meters below sea level. In this sequence, a basin with an elongated shape with a wider southern part than the northern part is seen.

Sequence C is equated with Volcanic Rocks (Figure 12) which are units representing the Camba and Molasa Formations of Sulawesi Langkowala Formation (lower to middle Miocene). In this basin there are several sediment thicknesses which reach 200 meters with a depth of about 3114 meters below sea level.

Sequence D which is equated with Limestone (Figure 12) represents the Tacipi Formation and belongs to the Sulawesi Molasa Eemoiko Formation which is middle Miocene. Has a sediment thickness of up to 700 meters. with a depth of about 3021 meters below sea level. This sequence forms a longitudinal basin with a north-south direction, where as the northward direction, the sediment is getting shallower and vice versa the southern direction the sediment gets deeper.

The Sandstone E Unit represents the deposits of the Walanae and Molasa Formations of Sulawesi. This unit has a thickness of up to 1000 meters at depths reaching 3278 meters below sea level. In this sequence there are also Quaternary basins which are at the top of the sequence and filled with new deposits. This sequence morphology has an elongated oval shape with the deepest part in the south and shallow in the north.

 

Bone Basin Stratigraphy

Neogen History of the Bone Basin

Gulf Bone has a complex history of formation in Neogene times, this formation is dominated by extension processes. The basic rocks of Bone Bay do not originate from the oceanic plate, but from some pre-Neogenous rocks. In the west it is composed of volcanogenic rocks, in the north composed of low-level and ofiolite metamorphic rocks, and in the east is composed of metamorphic and ultramafic rocks.

Basins began to form in the Early Miocene, the age of rock units also showed that the extension process began in the Middle Miocene even though the explosion had begun since the Early Miocene. Bone Bay is divided into several sub-basins and heights. Tinggian in the Gulf of Bone is a reflection of the shear fault zone which runs west west northwest-southeast. The direction of the shear fault is influenced by the bedrock structure, the active time of the shear fault also varies. The shear fault zone associated with Tinggian Basa has been active since the beginning of the basin formation, while the shear fault zone associated with Tinggian Kolaka has a younger age. This is evidenced by the movement in the Kolaka Fault which is on land in the Late Miocene to the Pliocene.

Faults that limit the sub-basin have an orientation north-west-south-southeast. These faults have striking vertical components so that they can be indicative of horizontal shifts. One of the faults is the Walanae Fault found in South Sulawesi and continuously up to Selayar Palung. The fault has been identified as a sliding fault, this is accompanied by a vertical component which is also found in Selayar Trench.

The sediments found in Bone Bay originate from the north, east and west of the basin. At the boundary of the basin there are carbonate deposits which are associated with deep sea deposits found in the center of the basin. The inconsistency between Unit D and E with the unit of rock above becomes a sign when Sulawesi Island undergoes a lift while at the same time Bone Bay experiences displacement. Movements in the Walanae Fault Zone and Bonerate Fault Zone cause inversion and leverage in the basin. These events caused siliciclastic sediments from the northern basin to enter the Gulf of Bone, this was followed by the formation of the southern-leading Bone Gorge, formation of exposure to carbonate rocks which showed a form of drowning at the edge of the basin, and back-stepping carbonate rocks.

References:

• Armstrong, F. S., 2012. Struktur Geologi Sulawesi. Institut Teknologi Bandung.
• Bemmelen, R.W.V., 1949, The Geology of Indonesia, vol. I A, Government Printing Office, The Hague.
• Camplin, D.J. dan Hall, R. 2013. Insight into the Structural and Stratigraphic Development of Bone Gulf, Sulawesi. Proceedings Indonesian Petroleum Association, 37th Annual Convention and Exhibition May 2013.
• Darman H., dan Hasan F. S., 2000. An Outline of The Geology of Indonesia, Published by IAGI- 2000, h. 101-120.
• Hamilton, W.H., 1970. Tectonic Map of Indonesia.USGS, Denver, Colorado.
• Lili Sarmili et. al., 2016, PROSES SEDIMENTASI CEKUNGAN BONE BERDASARKAN PENAFSIRAN SEISMIK REFLEKSI DI PERAIRAN TELUK BONE SULAWESI SELATAN, JURNAL GEOLOGI KELAUTAN Volume 14
• Sudarmono, 1999. Tectonic And Stratigraphic Evolution Of The Bone Basin,Indonesia: Insights To The Sulawesi Collision Complex. 27thProceedings, IPA Oktober 1999.