Those mechanisms are part of extensional and convergent global geotectonic events to which are related platform, fore-and back-arc basin sedimentation, and occurrence of volcanism. Offshore North Java, some extensional, half-graben and graben-like, transverse depressions, which are among the richest oil-provinces in the country (Sunda Basin, Arjuna Depression), locally extend to the land area where they merge into east-west back-arc basins.
WEST JAVA TECTONIC SETTINGThe West Java region currently marks the transition between frontal subduction beneath Sumatra, to the west. However, the region has been continuously active tectonically since rifting in the Eocene. The Eocene rifting, as throughout SE Asia, was probably related to the collision between India and Asia (e.g. Tapponier et al. 1986) and involved a significant influx of coarse clastic sediments. The Oligocene-Recent history is more dominated by subduction-related volcanism and limestone deposition. In general, West Java may be subdivided into the following tectonic provinces: (see Figure 4.2; modified after Martodjojo, 1975; Lemigas, 1975, and Keetley et al, 1997) • Northern basinal area: A relatively stable platform area, part of the Sundaland Continent, with N-S trending rift basins offshore and adjacent onshore, filled with Eocene-Oligocene non-marine clastics, overlain by Miocene and younger shallow shelf deposits. • Bogor Trough foreland basins composed of Miocene and younger sediments mostly deeper water sediment gravity flow facies. Young E-W trending anticlines formed during a recent episode of north-directed compressive structuring; • Modern Volcanic Arc: Active andesitic volcanism related to subduction of Indian Oceanic Plate below Sundaland Continent (Gede-Panggrango, Salak, Halimun, etc., volcanoes). • Southern slope regional uplift: mainly Eocene-Miocene sediments, including volcanic rocks belonging to the Old Andesite Formation. Structurally complex, N-S trending block faults, E-W trending thrust faults and anticlines and possible wrench tectonism. South-West Java contains a number of sedimentary basins that formed within the axial ridge and in the area between the volcanic arc and submerged accretionary prism associated with the northward subduction of the Indian Oceanic Plate. • Banten Block: The most western part of Java Island which may be subdivided into Seribu Carbonate Platform in the north, Rangkas Bitung sedimentary sub-basin, and Bayah High in the south. In the west there are minor low and highs so called Ujung Kulon and Honje High, and Ujung Kulon and West Malingping Low (Lemigas, 1975; Keetley et al, 1997).
Fig 2. Summary of west Java tectonic map ( from different sources)
NORTHWESTERN BASINAL AREA TECTONIC FRAMEWORK
The Northern offshore and adjacent onshore basinal area comprises two major basins so called North West Java Basin and Sunda-Asri Basinal area (Fig.3). The northern part of this area is dominated by extensional faulting with very minimum compressional structuring. The basins were dominated by rift related fault which contain several depocentres. In the NW Java Basin the main depocentres are called the Arjuna Basin North, Central and South and the Jatibarang Sub-basin. The depocentres are dominantly filled with Tertiary sequence with thickness in excess of 5,500 meters. The significant structures observed in the northern basinal area consist of various type of high trend area associated with faulted anticline and horst block, folding on the downthrown side of the major faults, keystone folding and drape over basement highs. Rotational fault blocks were also observed in several areas. The compressional structuring were only observed in the early NW-SE rift faults. These faults were reactivated during Oligocene time forming several series of downthrown structure associated with transpresional faulting in the Sunda area.
Although the Northwest Java basin area is currently positioned in a back arc setting, the West Java Sea rift systems did not form as back-arc basins. Extension direction fault patterns and basin orientation of the Northwest Java basins suggest that the sub-basinal areas are pull-apart basins at the southern terminus of a large, regional, dextral strike-slip system; i.e. the Malacca and Semangko fault zones propagating down to the west flank of the Sunda craton. Through both Eocene-Oligocene rift phases, the primary extension directions were NE-SW to E-W. Two observations support the interpretations that these basins are not back-arc related; 1) the extension direction for the WJS rifts is nearly perpendicular to the present subduction zone, 2) a thick continental crust is involved (Hamilton, 1979).
The NW Java depression is asymmetrical, with its deepest Arjuna Sub-basin lies at the foot of the Arjuna Plateau, separated by a major N-S trending fault. The basin opens southward into the onshore Ciputat, Pasir Putih and Jatibarang Sub-basins, separated by the Rengasdengklok and Kandanghaur – Gantar Highs, respectively. The sub-basins are characterised by the presence of alternating highs and lows bounded by extensional deep-seated faults which were active during sedimentation.
The Jatibarang Sub-basin( fig. 3) is bounded by the Kandanghaur - Gantar- horst-block to the west, and the Cirebon fault, east and north-eastwards. This major growth-fault is responsible for an important accumulation of Tertiary rocks including the Jatibarang volcanics, in the Jatibarang Sub-basin.
The Vera Sub-basin is a deep Mesozoic and Tertiary depression NE of Arjuna Sub-basin. This sub-basin is bounded by some major faults, especially to the south. The structures orientation is SW and SSW, similar to the direction of the Billiton Basin where Mesozoic (?) sediments are also known.
The Sunda-Asri basinal area consists of Sunda and Asri basin. This structural element is the westernmost basin of the northern basinal area of West Java. The Sunda Basin is a roughly northsouth depression with its main depocenter, the Seribu half graben, at its eastern edge, separated from the Seribu platform by steep flexures and faults. To the west, the basin is bounded by the Lampung High, to the south by the Honje High and to the north the Xenia arch separates the Sunda Basin from the Asri Basin. The Sunda Basin is the deepest basin in the northern basinal area of Java, where the basement is more than 3.8 second TWT, in the downthrown block of the Sunda/Seribu fault. A series of normal faults dissect the area in small horst and graben features.
The Asri Basin, located to the northeast of the Sunda Basin, is the second deep basin in the region with basement as deep as 3.0 sec. TWT. It is limited from the Sunda platform eastwards by a major normal fault. To the northwards and westwards, it is bordered by steep gradients and is dissected by normal faults.
Fig. 3. Half graben sub basin/depocenters within the Sunda, Asri
and NW Java BAsin areas ( Kohar et al, 1996)