The Geology of Indonesia/Java & Java Sea
Java, with a backbone comprising a subduction-induced volcano-plutonic arc, is considered classically as the southernmost leading edge of the continental Sunda Plate, overriding the oceanic Australia-Indian plate. In fact, the structural configuration is that of alternating highs and transverse depressions related to a more complex pattern, where discrete crustal blocks can be interpreted as pieces separated from the original monolithic craton. Two dynamic processes interact: • Collision of blocks in Pre-Tertiary times by closing of oceanic gaps is recorded or marked by roughly east-west ophiolitic belts (Ciletuh in West Java, Lok Ulo in Central Java) but the colliding pieces are not clearly identified. • Lateral displacement between blocks in Tertiary times is made by transcurrent faulting, components of large-scale strike-slip movement in response to the plate-convergence process itself. 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. The Java Island and the adjacent Java Sea is divided into two major provinces West and East Java. The dividing line between these two areas is chosen as a meridian-line, roughly joining the Karimun-Jawa Islands to Semarang continuing southwards on land (Fig. 4.1). The south Java outer arc-basin is also included within this chapter.
4.1. WEST JAVA
4.1.1. TECTONIC SETTING
The 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).
4.1.2 NORTHWESTERN BASINAL AREA
126.96.36.199 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. 4.3). 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 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.
The sediments of the West Java Sea basins are grouped into two very distinct sedimentary units which are the rift related sediment fills dominated by nonmarine / continental sedimentary sequences and the post-rift (sag) basin fills dominated by marginal marine and marine sedimentary sequences. In the following discussion, the sediment sequences are divided into five different tectonostratigraphic units based on their tectonic origins (Kohar et al, 1996).
The sedimentary sequence of the North West Java Sea basins rests on a multi-complexes of a Pre-Tertiary basement representing the continental crest of the Sundaland. The basement assemblage (Fig. 4.4) is composed of metamorphic and igneous rocks primarily of Cretaceous and old ages and subordinate limestones and clastic sediments of possible Early Tertiary age. This melange of low-grade meta-sedimentary, igneous, and meta-igneous rocks is the result of subduction-related accretionary processes associated with the Meratus Suture (Fig. 4.1) which was active during the Cretaceous and Paleocene. Metamorphic grade varies widely throughout the sub-basins indurated limestones to low grade metamorphic philites. Based on basement dating, regional metamorphism ended during the Late Cretaceous, while deformation, uplift, erosion and cooling continued into the Paleocene. Late Cretaceous to Paleogene calc-alkalic magmatism occurred throughout onshore and offshore Java due to normal subduction related processes. Andesitic magmatism continued into the early Eocene. Another important igneous event in the West Java Basin, was a Pliocene phase of alkali basalt magmatism which is preserved as either sills or dikes or as volcanic edifices. Based upon the deep going, mostly extensional-fault series, the basinal area could be divided into alternating graben-like sub-basin and positive ridge or platforms. Figure 4.3 displays the basin configuration of the West Java Sea basinal area.
188.8.131.52.2. Early Rift Fill (Paleocene ?/Eocene to Early Oligocene)
The early rift fills include the Banuwati Formation in the Sunda Basin and the Jatibarang Formation in the Arjuna Sub-basin. Continental and lacustrine systems dominated these sequences. The early rift fills are typically composed of immature clastics ranging from alluvial fanglomerate and conglomeratic sandstones to fluviatile sandstones and shales, culminated by anoxic lacustrine shales deposition in the Sunda Basin. Further east, in the Arjuna Sub-basin, the sequence is represented by alternating volcanic clastics and lacustrine clastics composed of andesitic volcaniclastics flow and tuff mixed with basement derived sediments (Gresko et. al.,1995). The early rift fills overlie basement and present in most of the deepest part of the Sunda, Asri and Arjuna Sub-basins. The alluvial fan facies which composed mainly of conglomerates, coarse to medium grained sandstones associated with basin margin fault. Its thickness ranges from 200 m to 30 m in a distance of 3 miles and until finally shales out to the south. It is interpreted that the alluvial fan deposition associated with a NW-SE trending basin margin fault, forms the early rift fill sediments, and progrades into a possible lake environment further south. The fluviatile sandstones and shales facies which onlap the alluvial fan facies. The fluviatile sandstones is interpreted as an axial channel fill if they are associated with alluvial fan and as a braided alluvial plain deposition on the western flank of the early rift graben (hanging wall fill). The third facies is transgressive deep lacustrine facies composed of black shales which covers the entire Banuwati area in the Sunda and Asri basins.
184.108.40.206.2. Syn-rift fills (Oligocene to Early Miocene)
Unconformably overlying the early rift fills is a thick syn-rift fill unit represented by the Talangakar Formation in the west and lower Cibulakan/Talangakar Formation in the east. This unit is present throughout the Nort East Java Basin, filling the series of half grabens of the West Java Sea Basin (Fig. 4.4). The Talangakar is divided into two members, the lower member of Zelda member and the upper member so called Gita member. The syn-rift fills include only the Zelda Member and are of economic importance as primary oil reservoirs in major oil fields (Cinta, Widuri, Zelda, BZZ) in the Sunda, Asri and Arjuna basins. The sequence is Oligocene to Early Miocene in age and dominated by non marine sediments composed of interbedded fluviatile sandstones, shales and coals. Overbank mudstones and occasionally shallow lacustrine mudstones fill the interchannel area. In the Arjuna area coals, limestones and marine shales are also present in the upper part of the syn-rift unit. The coal and carbonaceous mudstones have been typed as the main hydrocarbon source rock for the Arjuna crude (Gresko et. al., 1995, Sukamto et. al., 1995). Maximum thickness of this unit is 2000 m in Seribu Deep Basin and Asri Basin. Age determination is problematic in the syn-rift fill unit as diagnostic pollen and fossils are absent. The age determination was based on the overlying post-rift unit (Upper Talangakar) and the underlying Banuwati lacustrine unit and a thought that this unit has an Oligocene to Early Miocene age.
220.127.116.11.3. Early Sag Basin Fills (Post Rift, Early Miocene to Middle Miocene)
The early sag basin fills represent the overall transgressive setting in the Java Sea area related to the sea level rise during Early Miocene time. At this time the basin boundaries between the subbasins (Sunda, Asri, Hera and Arjuna) were not clearly defined. Basin bounding faults perhaps, were still active locally but subsidence had decreased significantly and rifting had ceased. Consequently, accommodation space was not entirely controlled by the movement of the faults for these post-rift sag successions. The overall depocentre shows a relatively symmetrical, work shape basin throughout the West Java Sea area. Non depositions continue to occur on paleohighs until Baturaja carbonate deposition commenced during Middle Miocene time, forming a bald area for the marginal marine deposition of the early syn-rift fills. The early sag basin fills (post-rift) include the previously described as Upper Talangakar (Gita and marine Talangakar Formation) and the carbonates of the Baturaja Formation and conformably overlie the syn-rift fills throughout the basin (Fig. 4.4). The lithology in the early sag basin fills is composed of interbedded sandstones, siltstones, mudstones and coal, and marine shales overlain by a continue succession of platform to reefal carbonates (Baturaja). The sandstones and reefal carbonates of the early sag basin fill unit contain importance hydrocarbon reservoirs for most of the oil and gas fields in the area. The non marine clastics are dominated by channel, point bar and marine bar sandstones deposited in a wide range of environments from low sinuosity channel on alluvial plain, distributary channel to marginal marine bars. Coals and overbank mudstones and siltstones filled the interchannel area, form an intraformational seal for the prolific fluvial sandstones of the early sag fills unit. As transgressive process continues fluviatile and deltaic sandstones, coals and non marine shales deposition ceased, marine environment gradually advanced onto the highs. Reefal carbonates grew on basement highs (i.e. Krisna, Bima, Rama) forming a fringing reef complex around the highs.
18.104.22.168.4. Main Sag Basin Fills (Middle Miocene-Late Miocene)
The main sag basin fills is dominated by shallow marine (neritic) to nearshore and deltaic facies include the Gumai, Air Benakat and Parigi Formation in the SE Sumatra area and most of the Upper Cibulakan Formation and Parigi Formation in the Northwest Java Basin (Fig. 4.4). During middle Miocene to Late Miocene the overall West Java Sea area were connected forming large sag basin. The lower part of the main sag fills occasionally onlaps the basin flank but by the end of Late Miocene shallow marine deposition covered the West Java Sea area. In the Sunda-Asri area the main sag basin fills are dominated by shallow marine clastics consisting of marine mudstones, calcareous and glauconitic sandstones and thin limestone stringers. The sequence is culminated by extensive platform carbonate deposition with some local carbonate build-up (reef) within the Air Benakat limestones. The Gumai-Air Benakat Formation sandstones are 10 to 70 feet thick and interbedded with shallow marine mudstones, they typically show a coarsening upward sequences. Locally, carbonate build-up also developed in the southern basin margin area. In the Rengasdengklok High/Seribu Shelf near the Northwest Java coastal area a series of thick reefal carbonates (Mid-Main carbonate) developed on a roughly N-S trending parallel to the regional basement fault blocks of the area. The carbonate build up consists of skeletal wackestone and packstone with the main grain constituents are corals, benthonic forminifera, bivalves, echinoderm fragments, red algae and minor quartz and glauconite grains. The age of this carbonate build up is thought to be Middle Miocene (NN5-NN9 age). Shallow marine carbonate sedimentation continued of reefal build-ups in the upper part of the main sag basin fills, previously called the Pre-Parigi and Parigi Formation Shallow marine mudstones, shales and glauconitic sandstones filled the inter-reef and open marine area. The distribution of the Pre-Parigi and Parigi build-ups shows a N-S and NW-SE elongation, these build-ups commonly grew on a basement high or on an underlying Baturaja build-up which caused only a slight topographic elevations (Fig. 4.5). The carbonate build-up comprises a combination of skeletal packstone, wackestone, and grainstone interbedded with mudstone lithofacies. On seismic section the geometry and distribution of these build-ups were clearly identified as well defined sub-elliptical build-ups.
22.214.171.124.5. Late Sag Basin Fills (Pliocene-Pleistocene)
Late sag basin fills represent the latest sedimentary sequence below the present day sedimentation of the West Java Sea area that include the Cisubuh Formation. In the west, the late sag basin fills composed of marine claystone and mudstone and culminated in the continental deposits of conglomerate and volcanic clastic sediments. The continental deposition occurred during the sea level low of the Pleistocene time, approximately 1.5 Ma ago, when the Sumatra and Java Islands were part of the main Sundaland to the north. Sandstones and conglomeratic sandstones interpreted as fluvitile sandstones and volcanic clastic are the main lithology of the Cisubuh continental. To the east, in the Arjuna basinal area, this unit is composed entirely of marine claystone and mudstone with thin sand stringers. Shallow marine deposition continued in the south eastern part of the Sundaland covering the western part of the North West Java Basin.
4.1.3. BOGOR TROUGH
126.96.36.199. TECTONIC FRAMEWORK
To the South of the northern basinal area, the north-south orientation of the structures, sub-basins and high is overprinted by an east-west feature of the Bogor Trough where the influences of the volcano-magmatic and its compressional effect are primordial (Fig. 4.2). The entire Bogor Trough is a thrust-fold belt and towards the north, the system is progressively younger in age, starting from Lower Miocene in the south to Plio-Pleistocene in the north. All sediments supplied from the North are shaling out here. Volcaniclastics were brought from the South. The Bogor Trough extends eastwards to the northern East Java region.
The Bogor Sedimentary Province (Fig. 4.5) is filled by 3 systems of sedimentation including the Ciletuh, Bayah and Jatibarang Formations. The mostly Middle to Late Eocene Ciletuh Formation (1400m) lies on top of a Late Cretaceous to Paleocene (possibly earliest Eocene?) subduction complex composed of mostly dismembered Pre- Tertiary oceanic crust and other rock units. Lower slope turbidites consisting of alternations of both volcaniclastics and conglomerates with fewer intercalations of volcanic and polymict breccia and claystone characterize the Ciletuh deposits. The second system consists of the transitional to shallow marine quartzose sandstones of the Bayah Formation which are also believed to be mainly Middle to Late Eocene in age. Intercalations of claystone and lignite are common. Marine sediments belonging to the Oligocene Batuasih Formation unconformably overlie this unit. These consists of marls, black claystones and shales which partly interfinger with the Oligo-Miocene Rajamandala Formation reefa1 limestones (90m). These are often thought of as equivalents of the Batu Raja Limestone. The third sedimentary system is characterized by volcanic sediment gravity flows. The lowermost of these is the Early Miocene Jampang Formation, consisting of breccias and tuffs up to 1000m thick. The name ”Old Andesite” is frequently used for this unit. Correlative with the Jampang and located further to the north is the Citarum Formation, consisting of tuffs and greywackes up to 1250m thick. These two formations are believed to represent contemporaneous components of the same deep marine fan system, where the Jampang Formation corresponds with the proximal fan deposits, and the Citarum Formation, the distal fan deposits. The Jampang is overlain by the Bojonglopang Formation limestone. In the northern areas of the Bogor Basin the Citarum is overlain by the Middle Miocene Saguling Formation which consists of breccias up to 1500m thick. This is overlain by claystones and greywackes of the Upper Miocene Bantargadung Formation (600m) which is followed by the gravity flow breccias of the Late Miocene Cantayan Formation. The sediments within the first and second systems were derived from the north, while the third system was derived from the south. (Schiller, 1993)
4.1.4. VOLCANIC ARC
The modern volcanic arc is an active andesitic volcanism related to subduction of Indian Ocanic Plate below Sundaland Continent (Gede-Pangrango, Salak, Halimun, etc., volcanoes). Results of previous work in West Java suggest the occurrence of volcanic producs of Late Tertiary magmatic activity; for example Pertamina (1988) recorded a K-Ar age of 12.0+ 0.1 Ma from a calc-alkaline pyroxene-andesite lava which represents part of the basement of the Quaternary Wayang Volcano. Pertamina (1988) study concluded those volcanic rocks in West Java range in age from 4.36+0.04 Ma to 2.62+0.03 Ma suggesting continuous magmatic activity during Pliocene time. The youngest age of volcanic rockwas obtainies from west of Pelabuhanratu (SW Java), where the K-Ar dating of the lava flow is 1.33+0.28 Ma (Soeria-Atmadja et al., 1994). See chapter 4.4 for further details on the magmatic arc.
4.1.5. SOUTHERN SLOPE REGIONAL UPLIFT
The southern mountains, some 50 km wide, extend from Pelabahanratu Bay to Nusakambangan Island. These represent the southern flank of the Java synclinal structure, an uplifted crustal block dipping to the south. The oldest rocks in the southern mountains are schists, phyllites and quartzites into which have intruded ultrabasic rocks. These rocks, which are exposed in the southwestern corner of island (the Jampang), are covered uncomformably by the Ciletuh formation of conglomerates and sandstone of late Eocene to early Oligocene- age (Baumann et al., 1973). Unconformably, on the top of Ciletuh formation, is the Jampang formation of early Miocene age. The Gabon formation in the eastern part of western Java is similar to this Jampang formation. The Jampang formation consists primarily of volcanic sed1ments such as brecciaous marl and clay. The underlying Ciletuh formation has been intruded by quartz porphyry, which might have brought the ore of the Cibitung gold mines (Nishimura & Hehuwat, 1980).
4.1.6. BANTEN BLOCK
188.8.131.52 TECTONIC FRAMEWORK
The Banten Block comprises several structural highs and lows (Fig. 4.2). The Seribu Platform has a rather thin Tertiary section (1.5 sec. TWT) which consists of Baturaja and mostly post-Baturaja sediments, located in the north of the Banten Block. It is separated from the Sunda Basin in the west by the major Seribu fault system, and gently plunges eastwards and northwards into the Arjuna Sub-basin and to the North Seribu basinal area, respectively. The later is a narrow deeper area affected by NS and NW-SE growth faults. Gentle drape over large basement high areas and reefal buildups are the main structures of the platform itself. Its onshore prolongation is known as the Tangerang High, which is separated from the Ciputat Sub-basin by a major NNW-SSE trending fault. The Bayah and Honje Highs are Tertiary structural highs located on the south coast of West Java, Indonesia, situated at the margin of the Malingping Low, the western extension of the Bogor Trough (Fig. 4.2.). The Honje High comprises mainly Miocene volcanoclastics flanked by Pliocene sediments to the west and Eocene strata to the east. Together with the adjacent Sunda Strait strike-slip basin, it probably formed in response to movement along the Sumatra strike-slip fault (Fig. 4.6). In the Sunda Strait and east and west of the Honje horst structure, and north and south of west Java (Malod et al 1996) are a series of moderately dipping half grabens which trend N-S. These are clearly visible on seismic to the south, offshore of the Honje High (Fig. 4.6). The Bayah High comprises large E-W trending anticlines cored by Eocene clean coarse-grained sandstones (Keetley et al, 1997).
The Banten Sedimentary Province consists of 3 main cycles of sedimentation (Fig. 4.5). The oldest part of the first system is dominated by Paleocene? volcanic and igneous rocks equivalent to the Jatibarang Formation. These are overlain unconformably by sha11ow marine to terrestrial deposits belonging to the mostly Eocene Bayah Formation. The lower portion consists of mostly black shales with some larger foram-rich limestone lenses which have been interpreted as prodelta deposits (at least 300m thick). The upper portion of the Bayah Formation consists of quartzose sandstones and pebbly sandstones with thin coal lenses (maximum 110 cm thick). The tota1 thickness of this unit is approximately 800m. The second cycle unconformably overlies the Bayah Formation, and is comprised of volcanic breccias and sandstones with some claystone belonging to the Cicarucup Formation. These are interpreted as breccias deposited as the basal portion of an alluvial fan sequence. These are followed by the mostly Oligocene to Early Miocene limestones of the Cijengkol Formation which are often rich in larger benthonic forams. Sudden massive influx of volcanics from the south consisting of tuffs and breccias deposited by sediment gravity flows belong to the Miocene Cimapag Formation (about 1500m thick). The third cycle is entirely composed of shallow to transitional marine sediments which correspond with the Saraweh and Badui Formations (about 1000m thick). The youngest marine-influenced sediments are from the Middle Miocene Bojongmanik Formation which consists of claystones and sandstones with some lignite lenses. These are unconformably overlain by Pliocene sediments (Schiller, 1993).
4.2. EAST JAVA
4.2.1. TECTONIC SETTING
The structural history of the East Java can not be separated from the structural history of the western part of the island and the tectonics of the SE Asia region. This area is located on the southeastern edge of the Sundaland craton where basement is Cretaceous to basal Tertiary melange. This old continental margin has a northeast to southwest structural trend that is clearly seen on offshore north Java seismic data.
In general, the East Java region can be grouped into five tectonic provinces (Fig. 4.7; modified after Yulihanto et al, 1995), from north to south are: • Northern slope includes the stable Rembang continental shelf and Randublatung transitional zone • Kendeng Trough, the eastern extension of Bogor Trough, a labile deep sea basin. • Modern Volcanic Arc • Southern slope regional uplift
4.2.2. NORTHERN SLOPE
184.108.40.206 GEOLOGICAL FRAMEWORK
The Northern Slope covered the Northeast Java Basin which lies between the Sunda Craton to the north and a volcanic arc to the south (the Java Axial Range). The basin can be classified as a classic back-arc basin. It consists largely of a foreland shelf dipping gently southward, which is covered by a relatively thin stratigraphic section (averaging less than 1850 meters). In contrast, the deep basin area contains more than 9000 meters of sediments. The structural configuration of the western part of the onshore NE Java Basin incluse subbasins with two different orientation. The Pati Trough trends NE-SW, whereas the Cepu and Bojonegoro subbasins are aligned E-W. The NE-SW orientation of the Pati Trough typifies the development of assymmetrical half graben structures (Yulihanto et al, 1995).
The Northern Slope stratigraphy, represented by the Rembang and Randublatung zones are dominated by stable continental shelf to basinal slope sediments. Stratigraphic and structural analyses by Yulihanto et al. (1995) show four depositional cycles within the Tertiary sediments of this area: a Late Oligocene-Early Miocene extensional phase, followed by Early Miocene basin subsidence, a Middle Miocene extentional phase, and Upper Miocene-Pliocene basin subsidence (Fig. 4.8).
220.127.116.11.1. Late Oligocene - Early Miocene extensional phase
The initial extensional phase is characterized by the formation of NE-SW oriented asymmetrical half grabens. These occur in association with left lateral motion along a NE-SW fault system that can be traced from the NE Java Basin across to south Kalimantan (Barito and Asem-Asem basins). Three depositional sequences can be recognized in this phase (Figs. 4.8):
1. Ngimbang Formation - lowstand systems tract: the early phase of deposition started with the Late Oligocene-Early Miocene sea level drop and includes a basin - floor and progradational slope complex. Basin floor deposits formed mainly by carbonate debris - flows resulting from the collapse of the eastern margin fault scarp. The progradational complex developed during the final phase of eustatic drop and consists of wacke - packstone lenses.
2. Kujung Formation - transgressive systems tract: the Late Oligocene-Early Miocene sea level drop was followed by a rise in relative sea level. The associated transgressive systems tract consists of fine grained sediments in the lower part of the Kujung Formation. The dominant lithology is marl interbedded with thin bedded green fossiliferous sandstone and limestone, and it contains larger forminifera, algae, and coral debris. In the upper part of the Kujung, the monotonous marl is intercalated with bioclastic limestone. At the type locality, the Kujung is 500 m thick. It was deposited in a deep, open marine environment during the Late Oligocene.
3. Prupuh Formation - highstand systems tract: The final extensional phase is topped by bioclastic limestone of the Prupuh Formation. It consists of interbedded reefal bio-clacarenite, bio-calcilutite, and blueish gray marl. These accumulated in outer neritic environments during the Late Oligocene.
18.104.22.168.2. Early Miocene basin subsidence phase
Early Miocene subsidence developed a ramp-type depositional platform (Figs. 4.8). Sedimentation began in the Early Miocene with progradation of a fine grained complex of lower shoreface or offshore deposits in a lowstand systems tract (Tuban Formation). These may be associated in some places with development of incised valley fill. A transgressive phase accompanied the subsequent sealevel rise, with accumulation of fine grained shale and marl in the Tawun Formation. Basinal subsidence closed in the Early Miocene with accumulation of bioclastic limestone in a highstand systems tract (upper part of Tawun Formation). The type locality of this formation is in Tawun Village and its thickness is about 730 m. The lower part of the formation is dominated by black-gray claystone and marl, changing gradually upward to gray siltstone. The siltstone intercalates with bioclastic limestone, consisting of orbitoid wackstone-grainstone with large forams, coral fragments, algae and molluscs. An upward increase in the bioclastic content of the limestone indicates an isolated shallow marine environment.
22.214.171.124.3. Middle Miocene extensional phase
The Middle Miocene extensional phase is characterized by formation of a NE-SW asymmetric half graben, which appears to have migrated eastward from the Late Oligocene-Early Miocene graben (Fig. 4.8). This second extensional phase is interpreted to result from rejuvenation of NE-SW left-lateral fault movement due to Middle Miocene oblique subduction of the oceanic Wharton plate under the continental Sunda plate. Four depositional sequences developed during this phase: (Tim Studi Cekungan Tersier, 1994; Figs. 4.8). The first sequence consists dominantly of slope-front fill seismic facies, which are interpreted as slope-fan deposits of a lowstand system tract. It can be correlated with the lower part of the Ngrayong Member. Subsequent sea-level rise resulted in development of a transgressive system tract, including beach to shallow open marine deposits in the middle part of the Ngrayong Member(Figs. 5-9). Sea-level rise ended with development of a highstand systems tract of coastal plain and deltaic deposits. These are included in the upper part of the Ngrayong Formation. The second sequence is less well developed. This sequence consists mainly of transgressive and highstand systems tracts. These correlate with the Bulu Formation, which mainly consists of bedded grainstone and wackstone, and the lower part of the Wonocolo Formation, composed of interbedded fossiliferous sandy marl and thin bedded gray fossilliferous calcarenites. Similar to the second sequence, the third sequence consists mainly of transgressive and highstand systems tracts (Fig. 4.8). The upper part of the Wonocolo Formation is interpreted as the transgressive system tract of the third sequence, consisting of shale with intercalations of calcarenite. The third sequence highstand systems tract is characterized by progradational sediments in the lower part of the Ledok Formation. The type locality is in Ledok Village, Cepu, where the thickness of this formation ranges from 100 to 250 m. The Lekok consists of thickening upward units of glauconitic, fossliferous, greenish-gray calcareous sandstone, interbedded with thinning upward beds of fossiliferous, greenish-gray sandy marl. The upper part of the Ledok Formation is characterized by bioturbation and large cross bedding, indicating outer to inner neritic environments. Seismic stratigraphic analysis of the fourth sequence indicates that the middle part of the Ledok Formation corresponds to progradational reflector patterns of a highstand systems tract (Figs. 4.8).
126.96.36.199.4. Upper Miocene - Pliocene basin subsidcnce phase
An erosional or unconformity surface separates Middle Miocene from the overlying Upper Miocene-Pliocene section, associated with the formation of incised valley fill in many places (e.g., Cepu and Bojonegoro areas, Yulihanto, 1993). The depositional history of the study area ended with sedimentation of the Mundu Formation, which consists of marl and shale that accumulated in association with the Pliocene sea level rise. Fossiliferous, greenish-gray marl dominates the lower part of the Mundu, while the upper part includes interbedded fossiliferous, greenish-gray sandy marl of the so-called Selorejo Member. The formation was deposited in outer neritic environments during the Late Miocene to Pliocene.
4.2.3. KENDENG TROUGH
188.8.131.52 GEOLOGICAL SETTING
The Kendeng Trough is a strongly folded and sometimes heavily faulted region, located to the south of the northern slope. Structuring is very recent and is probably still active. Fold axes are oriented in an east to west direction; an indicator that the adjacent and parallel volcanic chain is, at least in part, responsible for the compression. The Kendeng Zone can be subdivided into eastern and western areas, roughly split at the location of the Solo River outcrop sections at Ngawi. East of here folds are tight but not usually faulted, at least not on surface. Note that going east from Ngawi the age of sediments outcropping in this zone gets steadily younger. In the east, south of Surabaya, the folds are nearly lost under recent alluvium and even Pleistocene rarely crops out. West of Ngawi, towards Semarang, the folds expose rocks as old as Early Miocene and much faulting has been mapped. This east - west variation in structuring reflects a gravity anomaly trend, with the lowest gravity values in the west of the zone. The complexity and thickness of the Tertiary sediments in the western part of the Kendeng Zone, as well as surface undulation, are recognized from seismic.
The Kendeng Zone represents the central deep of the East Java Basin. Most lithological features show deep marine influence. The stratigraphy of the Kendeng zone is shown in figure 4 and includes the following units:
184.108.40.206.1. Pelang Formation
The type locality for this formation is in Pelang Village, south of Juwangi. The Pelang Formation there consists of 125 m. of alternating massive to bedded fossiliferous gray marls and gray claystones with intercalations of bioclastic limestones. These strata accumulated in neritic environments during the Early Miocene.
220.127.116.11.2. Kerek Formation
The name of Kerek comes from Kerek Village, in the vicinity of the Solo River (Bengawan Solo). The formation consists of about 800 m. of turbidites, made up mostly by fining and thinning upwards beds with sedimentary structures typical of density flows. Lithologies include gray tuffaceous sandstones and gray claystones or marls.
18.104.22.168.3. Kalibeng Formation
This formation has a type locality along the Kalibeng River, north of Jombang. It consists of massive fossiliferous greenish gray marl intercalated with thin bedded tuffs. These sediments accumulated in a bathyal environment during Pliocene time. The upper part of the Kalibeng (Atasangin Member) is composed of interbedded white tuffaceous fine to coarse sandstones, white tuffs, and brown volcanic breccias. These were deposited as turbidites. Other facies of the Kalibeng are the Cipluk Member, with marl and claystone (200-500 m.); The Kapung Member, which is composed of bioclastic wackstone and grainstone; and the Kalibiuk Member, characterized by claystone and balanus marl.
22.214.171.124.4. Sonde Formation
The type locality is in Sonde Village, west of Ngawi, where the thickness is 260 m. The lower part of this formation (Klitik Member) is dominated by sandy marl interbedded with calcareous sandstones and white tuffs, while the upper part consists of balamnus packstone and grainstone. The formation was deposited in shallow marine environments during Pliocene time.
126.96.36.199.5. Pucangan Formation
Type locality for the Pucangan Formation is at Gunung Pucangan, north of Jombang. It includes 323 m. of conglomeratic-coarse sandstones, tuffaceous sandstones, volcanic breccias, and black clay containing fresh water molluscs. This formation was deposited in a limnic environment during Late Pliocene to Pleistocene time.
188.8.131.52.6. Kabuh Formation
Kabuh Village, north of Jombang, has the type locality for this formation. The formation is 150 m. thick, more or less, and it consists of interbedded coarse sandstones with cross bedding, vertebrate fossils, lenses of conglomerates, and yellow tuffs. These accumulated in continental, fluvial and limnic environment during the last 0.75 MY.
4.2.4. VOLCANIC ARC
In the Central and East Java region the Tertiary volcanic arc has been recorded as having three distinct phases of activity. Based on groupings of radiometric ages (Bellon et al., 1990) and the stratigraphic occurrence of volcanic beds, the following phases can be recognized: 1. An early active volcanic phase from about 50 to 19 Ma (mid Eocene to mid Early Miocene). 2. A period of relative quiescence from about 19 Ma to about 11 Ma (late Middle Miocene). 3. A considerable increase in volcanic activity at about 11 Ma, with the volcanic chain moving about 50 kilometers north to its present position. 4. At about 3 Ma the volcanism changed with a new series of active volcanoes along the main arc, but also more K-rich volcanoes lying off the arc trend (e.g. Gunung Muria [1.1-0.4 Ma], offshore to the north on Bawean Island [0.8-0.3 MYBP], and Gunung Lasem [1.6-1.1 Ma, but not especially K-rich]). DSDP holes in the Indian Ocean west and south of Java yield data supporting the end of the second, the third and the last phase listed above. These wells contain tuffs dated as 11 MYBP and younger, with a notable increase in pyroclastic content in Late Pliocene or basa1 Quaternary times (about 2-3 Ma). The location of these sites on a northwards drifting oceanic plate precludes them recording Javanese volcanic activity much before 11 MYBP. For instance at 19 MYBP, when the ”Old Andesite” phase came to an end, ’ the DSDP sites would have been some 400 kilometers further south of the volcanic arc. Note that between,’ these main volcanic events there was still some continuing background volcanism, as seen by the tuffs present in Middle Miocene beds in the south of Java (Lunt et al, 1996). See chapter 4.4 for further details on magmatic arc.
4.2.5. SOUTHERN SLOPE REGIONAL UPLIFT
The southern slope regional uplift is also known as the southern mountains, consist of the ”old andesite” volcanic and volcaniclastic suite, initially interbedded with and then more completely overlain by Miocene limestones. These limestones often develop as reefal facies such as in the area south of Malang, the island of Nusa Barung, the Puger area and the Blambangan Peninsula. The southern mountains today are the site of dramatic karstified topography that is relatively young, i.e. it is probably the result of Quaternary uplift on the southern flanks of the modern volcanic chain. The most extensive Miocene reefal facies are in the south and east of Java. Also in the eastern area, in addition to the andesitic extrusives, there is reported to be a granite batholith near Merawan. This granite and associated dikes intrude and reported alter some older Miocene limestones and andesites but are then covered by the reefal limestones. Detailed data on the granite and the reefal limestones in this area is scarce but Van Bemmelen deduced that the limestones that follow the intrusion are equivalent to the reefal Wonosari Limestones further west in the Southern Mountains. The western Wonosari Limestones are probably latest Early to Middle Miocene in age. It would therefore appear that the Merawan granite is related to the older, 19 to 50 MYBP, volcanic phase, although there is still a question of how a ”granite” occurs so far from a continental margin, and intrudes at such shallow depths (Lunt et al., 1996). There are many signs pointing to a southerly quartz provenance that is separate from the Ngrayong sands of the north. These include the petrographic data in Muin (1985) that consistently records nearly 30% of sand grains as quartz in the Early to mid-Middle Miocene volcaniclastics Kerek Beds. In addition papers such as those by Kadar and Storrs Cole (1975) from the later Early Miocene of the Southern Mountains note biostratigraphy samples containing abundant quartz grains along with the transported larger forams they were studying (Lunt et al, 1996).
4.3 SOUTH CENTRAL JAVA BASINS
4.3.1. TECTONIC SETTING
The South Central Java basinal area lies south of Central Java on the northern flank of a major present day elongate bathymetric basin lying between the volcanic arc of Java itself (and its extensions NW and E) and the non-volcanic outer ridge bounding the Java Trench on its north flank. In broad tectonic setting this area is classified as outer arc basin, and it is a megatectonic feature associated with all island arc systems and may vary considerably in its complexity. The area contains two Neogene sedimentary basins whose structural outlines were determined during a Late Oligocene phase of folding, faulting and volcanism. The basins were filled with clastics of deep marine facies. The high areas surrounding the depocenters were covered mainly by an incomplete section of Neogene shallow marine limestones (including reefs). Three Neogene tectonic events of possibly regional importance are deduced from stratigraphic and seismic records: a minor Early Miocene event, a Mid Miocene event, and a Late Pliocene event. None of these events however, has considerably deformed the offshore Neogene. South of Central Java the deeper part of the outer arc basin proper shallows steadily northwards and seismic records show that a “basement” ridge and sediment filled basin are traversed before reaching the Java coast. A simplified mega-structural sense be considered part of the “southern mountains” of west and east Java which in the broad embayment south of Central Java runs beneath the sea (Bolliger & De Ruiter, 1974).
(south of Purwokerto) by the Nusa Kambangan ridge. South of this ridge an east-west trending depression - the ”western basin” - contains over 10,000 feet of undeformed sediment. Still further south an extensive high platform lies between the ”western basin” and the slope to the present day outer arc basin. The central province is the extension of the Kebumen Basin on land. It is characterized by a greater thickness of Neogene (over 15,000’) and the absence of a distinct unconformity at the base of the Miocene. Deeper seismic horizons, conformable with the base Miocene, could be mapped over most of the area down to a depth of over 25.000’. This basin is again separated from the outer arc basin by a broad but deeper ”basement” ridge. The eastern province is the offshore continuation of the Gunung Sewu plateau (south of Yogyakarta) which consists of flat lying Miocene limestones in outcrop. This limestone plateau covers most of the coastal iegions of eastern south Java and can be traced east at least as far as Lombok Island. In the offshore area, large carbonate build-ups, are found and one was drilled (ALV-1). As in the western province an angular Base-Miocene unconformity occurs. The Neogene sedimentary sequence dips gently to the south. Seismic lines (figs. 6 – 8) and structural cross sections (fig. 9) give an impression of the structural style of the various provinces.
A stratigraphically oriented field survey on South Central Java, the results of two wells, drilled offshore in deep water, and good quality seismic data allowed a tentative reconstruction of the sedimentary history of the area. The main tool for the stratigraphic correlations was the well-established zonation of planktonic foraminifera. The ages of the shallow marine sections, which in general do not contain planktonics, was based on the less accurate larger foram zonation.
Few Paleogene sections are known from southern Central Java. In the Jiwo Hills and at Nanggulan the oldest Paleogene sediments are of Middle Eocene age. They were initially deposited in a shallow marine environment (limestones and clastics), and grade into a deep marine facies over a relatively thin vertical interval. Upper Eocene was found in bathyal development in both areas. In the geographic center of Java (Lok Ulo, Banjarnegara area) an interesting melange of shallow and deep deposits is present, ranging in age from Upper Cretaceous (Cenomanian/ Turonian), over Paleocene to Upper Eocene. Most probably we are dealing here with an olistostromal mixture, which was emplaced into a trough during the Late Eocene. These few observations of Eocene sediments indicate a tectonically active period, involving not only fast subsidence and transgression but also pronounced topographic gradients. The Paleogene history was terminated by a regional tectonic event of Late Oligocene age. It is expressed as a phase of strong faulting and subsequent subsidence on the Sunda Shield and as a major folding phase in East Kalimantan. In the area under discussion it involved block tectonics, probable transcurrent movements and widespread volcanic activity. The ”Old Andesites” of South Java may be attributed to this phase. During that time the structural setting was created which was to control the Neogene sedimentary pattern.
The facies distribution of the Neogene appears to be controlled by the position of pre-existing high areas and the intervening depressions. Such highs originated during the Late Oligocene phase either by simple volcanic activity, or were the result of uplift and tilting of extensive tectonic blocks. The Karangbolong high, the West Progo Mountains and some smaller offshore highs, we would categorize as relicts of simple volcanic build-ups. On the other hand Nusa Kambangan and the western offshore province, the Gunung Sewu high and the eastern offshore province have to be considered as uplifted high areas. Here Oligocene, originally deep marine, sediments emerged and were truncated by erosion in Late Oligocene and Early Miocene time. Among the depressions the central offshore basin with its extension onshore (the Kebumen basin) and the depression of Yogyakarta appear to have been persistently deep. The Late Oligocene tectonic event is not expressed as an angular unconformity in the central basin. In contrast to this, the western offshore basin and possibly the Banyumas basin onshore started to subside only in the Early Miocene. The Neogene sedimentary sequence on the highs is incomplete and consists mainly of Early: to Mid Miocene shallow marine limestone ’ which overlies unconformably the so called ”Old Andesite”. The basinal areas are filled with generally deep marine clastics of variable composition. Clastic material of volcanic origin, ranging from fine-grained tuffs to boulder beds is found as well as deep marine day, sometimes interbedded with calci-turbidites. The presence of so much volcanic material suggests different phases of active volcanism during the Neogene. The calciturbidites are presumably derived from the areas where shallow marine limestone was deposited on highs that were volcanically less active. Thc relation between a high and a low area can be best illustrated from the well data of Alveolina (ALV-1) and Borelis (BOR-1) drilled offshore, in the Eastern Province and Central Province respectively (fig.10). ALV-I encountered a section consisting of deep marine Pliocene clay, overlying some 1000’ of shallow marine Middle Miocene limestone. ’The latter rests unconformably on strongly dipping, Upper Oligocene tuff and clay. The well bottomed in undatable volcanic agglomerates. The BOR-1 section consists of deep marine, Pliocene and Miocene clay. The well bottomed in undated basalt. The Miocene section is not complete owing to local faulting. It is of interest that the Lower Miocene deep marine clay of BOR-1 correlates seismically with the down flank extension of the Mid Miocene carbonates of ALV-1. This suggests that limestones started to be deposited on the flank of the Alveolina high already during the Early Miocene and transgraded the high fully only during the Mid Miocene, when they covered the former non-depositional/erosional area. Limestone deposition stopped later during the Middle Miocene, following a period of increased subsidence resulting in water depths too great for limestone production. As the carbonate build-up still stood out as a pronounced high on the sea bottom, during Late Miocene time it became non depositional. Fine Upper Miocene clastics were deposited around it until the bathymetric lows were filled and the crest of the high became covered by sediment at about beginning of the Pliocene. The sedimentary development of south Central Java, derived from surface sections and wells, is summarized in a time/ facies diagram (fig. 1 l). The essence of all our stratigraphic knowledge is given in fig. 12. By applying the sedimentary model described above, and with the help of seismic data, it was possible to make tentative facies maps over the South Central Java area (fig. 13 – 15). Two major and one minor regional tectonic events are reflected in various ways in the Neogene sedimentary sequence (fig. I l, 12). Early Miocene tectonism is reflected by the rapid subsidence of the western offshore basin and possibly the onshore Banyumas basin. It involved faulting and volcanism. The only clearly dated (by paleontology) volcanics of this time occur in the Baturng Mountains, SE of Yogyakarta. However, areas of older volcanic activity were probably reactivated: West Progo Mountains (van Bemmelen, 1949), Gabon volcanics (Mulhadiyono, 1973). A mid Miocene tectonic phase appears to have had a major regional effect. It is reflected by gaps in sedimentation not only on all the highs, but also in some depressions (Yogyakarta area). It was following this event that the limestones on the offshore ”Alveolina”–high were drowned and sedimentation ceased. On Java a new phase of strong volcanicity was triggered. A major tectonic event of Late Pliocene age caused the first phase of regional up- lift at Java It was accompanied by folding and widespread volcanicity.
4.4. MAGMATIC ARC
Java has often been referred to as a classical example of the relationship of calc-alkaline magmatism to subduction. Subduction of the Indian Ocean beneath the Sunda arc is considered to have been active since at least Eocene ~ time, according to geodynamic reconstructions (Hamil- ton 1979, Katili 1975, Rangin et al. 1990). The geology and petrology of the Quaternary Sunda arc volcanoes have been the subject of many investigations (Hutchison 1982, Wheller et al. 1987) but much less is known about Tertiary magmatism. Exposures of the oldest known volcanic rocks in Java occur as fragments of calc- alkaline lavas of late Cretaceous - Eocene age in the melange-type rock formations, e.g. Karangsambung (Suparka et al., 1990, Suparka and Soeria-Atmadja, 1991). Exposures of the younger calc-alkaline volcanic rocks, considered as Oligo-Miocene age (van Bemmelen 1949), are more widely distributed. They are exposed mostly along the southern coast of Java, and are referred to as the ”Old Andesites”. The more recent and active volcanoes of Java often overlie volcanic- and/or intrusive-rock units. Volcanic rock units are intercalated with Neogene sediments, and intrusive rocks cut these sediments. However, available radiometric or fission track ages on these Tertiary magmatic rocks are relatively scarce (Hehuwat 1976, Nishimura et al. 1978). It seems that the location of the axes of the successive magmatic arcs in Java has shifted not more than 60 km northwards to the present position of the Quaternary Sunda arc since Eocene/Oligocene time. Investigations by Bellon et al. (1989) and Soeria-Atmadja et al. (1990) have shown that Tertiary magmatic activity in Java took place in two distinct periods: Late Eocene – Early Miocene and Late Miocene - Late Pliocene. The products of the earlier event have built up the ”Old Andesites”, whereas those of the latter may be related to the early stages of magmatic activity of the modern Sunda arc (Bellon et al. 1989). K-Ar datings of the magmatic rocks in Java by Soeria-Atmadja et al (1994) indicate that two stages of volcanic activity may be distinguished throughout the Tertiary period. The earlier one took place from 40Ma (Karangsambung and Pacitan) to 19 – 18 Ma (Pacitan and Pangandaran). The following volcanic activity occurred between 12 Ma (Pertamina 1988) or 11 Ma (Bobotsari) to 2 Ma (Jatiluhur) and were succeeded by the Quaternary volcanism of the Sunda arc. The possible existence of a real break in volcanism between 18 and 12 Ma is questionable as new data on K-Ar ages point to volcanic activity at 13.7 Ma (JM-61, Bayah) and 15.3 Ma (PC-3, Pacitan). Perhaps we are only dealing with a relative paucity within the 18 - 12 Ma range.
4.5. QUATERNARY OF JAVA
Quaternary rocks in Java could be divided into non-volcanic and volcanic products. The non-volcanic products represented by Lower-Middle Pleistocene sediments of mostly non marine, and only little amount of marine sediments. The volcanic products are mainly as the results of Middle Pleistocene to Recent volcanic activities. However, little amount of Plio-Pleistocene to Lower Pleistocene volcanic materials have also been found in certain areas as the result of old quaternary volcanic activities. The quaternary sediments are exposed almost in all regions in Java, particularly at the middle and northern part of this island. In West Java, the quaternary sediments belong to Citalang, Tambakan and Ciherang Formations were deposited in non-marine environment. Tambakan and Citalang Formations are distributed in central west Java, and Ciherang Formation in northeast Java. Fresh water molluscs and vertebrate fossils are found within these formations, but no homminid fossils. Based on vertebrate fossils, the age of these formations are Lower to Middle Pleistocene. Upper Pleistocene to Recent volcanic products covered the sediments of those formations. Towards the east of the West Java region, the quaternary rocks are well exposed in Bumiayu Area, known as Bumiayu Basin. The oldest rocks are non marine sediments of Cisaat Formation (regrouped from formerly of Kaliglagah and Mengger Formations) of Lower Pleistocene, followed by Gintung Formation of Middle Pleistocene. These formations then covered by Upper Pleistocene to Recent volcanic products of Linggopodo Formation and from the activities of Slamet Volcano. The fresh water molluscs and vertebrate fossils were found in this area, but no homminid found from these formations. The most important quaternary in Java is found in Sangiran, Central Java and in Kendeng Zone of East Java. Sangiran area is situated at about 20 Km north of Solo, is a dome in elongated form, and the axis of this dome is of north-south ward, with mud volcano and several block faults in the center of the dome. The Sangiran dome is dissected by some rivers, with the biggest is Kali (river) Cemoro in the middle part of the dome, flowing from west to east direction. The rivers were denudated the area form the low undulated hills and valleys where the sediments are cropped out in this dome. In Sangiran area and in Kendeng Zone of East Java, the oldest sediments are belong to Kalibeng Formation of Late Pliocene in age. This formation consists of calcareous grey clays and marls were deposited in shallow marine environment. Above the Kalibeng Formation were deposited Pucangan Formation, consists of Iaharic breccias at the lower part and black and bluish grey of clays with intercalation of thin layers of tuff, diatomae and molluscs beds, were deposited in the swamps, lake and shallow marine environments during Early Pleistocene. Many vertebrate and Homo erectus fossils have been found in the black clays of Pucangan Formation in Sangiran area. The Pucangan Formation is overlain by Kabuh Formation, consisting of fine to very coarse tuffaceous sandstones with lenses of pumiceous conglomerate intercalated by silt and black clay. Cross bedding, parallel bedding and scouring structures are often found within sandstones and conglomerates. In Sangiran, the calcareous conglomerate is compacted, dense and rich with vertebrate and homminid fossils, was found at lower part of the Kabuh Formation, is well known as “Grenzbank Layer”. The Kabuh Formation is rich with vertebrate and Homo erectus fossils of Middle Pleistocene in age then covered by Upper Lahar of Notopuro Formation. The Notopuro Formation overlain by a sequence of alternating of tuffaceous sandstones, conglomerate and clays, and lahar layer at the uppermost part of this sequence which are belong to River Terraces Unit. Many vertebrate fossils were found in Java, e.g. Stegodont trigonocephalus VK., Hippopotamus namadicus, Rhinoceros palaeosondaicus, Bubalus (Buffaloes) c.f paleokarabau etc.. Hominid fossils, are found mainly from Sangiran area, and little amount from Sambungmacan (Sragen) and Patiayam (Central Java), from Kedungbrubus, Trinil, Ngawi, Ngandong and Perning (Mojokerto), East Java. The hominid fossils consist of Meganthropus paleojavanicus, Homo (Pithecanthropus) erectus, Homo erectus mojokertensis, and Homo erectus ngandongensis.