An Outline The Geology of Indonesia

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4. JAVA AND JAVA SEA

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. 4.4) 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 pre-Tertiary 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 deepest basin within the region with basement as deep as 3.0 sec. TWT. It is bounded from the Sunda plat-form eastwards by a major normal fault. To the northwards and westwards, it is bordered by steep gradients and is dissected by normal faults.

4.1.2.2 STRATIGRAPHY 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 sedi-mentary sequences and the post-rift (sag) basin fills dominated by marginal marine and marine sedimentary sequences (Fig. 4.5). In the following discussion, the sediment sequences are divided into five different tectonostratigraphic units based on their tectonic origins (Kohar et al, 1996).

4.1.2.2.1 Basement The sedimentary sequence of the North West Java Sea basins rests on a multicomplexes of

a Pre-Tertiary basement representing the continental crust of the Sundaland. The basement assemblage (Fig. 4.5) is composed of metamorphic and igneous rocks pri-

marily of Cretaceous and older ages and subordinate limestones and clastic sediments of pos-sible Early Tertiary age. This melange of lowgrade meta-sedimentary, igneous, and meta-igneous rocks is the result of subductionrelated accretionary processes associated with the Meratus Suture (Fig. 4.1 & 4.2) which was active during the Cretaceous and Paleocene. Metamorphic grade varies widely througbout 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 calcalkalic magmatism occurred throughout on-


shore 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 di-vided into alternating graben-like sub-basin and positive ridge or platforms. Figure 4.4 dis-plays the basin configuration of the West Java Sea basinal area.

4.1.2.2.2. Early Rift Fill 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 sand'stones 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. a1.,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 NWSE trending basin margan 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 fluvia-tile 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.

4.1.2.2.3. Syn-rift fills 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 North East Java Basin, filling the series of half grabens of the West Java Sea Basin (Fig. 4.5).

The Talangakar is divided into two members, the lower member and the upper member. The syn-rift fills include only the lower member and are of economic importance as primary oil reservoirs in major oil fields (Cinta, Widuri, Ze(da, 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 syn-rift unit is 2000 m in Sunda 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 Talanga-kar) and the underlying Banuwati lacustrine unit and a thought that this unit has an Oligocene to Early Miocene age.

4. JAVA AND JAVA SE

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4.1.2.2.4. Early Sag Basin Fills The early sag basin fills represent the overall transgressive setting in the Java Sea area re-

lated to the sea level rise during Early Miocene time. At this time the basin boundaries be-tween the subbasins (Sunda, Asri, 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 move-ment of the faults for these post-rift sag successions. The overall depocentre shows a rela-tively symmetrical, work shape basin throughout the West Java Sea area. Non depositions continue to occur on paleohighs until Baturaja carbonate deposition commenced during Mid-dle Miocene time, forming a bald area for the marginal marine deposition of the early syn-rift fills (Fig. 4.5). The early sag basin fills (postrift) include the previously described as Upper Talangakar and the carbonates of the Baturaja Formation and conformably overlie the syn-rift fills throughout the basin (Fig. 4.5).

The lithology in the early sag basin fills consists of interbedded sandstones, siltstones, mudstones and coal, and marine sliales 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 within the area. The non marine elastics are dominated by channel fill, point bars and marine bar sandstones deposited in a wide range of environments from low sinuosity channels on allu-vial plain, distributary channels to marginal marine bars. Coals and overbank mudstones, and siltstones filled the floodplain area, forming 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.

4.1.2.2.5. Main Sag Basin Fills 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. 4.5). 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 elastics consisting of marine mudstones, calcareous and glauconitic sandstones qnd 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 foraminifera, bivalves, echinoderm fragments, red algae and minor quartz and glauconite grains. The age of this carbonate build up is thought to be Middle Miocene.

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 are clearly identified as well defined sub-elliptical mounding features.

4.1.2.2.6. Late Sag Basin Fills Late sag basin fills represent the latest sedimentary sequence below the present day sedi-

mentation 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, approxi-mately 1.5 Ma, 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 entirely composed 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 4.1.3.1..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 eastwest feature of the Bogor Trough where the in-fluences of the volcano-magmatic and its compressional effect are primordial (Fig. 4.3). 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.

4.1.3.2 STRATIGRAPHY The Bogor Sedimentary Province (Fig. 4.6) is filled by 3 systems of sedimentation includ-

ing the Ciletuh, Bayah and Jatibarang Formations. The Middle to Late Eocene Ciletuh For-mation (1400m) lies on top of a Late Cretaceous to Paleocene 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 volcanics, 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 reefal 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

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4.1.5. SOUTHERN SLOPE REGIONAL UPLIFT The southern mountains 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 for-mation 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 sedlments 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 4.1.6.1 TECTONIC FRAMEWORK The Banten Block comprises several structural highs and lows (Fig. 4.3). 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 Subbasin 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. 4.3). 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. 4.7). 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. The Bayah High comprises large E-W trending anticlines cored by Eocene clean coarse-grained sandstones (Keetley et al, 1997).

4.1.6.2 STRATIGRAPHY

The Banten Sedimentary Province consists of 3 main cycles of sedimentation (Fig. 4.8). 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 shallow marine to terrestrial deposits belonging to the 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 total thickness of this unit is approximately 800m.

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A C SETTING tory of the Ehe island andge of the Shis old contn offshore noast Java regianto et al, 19includes the

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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.

4.2.2.2.2. Early Miocene basin subsidence phase Early Miocene subsidence developed a ramp-type depositional platform (Fig. 4.11).

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.

4.2.2.2.3. 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. 4.5). 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). 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 (Fig. 4.5). 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(Fig. 4.5).

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 trans-gressive 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 fossilferous sandy marl and thin bedded gray fossilliferous calcarenites.

Similar to the second sequence, the third sequence consists mainly of transgressive and highstand systems tracts. The upper part of the Wonocolo Formation is interpreted as the transgressive system tract of the third sequence, consisting of shale with intercalation 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 Ledok 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.

4.2.2.2.4. Upper Miocene - Pliocene basin subsidence 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 4.2.3.1 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.

4.2.3.2 STRATIGRAPHY 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 includes the following units:


- Pelang Formation: 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.

-Kerek 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.

- Kalibeng Formation: 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.

- Sonde Formation: 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.

- Pucangan Formation: 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.

- Kabuh 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:

An early active volcanic phase from about 50 to 19 Ma (mid Eocene to mid Early Miocene).

A period of relative quiescence from about 19 Ma to about 11 Ma (late Middle Miocene). A considerable increase in volcanic activity at about 11 Ma, with the volcanic chain moving about 50 kilometers north to its present position.

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 basal Quaternary times (about 2-3 Ma). The location of these sites on a northwar,ds 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 reFfal facies arg 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 con-sistently 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 con-taining abundant quartz grains along with the transported larger forams they were studying (Lunt et al, 1996).

4.3 SOUTH CENTRAL JAVA BASINS (Adapted from Bollinger & de Ruiter, 1975) 4.3.1. TECTONIC SETTING The South Central Java basinal area lies south of Central Java (Fig. 4.12) 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 fore 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 north-wards and seismic records show that a "basement" ridge and sediment filled basin are trav-ersed before reaching the Java coast (Fig. 4.13). A simplified mega-structural sense be

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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 DEPOSITS Quaternary rocks in Java could be divided into non-volcanic and volcanic products.

The non-volcanic products represented by LowerMiddle 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. Fri~sh 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 Bu-miayu Area, known as Bumiayu Basin. The oldest rocks are non marine sediments of Ci-saat Formation (regrouped from formerly of Kaliglagah and Mengger Formations) of Lower Pleistocene, followed by Gintung Formation of Middle Pleistocene. These forma-tions 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 hom-minid found from these formations.

The most important Quaternary in Java is found in Sangiran, Central Java and in Ken-deng 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 dorrLe.

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 denu-dated 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 pumice.ous 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.

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An Outline The Geology of Indonesia