IPA07-G-037

PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION

Thirty-First Annual Convention and Exhibition, May 2007

CRETACEOUS TO LATE MIOCENE STRATIGRAPHIC AND TECTONIC EVOLUTION OF WEST JAVA

Benjamin Clements*

Robert Hall*

ABSTRACT Palaeogeographic maps for intervals between the Cretaceous and Late Miocene illustrate the complex evolution of West Java. Basement is of Mesozoic age and in West and Central Java there are ophiolitic and arc rocks accreted to the margin of Sundaland in the Late Cretaceous. The oldest Cenozoic rocks in West Java are Middle Eocene formations in the Ciletuh Bay area that formed in quite different settings. There are volcanogenic turbidites and breccias containing abundant basaltic material that we suggest are deep water deposits, associated with the onset of subduction, formed close to a new arc or in its forearc. Nearby are quartz-rich sandstones deposited predominantly in a shallow marine shelf edge environment interpreted to be derived from basement highs. We assign these rocks to different formations and their present juxtaposition is suggested to be due to thrusting. We interpret there to have been a large southerly prograding delta system in SW Java during the Late Eocene. There is a considerable thickness of quartz-rich sandstones, forming an overall shallowing-up sequence, sourced from the north and probably derived from Sundaland. The Oligocene of West Java includes terrestrial quartz-rich sandstones, reefal and foraminiferal limestones and volcanogenic sediments deposited in fluvial to deeper water marine environments. The Early Miocene saw an important phase of explosive arc volcanism in south Java. By the Middle Miocene volcanism had diminished or ceased, allowing carbonates to be deposited on the arc rocks. In the Late Miocene volcanism resumed further to the north resulting in a new phase of volcanogenic turbidite deposition. It is not certain when subduction began beneath West Java and where the arc was situated. Except at Ciletuh the volcanic component of Paleogene * SE Asia Research Group, Royal Holloway

University of London

sequences is relatively minor. This has suggested that subduction-related volcanism did not commence until the Late Oligocene. However, we suggest that subduction-related volcanism began in the Eocene, but the arc did not become emergent until the end of the Oligocene. Loading by the volcanic arc formed a broadly E-W trending flexural basin to the north of the arc which filled with volcanogenic material from the south and continental clastic debris from the north. The distance between the Paleogene quartz-rich shelf sequences and the volcanic arc has been reduced by Neogene thrusting. INTRODUCTION Java is situated within the Indonesian archipelago at the southern margin of Sundaland and the Eurasian Plate (Figure 1). Sundaland is the continental core of SE Asia (e.g. van Bemmelen, 1949; Hamilton, 1979) formed by the accretion of blocks to the Eurasian margin, and had been assembled by the Late Triassic. Basement rocks include granites and metamorphic rocks of Palaeozoic and Mesozoic age, exposed in Borneo, Sumatra and the Malay Peninsula (Audley-Charles et al., 1988; Hutchison, 1989; Cobbing et al., 1992; Metcalfe, 1996). Ophiolitic and arc rocks were accreted in the Mesozoic at the periphery of Sundaland in Sumatra, Java and Borneo (Sukamto 1975; Wakita et al., 1994a and b, 1998; Parkinson et al., 1998 and Wakita 2000), and a Gondwana fragment was added to East Java and West Sulawesi in the Late Cretaceous (Smyth, 2005; Smyth et al., 2007). By the end of the Cretaceous and during the Paleocene much of Sundaland was an emergent continental region, probably with a passive margin south of Java, and a subduction margin to the west south of Sumatra accommodating northward movement of India. Rapid northward movement of Australia began in the Eocene and the present subduction south of Java was established in the early Cenozoic. Northward subduction of the Indo-Australian Plate beneath the Eurasian Plate has probably been

continuous since the early Paleogene although associated volcanism may not have been. Almost all the rocks exposed on Java are Cenozoic, and they include igneous intrusions, volcanic products, siliciclastic sedimentary rocks and shallow marine carbonates. In places they are significantly deformed. The island has a complex Cenozoic geological history and its relationship to regional plate movements and tectonic history is not well understood. We have been conducting field studies in Java, in collaboration with Indonesian colleagues, attempting to understand better the Cenozoic development of the island, and the interplay between igneous activity, sedimentation and deformation, and their larger-scale tectonic context. In this paper we present palaeogeographic maps for intervals between the Late Cretaceous and Late Miocene to illustrate the tectonic and stratigraphic evolution of West Java. The maps are based predominantly on observations from the field. We summarise the Cenozoic evolution of the western part of the island, the timing and consequences of deformation, and consider potential sources of sediments in West Java. METHODOLOGY This work is based on the results of several months of fieldwork carried out during three field seasons between 2004 and 2006 together with the results of laboratory work including petrographic studies, studies of light and heavy minerals, U-Pb dating of detrital zircons and biostratigraphic analyses. Palaeogeographic maps include the present-day coastline of West Java for reference; different colours are used to convey topography and bathymetry. We have not discussed the effects of global eustatic sea level change on the palaeogeographic maps. The Haq et al. (1987) curve shows long-term sea level almost constant or slightly falling throughout the Paleogene. The Kominz et al. (1998) and Miller et al. (2005) curves show sea level to be falling throughout the same period. All curves show long-term changes of less than 100 m and even third order sea level changes are less than 150 m. We feel that tectonic effects far exceed eustatic changes and therefore have not discussed sea level further. LATE CRETACEOUS There was subduction beneath Sundaland in the Early Cretaceous along a zone which ran from SW Java to the Meratus Mountains of Kalimantan. Accretionary-collision complexes resulting from

subduction (Sukamto, 1975, Wakita et al., 1994a and b, 1998; Parkinson et al., 1998; Wakita, 2000) include tectonic units formed by oceanic spreading, arc volcanism, oceanic and forearc sedimentation, and metamorphism. They include serpentinised ultrabasic rocks, basalts, cherts, limestones, siliceous shales, shales, volcanic breccias, and high pressure-low temperature and ultrahigh pressure metamorphic rocks (Parkinson et al., 1998; Wakita, 2000). The collision of a continental fragment of Gondwana origin (Smyth et al., 2007b) terminated subduction, probably in the Late Cretaceous (Figure 2), and this fragment now forms part of the basement of East Java. The accretionary rocks are well known from the Lok Ulo Complex of Central Java. In West Java similar rocks are exposed to the south of Ciletuh Bay (Figure 3) and include serpentinised peridotites (Figure 4a), gabbros, pillow basalts, and rare metamorphic rocks such as quartzite and amphibolite. Modern sands in the Cimadur River, near Bayah, contain ultrabasic grains suggesting undiscovered ophiolitic rocks inland. Details of the Late Cretaceous palaeogeography are speculative. There are Cretaceous granites, perhaps associated with subduction, exposed in Sumatra. To the west of Sumatra there are a number of linear high velocity anomalies in the lower mantle interpreted to represent Tethyan lithosphere subducted during Indias northward movement (van der Voo et al., 1999) and since overridden by India. We interpret the southernmost of these (van der Voo et al., 1999; Hafkenscheid et al., 2006) to record Cretaceous subduction which extended from north of India, east beneath Sumatra and West Java, into the West Pacific. MIDDLE EOCENE After Cretaceous collision of the Australian microcontinental fragment with the Java-Meratus subduction system subduction ceased (Parkinson et al., 1998; Smyth et al., 2007b), and there was a passive margin south of Java until the Eocene. In the Middle Eocene subduction resumed, and a new arc developed south of the Sunda Shelf. Subduction was re-established along the Java margin at this time. In West Java there are two sedimentary sequences of Middle Eocene age exposed in the Ciletuh Bay area (Figure 3). These represent the oldest sequences above basement. The relationships between these rocks and with the basement are complicated and stratigraphic contacts are not observed. Most authors have interpreted the contact

to be unconformable, although Martodjojo et al. (1978) suggested that the Ciletuh Formation rested conformably on the melange complex and van Bemmelen (1949) reported local thrust contacts between the pre-Tertiary and the Ciletuh Formation. Traditionally, two distinct lithofacies, volcaniclastic and quartzose, have been assigned to the Ciletuh Formation (e.g. van Bemmelen, 1949; Sukamto, 1975; Schiller et al., 1991). However, they are so different that we assign the volcaniclastic lithofacies to the Ciletuh Formation and the quartzose lithofacies to the Ciemas Formation. The Ciletuh Formation consists of coarse polymict breccias, volcanogenic debrites and turbidites (Figure 4b). These are best seen at Pulau Kunti where at least 100m of section is exposed. They contain abundant volcanic clasts (basalt and andesite) as well as laminated volcaniclastic clasts, several types of shallow water limestone clast and a small number of dacite, granite, and metamorphic clasts such as epidote amphibolite. Clasts can be up to ten metres across and in general are highly angular. Some basaltic blocks appear to have been extruded contemporaneously with the deposition of the breccia, with irregular smooth lava boundaries directly in contact with coarse angular breccias. Grey-green fine to medium grained volcaniclastic sandstones become more abundant up section. Assilina spp., Nummulites spp., Miliolid spp., Discocyclina spp. (M. BouDagher-Fadel, pers. comm., 2006) from limestone clasts (Figure 4c) within the breccias indicate an Early to Middle Eocene age. These clasts were only partly lithified when incorporated indicating this is the age of deposition of the breccias (Figure 4c). The Ciemas Formation is remarkably different in almost every way to the Ciletuh Formation. It comprises quartz-rich sandstones, pebbly sandstones and conglomerates. Pebbly material is predominantly vein and/or metamorphic quartz and is usually highly rounded, interpreted to represent the reworking of older fluvial/beach sediments of pre-Cenozoic age. Sandstones are typically texturally immature and generally poorly sorted despite being composed predominantly of quartz, much of which is of metamorphic origin. The formation is locally well bedded and sedimentary structures such as steeply scoured bases and fluidized slumps suggest rapid deposition. Rare redeposited coals are also present. The Ciemas Formation was deposited in relatively shallow water, perhaps on a narrow shelf edge, although there are some quartz-rich turbidites that indicate some deeper water deposition (Figure 4d).

Previous accounts (e.g. Schiller et al., 1991) of the Ciletuh Formation (our Ciletuh and Ciemas Formations) suggest sediment was carried from a continental shelf into deep water over more than 50 kilometres. Our observations suggest a number of problems with this interpretation. There is no mixing of quartzose and volcanic material, the coarsest and most angular material is the most distal, and this distal material (our Ciletuh Formation) is almost all derived from volcanic sources and ophiolitic basement. Many features of the Ciletuh Formation indicate active tectonism. The size and angularity of blocks require steep submarine fault scarps, exposing basement, into which were carried partly lithified volcaniclastic material. We interpret these deposits to indicate active faulting in deep water, accompanied by basaltic volcanism, with rare debris, such as Nummulitic limestones carried in from shallow water areas. We suggest that these deposits represent deformation and extension in a deep marine forearc setting accompanying the onset of subduction and the initiation of the volcanic arc (see Hall et al., 2007), as can be observed today in places such as Tonga, or the Izu-Bonin-Marianas arcs. In contrast, the Ciemas Formation was sourced from basement to the north. Large braided rivers draining an elevated Sundaland would have fed quartz-rich material across a narrow shelf (Figure 5a). Potential source areas are the Karimunjawa and Bawean Arches, local basement highs in West Java, Cretaceous granites and metamorphic rocks of SW Borneo, granites of the Malay Peninsula and Tin Belt, pre-Cenozoic rocks of Sumatra and elevated basement rocks of the Sunda Shelf. LATE EOCENE The Late Eocene in West Java was dominated by terrestrially-derived clastic sediments deposited predominantly by large braided rivers. Quartz-rich sandstones, pebbly sandstones and conglomerates dominate and were typically sourced from the north. In the NW Java Sea area alluvial fans formed in the early stages of basin development near to these fluviatile deposits. Conglomeratic material is predominantly well rounded vein/metamorphic quartz, and sandstones are fine to coarse grained and often texturally immature. Coals and carbonaceous mudstones are commonly associated with these rocks. The lower part of the succession is characterised by dark, pyrite rich marine mudstones and is interpreted as deeper water delta-slope

deposits (P. Lunt, pers. comm., 2006) which gradually coarsen up into the sandstones described above. Volcanogenic sandstones make up a minor part of the formation. These sediments are known as the Bayah Formation (Figure 4e) and are exposed over a wide area from Malingping in the west to Sukabumi in the east (Figure 3) and certainly extend further within the sub-surface. The Bayah Formation is more than 1000m metres thick and source areas for most of the sediments are likely to be similar to those of the Ciemas Formation. Rounded conglomeratic material in the Bayah Formation is interpreted to represent the reworking of older fluvial/beach deposits, probably of pre-Cenozoic age, derived from the north, whereas volcanogenic material was derived from a largely submerged volcanic arc to the south. Overall a coarsening upwards trend is observed and we interpret this to represent a delta system prograding southwards. A greater thickness of Upper Eocene terrestrially-derived sediments compared to the Middle Eocene suggests either an increase in sediment influx, an increase in accommodation space or both. We suggest that this was related to extension associated with subduction to the south, and regional extension of Sundaland to the north. In the offshore Malingping block both N-S and E-W faults were active in different parts of the block (Yulianto et al., 2007) at this time. E-W extension of Sundaland, manifested as rifting in the NW Java Sea area, contributed to subsidence creating accommodation space. E-W extensional faults further south are interpreted as the result of subduction-related extension south of Java. The region to the north of West Java was largely elevated (Figure 5a and b) and interpreted to be crossed by large braided rivers that flowed south from highlands to the north, and possibly between highlands to east and west. The climate was much more seasonal and possibly dryer during the Paleogene than the present day (R.J. Morley, pers. comm., 2006). Both of these factors may have encouraged the development of large braided rivers which supplied much of the clastic sediment to West Java. Provenance of Eocene sediments Zircons from sandstones have been analysed at the University of London using a Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA-ICP-MS). This work is in progress and here we

report preliminary findings. Terrestrially-derived sandstones of the Ciemas and Bayah Formations contain zircons which range in age from Cenozoic to Archean. Figure 6 shows U-Pb ages for detrital zircons from the Bayah and Ciemas Formations. The important peaks on the relative probabilityage plot (Figure 6A) are interpreted to represent zircons derived from different sources. A volcanic contribution is suggested by zircons of Paleogene age and the youngest of these zircons indicate a maximum depositional age of Late Eocene consistent with biostratigraphic data. There is a Mesozoic cluster of ages (c. 65-160 Ma) in which most of the ages are between 70 and 100 Ma, and interpreted here to indicate a Sundaland source. Upper Cretaceous granites with ages between 70 and 100 Ma are exposed in SW Kalimantan and are known to have been elevated during the Cenozoic, providing material to sediments of northern Borneo (van Hattum et al., 2006). Cretaceous granites of similar age are also reported from the Sunda Shelf (Hamilton, 1979) and the Thai-Malay peninsula (e.g. Bignell and Snelling, 1977; Beckinsale et al., 1979; Krhenbuhl, 1991). The zircon ages suggest that some of this material was also being transported south to West Java. There is also a 200270 Ma peak on the probabilityage plot (Figure 6A). This age range includes the most important episodes of Permian and Triassic granite magmatism in the Malay-Thai Tin Belt (e.g. Bignell and Snelling, 1977; Liew and Page, 1985; Seong, 1990; Krhenbuhl, 1991; Cobbing et al., 1992) suggesting a Malay peninsula source. There are some plutonic rocks with ages of 180 to 230 Ma in Sumatra (McCourt et al., 1996) which could have contributed material to West Java. There is also a small peak at 480 to 540 Ma (Figure 6A). The source of these zircons is not known. Lower Palaeozoic metamorphic and igneous rocks that could contain zircons of this age are known from west Kalimantan (Amiruddin and Trail, 1993; de Keyser, and Rustandi, 1993; Pieters and Sanyoto, 1993), Sumatra (Pulunggono and Cameron, 1984; Barber and Crow, 2005) and the Malay Peninsula (Jones, 1968). About 20% of the zircons have Precambrian ages and we interpret these to represent a Sundaland basement source; Precambrian zircons found in north Borneo sediments (van Hattum et al., 2006) were interpreted to be derived from SW Borneo or the Malay Peninsula. There are insufficient data to interpret the provenance of Java sediments in great detail. However, the West Java Phanerozoic zircon ages are more similar to those from north Borneo sandstones (van Hattum et al., 2006) for which a Sundaland source is clear, than to those from East

Java (Smyth, 2005; Smyth et al., 2007) where a Sundaland contribution is much less important. EARLY OLIGOCENE E-W extension continued into the Early Oligocene (e.g. Butterworth and Atkinson, 1993; Cole and Crittenden, 1997) and there was associated volcanism that was centred on the Jatibarang area of NW Java (Figure 7a). Volcanic rocks erupted into N-S trending graben and associated with lacustrine deposits now make up the Jatibarang Formation. The volcanic activity is very far from the likely subduction zone, and apparently not associated with composite volcanoes, but basaltic flows associated with lacustrine deposits, suggesting fissure eruptions more typical of rift settings. The formation of extensional structures perpendicular to the subduction axis is also unusual. We therefore suggest this extension was unlikely to be related to subduction. In other parts of the NW Java Sea lacustrine deposits of the Banuwati Formation were also deposited in depressions formed by fault graben. In West Java clastic sediments previously assigned to the Rajamandala Formation by Sukamto (1975) and the Ciletuh Formation by Kusumahbrata (1994) are referred to here as the Cikalong Formation; they are exposed at Padalarang, in the Cisukarama valley and at Cikalong (Figure 3). These comprise quartz-rich sandstones, pebbly sandstones, conglomerates and carbonaceous siltstones. Pebbles are highly rounded and thin channels, load casts, normal grading and fluid escape structures suggest rapid deposition. At Cikalong, Warungkiera these sediments are more than 500m thick although observed repetition by folding and faulting means that thickness estimates are uncertain. These sedimentary rocks are sometimes associated with rare limestone olistoliths of similar age, for example at Cikalong and in the Cisukarama valley, south of Cianjur. The olistoliths are typically coralline limestones with abundant foraminifera of similar age to the Cikalong Formation (P. Lunt, pers. comm., 2006). The presence of deep water agglutinated foraminifera in Cikalong Formation mudstones (P. Lunt, pers. comm., 2006) suggests a significantly different depositional environment to that of the shallow water limestones. We suggest that limestone blocks were transported as olistoliths into deeper water possibly in response to deformation associated with the building of the volcanic arc to the south. The similar lithological character of the Cikalong Formation to that of

Eocene sediments (well rounded pebbly material, immature quartzose sandstones) indicates that the Cikalong Formation was potentially sourced from these older Eocene sediments. Minor volcanic debris (fresh plagioclase feldspar, euhedral apatite and elongate zircon) indicates distal contemporaneous volcanism. To the west of Sukabumi there are thin units of poorly fossiliferous grey/green siltstones called the Batu Asih Beds. These lie above the Upper Eocene Bayah Formation and are poorly exposed over a small area at Batu Asih. Marine fauna are present (P. Lunt, pers. comm., 2006) as are brackish to freshwater palynomorphs (R. J. Morley, pers. comm., 2006) suggesting these were probably deposited in a shallow marine or lagoonal setting. The Lower Cijengkol Formation in the Bayah Dome is composed of quartz-rich sandstones and conglomerates. These sandstones and conglomerates are sometimes associated with coralline limestones and are interpreted to have been deposited in terrestrial to shallow marine conditions. Volcaniclastic rocks are also present in the lower part of the Cijengkol Formation but are rarely mixed with terrestrially-derived quartzose material suggesting different sources and different paths of sediment distribution. We suggest that the quartzose material was derived from Sundaland to the north and the volcaniclastic debris was derived from a submerged arc to the south. Volcaniclastic material may also have been transported from the Jatibarang area, to the east (Figure 7a). LATE OLIGOCENE By the Late Oligocene extension in the NW Java Sea had ceased and with it the volcanism around Jatibarang. Clastic sediments derived from the north formed thick deposits known as the Talang Akar Formation and much of the NW Java Sea area remained continental (Figure 7b) although there were short-lived marine incursions in the Arjuna area during the Late Oligocene (Pertamina 1996). Further south carbonates were being deposited along the shelf edge. These are now seen as reefal, algal and foraminiferal limestones which extend from Bandung in the east to Bayah in the west (Figure 6b). They form a prominent ridge at Padalarang (Figure 4f) and are assigned here to the Rajamandala Formation and further to the west to the Citarete and Cijengkol Formations. This Carbonate deposition may have continued through into the earliest Miocene (M. BouDagher-Fadel, pers. comm., 2007).

In addition to the limestones there are also clastic sedimentary rocks that make up the upper part of the Cijengkol Formation in the Bayah area. These are shallow marine to terrestrial deposits of quartz-rich sandstones, pebbly sandstones and conglomerates which correspond to the Talang Akar Formation in the NW Java Sea. This suggests that an area of shallow marine/terrestrial deposition extended from the NW Java Sea to this part of SW Java. Arc-derived volcanogenic material of Late Oligocene and Early Miocene age on land in West Java is much more widespread than that of material of Eocene and Early Oligocene age. Volcanic rocks range from basalts to rhyolites and exist as lavas, breccias, ignimbrites and tuffs as well as volcanogenic turbidites and debrites. The apparent lack of Paleogene material has led to suggestions that volcanic activity did not begin until the Late Oligocene (e.g. Hamilton, 1988) although van Bemmelen (1949) suggested that volcanic activity began in the Eocene in the Bayah area earlier than in the rest of Java. We suggest that the apparent increase in volcanic activity was because towards the end of the Oligocene the volcanic arc became emergent for the first time in West Java; before this it had been submerged and relatively non explosive. However, there may also have been an increase in volcanism during the Late Oligocene and Early Miocene. EARLY MIOCENE In the NW Java Sea widespread carbonate deposition of the Batu Raja Formation commenced (Figure 8). Reefal and platform limestones were deposited on fault block highs and carbonate muds in graben lows (Pertamina 1996). In the area of present-day West Java several kilometres of Lower Miocene volcanogenic turbidites and debrites of the Citarum Formation lie conformably above the limestones of the Rajamandala Formation. These were derived from the newly emergent volcanic arc to the south. The Oligo-Miocene volcanic arc rocks and limestones are assigned to the Jampang and Gabon Formation (Figure 4g) and make up much of the Southern Mountains. Dutch workers estimated the thickness of the Jampang Formation to be as much as 5km (van Bemmelen, 1949) and these rocks extend from Ciletuh in the west to Pangandaran in the east (Figure 3). Similar rocks such as the Cimapag Formation (Figure 4h) are exposed over a wide area in the Bayah Dome and comprise volcanic breccias, ignimbrites and epiclastic sediments.

Accumulations of thick sequences of arc-derived material of the Citarum Formation represent rapid subsidence and the formation or widening of a basin. The limestones of the Rajamandala Formation formed along the shelf edge and to the south was the volcanic arc (Figure 7b). We suggest that a basin had already started to form between the arc and the shelf edge in response to loading by the volcanic arc in the Late Eocene. During the Late Oligocene and Early Miocene the arc became emergent and the limestones of the Rajamandala Formation which had previously marked the shelf edge rapidly subsided as Early Miocene volcanogenic detritus smothered the limestones. Similar thick volcanogenic sequences exist in the flexural Kendeng basin (Waltham et al., 2007) in East Java and we suggest that this subsidence may reflect a period of increased volcanism. MIDDLE MIOCENE In SW and Central Java limestones of the Kalipucang and Pamatuan Formations lie unconformably above the Oligo-Miocene volcanic rocks of the Jampang and Gabon Formations. These are coralline and algal limestones and their associated slope deposits. Immediately to the north of the western part of the Southern Mountains Arc are limestones, epiclastic terrestrial and shallow marine conglomerates, sandstones and marls assigned to the Cimandiri Formation (Sukamto, 1975). Similar rocks are exposed further to the west in the north of the Bayah Dome. Further east around Majalengka (Figure 3) there are several hundred metres of calciturbidites and mudstones of the Cinambo Formation that were sourced from the north and west and similar rocks of the Rambatan Formation crop out to the north of Bumiayu, Central Java (Figure 3). Volcanic and volcaniclastic rocks of Middle Miocene age appear to be absent in West Java. Deposition of carbonates on top of the inactive Southern Mountains Arc during the Middle Miocene suggests volcanic activity had diminished or ceased. The shallow water terrestrial deposits of the Cimandiri Formation probably represent the final stages of sediment fill in the flexural basin to the north of the arc in the area to the west of present day Sukabumi. Further to the east deeper water conditions persisted as calcareous debris was shed into the basin from the shallow seas to the north and west. In the NW Java Sea deposition of the Upper Cibulakan Formation commenced and included the

Massive and Main clastic units as well as the Mid Main and Pre Parigi carbonates (Pertamina 1996). The diminution of volcanic activity in the Middle Miocene is observed in Java, and in a more extensive region to the east. It has been interpreted as the result of subduction hinge advance (Macpherson and Hall, 2002) which has been related to counter-clockwise rotation of Borneo and Java following the beginning of Australian collision in East Indonesia (Hall, 2002). LATE MIOCENE In the present day NW Java Sea extensive Upper Miocene limestones of the Parigi Formation (Pertamina 1996) indicate marine conditions persisted over much of the area during the Late Miocene. To the south, in Central Java around the towns of Bumiayu and Purwokerto (Figure 3) several kilometres of Upper Miocene volcanogenic turbidites (Figure 4i) and debrites of the Halang Formation are exposed. These comprise volcanic breccias and conglomerates, volcaniclastic sandstones and mudstones and in the Southern Mountains, lying unconformably above the Oligo-Miocene Jampang Formation are primary and epiclastic volcanogenic deposits and limestones. These were deposited in shallow marine conditions and are assigned to the Bentang Formation. Thick deposits of Upper Miocene debrites and turbidites of the Halang Formation and primary volcanic and volcaniclastic material deposited over large parts of the Southern Mountains indicate a resurgence in volcanic activity during the Late Miocene (Figure 9). The distribution of Upper Miocene primary and epiclastic volcanic rocks, and carbonates, in the Southern Mountains suggests that volcanism did not resume at the position of the Early Miocene volcanic arc. Upper Miocene volcanic rocks south of Bandung, in the northern central part of the Southern Mountains, suggest that the new arc was located to the north of the Paleogene volcanic arc. In western Central and eastern West Java the modern volcanoes of G. Slamet and G. Ciremai (Figure 3) are constructed on the deformed Upper Miocene deep water volcanogenic sediments of the Halang Formation. These were sourced from the Late Miocene arc. The Late Miocene arc was therefore not in the same position as the modern volcanoes nor was it located to the north as no volcanic arc rocks are known there. Two possibilities therefore exist as regards the position

of the Late Miocene volcanic arc in Central Java. Firstly, the arc volcanoes were south of the present day arc and Halang Formation rocks and are no longer observed on land since they have been removed by erosion, or secondly that a gap existed in the arc in Central Java and no Late Miocene volcanoes were present. SIGNIFICANCE FOR HYDROCARBON EXPLORATION The suggestion that Paleogene sediments were sourced from the Sunda Shelf and were deposited at its southern margin is of interest to the petroleum industry. Quartz-rich clastic rocks with abundant coals are exposed in SW Java and these rocks most likely extend within the subsurface northward, beneath much of West Java, toward the NW Java Sea. The presence of both source and reservoir rocks provide a possible petroleum system of relevance to future exploration in the region. The hypothesis of significant northward thrusting also offers new play concepts in SW Java. In this model Paleogene quartz-rich clastic sedimentary rocks would lie beneath the overthrust volcanic arc. Structural and stratigraphic traps may also exist beneath the thrust pile which may prove to contain hydrocarbon accumulations. CONCLUSIONS During the Late Cretaceous a continental fragment of Gondwana origin collided with the southern margin of the Eurasian continent causing subduction along the Java-Meratus subduction zone to cease. This contributed to the elevation of much of Sundaland at the end of the Cretaceous. We suggest a passive margin formed along the Java margin in response to this collision. The resumption of subduction is marked by coarse polymict breccias of the Middle Eocene Ciletuh Formation which we interpret to have been deposited in a deep marine forearc setting. Some distance to the north of the Ciletuh Formation there were quartz-rich sands derived from the north were deposited by braided rivers in West Java during the Middle and Late Eocene. These rocks are part of the Ciemas Formation and were deposited on the shelf, shelf edge and slope south of it. The continuation of Paleogene sediments in the subsurface beneath much of West Java combined with significant northward thrusting of the arc may provide new hydrocarbon exploration possibilities in West Java. There was volcanism associated with extension in parts of what is now the NW Java Sea during the

Early Oligocene. Volcaniclastic and lacustrine rocks make up the Jatibarang Formation. In the area that is now to the south of Sukabumi siliciclastic sediments of the Cikalong Formation were deposited in deep water and shallow water limestones were transported as olistoliths down a steep slope to the south of a narrow shelf. Further to the west volcaniclastic material which was sourced from the submerged arc contributed to what is now the Cijengkol Formation. In the Late Oligocene and earliest Miocene carbonates of the Rajamandala, Citarete and Cijengkol Formations were deposited close to the shelf edge. We suggest that during the Late Oligocene to Early Miocene the volcanic arc became emergent for the first time and there may have been an increase in volcanism. The Oligo-Miocene carbonates were smothered by material derived from the arc in the Early Miocene as the arc increased in size causing flexure and subsidence to the north. By the Middle Miocene there was a diminution or cessation of volcanism and carbonates were deposited in places on top of the inactive Southern Mountains volcanic arc. Shallow marine and terrestrial deposits of the Cimandiri Formation represent the final stages of basin fill within a flexural basin behind the arc in the west. Further to the east subsidence continued as calciturbidites of the Cinambo Formation were deposited. To the north deposition of the Upper Cibulakan Formation commenced and included the Massive and Main clastic units as well as Mid Main and Pre Parigi carbonates. During the Late Miocene volcanism resumed and several kilometres of volcanogenic material was deposited by turbidity currents and debris flows in a basin which covered an area over much of the western part of Central Java. The position of the Late Miocene arc is uncertain and we suggest that it was either located to the south of the present day Central Java, in a different position to the Paleogene and present day volcanic arcs or a gap in the arc existed in Central Java and no volcanoes had formed. To the north in what is now the NW Java Sea there was widespread deposition of carbonates. ACKNOWLEDGMENTS The SE Asia Research Group has funded this work. We thank many geologists at ITB and Pusat Survei Geologi (formerly GRDC) for collaborating in fieldwork and for many interesting discussions and

advice, and LIPI for much help with fieldwork permission requests. Ivan Yulianto and Edy Slameto who provided invaluable field support. We are very grateful to Peter Lunt for considerable help with planning, practical assistance and lengthy discussions regarding the geology of West Java, and Marcelle BouDagher-Fadel, Bob Morley, Bernhard Seubert and Moyra Wilson for help and advice. REFERENCES Amiruddin, and Trail, D.S., 1993, Geology of the Nangapinoh Sheet area, Kalimantan, Geological Research and Development Centre, Bandung, 49 p. Audley-Charles, M.G., Ballantyne, P.D., and Hall, R., 1988, Mesozoic-Cenozoic rift-drift sequence of Asian fragments from Gondwanaland, Tectonophysics, v. 155, p. 317-330. Barber, A.J., and Crow, M.J., 2005, Chapter 4: Pre-Tertiary stratigraphy, in A. J. Barber, M. J. Crow, and J. S. Milsom, eds., Sumatra, Geology, Resources and Tectonic Evolution, Geological Society London Memoir, v. 31, p. 24-53. Beckinsale, R.D., Suensilpong, S., Nakapadungrat, S. & Walsh, J.N., 1979, Geochronology and geochemistry of granite magmatism in Thailand in relation to a plate tectonic model, Journal of the Geological Society of London, v. 136, p. 529-540. Bignell, J.D. and Snelling, N.J., 1977, K-Ar ages on some basic igneous rocks from peninsula Malaysia and Thailand, Bulletin of the Geological Society of Malaysia, v. 8, p. 89-93. Butterworth, P.J., and Atkinson, C.D., 1993, Syn-rift deposits of the Northwest Java Basin, Fluvial sandstone reservoirs and lacustrine source rocks: In Atkinson, C.D., Scott, J., and Young, R., eds., Clastic rocks and reservoirs in Indonesia, IPA core workshop notes, Indonesian Petroleum Association 211-229. Cobbing, E.J., Pitfield, P.E.J., Darbyshire, D.P.F., and Mallick, D.I.J., 1992, The granites of the South-East Asian Tin Belt, British Geological Survey, Overseas Memoir 10, 369 p. Cole, J.M., and Crittenden, S., 1997, Early Tertiary basin formation and the development of lacustrine and quasi-lacustrine/marine source rocks on the Sunda Shelf of SE Asia, in Fraser, A.J., Matthews, S.J., and Murphy, R.W., eds., Petroleum Geology of

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Figure 1 - SE Asia map showing Sundaland block, subduction zones and political boundaries.

Figure 2 - Late Cretaceous (80 Ma) palaeogeographic reconstruction for SE Asia. A continental fragment of Gondwana origin collided with the southern margin of Sundaland causing a major plate reorganisation and cessation of subduction at the Meratus subduction zone.

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Figure 4 - a) Basement rocks (serpentinised peridotites) to the south of Ciletuh Bay. b) Polymict breccias

and interbedded volcanogenic turbidites at Pulau Kunti, Ciletuh Bay. c) Nummulitic limestone boulder within the Pulau Kunti breccia. d) Well bedded turbidites of the Ciemas Formation, south of Ciletuh Bay. e) Spectacular trough cross bedding within deltaic rocks of the Bayah Formation, Karantarage, Bayah. f) Rajamandala Limestone cliffs at Padalarang. g) Massive andesitic breccias of the Gabon Formation, Central Java and h) A flame structure with pen for scale from ignimbrites of the Cimapag Formation, Bayah. i) Part of more than 800m of sub-vertical volcanogenic turbidites of the Halang Formation, near Bumiayu.

Figure 5 - Middle and Late Eocene palaeogeography of West Java.

Figure 6 - A. Probability-density plot showing zircon populations for the Ciemas and Bayah Formations (n=109). Grains plotted are

Figure 7 - Early and Late Oligocene palaeogeography of West Java.

Figure 8 Early and Middle Miocene palaeogeography of West Java. Ash plumes represent dacitic and

rhyolitic volcanic centres where explosive volcanism is inferred.

Figure 9 - Late Miocene palaeogeography of West Java.