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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
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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
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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
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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
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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).
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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
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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
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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.
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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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Figu
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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.
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Figure 5 - Middle and Late Eocene palaeogeography of West
Java.
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Figure 6 - A. Probability-density plot showing zircon
populations for the Ciemas and Bayah Formations (n=109). Grains
plotted are
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Figure 7 - Early and Late Oligocene palaeogeography of West
Java.
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Figure 8 Early and Middle Miocene palaeogeography of West Java.
Ash plumes represent dacitic and
rhyolitic volcanic centres where explosive volcanism is
inferred.
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Figure 9 - Late Miocene palaeogeography of West Java.