Neogene Structures and Sedimentation History Along the Sunda Forearc Basins Off Southwest Sumatra and Southwest Java

Marine Geology 219 (2005) 133–154 www.elsevier.com/locate/margeo

Neogene structures and sedimentation history along the Sunda forearc basins off southwest Sumatra and southwest Java

Susilohadi Susilohadia,b, Christoph Gaedickea,*, Axel Ehrhardta

aFederal Institute for Geosciences and Natural Resources, Stilleweg 2, D-30655 Hannover, Germany bMarine Geological Institute, Bandung, Indonesia Received 14 December 2004; received in revised form 27 April 2005; accepted 5 May 2005

Abstract

Twenty multi-channel seismic lines in the southwest Sunda arc margin between Manna and west Java have been studied. This study depicts the structures and stratigraphy of the fore-arc basin since Late Paleogene in relation to the regional tectonic events. The paleomorphology of the Cretaceous continental margin persisted until the Oligocene and the paleoshelf margin of the continent extended north-westward off Sumatra. A residual basin filled with turbidite deposits developed just offshore of this margin. Subsequent fore-arc basin evolution was related to the slow down of subduction rate due to the collision of the Indian and Eurasian Plates in the Eocene. The rising Himalayan orogenic zone shed large amounts of sediment to the Indian Ocean and Sunda Trench beginning in the Late Paleogene which led to the growth of the accretionary prisms and the development of the Neogene fore-arc basin. At least two major structural events can be recognized in the fore-arc basin between Late Oligocene and Pliocene. Back thrust-faulting along the southern border of the fore-arc basin and initiation of the Cimandiri Fault Zone occurred in Late Oligocene, whereas the development of the Sumatra and Mentawai Fault Zones was initiated in Pliocene. Four Neogene sedimentary units can be recognized representing three main transgressive–regressive cycles and basin-fill deposits. The cycles resulted from a complex interplay of tectonically induced basin subsidence, eustatic sea level change and sediment supply related to volcanic activity that became abundant since late Middle Miocene. Turbidite deposition was common along and seaward of the basin slope during sea level lows in late Middle Miocene and Late Miocene. Sediment aggradation and progradation occurred during high sea level still stand and fall, respectively. Basin fills occurred mainly during the Pleistocene. D 2005 Elsevier B.V. All rights reserved.

Keywords: Fore-arc basin; Sunda arc; Sumatra fault zone; Mentawai fault zone; Java; Sumatra

1. Introduction * Corresponding author. Tel.: +49 511 643 3790; fax: +49 511 643 3663. A joint Indonesian–German project—the Geoscien- E-mail address: [email protected] (C. Gaedicke). tific Investigation of the Active Convergence zone

0025-3227/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2005.05.001 134 S. Susilohadi et al. / Marine Geology 219 (2005) 133–154 between the Eurasian and Indo-Australian Plate off ments, gravity profiling, swath bathymetric and Indonesia (GINCO 1)—was undertaken in 1998 to sediment echographic recordings. The main scientific acquire bathymetric and geophysical survey lines off objectives of this project were to study: southeast Sumatra and southwest Java (Figs. 1 and 2). The survey included digital multi-channel seismic re- – The geologic-tectonic development of the plate mar- flection profiles (MCS), gradient magnetic measure- gins and the structure of the subduction zone; and

Fig. 1. Generalized tectonic map of western Indonesia and location of study area. Structures on Sumatra are based on Sieh and Natawijaya (2000) and the Mentawai Fault Zone is based on Diament et al. (1992) and the present study. Limit of Cretaceous continental crust is taken from Hamilton (1979). CFZ=Cimandiri Fault Zone. S. Susilohadi et al. / Marine Geology 219 (2005) 133–154 135

102o E 103o E 104o E 105o E 106o E 107o E A-1X A-2X-1 Manna 7-14 13 SUMATRA 5 SO 00 200

1000 1500 SO137-10 Fig.10 SO137-09 o 1000 5 S

1 200 500 2000 Kota Agung SERIBU PLATFORM ENGGANO SO137-12 500 SO137-11 SO137-20

STRAIT 6oS Fig.7 1500 SUNDA SO137-19 6 2000 7-3 13 C-1SX SO PANAITAN SO137-06 JAVA 3 SO137-0 000 Fig.6 1500 UJUNG KULON ACCRETIONARY WEDGE 4 SO137-42 UK-1 C-1 Bayah SO137-27 Pelabuhan 2000 7oS Fig.3 2000 Ratu 1000 3500 SO137-30 SO137-33 Ciletuh 3000 SO137-31 JAMPANG 5000 45 00 400 0 SO137-29 500 200 6000 1500 SO137-34 2500 2500 5500 Fig.4

3000 Fig.5 o 6500 8 S

SUNDA TRENCH 2500 5500 6000

2000 SO137-01 SO137-03 Legend

Tectonic deformation front 9oS

5000 5500 Bathymetric contour (m)

A-2X-1 Drill hole Reflection seismic line, 0

50 SO137-30 Solid line used for Figure 5 10oS

500

0 100 km 4

5 00

Fig. 2. Bathymetric map of study area modified from Etopo2 Global 2’ Elevation (Smith and Sandwell, 1997) and the location of the SO137 (GINCO) seismic lines used for this study. Bold lines are used for Figs. 3-7 and Fig. 10.

– The facies distribution, stratigraphy and thick- The Sunda Arc is characterized by deep fore-arc nesses of accumulated sediments along the fore- basins, which extend between an outer arc high and arc region. the island arc. The hydrocarbon potential and basin 136 S. Susilohadi et al. / Marine Geology 219 (2005) 133–154 evolution of the basins off northern Sumatra is well 1975; Hamilton, 1979), and became very active during constrained (Izart et al., 1994) while the evolution of the Paleocene when the subduction rate was greater the fore-arc basins off southern Sumatra and south- than 15 cm/yr (Molnar and Tapponnier, 1975; Karig et west Java are poorly understood. The study presented al., 1979). A slow down of the convergence rate to 3 here focuses on the seismic stratigraphy and the de- cm/yr occurred in the Middle Eocene when the Indian velopment of Neogene structures in the fore-arc basin. continent started to collide with Eurasia (Karig et al., The study is based on the interpretation of 20 MCS 1979). The decrease in the convergence rate led to the lines (Fig. 2), which cover an offshore area between development of many extensional basins in Southeast west Manna and southwest Java together with multi- Asia (Daly et al., 1991; Hall, 1996, 1997). In the Late beam bathymetric data off southwest Java. However, Eocene–Early Oligocene a renewed spreading in the this study should be regarded as a preliminary study Indian Ocean led to the change of convergence direc- due to the lack of other controlling data such as well tion to nearly NE and to an increase in the subduction data. In addition, most of the seismic lines are not rate along the Sumatra and Java margins to a steady 5– closely spaced and a lack of tie-lines resulted in 6 cm/yr (Liu et al., 1983; Karig et al., 1979; Daly et al., interpretation uncertainties. 1987; Hall, 1996, 1997). This, in turn, initiated the Neogene fore-arc basin development along the margin of the Sunda arc. The Late Oligocene–Early Miocene 2. Regional geological setting collision of India and Eurasia caused massive amounts of terrigenous sediment to be dumped into the Indian The Sunda Arc comprises the Sunda Trench, outer Ocean and Sunda Trench. These sediments were rap- arc high or fore-arc ridge, the fore-arc basins, the idly accreted, creating the large accretionary prism active volcanic arc and the Cenozoic foreland on (Matson and Moore, 1992). northeast Sumatra and northern Java (Hamilton, Two models are discussed to explain the oblique 1979). Parts of the outer arc high rise above sea subduction along the western Sunda Arc margin: (1) a level and form outer arc islands off western Sumatra, significant counter-clockwise rotation of Sumatra, whereas they lie below sea level south of Java. Karig southern Malaya and Kalimantan in the Middle Mio- et al. (1980) and Moore and Karig (1980) have attrib- cene (Ninkovich, 1976; Hall, 1997) and (2) the re- uted the rapid outgrowth of the fore-arc ridge in the newal of spreading in the Indian Ocean accompanied western part of Sumatra to the accretion of thick by the change of convergence direction of Indian Plate Bengal and Nicobar fan sediments since the Late with respect to the Eurasian Plate to NE (Huchon and Miocene. The fore-arc basins extend from Burma in Le Pichon, 1984; Jarrard, 1986; Malod et al., 1995). the north to eastern Indonesia in the south (Moore et Both models led to increased obliquity that was ac- al., 1980, 1982). They are bordered by the outer arc companied by initiation of strong magmatic activity high and by the margin of the Sunda Island Arc. This (Simanjuntak and Barber, 1996; Hall, 1996, 1997). system resulted from plate convergence along the The oblique subduction beneath Sumatra caused the subducting oceanic Indian–Australian Plate beneath partition of strain into an orthogonal component the continental Eurasian Plate (Hamilton, 1979). resulting in thrust faulting in the accretionary wedge The relative movement between the Indo-Austra- and a right-lateral strike-slip component expressed by lian and Eurasian Plates during the Cenozoic is well the Sumatra Fault (Katili, 1973; Hamilton, 1979; constrained by paleomagnetic data and ocean floor Moore et al., 1980; McCaffrey, 1991, 2000; Malod magnetic anomalies on which various regional plate et al., 1995) and Mentawai Fault (Diament et al., tectonic reconstructions have been proposed (e.g. Daly 1992) systems. The Sunda Strait region represents et al., 1987; Rangin et al., 1999; Longley, 1997; Hall, the transition zone from the oblique convergence 1996, 1997). Since the Early Cenozoic India and Aus- along Sumatra to nearly normal convergence off tralia became a single plate and moved northward southern Java. It is interpreted either as related to against the Eurasian Plate (Liu et al., 1983; Hall, rotation of Sumatra relative to Java in the Late Ceno- 1997, 1998). The subduction system along the south- zoic (Ninkovich, 1976; Zen, 1985), or as an exten- west Sunda margin was initiated in Cretaceous (Katili, sional feature (Huchon and Le Pichon, 1984) resulting S. Susilohadi et al. / Marine Geology 219 (2005) 133–154 137 from the northwestward displacement of the southern were applied and a significant improvement of inter- Sumatra block along the Sumatra Fault. pretation results was achieved. In addition swath bathymetric data of the RV SONNE cruises SO137, SO138 and SO139 were 3. Data acquisition and processing loaded. By comparison of the seabed morphology with seismic imaging it is possible to cross-check Surveying was done along 45 traverses with a the CMP-locations and to determine the strike and digital multichannel reflection seismic system dip of structures and faults if they are visible at the (MCS). Seismic data with a total length of N4100 seafloor. Due to the fact that the hydrosweep data km and magnetic, gravity and bathymetric data with have some extension to both sides of the cruise lines a length of N5500 km were acquired. This study bases the strike of identified faults were recognized and on 20 MCS profiles with a length of 2560 km (Fig. 1). afterwards extrapolated to adjacent lines. After the loading of the seismic field data corrections for the true shotpoint-hydrophone geometry were done and common mid-point gathers (CMP) 4. Fore-arc basin created for quality control and for first interactive velocity analyses. The determination of the pre-stack The western Indonesian fore-arc basins extend parameters were performed by the FOCUSk software more than 1800 km from northwest of Aceh to south- modules, including e.g. bad trace editing, band pass west Java. The width of the basins varies from less filtering, deconvolution, velocity analyses. than 70 km south of the Sunda Strait to about 120 km The seismic processing was optimised in order to in the west off northern Sumatra (Fig. 1). The basins image deep crustal reflectors and to receive a suffi- form a strongly subsiding belt between the elevated cient resolution in the sedimentary cover. Hence, the Sumatra Paleozoic–Mesozoic arc massif cropping out multiple suppression was one of the main topics along Sumatra and Java, and the rising outer arc high during the processing. One or more of the following (Karig et al., 1980; Schlu¨ter et al., 2002). steps for multiple suppression were applied: 4.1. Basement ! FK-filtering after NMO overcorrection to attenuate the multiple-fartrace energy, The acoustic basement beneath the eastern side of ! Mute of the inner traces (neartrace) to attenuate the the northern Sumatra fore-arc basin is composed of multiple neartrace energy, Paleogene and older metasedimentary and metamor- ! Stacking to attenuate residual multiple energy. phic rocks (Beaudry and Moore, 1985; Cameron et al., 1980). Paleozoic metasedimentary and metamor- In the post stack domain an extra filtering and fk- phic rocks intruded by Permo-Triasic granites occur migration was done on all profiles. Instead of the fk- on the Barisan complex along northern Sumatra migration an Omega-x migration was applied on se- (Katili, 1973; Cameron et al., 1980). The exposed lected profiles in order to enhance the results in cases me´lange complex on the Banyak Islands (halfway of lateral velocity changes. Then the processed data between Nias and Sumatra, Fig. 1) led Karig et al. were converted into a SEG-Y format and loaded to the (1979) to suggest that in the central Sumatra region Schlumberger-GeoQuest Geoframe/IESXk system the subduction system has been continuously devel- for interactive interpretation. oped since the Cretaceous and has migrated westward A main difficulty of interpretation was the far grid across the Sumatra margin. line spacing in the prospect area as well as the lack of In the study area off south-western Sumatra, fea- tie lines for tracing seismic layers from one line to tures of the me´lange complex underneath the fore-arc another. Especially due to the geologic situation with basin are poorly imaged on seismic sections. A wide strong deformation and separated sub-basins tracking angle refraction study (Lelgemann et al., 2000; Kopp of seismic horizons was complicated. To overcome et al., 2001) revealed a layer with velocities ranging these difficulty different tools of the IESXk system from 5.3 to 5.7 km sÀ 1 that increased in thickness 138 S. Susilohadi et al. / Marine Geology 219 (2005) 133–154 from 1.5 km beneath the shelf to 4 km seaward, direction, the present bathymetric depth is more than suggesting highly consolidated sediments. The pre- 3000 m, suggesting a higher subsidence rate in this Cenozoic succession exposed in the Manna onshore area which resulted in thick deposits of Neogene region consists of Jurassic to Cretaceous sedimentary sediments (Figs. 2 and 5). and volcanic rocks comparable with the metasedi- The present depth of the base of the Neogene ments exposed in northern Sumatra (Gafoer and sedimentary succession is shown in a time contour Amin, 1993; Cameron et al., 1980). In contrast to map (Fig. 9). The broad shelf dips gently to the this continental type basement, the basement exposed southwest and stretches along the eastern side of the in the coastal region of Ciletuh, west Java (Fig. 2), is basin in the western region of south Sumatra. Here, of ophiolitic origin. It consists of peridotites, gabbros, the Early Neogene shelf edge occurs about 55 km pillow basalts and serpentinites that crop out in close southwest of the present coastline at a depth between association with metamorphic rocks including schists, 5.0 to 5.5 s two-way time (TWT). In front of this shelf phyllite and quartzite (Sukamto, 1975; Schiller et al., edge the acoustic basement submerges to N5.5sTWT 1991). Hamilton (1979) and Wakita (2000) have inter- and underlies a NW–SE striking elongated basin, preted this rock suite as part of the Cretaceous accre- running parallel to the subduction zone. This outer tion–collision complexes, which extend northeasterly basin may be regarded as a residual basin (Dickinson toward central Java and southeast Kalimantan (Fig. 1). and Seely, 1979) that evolved as a composite type The offshore extent of the Ciletuh complex is indicat- during the growth of the accretionary prism. The ed by a rough basement surface associated with strong Mentawai Fault Zone (MFZ) cuts through this basin faulting on the nearby seismic sections SO137-31 , (Figs. 7 and 8). The NW–SE trending accretionary SO137-33 and SO137-34 (Figs. 3 and 4). This inter- complex borders the southwestern end of the fore-arc pretation is supported by a high velocity layer (N5.9 basin, and is separated from the MFZ by a NW–SE km sÀ 1) beneath the fore-arc basin south of Jampang, trending smaller basin (Fig. 7). The top of the accre- west Java (Kopp et al., 2001, Schlu¨ter et al., 2002). tionary complex progressively deepens from Enggano Therefore, the continental margin and the subduction Island to more than 3.5 s TWT in the region south of zone have shifted southward since the Cretaceous, and the Sunda Strait and off Ujung Kulon on Java (Fig. 2). were located south of Jampang by Late Oligocene– A steep slope borders the northeastern basin in the Early Miocene (Hamilton, 1979; Fig. 2). vicinity of the Sunda Strait (Fig. 2) and is related to the southwestward tilting of the basement (Fig. 6). A 4.2. Neogene fore-arc basin structures steep slope gradient also occurs along the south off Ujung Kulon to Bayah, and probably follows the The morphology of the fore-arc basins is asymmet- Cretaceous–Paleocene trend of the continental margin rical (Fig. 2, Figs. 5-7). The inner slope near the island (Fig. 1). The Cimandiri Fault Zone (CFZ) separates of Sumatra dips gently southwestward toward the these regions from the Ciletuh–Jampang block, which accretionary complex where the water depth exceeds is underlain by the Cretaceous–Paleocene accretionary 2000 m. South of the Sunda Strait, the bathymetry is complex. The later was inverted during the Late Ol- strongly controlled by fault structures. Steep slopes on igocene when subduction in the Indian Ocean was the southern Sunda Strait represent southward plung- renewed. A rather narrow Neogene shelf area is pres- ing grabens caused by the transtensional Sumatra ent along the southern Jampang region (Figs. 2 and 5), Fault Zone (SFZ) (Lelgemann et al., 2000; Fig. 8), and corresponds to the southward tilting of the Cile- while the bay off southwest Java is governed by the tuh–Jampang block during the Neogene. The outer extensional tectonics of the Pelabuhan Ratu Fault basin spreads south of the Neogene shelf edge, where Zone (PRFZ) in association with the Cimandiri the top of the acoustic basement reaches depths of Fault Zone (CFZ). Water depth south of the Sunda more than 7.5 s TWT. The southern border of the Strait is less than 1500 m, but increases to more than basin is the hanging wall block of a thrust-faulted 2000 m off southwest Pelabuhan Ratu. South of the system, which possibly has developed since Middle Jampang region, where the basin axis is nearly or- Miocene. The upper part of the mid-Miocene section thogonal to the India–Australia plate convergence is folded by these thrusts and little thickness variation .Sslhd ta./Mrn elg 1 20)133–154 (2005) 219 Geology Marine / al. et Susilohadi S.

Fig. 3. Combined seismic lines SO137-30 and SO137-31 from off southwestern Java. Flower structure shown on SO137-30 has been generated by the Cimandiri and Sumatra Fault Zones (CFZ and SFZ) and occurred on Eocene–Oligocene submarine fan lobes. The faulting shown on SO137-31 represents extensional features of the Pelabuhan Ratu Fault Zone (PRFZ). Most of Neogene sediment thicknesses have been controlled by these faults. 139 140 .Sslhd ta./Mrn elg 1 20)133–154 (2005) 219 Geology Marine / al. et Susilohadi S.

Fig. 4. Seismic line SO137-34 from off southwestern Java showing southward wedging out of Neogene sediments. Unit 1 has been thrust-faulted southwards following renew subduction in the Oligocene. A structural unconformity observed on top of the Miocene Unit 3 indicates an increase in tectonic activity. .Sslhd ta./Mrn elg 1 20)133–154 (2005) 219 Geology Marine / al. et Susilohadi S.

Fig. 5. Seismic line SO137-03 off Jampang, west Java. Unit 1 has been distinguished from Unit 2 based on the strong structural onlaps related to the Late Oligocene back thrusting. The upper part of Unit 2 shows a northward progradation due to uplift of the accretionary prism. 141 142 S. Susilohadi et al. / Marine Geology 219 (2005) 133–154

Fig. 6. Seismic line SO137-36 from the southern Sunda Strait. Slope and basin fans are characteristic of forearc basins during relative sea level lows, but were not well developed during the late Middle Miocene relative sea level low, which may relate to a lack of sediment supply. indicates that the underlying folds existed when the Three major active faults occur in the fore-arc of mid-Miocene sediments were deposited. (Fig. 5). This the area studied (Fig. 8). Two dextral strike-slip fault system forms a prominent lineament striking north- zones aligned sub-parallel to the Sunda trench occur west-southeast across the Pelabuhan Ratu Bay (Fig. in the fore-arc, namely the Sumatra (SFZ) and Men- 8). Similar thrust-faulting is described by Van der tawai (MFZ) fault zones. These fault zones accom- Werff (1996) from the southern margin of the central modate the oblique convergence of the Indo- Java fore-arc basin in the Rama 44–46 section. Australian Plate with respect to the Eurasian Plate .Sslhd ta./Mrn elg 1 20)133–154 (2005) 219 Geology Marine / al. et Susilohadi S.

Fig. 7. Seismic line SO137-19 from the forearc basin west of Sumatra. The shelf has progressively shifted landward since Early Neogene. The basin is mostly filled with turbidite and basin facies deposits. 143 144 S. Susilohadi et al. / Marine Geology 219 (2005) 133–154

Fig. 8. Structural map of the study area illustrating the position of the Cretaceous–Paleogene shelf relative to the present structural configuration. Graben structures in the Sunda Strait are based on the study of GINCO MCS records and Lelgemann et al. (2000). Bathymetric lineations south of west Java are interpreted from multibeam bathymetric data. The lineations show the offshore extension of Sumatra Fault Zone (SFZ) and Late Oligocene thrust-faulting. CFZ=Cimandiri Fault Zone; PRFZ=Pelabuhan-Ratu Fault Zone.

(Malod and Kemal, 1996; Samuel and Harbury, ulate that the CFZ may have been active since Early 1996; Baroux et al., 1998). The third major fault Paleogene. Malod et al. (1995) assumed that the CFZ zone is the sinistral Cimandiri Fault Zone (CFZ) that extends towards Pelabuhan Ratu Bay in slightly cuts in a N708 E direction along the Cimandiri River different direction, which they referred to as the near Pelabuhan Ratu (Fig. 8) into the Java fore-arc Pelabuhan Ratu Fault Zone (PRFZ). Lines SO137- basin (Dardji et al., 1994). This fault separates the 30, SO137-31 and SO137-34 indicate that CFZ may continental crust to the north from the Cretaceous– splay in the Pelabuhan Ratu Bay into several faults, Paleocene accretionary complex to the south (Hamil- and accommodate extensional forces since the Late ton, 1979, Fig. 1). The occurrence of a Middle Miocene (Figs. 3 and 4). Eocene–Upper Oligocene submarine fan complex The SFZ striking parallel to the volcanic arc on along this border led Schiller et al. (1991) to spec- Sumatra connects the Andaman transform faults in the S. Susilohadi et al. / Marine Geology 219 (2005) 133–154 145

Fig. 9. Depth contour map of acoustic basement in two way travel time. Deep basement positions correspond with thick Neogene sedimentary deposits. north with the Sunda Strait graben system. The area of the Sunda Strait. Reflection data show that the SFZ between the trench and the SFZ has been referred to as and MFZ, and their associated fold structures, might the Sumatra fore-arc sliver by Jarrard (1986). In the have occurred since the Pliocene as defined by onlap- fore-arc basin south of west Java the NW–SE linea- ping strata. Therefore the origin of the fault system tions recorded in multibeam bathymetric data are may be linked to the increase of tectonic activity in the believed to represent the southern extension of the Pliocene. SFZ (Fig. 8). A positive flower structure is associated with the main fault zone (e.g. Fig. 4). To the southeast of Ujung Kulon (Fig. 8) a southern splay of the SFZ is 5. Chronostratigraphy recognized. This splay is associated with a flower structure that can be traced westward. However, the The definition of a chronostratigraphic framework structure diminishes before reaching the seismic line in the study area is problematic due to several reasons. SO137-36, but it may resume as the MFZ to the west Only few oil companies have carried out hydrocarbon 146 .Sslhd ta./Mrn elg 1 20)133–154 (2005) 219 Geology Marine / al. et Susilohadi S.

Fig. 10. Seismic line SO137-10 from off west Sumatra. The structure on the rightmost part is the MFZ flower structure. The upper part of Unit 3a shows a chaotic character that may represent crept or slumped deposition. S. Susilohadi et al. / Marine Geology 219 (2005) 133–154 147 exploration, such as seismic surveys or well drilling, Miocene turbiditic sediments (Amin et al., 1993). and published reviews based on these data are few. Basinward, the late Middle Miocene unconformity is Moreover, most of the available well data are from less pronounced, merging into a conformable succes- shelf areas that are not covered by the seismic lines sion between Middle and Upper Miocene sediments. used in this study. Across and inter-basinal strati- The Plio–Miocene boundary is well constrained by graphic correlations are also difficult due to the struc- a Upper Miocene unconformity on seismic sections tural complexity of the area and the fact that seismic from southwest Sumatra (Figs. 6, 7 and 10). The tie-lines are lacking. In this study, the chronostrati- boundary extends along the shelf, but shows rather graphic framework and stratigraphic facies definitions conformable bedding in the basin. A remarkable Plio– rely on previous fore-arc basin studies such as those Miocene unconformity is also observed in some by Bolliger and De Ruiter (1975), Noujaim (1977), regions within Pelabuhan Ratu Bay and the outer Karig et al. (1980), Beaudry and Moore (1985), Har- arc south of it. In the latter area it is now lying bury and Kallagher (1991), Schiller et al. (1991) and more than 3.5 s TWT deep (Figs. 3 and 4). However, Matson and Moore (1992), in addition to the onshore the unconformity might not be due to subaerial expo- geological maps which are particularly helpful in sure but rather suggests a strong influence of local interpreting nearby seismic lines. tectonics due to the initiation of SFZ and PRFZ. In the Ciletuh region the Eocene sediments uncon- The Plio–Pleistocene boundary has been defined formably lie on the pre-Cenozoic me´lange complex with reference to well C-1 SX data from the Sunda (Van Bemmelen, 1950; Sukamto, 1975; Schiller et al., Strait (Noujaim, 1977). Here, the Pliocene and Pleis- 1991). The suspected seaward extension of this Lower tocene sediments are unusually thick. Subsidence is Cenozoic unconformity in the vicinity of the Ciletuh probably linked to graben formation and the loading region is well imaged, but on the shelf area off effect of the Krakatau volcaniclastics. The southward western Sumatra it may have been superimposed by extension of the Plio–Pleistocene boundary is difficult the Upper Oligocene unconformity leaving patchy to trace due to the presence of basement highs. In the areas occupied by Paleogene sediments. fore-arc basin the boundary is tentatively placed on The Upper Oligocene unconformity marks the base the latest observable onlapping pattern, which usually of Neogene deposition. This erosional surface is is associated with young (Pliocene) deformation. marked by a very prominent angular unconformity of regional extent traceable in the fore-arc basins from northern Sumatra to the south of central Java 6. Seismic stratigraphic interpretation (Bolliger and De Ruiter, 1975; Karig et al., 1980; Beaudry and Moore, 1985; Rahmat and Oemar, Five main seismic units can be identified, referred 1998). The unconformity is well recognized on almost to as Units 1 to 5, consisting of Paleogene, Lower to all seismic sections. It indicates that prior to the Middle Miocene, Upper Miocene, Pliocene and Pleis- Neogene the shelf area was dominated by subaerial tocene deposits, respectively. exposure or by shallow water conditions. A pre-Neo- gene sea level below the shelf edge was also sug- 6.1. Paleogene unit 1 gested by Beaudry and Moore (1985) and Matson and Moore (1992) for the northern Sumatra fore-arc basin Paleogene Unit 1 spans from Middle Eocene to area. Late Oligocene during the phase of slow subduction. The upper Middle Miocene unconformity is well Unit 1 was deposited mostly beyond the paleoshelf as imaged on seismic sections from the shelf area, par- a turbidite fill. Off southwest Sumatra Unit 1 builds ticularly near the present coast (Fig. 6). Here, the late up a wedge body along the residual (outer) basin (Fig. Middle Miocene erosion has also truncated the Upper 7). The fill extends and laps out southward, but the Oligocene unconformity. This unconformity is also lack of an accretionary restricting barrier during the observed in the onshore region north of Manna slow down of subduction in Paleogene led the fill to (Figs. 7 and 10), where Middle–Upper Miocene shal- form as a sedimentary envelope over the accretionary low marine clastics overlie Upper Oligocene–Lower complex. In this area, the fill is highly deformed by 148 S. Susilohadi et al. / Marine Geology 219 (2005) 133–154 folds and thrusts rooted from the imbricate thrusts of 1999). The onshore and offshore data suggest that the complex, presumably since the increase of sub- the paleoshelf edge was situated just south of duction rate in the Late Eocene. Southwest of the Bayah, and probably coincident with the Cimandiri Sunda Strait a complex external mounded form char- Fault Zone. acterizes the residual basin (Fig. 6) and has been Along the south and southwest basin margin, par- interpreted as a submarine fan complex. ticularly off south Java, most of Unit 1 has been Sediments of similar settings in the northern Suma- folded and thrust-faulted in the Middle to Late Mio- tra fore-arc have been described as turbidite fills cene (Figs. 5 and 7). This deformation event is over- (Karig et al., 1980). In some areas of the shelf, Unit printed by Pliocene to Pleistocene deformation 1 is difficult to distinguish from the underlying base- associated with the Mentawai Fault Zone along the ment. The uppermost part of the basement probably suspected boundary between the continental crust and represents Unit 1 on Line SO137-19 (Fig. 7). It shows accretionary zone. a sub-parallel reflection pattern with strong amplitudes and low frequency indicative of a non-marine 6.2. Miocene units 2 and 3 to shallow marine depositional environment. A similar setting is also assumed by Karig et al. (1979) for the In the shelf regions the Lower–Middle Miocene rock cropping out along the west coast of north Unit 2 unconformably overlies the basement or sedi- Sumatra, which consists mainly of fluviatile to littoral mentary Unit 1. It was deposited in response to the conglomerates, sands and carbonaceous shales. A well subsequent relative sea level rise and following pro- from the shelf off northwest Manna drilled into Oli- longed still stand. Unit 2 shows a sheeted depositional gocene shallow marine to littoral sediments with some pattern in coastal areas off southwest Sumatra. It is indications of terrestrial influences and debris flows, characterized by medium to strong amplitudes and a suggesting tectonically unstable conditions during the parallel reflection pattern. Unit 2 gradually thickens time of deposition (Rahmat and Oemar, 1998). landward (Figs. 7 and 10). Its lower part downlaps the In the region off southwest Java Unit 1 is repre- underlying units during the sea level rise (Fig. 10). The sented by thick layers of subparallel, chaotic to hum- seaward oblique progradation pattern, low angle mocky reflection patterns with medium amplitudes. downlap contact and the development of shelf edge Here, Unit 1 locally shows a giant mounded external reefs prove the basinward progadation of Unit 2 during architecture with a downlap contact onto the sus- the subsequent high sea level still stand (Figs. 6 and 7). pected Cretaceous and Lower Paleogene accretionary In Pelabuhan Ratu Bay the lower part of Unit 2 is complex (Fig. 3). These external forms are interpreted mostly developed in the deeper part of the basin, to represent the body of a fan system. The fan system which is suspected to be a former residual basin. It thins towards Pelabuhan Ratu Bay, but extends south- cannot be differentiated clearly from Unit 1 due to the ward, downlapping onto the accretionary complex complexity of the structures (Fig. 3), but they may (Fig. 4), and extends southeast towards the basin have been continuously developed as low stand south of Jampang (Fig. 5). The sedimentary rocks deposits during the low sea level still stand in the overlying the Cretaceous–Paleocene accretionary Late Oligocene. During the subsequent sea level rise, complex in the Ciletuh region (Fig. 2) consist mainly Unit 2 developed mostly on the morphologically high of Middle to Upper Eocene sand-dominated subma- areas, which may have been developed as shelf depos- rine fan deposits (Schiller et al., 1991) that are prob- its in the Early to Middle Miocene (Fig. 3). ably contemporaneous with Unit 1. The mineralogical South of Jampang the thick Unit 2 has been dif- composition suggests that these fan deposits have ferentiated from Unit 1 based on the presence of onlap been derived mainly from the exposed Mesozoic gra- contacts which get clearer along the southern basin nitic rocks in the Seribu platform north of Java. The margin (Fig. 5). The structures, which are accompa- coeval sediments recovered from wells UK-1A, C-1 nied by back and seaward thrusting, are believed to and onshore outcrops (Bayah area, Fig. 2) indicate a have developed since the Late Oligocene when sub- shallow marine environment of inner shelf grading duction renewed. In the basin centre, both Units 1 and into deltaic and marshy conditions (British Gas, 2 were continuously developed and show strong S. Susilohadi et al. / Marine Geology 219 (2005) 133–154 149 amplitudes, low frequencies, and a subparallel to par- Unit 3 in Pelabuhan Ratu Bay mostly occurs in allel reflection pattern onlapping the basement in a topographic lows and is believed to represent mass- landward direction. The upper part of Unit 2 is dom- transported or high energy turbiditic deposits. Fur- inated by high to medium amplitudes and a parallel thermore, the Late Miocene tectonic activity on the reflection pattern. It shows northward prograding re- PRFZ is most likely responsible for the instability of flection patterns indicating the continuous uplift of the the region and controlled the subsidence and thick- accretionary prism. Overall, off southern Jampang, the ness of Unit 3 (Fig. 3). deposition of Neogene Unit 2 prograded and shifted northward. 6.3. Pliocene and Pleistocene units Unit 3 of Late Miocene age can be subdivided into Unit 3a and 3b in the fore-arc basin west of Sumatra. A major sea level fall at the end of the Miocene Unit 3a was developed mostly in the outer basin (Figs. finished deposition of Unit 3. The seismic reflection 7 and 10). It is relatively thick in the fore-arc basin patterns of the Pliocene and Pleistocene units, and west of Sumatra, but is nearly absent in the fore-arc their lateral distributions, are less variable than the basin west of the Sunda Strait (Fig. 6). It onlaps onto Miocene units. The Pliocene and Pleistocene units are Unit 2 in a landward direction and downlaps onto Unit commonly characterized by high continuity, medium 1 and the accretionary complex seawards. It is char- amplitudes and high frequencies of parallel reflec- acterized by a strong amplitude, parallel reflection tions, typical of submarine fan deposition and basin- pattern and by divergent external forms that may be fill facies. Overall thickness of the Plio–Pleistocene associated with small mounded bodies (Fig. 7). A units may reach 2.0 s TWT, particularly along the widespread chaotic pattern associated with relict bed- outer basin area (Figs. 6 and 7). This sediment thick- ding is observed on the upper part of Unit 3a in ness indicates high rates of sediment supply possibly seismic sections SO137-12 (Fig. 2) and SO137-10 caused by an increase in volcanic activity along (Fig. 10), indicating that the sediments have crept or Sumatra and Java islands or by rapid uplift of the slumped downslope in response to slope failure or island arc. The Pliocene and Pleistocene succession have been deposited by a succession of turbidity can be subdivided into three main units based on current flows. Its external form, associated with its seismic horizon terminations and geometry, these thick deposits, suggests that the basin formation has are: Units 4a and 4b of Pliocene age, and Unit 5 of been more intense combined with a high sediment Pleistocene age. supply, possibly due to increased volcanic activity. Unit 3b occupies the shelf. It was deposited fol- 6.3.1. Unit 4a lowing a relatively rapid sea level rise, and a subse- Deposition of the Lower Pliocene Unit 4a was quent highstand prior to the sea level fall again in the concentrated along the outer basin area following a Late Miocene. Unit 3b is represented by a thick relative sea level fall at the end of the Miocene. It is vertical aggradation of shelf deposits off west Sumatra regarded as a lowstand deposit. South of the Sunda and the Sunda Strait. It exhibits medium amplitudes Strait Unit 4a onlaps the Late Miocene Unit 3. It and parallel reflectors with high lateral continuity wedges out on the landward (northeastward) slope, (Figs. 6, 7 and 10). The upper part of Unit 3b dips but reaches a thickness of nearly 1.0 s TWT in the slightly southwestward. The sigmoidal pattern indi- centre of the basin (Fig. 6). The unit also tapers off on cates a slow regression on a low angle slope (Figs. 6, the accretionary prism and thins laterally northwest- 7 and 10). ward to less than 0.5 s TWT off southwest Sumatra, In Pelabuhan Ratu Bay and south of Jampang and less than 0.2 s TWT south of Ujung Kulon. In part both faulting and limited data hinder the subdivision of seismic section SO137-04 from this area, Unit 4a of Unit 3. Most of Unit 3 probably consists of Unit exhibits a stack of hummocky to chaotic packages, 3a, which is characterized by widespread chaotic which laterally turn into medium amplitude parallel reflection patterns in the Pelabuhan Ratu Bay region reflectors with high continuity. The variation in thick- (Fig. 3) and by a medium amplitude parallel reflec- ness of Unit 4a south of the Sunda Strait corresponds tion pattern in the region south of Jampang (Fig. 5). to the initiation of the SFZ and the opening of the 150 S. Susilohadi et al. / Marine Geology 219 (2005) 133–154

Sunda Strait. The developing graben allowed a turbi- controlled by the CFZ (Fig. 3). In the southern Jam- dite fan to be deposited. pang region lenses of chaotic patterns occur locally Unit 4a is well developed in deeper parts of the within Unit 5, the upper part is often characterized by basin southwest of Pelabuhan Ratu. It is characterized southward dipping slump structures (Fig. 5) suggest- by high amplitudes and frequencies, and by a contin- ing a high seismicity that may cause slope failure. uous parallel reflection pattern. The PRFZ strongly controlled the thickness variation of both Units 4 and 5 in the Pelabuhan Ratu Bay area. In the region south 7. Discussion of Jampang a pronounced southward downlapping contact indicates sediment supply from the north 7.1. Paleomorphology and structural development that differed from the underlying units. Table 1 compiles the fore-arc basin stratigraphy, 6.3.2. Unit 4b the regional and local tectonic events and the global Subsequent to the deposition of Unit 4a, a relative sea level changes (Haq et al., 1988). The morphology rapid sea level rise occurred that shifted sedimentation of the Cretaceous continental margin described by onto the shelf. A thick vertical aggradation of Unit 4b Hamilton (1979), which ran from southeast Kaliman- is characterized by a low angle sigmoidal prograda- tan to northwest Java (Fig. 1), persisted until the tional seismic pattern particularly along the west Oligocene. The seismic data indicates that the Sumatra shelf. Reflection patterns are dominated by paleoshelf margin of this continent extended north- highly continuous patterns with medium amplitudes westward off Sumatra to a distance of about 55 km and high frequencies (Figs. 6 and 10). Basinward, in from its present position (Fig. 8). A curvilinear resid- most areas Unit 4b cannot be separated from the ual basin developed just seaward of this margin. The underlying basin-fill facies resulting from the persistence of this morphology is probably related to expected Pliocene regressive phase (Figs. 5–7 and the slow down of subduction associated with the 10). However, on some sections west of Sumatra collision of the Indian and Eurasian Plates in the (e.g. SO137-09) and south of Jampang, a progressive Eocene. As a result, turbidite and submarine fan basinward downlap of Unit 4b is observed, indicating deposits are widespread along the continental slope a high sediment supply which is probably associated and in the residual basins. In contrast, erosion or with Late Pliocene volcanism in the southern parts of shallow marine deposition is common on the shelf. Sumatra and west Java. This observation differs from studies of Schiller et al. (1991) who suggested that the CFZ had been active 6.3.3. Unit 5 since the Early Paleogene and induced a remarkable Continuing subsidence has led to a landward mi- slope to allow turbidite deposition along the southern gration of the fore-arc basin axis and shelf area. The margin of west Java. Furthermore, the Paleogene was Pleistocene Unit 5 consists mostly of basin-fill sedi- also characterized by a lack of magmatic activity, as ments. The limited extent of seismic sections in the indicated by the petrographic study on Paleogene landward direction hinders the study of Pleistocene sediments from the Ciletuh area by Schiller et al. sedimentation on the shelf region. (1991). The clastic deposits are dominated by quartz, West of Sumatra the development of the MFZ which was possibly derived from the Seribu platform, since the Pliocene has subdivided the fore-arc basin north of Java. into two smaller linear basins. The basin-fill Unit 5 The Late Eocene–Early Oligocene renewal of laps out against the flanks of this flexural structure spreading in the Indian Ocean led to (1) the change (Figs. 7 and 10). Southwest of the Sunda Strait, where of convergence direction to nearly NE, (2) the uplift of the MFZ is absent, Unit 5 onlaps the flanks of Unit 4b the Himalayas and the subsequent delivery of massive (Fig. 6). It is commonly associated with low to me- amounts of sediment to the Sunda Trench, and (3) an dium amplitudes, high frequencies and a continuous increase in the subduction rate to 5–6 cm/yr along the parallel reflection pattern. In the region of Pelabuhan Sumatra and Java margins (Liu et al., 1983; Karig et Ratu Bay, Unit 5 shows onlapping contacts that are al., 1979; Daly et al., 1987; Hall, 1997, 1998). The Table 1 Compilation of tectonic and depositional events in the study area and their relation to regional tectonic events and the global eustatic sea level curve

a) Eustatic curve Tectonic events W off Sumatra SW off Sunda Strait Pelabuhan Ratu Bay S off Jampang (M Epoch (Haq et al.,1988) Regional Local Shelf Outer basin Shelf Outer basin Shelf Outer basin Age 250 200 150 100 50 5 m 0 Pleistocene Basin fill Basin fill (Unit 5) (Unit 5) Basin fill Basin fill Late Increase of Prograding shelf Volcanic Initiation of deposits Submarine fan Submarine fan (Unit 4 & 5) & slump Early Pliocene SFZ & MFZ (Unit 4a) (Unit 4a) (Unit 4 & 5) 5 activities (Unit 4b)

Prograding shelf Prograding shelf 133–154 (2005) 219 Geology Marine / al. et Susilohadi S. Rapid outgrow deposits deposits Turbidite deposits Turbidite deposits Late of accretionary (Unit 3b) (Unit 3b) (Unit 3) (Unit 3) prism 10 Submarine fan ? No deposition (Unit 3a) Middle ?Relative Rotation 15 of Sumatra Prograding Prograding shelf deposits shelf deposits Miocene (Unit 2) (Unit 2b)

20 Early ?Turbidite deposits Turbidite deposits (Unit 2) (Unit 2) Initiation of forearc basin 25 Shelf Shelf Late erosion erosion Shelf erosion Shelf erosion

30 Initiation of CFZ

Oligocene Early Development of accretionary 35 prism ?Shallow Turbidite Increase of marine deposits Submarine fan Submarine fan Submarine fan Late subsidence rate (Unit 1) (Unit 1) (Unit1) (Unit1) (Unit1) renew spreading 40 in Indian Ocean Slowdown of convergence rate Eocene Middle Initial contact 45 Indian & Eurasian Plates 151 152 S. Susilohadi et al. / Marine Geology 219 (2005) 133–154 active subduction zone became established in front of in the Pliocene as indicated by onlaps and sediment Java and west Sumatra. The origin of the southeastern thickness variations around the fault zones. These part of Java between the Cretaceous–Paleogene con- structures, particularly the SFZ, have displaced the tinental margin and the Oligocene active subduction is former structural grains, including the Cretaceous– still uncertain. This area may have developed as a Paleogene paleo-shelf and those resulting from rota- sliver when its northern border, the sinistral Cimandiri tion-related tensional stress. In addition, submarine Fault, was initiated to accommodate the nearly NE fan (Unit 4a) deposits southeast of the Sunda Strait plate convergence direction. The accretion of the thick give evidence that the Sunda Strait opened during the terrigenous sediments also caused a rapid growth and Pliocene. rise of accretionary complexes off Java and Sumatra (Karig et al., 1979; Schlu¨ter et al., 2002). Therefore, 7.2. Sedimentary sequences the Indonesian fore-arc basin evolution is promoted by two factors: (1) the growth and uplift of the accre- Thick continental slope and basin turbidite deposits tionary prisms that act as a barrier to the deep sea; and (Unit 1) dominated prior the development of the (2) the subsidence of the basement induced by tecton- Neogene fore-arc basins. The extent of Unit 1 was ics, as indicated by tilting and landward shift of shelf controlled by the paleomorphology which followed margin. Both factors are controlled by the mode of the Cretaceous to Paleogene continental margin (Fig. subduction. Thus the reorganization of plates in the 8). Therefore, shelf deposits were limited off west Neogene has affected the evolution of the fore arc Sumatra and Java. The eustatic sea level fall in the basins. Late Oligocene has enhanced turbidite deposition by South of Jampang along the southern border of the augmenting shelf and onshore erosion. fore-arc basin, back thrust-faulting developed during Two transgressive–regressive sequences, Units 2 the early stage of basin development in the Late and 3, have been recognized to occur during the Cretaceous (Figs. 5 and 8). Faulting was followed Miocene. Deposition of Unit 2 commenced when by tilting of the Ciletuh-Jampang block towards the eustatic sea level rose in the Early Miocene combined southwest. Its displacement and subsidence with re- with basin subsidence induced by renewed subduc- spect to the continental crust in the north is accom- tion. A well developed seaward progradation was modated by the CFZ that splays into several faults in characteristic of deposition along the shelf margin Pelabuhan Ratu Bay. off west Sumatra during a relative sea level still In the Middle to Late Miocene, a precise geody- stand and fall in the Middle Miocene. In contrast, namic evolution of western Indonesia depends on the tectonic stress led to greater basin subsidence and postulated rotation of Sumatra (Ninkovich, 1976; growth of the accretionary prism off west Java. Sea Hall, 1997) which induced tectonic activity, rapid level fall at the end of the Middle Miocene (Haq et al., basin subsidence and increasing volcanism on Suma- 1988) ceased deposition on the shelf. Thick and wide- tra and Java. However, such rotation is strongly ar- spread mass transport and high-energy turbidite gued (Huchon and Le Pichon, 1984; Jarrard, 1986; deposits (Unit 3a) are indicative of a steep relief and Malod et al., 1995), many features which are presum- an increase in tectonic activity during the lowstand. ably related to the rotation, e.g. opening of the Sunda Unit 3b records continuous basin subsidence and Strait, may also be attributed to renewed spreading in abundant sediment supply which resulted in shelf the Indian Ocean that led to the change of conver- aggradation along Sumatra, the Sunda Strait and pos- gence direction to nearly NE and the increase of sibly off west Java. A relative sea level fall occurred at subduction obliquity along the western Sunda Arc the end of the Late Miocene and was followed by the margin. The seismic data are inadequate to substanti- deposition of turbidites (Unit 4a). Thick deposits oc- ate the deduction of rotation. Most structures devel- curred, particularly in the basin south of Sunda Strait oped in the vicinity of the Sunda Strait prior to and west Java, suggesting a higher sediment supply Pliocene have been overprinted by the Sumatra which may have resulted from increased volcanic Fault Zone. The arc parallel strike-slip faults, the activity. A continuing high sediment supply combined Sumatra and Mentawai Fault Zones, were initiated with eustatic sea level rise and basin subsidence in the S. Susilohadi et al. / Marine Geology 219 (2005) 133–154 153

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