Cretaceous accretionary–collision complexes in central Indonesia
K. Wakita
Geological Survey of Japan, 1-1-3 Higashi, Tsukuba, Ibaraki 305, Japan
Received 26 April 1999; accepted 15 January 2000
Abstract
The geology of Cretaceous accretionary –collision complexes in central Indonesia is reviewed in this paper. The author and his colleagues have investigated the Cretaceous accretionary–collision complexes by means of radiolarian biostratigraphy and meta-morphic petrology, as well as by geological mapping. The results of their work has revealed aspects of the tectonic development of the Sundaland margin in Cretaceous time. The Cretaceous accretionary–collision complexes are composed of various tectonic units formed by accretionary or collision processes, forearc sedimentation, arc volcanism and back arc spreading. The tectonic units consist of chert, limestone, basalt, siliceous shale, sandstone, shale, volcanic breccia, conglomerate, high P/T and ultra high P metamorphic rocks and ultramafic rocks (dismembered ophiolite). All these components were accreted along the Cretaceous convergent margin of the Sundaland Craton. In the Cretaceous, the southeastern margin of Sundaland was surrounded by a marginal sea. An immature volcanic arc was developed peripherally to this marginal sea. An oceanic plate was being subducted beneath the volcanic arc from the south. The oceanic plate carried microcontinents which were detached fragments of Gondwanaland. Oceanic plate subduction caused arc volcanism and formed an accretionary wedge. The accretionary wedge included fragments of oceanic crust such as chert, siliceous shale, limestone and pillow basalt. A Jurassic shallow marine allochthonous formation was emplaced by the collision of continental blocks. This collision also exhumed very high and ultra-high pressure metamorphic rocks from the deeper part of the pre-existing accretionary wedge. Cretaceous tectonic units were rearranged by thrusting and lateral faulting in the Cenozoic era when successive collision of continental blocks and rotation of continental blocks occurred in the Indonesian region.q2000 Elsevier Science Ltd. All rights
reserved.
1. Introduction
The Indonesian region includes components which record various tectonic scenarios, i.e. oceanic plate subduction, sediment accretion, back arc rifting, forearc basin develop-ment, continental accretion and collision, lateral fault displacement, thrust movement and metamorphic rock exhumation.
In this paper, the author will review the structure and tectonic history of the Cretaceous accretionary–collision complexes in Indonesia. He and his colleagues have investigated the zone since 1992 under a cooperative project between the Geological Survey of Japan, Univer-sity of London, Research and Development Centre for Geotechnology, and Geological Research and Develop-ment Centre, Bandung. The author and his colleagues published and reported their research results on the geology and tectonics of each complex. Here the author will try to subdivide the tectonic elements in the complex in order to describe the characteristic elements, and will explain the Cretaceous tectonics in the central Indonesian region as a whole.
2. Outline of geology
The main lithotectonic components of the central Indo-nesian region comprise pre-Tertiary accretionary complexes, continental fragments, ophiolite massifs and island arcs which are distributed between the western margin of the Sundaland craton and the north and north-eastern margins of the Australian craton (Fig. 1). They are formed by subduction, accretion and collision processes from the Cretaceous to present.
The Cretaceous accretionary–collisional complexes in central Indonesia are assemblages of tectonically disrupted rocks and formations generated by various processes. They are distributed mainly in West and Central Java, South and Central Sulawesi, and West and South Kalimantan (Asikin, 1974; Sukamto, 1975, 1978, 1982, 1986; Ketner et al., 1976; Sikumbang, 1986; Suparka, 1988; Sikumbang and Heryanto, 1994; Heryanto et al., 1994; Wakita et al., 1994a,b, 1996, 1998; Miyazaki et al., 1996, 1998; Wakita, 1997; Parkinson et al., 1996, 1998). The major complexes are the Luk Ulo Complex of Central Java (Asikin, 1974), the Bantimala Complex of South Sulawesi (Sukamto, 1982) and
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the Meratus Complex of South Kalimantan (Sikumbang, 1986) (Fig. 2). These complexes are composed of similar lithological components, including radiolarian chert, pillow basalt, clastic sedimentary rocks, low-grade schist and pelite-matrix melange (Fig. 3).
The Luk Ulo Complex (Asikin, 1974; Ketner et al., 1976; Suparka, 1988; Wakita et al., 1994a) is exposed in the Karangsambung area of Central Java (Fig. 2). The complex is an assemblage of E–W trending tectonic blocks composed of crystalline schist, phyllite, marble, rhyolite, dacite, mafic and ultramafic rocks, limestone, chert, silic-eous shale, shale, sandstone and conglomerate, occurring as tectonic slices and fault-bounded blocks (Fig. 3). The complex is unconformably overlain by the Eocene Karang-sambung Formation.
The Bantimala Complex is located about 40 km NE of Ujung Pandang in South Sulawesi (Fig. 2). The Bantimala Complex (Sukamto, 1975, 1978, 1982, 1986; Wakita et al., 1994b, 1996; Miyazaki et al., 1996; Parkinson et al., 1998)
is a tectonic assemblage of slabs and blocks consisting of sandstone, shale, conglomerate, chert, siliceous shale, basalt, ultramafic rocks, schist, schist breccia and felsic intrusive rocks (Fig. 3). The tectonic slabs are oriented NW–SE and dip steeply to the east. Stratigraphic and struc-tural relationships among the components are unclear, except for that between schist and chert. The ages of the components range from Jurassic to Late Cretaceous. The oldest lithotectonic unit is a Jurassic shallow marine forma-tion, the Paremba Sandstone. The complex is unconform-ably overlain by Palaeocene volcanic rocks and younger formations.
al., 1994; Wakita et al., 1998). The ages of components of the complex range from Jurassic to early Late Cretaceous (Fig. 3). These rocks are unconformably overlain by Late Cretaceous volcanic rocks and turbidite, such as the Pitap (Alino) and Haruyan (Pudak) Formations. All these Meso-zoic rocks are unconformably covered by Eocene and younger formations.
Similar complexes crop out in the Latimojong, Barru and Pompangeo areas, Sulawesi, and the Ciletuh and Jiwo Hills areas in Java. In the Sintang area of West Kalimantan, the Late Cretaceous Selangkai Group includes Cretaceous radiolarian chert, schist and limestone as olistoliths within turbidite-olistostrome sequences. These olistoliths are remnants of a former accretionary wedge similar to that which generated the Luk Ulo and Meratus Complexes.
The components of these complexes can be subdivided into accretionary units, collisional units, volcanic arc units, ophiolitic units, forearc basin units, intrusive and effusive units and cover formations, which reflect differing origins and tectonic histories.
3. Accretionary units
Accretionary complexes are characterized by the presence of melange, stacked tectonic slices, radiolarian chert underlain by pillow basalt, ophiolite, and recon-structed “oceanic plate stratigraphy”. Among them, the oceanic plate stratigraphy (OPS) is the most important feature for the recognition of ancient accretionary complexes.
3.1. Oceanic plate stratigraphy
using radiolarian biostratigraphy (Wakita et al., 1994a, 1998).
This type of succession is called ‘Oceanic Plate Strati-graphy’ (OPS) (Isozaki et al., 1990; Matsuda and Isozaki, 1991; Wakita, 1997). It is compiled by the following sequence of processes: generation of oceanic plate at the oceanic ridge; formation of volcanic islands near the ridge, covered by calcareous reefs; sedimentation of calci-lutite and radiolarian chert on the flank of the volcanic island; deposition of radiolarian skeletons on the oceanic plate in a pelagic setting; sedimentary mixing of radiolarian remains and terrestrial grains to form siliceous shale in a hemipelagic setting, and the sedimentation of coarse-grained sandstone and shale at or near the trench of the convergent margin.
Reconstructed OPS provides us with information on the age range of oceanic subduction, and the age and width of the
subducted oceanic plate. The ‘Luk Ulo oceanic plate’ was generated before the earliest Cretaceous, and seamounts covered by reef limestone appeared in the Early Cretaceous. Radiolarian biostratigraphy of chert indicates that stable pela-gic conditions continued from the Hauterivian to Campanian. The overlying siliceous shale and shale indicate the duration of subduction and sediment accretion at the trench.
3.2. Pillow basalt
the lowest part of the OPS. It is considered to represent fragmented upper sections of accreted seamounts.
3.3. Chert
Ribbon chert and associated siliceous shale are mostly reddish brown in color, but some of them are gray and green. The chert is made up mostly of radiolarian skeletons and and their fragments, and thickness of beds ranges from 1 to 20 cm. The lower part of the chert sequence is sometimes interbedded with light gray limestone. The chert and lime-stone is underlain by pillow basalt. Chert grades into silic-eous shale toward the stratigraphic top in some localities. The ages of the chert components are well defined by radi-olarian biostratigraphy (Okamoto et al., 1994; Wakita et al., 1991, 1994a,b, 1997, 1998; Munasri, 1995; Wakita, 1997). The chert of the Meratus Complex ranges in age from early Middle Jurassic to late Early Cretaceous, while the chert of the Luk Ulo Complex ranges from Early Cretac-eous to latest Late CretacCretac-eous (Fig. 3). The radiolarian chert of the Luk Ulo and Meratus Complexes was originally pelagic sediment deposited as radiolarian ooze on the ocean floor.
3.4. Melange
The melange of the Luk Ulo Complex in the Karangsam-bung area, central Java includes clasts of sandstone, shale, siliceous shale, chert, limestone, basalt, rhyolite and schist within a shale matrix, which is locally sheared. The clasts range in size from 1 mm to several meters, and sometimes include larger tectonic blocks. The dominant clast type is sandstone, which contains angular to subrounded fragments of quartz, feldspar and mica, as well as fragments of felsic to basic volcanic rocks. Although the complex itself is inferred to be a melange, the complex is just an assemblage of various kinds of tectonic blocks. Melanges containing clasts of various rock types, with highly sheared matrix are loca-lized in some places. They are highly tectonized pebbly mudstone which stratigraphically grades into a turbidite formation. The shale matrices of the melange were sheared but not pervasively sheared, and slaty cleavages are devel-oped within the matrix only locally.
Polymict melange in the Bantimala Complex generally occurs in narrow zones between the tectonic slices, and includes clasts and blocks of chert, sandstone, and siliceous shale with subordinate basalt, limestone and schist embedded within a variably sheared shale matrix. Frag-ments of metamorphic rocks are very rare. The clasts are subrounded to subangular, and rhomboidal, spherical, blocky or irregular in shape. Long axes of clasts range from several millimeters to several hundred meters.
Melange of the Meratus Complex crops out on Pulau Laut. The melange includes clasts and blocks of Jurassic to Cretaceous chert, siliceous shale, basalt, limestone, marl and manganese carbonate nodules embedded within a sheared shale matrix. Sandstone or other coarse-grained
terrigenous sediments are lacking in the melange. Chert and limestone are thinly bedded. Basalt is mainly lava, and pillow structures are sometimes preserved. Limestone clasts are locally dominant in the melange. Fragments of manga-nese carbonate nodules are rare. The clasts are subrounded to subangular, lenticular to blocky in shape. Clast size ranges from several millimeters to several hundred meters long. Chert sometimes includes well-preserved radiolarians, ranging in age from Middle Jurassic to Early Cretaceous (late Albian to early Cenomanian) age. Siliceous shale clasts include radiolarians of Early Cretaceous age. The age of melange formation is estimated as slightly younger than the youngest age of the components of the melanges (Fig. 3).
4. Collisional units
The Bantimala Complex is composed mainly of sand-stone, conglomerate, shale, siliceous shale, chert, basalt, ultramafic rocks, schist, schist breccia and felsic intrusive rocks. Most of the components are similar to those of the Luk Ulo and Meratus complexes. The Jurassic Paremba sandstone and schist breccia is unique to the Bantimala Complex. An “unusual unconformity”, with chert atop a schistose basement (Haile et al., 1979), the Jurassic Paremba Sandstone (Sukamto and Westermann, 1992), and ultra-high pressure rocks (Parkinson et al., 1996, 1998) are the keys to understanding the tectonics of the Bantimala Complex.
The chert of the Bantimala Complex was deposited in a forearc setting, near the provenance of schist breccia and coarse-grained sandstone, in a relatively short period from the late Albian to early Cenomanian, and is contempora-neous with the coarse-grained flysch sequence of the Balangbaru Formation. These features suggest that the chert of the Bantimala Complex was formed in a back-arc or fore-arc setting.
4.1. Allochthon
The oldest lithotectonic unit of the Bantimala Complex is a Jurassic shallow marine formation, the Paremba Sand-stone (Fig. 3). The lower part of the Paremba SandSand-stone is composed of thinly bedded sandstone and shale, intercalated with thin limestone layers. Some shallow marine sedimen-tary structures such as ripple and convolute laminations are recognized. The upper part of the Paremba Sandstone includes substantial conglomerate layers containing pebbles of basalt and schist. Ammonites (Middle Liassic Fucini-ceras), gastropods and brachiopods of Early and Middle Jurassic occur in the Paremba Sandstone (Sukamto and Westermann, 1992).
Sandstone is older than the high P/T metamorphic rocks, the formation is older than the accretion and collision stage. The sandstone is incorporated within the tectonic assemblage in the Bantimala Complex as tectonic slices detached from a colliding microcontinent in late Cretaceous time.
4.2. High and very high P/T metamorphic rocks
Metamorphic rocks in the Cretaceous suture zone are phyllite, quartz-mica schist, greenschist, glaucophane schist, and eclogite. They are mostly of high P/T type, shown by high P/T mineral assemblages (Miyazaki et al., 1998; Parkinson et al., 1998).
The major types of metamorphic rock in the Luk Ulo, Bantimala, Meratus and Pompangeo complexes are described as follows:
Lok Ulo:
Amphibolite-grade schist (predominantly garnet-mica-quartz schist) is tectonically intercalated within larger tectonic slices and blocks of sedimentary rocks. Small amounts of glaucophane rock, garnet amphibolite, lawso-nite eclogite and jadeite-quartz-glaucophane rock occur as small tectonic blocks in sheared serpentinite along fault zones (Miyazaki et al., 1998). The latter two types were recrystallized at pressures.18 kbar.
Bantimala:
Low grade greenschists and glaucophane-bearing schists occur as imbricate slices in the Bantimala Complex. Peak P–T conditions have been estimated at around 350– 4508C and 5–8 kbar (Miyazaki et al., 1996; Parkinson et al., 1998), and K–Ar ages are generally in the range 111–114 Ma (Hamilton, 1979; Hasan, 1990; Wakita et al., 1996; Parkinson et al., 1998). Very high pressure metamorphic rocks also occur, but are much less abun-dant. They occur as small tectonic blocks and slabs asso-ciated with serpentinite and comprise eclogite and garnet-glaucophane rock (P18–24 kbar, T580–6208C), coesite-bearing jadeite quarzites (P.27 kbar, T720–7608C and garnet amphibolite (Miyazaki et al., 1996; Parkinson et al., 1998). K–Ar ages of phengite for the VHP rocks are generally older than for the low-grade schist country rocks: 132^7 Ma, 113^6 Ma and 124^6 Ma (garnet-glaucophane rock; Wakita et al., 1996) and 137^3 Ma (eclogite; Parkinson et al., 1998). Meratus:
The complex is a tectonic assemblage of slabs and blocks consisting of high-pressure metamorphic rocks (Hauran Schist and Pelaihari Phyllite) distributed in the SW part of the Meratus Mountains. They occur as wedge-shaped tectonic blocks in fault contact with ultramafic rocks and Cretaceous formations. The metamorphic rocks are divided into two types, low grade “Pelaihari Phyllite”, and relatively higher grade “Hauran Schist”. The Hauran Schist includes glaucophane schist, chloritoid-quartz schist, kyanite-quartz-phengite-chloritoid schist, garnet
mica schist, quartz-mica schist, piemontite schist and amphibolite. The protoliths of the Hauran Schist were predominantly pelitic and basic rocks, although the chlor-itoid and kyanite-bearing schists are probably derived from bauxites and evaporites. K–Ar ages of micas range from 110 to 180 Ma (Wakita et al., 1998). Some of the metamorphic rocks from the Meratus Complex of SE Kalimantan are clearly of continental origin. Kyanite-quartz and chloritoid-quartz schists of the complex are probably derived from continental sedi-mentary cover rocks such as laterite. The same parentage was proposed for similar metamorphic rocks of the Pompangeo Complex in Central Sulawesi (Parkinson, 1991, 1996, 1998a,b).
4.3. Breccia-sandstone-chert sequence
Metamorphic rocks in the Bantimala area are unconform-ably overlain by schist breccia, which grades into radiolar-ian chert through sandstone interbedded with chert (Fig. 3; Haile et al., 1979; Wakita et al., 1994b). The schist breccia is mostly of sedimentary origin and contains angular frag-ments of schists within a sandstone matrix. The size of fragments ranges from several centimeters to several tens of centimeters. The metamorphic grade of the schist frag-ments is the same as that of the regional schists in the Bantimala Complex. The overlying sandstone is rich in mica and quartz fragments, and sometimes contains radi-olarian remains.
Thin beds of radiolarian chert are intercalated within coarse-gained sandstone. The number of chert beds increase towards the top of the stratigraphic column. The chert layers range from 1 to 20 cm thick and are interbedded with thinner shale layers less than 1 cm thick. The chert is rather muddy compared with cherts in other orogenic belts, and is composed mainly of skeletons and fragments of radiolarians with a small amount of shale. The chert sometimes includes well-preserved radiolarians of middle Cretaceous (late Albian to early Cenomanian) age, including Holocryptoca-nium barbui, Thanarla conica, Archeodictypomitra vulgaris and Phopalosyringium majuroensis.
4.4. Forearc basin unit
The Cretaceous forearc basin unit is developed over a wide area, including South and West Kalimantan and South and West Sulawesi (Figs. 2 and 3). The Balangbaru Formation of South Sulawesi and the Pitap Formation of South Kalimantan have been well investigated (Hasan, 1990, 1991; Sikumbang, 1986; Sikumbang and Heryanto, 1994).
Formation is contemporaneous with the aforementioned radiolarian chert in the Bantimala Complex (Wakita et al., 1996), although Hasan (1990) reported Late Cretaceous foraminifers from the formation. The equivalent of the Balangbaru Formation is located in the Barru and Latimo-jong areas of South Sulawesi.
The sandstone of the Balangbaru Formation is composed of quartz, micas, plagioclase and rock fragments of meta-morphic, shale, and chert. The main provenance for the sandstones of the Balangbaru Formation was the meta-morphic component of the Bantimala Complex.
The Pitap Formation consists mainly of flysch-type sedi-mentary rocks such as sandstone, siltstone, conglomerate and shale with subsidiary limestone layers and blocks which contains the foraminifera Orbitolina cf. oculata of Aptian–Albian age (Sikumbang and Heryanto, 1994).
The Pitap Formation of the Meratus Complex and the Barangbaru Formation of the Bantimala Complex were deposited in a forearc basin. The Selangkai Group of West Kalimantan is equivalent to the Pitap and Barangbaru Formation. The forearc sedimentary units in West and South Kalimantan and South Sulawesi were accumulated contemporaneously as terrestrial deposits in the wide region of the forearc basin (Fig. 3).
5. Volcanic arc unit
5.1. Late Cretaceous volcanic suite
An andesitic to basic volcanic unit is present in South Kalimantan (Figs. 2 and 3), and similar rocks are inferred as a provenance of the sandstones of the Luk Ulo Complex. The Haruyan Formation in the Meratus area, South Kali-mantan is a product of Late Cretaceous volcanic activity (Heryanto and Sanyoto, 1994; Heryanto et al., 1994). The Haruyan Formation consists mainly of basic to andesitic volcanic rocks, such as lava, tuff and tuff breccia. The lavas sometimes show pillow structures indicating submarine volcanism (Wakita et al., 1998). Tuff breccia consists of feld-spar phenocrysts, pumice, lava fragments and irregular-shaped fragments of pale-colored chert within a light purple colored tuff matrix. One of the chert samples has yielded Cenomanian radiolarians. The Haruyan Formation is interfin-gered with the Pitap Formation which was deposited in a fore-arc basin (Haryanto and Sanyoto, 1994; Heryanto et al., 1994). Andesitic volcanic activity, represented by the Haruyan Formation of the Meratus Complex may be partly contem-poraneous with the felsic volcanic activity recorded by the rhyolite within the chert beds of the Bantimala Complex. The andesitic volcanic rocks of the Haruyan Formation are a possible provenance of andesitic volcanic fragments in the sandstone of the Luk Ulo Complex.
Petrography of sandstones in the Luk Ulo Complex suggest that the provenance was an immature volcanic arc (Wakita et al., 1998). The sandstone is lithic wacke
composed mainly of angular to subrounded fragments of feldspars and rock fragments of intermediate to basic volca-nic rocks. This suggests that the most important provenance of the sandstone was a volcanic terrain of intermediate to basic composition.
5.2. Cretaceous intrusive rocks
Leucocratic rocks, classified as “plagiogranite”, occur in the Meratus Complex closely associated with ultramafic rocks (Sikumbang, 1986). Granite and granodiorite have also been recorded from a few localities in the Meratus Mountains (Sikumbang and Heryanto, 1994). Granodiorite is intruded into the Pitap Formation. K–Ar dating of the granite yields an age of 115 Ma (Heryanto et al., 1994). The granitic rocks are possibly products of Late Cretaceous volcanic activity in an immature volcanic arc located in the Meratus area.
5.3. Ophiolitic unit
Ophiolite units are recognized in the Luk Ulo, Bantimala, Meratus, Ciletuh, Jiwo, Barru, Latimojong and Sangkuriang areas (Figs. 2 and 3). They represent obducted oceanic plate and are dismembered by tectonic disruption. They occur as tectonic blocks, consisting mainly of altered ultramafic rocks, associated with pillow lava and chert.
Dismembered ophiolite crops out in the Karangsambung area of Central Java. It occurs as a tectonic block which is elongated more than 10 km in an E–W direction with 2 km thickness in N–S direction. It is separated from melange and schists by faults. The ophiolite consists of pillow basalt, dolerite, gabbro, serpentinized peridotite and lherzolite; these rocks have suffered zeolite to greenschist facies meta-morphism (Suparka, 1988).
Ultramafic rocks of the Bantimala Complex, South Sula-wesi are dark green in color, and mostly serpentinized peri-dotite, with local chromite lenses. The age of the ultramafic rocks is unknown. Two massifs of sheared, faulted and variably serpentinized ultramafic rocks crop out extensively in the Meratus Complex, South Kalimantan. They are called the Meratus and Bobaris ophiolites. They comprise serpen-tinized peridotite, harzburgite and dunite with minor pyrox-enite, and are intimately associated with gabbro and amphibolite. The ultramafic rocks are variably affected by low-grade metamorphism. Chromite is sometimes present but is a minor constituent.
No age data is available for the ultramafic rocks and pillow basalt in the three complexes. If the rocks originated from the oceanic plate or seamounts born near ridges, their ages would be older than the age of the radiolarian chert (Fig. 3).
6. Overlap assemblage
sedimentary rocks cover both the Cretaceous suture zone and its neighbouring continental margins (Fig. 2).
6.1. Paleogene effusive rocks
Tectonically brecciated rhyolitic lava, with K-feldspar phenocrysts, and rhyolitic tuff, containing pumice are tecto-nically intercalated with sedimentary rocks of the Luk Ulo Complex, although exact relationships are unclear. The age of the rhyolite (which was formerly reported as quartz porphyry) has been reported to be 65 Ma by the fission track method (Ketner et al., 1976). The Bantimala Complex is unconformably overlain by Palaeocene propylitized volcanic rocks (Sukamto, 1986; Wakita et al., 1994, 1996). Paleocene to Eocene volcanic activity can be traced from South Sulawesi to the southwestern coast of Sumatra via Java island (Soeria-Atmadja et al., 1998). Calc-alkali volcanic rocks are scattered throughout South Sulawesi (Yuwono et al., 1988), South Kalimantan (Soeria-Atmadja et al., 1998) and Central Java (Suparka and Soeria-Atmadja, 1991). These data suggest that subduction-related magma-tism occurred all along the southeastern margin of Sunda-land at this time (Soeria-Atmadja et al., 1998).
6.2. Cenozoic cover sequence
Cenozoic sediments cover all the accretionary–collision complexes of the Cretaceous suture zone (Fig. 3). Quatern-ary formations unconformably cover all of the older units. The Luk Ulo Complex of the Karangsambung area is uncon-formably overlain by the Eocene Karangsambung Forma-tion and the Miocene Waturanda, Penosogan and Halang Formations. The Meratus Complex is unconformably covered by the Eocene Tanjun Formation, the Oligocene-Early Miocene Berai Formation, the Middle to Late Miocene Warukin Formation, the Pliocene to Early Pleisto-cene Dahor Formation and Quaternary sedimentary cover. The Bantimala Complex is overlain by the Eocene Malawa Formation, the Eocene to Middle Miocene Tonasa Forma-tion, the Middle to Late Miocene Camba FormaForma-tion, and Quaternary sedimentary cover.
7. Discussion
Cretaceous accretionary–collision complexes in central Indonesia were the products of the interaction of a continent and an ocean. What kind of ocean is represented in the Cretaceous accretionary–collisional complexes in central Indonesia? How old was the ocean? What is the relationship between the ocean and the continents? Was it an active margin? Which mechanism was dominant in the margin: tectonic erosion or sediment accretion? What kinds of accretionary wedge developed? Was there a marginal sea between the arc trench system and the continent? What kind of volcanic activity occurred along the convergent margin? Were there special tectonic events, such as continental
colli-sion or hot spot volcanic eruption, during the final stage of formation of the accretionary–collision complexes? When and how were the complexes deformed? What is the evidence for the continental collision? All these questions will be discussed in the following section.
Although ophiolite is one possible remnant of an ancient oceanic plate, it is difficult to discern the age of an oceanic plate directly from ultramafic rocks and basalts. Chert, the upper part of an ophiolite suite is the most useful tool to understand the age of the paleo-ocean, because chert often includes radiolarians and other marine microfossils. The chert occurs mainly as fragments in melanges or as tectonic blocks in ancient accretionary wedges rather than as the sedimentary cover of the ophiolite suite. The age of the chert records the period of time between the birth of the oceanic floor to the time when they were overlain by detrital sediments derived from the continents. Cherts of the Indo-nesian accretionary–collision complexes range in age from late Early Jurassic to latest Cretaceous (Wakita et al., 1994, 1998). The age indicates that the oceanic plate was gener-ated prior to the late Early Jurassic.
Accretionary wedges were developed along the Sunda-land margin in the Cretaceous. The main part of the Luk Ulo Complex is a typical accretionary wedge formed by oceanic plate accretion in the Cretaceous. Evidence for the paleo-accretionary wedge is the presence of ‘Oceanic Plate Strati-graphy’ (OPS) (Isozaki et al., 1990; Matsuda and Isozaki, 1991; Wakita, 1997) consisting of pillow lava, limestone and chert alternation, chert, siliceous shale and sandstone and shale alternation in ascending order. As mentioned above, the oldest age of the chert indicates the age of forma-tion of the oceanic plate, while the age of siliceous shale and sandstone/shale alternation indicates the time of arrival of the oceanic plate at the trench. The age of siliceous shale and sandstone/shale alternation ranges from Early Cretac-eous to latest CretacCretac-eous in the Luk Ulo and Meratus Complexes. The data suggest that the sediment accretion continued from the Early Cretaceous to latest Cretaceous in central Indonesia.
dismembered ophiolites of the Meratus and Luk Ulo Complexes are possible remnants of the basement of the back arc basin (Fig. 4).
The collision of a continental block is the most important tectonic event recorded in the Cretaceous accretionary– collision complexes (Fig. 4). Evidence of the collision is preserved in the Bantimala Complex. Although subduction of an oceanic plate can cause high P/T metamorphic rocks in the convergent margin, the collision of a continental block played an important role in the exhumation of high P/T, especially very high to ultra-high pressure, metamorphic rocks.
The scenario of the collision of a continental block is supported by the following evidence. The presence of very high and ultra-high pressure metamorphic rocks, high aluminous protolith of the metamorphic rocks, an allochtho-nous Jurassic formation which is older than the age of subduction, and chert unconformably overlying the high
P/T schist. These rocks are components of the collisional unit in the Cretaceous accretionary–collision complexes in central Indonesia. The timing of the collision is middle Cretaceous because the K–Ar age data from micas of high P/T metamorphic rocks are concentrated between 110 and 115 Ma. The metamorphic rocks were pushed upward by the buoyant continental block, and were exhumed to shallower levels where the temperature was cooler than the closure temperature of muscovite which is used to determine their age by the K–Ar method.
oceanic plate would have moved southward and subducted under the volcanic arc on the oceanic plate (Fig. 4).
Late Cretaceous forearc sediments were widely deposited on the Cretaceous accretionary wedges. The Balangbaru Formation of the Bantimala Complex, the Pitap Formation of the Meratus Complex, and the Selankai Formation of West Kalimantan were deposited in a forearc setting in the Late Cretaceous. These formations cover unconform-ably the accretionary–collision complexes in the Kaliman-tan and Sulawesi regions. Sandstones of the formations are composed of quartz, micas, plagioclase and rock fragments of metamorphic, shale, and chert, as well as fragments from a granitic terrane. These are provided from the accretionary and collision unit of the Cretaceous accretionary–collision complexes and partly from the Southwest Borneo terrane. After the collision of the continental block, part of the accre-tionary wedge was uplifted, and the detrital sediments were eroded from the Sundaland continent and transported into the forearc basin on the wedge. The sandstone petrography data suggests a change of tectonic setting in Late Cretaceous time, after the collision of a continental block. Not all areas changed their tectonic setting, however, as the oceanic plate stratigraphy indicates continuous subduction throughout the Cretaceous in the Luk Ulo Complex. All the accretionary– collision complexes, as well as the Cretaceous forearc basin formations, are unconformably overlain by Eocene formations.
All the Cretaceous accretionary–collision complexes in Indonesia were tectonically overprinted by Cenozoic tectonics. The Banggai-Sula and Tukang Besi detached microcontinental blocks collided in the Miocene and some metamorphic rocks were exhumed, and the East Sulawesi ophiolite was obducted (Simandjuntak, 1990; Parkinson, 1991; Simandjuntak and Barber, 1996).
The tectonic events recorded in the Cretaceous accretion-ary–collision complexes in central Indonesia are subduc-tion, accretion of fragments of an oceanic plate and microcontinents, sediment accretion and melange forma-tion, collision of continental blocks, exhumation of high P/T metamorphic rocks, obduction of oceanic plate (ophio-lite) and rearrangement of the components by later faulting.
Acknowledgements
I wish to thank to Dr. Ir. S. Suparka, vice president of LIPI (formerly director of RDCG) and Dr. Jan Sopaheluwa-kan , director of RDCG for their help and support during my field research. I also express thanks to Dr. I. Metcalfe of New England University for kindly offering the opportunity of submitting paper in this volume. I am grateful to Dr. A.J. Barber of Royal Holloway, University of London for his effective suggestions and discussion of the geology of this area. I would like to thank Dr. Chris Parkinson, Tokyo Institute of Techonoloy for his detailed review of my manuscripts.
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