Tectonic implications of new age data for the Meratus Complex of south Kalimantan, Indonesia

The Island Arc (1998) 7, 202±222

Thematic Article Tectonic implications of new age data for the Meratus Complex of south Kalimantan, Indonesia KOJI WAKITA1, KAZUHIRO MIYAZAKI1, ISKANDAR ZULKARNAIN2, JAN SOPAHELUWAKAN2 PRIHARDJO SANYOTO3

AND

1 Geological Survey of Japan, Tsukuba, Ibaraki, 305 Japan, Research and Development Centre for Geotechnology, Bandung 40135 and 3 Geological Research and Development Centre, Bandung, 40122, Indonesia

2

Abstract Cretaceous subduction complexes surround the southeastern margin of Sundaland in Indonesia. They are widely exposed in several localities, such as Bantimala (South Sulawesi), Karangsambung (Central Java) and Meratus (South Kalimantan). The Meratus Complex of South Kalimantan consists mainly of meÂlange, chert, siliceous shale, limestone, basalt, ultrama®c rocks and schists. The complex is uncomformably covered with Late Cretaceous sedimentary-volcanic formations, such as the Pitap and Haruyan Formations. Well-preserved radiolarians were extracted from 14 samples of siliceous sedimentary rocks, and K±Ar age dating was performed on muscovite from 6 samples of schist of the Meratus Complex. The radiolarian assemblage from the chert of the complex is assigned to the early Middle Jurassic to early Late Cretaceous. The K±Ar age data from schist range from 110 Ma to 180 Ma. Three samples from the Pitap Formation, which unconformably covers the Meratus Complex, yield Cretaceous radiolarians of Cenomanian or older. These chronological data as well as ®eld observation and petrology yield the following constraints on the tectonic setting of the Meratus Complex. (1) The meÂlange of the Meratus Complex was caused by the subduction of an oceanic plate covered by radiolarian chert ranging in age from early Middle Jurassic to late Early Cretaceous. (2) The Haruyan Schist of 110±119 Ma was affected by metamorphism of a high pressure±low temperature type caused by oceanic plate subduction. Some of the protoliths were high alluminous continental cover or margin sediments. Intermediate pressure type metamorphic rocks of 165 and 180 Ma were discovered for the ®rst time along the northern margin of the Haruyan Schist. (3) The Haruyan Formation, a product of submarine volcanism in an immature island arc setting, is locally contemporaneous with the formation of the meÂlange of the Meratus Complex. Key words: accretionary complex, chert, Cretaceous, Indonesia, Kalimantan, meÂlange, Meratus, radiolarian, schist, ultrama®c rocks. INTRODUCTION The southeastern spur of the pre-Cretaceous continental basement of Sundaland extends into West Kalimantan (Hutchison 1989). Cretaceous Accepted for publication May 1997.

granites were intruded into the basement of central and western Kalimantan, Sumatra and the western part of the Java Sea (Hamilton 1979). Cretaceous subduction complexes surround the southeastern margin of Sundaland. Cenozoic sedimentary and volcanic cover rocks are

Tectonic implications for the Meratus Complex 203

extensive, and the complexes are exposed only in the Bantimala (South Sulawesi), Karangsambung (Central Java) and Meratus (South Kalimantan) areas (Fig. 1). Prior to Neogene foreland subsidence of the Makassar Strait (Bergman et al. 1996), the Cretaceous complexes of the Meratus and Bantimala areas were located close together. Tertiary subduction complexes and obducted ophiolite are distributed to the east in Central and East Sulawesi (Simandjuntak 1990; Parkinson 1991; Cof®eld et al. 1993; Bergman et al. 1996), and the Late Miocene collision of the Banggai±Sula Platform has resulted in west-directed overthrusting throughout Sulawesi (Fig. 1). Cretaceous subduction complexes of the Indonesian region are characterized by ultrama®c rocks, metamorphic rocks and meÂlanges containing radiolarian chert. These complexes have not previously been studied in detail. The joint GSJ±LIPI research on Indonesian Cretaceous subduction complexes has been conducted since 1993. During 1993 and 1994, the age, petrology, geochemistry and structures of the complexes of Central Java and South Sulawesi were investigated (Wakita et al. 1994a,b; 1996;

Fig. 1 Distribution of Cretaceous subduction complexes in Indonesia.

Miyazaki et al. 1996). The differences and similarities between the two subduction complexes of Central Java and South Sulawesi were noted. Wakita et al. (1994a) determined the history of the accretionary process to form the Luk±Ulo MeÂlange Complex of the Karangsambung area, central Java on the basis of radiolarians extracted from siliceous and argillaceous rocks. The detailed age data of the sedimentary rocks suggests that subduction continued from Early to Late Cretaceous. Oceanic materials such as chert, limestone and pillow basalt travelled on the oceanic plate, and were accreted with terrigenous materials at the `Karangsambung Trench'. Wakita et al. (1994b; 1996) studied ages and the stratigraphical relationship of radiolarian chert, metamorphic rocks and other components of the Bantimala Complex of South Sulawesi, and reported the mid-Cretaceous collision of a microcontinent covered by Jurassic sedimentary rocks, the exhumation of very high pressure metamorphic rocks (18±24 kbar: Miyazaki et al. 1996), and successive chert sedimentation (Wakita et al. 1996). Although various authors have discussed the relationship between the three complexes of

204 K. Wakita et al.

Central Java, South Sulawesi and South Kalimantan, detailed data on the subduction complex of South Kalimantan are sparse compared with that for the other complexes. Detailed investigation of the complex is very important to understand the tectonic setting of the Indonesian region in Cretaceous times, and to understand the type of orogenic belts in this region. In this paper new data of radiolarian biostratigraphic and K±Ar age data for the preTertiary complex in South Kalimantan are presented, and tectonic development of this region is discussed. OUTLINE OF GEOLOGY The Meratus Mountains and Laut Island (Meratus area) are located in the South Kalimantan

province of Indonesia (Fig. 2). Few works on the geology of these areas have been published except for the geological 1:250 000 sheet maps of the Geological Research and Development Centre (GRDC); for example those of Supriatna (1989) Supriatna et al. (1983), Heryanto & Sanyoto (1994), Heryanto et al. (1994) Rustandi et al. (1981; 1984; 1995) among others. We also have a short description of the geology of Hamilton (1979), and Sikumbang (1986, 1990). The basement in the Meratus area, namely the Meratus Complex is composed of high pressure metamorphic rocks (Hauran Schist and Plaihari Phyllite), ultrama®c rocks (Meratus Ophiolite) and meÂlanges including clasts of chert, limestone and basalt within shale matrices. These rocks are uncomformably overlain by Late Cretaceous formations, such as the Pitap and Haruyan Formations, or the Alino and Pudak Groups (Fig. 3).

Fig. 2 Geologic map of the Meratus area, South Kalimantan, Indonesia. Upper left inlet shows the localities of schist samples for K±Ar age dating (BB±30A, BBII±30A, BBII±5, BBII±3B, BBII8A, BBII±11) and locality of SK6A, a chert clast in tuff breccia of the Haruyan Formation. Stars indicate the localities of radiolarian occurrences in meÂlanges of Laut Island.


Fig. 3 Summary of the stratigraphy in the Meratus area based on the chronological data of this work.

All these Mesozoic rocks are unconformably covered by Tertiary formations. These comprise in ascending order: the Tanjung Formation (Eocene), the Berai Formation (Oligocene to Early Miocene), the Warukin Formation (Middle to Late Miocene), the Dahor Formation (Pliocene to early Pleistocene) and Quaternary sedimentary cover. Although the Dahor Formation unconformably overlies the Warukin Formation, the other Cenozoic formations lie conformably on the older formations. The Tanjung, Berai and Warukin Formations are tectonically deformed, and are locally overturned near the lithotectonic units of the Meratus Complex. PITAP AND HARUYAN FORMATIONS The Pitap and Haruyan Formations (Heryanto & Sanyoto 1994; Heryanto et al. 1994) are Late Cretaceous sedimentary and volcanic covers of the Meratus Complex. They are unconformably underlain by ultrama®c rock, metamorphic rocks and meÂlanges of the Meratus Complex. The Haruyan Formation in this paper includes Late Cretaceous basic to intermediate volcanic rocks distributed in the Meratus Mountains and Laut Island. Therefore, the Pitanak Group in the southwestern part of the Meratus Mountains (Sikumbang & Heryanto 1994) are described as

the Haruyan Formation in this paper. The Haruyan Formation consists mainly of basic to andesitic volcanic rocks, such as lava, tuff and tuff breccia (Fig. 4a,b). Lava sometimes shows pillow structures indicating submarine volcanism. The Haruyan formation is inter®ngered with the Pitap Formation. Although volcanic breccia and lavas in the southern part of the Meratus Mountains are described as the Alino Group (Sikumbang & Heryanto 1994), they belong to the Haruyan Formation in this paper. The tuff breccia consists of feldspar crystals, pumice, lava fragments and irregular-shaped fragments of pale-colored chert within a light purple colored tuff matrix (Fig. 4b). One of the chert samples, SK6a, yields Cenomanian radiolarians. In this paper, the Pitap Formation (sensu lato) includes all Late Cretaceous marine sedimentary formations in the Meratus area, since the Alino Formation (Supriatna 1989) or the Alino Group and Batununggal Formation in the Banjarmasin area (Sikumbang & Heryanto 1994) are equivalent to the Pitap Formation in the Kotabaru, Amuntai, and Sampanahan areas (Rustandi et al. 1981; Heryanto & Sanyoto 1994; Heryanto et al. 1994). The Pitap Formation consists mainly of ¯ysch type sedimentary rocks such as sandstone, siltstone, conglomerate and shale with subsidiary limestone layers and


blocks (Fig. 4c). The limestone contains foraminifera Orbitolina cf. oculata of Aptian±Albian age (Sikumbang & Heryanto 1994), and occurs as olistostromal blocks.

This formation includes various rock facies such as deep marine turbidite, shallow marine calcareous mudstone with Cenomanian molluscs (Turritella: Sikumbang & Heryanto 1994), and

Fig. 4 Photographs showing lithology: (a) pillow lava of the Haruyan Formation, east of Kandagan; (b) tuff breccia with chert clasts in the Haruyan Formation; (c) ¯ysh of the Pitlap Formation, east of Kandagan, Mandikapau, southeast of Marutapura; (d) meÂlange of the Meratus Complex, including angular chert blocks, Sekoyang, Laut Island; (e) bedded chert of the Meratus complex, Sekoyang, Lauf Island; (f) bedded very siliceous shale of the Meratus Complex, Sekoyang, Laut Island.


conglomerate rich in clasts of ophiolite origin. Volcanic lava and breccia in the Alino Group (Sikumbang & Heryanto 1994) are excluded from the Pitap Formation of this paper. MERATUS COMPLEX The Meratus Complex is a tectonic assemblage of slabs and blocks consisting of sandstone, shale, conglomerate, chert, siliceous shale, basalt, ultrama®c rocks and schist. The ages of components range from Jurassic to early Late Cretaceous. MEÂLANGE

MeÂlanges do not occur in the Meratus area but are distributed on Laut Island (Fig. 2). The meÂlanges are de®ned as assemblages of tectonic slabs with various lithologies and stratigraphic formations ranging in age from Jurassic to Cretaceous. The meÂlanges are unconformably overlain by or in fault contact with Late Cretaceous, Tertiary and Quaternary formations.

The most distinct outcrop of meÂlange occurs along the southwestern coast of Laut Island (Fig. 5). The meÂlange includes clasts and blocks of chert, siliceous shale, basalt, limestone, marl and manganese carbonate nodules embedded within a sheared shale matrix (Fig. 4d). It is signi®cant that sandstone or other coarsegrained detrital sediments are lacking in the meÂlange. The detailed structure of the meÂlanges are unclear, because of limited exposures in this region. The shale matrix is usually sheared to some degree. In the Sekoyang area, tectonic slabs and blocks are tectonically mixed with mudstone dominant matrices. The dip of the foliation of sheared matrix ranges from 20° to 80° toward the northwest or north (Fig. 5). Major clasts include chert, siliceous shale, limestone and basalt. Chert and limestone are thinly bedded. Basalt is mainly lava, and pillow structures are sometimes preserved. Limestone clasts are locally dominant in the meÂlange. Fragments of manganese carbonate nodules occur rarely. The clasts are subrounded to subangular, lenticular to blocky in shape. Clast size ranges from several millimeters to several hun-

Fig. 5 Geologic sketch map of the meÂlange, Sekoyang, Laut lsland. Shore line is subparallel to the general trend of the meÂlange. The meÂlange is folded in a meter to several tens of meters order.

208 K. Wakita et al. Table 1 K±Ar age data of schist of the Meratus Complex. See Fig. 2 for the sample localities Sample no.

Material analyzed

Rock type

Locality

BB-30a

White mica

Schist

Batuditabang

BBII-30a

White mica

Schist

Batuditabang

BBII-5

White mica

Schist

Manunggul

BBII-11

White mica

Schist

Damargusang

BBII-3b

White mica

Schist

Tiwingan

BBII-8a

White mica

Schist

Pamaton

dred meters long. Clasts in the meÂlange are usually less than 1 m in long axis, but sometimes reach several metres long. Chert is the dominant rock type in the meÂlange. Chert layers range from 1 to 20 cm thick and are interbedded with <1 cm thick shale layers (Fig. 4e). The bedded chert is mostly red or reddish-brown in color, and sometimes light gray or greenish-gray in color. The chert is composed mainly of skeletons of radiolarians, their fragments and a small amount of shale. The chert sometimes includes well-preserved radiolarians ranging in age from Middle Jurassic to Early Cretaceous (late Albian to early Cenomanian) age. Siliceous shale clasts are light gray, gray or reddish brown in color, and composed of terrigenous fragments, radiolarian skeletons and other detrital materials (Fig. 4f). Some of them (SK60A, B) include radiolarians of Early Cretaceous age. METAMORPHIC ROCKS

Metamorphic rocks are distributed in the southwestern part of the Meratus Mountains. They occur as wedge-shaped tectonic blocks in fault contact with ultrama®c rocks and Cretaceous formations. The metamorphic rocks include glaucophane schist, garnet mica schist, quartz mica schist, piemontite schist, amphibolite and phyllite. Lower grade metamorphic rocks called Pelaihari Phyllite (poorly distributed around Pelaihari village) and higher grade schist, called Hauran Schist, are rather widely distributed in the southern part of the Meratus Mountains. The metamorphic rocks include schists of high pressure±low temperature type, consisting of glaucophane (crossite), quartz and small amounts of epidote, apatite and hematite.

Isotopic age (Ma) Average age (Ma) 181. 180. 165. 165. 119. 118. 116. 116. 116. 115. 112. 108.

9. 9. 8. 8. 6. 6. 6. 6. 6. 6. 6. 6.

180. 9. 165. 8. 119. 6. 116. 6. 115. 6. 110. 6.

The major protoliths of the metamorphic rocks were pelitic and basic rocks. Chloritoide±quartz schist and kyanite±quartz schist are characteristic of the Haruyan Schist. Chloritoide±quartz schist consists only of chloritoid and quartz, while kyanite±quartz schist contains kyanite, quartz, hematite and small amounts of white mica. These mineral assemblages indicate that protoliths of the schists had enormously high contents of aluminium. The origin of high aluminous metamorphic rocks could be high aluminous detrital sediments derived from a tropical continent covered by laterite. K±Ar age date for muscovite was obtained from six samples of (garnet±)quartz±muscovite schist from the Meratus Mountains (Fig. 2, Table 1). These samples were analyzed by Keith Noyes of Teledyne Brown Engineering. K±Ar age data of micas from the schists range from 110 to 180 Ma. The samples yield K±Ar ages between 110 and 119 Ma except for BB-30a and BBII-30a. These data are consistent with data previously reported (113 Ma: K±Ar age of hornblende schist; Sikumbang & Heryanto 1994). Older ages, such as 165 and 180 Ma were obtained from the samples BB-30a and BBII-30a which were collected at the same locality on the northern margin of the schist distribution. The apparently older metamorphic rocks may belong to a different tectonic block from the main part. ULTRAMAFIC ROCKS

Ultrama®c rocks are widely distributed in the Meratus area (Fig. 2). The ultrama®c rocks are dark green in color, are mostly serpentinized, sheared and faulted. They comprise serpentinized peridotite, harzburgite and dunite with minor pyroxenite, and are intimately associated


with gabbro and amphibolite. The ultrama®c rocks are variably affected by low grade metamorphism. Chromite is sometimes present but is a minor constituent. The K±Ar radiometric age of a metadolerite dyke in the upper stream of the Satui River was reported as 116 Ma (Sikumbang 1986). The ages of ultrama®c rocks in Laut Island are estimated from the age of chert which originally overlay the ultrama®c rock, together with basic igneous rocks such as basalt and gabbro. The oldest chert in Laut Island, of early Middle Jurassic, indicates the age of ultrama®c rocks is older than early Middle Jurassic at Laut Island. INTRUSIVE ROCKS

Leucocratic rocks in an ultrama®c unit include quartz diorite and trondhjemite which are closely associated with dolerite and gabbro (Sikumbang 1986). These rocks are classi®ed as `Plagiogranite'. Granite and granodiorite have been recorded from a few localities in the Meratus Mountains (Sikumbang & Heryanto 1994). Granodiorite intruded into the Pitap Formation. K±Ar dating of the granite yields an age of 115 Ma (Heryanto et al. 1994).

RADIOLARIAN BIOSTRATIGRAPHY The following samples and extracted radiolarians were collected: one sample from the Haruyan Formation at Mandikapan and three from the Haruyan Formation near Mount Baturung, 16 samples from the Pitap Formation along the road between Kandagan and Batulicin, ®ve samples from the Pitap Formation east of Kotabaru, Laut Island, one shale sample from the Haruyan Formation east of Kotabaru, Laut Island, three samples from chert on the ophiolite at Batricin, three samples from chert east of Kotabaru, 20 samples from chert in meÂlange at Sekoyang, 10 samples from siliceous shale in the meÂlange at Sekoyang, nine from a shale matrix of meÂlange at Sekoyang and ®ve samples from manganese carbonate nodules in meÂlange at Sekoyang. Among the samples mentioned above, 14 samples in the Laut MeÂlange, one sample from the Haruyan Formation and two from the Pitap Formation yielded diagnostic radiolarians for age determination. Radiolarians were chemically extracted from chert and siliceous shale using hydro¯uoric acid as discussed by Pessagno & Newport (1972). The associations recognized are shown in Figs. 7±13 and the Appendix. These associations range in age from early Middle Jurassic to late Early Cretaceous (Fig. 6) based on

Fig. 6 Age of chert and siliceous shale based on the ranges of yielding radiolarians.


Fig. 7 (1) Archaeodictyomitra sp.; (2) Hsuum sp.; (3) Hsuum sp.; (4) Transhsuum hisuikyoense (Isozaki and Matsuda); (5) Transhsuum hisuikyoense (Isozaki and Matsuda); (6) Unuma sp.; (7) Nassellaria gen. and sp. indet.; (8) Tricolocapsa sp.; (9) Hsuum sp.; (10) Cyrtocapsa sp. aff. mastoidea Yao; (11) Cyrtocapsa sp. aff. mastoidea Yao; (12) Archicapsa (?) pachydema (TAN); (13) Archicapsa (?) pachyderma (Tan); (14) Eucyrtidiellum sp.; (15) Tricolocapsa sp.; (16) Praeconocaryomma sp. Scale bar 0.1 mm.


Fig. 8 Thanarla sp.; (2) Thanarla sp.; (3) Thanarla sp.; (4) Archaeodictyomitra apiarium (Riist); (5) Archaeodictyomitra minoensis Mizutani; (6) Archaeodictyomitra apiarium (Rust); (7) Cinguloturris cylindra Kemkin and Rudenko; (8) Cinguloturris cylindra Kemkin and Rudenko; ((9) Xitus sp.; (10) Pseudodictyomitra carpatica (Lozyniak); (11) Parvicingula mashitaensis Mizutani; (12) Parvicingula mashitaensis Mizutani; (13) Protunuma japonicus Matsuoka and Yao; (14) Stichocapsa sp.; (15) Tricolocapsa (?) sp.; (16) Podobursa sp.; (17) Podobursa sp.; (18) Sethocapsa sp.; (19) Eucyrtidiellum pyramis (Aita); (20) Eucyrtidiellum pyramis (Aita); (21) Stichocapsa sp.; (22) Saitoum sp.; (23) Pseudoeucyrtis (?) sp.; (24) Pantanellium sp.; (25) Pantanellium sp.; (26) Spumellaria gen. and sp. indet.; (27) Spumellaria gen. and sp. indet.; (28) Stichocapsa sp.; (29) Sethocapsa sp. Scale bar 0.1 mm.

numerous bio-stratigraphic works (Pessangno 1976; 1977a, b; Schaaf 1981; 1984; Taketani 1982; Matsuoka 1983; 1986; Isozaki & Matsuda 1985; Matsuoka & Yao 1985; 1986; Teraoka & Kurimoto 1986; Aita 1987; Carter et al. 1988; Tumanda 1989; Hori 1990; Qun 1993; GoricÏan 1994; Jud 1994; O'Dogherty 1994; Baumgartner et al. 1995a,b).

MEÂLANGE AT SEKOYANG

Figure 6 is a geologic sketch map indicating fossil localities and fossil ages. The components of the meÂlange such as chert, limestone and basalt are tectonically disrupted, and are fault bounded. Diagnostic radiolarians were extracted from one

or two chert samples in one tectonic slab of the meÂlange. Therefore the occurrence of radiolarians does not show the biostratigraphic relationship in the meÂlange. What we can interpret from the radiolarian data, however, is a reconstructed succession of the protolith which was dismembered during meÂlange formation. The oldest radiolarian assemblage in this meÂlange is of early Middle Jurassic (Fig. 6; Fig. 7). The assemblage from the sample SK47C includes Transhsuum hisuikyoense Isozaki & Matsuda and Archicapsa (?) pachyderma (Tan). Unuma sp. A. (?) pachyderma is restricted to the early to late Bajocian (Baumgartner et al. 1995a). Sample SK52A is from the red shale matrix of chert breccia, while Sample SK52B is a red chert


clast of the same chert breccia. The former includes a late Tithonian assemblage such as Archaeodictyomitra apiarium, Cinguloturris cylindra, Eucyrtidiellum pyramis, Parvicingula mashitaensis, and Protunuma japo-

nicus (Fig. 8, Appendix), whereas the latter yields a Middle Jurassic assemblage such as Eucyrtidiellum unumaense, Protunuma c.f. turbo, Stichocapsa himedaruma and Hsuum spp (Fig. 9, Appendix).

Fig. 9 (1) Archaeodictyomitra sp.; (2) Archaeodictyomitra sp.; (3) Parvicingula sp.; (4) Hsuum sp.; (5) Hsuum sp.; (6) Hsuum sp.; (7) Nassellaria gen. and sp. indet.; (8) Parvicingula sp.; (9) Eucyrtidiellum unumaense (Yao); (10) Eucyrtidiellum unumaense (Yao); (11) Unuma sp.; (12) Hsuum sp.; (13) Unuma sp.; (14) Protunuma cf. turbo Matsuoka; (15) Stichocapsa himedaruma Aita; (16) Tricolocapsa sp.; (17) Tricolocapsa sp.; (18) Parvicingula sp.; (19) Sethocapsa (?) sp.; (20) Sethocapsa (?) sp.; (21) Cryptamphorella sp.; (22) Cryptamphorella sp. Scale bar 0.1 mm.


A chert sample SK41X includes a variety of Spumellaria showing delicate structures: these include Tritrabs, Triactoma, Emiluvia, Higmastra and Alievium together with species of Nassellaria, such as Eucyrtidiellum ptyctum (Fig. 10, Appendix). The dignostic species, E. ptyctum and Emiluvia prenyogii indicate the age of the sample is Middle Jurassic (Fig. 6). Samples of SK47B, SK57 and SK58 are Middle Jurassic based on the radiolarian assemblages. Samples of SK47A, B and C are obtained from south to north with about 1 m distances in a continuous sequence.

Fig. 10 (1) Thanarla brouweri (Tan); (2) Archeodictyomitra sp.; (3) Eycyrtidiellum ptyctum (Riedel and San®lippo); (4) Podobursa sp.; (5) Podobursa sp.; (6) Nassellaria gen. and sp. indet.; (7) Parvicingula sp.; (8) Parvicingula sp.; (9) Mirifusus sp.; (10) Triactoma sp.; (11) Tritrabs rhododactylus Baumgartner; (12) Pantanellium sp.; (13) Archaeospongoprunum sp.; (14) Pantanellium sp.; (15) Emiluvia sp.; (16) Emiluvia prenyogii Baumgartner; (17) Spumellaria gen. and sp. indet.; (18) Alievium sp.; (19) Higmastra sp.; (20) Nassellaria gen. and sp. indet.; (21) Spumellaria gen. and sp. indet. Scale bar 0.1 mm.

The meÂlange locally includes light gray chert with rough surfaces, although most of the chert is red or reddish brown in color. The former contains various obscure fragments which might be components of ash. It has been termed, `tuffaceous chert'. The beds of the chert are relatively thicker than in the reddish brown bedded chert. The samples, SK50A and SK50B, contain Pseudodictyomitra carpatica, Sethocapsa uterculus, Xitus gifuensis, Pantanellium lanceola and others ranging in age from late Kimmeridgian to late Valanginian (Fig. 11, Appendix).


Fig. 11 (1) Thanarla brouweri (Tan); (2) Archaeodicyomitra sp.; (3) Archaeodicyomitra sp.; (4) Pseudodictyomitra carpatica (Lozyniak); (5) Pseudodictyomitra carpatica (Lozyniak); (6) Pantanellium lanceola (Parona); (7) Xitus gifuensis Mizutani; (8) Xitus sp.; (9) Pseudodictyomitra sp.; (10) Cryptamphorella shpaerica (White); (11) Cryptamphorella sp.; (12) Cryptamphorella sp.; (13) Paronaella (?) sp.; (14) Sethocapsa cf. uterculus (Parcona). Scale bar 0.1 mm.


Fig. 12 (1) Thanarla paci®ca Nakaseko and Nishimura; (2) Thanarla broweri (Tan); (3) Thanarla broweri (Tan); (4) Thanarla lacrimula (Foreman); (5) Pseudodictyomitra carpatica (Lozyniak); (6) Pseudodictyomitra sp.; (7) Stichomitra mediocris (Tan); (8) Parvicingula sp.; (9) Dictyomitrella (?) puga Schaaf; (10) Dictyomitrella (?) puga Schaaf; (11) Nassellaria gen. and sp. indet.; (12) Stichocapsa cf. japonica Nakaseko and Nlishimura; (13) Cryptamphorella shpaerica (White); (14) Cryptamphorella shpaerica (White); (15) Cryptamphorella cf. clivosa (Aliev); (16) Nassellaria gen. and sp. indet.; (17) Sethocapsa sp.; (18) Pseudoeucyrtis cf. hanni (Tan); (19) Eucyrtidiellum sp.; (20) Archaeodictyomitra sp.; (21) Hiscocapsa grutterinki (Tan); (22) Pantanellium lanceola (Parona); (23) Acaeniotyle umbilicata (Rust); (24) Deviatus sp.; (25) Sethocapsa uterculus (Parona); (26) Sethocapsa uterculus (Parona); (27) Bistrkum sp.; (28) Crucella sp.; (29) Crucella sp. scale bar 0.1 mm.

SK50B is sampled at about 3 m north of SK50A with a 2 m lack of outcrop in between. Demonstrably the youngest rock in the meÂlange at Sekoyang is a very siliceous shale. The shale consists of light greenish gray very siliceous beds of 1±15 cm thick interbedded with thinner dark gray shale partings. The sample SK60A is a very siliceous shale part, while SK60B comes from a dark gray shale parting. They yield similar Early Cretaceous assemblages ranging from late Valanginian to early Aptian. The assemblage contains Acaeniotyle umbilicata, Cyptamphorella shaerica, Pantanellium lanceola, Pseudodictyomitra carpatica, Sethocapsa uterculus, Stichomitra dediocris and Thanarla lacrimula (Fig. 12, Appendix).

The shale matrix of the meÂlange and manganese carbonate nodules show a lack of radiolarians or include only very poorly preserved radiolarians. MEÂLANGE AT EAST OF KOTABARU

Sample SK34, pale green chert, was obtained east of Kotabaru (Fig. 2). It includes Rhopalosyringium adriaticum which ranges from Middle Albian to Cenomanian (Fig. 6). HARUYAN FORMATION

Fragments of light yellowish or milky colored chert are embedded in basaltic tuff breccia south


Fig. 13 (1) Archaeodictyomitra sp.; (2) Thanarla sp.; (3) Dictyomitra sp.; (4) Dictyomitra sp.; (5) Stichomitra (?) sp.; (6) Thanarla brouweri (Tan); (7) Dictyomitra sp.; (8) Dictyomitra sp.; (9) Dictyomitra sp.; (10) Xitus sp.; (11) Stichomitra communis Squinabol; (12) Eucyrtidiellum sp.; (13) Thanarla sp.; (14) Rhopalosyringium sp.; (15) Rhopalosyringium sp.; (16) Stichomitra communis Squinabol; (17) Archaeodictyomitra cf. obesa (Squinabol); (18) Thanarla sp.; (19) Dictyomitra sp.; (20) Novixitus sp. Scale bar = 0.1 mm.

of the Meratus Mountains (Fig. 2). Chert sample, SK6A, which occurs at the foot of Mount Baturung, includes Dictyomitra cf. formosa, R. adriaticum, Stichomitra communis, Thanarla brouweri, Xitus sp. and other species (Fig. 13, Appendix). The assemblage indicates an Early Cretaceous to Cenomanian age (Fig. 6). PITAP FORMATION

Two samples, SK24E and SK36B of shale alternating with thinner sandstone beds in the Pitap

Formation contain (Nassellaria).

Cretaceous

radiolarians

DISCUSSION The Meratus Complex is characterized by high pressure±low temperature metamorphic rocks, ultrama®c rocks, and meÂlanges including clasts of radiolarian chert, pillow basalt and limestone. The chronological, stratigraphical and petrological data presented in this paper give us the new


Fig. 14 Middle to Late Cretaceous tectonic setting of the Meratus area (South Kalimantan).

view of tectonic evolution of the Meratus Complex in Cretaceous time. Radiolarian biostratigraphic studies on the Â melange in Laut Island revealed that the cherts in the meÂlange range in age from Bajocian to Cenomanian, although previous works recognized only cherts including Early Cretaceous radiolarians. The data suggests that the subducted oceanic plate covered by these cherts was at least older than early Middle Jurassic. The oceanic plate evolved at some time before early Middle Jurassic, migrated toward the Sundaland Continent, and ®nally subducted in middle Cretaceous time. Granite, granodiorite, diorite and gabbro intruded the Meratus Complex. The radiometric age of granite is 115 Ma (Heryanto et al. 1994). These igneous rocks may have been caused by the subduction of the oceanic plate already mentioned beneath the marginal sea along the Sundaland margin. Basaltic to andesitic lava and tuff breccia of the Haruyan Formation and Pitanak Group are products of submarine volcanism, because chert fragments in the tuff breccia include deep marine fauna, radiolarians. These submarine volcanic products are often recognized near immature island arcs caused by the interaction between two oceanic plates. The lower part of the Haruyan Formation and Pitanak Group are formed in an immature island arc setting. Contemporaneously, the meÂlange of Laut Island was formed by the subduction of the oceanic plate during late Early Cretaceous time.

The meÂlange of Laut Island is characterized by a lack of coarse-grained detrital clastic sediments such as sandstone and conglomerate. The sediment supply from the continental side is absent or very poor, although pelagic sediments to trench and fragments of seamounts were derived from the oceanic plate and accreted on the continental margin. This evidence suggests that the trench was far from the continent and that mountain building did not proceed near the trench. In the Late Cretaceous, detrital clastic sediments of the Pitap Formation covered the meÂlanges, metamorphic rock and ultrama®c rocks in a shallow marine environment. This signi®es that the Meratus Arc was mature enough to provide coarse-grained detrital clastic rocks on the continental slopes and in the forearc basins. Glaucophane schist of the Hauran Schist is caused by oceanic plate subduction along the trench. Judging from the petrological studies, however, the protoliths of some schists are different from the products of normal subduction metamorphism such as the Sambagawa Metamorphic Rocks in Japan. The presence of schist consisting only of quartz and chloritoid and the common occurrence of kyanite in the schists indicate that some of the protoliths had high aluminium contents. The origin of highly aluminous metamorphic rocks could be continental cover or margin sediments. Various sizes of continental fragments drifted northward and accreted along the Asian continental margin following the break-up of the Gondwanaland (Nur & BenAvraham 1983; Maruyama et al. 1989). The high


aluminous sediments could have been derived from the surface of a continental fragment (a microcontinent), detached from the margin of the Gondwana super-continent. This is a similar situation to that of the Bantimala Complex, South Sulawesi (Wakita et al. 1996). The Jurassic shallow marine sedimentary rocks incorporated in the Bantimala Complex were evidence of the microcontinent collision and accretion (Wakita et al. 1996). The older metamorphic rocks of 165 and 180 Ma occurred as a small tectonic block along the northern margin of the Hauran Schist Block. They are not high pressure type metamorphic rocks like the other metamorphic rocks, but of intermediate pressure type. They were exhumed and tectonically mixed with other components of the Meratus Complex during the processes of subduction, collision and accretion of oceanic plate and micro-continent. Major tectonic events are recorded in three stages of unconformity in the Meratus area; that is the Middle Cretaceous, Paleocene and Late Miocene (Fig. 14). Ultrama®c rocks, high-pressure schist and meÂlanges were locally exhumed and provided their fragments into the Pitap Formation. Before the deposition of the Eocene Tanjung Formation, ultrama®c rocks, schists and meÂlanges were tectonically juxtaposed with Late Cretaceous sedimentary±volcanic formations such as the Pitap and the Haruyang Formations. This Cretaceous±Paleocene event is contemporaenous with rearrangement of the components of Cretaceous island arc systems along the Sundaland margin such as Karangsambung, Central Java (Wakita et al. 1994a) and Bantimala, South Sulawesi (Wakita et al. 1994b; 1996). Finally, Tanjung, Berai, and Warukin Formations were tectonically deformed until they were covered by the Pliocene Dahor Formation. The latest tectonism could be related to the westward obduction of the East Sulawesi ophiolite in Oligocene time and Miocene to Pliocene collision of the Sula microcontinent (Parkinson 1991; Cof®eld et al. 1993, Bergman et al. 1996). SUMMARY (1) The Meratus Complex is a product of oceanic plate subduction and successive collision of microcontinents during Cretaceous time.

(2) Radiolarian biostratigraphic studies reveal that the meÂlange of the Meratus Complex includes chert ranging in age from early Middle Jurassic to late Early Cretaceous. (3) The Haruyan Schist of 110±119 Ma was a high pressure-low temperature type caused by oceanic plate subduction. Some of the protoliths were high alluminous continental cover or margin sediments. Intermediate pressure type metamorphic rocks of 165 and 180 Ma were discovered along the northern margin of the Haurun Schist. (4) The Haruyan Formation is a product of submarine volcanism in an immature island arc setting, locally contemporaneous with the formation of the meÂlange of the Meratus Complex. ACKNOWLEDGEMENTS This paper is one of the results of the joint project between the Research and Development Centre for Geotechnology (RDCG) in Bandung, University of London and the Geological Survey of Japan (GSJ) under the ITIT program `Research on Mineral Resources Assessment of Oceanic Plate Fragments'. The authors wish to thank to Ir. S. Suparka, vice president of LIPI for his helpful support during our geological survey. We also express thanks to Dr A. J. Barber of Royal Holloway, University of London for his effective suggestions and discussion of the geology of this area. We are grateful to Dr C. D. Parkinson, STA fellow of GSJ, and Dr C. Kurimoto for their critical review of our manuscripts. REFERENCES AITA Y. 1987. Middle Jurassic to Lower Cretaceous Radiolarian Biostratigraphy of Shikoku with Reference to Selected Sections in Lombardy Basin and Sicily. Science Reports of the Tohoku University. Second Series 58, 1±91. BAUMGARTNER P. O., BARTOLINI A., CARTER E. S., et al. 1995a. Middle Jurassic to Early Cretaceous Radiolarian Biochronology of Tethys Based on Unitary Associations. In Baumgartner P. O., O'Dogherty L., Gorican S., Urquhart E., Pillevuit A. & De Wever P. eds. Middle Jurassic to Lower Cretaceous Radiolaria of Tethys: Occurrences Systematics. Biostratigraphy, Memoires de Geologie (Lausanne) 23, 1013±43.

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Tectonic implications for the Meratus Complex 221 APPENDIX 1. Fossil details


Tectonic implications of new age data for the Meratus Complex of south Kalimantan, Indonesia

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