Thematic ArticleTectonic implications of new age data for the Meratus Complex

of south Kalimantan, Indonesia

KOJIOJI WAKITAAKITA1, KAZUHIROAZUHIRO MIYAZAKIIYAZAKI1, ISKANDARSKANDAR ZULKARNAINULKARNAIN2, JANAN SOPAHELUWAKANOPAHELUWAKAN2 ANDANDPRIHARDJORIHARDJO SANYOTOANYOTO3

1Geological Survey of Japan, Tsukuba, Ibaraki, 305 Japan,2Research and Development Centre for Geotechnology, Bandung 40135 and3Geological Research and Development Centre, Bandung, 40122, Indonesia

Abstract Cretaceous subduction complexes surround the southeastern margin of Sun-daland 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 melange, chert, sili-ceous shale, limestone, basalt, ultramac rocks and schists. The complex is un-comformably covered with Late Cretaceous sedimentary-volcanic formations, such asthe Pitap and Haruyan Formations.

Well-preserved radiolarians were extracted from 14 samples of siliceous sedimentaryrocks, and KAr age dating was performed on muscovite from 6 samples of schist of theMeratus Complex. The radiolarian assemblage from the chert of the complex is assignedto the early Middle Jurassic to early Late Cretaceous. The KAr age data from schistrange from 110 Ma to 180 Ma. Three samples from the Pitap Formation, which un-conformably covers the Meratus Complex, yield Cretaceous radiolarians of Cenomanianor older.

These chronological data as well as eld observation and petrology yield the followingconstraints on the tectonic setting of the Meratus Complex.

(1) The melange of the Meratus Complex was caused by the subduction of an oceanicplate covered by radiolarian chert ranging in age from early Middle Jurassic to lateEarly Cretaceous.

(2) The Haruyan Schist of 110119 Ma was affected by metamorphism of a highpressurelow temperature type caused by oceanic plate subduction. Some of theprotoliths were high alluminous continental cover or margin sediments. Intermediatepressure type metamorphic rocks of 165 and 180 Ma were discovered for the rst timealong the northern margin of the Haruyan Schist.

(3) The Haruyan Formation, a product of submarine volcanism in an immature is-land arc setting, is locally contemporaneous with the formation of the melange of theMeratus Complex.

Key words: accretionary complex, chert, Cretaceous, Indonesia, Kalimantan, melange,Meratus, radiolarian, schist, ultramac rocks.

INTRODUCTION

The southeastern spur of the pre-Cretaceouscontinental basement of Sundaland extends intoWest Kalimantan (Hutchison 1989). Cretaceous

granites were intruded into the basement ofcentral and western Kalimantan, Sumatra andthe western part of the Java Sea (Hamilton1979).

Cretaceous subduction complexes surroundthe southeastern margin of Sundaland. Cenozoicsedimentary and volcanic cover rocks are

The Island Arc (1998) 7, 202222

Accepted for publication May 1997.

extensive, and the complexes are exposed only inthe Bantimala (South Sulawesi), Karangsambung(Central Java) and Meratus (South Kalimantan)areas (Fig. 1).

Prior to Neogene foreland subsidence of theMakassar Strait (Bergman et al. 1996), the Cre-taceous complexes of the Meratus and Bantimalaareas were located close together. Tertiary sub-duction complexes and obducted ophiolite aredistributed to the east in Central and East Sul-awesi (Simandjuntak 1990; Parkinson 1991; Cof-eld et al. 1993; Bergman et al. 1996), and theLate Miocene collision of the BanggaiSulaPlatform has resulted in west-directed over-thrusting throughout Sulawesi (Fig. 1).

Cretaceous subduction complexes of the In-donesian region are characterized by ultramacrocks, metamorphic rocks and melanges con-taining radiolarian chert. These complexes havenot previously been studied in detail.

The joint GSJLIPI research on IndonesianCretaceous subduction complexes has been con-ducted since 1993. During 1993 and 1994, the age,petrology, geochemistry and structures of thecomplexes of Central Java and South Sulawesiwere investigated (Wakita et al. 1994a,b; 1996;

Miyazaki et al. 1996). The differences and simi-larities between the two subduction complexes ofCentral Java and South Sulawesi were noted.Wakita et al. (1994a) determined the history ofthe accretionary process to form the LukUloMelange Complex of the Karangsambung area,central Java on the basis of radiolarians ex-tracted from siliceous and argillaceous rocks. Thedetailed age data of the sedimentary rocks sug-gests that subduction continued from Early toLate Cretaceous. Oceanic materials such aschert, limestone and pillow basalt travelled onthe oceanic plate, and were accreted with ter-rigenous materials at the `KarangsambungTrench'. Wakita et al. (1994b; 1996) studied agesand the stratigraphical relationship of radiolarianchert, metamorphic rocks and other componentsof the Bantimala Complex of South Sulawesi, andreported the mid-Cretaceous collision of a mic-rocontinent covered by Jurassic sedimentaryrocks, the exhumation of very high pressuremetamorphic rocks (1824 kbar: Miyazaki et al.1996), and successive chert sedimentation(Wakita et al. 1996).

Although various authors have discussed therelationship between the three complexes of

Fig. 1 Distribution of Cretaceous subduction complexes in Indonesia.

Tectonic implications for the Meratus Complex 203

Central Java, South Sulawesi and South Kali-mantan, detailed data on the subduction complexof South Kalimantan are sparse compared withthat for the other complexes. Detailed inv-estigation of the complex is very important tounderstand the tectonic setting of the Indonesianregion in Cretaceous times, and to understandthe type of orogenic belts in this region.

In this paper new data of radiolarianbiostratigraphic and KAr age data for the pre-Tertiary complex in South Kalimantan are pre-sented, and tectonic development of this region isdiscussed.

OUTLINE OF GEOLOGY

The Meratus Mountains and Laut Island (Me-ratus area) are located in the South Kalimantan

province of Indonesia (Fig. 2). Few works on thegeology of these areas have been published ex-cept for the geological 1:250 000 sheet maps ofthe Geological Research and Development Cen-tre (GRDC); for example those of Supriatna(1989) Supriatna et al. (1983), Heryanto & San-yoto (1994), Heryanto et al. (1994) Rustandi et al.(1981; 1984; 1995) among others. We also have ashort description of the geology of Hamilton(1979), and Sikumbang (1986, 1990).

The basement in the Meratus area, namely theMeratus Complex is composed of high pressuremetamorphic rocks (Hauran Schist and PlaihariPhyllite), ultramac rocks (Meratus Ophiolite)and melanges including clasts of chert, limestoneand basalt within shale matrices. These rocks areuncomformably overlain by Late Cretaceousformations, such as the Pitap and Haruyan For-mations, or the Alino and Pudak Groups (Fig. 3).

Fig. 2 Geologic map of the Meratusarea, South Kalimantan, Indonesia.Upper left inlet shows the localities ofschist samples for KAr age dating(BB30A, BBII30A, BBII5, BBII3B,BBII8A, BBII11) and locality of SK6A,a chert clast in tuff breccia of the Ha-ruyan Formation. Stars indicate thelocalities of radiolarian occurrences inmelanges of Laut Island.

204 K. Wakita et al.

All these Mesozoic rocks are unconformablycovered by Tertiary formations. These comprisein ascending order: the Tanjung Formation (Eo-cene), the Berai Formation (Oligocene to EarlyMiocene), the Warukin Formation (Middle toLate Miocene), the Dahor Formation (Pliocene toearly Pleistocene) and Quaternary sedimentarycover. Although the Dahor Formation uncon-formably overlies the Warukin Formation, theother Cenozoic formations lie conformably on theolder formations. The Tanjung, Berai andWarukin Formations are tectonically deformed,and are locally overturned near the lithotectonicunits of the Meratus Complex.

PITAP AND HARUYAN FORMATIONS

The Pitap and Haruyan Formations (Heryanto &Sanyoto 1994; Heryanto et al. 1994) are LateCretaceous sedimentary and volcanic covers ofthe Meratus Complex. They are unconformablyunderlain by ultramac rock, metamorphic rocksand melanges of the Meratus Complex.

The Haruyan Formation in this paper includesLate Cretaceous basic to intermediate volcanicrocks distributed in the Meratus Mountains andLaut Island. Therefore, the Pitanak Group in thesouthwestern part of the Meratus Mountains(Sikumbang & Heryanto 1994) are described as

the Haruyan Formation in this paper. The Ha-ruyan Formation consists mainly of basic toandesitic volcanic rocks, such as lava, tuff andtuff breccia (Fig. 4a,b). Lava sometimes showspillow structures indicating submarine vol-canism. The Haruyan formation is interngeredwith the Pitap Formation.

Although volcanic breccia and lavas in thesouthern part of the Meratus Mountains are de-scribed as the Alino Group (Sikumbang &Heryanto 1994), they belong to the HaruyanFormation in this paper. The tuff breccia consistsof feldspar crystals, pumice, lava fragments andirregular-shaped fragments of pale-colored chertwithin a light purple colored tuff matrix(Fig. 4b). One of the chert samples, SK6a, yieldsCenomanian radiolarians.

In this paper, the Pitap Formation (sensulato) includes all Late Cretaceous marine sedi-mentary formations in the Meratus area, sincethe Alino Formation (Supriatna 1989) or theAlino Group and Batununggal Formation in theBanjarmasin area (Sikumbang & Heryanto 1994)are equivalent to the Pitap Formation in theKotabaru, Amuntai, and Sampanahan areas(Rustandi et al. 1981; Heryanto & Sanyoto 1994;Heryanto et al. 1994). The Pitap Formationconsists mainly of ysch type sedimentary rockssuch as sandstone, siltstone, conglomerate andshale with subsidiary limestone layers and

Fig. 3 Summary of the stratigraphy inthe Meratus area based on the chro-nological data of this work.

Tectonic implications for the Meratus Complex 205

blocks (Fig. 4c). The limestone contains fora-minifera Orbitolina cf. oculata of AptianAlbianage (Sikumbang & Heryanto 1994), and occurs asolistostromal blocks.

This formation includes various rock faciessuch as deep marine turbidite, shallow marinecalcareous 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 theHaruyan Formation; (c) ysh of the Pitlap Formation, east of Kandagan, Mandikapau, southeast of Marutapura; (d) melange of the MeratusComplex, including angular chert blocks, Sekoyang, Laut Island; (e) bedded chert of the Meratus complex, Sekoyang, Lauf Island; (f) beddedvery siliceous shale of the Meratus Complex, Sekoyang, Laut Island.

206 K. Wakita et al.

conglomerate rich in clasts of ophiolite origin.Volcanic lava and breccia in the Alino Group(Sikumbang & Heryanto 1994) are excluded fromthe Pitap Formation of this paper.

MERATUS COMPLEX

The Meratus Complex is a tectonic assemblage ofslabs and blocks consisting of sandstone, shale,conglomerate, chert, siliceous shale, basalt, ul-tramac rocks and schist. The ages of compo-nents range from Jurassic to early LateCretaceous.

MELANGE

Melanges do not occur in the Meratus area butare distributed on Laut Island (Fig. 2). Themelanges are dened as assemblages of tectonicslabs with various lithologies and stratigraphicformations ranging in age from Jurassic to Cre-taceous. The melanges are unconformably over-lain by or in fault contact with Late Cretaceous,Tertiary and Quaternary formations.

The most distinct outcrop of melange occursalong the southwestern coast of Laut Island(Fig. 5). The melange includes clasts and blocksof chert, siliceous shale, basalt, limestone, marland manganese carbonate nodules embeddedwithin a sheared shale matrix (Fig. 4d). It issignicant that sandstone or other coarse-grained detrital sediments are lacking in themelange. The detailed structure of the melangesare unclear, because of limited exposures in thisregion. The shale matrix is usually sheared tosome degree. In the Sekoyang area, tectonicslabs and blocks are tectonically mixed withmudstone dominant matrices. The dip of the fo-liation of sheared matrix ranges from 20 to 80toward the northwest or north (Fig. 5).

Major clasts include chert, siliceous shale,limestone and basalt. Chert and limestone arethinly bedded. Basalt is mainly lava, and pillowstructures are sometimes preserved. Limestoneclasts are locally dominant in the melange.Fragments of manganese carbonate nodules oc-cur rarely. The clasts are subrounded to suban-gular, lenticular to blocky in shape. Clast sizeranges from several millimeters to several hun-

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

Tectonic implications for the Meratus Complex 207

dred meters long. Clasts in the me lange areusually less than 1 m in long axis, but sometimesreach several metres long.

Chert is the dominant rock type in the melange.Chert layers range from 1 to 20 cm thick and areinterbedded with

with gabbro and amphibolite. The ultramacrocks are variably affected by low grade meta-morphism. Chromite is sometimes present but isa minor constituent. The KAr radiometric ageof a metadolerite dyke in the upper stream of theSatui River was reported as 116 Ma (Sikumbang1986).

The ages of ultramac rocks in Laut Island areestimated from the age of chert which originallyoverlay the ultramac rock, together with basicigneous rocks such as basalt and gabbro. Theoldest chert in Laut Island, of early MiddleJurassic, indicates the age of ultramac rocksis older than early Middle Jurassic at Laut Is-land.

INTRUSIVE ROCKS

Leucocratic rocks in an ultramac unit includequartz diorite and trondhjemite which are closelyassociated with dolerite and gabbro (Sikumbang1986). These rocks are classied as `Plagiogran-ite'.

Granite and granodiorite have been recordedfrom a few localities in the Meratus Mountains(Sikumbang & Heryanto 1994). Granodiorite in-truded into the Pitap Formation. KAr dating ofthe granite yields an age of 115 Ma (Heryantoet al. 1994).

RADIOLARIAN BIOSTRATIGRAPHY

The following samples and extracted radiolarianswere collected: one sample from the HaruyanFormation at Mandikapan and three from theHaruyan Formation near Mount Baturung, 16samples from the Pitap Formation along the roadbetween Kandagan and Batulicin, ve samplesfrom the Pitap Formation east of Kotabaru, LautIsland, one shale sample from the HaruyanFormation east of Kotabaru, Laut Island, threesamples from chert on the ophiolite at Batricin,three samples from chert east of Kotabaru, 20samples from chert in melange at Sekoyang, 10samples from siliceous shale in the melange atSekoyang, nine from a shale matrix of melange atSekoyang and ve samples from manganesecarbonate nodules in melange at Sekoyang.

Among the samples mentioned above, 14 sam-ples in the Laut Melange, one sample from theHaruyan Formation and two from the PitapFormation yielded diagnostic radiolarians for agedetermination. Radiolarians were chemically ex-tracted from chert and siliceous shale using hy-drouoric acid as discussed by Pessagno &Newport (1972). The associations recognized areshown in Figs. 713 and the Appendix. Theseassociations range in age from early Middle Jur-assic to late Early Cretaceous (Fig. 6) based on

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

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Fig. 7 (1) Archaeodictyomitra sp.; (2) Hsuum sp.; (3) Hsuum sp.; (4) Transhsuum hisuikyoense (Isozaki and Matsuda); (5) Transhsuumhisuikyoense (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.

210 K. Wakita et al.

numerous bio-stratigraphic works (Pessangno1976; 1977a, b; Schaaf 1981; 1984; Taketani 1982;Matsuoka 1983; 1986; Isozaki & Matsuda 1985;Matsuoka & Yao 1985; 1986; Teraoka & Kurimoto1986; Aita 1987; Carter et al. 1988; Tumanda 1989;Hori 1990; Qun 1993; Gorican 1994; Jud 1994;O'Dogherty 1994; Baumgartner et al. 1995a,b).

MELANGE AT SEKOYANG

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

or two chert samples in one tectonic slab of themelange. Therefore the occurrence of radiolari-ans does not show the biostratigraphic relation-ship in the melange. What we can interpret fromthe radiolarian data, however, is a reconstructedsuccession of the protolith which was dismem-bered during melange formation.The oldest radiolarian assemblage in this mel-ange is of early Middle Jurassic (Fig. 6; Fig. 7).The assemblage from the sample SK47C includesTranshsuum hisuikyoense Isozaki & Matsudaand Archicapsa (?) pachyderma (Tan). Unumasp. A. (?) pachyderma is restricted to the early tolate Bajocian (Baumgartner et al. 1995a).

Sample SK52A is from the red shale matrix ofchert breccia, while Sample SK52B is a red chert

Fig. 8 Thanarla sp.; (2) Thanarlasp.; (3) Thanarla sp.; (4) Arch-aeodictyomitra apiarium (Riist); (5)Archaeodictyomitra minoensis Mizu-tani; (6) Archaeodictyomitra apiarium(Rust); (7) Cinguloturris cylindraKemkin and Rudenko; (8) Cingulo-turris cylindra Kemkin and Rudenko;((9) Xitus sp.; (10) Pseudodictyomitracarpatica (Lozyniak); (11) Parvicingulamashitaensis Mizutani; (12) Par-vicingula mashitaensis Mizutani; (13)Protunuma japonicus Matsuoka andYao; (14) Stichocapsa sp.; (15) Tri-colocapsa (?) sp.; (16) Podobursa sp.;(17) Podobursa sp.; (18) Sethocapsasp.; (19) Eucyrtidiellum pyramis (Aita);(20) Eucyrtidiellum pyramis (Aita);(21) Stichocapsa sp.; (22) Saitoumsp.; (23) Pseudoeucyrtis (?) sp.; (24)Pantanellium sp.; (25) Pantanelliumsp.; (26) Spumellaria gen. and sp.indet.; (27) Spumellaria gen. and sp.indet.; (28) Stichocapsa sp.; (29)Sethocapsa sp. Scale bar 0.1 mm.

Tectonic implications for the Meratus Complex 211

clast of the same chert breccia. The former in-cludes a late Tithonian assemblage such asArchaeodictyomitra apiarium, Cinguloturriscylindra, Eucyrtidiellum pyramis, Par-vicingula mashitaensis, and Protunuma japo-

nicus (Fig. 8, Appendix), whereas the latteryields a Middle Jurassic assemblage such asEucyrtidiellum 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.

212 K. Wakita et al.

A chert sample SK41X includes a variety ofSpumellaria showing delicate structures: theseinclude Tritrabs, Triactoma, Emiluvia, Higm-astra and Alievium together with species ofNassellaria, such as Eucyrtidiellum ptyctum(Fig. 10, Appendix). The dignostic species,E. ptyctum and Emiluvia prenyogii indicate theage of the sample is Middle Jurassic (Fig. 6).Samples of SK47B, SK57 and SK58 are MiddleJurassic based on the radiolarian assemblages.Samples of SK47A, B and C are obtained fromsouth to north with about 1 m distances in acontinuous sequence.

The melange locally includes light gray chertwith rough surfaces, although most of the chertis red or reddish brown in color. The formercontains various obscure fragments which mightbe components of ash. It has been termed, `tuff-aceous chert'. The beds of the chert are rela-tively thicker than in the reddish brown beddedchert.

The samples, SK50A and SK50B, containPseudodictyomitra carpatica, Sethocapsa ut-erculus, Xitus gifuensis, Pantanellium lanceolaand others ranging in age from late Kimme-ridgian to late Valanginian (Fig. 11, Appendix).

Fig. 10 (1) Thanarla brouweri (Tan);(2) Archeodictyomitra sp.; (3) Eycyrti-diellum ptyctum (Riedel and Sanlippo);(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 Ba-umgartner; (12) Pantanellium sp.; (13)Archaeospongoprunum sp.; (14) Pant-anellium 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. Scalebar 0.1 mm.

Tectonic implications for the Meratus Complex 213

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) Pseudodictyomitrasp.; (10) Cryptamphorella shpaerica (White); (11) Cryptamphorella sp.; (12) Cryptamphorella sp.; (13) Paronaella (?) sp.; (14) Sethocapsa cf.uterculus (Parcona). Scale bar 0.1 mm.

214 K. Wakita et al.

SK50B is sampled at about 3 m north of SK50Awith a 2 m lack of outcrop in between.

Demonstrably the youngest rock in the me-lange at Sekoyang is a very siliceous shale. Theshale consists of light greenish gray very sili-ceous beds of 115 cm thick interbedded withthinner dark gray shale partings. The sampleSK60A is a very siliceous shale part, whileSK60B comes from a dark gray shale parting.They yield similar Early Cretaceous assemblag-es ranging from late Valanginian to early Aptian.The assemblage contains Acaeniotyle umbili-cata, Cyptamphorella shaerica, Pantanelliumlanceola, Pseudodictyomitra carpatica, Setho-capsa uterculus, Stichomitra dediocris andThanarla lacrimula (Fig. 12, Appendix).

The shale matrix of the melange and manga-nese carbonate nodules show a lack of radiolari-ans or include only very poorly preservedradiolarians.

MELANGE AT EAST OF KOTABARU

Sample SK34, pale green chert, was obtainedeast of Kotabaru (Fig. 2). It includes Rho-palosyringium adriaticum which ranges fromMiddle Albian to Cenomanian (Fig. 6).

HARUYAN FORMATION

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

Fig. 12 (1) Thanarla pacica Nak-aseko and Nishimura; (2) Thanarlabroweri (Tan); (3) Thanarla broweri(Tan); (4) Thanarla lacrimula (Fore-man); (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. andsp. indet.; (12) Stichocapsa cf. japo-nica Nakaseko and Nlishimura; (13)Cryptamphorella shpaerica (White);(14) Cryptamphorella shpaerica(White); (15) Cryptamphorella cf. cli-vosa (Aliev); (16) Nassellaria gen. andsp. indet.; (17) Sethocapsa sp.; (18)Pseudoeucyrtis cf. hanni (Tan); (19)Eucyrtidiellum sp.; (20) Arch-aeodictyomitra sp.; (21) Hiscocapsagrutterinki (Tan); (22) Pantanelliumlanceola (Parona); (23) Acaeniotyleumbilicata (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.

Tectonic implications for the Meratus Complex 215

of the Meratus Mountains (Fig. 2). Chert sample,SK6A, which occurs at the foot of Mount Batu-rung, includes Dictyomitra cf. formosa, R. ad-riaticum, Stichomitra communis, Thanarlabrouweri, Xitus sp. and other species (Fig. 13,Appendix). The assemblage indicates an EarlyCretaceous to Cenomanian age (Fig. 6).

PITAP FORMATION

Two samples, SK24E and SK36B of shale alter-nating with thinner sandstone beds in the Pitap

Formation contain Cretaceous radiolarians(Nassellaria).

DISCUSSION

The Meratus Complex is characterized by highpressurelow temperature metamorphic rocks,ultramac rocks, and melanges including clasts ofradiolarian chert, pillow basalt and limestone.The chronological, stratigraphical and petrologi-cal data presented in this paper give us the new

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.

216 K. Wakita et al.

view of tectonic evolution of the Meratus Com-plex in Cretaceous time.

Radiolarian biostratigraphic studies on themelange in Laut Island revealed that the chertsin the melange range in age from Bajocian toCenomanian, although previous works recog-nized only cherts including Early Cretaceousradiolarians. The data suggests that the sub-ducted oceanic plate covered by these cherts wasat least older than early Middle Jurassic. Theoceanic plate evolved at some time before earlyMiddle Jurassic, migrated toward the SundalandContinent, and nally subducted in middle Cre-taceous time.

Granite, granodiorite, diorite and gabbro in-truded the Meratus Complex. The radiometricage of granite is 115 Ma (Heryanto et al. 1994).These igneous rocks may have been caused bythe subduction of the oceanic plate alreadymentioned beneath the marginal sea along theSundaland margin.

Basaltic to andesitic lava and tuff breccia ofthe Haruyan Formation and Pitanak Group areproducts of submarine volcanism, because chertfragments in the tuff breccia include deep marinefauna, radiolarians. These submarine volcanicproducts are often recognized near immatureisland arcs caused by the interaction betweentwo oceanic plates. The lower part of the Ha-ruyan Formation and Pitanak Group are formedin an immature island arc setting. Contempora-neously, the melange of Laut Island was formedby the subduction of the oceanic plate during lateEarly Cretaceous time.

The melange of Laut Island is characterized bya lack of coarse-grained detrital clastic sedimentssuch as sandstone and conglomerate. The sedi-ment supply from the continental side is absent orvery poor, although pelagic sediments to trenchand fragments of seamounts were derived fromthe oceanic plate and accreted on the continentalmargin. This evidence suggests that the trenchwas far from the continent and that mountainbuilding did not proceed near the trench.

In the Late Cretaceous, detrital clastic sedi-ments of the Pitap Formation covered the me-langes, metamorphic rock and ultramac rocks ina shallow marine environment. This signies thatthe Meratus Arc was mature enough to providecoarse-grained detrital clastic rocks on the con-tinental slopes and in the forearc basins.

Glaucophane schist of the Hauran Schist iscaused by oceanic plate subduction along thetrench. Judging from the petrological studies,however, the protoliths of some schists are dif-ferent from the products of normal subductionmetamorphism such as the Sambagawa Meta-morphic Rocks in Japan. The presence of schistconsisting only of quartz and chloritoid and thecommon occurrence of kyanite in the schists in-dicate that some of the protoliths had high alu-minium contents. The origin of highly aluminousmetamorphic rocks could be continental cover ormargin sediments. Various sizes of continentalfragments drifted northward and accreted alongthe Asian continental margin following thebreak-up of the Gondwanaland (Nur & Ben-Avraham 1983; Maruyama et al. 1989). The high

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

Tectonic implications for the Meratus Complex 217

aluminous sediments could have been derivedfrom the surface of a continental fragment (amicrocontinent), detached from the margin of theGondwana super-continent. This is a similar sit-uation to that of the Bantimala Complex, SouthSulawesi (Wakita et al. 1996). The Jurassicshallow marine sedimentary rocks incorporatedin the Bantimala Complex were evidence of themicrocontinent collision and accretion (Wakitaet al. 1996).

The older metamorphic rocks of 165 and180 Ma occurred as a small tectonic block alongthe northern margin of the Hauran Schist Block.They are not high pressure type metamorphicrocks like the other metamorphic rocks, but ofintermediate pressure type. They were exhumedand tectonically mixed with other components ofthe Meratus Complex during the processes ofsubduction, collision and accretion of oceanicplate and micro-continent.

Major tectonic events are recorded in threestages of unconformity in the Meratus area; thatis the Middle Cretaceous, Paleocene and LateMiocene (Fig. 14). Ultramac rocks, high-pres-sure schist and melanges were locally exhumedand provided their fragments into the PitapFormation.

Before the deposition of the Eocene TanjungFormation, ultramac rocks, schists and me-langes were tectonically juxtaposed with LateCretaceous sedimentaryvolcanic formationssuch as the Pitap and the Haruyang Formations.This CretaceousPaleocene event is contempo-raenous with rearrangement of the componentsof Cretaceous island arc systems along the Sun-daland margin such as Karangsambung, CentralJava (Wakita et al. 1994a) and Bantimala, SouthSulawesi (Wakita et al. 1994b; 1996).

Finally, Tanjung, Berai, and Warukin Forma-tions were tectonically deformed until they werecovered by the Pliocene Dahor Formation. Thelatest tectonism could be related to the west-ward obduction of the East Sulawesi ophiolite inOligocene time and Miocene to Pliocene collisionof the Sula microcontinent (Parkinson 1991;Cofeld et al. 1993, Bergman et al. 1996).

SUMMARY

(1) The Meratus Complex is a product of oceanicplate subduction and successive collision of mic-rocontinents during Cretaceous time.

(2) Radiolarian biostratigraphic studies revealthat the melange of the Meratus Complex in-cludes chert ranging in age from early MiddleJurassic to late Early Cretaceous.(3) The Haruyan Schist of 110119 Ma was a highpressure-low temperature type caused by oce-anic plate subduction. Some of the protolithswere high alluminous continental cover or mar-gin sediments. Intermediate pressure typemetamorphic rocks of 165 and 180 Ma were dis-covered along the northern margin of the Hau-run Schist.(4) The Haruyan Formation is a product of sub-marine volcanism in an immature island arcsetting, locally contemporaneous with the for-mation of the melange of the Meratus Complex.

ACKNOWLEDGEMENTS

This paper is one of the results of the joint pro-ject between the Research and DevelopmentCentre for Geotechnology (RDCG) in Bandung,University of London and the Geological Surveyof Japan (GSJ) under the ITIT program `Re-search on Mineral Resources Assessment ofOceanic Plate Fragments'.

The authors wish to thank to Ir. S. Suparka,vice president of LIPI for his helpful supportduring our geological survey. We also expressthanks to Dr A. J. Barber of Royal Holloway,University of London for his effective sugges-tions and discussion of the geology of this area.We are grateful to Dr C. D. Parkinson, STAfellow of GSJ, and Dr C. Kurimoto for theircritical review of our manuscripts.

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APPENDIX 1. Fossil details

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