Journal of the Geological Society, London, Vol. 145, 1988, pp. 577-590, 12 figs Printed in Northern Ireland

Late Palaeogene-Quaternary geology of Halmahera, Eastern Indonesia: initiation of a volcanic island arc

R.HALL,l M. G. AUDLEY-CHARLES,' F. T. BANNER,' S. HIDAYAT2 &L S. L. TOBING~ Department of Geological Sciences, University Collcge London, Gower Street, London WClE 6BT, UK Geological Research and Development Centre, Bandung, Indonesia

Abstract The Late Palaeogene-Quaternary stratigraphy of Halmahera is described, and new forma- tion names are proposed, based on recent field investigationsof the NE and central partof the island. This stratigraphic information provides new insights into the Neogene history of Halmahera and the development of the present island arc. The Late Palaeogene and younger rocks rest unconformablyon an ophiolitic Basement Complex which formed part of a Late Cretaceous-Early Tertiary fore-arc. After volcanic arc activity ceased in the Eocene the former fore-arc terrane was uplifted and deeply erodedin the Late Palaeogene. Some of theLate Palaeogene-Early Miocene river valleys are currently being re-excavated by the present rivers. Slow subsidence began in the mid-late Oligocene and by theend of theMiocene all easternHalmahera was thesite of shallow-watercarbonate deposition. There is no evidence for arc volcanism in central Halmahera at this time and the reported Oligo-Miocene volcanism in nearby regions is interpreted as volcanism related to the Sorong Fault system. The Miocene shallow water region subsided rapidly in the Early Pliocene and the sedimentary basin formed was filled with mark succeeded by siliciclastic turbidites, with increasing amounts of calc-alkaline volcanic debris from a Pliocene volcanic arc built on the western arms of Halmahera, probably on the eroded Early Tertiary arc. This phase of rapid subsidence in the Pliocene back-arc region resulted from the initiation of subduction of the Molucca Sea lithosphere eastwards beneath Halmahera. Differential subsidence on NW-SE and NE-SW sets of faults in the region immediately behind the active arc led to the formation of deep sediment-filled basins adjacent to the eastern arms. A major deformation event in the Pleistocene resulted in folding and local thrusting at the junction betweeneastern and western Halmahera and volcanism ceased in thePliocene arc. Thethird Halmahera arc, the Quaternary arc, currently active in the northern partof the islands, began activity within the last 1 Ma and is built upon the deformed and partly eroded Pliocene arc. The Pleistocene deformation event and shift in positionof the arc are interpreted as the result of the interaction of the eastward-dipping Molucca Sea plate with adjacent plates, either with a fragment of the Australian continent in the Sorong Fault zone and/or with the Philippine Sea plate beneath northern Halmahera.

At the end of the Eocene a major plate reorganization event 1000 km east of Sulawesi beforesubduction began. As occurred in the western Pacific which is recognizable over a Molucca Seasubduction proceeded, Australia moved verywide region (Hayes & Lewis1984) along the Pacific obliquely northwards with respect to the Pacific but regional margin and in SE Asia. On Halmahera (Fig. 1) this event geological and palaeomagnetic evidence is not yet sufficient led to imbrication of ophiolites,metamorphic rocks and to lix the past position of Halmahera relative to Australia. sediments which hadformed part of aCretaceous-Early The history of the region after the Late Eoceneis at present Tertiaryfore-arc terrane (Hall er al. 1988) traceable very poorly known since the geology of the regions around northwardsinto the Philippines at least as faras east the complex knot of Halmahera has not yet been Mindanao. This fore-arc terraneis the Basement Complex of investigated indetail. Marine geophysical studies of these theNE and SE arms of Halmahera.The present and regions provideimportant constraints on the recent plate recently active volcanic arc built on the NW arm and islands tectonic history but information from land-based studies is off westernHalmahera is situatedabove aneast-dipping essential to extend our knowledgeback beyond afew subduction zone (Fig. 2). Thus the Early Tertiary fore-arc millionyears. The geological setting of Halmaheraand terrane, which forms the east Halmahera basement, is now previousknowledge of the region is summarized in our situated in a back-arc position relative to the present arc. On earlier paper dealing with the Basement Complex (Hall et the eastside of the Molucca Seaeastward subduction al. 1988); here we describe the cover rocks to the Basement beneath Halmahera has formed the Halmaheravolcanic arc, Complex and relate new field geological information to the and on the west side oceanic lithosphere has been subducted development of the Halmahera volcanic arc. westwards beneathnorth Sulawesi, forming the Sangihe volcanic arc (Fig. 1). Studies of recent seismicity show that ,theMolucca Sealithosphere has an inverted U-shaped Late Palaeogene and Neogene stratigraphy of configuration (Fig. 3, Hatherton & Dickinson1969; Halmahera Cardwell et al. 1980)with aminimum of loo0 km of subductedlithosphere. Therefore, the eastHalmahera Halmahera is covered by tropical rainforest and therefore BasementComplex must havebeen situated at least field geological investigations are accomplished by geologists 577

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E 120° 130° I I I

0 km 500 j P

Fig. 1. Location of Halmahera, Bacan and principal bathymetric features of the adjacent regions after Mammerickx ef al. (1976).

and teams of porters, carrying equipment and food, making tion of Halmahera is still at a reconnaissance level and the traversesin areas selected by aerialphotographic study. difficulties of fieldwork preclude the establishment of These traverses have a duration of several days, are entirely detailed measured type sections; the localities from which on foot, follow riverswhere possible but are modified en the new formations have been describedare shown in Fig. 4. route by geology, terrain,weather, time, difficulty and This work forms part of a joint project between UCL and supplies. Details of the traverses and localities referred to in the GeologicalResearch andDevelopment Centre the text are shownin Fig. 4. Inorder to describe the (GRDC),Bandung, Indonesia, and asthe investigation succession Late Palaeogene and Neogene rocks have been proceeds we expectthat changes will berequired tothe assignednew formationnames. Some stratigraphical stratigraphydescribed here, to take account of new terminologyhas been introduced in earlier publications discoveries andstratigraphical variation within the large (Apandi & Sudana 1980; Supriatna 1980; Sukamto et al. areaunder investigation. One aim of the project is to 1981) and comparison of the stratigraphy described here and produce a new geological map of the islands but it is not yet that of previous authors is shown in Fig. 5. As a result of possible to show the distribution of the formations described our work we have been able to date much of the Neogene here. We have therefore modified the published geological sequence more precisely and subdivide it lithostratigraphi- maps (Apandi & Sudana 1980; Supriatna 1980; Yasin 1980) cally ingreater detail. This stratigraphy provides new to summarizepresent knowledge of the geology and insights intothe Neogene history of Halrnaheraand the structure of Halmahera (Figs 6 & 7) which incorporates the development of the present island arc. However, the new results of the project so far. Sample numbers referred to in formation names are necessarily provisional since investiga- the text and shown in Fig. 4 are stored in the Department of

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PHILIPPINE

Fig. 2. Principal tectonic features of the Hal- BANDA SEA mahera region after Hamilton (1979) and Silver (1981). Solid triangles represent active volcanoes of the Halmahera and Sangihe arcs. Slip rates AUSTRALIAN along the Philippine Trench from Ranken er al. (1984).

GeologicalSciences, University College London(UCL). established in the Early Miocene (see below) but the NE Fauna1 identifications were based on thin section determina- part of the NE arm must have been the site of carbonate tions. In certain cases the ages of samples have been given reefs by the Late Oligocene since high energy, inner shelf in the text using the P and N zones of Blow (1969,1979) and limestones sampled in theOnat Rivercontain reworked the East Indies Letter Stages and the approximate positions coralgal material of Late Oligoceneage (Tel-4) with no of these stage boundaries are shown in Fig. 5. evidence of stratigraphic admixing (HA72A: Lepidocyclina (Nephrolepidina), Rotalia gr. trochidiformis, Borelis pygmaeus, Halkyardia sp., with abundantdebris of Onat Mar1 Formation (mid-late Oligocene) rhodophyticcoralline algae, rarer bryozoa and molluscs). The Neogene sequence exposed on Halmahera shows some West of the Onat River the Onat Marl Formation has not variation across the island reflecting the history of uplift, been recognized although in the Subaim Fault zone there erosionand re-submergence following themajor event in are slices of redeposited calcisiltites and calcilutites of Late Eocene which uplifted the Basement Complex (Hall er possible Palaeogene age. al. 1988). Submergence appears to have begun earlier in the eastern part of the NE arm where, unconformably above the Basement Complex, the Onat Marl Formation is a relatively Jawali Conglomerate Formation (?Oligocene-Early deep water off-reef facies. Inthe type area in theupper Miocene) Onat River (Fig. 4) the formation includes soft white marls, West of theOnat Riverthe oldest Neogene rocks grey mudstonesand siltstones interbedded with graded recognizedbelong to the Jawali Conglomerate Formation. calcilutites and calcisiltites which are of middle tolate The type area for the formation is the upper valley of the Oligocene age (HA99: Chiloguembelina spp., Jawali River (Fig. 4) where there are well exposed, coarse Dentoglobigerina, Tenuitella spp., Cassigerinella chipolensis, boulderconglomerates containing clasts of ophiolitic note absence of Pseudohmtigerina). material. The clasts are poorly sorted and well rounded and West of the Onat River, reef limestone deposition was range in size up to 0.5 m across. The conglomerates are

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of matrix decreases, as does the proportion of carbonate in the matrix.Stratigraphically higher in the conglomerates, where there is more carbonate detritus, there are raresmall, conical gastropods in the sandy matrix. The thickness of the Jawali ConglomerateFormation in the Jawalivalley is approximately 400 m. The conglomerates thin laterally away from the present valley bottoms and cannot be traced far from the rivers. The regular change in clast size, matrix character and ophiolitic debrisindicates that they are alluvial or fluvial deposits which accumulated on an irregular surface of the underlying Basement Complex. In the Saolat and Jawali Rivers there is atransition from these conglomerates into the overlying limestones indicating marinea transgression andthis limestone onlap has probably covered the conglomerates in MOLUCCA SEA PLATE other areas. For example, in some of the tributaries of the DodagaRiver huge gabbro and cumulate ultrabasic boulders (>l0 m across)occur asfloat. The size and abundance of theseboulders increases going upstream i indicating approach to the source region but there are no exposures of these rocks in situ, and in the highest parts of Fig. 3. Present configuration of the Molucca Sea Plate in the region these steep and narrow river valleys there are outcrops of between Halmahera and the Sangihe Arc after Cardwellet al. Miocenelimestones. The size of this float isconsistently (1980). Halmahera is located on a sub-plate which is being largerthan the float derivedfrom the exposed basement underthrust from the west by the Molucca Sea Plate and from the NE by the Philippine Sea Plate. The southern boundaryof this rocks, which are dominantly microgabbros with occasional sub-plate is the Sorong Fault system. The natureof the sub-plate serpentinites, limestones and Dodaga Formation (Hall et al. boundary east of Halmahera is still uncertain (Hall 1987). 1988) breccias, suggesting that the huge boulders represent re-eroded boulders from river valley conglomerates and that some of thepresent valleys are excavating a pre-existing clast-supported and have a matrix of poorly sorted pebbly topography. Similar relationships are observed in the Onat sandstone withsimilar ophioliticdetritus andsome River near the junction with the Geledongan River (about carbonate clasts. The contact with the Basement Complex is 25 km SE of Dodaga village) where small conglomerate not visible but as the basement rocks are approached the deposits in the valley bottom fill elongate depressions in the clasts in the conglomerate become larger and the proportion BasementComplex parallel tothe main valley. The

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Fig. 4. Location of traverses (dotted lines) made in central andNE Halmahera. Localities of sample numbers given in text (H andHA numbers) are also shown. Type areas for new formations are indicated by cross-hatching.

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YOUNGER SEDIMENTARY ROCKS

YOUNGER MLCANIC ROCKS

OLDERSEOIMfNTARYWCKS

OLDER VOLCANIC ROCKS I BASEMENT COMPLEX I

Fig. 5. Stratigraphy of Halmahera based on this work and comparison to thatof previous authors. The approximate positionof P and N zone boundaries (Blow 1969, 1979) and East Indies Letter Stage boundaries are from Jenkins er al. (1985).

Fig. 6. Sketch geological mapof Halmahera based on Apandi& Sudana (1980), Silitongaet al. (1981), Supriatna (1980) and Yasin (1980) and modified after our own observations. Halmahera is divided into two provinces by the nature of the underlying basement rocks.

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R. 8. (a) Ophiolitic boulders in Jawali Conglomerate Formation. Onat River. Chisel for scale. (b) Echinoid and leaf imprint in mark of Saolat Mar1 Formation. Saolat River. Pen for scale.

conglomerates (Fig. 8a) are composed mainly of boulders River. The base of the limestone was not observed in any and pebbles of the Basement Complex, cemented by calcite other areas although float samples indicate that, as in the and locally containing limestone pebbles. The clasts are well mountainsaround the Jawali River,there is a pebbly roundedand the conglomerates appear to be fluviallag limestone with ophiolitic clasts at the base. It is probable deposits. One clast of reef limestone has been dated as that this limestone rests directly on the Basement Complex, Eocene (HA86: Discocyclina spp., Asterocyclina, Gypsina) andprobably on otherpre-Neogene rocks, without a and another as probableEarly Miocene (HA83: significant thickness of conglomerateover most of the Lepidocyclina,Globorotalia cf. zealandica). This suggests northern part of the NE arm. that the conglomerate was deposited by an early Neogene The character of the limestones varies across the area river system erodinga landscape exposing theBasement from fore-reef and reef clastic limestonesto back-reef clastic Complex,Eocene reef limestones of theGeledongan limestonesand lagoonal limestones. The distribution of Formation (Hall et al. 1988) and Early Miocene limestones. thesefacies deduced from thetraverses suggests thatthe This older valley has been re-excavated by the Onat River. reef edge was roughly parallel to the northern coast of the The conglomerates locally pass upwardsinto a finer NE arm but about 5 km inland from the present coastline. grainedbreccio-conglomerate with roundedand angular The limestones vary in thickness,although the exact clasts of basic and ultrabasic rocks in a calcareous pebbly thickness is difficult to determine because of faulting and sand matrix. On the mountain sides above the Jawali River folding. In the Saolat and Talawi Rivers the limestones are and in the Saolat River this breccio-conglomerate horizon about 200 m thick but further north, near Subaim, are about rests directly on brecciated and blocky basic and ultrabasic 500 m thick. The oldest limestones above the unconformity rocks of the Basement Complex with no intervening are EarlyMiocene, and none are younger than Late conglomerate. The proportion of ophiolitic debris in these Miocene-Early Pliocene. The reef and fore-reef limestones calcareous rocks varies;some beds are strictly calcareous vary from massive to thickly bedded and contain abundant micro-conglomerates or micro-breccias, while othersare large corals, algal, bryozoan, echinoid and mollusc debris. merely pebbly limestones with ophiolitic pebbles. Because (HA49A: Spiroclypeus, Eulepidina spp., Miogypsinella, of incompleteexposure it isdifficult todetermine the very primitive Miogypsina. EarliestMiocene. HA5: thickness of this transition into limestones but it is nowhere Miogysinoides dehaarti, Miogypsina, Operculinella, more than 50 m thick and may be as thin as a few metres in Lepidocyclina, Spiroclypeus, Sphaeroidinellopsis, Globigeri- places. noides. LateEarly Miocene. HA125: Lepidosemicyclina, Sontes. Late Early Miocene. HA133: Sorites, Heterostegina, Eulepidina? LateEarly Miocene. H188: Triloculina, Subaim Limestone Formation (Early Miocene-Early Planorbulinella, Globigerinoides obliquus. Early Miocene or Pliocene) younger. H189: Parrelinna, Planorbulinella, Operculinoides, This formationrests unconformably on theBasement Globigerinoides. EarlyMiocene or younger. HA123: Complex over a very wide area of the northern part of the Miogypsina, Lepidocyclina. Late Early or Middle Miocene. NE arm of Halmahera. The formation is named from the H47: Parrellina,Operculinella, Pararotalia, Miogypsina, upper Subaim River (Fig. 4) where the limestones are well Lepidocylina(Nephrolepidina), Sphaeroidinellopsis semi- exposed. Reef limestonesand fore-reef clastic limestones nulina. Tf, probablyMiddle Miocene. H48: Cycloclypeus, cap the range of mountains running SW from Subaim where Lepidocyclina clasts, Globigerinoides, Sphaeroidnellopsis they are exposed at heights up to 1084 m. North of Subaim, multiloba, S. seminulina, Globorotalia fohsi. N12 Middle in the Titilegan River near to Lolobata, and south of Ekor, Miocene (Tf2) (Serravallian). H50: Cycloclypeus, the reefal facies is exposed at topographically lower levels Lepidocyclina, Heterostegina, Globigerinoides, due to major NW-SE faults. The transition from the Jawali Sphaeroidinellopsis, Globorotaliafohsi robusta, G. cf. Conglomerate Formation was observed only in the Jawali menardii. Tf2, late N12 Middle Miocene (Serravallian).

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HA6:Rhodophyte algae, rare codiacea, Miogypsina, imprints and at one locality in the Saolat River, a perfectly Heterostegina, Lepidocylina,Cycloclypeus, Operculinella preservedechinoid about lOcm across (Fig. 8b). Plant and many redeposited clasts derived from Early and Middle remains, particularly leaves, areabundant andsome thin Miocenetogether with Orbulina, Sphaeroidinellopsis, layers containabundant small, thin-shelledbivalves. Few Globigerinoides, Dentoglobigerina altispira, Globorotalia cf. sedimentary structures were found, and the preservation of plesiotumida, G. menardii, Sphaeroidinella, Pulleniatina. the leaves and echinoids indicates either very rapid burial or Late Miocene or Early Pliocene. HA120: Orbulina suturalis, little biological activity; a few sand-filled tubes occur locally. Globigerinoides, Sphaeoidinellopsis, Globigerina spp., Because of the very low dip on the mark and theslight open ?Pulleniatina. Probably Late Miocene). folding of these rocks the total thickness of marlsis Most of the fore-reef limestones in the Subaim region uncertain; it is at least 100 m. contain small amounts of siliciclastic debris, evidently The sequence above the marls is very poorly exposed in derived from basic and ultrabasic rocks. Interbedded with the Saolatand Jawali Rivers.Intermittent exposures thesecarbonate clastics in places aredark calcareous indicate thatthere is a gradualtransition to coarser- sandstonescontaining very angularbut well sorted grained siliciclastic sediments. These are thin- to medium- siliciclastic material. The grainsinclude single mineral beddedsandstones and siltstones withoccasional thin fragments of plagioclase, pyroxene,hornblende and conglomeratebeds. The sandstones are locally channelled serpentine, all very fresh. The angularity and freshness of and in theSaolat River the channels have a N-S such unstable mineral grains, together with the admixture of orientation. All of the debrisappears to be well-rounded carbonateand non-carbonate debris, all well sorted, and well-sorted. Some broken shelly material is present in suggests thatthis material was redeposited,probably in the sandstones,and abundant organic material, some of front of the reef ina similar environmentto off-shore, which is coalified. In interbedded mark and shales the plant present-day Halmahera with a similar ophioliticsource material, which in hand specimen resembles leaf and twig region surrounded by carbonate reefs. Notably absent from debris, is less mature. There is no sign of pressure solution the debris is calc-alkaline volcanic material. or recrystallization such as might be expected by the degree Furtherinland, to the east of the coastalmountains, of coalification observed in the plant material. The coalified limestones form a karstic capping to the mountains drained material may bederived by erosion of stratigraphically by theDodaga River. In this region they are athinner lower levels in the sequence onHalmahera (fieldwork in bedded back-reef facies equivalent to the reefal limestones 1987 discovered thin coal seams of probable Eocene age at of the coastmountains. The ages are apparently slightly the east end of the SE arm) or may be debris from forest younger but this probably reflects a bias towards sampling of fires. In the Talawi River the structural position of a similar stratigraphically higher parts of the Subaim Limestone inthe sequenceindicates stratigraphically higher levels in the Dodaga region compared to its lower parts in the Subaim SaolatMarl Formation. The Talawirocks are bluish and Saolat-Talawi regions. (H165: Nephrolepidina, turbiditesandstones and siltstones, in places arkosic, with Operculinella, abundant Operculina. Late Early Miocene to well-developed normal grading, load structures and with a Middle Miocene. H70: Alveolinella quoyi, Sorites martini, low sand/shale ratio (Fig. sa). Some of the mudstone units Operculina with coral and codiacean algal debris and rare are markedlybioturbated and organic material is moder- cheilostone bryozoa. Middle Miocene TM or Late Miocene atelycommon. A calcareoussandstone with reworked Tg-probably late Serravallian/Tortonian. H164: Codiacea, pelagicmicrite clasts, allochthonouscalcareous algae and corals, rarerhodophytes and bryozoa, ?Marginopora. debris from basic volcanic and plutonic rocks is of probable Probably Middle Miocene or younger). Pliocene age (Ha: Globigerinoides, Pararotalia, Globoro- talia cf. tumida, Sphaeroidinellopsis).

Saolat Marl Formation (EarlyPliocene) Wasile Sandrtone Formation (Early? Pliocene) In the coastal mountains there is a relatively sudden change Inthe upper valley of the Wasile River the Basement from limestones to dark grey-green coloured marls of the Complex is in contact with the Neogene rocks to the south; Saolat Marl Formation which are well exposed in the type the contact is a major steep fault marked by a wide zone of section in the Saolat River (Fig. 4). At the base of the mark sheared serpentinite. To the south of the serpentinites is a in the Subaim Mountains there are horizons of thin, well zone of steeply-dipping turbidite sandstones and mudstones laminated calcisiltities which are locally channelled into the whose exact stratigraphic position is uncertain. They are of marls. One of these close to the limestone-mar1 boundary is probablePliocene age (H38: Intraclasts include Neogene of Late Miocene-Early Pliocene age (H53: Uvigerina spp., pelagicmicrite containing Globigerinoides, Sphaeroidi- Lenticulina, Globigerinoides quadrilobatus group, Orbulina, nellopsis. H39: Globorotalia cf. menardii, Sphaeroidinello- Globorotalia menardii group, G. cf. ungulata, psis, ?Pulleniatina, Globigerinoides cf. obliquus, Globorota- Dentoglobigerina, ?Globoquadrina ; allochthonous clasts lia cf. ungulata. Pliocene.H41: Globorotalia menardii, with Amphistegina and Operculinella). The interbedding Globoquadrina, Sphaeroidinellopsis, Orbulina,Globorotalia of redepositedlimestones (with reefaldebris) and mark scitula) andare assigned theto Wasile Sandstone suggests that the boundary is likely to be diachronous. This Formation, named from a section in the Wasile River (Fig. is supported by the thinning of theSubaim Limestone 4), above the Saolat Marl Formation. The most complete, Formation from NE to SW between Subaim and Ekor and and type, section is exposed over a few hundred metres in by the 1:250 000 geological map(Yasin 1980)which the headwaters of the Wasile River, north of the mountain suggests that Miocene limestones are not present in the SW crest. The oldestrocks are grey-blue,medium-bedded arm.The mark clearly represent verycalm depositional sandstonesalternating with silts and silty mudstones. The conditions since many beds contain perfectly preserved leaf sandstones have flat bases, although on loose blocks in the

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Fig. 9. (a) Graded bedding in upper part of Saolat Mar1 Formation. Talawi River. (b) Tuff in lower part of Tapaya Volcanic Formation. Tapaya River.

stream grooves and small flutes arepresent. The sand/shale Wasile SandstoneFormation is thedeeper water lateral ratio is low, approximately l :4. Thispart of thesequence equivalent of theSaolat Marl Formation. dips northwardsat a very high angleand is overturned. Stratigraphically higher in the section the beds dip steeply SE and are right-way up. The sequence is similar to that Tapaya Volcanic Formation (Early -Late Pliocene) below except that fine sandstones are normally graded and In the region south of Ekor drained by the rivers Parama, mudstone units up to 30 cm thick contain mudstoneclasts up Tapaya, Kiloting, Pettigoagoa and their tributaries there is a to lOcm long suggesting slumping. Some of the mudstones transitionfrom Early Miocene reef carbonates of the and siltstones contain abundant plant material. The highest Subaim LimestoneFormation through the Saolat Marl part of the sequence is significantly coarser than that below, Formationinto Pliocene siliciclastic turbidites inwhich with an increase in the sand/shale ratio. Sandstone beds up volcanic detritus makes an increasing contributionLower from to 1m thick alternate with mudstones of about the same toUpper Pliocene. Mark and marly limestonesare thickness. Some sandy beds contain abundant large elongate interbedded with a volcaniclastic turbidite and tuff sequence rip-up clasts of mudstone, while others have erosional bases whichgives way to tuffs and lavas. The volcaniclastic and andare channelled into the underlying mudstones. The associated sediments are assigned to the Tapaya Volcanic lower parts of thechannelled sandstones have large-scale Formation for which the type area is the headwaters and cross-laminationand grade upwards into laminated silt- upper reaches of the Tapaya River (Fig. 4). In this region stones. At the top of this part of the sequence are coarse theSubaim Limestone Formation andSaolat Marl pebbly and boulder-bearing sandstones with abundant coaly Formationare succeeded by aturbidite sequence about material. The boulders and pebbles are reworked siliciclastic 300m thickcomposed of shales,siltstones and greenish rocks. Most of the plant material is elongate and parallel to sandstones. In the Magdalena River about 7 km south of beddingbut there are somerounded coal clasts; atleast Ekor a seriesof dark greenish sandstones, with calcarenites, some of this material thus appears to have been coalified shales, mark and siltstones with very steep dips is folded beforedeposition into these sandstones. All of the into a tight syncline. These apparently deep-water deposits sandstones in this section are calcareous and the proportion are assigned tothe Wasile SandstoneFormation and a of carbonate decreases up-section. The siliciclastic material sample of pelagic micrite is of probable Late Miocene age present is angularand includes fresh grains of green (HA114: Orbulina suturalis, Sphaeroidinellopsis, Globigeri- clinopyroxene and plagioclase, and clasts of volcanic rocks, noides spp.) They are succeeded by rocks in which volcanic microgabbrosand serpentinites. This section suggests a debris becomes gradually more important (Fig. 9b). Beds of prograding submarine fan, with the higher beds representing conglomerate up to 10 m thick with a tuff matrix alternate upper-fan channel-fill deposits. Thesequence has amore withthinly bedded siliciclastic sandstone-shaleturbidites proximal character than the turbidites assigned above to the making a sequence at least 100 m thick that is succeeded by upperpart of theSaolat Marl Formation and this is a shale-tuff sequence. The turbidites have erosional bases consistent with the coarser turbidites of the Wasile River and parallel laminated tops and individual turbidite units are being younger than the Saolat Marl Formation. .In view of 10-25 cm thick. They dip steeplyandare locally overturned. the considerable field evidence for redeposition of sediments Pelagic limestonesandprobable slumped limestone in this sequence the youngest age (i.e. Pliocene) has been conglomerates with pelagicand derived reef material of accepted as the age of deposition and the older dates are EarlyPliocene age occur in thesequence (HA135: assumed to be dueto reworking of oldermaterial. It is rhodophytealgae with Orbulina, Globigerinoides cf. however possible thatthis section is nota single intact obliquus,Globoquadrina, Dentoglobigerina altispira s.l., sequence,but is tectonicallycondensed; this is consistent Globorotalia margaritae, GI. tumida., menardii, Sphaeroidin- with the steep dips in this narrow zone close to the major ellopsis). Locally pelagic micrites are interbedded with tuffs fault to the NW. This interpretation could mean that the andcontain a middle tolate Pliocene (N20/21) fauna

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Fig. 10. (a) Flow-banded basalts and basaltic andesites of the Tafongo Volcanic Formation. (b) Hydrothermally-alteredvolcanic rocks forming the basement of the western arm of Halmahera.

(HA140: Orbulina, Pulleniatina, Globorotalia cf. menardii Quaternary volcanic rocks cultrata, Sphaeroidinellopsis, G1. menardii). The Quaternary volcanic arc is active north of Makian and recently inactive on the islands to the south (Fig. 6). The Tafongo Volcanic Formation (Late Pliocene- volcanoes arestratiform cones typical of calc-alkaline ?mid-Pleistocene) volcanoes andtheir products are basaltic to andesitic The increasing volcanic contribution with diminishing age is pyroclastics and lavas. The chemistry of the volcanic rocks is expressed by the westward increase in volcanic rocks in the typical of a calc-alkaline intra-oceanic arc except on Bacan region west of Ekor. Thus the western part of Halmahera wherethere is evidence of eruptionthrough continental (west of theTapaya River) is composed mainly of the crust (Morris et al. 1983). In central Halmahera the present products of theLate Pliocene-Pleistocene volcanic arc. active volcanic arc passing through the islands of Ternate Sediments of the Tapaya Volcanic Formation are overlain andTidore isnow west of the west arms of Halmahera stratigraphically by volcanic conglomerates,porphyritic indicatinga westward shift of volcanic activity by about basalts/andesites and consolidated tuffs of Late Pliocene to 30km that must have occurred in the mid orLate presumedEarly Pleistocene age assigned tothe Tafongo Pleistocene. Northwards the arc follows the NW arm and Volcanic Formation. West of the Tapaya River calc-alkaline the currently active volcanoes retain a perfect conical form lavas become much moreimportant around the western indicating their youth and are built upon tilted fault blocks shore of Kau Bay between the mouth of the Tapaya River (Verstappen 1964) of pre-Quaternaryrocks. The present and Bobaneigo village. The formation takes its name from active volcanic arc is no older than mid-Pleistocene (1 Ma). the cape (Tanjong) and coastal exposures near thevillage of Tafongo on Kau Bay where the lavas are particularly well exposed (Fig. 4). These lavas and tuffs, which make up a Structure large part of the western arms of central Halmahera, are Geologically Halmahera can be divided into two provinces probably Late Pliocene to Pleistocene inage. A tuff (Fig. 6): aneastern province with anophiolitic Basement intercalation is reportedto contain a Pleistocene fauna Complex forms the NE and SE arms whereas the western (Apandi & Sudana 1980). The Tafongo Volcanic Formation province is composed largely of Pliocene-Recent volcanic also contains much coarse volcaniclastic material in the form rocks which formthe NW and SW arms.Early Neogene of volcanic breccias of vesicular lavas. Many exposures of sediments which unconformablyoverlie theeastern the lavas display sheeting and flow banding (Fig. 10). On the HalmaheraBasement Complex can be tracedinto the west coast of the western arm in the rivers around Guraping westernprovince and under the Pliocene volcanic rocks village porphyritic lavas and vesicular lavas are associated through a junction zone between the two provinces which with volcanic conglomerates, volcanic breccias and tuff forms a topographic depression southof Ekor. conglomerates. Weathering is a least 1.5 m deep in many places. The easternprovince Quaternary coral reef Thestructure of theeastern province is dominated by In the valley of the Kiloting River about 11 km south of vertical faults. A major angular discordance, often marked EkorQuaternary reef limestonesresting uncomformably by a coarse conglomerate, is present at the baseof the Late uponolder rocks contain abundant fragments of fresh Palaeogeneand Neogene rocks overlying theimbricated plagioclase andrare pyroxene with lithic fragments of Basement Complex. In the Subaim region of the NE arm fine-grainedandesite (HA126: bryozoa,coralline algal the rocks above the unconformityare deformed into upright debris, ostracods, Planorbulinella, Amphistegina, Globigeri- open folds with a wavelength of several kilometres (Fig. 7, noides cf. ruber, Globorotalia tumida, G/. inflata). section 2). Significant post-Miocene uplift is indicated by the

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fact that the unconformity at the base of the limestones is importantand intense deformation where the Neogene now exposed at up to 1 km above sea-level. Locally the dip rocks have been locally strongly deformed,perhaps in on the Neogene rocks in this part of the NE arm becomes discretezones. In the valley of the Kiloting River are a very steep and the Neogene rocks are cut by major faults series of lowridges composed of SubaimLimestone withtwo principal trends: NE-SW and NW-SE. Some of Formation which hasbeen locally overturnedand the faults are marked by sheared serpentine and overturned overthrust as flat sheets. In the region south of Ekor the soft Neogene sediments in zones at least several hundred metres sediments, volcaniclastic and volcanic rocks have been wide. Although we cannotrule out strike-slip motion on strongly folded into tight folds with steeply dipping (70-90") these faults they evidently have an important component of limbs and axes trending broadly northwards. Turbidites at vertical movement and NW-SE faults account for changes the base of theTapaya Volcanic Formationdip steeply, in height of the Subaim Limestone Formation between Ekor generally between 80" and 90", and are locally overturned. and Lolobata indicating post-Miocene vertical displacements South of the Kiloting River across a major ridge into the of at least several hundred metres. valley of the Pettigoagoa thesame formation has been The NE-SW set of fractures appears to be even more tightly foldedand locally shearedinto the Subaim important. The Kau basin, separating the NW and NE arms, Limestone Formation and in the Magdalena River turbidites is shown by Hamilton (1979) as containing 3km of sediment are foldedinto a tight syncline. TheQuaternary reef at its eastern end and we have identified a major fault zone, limestones are locally unconformable upon older rocks and the Subaim Fault, running NE-SW on the southern side of sincePleistocene rocks arepresent in the deformed the basin (Figs 6 & 7). The age of the fault is uncertain but basement tothe Quaternary volcanoes the deformation it must have been active since the Early Pliocene as it brings must be Pleistocene in age and is probably no older than the Basement Complex to almost 2 km above sea-level and mid-Pleistocene. juxtaposesthe Basement Complex and steeplydipping Pliocenerocks of the Wade SandstoneFormation in the Wasile River. Our fieldwork andexamination of aerial Late Palaeogene-Recent history of Halmahera photographsindicates that theeastern part of the The pre-Neogene basement of the eastern province is well Halmahera K-shapeis determined by these two sets of exposedand consists of ophiolitic rocksimbricated with majorsteep fractures. This interpretation is supported by Mesozoic andEarly Tertiary sediments (Hall et al. 1988). the map of sediment isopachs (Hamilton 1979) in the three The basement of the western province is largely covered by marine basins around eastern Halmahera which shows more Neogene-Recentsedimentary and volcanicrocks and than 1km of sediment in two of them (Kau basin and Buli remains poorly known. The oldest rocks on the 1:250 000 basin) and more than 5 km of sediment in the Weda basin. geological maps(Apandi & Sudana 1980; Supriatna 1980; The beginning of activity on these faults cannot yet be Yasin1980) are shown as the BacanFormation and dated. Since theydeform Pliocene rocks they musthave tentatively datedLateas Oligocene-EarlyMiocene. been active in Plio-Pleistocene time but they may be older However,the formation includes anumber of unrelated structures. The position of the Miocene reef edge parallel to units such as Late Cretaceous breccias of theDodaga the present fault-controlled SW edge of the Kau basin may Breccia Formation (Hall et al. 1988) which are imbricated in indicate that subsidence in this basin began in the Miocene. theBasement Complex in easternHalmahera as well as Hydrocarbons in the form of oil seepages have been undeformed,probably Palaeogene, volcanicbreccias on reported from Halmaheraand we observed oil and gas Bacan (Yasin 1980; Silitonga er al. 1981). The volcaniclastic seeping vigorously into a village water well in Lolobata. The rocks forming the basement of thewestern province are oil is anatural light condensate which we suspect tobe typically unfossiliferous and include pyroclastic rocks, lava escaping into Quaternary sediments of the Kau basin along breccias andsubaerial conglomerates, locally hydrother- the Subaim Fault zone. mally-altered anddeeply weathered (Fig. lob) and consequentlyextremely difficult date.to In central Halmahera the basement of the western province includes The western province and the junction zone volcanicrocks of thelate Mesozoic-Early Tertiaryarc. The nature of deformation in the western province is less South of Gurapingon the westerncoast of central clear. We examined only limited areas of the western arms Halmahera (Fig. 4) clasts in a volcanic conglomerate include in the field and poor exposure, plus the fact that many of the possible rudist fragments suggesting a Late Cretaceous age. volcanic rocks lack good stratification, are major handicaps. Close tothe TapayaRiver volcaniclasticrocks, tuffs and Much of the NW arm isalso covered by the products of volcanic conglomerates areinterbedded with calcareous Recent volcanism. However, the(probable) Early Pleis- mudstonesand mark containingplanktonic forams of tocene volcanicrocks onthe western coast of central Middle Eocene age (HA138: Acarinina cf. pseudotopilensis, Halmaheranear Guraping have been folded and dips of Morozouella cf. spinulosa, Morozouella sp.) This suggests 40-60" are common. In contrast, the volcanic rocks of the that the younger volcanic arcs which built the western arms active and recently active arc in the islands of Ternate and of Halmahera cover theeroded Late Cretaceous-Early Tidore have not been significantly deformed. Verstappen's Tertiary volcanic arc. (1964) aerialphotographic study showed that volcanic At the end of the Eocene the arc and fore-arc terrain activity in the NW arm is concentratedin a graben zone forming theHalmahera basement wasstrongly deformed situatedbetween tilted fault-blocks andthe Quaternary causing imbrication and uplift (Hall et al. 1988). The cause volcanoes are built onthese blocks. The youngestrocks of this deformationremains obscure but the event is dated in this tilted basement are Pleistocene. recognizable over a widespread region between New Guinea The junction between the eastern and western provinces (Kroenke 1983) andthe western Pacific (Hayes & Lewis in the narrow neck of theHalmahera K is azone of 1984) and may be related to a major change in direction of

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motion of the Pacific plate atabout 40Ma (Uyeda & E 127' 129O Ben-Avraham 1972). The Oligocene was a period of uplift and deep erosion of the Basement Complex, as indicated by the deepvalleys containing fluviatile ophiolitic conglomerates now beingre-excavated by thepresent-day rivers. Slow subsidencebegan in theLate Oligocene, first in NE Halmahera, leading to deposition of mark, whereas further SW carbonatedeposition began in the EarlyMiocene. Hamilton (1979) suggested,apparently onthe basis of earlierreconnaissance studies in theHalmahera region summarized by VanBemmelen (1970), thatbetween the Oligocene and Early Miocene Halmahera was an east-facing island arc and that a flip in subduction polarity led to the present tectonic configuration. However, in our fieldwork in theNE arm and central Halmahera we havefound no evidence for an Oligo-Miocene volcanic arc. Biostratigraph- ical evidence indicates no major breaks in the sequence and between the Late Oligocene and Early Pliocene there are no volcanic rocks,and calc-alkaline debris is notablyabsent fromthe small butconstant 'background' of siliciclastic debris found in Oligo-Miocene carbonates. The composition of this debris indicates erosion of the underlying Halmahera ophioliticBasement Complex. Oligocene-Early Miocene volcanism is reported from reconnaissance work on Waigeo (Van der Wegen 1963) and on Bacan (Yasin 1980; Silitonga et al. 1981). Both of these islands are close to the Sorong Fault system which is a transform fault zone with a history of volcanic activity (Morris et al. 1983; Dow & Sukamto 1984) and Bacan is situated on a splay of the Sorong Fault marked by recent volcanic activity (Hall et al. 1988) which is probably the extension of the Molucca-Sorong Fault Fig. 11. Map of active and inactive Quaternary volcanoesof (Letouzey et al. 1983). It is one of several splays of the Halmahera region showing Benioff zone contoursfrom Cardwell et SorongFault zone identified byseismic reflection work al. (1980) and Morris et al. (1983). (Letouzey et al. 1983) between Halmahera and Seram. We consider it moreprobable that Oligo-Miocenevolcanic activity was related to fault motion south of Halmahera at formed was filled by a overall coarsening-upwards sequence the Pacific-Australian plateboundary rather than to a with anincreasing volcaniclastic component,marking volcanic arc on Halmahera. shallowing of the basin and increasing arc activity, with lavas In the early Pliocene there was a change from the stable andsubaerial volcanicbreccias andconglomerates atthe conditions of carbonate deposition across east and central highest levels. Our, admittedly slight, evidence suggests that Halmahera with atransition from limestones tomark, the Pliocene arc was built on the eroded basement of the followed rapidly by an increase in the amount of siliciclastic EarlyTertiary arc. If this is correct,and more work is debris whichwas depositedas submarine fan turbidites. required to establish the nature and history of the basement Calc-alkaline volcanic debris appeared in the mid-Pliocene of the westernprovince, the position in which the and after this there was a gradual increase in the amount of lithosphere fractured leading to subduction of the Molucca volcanic material, initially as tuffs and volcaniclastic Sea plate may have been determined by the thickened crust turbiditesand later as lavas. We interpret this rapid beneath the older arc. transition as the result of the initiation of subduction of the We suggest that the NW-SE and NE-SW sets of major Molucca Sea lithosphere to the west of Halmahera, causing vertical faultswere initiated in the immediate back-arc subsidence in easternHalmahera, followedby the region at this time. Although we have no direct evidence for formation of a Pliocene volcanic arc in the westernprovince. theearliest movements on these faults, vertical displace- This interpretation is in good agreement with the amount of ments of greaterthan 1km can beproved which are subductedlithosphere beneath theHalmahera arc. If definitely post-Early Pliocene and the sediment thickness in subduction was initiated atabout 5 Ma (Late Miocene- the basins surrounding the eastern arms suggest subsidence Early Pliocene) volcanism would have begun at about 3 Ma onthese faults began in the Pliocene, if notearlier. (mid-Pliocene)when the slabreached 100 km (the active Hamilton (1979)shows over 5 km,and publishedseismic volcanoes of the present arc (Fig. 11) are all situated more reflection profiles (Letouzey et al. 1983) indicate up to 8 km, than 100 km above the Benioff zone) and with subduction of sedimentin the Weda basin. The presence of continuing atthe same rate the slab would amveat its hydrocarbonseeps along the Subaim Fault zone on the present depth of 250 km. It appears that theforces which led southern edge of the Kau basin suggests maturation of the torupture of the lithosphere first producedsuddena organic-rich Miocene or Early Pliocene sediments and the downwarping of the crustbeneath east Halmahera, distribution of carbonate facies in the SubaimLimestone immediately behind the arc, resulting in rapid subsidence of Formation suggests afault-controlled margin to the Kau the Miocene reef limestones. The sedimentary basin which basin as early asthe Miocene. At present we have no certain

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explanationfor thepattern of faultingand differential PHILIPPINE - subsidence in the region immediately behind the active arc. The faultingis oblique tothe ‘grain’ of the Basement Complexwhich is formed ofN-S oriented slices and therefore a basement control seemsunlikely. If fault activity began inthe Late Miocene or Early Pliocene thefault pattern may be related to stresses produced in the bending lithosphere associatedwith rupturing of the plateand initiation of subduction of the Molucca Sea Plate. We note that the fault pattern has been exaggerated in the present active arc with horst blocks forming the NE and SE arms at upto 2 kmelevation separated by deep sediment-filled basins. We do not yet know if this marked differential relief is typical of island arcs or is due to Halmahera’s especially complex position at a knot of plate boundaries, A majordeformation event affected thearc in the Pleistocene.This caused tilting of majorfault-bounded blocks in the eastern and western provinces, intense folding with overthrusting in the junction zone of the western and eastern provinces, andprobably caused local folding and faulting in discrete zones in botheastern and western provinces. We suggest thatthe unconformitywithin the sedimentarysequences shown onthe seismic profiles (Letouzey et al. 1983) across the Weda basin is Pleistocene in age. The ages of the Weda basin sequences are unknown but the sedimentsrest unconformably onthe ophiolitic b basement and the intra-sequence unconformity is known to c post-date a phase of compressional tectonics (Letouzey et al. 1983). In central and NE Halmahera the Pleistocene event is the only such compressional phase. Arc volcanism appears to have ceased, albeit briefly, and the present active arc was built unconformably on the deformedolder rocks aftera ‘\ ‘. C shift of position of the active volcanoes westwards by about 30km in centralHalmahera. One explanation of this W deformation event could be a cessation of subduction in the Fig. 12. Cartoon to illustrate interference of Philippine Sea plate Late Pliocene-EarlyPleistocene followed by renewal of with Molucca Sea plate beneath northern Halmahera causing shift subductionin the mid-latePleistocene butthere is no in volcanic axis. The present configuration of plates is shown in (a) indication of agap in the seismicity beneathHalmahera modified from Cardwell et al. (1980); (b) shows a profile before (Cardwell et al. 1980) to suggest a break in the subducted approximately 1 Ma and (c) shows present situation. slab. An alternativeexplanation is thatthe deformation marks a tectonic event at one of the existing or developing the splay passing through Bacan (Fig. 6, Hall et al. 1988) the plate margins in this unusually complex region. There are Quaternary arc is parallel to the Benioff zone contours (Fig. several possible locationsfor such anevent (Fig. 2). One 11) whereas on Bacan the line of Quaternary volcanoes is possible location is tothe north of Halmaheranear the almost at right angles to the contours projected by Cardwell Philippine Trench.The Philippine Trench isvery young et al. (1980). The shift in position of active volcanicity could (Cardwell et al. 1980) and seismicity ceases east of the NE thereforemark westwards motion of the continental arm of Halmahera at about 2%; the Philippine Trench is fragment south of this splay dragging the east-dipping slab evidently propagatingsouthwards (Hall 1987) butthe of the Molucca Sealithosphere westwards.Such motion history of its development is uncertain.However, the would steepenthe east-dipping limb of the MoluccaSea amount of lithospheresubducted is less than 150 km and plateand shift the active arc westwards abouta pivot in beneath northern Halmahera the east-dipping Molucca Sea northHalmahera. This explanation would betested by plate and the west-dipping Philippine Sea plate appear to be dating the metamorphosed rocksassociated with the fault in collision at a depth of 100-150 km (Fig. 3, Cardwell et al. zone passing throughBacan and workis currently in 1980). The lengths of the subducted slabs suggest that the progress to do this. Philippine platehas collidedwith the Molucca Seaplate More complex schemes can be envisagedincluding beneath northern Halmahera at some time in the last 1Ma neither,either or both of the two possibilities described and the likely effect of such a collision would be to steepen above and including deformation at other plate boundaries the Molucca Sea plate whichwould be forced westwards, in the region,for example thesouthern extension of the thus shifting the axis of volcanicity in the same direction Philippine Fault system. Nakamura et al. (1984)suggest a (Fig. 12). change in the convergence direction between the Eurasian A second possible location for the tectonic event is along and Philippine Sea plates at about 1Ma and this also may the Sorong Fault system separating the Philippine Sea and have caused thedeformation event. The number of AustralianPlates. The SorongFault system forms the possiblilities reflects the unusually complex pattern of plate southern boundary to the Molucca Sea region and north of boundaries in the Halmahera region, the changing position

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and nature of these boundaries and the development of new evolution. Journal of Geophysical Research, 89,9171-95. plateboundaries during the last fewmillion years (Hall JENKINS,D. G., BOWEN,D. Q., ADAMS,C. G., SHACKELTON,N. J. & 1987) and, not least, our relatively slight knowledge of this BRASSELL,S. C. 1985. The Neogene.Part 1. In: SNELLING,N. J. (ed.) The Chronology of the Geological Record. Geological Society, London, complex region. TheHalmahera region is remarkable in Memoir, 10, 199-210. preservingboth a stratigraphic record of the progress of KROENKE,L. W. 1983. Cenozoic development of thesouthwest Pacific. subduction and alithospheric record whichcan still be UnitedNations Economic and Social Commicsion for Asia andthe interpretedfrom seismic andother geophysical studies. Pacific, Committee for Co-ordination of Mineral Prospecting for Mineral Resourcesin South Pacific offshore areas (CCCOPISOPAC) Technical Evidence of present-day volcanicity and seismicity, and Bulletin, 6. marine geophysical studies have provided an insight into the LETOUZEY,J., DE CLARENS,J., GUICNARD,J. & BERTHON,J.-L. 1983. most recent development of the region but more land-based Structure of the North Banda-Molucca area from multichannel seismic geological studies are desperatelyneeded to providea reflection data. ProceedingsIndonesian Petroleum Association, lzth longer time-scale. The apparently simple picture revealed by Annual Convention 1983, 143-56. MAMMERICKX,J. FISHER,R. L., EMMEL, F. J. & SMITH,S. M. 1976. the geophysical studies clearly developedin a complex Bathymetry of the east and southeast Asian seas. Geological Society of manner. America, Map and Chart Series, MC-17. MORRIS,J. D., JEZEK,P. A., HART,S. R. & GILL, J. B.1983. The Halmaheraisland arc, MoluccaSea collision zone, Indonesia: a Financial support for our work in Indonesia was provided by the geochemical survey. In: HA~ES,D. E. (ed.) The Tectonic and Geologic RoyalSociety, Amoco International, British Petroleum and the Evolution ofSouth-east Asian Seas andIslands. Part 2. American University of London Consortium for Geological Research in SE Geophysical Union Monograph, 23, 373-87. Asia. GRDC Bandung provided aerial photographs and invaluable NAKAMURA,K., SHIMAZAKI,K. & YONEKURA,N. 1984. Subduction, bending practicalassistance in Indonesia.We thank P. Ballantyne for his and education. Present and Quaternary tectonics of the northern border work on this project. of the Philippine Sea plate. Bullefin de la Socittk Giologique de France (7), 26, 221-43. RANKEN,B., CARDWELL,R. K. & KARIG,D. E. 1984.Kinematics of the References Philippine Sea Plate. Tectonics, 3, 555-75. SILITONGA,P. 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Received 27 March 1987; revised typescript accepted 27 January 1988

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