nichols hall, 1991 (basin formation and neogene sedimentation in a backarc setting, halmahera,...

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B a sin f o r m a t i o n a n d N e o g e n e s e d i m e n t a t io n ina b a c k a r c s e t t in g , H a l m a h e r a , e a s t e r n I n d o n e s i aGar y J , N icho lsD e p a r t m e n t o f G e o l o g y , R o y a l H o l l o w a y a n d B e d f o r d N e w C o l le g e , U n i v e r s i t y o fL o n d o n , E g h a m , S u r r e y T W 2 0 0 E T, U Kand Rob er t Ha llD e p a r t m e n t o f G e o l og i c a l Sciences, U n i v e r s i t y C o l l e g e L o n d o n , G o w e r S t r e e t , L o n d o nW C I E 6 B T , U KReceived 27 Octob er 1989; rev ised 15June 1990; accepted 6Ju ly 1990I t has been proposed that basins in backarc sett ings form in associat ion wi th subduct ion byth inn ing of cont inenta l crust, by backarc spread ing in oceanic crust, by com press ion ( retro-arcbasins) or by trapp ing of p ieces of oceanic p late behind an arc. The Halm ahera Basin in easternIndones ia deve loped in a backarc sett ing bu t does no t fa l l in to any of these categor ies; i t form edby subs idence o f th ickened crust made up o f imbr ica ted Mes ozo ic-Pa laeo gene a rc and oph io l i terocks. Halmahe ra l ies at the wes tern edge of the Phi l ipp ine Sea Plate in a com plex zo ne ofconvergence be tween the Euras ian marg in , the ocean ic p la tes o f the West Pac if ic and theAu st ra l ian - In d ian P late to the sou th . The basem ent i s an imbr ica ted c om plex o f Mesozo ic toPalaeogene oph io l i te , arc and arc- re lated rocks. Dur ing the Miocene th is baseme nt com plexform ed an area of th icken ed cru st on which carbona te reef and reef-associated sed imen ts weredepos i ted . We in te rp re t th is sha l low mar ine reg ion o f th ickened crust to be s imi la r to m any o f theoceanic p lateaux and r idges found wi th in the Phi l ipp ine Sea Plate today. In the Late Miocene,convergence between the Phi l ipp ine Sea Plate and the Eurasian margin resul ted in the formationo f the Ha lmahera T rench to the we st o f th is reg ion o f th ickened crust . Subduct ion o f the Mo luccaSea Plate at the trench caused the d eve lopm ent of a volcan ic is land arc. Subs idence in thebackarc area produced a broad sedimentary basin f i l led by clast ics eroded from the arc and fromup l i fted basem ent and cov er rocks. The bas in w as asym metr ic w i th the th ickest sed imenta ry f i l lon the weste rn s ide , aga inst the vo lcan ic a rc . The Ha lmahera Bas in was mod i f ied in thePl io -P leistocene by eas t -w es t com press ion as the Mo lucca Sea P late was e l im ina ted bysubduct ion .Keyw ords: Halm ahera , eastern Indonesia ; backarc bas in ; bas in forma t ion

I n t r o du c t i o nBackarc marginal basins are thought to form typicallyby trench rollback causing backarc spreading (Dewey,1980) and the generation of new crust (Karig, 1971).Basins in this setting may also form by the rifting ofcontinental crust, for example the Sea of Japan and theOkinawa Trough (Kobayashi, 1985), or by the trappingof oceanic crust behind an arc (for example, the BeringSea, where no backarc spreading has occurred (e.g.Scholl e t a l . , 1986)). The crust underlying marginalbasins is normally thicker than typical oceanic crust,but has an oceanic character (Mitchell and Reading,1986). Retro-arc basins (Dickinson, 1974) form ascompressive foreland basins in continental crust behinda continental margin volcanic arc.The Halmahera Arc in eastern Indonesia ( F i g u r e 1 )is a volcanic arc resulting from the eastward subductionof the Molucca Sea Plate. Unlike many other arcs in thewestern Pacific, the Halmahera Arc is a relativelyyoung feature which was initiated in the Late Miocene.It is therefore possible to document the evolut ion of anarc and backarc region without the complications of along history of arc-trench tectonics. A thicksedimentary sequence accumulated in a sedimentarybasin behind this volcanic arc during the Late Miocene0264-8172/91/010050-121991 Butterworth-Heinemann Ltd

and Pliocene and this sequence is now partly exposedon land as a result of Pliocene or younger uplift anddeformation. We term this sedimentary basin theHalmahera Basin (F i g u r e 2 ) .The Halmahera Basin was and is situated in abackarc setting, but is not underlain by newly formedoceanic crust and does not show the characteristics of aretro-arc, foreland-type basin. It formed in the lateNeogene in an area underlain by anomalously thickcrust composed o f ophiolitic and volcanic arc rocks andassociated sedimentary rocks. The Halmahera Basinformed in such a position because the thickened crustinfluenced the Neogene development of subduction inthis region (Hall and Nichols, 1990a). Although notpreviously described, backarc basins formed in thistype of setting may not be unusual.The island of Halmahera is an excellent place toexamine the entire evolution of a young, short-livedarc-trench system. In this paper we document theNeogene evolution of Halmahera and the adjacentoffshore areas using new stratigraphic and structuraldata from the region. These dat a are integrated into theregional tectonic framework to illustrate the evolutionof the Halmahera backarc basin.

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EIlO

8o

Backarc set t ing basin format ion and sedimentat ion: G. J . Nichols and R. Hal l130 150

3I 'PHILIPPINE SEA l / :~// ."PLATE / . , ' // i o. J ' -

~ - -/ /~ C A R O L I N E P L A T E~ M O L U C C A SEb ~Q ~ ~/ /~COLLISIONZONE"~I~IHALMAHERA ,Q ~ ~ 4 ~ r # ~ U / & ~

BANDA SEA

a~INDIAN OCEAN - AUSTRALIAN P L A T E ~ j . _ ~ _ _ ~

Figure 1 P l a te bounda r i es i n the H a l m ahera reg i on ( f rom N i cho l s e t aL , 1990)- o 1km

Geolog ica l and t ec ton ic se t t ingHalmahera is over 300 km from north to south and125 km east to west and has a distinctive K-shape offour arms separated by bays ( F i g u r e 2 ) . The island is

Outcrop of Neogel~ o f the Halmahera EVolcanoes of the H

" ~ Thrusl~sMai or faults

L 0 o

N o r t hW e s t T ~ r m , , ~ ' ~ / ~ 1

,: rth rm~ ~ ' j " ~ 'I B U L I BAH A L M A ~ - A t ~ a',-.

A.~ i l S o ut h E o s t ~- L , o F o ' A n

50 kn]28I

F i gur e 2 T h e m a i n s t ru c t u r a l f e a t u r e s o f s o u t h e r n H a l m a h e r aa n d t h e p r i n c i p a l l o c a li t ie s m e n t i o n e d i n t h e t e x t

totally covered by rainforest and the terrain is generallysteep and rugged. Because o f the difficulties of accessand the remoteness of the island, Halmahera hasreceived relatively little attention. The firstcomprehensive maps showing any degree of detail werepublished by the Indonesian Survey (GRDC) in 1980(Apandi and Sudana, 1980; Supriatna, 1980; Yasin,1980; Sukamato e t a l . , 1981). The stratigraphyestablished by these authors was revised in subsequentpapers by Hall (1987) and Hall e t a l . (1988a, b). In thelight of new information obtained during fieldwork in1987, we are able to present a more detailedstratigraphy ( F i g u r e 3 ) and a revised geological map ofthe southern part of the island ( F i g u r e 4 ) whichsupersedes and adds to previously published work.Information from a number of multi-channel seismicprofiles across Weda Bay shot by the IFP (InstitutFran~ais du P6trole) has been incorporated togetherwith new bathymetric data obtained on cruise CD30aboard the C h a r l e s D a r w i n in 1988.Recent studies of the tectonic evolution of this area(Hall, 1987; Nichols e t a l . , 1990) have shown thatHalmahera is situated at the southern end of thePhilippine Sea Plate ( F i g u r e 1 ) . The Philippine Trenchis propagating southwards through this part of thePhilippine Sea Plate whilst some of the convergencebetween the Philippine Sea Plate and the Eurasianmargin is transferred through a dextral strike-slipsystem into the Molucca Sea Collision Zone (Nichols e ta l . , 1990). Volcanism in the Ha lmaher a Arc ( F i g u r e 2 )is related to subduction at the Halmahera Trench(Hatherto n and Dickinson, 1969; Katili, 1975) of theMolucca Sea Plate (Cardwell e t a l . , 1980; McCaffrey,1982). The Halmahera Arc includes a chain of volcanicislands offshore western Halmahera and volcanoes inthe NW arm of the island ( F i g u r e 2 ) . To the south, theSorong Fault Zone ( F i g u r e 1 ) is a major sinistralstrike-s lip system which separates this complex zone of

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B a c k a rc s e t ti n g b a s in f o rm a t i o n a n d s e d ime n t a t i o n : G . J . N i c h o l s a n d R . Ha l lI I s . , . , , 0 , . , = . , , M S Io u , , , . . , . , . , , v o . . c , . . = , , . . ,

T A F O N G O V O L C A N I C F O R M A T I O NLate PIIocene mid-Pliocenevolcanic and volcaniclastic rocks

L O K U F O R M A T I O N 1 0 00 m +Late Miocen e (- E~lrlyPliocane).

K U L E F U F O R M A T I O N 2 0 0m +( ? P l i o c a n e - P l e i s t o ce n e )Tuffaceous sandstoneG O L A F O R M A T I O N 3 0 0m +

(Late Miocene -) PlioceneLin~stone an d calcareous mudstoneD U F U K F O R M A T I O N 1 0 0 0 m - 1 5 0 0m(Late Miocene -) Early PlioceneShalow marine mudstone,sandstone, conglomerateA K E L A M O F O R M A T IO NLate M iocene (- E arly Pliocene)Carbonaceous mudstone 0 -S U P E R A K F O R M A T I O N 1 0 0 0m - 1 5 0 0mLate Miocene (- EarlyRiocene)Shallowmarineconglomerate nd sandstone

F i g u r e 3 Th e s t ra t i g ra p h y o f Ha lm a h e ra

Nt

S A O L A T F O R M A T I O N 1 0 0 0m +Late Miocene PlicoeneMarletone, edeposited eef imestone ndlittoral sandston e

S U B A I M U M E S T O N E F O R M A T I O N 5 0 0m +Early- Late Miocene ? Early Riocane)Reef and reef-associated limestoneJ A W A U & G E M A F C O N G L O M E R A TE \F O R M A T I O N S 0 - 4 ( Xl m \(? EadyMiocene)Ruvial& littoral \conglomerateand sandstoneO P H I O B T IC B A S E M E N T C O M P L E X(a) BUU GRO UP - forsarc volcanic,volcaniclasticand sedimentary ocksLate Cretaceousand Eocene(b) Ultrabasic nd basic rocksPre - Late Cretaceous

,mary Umestones

~ S a o l a t Fro,

ubaim LimestoneFm.emaf & JawalionglomerateFms,phiolitic Basementomplex

k m 5 0i i i

F i g u r e 4 Ge o lo g ic a l m a p o f s o u th e rn Ha lm a h e ra b a s e d o nn e w f i e ld d a ta , a e r ia l p h o to g ra p h ic i n te rp re ta t i o n a n d n e wp a la e o n to lo g ic a l d a ta

E-W convergence from the Australian Plate and theBanda Sea area (Hamilton, 1979). The southern part ofthe island of Halmahera lies adjacent to one of themain strands of the Sorong Fault System ( F i g u r e 1 ) .Pre-Neogene h istoryThe oldest rocks of eastern Halmahera ( F i g u r e 4 ) areophiolitic rocks (described by Hall e t a l . , 1988a)overlain unconformably by the Bull Group whichconsists of Upper Cretaceous to Lower Tertiaryvolcanic and volcaniclastic rocks interpreted as part of aforearc sequence (Hall e t a l . , 1988a and new data).These ophiolitic and forearc rocks (the OphioliticBasement Complex) were deformed and imbricated inthe Late Palaeogene and form the basement to theNeogene succession. Oligocene marls and marlylimestones previously assigned to the Onat MarlFormation are exposed in the NE arm where they wereinterpreted to rest unconformably on the OphioliticBasement Complex (Hall e t a l . , 1988b). These rockshave been found in only one small area with no clearcontact relationships and have not so far proved to be amappable unit. Therefore they are not shown as aseparate formation on F i g u r e 3 .The basement of western Halmahera is composed ofvolcanic arc rocks (the Oha Volcanic Formation) whichare petrographically and chemically similar to thevolcanic component of the forearc sequences exposedon the SE and NE arms. They are typically aphyricbasaits and basaltic andesites, lacking hornblende(Hakim, 1989 ) and are very different from theporphyritic pyroxene and hornblende andesites of

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B a c k a rc s e t t i n g b a s in f o rma t i o n a n d s e d im e n t a t i o n : G . J . N i c h o l s a n d R . Ha l lNeogene to Recent age. We interpret the Oha VolcanicFormation as part of the volcanic arc which suppliedthe forearc sequence of the Buli Group. The OhaVolcanic Forma tion forms mountains along the westernside of the SW arm extending into central Halmahera.Similar rocks are reported from the northern tip of theNW arm and Morotai (Supriatna, 1980).

Neogene stratigraphyThe new stratigraphy for the Neogene of Halmahera isshown in F i g u r e 3 . All Miocene limestones are assignedto the Subaim Limestone Formation (Hall e t a l . ,1988a). We have been able to subdivide the WedaFormation of Apandi and Sudana (1980), Supriatna(1980) and Yasin (1980) into a number of formationsforming part of a new group, the Weda Group. TheWeda Group includes the Saolat Marl Formation ofHall et al . (1988b).The nature of exposure in the heavy rainforestterrain of Halmahera is such that it is not possible toprovide logged sections of all formations or defineupper and lower boundaries in every case. However,the formations are all mappable units with consistentlithological characteristics and age ranges. Ages arebased on determinations of both foraminifera andcalcareous nannofossils.J a w a l i C o n g l o m e r a t e F o r m a t i o nThe Jawali Conglomerate Formation is a sequence ofcoarse boulder conglomerates and sandstones up to400 m thick interpreted to be fluviatile by Hall et al .(1988b). The conglomerates contain abundant clasts ofophiolitic material and are exposed in river sections atthe southern end of the NE arm. The JawaliConglomerate Formation rests directly on theOphiolitic Basement Complex and is conformablyoverlain by the Subaim Limestone Formation(Miocene).G e m a f C o n g lo m e r a t e F o r m a t io nAt the western end of the SE arm near Sagea (F igures 2and 4) dark sands and conglomerates are well exposedalong the coast and patchily exposed inland. They arecomposed dominantly of serpentinite and weatheredultramafic material with rare clasts of gabbro and redchert. The conglomerates are well bedded with verywell sorted, well rounded clasts. They are interbeddedwith well sorted sandstones which are typically lithicarenites made up of grains of serpentinite with smallamounts of chrome spinel and rare chert.The Gemaf Conglomerate Formation is interpretedas having been deposited in a littoral environment onthe basis of the very well rounded nature of the clasts(at all sizes) and the very good sorting of the materialinto beds of different grain sizes; there are no signs ofchannelling and graded beds are rare. The veryrestricted clast types indicate that the material wasderived from a terrain of ophiolitic material. No faunahave been found in the present study. Brouwer (1923)describes a thick bed of limestone with a LateOligocene-Early Miocene fauna, including benthonicforaminifera, resting on conglomerate beds from alocality near Sagea. His description implies that thelimestone bed is conformable with the conglomeratesequence and his map (Brouwer, 1923, Kaart 1) shows

this limestone as Aquitanian. We interpret thissequence as a conformable transition to the SubaimLimestone Formation.S u b a i m L i m e s t o n e F o r m a t i o nThe Subaim Limes tone Format ion was defined by Hallet al . (1988b) as a sequence of reef and reef-associatedlimestones found in outcrops along the NW side of theNE arm. Further extensive areas of outcrop of thisformation occur in central Halmahera and in thesouthern and eastern parts of the SE arm where it maybe up to 500 m thick. On the SW arm similarlimestones occur in a small outlier above older rocksand as clasts in younger sedimentary rocks. Where thebase of the Subaim Limestone Formation is seen, it iseither an unconformable contact with the OphioliticBasement Complex or a conformable transition fromconglomerates which rest directly on the OphioliticBasement Complex. The oldest limestones above theunconformity are Early Miocene, and none areyounger than Late Miocene-Early Pliocene. Thecontact with the overlying Saolat Formation in the SEarm is transitional. Rocks of the Saolat Formationimmediately above the Subaim Limestone Formationalso yield Late Miocene-Early Pliocene ages. Work isin progress to obtain better stratigraphic resolution butat present we consider the Subaim LimestoneFormation to be entirely restricted to the Miocene.In most exposures, the limestones are thickly beddedpackstones and wackestones of foraminifera,bryozoans, echinoderms, molluscs, corals, corallinealgae and lamellar algae. True coral boundstones havenot been found in the SE arm, although there areexamples of algal boundstones. There are rare clastichorizons of lithic arenites in the central fold and thrustbelt of Halmahera. The abundance of bioclastic debrissuggests that, in general, these limestones weredeposited off a reef, probably in a fore-reef setting.L o k u F o r m a t i o nIn the western part of the SW arm sedimentary rocks ofthe Loku Formation are exposed in areas between theregions of the Oha Volcanic Formation which form thehighest parts of the arm. These sedimentary rocksinclude breccio-conglomerates, well bedded sand-stones, silty mudstones and redeposited limestones.The conglomerates are disorganised, matrix-supportedand form beds up to 10 m thick. Some of the clasts arevolcanic and volcaniclastic rocks containing pyroxene-and horneblende-andesite debris; such andesites aretypical of the Neogene sequences and mineralogically,chemically and texturally similar to the Pleistocene andpresent day volcanic rocks of the Halmahera volcanicarc (Hakim, 1989). There are also abundant clasts witha shallow marine fauna which are reworked fromLower, Middle and Upper Miocene limestonestogether with semi-lithified calcareous mudstones withlate Late Miocene-Early Pliocene nannofossils andforaminifera. These beds have the characteristics ofdebris flows. The finer sedimentary rocks of the LokuFormation occur mainly in decimetre thick, normallygraded units; these beds have sharp bases and mayinclude a lag of mud-chip conglomerate. Full Boumasequences are rare, but the general character of thesebeds indicates deposition as turbidites. Nannofossilages from the mudstone beds which do not containreworked fossils indicate a Late Miocene age for the

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Backarc se t t ing bas in fo rm at ion a nd sed im enta t ion : G.Loku Format ion. In the type area of the formation nearLoku (Figure 2) and to the north along the western sideof the SW arm this formation is tightly folded, withlocally overturned fold limbs, about steeply dipping,N-S axial planes. The repetition of the formation bythese folds makes estimation of the thickness of theformation difficult, but a total thickness of at least1000 m is considered likely.W e d a G r o u pThis thick sequence o f Neoge ne sedi mentary rocks fillsthe Halmahera Basin. They are exposed in the SWarm, in the central part of the SE arm, in the fold beltof central Halmahera and along the north side of theNE arm (Figure 4) . Most of this outcrop area wasformerly assigned to the Weda Formation by Apandiand Sudana (1980), Supriatna (1980) and Yasin (1980).However, detailed aerial photographic interpretationof southern Halmahera shows that the sedimentaryrocks can be subdivided on the basis of the character ofthe topography. Traverses across the SW arm and thesouthern side of the SE arm showed that units mappedfrom aerial photographs could be recognised on theground by their distinctive lithological characteristicsand are formations, although boundaries aretransitional.S u p e r a k F o r m a t i o nThe Superak Formation rests unconformably on theOha Volcanic Formation in central Halmahera. In theSW arm it overlies the Loku Formation; the contactmay be an unconformity or a fault. Two members arerecognised: a conglomeratic member whichpredominates in the north of the outcrop area and asandy member which is more common in the south.The conglomerate member is exposed in an extensivearea in the northern part of the SW arm where it is atleast 1000 m, and may be over 1500 m, thick. The typesection is in the Superak River in the northern part ofthe SW arm (Figure 2) . The co nglomerates exposed arenormally graded, or occasionally reverse graded, withbeds ranging from 0.5 m to several metres in thickness.Clast-supported beds of moderately rounded pebbleand cobble conglomerates form most of the sequence,but some of the thicker beds locally include moreangular clasts and boulders up to 4 m in diameter. Thebases of the beds are generally sharp and erosional withclear channel forms evident in places. They gradelocally into pebbly sandstones which contain plantfragments, benthonic foraminifera and fish otoliths.Volcanic rocks, reworked sandstones and, to a lesserextent, coral limestones, are the principal clast types.The volcanic clasts include abundant aphyric basalts inthe lower part of this formation; porphyritic andesiteclasts become more common upwards (Nichols et al.,1991). The aphyric basalt clasts are mineralogically,texturally and chemically (major, trace and rare earthelements) very similar to the Oha Volcanic Formation,whereas the porphyritic andesite clasts closely resembleNeogene-Recent volcanic rocks (Hakim, 1989).The sandier outcrops of the Superak Formation inthe southern half of the SW arm are rhythmicallybedded, sharp-based and sharp-topped beds ofsandstone and siltstone. The sandstones are medium tovery coarse in grain size and contain abundantbioclastic and plant material; mud clasts are common,

d . N i c h o l s a n d R . H a l lforming beds of intraformational mud-chipconglomerates with a sandy matrix. These sandstonesare composed predominantly of volcanic lithic clasts,plagioclase feldspar and foraminifera in variableproportions in a carbonate mud matrix.The sedimentary features of the conglomeratemember indicate deposition by channellised tractioncurrents. A subaerial environment can be ruled outbecause the conglomerates are commonly interbeddedwith sandstones containing a fully marine fauna. Afan-delta setting is interpreted for the conglomeratemember whereas the sandy member is considered to bea lateral equivalent deposited in a shallow marinesetting. Almost all samples of fine grained materialfrom this formation yield ages, based on bothnannofossils and microfossils, within the late LateMiocene-Early Pliocene. However, one sample(HG 102) from the headwaters of Air Gola (Figure 4)yielded a Late Miocene age based on nannoflora.A k e l a m o F o r m a t i o nThe type area of the Akelamo Formation is in thevalley of the Kali Akelamo , in the centre of the SW arm(Figure 2); here, and in further outcrops at the northernend of this arm, the formation is poorly and patchilyexposed in areas of subdued, often almost flat,topography. The total thickness of the AkelamoFormation is about 500 m. It consists of thickly beddedcalcareous mudstones with little internal structure.Wood, leaf and other plant debris are abundant insome exposures and concretionary iron oxides formnodules in places. Nannofossils from several samplesindicate a Late Miocene-Early Pliocene age, with onesample indicating a specific Late Miocene age on thebasis of the presence of a small number of Discoaster cf.neorectus. Fully marine conditions close to a source ofterrigenous plant material are indicated by the fossilcontent. The exclusively fine nature of the sedimentsindicates that the areas of deposition were shelteredfrom sources of clastic input which formed coarserdeposits in the laterally equivalent Dufuk Formation. Itis clearly conformable with the Superak Formation inthe type area.D u f u k F o r m a t i o nThis formation of sandstones and mudstones with rareconglomerate beds has its type area in the Dufuk Riverin the northern part of the SW arm (Figure 2) . Thethickness in the type area and in other sections is atleast 1000 m and may be around 1500 m. W here thebasal contact is seen it is conformable with theunderlying Superak Formation. Clast-supportedconglomerates occur mainly in the lower part of thesequence; they form beds less than a metre thickcontaining well sorted pebbles of volcanic rocks andintraformational sandstones and muds tones, in a sandymatrix. The bases are erosional and have a channel-fillform in places. The sandstones are greenish in colour,and contain gastropods, small bivalves, foraminiferaand abundant plant material. They occur in centimetreto decimetre beds which locally show a normal gradingfrom a sharp base. Sedimentary structures aremoderately common with parallel lamination andseveral instances of decimetre-scale troughcross-bedding. Low angle cross-stratification and ripplecross-lamination are also found. Bioturbation is

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Backarc set t ing bas in format ion and sed imentat ion: G. J . Nicho ls and R. Hal lcommon; vertical S k o l i t h o s traces up to 20 cm long mixlithologies between layers, C h o n d r i t e s and otherhorizontal forms can be seen on some bedding surfaces.The sandstones are typically rich in plagioclasefeldspar, bioclastic debris, carbonaceous material andlithic clasts. The lithic fragments are principallyporphyritic andesites which are texturally andchemically similar to the volcanic rocks of LateNeogene age (Hakim, 1989); packstones andwackestones of coral debris are also present. Thedepositional features indicate a shallow marineenvironment supplied with sediment from a vegetatedvolcanic terrain. Determinations of the nannofossilsand foraminifera from samples of the Dufuk For mationindicate a Late Miocene-Early Pliocene age range butfour samples from the SW arm indicate ages not olderthan Early Pliocene.G o l a F o r m a t i o nAlthough this formation occurs over a very large areaof the SW arm, exposur e of the Gol a Formati on is verypoor because the topography is low and there are nodeeply incised valleys. The type section is in the valleyof the Air Gola in the southern half of the SW arm( F i g u r e 2 ) . The principal lithology is a very pale greycalcareous mudstone with variable proportions of siltand clay in different beds. Fine shelly material isabundant. All the exposures are more indurated thanmudstones of other formations in the Weda Group.The karstic character of the Gola Formation seen onaerial photographs indicates that it is calcareousthroughout. The formati on is at least 300 m thick;neither upper or lower contacts are seen.Lithologies include foraminiferal wackestonescontaining some fine carbonaceous material and verysmall amounts of siliciclastic debris (corrodedplagioclase feldspars). Nannoflora from samplescollected in Air Gola indicate a late LateMiocene-Early Pliocene age range but also includereworked Cretaceous and Early-Middle Mioceneforms. The foraminifera found in the same sectionindicate a Pliocene-Recent age. The age of theformation is therefor e considered to be Early Pliocene.The depositional environment was fully marine, withthe small amounts of siliciclastic detritus indicating thatit was remote from any terrigenous source.

S a o l a t F o r m a t i o nThe type area for the Saolat Marl Formation, definedby Hall e t a l . (1988b), is on the NE arm of Halmaheranear the sout hern end o f Kau Bay. Similar sequences ofgrey marls are exposed within a large area of the SEarm and are assigned to the same formation. In the SEarm there are some beds of sandstone andconglomerat e within the marls, so the earlier name hasbeen amended to 'Saolat Formation'. The stratigraphicposition and ages of these rocks indicate that the SaolatFormation is coeval with the Weda Group formationsexposed in western Halmahera. The thickness isestimated to be in excess of 1000 m.The Saolat Formation has been examined in fiveparts of the outcrop area in the SE arm ( F i g u r e 2 ) .There is a remarkabl e consistency in the facies in theseexposures, which are spread over a large area. Thecarbonate mudstones are pale grey, well bedded incentimetre to deci metre thick, structureless or normallygraded units: the bedding is picked out by variations in

the concentrations of bioclastic debris andcarbonaceous detritus (mainly leaf debris). They arelocally intensely bioturbated, with both vertical andhorizontal traces resulting in mixing between layers.The mudstones are rich in planktonic foraminifera, andsome of the 'sandier' mudstones are foraminiferaiwackestones or packstones. One sample from AirMdolf indicates a Late Miocene age. Foraminifer a andcalcareous nannofossils from all other exposures in theSE arm indicate a Late Miocene-Pliocene age range.Sandstones previously assigned to the WasileSandstone Formation (Hall e t a l . , 1988b) and hereincluded as part of the Saolat For mation yield Plioceneages. We therefore interpret the Saolat Formation toextend in age from the Late Miocene to the EarlyPliocene.Near Waleh, in the middle of the southern side of theSE arm ( F i g u r e 2 ) , very much coarser limestones areexposed as thick fining-up sequences of beds ofcalc-lithic rudites and arenites. The basal beds arechaotic breccio-conglomerates containing bioclasticlimestone clasts with sharp erosive bases; these pass upinto fine conglomerates and then sandstones of thesame material overlain by carbonate mudstones. Thissequence is considered to be the result of individualmass-flow events which interrupted the otherwise quietconditions of sedimentation.Black conglomerates and sandstones occur as highlydistinctive beds amongst the carbonate mudstones onthe coast near Waleh and in a section of the Air Mdolfat the eastern end of the SE arm ( F i g u r e 2 ) . Theconglomerates form beds up to 0.5 m thick, they arewell sorted and made up of well rounded clasts ofserpentinite with rare clasts of gabbro, red chert andgrey intraformational limestone. The sandstones arelithic arenites, composed of serpentinite grains (insome cases exclusively), opaque minerals, chromespinel and variable proportions of bioclastic(foraminiferal) debris; clastic material which could bederived from Neogene volcanic sources is notablylacking in the Saolat Formation. The sandstones have amatrix of carbonate mud or are cemented by sparrycalcite. They are moderately to well sorted, laminatedand occur in centimetre to decimetre thick beds. AtWaleh, the sandstones have sharp-based beds whichshow parallel lamination in the lower part and waveripples at the top. The symmetrical ripple forms aredraped by carbonate mudstones. The roundness of theconglomerate clasts, sorting of the clastic material andthe presence of wave ripples are very strong evidencefor a shallow marine to littoral environment ofdeposition for these clastic units.U n d i f fe r e n t i a te d W e d a G r o u pUpper Miocene-Lower Pliocene and Pliocene rocksare exposed in a N-S belt across central Halmahera( F i g u r e 4 ) . In this region the rocks are folded into tight,upright or west-verging folds with approximately N- Saxes to form the central fold and thrust belt ofHalmahera which separates the western and easternprovinces of the island. A variety of lithologies areexposed, principally calcareous sandstones, mudstonesand thin-bedded limestones. These closely resemblerocks of the Weda Group in the SW arm andpetrographic examination indicates a similar sourcearea (Nichols e t aI . , 1991). This area is mapped asundifferentiated Weda Grou p because the tight folding

Ma r ine and Pet ro leum Geology , 1991, Vo l 8 , February 55


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B a c k a rc s e t t i n g b a s in f o rm a t i o n a n d s e d ime n t a t i o n : G .and the nature of the exposure in th is bel t precludedivision of the sequence into separate formations. Thepreviously defined Tapaya Volcanic Formation (Hall e ta l . , 1988b) is tightly folded and is here included withthe undifferentiated Weda Grou p of the Central Zone.These rocks are Pliocene in age and a N20/N21(Mid-Late Pliocene) date reported by Hall et a l .(1988b) from one sample provides the best constrainton the age of the post-Weda Group deformation event.P l i o c e n e - P l e i s t o c e n e v o l ca n i c r o c k sAt the northern end of the SW arm there are exposuresof laminated siltstones, bedded sandstones and pebblysandstones made up largely of very fresh volcanicdetritus. These rocks contain a rich ichnofauna ofR h i z o c o r a l l i u m and C h o n d r i t e s , and plant fragmentsare common in some layers. These characteristicssuggest a shallow marine environment near to a sourceof abundant volcanic material. The rocks arepetrographically different from other arenites in theWeda Group; they contain biotite and quartz andabundant lithic volcanic grains including pumice andquartz-biotite dacite clasts. Because of thesedifferences we assign them to a separate formation, theKulefu Formation, with a type section on the island ofKulefu in Weda Bay ( F i g u r e 2 ) . In the Pajahe arearocks assigned to the Kulefu Formation restunconformably on the folded rocks of the WedaGroup. No fauna has been found in the KulefuFormation. At the north end of the Central Zone,volcaniclastic rocks and pyroxene andesites of theTafongo Formation (Hall e t a l . , 1988b) dip at a low

J . N icho ls and R. Ha l langle and a tuff intercalation is reported to contain aPleistocene fauna (Apandi and Sudana, 1980).S e i s m i c r e f l e c t io n p r o f i le sSeismic reflection profiles covering the Weda Bay area(Letouzey e t a l . , 1983) consist of five widely-spacedlines shown in F i g u r e 5 . Our preliminary interpretationof these has identified four seismo-stratigraphic units(A-D) above the basement. The highest unit (A) is apackage of sediments which are considered to representQuaternary deposition in the bay. In the centre ofWeda Bay, unit A is consistently between 250 and500 m thick (assuming a seismic velocity of 1.8 km -~ sfor soft calcareous sediments). Unit A is thinner closeto the margins of the bay except near the tip of the SEarm where slumping, probably associated with activefaulting along the north side of the bay, has occurred.Areas of non-deposition and erosion are foundassociated with active faults near the Widi Islands onthe south side of the bay.

Beneath a reflector which marks the base of unit Athere is a unit of variable thickness (B) which occurseverywhere except in the regions of faulting anderosion on the northern and southern margins of WedaBay. The seismic character of unit B suggests that it is aheterolithic sequence of mudstones, sandstones andcarbonates. Using an average seismic velocity of2.0 km -1 s, the distr ibution of unit B on the profilessuggests a general thickening from the northern andsouthern edges of the basin towards a depocentremid-way between the tip of the SW arm and Gebewhere there is up to 2 km of unit B ( F i g u r e s 5 and 6).1128 h29 o

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56 Ma r ine and Pe t ro leum Ge o logy , 1991 , Vo l 8 , Feb rua ry


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B a c k a r c s e t ti n g b a s i n f o r m a t i o n a n d s e d i m e n t a t i o n : G . J . N i c h o l s a n d R . H a l lN

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F i g u r e 6 I n t e r p r e t a t i o n o f m u l t i - c h a n n e l s e i s m i c l i n e P A C 3 1 3 , l o c a t e d on Figure 5. U n i t A is c o n s i d e r e d t o b e Q u a t e r n a r y d e p o s i t s ,U n i t B t h e L a t e M i o c e n e - P l i o c e n e W e d a G r o u p , U n i t C is t h e M i o c e n e S u b a i m L i m e s t o n e F o r m a t i o n a n d U n i t D is a s e q u e n c e o fl a y e r e d s e d i m e n t s o f u n k n o w n a f f i n it i e s p r e s e r v e d in a f a u l t b l o c k in t h e b a s e m e n tW e i n t e r p r e t u n i t B a s e q u i v a l e n t t o t h e L a t eM i o c e n e - P l i o c e n e s e d i m e n t s o f t h e W e d a G r o u p . A tt h e b a s e o f u n i t B i s t h e s t r o n g e s t c o n s i s t e n t r e f l e c t o ro b s e r v e d o n t h e p r o f i l e s . U n i t C h a s a c o n s i s t e n t T W To f b e t w e e n 0 . 5 a n d 1 s e c o n d (Figure 6) a n d i t s b a s e i sm a r k e d b y a n o t h e r s t ro n g r e f le c t o r . T h e s ec h a r a c t e r i s t i c s a r e c o n s i s t e n t w i t h a n i n t e r p r e t a t i o n o fu n i t C a s s e d i m e n t s e q u i v a l e n t t o t h e S u b a i mL i m e s t o n e F o r m a t i o n . I n th e p a r t s o f t h e p ro f i le s w h e r et h e l o w e r p a r ts o f t h e s e c t i o n c a n b e i n t e r p r e t e d , u n i t Ce i t h e r li es d ir e c t ly o n a h o m o g e n o u s b a s e m e n t , o r o n as e q u e n c e o f w e ll s t ra t i fi e d o l d e r d e p o s i t s ( u n i t D ) i n t h ec e n t r e o f t h e b a y (Figure 6). T h e s e s t r a t i g r a p h i cf e a t u r e s a l s o s u p p o r t t h e i d e n t i f i c a ti o n s o f u n i t C a s t h eS u b a i m L i m e s t o n e F o r m a t i o n .T h i c k n e s s v a r i a t i o n s i n u n i t D s u g g e s t d e p o s i t i o nd u r i n g a r i f ti n g p h a s e . A s s u m i n g u n i t C is t h e S u b a i mL i m e s t o n e F o r m a t i o n , u n i t D m u s t b e p re - M i o c e n e ina g e . U n i t D c o u l d b e e q u i v a l e n t t o r o c k s o f t h e B u l lG r o u p , a s i n p l a c e s i n t h e S E a r m t h e s e r o c k s a r e n o ts t ro n g l y d e f o r m e d . H o w e v e r , t h is s e q u e n c e c o u l d a ls or e p r e s e n t p o s t - B u l i G r o u p a n d p r e - S u b a i m L i m e s t o n eF o r m a t i o n r o c k s w h i ch h a v e n o t b e e n f o u n d o nH a l m a h e r a . I f t h e i n t e r p r e t a t i o n o f t h e a g e s o f u n i t s

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F i g u r e 7 M a p o f t h e g e n e r a l f a c i e s a n d e n v i r o n m e n t r e l a t io n -s h i ps f o r th e L a t e M i o c e n e t o P l i o c e ne W e d a G r o u p s e d i m e n t a r yr o c k s o f t h e H a l m a h e r a B a s i n

A - D i s c o r r e c t t h e r e is l i tt le e v i d e n c e o f i m p o r t a n tN e o g e n e e x t e n s i o n i n t h e W e d a B a y a r e a .B a s in ev o lut io nStage 1 . Pre-NeogeneT h e e v i d e n c e t h a t w e h a v e o b t a i n e d f r o m e a s t e rn a n dw e s t e r n H a l m a h e r a s u g ge s ts t ha t e a s t e rn H a l m a h e r a isu n d e r l a in b y a b a s e m e n t c o m p l e x in c l ud i ng E o c e n e a n do l d e r r o c k s. T h e b a s e m e n t c o m p l e x i n c lu d e s o p h io l i ti cr o c k s, a n d L a t e C r e t a c e o u s t o E o c e n e a r c v o lc a n ic ,v o l c a n i c la s t ic a n d s e d i m e n t a r y r o c k s o f t h e B u l i G r o u p .W e i n t e r p re t t h e O h a V o l c a n i c F o r m a t i o n , w h i c h fo r m st h e b a s e m e n t o f w e s t e r n H a l m a h e r a , t o b e th e v o l c a ni ca r c w h ic h s u p p l i e d d e b r i s t o t h e f o r e a r c s e q u e n c e o f t h eB u li G r o u p . T h e s e r o c k s w e r e d e f o r m e d a n d u p l i ft e db e f o r e t h e e n d o f th e P a l a e o g e n e .T h e f lu v i a ti l e a n d l i tt o r a l c o n g l o m e r a t e s o f t h e J a w a l ia n d G e m a f C o n g l o m e r a t e F o r m a t i o n s i n d i ca t e t h a tm o s t o f t he e a s t e r n H a l m a h e r a w a s c lo s e to o r a b o v es e a l e ve l b y t h e b e g i n n i n g o f t h e M i o c e n e . O u ri n t e r p r e t a t i o n o f t h e W e d a B a y s e i s m i c p ro f i l e , w h i c h ist h a t th e S u b a i m L i m e s t o n e F o r m a t i o n e x t e n d s a ll t h ew a y a c r o s s W e d a B a y , i m p l i e s t h a t t h e w h o l e o f t h es o u t h e r n H a l m a h e r a r e g i o n w a s c l o s e t o s e a l e v e l .S ta g e 2 . E a r l y - M i d M i o c e n eT h e J a w al i C o n g l o m e r a t e F o r m a t i o n i n t h e n o r th a n dt h e l at e ra l ly e q u iv a l e n t G e m a f C o n g l o m e r a t e F o r m a -t i o n f u r t h e r s o u t h p a s s u p i n t o t h e S u b a i m L i m e s t o n eF o r m a t i o n . I n o t h e r a r e a s r e e f a n d r e e f - a s s o c i a t e dc a r b o n a t e s e d i m e n t s o f th e S u b a i m L i m e s t o n eF o r m a t i o n w e r e d e p o s i t e d d i r ec t ly o n t h e b a s e m e n tr o c k s . M i o c e n e l i m e s t o n e s e x t e n d f r o m M o r o t a i(Figure 2) i n t h e n o r t h ( S u p r i a t n a , 1 9 8 0 ), a c ro s s e a s t e r na n d s o u t h e r n H a l m a h e r a , a t le a st a s f a r a s W a i g e o a n dt h e B i r d ' s H e a d o f I r i a n J a y a ( V i s s e r a n d H e r m e s ,1 9 6 2) , in d i c a t i n g t h a t t h e M i o c e n e w a s a p e r i o d o f q u i e ts t a b l e c o n d i t i o n s o v e r a w i d e a r e a o f e a s t e r n I n d o n e s i a .T h e t h i c k n e s s o f th e M i o c e n e l i m e s t o n e is f a i rl yu n i f o r m a c r o s s H a l m a h e r a a n d t h e r e a r e n o a p p a r e n ts t r u c t u r a l c o n t r o l s o n t h e d i s t r i b u t i o n o f th i s u n it , w i t ht h e p o s s i b le e x c e p t i o n o f p a r t s o f th e S W a r m w h e r eM i o c e n e l i m e s t o n e i s n o w a b s e n t .Stage 3 . Late MioceneI n t h e L a t e M i o c e n e e a s t w a r d s u b d u c t i o n o f t h eM o l u c c a Se a P la t e b e g a n a t t h e H a l m a h e r a T r e n c hr e su l ti n g in t h e f o r m a t i o n o f t h e H a l m a h e r a V o l c a n i cA r c . T h i s e v e n t is d a t e d b y t h e a p p e a r a n c e o f f r e s ha n d e s i t i c d e t r i t u s i n t h e L a t e M i o c e n e s e d i m e n t a r yr o c k s o f H a l m a h e r a a n d i m p l i e s t h a t s u b d u c t io n b e g a ne a r l i e r t h a n p r e v i o u s l y s u g g e s t e d b y H a l l ( 1 9 8 7 ).I n t h e w e s t e r n p a r t o f th e S W a r m s u b m a r i n e d e b r i sf lo w s an d t u r b i d it e s o f t he L o k u F o r m a t i o n a r e t h o u g h t

Ma r ine and Pe t ro leum Geo logy , 1991 , Vo l 8 , Feb rua ry 57


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B a c k ar c s e t t in g b a s i n f o r m a t i o n a n d s e d i m e n t a t i o n : G .to rest unconformably on the Oha Volcanic Formation.The Loku Formation contains andesite debris andclasts of reworked Lower, Middle and Upper Miocenelimestones. The Loku Formation ~s thereforeinterpreted to indicate major uplift of a shallow waterlimestone region at the same time as the volcanic arcbegan its activity. This uplift was accompanied bysignificant subsidence with the eroded materialdeposited as debris flows and turbidites on a submarineslope or in deep water. Limi ted palaeocurre nt evidencefrom the turbidite sequence suggests west-directedtransport of sediment.On the eastern side of the SW arm and in centralHalmahera the Superak Formation rests directly on theOha Volcanic Formation and is a shallow marinesequence of conglomerates and sandstones. Thedistribution of the coarser material clearly indicates asource in the region of the western part of centralHalmahera (Figure 7) . The lower part of the SuperakFormation contains common aphyric basaltic clastsinterpreted to be derived from the Oha VolcanicFormation. Fresh pyroxene and hornblende andesitedebris dominates the clastic compo nent in rocks of theWeda Group (Nichols e t a l . , 1991) and is interpretedto be derived from the active late Ne ogene volcanic arc.In eastern Halmahera reef-associated limestones ofthe Subaim Limestone Formation pass up into shallowmarine, but probably deeper, marls and foraminiferallimestones of the Saolat Formation. The minorsiliciclastic compon ent was derived exclusively from theOphiolitic Basement Complex.We suggest that the Loku Formation representsslope sedimentation close to the active arc, possibly inthe forearc, whereas the Superak Formation representscoeval sedimentation close to the arc, but in thebackarc. The area of eastern Halmahera lay in thebackarc but too distant from the arc to receive arcvolcanic debris. The sediments of the Hal mahera Basinwere deposited in this backarc setting.S ta g e 4 . L a t e M i o c e n e - E a r l y P l i oc e n eIn the western part of the Halmahera Basin theSuperak Formation is overlain by shallow marinedeposits of the Akelamo, Dufuk and Gola Formations.We estimate the total thickness of the Weda Group inthe SW arm to be between 2800 and 3800 m. Overall,the amoun t of clastic input from the arc decreased withtime and the sequence becomes finer grained and richerin carbonate up-section. This may in part reflectdecreased volcanic activity. In the eastern part of thebackarc, marl and limestone deposits of the SaolatFormation continued to accumulate throughout this

J . N i c h o l s a n d R . H a l lperiod. Andesitic volcanic debris is absent in the SaolatFormation, although clastic material was eroded fromuplifted blocks of the Ophiolitic Basement Complexand carbonate reefs. The total thickness of the SaolatFormation is probably between 1000 and 1500 m.S ta g e 5 . M i d - L a t e P l i o ce n eThe Weda Group was deformed after the Mid-LatePliocene (N20/N21). It appears that volcanic activitymay have been renewed at about this time, and brieflychanged character. In the Central Z one Pliocene rocksof the Weda Group contain an increasing amount ofvolcanic debris up-section (Hall et al . , 1988b) and aretightly folded. In some parts of the northern SW armthe Kulefu Formation rests unconformably on foldedWeda Group rocks and includes unusual dacitic crystaland lithic debris.B a s i n f o r m a t i o n a n d g e o m e t r yImportant uplift and subsidence of western Halmaherabegan in the late Late Miocene (c. 8 Ma). Uplift of thevolcanic basement was quickly followed by subsidenceand deposition of at least 1000 m of submarine slopedeposits (the Loku Formation) on the western side ofthe SW arm. The Loku Formation contains volcanicdebris typical of the Neogene volcanic arc andreworked Miocene limestones suggesting derivationfrom the east; palaeocurrent indicators also suggestwestward transport. We interpret the Loku Formationas probable forearc deposits.In the same Late Miocene interval shallow watersediments of the Superak Formati on were deposited onthe eastern side of the SW arm. These are the firstdeposits of the Halmahera Basin. At least 2800 m ofshallow marine deposits form the Weda Group whichfines up from fan-delta conglomerates to sandstones,mudstones and limestones. In the eastern part of theHalmahera Basin there is no unconformity between theMiocene Subaim Limestone Formation and the WedaGroup which is represented by the shallow marinedeposits of the Saolat Formation. The SaolatFormation is at least 1000 m thick. The Mio-Pliocenesequence interpreted from the seismic sections acrossWeda Bay is up to 2000 m, but typically about 1000 mthick. The basin was hence markedly asymmetric, withthe greatest thickness of sediment adjacent to theHalmahera Arc on the western side of the basin(Figure 8) . As the Miocene limestone represents anapproximate sea level datum and covers the wholearea, this indicates that subsidence was asymmetric.The evidence of a shallow marine environment of

HALMAHERA ARC HALMAHERA BASINW E

t /v v v v v v v v v v v v v v v v v v v v v v w ~ b~ ~,~ -_ h

Tafongo& KulefuFms.~ Loku Fm,~ O h a Volcanics

~ Gola Fm.~ DufukFm .~ AkelamoFro.~ SuperakFm.

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( v e r f f c a l e x a g g e r a O o n x 5 )~ SaolatFrn.~ Subaim imestoneFin.~ OphioliticBasementComplex

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I : i gu te $ Schem at i c E -W p ro f i l e ac ross the H a l m ahera Basi n i n the P l i ocene (p r i o r to de fo rm a t i on by E -W c o m p r e s s i o n )58 Mar ine and Pet ro leum Geo logy , 1991, Vo l 8 , February


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B a c k a rc s e t t i n g b a s in f o rma t i o n a n d s e d im e n t a t i o n : G . J . N i c h o l s a n d R . Ha l ldeposition for almost all the Weda Group sedimentaryrocks indicates that subsidence continued into thePliocene.Andesitic volcanic rocks are abundant as clasts in theUpper Miocene to Pliocene sequence of the SW arm.These volcanic clasts are different from the basalticvolcanic rocks forming the basement of the westernpart of the island (Hakim, 1989). The andesite clastsare petrographically and chemically similar to volcanicrocks of the present Halmahera Arc. The appearanceof these clasts low in the W eda Grou p indicates that theHalmahera Arc was active by late Late Miocene timesand that subduction at the Halmahera Trench musthave commenced prior to 8 Ma. This is earlier thansuggested by Hall (1987) and Hall e t a l . (1988b). Weconsider the uplift of the western side of Halmaheraand the subsidence of the backarc region to beintimately related to the development of theHalmahera Trench and volcanic arc. The HalmaheraBasin therefore formed by asymmetric subsidence ofthe backarc region ( F i g u r e 8 ) .

The western side of the basin must have beensubsiding faster and receiving more sediment than theeastern side of the basin. The Halmahera Basin wasactively subsiding from the Late Miocene until sometime in the Mid -L at e Pliocene, a period of up to 6 My.During this interval the western side of the basin,adjacent to the arc, subsided at least 2.8km,representing an average subsidence rate of at least47 cm per 1000 y. Subsidence rates for the eastern partof the basin, abou t 100 km f rom the arc, wereapproximately 17-34 cm per 1000 y.The provenance of material is different on thewestern and east ern sides of the basin. T he western sideof the basin was supplied with clastic debris from thevolcanic basement and active volcanic arc but lacksmaterial derived from the Ophiolitic BasementComplex. In contrast, the eastern side of the basincontains abundant ophiolitic debris and lacks debrisfrom the Halm ahera Arc. Local uplift of the OphioliticBasement Complex in the eastern part of the basin isindicated by the ophiolitic debris found in littoralsediments of the Saolat Formation. At present thereare blocks of Ophiolitic Basement Complex on thesouthern side of the SE arm bound ed by faults trending

WNW-ESE and offshore there are basement blocksbounded by steep faults with a similar orientation. Theuplift of these blocks may be associated with E-Wshortening, with the ES E - W NW faults acting as lateralramps to thrusts, but there is no other evidenceconfirming Mio-Pliocene thrusting in this part of thebasin.P l i o - P l e i s t o c e n e m o d i f i c a t io n o f t h e b a s inIn central Halmahera Late Miocene to Pliocenesedimentary rocks are defor med in a N- S trending foldand thrust belt. Furt her to the east the Miocene SubaimLimestone Formation was thrust westwards over theyounger Saolat Formation ( F i g u r e 4 ) . Structuralrelationships indicate that at least 60 km of E- Wshortening has occurred in central and easternHalmahera. In the SW arm of the island thesuperposition of gently-dipping sediments of theSuperak For mation on tightly folded rocks of the LokuFormation suggests that there is a thrust between thesetwo units. This thrust may be the southward extensionof one of the major thrusts of the central fold and thrustbelt. East-west shortening across the island is relatedto subduction of the Philippine Sea and Molucca SeaPlates (Hall and Nichols, 1990a) and the developmentof the Molucca Sea Collision Zone (Silver and Moore,1978; Moore and Silver, 1983).The thrusting in southern Halmahera and continueduplift along the line of the Halmahera Arc, haveexposed Weda Group sedimentary rocks on land,whereas in Weda Bay there is evidence of recentsubsidence. The deepest part of Weda Bay is almost2000 m be low sea level ( F i g u r e 9 ) and is an areahounded to the north and south by steep submarinescarps. These scarps have a WNW-ESE trend andrecent bathymetric profiles across Weda Bay obtainedon cruise CD30 of the research vessel C h a r l e s D a r w i n(Masson, 1988) indicate that they are active faults( F i g u r e 9 ) . These faults are parallel to a prominentstructural trend identified in southern Halmahera andthe adjacent region (Nichols et a l . , 1990). They areconsidered to be sinistral faults which are splays off theSorong Fault to the south (Hall and Nichols, 1990b).These faults control the form of the present daydepocentre in Weda Bay.

~ L 5 0 k m

/ 4 0 0 ~

(B) A0 k m 5 0I I

B

F i g u r e 9 (A) The bathymetry of Weda bay (com piled from GEBCO Indonesian hydrographic survey charts and Ch a r l e s Da r w in cr u isedata). (B) Bathymetric p r o f i l e a c r o s s Weda Bay ( Ch a r le s Da r w in Cruise data)M a r i n e a n d P e t ro l e u m G e o l o g y , 1 99 1 , V o l 8 , F e b ru a ry 5 9


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B a c k a rc s e t t in g b a s i n f o r m a t i o n a n d s e d i m e n t a t i o n : G .D i s c u s s i o nUnlike many other arcs in the western Pacific, theHalmah era Arc is a relatively young feature, formed asa consequence of eastward subduction of the MoluccaSea Plate beneath a part of the Philippine Sea Plate. Itis hence possible to document the evolution of an arcand backarc region without the complications of a longhistory of arc-trench tectonics.The region east of the Halmahera Arc is apparentlynot a 'typical' backarc region. The crust is not normaloceanic crust, but is a complex of ophiolitic and islandarc igneous rocks and related forearc sediments whichwere imbricated together and uplifted in the LatePalaeogene. The Halmahera basement resembles ridgeand plateau regions within the Philippine Sea Plate(Hall and Nichols, 1990a). In the north part of thePhilippine Sea Plate the Daito Ridge province,including the Oki-Daito and Daito Ridges, and theAmami Plateau, have been interpreted as remnant arcs(Murachi e t a l . , 1968; Karig, 1975; Shiki e t a l . , 1977;Mizuno e t a l . , 1978; Klein and Kobayashi, 1981; Lewise t a l . , 1982; Tokuya ma e t a l . , 1986) and continentalfragments (Nur and Ben-Avraham, 1982). Such ridgeand plateau regions or other regions of anomalouslythick crust may play an important role in the evolutionof convergent plate boundaries in oceanic regions (Nurand Ben-Avraham, 1982; Hall and Nichols, 1990a). Asthey are crustal edifices several kilometres higher thannormal oceanic crust, they will not be subducted easilyand may cause either the direction of subduction toreverse or the subduction zone to step out around theplateau. Similarly, a propagating trench system willtake a path around such a plateau or ridge rather thanthrough it. It is therefore to be expected that thesethickened crustal fragments will eventually lie on thearc side of a trench and form part o f the backarc region.Basins may form in backarc regions by a variety ofmechanisms and we prefer to use the term backarcbasin in a purely descriptive, rathe r than genetic, sense.The Neogene Halmahera Basin formed in backarcsetting and is therefore a type of backarc basin; it is abackarc basin formed by subsidence in a region ofthickened crust. Some distinctive characteristics of thisbasin are: (1) Shallow marine sedimentary rocks restdirectly on arc-related and ophiolitic basement rocks.Prior to basin initiation the region was at, or close to,sea level. (2) Shallow marine sedimentation persistedthroughout the basin history. Subsidence wascontinuous and the rate of sediment supply neverexceeded the rate of subsidence. (3) The basin has anasymmetric profile with the greatest subsidence on theside adjacent to the volcanic arc. (4) Clastic input camefrom the active volcanic arc and uplifted parts of thebasement. Some of this uplift is related to the initiationof the arc (on the arc side of the basin). (5) Differentparts of the basin have different and distinctiveprovenances. Clastic debris from the active arcdomina ted on the arc side of the basin but large areas ofthe basin did not receive detritus from this source.

This type of basin differs from other basins inbackarc settings as follows: (1) There is no indication ofthe formation of new oceanic crust by backarcspreading. (2) There is no evidence for continentalcrustal basement in the backarc region. (3) There is noevidence that subsidence was related to crustalextension. (4) There is no evidence for thrusting before

J . N i c h o l s a n d R . H a l lo r d u r i n g s e d i m e n t a t i o n . T h r u s t i n g , u n r e l a t e d t o a r ce v o l u t i o n , o c c u r r e d a f t e r b a s i n f o r m a t i o n a n d w a sd i r e c t e d t o w a r d s t h e v o l c a n i c a r c.Basin development was coeval with and dependenton volcanic arc development. Basins of the Halmaheratype will form where a subduction zone developsadjacent to a region of thickened crust and thecharacteristics outlined above distinguish these basinsfrom backarc basins formed by the generation of newcrust and from retro-arc basins formed by foreland-typethrust tectonics behind an arc.AcknowledgementsFinancial support for our work in Indonesia wasprovided by Amoco International, Enterprise Oil,Total Indonesie and Union Texas. GRDC Bandungprovided invaluable support for the field programme inIndonesia. We thank Paul Ballantyne, LawrenceGarvie, Sufni Hakim and Kusnama for theircontributions to this project. We thank Jean Letouzey(IFP, Paris) for allowing us access to the seismicreflection profiles across Weda Bay. The RRS C h a r l e sD a r w i n cruise CD30 was funded by NERC. We thankEli Silver and Dave Scholl for helpful and constructive~reviews.

ReferencesApandi, T. and Sudana, D. (1980) Geologic ma p of the Ternatequadrangle, Nor th Maluk u Geolog ica l Rese arch andDevelopment Centre, Bandung, IndonesiaBrouwer, H. A. (1923) Geolgische onderzoe kingen op het e i landHalmaheira Jaarboek Mi imw ese n Nederlandsch Oost-lndie

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exp l o ra t i on fo r o i l in N e the r l ands N e w G u i nea Konik l i jkNeder lands Mi jn bou wk und ig Genootschaft Verhandlingen,Geologische Serie 20, 265 pp.Yasin, A. (1980) Geologic ma p of the Bacan quadrangle, Nor thMa luku G eo l og i ca l R esea rch a nd D eve l op m e n t C en t re ,B a n d u n g , I n d o n e s ia

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nichols hall, 1991 (basin formation and neogene sedimentation in a backarc setting, halmahera,...

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