J. geol. Soc. London, Vol. 136, 1979, pp. 547-568, 9 figs., 2 tables. Printed in Northern Ireland.

Reinterpretation of the geology of Seram: implications for the Banda Arcs and northern

M. G. Audley-Charles, D. 1. Carter, A. J. Barber, M. S. Norvick & S. Tjokrosapoetro

SUMMARY:Reconnaissance field traverses in Seramhave led to major revisions in the stratigraphy,structure, tectonic history and geological maps. The island is composed of 4 principalstratigraphical-structural elements: (1) metamorphiccontinental - I of uncertain structural status and palaeogeographical affinity,(2) an entirely marine early Tiiassic- Miocene imbricate succession regarded as para-autochthonous, (3) an allochthon composed of severaldifferent thrust sheets, including metamorphic rocks, Triassic limestone and a late Miocene olistostrome, (4) a Plio-Pleistocene post-orogenic autochthon. The apparently over- thrust slices of metamorphic basement complex can be interpreted as derived from either the Asian or Australian craton. The Australian shelf, slope and rise sediments, possibly including some oceanic sediment, are regarded as para-autochthonous. Remarkably close correlation is demonstrated between the stratigraphical breaks reported from the Mesozoic-Cenozoic succes- sion of the NW Australian shelf, from Misool, and the para-autochthonous rocksof Seram and Timor. This emphasizes the presence of the Australian craton underlying these 3 islands. Close correlation is also found between the allochthonous rocksof Seram and Timor. Some of these thrust sheets are interpreted as having been derived from the Asian continental margin. The ultrabasic rocks of SW Seram and Ambon seem to form the highest thrust sheets. The main period of orogenesis, involving over-thrusting, olistostrome emplacement and imbricationof the underlyingAustralian cover-rock sequences occurred in the late Miocene-early Pliocene (N.18). Thestructural position of thevolcanic rocks of Ambon is uncertain.A tentative interpretation is that they are in sim, having been extruded from deep-seated fractures that penetrate the 'Asian' thrust sheets and the underlying Australian continental basement.

Inour continuing study of theBanda Arcs, Seram metamorphic rocks appearedlikely to provide clues to (Fig. 1) was considered the most important island to the structure and geological history and to assist com- investigateafter Timor, since it lies on the opposite parison with Timor. Additional traverses were planned side of the 180" curved Banda Arc, is the next largest around Nief in NE Seram which it was thought would island (being 400 X 75 km), and has geological sketch prove to be similar in stratigraphy and structure to the map coverage (with sample descriptions), which allow Kolbano, Aliambata and Iliomar of southern reinterpretation onthe basis of airphotos, ERTS Timor. photos and field studies. The geology of Seram is a Our geological traverses in central, NE and S Seram mirror image of the geology of Timor in many impor- are shown on Fig. 2. On thebasis of field traverses and tant respects, such as the supposed directions of over- micropalaeontology, stratigraphical/structural sections thrustingand the contrastingMesozcic faunas and have been drawn (Figs. 3,4,5). The new stratigraphi- facies showing affinities with either Australian (base- cal data for the pre-Neogene rocks has allowed the ment) or 'Asian' (overthrust) elements. Published re- geological sketch maps, sections and sample descrip- ports on thegeology of Seram (Valk 1945; Germeraad tions of Central Seram (Germeraad 1946) to be con- 1946; van der Sluis 1950;Zillman & Paten1975) siderably revised (Fig. 6),and those of westernand suggested that lithologies and faunas were similar to easternparts of Seram (Valk 1945; van der Sluis those in Timor.Compared with Timor(Audley- 1950) to be reinterpreted (Fig. 7). These interpreta- Charles 1968; Carter et al. 1976; Barber er al. 1977), tions were aided by vertical and oblique airphotos and accounts of thepre-Neogene stratigraphy and struc- the ERTS photos of Seram. ture of Seram seem confused, with apparently unre- lated stratigraphical divisions and dissimilar structural Outline of geology elementslumped together unconvincingly. Further- more, no unambiguous evidence of overthrusting had Thestratigraphical nomenclature used throughout is beenfound by earlierworkers, although they sus- informal for all the pre-Pliocene rocks. Where stratig- pected such structures were present. raphical divisions have been studied in the field, some Central Seram appeared to contain thewidest range new informal names have been proposed, but other- of stratigraphical divisions and was therefore selected wise the published namesare retained. The Plio- as the primary target of our 1975-6 expeditions. The Pleistocene strata were formally described by Zillman

0016-7649/79/090&0547$02.00 01979 The Geological Society 548 M. G. Audley-Charles et al.

l35 E

S'N PACIFIC

OCEAN

0'

10's

INDIAN

OCEAN 15's

NEOGENE IMBRICATE ZONE

135.E

FIG. 1. Location map for the Banda Arcs, eastern . The A-zone (northern margin ofthe imbricate zone) is plotted from Bally (1975) at the boundary of the imbricate zone with the gently deformed Australian craton and cover, except in the northern Banda -E Sulawesi , where young rifting may have separated it from and the Sula islands. The position of the Benioff zone, not indicated, is controversial. The dotted line marks the edge of the Australian continental shelf.

Il8.E 13dE, 0 129*E 2Fl 10 32-2

3'5

-5

CER a EU SAMPLES IAUOLEY-CHARLES) CE SAMPLES IBAREERI

CC SAMPLESICARTER)

1-1 10 82-SAMPLESINORVlCKl

12 8'E 129*E SERAM 1?dE FIG.2. Traverse and sample location map of Seram. The widespread and numerous samples collected by Rutten &. Hotz are described and located by Valk (1945), Germeraad (1946) and van der Sluis (1950). Reinterpretation of the geology of Seram 549 & Paten (1975) in terms of two divisions (Wahai Beds allochthonouselements because there is evidence of and Fufa Formation). their having been overthrust to their present position The repetition of stratigraphical divisions with some above the para-autochthon (Figs. 3,4,5) and because invertedages (Figs. 3,4,5), involvingrocks ranging they have strikingly different facies and faunas from from early Triassic to late Miocene, implies an imbri- Mesozoicrocks of thesame age in thepara- cated para-autochthon, whose succession, facies, and autochthon. The conclusion that these allochthonous faunas have a strong affinity with Australian continen- elements were derived, from a continental marginNW tal margindeposits in Misool(van Bemmelen 1949; of Seram, before the Banda Arc acquired its sinuosity, Froidevaux 1975), (Visser & Hermes together with theirremarkable similarities with the 1962;Harrison 1969; Davies & Norvick1974), the allochthon of Timor leads to their being regarded as NWAustralian shelf (Powell 1976) andTimor having been derived from the Asian continental mar- (Audley-Charles1968, 1978). gin possibly in the vicinity of easternmost Java or S Somestratigraphical divisions areplaced in Sulawesi (Audley-Charles 1978; Carter et al. 1976).

LOEMOETEMOUNTAIN RANGE (ASINEPE TRAVERSE) N S ASINEPE

SCALE

FACIES

R SALAWLI l

/ DWAKUKU BEDS ITRIASI ’

SELEMAN BAY TRAVERSE W E SELEMAN BAY

rSKM

I KM

MANUSELAMOUNTAINS TRAVERSE

FIG.3. Stratigraphical-structural sections in Mt. Asinepe, Seleman Bay, and Manusela mountains of central Seram.

3 M. G. Audley-Charles et al. 550 - SCHEMATIC SIRATIGRAPHICSECIION WAl WAKUKU GEOLOGICAL MIDDLE WAl WAKUKU SKETCH MAP

1 1 c L

E

LI

E

AxBEDS NlEF l NlEF BEDS

FIG. 4. Stratigraphical section and sketch map of the Wai Wakuku, central Seram.

In parts of central northern and E Seram, the Salas Stratigrapby of the Block Clay formation above the overthrust elements Australian shelf, slope and rise and the para-autochthon appears to have been emp- fasies (para-aubochthon) laced in its present position as an olistostrome during the late Miocene (N.18).It is best regarded as part of 3 mainlithostratigraphical divisions of thepara- the allochthon, although most of its material, in the autochthonare recognized: the Nief beds (late limitedregions where it hasbeen studied, has been Jurassic-late Miocene), the Saman Saman Limestone derivedfrom the para-autochthon and onlya small (Triassic) andthe Wakuku beds (early Triassic- proportion of theexotic blocksbelong tothe Jurassic). These divisions are of formation status but allochthonous 'Asian' elements. Elsewherein Seram the are described here as informal divisions owing to the proportion of 'Asian' exotics may be higher. The Salas reconnaissance nature of our investigation. They are BlockClay is overlainunconformably by the widespread throughout Seram, and are imbricated so autochthonous,post-orogenic sediments of early thatthe Nief bedshave been found in repetitive Pliocene-Quaternary age (Zillman & Paten 1975). sequenceswith the Wakuku beds (Figs. 3, 4). The The overallstratigraphic sequence and its 3 main Saman Saman Limestone is well developed onlyin the elements, the late Cenozoic autochthon,Mesozoic and Manusela Mountains of S central Seram, but appears Cenozoic para-autochthon, and the apparently mainly locally to extend NW along the strike of these moun- Mesozoic allochthon, which may extend down into the tains into the northern part of W Seram (Valk 1945). Palaeozoic,are remarkably similar to the sequences The SamanSaman limestone facies is found in thin reported from Timor and other islands of the Outer intercalations in theWakuku beds, suggesting that Banda Arc (Tables 1, 2). theirages overlap. There is someindication of a Reinterpretation of the geology of Seram 551

FIG. 5. Stratigraphical sections and sketch map of the Nief area of NE Serarn. stratigraphicalgap between thetop of the Saman though exposure is poor, being largely masked by talus Saman-Wakukudivisions and the base of the Nief from the high mountainranges to the S. Scattered beds, with the Middle and possibly some of the early exposures in creek beds suggest that the structure is Jurassicbeing unrepresented by any strata in the complicated by numerous folds and faults. The most para-autochthon. This would not be a surprising gapin complete section is in the Wai (=river) Wakuku, SE Australiancontinental margin deposits. The Middle of Manusela in central Seram. The base of the section and late Jurassic and locally the early Cretaceous are is not exposed, but vertical shearing suggests that here often missing fromthe basins around the NW Au- it is steeply faulted against the Nief beds. The Wakuku stralia offshore region (Powell 1976). Part of the Mid- bedsconsist of thinlybedded and laminated, hard, dle and late Jurassic are missing from Timor (Carter et dark grey, carbonaceous and micaceous siltstones and al. 1976) and from Misool (van Bemmelen 1949). This mudstones, with small lenticles of fine, silty sand- erosionalphase in the Mesozoic of theAustralian stones.These are interbedded with fine, quartzose, margin was ascribed by Powell (1976), following Fal- micaceous, carbonaceous sandstones,10 cm-l m thick. vey (1972), to the break-up unconformity associated Sedimentarystructures include indistinct ripple-drift with the rifting and block faulting related to the de- lamination and extensive bioturbation. Deposition was velopment of theWharton Basin(eastern Indian probably under fully marine, low energy conditions in Ocean) and to the disruption of Gondwanaland. The moderatelydeep water. Rare Halobia-type thin- stratigraphy of thepara-autochthon of Seramand shelled bivalves occur in the siltstones. Two samples Timor,and that of Misooland the NW Australian have yielded early Triassic palynomorphs (Price 1976). offshore area is summarized in Table 1. Elsewhere in the central valley region, the Wakuku beds were found in the Wai Lotu W of Kaniki and in the Wai Ai NE of Hatuolo (Fig. 2). Grey laminated siltstones are interbedded with thinmicaceous sand- Wakuh beds stones.Float with Halobia-type bivalves suggests The Wakuku beds take their informalname from further similarities with the Wai Wakuku. In the Wai the Wakuku River S of Manusela village (Figs. 2,4). Lotu,turbidite structures (groove casts, prodmarks They comprise a series of dark grey siliciclastics. They and flute moulds) occur on the bases of the sandstone probablyextensively underlie the central valley, al- beds. 552 M. G. Audley-Charles et al.

PARA-AUTOCHTHON (dlstal faces) AUTOCHTHON (proxlmal faces)

BIOSTRATIGRAPHH -OFFSHORE DIVISIONS SERAM TIMOR MISOOL CANNING BASIN M P110 N2( Shelf deposlt E P110 Nlt -Kolbano facles I llmestone dolomlte g L M~oc N1; llmestone M M~ocN1: basal sandstone Bz E M~oc N4 Nlef Beds Kolbano facles L Ollg N3 I- E Ollg Nlef Beds - L Eoc Kolbano facm shelf calcarenltes M Eoc nerltlc 1st W I= E Eoc Nlef Beds deep water claystones and L Pal -Kolbano facles calclslltltes 2 M Pal Nlef Beds E Pal uIImmmm Maestr deep water Campan Nlef Beds Kolbano facles bathyal facles claystones and Globobuncana Santon s~ltstone marls wlth shales Conlac lhmestone and mar1 2 Turon Cenom claystone wlth 2 Alblan Kolbano W B sandstone facles 6 Aptlan L P Barrem L bathyal facles Hauter Nlef Beds Wal Bua major transgresslve facles sequence Valang Rvaz I Tlthon -Lower Fatlet Lst Klmmer nerltlc facles t ransgresslve sequenc Oxford Llllnta Beds Callov 0 Bathon Wal Lull Fm I1 2 nerltlc faces Q LT Baloc 3 mark 7 Toarc Wakuku Beds deltalc and lhmestones eplcontmental Plelns shale marmesequence Smemur sandstones Hettana Rhaet T Saman Saman Lst ? 0 Norlan Altutu Lst g- Carman l U Babulu Keskam Beds Ladm Wakuku Beds Member deltalc and eplcontlnental Anwan TABLE 1: Comparison of thestratigraphical -7 divisions andbiostratigraphical breaks in the para- autochthonous Mesozoic-Pliocene successions of Seram and Timor with the autochthonous Mesozoic- Pliocene successions of Misool and offshore Canning Basin. The data for the offshore Canning Basin is based on Powell (1976). The biostratigraphic data for Misool is based on re-eualuating the ages of the faunas describedby van Bemmelen (1949) and on the many strongsimilarities in lithofacies between Misool and Tirnor-Seram deposits. The Norian limestones and mark of Misool are interpreted as possibly allochthonous and hence omitted from Table 1. In Table 2 these limestones and marls are included with the Misool autochthon. Reinterpretation of the geology of Seram 553

Inthe Loemoete mountains, near the N coast of ous calcilutite or induratedmarl, full of thin-shelled centralSeram, the Wakuku beds outcrop in similar planktonicHalobia-like bivalves and small spherical turbiditefacies, locally with calcareousnodules and radiolariaare alsopresent, dated as probably late cone-in-cone nodules. In the section just S of Seleman Triassic or early Jurassic. Bay,quartz arenites occur locally with small quartz The thickness of theSaman Saman Limestone is pebbleconglomerates. The relative stratigraphical estimatedto be about 1000 m, but this is tentative position of thesearenites with conglomerates within owing to the possible presence of faulted repetitions. the Wakuku sequence is not known. TheSaman Saman Limestone behavedas the most The Wakuku beds of the Nief region, NE Seram, competentmember of thepara-autochthon. It does arepredominantly mudstones with subordinate are- notappear to have been involved in the complex nites. Theyhave been dated by palynomorphsas imbrication characterizing the Nief and Wakuku beds. ranging fromearly Triassic to earlyJurassic (Price The main outcrop is in the Manusela mountains and 1976). its NW extension along strike in NW Seram. A similar Thesketch map and sample descriptions of Valk facies is foundas thin intercalations in theWakuku (1945) suggested thatthe Wakuku beds and Saman beds. Saman Limestone outcrop widely in the NW part of The uniformly fine grain-size of the sediments, with Seram,where they have been strongly folded and their lack of terrigenous material except clay, and the faulted together. predominantlyplanktonic fauna, the absence of a On the oblique and vertical trimetrogon airphotos shelly benthos, and the fine lamination, suggest that of the E end of the central valley, the Wakuku beds theSaman Saman Limestone was deposited in very appear to be structurally overlain by early Cretaceous deep water beyond the shelf, probably on the conti- to Miocene Nief beds. Onthe ground, the contacts nental rise of the northern Australian margin, as prop- appear to be controlled by nearly vertical faults. Loc- osedfor the equivalent Aitutu Limestone in Timor ally, as in the Wai Lotu, chevron folds are developed. (Audley-Charles 1968). However, on Gunung Asinepe (Fig. 3) and SW of the Manusela Boundaryfault (Figs. 4,9), there is clear evidencethat the Wakuku beds are imbricated with Nief beds Nief beds. It is likely that the Nief beds are locally thrust over the Wakuku beds in the central valley. (One of theauthors regards the Nief bedsas al- The age of theWakuku beds throughout Seram lochthonous, a view expressed in Table 2). appears to range from early Triassic to early Jurassic Wherever seen in Seram, the Nief beds are strongly according to palynological evidence (Price 1976). imbricated and sheared. The unit takes its name from Triassic-Jurassic rocks, lithologically similar to the the River Nief in NE Seram and is dated late Jurassic- Wakukubeds, occur in Timor,western Papua New lateMiocene. The Nief bedsare deep water sedi- Guinea, Irian Jaya, Misool and on the NW Australian ments,contain virtually no terrigenous detritus and shelf (Table l), and constitute an Australian siliciclas- carry rich planktonicfaunas. They can be separated tic shelf (basin) facies. into17 biostratigraphical-lithological subdivisions: Thetrue thickness of theimbricated, folded and breaks in deposition span most of the Oligocene, the faulted Wakuku beds, whose base has not been seen, Senonian/Palaeoceneboundary latethe and is estimatedas approximately 1000 m. In Misool a Cenomanian-early Senonian interval. similar facies of Triassic age (the Keskain Beds) is said The 5 highest subdivisions are grey,argillaceous, to be more than 2000 m thick and to overlie uncon- occasionally procellaneouscalcilutites, becoming formably aslightly metamorphosed flysch-type facies violet-purple and fissile near the base where they are (Froidevaux 1975). associated with silty, argillaceouslimestone. These bedsrepresent continuous deposition from latest Oligocene (N.3) tolate Miocene(N.16 or N.17). Saman Saman Limestone Below the underlying stratigraphicalbreak 5 further Thisformation takes its informal name from Mt. subdivisions are recognized: theseconsist of cream Saman Saman in the Manusela Range of central and white calcilutites, reddish or purple, usually fissile, Seram. It consists of mushroom coloured, laminated argillaceous calcilutites and red and green mark. Their marl, full of moulds of radiolaria and relict fragments ages range from late Eocene to early Palaeocene and of Halobia-type bivalves, interbedded with brittle, the basalbed contains derived, lateCretaceous buff-greyfinely recrystallizedcalcilutites, showing material. burrows infilled with darkermaterial, violet-grey One late Senonian subdivision has been separated. streaked calcilutites with radiolaria,and beds and This consists of dense, brittle, often porcellaneous and nodules of black chert. These rocks are similar to the occasionally finely recrystallizedcalcilutite with or Aitutu Limestone of Timor, dated as Middle and late withoutcherts and carrying rich, Maestrichtian- Triassic. Pale greenish-grey, calcite-veined,argillace- Campanian Globotruncana assemblages. The calcilu- 554 M. G. Audley-Charles et al.

NOHlH30lflV-VIVd NVllVUSnV. Reinterprefationof the geology of Seram 555

NOHlH30lflV VllVdNVIlVIlSflV

Y 556 M. G. Audley-Charles et al. tites,which often show slump structures, are white, ic), KaiboboComplex (probably Triassic in part), cream, violet or pink and the cherts may be black, red TehoruFormation (possibly Palaeozoic in part),the or grey. The former always show strongly developed, Kobipoto Complex (possibly Precambrian in part) and parallel or sub-parallel stylolites which are sometimes theSalas Block Clay (a Neogeneolistostrome). The filled withchocolate-coloured clay. As in Timorno serpentinitesand igneous rocks of W Seram and beds of earlySenonian or Turonianage have been Ambon that are also interpreted as thrust sheets are encountered: in Seramthe late Cenomanian also is discussed later with the igneous rocks. missing. 6 pre-late Cenomanian subdivisions are recognized in the Nief beds. The youngest is apurple-brown, Asiepe Limestone argillaceous, Albian-early Cenomanian calcilutite carr- ying abundant smallHedbergella and Heterohelix Thishas been named after Mt. Asinepe in the moremanni. Others consist of bioturbated, sometimes Loemoete Mountain range of N central Seram (Fig. 3). siliceous, grey andpurple calcilutites with abundant It is composed of a grey, sometimes crystalline, oolitic radiolaria,Inoceramus prisms, ostracods and andbioclastic grainstone, containing crinoid ossicles, Stomiosphaera,etc. and low density,deep pink, calcareous algae, brachiopods, echinoids, sponges and manganese-filmed radiolarites. Probably the oldest is a corals. It represents a reef or sub-reef facies of early reddish-brown topurple mudstone carryingthin- Mesozoic (probably Triassic) age. Van derSluis (1950) shelled, Monotis-like bivalves, Inoceramus prisms, os- regarded this fauna as Jurassic, largely on the basis of tracods,abundant radiolaria (including speciesa Loucenipora sp. However, corals and associated fauna characteristicallyassociated with Stomiosphaera and (suchMontliualtia,as Thecosmilia, Isastrea, As- Cadosina in Timor and recently reported from the late traeomorpha, Stylophyllopsis, plocyclolites, Myriophyl- JurassicFranciscan of California) andslumped, lia and Spongiomorpha), in anallochthonous lime- shallow-water, late Jurassic material concentrated on stonefrom Timor closelyresembling theAsinepe manganese-filmedbedding laminae. Many of these Limestone in facies and fauna, havebeen dated as late calcilutites and radiolarites are faunally and lithologi- Triassic (Yamagiwa 1963). A Triassic age for at least cally indistinguishable from those of similar age found part of the Asinepe Limestone therefore seemslikely. in the Kolbano thrust sheet of Timor (Audley-Charles The Asinepe Limestone, like its Triassic equivalent 1968; Carter et al. 1976). which formspart of the mainly Permian Maubisse The base of the Nief beds has not been seen. The Formation of Timor, is a shallow, warm water deposit middle and early Jurassic may be missing, as in many and hence has close affinities with the Asian continen- partsof the offshorebasins round NW Australia tal margin facies and faunas (Kanmera & Nakazawa (Table 1). 1973). It forms a striking contrast in facies with the The many repetitions of sequences indicates (Figs. TriassicSaman Saman Limestone, which it overlies 3,4,5) thatthe Nief bedshave been strongly de- tectonically in S Seram (Fig. 3). formed and structurally thickened by imbrication. The The Asinepe Limestone outcrops in the main cen- late Cretaceous limestone may have acted as a compe- tral ranges, extending from the Manusela mountainsof tentbed, although subject to pressure solution and S Seram to the Loemoete mountains in the N and NW some internal deformation, as revealed by the stylo- (Figs. 3,6,7). On the airphotos it clearly forms one or lites andthe stretching of cherts.However, the red more overthrust sheets. It rests tectonically on rocksof andgreen argillaceous calcilutites, whichresemble strikinglydifferent facies, and on variousimbricated shales in the field, probably behaved as an incompe- strata ranging in age from Triassic to Miocene, some tent envelope to the limestone and allowed extensive of which have steeper dips than the overlying Asinepe slippage. Limestone.In the Kabau range of centralSeram, it The fauna1and lithological characteristics of the appears to forma thrust sheet, resting on this same Nief beds suggest that the unit was laid down under imbricated succession of Nief and Wakuku beds. bathyal conditions, perhaps on the continental rise of TheAsinepe Limestone alsooccurs in parts of the northern Australian margin and also on the deep western and eastern Seram as much smaller outcrops. ocean floor. Thishypothesis is consistentwith the Theseappear to besmall klippe in the NW(Valk condensed nature of the sequence, in which rocks of 1945), where they have been much affected by post- late Jurassic-late Miocene age occur within a very thin thrusting steep faults. In theE, the Asinepe Limestone section. seems to form ‘fatus’ (local name for isolated steep- sided limestone hills), which are probably large exotic Stratigraphy of the thrust blocks in theSalas Block Clay. It alsooccurs as sheets (allochthon) boulderand pebble size exotics in the SalasBlock Clay of N Seram. 5 majorstructural elements areinterpreted as Thetrue thickness ofthe Asinepe Limestone is allochthonous in Seram: the Asinepe Limestone (Triass-estimated to be about 1500 m. Reinterpretation of the geology of Seram 557

NOTE MANYNOTE NIB THRUSTSHAVE m NlEF BEDS 8 WAKUKU BEDS BEEN DISPLACED BY WAKUKU BEDS LATER STEEPFAULTS m

SAMANSAMAN LIMESTONE l2dE

FIG. 6. Geological sketch map of Central Seram based on field traverses, interpretation of airphotos and ERTS photos, and revision of the geological sketch map by Germeraad (1946) from re-assessment of his sample descriptions.

Kaibobo Complex Formation’.Slate Glossy and The rockspredomin- are This informalterm,named after Kaibobothe penin- antly glossy broken lenticular sula of westernSeram where these rocks areexten- shear surfaces coated with graphite, and interbedded with greenschists composed of albite, epidote, chlorite sively exposedacross their strike, is proposed to in- clude rocks referred to by earlier workers from west- and actinolite. The greenschists are sometimes banded,probably derived from tuffs, orare more ern and central Seram as Greywacke and Glossy (or massive, representing lava flows. This assemblage of Lustrous)Slate Formation and as the Crystalline rocks closely resembles the low grademetamorphic Schists, Gneissesand Amphibolites of Valk (1945) rocks included in the Lolotoi Complex of Timer andGermeraad (1946). The reclassification of these(Barber Audley-Charles 1976). rocks is reflected in the geologicalsketch map of The age of the ‘Greywacke and Glossy Slate Forma- Seram (Figs. 6,7). tion’ is uncertain but may be Triassic at least in part, because of the reported presence of limestones closely ‘Greywacke and Glossy Slate Formation’ resembling the allochthonous Triassic Asinepe Lime- I A traverse was made on the N side of TalutiBay stone. from Japutih to Piliana, across an area shown on the Theoverthrust position of the ‘Greywackeand mapprepared by Germeraad(1946) as‘Greywacke GlossySlate Formation’ is interpretedfrom Valk FIG. 7. Geological sketch map of Seram, Ambon and the Uliassers based on revisions of the maps by Valk (1945), Germeraad (1946) and van der Sluis (1950) from re-assessments of their sample descriptions, field traverses, and photogeological interpretation. Identification of the A-zone follows Bally (1975). It represents the northern margin of tne imbricate zone and is based on the reinterpretation of the Phillips Petroleum seismic reflection line 143 by Audley-Charles & Carter (1978). Note: Some of the present authors regard Kobipoto Complex as more likely to represent upthrust horsts.

(1945), who remarked that it is everywhere‘pushed Slate Formation’, are here regarded as possioie equi- forward against or over the Upper Triassic’. valents tothe Saman Saman Limestone. Inwestern Seram, these ‘Triassic’ radiolarian limestones and ar- gillaceous limestones are described by Valk (1945) as Possible equivalents of the SamanSaman ‘interbeddings in theUpper Triassic flysch’. He dis- Limestone tinguished them from the ‘enormous limestone moun- On the basis of ourstudies in centralSeram, the tains’ of NW Seram, which we interpret as part of the hornstonesand radiolarites, dark greyradiolarian Asinepe Limestone on the basis of the airphotos (Figs. limestones and argillaceous limestones with thin shel- 6,7). The reported ‘interbeddings’ of these rocks with led lamellibranchs of uncertain age, which Valk(1945) ‘Upper Triassic flysch’ also suggests comparison with includedwith his Triassic‘Greywacke and Lustrous the Saman Saman Limestone. We suspect that these Reinterpretation of the geology of Seram 559

rocks are slices of Saman Saman Limestone that have corals, algal limestone and crinoids. The organic debris becometectonically associated with theKaibobo is deformed and recrystallized, but still clearly recog- Complex by faulting. nizable. The crinoid ossicles are of Palaeozoic type. The metamorphic grade also increases in a S or SW direction; argillaceous rocks are slates at Tehoru and High grade metamorphic rocks N of Sahulau mica schists at Sepa. A similar increase of metamor- (i.e. Valk’s‘Crystalline Schists’) phic grade is also seen along the W shore of Elaputih Exposures of theserocks were not studied in the Bay from Sahulau to Tahulale. In the areas of higher field. Their description is based on Valk (1945) and on grade rocks, a dominant schistosity is defined by mus- float in rivers draining S from western Seram. Their covite and biotite flakes, although these minerals have presence on the sketch map (Fig. 7) is derived from frequentlybeen affected by laterdeformation, and Valk’s map modified by our reconnaissance, the ERTS biotite may be retrogressed to chlorite. Garnet occurs and airphotos. in boththe grits andthe schists; the crystals are Duringa traverse up the Wai Mala to the N of frequently enclosed in augen and may include inclu- Sahulauthrough the ‘Phyllite Formation’(Tehoru sion trails of quartz or graphitedefining anearlier formation-seelater) themetamorphic grade in the schistosity. slates, grits and limestones was observed to be declin- All the rocks have been affected by multiple phases ing. However, the float coming downstream indicated of deformation, refolding earlier cleavage and foliation that highergrade metamorphic rocks occur in the surfaces. The later folds may themselves develop sec- headwaters of this river. The highergrade rocks in- ondarycrenuiation cleavages, often with apressure cludehornblende schists andgneisses, serpentinites solution component, especially in the grit beds. Later andone specimen of garnet-pyroxenegranulite folds also affect earlier quartz veins and segregations. (metagabbro). This rock is at least of upper amphibo- Slatyand schistose rocks arefrequently affected by lite metamorphic grade, but has been partially retrog- kink band folding. The higher grade psammitic rocks ressedunder lower amphibolite facies conditions, aroundSepa and Tahulale are similar in lithology, forming reaction rimsof hornblende around the garnet structure and metamorphic state to much of the Moi- crystals. nian of the NW Highlands of Scotland, although the The assemblage of high grade metamorphic rocks in metamorphism in the PhylliteFormation never the float of the WaiMala is incompatible with the reached thetemperatures at which partialmelting metamorphicgrade of the ‘Phyllite Formation’ex- takes place. posed in the banks of the river. A meta-anorthosite TheTehoru formation maybe compared directly similar to the metagabbro collected in the Wai Mala with rocks of the Aileu-Maubisse Overthrust Unit in has been described from the Lolotoi Complex of the E Timor (Barber et al. 1977). In Seram, however, the Booi massif in W Timor. clastic rockshave theopposite polarity, having evi- Inconclusion, themetamorphic rocks of the dently been derived from the S, and being more highly Kaibobo Complex appear to be similar to those in the metamorphosed in the same direction. In Timor, the Lolotoi Complex of Timor (Barber & Audley-Charles clastic rocks are derived from the N and the highest 1976) in their range of metamorphic grade and lithol- grademetamorphic rocks occur along the N coast ogy, as well as in their structural history and present (Barber et al. 1977). In Seram, calcareous rocks con- position with respect to both the oceanic taining Palaeozoic fossils are interbedded in the suc- and Australian continental margin. cession towards the N. Valk (1945) suggested that the phyllitespass northwards into the unmetamorphosed ‘Greywackeand Glossy Slate Formation’, which he Tehoru formation describedas interbedded with themassive Triassic This formation takes its name from Tehoru village limestone(Asinepe of this account).Similarly, in on the western shore of Taluti Bay. Much of southern Timor,the silicilastic AileuFormation passes south- and SW Seram is occupied by an extensive series of wardsinto thePermian crinoidal limestones of the metamorphosed grits and interbedded argillaceous de- MaubisseFormation around the village of Maubisse posits, previously described as ‘Phyllite Formation’ or (Barber et al. 1977). ‘Amphibolites’ in the Wallace Mountains (Valk 1945; Germeraad 1946). The grits becomemore prevalent Kobipoto Complex of Central Seram and thicker towards the S coast (e.g. grits up to 3 m at Paulohi).They are also coarser-grained and contain Highgrade metamorphic rocks of unknownage large mudflakes up to 30 cm in length. This sequence wereexamined in 2 areas: betweenKaniki and Wai appears to be a turbidite one. N of Sepa some of the Tuh in the W, and in the Wai Isal gorge near Hatuolo grits becomecalcareous. N of Sahulau,amassive in the E. Germeraad (1946) showed that metamorphic limestone band up to 9 m thick was mapped, interbed- rocks occur extensively between these localities; they ded with slates and grits, and containing fragments of outcropover a largearea N of thecentral valley, 560 M. G. Audley-Charles et al. formingan upland region called here the Kobipoto Inthe gorge of theWai Tuh, granitic rocks are massif after the mountain range of that name. How- intimatelyassociated with serpentinitebreccias. Un- ever, in theKobipoto massif the highgrade fortunately,the relationships between these 2 rock metamorphic rocks areoverlain unconformably by types was not clear,but the contacts could be tectonic. unmetamorphosed sedimentary rocks. Similar cordierite gneisses to those in the Kobipoto Complex were recorded by Rutten & Hotz from other parts of Seram, e.g. the northern part of the Hoamoal Metamorphic rocks of the Kobipoto Complex peninsula near Salah, the Wai KawaN of Piru, and the The Kobipoto Complex in central Seram includes a Kaibobo peninsula (Valk 1945). They continue west- variety of highgrade metamorphic rock types. Near wards into the neighbouring island of Buru (van Bem- Kaniki, the southern margin of the complex is com- melen 1949). An extensive outcrop of cordierite gneis- posed of pelitic and semi-pelitic schists, quartzites and ses is shown by van der Sluis (1950) in the SE part of rarer calc-silicate rocks. Pebbles of hornblende gneiss Seram. These rocks are also migmatitic. Van Bemme- and schist, found by us in the Wai Sai, and by Rutten len (1949) recordedcordierite-bearing granitic rocks & Hotz (Germeraad 1946) in the Wai Ibi, may also from the islands of the Outer Banda Arc lying to the have come from this complex. Further N, in the gorge SE of Seram, including Manawoka, Kasiui and Tioor, of the Wai Tuh, the schists pass into migmatitic gneis- andsimilar rocks were collected during the ’1976 ses with quartzo-feldspathic segregations and complex Banda Sea Program’ from Kur and Fadol in the Kai folding(metatexites, Brown 1973). Onthe northern Islands. In many of theseoccurrences, serpentinite margin of the complex, the gneisses are associated accompanied the gneisses and granitic rocks. with coarse-grained granitic rocks containing gneissose Adiscontinuous belt of cordieritegneiss, granite xenoliths (diatexites, Brown 1973). andserpentinite extends from Buru, through Seram Characteristicminerals in the schists andgneisses and around the Outer Banda Arc. The association of include garnet,staurolite, andalusite and cordierite. granitesand serpentinites is unusual,and together Thelatter is usuallypseudomorphed by whitemica with the highgrade of themetamorphic rocks, sug- (pinite). The biotiteis a red-brown variety with intense gests that the assemblage was derived from the base of pleochroichaloes around zircon inclusions. Some of the continental crust. Rapid uplift while high tempera- thexenoliths within the diatexites are composed al- tures were maintained is indicated by the replacement mostentirely of aluminium silicate mineralsand in- of earlieraluminium-silicate minerals by sillimanite cludedense felts of sillimanite (fibrolite) withgreen and spinel, and by the development of an andalusite- spinel which appear to be pseudomsrphing some ear- cordierite, high temperature/low pressure assemblage. lier aluminium silicate mineral(possibly kyanite). There are 3 possible interpretations for the origin and Biotite is frequently retrogressed to chlorite, iron ox- emplacement of thesecontinental rocks: they may idesand sagenite (needle-like crystals of rutile). Or- havebeen overthrust from the outer margin of the thoclase feldspar and sillimanite may be retrogressed Australian continental plate, overthrust from the lead- to sericitic white mica. ing edge of the Asiatic plate (Figs. 8,9), or upthrust as The metasediments of the Kobipoto Complex were horsts through the overlying sediments from the Au- metamorphosed at a very high grade, reaching temp- stralianbasement beneath. From the available evi- eratures at which a substantial proportion of the grani- dence the last explanation seems least likely to some tic material in the rock became molten, leaving behind of the authors. a residuum of quartzose and aluminous material, now preservedas xenoliths in thegranite. Such tempera- Unmetamorphosed Sedimentary rocks as- tures are generally reached in the upper part of the amphibolite facies. However, the granitic rocks show sociated with the Kobipoto Complex peculiaritieswhich suggest that they have been Wakuku beds. Althoughno contacts have been - metamorphosed atan even higher grade; e.g. the served,the metamorphics are believed to be faulted granitescommonly contain garnetsand cordierite, against Wakuku beds in the S. Outcrops of Wakuku neither of which areconstituents of normalgranitic beds adjacent to the metamorphics in the Kaniki area rocks, and although the granites are pale in colour on are heavily sheared. Fault lenses metamorphics are weathered surfaces, they are very dark when freshly of broken.These features are characteristic of rocks present in someshear zones. The shear zones are whichhave been metamorphosed in thegranulite steeplydipping or vertical, andparallel toa major facies. The diagnosticmineral of this facies, hyper- photogeological lineament at the edge of the Kobipoto massif. sthene,had not been identified in any of the thin sectionsstudied. However, Germeraad (1946) re- ported cloudy‘enstatite’ from one of his specimens Wai Tuh beds. At the westernend of theKobipoto from this area and unspecified ‘pyroxenes’ in several massif,aseries of greysiltstones, sandstones and others. conglomerates are steeply faulted inwith high grade Reinterpretation of the geology of Seram

S N

MANUSELA MT. RANGE KOBIPOTO MASSIFSERAM BASIN WAHAI SE A -_ WAl TUH BEDS ?LY NIOCENE Lsr' WAHAI VOLCANICS WAH,Al

FIG. 8. Sketch cross section through central Seram interpreting the Kobipoto Complex as an 'Asian' thrust sheet, much broken by steep faults in the Plio-Pleistocene. The position of the Wai Tuh beds and the early Miocene limestone are schematic only.

ZONE A-ZONESW E - ZONE NE ---v-- BANDA SEA AMBON SERAM SERAMSERAM AMBON SEA BANDA TROUGH MISOOL

klAN' ALLOCHIHON VOLCANIC BANDA ARC

FIG.9. Schematic cross section through the volcanic Banda Arc, Ambon, Seram to Misool. The main thrust sheets T 1-6 may be identified from Table 2 although the numbering system is different in the Table. For simplicity and clarity most of the effects of the Plio-Pleistocene strong phase of steep angled faulting have been omitted. The Benioff zone (B-zone) and the A-zone (Bally 1975) were active during the late Miocene to late Pliocene but now appear to be relatively inactive (see Hamilton 1974, earthquake map). metamorphics. Similar clastic rocks were described by conglomerate.The conglomerate contains clasts of Germeraad (1946) in streamsdraining intothe Wai high grade metamorphics, ultramafics and volcanics, as Isal further E. The clastics are similar to the Wakuku well as the Senonian limestone mentioned above. Vol- beds,but the presence of late Senonianlimestone canics of this typehave yet to berecorded from clasts in theconglomerates of the WaiTuh indicate outcrop in central Seram. No estimate of thickness can that they are considerably younger than the Wakuku be made for the Wai Tuh section. The Wai Tuh beds in age. Further palaeontologicalwork is neededto are believed to rest unconformably on the metamor- date the clastics. phics. Exposures in the Wai Tuh include poorly consoli- It is tempting to compare in terms of both age and dated, dark grey, laminated siltstones, with thin strin- lithology the Wai Tuh beds with the upper part of the gers of fine sandstone. The sandstones contain large Palelo Group of western Timor (Audley-Charles et al. bronze mica flakes, abundant plant remains, load cast- 1975). The key to this correlation lies in the age of ing and flame structures. Also present are thicker, pale associated siliciclastics. If theseprove to belate green,medium tocoarse-grained, sometimes quartz- Cretaceous-Palaeocene, then the Wai Tuh beds would ose sandstones and unsorted beds of angular polymict correlate very closely with the Palelo Group of Timor. 562 M. G. Audley-Charles et al. AsinepeLimestone and earlyMiocene Limestone. In Ltd.), and appears to thicken offshore. It seems proba- the Gunung Kobipoto area, near the Wai Isal gorge, ble that much of the sub-Wahai ‘basement’ of Zillman white cliffs of limestones appear to structurally overlie & Paten (1975) in the Bula-Nief region of NE Seram themetamorphics. Float boulders suggest that both is the Salas Block Clay olistostrome. TriassicAsinepe Limestone and early Miocene cal- A low level air reconnaissance shows much of the carenites and calcirudites, similar to the Cablac Lime- northern half of easternSeram to consist of large stone of Timor, might bepresent above the landslips,described by earlier workers as a ‘soup of metamorphics. shales’(van der Sluis 1950),and the occurrence of manyscattered large blocks (between c. 0.5 and 1.0 km diameter) which closely resemble the ’fatus’ of Sdas Block Clay (olistostrome) Timor(Molengraaff 1915;Audley-Charles & Carter This takes its name from the Salas River near the N 1972). The ‘fatus’ in Timor are exotic blocks of the coast of E Seram between Bula and Nief (Fig. 2). It Bobonaro Scaly Clayolistostrome (Audley-Charles has a greyish, somewhat waxy clay matrix, only locally 1968). However, it must be emphasised that the Salas with a scaly texture, containing exotic blocks of vari- Block Clay, where studied in the field, differs from the ous types and sizes ranging up to 6 m diameter. The Bobonaro Scaly Clay in being much thinner, in lacking larger blocks are angular or subangular, mainly quartz thesame highproportion of exoticblocks, and in arenitesand siltstones, often micaceous and car- lacking the predominantly scaly waxyclay matrix of bonaceous,with interbedded shale and argillaceous the Bobonaro. The Salas Block Clay may therefore be limestones. The blockscan be identifiedwith the onlyan erosional remnant of aonce much thicker Wakuku beds of early Triassic-early Jurassic age, and deposit. are mixed with blocks of probably Triassic oolitic and The youngest member of the Nief beds below the bioclasticlimestones with acoral-brachiopod reefal Salas Block Clay is late Miocene N.17, and the oldest fauna, whichcan becompared closelywith the rocks above the Salas Block Clay are the Wahai beds Asinepe Limestone. Other blocks in this river section of earlyPliocene age, N.19. The age of theSalas include limestones of the Saman Saman type and red BlockClay is thuseither latest Miocene orearliest lutites of Albian age with a very deep water micro- Pliocene, N.18. fauna belonging to the Nief beds, as well as calcilu- tites of Palaeogeneage containing an abundant de- rived microfauna and basic volcanic clasts. Stratigraghy of the exotic manganese nodule- In the lower course of the Wai Tuh river of central bearing red mudstones Seramenormous exotic blocks of limestone (closely reminiscent of the Timor ‘fatus’) occur. These blocks On a hill at Bula is an isolated outcrop of clays with musthave been carried across the Kobipoto massif manganese nodules (reddish-brown, yellow, grey and because they have a similar lithologyto the limestones white clays, some of which contain large numbers of of the mountain ranges S of the Kobipoto range and small (up to 30 mm diameter) sub-rounded, concentri- are clearly out of place in this valley, as no other rocks cally banded, earthy manganese nodules). Other small of this type from which they could have been derived clasts includesiltstone, sandstone, calcilutite, chert, areknown. Other large exoticsseen in this valley olive-green shale and grey mudstone, quartz silt and a included a very large block of pillow lava, unknown in little whitemica. Thefauna consists of corroded, outcrop, which was upside-down. sphericalradiolaria (not replaced by calcite), white The Salas Block Clay outcrop is marked by much opaline sponge spicules, fragments of the deep-water landslipping,producing muddy exposures often co- foraminiferan Bathysiphon, a specimenof Dorothia sp. vered by grass andbamboo. The fact that previous resembling the Albian-Cenomanian D. gradata,a workers did not recognize this as a formation testifies fragment of Nodosaria, and a small corroded fragment to the difficulty of distinguishing this as a stratigraphi- of an echinoid spine. The unaltered nature of the clay cal division or structural element,which may be partly and the intense corrosion of the radiolaria suggest that explained by the predominant exotic block constituent this depositmay have been laid downclose to the being Wakuku beds,so that the frequentlandslips give compensation depth for silica at about 4000 m. The the impression that is an outcrop of Wakuku. The 2 microfauna in the associated clays without manganese main clues to its proper recognition are: (1) the pres- nodules suggests an early Cretaceous age. ence of mixed exotic blocks of widely different ages, Thestratigraphical and structural relationships of those of the same age being of widely different facies, theserocks are veryuncertain. The lack of internal and (2) thepresence locally of the scaly, waxyclay deformationdistinguishes them from the Nief beds, matrix. with which in other respects they have close affinity. The Salas Block Clay probably unconformably over- Perhapstheir particular clay compositionallowed lies the Wakuku beds at Nief. It can be identified on them to escape deformation and that they are part of the seismicreflection records (line 74-18 of AAR the Nief beds.Alternatively, they might represent a Reinterpretation of the geology of Seram 563

large exotic block within the Salas Block Clay (olis- the cherty limestone of the Triassic para-autochthon tostrome). It seems unlikely that such soft beds could andthe ‘Greywacke and GlossySlate Formation’, be part of a separate thrust sheet. which implies the dykes are post-thrusting (i.e. post- These beds are the deepest water depositsin Seram lateMiocene, N.18), (5) widespreadblack vesicular andcomparable with the manganese nodule-bearing lavas that are either post-Triassic because they locally red clays that occur as exotic blocks in the Bobonaro bake the Triassic strata, or possibly Triassic and re- Scaly Clay olistostrome of Timor, and which formed lated to the volcanics in the Wakuku beds of central on the floor of the Cretaceous Tethys ocean (Margolis Seram. et al. 1978). Apartfrom the blackvesicular lavas, all theW Seram igneous rock types are found around the north- ern end of Piru Bay, where they are associated with a Stratigraphy of the autochthon largepositive Bouguer gravity anomaly (Milsom 1977), which implies that they extend deep into the Zillman & Paten (1975) described the autochthonous crust (c. 3 km). Basic igneous rocks are also reported rocks of Seram in terms of 2 principal divisions: from the Hoamoal peninsula (Fig. 8). All the igneous rocks, except the ijolite dykes, lack Fufa Formation. Thisconsists of sands,conglomer- metamorphicaureoles and clear intrusive contacts ates,reefal and other carbonate sediments, marine against non-igneous rocks. Reported contacts are de- mudand lignite representing neritic, paludaland scribedas tectonic. The availablesketchy evidence fluviatile deposits. Its maximum recorded thickness in suggests that they form part of a thrust sheet, or more anonshore wellwas 600m; it probablythickens likely, several thrust sheets broken up by later steep offshore in the Seram Sea. Theage of the Fufa Forma- faults (Figs. 7,9). tion is Pleistocene, N.22 and N.23. It lies unconforma- There is much uncertainty about the relative struc- bly on the Wahai beds and locally oversteps onto the tural positions of the basic andultrabasic igneous Salas Block Clay (olistostrome). rocks of W Seram, where some may form the highest thrustsheet, considerably affected by post-Miocene Wahai beds. Theseare mainly composed of mud- steep faulting. The granites and cordieritegneiss of the stones and siltstones, with a maximum recorded thick- KobipotoComplex are associated with serpentinites. ness of 156 m,although they probably thicken This suggests comparison with the granulite, amphibo- offshore in theSeram Sea. Zillman & Paten(1975) lite and greenschist rocks of the Lolotoi complex of W considered theWahai beds to have been deposited Timor, which in Mollo and Mutis rest on a platform of initially in bathyal conditions, which became shallower aserpentinite (G.S.I., 1: 250,000 Geological Map towardsthe upper part of the division. TheWahai Atambua, in prep.). In Ambon, the ultrabasic igneous beds are of Pliocene and Pleistocene age (N.19-N.21), rocks appear to be overlain by the volcanics forming and are unconformable on the Salas Block Clay (olis- much of the islands of the Uliassers (van Bemmelen tostrome). Locally, afacies of the Wahai beds, contain- 1949). inga significant proportion of tuffs andre-worked Thegranites of WSeram and the volcanics of tuffs, unconformablyoverlies the high grade Ambon carry cordierite, suggesting an affinity between metamorphic rocks of the Kobipoto massif (Fig. 8). these rocks. The interpretation favoured here is that they are in situ. They may be related to deep fractures, which have penetrated the thrust sheets and the un- Igneous rocks derlyingAustralian continental basement. Emplace- ment through continental basement could have led to The majority of igneous rocks occur in W Seram. The the assimilation of countryrocks, resulting in the exposures in centraland E Seramare mainly loose acquisition of cordierite. The effusive rocks of Ambon blocks,many of whichmay be interpreted as exotic are Plio-Pleistocene in age (van Bemmelen 1949), and blocks in the Salas Block Clay olistostrome. There are a dacite at Liang, Ambon,of Pliocene age (M. Abbott, 2 exceptions: (1) minorandesite and dacite effusive pers.comm.), indicates that they are post-thrusting, rocks in the Wakuku Beds, and (2) tuffs and tuffaceous whichwould accord with their being in situ and re- siltstones of Neogeneage in theKobipoto massif, latedto the ijolite dykes of Seram.The available regarded as a facies of the Pliocene Wahai Beds (Fig. evidence suggests that these volcanics and dykes are 8). associated with a fracture system that developed after 5 main types of igneous rocks were reported from the thrust tectonics, so that these igneous rocks do not W Seram by Valk (1945): (1) granites, granite aplites appearto be related to subduction processesas- and pegmatites and quartz diorites containing cordier- sociated with the volcanic Banda Arc (Fig. 9). Pending ite, (2) peridotites(harzburgites, lherzolites anddu- more detailed field study of this complex region, in- nites, some with numerous veins of hornblende gab- terpretation of the igneous rocks can only be made in bro), (3) gabbro and diabase, (4) ijolite dykes cutting simplified and speculative terms. 564 M. G. Audley-Charles et al. Ambon and the Uliassers Imbrication in the para-autochthon The ‘basement’ of theseislands is composed of The stratigraphical and structural evidence for im- unmetamorphosed Triassicsandstones. S of Ambon brication in the Nief beds and Wakuku beds is clear these occur together with serpentinite and granite of (Figs. 3-5). The imbricate sections in the Nief beds of unknown age. Hutchison (1976) reported that at Tan- E and central Seram are remarkably similarto sections jung Seri, serpentinite overliesgranite and proposed exposed in southernTimor at Kolbano and Iliomar that mantle material had been obducted over granitic (Barber et al. 1977, fig. 3). continental crust. The critical locality was visited dur- The para-autochthonous rocks are similar in lithol- ing the ‘Banda Sea Program’ by A. J. Barber and Mr ogy, facies and faunas to those reported from Misool Suharsono.They found that veins of granitewere (van Bemmelen 1949; Froidevaux 1975), New Guinea intruded into the serpentinite and that blocks of ser- (Visser & Hermes1962; Davies & Norvick1974), pentinite were enclosed as xenoliths within the granite. NWAustralian shelf (Lofting et al. 1975; Powell If theserpentinite reached its present position by 1976). The term para-autochthonous is employed be- overthrusting, it was subsequently intrudedby granite. cause these rocks, which are interpreted on the basis The ‘basement’ is overlain by volcanic rocks forming of their lithology, fauna and microflora as Australian 2 major volcanicstructures in thenorthern part of slope and rise facies and which may be traced around Ambon. Volcanic dacite from Liang has been dated by the Outer Banda Arc, northern Australia,New Guinea K-Armethods at 4.5Ma (M. Abbott,pers. comm. andMisool (Table l), havebeen structurally dislo- 1977). The volcanic structures are extensively eroded catedfrom their basement in theOuter Banda Arc and are overlain by raised coral reefs. The volcanoes islands but can be traced towards the Australian cra- of Ambon were evidently active in Pliocene times, but ton, where they are clearly autochthonous. A case can are now no longer active. be made for interpreting the Nief beds as allochthon- The islands of Ambon, Ambelau and the Uliassers ous (Table 2). occupy the same position in relation to Seram that the Some members of this imbricated para-autochthon islands of Alor, Atauro and Wetar bear to Timor on havesuffered penetrative deformation such as de- theopposite side of theBanda Arc. As at Ambon, velopment of stylolitic cleavage and stretching of chert theseother islandswere volcanically active during nodules. Pliocene times but subsequently became inactive. Pre- The age of the imbrication in Seram can be dated as sent volcanic activity in the Banda Arcs is restricted to post-N.17on the basis of the youngestmember in- the segment between Damar and the Banda Islands. volved, and pre-N.19 because the basal unimbricated Thepresent spatial relationship between Ambon, Wahai beds overlie the imbricated Nief beds uncon- Uliassers and Seram may not represent their original formably. This dates the imbrication as late Miocene configuration. Major strike-slip faults appear to have (N.18). The imbrication in Timor was dated as early been active during the late Neogene and Quaternary. Pliocene (N.19-N.20) because the youngest rocks in- volved in the imbricated thrust sheets areof N.18 age. Structure Overthrusting of the allocbthonous elements A geological sketch map of Seram was produced by Valk(1945) for W Seram, Germeraad (1946) for The stratigraphical and topographical evidence for central Seram, and van der Sluis (1950) for E Seram, majoroverthrusting is notclear (Figs. 3-5) partly using the field notebooksand samples collected by because numerous later steep faults have displaced the Rutten & Hotz, whocarried out the field work in thrust sheets (Figs. 6,7). The intensity of these steep 1917-19.Valk, Germeraad,and van der Sluis all faults has obscured to some extent the basic overthrust inferredmajor overthrusting and structural vergence sheet structure, just as in Timor (Carter et al. 1976; towards the N, although overthrustingwas not proven. Barber et al. 1977). The principalcontributions of our1975 expedition One of the principal stratigraphical criteria confirm- have been (a) to provide sufficient stratigraphical and ing the presence of allochthonous elements in Seram is structuralevidence to provethe presence of major the distinction between 2 different limestone facies of overthrusts composed of allochthonous elements; (b) late Triassicage occurring above each other in the the recognition of intense imbrication in the underly- Manusela mountains of S central Seram (Fig. 3). The ingsection of Triassic-Miocene strata forming the higherstructural element is theAsinepe Limestone, para-autochthon, and (c) the identification of a wide- with a rich fauna of corals, brachiopods, sponges and spread SalasBlock Clay olistostrome in centraland crinoids in abioclastic shallow, warm-water facies. easternSeram. This previouslyunrecognized olistos- Immediately underlying this allochthonous element is trome was often described by earlier workers either as theSaman Saman Limestone, a calcilutite with ‘dreadful ground’, meaning deep sticky mud with scat- radiolaria and an Halobia-type planktonic fauna prob- tered boulders, or as a ‘soup of shales’. ablydeposited in adeep-water distal facies. The Reinterpretation of the geology of Seram 565

Wakukubeds, a siliciclastic flysch facies, whoseage of the Trough. They interpreted the earthquake data overlapsthe Saman Saman Limestone, also occurs in theAru regionas indicating thatthe Tarera beneath the thrust sheets. Aidoenafault zone of Irianextends westwards, This distinction between 2 Triassic limestone facies separating the Banda Arc into 2 subducting slabs, one is important in interpretingthe structure of Seram associated with the Seram Trough and the other with (Figs. 3,9) and its palaeogeographical evolution. Ear- the Timor Trough. Such a tear in the hinge region of lier workers,who found these different but coeval the Banda Arcs may be related to the curvature of the types of limestone faulted together (Valk 1945), did Banda Arcs that must be post-overthrusting and may not separate them into differentstructural elements, be associated also with extensional tectonics reported partly perhaps because they were unable to distinguish in the Aru Trough (Bowin et aZ., in press). It is also the important effects of the post-thrusting steepfaults. possible that this extensionalregime led to the de- 2 other examples of allochthonous elements having velopment of the Weber Deep after the collision be- a similar age to different facies of the para-autochthon tween the volcanicarc and theAustralian occur in Seram.The post-Senonian(possibly and is also associated with the post-collision acquistion Palaeocene)Wai Tuh siliciclastics in theKobipoto of curvature by the arc. Complex contrast with the bathyal and distal facies of the para-autochthonous Nief beds that range from late JurassicMiocene. The shallow water grainstones and Gravity data for Seram, calcirudites of theCablac-type limestone of early Ambon and Uliassers Miocene age, also found in the Kobipoto massif, con- traststrongly with the earlyMiocene bathyal and Milsom (1977) pointed out that the Bouguer anomaly distal facies of the para-autochthonous Nief beds. map of Seram is similar to that of Timor (Chamalaun Thearguments for interpreting the allochthonous et al. 1976; Milsom & Richardson1976). In both elements as having been derived from the Asian conti- islands the gravity contours follow the long axes, with nental margin have already been set out. the Neogene-Quaternarybasins apparently well defined on the Australian-New Guinea side and with high values and steep gradients along the Banda Sea Earthquake data for coasts. More gentle gravity gradients are found on the Seram-Ambon area Australian-New Guinea side of the Outer Banda Arc islands. The steep gradients of the Banda Sea side of The earthquake map (Hamilton 1974) shows 2 promi- these islands is interpreted as evidence of the Outer nentfeatures in theSeram area: (1) there is avery Banda Arc islands being underlain by Australian con- widescatter of shallowfocus earthquakesextending tinental crust which extends to their Banda Sea coasts. c. 200 km from the N coast of Seram to the submarine The Piru Bay local gravity high corresponds to the ridges S of Ambon.No intermediate or deep focus presence of ultrabasicigneous rocks, including shocks arerecorded in this regionand the Benioff harzburgitesand lherzolites (Valk 1945), probably zone cannot be recognized below 100 km; (2) - extending down to at least 3 km depth (Milsom 1977). quakesare concentrated throughout central Seram Thesedense rocks may beassociated with major and along the N coast of E Seram, with very few in W, strike-slip faultingwhich post-dates the overthrusting. E and S Seram. One possible explanation of this distribution is that most of theearthquakes are related to important strike-slip faults. ERTS imagery and airphotos show a ACKNOWLEDGMENTS.Our expeditions (1975-76) were spon- distinctive trapezoidalpattern of complexlarge and sored by the Indonesian Geological Survey, with Dr H. M. S. smallscale lineaments, whichin otherparts of the Hartono acting ascoordinator. Financial support was pro- vided by the Natural Environment Research Council and BP world is usuallycharacteristic of transcurrent fault Petroleum Development Ltd. Local transport and base camp terrains. Such a transcurrent fault system couldwell be facilities in Seram, as well as general assistance in Ambon, associated with the recent acquistion of curvature of were provided by AAR Ltd. We are particularly indebted to this easternpart of theBanda Arc (Fitch 1972). Mr G. Thomas andDr T. D. Adams of BP Petroleum Cardwell & Isacks (1978) described 2 underthrusting Development Ltd.and Mr R. Patenand Mr G. Evans of mechanisms for earthquakes in the region of Seram S AAR Ltd.

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