Stratigraphy of Western Irian Jaya: D.S. D, B. Dow R [PDF]

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© IPA, 2006 - 12th Annual Convention Proceedings, 1983



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PROCEEDINGS INDONESIANPETROLEUM ASSOCIATION Twelfth Annual Convention,, June 1983



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THE STRATIGRAPHY OF WESTERN IRIAN JAYA P.E. Pieten*, C.J. Pigram* D.S. Trail*, D,B. Dow* N. Ratman**, R. Sukamto** INTRODUCTION From 1978 to 1982 the Geological Research and Development Centre of the Indonesian Department of Mining and Energy, and the Australian Bureau of Mineral Resources have collaborated in the Irian Jaya Geological Mapping Project. This is a co-operative project which carried out the systematic 1:250,000 scale geological map ing of western Irian Jaya, as far east as longitude 136B30’ East (Figure 1). The pre-existing stratigraphy of this region was established by geologists of the Nederlandsche Nieuw Guinee Petroleum Maatschappij and is described by Visser and Hermes (1962). The detailed and painstaking mapping wried out by these geologists between 1935 and 1960 in conditions of great hardship, produced a highly reliable stratigraphy of the area under consideration. However, systematic mapping, albeit at 1:250,000 scale, requires a regional rationalisation of the stratigraphy which is not essential in petroleum exploration, and we have also had to correct the common tendency to assume that biostratigraphic units, identified by their fossil content, can be mapped and classified as if they were lithostratigraphic units defined by their constituent rock types. Dr. Alan Lloyd has provided invaluable assistance in the considerable problem of correlating the Visser and Hermes time sub-divisionwith standard Tertiary stages. The stratigraphic units employed in the new 1:250,000 scale geological maps of western Irian Jaya are listed and described below. The nomenclature is summarized in tables and the relationships of the various units are displayed in several time-space diagrams. A synthesis of the stratigraphy is attempted in a summary geological history of western Irian Jaya, and a note is added on the petroleum potential of the region. We acknowledge the assistance of all personnel who have worked with the Irian Jaya Geological Mapping Project, and we are grateful that the Indonesian Petroleum Association has provided a forum at which we could discuss our emerging results. It is a pleasure to acknowledge the encouragement and unfailing co-operation of Mr. Hartono, Director of the Indonsian Geological and Development Centre, who has permitted publication of this paper. Permission of the Director, Bureau of Mineral Resources,



* Bureau of Mineral Resources, Camberra, Australia ** Geological Research and Development Centre, Bandung, Indonesia .



Australia, has also been obtained for publication by the Australian authors.



REGIONAL GEOLOGICAL SETTING The mainland of Irian Jaya and Papua New Guinea may be subdivided into three east-trending zones (Figure 2b) which differ characteristically in stratigraphic, tectonic and magmatic history. A northern Oceanic Province of ophiolite and island-arc volcanics is separated from a Continental Province with sediments overlying a relatively stable basement by a Transition Zone with strongly deformed and regionally metamorphosed rocks. The transition zone forms a belt exposed along the north flank of the Central Range and is separated from the other provinces by major thrust faults, and transcurrent faults. The distribution and contrasting geology of the provinces is the result of interaction between the Australia-India and Pacific plates which probably dates back to the Early Jurassic. Following cessation of island-arc volcanism in early Miocene times, a major orogeny has resulted from collision of the Australian continent with an island arc overlying the Pacific plate and from continued convergence after collision. This relatively simple zonation in the mainland is not easily applied further west in the Birds Head and Birds Neck. This region is an amalgamhtion of widely diverse terrains with oceanic as well continental affinities and with distinct geological histories. The terrainsare juxtaposed along sutures which are commonly recognised as major faults. Fault-bounded fragments of ophiolite, Paleogene islandarc volcanics and post-volcanic sediments were mapped on Waigeo, Batanta, Supiori, Biak and Yapen Islands and in the Tosem and Arfak Mountains. These allochthonous blocks fall into the Oceanic Province and probably made part of an original continuous magmatic belt which was disrupted by transcurrent faulting during late Miocene and Pliocene time, The central and southern Birds Head has a basement of folded and regionally metamorphosed Siluro.Devonian turbidites and the western Birds Neck area is almost certainly also underlain by basement of continental affinities. However, each of these terrains is characterized by a unique stratigraphy, and history of deformation of the sedimentary cover. Misool Island, Onin and Kumawa Peninsulas are also categorized with the Continental Province. The rocks of the Tamrau Mountains, the Wandamen Peninsula and Wondiwoi Mountains in the east Birds Neck



230



and Gf the Weyland Mountains and northern Central Range are strongly and complexly deformed by folding and faulting and have been subjected to regional metamorphism. These terrains are lumped together in the Transition Zone. Lithostratigraphic units confined t o the Sorong and Ransiki Fault Zones, although not metamorphosed are also grouped in this Zone. STRATIGRAPHY Distribution of the stratigraphic units is best displayed on the new 1:250,000 scale preliminary geological maps of trian Jaya, available from the Geological Research and Development Centre in Bandung. The Oceanic Province, floored by oceanic crust, and the Continental Province, founded on continental crust are stratigraphically distinct in pre-Miocene units at least. A Transition Zone between the two also contains several unique stratigraphic units though there is considerable overlap in places and it contains many rock types clearly derived from the Continental Province in particular (Figure 2b). CONTINENTAL PROVINCE In the Continental Province metamorphic basement is exposed in the Birds Head and Misool Island, but in the Central Ranges the exposed pre-Carboniferous sediments are not metamorphosed. A platform sequence of Late Paleozoic and Mesozoic clastic sediments is overlain by Tertiary limestone and young Cainozoic molasse deposits in the Central Range and Birds Neck. In the Birds Head, the Mesozoic sequence is t h n , interrupted and locally absent. On Misool Island a unique and almost continuous sequence of deep-water and shallow marine sediments extends from Triassic times to the present-day. he-Carboniferous Basement Kemum Formation The Kemum Formation (Visser SC Hermes, 1962) forms a basement block in the central part of the Birds Head where it is bounded by the Sorong Fault Zone to the north and the Ransiki Fault Zone to the east (Figure 2a). To the south and southwest, rocks of Late Paleozoic, Mesozoic and Cainozoic age, overlie the basement block with angular unconformity . The lower contact of the Kemum Formation is not exposed and the unit has a minimum thickness of a few thousand metres. The age of the unit is based on sparsely distributed Silurian graptolites and Devonian ostracods. A K-Ar age of about 1250 my for a granodiorite pebble in a meta-conglomerate indicates a Precambrian provenance. The Kemum Formation is intruded by Late Carboniferous and Permo-Triassic plutons of the Anggi Granite, and by dykes of basaltic or andesitic composition yielding Pliocene K-Ar ages (Figures 3 & 4; Table 1). The unit consists dominantly of low-grade metamorphic rocks comprising thinly interbedded pelitic and psammitic layers with sedimentary textures and structures typical of distal turbidites. The main rock types are slate, slaty shale, argillite and metawacke; meta-arenite and metaconglomerate



are less common. Thin intercalations of recrystallised limestone and dykes or sills of metavolcanics are rare. A much less widespread sandy facies consists of locally calcareous quartz-rich metawacke and meta-arenite, and siliceous slate or argillite. The rocks are deformed by tight to isoclinal mesoscopic and megascopic folds, and are regionally metamorphosed to the chlorite zone. High-temperature metamorphism overprints the low-grade metasediments immediately adjacent to the Ransiki Fault Zone, giving rise t o knotted slate, phyllite, schist, gneiss and gneissic granite ranging from the biotite zone through the andalusite t o the sillimanite zone of metamorphism. These rocks are intruded by swarms of veins, dykes and other small bodies of granite and pegmatite. The greatest density of granitoid bodies occurs in the sillimanite zone indicating that the plutonics were formed by anatexis of the high-grade metamorphics: Based on stratigraphic relationships, the regional deformation and lowgrade metamorphism occurred some time during the Devonian or Early to Middle Carboniferous. The age of the high-temperature overprinting metamorphism straddles the boundary of the Permian and Triassic systems, as indicated by K-Ar dating of metamorphic minerals in gneissic granites. Aisasjur Formation The Aisasjur Formation (Pigram & Sukanta, 1982) is only locally exposed in the Birds Head. The unit is deformed like the Kemum Formation, but evidence for pervasive low-grade metamorphism is not clear. It is overlain with an angular unconformity by Middle or Late Carboniferous basal sediments of the Aifam Group, and is possibly younger than the Kemum Eormation. The thickness is unknown and no fossils have been found in it (Table 1). The rocks of the Aisasjur Formation coniprise micaceous wacke and arenite, with clasts of quartz, low-grade metasediments and fledspar, interbedded with siltstone, shale and slate. The composition of these rocks indicates deposition as proximal turbidites and a provenance with extensive granitoids. Modio Dolomite The Modio Dolomite (Pigram & Panggabean, 1983) crops out beneath the Permo-Carboniferous Aiduna Formation in the southern fall of the western Central Ranges of Irian Jaya. A Silurian to Early or Middle Devonian age is indicated by the general aspect of a poorly preserved conodont fauna. The section is incomplete and the minimum thickness is about 1000 m. The unit consists of well-bedded dolostone and dolomite limestone with chert and pyrite nodules in places. Shale, siltstone and calcareous quartz sandstone occur near the t o p (Figure 7; Table 1). The Modio Dolomite may be stratigraphically equivalent to dolomite, dolomitc limestone and minor clastic sediments at total depth in the ASM-I well offshore in the Arafura Sea. This dolomitic sequence is overlain by Permian carbonaceous sediments of the Aifam Group. The Modio Dolomite resembles the Mangguar Formation exposed along the southeast coast of the Birds Neck, and is also similar to



23 1 the Brug Formation described by Visser and Hermes (1962) from eastern Irian Jaya. The Brug Formation, however, is reported to grade into the overlying Mesozoic Kembelangan Group. Tuaba Formation The Tuaba Formation (new name) consists of highlyindurated, well-bedded mudstone, shale, quartz siltstone and quartzite exposed along the south flank of the Central Range on the eastern boundary of the area mapped(Figure 7;Table 1).Although they are barren, and stratigraphic relationships are obscured by faulting, it is thought that these sediments occur beneath the Aifam Group. The unit probably extends eastwardsalmost to the Papua New Guinea border and may be correlated, with the ‘overthrust mass’, ‘Lower Otakwa formation’, and ‘Simpang series’ as mapped by the NNGPM geologists (Visser and Hermes, 1962). The rocks of this unit resemble some of the undifferentiated Paleozoic and Mesozoic sediments in the Birds Neck. However, stratigraphic correlations are highly speculative as the sediments in the Birds Neck are strohgly deformed and also unfossiliferous. Farther east along the southern slopes of the Central Range, Visser and Hermes record pebbles of limestone with tabulate corals of Late Silurian age, and of micaceous or calcareous sandstone and sandy limestone with Devonian brachiopods, bivalves and tabulate’ corals. The pebbles may be reworked from Pliocene and Pleistocene conglomerate. In the same area silicified basaltic volcanics and dark limestone of the Awitagoh Formation, .and dark pelitic and dolomitic rocks of the Kariem Formation were placed tentatively in the Cambrian by Visser and Hermes (1962).



Lip Metamorphics The Ligu Metamorphics (Pigram et al., 1982) crop out along the south coast of Misool Island. They consist of slate and phyllite with minor metasandstone, argillite, quartzite and greywacke. Thickness and age are unknown (Figures 4 & 5; Table 1). PennoCarboniferous Sediments Aifam Group The Aifam Group was defined by Pigram and Sukanta (1982) who upgraded the original definition of the Aifam Formation of Visseg and Hermes (1982). The subdivision of the Group and the names applied to each of the formations are shown inTable 5. The type area for the group is the Aifam River, a tributary of the Aifat (Kamundan) River, in the central Birds Head. The Aifam Group crops out in the Birds Head, southern Birds Neck, along the southern margin of the Central Range and is known from a few petroleum exploration wells. In the Birds Head region the Aifam Group crops out along the south side of the Warsamson Valley, and as a belt extending eastwards from the Aifat River to the Mios River; subsurface it is known in the Klamogun, Klawilis 1, West Klamono, Sele 39, Puragi 1, TBE-1 and P.T. 371-420 wells. In the Birds Neck the Aifam Group is restricted to thin metamorphosed slivers along the west flank of the Wondiwoi



Mountains; it is also known from the Kembelangan 1 well and possibly in the ASF-I well. Along the southern flank of the Central Range the group is found throughout the Charles Louis Mountains and southeast of the Wissel Lakes(Figures3,4,6 &7). In the Warsamson Valley the Aifam Group is undifferentiated and consists of a basal arkose overlain by well-bedded quartz sandstone, calcareous shale and shaley limestone in turn overlain by black shale. The group appears to rest on the Early Carboniferous Melaiurna Granite. However, a sample of limestone in float yielded thelodont fish scales of Devonian aspect (Young and Nicoll, 1979). The Aifam Group in this outcrop is overlain unconformably by the Eocene Faumai Limestone, as it is in the Klawilis, West Klamono, Klamogun and Sele 39 wells: the base of the group has not yet been penetrated by an exploration well. In the central Birds Head the Aifam Group is divided into three formations (Table 1). The lowest is the Aimau Formation and consists of basal thin red conglomerate, sandstone and shale with silicified wood, overlain by a sequence of well-bedded siliceous sandstone and greywacke interbedded with shale, siltstone and grey limestone. The overlying Aifat Mudstone consists of black calcareous mudstone with abundant concretions, minor dirty limestone and rare thin quartz sandstone beds. The uppermost Ainim Formation consists of interbedded carbonaceous silty mudstone, quartz sandstone, greywacke and siltstone, and contains coal seams up to 1 m thick. The Aifam Group rests unconformably on the Siluro-Devonian Kemum Formation and is both paraconformably and disconformably overlain by the Triassic-Jurassic Tipuma Formation. In the southern Wondiwoi Mountains, the metamorphosed Aifam Group consists of quartzite, quartzose metasandstone, conglomerate and marble. The Aiduna Formation is the only unit in the Aifam Group along the southern fall of the Central Range. It consists of well-bedded feldspathic and micaceous lithic sandstone interbedded with carbonaceous shale and siltstone, and contains a little fossiliferous biocalcarenite, polymict conglomerate and coal. The Aiduna Formation appears to rests paraconformably on the Siluro-Devonian Modio Dolomite and is overlain conformably by Triassic-JurassicTipuma Formation. The maximum thickness of the Aifam Group is 2950 m in the type section. The Aifam Group ranges in age from Middle Carboniferous to Late Permian at the type locality. Numerous fossils throughout the group include silicified wood, plant fossils, conodonts, corals, bryozoa, brachiopods, ammonoids, fusulinids, crinoids and a single trilobite. Good reservoir rocks are not present in the Aifam Gr,oup, but source rocks may occur in the Aifat Mudstone, Ainim and Aiduna Formations. Paleozoic Granitoids The Late Paleozoic to Early Mesozoic granitic rocks generally occur in the Transition Zone, between the Continental and Oceanic Provinces. The Anggi Granite Suite, however, is emplaced in the Kemum Formation of the Continental Province in the eastern Birds Head, close to the Ranslki Fault Zone.



232 Anggi Granite Suite The Anggi Granite Suite (new name) comprises a number of plutonic bodies in the eastern part of the Birds Head where they are emplaced in the Kemum Formation (Figure 4; table 1). The plutons show little variation in composition and texture and consist mainly of medium-grained biotite and biotiete-muscovite granite, with late-phase aplite and pegmatite dykes and veins along the margins. The contact with the Kemum Formation is partly intrusive and partly fault-controlled, with contact aureoles developed in places. Four K-Ar ages on biotite (l), muscovite (1) and hornblende (2) range from 225 my to 243 my (latest Permian to earliest Triassic); two other granitoids from the sandy facies of the Kemum Formation gave K-AI biotite ages of about 295 my (Late Carboniferous). The Anggi Granite Suite is interpreted as S-type granite formed by the partial melting of pelitic and psammitic metasediments. Triassic-Early Jurassic Sediments Tipuma Formation The Tipuma Formation was defined by Visser and Hermes (1962): It crops out in a narrow continuous belt extending from the central Birds Head along the southern margin of the Kemum Block through the Lina Mountains, to the east coast of the Birds Head and Rumberpon Island (Figures 3-7; Table 1). In the Birds Neck the Tipuma Formation crops out along the west side of the Wondiwoi Mountains and around Lake Jamur. To the southeast the Tipuma Formation crops out throughout the Charles Louis Mountains and around the Wissel Lakes. It has also been reported in the Puragi 1, Kembelangan I , and ASF-I expIoration wells and possibly in TBE-1. The Tipuma Formation is a distinctive suite of red sediments. In the Birds Head and on Rumberpon Island it consists of thick-bedded, red or locally green silty mudstone, minor micaceous quartz greywacke, sandstone and limestone. Mottling and bleached spots or horizons are common. In the Wondiwoi Ranges of the southern Birds Neck the Tipuma Formation is metamorphosed to red- and green slate, phyllite and quartzite. Southeast of Lake Jamur, in the Charles Oouis Mountains and around the wissel Lakes the Tipuma Formation consists of well-bedded red or rarely green shale, red, green, grey or white sandstone and minor conglomerate. Bleached spots and horizons are common. The sandstones may be micaceous or feldspathic. South of Wissel Lakes a tuff horizon, volcanoclastic sandstone and minor limestone occur. Maximum thickness of the Tipuma Formation ranges from 300 m in the Birds Head to 500 m in Kembelangan 1 well. The Tipuma Formation rests partly conformabIy and partly disconformably on the Aifam Group and is overlain paraconformably by the Jurassic-Cretaceous Kembelangan Group. No fossils are known from the Tipuma Formation. It is usually assigned a Triassic to Early Jurassic age because of its stratigraphic position between the Permo-Carboni-



ferous Aifam Group and Middle Jurassic to Cretacous Kembelangan Group. The lack of organic matter and the highly oxidised state of the shale makes the Tipuma Formation an unlikely source rock. Some sand bodies in the Central Range may have reservoir potential. Keskain Formation The Keskain Formation was defined by Pigram et al. (1982). It crops out along the southern side of Misool Island and on a few adjacent small islands. It consists of rhytmically-bedded black mudstone, quartz sandstone and rare thin dark grey limestone. The formation is at least 1000m thick (Figures 4 & 5; Table 1). The Keskain Formation rests unconformably on the Ligu Metamorphics and is overlain unconformably by the Late Triassic Bogal Limestone. The age range of the formation is not known but Middle to Early Late Triassic fossils (Daonella sp., Halobia sp. and a single ceratitic ammonoid) have been found at the top of the sequence. The Keskain Formation may have some potential as a source rock, but has no potential as a reservoir. Bogal Limestone The Bogal Limestone was defined by Pigram et al. (1 982). It is only known in outcrop from the east part of Misool Island and a few small nearby islands. It may be present at the base of the TBJ-1 well,(Figures 4 & 5; Table 1). It consists of grey nodular coralgal limestone and biocalcarenite, which are highly fossiliferous (brachiopods, bivalves, corals) and indurated, together with minor sandy calcarenite and marl. The formation rests unconformably on the Keskain Formation and is unconformably overlain by the Fageo Group. It is Norian (Middle Late Triassic) in age. The Bogal Limestone may have some reservoir potential.



Jurassic-CretaceousSediments Kembelangan Group The Kembelangan Formation was originally defined by Visser and Hermes (1962) and raised to group status by Pigram and Sukanta (1982). The names applied to each of the formations within the group are shown in Table 6. The Kembelangan Group crops out throughout the eastern Birds Head, Birds Neck and Central Range. In the Birds Neck and eastward into the Central Range, where it from the Aifat River in the central Birds Head to the east coast. It is found on Rumberpon, Mios Waar and Yop Islands and as discontinuous outcrops throughout the Birds Neck and eastward into the Central Range, where it is well exposed around the Wissel Lakes. The Kembelangan Group has been penetrated in Puragi 1, TBE-1, Besiri 1, ASF-1, ASM- 1, Kembelangan 1, Rawarra 1, and Wasian 1 wells (Figures 3-7). In the Birds Head the Kembelangan Group contains only the Jass Formation @gram & Sukanta, 1982) which consists of black to brown partly calcareous and micaceous mudstone, lithic sandstone, muddy sandstone and limestone,



233 with a little quartz sandstone, and quartz or polymictic conglomerate. The Jass Formation was intersected in the Puragi 1, TBE-1, Rawarra 1 and Wasian 1 wells. The maximum thickness is approximately 400 m. The Jass Formation rests disconformably on the Tipuma Formation and is conformably overlain by the New Guinea Limestone Group. It ranges in age from Early to Late Cretaceous. In the Birds Neck the Kembelangan Group is exposed in the cores of tight anticlines of the Lengguru Fold Belt. In the west and centre the group consists of alternating sandstone and mudstone which are progressively metamorphosed in an eastward direction. Along the eastern coast of the Birds Neck and in the islands offshore in the Transition Zone between Continental and Oceanic Provinces, the Kembelangan Group is dominated by mudstone which has also been metamorphosed to slate. In the west the basal unit of the Kembelangan Group is the Kopai Formation* of Middle to Late Jurassic age. It consists of a basal conglomerate, well-bedded calcareous black shale and black limestone. In the centre, the Kopai Formation is metamorphosed to black slate, marble and metaconglomerate. The overlying Woniwogi Sandstone of Late Jurassic to Early Cretaceous age consists of thick-bedded, glauconitic and partly micaceous orthoquarzite with thin shale beds near the top. Eastwards this formation has been metamorphosed to micaceous quartzite with minor slate. The overlying Early to Late Cretaceous Piniya Mudstone consists of micaceous, glauconitic and partly calcareous mudstone, muddy glauconitic sandstone and siltstone. To the east it is metamorphosed to black slate. The youngest formation is the Ekmai Sandstone which consists of massive to thick-bedded quartz sandstone and siltstone, glauconitic sandstone, carbonaceous sandstone, siltstone and minor mudstone; to the east it also has been metamorphosed to quartzite and slate. Along the eastern side of the Wondiwoi Mountains and on Yop and Mios Waar Islands, the Jurassic to Cretaceous Kembelangan Group consists of black slate with minor micaceous quartzite. Further north on Rumberpon Island and onshore, opposite the island, the group consists of black, partly calcareous mudstone and minor quartz sandstone. In three exploration wells in the Birds Neck it is overlain conformably by the New Guinea Limestone Group, but the contact between the base of the Kembelangan Group and the underlying Tipuma Formation is not seen in the Besiri-1,ASF-1 or Kembelangan 1 wells. In the Central Range around the Wissel Lakes, the Kembelangan Group consists of alternating sand and shale in the south and a sequence dominated by mudstone and partly metamorphosed in the north, largely in the Transition Zone between the Oceanic and Continental Provinces. The same nomenclature that was applied to the formations in the Birds Neck has been used in the southern region. The Middle to Upper Jurassic Kopai Formation consists of light grey quartz sandstone which is argillaceous, glauconitic and calcareous, interbedded with black to grey



*



The formation of the Kembelangan Group were defened by Pigram and Panggabean (1983) in the Western Central Range. This subdividion was adopted by Robinson et al. (1982, 1983) in maps of the Birds Neck.



silty mudstone, minor conglomerate, calcarenite, calcilutite and greensand. The overlaying Woniwogi Sandstone, which is probably Late Jurassic to Early Cretaceous, consists of well-bedded to massive glauconitic, pyritic orthoquartzite (micaceous in part) with minor siltstone and thin-bedded black mudstone near the top. The Woniwogi Sandstone is overlain by Early to Late Cretaceous Piniya Mudstone which consists of thin-bedded to massive, partly calcareous and glauconitic black mudstone, minor thin glauconitic quartz sandstone, muddy siltstone and red weathering marlstone. This formation was also penetrated in the ASM-1 well offshore. The overlying Late Cretaceous Ekmai Sandstone consists of greenish grey, thick-bedded to massive, well sorted quartz sandstone, which may be glauconitic, carbonanceous or calcareous. Locally, the interbedded shale is green or red. The maximum thickness of the Kembelangan Group in the southern region is 1900 m. To the north of the Wissel Lakes the Kembelangan Group is dominated by Middle Jurassic to Cretaceous black mudstone, with minor quartz sandstone and thin limestone. Farther north it passes into slate in the Transition Zone. The Kembelangan Group is thought to have considerable petroleum potential. The Kopai Formation, Piniya Mudstone and mudstone of the undivided Kembelangan Group are potential source rocks and the Woniwogi and Ekmai Sandstones may have some reservoir potential. Fageo Group The Fageo Group was defined by Pigram et al. (1982). The names of the formations making'up the group and their correlation with the Kembelangan Group are shown in Table 1. The Fageo Group crops out on Misool Island and on small islands nearby; it may be present in TBJ-1 well (Figures 4 & 5). In the Misool Archipelago the Fageo Group comprises three formations. The basal Yefbie Shale of Early to Middle Jurassic age consists of grey calcareous shale and siltstone with thin quartz sandstone, pebbiy conglomerate and limestone conglomerate at the base. Very large concretions and carbonaceous material are common in the lower part of the section. The Yefbie Shale is about 80 m thick and contains many ammonites, belemnites, bivalves and brachiopods. It is overlain by the Upper Jurassic &mu Formation which consists of well-bedded grey, silty algalmat limestone, marlstone and thin shales. The formation is approximately 80 m thick and contains abundant belemnites, ammonites and bivalves. The Demu Formation is overlain by the Upper Jurassic Lelinta Shale which consists of grey, partly calcareous shale that weathers to small pieces. It is approximately 100 m thick and is also highly fossiliferous. In the TBJ-1 well the Fageo Group consists of a basal calcareous shale and detrital limestone with sandstone and a little coal, overlain by interbedded shales and limestone. The Fageo Group rests unconformably on the Bogal Limestone and is overlain conformably by the late Late Jurassic to early Late Cretaceous Facet Limestone Group . The Fageo Group may have some pQtentia1as a petroleum source rock.



234 Facet Limestone Group The Facet Limestone Group was defined by Pigram et al. (1982). The names of the formations within the group are shown in Table 1:The Facet h e s t o n e Group crops out in southern Misool Island and on several small islands to the southeast. It is present in the TBJ-1 well and may be present at total depth in the CS-1 well and the TBF-1 well (Figures 4 & 5). In the Misool Archipelago the lower unit of the Facet Limestone Group is the Late Jurassic to Early Cretaceous Gamta Limestone which consists of well bedded, white calcilutite with chert nodules, separated by very thin calcareous shale horizons in the lower part and by chert beds in the upper part. The formation is about 80 m thick and is highly fossiliferous with belemnites, ammonites and bivalves. The upper unit is the Early to Late Cretaceous Waaf Formation which consists of thin-bedded maroon tuffaceous calcilutite, shale and marlstone, some grey nontuffaceous shale and red chert. The formation is approximately 80 m thick and contains the large bivalve Inoceramus sp., and formanifiera. The Face Limestone Group rests conformably on the Fageo Group and is overlain conformably by the Late Cretaceous Fafanlap Formation. Fafanlap Formation The Fafanlap Formation was defined by Pigram et al. (1982) (Table 1). It crops out on Misool Island and on several islands to the east and may be present in TBF-1 and CS-1 wells (Figures 4&5). The lower part of the Late Cretaceous Fafanlap Formation consists of well-bedded, grey to brown calcareous siltstone, greywacke and shale with maroon, tuffaceous shale and siltstone at the base. The top 50 m of t.he formation consist of thin-bedded nodular limestone. The formation is at least 200 m thick and contains rare bivalves (Inoceramus sp.) and small ammonites. The Fafanlap Formation rests conformably on the Facet Limestone Group and is overlain conformably by the Late Cretaceous to Paleocene Daram Sandstone. Puragi Formation In the Puragi 1 and Wasian 1 wells iii the southern part of the Birds Head a sequence of dolomite, limestone, and anhydrite-bearing shale and sandstone, named by us the Puragi Formation, underlies the New Guinea Limestone and in Puragi I overlies Kembelangan Group sediments of Late cretaceous age (Visser & Hermes, 1962) (Table 2). This sequence is 320 m thick in Wasian 1 well. It probably represents a sebkha or evaporitic back reef facies, formed in a shallow, periodically drying lagoon in an arid climate (Figures 3 & 4). Anhydrite has also been recorded within the lower part of the New Guinea Limestone Group, south of the Central Range. New Guinea Limestone Group The New Guinea Limestone Group (Visser & Hermes, 1962) embraces the several limestone formations deposited



in western Irian Jaya between Paleocene and middle Miocene times and includes a few thin clastic deposits. The group was defined by Visser and Hermes as consisting only of shoal limestone of Tertiary and uppermost Cretaceous age. We have widened the definition to accommodate the pelagic Imskin .Limestone of the Birds Neck, which is clearly a lateral equivalent of the shoal limestone, and other Tertiary limestones not included by Visser and Hermes. The constituent units of the group were all named "Formations" by Visser and Hermed, regardless of constituent rock type. Where the dominant rock type is limestone, we have replaced "Formation" in the name with "limestone", to convey immediately to the user the composition of the unit. Waripi Formation The Waripi Formation (Visser & Hermes, 1962) crops out in the western mountains of the Central Range from where it extends westwards into the southern extremity of the Birds Neck (Table 2; Figure 7). The formation consists of well-bedded, sandy oolitic calcarenite and biocalcarenite, talcareous quartz sandstone and red-brown oolitic biocalcarenite. The limestone is commonly dolomitic and in many places contains miliolid foraminifera. The maximum estimated thickness of the Waripi Formation is 700 m in the upper Baupo River; Visser and Hermes (1962) estimate a thickness of 380 m at the west end of its distribution range but state that the formation thins and disappears in eastern Irian Jaya. The Waripi Formation contains no age-diagnostic fossils but lies with gradational contacts between the Eocene Yawee Limestone and Late Cretaceous Ekmai Sandstone. The Waripi Formation is probably of Paleocene age, but may partly be a lateral equivalent of the Ekmai Sandstone. The clastic detritus in the formation was probab!y derived from the south; the oolites suggest a shallow carbonate bank and the formation was probably deposited on a very shallow shelf. The Waripi Formation may form a suitable reservoir for petroleum, if capped. The presence of small quantities of mudstone within the unit may provide local seals, and Visser & Hermes (1962) note-that thick sandy clay forms the top. of the formation east of the Steenboom River. Yawee Limestone The name Yawee Limestone was applied by Pigram and Panggabean (1983) to the undivided part of the New Guinea Limestone Group overlying the Waripi Formation in the southern part of Irian Jaya'(Table2). The Yawee Limestone extends from the eastern limit of the area mapped (Figures 6 & 7) westwards to the southern part of the Birds Neck. The Yawee Limestone consists of well-bedded to massive calcarenite, micrite and calcirudite; much of it is biocalcarenite and biomicrite. A little chalk (calchtite), oolitic calcarenite and sandy calcarenite occur in a few places. The base of the limestone commonly contains sub-angular quartz grains. The maximum thickness of the limestone is



235



approcimately 1200 metres. The age ranges of foraminifera from Yawee Limestone fall into two distinct clusters : - an older cluster corresponding to the Eocene (Ta2 to Tb), and - a younger cluster ranging from late Oigocene to middle Miocene (lower Te to lower Tf). The gap in early Oligocene times probably represents a hiatus iq deposition. The earlier cluster corresponds to the fauna of the Faumai Limestone. The Yawee Limestone has a gradational contact with the Waripi Formation, and is conformably overlain by the Buru Formation, generally by mudstone. It is a platformfacies limestone deposited in a shelf environrnenti no reefs were observed in it. The Adi Member of the Yawee Limestone was defined by Panggabean (1982). It is a thin and discontinuous, little known unit, probably composed of sandstone and mudstone with a little lignite, and ranges up to 150 m in thickness. It is believed to be late Oligocene to early Miocene and probably represents the drop in sea-level of late Oligocene times p a i l & Mitchem, 1979).



Faumai Limestone The Faumai Limestone (Faumai Formation of Visser & Hermes, 1962) can be recognized in outcrop only in the eastern part of the Birds Head, where it is overlain by the clastic S i r e Formation and is separated by it from the later, Miocene part of the New Guinea Limestone Group. The outcrop of the Faumai Limestone extends from the eastern side of the Ayamaru Plateau eastwards to the coast of Cenderawasih Bay (Table 2; Figures 3 & 4) The Faumai Limestone is a well-bedded arenacous limestone consisting of calcarenite which is commonly muddy. It is about 250 m thick. The limestone represents carbonate bank and shoal deposits. It contains abundant larger foraminifera which date it as Ta to Tb or middle Eocene to Oligocene. Lateral equivalents of the Faumai Limestone are present in h e New Guinea Limestone Group throughout western Irian Jaya, e.g. in the Yawee Limestone, but the limestone is recognized as a lithostratigraphic unit only in the Birds Head where it is capped by the clastic Sirga Formation. The Faumai Limestone rests on the Jass Formation probably disconformably, it is also disconformable on the Aifam Group in the upper Aifat River and in Klawilis 1 and West Klamono 2 wells. Sirga Formation The outcrop of the Sirga Formation (Visser & Hermes, 1982) extends from the Ayamaru Plateau across the 'northern margin of the Bintuni Basin for over 150 km to the headwaters of the Muturi River, 20 km west of the shore of Cenderawasih Bay. It is also found subsurface in the Salawati Basin west of the ayamaru Plateau (table 2; figures 3.8~4). The predominant rock types in the Sirga Formation range from siltstone and mudstone in the west and south to quartz sandstone and conglomerate in the north and east. It appears to have been derived from a landmass



occupying the present-day outcrop of the Kemum Formation, and to form a lens-like sheet thinning both north and south from a rnaxhum thickness of 200 m in the Aifat River. Large and small foraminifera in the Sirga Formation yield an early Miocene age. The formation is probably transgressive and deposited in shallow water as sea-level rose ' after the world-wide drop recorded by Vail and Mitchem (1979) late Oligocene times. The Sirga Formation lies conformably on the Faumai Limestone and disconformably on the Aifam Group near the Ayamaru Plateau. It is conformably overlain by Kais Limestone or, in some exploration wells in the Salawati Basin, by Klamogun Limestone.



hskin Limestone The Imskin Limestone (Imskin Formation of Visser & Hermes, 1962) crops out along the eastern side of the Birds Neck extending from Rumberpon Island southwards for 250 km to Lake Jamur. The Imskin Limestone is a well-bedded dense calcilutite commonly with abundant pelagic foraminifera, and with beds of chalk, calcarenite, marl and pyritic limestone in places. The limestone is exposed in the strongly deformed terrain of the Lengguru Fold Belt where it is tightly folded and repeated by thrust faults. Visser and Hermes (1962) estimate a thickness of approximately 1000 m for the limestone (Figures 46). We have found that the Imskin Limestone ranges in age from Paleocene to middle Miocene. Visser and Hermes (1962), however, are emphatic that the Imskin Limestone in the northern part of its range is only of Late Cretaceous age. The Imskin Limestone represents a deep-sea environment; gradation into shallow-water limestone is common and it overlies the shallow marine Sirga Formation west of Rumberpon Island. Visser and Hermes (1962) record that the Imskin Limestone overlies and is intercalated with the Late Cretaceous Ekmai Sandstone. Klamogun Limestone The Klamogun Limestone (Klamogun Formation of Visser & Hermes, 1962) occurs subsurface throughtout the Salawati Basin, but does not crop out. It is described by Visser and Hermes (1962) as a dense, more or less marly, well-bedded to flaggy limestone with common to abundant pelagic foraminifera; it contains beds of marl. The maximum reported thickness is 1159 m in the Klamogun 1 well. The limestone was deposited ini the open sea, probably in deep water. The age of the Klamogun Limestone is early to middle Miocene. The Klamogun Limestone lies conformably on the Sirga Formation and is laterally equivalent to the Kais Limestone, Sekau Formation and Klasafet Formation; it is also overlain by the Klasafet Formation. The Klamogun Limestone contains bitumen, and traces of oil and gas have been reported from it. Both Visser and Hermes (1962) and Vincelette and Soeparjadi (1976) believe it to be the source of the oil in the Salawati Basin. Recently it has been suggested that thermal maturation of the limestone is not sufficient to generate oil.



236 Kais Limestone The outcrop of the Kais Limestone (Kais Formation of Visser & Hermes, 1962) forms a broad belt crossing the Birds Head from west to east (Table 2; Figure 3-5). It consists of calcarenite and muddy calcarenite; the patch reefs of the Salawati Basin and the southern margin of the Ayamaru Plateau are formed largely by boundstone or reef material in the position of growth. The thickness of the limestone changes considerably, over short distances; the maximum reported thickness is 557 m in Klamono 39 well, but the limestone forming the Ayamaru Plateau is probably thicker than this. The Kais Limestone represents a reef complex comprising platform and patchreef facies. The patch reefs are largely confined to the Salawati Basin. The age of the Kais Limestone is most probably early to middle Miocene. The Kais Limestone rests conformably on the Sirga Formation and unconformably on the Aifam Group. It is laterally equivalent to the Klamogun Limestone, Sekau Formation, and Klasafet Formation. It is overlain conformably by the Steenkool Formation. The large productive patch reef reservoirs of the Kais Limestone in the Salawati Basin are well described by Vincelette and Soeparjadi (1976). Sekau Formation The Sekau Formation (Sekau Member of Visser & Hermes, 1962) is a discontinuous unit which occurs along the southern margin of the Ayamaru Plateau and is patchily developed east and west of the plateau, in close association with the Kais Limestone (Table 2; Figure 4). The Sekau Formation is a calcirudite consisting of limestone clasts (in places coral in position of growth) supported by a muddy or marly calcareous matrix. Visser and Hermes (1962) estimate a thickness of 30 to 50 m for the formation. The Sekau Formation represents coral growing in or smothered by muddy water. coral debris deposited below wave-base at the base of a reef face, or a dying reef margin encroached on by marly deep-water sediment. The Sekau Formation is early to middle Miocene and is laterally equivalent to the Klamogun Limestone, Kais Limestone, and Klasafet Formation. It is overlain by the Steenkool Formation. Klasafet Formation The Klasafet Formation (Visser & Hermes, 1962) crops out discontinuously across the Birds Head from west to east, though it appears to be almost continuous subsurface in the Salawati Basin at least. The formation consists of massive to well-bedded marl, micaceous and calcareous siltstone and a little limestone (Table 2; Figures 3 , 4 & 6). Visser and Hermes estimate the thickness of the Klasafet Formation to be approximately 1900 m. The formation is 500 m thick in the Klamono oil field. The Klasafet Formation is contemperaneous with the Kais Limestone and is a facies deposited in deeper water below wave-base in the same basin in which abundant reefs grew and merged in shallow water to form the patch reefs and platforms of the Kais Limestone. The marly sediment eventually built up to the level of the reefs and smothered them. Visser and



Hermes (1962) note that the youngest sediments are shallow-water deposits and that a southward decrease in clastic material in the Klasafet Formation indicates a northern source for the material. The age of the Klasafet Formation is early t o middle Miocene; it may range into the late Miocene. The Klasafet Formation overlies and is probably also partly equivalent to the Klamogun Limestone; it is equivalent to the Kais Limestone and Sekau Formation, and is conformably overlain by the Steenkool Formation. The Klasafet Formation seals the oilbearing patch reefs of the Salawati B a s h Sediments of the Onin and Kumawa Peninsulas. The Onin and Kumawa Peninsulas were not covered by the field investigations of our project. However, pre-1962 and later oil exploration data have been re-interpreted on new comprehensive air photography and 1:250,000 scale geological maps are being compiled. The predominantly calcareous Paleocene to late Miocene sediments of the peninsulas have been placed in the expanded New Guinea Limestone Group. They are clearly lateral equivalents of the original members of this group and there is a resemblance between the Miocene facies of the Onin Peninsula and the Miocene facies of the Birds Head, including the Salawati Basin. On the Kumawa Peninsula the Kumawa Limestone (Table 2 ) (Kumawa Formation of Visser & Hermes, 1962) displays two facies - a coralline member consisting of massive reef limestone and a bedded member of flanking near-reef deposits. Formanifera range from early Oligocene to late Miocene; an estimate of thickness derived from Visser and Hermes (1962) suggests about 1500 m of limestone. The base of the Kumawa Limestone is not seen; the bedded member is overlain by marl of the Klasafet Formation. In the Onin Peninsula (Table 2, Figure 5) the Paleocene Baham Formation (Visser & Hermes, 1962) consists of light grey chalky to granular limestone with some glauconite, glauconitic sandstone and dark green shale. In exploratior, wells it is conformable on the Kembelangan Formation and is conformably overlain by the Ogar Limestone Visser and Hermes estimate the thickness of the formation as 250 m. The Onin Limestone (Onin Formation of Visser & Hermes, 1962) consists of fine-grained well-bedded limestone with abundant planktonic formanifera. It is probably a deep-water deposit similar to the Imskin Limestone of the Birds Neck and Visser and Hermes estimate its thickness as 2000 m. Foraminifera range from Oligocene to early Miocene; its stratigraphic relationships are not celar. The Ogar Limestone, Rumbati Limestone and Tawar Marl are all of middle to late Miocene age and appear to be facies equivalent. The Ogar Limestone is a massive reef limestone named Ogar Formation by Visser and Hermes (1962); they estimated its thickness as certainly more than 5-00 m. The Rumbati Limestone (new name) is a brown and grey fine-grained dolomitic limestone which is locally pyritic. It is thought to represent a back-reef facies. It conformably overlies the Tawar Marl but i s also of equivalent age. The Tawar Marl (new name) consists of soft



237 fine-pined limestone and marl. It is faulted against the Onin Limestone.



Cainozoic Sediments of Misool Island Cainozoic Sediments of Misool Island are almost entirely calcareous and have been placed by us in the expanded New Guinea Limestone Group, originally defied by Visser and Hermes (1962). Previously, no limestone younger than Miocene has been accommodated in the group, because as A. Lloyd (pers. comm. 1982) pointed out, limestone effectively disappears from the stratigraphic column throughout Irian Jaya after middle Miocene times. In the Continental Province, only Misool provides an environment for Late Cainozoic limestone deposition (Table 2; Figures



4&5). The Daram Sandstone was defied by Pigram et al. (1982). It crops out on many small islands in the eastern part of Misool Archipelago and consists of thick-bedded calcareous quartz sandstone, siltstone and sandy limestone with rare silicified wood. The formation is approximately 50 m thick and is probably Paleocene to early Eocene in age. The Daram Sandstone rests conformably on the Late Cretaceous Fafanlap Formation and is overlain conform. ably by the (?) Palaeocene to Oligocene Zaag Limestone. The Zaag Limestone was defined by Pigram et al. (1982) and crops out extensively on Misool Island and on islands to the east and southeast. It consists of thick-bedded grey, oolitic biocalcarenite and shoal limestone. The formation is at least 150 m thick. The Paleocene to Oligocene Zaag Limestone rests conformably on the Daram Sandstone and is overlajn partly unconformably and partly paraconformably by the early Miocene Kasim Marl. The Zaag Limestone is equivalent to the Faumai Limestone of western Irian Jaya. The Kasim Marl was defined by Pigram et al. (1982) and forms an east-trending depression on Misool Island. The formation is of early Miocene age and consists of well. bedded marl and sandy limestone; it is about 50 m thick The Kasim Marl rests partly unconformably and partly paraconformably on the Zaag Limestone and is overlain conformably by the Openta Limestone. The Kasim Marl is probably equivalent to the Sirga Formation and Adi Member of the Yawee Limestone in western Irian Jaya. The Openta Limestone was defined by Pigram et al. (1982). It crops out extensively on the northern part of Misool Island and consists of poorly bedded shoal and reefal limestone. It is at least 50 m thick and is of early to middle Miocene in age. The Openta Limestone rests conformably on the Kasim Marl and unconformably on the Fafanlap Formation and Facet Limestone Group. It is overlain partly unconformably and partly paraconformably by the Atkari Limestone. The Openta Limestone is equivalent to the Kais Limestone of Western Irian Jaya. The Atkari Limestone was defined by Pigram et al. (1982) and crops out extensively along the north coast of Misool Island. It consists of raised Plio-Pleistocene reef complexes and poorly consolidated neritic sand bodies. It rests partly unconformably and partly conformably on the Openta Limestone.



Pliocene Intrusive Timepa Monzonite The Timepa Monzonite (Pigram & Panggabean, 1983) is exposed in stocks which are emplaced in the Aiduna Formation (Aifam Group) and Kembelangan Group along the southern fall of the western Central Range. Similar plutonics intruding the Derewo Metamorphics may also belong to this unit. Where the contact is not faulted the intrusive bodies are accompanied by a contact aureole. The main rock types are monzonite, diorite, quartz diorite, adamellite and aplite. Some of the rocks show a foliated structure. K-Ar determinations on hornblende and biotite gave Pliocene ages respectively of 7.09 my and 2.11 my. The unit is tentatively correlated with similar plutonics further to the east along the Central Range where they are associated with skarn-type copper and gold mineralization at Tembagapura. Late Cainozoic Clastic Sediments The three major Late Cainozoic clastic formations are precise equivalents in both rock type and age throughout western Irian Jaya. These are the Klasaman, Steenkool and Buru Formations which occur respectively in the Salawati and Bintuni Basins and in the southern part of the Central Range. These formations are overlain in places by distinctive younger restricted formations. However, many Quaternary deposits of clastic sediment h*- not been described. Klasaman Formation The Klasaman Formation was defined by Visser and Hermes (1962). It crops out over a large area of Salawati Island in the western Birds Head and along the southern side of the Ayamaru Plateau as far east as the Kais River. The Klasaman Formation has been penetrated in many wells drilled in the Salawati Basin (Table 2; Figures 3 & 4). The late Miocene to Pliocene Klasaman Formation consists of interbedded sandy, partly calcareous mudstone and muddy, partly calcareous sandstone. In the upper part conglomerates and lignite seams occur. Minor molluscan coquina beds are also present. Conglomerates are more common to the north. The maximum thickness is about 4500 m. Benthonic and pelagic foraminifera, molluscs and bryozoa are the most common fossils. The Klasaman Formation rests conformably on the Klasafet Formation to the south and disconformably on it in the. north. The Kalasaman Formation is overlain unconformably by the Quaternary Sele Conglomerate. The Klasaman Formation is an immature source rock. Some of the coarse clastic beds near the northern parts of the Salawati Basin may have reservoir potential. Steenkool Formation The Steenkool Formation was defined by Visser and Hermes (1962). The Steenkool Formation crops out extensively in the southeastern Birds Head and beneath the plains in the Bintuni Basin that extend southward to the Seram Sea, Small inliers are known within the



238



Lengguru Fold Belt to the east of Arguni Bay and Kaimana (Table 2; Figures 4-6). The formation has been intersected in many of the wells drilled in the Bintuni Basin. The facies within the Steenkool Formation change considerably, over short vertical and horizontal distances. In the northern part the formation comprises interfingering sandy and muddy facies. The sandy facies consists of lithic sandstone with interbedded mudstone, siltstone and conglomerate with minor calcarenite and lignite. The muddy facies consists of mudstone and siltstone with minor interbeds of sandstone and rare conglomerate. The mudstone is partly calcareous at the base. Both facies are overlain by a unit composed of poorly consolidated ferruginous sandstone, conglomerate and mudstone. South of Bintuni Bay the muddy facies predominates in the Steenkool Formation and coarse clastic detritus is rare. It consists mainly of micaceous mudstone, which is locally calcareous, with minor sandstone and conglomerate. Plant remains, lignite beds, mollusc fragments and reworked formanifera are common. The maximum thickness of the formation is approximately 3500 m. The age of the Steenkool Formation is late Miocene to Pliocene. It rests Conformably on the New Guinea Limestone Group and is overlain by the Quaternary Upa Conglomerate in the Central Birds Head and by Recent alluvium throughout the southern Bintuni Basin. The Steenkool Formation in the deepest parts of the Bintuni Basin may have some source rock potential, and some of coarse clastics may have reservoir potential.



thickness is 120 m. No diagnostic fossils have been found in the formation, but it rests disconformably on the Klasaman Formation and is therefore younger than Pliocene.



Bum Formation The Buru Formation was defied by Visser and Hermes (1962). The Buru Formation crops out around the isolated Buru Mountains, along the southern fall of the Central Range and around the Wissel Lakes (Table 2; Figure 7). The formation is of late Miocene to Pliocene age and consists of a basal calcareous mudstone and sandy shale overlain by well-bedded, micaceous and partly carbonaceous lithic sandstone and mudstone in which she11 beds are common. The upper part of the sequence consists of carbonaceous lithic sandstone, well-bedded shale, marine limestone, and conglomerate, with lignite beds up to 1 m thick. The maximum thickness of the formation is approximately 3000 m. Foraminifera, molluscs, and plant remains are common. The Buru Formation rests conformably on the New Guinea Limestone Group and is overlain by Quaternary fanglomerates and flood plain deposits. The Buru Formation may have source rock potential deep in the molasse basin south of the mai-n range, and some of the coarse clastics beds may have reservoir potential.



Metamorphic Rocks The metamorphic rocks of the Transition Zone are generally thought to be timeequivalents to the Late Paleozoic and Mesozoic formations of the Continental Province, and to have been metamorphosed during Tertiary times (Table 3). The undifferentiated metamorphics of the Birds Neck may contain rocks which were metamorphosed in Middle Paleozoic times.



Sele Conglomerate The Sele Conglomerate was defined by Visser and Hermes (1962). It crops out on Salawati Island and in the western Birds Head, east of Sorong. It consists of polymictic conglomerate with thin claystone and sandstone intercalations. Plant remains are common. Maximum



Upa CongIomerate The Upa Conglomerate was defined by Pigram and Sukanta (1982). It crops out in the central Birds Head between the Kais and Rawarra Rivers and consists of poorly consolidated, carbonaceous polymictic conglomerate and lithic sandstone. The formation contains no detritus from the New Guinea Limestone Group. The maximum thickness is approximately 100 m (Table 2; Figure 4). The Upa Conglomerate rests conformably on the Steenkool Formation and is overlain disconformably by alluvial terrace and flood plain deposits. It is probably Quaternary. Unnamed Fanglomerate An extensive belt of unnamed Quaternary fanglomerate occurs along the southern flank of the Central Range. It consists of limestone conglomerate with minor sandstone and rare mudstone. The formation rests unconformably on parts of the Kembelangan and New Guinea Limestone Groups and the Bum Formation. A Pleistocene age is indicated by the stratigraphic position and geomorphology of the unit.



TRANSITION ZONE



Undifferentiated Metasediments In the east part of the Birds Neck, south of the Wandamen Peninsula, several elongated fault blbcks are made up of strongly deformed, low-grade metamorphics. The most common rock types are dark slate, phyllite, metasiltstone, metawacke, and quartzite; others include chlorite schist (derived from tuff and volcanogenic sediment), marble, calcareous slate, metachert, and metamorphosed intermediate to basic intrusive rocks.'The interbedded slate, metawacke and metasiltstone are probably distal turbidites (Table 3; Figure 6). The metamorphic assemblages represent the chlorite zone of the greenschist facies, and all rock types are tightly folded and cleaved. The lack of fossils and the intense deformation preclude reliable stratigraphic correlation. The rocks may represent the Kemum Formation, the Paleozoic sediments of southeastern Irian Jaya or the open-marine facies of the Mesozoic Kembelangan Group. The rocks are locally intruded by dykes of pegmatite and aplite; some aplite dykes are ptygmatically folded. To the south the



239 rocks of the unit are partly in fault contact and partly intruded by the Kwatisore Granite which has a minimum age of 197 my. or Late Triassic. Mangguar Formation The Mangguar Formation (new name) is a folded thinbedded marble which may be dolomitic. It is almost certainly interlayered with fine-grained clastic sediment. The Mangguar Formation crops out on the east coast of the Birds Neck; similar rock types occur further south, intruded by the Kwatisore Granite which has a Late Triassic minimum age. 'The carbonate content of the Mangguar Formation suggests a correlation with, the Siluro-Devonian Modio Dolomite of the western Central Range (Table 3; Figure 6). Wandamen Gneiss The Wandamen Gneiss (new name) consists of medium to high-grade regional metamorphic rocks which underlie a north-trending elongate domal structure forming the Wandamen Peninsula and Roon Island (Table 3; Figure 5). The main rock types are muscovite-biotite quartzofeldspathic gneiss with minor pelitic schist, amphiboiite and metacarbonate. These rocks are mainly derived from a sedimentary protolith, although some may have a granitic parentage. The rocks are metamorphosed in the amphibolite facies and metamorphism is of the relatively high-pressure kyanite-type. The age of metamorphism is Pliocene, based on K-Ar dating of biotite and hornblende. The metamorphics are tightly deformed but, where observed, the lithological layering appears to be inherited from sedimentary rocks. The domal structure is tentatively interpreted as an exhumed folded thrust surface with remnants of the overlying slab preserved in places. The overthrust rocks belong to the undifferentiated Paleozoic and Mesozoic metasediments of the Birds Neck. Derewo Metamorpnm The Derewo Metqorphics (Dow & Hamonangan, 1981) form a continuous belt along the north flank of the Central Range. In western Irian Jaya only the westernmost segment of this belt has been examined @ab.el 3; Figure 7). The unit comprises a wide variety of dominantly lowgrade regional metamorphics. The most common rock types are'slate, phyllite, metasiltstone and metawacke which are generally dark grey or black. Metamorphosed intermediate to basic volcanics and volcanogenic sediments are an important component, in places associated with metaintrusive rocks of similar compostion. Interbeds of quartzite and recrystallized limestone are sporadic but widespread. In the Weyland Mountains partly recrystallized massive acidic volcanics and thick lenses of marble and calc-silicate rocks are intercalated with slate, phyllite and quartzite. The rocks are metamorphosed in the greenschist facies mainly in the chlorite zone and locally up into the biotite zone. However, mineral assemblages typical of the glaucophane-lawsonite schist facies have been reported further east outside our map area.



The rocks are deformed by tight or isoclinal folds with wavelengths ranging from centimeters to tens of metres. TWO or three cleavage types are associated with at least three phases of folding. Poorly preserved Mesozoic ammonites and belemnites occur in slates north of the Wissel Lakes. No direct evidence for the age of metamorphism has been obtained, but foliated rocks from the Utawa Diorite, which intrudes the Derewo Metamorphics, have yielded Miocene K/Ar ages. The rocks of this unit are in fault contact with ultramafic and mafic slivers to the north and fine clastics of the Kembelangan Group to the south. The unit is intruded by the middle Miocene Utawa Diorite and possibly by the Pliocene Timepa Monzonite. Plutonic Rocks Plutonic rocks are characteristic of the Transition Zone and the plutons are largely bounded by faults. The only occurrence outside the zone is the Anggi Granite Suite close to the Ransiki Fault Zone. Melaiurna Granite The Melaiurna Granite (Table 1) (Visser & Hermes, 1962) forms an irregular body within the Sorong Fault Zone in the Warsamson Valley of the western Birds Head. It consists of coarse-grained, commonly porphyritic granite with phenocrysts of pink orthoclase up to 10 cm long. The grainite intrudes the Kemum Formation and both units are in contact with arkose, quartz sandstone, limestone, mudstone and minor conglomerate which resemble the Aifam Group but may be partly of Devonian age. Two K-Ar ages on biotite from the granite gave an average value of about 326 my. (Carboniferous). Sorong Granite The Sorong Granite (Table 3) (Visser & Hermes. 1962) forms an elongate body in the Sorong Fault Zone where it intersects the northwest coast of the Birds Head. The small pluton consists of equigranular granite and syenite containing xenoliths of metasediments, and is cut by aplite and quartz veins. The rocks are in places mylonitized or cataclastically deformed along fault zones. The only age constraint is that the pluton intrudes the Kemum Formation. Netoni Intrusive Complex The Netoni Intrusive Complex (Table 3) (Pieters, Hartono & Amri, 1982) forms a fault-bounded, composite, plano-convex batholith (60 km long by 10 km wide) immediately north of the main strand of the Sorong Fault Zone ini the northeast part of the Birds Head. The rocks are I-type granitoids with a modal composition which reveals two suites. The main suite ranges from quartz syenite and quartz monzonite to granite and adamellite; the other suite is restricted to quartz diorite and diorite. Xenoliths mostly consist of gabbro, hornblende diorite, amphibolite and hornblende schist. Late phase pegmatite and aplite occur locally as dykes and veins. The pluton forms a large allochthonous fault block and



240 is juxtaposed against the Tamrau Formation in the Transition Zone. At the western end of the batholith the fault between the two units may be overlain by the ? Late Cretaceous Amiri Sandstone. Six K-Ar age determinations on hornblende and biotite range from 225 my. to 245 my. (latest Permian to earliest Triassic).



Zone. The unit consists dominantly of diorite, locally grading into gabbro and is cut by numerous acidic veins and dykes. The adjacent country rocks (Kemum Formation) shows various degrees of assimilation by the plutonic body. A K-Ar analysis on hornblende yielded an age of 15.4 2 0.5 my (middle Miocene).



Maransabadi Granite The Maransabadi Granite (Table 3; Figure 5)(new name) is exposed in the Auri Island Group, in western Cenderawasih Bay, where it is emplaced within undifferentiated Paleozoic and Mesozoic metasediments of the Transition Zone. The rocks consist mainly of medium-grained biotite granite and granodiorite with minor quartz diorite, gabbro and rare tourmaline pegmatite. K-Ar determinations on hornblende and biotite respectively yielded ages of 278 5 4 my. and 231 5 3 my. The biotite age is likely to reflect a post-recrystallization event, and the Early Permian date from hornblende probably indicates the minimum age of emplacement.



Volcanic Rocks



Kwatisore Granite The Kwatisore Granite (Table 3; Figure 6) (new name) forms a large irregular pluton in the southeast part of the Birds Neck. The rocks of this unit are in fault contact with and intrude undifferentiated Paleozoic and Mesozoic metasediments of the Transition Zone. The pluton is overlain by Miocene limestone and by quartz sandstone which may be the Late Cretaceous Ekmai Sandstone. The main rocktypes are biotite granite and granodiorite. The rocks are locally flow-foliated, particularly where xenoliths are abundant. A Triassic K-Ar age of 197 3 my. was obtained from biotite, but this probably reflects a post-crystalization event. Utawa Diorite The Utawa Diorite (Dow & Hamonangan, 1981) is a batholith 100 km long and about 40 km across exposed in the upper plate of the Derewo Overthrust in the northern fall of the Wayland Mountains at the head of Cenderawasih Bay (Table 3; Figure 7). The batholith is primarily composed of diorite and granodiorite but granitic rocks are also common and amphibolite is associated with ultramafic rocks intruded by the diorite. Several isotopic dates forms a tight cluster in middle Miocene times. A Silurian age obtained from a granite sample suggests that the batholith contains partly remelted Early Paleozoic rocks. A thin remnant of what was perhaps an extensive superstructure of intermediate volcanics is intruded by the batholith along its northern contact; it also intrudes ultramafic rocks in this region. On the south it is emplaced in schist of the Derewo Metamorphics, and is bounded by faults on east and west sides. Lembai Diorite The Lembai Diorite (Robinson & Ratman, 1978) is exposed on the inner side of the curved fault segment which connects the Sorong Fault Zone with the Ransiki Fault



Moon Volcanics The Moon Volcanics (Pieters, Hartono & Amri, 1982) form a belt over 100 km long in the north-central Birds Head between the Sorong and Koor Fault Zones(Tab1e 3, Figure 3). The volcanics consists of intermediate pyroclastics and lava with minor acid and basic members and rare intercalated limestone. Cogenetic dykes and stocks are widespread. Foraminifera indicate a middle Miocene age. The volcanics lie unconformably on the Mesozoic Tamrau Formation and the Eocene to middle Miocene Ajai Limestone, interfinger with the early to late Miocene Koor Formation and are probably overlain unconformably by the Plio-Pleistocene Opmarai Formation. Dore Home Volcanics On north Salawati Island the Dore Home Volcanics (Amri et al., in prep) are made up of andesite and minor basalt lava, lava breccia, aggomerate and tuff, with volcanogenic sediments, cut by diorite dykes. The unit is partly a lateral equivalent of the Serewin Limestone which consists of calcarenite, calcilutite, calcareous silstone and sandstone, with minor intercalations of volcanogenic sediments and volcanics at the base. Foraminifera indicate a early to middle Miocene age for the calcareous rocks, and the Dore Home Volcanics and Serewin Limestone may be correlated with respectively the Moon Volcanics and Koor Formation in the northern part of the Birds Head.



Berangan Agglomerate The Berangan Agglomerate (Robinson & Ratman, 1978) forms a symmetrical humped mountain rising up from the alluvial lowlands in the far northeastern part of the Birds Head. The rocks consist of fresh basaltic agglomerate and lava breccia. The morphology df the mountain suggests a Pliocene age for the volcanic activity.



Quaternary Basalt Along the northern flank of the Weyland Range occur faulted remnants of biotite and hornblende bearing olivineaugite basalt. These potassium-rich basic volcanics are interpreted as valley-fill lava flows, probably of Pleistocene age.



Jamur Volcanics The Jamur Volcanics (new name) form a well-preserved cone, about 3 4 km wide which overlies a prominent fault in the southern part of the Birds Neck. Jamur lake probably owes its existence to damming by the volcano. The volcano is built up of lava, pyroclastics and lahar deposits of



24 1 potassic and intermediate to basic composition. A Quaternary age for the volcano is inferred from its state of preservation.



Sedimentary Rocks The sedimentary rocks of the Transition Zone are either strongly deformed and partly metamorphosed, pre-Pliocene fine-grained sediment or Plio-Pleistocene fine and coarsegrained, predominantly clastic sediment deposited in a shallow marine or fluviatile environment. UndifferentiatedKembehngan Group Undifferentiated sediments of the Kembelangan Group (Visser & Hermes, 1962) crop out in the eastern part of the Birds Neck and in the northern foothills of the Central Range (Table 1; Figures 4827). They are almost entirely composed of massive to cleaved mudstone with beds of siltstone and muddy sandstone. Only Jurassic ammonites have been recovxed from these sediments but they may extend into the Cretaceous. On the eastern margin of the area mapped, an isolated micritic limestone overlying the mudstone yielded Paleocene foraminifera. The mudstone is equivalent to the closely similar Tamrau Formation of the Transition Zone in the Birds Head. The undifferentiated mudstone appears to be very thick, well in excess of 1000 m. Its base is not exposed, but it is overlain by undifferentiated New Guinea Limestone Group north of the Wissel Lakes. The mudstone appears to be carbonaceous, but its source rock potential has not been tested.



Tamrau Formation The Tamrau Formation was originally defined by Lehner (1954), but the name was dropped by Visser and Hermes (1962). Pieters et al. (1983) have resurrected the name for a unit of fme-grained sediments found within and to the north of the Sorong Fault Zone. The Tamrau Formation crops out as a continuous belt across the northern Birds Head. It consists of dark shale, siltstone and slate with interbedded quartz wacke, minor quartz arenite and calcilutite. In the east some medium to high-grade metamorphics have been included in this formation. The formation is probably at least 1000 m thick. A few ammonites, bivalves and planktonic foraminifera are found in the formation, and it appears to be Middle Jurassic to Late Cretaceous in age. The base of the Tamrau Formation is not seen. It is conformably overlain by the Tertiary Ajai Limestone, Miocene Koor Formation and Miocene Moon Volcanics. The Tamrau Formation may have some potential as a petroleum source rock. The unit consists of extensively recrystallized barren calcilutite with discontinuous beds of poorly sorted, polymictic conglomerate near the base. The limestone was deposited some time during Paleogene and lower Miocene as the unit unconformably overlies the Tamrau Formation and is overlain by the Moon Volcanics. The unit is possibly equivalent to the Imskin Limestone.



Allochthonous Units in the Sorong and Ransiki Fault Zones The Sorong Fault Zone ranges in width from a few hundred metres to 10 km and contains an assembly of little deformed tectonic blocks, slabs and other fragments, as well as sheared and crushed rock of widely diverse composition, origin and age. Occurrences too small to be shown on 1:250,000 scale maps were lumped together under the informal name T O Wmegabreccia by Pieters, Hartono L+ Amri (1983). The larger part of this tectonic unit is a chaotic, strongly deformed mixture of mudstone, calcareous mudstone, argdlaceous and arenaceous limestone, pure limestone, calcarenaceous sandstone and siltstone, and polymict conglomerate or breccia, all ranging in age from Late Cretaceous to middle Miocene. Tectonically intercalated with this fault melange are small fault slivers of serpentinite and peridotite, gabbro, intermediate to basic volcanics, dense calcilutite, granite and aplite, and representatives of the units described below. Although they are invariably fault-bounded, it is possible to delineate and define lithostratigraphic units within the Sorong Fault Zone. Some of these can be correlated with previously named units from the Continental and Oceanic Provinces and Transition Zone such as the Kemum Formation, Aifam Group, Tipuma Formation, Tamrau Formation, Manambar Formation, Faumai Formation, Sirga Formation, h i s Formation and Klasafet Formation. Other units are new and include the Senopi Sandstone, Kebar Limestone, Jefman Breccia, Asbakin Limestone, and several granitoid bodies described above. The Senopi sandstone consists dominantly of feldspathic quartz arenite, and although it is unfossiliferous it is thought to be equivalent to the Sirga Formation. The Kebar Limestone ranges in age from early to middle Miocene and its main rock types are biocalcarenite and biocalcilutite commonly with admixtures of arenaceous or argillaceous materib al. The Jefman Breccia is a very distinctive unit restricted to the western part of the fault zone. It is composed of massive, moderately well sorted, angular to subrounded polymict fragments without matrix. Except for widelyspaced shear zones the breccia is not much deformed. It is probably a fanglomerate deposit formed adjacent to fault scarps rather than a tectonic breccia. Based on the composition of the fragments the maximum age of this unit is middle Miocene. The Asbakin Mestone is made up of early to middle Miocene calcilutite and minor limestone breccia. The Ransiki Fault Zone is at the most 2.5 km wide and is also occupied by a fault melange with a wide assortment 3f rock types. Of the units juxtaposed along the fault zone, the island-arc volcanics (Arfak Volcanics) and possibly the calcilutite of the Oceanic Province are much more sheared and otherwise deformed than the metasediments (Kemum Formation) and granitoids (Anggi Granite Suite) of the Continental Province. In addition to these, the fault zone also includes lenticular blocks of fossiliferous calcareous sandstone and mudstone, arenaceous and argiUaceouslimestone, and calcareous conglomerate of the Late Cretaceous Rafi Formation and biocalcarenite, conglomerate and minor calcareous mudstone of the early to middle Miocene Wai Limestone.



242



OCEANIC PROVINCE Ophiolite and Paleogene to early Miocene island-arc volcanics, intrusives and contemporaneous sediments are extensively exposed between the northern foothills of the Central Range and the north coast of Irian Jaya. A similar association occurs further westward in fault-bounded fragments which form the basement of Yapen and Num Islands, Arfak Mountains (east Birdshead), Tosem Mountains (north Birdshead). Batanta Island, Waigeo Island, and Biak and Supiori Islands. In the waning stages and after cessation of magmatic activity limestone deposition prevailed from early to middle Miocene and locally late Miocene. During latest Miocene and Pliocene the island-arc volcanic terrain was deformed, uplifted and fragmented by transcurrent as well as thrust faulting. Subsequent erosion and sedimentation gave rise to Pliocene-Quaternary near-source deposits mantling the tectonic highs and thick deposits of fine-grained detritus in local basins (Table 4;Figs.3,7 & 8). Latest descriptions of the stratigraphy in the Oceanic Province are by Masria, Ratman & Suwitodirdjo (1981) for Biak; Supriatna, Apandi & Simandjuntak (in press) for Waigeo; Atmawinata, Ratman & Pieters (in prep.) for Yapen; Pieters, Hartono & Amri (1983) for the Tosem Mountains; Pieters, Hakim & Atmawinata (in prep.) for the Arfak Mountains; Dow et al. (in prep.) for the Nabire area and Minimitara Mountains. Ophiolite Waigeo Island is founded on oceanic crust and upper mantle rocks which form the major part of an ophiolite complex (Table 4). The mafic and ultramafic rocks comprise dunite, harzburgite, pyroxenite, serpentinite, gabbro and pillow basalt, and are associated with minor red shale and chert. This complex is locally intruded by andesite, diorite, basalt and hornblende gabbro. It is overlain by the Tanjung Bomas Formation composed of calcareous wacke sandstone and siltstone, red shale and calcareous wacke sandstone and siltstone, red shale and chert. The age of these open-sea sediments is thought to be Late Jurassic based on pelagic foraminifera, Both units are unconformably overIain by and also supplied the detritus for the Lamlam Formation with sandstone, siltstone and polymict breccia as main rock types. These rocks form a turbidite sequence and have an Eocene, Paleocene or slightly older age. On Gag and Fam Islands, respectively 50 km west and 30 km southwest of Waigeo Island, ultramafic and mafic rocks are also exposed. Brecciated serpentinite, pyroxenite, gabbro, basalt, diorite and andesite of Fam Island were named Waijar Ophiolite. The rocks on Gag Island are subdivided into massive, partly serpentinized harztjurgite with veins and dykes of orthopyroxenite, and a sheeted dolerite dyke complex. Serpentinized peridotite recorded from Biak Idand is close to a small chert deposit of unknown age. The Jobi Ophiolite Breccia(Tab1e 4jFigure 8) is exposed in the easternmost part of Yapen Island but is separated from the island-arc volcanics by a major fault zone. The breccia contains serpentinite, layered gabbro, pegmatitic gabbro, diorite and basalt which in many places are disturbed by fracturing.



In the Nabire area elongate bodies of ultramafic and mafic rocks are in fault contact with the Tobo Volcanics which are thought to belong to the island-arc volcanic suite (Table4). The rocks are unconformably overlain by Pliocene sediments such as the Bumi Mudstone and Legare Limestone. The main rock types in this unit are peridotite, pyroxenite and gabbro commonly with supehposed metamorphic textures, serpentinite, amphibolite and hornblende schist. Discontinuous fault slivers of ultramafic and mafic rocks (Table4,Figure 7) were mapped in the northern Minimitara Mountains. They are thrust over and possibly also interfaulted with the Derewo Metamorphics. The principal rock types are peridotite, pyroxenite and gabbro with metamorphic textures, and serpentinite. Island-Arc Volcanics Volcanics of island-arc type are locally exposed on Waigeo Island as a volcanic member in the Rumai Formation (Table 4). Basalt pillow lava and andesite to basalt lava-breccia with intercalations of tuffaceous sandstone interfinger with mudstone, tuff, and minor sandstone and conglomerate. The sediments yielded planktonic foraminifera ranging in age from Eocene to Miocene, and indicating an open marine environment. On Batanta and Kofiau Islands the volcanic suite is known as the Batanta Formation and Y d i Formations (Table 4) respectively. The Batanta Formation forms a sequence of agglomerate, basic to intermediate lava and lava breccia with intercalations of tuff, volcanogenic sediments and limestone which is intruded by dykes of dolerite, gabbro and diorite. In the Yarifi Formation volcanogenic sediments are much more prominent comprising tuffaceous sandstone, wacke sandstone, siltstone, agglomerate and locally conglomerate and limestone. The age range of Oligocene to early Miocene is based on foraminifera. In the Tosem and Arfak Mountains the volcanics are assigned respectively to the Mandi Volcanics (Table 4; Figure 3) and Arfak Volcanics. In the Tosem Mountains these are mainly tuff, agglomerate, lava breccia, massive lava and pillow lava of basalt to andesite composition, cut by dykes and stocks of basalt porphyry, gabbro and dolerite, and one body of diorite and granodiorite. Except for the granodiorite, the igneous components of the Man& and Arfak Volcanics are very similar, but in the Arfak Mountains these rocks pass eastward into volcanogenic wacke sandstone, mudstone and breccia or conglomerate, tuffaceous sandstone and impure limestone. In both units formanifera from calcareous beds range in age from late Eocene to early Miocene, and four K-Ar ages on hornblende and one on plagioclase fall within the Oligocene to Miocene bracket. On Biak and Supiori Islands small outcrops of altered basalt and andesite lava, lava breccia, volcanic conglomerate and tuff belong to the island-arc volcanic suite. These rocks were name the Auwewa Formation (Table 4; Figure 8) although the type area of this unit is located 300 km to the southeast, in the northern foothills of the Central Range. The island-arc volcanic suite is well represented on Yapen Island (Table 4; Figure 8) as the Yapen Volcanics. The unit consists of basalt and minor andesite agglomerate, volcanic



243 breccia and tuff with intercalations of massive lava, lava breccia and pillow lava, wacke sandstone, tuffaceous sandstone and calcareous tuff; locally they contain lenses and beds of limestone, and comagmatic dykes of microdiorite and andesite porphyry. The formation includes two members: the Manupang Member with dominantly crystal and lithic tuff, and the Ambai Member with calcareous wacke sandstone, tuffaceous sandstone, siltstone, mudstone, tuff and minof limestone, breccia and conglomerate. Both planktonic and larger benthonic foraminifera in these units range in age from late Eocene to early Miocene. In the Nabire area, the Tobo Volcanics (Dow et al., in prep.) possibly represent island-arc volcanics. They consist of altered and very indurated basic agglomerate and lava and volcanogenic sediments, cut by gabbroic dykes. The age of the unit is not known; its rock types are closely similar t o those of the Yapen Volcanics and of the Auwewa Formation (visser & Hermes, 1962) north of the Minimitara Mountains. Volcanics exposed in the northern foothills of the Minimitara Mountains are also assigned to the Tobo Volcanics by Dow and Hamonangan (1981). Prevously Visser and Hermes (1962) included these in the Auwewa Formation. The Nabire Volcanics, northeast of Nabire town, may be of two ages. Near the town, basic tuff, agglomerate and lava breccia, commonly sheared, grade downwards through calcareous tuffaceous sediments into the Nanamajiro Limestone. The limestone consists mostly of calcilutite with some tuffaceous material and contains large foraminifera ranging in age from middle Oligocene to early Miocene. The transition probably represent the waning stages of island-arc volcanicity and the beginning of limestone deposition recorded elsewhere at this time. The bulk of the Nabire Volcanics form two large outcrops separate from the above rocks and consist of agglomerate, tuff and lava breccia ranging in composition from alkali basalt and spilitic basalt to andesite; they are intruded by dykes of porphyritic andesite and microdiorite. Intercalations of tuffaceous sandstone, volcanogenic conglomerate and sandy calcareous mudstone occur in places. The mudstone contains foraminifera with a late Miocene to Pliocene age, consistent with a late Miocene K/Ar date of 5.62 my obtained on hornblende from an intrusive rock. Fault-bounded outcrops of bedded greenschist, metawacke sandstone with intercalations of purple schist, metavolcanic breccia and metatuff and intrusions of metagabbro and metabasalt (Table 4,Figure 8) were mapped as the Saranami Schist on Batanta Island, the Korido Metamorphics on Supiori Island and the Rosburi Schist on Yapen Island, and greenschist was also penetrated in the Niengo 1 well, 150 km east of Biak, on the north coast of the Irian Jaya mainland. The relationship of the greenschist to the island-arc volcanics is unclear and controversial. Pieters believes it represents on earlier metamorphosed volcanic association, possibly of Cretaceous age.



Post-volcanic Sediments On Waigeo Island (Table4) the ophiolite and island-arc volcanics are unconformably overlain by limestone with intercalations of arenaceous and argillaceous limestone of



the Waigeo Formation. Planktonic as well as larger benthonic foraminifera indicate a Miocene age. This unit in turn is overlain by late Miocene sandstone with interbeds of argillaceous limestone, and late Miocene to Pliocene limestone. The island-arc volcanics on Batanta and Kofiau Islands (Table 4) are unconformably overlain by the early to middle Miocene Dajang Limestone. The unit consists of calcarenite, calcarenaceous sandstone and calcareous mudstone with locally, a basal conglomerate containing volcanic clasts. This unit is separated by a hiatus from the conglomerate and sandstone with minor mudstone of the Marchere Formation. The age of this clastic deposit is Pliocene and the detritus is derived from basic to intermediate volcanic as well as granitic source areas. In the Tosem Mountains(Tab1e 4; Figure 3) the overlying limestone is only preserved in small outliers. The Pliocene to Pleistocene polymict conglomerate, muddy sandstone and calcareous mudstone of the Opmarai Formation, although not in stratigraphic contact with the volcanics, appear to be largely derived from this unit. The ArfakVolcanicslareunconformably overlain(Tab1e 4) by fossiliferous calcilutite and calcarenite and argdlaceous limestone of the M m n i Limestone. Assemblages of larger benthonic and planktonic foraminifera give an early to middle mocene age. In the northern part of the Arfak Mountains both the Arfak Volcanics and Maruni Limestone are onlapped by the late Miocene to Pleistocene Ekfoor Formation of the Manokwari Basin. The main lithologies are sandstone, siltstone and mudstone with minor polymict conglomerate which were deposited in a shallow marine to estuarine environment with the detritus derived from the Kemum Block as well as the Arfak Volcanics. Reeflimestone, massive and porous algal-foraminifera1biomicrite, calcilutite and calcarenite, more or less heavily contaminated by terrigenous detritus, and sandstone and polymict conglomerate rest locally on the Befoor Formation or directly on the Arfak Volcanics. These sediments have a Pleistocene age and belong to the Manokwari Formation. An intercalation of weathered vesicular volcanics may be correlated with the Berangan Agglomerate of the Transition Zone. On Biak and Supiori Islands (Table 4; Figure 8) the island-arc volcanics are unconformably overlain by the late Oligocene to early Miocene Wainukendi Formation and the early to middle Miocene Wafordori Formation. The Wainukendi Formation is composed of limestone with lenses of volcanogenic conglomerate and interbeds of argillaceous limestone and fossiliferouslimestone. The Wafordori Formation is made up of argillaceous limestone, locally tuffaceous, with intercalations of sandstone and fossiliferous limestone. The Warfordori Formation partly interfingers with and partly overlies the Wainukendi Formation and both units are conformably overlain by limestone with intercalations of conglomerate, breccia, arenaceous limestone, sandstone and argillaceous limestone of the early to middle Miocene Napisendi Formation. The Mokmer Formation of Pleistocene and probably Holocene age forms terraces of coralline limestone and chalk. On Yapen Island (Table 4; Figure 8)i the cessation of island-arc volcanic activity is marked by the early to middle Miocene W w i Limestone. This unit consists of bedded and massive fossiliferous calcarenite and calcilutite with inter-



244 calations of argllaceous limestone, chalky limestone and calcareous sandstone. The base is locally characterized by conglomerate with mixtures of volcanic and limestone detritus. Both the Wurui Limestone and the Yapen Volcanics Sumboi Marl and the Pliocene to Pleistocene Ansus conglomerate. The lithologies in the Sumboi Marl are globigernia-bearing argdlaceous limestone with intercalations of limestone, wacke sandstone and pebbly sandstone forming deeper marine sediments which are partly lateral equivalents of the dominantly polymict conglomerate, breccia, pebbly sandstone and calcareous sandstone of the Ansus Conglomerate. Towards the east of Yapen Island the Jobi Ophiolite Breccia, Yapen Volcanics and Wurui Limestone are onlapped by the Pliocene to Pleistocene Kurudu Formation. The interbedded sandstone siltstone, mudstone and claystone of this formation were deposited in a shallow marine enviroment and are correlated with a sequence of similar sediments, at least 3000 m thick, penetrated in five Pertamina-Tesoro wells located along the eastern side of the Cenderawasih Bay. These sediments make up part of the Pliocene-Quaternary Waipoga Basin which is marked by a distinct northwest-trending gravity anomaly with Bouguer anomaly vlues as low as-100 mgal.



GEOLOGICAL HISTORY Irian Jaya contains two distinct stratigraphic provinces with radically different pre-Miocene histories. They are an Oceanic Province in the north, floored by oceanic crust, and a Continental Province in the south, founded on continental crust. The two axe separated by a Transition Zone which also contains unique stratigraphic units and which ranges from 75 km across in te northern part of the Central Range to the sharply defined Ransiki Fault Zone in the eastern part of the Birds Head (Fig. 2). OCEANIC PROVINCE The geological history of the Oceanic Irovince is brief and simple. Island-arc volcanics were built up in Early Tertiary times over oceanic crust and locally over pelagic sediments. By the end of the early Miocene the volcanoes were extinct and had been reduced by erosion to platforms on which coral reefs flourished. Uplift began in late Miocene times and has continued to the present day; the products of erosion form a patchy cover of molasse.



CONTINENTAL PROVINCE A granite pebble dated as Precambrian in conglomerate of the Kemum Formation indicates that these SiluroDevonian turbidites, perhaps together with similar sediments on Misool, were deposited on the continental slope or rise of an ancient land that was almost certainly the part of Gondwana that later became the Australian continent. The Modio Dolomite of the Central Ranges was also deposited in Siluro-Devonian times and the pre-Carboniferous turbiditic sediment (Tuaba Formation) about 100 km east of Modio may be of the same age.



Deformation, metamorphism and intrusion of the turbidites of the Kemum Formation followed, presumably at a convergent plate margin, in Late Devonian or Early Carboniferous times. The Modio Dolomite and the Tuaba Formation were neither metamorphosed, nor intruded by granitoids, and are only moderately deformed. A stable sedimentary environment prevailed from the Middle Carboniferous in the Birds Head and from Permian times in the Central Range. Fine to moderately coarse clastic sediments were deposited on flood plains, in deltas and in shallow seas fringed by densely vegetated coal swamps; periodic incursions of the sea are marked by thin limestones. In the Birds Head only, however, granitoid plutons were emplaced in Late Permian to Triassic times accompanied by high-temperature/low-pressure metamorphism. During Triassic to Early Jurassic times an arid climate prevailed, and red beds were deposited throughout western Irian Jaya. Some volcanic activity is recorded in the sediments of the Central Range. On Msool Island a unique situation had developed; the deposition of turbiditic sediment began at an unknown date and continued into Middle or Late Triassic times. This was followed by shallow-water limestone also in the Late Triassic. The Middle Jurassic was a period of regional marine transgression in the Birds Neck and western Central Ranges, and this is seen by Pigram and Panggabean (1981) as marking the break-up of the ancient northern margin of the Australian part of Gondwana, and as preceding the opening of the Indian Ocean. Shelf sediments consisting of alternating sand and mud were deposited on the continental platform .in the Birds Neck and western Central Range throughout the remainder of Mesozoic times. However, Mesozoic shallow marine sediments in the Birds Head'are restricted to the Cretaceous. On Misool Island, Early and Middle Jurassic calcareous sediments were deposited in a shallow sea with little clastic sediment, but succeeding Late Jurassic to Late Cretaceous sediments were possibly all deposited under bathyg conditions. Evaporitic sediments followed by dolomite were deposited, probably in restricted basins, in the central Birds Head and perhaps south of the Central Range between the end of Late Cretaceous times and the early Eocene. During the same period oolitic and bryozoal limestone were deposited on a shallow carbonate platform south of the Central Range and in the southern Birds Neck. By the beginning of Eocene times, no clastic sediment was being deposited anywhere in western Irian Jaya, and the depositon of generally shallow-water reef and shoal limestones continued into the middle Miocene, apart from a short break in the Oligocene and earliest Miocene recorded by clastic sediments on Misool Island, the western and central Birds Head on the platform south of the western Central Ranges. In the Birds Neck the platform limestone interfingers eastward with deep-water limestone deposited from Eocene or even Late Cretaceous times until the early or middle Miocene. Rapid uplift beginning in Late Miocene time halted the depositon of limestone throughout western Irian Jaya.



245 TRANSITION ZONE The Transition Zone between the Continental and Oceanic Provinces is probably entirely composed of faultbounded blocks derived from more or less distant sources. The Paleozoic metamorphics of the Birds Neck may largely represent distal turbidites, though a tightly folded thin-bedded limestone or doIomite may be an equivalent of the Modio Dolomite. The ages of none of these rocks is known, but they are intruded by the Kwatisore Granite, which has yielded a Triassic isotopic age. Other dislocated granitic plutons in the Transition Zone range from Carboniferous t o Triassic in age. Mesozoic history throughout the Transition Zone is remarkably consistent. A thick sequence of mudstone and minor sandstone with intercalations of intermediate to basic volcanics is found, more or less metamorphosed, in the northern Birds Head between the sorong and Koor Fault Zones, in the eastern part of the Lengguru Fold Belt, and in the mountainous terrain north of the Central Ranges. These rocks are largely slate and have been variously mapped as Tamrau Formation, undifferentiated Kembelangan Group and Derewo Metamorphics. Acidic volcanics within these sediments in the Weyland Mountains are of unknown age. The great thickness of the black mudstone unit and the total absence of shallow-water fauna suggests deposition off the continental shelf. In the Transition Zone, limestone is less prominent in the Tertiary sequence. Paleogene and Miocene Limestone are both present in the Birds Head north of the Sorong Fault Zone, but middle Miocene intermediate volcanics are prominent in the same area, and at the head of Cenderawasih Bay a dioritic batholith of middle Miocene age may originally have carried a large volcanic superstructure. The cessation of island-arc volcanism in early Miocene times is believed to mark the collision of the Australian continent with the island arc of the oceanic province. Metamorphism of the black slates at the convergent plate Joundary may pre-date this, as in Papua New Guinea where orogenesis begins in Oligocene times. The intrusion of the middle Miocene Utawa Diorite into ophiolitic rocks of the Oceanic Province indicates that the latter had been displaced prior to this event. I



LATE CAINOZOIC COMMON HISTORY From late Miocene times, the Oceanic and Continental Provinces; together'with the Transition Zone, have had a common geological history and appear to have been contiguous. The late Miocene was marked by dramatic uplift throughout western Irian Jaya, and the Late Cainozoic history is characterized by the rapid development of large and small basins whch trapped the clastic sediment produced by erosion of the rising terrain. Only Misool and Biak Islands remained starved of clastic detritus and the depositon of platform limestone has continued there to the present-day. Post-orogenic volcanism has scattered and small representatives in the Pliocene alkali basalt of Nabire, the olivine basalt of the Weyland Mountains, the andesite of Mount Berangan and the Recent small cone of potassic volcanic$ at Lake Jamur.



PETROLEUM PROSPECTS In the Oceanic Province, the sedimentary section is patchily developed and commonly thin. The thick Kurudu Formation north of Yapen has been unsuccessfully tested by Pertamina-Tesoro. The Transition Zone is characterized by deformation, metamorphism and igneous intrusion, but the source rock potential of the Mesozoic black mudstones deserves to be investigated. The Continental Province is most promising for petroleum prospects and of course contains the productive Salavati Basin. Our knowledge is summarised below : - The Modio Dolomite contains altered conodonts which indicate it is overmature. - The Aifm Group contains graphitic coal at Tembagapura, but elsewhere the coals appear t o be sub-bituminous and probably rich in inertinite. In the Birds Head the conodont colour alteration is only 2, and favourable for oil generation. Sandstones appear to be tight because of silica and carbonate cement. - The Kembelangan Group contains mudstones in the shallow-water facies but random samples have returned disappointing results from source-rock investigations. Mixed but better results have been obtained from reservoir investigations of random samples from &hesandstones. - The New Guinea Limestone Group was believed t o be the source of oil in the Salawati Basin, but is now thought to be immature. Deep burial of suitable facies in e.g. the Bintuni Basin may generate oil. The limestone facies of the Onin Peninsula bear a striking reszm5larc:e tc those containing oii pools in the Salawati Basin. The occurrence of gas in the Sirga Formation suggests that clastic intercalations in the limestones may be suitable targets. - Young clastic sediments of the Steenkool and Buru Formations may both generate and hold oil at depths in excess of 4000 m in the young molasse basins. - Young and active deformation in Irian Jaya necessitates a close study of the time of folding, oil migration and the deposition of a suitable cap rock before testing structural traps in Irian Jaya. Though Visser and Hermes were over-conservative in their assessment of the Salawati Basin, their comments on the necessity for true oil-source rocks, suitable carrier rocks for migration, and the presence of a timely seal, are still valid today.



BIBLIOGRAPHY AMRI CHAIRUL, PIETERS, P.E., PRIHADJO and SUPRIATNA, S., (in prep.). Systematic geological map, Sorong 1 : 250,000 sheet. Indonesian Geological Research and Development Centre. Preliminary Edition. ATMAWINATA, S., RATMAN, N. and PIETERS, P.E. (in prep.). Geological Report of the Yapen 1:250,000 sheet, Irian Jaya. Indonesian Geological Research and Development Centre Open fde report.



246 DOW, D.B., and HAMONANGAN, B., 1981 - Systematic geological map, Indonesia; Enarotali 1:250,000 sheet. Indonesian Geological Research and Development Centre, preliminary edition. DOW, D.B., 1983. Interaction during the Pliocene between the Australian and Pacific Plates in Western Irian Jaya. Abstract, 6th Australian Geological Convention, Canberra. DOW, D.B. and HARTONO, UDI (in prep.) The nature of the crust beneath Geelvink Bay. Proceedings Indonesian Petroleum Association, Eleventh Annual Convention. DOW, D.B. and RATMAN, NANA (in prep.) Neogene tectonism, metamorphism, and magmatism in the Kepala Burung Region of Irian Jaya, Indonesia. The Fourth Regional Conference on Geology, Mineral and Energy Resources of Southeast Asia. DOW, D.B., TRAIL, D.S., RATMAN, NANA & HAMONANGAN, BHAKTI (in prep.) Geological Data Record Enarotali 1 :250,000 sheet area, Irian Jaya. Indonesian Geological Research and Development Centre Open File Report (unpublished). FORESMAN, J.B., PERKINS, E.H., FROIDEVAUX, C.M., MORRIS, D.A., 1972. Geologic study of the onshore Arafura Sea Contract Area, West Irian, Indonesia. Phillips Petroleum Company, Exploration Projects Group, Surface Projects section Report (unpublished). FROIDEVAUX, C.M., 1974. Geology of Misool Island (Irian Jaya). Proceedings Indonesian Petroleum Association 3rd Annual Convention. HAMILTON, W., 1979. Tectonics of the Indonesian Region. United States Geological Survey Professional Paper 1078, p. 345. MASRIA, M., RATMAN, N., SUWITODIRDJO, K., 1981. The Geology of the Biak Quadrangle Irian Jaya. Explanatory Notes. Geological Research and Development Centre, Department of Development and Energy, Indonesia. NICOLL, ROBERT, S., 1981. Irian Jaya Conodont age determinations, 1981. Bureau Miner1 Resources Profesional Opinion 1981/20 (unpublished). PANGABEAN, HERMES (1 982) Report on the Geology of the Omba 1:250,000 sheet area, Irian Jaya, Indonesia. Geological Research and Development Centre Open file report. PIETERS, P.E., RYBURN, R.J. and TRAIL, D.S., 1979. Geological Reconnaissance in Irian Jaya, 1976 and 1977. Bureau Mineral Resources Geology and Geophysics Australia Record 1979/19 (unpublished). PIETERS, P.E., HAKIM, A.S.' and ATMAWINATA, S., 1982. Systematic geological map, Indonesia. Ransiki 1:250,000 sheet. Indonesian Geological Research and Development Centre, preliminary edition. PIETERS, P.E., HARTONO, U. and CHAIRUL AMRI, 1983. Geological Data Record : Mar 1:250,000 sheet



area, Irian Jaya. Indonesian Geological Research and Development Centre, Open fde report (unpublished). PIGRAM, C.J., CHALLINOR, A.B., HASIBUAN, F., RUSMANA, E. and HARTONO,. U., 1982. Ethostratigraphy of the Misool Archipelago, Irian Jaya. Geologie en Mijnbouw, 61,245-279. PIGRAM, C.J. and PANGGABEAN, H., 1983. Geological Data Record: Waghete (Yapekopra) 1:250,000 sheet area, Irian Jaya. Indonesian Geological Research and Development Centre Open file report (unpublished). PIGRAM, C.J. and SUKANTA, U., 1982. Geological Data Record T a d a b u a n 1:250,000 sheet area, Irian Jaya. Indonesian Geological Research and Development Centre Open file report (unpublished). PIGRAM C.J., ROBINSON, G.P. and LUMBAN TOBING,S. (in press). Late Cainozoic Origin for the Bintuni Basin and adjacent Lengguru Fold Belt, Irian Jaya. Proceedings Indonesian Petroleum Association 11th Annual Convention. ROBINSON, G.P. and RATMAN, N., 1978. Stratigraphic and tectonic development of the Manokwari area, Irian Jaya. BMR Journal of Australian Geology and Geophysics, 3, 19-24. ROBINSON, G.P., RYBURN, R.J., TOBING, S.L. and ACHDAN, A., 1983. Systematic geological map Indonesia; Steenkool (Wasior) 1:25Q,OOO sheet. Indonesian Geological Research and Development Centre, preliminary edition. ROBINSON, G.P., RYBURN, R.J. and TOBING, S.L., 1982. Systematic geological map, Indonesia; Kaimana 1:250,000 Sheet. Indonesian Geological Research and Development Centre, preliminary edition. SUPRIATNA, S., APANDI, T. and SIMANDJUNTAK, W. (in press). Geological Report on the Waigeo 1:250,000 sheet area, Irian Jaya. Indonesian Geological Research and Development Centre Open file report. VAIL, P.R. and MITCHUM, Jr., R.M., 1979 - Global cysles of relative changes of sea level from seismic stratigraphy, In Watkins, Montadert & Dickson eds Geological & geophysical investigations of continental margins. American Association of Petroleum Geologists Memoir 29. VINCELETTE, R.R., and SOEPARDJADI, R.A., 1976 Oil-bearing reefs of the Salawati Basin of Irian Jaya, Indonesia. American Association of Petroleum Geologists Bulletin 60,1448-1462. VISSER, W.A. and HERMES, J.J., 1962. Geological results of the exploration for oil in Netherlands New Guinea. Government Printing office, the Hauge. YOUNG, G.L. and NICOLL, R.S., 1979. Report on samples from Irian Jaya submitted for micropalaeontological examination. Bureau Mineral Resources Professional Opinion. Geol. 79/028 (unpublished).



247



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intermediate vo Ic an ics



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plat form I imestone



LEGEND FOR FIGURES 3-8



evaporites



metamorphics



dolomite



lggq mudstone



sandstone



conglomerate



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FIGURE 4. T ME -SPACE DIAGRAM



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FlGU RE 6. TIME-SPACE DIAGRAM FOR SECTION D-D



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Palaeozoic & Mesozoic sedimentary protoliths



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FIGURE 7. TIME-SPACE DIAGRAM



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Miocene



Pliocene



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SPACE DIAGRAM



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protoliths)



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N o m e n c l a t h r e f o r t h e P a l a e o z o i c and. Mesozoic u n i t s of the c o n t i n e n t a l p r o v i n c e



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TABLE 1.



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TABLE 2.



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IMSKIN Lst



STEENKOOL FIMl



CENTRAL NECK



I



I



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BAHAM



ONIN Lst



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KUMAWA



PENINSULA



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N o m e n c l a t u r e f o r the C a i n o z o i c units of the continental province.



L



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B I R D SHEAD CENTRAL



I



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AGE



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Stratigraphic nomenclature of the Transition Zone.



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Kwa t iso re



I



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I



Undiffekent i a t e d Paleozo i c & Mesozoic m e ta- sediments



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c o r a l 1st & alluvium



Mangguar Fmn '



-



Table 3.



Complex



Tamrau Fmn



Amifi Sst



-limeitonel



Fmn



alluvium & c o r a l 1st



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c o r a l 1st & alluvium



N. SALAWATI ISL



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SORONG FAULT ZONE



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Coral 1 s t & alluvium



Table 4 .



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Coral 1 s t b alluvium



YMEU 6



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LXa*UpaUg Wr



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Job1 'Ophiolite Breccia



1



--



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allwfum h coral 1st



NUPI ISL



S t r a t i g a p h i c nomenclature of the Oceanic Province.



Befoor Rn



Hanokwari



Coral 1st 6 alluvium



b



supIoBz 1%



BUK



ultrbafics. ultrkfics a f i c s b amphibolimafics te



'3



.__.



alluvium



I



?-



alluvium 6 coral l e t



6,



N o\



0



26 1



I



1



c



AINIM



I



AIFAT MUDSTONE



A



AIMAU F O m T I ON



MEMBER



Table 5.



Nomenclature of the Fermo-Calrt.;-.~i'ercus A i f a m Group compared with ~ h a r~ s r d b;\Visser & Hermes (1962)



KOPAI FORMATIOIJ (Kembelangan E'xm - A member)