A review of the timing of the major tectonic events in the New Guinea Orogen

Journal of Southeast Asian Earth Sciences, Vol. 6, No. 3/4, pp. 307-318, 1991 Printed in Great Britain

0743-9547/91 $3.00 + 0.00 Pergamon Press Ltd

A review of the timing of the major tectonic events in the New Guinea Orogen C. J. PIGRAM and P. A. SYMONDS Bureau Mineral Resources, Australia PO Box 378, Canberra, ACT, 2601 Australia

(Received 4 August 1990; accepted for pubfication 5 May 1991)

Abstract--Three major tectonic events have shaped the gross structural and sedimentary architectural of the New Guinea Orogen in the western Pacific. The Mesozoic tectonic history of the orogen was extensional. Rifting during the Triassic and the Early Jurassic led to the formation of a passive margin along the northern edge of the Australian craton. A siliciclastic sag phase sequence was draped across this margin during the Jurassic and Cretaceous. A second phase of rifting in the Late Cretaceous dismembered the eastern part of the margin and led to the opening of the Coral Sea Basin and another contemporaneous ocean basin to the north during latest Cretaceous to Eocene time. The Early Tertiary sag phase sequence is dominantly carbonate. The third major tectonic phase in the development of the orogen was the initiation of mountain building. Analysis of the foreland basin history of the New Guinea Orogen shows that the flexing of the Australian craton, as a consequence of the emplacement of an allochthonous thrust mass, first occurred in Mid Oligocene time. In the basin this event is also marked by a switch in the direction of clastic sediment supply from the south to the north, a marked increase in sedimentation rates and the development of profoundly diachronous sequences.

INTRODUCTION Tr~ NEW GUINEAOrogen occurs in the southwest Pacific and occupies the island of New Guinea and several small nearby islands (Fig. 1). The orogen is forming as a consequence of an ongoing process whereby a series of forearc, island arc, oceanic and continental terranes have docked and are being thrust over the former northern passive margin of the Australian craton by continuing northward movement of the Australian craton (Pigram and Davies 1987). The orogen, which is up to 400 km across and 3000 km long (comparable in size to the Appalachian Orogen) may be subdivided into three zones: a northern zone of allochthonous terranes (the mobile belt of Dow 1977); a central zone of paraautochthonous strata that form a fold and thrust belt, called the Papuan Foldbelt in Papua New Guinea (PNG); and a southern foreland (the Fly and Arafura Platforms) (Fig. 1). A knowledge of the timing and nature of the major tectonic events in the New Guinea orogen is essential to establishing the tectonic framework of the basins in the region and to understand the major controlling factors in the development of petroleum plays in those basins. The major tectonic events also represent times at which continental blocks or fragments may have been removed from the margin. A knowledge of the history of the New Guinea Orogen is necessary if we are to identify both the source terrane and the times of departure of the many former Gondwana fragments that are now incorporated into southeast Asia. Correctly identifying the timing of major events along this northern margin has wider implications for deciphering the effects of intraplate stress throughout Australian basins where many of the oil fields are trapped in reactivated structures of Tertiary age. This basin reactivation is thought to be related to intraplate stress effects which reflect changes in plate boundary conditions particularly along the northern margin of the Indian Australian plate.

In this paper we review the evidence for, and the timing of, the three major tectonic events that have shaped the gross architecture of the New Guinea Orogen. These major tectonic events are: (1) Early Mesozoic extension leading to the formation of the northern passive margin in Australia; (2) Late Mesozoic extension and strike-slip faulting of the eastern part of the margin leading to the opening of the Coral Sea Basin and the formation of the Eastern and Papuan Plateaus and; (3) Mid Oligocene collision which initiated mountain building along the northern margin of the Australian continent and led to the development of the New Guinea Orogen. This paper does not contain a detailed and comprehensive compilation of the geology of New Guinea. Rather it summarises the current knowledge of the nature and the timing of the principal tectonic events that have shaped the northern margin of the Australian craton. Our intention is to highlight the significant advances in our understanding in the evolution of the margin that have occurred in the last decade and to suggest models and identify problems that can be tested by future research.

EXTENSIONAL TECTONISM/PASSIVE MARGIN FORMATION

Early Mesozoic rifting Early analyses of the New Guinea Orogen in terms of geosynclinal theory recognised the major geological differences between northern and southern parts of New Guinea and that the miogeocline was part of Australian craton (Visser and Hermes 1962, Thompson and Fisher 1967). However, it was not until the late 1970s that any attempt was made to analyse the tectonic and stratigraphic history of the former margin to determine when

307

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Fig. ]. Location and distribution of the New Guinea Orogen. The orogen occupies the island of New Guinea and adjacent small islands. It consists of a northern zone comprising allochthonous terranes and a southern zone of para-autochthonous thrust sheets that form a fold and thrust belt. PFB = Papuan Fold Belt.

it had formed. Hamilton (1979) assumed the sea-floor spreading was initiated during the Middle Jurassic along the entire margin whereas Pigram and Panggabean (1982, 1984) suggested that the timing of the onset of sea-floor spreading was diachronous, starting in the east during the Early Jurassic and propagating westward and then to the south to begin the process of excising Australia from Gondwana (Fig. 2). Both analyses incorrectly placed the eastern limit of the margin at about 145°E thereby excluding that part of the margin formed by the marginal plateaus to the north of the Coral Sea Basin. The history of this part of the margin is discussed in the following section. The analysis of passive margin formation by Pigram and Panggabean (1982, 1984) was based on the model developed from Australian margins by Falvey and Mutter (1981). In this model they suggested that margin evolution followed a sequence of events with each phase marked by a unique association of sedimentation, tectonics and subsidence regimes. In recent years, as the understanding of the structural evolution of passive margins has evolved, and as more information has become available from many passive margins around the world, it has become clear that this simple association is no longer valid. Sea-floor spreading can be delayed well into the sag phase of subsidence and indeed the subsidence history of a margin is dependent on whether the margin is an upper or lower plate margin (Lister et al. 1991). Therefore care needs to be exercised in applying simple models to interpret the history of a margin in an attempt to establish the timing of the onset of sea-floor spreading. In New Guinea it is clear that the northern part of the Australian craton underwent extensional tectonism and rift related volcanism during the Triassic and Early Jurassic. The Middle Jurassic to Early Cretaceous period was tectonically quiet and one in which a classic sag

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Timing of the major tectonicevents in New Guinea phase megasequence comprising marginal marine to deep water clastic sequences (Brown et al. 1980, Home et al. 1990) were deposited. It seems reasonable to assume that the margin was formed by rifting and that as a consequence sea-floor spreading was initiated during the Early Mesozoic. But as the oceanic basin has since been consumed estimates of its age will always be inferred rather than directly dated. In summary, the development of the former passive northern margin of Australia was initiated by Mid Triassic time. A sag phase sedimentary sequence was deposited during Middle Jurassic to Cretaceous time implying that sea-floor spreading was also initiated during this period, probably by Middle or Late Jurassic time. Several major problems relating to this aspect of margin development need to be resolved. These include determining the nature of the extensional tectonism. Was the margin an upper or lower plate margin? Did the tectonic style change along strike? Was rifting and the onset of sea-floor spreading truly diachronous along the margin? What control do the Early Mesozoic structures exert on the later compressional structures related to the collisional events? Although these problems are being addressed, the lack of data from eastern Irian Jaya remains a major limitation in any analysis of the region. Pigram and Panggabean (1984) suggested that the presence of a Mesozoic "rift drift" sequence on many of the microcontinents in eastern Indonesia was evidence that they had been detached from the northern margin of Australia during the Early Mesozoic. However, the presence of a similar stratigraphic history does not provide clear evidence of when the fragments were removed from the margin. The Early Mesozoic oceanic basin that formed to the north of the margin was the far eastern part of the Tethys Ocean (Pigram and Panggabean 1984). Continental fragments detached from the margin during the Early Mesozoic would have ended up on the northern side of the Tethyan Ocean. With the consumption of this ocean beneath the Laurasian margin it would seem most likely that continental fragments would have been accreted to the Laurasian landmass. For this reason we no longer believe that the microcontinents now found in eastern Indonesia represent the continental fragments from the northern margin of Australia during the Early Mesozoic. L a t e M e s o z o i c rifting and s t r i k e - s l i p f a u l t i n g

In the eastern Papuan Basin of PNG there is a major unconformity which separates Tertiary and Mesozoic sediments (Brown et al. 1980, Home et al. 1990). In places all the Cretaceous and part of the Jurassic section was removed. Following the drilling of three holes in the Coral Sea Basin by the Deep Sea Drilling Program (Andrews et al. 1975), Weissel and Watts (1979) were able to show that the basin was floored by oceanic crust that had formed by sea-floor spreading during the Paleocene to Middle Eocene. This age of sea-floor spreading implied that extensional tectonism must have occurred in the region during the Cretaceous. Geologists

309

working in the Papuan Basin then assumed that the extensional tectonism that led to sea-floor spreading in the Coral Sea Basin was also responsible for the late Mesozoic unconformity in the Papuan Basin (Brown et al. 1980). The kinematics of the relationship between the two has rarely been investigated. With the identification of fan shaped magnetic anomalies in the Coral Sea Basin (Weissel and Watts 1979) it became apparent that opening had occurred about a pole of rotation located in the Papuan Basin to the west. This implied that the western plate boundary during the development of the Coral Sea Basin was a sinistral strike-slip fault system. Taylor and Falvey (1977) addressed this problem and suggested that a strike-slip fault was located between the Eastern and Papuan Plateaus and passed into the Central Highlands via the Moresby and Aure Troughs. They linked this fault zone to the Lagaip Fault system which was then thought to be active at this time (Dow et al. 1972). Subsequent work has shown that the Lagaip Fault is a Late Cenozoic thrust system (Rogerson et al. 1987b) and that the Moresby and Aure Troughs did not exist in the Late Cretaceous and Early Tertiary (Pigram and Symonds 1988). Symonds et al. (1984) pointed out that the Coral Sea Basin was surrounded by continental crust so that the New Guinea Orogen in eastern Papua New Guinea is separated from the Paleogene oceanic crust of the Coral Sea Basin by a prolongation of thinned continental crust that forms the Eastern and Papuan Plateaus (Fig. 3). These plateaus had undergone considerable crustal thinning by extensional tectonism during the Cretaceous (Symonds et al. 1984). The recognition of Late Cretaceous extensional structures on the Eastern Plateau which is the most westerly of the plateaus was important because it showed that the extensional tectonism that led to the formation of the Coral Sea Basin extended much further west than had been assumed by Taylor and Falvey (1977). Furthermore investigations of the western Coral Sea by a joint Bundesanstalt ffir Geowissenshaften und Rohstoffe (BGR)/Australian Bureau Mineral Resources (BMR) program using the R. V. Sonne showed the Osprey Embayment to be highly extended continental crust and that locally, sea-floor spreading may have occurred (Symonds et al. 1984). This data is significant because it shows that the plate boundary, that was active during the extensional regime, had to be to the west of the Eastern Plateau. Furthermore, it also shows that the plate boundary had to be a left lateral or sinistral strike slip system because of the northward movement of the Eastern Plateau relative to the Australian Craton. Positioning the plate boundary this far west at the eastern margin of the Papuan Basin provides a mechanism for the formation of the Late Mesozoic unconformity in the eastern Papuan Basin. The most likely position for this structural system was through the Bligh and Pandora Troughs (Figs 3, 4). In 1985 the BMR carried out an investigation of this region using the R. V. Rig Seismic. The results of this work (Pigram and Symonds 1986) demonstrated that the Bligh and Pandora Troughs

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Fig. 3. Major structural elementsof the western Coral Sea region. The Pandora and MoresbyTroughs are Late Cainozoic foreland basins. The Pandora and Bligh Troughs were originallytranstensional basins formed by strike slip faulting during the Late Cretaceousto Paleocene.The Osprey Embaymentis an area of highly extendedcontinentalcrust with local areas of volcanicsthat may be oceanicbasalts. The deformationfront marks the present day limit of thrusting along the northern edge of the Moresby and Pocklington troughs. AC = Anchor Cay 1 well. were initiated during the Late Mesozoic by transtensional faulting. The Pandora Trough was subsequently reactivated by Mid Tertiary convergent tectonism. The Late Mesozoic to Early Tertiary configuration of the plate boundaries in the eastern part of northern Australia are shown schematically in Fig. 4. In this reconstruction, margin extension during the Cretaceous and Paleocene has formed the Queensland, Eastern and Papuan Plateaus. The Eastern and Papuan Plateaus have been detached from the Queensland Plateau by sea-floor spreading in the Coral Sea Basin and possibly in the Osprey Embayment. The western plate boundary to this system is shown as a north trending sinistral strike fault system located along the western side of the Eastern Plateau. It was uplift along this boundary that led to the extensive erosion of the Mesozoic section in the eastern Papuan Basin. Conjugate W N W trending dextral faults are also shown on the Fly Platform. These faults are poorly known and their history of movements remains to be investigated as more subsurface data becomes available onshore. They are shown on Fig. 4 to indicate that if they were active at this time then they had to be dextral wrench systems. In Fig. 4 we also postulate the presence of another contemporaneous spreading centre to the north of the

Eastern and Papuan Plateaus. The reason for this is discussed in the following paragraph. Late Cretaceous to Eocene tholeiitic basaltic rocks and ultramafics of ocean floor character occur along the southern side and in the eastern part of the Papuan Peninsula (Davies and Smith 1971, Smith and Davies 1976). The rocks of this association are now known to extend along the eastern side of the Aure Trough as far north as the Markham Valley (Rogerson and Hilyard 1990). Therefore these fragments of oceanic crust that now occur to the north of the plateaus, must have formed in an oceanic basin that was opening at the same time as the sea-floor spreading was occurring in the Coral Sea Basin, but separated from it by the Eastern and Papuan Plateaus as shown schematically in Fig. 4. The consumption of this oceanic crust along a northward dipping subduction during the Tertiary led to the emplacement of these fragments of oceanic crust into a forearc assemblage that now forms the southern and western sides of the East Papuan Composite terrane (EPCT) (Pigram and Davies 1987). Ultimately the closure of this basin led to the collision of the EPCT with the northern side of the Eastern and Papuan Plateau (Pigram and Davies 1987). The deformation front of this collision extends from the western side of the Aure

Timing of the major tectonic events in New Guinea

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Fig. 4. Schematic map of the plate boundary configuration in Eastern New Guinea during the latest Cretaceous and Early Tertiary. The sinistral strike--slip fault along the western side of the Eastern Plateau carried the plateau to the north as the Coral Sea Basin opened. Transtensional movement along this fault created the proto-Pandora Trough. The evidence for the oceanic basin to the north of the Eastern and Papuan Plateaus is discussed in the text. The left lateral oblique spreading centre along the northern margin of the craton may provide a mechanism for detaching and moving microcontinents westward (see text for discussion).

Trough along the northern side of the Moresby Trough (which is the foredeep of the foreland basin formed on the plateaus by the collision) and Papuan Plateau to the Pocklington Trough (Fig. 3). Figure 4 also shows an oblique sea-floor spreading system along the northern edge of the Australian craton and a series of microcontinents being detached and transported westward. This Late Mesozoic to Early Tertiary tectonism along the eastern part of the northern Australian margin is important in relation to the problem of the origin of the microcontinents in Eastern Indonesia because it represents a further opportunity for removing continental fragments from the Australian margin. Figure 4 is an attempt to show how this may have occurred and provides a mechanism for both removing and transporting the fragments westward relative to the Australian margin. Such a mechanism related to the opening of the Coral Sea Basin is appealing in that spreading in that basin ceased in the Middle Eocene (as it did along the entire eastern side of Australia) (Weissel and Watts 1979). If we speculate that the entire plate boundary system, as portrayed in Fig. 4 also ceased operating at this time, then the distance that the microcontinents could have been moved from the margin is limited, thus keeping the microcontinents in relatively close proximity to the Australian margin. This speculation about the possible detachment of some of the microcontinents during Late Mesozoic is

supported by the palaeomagnetic results from Misool (Wensink 1987, 1990, Thrupp et al. 1988) which show that the Misool terrane was on or near the Australian margin but undergoing rotation during the Cretaceous. Furthermore the Cretaceous stratigraphy of Misool shows a marked change in depositional environments from Mid Cretaceous bathyal marine carbonates (Fageo Group) to shallow water marine clastics of the Fafanlap Formation in the Late Cretaceous (Pigram et al. 1982). This change, indicative of both uplift and the creation of a new source area in the Misool region, when combined with the palaeomagnetic results suggests that the terrane was tectonically active during the Cretaceous. This teetonism may have been related to the detachment of the Misool terrane from the Australian margin. The Late Mesozoic unconformity on the Sula Platform which marks a change from bathyal conditions to erosion followed by shallow water deposition (Pigram et al. 1984, 1985, Garrard et al. 1988) may have similar significance. Most of the ideas presented in this section are as yet untested. Research needs to be directed at determining the structural style and movement history of the Cretaceous to Early Tertiary plate boundary in the western Coral Sea. The determination of the movement history of the microcontinents in eastern Indonesia remains an outstanding problem, although one that is being worked on.

312

c . J . PIGRAM and P. A. Sw,lom~s

COLLISIONAL MARGIN FORMATION From the days of Wegener (1929) the New Guinea Orogen has been recognised as a product of convergent tectonism along the northern edge of the Australian craton. With the advent of plate tectonics the orogen was viewed as the product of an arc/continent collision and was used by Dewey and Bird (1972) as their type area for such processes. It was argued that the collision of the craton with the island-arc complex led to the formation of the orogen by the emplacement of forearc and arc material and the telescoping of the craton margin and sediment cover. The collision was generally seen as a synchronous event along the margin east of Sarera (Cendrawasih) Bay (Visser and Hermes 1962, Thompson 1967, Davies 1971, Davies and Smith 1971, Page 1971, 1976, Dow 1977, Jaques and Robinson 1977, Hamilton 1979). However, Davies (1982) and Kroenke (1984) argued that the orogen developed as a consequence of a series of arc collisions while Pigram and Davies (1987) pointed out that the evolution of the orogen was complex and involved the episodic accretion of continental and oceanic fragments as well as island-arc complexes. The timing of the initiation of mountain building that led to the formation of the New Guinea Orogen has been the subject of considerable speculation. Suggested ages for the initiation of collision range from Eocene to Late Miocene--a span of 30 Ma (Visser and Hermes 1962, Thompson 1967, Davies 1971, Davies and Smith 1971, Page 1971, 1976, Dow 1977, Jaques and Robinson 1977, Hamilton 1979, Kroenke 1984, Pigram and Davies 1987, Rogerson et al. 1987a, Hilyard et al. 1988, Francis 1990, Pigram et al. 1990). In this paper the initiation of mountain building is taken as the time when the Australian craton first felt the effect of the emplacement of a thrust mass along its northern margin. Most attempts to identify the initiation of collisional processes are based on work carried out within the orogen and consequently run the risk of not identifying the oldest event because of overprinting caused by younger tectonic events or incorrectly attributing some older tectonic events to initiation of orogenesis when in fact they did not involve the Australian craton. As Pigram and Davies (1987) pointed out several of the terranes that docked with the Australian margin were composite and had amalgamated prior to collision with the northern margin of Australia. Consequently, there are records of deformation events preserved in the orogen, that are a product of the amalgamation of terranes far from the Australian margin. These events do not therefore mark the beginning of the compressional deformation of the northern margin of Australia. Perhaps the best example is the Eocene deformation associated with the amalgamation of the East Papuan Composite Terrane prior to its docking with the Australian margin in the Miocene (Pigram and Davies 1987). The most reliable record of the initiation of collisional processes in an orogenic belt is contained in the adjacent

foreland basin. Foreland basins form as a result of continental margin loading by the emplacement of a thrust m a s s , a s a consequence of compressional tectonics (Price 1973, Dickinson 1974, Beaumont 1981, Jordan 1981). The resulting flexure caused by loading creates an asymmetric basin bounded on the cratonic side by a peripheral forebulge (Fig. 5). The flexural history of the basin is controlled by migration of the thrust mass across the passive margin onto the craton. Both the foreland basin and the forebulge migrate toward the craton in response to the encroaching thrust mass. The deepest part of the foreland basin, the proximal foredeep, occurs adjacent to the thrust mass (Price 1973, Dickinson 1974, Beaumont 1981, Jordan 1981, Tankard 1986). Deposition in a foreland basin is initially characterised by an underfilled phase, where the basin is able to accommodate all the detritus shed from the emerging mountains (the thrust mass). During the early part of the underfilled phase the thrust mass lies largely below sealevel, loading the thin outer edge of the continental margin and producing a deep, starved, narrow basin in which terrigenous muds and pelagic sediments are deposited in the proximal foredeep. During this stage the peripheral forebulge may raise the former passive margin shelf above sea-level, so that the passive margin shelf sequence is separated from the foreland basin sequence by an unconformity (Jacobi 1981, Stockmat et al. 1986).

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Fig. 5. Cartoon illustrating the development of a foreland basin through time. Note that the margin flexes in response to the emplacement of a thrust load and that the basin broadens with time in response to the loading of thicker continental crust.

Timing of the major tectonic events in New Guinea

As the thrust mass migrates across the former margin, it grows by incorporating both the basement and the passive margin sediments, gradually emerges and begins to shed large volumes of detritus into the proximal foredeep. Deposition in the proximal part of the foreland basin is dominated by material derived from the thrust mass. In contrast the distal basin, adjacent to the peripheral forebulge, may be shallow and distant from major source areas and thus receive little terrigenous irput. Accordingly these areas constitute potential sites for carbonate sedimentation and the initiation of platform development. If carbonate sedimentation is initiated in a foreland basin, deposition will continue until near the end of the underfilled phase, after which the sediments will be buried by the overflow of terrigenous detritus from proximal to distal parts of the basin as the basin passes into the overfilled phase (Pigram et al. 1989). The subsequent history of the basin will be dominantly non-marine, although there may be periods of minor deepening and possible shallow marine deposition resulting from load relaxation during tectonically quiet periods (Tankard 1986). In summary foreland basin development and hence the initiation of collision, will be marked by:

313

Foreland basin development in N e w Guinea

Studies of the development of the foreland basin in New Guinea have been confined to PNG and western Irian Jaya. There has been no published modern study of the basin in eastern Irian Jaya although some broad conclusions can be drawn from the information in Visser and Hermes (1962). The terrane analysis of Pigram and Davies (1987) shows that the first docking event occurred in the PNG sector of the orogen so the discussion below focuses on the Cenozoic history of the Papuan Basin which records the transition of the margin from passive to collisional. The Mesozoic sediments of the Papuan Basin were deposited across a passive margin that had been affected by rifting episodes during the Early and Late Mesozoic (as discussed above). Following separation from Antarctica in the Cretaceous (Weissel and Hayes 1972, Cande and Mutter 1982), rapid northward movement of the Australian continent from the Eocene carried the Papuan Basin from temperate, through subtropical, to tropical climatic zones (Davies et al. 1987, Feary et al. 1991). The Cainozoic geological history of the Papuan Basin has been discussed by Pigram et al. (1990) and Francis (1990) and is briefly summarised below and in Figs 6 and 7.

(i) flexing of the margin which will be characterised by a time transgressive increase in the subsidence of the margin and a marked thickening in the sedimentary column; (ii) a marked change in provenance as sediment are derived from the emerging orogen for the first time; (iii) the onset of diachronous deposition in the foreland basin; and (iv) an areally restricted unconformity across the site of the former passive margin shelf (Jacobi 1981).

Middle Eocene to Early Oligocene

During the Middle Eocene to Early Oligocene, up to 500 m of temperate, shallow water, carbonate sediments (Mendi Group; Fig. 6) were deposited on the shelf and slope, which was up to 250 km across (Fig. 7A). Pelagic sediments were deposited on the rise and abyssal plain (Port Moresby Association: Fig. 7A).

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Following a rise in sea-level during the Late Miocene, clastic deposition encroached further southward, burying most of the carbonate platform. Renewed carbonate sedimentation following the sea-level rise was restricted to the most southerly part of the basin (Fig. 7C) (unnamed limestone in Fig. 6). Late Pliocene and Quaternary

Rapid southward migration of clastic deposition resuited in burial of the entire carbonate platform beneath shallow marine and fluvioclastic sediments of the Fly Platform and Gulf of Papua (Era Beds; Fig. 6). Carbonate sedimentation ceased in the foreland basin, but continued on the northeast Australian shelf to the south \ * |?--::~?,?:~i~:::where the Great Barrier Reef was flourishing (Davies + /| : . . . . . et al. 1987, 1989). n o ~ h e r n Great Barrier Reef Previous attempts to recognise foreland basin facies in Section New Guinea have been based on the expectation that it ~4L sec,,ooo-0' ~-~ would be a clastic depocentre and that the limestones of 23/09/142 the Papuan Basin were part of the passive margin ~ Clasticsediments sequence (Brown and Robinson 1982) or were deposited Carbonate in a backarc environment (Home et al. 1990). A major sediments change in basin sedimentation which occurred after 0 250 km Mid-Oligocene time has been interpreted by Pigram et Fig. 7. Generalised Tertiary palaeogeographyof the Papuan Basin, al. (1989, 1990) as signifying the start of a foreland basin PNG. Present day coastline is shown for reference(after Pigram et al. development in New Guinea. The recognition that 1990). the Late Oligocene to Early Miocene Darai Limestone platform was deposited in foreland basin (Pigram et al. Middle Oligocene 1989, 1990) was the critical breakthrough in establishing Latest Early and Middle Oligocene sediments have the time of initiation of orogenesis in the New Guinea not been identified within the region; an unconformity Orogen. To further test this interpretation we have investigated separates the Mesozoic to Early Oligocene shelf the flexural history of the margin using geohistory deposits from the overlying Late Oligocene and younger analysis (Van Hinte 1978, Falvey and Deighton 1982) to sediments (Fig. 6). obtain basement subsidence curves along a transect perpendicular to the orogen. Figure 8 shows the baseLate Oligocene to Middle Miocene ment subsidence curves for three wells in the offshore During the Late Oligocene to earliest Miocene, car- eastern Papuan Basin. Uramu 1 is located close to the bonate sedimentation was re-established in the southern fold and thrust belt while Anchor Cay 1 is located in a part of the basin (Darai Limestone; Fig. 6). Initially a distal foreland position. The wells show progressive condensed section of pelagic carbonates and shale (e.g. onset of accelerated subsidence from north to south Kera Formation; Fig. 6) were deposited to the north but beginning in the Late Oligocene at Uramu 1, in the Late a rapid influx of clastic sediment derived from the north Miocene in Kusa 1 and not until the Mid-Pliocene in soon followed (Aure Group; Fig. 6) (Brown et al. 1975). Anchor Cay 1. The exact time at which the basement at By the Early Miocene (Fig. 7B), an extensive tropical, Uramu 1 and Kusa 1 began to subside more rapidly is rimmed, carbonate platform up to 500 km across had not clear because of missing section. Uramu 1, while developed (Darai and Nipa Group; Fig. 6) and coexisted being the most northerly of the wells examined, is still a considerable distance from the former edge of the with the clastic sediments to the north. passive margin. To make an estimate of just how far from the margin depends on assumptions about how Middle and early Late Miocene much telescoping of the former margin has occurred. A A major sea-level fall spanning the Middle to early figure in the order of 100-150km may be possible Late Miocene (Haq et al. 1987) terminated carbonate (Hobson 1986) implying that the Uramu 1 well may not 7°--

°6f

Shelft m Reef Slope

~

.

i

Emergent

Timing of the major tectonic events in New Guinea 160

Tim Ilhi

120

I

I

I

I

,

O0 I

315 40

4

f

0 I

I

mu

I

0

I -

=

.

-

Anchor

~-4

1-=

il

i

1

Urlmu 1 '~

i /

I

Ira-

I I

Extension

Sag

Foreland basin

Fig. 8. Basement subsidence curves for three wells in the Papuan Basin which form a transect perpendicular to the orogenic belt. Each well shows an accelerated subsidence phase and the onset of this phase is younger to the south away for the orogen.

show evidence of the earliest phase of flexuring. Furthermore the times of onset of rapid subsidence in both wells are masked by unconformities which may be related to major sea-level falls• Both the Mid-Oligocene and Middle to early Late Miocene were times of major sea-level falls (Haq et al. 1987). Faster subsidence at Kusa may have begun as early as Middle Miocene time. If this were so then the onset of faster subsidence across the foreland would show a more linear progression to the south• The time transgressive nature of the flexuring is consistent with it being caused by loading and suggests that the collision that formed the orogen had been initiated by Late Oligocene time. Foreland basin

1500

1250

I000

~

750

Passive margin

500

A v e r a g e rate 125 m/my 250

80

50

40

30 Time (million years)

20

1'0

0 23/0a/146

Fig. 9. Comparison of sedimentation rates for the carbonate deposition during passive margin and foreland basin phases of the Papuan Basin• The much greater rates of sedimentation during the foreland basin phase are a consequence of the additional space created by the flexing of the margin and the tropical carbonates high productivity.

A further indication that the margin was being flexed can be seen in the relative sedimentation rates for the two carbonate units in the basin. The extra space created by the flexuring is recorded in the marked increase in sedimentation rates during the foreland basin phase. Figure 9 compares the sedimentation rates for the Eocene-Early Oligocene Mendi Group with that of the Late Oligocene-Early Miocene Limestone and shows the marked increase in sedimentation rates. The initial phases of basin deposition are recorded in the pelagic condensed sequence of the Late Oligocene Kera Formation (Davies 1983). The marked change in provenance that characterises the emergence of the overthrust mass is shown by the influx of Late Oligocene--Miocene Aure Group sediments (Fig. 6) which were derived from the north and northeast (Brown et al. 1975)• These sediments, which were deposited in the proximal foredeep of the foreland basin, are over 3000 m thick, shallow upwards and eventually bury the carbonate platform of the Darai Limestone. The fourth criteria of an unconformity separating the former passive margin shelf sediments and the foreland basin section is also present• The youngest age from the Mendi Group is Tc while the oldest age in the Darai Limestone and Aure Group sediments is Te. Td sediments have not been found and appear to be entirely absent. This type of unconformity is attributed to the passage of peripheral forebulge across the former shelf region (Jacobi 1981, Stockmal et al. 1986). However, in New Guinea this Mid-Oligocene unconformity formed at the time of a major fall in sea-level (Haq et al. 1987) so the unconformity may in fact be the product of several events working in concert. In summary the major criteria for identifying the onset of foreland basin development--increased subsidence due to flexing and a change in provenance--suggest that the initiation of orogenesis in PNG occurred in the Mid Oligocene at about 30 Ma ago. The subsequent development of the orogen was complex and involved the addition of numerous allochthonous terranes (Pigram and Davies 1987) (Table 1).

C. J. PIORAM and P. A. SYMONDS

316

Table 1. TECTONIC

AGE

EVENT

Q P

<5

NEW GUINEA OROGEN

MAJOR TERRANE <4 M

<3

DOCKING EVENTS

and

FORELAND

>-

<2

BASIN FORMATION

.<

-

o

I-

30my < C O L L I S I O N

COMMENTS

Seafloor spreading in the Bismarck Sea 5 = Docking of the Seram Terrane Seafloor spreading initiated in the Woodtark Basin 4 = Docking of northern Island Arc terranes 3 = Docking of Western Irian Jaya Composite terrane ?Seafloor spreading in the Solomon Sea 2 = Docking of East Papuan Composite terrane Initiation of collisional margin with the accretion of the South Sepik Composite Terrane

=,

Caroline Plate seafloor spreading (mag An 12-10) uJ

E

55 my

SAG P •

.

NORTHERN AUSTRALIAN

•

65

my

....

PASSIVE

RIFTING CRET

?100 my

---

SAG

JUR TRIAS

Seafloor spreading in the Coral Sea Basin (mag. anomalies 27-24) and region to the north of the Eastern and Papuan Plateaus. Northern oceanic basin consumed during ?Late Eocene to ?early Miocene time

?200 my

MARGIN FORMATION

Rifting confined to the eastern part of the northern Australian margin. Formation of Eastern and Papuan Plateaus, Osprey Embayment, proto-Bligh and Pandora Troughs Seafloor spreading to north of Australia to form eastern Tethys ocean which has since been consumed, probably during the late Mesozoic and early Tertiary

....

RIFTING PERM

Breakup of northern Gondwana

Dating and identifying the style of major tectonics events that have shaped the development of the New The major events that have controlled the develop- Guinea Orogen while obviously important in underment of the gross architecture of New Guinea are standing the evolution of the Orogen, also has wider summarised in Table 1. These events are: implications because the northern margin of Australia is (1) Early Mesozoic rifting and formation of the north- thought to be the source area for many of the microern margin of the Australian craton. Sea-floor spreading continents in Eastern Indonesia. Traditionally there have been two favoured mechanto the north of the margin during the Mesozoic created isms for the derivation of the microcontinents of eastern an eastern extension of the Tethyan Seaway but this Indonesia from the northern margin of Australia. The oceanic floor was subsequently consumed, probably suggestion that microcontinents where detached during during the Late Mesozoic and Palaeogene. early Mesozoic rifting (Pigram and Panggabean 1984) (2) Late Cretaceous rifting of the eastern part of the northern margin of Australia. This tectonism which led seems unlikely because the microcontinents would have to the opening of the Coral Sea Basin and an unnamed been lost across Tethys and accreted to the Laurasian basin to the north. The western boundary of this tecton- continent. The other mechanism, detachment of the ism was a sinistral strike-slip fault system that extended fragments by strike-slip faulting, during the developfrom the Osprey Embayment northward through the ment of the New Guinea Orogen (Hamilton 1979) proto-Bligh, Pandora and Aure Troughs. The nature of remains a possibility. However, the docking of terranes this plate boundary further north beyond the margin of along the margin to create the orogen must also have the the continent is not known. However, it seems likely that effect of locking in cratonic fragments. Therefore mait may have linked up with the active spreading centre terial removed from the region during collision would be of another marginal basin that was, at least in part, more likely to consist of island arc related or oceanic contemporaneous with, but separated from the Coral assemblages derived from the allochthonous terranes. Sea Basin, by the continental prolongation that now The third possibility suggested here, of an oblique plate boundary along the northern margin of Australia during forms the Eastern and Papuan Plateaus. (3) The final major event to shape the margin was the the Late Cretaceous appears to have several advantages. initiation of mountain building in Mid Oligocene time. It provides a mechanism to both detach and displace the The emplacement of forearc material across the former fragments westward while keeping them in reasonable passive margin initially bowed up the former shelf areas proximity to the margin. The global plate boundary of the margin during Td time. With the passage of the reorganisation during the Eocene may have then led to peripheral forebulge the flexing of the margin produced the consumption of the Cretaceous and Early Tertiary a foreland basin. The marked change in sedimentary ocean basin created at that time thereby bringing the style and facies linked to the flexure of the margin point Australian margin and the fragments into close proximto a Mid Oligocene (about 30 Ma) start for the develop- ity again. This idea needs to be tested with detailed stratigraphic and facies studies of the sequence on the ment of the New Guinea Orogen. CONCLUSIONS

Timing of the major tectonic events in New Guinea

microcontinents to establish the timing of events such as a marked change in depositional style or uplift. Careful and detailed palaeomagnetic analysis is also essential. Acknowledgements--Published with permission of the Director of the

Bureau of Mineral Resources. Brian Taylor, John Milsom and Gary Nichols are thanked for their constructive reviews.

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