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Appendix I

Texte intégral

Report on the Geology of New Britain, by Robert J. Ryburn 1970 (“draft copy”)

1New Britain and New Ireland have a history of volcanism and tectonism dating back to the Lower Tertiary. Their geology resembles that of the Solomon and New Hebrides Islands and to some extent that of the Huon Peninsula in mainland New Guinea. New Britain has many characteristics of an island arc. It has an elongated arcuate shape, an oceanic trench paralleling the south coast (Planet Deep), a zone of earthquake foci dipping steeply northwards under the island to a depth of over 500km (Denham 1969; Johnson 1970) and a belt of active volcanism along the north coast. New Ireland does not have a closely associated trench and has no active volcanoes although the chain of volcanic islands paralleling the northwest coast probably dates from the quaternary. In many respects, the geology of the two islands is similar. However New Ireland and the Gazelle Peninsula seem to have been modified by Quaternary crustal movements associated with the northwest Solomon Islands trend and now constitute a separate tectonic province from the rest of New Britain.

New Britain

2The geological evolution of New Britain appears to have started in the Eocene with an episode of island arc volcanism. This resulted in the accumulation of a thick pile of volcanic rocks and derived sediments, which now underlie much of the island. These rocks, for which Macnab (1970) has proposed the name “Baining Volcanics”, are exposed in parts of the Gazelle Peninsula and in eastern and central New Britain. Rock types are principally basic to intermediate lava and pyroclastics, volcanic conglomerates, greywacke, silt stone and shale, the proportions of which vary considerably. Eocene foraminifera have been identified from rare limestone lenses and from limestone clasts in conglomerates. The thickness of the sequence cannot be measured, but is undoubtedly greater than 2,000m. The provenance is entirely volcanic and the environment of deposition partly terrestrial, partly marine. Pillow lavas, however, are rare.

3Following their deposition in the Eocene, the Baining Volcanics were subject to folding, faulting, uplift and erosion; younger rocks, the oldest of which are Upper Oligocene, rest with marked unconformity on an irregular erosion surface. Unlike the younger volcanics, the Baining Volcanics are very indurated and extensively jointed. There are signs of incipient metamorphism in many outcrops and in the central Baining Mountains of the Gazelle Peninsula there is a narrow zone of actinolite and biotite schist. Due to the scarcity of bedding exposures, little is known of the style of deformation but, where seen, dips are generally moderate to steep (in some instances overturned). Faulting and shearing are common in many outcrops. Intrusive rocks of basic to intermediate composition are widely distributed in the Baining Volcanics. The larger plutons, up to 15km across, tend to occur along the axis of the island. They are composed of diorite, gabbro, tonalite, granodiorite, adamellite and monazite. Some of the hypabyssal intrusions may have been emplaced before or during deformation of the Baining Volcanics as they show signs of alteration. Most of the larger bodies are relatively fresh and are probably post orogenic but older than overlying Middle Miocene limestones. Dykes, sills and minor intrusions of equivalent but finer grained lithologies are widespread.

4A further period of volcanism in the Upper Oligocene and Lower Miocene is represented by interbedded pyroclastics, tuff, volcanic conglomerate, lava, calcareous sediments and impure limestone, which occur in the Open Bay area and in West New Britain southwest of the Willaumez Peninsula. These rocks are similar with the Baining Volcanics but they are less indurated, less jointed, and are tilted only gently to moderately. They unconformably overlie the Baining Volcanics and are overlain by Middle Miocene limestone. The marine sediments have yielded foraminifera of Upper Oligocene and Lower Miocene ages (“e stage”). In Middle Miocene times, New Britain underwent a period of slow subsidence during which considerable thicknesses of reef limestone were deposited over much of the island. Large areas of cream to white, compact limestone are preserved as cappings in the Whiteman Ranges of West New Britain, the Nakania Mountains north and west of Jacquinot Bay and in the northwestern Gazelle Peninsula. The thickness, which varies considerably, is commonly greater than 300m, but in places it exceeds 1,500m. Extensive sampling for microfauna has shown the bulk of the limestone to be Middle Miocene. Locally the base and top of the limestone extend into the Lower and Upper Miocene respectively.

5Following the Middle Miocene limestone deposition, volcanic activity recommenced in the Upper Miocene. In the Gazelle Peninsula, andesitic and basaltic lava, agglomerate and tuff with horizons of tuffaceous marine sediments containing Upper Miocene to Pliocene microfauna crop out over a large area east of the southern portion of the Baining Fault (Map 3). These rocks are semi-consolidated, poorly jointed and widely disturbed by faulting. West of the Baining Fault, rocks of equivalent age comprising a sequence of up to 500m of poorly consolidated terrestrial and marine tuffaceous sediments of intermediate volcanic derivation dip tingly southwestwards from a height of 1,700m on Mt Sinewit in the Central Baining Mountains. In the Mevlo river headwaters, a basal ash-flow tuff member rests uncomformably on Baining Volcanics. In the Wide Bay-Open Bay isthmus, the sequence overlies “e stage” volcanics and is overlain by Pliocene limestones. In Central New Britain, to the south of Kimbe Bay, occur unconsolidated intermediate to acid tuffs and tuffaceous sediments, which are tentatively correlated with the Upper Tertiary Gazelle Peninsula volcanics. Here the sequence is at least 300m thick and overlies Baining Volcanics or, in places, Middle Miocene limestones. Since deposition these rocks have been uplifted to a maximum of 600m above sea level. Volcanics of Upper Tertiary age (not shown on map) may overlie the “e stage” volcanics on the north coast, west of the Willaumez Peninsula. In the same vicinity, volcanic plugs of probably Upper Tertiary age (shown on map) form the prominent twin peaks of Mt. Penk.

6Much of the present mountainous topography of New Britain is the result of block faulting and regional uplift, which seems to have started in the late Pliocene or early Pleistocene and is probably continuing today. Evidence of uplift is to be seen in the presence of Upper Tertiary marine sediments now exposed at considerable altitudes in Central New Britain and the Gazelle Peninsula. Southeast of Open Bay, soft calcareous sediments of Pliocene or younger age lie on the downthrown northern side of a major fault which bounds the uplifted mountains to the south. The lack of coarse detritus in these sediments shows that the vertical uplift of the southern mountain block has occurred since their deposition. Thick wedges of fanglomerate and alluvium deposited on the downthrown, northeast side of the Wide Bay Fault (see map) were derived from the rising mountain block to the southwest. Continued movement on the fault has tilted these to a maximum angle of 60° degrees northeast in close proximity to the fault and to progressively smaller angles away from the fault. Similarly, tilted conglomerates lie to the northeast of the Baining Fault in the central Gazelle Peninsula. Here the conglomerates grade into finer, terrestrial sediments away from the fault with pumiceous and tuffaneous detritus from the Rabaul volcanoes appearing towards the top of the sequence. Regional uplift of the south coast of New Britain and the east coast of the Gazelle Peninsula has produced series of raised coral reefs in many places. Up to six terrace levels can be recognized in places. The uplift has not been uniform along the length of the coast as terrace altitudes vary laterally. In Jacquinot Bay reef terraces can be recognized up to a height of 500m. In many places, recently formed reefs have been elevated a few metres above sea level, clearly indicating that uplift is still occurring.

7Unlike the south coast, the north coast does not display evidence of Quaternary uplift and may in fact have been sinking. Coral reefs are now forming over large areas off the north coast and it is likely that considerable quantities of volcanic detritus are also being deposited in the same region. At about the same time as the renewal of tectonism in the late Pliocene or early Pleistocene, volcanism broke out once again in New Britain. Quaternary volcanic rocks are found in three areas: at Rabaul, in the east; along the north coast and on Willaumez Peninsula; and at the western end of the island, south of Cape Gloucester. For the most part, these rocks are the products of central-type stratavolcanoes many of whidh are still active (Fisher 1957). Rock compositions range from tholeiitic basalt to rhyolite, and the most abundant lava type appears to be of intermediate composition (53-63 weight percent Si02). These intermediate rocks show similar petrographic and chemical characteristics to “andesites” (sensu lato) described from several island-arc areas in the Western Pacific region. At Rabaul, a caldera open to the sea on its southeastern side exposes lava flows as well as air-falls and ash-flow pyroclastic materials. These deposits were erupted from several centres, many of which are preserved as remnant cones on the caldera rim. Post-caldera volcanoes are also present; two of these, Tavurvur and Vulcan, erupted in 1937 causing the temporary evacuation of Rabaul township (Fisher 1939). Most of the Quaternary volcanoes in New Britain are found in an east-west arc along the north coast and on Willaumez Peninsula. The peninsula is a north-south chain of volcanoes which probably delineates a major fracture at right-angles to the long axis of New Britain. The volcanoes range from small cones only a few tens of metres in height to towering peaks over 2000 m above sea-level. Calderas are present on at least four volcanoes. Ulawun, Lalobau and Pago volcanoes have all erupted this century. Ulawun produced lava flows and nuees ardentes when it last erupted in January 1970 (Davies 1970).

8Most of the Quaternary volcanic rocks at the western end of New Britain are the products of stratovolcanoes Tangi and Talawe, both of which rise over 1500 m above sea-level. There are smaller eruptive centres on the saddle between these two cones, and the crater cluster of Langila has erupted explosively several times this century (for a description of activity between 1852 and 1956, see Taylor, Best and Reynolds 1957). East of Tangi and Talawe volcanoes lie two extensive volcanic centres, Schrader in the west and Andewa in the east. The dissection of both these volcanic complexes is much more advanced than on any of the other Quaternary volcanoes in New Britain, suggesting an early Pleistocene age. It is doubtful if either centre has been active in recent times, and both may have commenced activity at the end of the Pliocene. For convenience, however, Schrader and Andewa are grouped with the rest of the Quaternary volcanoes on the accompanying geological map.

Map 7. Geological Map (prepared by the Bureau of Mineral Resources, Canberra)

Map 7. Geological Map (prepared by the Bureau of Mineral Resources, Canberra)

Source: https://ecat.ga.gov.au/​geonetwork/​srv/​eng/​search#!a05f7892-79e4-7506-e044-00144fdd4fa6

Bibliographie

References

Davies, R.A. 1970. The 1970 eruption of Mt Ulawun, New Britain. Report from Smithsonian Institution Center for Short-Lived Phenomena, May 15th 1970.

Denham, D. 1969. “Distribution of earthquakes in the New-Guinea – Solomon Islands Region”. Journal of Geophysical Research, 74(17): 4290-4299.

Fisher, N. H. 1939. Geology and volcanology of Blanche Bay and the surrounding area, New Britain. Commonwealth Govt Printer: Geological Bulletin (New Guinea Territory) 1, 68 pages.

Fisher, N. H. 1957. Catalogue of the active volcanoes of the World including solfatara fields. Part V. Melanesia. Napoli: International Volcanic Association, 105 pages.

Johnson, R. W. 1970. Seismicity in the Bismarck Volcanic Arc. Commonwealth of Australia (Department of National development), Bureau of Mineral Resources, Geology and Geophysics, Record 70/35, 5 pages.

Macnab, R. P. 1970. The geology of the Gazelle Peninsula T.P.N.G. Commonwealth of Australia (Department of National development), Bureau of Mineral Resources, Geology and Geophysics, Record 70/63, 126 pages.

Taylor, G. A., J. G. Best and M. A. Reynolds 1957. Eruptive activity and associated phenomenon, Langila volcano, New Britain. Commonwealth of Australia (Department of National development), Bureau of Mineral Resources, Geology and Geophysics, Report 26, 53 pages.

Table des illustrations

Titre Map 7. Geological Map (prepared by the Bureau of Mineral Resources, Canberra)
Légende Source: https://ecat.ga.gov.au/​geonetwork/​srv/​eng/​search#!a05f7892-79e4-7506-e044-00144fdd4fa6
URL http://books.openedition.org/pacific/docannexe/image/789/img-1.jpg
Fichier image/jpeg, 252k

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