Geology of New Ireland, Papua New Guinea

New Ireland is a narrow island 360 km from northwest to southeast and up to 48 km wide. The broader southern part of the island is steeply mountainous, with peaks to 2400 m above sea level in the Hans Meyer Range, and is diagonally bisected by the Kamdaru and Weitin valleys. The central and northwestern parts of the island are extensively capped by limestone plateaux which are tilted to the north-northeast. The plateau in the northwest is known as the Schleinitz Range, and the higher plateau in the central region is the Lelet Plateau. A low saddle separates the Lelet Plateau from the southern mountains. Outcrop is generally poor and extensively weathered beneath the dense primary rainforest that blankets the island, though entrenched river gorges provide some good, though relatively inaccessible, sections. The oldest rocks are the lower to middle (or lower upper) Oligocene Jaulu Volcanics. These consist of lapilli tuff, agglomerate, and subordinate porphyritic pyroxene andesite lava, and are intruded by gabbro, norite, diorite, tonalite, trondhjemite, granodiorite, and leucocratic dyke rocks, which have been named the Lemau Intrusive Complex. Some or all of these intrusives may be related to the Jaulu Volcanics; K/Ar ages are 31.8Â} 1.0 m.y., 17.5 Â} 0.6 m.y., and 13.8 Â} 0.5 m.y. The Jaulu Volcanics and Lemau Intrusive Complex are best exposed in southern New Ireland, where erosion has been deepest. Elsewhere they are exposed only along the southwestern fall of the ranges, where the limestone plateaux have been removed by erosion. The upper lower Miocene Lossuk River Beds are a thin series of clastic sedimentary rocks derived from the Jaulu Volcanics, which they unconformably overlie, and are found only in the northwest. The main limestone units are the Lelet Limestone, which forms the plateaux in the centre and northwest, and the Surker Limestone in the south. The two were probably partly lateral equivalents, but the Lelet Limestone has a longer range (lower Miocene to Pliocene or Pleistocene, compared with lower to middle Miocene). The narrow neck of land between the outcrop areas of Lelet Limestone in the northwest and Surker Limestone in the southeast is largely occupied by uppermost Miocene volcaniclastic (partly turbidite) and biogenic ooze sediments of the Rataman Formation, which are probably deep-water contemporaries of higher beds of the Lelet Limestone. The white, chalky Punam Limestone unconformably overlies the Rataman Formation along a narrow strip of the foothills near the northeastern coast of south-central New Ireland. It is Pliocene or younger. Embayments in the Punam Limestone are filled with Plio-Pleistocene sediments of the Uluputur Beds, which comprise intraformational conglomerate, lithic sandstone, and siltstone. A thick succession of fanglomerate and beach sands, in places cemented to conglomerate and sandstone, flanks eastern and western coasts of the southern, mountainous part of the island. These sediments, named the Maton Conglomerate, unconformably overlie the Jaulu Volcanics, from which they were derived, and also overlie in places the Rataman Formation and Surker and Punam Limestones. The conglomerate is overlain only by Pleistocene to Holocene coral terraces. The Weitin and Sapom Faults are the major structural features on New Ireland. Movement on the Weitin Fault may be left-lateral. New Ireland developed presumably on oceanic crust by seafloor (and possibly subaerial) volcanism in the early and middle Oligocene. A landmass emerged in the early Miocene, when the Lossuk River Beds were deposited, and then probably subsided steadily throughout the Miocene and early Pliocene and was rapidly uplifted in the late Pliocene and Quaternary, when a flight of terraces on the northeastern fall of the Lelet Plateau was probably cut by wave-erosion. Left-lateral faulting and crustal extension (?transform faulting) in the late Miocene may have opened a rift between central and southern New Ireland, in whic

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Organization: Geoscience Australia Data