Geology of Eastern Papua

Abstract

Eastern Papua is a southeasterly trending mountainous peninsula with islands to the east and southeast, between 7° to 12° S. and 146° to 155° E. The peninsula and islands consist of a linear core of Mesozoic sialic metamorphic rocks flanked by Mesozoic and younger mafic rocks, and partly concealed by still younger sedimentary and volcanic rocks. The Mesozoic sialic rocks are exposed in a belt 900 km long and up to 60 km wide. They consist of Cretaceous (and possibly older) sediments which were metamorphosed during the Paleocene or Eocene. Metamorphic grade is generally greenschist facies, but is higher in the D9Entrecasteaux Islands and on part of Misima Island; lawsonite occurs mainly within a few kilometers of the Owen Stanley fault which bounds the mainland metamorphics on the north and northeast. Mesozoic mafic rocks include peridotite, gabbro and basalt of the Papuan ultramafic belt, metabasalt exposed in the Suckling-Dayman mountain block and on Normanby and the Deboyne Islands, and unmetamorphosed basalt elsewhere on the mainland. These rocks are, at least in part, Cretaceous. Younger mafic rocks include Eocene basalt on the southeastern mainland, and upper Oligocene and lower and middle Miocene tuff and lava at scattered localities. Eocene sediments consist of chert and deep-water limestone along the south coast of the mainland at Port Moresby and Magarida, and Eocene clastic sediments inland from Port Moresby and near Tapini. No lower or middle Oligocene rocks are known. Upper Oligocene, Miocene and Pliocene sediments in the Aure Trough (145° to 146° E.) have a maximum thickness of about 12,000 m; the sediments are mainly alternating mudstone and graywacke, and are probably turbidites. Middle Miocene reef limestone on the eastern hinge line of the trough at 146° E. is 1,000 m thick. Middle Miocene and younger clastic sediments in the Cape Vogel Basin (150° E.) are at least 4,000 m thick. Pliocene and Quaternary volcanic rocks range in composition from basalt to rhyolite and include some potash-rich rocks. Intrusive rocks include Eocene tonalite in the ultramafic belt, Oligocene? gabbro near Port Moresby, middle Miocene granodiorite west of Salamaua, middle Miocene potash-rich intrusives on the southeastern mainland, Pliocene andesitic porphyries near Wau, and Pliocene granodiorite in the D9Entrecasteaux Islands. The Cretaceous (and possibly older) sialic sediments are thought to have been metamorphosed during the early Eocene at the time of emplacement of the Papuan ultramafic belt. The ultramafic belt is thought to be part of a thrust slice of oceanic mantle and crust which rode over or was underridden by the Cretaceous sediments on a low-angle fault (the Owen Stanley fault). The metamorphic rocks were partly exposed during the middle or late Eocene but there was no major land mass in the area until the upper Oligocene. During the upper Oligocene, Miocene and Pliocene the sialic metamorphic rocks emerged, perhaps isostatically. Erosion of the resulting mountain block and contemporary volcanism and limestone development contributed great volumes of sediment to the Aure Trough and lesser volumes to the Cape Vogel basin. The history of the area can be interpreted in terms of interaction between Australian and Pacific lithospheric plates and opening of the Coral Sea by rifting.