Late Cainozoic origin for the Bintuni Basin and adjacent Lengguru fold belt, Irian Jaya

Abstract

Abstract Unlike the older, highly prospective Salawati Basin of Irian Jaya, the adjacent Bintuni Basin and the complementary Lengguru Fold Belt are young features formed in Late Cainozoic times. They may have been produced by collision between the West Irian Jaya microcontinent and the northwestern margin of the Australian continent, along a shortlived east-facing subduction zone extending along the west side of Sarera Bay north of the newly discovered Recent potassic volcano at Jamur Lake. Deposition of shallow-water carbonates (New Guinea Limestone Group) in Irian Jaya ceased in Middle Miocene times and this limestone forms a basement to the Late Cainozoic clastic sediments filling the highly asymmetrical Bintuni Basin. From outcrop in the western margin of the basin the limestone plunges eastwards to a maximum depth of 4000 m, then again crops out in the Lengguru Fold Belt just east of the deepest point in the basin. The Lengguru Fold Belt comprises Mesozoic clastics and Tertiary limestones of shelf facies and is an accuate belt of asymmetric to overturned en echelon folds which are truncated to the south by the Tarera–Aiduna Fault Zone and overlapped by the Bintuni Basin to the north. The intensity of folding and deformation in the fold belt increases eastwards, and along its eastern margin the folded sediments are largely low-grade metamorphics faulted against the Late Cainozoic gneisses of the Wandamen Peninsula. The large volume of sediment underlying the fold belt is revealed by a large negative gravity anomaly which extends from the northern Bintuni Basin south–eastwards along the eastern margin of the fold belt and not as expected southwestward into the basin itself. This indicates that the deformed sediments may be stacked over a relatively undeformed sequence of similar sediments, and may be separated from them by a major decollement.