Benyamin Sapiie

Late Neogene History of the Bird's Head Area, West Papua, Indonesia: An Insight From Detrital Zircon

This study presents a review of new results from the Upper Neogene Formation and no research has been carried out concerning its zircon geochronolgy prior to this project. Late Neogene sedimentation in the Birds Head was characterized by siliciclastic deposition that followed the widespread Early Neogene of the New Guinea Limestone Group. These siliciclastic rocks include the Klasafet, Steenkool, Klasaman Formations, and newly defined Konjah Formation. They are underlain by an extensive carbonate platform, the Kais Formation. In the Birds Head the important sedimentological change from carbonate to siliciclastic deposition occurred in the Middle to Late Miocene when the Kais Formation was conformably overlain by the Klasafet Formation. For the Phanerozoic zircons, there are three important age group; Pliocene (ca. 3 to 5 Ma), Miocene (ca. 12 to 20 Ma), and Permian–Triassic (ca. 205 to 275 Ma). The most abundant group is Pliocene, found only in the Konjah Formation. In the Precambrian, there are three important group ages. These groups are Neoproterozoic (ca. 0.9 to 1.2 Ga), Mesoproterozoic (ca. 1.4 to 1.6 Ga), and Palaeoproterozoic (ca. 1.8 to 2.0 Ga). There is an increase in the abundance of detrital Precambrian zircons from the Middle Miocene to Lower Pliocene formations. The much larger proportion of Precambrian zircons compared to Phanerozoic zircons in the Klasafet and Steenkool Formations indicates a new older source was available, and the older zircons were not simply derived by reworking of older sedimentary rocks. It is suggested that this increase in abundance of Precambrian zircons marks unroofing of the northeast part of the Birds Head, the Kemum High, during the deposition of the Upper Neogene siliciclastic rocks. This caused the Silurian–Devonian Kemum Formation and Upper Carboniferous to Triassic granitic rocks to become available for erosion. There was a second phase of acid igneous activity during the Pliocene in the area which contributed material to the Middle Pliocene Formation. This activity is indicated by Lower to Middle Pliocene zircons found in the Konjah Formation and was never reported previously. All observations support the suggestions there is a Middle Pliocene unconformity in the Birds Head that continues offshore to the Seram area. This unconformity is probably related to the Sorong Fault movement and predates Seram Trough development. The Konjah Formation was deposited above this unconformity in a fluvial setting.